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Concentrating solar power for seawater thermal desalination
Desalination 396 (2016) 70–78
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Concentrating solar power for seawater thermal desalination☆ Osman Ahmed Hamed a,⁎, Hiroshi Kosaka b, Khalid H. Bamardouf a, Khalid Al-Shail a, Ahmed S. Al-Ghamdi a a b
Desalination Technologies Research Institute (DTRI), Saline Water Conversion Corporation (SWCC), P.O. Box 8328, Al-Jubail 30951, Saudi Arabia Hitachi Zosen Corp., Osaka, Japan
H I G H L I G H T S • Operational performance of Fresnel concentrating solar power (CSP) system. • Cost effectiveness of a commercial solar assisted thermal desalination plant • Impact of DNI, thermal energy storage and fuel cost on the feasibility of CSP assisted thermal desalination plant.
a r t i c l e
i n f o
Article history: Received 29 December 2015 Received in revised form 18 May 2016 Accepted 11 June 2016 Available online xxxx Keywords: CSP Fresnel Desalination Cost effectiveness
a b s t r a c t Extensive pilot plant experimental studies for a period of one year were carried out to study the impact of climatic conditions on the operational performance of an innovative Fresnel solar collecting system. The solar measurements revealed that the total yearly Direct Normal Irradiance (DNI) on the tested site amounts to 1132 kWh/ m2. The thermal collector efficiency, which depends on climatic conditions such as solar insolation, ambient temperature, receiver temperature as well as heat losses, ranges from 60% to 80%. The cost effectiveness when the tested Fresnel solar collection system with solar multiple of 1.0 (limited to day time operation) is combined with a commercial thermal desalination plant is compared with one completely run by fossil fuel. The breakeven fuel cost whereby the levelized cost of water of the two cases will be equal is yielded at a fuel cost of $92/bbl. When the tested Fresnel solar collection system is run at a location with a relatively high annual DNI level (1937 kWh/m2), the fuel breakeven cost falls to $52/bbl. This study also revealed that combining a Fresnel solar collection system with an MED thermal desalination plant under specific climatic conditions is considered more cost effective when operated without thermal energy storage. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The Saline Water Conversion Corporation (SWCC) of Saudi Arabia is currently operating small scale single purpose thermal desalination plants with water production capacities ranging from 250 to 9000 m3/ day. One of the major problems that impede the cost effectiveness of a single purpose thermal desalination plant is its high fuel energy consumption. Techno-economic feasibility of small scale MSF or MED thermal desalination plants driven directly by boilers can be greatly enhanced when solar energy is employed to provide all or part of the thermal energy consumption.
☆ Part of the paper is presented at the International Desalination Association (IDA) Conference held at San Diego, USA August 30–4 September, 2015. ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (O.A. Hamed), [email protected] (H. Kosaka).
http://dx.doi.org/10.1016/j.desal.2016.06.008 0011-9164/© 2016 Elsevier B.V. All rights reserved.
A solar distillation plant may consist of one integrated system (direct solar desalination) or two separate devices, the solar collector/accumulator and distiller (indirect solar desalination). Direct use of solar energy is through the direct heating of salty water by the sun through conventional solar stills for low water production [1]. Another example of direct use of solar energy is achieved by combination of the principle of humidification-dehumidification with solar desalination using air as a heat carrier [2,3]. For relatively large water production capacities, the solar energy is indirectly used to drive thermal desalination plants by capturing solar radiation through one of the modern technologies which transform the solar energy into heat using means such as parabolic trough and linear Fresnel collectors, evacuated tube collectors and salinity gradient solar ponds [4–20]. Concentrated solar power (CSP) technology stores the energy from solar radiation in a working fluid in the form of heat. This heat can then be used directly to run a conventional thermal desalination multistage flash ((MSF), multi-effect distillation (MED), multi-effect distillation with thermal vapor compression (MED/TVC) plant. The thermal energy can alternatively be converted into electrical energy through a conventional power generation
O.A. Hamed et al. / Desalination 396 (2016) 70–78
Nomenclature A total collector aperture area (m2) ATAN inverse tangent value CAPEX total capital expenditure of solar field ($) COSІθdІ cosine absolute value of sun's direction angle Cp specific heat (kJ/(kg °C)) CRF capital recovery factor DNI direct normal irradiation (kW m−2) actual or effective received irradiation (kW/m2) DNIeff ΔT oil temperature rise (°C) K_∥(Θ_∥) longitudinal correction factor K_⊥(Θ_⊥) transversal correction factor LCOW levelized cost of water ($/m3) n amortization period (years) OPEX total annual operational and maintenance expenditure ($) heat loss from receiver per unit length (W/m) Ploss net (useful) thermal energy absorbed by the receiver Pth (kW) q oil volumetric flow rate (m3/s) SINІθdІ sine absolute value of sun's direction angle average of inlet and outlet temperature of heat transfer Tabs fluid (°C) TAN tangent value MED annual water production (m3/year) Wc Z discount rate (%) optical efficiency at zero η0 overall solar system efficiency (%) ηsy thermal efficiency (%) ηth Θa sun's measured height angle (°) θd sun's direction angle (°) θII sun irradiation longitudinal angle (°) θL sun irradiation transversal angle (°) Π numerical value of PI (3.142) ϱ density (kg/m3)
plant and can then be used to run a reverse osmosis (RO), electrodialysis (ED) or mechanical vapor compression (MVC) desalination plant. CSP collectors developed and tested so far can be broadly divided into two categories: Line focus collectors and point focus collectors [21]. Line focus collectors include both the parabolic trough (PT) and Linear Fresnel (LF) types which use a single axis tracking system and can yield moderate temperatures up to 400 °C, while point focus collectors include parabolic dish and central receiver collectors with dual-axis tracking systems generating temperatures as high as 1000 °C or more. CSP technology offers two main advantages [21]. First, all CSP technologies can be combined with thermal energy storage systems. Second, CSP plants can be operated with fossil fuel backup (hybrid operation). The use of thermal energy storage systems and/or hybrid operation provides the possibility of continuous 24 h operation of solar assisted desalination plants. A number of studies have been reported comparing between parabolic trough and linear Fresnel applications [22–32]. Lined focused parabolic and linear Fresnel solar concentrators [22] both consist of a long reflector, which act as the only concentrator aligned on a north–south axis. One advantage of these systems is the tracking which is primarily only in one dimension. The reflector is rotated to track the sun's movement and it's reflected solar energy is concentrated along a focal line and captured by receiver tube containing a heat absorbing fluid that absorbs the concentrated heat. One-axis solar concentration provides a simple operation and highly reliable system to reach maximum operation temperatures about 400 °C. Normally, medium concentration ratios
