The application of solar energy for large-scale seawater desalination

Desalination,

115

89 (1992) 115- 184

Elsevier Science Publishers B.V., Amsterdam

The application of solar energy for large-scale seawater desalination* Daniel Hoffman ADAN Technical and Economic Services, Ltd., P. 0. Box 18294, Tel Aviv (Israel) (Received April 16, 1992)

CONTENTS

116 116 119 125

142

167

172

180 182 182

Summary Introduction Executive Summary Applicable Seawater Desalination Technologies 125 Overview I28 Process descriptions 130 Technical and economic considerations in designing thermal desalination plants 134 Current technology - performance and economics Applicable Solar Collection Technologies 142 Overview 145 Systems descriptions 152 Current technology - performance and economics Matching Desalination and Solar Collection Systems 168 Siting considerations 169 Energy transport considerations 170 Energy storage and back-up supply considerations 171 Optimal temperature Optimal Solar Desalination Systems and Economics 172 Optimal solar collection and desalination systems 174 Expected economics Conclusions Abbreviations Bibliography

*Under Contract No. 75.05.02.01 Engineering Division. 001 l-9164/92/$05.00

of the Israel Ministry

of Energy and Infrastructure,

Q 1992 Elsevier Science Publishers B.V.

All rights reserved.

116 SUMMARY

Seawater desalination and solar energy collection systems suitable for large-scale regional water supply projects are described and their economics analyzed. Four potential combinations of these systems, within 100,000 m3/d plants, are discussed and compared. The combination of solar pond and low temperature multi-effect distillation (LT-MED) provides the lowest solar desalinated water costs. These range from 67-88c, depending on interest rate, local insolation, actual pond cost and performance, and are competitive with lowest cost desalinated water from a fossil fueled plant, an LT-MED within an electric power cogeneration scheme, when heavy fuel (No. 6) prices are above $150-220/tan.

INTRODUCTION

Ever since the early 195Os, water planners in Israel have realized that it was only a matter of time before water demand would exceed conventional water supply sources and that water shortages would limit the country’s development. The need to turn, sooner or later, to non-conventional water supply sources, including brackish and, eventually, seawater desalination, was clear. The development of economically viable desalination processes was, therefore, given a high national priority. A strong governmentsupported research and development (R&D) effort resulted in the engineering of novel seawater distillation processes, which operated reliably and efficiently with a variety of energy sources. Plants based on these processes were sold commercially worldwide and gained Israeli desalination industry high credits and a well earned reputation. The development of solar energy, and renewable energy sources in general, was, likewise, a top priority item in Israel. Israel imports its tota fossil fuel requirements. Even before the 1973 fuel crisis, Israeli government support of solar energy R&D was fairly extensive. The pioneering efforts of Dr. Harry Tabor on flat plate collectors and solar ponds and the work of Prof. Israel Dostrovsky on solar towers are well known throughout the world. The State’s recent concern with environmental issues, particularly with increasing levels of air pollution, has given solar energy research fresh impetus. Over the past few years, the national and, in fact, the entire regional water supply situation has deteriorated sharply due to the accumulated effect

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of a continuous five-year drought. This has led to a reexamination of radical solutions, including large-scale desalination. Concrete regional projects with international support and financing are being seriously considered now, on their own and as part of the Middle East peace-making process. Water is certainly a common concern and a potential stimulus for peaceful cooperation. Seawater desalination utilizing non-polluting solar energy appears, on the face of it, to be an ideal solution for the region. Climatic conditions in the Middle East are conducive to solar heat collection, and large water consuming population centers are located near the Mediterranean and Red Seas and the Persian-Arabian Gulf, which will serve as the saline water “raw material” source. ADAN Technical and Economic Services Ltd. was commissioned by the Israel Ministry of Energy and Infrastructure to review and study the technical potential and the resulting economics for the application of solar desalination in the region and in other typical water short coastal areas worldwide. The study’s terms of reference were as follows: 1. Review state-of-the-art technology and projected developments and improvements in both solar heat collection and seawater desalination, and identify the most efficient and economical systems suitable for their combination. The study was to focus, specifically, on those technologies applicable for the very large scale plants required for regional water projects. 2. Perform an initial study to compare the relevant technologies and resultant economics of the various solar energy and desalination systems, and determine the preferred systems and their best mode of linkage. 3. Discuss the preferred design and its economics at current equipment, fuel, and capital costs, and compare it with desalination systems utilizing non-solar energy sources. The following technical and economic considerations guided ADAN in its performance of the study: 1. It is generally accepted that at today’s fossil fuel prices, US $18-22 per barrel of oil, soIar energy is hardly competitive. The long-term prognosis for fuel costs, however, is a steady rise. Thus, even if the study would reconfirm that solar energy in general, and solar desalination in particular, were not competitive at current fuel prices, it would serve a useful purpose by indicating the break-even point at which they would be competitive. 2. Solar energy collection and seawater desalination projects are capital intensive enterprises. Availability of low-cost money (e.g., governmental or

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from international development financing sources) is crucial to making solar desalination viable. It was clear that one of the important objectives of the study should be to explore, through a sensitivity analysis, at which financing rates solar desalination on a regional scale would be economical. 3. It is obvious that use of advanced design concepts with clear economic benefits would have to be considered in the study if water costs are to be brought down. On the other hand, to be immediately applicable, the study had to avoid untested ideas and designs and limit itself only to technologies which are already developed, proven and commercially available. The study chose, therefore, a balanced approach and used reasonably economic projections for next generation designs, providing these were merely extensions of existing technologies. 4. The ability to benefit from economies of scale is critical for any technology adopted for regional solar desalination projects. Technologies unsuitable for such large-scale projects were eliminated from consideration. A 100,000 m3/d plant was chosen as the reference size for the study - an output sufficient, at a typical household per capita water consumption of 125 l/d, for over 800,000 inhabitants. Suitable technologies not applied to date on such a scale had their costs carefully extrapolated. 5. Too many variables, many of them site-specific, will affect the cost of any concrete project. For each such variable (e.g., solar collection and desalination plants efficiencies, equipment investments and operating costs, local infrastructure requirements and costs), there is a range of figures which can apply. The study could, therefore, only provide indicative “ball-park” figures. It was decided, however, that these figures should explore the limits of current technology and project the target potential of solar desalination. The best figures from any variable range were, therefore, selected for the economic calculations. For solar energy plants’ costs and performance, the figures used were those supplied by their manufacturers. Desalination plant figures are based on published data and ADAN’s calculations and projections. 6. Local solar insolation rates will also vary from site to site and directly affect economics. Total annual solar insolation rates on the horizontal along the Israeli Mediterranean coast are on the order of 1,800-l ,900 kWh/m2/y. In the south of Israel, from the Dead Sea to the northern tip of the Gulf of Eilat, these rates range from 1,900-2,100 kWh/m2/y. Further south, beyond Israel’s borders, these figures increase to about 2,500 kWh/m2/y. Since it seems probable that the majority of solar desalination plants put up in the Middle East would be sited along the Red Sea and the Persian-Arab Gulf, a typical insolation figure for these areas of 2,400 kWh/m2/y, was used in all study calculations.

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7. Due to the diffusive, low-intensity nature of solar radiation, land requirements for solar energy projects are inherently large. In densely populated areas land costs could be high, and this might prohibit large solar desalination projects. This is also a site-specific factor. The land requirements were, therefore, calculated and noted in the study for each scheme, but their costs were not included in the economic calculations.

EXECUTIVE

SUMMARY

Five principle desalination processes which can be operated with solar energy inputs have been described: multi-stage flash (MSF) distillation, low temperature multi-effect distillation (LT-MED), thermal vapor compression (TVC) distillation, seawater reverse osmosis (SWRO) and mechanical vapor compression (MVC) distillation. Of these five desalination processes, three are shown to be the most suitable for the large-scale plants of interest for regional projects, each most economical for a specific type and grade of energy input: TVC for medium pressure steam inputs, LT-MED for low pressure steam inputs, and SWRO for electric power input. The specific investments and performance of high-efficiency, large, 100,000 m3/d plants based on these three processes were presented (Table I). Special emphasis was put on energy requirements, translated to “equivalent specific fossil fuel consumptions”.

TABLE I

Investments and performance of high efficiency plants Process

Specific investment

Energy requirement

Equivalent specific fuel requirement (kg fuel/m3)

($/m3/d) TVC

1100

67 kg steam @ 4 bar; 1.2 kWh/m3 product

5.3

LT-MED single purpose

1000

77 kg steam @ 0.55 bar; 1.2 kWh/m3 product

6.0

LT-MED dual purpose

900

100 kg steam Q 0.4 bar; 1.2 kWh/m3 product

1.7

SWRO

700

5 kWh/m3 product

1.1

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Corresponding energy component costs and overall water costs for such plants with fossil fuel energy inputs at a fuel cost of $lOO/ton (about $2.5/MBtu), a power cost of Sc/kWh, and at interest rates varying from 6-10% are given in Table II.

TABLE II Energy component costs and overall water costs Process

Interest rate

Total water cost

Energy cost component

(TWC)

(%I

(C/m3)

(C/m3)

(% of TWC)

TVC

6 10

91 102

56 56

62 55

LT-MED single purpose

6 10

96 105

64 64

67 61

LT-MED dual purpose

6 10

52 61

21 21

40 34

SWRO @ 5 km/m3

6 10

64 71

25 25

39 35

The most economic fossil fuel energy desalination scheme is the dual purpose (water and power cogeneration) LT-MED. If no such scheme is possible, then the most economical single-purpose desalination plant is SWRO. Three solar energy systems were selected, each best suited to generate on a large scale the type and grade of energy required by the three chosen desalination processes: 1. High-temperature parabolic mirror concentrators, as developed by the Luz Company, within its solar electric generating system (SEGS). SEGS stations will produce power to drive a SWRO plant, or, within a cogeneration scheme, will supply exhaust steam from a back-pressure turbine to an LT-MED plant and export its power to a utility. 2. Intermediate temperature parabolic mirror concentrators in a low-cost design by the Paz-Pimat Company of Israel to generate 2-4 bar steam for a TVC plant.

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3. Salinity gradient solar ponds, as developed by the Ormat Company of Israel, for producing low-grade heat or steam (at 0.55 bar) for use by a single-purpose LT-MED. “Thermal diode” solar ponds, utilizing the Are1 Company’s transparent absorber-insulator honeycomb panels, may also be applicable after additional development, but their economics were not examined in the study. The three selected solar energy systems have been described and their economics analyzed. All systems, except for solar ponds, require fuel firing to supply backup heat and some heat storage to assure a continuous energy supply. Highest annual utilization is critical in solar desalination (to reduce all capital cost components). Solar ponds accommodate for the variable nature of solar insolation (nighttime, cloudy periods and winters) by oversizing catchment areas and increasing hot water layer depth, i.e., by a built-in heat storage capacity. With high temperature and intermediate temperature solar energy systems, solar heat represents only 23-35% of their outputs. The balance is derived through fossil fuel firing. Average energy costs are, therefore, dominated by fuel costs. At today’s fuel costs, the solar component is 1.5-2 times higher in cost than the fossil fuel component. The average energy costs from the various solar systems at a fuel cost range of $lOO-140/tori ($2.5-3.5/MBtu) are shown Table III. These costs do not include site-related operating and indirect costs.

TABLE III Average

energy costs for the various solar energy systems

Solar system

Type of energy

Energy cost @I 6% interest

@ 10% interest

Luz type SEGS, power only

Electric power

5.1-5.9 C/k?Vh

5.9-6.6

CfkWh

Luz type SEGS, cogeneration

Electric power 0.4 bar steam

5.1-5.9 ClkWh 2.1-2.6 $/ton

5.9-6.6 CfkWh 2.5-2.8 $/ton

IT parabolic mirrors

4 bar steam

8.1-10.1 $/ton

9.8-l 1.5 $/ton

Solar ponds

0.55 bar steam

1.8-2.6 $/ton

2.4-3.5 $/ton

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It is seen from this table that low-grade heat from solar ponds costs the least. Solar pond hot layer pumping costs contribute about 30C/ton to indicated solar pond steam costs. However, the value of this steam’s condensate, which, unlike other systems, is added to the desalination plant product, will in itself be worth 67-88C/ton. Siting of solar desalination plants is an important consideration, as it affects auxiliary system costs and energy requirements. There are conflicting demands. The solar collection system’s preference is for an inland site where insolation is higher and land costs lower. The desalination plant requires siting close to the sea to minimize seawater supply and brine reject system investment and pumping costs. The desalination plant should be close to the solar system to reduce the heat transfer fluid system’s investment, pumping costs and heat losses. The whole system should be close to its consumers to minimize product delivery system’s investment and pumping costs. Overall preference is for a site close to the sea. Suitable sites have to be identified and reserved by the concerned Water Authorities. Reasonable leasing rates must be assured if land costs are too high. To achieve lowest desalinated water costs, the design of all solar collection and desalination plant system elements and their interfacing must be optimized. Optimal design parameters here (e.g., solar energy delivery temperatures, plant investment efficiency trade-offs) may not be the same as for their conventional, non-combined applications. Land area requirements (X 1000 m2) for a 100,000 m3/d solar desalination plant for the various solar desalination combinations are: SEGS and LT-MED*

3,000-4,000

Solar pond and LT-MED

3,300-4,200

IT troughs and TVC

l,lOO-1,500

SEGS and SWRO

320-420

*Including requirements for a 200 MW net power output station.

The LT-MED plant combinations

require most land area.

The expected “hard cost” investments required for various 100,000 m3/d solar desalination stations, excluding site-related and “soft costs” such as interest and insurance during construction, legal fees, etc., are given in Table IV.

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TABLE IV Hard cost investments SEGS and LT-MED*

Solar pond and LT-MED

IT troughs and TVC

SEGS and SWRO

Solar system Desalination Civil works

440* 90 10

45-65 95 10

260 110 10

50 70 10

Total

.540*

150-170

380

130

*Includes requirements for a 200 MW net power output station.

The solar pond and LT-MED and the SEGS-SWRO systems require the least capital investment. Estimated water costs from 100,000 m3/d solar desalination stations at various interest rates and for a range of fuel costs from $lOO-140/tori ($2.5 3.5 MBtu) including assumptions and “guesstimates” for additional siterelated costs, which vary from site to site, and for indirect costs at 12-15c/m3 are given in Table V. Lowest cost solar desalinated water is obtained from the solar pond and LT-MED system. TABLE V Estimated water costs from 100,000 m3/d solar desalination stations Interest rate (%)

Luz SEGS and LT-MED (C/m3)

Solar pond and LT-MED (C/m3)

IT troughs and TVC ( C/m2)

Luz SEGS and SWRO (C/m3)

6 8 10

73-79 80-84 87-91

67-73 74-81 80-88

112-123 123-134 132-144

77-81 82-86 88-91

These solar pond and LT-MED water costs are competitive with those from a dual-purpose LT-MED plant operating with fossil fuel energy at fuel prices from $150-210/tan. It should be stressed, however, that these are target figures - the best that can be achieved. They are based on the performance of an advanced, high-efficiency LT-MED plant and a solar pond

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operating

at a very high average annual insolation rate, 2400 kWh/m*/y.

While this latter figure may be typical at low Red Sea and Persian-Arabian Gulf latitudes, at Israel’s Red Sea coast it will be 20% lower. Along its Mediterranean coast insolation will be 25% lower. The product from the thermal desalination plants (LT-MED and TVC) will be distilled with less than 25 ppm TDS. The product from the membrane plant (SWRO) will be at 400-600 ppm TDS. The World Health Organization allows 1,000 ppm TDS water to be considered potable, but recommends 500 ppm TDS as the top limit. It is possible, therefore, to blend the distillation plant product (only) with any available low-cost, low-salinity (up to 1,500 ppm TDS) brackish water. If we assume such brackish water is available at 10C/m3, the costs for blended water costs will be as shown in Table VI.

