Electricity and potable water from a solar tower power plant

Renewable Energy, Vol. 14, Nos. I-4, pp. 23-28, 1998

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Pergamon

© 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0960-1481 (98) 00042-1 0960-1481/98 $19.00+0.00

PII:

ELECTRICITY AND POTABLE W A T E R F R O M A SOLAR T O W E R P O W E R PLANT Dr.-Ing. Jt~rgen Rheinl~nder, Dr.-Ing. Frank Lippke * Dipl.-Ing. Manfred Schmitz-Goeb, L+C Steinmfiller, Gummersbach, Germany Dipl.-Ing. G~nter F. Tusel, G.F.T. GmbH, Homburg an der Saar, Germany * Center for Solar Energy and Hydrogen Research, Baden-Wuerttemberg (ZSW), Hessbruehlstr. 21 c, D-70565 Stuttgart, Germany

ABSTRACT The cogeneration of electricity and potable water utilising solar energy is studied, assuming solar tower power plants with the open volumetric PHOEBUS receiver. The results for alternative plant configurations show that the water production cost is about the same or even lower than the cost of water produced by conventionally fired systems. Furthermore, the integration offers a reduction of CO2-emissions related to the water production of up to 50%, additionally to the environmental benefits of solar electricity production. © 1998 Elsevier Science Ltd. All rights reserved. KEYWORDS Solar Energy, Solar Thermal Electricity, Solar Tower, Desalination, MED, Cogeneration

INTRODUCTION For Arab countries, generation of electricity and potable water is of high priority. The utilisation of solar energy as a primary energy source is attractive because of the generally favourable meteorological conditions. At the same time this concept is well foresighted as it will help to make fossil energy resources last longer in these countries. Solar tower power technology is mature for large scale application in units of 30 MWe or larger. Combination of solar electricity generation with the desalination of seawater is especially convenient in the case of the solar tower with volumetric air receiver. The hot air leaving the steam generator can be used to heat the desalination unit without need for extra fuel. This advantage is valid for both, the solar only tower with Rankine-steam process and the hybrid fossil/solar integrated combined cycle (ISCCS) with Brayton and Rankine processes. For exemplary cases the advantages and disadvantages of various process configurations are shown here as well as potential specific costs of water and electricity.

23

24

J. RHEINLANDER et al. THE PHOEBUS SOLAR TOWER PLANT

The PHOEBUS concept is based on a volumetric air receiver which has been throuroughly tested in pilot plants. It copes very well with severe and rapid transient solar conditions. The incident sunrays are reflected and concentrated by a mirror (heliostat) field onto a central receiver, which is mounted on a tower. The irradiation penetrates into this volumetric, steel wire mesh receiver, and is mainly absorbed and transformed into heat by the wires deeper within the absorber. The heat is transferred to the non-polluting, non-corrosive, non-inflammable, and universally available air, which is sucked through the knitted mesh at temperatures up to 700 °C. The hot air is fed into a conventional waste heat recovery steam generator (WHRSG) and the heat transferred drives a conventional power station. Since 1993 a test circuit with the air receiver and thermal storage and a thermal power of 3 MWth has been on test at the European Solar Research Centre Plataforma Solar de Almeria. The test results are excellent, showing that the receiver can be started in the morning at sunrise very quickly, since there are no large masses to be heated up. The solar power plant can be equipped at comparatively small extra expense with a burner system firing natural gas or light fuel oil. The burners are installed in the hot air duct behind the receiver and act as fossil back-up for the steam generator. With that, the PHOEBUS power tower can be operated either purely solar, fossil/hybrid, as well as fossil only. In principle several process circuit options for Phoebus are possible and have been evaluated in the past: • The basic concept of directly using the solar heated air for steam generation in a Rankine cycle. A lack of solar power or additional power demand can be compensated by the duct burner. Solar fractions up to 90% are possible. • The application of thermal energy storages in conjunction with a steam generator and steam turbine for solar-only operation. •

Direct integration of the solar air cycle with the gas-turbine outlet of a combined-cycle system. Depending on the operating mode solar fractions from 4% to 30% can be attained. Given suitable site conditions this represents an option for future Phoebus plants as far as levelized energy cost (LEC) is concerned.