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between 15 and 40 are attainable; therefore, one-axis sun tracking is required [23]. Synthetic oils are used as heat transfer fluid in conventional solar PT collectors, which limits the top temperature. Nevertheless, the synthetic oil may be replaced by water in order to generate steam directly into the absorber pipe and temperatures up to 400 °C may be allowed. Direct steam generation (DSG) offers the potential for higher performance of the plants and for cost reduction [24]. Parabolic trough collector (PTC) using DSG has identical collector structures as for thermal oil. On the other hand, linear Fresnel collectors (LFC) with DSG use a potentially cheaper design mainly due to the use of flat mirrors and structural advantages, however with a lower optical efficiency. DSG avoids the costs of heat transfer fluid and the central oil heated steam generator. The DSG system is not without its technical challenges, with the risk of overheating tubes and potential flow instabilities [25]. Sophisticated controls are required to accommodate the use of the two-phase flow of water and steam. Compared with parabolic troughs, linear Fresnel collectors suffer from lower optical efficiency [26]. However, the low-profile setting of the linear Fresnel collector poses no mechanical difficulty to maximize the collector geometrical concentration ratio (the ratio of mirror aperture to receiver aperture), which enables high temperature output. The high temperature output would give rise to high power cycle efficiency and accordingly a great reduction of storage system cost. The low-profile setting of the mirrors also leads to a lower wind load requirement and thus lower-cost mirror assembly design. Further, it will also help in lowering the O&M cost for a power plant. The fixed receiver assembly greatly reduces the risk of heat transfer fluid (HTF) leakage and the resulting maintenance labor. A comparison has been made between the optical performance of parabolic trough collectors and linear Fresnel reflectors using multi-tube receivers and secondary [27]. The results reveal that PTC efficiency is higher than the efficiency of LFCs, either with multi-tube or secondary reflector receiver. This was due to the fact that PTCs conform a perfect parabola with its aperture perpendicular to the impinging beam, in the transversal plane, at all moments. However, LFC are characterized by a simpler configuration: narrower mirrors, and thus lighter structure, fixed receiver, and leakages avoidance. Comparison of the annual performance and economic feasibility of Integrated Solar Combined Cycles (ISCC) using two solar concentration technologies: parabolic trough collectors and linear Fresnel reflectors, is reported [28]. Results show that the thermal contribution is higher with PTC, but LFR may improve the economic feasibility of the plant. Existing commercial CSP plants are mainly used for electricity generation rather than water production. The National Renewable Energy Laboratory (NREL) [33], has compiled data on concentrating solar power (CSP) projects around the world that are either under operational, construction, or development stage. CSP technologies include parabolic trough, linear Fresnel reflector, power tower, and dish/engine. The majority of solar assisted power generation plants are using parabolic trough collectors with planned electricity generation per plant in the range 1800 to 175,000 MWh/year and are equipped with thermal storage system of molten salt (60% sodium nitrate, 40% potassium nitrate). There are seven operational solar power projects which are employing Fresnel solar collectors with plant generated electricity in the range of 2000 to 280,000 MWh/year and the majority of which are without storage. Around 24 concentrating solar power (CSP) projects that are either operating or under construction use power tower systems and molten salt for storage. Only one CSP project is under construction that uses dish/engine systems with at turbine capacity of 1 MW and without storage system. A number of parabolic collector desalination demonstration plants have been implemented and tested [6]. At the plataforma Solar de Almeria, Spain, a parabolic trough collector field was connected to an MED plant with a water thermal storage system. At the second phase of the project, a double-effect absorption heat pump was coupled with the solar desalination plant. Subsequently, the thermal energy
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consumption of the desalination plant decreased from 63 kWh/m3 to 36 kWh/m3. A comprehensive review of the salient design and operating features of solar assisted thermal desalination plants were reported [6,20]. Linear Fresnel systems have recently been developed to achieve a more simple design and lower cost than the parabolic trough. Compared to the existing parabolic trough, the linear Fresnel collector system designed by Novatec-Biosol [34] shows a weight reduction per square meter of 80% resulting in both lower capital cost and reduces life cycle emissions. Also, land use efficiency with a linear Fresnel is about 2 times higher than that of a parabolic trough. The use of Fresnel collector in desalination is limited. One of the problems experienced with conventional slightly curved Fresnel mirrors is that the focal point of reflected sunlight reaching the receiver can vary depending on mirror location and sun angle. This study was carried out jointly by the Desalination Technologies Research Institute (DTRI) of SWCC and Hitachi Zosen Corporation (Hitz) of Japan to explore the prospects of the application of an improved Fresnel CSP collector system when integrated with a thermal desalination plant under the climatic conditions of Saudi Arabia. The improved Fresnel CSP collector is equipped with a new mechanism that controls the curve of each mirror, together with sun tracking to focus all reflected sunlight on the receiver tubes and consequently improve the solar collector thermal and optical efficiencies. A fully controlled innovative Fresnel solar collector system was constructed and installed at DTRI premises in Al-Jubail city, Saudi Arabia. Extensive experimental tests covering a one year period were carried out to assess the operational performance of the solar collecting system under the Saudi Arabia site specific solar radiation conditions and ambient temperatures. The experimental test results are then utilized to explore both the technical and economic potential of a commercial solar assisted multi-effect desalination plant. 2. Plant description As shown in Fig. 1, the CSP demonstration plant consists of a solar concentrating system, a steam generator and a condenser. The solar concentrating and collecting plant consists of innovative solar Fresnel reflectors, receiver tube, and low and high temperature heat transferred fluid oil accumulator tanks. The solar reflector consists of 6 parallel rows. Each row consists of 92 controlled curved mirrors positioned in south-north horizontal axis and with a total aperture area of 662.4 m2. The mirrors focus the sun's direct beam radiation onto a linear absorber pipe at the focus of the Fresnel mirrors. The absorber pipe is formed from 24 SCHOTT PTR®70 receivers which are designed to operate with oil-based heat transfer fluids at temperatures up to 400 °C and having more than 95% of heat absorption coefficient and less than 10% of emissivity characteristics. Sun.'s rays on Fresnel reflectors are concentrated more than seventy times on the receiver tube positioned at a