TABLE VI

Costs of blended water Brackish Interest salinity rate (ppm TDS) (so)

Luz SEGS and LT-MED ( C/m3)

Solar pond and LT-MED ( C/m3)

IT troughs and TVC (C/m3)

Luz SEGS and SWRO ( C/m3)

1,ooo

6 8 10

42-45 45-47 49-52

39-42 42-46 45-49

61-67 67-73 71-77

78-93 82-97 88-91

1,500

6 8 10

52-56 57-59 61-64

48-52 53-57 57-62

78-85 85-93 91-99

78-93 82-97 88-91

The monthly cost of producing water for a family of four with an average per capita water consumption of 125 l/d (33 gal/d) with a solar pond and LT-MED system (at plant battery limits, excluding product pumping, chlorination, storage and distribution) is $6-9 for blended water and $1 l-14 for high quality unblended water. Solar desalination is a capital-intensive enterprise. Capital recovery charges represent about two-thirds of overall water cost with the solar pond and LT-MED solar desalination system - a dominant component. Increasing the interest rate by a full 1% within a range of 6-10% will raise the total desalinated water costs by 3.5-4c/m3, or 4.5-5%. Low-cost money is,

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therefore, the key to reducing water cost. Tax incentives, subsidized credits, and/or grants will have to be made available by government or international development agencies if the private sector is to be induced to participate on a commercial basis. Government and international development agencies’ assistance in bringing down solar desalination water costs is also required to finance the engineering development, construction, and long-term testing of large-scale demonstration plants. Continuous large-scale experience is lacking for both the solar gradient and thermal diode ponds. Further development and the large-scale production of thermal diode pond honeycomb panels can reduce their costs significantly. LT-MED technology, while tested and proven, can benefit from additional improvements in large-plant design and efficiency. There are strong institutional considerations beyond economics which work in favor of solar desalination, such as its environmental cleanliness and safety. These community benefits, if articulated and quantified, will lead to public acceptance and support of a solar energy desalination program, providing its economics are within range of alternative options.

APPLICABLE

SEAWATER DESALINATION TECHNOLOGIES

overview Seawater vs. brackish water desalination Brackish waters are normally defined as inland waters with salinities of l,OOO-10,000 ppm TDS. Seawaters vary from about 34,000-47,000 ppm TDS (the higher values existing in the Red Sea, the Persian-Arabian Gulf and the Indian Ocean). Mediterranean seawater salinity along Israel’s coastline is between 38,000-41,000 ppm TDS. Seawater desalination is costlier than brackish water desalination. The higher seawater salinities dictate higher specific energy inputs and higher cost equipment for seawater desalination than for brackish water desalination. However, whereas seawater composition is basically the same throughout, and about 90% of its salinity is composed of NaCl, which is a nonscaling salt, inland brackish waters vary considerably in composition and normally contain significant amounts of problematic carbonate and sulfate scaling salts, organic materials, and other contaminants. These can adversely affect desalination plant performance. Brackish water desalination plants, therefore, often require complicated feed pretreatment sections.

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Brackish water desalination is performed best through membrane processes such as RO and ED. These processes utilize electrical energy at a rate of about 1.5-2.0 kWh/m3 product with brackish waters and convert from 50-85% of the brackish feed into a potable product, leaving the rest to be disposed of as brine. Inland brackish water desalination, therefore, also entails solving a 15-50% brine disposal problem. Desalinating brackish waters, if these are available near population centers, will always be cheaper than seawater desalination. However, as will be discussed below, if available brackish waters are limited in quantity and are of sufficiently low salinity (below 1,500 ppm TDS), their total use, through mixing with a high purity distilled product from thermal seawater desalination plants, should be considered. Thermal vs. electrical desalinationprocesses Thermal desalination processes generate through simultaneous evaporation and condensation of the saline feed (i.e., through change of phase separation) a pure distilled product of 2-25 ppm TDS. This is a much higher water quality than is necessary for municipal consumption. WHO recommends a top limit of 500 ppm TDS and allows a top limit of 1,000 ppm TDS. The major thermal processes employed today are MSF, MED, and TVC distillation processes. All of these recycle their heat inputs repeatedly within their systems to reduce specific energy requirements and require some electric power for process pumping. As will be discussed later, the effcienties of plants based on these processes are a function of top operating temperature, the number of stages or effects, and the amount of heat transfer surface areas designed into the plants. Specific heat consumptions for typical large plants built to date with fossil fuel energy inputs are on the order of 40-80 kcal/kg product. Pumping power requirements (excluding seawater intake pumping, which varies from site to site, depending on plant distance and elevation relative to sea front) are about 2.5-4.0 kWh/m3 for MSF plants and 1.2-2.0 kWh/m3 for MED and TVC plants. The plants require a heat sink for heat rejection and must be provided with seawater coolant (including feed) at a rate of anywhere from 4.5-10 m3 coolant per 1 m3 of product. Thermal desalination plants benefit strongly from economies of scale, and single units as large as 11,000 m3/d for TVC, 25,000 m3/d for MSF, and

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20,000 m3/d for MED have been built and operated. Designs have been completed for plants with capacities as high as 100,000 m3/d per single MSF unit and 40,000 m3/d per single MED unit. The major electric power consuming seawater desalination process is SWRO. It generates a product of 400-600 ppm TDS in a single pass and about 200 ppm TDS in a two-pass design. SWRO plant design can be simplified and its economics improved significantly if allowed product salinity limits are raised to, say, 1,000 ppm TDS, as would be acceptable for blending with thermal plant distilled product water. SWRO plants use membrane separation (under pressure), require strictly engineered and operated feed pretreatment to protect the membranes from various possible contaminants, suffer from membrane degradation (deterioration in plant performance over time, in both product water output and quality), and require periodic membrane replacements. The time between replacements is a critical factor affecting product cost. Specific energy consumption with SWRO plants has been reduced through use of energy recovery turbines and are in the range of 5-7 kWh/m3 product, excluding seawater pumping. Seawater feed requirements are from 2-4 times product. SWRO plants are limited by present membrane module sizes. Large plants incorporate large batteries of membranes, and there are very limited economies of scale. Beyond a capacity of about 6,000-7,000 m3/d, plants consist of multiple units operating in parallel, and even pumps, controls and instrumentation repeat. An alternative electric consuming process is the MVC distillation process, which, as its name implies, involves change of phase separation and generates a distilled product. MVC plants operate on the heat pump principle to reduce energy consumption, and utilize mechanical compressors. Present plants consume 9-12 kWh/m3, with projections for future designs given (by manufacturers) at 7.5-8 kWh/m3 (i.e., somewhat higher than with SWRO plants). MVC plants, however, do not require a critical feed pretreatment system and periodic membrane replacements. On the other hand, their initial investments are significantly higher than for SWRO plants. Current commercial MVC plants are limited by available compressor sizes and costs to unit capacities of 2,000-2,500 m3/d max. (i.e., very large plants must incorporate multiple unit installations with very limited economies of scale). However, these current plants utilize simple, moderate cost, single-stage centrifugal compressors. Much larger units have been designed in paper studies, based on the use of high-speed, high-volumetric flow axial compressors.

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Process descriptions Even though these desalination processes are well known to readers of

Desalination, we are reviewing them briefly for reference. Multi-stage flash distillation MSF plants consist of a number of “stages,” normally 20, and as many as 46, each of which consists of a flash chamber, a vapor droplet separator, and pre-heating condensation tubes. The combined group of stages is called the “heat recovery section.” The process takes advantage of the fact that a hot water stream will flash off some vapor when introduced into a chamber at a lower pressure than the liquid-vapor equilibrium pressurecorresponding to thestream’s temperature. If this pressure driving force is maintained, such as by condensing the generated vapor through a continuously supplied coolant stream, the hot water stream flowing through the chamber evaporates continuously. A steady-state pressure is established in the chamber, intermediate between the liquid-vapor equilibrium pressures corresponding to the hot water and coolant stream temperatures, so as to provide fixed driving forces for both flashing and condensation. Such a continuous heat exchange actually occurs in each stage of the MSF plant with exiting hot seawater cooling off and becoming progressively more concentrated through evaporation, and entering seawater recovering the latent heat given off by the condensing vapors and heating up sensibly. The resultant salt-free condensate is the MSF plant’s product.

Multi-eflect distillation MED derives its name from the fact that it utilizes a number of effects, normally 8-12, but also as high as 18, to produce a multiple amount of distilled water from each unit of input steam. In each effect there is simultaneous evaporation of seawater, flowing as a film on one side of a heat transfer surface, and condensation of vapor, generated in a preceding effect, i.e., latent heat transfer (there may be also some minor sensible heat transfer to the seawater). The evaporation-condensation process occurs in each succeeding effect around a temperature-pressure liquid-vapor equilibrium point lower than in the preceding effect. Heat input is through condensation of heating steam in the first effect. This latent heat is recycled from effect to effect, each recycle producing

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additional product. The group of effects in a plant are called, therefore, the “heat recovery section”. The heat is finally rejected in the heat rejection condenser to the incoming seawater, which serves as the coolant “heat sink” and is preheated sensibly. l%ermal vapor compression distillation

TVC is similar to the MED process, except that it operates on higher pressure input steam and utilizes this higher pressure through a steam-jet thermocompressor to increase the system’s efficiency or economy ratio (ER). The motive steam expands in the thermocompressor, sucks low pressure vapor from one of the cooler, lower pressure effects, compresses and recycles it to the first, warmest effect. The recirculated vapor thus adds its latent heat to the latent heat discharged in the first effect by the motive, input steam. Though TVC plants may employ the same number of effects as an MED plant, only 4-7 effects are normally included within the vapor recycle loop.

Seawater reverse osmosis

SWRO is based on the natural phenomenon that every saline solution has an “osmotic” pressure which is proportional to its concentration. When a semi-permeable membrane separates two saline solutions with different concentrations and osmotic pressures, the “osmotic” pressure differential forces the solvent, and due to membrane imperfections also part of the solute, to flow through the membrane from the dilute solution to the concentrated one. In the RO process this normal osmosis is reversed through the application, by way of a high pressure pump, of a pressure higher than the (sea or brackish water) osmotic pressure differential. The process is dominated by the quality and characteristics of the semi-permeable membranes. Two parameters define their performance: “flux,” i.e., solvent flow per unit membrane area (affects required plant membrane areas and capital costs) and “salt rejection,” i.e., fraction of solute rejected by the membrane. The two characteristics are contradictory. A “loose” membrane will allow higher fluxes but will have a lower rejection, and a “tight” membrane vice-versa. High operating pressures will increase flux but will also increase pumping power and membrane compaction and lead to faster degradation with time

130

of performance. Seawater membranes operate at pressures of 60-70 bar and require salt rejections of 99.8% min. Plant “conversion ratio” (i.e., ratio of product water to feed) is also an important parameter. High conversion factors reduce pumping power requirements but increase average solution concentration and product salinity. SWRO conversions are 2550%. Membrane lifetime is a critical factor. Membrane lifetime and performance are sensitive not only to compaction but also to fouling and scaling. Therefore, RO plants require strict feed pretreatment. Typical SWRO membrane lifetimes are 3-7 years. Mechanical vapor compression MVC operates on the heat pump principle. It employs an electrically (or mechanically) driven mechanical compressor operating across one or more evaporator effects connected in series to pump vapor generated at a low pressure effect to the condensation section of a higher pressure effect. The evaporation-condensation latent heat is thus continuously recycled, and the only energy input, except for a small fraction required for process liquid pumping, is the mechanical energy invested through the compressor in raising the vapor’s enthalpy to a level sufficient to overcome all heat and mass transfer resistances, boiling point elevations, pressure drops, and other irreversibilities. The more efficient the evaporator (larger heat transfer areas, higher heat transfer coefficients), the lower the compressor’s pumping head and its energy consumption. Other necessary MVC plant sub-systems are: (a) feed heat-exchangers, which recuperate the sensible heat of the outgoing distillate and brine streams by transferring it to the feed (the feed is thus preheated close to process temperature); (b) a non-condensible gases removal system.

Technical and economic considerations in designing thermal desalination plants The optimal design configuration for any thermal desalination plant depends on the availability and relative costs of various types of energy or fuel at site, the cost of capital, and the required annual plant utilization factor. It is always possible to design a low-cost plant (i.e., with less heat transfer areas) at the expense of lower efficiency and a higher specific energy requirement. Such a design would, in fact, be optimal for low

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utilization factor sites (e.g., tourist serving locales with a seasonal demand). High fuel cost areas with a high utilization factor would require larger, costlier, but more efficient plants. Required plant performance is also dictated by the availability (or unavailability) of a nearby waste heat source or an industrial facility generating by-product process steam, or a power station which may be utilized in a cogeneration scheme. The costs of such by-product or cogenerated heat inputs are related to their temperatures or exergy (that part of the energy which may be converted to useful work). The lower the temperature the lower their associated cost. The only way to properly price these various grades of heat to the desalination plants, once local fuel costs are known, is by putting them on a common energetic basis: their “equivalent specific fuel consumption” in kg fuel/kcal. This figure must be calculated for each type of heat supply scheme according to the marginal amount of additional fuel required to generate the heat (or steam), allowing for all intermediate boiler, turbine, and power cycle efficiencies. Any other approach will distort the benefits of cogeneration, by-product, and/or process heat utilization schemes. Where no low-cost, low-grade heat sources are available, thermal desalination plants require raising steam with fuel fired boilers. Normally, raised steam pressures are in the range of 8-16 atm. Due to the high cost of this steam, the plants would be designed for their highest ERs, 10: 1 to 12: 1 for MSF, and 12: 1 to 14:l for TVC. (ER is defined as the ratio of distilled product to steam input.) At typical boiler efficiencies, 65 kg of fuel are required per ton of steam, and the TVC plant specific fire1consumptions will be 4.7- 5.5 kg/m3. (In the OPEC countries where the internal price for fuel used to be low, MSF plants with ERs of 8: 1 are common.) Obviously, with the high costs of solar heat, only top efficiency TVC plants with an ER as high as is practically possible - 15: 1 - should be considered for single-purpose, water only solar stations. Solar heat collection for such TVC plants would be via intermediate temperature (llO-200°C) collectors, and the optimal collection temperature will have to be established (see Section on Optimal Temperature). Aside from the limited and special cases where zero or close to zero cost industrial waste heat is available (normally at very low temperatures, no more than 5O”C), the lowest cost desalinated seawater from thermal desalination plants is obtained today in those plants operated within a dualpurpose water and power cogeneration station. In these stations, the steam raised at a high pressure expands first in a condensing-extraction (“passout”)