All these process concepts are suitable for integrating a desalination plant. Operating the steam cycle in back pressure mode allowing for the heat of condensation to be used as heat supply to desalination is a well known concept, but it reduces the efficiency and the power output of the power plant. Using the exergy of the hot air leaving the steam generator is a cheap heat source for driving a Multi-Effect-Distillation (MED)-plant and does not restrict power output.. THE MED TECHNOLOGY For the current analysis, data on the MED technology will be used which corresponds to low temperature applications. The number of effects depends on the flue gas temperature and on the design of the heat exchanger needed to recover heat in the WHRSG. The specific investment cost is adapted to the varying capacities of the applications by the following equation:

__c = 0.7 + 0.3./~--eCo

Vv

with Vo=12000 m3/d and Co=2550 DM/(m3d) for the reference case with 25 effects.

(1)

Electricity and potable water from a solar tower power plant

25

COGENERATION OF ELECTRICITY AND POTABLE W A T E R BY PHOEBUS AND MED Within the PROSOL study (Lippke et al., 1996), detailed thermodynamic designs and comparisons of Rankine cycles with integrated solar combined cycle systems (ISCCS) were performed for power tower processes with a PHOEBUS receiver. Based on these results, the integration of an MED plant into both systems was studied. One important question to be answered by the investigation is how to operate the MED at night, when no solar energy is available. This may either be done by thermal storage or by extra fossil boilers which are used at night only. Generally, the comparison of systems with storage to those with a fossil back-up yields lower water production cost for the latter one. Main reason for this is the low capacity of the MED plant resulting in high specific investment cost, resulting from storing the heat. Combination of the PHOEBUS Receiver with Rankine Cycle and MED Process Scheme: The investigation of different possibilities for the integration of an MED plant into the PHOEBUS Rankine power plant showed that the most promising option is the production of additional low pressure steam in the WHRSG (Figure 1). With respect to the base Rankine power tower concept, the additional low pressure steam generator can be introduced into the WHRSG without penalising the efficiency of the electricity production. By doing so, an additional thermal power of 7.4 MWth can be utilised to produce low pressure saturated steam of 2 bar, which then feeds the MED plant. Since the solar power process is mainly operated during the day, the MED plant must be operated by means of an fossil boiler during the night. Performance & Cost Calculation: The first column in Table l summarises the results from the calculations on performance as well as on cost. It is assumed that the solar power plant is operated for 3000 hours a year and that the MED will be operated for 8400 hours (5400 by means of the back-up boiler). About 5920 m3/d of water will be produced by the MED. The back-up boiler consumes 47012 MWhth/a of fossil energy and the MED requires 3108 MWhe of electricity. About 22200 MWhth/a are supplied by the solar power tower (7.4 M W for 3000h). Altogether, the total investment cost of all MED-related components is about 20.4 million DM. Considering a depreciation period of 20 years, a depreciation rate of 7 %, 5 % of O&M cost, a fuel price of 0.02DM/kWhth for natural gas, and electricity cost of 0.245 DM/kWe ~, solar generated and 0.132 DM/kWhe from the net at night, the water production cost results in 2.13 DM/m 3 or 1.25 US$/m 3.

Combination o f the PHOEBUS Receiver with Combined Cycle and MED Process Scheme: For a solarised combined cycle which is operated throughout the day, the MED process can only be introduced at the outlet of the WHRSG (see Figure 2). This is due to the fact that for such a system, a two (or three)-pressure steam generator should be implemented in order to achieve the best heat recovery from the flue gas entering the WHRSG at a temperature lower than the hot air from the PHOEBUS receiver (700°C). As a result, the upper temperature of the MED has to stay well below 96 °C.

zThese 0.245 DM/kWhe is the cost predicted for the PHOEBUS Rankine power plant for the wheather conditions of Jordan

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J. RHEINLANDER et al.