height of 3.8 m and then transferred to the oil-based heat transfer fluid (HTF) and flowing inside the receiver tube. Heated HTF is accumulated in the high temperature tank and sent to steam generator as a heat source. Low temperature HTF discharged from the steam generator is returned to the low temperature tank and sent to the receiver tube again by thermal HTF circulation pumps. The steam generated can then either be passed to a condenser that simulates an MSF brine heater or passed directly to an existing MSF pilot plant to supply part of its steam requirements. A pyrheliometer with a sensitivity of (8.24 ∗ 10∗∗6 V) / (W/m2) is used for measurement of direct normal irradiance (DNI) and it is linked with a tracking system to follow the apparent movement of the sun and monitor sun direction and height angles with a 0.01 °C accuracy. Temperatures are measured by platinum thin film RTD class B sensors with tolerance of ±0.3 °C. The oil flow rate is measured with a flow meter with a measured error of ±0.05%. The pressure transmitters are having an accuracy of ± 0.25%. Steam flow rate and pressure are measured by a flow meter equipped with dual sensors measuring pressure differential and pressure simultaneously and separately with a high accuracy specification. 3. Methodology The main objective of the experimental study is to assess the thermal and optical performance characteristics of the modified Fresnel and to what extent it is influenced by the local site climatic conditions. Site measurements which are utilized to assess the operational performance of the Fresnel solar collecting system included daily variation in ambient conditions such as temperature, direct normal irradiation, sun direction and height angles. The inlet and outlet temperature of the oil passing through the receiver as well its volumetric flow rates were also monitored together with the quantity, pressure and temperature of the generated steam. A data acquisition system integrated with solar system collects and stores all the monitored and measured data every 10 s. The 10 s readings are then used as input to an excel program to calculate the quantity of thermal energy absorbed and lost by the receiver and the actual amount of solar incidence received by the collector. For each calculated parameter, an average daily value of the 10 s readings is obtained by dividing the summation of all the daily 10 s readings by the number of readings. The monthly readings are then obtained by the summation of the corresponding daily readings. 3.1. Overall solar system efficiency The overall solar system efficiency (ηsy) represents the ratio of total thermal energy absorbed by the receiver (Pth/A) to the incident direct normal solar irradiation (DNI). It excludes sun position and height by assuming that all mirrors are perpendicular to the impinging solar beam and disregard all other imperfections. Overall solar system efficiency = (useful absorbed energy / received sunlight) ηsy ¼ ðPth =AÞ=DNI
ð1Þ
A = Total collector aperture area. The net (useful) thermal energy absorbed by the receiver and transferred to the oil passing inside the receiver tube (Pth) is determined from the following equation: Pth ðWÞ ¼ q ϱ Cp ΔT Where
Fig. 1. Experimental setup of the solar collecting system.
q ϱ Cp ΔT
oil volumetric flow rate (m3/s) density (kg/m3) specific heat (kJ/(kg °C)) oil temperature rise (°C)
ð2Þ
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Fig. 2. Impact of variation of incidence angles in transversal and longitudinal dimension on the correction factors for the energy yield at the tested Solar Fresnel collector.
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Fig. 4. The daily variation of the overall solar system efficiency (ηsy).
determined from the following relationship [35]: Daily average overall thermal efficiency ¼ ∑ððPth =AÞ=∑DNI
ð3Þ
3.2. Collector thermal efficiency Out of the total amount of solar radiation incident on the solar collectors, only a fraction is converted into thermal energy while the rest is lost to the environment by the reflection of the incoming radiation and to the ambient by various heat transfer mechanisms. The collector thermal efficiency (ηth) is defined as the ratio of the useful thermal energy delivered to the actual energy incident on the collector aperture. ηth ¼ ðcollected thermal energy by HTF oilÞ=ðactual received sunlightÞ ð4Þ ηth ¼ Pth =ðA DNIeff Þ
ð5Þ
Where DNIeff = The actual or effective received irradiation. The effective received irradiation of the LFR system is limited by the angle at which the sunlight strikes the reflectors and because the mirrors must be oriented to reflect the irradiation to the receiver, they most often do not directly face the sun. This non-normal orientation towards the incoming sunlight is the primary optical loss for LFR, and the loss is incurred both with respect to the transversal plane (perpendicular to the axis of the collector) and longitudinal plane (parallel with the axis of the collector). DNIeff is conventionally
DNIeff ¼ DNI K ⊥ðΘ ⊥Þ K ∥ðΘ ∥Þ
ð6Þ
Where K_⊥(Θ_⊥) and K_∥(Θ_∥) are transversal and longitudinal correction factors respectively and depend strongly on the position of the sun with respect to the collector [35]. They are dependent on the sun's measured height angle (Θa) and sun's direction angle (θd). The transversal K_⊥(Θ_⊥) and longitudinal K_∥(Θ_∥) correction factors are usually described respectively as a function of the transversal (θL) and longitudinal (θII) solar incidence angles on the collector [35]. θL and θII are computed from the measured(Θa) and (θd) solar angles using the following equation: θL ¼ 90−ATANððTANðθa=180 πÞÞ=ðSINІθdІ=180 πÞÞÞ=π 180
ð7Þ
θII ¼ 90−ATANðІðTANðθa=180 πÞІ=ðCOSІθdІ=180 πÞÞÞÞ=π 180 ð8Þ The Transversal correction factors K_⊥(Θ_⊥) which accounts to the mirror shading and blocking and for the collector under study is computed from sun irradiation transversal angle (θL) using the following equation: K ⊥ðΘ ⊥Þ¼ −1 10ð−6Þ θL3 –4 10ð−7Þ θL2 þ ð0:0001 θLÞ þ 0:9978
ð9Þ
The longitudinal correction factors K_∥(Θ_∥) which accounts for the energy that does not impinge in the receiver due to the longitudinal component of the irradiance [27] is determined from irradiation
Fig. 3. The monthly variation of direct and effective irradiance, thermal energy absorbed by receiver and receiver thermal losses.
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Fig. 5. The daily variation of the thermal collector efficiency(ηth).
longitudinal angle (θII) using the following equation: K ∥ðΘ ∥Þ ¼ 7 10ð−10Þ θII5 –9 10ð−8Þ θII4 þ 2 10ð−6Þ θII3 –3 10ð−5Þ θII2 −ð0:005 θIIÞ þ 1:0001
Tabs is calculated from the average of inlet and outlet temperature of heat transfer fluid in °C.