132

turbine or in a back-pressure steam turbine and is then extracted or discharged, as the case may be, into the desalination plant. Water costs benefit here from the practice of charging power production with all investment and operating costs which would have accrued if the station would have been built and operated as a single-purpose, power only station. Water production is charged only with the marginal added costs due to the mating of the desalination plant. The key cost benefit for water production in a dual-purpose power and water plant, vis-a-vis costs in a single-purpose, water only plant (aside from the savings in common facilities and manpower) is in the highly reduced cost of the low temperature, low pressure steam utilized by it. Power production bears the major cost of generating this low pressure steam. Water production is charged only for the marginal increase in boiler fiel consumption incurred in raising the extra turbine throttle steam required to generate the same net power output with a steam extraction or a back-pressure turbine compared to a conventional power only station utilizing a condensing steam turbine. Obviously, the lower the pressure of the extraction or back-pressure steam, the lower the added fuel consumption and its cost to water production. On the other hand, the lower the temperature and pressure of the steam delivered to any thermal desalination plant, the lower the plant’s ER (i.e., less water can be generated per unit of steam input). ERs can be increased up to certain limits only by increasing heat transfer surface area and plant size (i.e., capital costs). The optimal discharge pressure in a dual-purpose station is, therefore, dependent on the desired power to water ratio, fuel and power costs, and the specific desalination plant’s process characteristics, design, and economics. The most economical thermal seawater desalination plants available today for dual-purpose water and power stations are low-temperature, horizontal tube, falling film, MED (LT-MED) plants, designed specifically to operate efficiently with very low pressure turbine exhaust steam. These plants incorporate very large areas of low-cost aluminum alloy heat transfer surfaces and use epoxy coated mild steel evaporator shells. With these materials of construction, the penalty in higher specific investment due to the larger plant size is relatively low. MSF plants, whose capacity and ER are strongly dependent on high temperature operation, are not competitive with LT-MED plants for dualpurpose stations. In most typical fuel and power cost situations, LT-MED plants are designed to operate at a back-pressure of 0.4 bar, equivalent to

133

saturated steam at 75”C, including allowance for vapor duct pressure drops. These LT-MED plants have 12 effects, a top brine temperature of 72°C and an ER of about 9.1: 1 (i.e., require about 110 kg of 0.4 bar back-pressure steam per m3 product). This is a specific energy consumption of about 63 kcal/kg product. The main point, however, is that these kcals are of such a low grade that the specific fuel consumption charged to water production, corresponding to the increase in fuel consumed by the boiler supplying motive steam to the power generating turbine, is only 1.5-2.0 kg fuel/m3 product. By comparison, an MSF plant operating with the same low pressure, low temperature steam would have an ER of only 4-5 (i.e., will have twice as large a specific fuel consumption). Alternatively, if the MSF plant is designed to operate at its most efficient top brine temperature of 120°C (a high figure from the point of view of scale and corrosion control), it can be designed for an ER of 11.0-l 1.5: 1. Its specific energy consumption will be only 50 kcal/kg product, but at a steam pressure of 2 bar, this will result in a specific fuel consumption of about 3 kg fuel/m3 product. In some cases, particularly where cogeneration is imposed on an existing power station, turbine extraction steam is available only at a given value, anywhere from 1.4-2.7 bar. In such cogeneration situations, TVC plants with ERs of 12: 1 to 14: 1 are preferable. Their specific energy consumptions are 40-47 kcal/kg, equivalent to specific fuel consumption of 2.3-2.8 kg fuel/m3. It is possible to design higher efficiency LT-MED plants with 16-18 effects and, perhaps, even feed preheaters, and a top brine temperature of about 80°C. This temperature is still acceptable from the point of view of corrosion and scaling, and ERs of about 13: 1 will be obtained. These higher efficiency LT-MED plants are the preferred choice for single-purpose solar energy desalination in combination with low temperature collectors (e.g., solar ponds). Their use in a cogeneration scheme with a turbine means a back-pressure of about 0.55 bar. Specific fuel consumption charged to water production is then 2.1 kg/m3 (i.e., a slightly higher figure than with plants designed for 70-72°C operation). Indeed, several independent studies have confirmed that 0.4 bar operation will give lowest specific fuel consumption. Since variations of klO”C from this optimal steam temperature increase LT-MED plant specific fuel consumption only marginally, they may be warranted where other water to power ratios are required.

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Current techn.ology - peformance and economics Desalination plant costs differ from manufacturer to manufacturer, from project to project, and are strongly affected by economies of scale and locally available energy sources and their costs. Table VII is based on published costs and performance for the larger desalination plants built for each process over the past 10 years and some averaging and extrapolations. The table demonstrates clearly why the TVC process is preferable to MSF for single-purpose plants and for cogeneration schemes with higher pressure turbine extraction steam situations. It not only has better economics, lower specific investment and energy consumption, but also allows operation with significantly lower top brine temperatures (i.e., at lower scaling and corrosion rates). The table also shows the superior economics of SWRO vis-a-vis MVC for electric consuming power seawater desalination plants. Though other important cost factors which weigh on SWRO such as membrane and filter replacements and pretreatment chemicals costs, as well as less quantifiable factors, such as membrane sensitivity to feed contamination and the higher quality of the MVC product, are not expressed in the table, SWRO’s lower specific investment and energy requirements prevail. It should also be remembered that the SWRO investment figures already include the costs of the membranes which will be used during the initial 3-7 years. These should really be amortized separately and charged as an operating expense. More importantly, the largest MVC plants are available today commercially only in capacities of 1,500-2,500 m3/d per unit. It is inconceivable to provide the capacities of interest for regional water supply schemes of, say, 100,000 m3/d and up in a battery of 40-65 MVC units operating independently in parallel, The mechanical compressors are, after all, large (in some cases also high speed) rotating machines, requiring maintenance, and have a limited reliability. Each such MVC unit would contain also at least three other rotating items of equipment, namely the brine, product, and recirculation pumps. If no central non-condensible gases removal system is installed, each MVC unit would also include a mechanical vacuum pump. Tables VIII, IX, and X, which provide water cost breakdowns for large, 100,000 m3/d seawater desalination plants, have been limited, therefore, to the three “surviving” processes deemed best-suited for large-scale solar desalination: TVC, LT-MED, and SWRO. The water costs are shown with fossil fuel energy inputs for various schemes. Fuel consumptions in cogeneration schemes were based on a typical 120 MW steam turbine power station with an overall heat rate in the condensing mode of operation of about

Thermal

Thermal

Electric power

Electric power

Electric power

TVC

LT-MED

SWRO with energy recovery

SWRO without energy recovery

MVC

Type of energy input

processes

90 80 70

130 100 80

.--

Heat input temperature

.._..

for desalination

Thermal

parameters

MSF

Process

Economic

TABLE VII

80 70 60

70 70

m

Top brine temperature

10-13 8-10 6-8

ER

5-6

7-8

8-12 14-16

-

-

1.2-1.8 1.2-1.8 1.2-1.8

1.2-1.8 1.2-1.8

2.5-4 2.5-4 2.5-4

+ kWh/m3)

-

42-55 56-70 70-94

36-45 45-55

44-53 60-78 1 IO-135

(kcal/kg

Specific energy consumption

1200-1800 1000-1250

600-800

700-900

900- 1300 900-1300 900-1300

1000-1300 1000-1300

1300-2000 1300-2000 1300-2000

($/m3/d)

_

-.

Specific investment for large plants

--

E

136 TABLE VIII TVC plant (100,000 m3/d) water cost breakdown

Single purpose

Cogeneration

15:l 5 kgf/m3 1.2 kWh/m3

12:l 2.3 kfg/m3 1.2 kWh/m3

$1, 100/m3/d

$1 ,000/m3/d

0.50 0.06 0.03 0.03 0.02 0.02

0.23 0.06 0.03 0.03 0.02 0.02

0.66

0.39

Capital recovery ($/m3) @ 90% annual plant utilization factor, 25 y amortization and interest at: 6% 8% 10%

0.25 0.31 0.36

0.23 0.28 0.33

Total water cost Interest at: 6% 8% 10%

0.91 0.97 1.02

0.62 0.67 0.72

Plant parameters: ER Specific fuel consumption Specific power consumption (excluding seawater pumping) Specific investment (installed, including civil works) Operating costs ($/m3): Fuel @ $lOO/ton Power @ SclkWh O&M labor @ $45,OOO/man/y Parts @ 1% of investment/y Chemicals G&A (insurance, overhead, etc.) Sub-total

2,400 kcal/kWh (i.e., a net thermal efficiency of about 35.8% and a specific fuel consumption, with 9,600 kcallkg LHV fuel oil of about 250 g fuel/ kWh). The plants’ main parameters (i.e., specific investment and energy consumptions) were taken as the lowest of the figures shown in Table VII). Fuel, power, and labor were based on the following rates: fuel at $lOO/ton,

137 TABLE IX

LT-MED plant (100,000 m3/d) water cost breakdown Single-purpose

Cogeneration

80°C 13:l 5.8 kgf/m3 1.2 kWh/m3

72°C 10: 1 1.5 kgf/m3 1.2 kWh/m3

$950/m3/d

$900/m3/d

0.58 0.06 0.03 0.03 0.02 0.02

0.15 0.06 0.03 0.03 0.02 0.02

0.74

0.31

Capital recovery ($/m3) @ 90% annual plant utilization factor, 25 y amortization and interest at: 6% 8% 10%

0.22 0.27 0.31

0.21 0.25 0.30

Total water cost Interest at: 6% 8% 10%

0.96 1.01 1.05

0.52 0.56 0.61

Plant parameters: Top brine temperature ER Specitic fuel consumption Specific power consumption (excluding seawater pumping) Specific investment (installed, including civil works) Operating costs ($/m3): Fuel @ $lOO/ton Power @ SClkWh O&M labor @ $45,OOO/man/y Parts @ 1% of investment/y Chemicals G&A (insurance, overhead, etc.) Subtotal

.

power at Sc/kWh average, and labor at $45,00O/man/y, average. These rates seem to represent current regional cost figures. Manpower requirements were taken as 20 for operator shifts and 5 for maintenance (electrician, instrumentation, and controls specialist, mechanics). Three interest rates were used to demonstrate water costs sensitivity to financing charges.

138 TABLE X SWRO plant (100,000 m3/d) water cost breakdown

Plant parameters: Specific power consumption (excluding seawater pumping) Membrane lifetime Specific investment (installed, including civil works) Operating costs ($/m3): Electric power @ SC/kWh O&M labor @ $45,OOO/man/y Membrane replacements Filters and parts replacements Chemicals (pretreatment, cleaning, and posttreatment) G&A (including insurance, overhead, etc.) Subtotal Capital recovery @ 90% annual plant utilization factor, 25 y amortization and interest at: 6% 8% 10% Total water cost Interest at: 6% 8% 10%

5 kWh/m3 3-7 y (per membrane guarantee) $700/m3/d

0.25 0.03 0.11 0.03 0.04 0.02 0.48

0.16 0.20 0.23

0.64

0.68 0.71

Use of “best figures” for both investment and energy consumption seems to be a contradictory demand in view of the comments in the section on Considerations in Designing Thermal Desalination Plants. However, we believe that, due to the large economies of scale possible in regional projects, properly designed plants can achieve such optimal figures concurrently with state-of-the-art technology. In fact, in some published studies, as well as in ADAN’s own design studies, even lower specific investment figures have been derived for the size of plants envisaged in this study.

139

Tables VIII, IX, and X demonstrate the dramatic improvement in thermal desalination plants’ energy and total water costs with cogeneration schemes and the rationale for seeking first such schemes. Single-purpose thermal plants at today’s fuel costs have all but disappeared, except for the OPEC countries. In non-OPEC countries, and lately more and more in the OPEC countries too, single-purpose desalination means a SWRO or MVC desalination plant, using grid supplied or self-generated (mostly by diesel) power. It should be remembered that the specific investments and the water costs shown above cannot, and should not, be compared to the costs of existing water sources in Israel and/or other countries in the region. These water projects were constructed at historical material, labor, equipment, and financing costs. The economics of any proposed new desalination plant. should be compared only to the current real costs today of building new alternative water supply projects on the same scale and under the same financing terms and levels of government benefits and support. The economics of fossil fueled desalination plants have been found in many locations throughout the world to be competitive with new alternative water supply schemes such as: 1. Transport of water from distant natural sources - in small capacities, above 50-100 km; in large capacities, above 500-600 km. 2. Sewage reclamation for non-agricultural uses (due to non-acceptance of reclaimed sewage by the public). 3. Brackish water desalination - where the brackish waters are hard scaling and/or contaminated and/or limited in quantity. In evaluating the potential for solar rather than fossil fuel energy inputs, the most significant parameters to examine in the economic analysis of each desalination process are the energy cost components. These are summarized below in Table XI in terms of (a) absolute contribution to unit product cost, (b) effect on total water cost, and (c) relative weight (percentage) in total water cost. Since the figures calculated above are specific to the fuel and power costs used ($lOO/ton fuel and Sc/kWh), prices which may be relevant today but are subject to market and politically driven fluctuations, they are also shown in the table for alternative, higher fuel and power costs. Table XI indicates, therefore, not only the potential effect solar (or any other) energy input can have on desalinated water costs at today’s energy costs, but also at prices anticipated for the future. It should be borne in mind that the long-term prognosis for fuel costs is a steady rise. It is this long-term projection, and not only environmental

5.0 5.0 5.0

2.3 2.3 2.3

5.8 5.8 5.8

1.5 1.5 1.5

-

TVC Cogeneration

LT-MED Singlepurpose

LT-MED Cogeneration

SWRO with energy recovery

-

100 150 200

100 150 200

100 150 200

100 150 200

1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 5.0 5.0 5.0

0.23 0.35 0.46 0.58 0.87 0.16 0.15 0.23 0.30 -

(kWh/m3)

Spec. cons.

Power

0.50 0.75 1.00

($/m3)

(kg/m3) ($/ton)

Spec. cost

Price

COllS.

Spec.

Fuel

TVC Singlepurpose

Interest rate

Process

Energy cost analysis for desalination plants

TABLE XI

0.05 0.07 0.09

0.05 0.07 0.09

0.05 0.07 0.09

0.05 0.07 0.09

0.05 0.07 0.09

($IkWh)

Price

0.56 0.83 1.11 0.28 0.43 0.57 0.64 0.95 1.27 0.21 0.31 0.41 0.25 0.35 0.45

0.06 0.08 0.11 0.06 0.08 0.11 0.06 0.08 0.11 0.25 0.35 0.45

($/m3)

0.06 0.08 0.11

($/m3)

Spec. cost

Total energy cost

10% 1.02 1.29 1.57 0.72 0.87 1.01 1.05 1.36 1.68 0.61 0.71 0.81 0.71 0.81 0.91

6% 0.91 1.18 1.46 0.62 0.77 0.91 0.96 1.27 1.59 0.52 0.62 0.72 0.64 0.74 0.84

($/m3)

Total water cost

39 47 54

40 50 57

67 75 80

45 56 63

62 70 76

6%

(W

Energy cost

35 43 49

34 44 51

61 70 76

39 49 56

55 64 71

10%

141

benefits, which has led to worldwide development of solar energy. In other words, even if this study establishes that solar energy is not competitive with fossil fuels at current fuel prices, it will indicate the break-even prices which would have to be reached in the future for it to be preferred for desalination. As can be seen, fossil fuel energy costs at current prices constitute 5562% of total desalinated water costs in single-purpose thermal plants and 35 40% in dual-purpose plants. More expensive energy will have a terminally negative effect on single-purpose plant water costs but is relatively tolerable with cogeneration plants. Each one percent increase in current fuel prices will increase product water costs in a dual-purpose LT-MED plant by only 0.35-0.4% vs. 0.55-0.6% in single-purpose plants. In absolute value terms, the doubling of fuel prices will affect water costs with the cogeneration scheme by only 20C/m3. In the single-purpose plant the increase will be 55C/m3. With electric power consuming SWRO plants, fossil fuel energy costs also currently comprise 35-39% of total water costs. However, since the specific fuel consumption with SWRO plants (about 1.1 kgf/m3*) is lower than with LT-MED cogeneration plants (about 1.7 kgf/m3*), the absolute increase in total desalinated water costs due to any increase in fuel cost will be slightly smaller. In any case, the least expensive fossil-fuel desalinated water, even with a doubling offuel prices, is obtained withLT-MED plants in a cogeneration scheme. It should be remembered, though, that the above figures were derived on the basis of a 90% annual utilization factor. In dual-purpose stations, desalination plant utilization also depends on power plant availability and load factor. Ninety percent overall utilization may be an ambitious but achievable goal with fossil fuel stations. With solar power stations, even if their mechanical reliability (complicated by the solar collector drive, control, and cleaning mechanisms - see description of the Luz system in the Solar Systems Descriptions section) is considered similar, solar heat input is limited on the average to less than 25% of annual hours. Back-up fossil fuel heat inputs, with or without some energy storage, must be provided. On the other hand, low-grade solar heat from simple and reliable solar ponds (which also have a built-in heat storage capacity, allowing continuous night and day operation and reasonable winter time outputs) may be sufficiently low in cost so as to make a single purpose LT-MED a viable and competitive option. These alternatives, and the relative costs of various grades of solar energy cost vis-a-vis fossil fuel energy, are examined below.