An investigation on the number of effects o f the MED showed that 16 should be selected to optimise its performance, although the upper temperature 70 °C of the MED reduces the amount o f heat recoverable at the outlet o f the WHRSG. In total, 2.2 MWth can then be withdrawn from the flue gases during the day. At night, this is reduced to 44l kWth since no solar heat is available then. In order to operate the MED plant continuously at night, a fossil fired boiler is needed again. Performance & Cost Calculation: The second column in Table 1 shows the results for MED and ISCCS. In this case, 982 m3/d of water will be produced. The total investment cost can be estimated as 3.65 million DM. Considering a price of electricity of 0.082 DM/kWhe, which is the result o f the PHOEBUS-study for the ISCCS (Lippke et al., 1996), the water production cost yield 2.30 DM/m 3 or 1.35 US$/m 3.

Receiver 1(?0 MW

Burner 0 MW

,!i

L

m k Is

241

Fig. 1: Process scheme for the integration of the MED into a PHOEBUS Rankine cycle system.

, 187

241! MEO-HX 217 MW

96

O

Fig. 2: Process scheme for the integration of the MED into a PHOEBUS solar/hybrid combined cycle system.

THE REFERENCE FOSSIL FIRED MED PLANT For comparison purposes, Table 1 also gives an idea on the water production cost of a fossil fired, 25 effect M E D plant. The calculation follows the same procedure as for the cases with solar energy. However, one difference lies in the electricity cost to be supplied from the net. These are set to a consumer's price of 0.132 D M / k W h e which is twice the levelised electricity cost predicted within the PROSOL-study for a natural gas fired combined cycle. Since the results highly depend on the capacity of the system, two cases are shown in Table 1. The first, "Boiler l", shows the results for a water production rate of 5920 m~/d which is the same as that for the Rankine system. The other, "Boiler 2", is valid for a production rate of 982 m3/d, the same as for the ISCCS system.

Electricity and potable water from a solar tower power plant

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A s c a n be seen, the w a t e r p r o d u c t i o n costs are close to t h o s e for M E D c o m b i n e d to the solar p o w e r tower p r o c e s s e s . For a c a p a c i t y o f 5920 m3/d, cost o f 2.14 D M / m 3 ( c o m p a r e d to 2.13 D M / m 3) is predicted, for 982 m3/d this b e c o m e s 2.79 D M / m 3 ( c o m p a r e d to 2.30 D M / m 3 for the ISCCS).

Table: 1:

P e r f o r m a n c e and Cost Calculation for the Integration o f the M E D into P H O E B U S R a n k i n e and I S C C S cycles, in c o m p a r i s o n to a c o n v e n t i o n a l fossil fired s y s t e m

OPERATION Daytime operation Night-time (fossil) operation Yearly solar Operation Yearly operation of MED

Rankine 10 14 300 350

ISCCS 10 14 300 350

Boiler 1 10 14

7.4 25 30 5920 1.5 3108 2874 0.24 0.132

2.17 16 53 982 2.3 791 2763 0.082 0.082

7.4 25 30 5920 1.5 3108 2874

1.23 25 30 982 1.5 516 4459

0.132

0132

7.4 0.02 47012

1.73 0.02 9961

7.4 0.02 73129

COST CALCULATION FOR WATER PRODUCTION Investment cost for MED 17.0 Low pressure heat exchanger (estimated) 2.6 Fossil fired boiler @ 105 DM/MWth 0.74 Total investment cost 20.35