4. Results & discussion
ð10Þ 4.1. Thermal and optical performance assessment Fig. 2 shows the impact of variation of sun irradiation longitudinal or transversal angle on the corresponding longitudinal or transversal correction factors. It also shows that the predicted values of the longitudinal or transversal correction factors for the Fresnel solar collector tested in this study are to a great extent comparable with Novtec published correction factors [34]. 3.3. Optical efficiency The optical efficiency depends on the optical properties of the materials involved, the geometry of the collector and the various imperfections arising from the construction of the collector with regard to receiver alignment to the focal line of the collector, and the system's tracking precision [36]. It is defined as the ratio of energy absorbed by the receiver to the energy incident on the collector's aperture as shown in the following relationship: η0 ¼ ððPth =AÞ þ Ploss Þ=DNIeff
ð11Þ
Where Ploss is the heat loss from receiver per unit length and is determined from the following empirical equation [37]: Ploss ðW=mÞ ¼ 0:141･Tabs þ ð6:48E−9Þ･Tabs 4
ð12Þ
The experimental results revealed that at the examined site, the maximum direct normal irradiation (DNI) from sun on the ground is relatively low and in most cases range from 350 to 500 W/m2. As shown in Fig. 3, the monthly measured DNI varies from as low as 65 kWh/m2 in January to about 125 kWh/m2 in June and the total yearly irradiation amounts to 1132 kWh/m2. The actual or effective energy incident on the collector's aperture is less than this maximum irradiation because all the mirrors will not be perpendicular to the impinging solar beam. As shown in Fig. 3, the monthly actual impinging solar irradiation flux varies from 32 to 107 kWh/m2 and around 13% to 57% of the direct solar irradiation is dissipated. The actual total yearly solar energy flux impinging on the solar collectors aperture is 835 kWh/m2 which represents 73.76% of the maximum impinging solar power. Fig. 3 also shows that the monthly thermal energy flux absorbed by oil varies from 21 kWh/m2 in January to 58.3 kWh/m2 in June. The total yearly thermal energy received by the oil is 414 kWh/m2 which represents 36.65 and 49.6% of the maximum solar power that would impinge onto the reflecting surface and the actual solar radiation striking the collector surface, respectively. Fig. 4 Shows the variation of the overall solar system efficiency (ηsy) with the year days. The temperature of the HTF leaving the receiver is initially set for the first two months at 340 °C. If the HTF outlet
Fig. 6. The daily variation of the optical efficiency(η0).
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Table 1 Performance characteristics of reported Fresnel solar collectors.
Collector reference condition External temperature Inflow temperature Outflow temperature No wind Direct normal irradiation Transversal angle θ_⊥ Longitudinal angle θ_// Optical efficiency at zero Thermal power output per unit area W/m2
The anticipated performance of the Fresnel collector evaluated in this study Optical efficiency at zero of Fresnel collector evaluated in this study Thermal power output per unit area W/m2 Percentage increase in thermal power output
Case 1 Novatec Solar [34]
Case 2 Novatec Solar [34]
Case 3 Industrial Solar GmbH reported [38]
40 °C 100 °C 300 °C No wind 900 W/m2 30° 10° 0.67 Novatec solar 537 W/ m2
40 °C 300 °C 500 °C No wind 900 W/m2 30° 10° 0.65 Novatec solar 525 W/m2
30 °C 160 °C 180 °C No wind 900 W/m2 30° 0° 0.663 Industrial solar 562 W/m2
0.73 Present study 620 W/m2 15.5%
0.73 Present study 567 W/m2 8.0%
0.73 Present study 632 W/m2 12.5%
temperature is below the set value, heated HTF oil is recycled and returned to the receiver tube. Otherwise it is passed to the steam generator. As a result of the prevailing site solar irradiance, it has been found that it is more appropriate to set the outlet temperature of the oil leaving the receiver at a relatively low value. It was first reduced from 340 °C to 250 °C and then finally to 225 °C. The overall system efficiency during the period when the outlet temperature is set to 225 °C, ranges from 30% to 63%. As shown in Fig. 5, the collector thermal efficiency which depends on climatic conditions such as solar insolation, ambient temperature, receiver temperature as well as heat losses ranges from 30 to 80%. When the receiver outlet temperature is set to 225 °C, the collector thermal efficiency improves and, in most cases, ranges from 60% to 80%. The optical efficiency at zero (η0 (eita zero) is an important design characteristic factor and is normally used to determine the energy yield of the solar collector field. The energy yield of the solar field (Pth) is the product of the primary reflector surface, the Direct Normal Irradiance (DNI) and the optical efficiency factor (η0), minus the heat losses from the receiver radiation (P). Pth ¼ A ðDNIeff η0 −Ploss Þ
ð13Þ
As shown in Fig. 6, the optical efficiency during the period when the receiver outlet temperature is set at 225 °C, ranges from 60% to 80% and with about 73% average optical efficiency. The optical performance of the linear Fresnel collector (LFC) tested in this study can be compared with commercial LFC developed by Novatec Solar [34] and Industrial Solar GmbH [38]. Novatec Solar has developed a patented solar reflector design based on Fresnel collector technology. The solar boiler uses parallel rows of flat glass mirrors to focus direct solar irradiation onto a linear receiver. Feed water is conveyed through an absorber tube to generate steam. The operating performance of the Novatec solar
collector for two different cases is shown in Table 1. In case 1, the LFC is operated at low evaporating temperature of 300 °C generating a thermal output of 537 W/m2, while in the second case it is operated at a high temperature of 500 °C, producing superheated steam using a vacuum receiver and with a thermal output of 525 W/m2. Under the same solar operating conditions the thermal power output per unit area expected to be produced by the modified Fresnel collector examined in this study, can be determined using the following relationship: Pth ¼ A ðDNI η0 K ⊥ðΘ ⊥Þ K ∥ðΘ ∥Þ−Ploss Þ
ð14Þ
Using the same operating conditions reported by Novatec and shown in Table 1, the performance characteristics of the collector system tested in this study K_⊥(Θ_⊥),K_∥(Θ_∥ and P loss are determined using Eqs. (9), (10) and (12) respectively. For each case as shown in Table 1, the thermal power output expected to be produced using the tested collector with optical efficiency (η_0) value of 0.73, is then obtained using Eq. (14). For case 1, the calculation results revealed that the thermal power expected to be generated by the tested collector is 620 W/m2 which is about 15.5% higher than that yielded by the Novatec solar collector. On the other hand, when the Novatec solar collector is operated at the conditions as shown in Case 2, the expected thermal yield is around 8.0% higher than that generated by the Novatec solar collector. Industrial Solar GmbH also reported [38] the operating conditions of linear solar Fresnel collector LF-11 as shown in case 3 of Table 1 for generating 562 W/m2 of process heat. Under the same operating conditions, it has been determined that the modified Fresnel collector generates thermal output of 632 W/m2, which is about 12.5% higher than that generated by the Industrial Solar Fresnel collector. It has thus been confirmed that the modified Fresnel collector tested in this study yields relatively high thermal power output per unit area as compared to the reported performance of industrial Fresnel solar collectors. This can be attributed to its high optical efficiency and low thermal loss. Industrial Fresnel solar collectors which are slightly curved can focus sunlight on receiver, but the focal point varies depending on mirror location and sun angle. However the modified Fresnel collector system
Table 2 Cost data.
Fig. 7. Configuration of solar assisted thermal desalination plant.