*Based on a 120 MW steam turbine power station with a heat rate of about 2,400 kcal/kWh.

142 APPLICABLE

SOLAR COLLECTION TECHNOLOGIES

Overview The development of solar energy technology received impetus after the oil price increases in the 1970s. With various government R&D funding support, great gains were made in improving collection surface materials and designs, raising temperatures and power conversion efficiencies, and reducing costs through the scale-up of plants. Except for the Luz company’s privately funded efforts in the United States, it appears that the large-scale application of solar energy and the further development of technology have been stymied by the relatively low fuel prices of the late 1980s. Though we have reviewed for this study all current solar energy technologies, our main interest was directed to those technologies that: l l l

are already developed, proven and commercially available today are suitable for large-scale application (multi MW thermal) have operating characteristics compatible for matching with candidate desalination processes and plants.

These requirements eliminated consideration of all the photonic conversion technologies such as the photovoltaic (PV), photo-chemical, bio-conversion, and photovoltaic-electro-chemical. Thermal conversion systems are normally grouped into three main groups according to their operating temperatures: (1) high temperature collectors, (2) intermediate temperature collectors, and (3) low temperature collectors. As with desalination plants, here, too, the investment to efficiency trade-off is clearly defined. As we shall see later, with “solar desalination” the determination of this optimal ratio is further complicated by its linkage to the desalination plant investment-efficiency equation. It is the overall combined solar energy and desalination plant investment-efficiency ratio including heat storage, heat transport, and plant siting considerations - that is important. All collection systems, except for solar ponds, are of relatively small modular design. Large plants require batteries of parallel modules. This means that there are only limited economies of scale in solar collection. Economies of scale are obtained mainly with larger power generating equipment in projects where electric power is the end product. Though fairly large solar ponds may be built, they too are limited in size for practical operational reasons to about 170,000 m2 per pond. Larger plants require multiple ponds. Power production from solar ponds, however,

143

does not benefit from economies of scale in equipment. The Organic Rankine Cycle (ORC) power generation turbines suitable for solar ponds are limited today to 1.2 MW. For the future 5 MW turbines are projected. The high temperature collectors (300-1050°C) are more complex and expensive, requiring continuous tracking of the sun’s position (i.e., auxiliary energy for tracking drive and better insulations to minimize heat losses). The higher temperature heat, however, enables a higher thermodynamic efticiency in its conversion to electric power (Carnot’s Law). These solar systems should be examined, therefore, only in the context of joint production of water and power in a dual-purpose station. As will be discussed in the section on Energy Transport Considerations, if siting conditions are such that it is necessary to locate the solar collectors and the desalination plant far apart, then efficient and cost effective solar energy transmission can be only in the form of electric power. “Solar desalination” will then automatically mean use of a power-consuming desalination system (i.e, a SWRO or MVC plant), and our techno-economic study is reduced to the trivial comparison of the economics of power from high temperature solar collectors to power from conventional fossil fuel power stations on the basis of delivered cost to the desalination plant’s battery limits. Of the four basic types of high temperature collectors - cylindrical parabolic mirror concentrators, parabolic dish “double curvature” mirror collectors, the Fresnel lens refractor or reflector collectors (using acrylic material lenses), and the distributed heliostat central receiver (“solar tower”) - only the first meets the three selection criteria noted above and is examined in our study. The parabolic dish is costly and limited in size, and even in multiple units is not suitable for large-scale power generation. The central receiver is very suitable for multi-MW stations. Conceptual studies have been prepared of stations generating up to several hundred MW, taking advantage of the very high temperatures which can be developed in “solar towers” (over 1000°C) and the high thermodynamic efficiencies possible with such high temperature heat input. However, this technology is not developed sufficiently and is thus not yet commercial. The intermediate temperature collectors (lOO-300°C) utilize a limited degree of solar concentration and periodic or continuous variation of tilt towards the sun. The reduced complexity of the intermediate temperature collectors and their ability to use lower cost materials have resulted in lower specific investments per kcal, making them more economic vis-a-vis the high temperature collectors for applications where only moderate process steam pressures or heat input temperatures are required. This makes them potential

144

candidates for supplying heat to single-purpose, water only, TVC (or MSF) desalination plants. Of the various intermediate temperature solar collectors, we have examined two: l

l

Parabolic trough collectors (using glass mirrors), which reach temperatures of 120-180°C with periodic tracking and have a collection efficiency of about 40% at 150°C; and Evacuated tube flat collectors (with a selective high absorber coating), which reach temperatures of up to 200°C.

Of the two, we have selected the low cost parabolic trough collectors because evacuated tube collectors are less suitable for scale-up to large plants. Beyond a certain size (lo-15 cm) the glass tubes become too expensive, so that a large number of tubes with many connections are necessary for even a moderate size collecting field. Though no large-scale solar energy plants using moderate temperature parabolic troughs have been built to date, this is due more to lack of commercial demand, at current fuel prices, for large quantities of process steam at resultant costs than to equipment limitations. The equipment is physically similar to and even simpler than Luz’s collectors, and the technology is sufficiently developed and proven to allow its application on a scale as large, at least, as Luz’s initial 13 and 30 MW stations. In any such large station, the current size of the parabolic mirrors will probably be scaled-up and an automatic cleaning device will have to be developed, but both tasks are technically feasible and not too difficult to engineer. The low temperature collectors (up to 100°C) are stationary and do not require optical concentration. The most common are flat plate collectors, parabolic trough collectors (with aluminum mirrors) and solar ponds. These systems are fully developed and commercial and are suitable for combining with desalination plants. In fact, small demonstration solar desalination plants have already been built and tested with both flat plate and solar pond collectors. There are two principal solar ponds suitable for desalination: the salinity gradient solar pond and the thermal diode pond. We have chosen to concentrate in this study on the salinity gradient solar pond, since at this point it is more developed than the thermal diode pond, is more economical, and has already been tested in large-scale projects. Nevertheless, we will also briefly discuss the thermal diode pond technology, as it offers features that could eventually qualify it for the solar desalination projects under consideration.

145

This will require, though, that the cost of the transparent honeycomb absorber-insulator plates, which are the principal cost element in thermal diode ponds, be reduced through further engineering development and economies of large-scale production, as projected by their developer. Satisfactory long-term operating experience must also be demonstrated somewhere in larger ponds than tested to date. Though the electric power conversion efficiency of the solar ponds is only a low 1.O-1.4% due to the low top temperature of the power generation cycle, solar energy collection efficiencies are 16-20% (of total incident insolation) for salinity gradient ponds and about 40% for thermal diode ponds. As will be shown, these figures are more than acceptable for largescale solar desalination applications, but, due to the large collection areas required, the economics of such projects will also depend on the availability of suitable low-cost flat land. The desalination plant combination is a particularly good fit for the salinity gradient type of solar pond because the desalination plant will supply the pond with two premier requirements for its operation: make-up of salt losses (due to diffusion) and make-up of cooling water losses (due to evaporation). As for the desalination plant’s operating economy, the solar ponds satisfy its most important requirement, a continuous energy supply. Their built-in heat storage capacity enables achieving a high annual utilization factor and adds to the overall system’s flexibility. Both the MSF and the MED desalination processes are suitable for combining with solar ponds. However, MED plants are preferable. With the 80-90°C heat available from solar ponds, MSF plants will have ERs of only about 6:l and will require 2.5-4 kWh/m3 for process pumping. Lowtemperature MED (LT-MED) plants, on the other hand, can be designed with ERs as high as 13: 1 and require only 1S-2.0 kWh/m3 for pumping.

System descriptions Of the various solar collection systems developed to date, only the three systems identified above as most relevant to large-scale solar desalination will be described. High temperature collection The one-axis tracking parabolic mirror system as developed by Luz International Ltd. is, without doubt, the most developed, tested, and scaled-up

146

high temperature collection technology. It is a technology which has been applied on a large scale in California under conditions very similar to those existing in the Middle East. Its performance, investment, and operating costs are well known and documented. The Luz systems concentrate the sun’s direct radiation through the parabolic mirror system on a heat collecting element (HCE) positioned at the focus of each collector. The HCE is a metallic pipe enclosed in a vacuumsealed glass pipe, which provides insulation and minimizes heat losses. An electronic sensing and control system tracks the sun and regulates the parabolic troughs’ angle to optimize heat input. Normally, an east-west axis orientation gives the highest annual average heat input; but Luz, in its California Mojave Desert project, utilizes a single north-south axis orientation, which provides higher peak hours collection. The major portion of Luz’s revenues are derived from the sale of power during peak hours at peak rates. Efficient long-term performance of the system requires periodic cleaning of the mirror surfaces. The actual cleaning cycle depends on local dust conditions and wind and rain patterns, but is typically once every two weeks. Luz has developed an automatic cleaning device, which operates continuously and eases the cleaning maintenance burden. In the early 30 and 80 MW stations installed by Luz, the solar heat is absorbed by a synthetic oil flowing through the HCE, which is heated to about 400°C. The oil then circulates through a heat exchanger to produce the superheated steam which drives the power generating turbine generator (see Fig. 1). In the advanced, larger scale 200 MW stations designed later (see Fig. 2), the steam was to be generated in the solar collector units themselves without the intermediate oil circuit. This would have reduced equipment costs and pumping requirements due to latent, rather than sensible heat transfer at the collectors and the fact that the latent heat of water is high. It would also generate higher pressure steam by eliminating the temperature loss due to the secondary oil to water heat transfer temperature driving force. However, this new design requires additional development to overcome engineering problems related to two-phase, high pressure flow, surge, vibration control, etc. Nevertheless, the examination of desalination systems based on high temperature solar energy collection in this study will utilize performance and economics projected by Luz for this latest state-of-the-art technology.

Intermediate temperature collection The intermediate temperature trough system chosen is the CS-112 Solar Energy System manufactured by the Paz-Pimat Company, Raanana, Israel

147

OOLING WATER

HTFLOOP

+

SOLAR STEAM LOOP

Fig. 1. Luz current solar electric generating system.

FLASH EVAPO

1

FUEL

BOilCI feed

J’ 0.4 bar arm Back-pressure

Fig. 2. Luz advanced solar electric generating system with combined cycle back-up.

148

Fig. 3. Paz-Pimat intermediate temperature trough system: The CS-112 solar energy system.

(see Fig. 3). The CS-112 system is capable of reaching temperatures as high as 2OO”C, and at the temperatures of interest for desalination - 1 lo-150°C - has an efficiency for solar beam radiation of 35-40%. The system is simple, rugged, and inexpensive, and requires little mechanical maintenance. It consists of modular parabolic glass mirrors which concentrate and reflect solar radiation onto an overhead receiver located in their focal line. The receiver is a selectively coated pipe enclosed in a glass pipe. The entire array is movable on a single, horizontal, east-west axis, but there is no continuous, automated tracking. Alignment is performed manually three to six times a year. Automated tracking is available, though, as an option at extra cost and should be considered for the Iarge fields envisaged for regional projects. The heat transfer fluid is water. If steam is required, it is generated in an adjoining flash evaporator.

LQWtemperature collection The world leader in salinity gradient solar pond development is the Ormat Company of Yavne, Israel. This company has built a series of ponds, each increasing in size and efficiency, ranging from a 7,500 m2 (150 kWe) pilot plant in 1975 to a 250,000 m2 (2.2-5 MWe) full-scale plant in 1982. Ormat’s main interest in the ponds was power generation through its Organic Rankine

149

SOUR

INSOIATION

HEAT EXTRACTION WIND A’ITENUATION

PUMP

NETS

GRADIENT

LAYER

HEAT STORAGE LAYER

IMPERMEABLE SYNTHETIC MEMBRANE LINING

Fig. 4. Non-convective,

salinity gradient solar pond.

Cycle (ORC) turbogenerator power plants. However, other potential uses for its low grade heat, such as desalination and air conditioning, were also investigated in its ponds through pilot and demonstration plants. A salinity gradient solar pond (see Fig. 4) is a shallow pond several meters deep in which a salinity and density gradient is maintained. The highest salinity layer is at the bottom. It is heated by solar radiation, and its high density suppresses an upward convective movement, which would mix the pool and dissipate the heat throughout its volume. The heat is thus trapped in this bottom layer, which can reach temperatures approaching its boiling point. Heat is removed from the pond by pumping out this bottom layer, passing it through a heat-exchanger, and returning it to the bottom of the pool to avoid loss of salts. The density gradient above this layer will not be disturbed by such recirculation. In combination with an LT-MED desalination plant, it may be preferable to flash the hot bottom stream directly to generate vapor rather than use a metallic surface heat-exchanger. Though the high salinity brine at about 25-27% TDS has a high boiling point elevation (BPE), i.e., loss of temperature driving force, the vapor generated will contribute to desalination plant product (equivalent to another LT-MED plant effect). The reduced quantity of higher salinity brine returned to the bottom of the pond will cause downward movement of the pond’s layers and help overcome the slow natural degradation of its salinity gradient through upward salt diffusion. (It

150

has been called “the falling pond” by Dr. H. Tabor, the pioneering developer of salinity gradient, non-convective solar ponds). Sea water and/or the desalination plant brine reject can be used to flush the pond’s top layer and make up for water evaporation losses. The desalination plant powered by a back-up boiler could also assist during pond construction in the initial building up of its salinity (and temperature) gradient. This initial start-up process has to be augmented either by: (1) an addition of purchased salt at a rate of anywhere from 400-750 kg/m2 of pond surface, depending on heat storage layer depth; (2) brine concentration through a special brine concentrator evaporator (BCE), utilizing steam and/or mechanical energy (the BCE will be hired or leased only for the startup phase and subsequently transferred to other projects or uses); (3) brine concentration through a separate shallow pond and spray system developed by Ormat. In any case, the start-up process will require a significant up-front investment (as high as 25% of pond construction cost) and can last as long as one full year. The big advantage of salinity gradient solar ponds is that it is possible to increase bottom layer depth from 1.5 m to anywhere up to 6.5 m (maximal total pond depth is 10 m), thereby creating a heat storage capacitance, so important for night time, cloudy weather, and winter operation, In spite of this capacitance, though, there will be a drop in pond heat output during the winter, and the whole pond will cool off. Top brine temperature normally stays at ambient temperature plus 55°C; with special salts this differential can be increased to 60°C. The two main operational problems with salinity gradient ponds are: (1) loss of transparency through growth of algae and/or through fouling by those wind-blown dirt particles which do not settle to the bottom and (2) disturbance of the salinity and temperature gradients through waves and currents created by wind shear forces. The algae problem is solved by addition of chemicals and the suspended dust problem through the addition of flocculants. Waves created by wind forces are attenuated by floating a low-cost plastic netting developed by Ormat. Other operation and maintenance concerns with salinity gradient solar ponds are: (1) salt loss due to diffusion - requires salt make-up or, with a nearby desalination plant, bottom layer concentration; (2) salt, water, and heat losses due to ground leakage - overcome by proper sealing of the pond’s impermeable plastic lining; (3) loss of water due to evaporation overcome by seawater or brine make-up.