2.72 0.8 0.17 3.65

17.0

4.38

0.74 17.76

0.12 4.50

10~ DM 106 DM 106 DM 106 DM

2.93 0.54 0.94 4.41

0.51 0.06 0.20 0.79

2.56 0.41 1.46 4.43

0.65 0.07 0.24 0.96

106 DM/a 106DM 106DM 106DM

2.13 1.25

2.30 1.35

2.14 1.26

2.79 1.64

DM/m 3 US$/m3

350

Boiler 2 10 h 14 h d/a 350 d/a

MED

power supply Number of effects specific heat consumption Pth.~o¢¢ Vwa,er spec. electricity consumption yearly electricity, cons. spec. cost spec. electricity cost (solar generated) spec. electricity cost (from net/at night)

MWth kWh/m3 m3/d kWh/m~ MWh DM/(m3/d) DM/kWh DM/kWh

FOSSIL FIRED BURNER/BOILER

capacity spec. fuel price annual fuel consumption

1.23 MWth 0.02 DM/kWhth 12131 MWhth

Annual cost (9.4% annuity, 5% spec. O&M cost)

annual capital cost annual electricity cost annual fuel cost total annual cost WATER PRODUCTION COST water prod. cost water prod. cost @ 1.7 DM/US$

ENVIRONMENTAL

BENEFITS

To c o m b i n e solar t o w e r p o w e r plants with desalination o f water exploits two o p p o r t u n i t i e s for environm e n t a l benefits. T h e P R O S O L - s t u d y s h o w e d that a P H O E B U S R a n k i n e cycle a v o i d s 5 2 . 2 % o f the C O 2 - e m i s s i o n s o f a natural g a s fired c o m b i n e d cycle (CC). For an I S C C S the a v o i d a n c e is 11.6%. T h e specific e m i s s i o n s o f t h e s e plants are 191 g / k W h e (solar Rankine), 351 g / k W h e (ISCCS), and 397 g / k W h e for the C C s . C o n s i d e r i n g t h e s e e m i s s i o n s , and 216 g / k W h t h in case o f natural g a s used in the fossil fired b a c k u p boiler, T a b l e 2 s u m m a r i s e s the results for the M E D plants. A s can be seen, a s i g n i f i c a n t reduction of

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J. RHEINL,~NDER et al.

CO2-emissions can be achieved. For the Rankine cycle the emissions are cut by approx. 37%, for the ISCCS a reduction of 14% is predicted. Table: 2:

CO2-Emissions of fossil driven and into solar tower power plants integrated MED plants

COz-Emissions specific CO2-Emissions (natural gas) annual CO2-Emissions from natural gas specific CO2-Emissions (electricity cons.) annual CO2-Emissions (electricity cons.) TOTAL ANNUAL CO2-Emissions SPECIFIC CO2-Emissions

Rankine 216 10155 191 594 10748 5187

ISCCS 216 2152 351 277 2429 7068

Boiler I 216 15796 397 1234 17030 8219

Boiler 2 216 2620 397 205 2825 8219

[g/kWhth] It/a] [g/kWhe] [t/a] [t/a] [g/m 3]

CONCLUSIONS Solar tower power stations represent an up-to-date and promising technology for the reduction of fossil energy consumption. Extensive work has led to technically reliable solutions for the key solar components receiver and heliostat. The use of air as the heat carrier medium means much simpler construction and maintenance for the plant and demands less highly-specialised personnel. The convenient operation of the plant which is an inherent advantage of the PHOEBUS power plant technology makes it extremely suitable for the use in sun belt countries. Cogeneration of water and electricity is sensible in the view of economics and sustainability. The bargain of combining PHOEBUS with a MED-plant is found in the surprising fact that the heat for the MED-plant is delivered without reducing the power plant output or efficiency. The PHOEBUS technology is being offered as a turnkey power plant by L. & C. STEINMOLLER, a leading power station constructor. REFERENCES Lippke, F., Schmitz-Goeb, M. and Finker, A. (1996). PHOEBUS Power Tower Processes with the Open Volumetric Air Receiver. 8th Int. Symposium on Solar Thermal Concentrating Technologies, Oct. 6-11, 1996, Cologne, Germany