Specific CAPEX solar field US$/m2
279 US$/m2
CAPEX MED desalination plant CAPEX Back-up boiler OPEX MED without energy OPEX Solar field OPEX boiler
$ 10,000,000 $600,000 $0.28/(m3 distillate) $5.4/(m2 year) $0.1/(m3 distillate)
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Fig. 8. Impact of fossil fuel cost on water production cost without storage.
examined in this study has the unique capability of controlling the curvature of each mirror together with sun tracking to focus all reflected rays on the receiver. 4.2. Potential of coupling the solar collecting system with a commercial desalination plant The solar field performance test results are utilized to assess the economic feasibility of a commercial solar assisted thermal desalination plant operating under the same specific climatic conditions. A small capacity MED-TVC standalone desalination plant as normally operated at relatively low top brine temperature (TBT) of 65 °C and water production capacity of 1 MIGD (4546 m3/d) is selected. The MED-TVC distiller will be integrated with the modified Fresnel CSP solar collector and combined with a fossil fuel run boiler to supply part of its thermal energy requirements. When the supply of the solar energy is interrupted, thermal energy requirements of the MED TVC desalination plant is supplied by the fossil fuel backup boiler to maintain constant heat supply and 97% plant availability. The MED-TVC pumping and electrical energy requirement which is around 2.0 kWh/m3 is supplied from the national grid as shown in Fig. 7. Based on an MED-TVC distiller with a performance ratio of 9 kg/ 2326 kJ, the specific energy requirement will be 258.44 kJ/kg. The total thermal energy requirements of 4546 m3/d (1 MIGD) distiller will be 13.6 MWth. The monthly average thermal energy absorbed by oil from the tested Fresnel CSP system under the specific site conditions with annual DNI of 1132 kWh/m2 and transferred to the steam generator as obtained from Fig. 3, is 40 kWh/m2 month. The average sunshine period at the tested site to maintain this value is around 5 h. Subsequently, the solar full load hours are 1770 h/year with a 20.8% solar share without thermal energy storage. The remaining 79.2% of the MED thermal requirements is covered by fossil fuel run boiler. The thermal energy flux that will be supplied by the solar field and gained by generated steam is 254 W/m2 assuming 90% thermal efficiency. Accordingly, the total Fresnel minor area required with a solar multiple of 1.0 and without thermal energy storage to supply the MED thermal energy requirements of 13.6 MWth is 55,737 m2. The levelized cost of water (LCOW) is determined from the following relationship:
CAPEX is total capital expenditure of solar field ,MED plant and backup fossil fuel boiler, OPEX is total annual operational and maintenance expenditure of solar field, MED plant and fossil fuel boiler as well as the annual energy cost of fossil fuel. WC = MED annual water production CRF is capital recovery factor (CRF) and determined from: CRF ¼
zð1 þ zÞn ð1 þ zÞn −1
Where is Z discount rate and (n) amortization period in years. The levelized cost of water (LCOW) is calculated based on discount rate of 5%/y, 30 years amortization period, 0.0751 capital recovery factor, 97% plant availability and $0.04/kWh electricity cost. A sensitivity analysis is carried when fuel cost is varied between $10 to $120 oil per barrel equivalent. The capital expenditure (CAPEX), operational and maintenance expenditure (OPEX) of solar field, MED plant and backup fossil fuel boiler are estimated and shown in Table 2. Fig. 8 shows the impact of variation of fuel cost on the levelized cost of water (LCOW) for two different scenarios. In the first scenario as shown by the dotted line, the MED-TVC thermal requirements are solely supplied by back-up fossil fuel run boiler. The LCOW ranges from $1.43/ m3 to $7.1/m3 as the fossil fuel price is increased from $10/barrel oil equivalent (bbl) to $120/bbl. In the second scenario, part of the MEDTVC system thermal energy requirement is supplied by the Fresnel solar collector, (which is not oversized) with a solar multiple of 1.0 and the remaining part is provided by a fossil fuel run boiler. The solar
0 B B LCOW B BUS @ m3Þ¼
Where
X
CAPEX CRF þ OPEX
ð16Þ
ð15Þ
WC
Fig. 9. Impact of fossil fuel cost on water production cost without storage.
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levelized cost of water of the two cases will be equal is yielded at a fuel cost of $92/bbl. The high value of the breakeven fuel cost is attributed to a low total yearly irradiance of 1132 kWh/m2 experienced at the specific test site. When the tested Fresnel solar collection system is operated at a location of a relatively high annual DNI level of 1937 kWh/ m2, the fuel breakeven cost at this location falls to $52/bbl. The economic study also revealed that combining a Fresnel solar collection system with an MED thermal desalination plant under the specific climatic conditions is more cost effective when operated without thermal energy storage.
References Fig. 10. Impact of fossil fuel cost on water production cost with 8 h thermal energy storage.
full load hours are 1770 h/year and fossil fuel load hours are 6727.2 h/ year. The LCOW varies from $2.24/m3 to $6.8/m3 for the same range of fuel cost. The breakeven fuel cost whereby the LCOW of the two scenarios will be equal is yielded at a fuel cost of $92/bbl. The obtained high breakeven fuel cost is attributed to the low total yearly irradiance of 1132/m2, the level which is exhibited at the specific test site. When the tested Fresnel solar collection system will be operated at a location with a relatively high annual DNI of 1938 kWh/m2, the fuel breakeven cost at this location reduces to $52/bbl as shown in Fig. 9. The cost effectiveness of integrating thermal energy storage (TES) with an oversized Fresnel solar field is also examined. An eight hour storage system is assumed to be integrated with the solar collection system. The solar collection area has to be selected to generate the required thermal energy to operate the MED TVC desalination plant for a 13 h period at the rated capacity. A fossil fuel run boiler will supply the additional heat to operate the desalination plant continuously. The required oversized solar collection area, which corresponds to a solar multiple of 2.6, is 144,917 m2 when the annual DNI is 1132 kWh/m2. Fig. 10 shows a comparison between a solar assisted desalination plant equipped with an eight hour thermal energy storage (TES) system with a100% fossil fuel operated desalination plant assuming that the specific capital cost of the thermal energy storage system is $35/(kWhth ). The solar full load hours are 4603 h/year and fossil fuel load hours are 3895 h/year In this case, 54% of the thermal energy requirements of the 1 MIGD MED desalination plant will be supplied by the solar system and the remaining 46% by fossil fuel. The breakeven fuel cost is found to be $104/bbl. Meanwhile the breakeven fuel cost without TES system as shown above is $92/bbl at annual DNI of 1132 kWh/m2. It can be concluded that combining the Fresnel solar collection system with MED thermal desalination plant under the specific climatic conditions considered in this study is more cost effective when operated without thermal energy storage. 5. Conclusions Extensive pilot plant tests covering one year period were carried out to assess the optical and thermal performances of an innovative configuration of a Fresnel CSP collector under Saudi Arabia climatic conditions. Performance comparison of the modified Fresnel collector tested in this study with those of industrial Fresnel solar collectors reported in literature reveals that it yields a relatively high thermal power output per unit area. This can be attributed to its relatively high optical efficiency and low thermal loss. The cost effectiveness when the modified Fresnel solar collection system with a solar multiple of 1.0 is combined with a commercial 1 MIGD MED-TVC thermal desalination plant is compared with one completely run by fossil fuel. The breakeven fuel cost whereby the
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Contents lists available at ScienceDirect
Desalination journal homepage: www.elsevier.com/locate/desal
Concentrating solar power for seawater thermal desalination☆ Osman Ahmed Hamed a,⁎, Hiroshi Kosaka b, Khalid H. Bamardouf a, Khalid Al-Shail a, Ahmed S. Al-Ghamdi a a b
Desalination Technologies Research Institute (DTRI), Saline Water Conversion Corporation (SWCC), P.O. Box 8328, Al-Jubail 30951, Saudi Arabia Hitachi Zosen Corp., Osaka, Japan
H I G H L I G H T S • Operational performance of Fresnel concentrating solar power (CSP) system. • Cost effectiveness of a commercial solar assisted thermal desalination plant • Impact of DNI, thermal energy storage and fuel cost on the feasibility of CSP assisted thermal desalination plant.