151

The thermal diode solar pond was developed by the Are1 Company of Israel to bypass most of the above O&M concerns and to avoid the need for a saline body of water. It operates normally with a non-saline pool, thus eliminating the need to purchase salt and/or to concentrate brine (initially, during pond construction and, later, for salinity gradient correction and maintenance). It can thus also utilize lower cost lining materials and less strict construction (welding) standards and precautions to protect against leakages. Leakages are more critical with salinity gradient type of ponds where they cause a costly loss of salt and, if the pond is built above an aquifer, its contamination. The Are1 thermal diode solar pond is marketed under the trade name “Thermolake”. It is similar in shape to the salinity gradient pond but uses a covering made of transparent honeycomb panels rather than the pool’s density gradient to trap solar energy. The panels designed by Are1 transmit a very high percentage of the incoming solar energy while suppressing both convective and radiative heat losses (i.e., act as absorbers-insulators). Current panel dimensions are 200 cm X 100 cm X 10 cm. The pond, which serves here, too, as the absorbing and heat storage medium, can similarly be designed for any size and depth. The panels protect it from dust and winds. With significantly lower top surface evaporative heat losses to the atmosphere and lower bottom layer heat losses to the ground (the hottest water in the thermal diode solar pond is at the top of the pool and not in the bottom layer), the Thermolake achieves an overall solar energy collection efficiency two to three times higher than a salinity gradient pond of equal size (about 40-45% vs. 16-20%). While normally the Thermolake’s body of water would be non-saline, it can also operate with saline waters. Any solar desalination project based on the Thermolake would use a pool constructed with seawater. As with a salinity gradient solar pond desalination project, the pool’s depth would be maximized to provide highest heat storage. Heat will be removed by flash cooling in the desalination plant, a stream taken from the hottest, in this case the top layer. This provides additional product and saves on heat exchanger costs. The flashing process will most likely be carried out in stages down to the lowest possible temperature to maximize heat removal and added product water per unit of pumped heat transfer fluid. To avoid build-up of the pool’s salinity, the cooled stream will be discharged and replaced by fresh, preheated seawater. Large Thermolakes, on the scale required for regional solar desalination projects, would have the thermal diode panels assembled on roof-like structures supported by floating barges.

152

Current technology - pe~ormance and economics The economics of solar energy collection - as a viable alternative to fossil fuel energy inputs to any desalination plant - boil down to the delivered cost per steady-state kcal or kWh at the desalination plant’s battery limits. Costs should include all investments and O&M requirements, not only for the collectors but also for the heat transport, storage, and back-up systems required to assure maximum utilization of the desalination plant. Since “cost per kcal heat” has no meaning unless the grade or temperature of this heat is defined, it is good engineering practice to convert all thermal energy inputs into a fossil fuel operated desalination plant to their * basic common denominator: “equivalent fuel consumption” (see the section on Technical and Economic Considerations in Designing Thermal Desalination Plants, above). Different ER plants, utilizing different grades of heat or steam (particularly within power cogeneration schemes), can thus have their true efficiencies measured and compared in terms of their “equivalent specific fuel consumption” in units of kg fuel/m3 product. The relative energy consumptions of electric power consuming desalination plants vis-a-vis thermal plants cannot thus be compared on a simplistic basis of 860 kcal/kWh. Electricity is high-grade energy. The comparison must take into account also the thermal conversion efficiencies of the stations generating their power, whether diesels at 40-48 % or large grid connected central stations at 37-40%, etc. Based on the heat content of the fuel feeding these stations (e.g., 6,500 kcallkg coal or 9,600 kcal/kg Bunker “C” fuel lower heat values [LHVs]), each electrical kWh will be charged for the fuel weight equivalent to 2-3 kWh thermal. The costs of all purchased power can be calculated accurately based on these station heat rates, fuel LHVs, and fuel costs. Solar energy costs to desalination plants utilizing power are similarly easy to derive. A kWh is a kWh, and its unit cost from a single-purpose solar power station (e.g., Luz’s SEGS) is known as accurately as if it came from a fossil fuel station. With thermal desalination plants using collected solar heat at various grades and within various schemes, the “equivalent specific fuel consumption” measure in kg fuel/m3 does not exist. Solar collector outputs as a function of heat delivery temperature (i.e., collection throughput and efficiency, including system heat losses) must be determined at each specific site (variable insolations, climatic conditions, etc.). Only then can the costs per kcal delivered to the desalination plant’s battery limits at any given

153

temperature be established. This figure is the bottom line for evaluating the solar collectors’ performance and economics. For solar cogeneration schemes (e.g., the Luz SEGS turbine operating at a back-pressure suitable for an LT-MED plant), the cost of low pressure steam input to the desalination plant must be calculated in the same manner as with fossil fuel cogeneration stations. The “equivalent fuel consumption” figure was derived therein as the marginal increase in boiler fuel consumed to generate the additional turbine throttle steam required to maintain the same net power output. For solar cogeneration this transforms into “the marginal increase in throttle steam cost (allowing for the proper cost-mix of both solar raised steam and backup fuel fired steam) required to maintain same net SEGS power output.” This cost has been roughly calculated below for Luz’s most advanced 200 MW reference station. As noted in the Introduction, land costs have been omitted from our economic analysis as they are site-specific. However, for each collection system we have identified its “specific land area” (not collection or “field” area) per kcal or kWh parameter. When the land costs at any candidate plant site are known, it will then be easy to introduce their contribution to unit product costs (see also Siting Considerations, below). Also ignored at this stage of our calculations is solar heat transfer fluid pumping. As noted below, pumping costs are an important consideration in desalination plant siting. Since the heat is transported in most cases sensibly, pumping requirements can be appreciable. (Luz has opted, therefore, in its latest designs for water rather than oil as its heat transfer fluid with in situ flashing and two-phase transport. With solar ponds, where hot layer recirculation can require about 6 kWh/ton of flashed low pressure steam, or about 0.6 kWh/m3 of product with a 1O:l ER LT-MED plant, the desalination plants must be sited right next to the ponds.) High temperature collection - electric power generation Luz International’s published performance and cost figures for the 30 and 80 MW SEGS installed by it in Southern California are summarized in Table XII. It is encouraging to note that these figures have improved significantly with plant size and date of installation due to technological developments (e.g., better HCE heat absorbing coating, higher solar field outlet temperature) and economies of scale (longer collectors, reduced number of flexible hoses, tracking microprocessors, temperature sensors,

154 TABLE XII Luz SEGS Station size

performance and costs

Temperature

Solar energy to power conversion efficiency

Efficiency

Solar collector

Steam turbine cycle

Average

Peak

Specific investment

Power cost

(MW

(“C)

(%)

(%)

(%)

(%)

WkW

(ClkWh)

30 30 80 200

349-oil 391-oil 391-oil 420-steam

40 45 45 -

30.6 37.5 37.6 -

lo-12 12-14 15-17

22.2 24.0 26-28

3860 2875 2000

11.5-12.0 7.5-8.5 5.8

and other auxiliary equipment). Nevertheless, at the fuel prices considered applicable to this study, costs are still higher than power costs from conventional fossil fueled power stations, making them economical only for peak power supply. As noted previously, our study will rely on expected.power costs from Luz’s state-of-the-art 200 MW station, also shown in Table XII. As can be seen, the costs claimed by Luz are almost competitive with fossil fuel power at non-peak hour rates. At certain sites remote from the grid they may even be cheaper than transported grid power. When fuel prices escalate, as is projected over the 2%year solar plant life, interest in solar power should grow. It should be borne in mind, however, that all costs shown below for Luz SEGS plants are predicated on the use of USA tax benefits and incentives applicable at the time of their publication. This is a clear indication that, as long as current fuel prices persist, solar desalination based on Luz-type equipment in other parts of the world will require similar economic benefits and incentives (or government subsidies) to be viable. Lower cost power and heat than shown above will be obtained if, according to Luz’s latest thinking, the annual utilization factor will be increased by operating a more efficient gas and steam turbine combined cycle system, rather than a simple gas fired boiler, during nights and low insolation times. The following figures were provided by Luz’s Israeli subsidiary for a 200 MWe solar power station using such a combined cycle back-up based on generalized site and weather conditions. No attempt was made to check and reconfirm them. Actual figures may vary according to specific site conditions.

155

Out of the installed capacity of 220 MW, 80 MW will be derived from the gas turbine and 140 MW from the steam turbine. Steam supply to the turbine will be 120 bar, 500°C. During operation, 80 MW will be derived from the gas turbine power output and 120 MW from the steam turbine output. (Note: the discharge of such a 120 MW steam turbine fits the 420-460 ton/h steam requirements of a 100,000 m3/d LT-MED plant - the reference size used herein to demonstrate LT-MED economics.) Of the 120 MW, during 2200 h/y, 100 MW will be due to the solar system’s input (i.e., annual power output attributable to the solar system will be 220,000 MWh/y), and 20 MW will be due to gas turbine exhaust gases heat recovery. The higher temperature exhaust gas heat will be used to increase solar raised steam temperature and power cycle efficiency. Combined cycle efficiency will then be 51%. It should be possible to increase the number of hours of such high efficiency combined cycle and solar system operation to about 4,400 h/y (50% annually) if a feasible heat storage scheme is developed and a solar heat storage capacity is added. During the remaining annual operating hours, solar heat input is substituted by supplementary firing of the waste heat boiler. The combined cycle efficiency will then be 44%. Station-specific investment will be $2,OOO/kW installed. Of this total investment, 50% will be due to the solar collection system. It is difficult to calculate the exact average cost of turbine throttle steam input to such a station, allowing for both solar and fossil fuel heat inputs at a 90% annual load factor operation. Nevertheless, we have tried to estimate it roughly to allow a comparison of a Luz SEGS and an LT-MED cogeneration scheme with other solar desalination schemes. Due to the superior economics of LT-MED in cogeneration with fossil fuel power stations (see section on Current Technology, above), the need for such a comparison is clear. The annual power output due to solar heat input will be 220,000 MWh/y, or 23.2% of total steam turbine annual output (at 90% utilization) of 0.90x 8760 h/y X 120 MW=946,000 MWh/y. (Note: Gut of the total 200 MW station output of 1,577,OOO MWh/y, solar collection provides only 14%! Solar contribution (“assistance”) is, indeed, quite marginal.) It is clear, therefore, that it is important, particularly for base loaded solar desalination applications, to invest in the most efficient and costeffective fossil fuel power generation section of plants. Its economics will dominate the Luz system and will determine if product water costs are viable. As shown in Table XI, the cost of electrical energy supplied to a SWRO desalination plant will comprise one-third to one-half of the total product water cost depending on the unit power cost. With an SEGS and

156

LT-MED cogeneration project, power must remain being the dual-purpose facility’s “main product,” subsidizing water production by bearing all indirect costs and enabling water output to be charged only with the station’s marginal costs. The specific investment in solar collection system per ton per year of throttle steam raised by it (at a specific investment in collection system of $l,OOO/kW and an assumed turbine heat rate of 2700 kcal/kWh (i.e., about 3.5 kg steam/kWh) is $26O/ton/y. For the purpose of this study, we will estimate the cost of solar generated steam based on its capital cost component only, excluding any participation in general operating costs. These costs will be later loaded on exported power cost only. The cost of solar heat raised steam at various interest rates for a 25-year amortization period and 2200 h/y annual utilization are given in Table XIII. TABLE XIII

Cost of solar heat raised steam Interest rate ( %I)

Capital recovery factor

Steam cost ($/ton)

6 8 10

0.07823 0.09368 0.11017

20.3 24.4 28.6

Since fossil fuel raised steam is dominant (77%), its cost will be approximated based on three fuel costs: $2S/MBtu, $3.O/MBtu, and $3S/MBtu (1 MBtu=l million Btu’s). The steam raised by supplementary firing of the waste heat boiler represents at (7,884-2,200) h/y x 100 MW=568,400 MWh/y, 60.1% of the steam turbine’s annual power output (946,000 MWh/y) and of its average annual throttle steam input. Its fuel cost (we will exclude boiler cost and maintenance), at a boiler efficiency of 88%, is: Cost of supplementary fired boiler steam: 2.5 Fuel cost ($/MBtu): 8.2 Steam cost ($/ton):

3.0 9.8

3.5 11.5

It is seen that Luz’s solar raised high pressure steam cannot compete with the steam raised by a fossil fuel fired boiler at any interest rate and fuel price. One wonders then if such a system has any justification at all as a power plant for desalination. An all-coal or heavy fuel fired conventional

157

power station would no doubt be simpler and, unless fuel prices really soar, also more economical. Steam generated from the gas turbine’s exhaust gases generates 20 MW continuously and represents 20 MW:120 MW= 16.7% of average turbine steam input. For the purpose of this initial approximate check, we will charge it with the cost of: 20 MW/@O MW +20 MW) =20% of the fuel consumed by the gas turbine. Based on available General Electric gas turbine heat rate data, about 10,400 Btu/kWh (2,400 kcal/kWh), the steam cost will be: Cost of waste heat raised steam: Fuel cost ($/MB@: Steam cost ($/ton):

2.5 5.7

3.0 6.9

3.5 8.0

The average unit cost of throttle steam input to the SEGS power generation steam turbine can now be calculated, as well as the cost of any 0.4 bar back-pressure steam supplied to an LT-MED plant operating in a cogeneration scheme with an SEGS back-pressure turbine. The increase in turbine heat rate due to such back pressure operation - vis-a-vis a condensing turbine generating the same net power output - translates to about 0.20 tons of additional throttle steam per ton of discharge steam. The costs of both 120 bars throttle and 0.4 bar discharge steams are shown in Table XIV. The low cost of the back-pressure steam again demonstrates the efficiency and energy cost advantage of an LT-MED plant cogeneration scheme. We can also now calculate the average costs of power produced from the entire SEGS, including both steam turbine and gas turbine power. These costs, shown in Table XV, will be required later for deriving solar desalination water costs from an SEGS and any adjoining electric power consuming desalination plant (e.g., an SWRO). Luz’s figure in Table XII of 5.8clkWh was based on a 30-year amortization period and unknown back-up fuel and money costs (including U.S. Federal energy tax and investment tax credits). Our calculation will be consistent with the ground rules used throughout this study: (1) a 25-year amortization period; (2) interest rates of 6%, 8%, and 10%; and (3) fuel rates ranging from !§2.5/MBtu to $3.5/MBtu. The 120 MW steam turbine’s energy cost component is calculated at 3.5 kg of throttle steam per kWh. The 80 MW gas turbine’s energy cost component is the 80% balance of its fuel consumption cost not charged to its exhaust gases heat. These are averaged.