a r t i c l e
i n f o
Article history: Received 29 December 2015 Received in revised form 18 May 2016 Accepted 11 June 2016 Available online xxxx Keywords: CSP Fresnel Desalination Cost effectiveness
a b s t r a c t Extensive pilot plant experimental studies for a period of one year were carried out to study the impact of climatic conditions on the operational performance of an innovative Fresnel solar collecting system. The solar measurements revealed that the total yearly Direct Normal Irradiance (DNI) on the tested site amounts to 1132 kWh/ m2. The thermal collector efficiency, which depends on climatic conditions such as solar insolation, ambient temperature, receiver temperature as well as heat losses, ranges from 60% to 80%. The cost effectiveness when the tested Fresnel solar collection system with solar multiple of 1.0 (limited to day time operation) is combined with a commercial thermal desalination plant is compared with one completely run by fossil fuel. The breakeven fuel cost whereby the levelized cost of water of the two cases will be equal is yielded at a fuel cost of $92/bbl. When the tested Fresnel solar collection system is run at a location with a relatively high annual DNI level (1937 kWh/m2), the fuel breakeven cost falls to $52/bbl. This study also revealed that combining a Fresnel solar collection system with an MED thermal desalination plant under specific climatic conditions is considered more cost effective when operated without thermal energy storage. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The Saline Water Conversion Corporation (SWCC) of Saudi Arabia is currently operating small scale single purpose thermal desalination plants with water production capacities ranging from 250 to 9000 m3/ day. One of the major problems that impede the cost effectiveness of a single purpose thermal desalination plant is its high fuel energy consumption. Techno-economic feasibility of small scale MSF or MED thermal desalination plants driven directly by boilers can be greatly enhanced when solar energy is employed to provide all or part of the thermal energy consumption.
☆ Part of the paper is presented at the International Desalination Association (IDA) Conference held at San Diego, USA August 30–4 September, 2015. ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (O.A. Hamed), [email protected] (H. Kosaka).
http://dx.doi.org/10.1016/j.desal.2016.06.008 0011-9164/© 2016 Elsevier B.V. All rights reserved.
A solar distillation plant may consist of one integrated system (direct solar desalination) or two separate devices, the solar collector/accumulator and distiller (indirect solar desalination). Direct use of solar energy is through the direct heating of salty water by the sun through conventional solar stills for low water production [1]. Another example of direct use of solar energy is achieved by combination of the principle of humidification-dehumidification with solar desalination using air as a heat carrier [2,3]. For relatively large water production capacities, the solar energy is indirectly used to drive thermal desalination plants by capturing solar radiation through one of the modern technologies which transform the solar energy into heat using means such as parabolic trough and linear Fresnel collectors, evacuated tube collectors and salinity gradient solar ponds [4–20]. Concentrated solar power (CSP) technology stores the energy from solar radiation in a working fluid in the form of heat. This heat can then be used directly to run a conventional thermal desalination multistage flash ((MSF), multi-effect distillation (MED), multi-effect distillation with thermal vapor compression (MED/TVC) plant. The thermal energy can alternatively be converted into electrical energy through a conventional power generation
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Nomenclature A total collector aperture area (m2) ATAN inverse tangent value CAPEX total capital expenditure of solar field ($) COSІθdІ cosine absolute value of sun's direction angle Cp specific heat (kJ/(kg °C)) CRF capital recovery factor DNI direct normal irradiation (kW m−2) actual or effective received irradiation (kW/m2) DNIeff ΔT oil temperature rise (°C) K_∥(Θ_∥) longitudinal correction factor K_⊥(Θ_⊥) transversal correction factor LCOW levelized cost of water ($/m3) n amortization period (years) OPEX total annual operational and maintenance expenditure ($) heat loss from receiver per unit length (W/m) Ploss net (useful) thermal energy absorbed by the receiver Pth (kW) q oil volumetric flow rate (m3/s) SINІθdІ sine absolute value of sun's direction angle average of inlet and outlet temperature of heat transfer Tabs fluid (°C) TAN tangent value MED annual water production (m3/year) Wc Z discount rate (%) optical efficiency at zero η0 overall solar system efficiency (%) ηsy thermal efficiency (%) ηth Θa sun's measured height angle (°) θd sun's direction angle (°) θII sun irradiation longitudinal angle (°) θL sun irradiation transversal angle (°) Π numerical value of PI (3.142) ϱ density (kg/m3)
plant and can then be used to run a reverse osmosis (RO), electrodialysis (ED) or mechanical vapor compression (MVC) desalination plant. CSP collectors developed and tested so far can be broadly divided into two categories: Line focus collectors and point focus collectors [21]. Line focus collectors include both the parabolic trough (PT) and Linear Fresnel (LF) types which use a single axis tracking system and can yield moderate temperatures up to 400 °C, while point focus collectors include parabolic dish and central receiver collectors with dual-axis tracking systems generating temperatures as high as 1000 °C or more. CSP technology offers two main advantages [21]. First, all CSP technologies can be combined with thermal energy storage systems. Second, CSP plants can be operated with fossil fuel backup (hybrid operation). The use of thermal energy storage systems and/or hybrid operation provides the possibility of continuous 24 h operation of solar assisted desalination plants. A number of studies have been reported comparing between parabolic trough and linear Fresnel applications [22–32]. Lined focused parabolic and linear Fresnel solar concentrators [22] both consist of a long reflector, which act as the only concentrator aligned on a north–south axis. One advantage of these systems is the tracking which is primarily only in one dimension. The reflector is rotated to track the sun's movement and it's reflected solar energy is concentrated along a focal line and captured by receiver tube containing a heat absorbing fluid that absorbs the concentrated heat. One-axis solar concentration provides a simple operation and highly reliable system to reach maximum operation temperatures about 400 °C. Normally, medium concentration ratios
71
between 15 and 40 are attainable; therefore, one-axis sun tracking is required [23]. Synthetic oils are used as heat transfer fluid in conventional solar PT collectors, which limits the top temperature. Nevertheless, the synthetic oil may be replaced by water in order to generate steam directly into the absorber pipe and temperatures up to 400 °C may be allowed. Direct steam generation (DSG) offers the potential for higher performance of the plants and for cost reduction [24]. Parabolic trough collector (PTC) using DSG has identical collector structures as for thermal oil. On the other hand, linear Fresnel collectors (LFC) with DSG use a potentially cheaper design mainly due to the use of flat mirrors and structural advantages, however with a lower optical efficiency. DSG avoids the costs of heat transfer fluid and the central oil heated steam generator. The DSG system is not without its technical challenges, with the risk of overheating tubes and potential flow instabilities [25]. Sophisticated controls are required to accommodate the use of the two-phase flow of water and steam. Compared with parabolic troughs, linear Fresnel collectors suffer from lower optical efficiency [26]. However, the low-profile setting of the linear Fresnel collector poses no mechanical difficulty to maximize the collector geometrical concentration ratio (the ratio of mirror aperture to receiver aperture), which enables high temperature output. The high temperature output would give rise to high power cycle efficiency and accordingly a great reduction of storage system cost. The low-profile setting of the mirrors also leads to a lower wind load requirement and thus lower-cost mirror assembly design. Further, it will also help in lowering the O&M cost for a power plant. The fixed receiver assembly greatly reduces the risk of heat transfer fluid (HTF) leakage and the resulting maintenance labor. A comparison has been made between the optical performance of parabolic trough collectors and linear Fresnel reflectors using multi-tube receivers and secondary [27]. The results reveal that PTC efficiency is higher than the efficiency of LFCs, either with multi-tube or secondary reflector receiver. This was due to the fact that PTCs conform a perfect parabola with its aperture perpendicular to the impinging beam, in the transversal plane, at all moments. However, LFC are characterized by a simpler configuration: narrower mirrors, and thus lighter structure, fixed receiver, and leakages avoidance. Comparison of the annual performance and economic feasibility of Integrated Solar Combined Cycles (ISCC) using two solar concentration technologies: parabolic trough collectors and linear Fresnel reflectors, is reported [28]. Results show that the thermal contribution is higher with PTC, but LFR may improve the economic feasibility of the plant. Existing commercial CSP plants are mainly used for electricity generation rather than water production. The National Renewable Energy Laboratory (NREL) [33], has compiled data on concentrating solar power (CSP) projects around the world that are either under operational, construction, or development stage. CSP technologies include parabolic trough, linear Fresnel reflector, power tower, and dish/engine. The majority of solar assisted power generation plants are using parabolic trough collectors with planned electricity generation per plant in the range 1800 to 175,000 MWh/year and are equipped with thermal storage system of molten salt (60% sodium nitrate, 40% potassium nitrate). There are seven operational solar power projects which are employing Fresnel solar collectors with plant generated electricity in the range of 2000 to 280,000 MWh/year and the majority of which are without storage. Around 24 concentrating solar power (CSP) projects that are either operating or under construction use power tower systems and molten salt for storage. Only one CSP project is under construction that uses dish/engine systems with at turbine capacity of 1 MW and without storage system. A number of parabolic collector desalination demonstration plants have been implemented and tested [6]. At the plataforma Solar de Almeria, Spain, a parabolic trough collector field was connected to an MED plant with a water thermal storage system. At the second phase of the project, a double-effect absorption heat pump was coupled with the solar desalination plant. Subsequently, the thermal energy
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consumption of the desalination plant decreased from 63 kWh/m3 to 36 kWh/m3. A comprehensive review of the salient design and operating features of solar assisted thermal desalination plants were reported [6,20]. Linear Fresnel systems have recently been developed to achieve a more simple design and lower cost than the parabolic trough. Compared to the existing parabolic trough, the linear Fresnel collector system designed by Novatec-Biosol [34] shows a weight reduction per square meter of 80% resulting in both lower capital cost and reduces life cycle emissions. Also, land use efficiency with a linear Fresnel is about 2 times higher than that of a parabolic trough. The use of Fresnel collector in desalination is limited. One of the problems experienced with conventional slightly curved Fresnel mirrors is that the focal point of reflected sunlight reaching the receiver can vary depending on mirror location and sun angle. This study was carried out jointly by the Desalination Technologies Research Institute (DTRI) of SWCC and Hitachi Zosen Corporation (Hitz) of Japan to explore the prospects of the application of an improved Fresnel CSP collector system when integrated with a thermal desalination plant under the climatic conditions of Saudi Arabia. The improved Fresnel CSP collector is equipped with a new mechanism that controls the curve of each mirror, together with sun tracking to focus all reflected sunlight on the receiver tubes and consequently improve the solar collector thermal and optical efficiencies. A fully controlled innovative Fresnel solar collector system was constructed and installed at DTRI premises in Al-Jubail city, Saudi Arabia. Extensive experimental tests covering a one year period were carried out to assess the operational performance of the solar collecting system under the Saudi Arabia site specific solar radiation conditions and ambient temperatures. The experimental test results are then utilized to explore both the technical and economic potential of a commercial solar assisted multi-effect desalination plant. 2. Plant description As shown in Fig. 1, the CSP demonstration plant consists of a solar concentrating system, a steam generator and a condenser. The solar concentrating and collecting plant consists of innovative solar Fresnel reflectors, receiver tube, and low and high temperature heat transferred fluid oil accumulator tanks. The solar reflector consists of 6 parallel rows. Each row consists of 92 controlled curved mirrors positioned in south-north horizontal axis and with a total aperture area of 662.4 m2. The mirrors focus the sun's direct beam radiation onto a linear absorber pipe at the focus of the Fresnel mirrors. The absorber pipe is formed from 24 SCHOTT PTR®70 receivers which are designed to operate with oil-based heat transfer fluids at temperatures up to 400 °C and having more than 95% of heat absorption coefficient and less than 10% of emissivity characteristics. Sun.'s rays on Fresnel reflectors are concentrated more than seventy times on the receiver tube positioned at a