158 TABLE XlV Luz SEGS high-pressure turbine throttle steam and 0.4 bar back-pressure steam costs Interest rate

Fuel cost

Solar steam cost

Suppl. firing steam cost

Exhaust gases steam cost

Average throttle steam cost

Back pressure steam cost

(23.2%)

(60.1%)

(16.7%)

(100%)

(@ 20%)

($/ton)

($/ton)

($hn)

($/ton)

(WMIW

(So)

($/ton)

2.5

6 8 10

20.3 24.4 28.6

8.2 8.2 8.2

5.7 5.7 5.7

10.6 11.5 12.5

2.1 2.3 2.5

3.0

6 8 10

20.3 24.4 28.6

9.8 9.8 9.8

6.9 6.9 6.9

11.7 12.6 13.8

2.3 2.5 2.8

3.5

6 8 10

20.3 24.4 28.6

11.5 11.5 11.5

8.0 8.0 8.0

12.8 13.7 14.7

2.6 2.7 2.8

TABLE XV Luz SEGS power costs 120 MW steam turbine energy cost

80 MW gas turbine energy cost

200MW average energy cost

200MW capital recovery cost

Other costs

($/MBtu) (Z)

(C/kWh)

(C/kWh)

(C/kWh)

(C/kWh)

(C/kWh) (C/kWh)

2.5

6 8 10

3.7 4.0 4.4

2.1 2.1 2.1

3.1 3.2 3.5

1.0 1.2 1.4

1.0 1.0 1.0

5.1 5.4 5.9

3.0

6 8 10

4.1 4.4 4.8

2.5 2.5 2.5

3.5 3.6 3.9

1.0 1.2 1.4

1.0 1.0 1.0

5.5 5.8 6.3

3.5

6 8 10

4.5 4.8 5.1

2.9 2.9 2.9

3.9 4.0 4.2

1.0 1.2 1.4

1.0 1.0 1.0

5.9 6.2 6.6

Fuel

Interest rate

Total power costs

159

Capital recovery costs have been calculated for the power generation section’s specific investment of $l,OOO/kW installed ($l,OOO/kW were already charged to solar energy collection). Other costs such as labor, maintenance, insurance, etc., have been estimated at a flat lc/kWh. According to Luz, the projected 200 MW station would have required 1 million m2 of field (collector) area. At a land area to field area ratio of 3-4: 1, this means 3-4 million m2 of land and a 15-20 m2/kW specific land requirement.

Intermediate temperature (IT) collection The specific investment in small to moderate scale Paz-Pimat CS-112 parabolic trough systems is about $200/m2 of solar field including all auxiliaries. For large systems on the scale of Luz’s SEGS, we estimate the figure should drop to about $150/m2. If continuous automated tracking is desired, it will add about $40-50/m2. Since we can expect the same limited number of solar insolation hours (2,200 h/y) as with the Luz station, the same considerations of fossil fuel backup and heat storage exist. Due to the lower steam pressures involved, heat storage should be easier. It will, nevertheless, represent a problem for any large-scale plant. Storage will also require the oversizing of collector areas to increase peak insolation hours energy catchment. Pressurized storage with insulated steel tanks will cost, installed, $250-400/m3 of tank. Such storage, if used, will contribute $6-lo/ton steam per year. Even if such storage is found justified to help overcome day-night intermittency, an auxiliary boiler will be required for winter-time operation. The system’s economics will be dominated, therefore, by fuel prices. Since a CS-112 and TVC solar desalination system will essentially operate as a single-purpose desalination station, this dominance will be more pronounced than with the Luz-LT-MED cogeneration scheme. Water costs will be correspondingly high. In fact, it is almost clear, apriori, that this solar desalination combination will not be any more competitive with its partial solar heat input than the single-purpose TVC plant was competitive with pure fossil fuel raised steam input. It should be noted, though, that the gas turbine used by Luz (albeit in an efficient combined cycle scheme) requires higher quality, higher cost fuel than the Bunker “C” fuel which would fire the boiler here. Also, the specific investment in the CS-112 solar collection areas is significantly lower than with Luz’s SEGS.

160

Thus, though exact calculation of solar derived steam costs (including some optimal storage capacity) and their averaging with back-up, auxiliary boiler raised steam costs, as done for the Luz SEGS, is not warranted for this study, we have made some approximations. The cost of solar raised steam by the CS-112 system depends on local insolation rates and the optimal steam pressure for the TVC desalination plant. With the 2,400 kWh/m*/y annual average insolation rate used throughout the study and a steam pressure of 4 bar (saturated - equivalent to 145”C), about 1.5 tons of steam are generated per m* area per year. The specific investment, using a manual alignment system, oversizing of field areas, some heat storage capacitance, and a back-up boiler is estimated at $180/m* field area, or $120/tori steam/y at a 40% collection efficiency. It is assumed that 35% of the total steam output will be solar derived and 65% fossil fuel derived. Boiler efficiency is 85% and fuel cost varying between $lOO-1401ton ($2,5/MBtu, $3.O/MBtu, and $3.5/MBtu), as for the Luz SEGS. Operating costs for the size of fields envisaged in this study are on the order of 80C/ton steam.

TABLE XVI Paz-Pimat system 4-bar steam costs Fuel cost

Interest rate

Capital recovery factor

Operating cost

Total steam cost

($/ton) ($/ton) ($/ton)

($/ton)

($/ton)

Capital cost component

Solar steam

($/MBtu) (I)

Boiler steam

Average: (.35 solar .65 boiler)

2.5

6 8 10

0.07823 0.09368 0.11017

9.4 11.2 13.2

6.7 6.7 6.7

7.6 8.3 9.0

0.8 0.8 0.8

8.4 9.1 9.8

3.0

6 8 10

0.07823 0.09368 0.11017

9.4 11.2 13.2

8.0 8.0 8.0

8.5 9.1 9.8

0.8 0.8 0.8

9.3 9.9 10.6

3.5

6 8 10

0.07823 0.09368 0.11017

9.4 11.2 13.2

9.3 9.3 9.3

9.3 10.0 10.7

0.8 0.8 0.8

10.1 10.8 11.5

161

Solar, boiler, and average system steam costs are then, at a 25year amortization and 90% overall utilization, as shown in Table XVI. It is seen that only at an interest rate of 6% and fuel costs of $3S/MBtu and up ($140/tori Bunker “C” fuel) can Paz-Pimat’s solar system steam be competitive with an all-fuel raised steam source. The ratio of land area to solar energy collection field area for this system is 2-3: 1. This translates into a specific land requirement of 1.3-2.0 m2/ton steam/y. In a system combined with fossil fuel firing, where solar raised steam comprises only 35 % of annual steam supply, specific area requirement per average annual steam output is OS-O.7 m2/ton steam/y. Low temperature collection The investments in salinity gradient solar ponds depend on site conditions and required earth works. As much as 40% of total pond costs can be attributed to such earth works. The availability of a flat level site and a handy clay source for pond construction is important. According to Ormat, in a typical site the breakdown of investments should be: $/m2 Earth works Synthetic membrane lining Wind attenuation nets Pumps, instrumentation, electrical, and other auxiliary equipment Start-up costs, including initial pumping and concentration of seawater fill and/or any salt purchases Total

3-6 3 1.5 2.5 2.5 12.5-15.5

We will approximate the cost of solar pond raised steam at 0.5 bar (saturated - equivalent to SOOC) by calculating its capital recovery cost component, as done for Luz SEGS and Paz-Pimat system solar raised steams, and adding pumping, labor, chemicals, maintenance, and other operating costs. For the scale of projects considered herein, several million m2 of ponds area, O&M costs should total 20c/m2/y or 26-32c/ton steam, depending on actual solar energy collection efficiency. Recirculated heat transfer brine pumping energy will add, at about 6 kWh/ton steam and SC/kWh, approximately 30c/ton steam. Other site-related costs will have to be worked out specifically for any candidate site and added on later, only on

162

the basis of actual site-related data (see Matching Desalination and Solar Collection Systems section, below). No fossil fuel back-up system will be used, and the pond will be oversized by 40% (at maximum depth) to allow a 90% annual utilization factor. In evaluating solar pond steam cost for desalination, it should be remembered that this steam, raised by flashing the pond’s hot brine layer, is added to the LT-MED’s product (i.e., the LT-MED plant’s ER must be increased by 1.0). Solar pond raised steam costs, corresponding to 16-20% collection efficiencies, an average annual insolation of 2,400 kWh/m2/y, and a 25year amortization period, are shown in Table XVII. These costs are, in fact, quite reasonable and make sense, even at current fuel prices, where only low-grade heat is required and no cogeneration scheme is possible. Without a cogeneration scheme, the exergy of high temperature fuel fired heat would be wasted, and solar pond raised heat becomes attractive. Realizing this, the developers of solar ponds have tried to identify all such applications requiring only low-grade heat. To date, only three applications have been found worth investigating: power production with Organic Rankine Cycle (ORC) systems, absorption type air-conditioning, and desalination. If the figures derived in this study are demonstrated

TABLE XVlI Solar pond 0.5 bar steam costs Specific investment

Collection efficiency

($h2)

(%)

12.5

16

20

15.5

16

20

Sp. steam production (ton/m2/y)

Interest rate

Operating costs ($/ton)

Steam costs

(%)

Capital costs ($/ton)

0.615 0.615 0.615 0.770 0.770 0.770

6 8 10 6 8 10

1.59 1.90 2.24 1.27 1.52 1.79

0.62 0.62 0.62 0.56 0.56 0.56

2.21 2.52 2.86 1.83 2.08 2.35

0.615 0.615 0.615 0.770 0.770 0.770

6 8 10 6 8 10

1.97 2.36 2.77 1.57 1.88 2.22

0.62 0.62 0.62 0.56 0.56 0.56

2.59 2.98 3.39 2.13 2.44 2.78

($/ton)

163

long-term in operation, it may very well be that desalination will turn out to be the solar ponds’ most promising, immediate, and largest scale application. The specific area requirement is 1.3-1.6 m*/ton steam/y. Summary

To bring all the figures derived in this section into focus, solar energy costs (at various grades and pressures) and specific area requirements for all candidate systems are summarized in Table XVIII. These figures will be used later in the section Expected Economics as inputs to calculate solar desalinated water costs. For the Luz and Paz-Pimat systems, which include fossil fuel back-up, the cost ranges shown correspond to a fuel cost range of $lOO-140/tori ($2S/MBtu to $3S/MBtu). The cost ranges shown for the solar ponds correspond to the ranges of specific investment and collection efficiency used in Table XVII. Luz system parameters are shown not only for the single- and dualpurpose power generation station (SEGS), but also for its solar collection field section alone, which could serve on its own as a high pressure steam raising station (for 2,200 h/y). The latter figures can thus be compared, just as a point of interest, to those for the Paz-Pimat station raising lower grade 4 bar steam. Paz-Pimat steam costs are about half of Luz’s. With the Luz cogeneration scheme, the 0.4 bar back-pressure steam is charged only for the marginal increase in land area due to the 20% additional solar collection surfaces required to increase solar energy derived throttle steam input. As seen in Table XVIII, the costs of low-grade steam from solar ponds are of the same order of magnitude as the cost of back-pressure steam from a Luz SEGS cogeneration scheme. The comparison between the two low pressure steam sources, however, favors solar ponds for the following reasons: l

l

l

l

Due to the need to use lighter, higher grade fuel with Luz’s combined cycle gas turbine scheme, only the higher fuel cost figures are relevant today. In the Luz scheme operating costs were absorbed fully by the power generating section, whereas with solar ponds they are borne fully within steam costs. The solar pond, which does not require any fossil fuel burning, is environmentally cleaner. Solar pond steam production does not entail the high investments and the complexity of operating and maintaining large dual-purpose power

Power

120 bar steam @ 2200 h/y

Power and 0.3-0.5 bar steam

4 bar steam @ 2750 h/y

4 bar steam @ 7885 h/y

Luz SEGS singlepurpose

Lu2 parabolic mirrors

Luz SEGS Cogen. scheme

Paz-Pimat parabolic mirrors

W boiler back-up

Solar ponds 0.3-0.5 bar steam

Type of energy

Solar system

1.3-l .6 1.3-1.6 1.3-l .6

OS-O.7 OS-O.7 OS-O.7

1.3-2 1.3-2 1.3-2

0.2 0.2 0.2

4-5 4-5 4-5

-

-

-

15-20 IS-20 15-20

15-20 15-20 15-20

13-21 13-21 13-21

120 120 120

120 120 120

200 200 200

260 260 260

6 8 10

-

1.8-2.6 2.1-3.0 2.4-3.4

8.4-10.1 9.1-10.8 9.8-l 1.5

10.2 12.0 14.0 6 8 10 6 8 10

2.1-2.6 2.3-2.7 2.5-2.8

20.3 24.4 28.6

6 8 10 6 8 10

-

$/ton stm

Energy costs

6 8 10

(%)

Interest rate

-

2000 2000 2000

$/kW

$/ton stmly

m”/ton stm/y

m2fkW

Specific investments

summary

Sp. area required

Solar energy system costs and area requirements

TABLE XVIII

-

-

-

5.1-5.9 5.4-6.2 5.9-6.6

-

5.1-5.9 5.4-6.2 5.9-6.6

ClkWh

165

l

stations (a mix of steam and gas turbines, exhaust gases heat recuperation, etc.) and the need to match loads and assure a buyer for the power output. Most importantly, within the context of solar desalination, it should be recalled that the condensate of the steam from any solar pond serving an LT-MED desalination plant will be added to the plant’s product. Its value, at resultant water costs, will be 67-8Wton (see Table XXIII)!

All in all, there is no doubt that the solarpond-LT-MED combination has a clear advantage over a Luz SEGS and LT-MED cogeneration scheme. It should be interesting now to compare the costs of solar pond steam also with the cost of 0.4 bar back-pressure steam derived from a fossil fuel operated cogeneration scheme. As explained in the section on Current Technology - Performance and Economics, the cost of such cogenerated “by-product” steam for the purpose of desalination can be approximated on the basis of the cost of its marginal equivalent specific fuel requirement. These steam costs, at an equivalent specific fuel consumption of about 15 kg fuel/ton steam and at various fuel prices, are shown in Table XIX. Table XX shows the break-even fuel prices at which the costs of solar pond raised steam over the ranges of specific pond investment, solar collection efficiency, and interest rates used in this study are equal to the cost of fossil fuel cogenerated steam. It is seen that under the best conditions, with a lowest cost pond at a specific investment of $12.5/m2 built with 6% interest on capital and operating at the highest expected collection efficiency of 20%) the break-even price for No. 6 fuel oil is $122/tori.. Solar pond raised steam cannot compete with fossil fuel raised cogenerated steam at current $lOO/ton fuel price. The break-even fuel price for the worst solar pond case (i.e., highest specific investment, $15.5/m2; lowest collection efficiency, 16%; and 10% interest) is $226/tori.. If interest for this worst case were to be reduced to 6%) the break-even fuel price would drop to $173/tori.. Low cost capital is certainly crucial to the viability of solar energy. Referring now to Table XI, which shows the effect of variations in fuel price over a range of $lOO-2OO/ton on desalted water costs, we can use the break-even fuel costs in Table XX to get a first insight on the economics to be expected from solar pond energized LT-MED plants. By interpolation we see from Table XI that under best solar pond conditions, with a break-even fuel cost of $122/tori at 6% interest and $157/tori at 10 % interest, LT-MED product water costs increase by 4c/m3 and 9c/m3, or 8 % and 15 % , respectively, vis-a-vis a conventional fossil fueled cogeneration scheme operating at today’s fuel price of $lOO/ton. These figures

166 TABLE XIX Steam costs at various fuel prices Cogenerated steam cost

Fossil fuel cost $/ton

$/~~tu

($/ton)

100 140 180 220

2.5 3.5 4.5 5.5

1.5 2.1 2.7 3.3

TABLE XX Solar pond steam break-even fuel costs Specific investment

Collection efficiency

Interest rate

Steam cost

Break-even fuel cost

($/m2)

(%I

@J)

($/ton)

($/ton)

12.5

16

6 8 10 6 8 10

2.21 2.52 2.86 1.83 2.08 2.35

147 168 191 122 139 157

6 8 10 6 8 10

2.59 2.98 3.39 2.13 2.44 2.78

173 199 226 142 163 185

20

15.5

16

20

are still better than those of a fossil fueled SWRO plant operating at any power cost. We also see that at the other extreme, under the worst solar pond conditions investigated in the study, with a break-even fuel cost of $173/tori at 6% interest and $226/tori at 10% interest, LT-MED plant water costs increase by 1 1C/m3 and 19C/m3, or 21% and 31%, respectively, vis-a-vis a fossil fueled cogeneration plant. These figures can compete with a fossil fueled SWRO plant when power costs are above 5C kWh and 7ClkWh respectively.

167

These comparisons, however, are preliminary and only approximations. They are based on the economics of an LT-MED plant designed for optimal turbine back-pressure steam operation and not optimal solar pond heat utilization, The optimal design and final economics will also reflect specific site conditions and other considerations which affect the proper matching and combining of the solar collection and desalination systems and overall costs. Some of these considerations will be discussed in the following section.

MATCHING

DESALINATION

AND SOLAR COLLECTION SYSTEMS

Solar desalination has been investigated continuously over the past thirty years. Initial plants were based on simple solar stills. Later plants were based on brackish water desalination processes such as ED and RO, driven by solar generated electric power, and seawater distillation processes such as MSF and MED, driven by solar heat. It is generally accepted today that at current fuel prices solar desalination water costs make these technologies applicable only for small remote communities where the alternative cost of water supplied through transportation is prohibitively high. In fact, only small-scale plants, on the order of tens to several hundreds of m3/d, at most, were actually built and tested. Reported desalinated water costs from these installations ranged from $2.5/m3 to $5.5/m3. Design studies for larger scale plants have been reported, but even these were for plants on the order of only several thousands m3/d. These economic studies have demonstrated that for solar desalination to be cost effective, it must maximize plant utilization through fuel firing backup. Just as “solar electric power generation” has become “solar assisted power generation, n so has the “solar desalination” concept become a “solar assisted desalination” concept. “Conventional” desalination plants (i.e., plants operating conventionally with fossil fuel energy sources) are utilized extensively today in arid, watershort areas throughout the world. Since most of these sites are, in fact, endowed with high solar insolation, it is clear that it is the high current cost of “collected” and delivered solar energy and the extreme variations in solar energy input rate (day vs. night, cloudiness, winter vs. summer) that has hindered the widespread application of solar desalination. It seems to us, nevertheless, that the costs of solar energy and overall desalinated water could be lowered in the large regional plants of concern in this study, if:

168 l

l

l

Both solar collection and desalination plant systems are properly designed and matched to suit each other, e.g., ERs and operating temperatures. Technical development and engineering efforts aimed at reducing specific investments are continued in both systems (e.g., adopting new materials of construction, improving heat transfer coefftcients and maximizing benefits due to economies of scale). Better heat storage and fossil fuel energy back-up schemes are developed.

Some of these aspects, related to matching solar and desalination systems on a large scale, particularly in the large regional plants envisioned in this study, are discussed below. The actual study and quantification of all these factors for specific sites is beyond the scope of this study.

Siting considerations

Siting of solar powered desalination plants is affected by various requirements, some of them outright conflicting: 1. The need to be near the sea to reduce both seawater supply and coolant and brine disposal lines and their pumping energy requirements. Long inland seawater supply and brine and coolant outfall lines may be problematic in regional projects sited near high density population centers. This is due not only because they may necessitate acquiring land passage rights from many private and public property owners, but also because, if the lines are buried to avoid landscape damage, there will always be the danger of eventual leakage and contamination of scarce underground waters (i.e., underground aquifers). The pipes will have to be properly coated (e.g., cement) and amply sized to reduce friction pressure losses (i.e., they will require a considerable investment). To appreciate the power consumption associated with pumping seawater to an inland thermal desalination plant, we estimate that, at a typical seawater to product ratio of 6: 1 (with a high ER plant), seawater supply specific pumping power consumption for each 10 km (6 miles) of distance and, say, 50 m of elevation, will be 4 kWh/m3 product. At Sc/kWh this will add 20c/m3 to desalinated water costs. Normally, desalination plant elevation is sufficient to discharge coolant and brine to the sea by gravity. 2. The need to be removed from the seafront where solar insolation is normally lower than inland. In some places the difference can reach 20-25 % .

169

3. The need for large solar energy collection areas and the high cost of sea-front real estate near population centers. Each $10/m2 of sea-front real-estate cost adds the following to desalinated water costs at a land lease rate of 4% per annum: l

l

l

l

With a solar pond and 13: 1 ER LT-MED system, at 0.1-O. 12 m2 area per m3/y of product - 4-4.8c/m3. With a Paz-Pimat parabolic collector and 15: 1 TVC system, at 0.0330.046 m2/m3/y of product - 1.3-1.8c/m3. With a Luz SEGS and a 5 kWh/m3 SWRO system, at 0.010-0.013 m2/ m3/y of product - 4-5c/m3. With a Luz SEGS and 1O:l ER LT-MED cogeneration system, at 0.02 m2/m3/y of product - 8c/m3.

4. The need to site the desalination plant as close as possible to the solar collection areas, to minimize heat losses and energy transport costs (particularly important for solar ponds). 5. The need to site the desalination plant as close as possible to the water consumers, to minimize product water transport and pumping costs. Product pumping from a desalination plant remote from the consumption center contributes about 0.35 kWh/m3 for each 10 km (6 miles) distance. At Sc/kWh this will add about 1.7c/m3 to product cost. Here too, property transfer rights for piping may also prove problematic and/or add costs. 6. For the case of a solar pond system, the need to consider site topography (to minimize earth moving works), the availability of clay (to reduce pond construction costs), and ground water table level and flow (to minimize heat losses). 7. The need to consider local dust conditions, wind velocities, directions and forces, and other weather related influences. These will affect collection surface structural supports and cleaning requirements. High winds are harmful to the maintaining of the salinity gradient in solar ponds. (Earthquake considerations, we assume, will be negligible for most, if not all, potential sites in the Middle East.)

Energy transpon considerations The easiest way to transport solar energy is in its electric power form by high voltage lines tied into the regional grid, where possible. Solar heat transport is costlier, requiring pumping a heat transfer fluid in insulated pipes to minimize heat losses. Solar heat transport costs depend not only on

170

the distance between the collectors and the desalination plant, but also on the chosen collection temperature and heat transport fluid temperature cooling range. Another factor to be considered and decided on is how to control heat transport to the desalination plant - for constant temperature, or constant thermal load, or some combination of these. This decision affects heat transfer fluid pumping requirements (costs), solar collector efficiency, and the desalination plant utilization factor. Energy storage or back-up supply considerations Solar insolation is available at variable rates only during about 3000 h/y. This is equivalent to about 2000-2,200 h/y at an average heat insolation rate, or 23-25% of 8760 h/y. Maximum utilization of the investment in any desalination plant requires, therefore, fossil fuel back-up for the remaining 6570% of the time, and, if practical and economic, also some heat storage capacitance. Indeed, as noted earlier, solar desalination, even more than solar power production (which is oriented to peak demand power supply), is in effect only a “solar assisted” operation. Winter-summer heat input variations, normally at ratios of up to 1:4, mean also that the systems must be oversized during the summer to allow for minimal outputs during winter time. This results in summer time “clipping” of system output with parabolic collectors through the idling or defocusing of some collectors. With solar ponds serving an LT-MED plant, however, there is no need for such clipping. LT-MED plants are capable of absorbing the higher heat inputs and, in fact, will increase water output, providing heat rejection capacity is also increased through a higher rate of seawater coolant supply. There is an optimal oversizing ratio for each system, and with fossil fuel energy back-up, also for each fuel price situation. With a solar pond the problem is less acute. High heat storage capacities can be built into the pond by increasing the bottom hot water layer to the maximum practical depth, 6.5 m. Though all pond temperatures drop during winter, this affects the performance of an associated LT-MED plant only somewhat since coolant “heat sink” temperature also drops. Average heat transfer coefficients are lower, but since LT-MED plant ERs are basically a function of their temperature differential, it is the differential which must be maintained. On an annual average, any wintertime decrease in water production will be compensated for by summertime over-production. Day-night and cloudy weather variations can certainly be handled by solar pond built-in storage.

171

With the intermediate temperature parabolic trough collectors and TVC plant combination, both short-term storage in insulated pressurized containers and supplementary steam raising with an auxiliary boiler will be required. The best combination and the entire back-up system have to be optimized. As discussed earlier, non-solar heat input and heat storage systems are most critical for the high temperature solar power and LT-MED cogeneration schemes. Here the intermittent nature of solar radiation affects not only the efficient utilization of the investment in the desalination plant, but also the efficient utilization of the investment in power generation equipment. Luz’s solution to this problem evolved from use of a gas fired boiler and storage combination to a more efficient gas turbine and waste heat recovery boiler combination with supplementary firing, backing the main power generating steam turbine. It is not clear to us at this point that this back-up scheme will also be optimal for any solar desalination cogeneration plant designed for regional projects. Other alternatives should be examined, comparing local gas turbine grade fuel prices to lower cost Bunker “C” fuel prices at various sulfur contents acceptable environmentally at any candidate site. Whicheverplant is eventuallychosen, it is clear thatserious consideration must be given to the proper design of its heat storage and back-up schemes to achieve XI% annual plant utilization.

Optimal temperature The development of most thermal solar energy systems to date has been oriented towards electric power production: how to improve its thermodynamic (Rankine cycle) efficiency and reduce its unit costs. This necessitated pushing solar energy temperatures higher and higher, above 400°C in the case of the Luz parabolic trough mirror system. The intermediate temperature collectors, unable to compete in power generation with high temperature collectors, found themselves a market niche as lower cost process steam suppliers. Their collection temperatures were dictated (albeit within a narrow range) by each specific process requirement. Solar ponds, producing power with low temperature ORC turbo-generators, succeeded in approaching temperatures just below maximum - boiling (100°C). Solar energy collection temperatures optimal for power and/or industrial process steam generation are not necessarily the optimal temperatures for solar desalination purposes. Offhand, we can say, though, that:

172 l

l

l

Since an LT-MED desalination plant in a cogeneration scheme benefits most when the power cycle efficiency is highest, there should not be any change in Luz’s efforts to maximize its solar collection temperature. With intermediate temperature parabolic collectors and TVC systems, steam throughput will increase and TVC thermocompressor performance will drop with temperature reduction. It will be necessary to optimize. With solar ponds, optimal temperature depends more on the heat transfer scheme (hot layer brine flashing or heat exchange) and less on heat losses considerations.

OPTIMAL SOLAR DESALINATION

SYSTEMS AND THEIR ECONOMICS

Optimal solar collection and desalination systems

Solar desalination will be characterized by large specific investments and low operating costs. Economies of scale will be manifested more within the desalination sections of any plant, less in the solar collectors. Lowest cost desalinated water will be obtained in a solar desalination plant when the combined specific investment (in $lm3/d) in both the solar energy collection system and the desalination plant is minimal and the annual equipment utilization factor maximal. Maximum desalination plant utilization in a solar energy system means provision for nighttime and some winter heat storage capability or back-up energy inputs, utilizing conventional fuel. The objective and main output of any actual design study willbe to define the solar energy and desalination system/processes, and their design parameters, which will achieve such a minimalspecific investmentand such a maximal utilizationfactor, under the actual conditions prevailing at the site. This optimization of design and economics of solar desalination plants for the region will be based on the following insolation, typical at the low latitudes of the Middle East: Yearly average

2,400 kWh/m2/y (note: inland figures could be as high as 2,500 kWhlm2/y)

Annual mean

280 w/m2

Daily continuous

6.75 kWh/m2

173

Four relevant solar collection and desalination plant combinations have been identified earlier in this study: 1. A high temperature Luz type parabolic troughs single-purpose solar power station (SEGS) with all its net power output going to an adjacent SWRO plant. The SWRO plant will be designed to produce a product with less than 500 ppm TDS to meet WHO standards. It will utilize the power station’s coolant reject as its preheated feed (membrane fluxes normally improve with increased seawater temperature, up to a limit of about 40°C) and will benefit from shared personnel and facilities. If prevailing fuel costs are high and the cost of SEGS power is competitive, the station power section may be oversized to export some power to gain overall economies of scale. Similar profitability considerations may lead to periodic reductions in the SWRO plant’s output to make more power available for export during peak demand hours when high tariffs are in force. 2. A dual-purpose Luz SEGS operating within a power and low pressure steam cogeneration scheme. The SEGS will employ a back-pressure turbine, which will exhaust its steam, at 0.4 bar (or some other pressure to be optimized), into an LT-MED plant. The power output from this station will be sold to the grid. Part of this power may be used to power an adjoining SWRO plant within a “hybrid” desalination station. The SWRO plant will be designed to produce lower quality, lower cost water, at 1,000 ppm TDS. This water will then be blended with the distilled (less than 25 ppm TDS) LT-MED product. As in case 1 above, the SWRO plant will utilize preheated feed (from the LT-MED coolant discharge) and its output may be reduced during peak demand hours to gain added revenues. According to industry studies, both capital and water costs from such a higher salinity product SWRO plant can be expected to be 12-15% lower than with a normal SWRO plant (larger fluxes, higher conversion ratios, longer membrane life-times). 3. IT Paz-Pimat type low-cost parabolic troughs generating moderate pressure steam at 150°C (or another temperature, to be optimized) for a TVC distillation plant. 4. Ormat type solar ponds supplying low grade heat to an LT-MED plant. As noted above, the optimal design parameters for these solar collector systems and desalination plant combinations (i.e. their operating temperatures, throughputs, efficiencies) will differ from those chosen for their normal “single-purpose” applications. These design optimizations are

174

beyond the scope of this study. We will, however, make some intuitive assumptions as to the optimal designs and estimate the costs for these designs. For example, it is fair to assume that, due to the high cost of solar heat, high ER TVC and LT-MED plants with a large number of effects and/or feed preheaters (i.e., a forward feed scheme) are warranted. Also, the designs will strive to reduce seawater and solar collector heat transfer fluid pumping requirements. It is also obvious that the high load or utilization factors necessary for maximizing annual water production and minimizing fixed charges per unit product will require designing for simplicity and high reliability. Not only must all technologies and equipment be well proven, but maintenance requiring standard items of equipment (SIE) (e.g., pumps, motors, and other rotating machines) must be designed for redundancy, i.e., include stand-bys and/or online maintenance. In large plants these SIE’s are less significant within total investment, and redundancy should contribute only negligibly to first cost. In evaluating each of the four chosen solar desalination combinations quantitatively, we exclude at this point land costs, seawater supply, coolant, brine disposal, solar energy transport fluid, and product water delivery systems and pumping costs. All these are site-related costs and should be introduced only after a search is conducted to identify potentially suitable sites in the region. Once the sites are identified and it is possible to evaluate their related auxiliary costs, these costs will be added to product costs from each of the four candidate solar desalination systems, as done in the section on Expected Economics, below.

Expected economics

Following are the estimated economics, excluding land and site-related costs (plant area requirements are noted separately) for the four chosen solar desalination systems. Energy costs are based on the solar systems’ steam and/or power costs shown in Table XVIII. Specific investments are per the Current Technology sections, above. A desalination plant with the same reference output of 100,000 m3/d (about 33 million m3/y) as used for Tables VIII-X was chosen for the comparison. As noted earlier, at an average per capita water consumption of about 125 I/d, it will suffice for an urban population of over 800,000. The scale of the required investments for the complete solar desalination projects is presented in Table XXI. Investments were taken as “hard costs”

175

TABLE XXI Estimated investments for solar desalination plants ($ million) Luz SEGS & LT-MED Solar system (w/o land costs) Desalination plant Civil engineering works

440*

Total

540”

90 10

Solar pond & LT-MED

IT troughs & TVC

Luz SEGS & SWRO

45-65

260

50

95 10

110 10

70 10

380

130

150-170

*Including requirements for generating 200 MW net.

only, excluding “soft costs” such as interest and insurance during construction, financing, legal costs, etc. For the Luz SEGS and LT-MED cogeneration scheme, investments include the complete cost of the 200 MW power station. It should be remembered, though, that almost all of these costs (at $2,OOO/kW) are related to power generation and are factored into the price of exported power. The desalination section imposes only a marginal 20% increase in the solar The Luz collectors section, at a cost of: 0.20 x $200,000,000=$40,000,000. SEGS associated with the SWRO plant is sized for the 21 MW required for SWRO plant power requirements only, at 5 kWh/m3. Land requirements (1000 m2) for the various solar desalination systems are: Luz SEGS & LT-MED

3000-4000 (including requirements for generating 200 MW net power output)

Solar pond & LT-MED

3300-4200

IT troughs & TVC

1100-1500

Luz SEGS & SWRO

320-420

Both LT-MED solar desalination systems require the most land area. Table XXII presents resultant water costs with solar energy inputs at various interest rates. Fuel cost variations from $lOO-140/tori ($2.53.5 MBtu) define the ranges of steam and power costs. Desalination plant

21

58-64

Capital recovery: @ 2%year amortization

Total water costs 72-76

30

42-46

37-43

Sub-total

25-28 7-8 3 3 2 2

21-26 6-7 3 3 2 2

Operating costs: steam Power O&M labor Parts Membranes Chemicals G&A (insurance, overhead, etc.)

10

6

Interest rate (%)

Luz SEGS 8~ LT-MED

52-58

22

30-36

2 2

14-20 6 3 3 -

6

65-73

31

34-42

18-26 6 3 3 2 2

10

Solar pond & LT-MED

Water cost breakdown for solar desalination plants (C/m3)

TABLE XXII

97-108

25

72-83

56-67 6 3 3 2 2

6

117-129

36

81-93

65-77 6 3 3 2 2

10

Paz troughs & TVC

65-69

--

16

49-53

26-30 3 3 11 4 2

6

76-79

23

53-56

30-33 3 3 11 4 2

10

Luz SEGS & SWRO

177

efficiencies (ERs and specific power consumptions) are as in Tables VIII-X. Capital recovery relates only to the investments in the desalination plants, as shown in Table XXI. Solar plant investments are accounted for in unit solar energy costs. It is seen that the solar pond and LT-MED combination provides lowest cost water, S-10% lower than with a 200 MW SEGS-LT-MED cogeneration plant. (And so, with solar desalination, unlike fossil fuel desalination, a single-purpose plant’s economics turn out to be better than its dual purpose counterpart’s.) Equally important, particularly when the task of raising capital to construct any solar desalination plant is faced, is the fact that the solar pond and LT-MED system also requires (with the SEGS-SWRO alternative) the lowest total investment: $1,500-1,700/m3/d. A more accurate economic study for all solar desalination systems, optimally designed and including site-related “soft” and all other costs omitted in this study, and a sensitivity analysis, will have to be prepared for each specific project before it is implemented. Nevertheless, to present as the final bottom-line outcome of this study, more complete water cost figures, and to enable a rough comparison with fossil fuel desalination, we will “guesstimate” these site-related, auxiliary-related, and “soft” costs, which were omitted so far from our calculations, and add them to the amounts given in Table XXII. We will assume that additional pumping requirements, covering seawater supply, product and other site-related fluid pumping, will be 1 kWh/m3 for the thermal plants (TVC and LT-MED) and 0.4 kWh/m3 for the SWRO plant (smaller seawater requirement). At SC/kWh, these additional pumping requirements will cost 5c and 2c/m3, respectively. All other costs, including any solar system costs neglected so far, are estimated conservatively at another 10c/m3. Total solar desalinated water costs, after including these 15c/m3 and 12c/m3 additional costs, are shown in Table XXIII. TABLE XXIII

Estimated total solar desalination water costs (C/m3) Interest (S)

Luz SEGS & LT-MED

Solar pond & LT-MED

IT troughs 8t TVC

Luz SEGS & SWRO

6 8 10

73-79 80-84 87-91

67-83 74-81 80-88

112-123 123-134 132-144

77-81 82-86 88-91

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To compare above total water costs for the solar pond and LT-MED combination with those from an LT-MED plant operating within a fossil fueled cogeneration scheme on the same total cost basis, we must add first an allowance of 5C/m3 for site-related seawater and product pumping (not included in Table XI) also to fossil fuel desalinated water costs. The comparison with such corrected Table XI figures shows that at 6% financing the break-even fuel costs are about $150/tori and at 10% interest about $200210/ton. Any study of the application of thermal seawater desalination plants in the region cannot be complete without considering schemes for utilizing the high purity of the distilled product (25 ppm TDS maximum, compared to 500 ppm TDS maximum recommended by the World Health Organization) to reduce unit product water cost. This means blending the distilled water with locally available low salinity brackish waters (l,OOO-1,500 ppm TDS maximum) or with higher salinity, but lower cost product from adjoining SWRO plants designed to specifically for such services. Table XXIV demonstrates the overall economies possible with such blending, using Table XXIII total estimated unblended water costs. For comparison, we have included also the SWRO water costs as given in Table XXIII. Blending does not apply to the higher salinity (400600 ppm TDS) SWRO plant product. Table XXIV, in effect, “does justice” to the thermal desalination systems by expressing and quantifying their advantage vis-a-vis SWRO, and in general, of producing a much higher quality product. This advantage would have been, otherwise, ignored. Brackish water cost is assumed at 1000-1500 ppm TDS to be 10C/m3. TABLE XXIV Estimated total water costs with blending (C/m3)

ppm TDS

Luz SEGS 8c LT-MED

Solar pond & LT-MED

IT troughs & TVC

Luz SEGS & SWRO

looo: 6% Interest 8% ” 10% ”

42-45 45-47 49-52

39-42 42-46 45-49

61-67 67-73 71-87

77-8 1 82-86 88-91

1500: 6% Interest 8% ” 10% ”

52-56 67-59 61-64

48-52 53-57 57-62

78-85 85-93 91-93

77-8 1 82-86 88-91

179

To appreciate the consequence of producing water at the above blended and unblended desalinated water costs, let us take a typical family of four, with an average per capita water consumption of 125 l/d (33 gal/d). The monthly cost of supplying such a family’s water consumption with our preferred solar pond and LT-MED system at the plant’s battery limits (excluding storage, chlorination, and distribution costs) is $6-9 for blended water and $11-14 for high quality unblended water. The dominating contribution of total fixed charges to overall water cost with the solar pond and LT-MED desalination plant combination demonstrates and reemphasizes the inherent and basic fact that solar desalination is a capital-intensive enterprise. A sensitivity analysis of the various factors making up these capital costs shows: 1. Solar pond investment is controlled directly in the same proportion by the two variables explored in Table XVII, specific construction costs and solar collection efficiency. These could vary by about 25% - $12.515.5/m* and 16-20%, respectively. Each 1.0% increase in pond construction cost or 1.0% decrease in collection efficiency increases solar pond steam costs by 0.83% and desalinated water costs from the solar pond and LT-MED combination by about 0.2%. 2. All calculated figures are based on a reference average annual insolation rate of 2,400 kcal/m*/y. Insolation rates vary in the Middle East within a similar range of about 25% (from 2,000-2,500 kcal/m*/y), and they directly affect solar pond investments in the same manner as the variables noted in above. Therefore, the same sensitivity ratios also apply to this variable. 3. The overall specific investments in the solar pond and LT-MED combination ranged from $1,500-1,700/m3/d. The effects of specific investments and interest rates on water costs with a 25-year amortization period and a 90% annual utilization factor are illustrated in Table XXV. 5c/m3 were added to the plant capital cost component as our estimate for the capital cost portion of the 15c/m3 allotted in Table XXIII for site-related, auxiliary-related and “soft” indirect costs. It is seen that capital costs comprise about two-thirds of total desalinated water costs with the solar pond and LT-MED combination. Each 1% increase in interest rate adds 3.5-4c/m3, or about 4.5-5%, to water costs. Low cost money is the key to reducing water cost. Tax incentives, subsidies, and/or grants from local governments and/or international aid will be necessary if the intention is to involve the private sector and attract investors.

180 TABLE XXV

Solar pond and LT-MED capital cost analysis Plant capital costs

Interest rate

($ mil.)

(%6)

150 150 150 170 170 170

6 8 10 6 8 10

Fixed charge factor

0.07823 0.09368 0.11017 0.07823 0.09368 0.11017

Capital cost component Total

Total water cost

Capital cost of water cost

Plant

Site

(C/m3)

(C/m3) (C/m3)

(C/m3)

VJ)

36 43 50 41 49 57

5 5 5 5 5 5

67 74 80 73 81 88

61 65 69 63 67 70

41 48 55 46 54 62

There are strong institutional considerations, beyond economics, which work in favor of solar desalination, such as its environmental cleanliness and safety. These considerations and benefits, properly articulated and quantified, will lead to public acceptance and support of a solar energy desalination program, providing its economics are within range of alternative options.

CONCLUSIONS

1. The solar pond and LT-MED combination appears to be the most economical solar desalination system. It requires the lowest overall investment and results in the lowest cost water. 2. This system is also the only one that can be all solar (i.e., will not require fuel fired heat backup to assure the high annual utilization factor required for solar desalination). It will be, therefore, totally non-polluting and environmentally acceptable near residential property and unaffected by future fuel cost escalations. 3. The system will require large land areas, about 33-42 m2/m3/d capacity. 4. The best siting for such a plant would be near the sea, not too far from existing and/or developing population centers. Interested water authorities will have to act as soon as possible, while space may still be available and uncommitted, to search, locate, purchase, and reserve suitable seafront land

181

for such future solar desalination plants. To avoid inflating solar desalination water costs, reasonably low land leasing rates will have to be assured. 5. The specific investment in large-scale solar pond and LT-MED systems with current technology is $1,500-l ,700/m3/d. 6. Low-cost money is critical to reduce the capital cost component associated with this investment. Government and/or international aid via grants, subsidized financing, and/or tax benefits have to be offered if the private sector is to be involved in such projects. 7. Water costs from the solar pond and LT-MED system, including estimates for site-related costs, range from 67-73C/m3 at 6% interest to 80-88c/m3 at 10% interest. 8. Due to the high purity of the system’s product, it is possible to blend it with low-cost, low salinity brackish water, if available. Water costs will then be reduced by 40-45 % with 1,000 ppm TDS brackish water blending, and by about 30% with 1,500 ppm brackish water blending. Blended water costs will then range from 39-62C/m3, depending on money cost and brackish water salinity. 9. The cost of producing the monthly water requirements (at plant battery limits, excluding chlorination, storage, and distribution) of an average family of four with a daily per capita water consumption of 125 l/d (33 gal/d), will be $ 6-g/month for blended water, and $ 11-14/month for extra high quality water. 10. These figures represent the limits of current technology in a high insolation location. It should be possible to demonstrate them in a large-scale prototype project sited in the lower latitudes of the Middle East (or elsewhere where good solar conditions exist). Further development of the solar pond and the LT-MED desalination system may even improve these figures somewhat. LT-MED technology is developed and proven, but design improvements for plants on the scale envisaged for regional projects can still be significant. Solar pond technology could benefit from further engineering development and the gaining of longer term operating experience with largescale ponds. No such experience exists to date. The main drawback with salinity gradient ponds is their susceptibility to upsets in the salt and temperature gradients due to extreme weather conditions. Such upsets require long-term shutdowns (up to a full year) to build up the proper gradients. Other solar pond techniques, such as the thermal diode pond, offer greater stability than salinity gradient ponds and several other advantages. However, the current high cost of the absorber-insulator plates for these ponds and the lack of large-scale operating experience prohibit their consideration today. Further development of this technology and large

182

scale production of the thermal diode honeycomb panels can reduce these costs greatly. Government and international funding should be enlisted to support all these development effort.

ABBREVIATIONS

ED ER IT LT-MED MSF MVC RO SEGS SWRO TVC

-

-

-

electrodialysis economy ratio (distilled water to steam input) intermediate temperature low temperature multi-effect distillation process multistage flash distillation process mechanical vapor compression distillation process reverse osmosis solar electric generating system by Luz seawater reverse osmosis process thermal vapor compression distillation process

BIBLIOGRAPHY 1 D. Hoffman, Low Temperature Evaporation Plants, presented at the 73rd A.1.Ch.E. Annual Meeting, Chicago, Nov. 16,1980, and printed in the October 1981 issue of CEPChemical Engineering Progress. 2 D. Hoffman, Low Temperature Distillation Plants - A Comparison with Seawater Reverse Osmosis, presented at the Ninth Annual Conferenceof The National Water Supply Association, Washington, D.C., May 31, 1981. 3 A. Zfati and D. Hoffman, Advanced Designs for Large Scale Low Temperature Multi Effect Seawater Distillation Plants, Internal ADAN Ltd. Study No. 102, September 1989. 4 G.F. Leitner, Total Water Cost on a Standard Basis for Three Large Operating S.W.R.O. Plants, presented at the Malta Desalination Conference, 1991; Desalination, 81 (1991) 39-48. 5 H.Z. Tabor, Solar Power Report 1989, prepared for the World Energy Council, January 1990. 6 P. Glueckstern and I. Spiewak, Israeli Experience in Solar-Thermal Heating and Cooling, presented at the U.N./USSR Workshop on Solar Energy, Moscow, September 21, 1987. 7 P. Glueckstern and G. Carmel, Solar Energy Development and Utilization in Israel Status and Prospects, Israel Ministry of Energy and Infrastructure paper, 1991. 8 A.A. Delyannis and E. Delyannis, Solar Desalination, Desalination, 50 (1984) 71-82. 9 H. Tabor, Solar Ponds as a Heat Source for Low Temperature Multi Effect Distillation Plants, presented at the 1 lth Israel Symposium on Desalination, March 1975.

183 10 H. Tabor, Solar Ponds - A Review Article, presented at the U.N. Conference on New and Renewable Energy Sources, August 1981. 1 I D. Zaslavsky, Thermolake - The Are1 Solar Pond, Technical Description, September 1986. 12 M. Rommel and V. Wittwer, Transparently Insulated Solar Pond, presented by the Fraunhofer institut fiir Solare Energiesysteme, Freiburg, FRG, at the Solar World Conference, Denver, 1991. 13 Y. Gilon, Luz Industries Israel Ltd., company literature and personal communication, December 1990. 14 D. Jaffe, S. Friedlander and D. Kearney, The Luz Solar Electric Generating Systems in California, Luz Publication, Tel Aviv, 1987. 15 B. Doron, U. Fischer, J. Weinberg, Ormat Turbines Ltd, company literature and personal communication, January 1991. 16 D. Foniman, Paz-Pimat Ltd., company literature and personal communication, February 1991. 17 S. Klier, Are1 Energy (1982) Ltd., company literature and personal communication, November, 1991. 18 D. Chorna, The Ben Gurion Test Center for Solar Electricity Generating Technologies at Sde-Boqer, Israel, inspection visit and discussions. 19 I. Spiewak, personal communication and discussions following critical review and comments on study draft, January 199 1.