height of 3.8 m and then transferred to the oil-based heat transfer fluid (HTF) and flowing inside the receiver tube. Heated HTF is accumulated in the high temperature tank and sent to steam generator as a heat source. Low temperature HTF discharged from the steam generator is returned to the low temperature tank and sent to the receiver tube again by thermal HTF circulation pumps. The steam generated can then either be passed to a condenser that simulates an MSF brine heater or passed directly to an existing MSF pilot plant to supply part of its steam requirements. A pyrheliometer with a sensitivity of (8.24 ∗ 10∗∗6 V) / (W/m2) is used for measurement of direct normal irradiance (DNI) and it is linked with a tracking system to follow the apparent movement of the sun and monitor sun direction and height angles with a 0.01 °C accuracy. Temperatures are measured by platinum thin film RTD class B sensors with tolerance of ±0.3 °C. The oil flow rate is measured with a flow meter with a measured error of ±0.05%. The pressure transmitters are having an accuracy of ± 0.25%. Steam flow rate and pressure are measured by a flow meter equipped with dual sensors measuring pressure differential and pressure simultaneously and separately with a high accuracy specification. 3. Methodology The main objective of the experimental study is to assess the thermal and optical performance characteristics of the modified Fresnel and to what extent it is influenced by the local site climatic conditions. Site measurements which are utilized to assess the operational performance of the Fresnel solar collecting system included daily variation in ambient conditions such as temperature, direct normal irradiation, sun direction and height angles. The inlet and outlet temperature of the oil passing through the receiver as well its volumetric flow rates were also monitored together with the quantity, pressure and temperature of the generated steam. A data acquisition system integrated with solar system collects and stores all the monitored and measured data every 10 s. The 10 s readings are then used as input to an excel program to calculate the quantity of thermal energy absorbed and lost by the receiver and the actual amount of solar incidence received by the collector. For each calculated parameter, an average daily value of the 10 s readings is obtained by dividing the summation of all the daily 10 s readings by the number of readings. The monthly readings are then obtained by the summation of the corresponding daily readings. 3.1. Overall solar system efficiency The overall solar system efficiency (ηsy) represents the ratio of total thermal energy absorbed by the receiver (Pth/A) to the incident direct normal solar irradiation (DNI). It excludes sun position and height by assuming that all mirrors are perpendicular to the impinging solar beam and disregard all other imperfections. Overall solar system efficiency = (useful absorbed energy / received sunlight) ηsy ¼ ðPth =AÞ=DNI
ð1Þ
A = Total collector aperture area. The net (useful) thermal energy absorbed by the receiver and transferred to the oil passing inside the receiver tube (Pth) is determined from the following equation: Pth ðWÞ ¼ q ϱ Cp ΔT Where
Fig. 1. Experimental setup of the solar collecting system.
q ϱ Cp ΔT
oil volumetric flow rate (m3/s) density (kg/m3) specific heat (kJ/(kg °C)) oil temperature rise (°C)
ð2Þ
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Fig. 2. Impact of variation of incidence angles in transversal and longitudinal dimension on the correction factors for the energy yield at the tested Solar Fresnel collector.
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Fig. 4. The daily variation of the overall solar system efficiency (ηsy).
determined from the following relationship [35]: Daily average overall thermal efficiency ¼ ∑ððPth =AÞ=∑DNI
ð3Þ
3.2. Collector thermal efficiency Out of the total amount of solar radiation incident on the solar collectors, only a fraction is converted into thermal energy while the rest is lost to the environment by the reflection of the incoming radiation and to the ambient by various heat transfer mechanisms. The collector thermal efficiency (ηth) is defined as the ratio of the useful thermal energy delivered to the actual energy incident on the collector aperture. ηth ¼ ðcollected thermal energy by HTF oilÞ=ðactual received sunlightÞ ð4Þ ηth ¼ Pth =ðA DNIeff Þ
ð5Þ
Where DNIeff = The actual or effective received irradiation. The effective received irradiation of the LFR system is limited by the angle at which the sunlight strikes the reflectors and because the mirrors must be oriented to reflect the irradiation to the receiver, they most often do not directly face the sun. This non-normal orientation towards the incoming sunlight is the primary optical loss for LFR, and the loss is incurred both with respect to the transversal plane (perpendicular to the axis of the collector) and longitudinal plane (parallel with the axis of the collector). DNIeff is conventionally
DNIeff ¼ DNI K ⊥ðΘ ⊥Þ K ∥ðΘ ∥Þ
ð6Þ
Where K_⊥(Θ_⊥) and K_∥(Θ_∥) are transversal and longitudinal correction factors respectively and depend strongly on the position of the sun with respect to the collector [35]. They are dependent on the sun's measured height angle (Θa) and sun's direction angle (θd). The transversal K_⊥(Θ_⊥) and longitudinal K_∥(Θ_∥) correction factors are usually described respectively as a function of the transversal (θL) and longitudinal (θII) solar incidence angles on the collector [35]. θL and θII are computed from the measured(Θa) and (θd) solar angles using the following equation: θL ¼ 90−ATANððTANðθa=180 πÞÞ=ðSINІθdІ=180 πÞÞÞ=π 180
ð7Þ
θII ¼ 90−ATANðІðTANðθa=180 πÞІ=ðCOSІθdІ=180 πÞÞÞÞ=π 180 ð8Þ The Transversal correction factors K_⊥(Θ_⊥) which accounts to the mirror shading and blocking and for the collector under study is computed from sun irradiation transversal angle (θL) using the following equation: K ⊥ðΘ ⊥Þ¼ −1 10ð−6Þ θL3 –4 10ð−7Þ θL2 þ ð0:0001 θLÞ þ 0:9978
ð9Þ
The longitudinal correction factors K_∥(Θ_∥) which accounts for the energy that does not impinge in the receiver due to the longitudinal component of the irradiance [27] is determined from irradiation
Fig. 3. The monthly variation of direct and effective irradiance, thermal energy absorbed by receiver and receiver thermal losses.
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Fig. 5. The daily variation of the thermal collector efficiency(ηth).
longitudinal angle (θII) using the following equation: K ∥ðΘ ∥Þ ¼ 7 10ð−10Þ θII5 –9 10ð−8Þ θII4 þ 2 10ð−6Þ θII3 –3 10ð−5Þ θII2 −ð0:005 θIIÞ þ 1:0001
Tabs is calculated from the average of inlet and outlet temperature of heat transfer fluid in °C.
4. Results & discussion
ð10Þ 4.1. Thermal and optical performance assessment Fig. 2 shows the impact of variation of sun irradiation longitudinal or transversal angle on the corresponding longitudinal or transversal correction factors. It also shows that the predicted values of the longitudinal or transversal correction factors for the Fresnel solar collector tested in this study are to a great extent comparable with Novtec published correction factors [34]. 3.3. Optical efficiency The optical efficiency depends on the optical properties of the materials involved, the geometry of the collector and the various imperfections arising from the construction of the collector with regard to receiver alignment to the focal line of the collector, and the system's tracking precision [36]. It is defined as the ratio of energy absorbed by the receiver to the energy incident on the collector's aperture as shown in the following relationship: η0 ¼ ððPth =AÞ þ Ploss Þ=DNIeff
ð11Þ
Where Ploss is the heat loss from receiver per unit length and is determined from the following empirical equation [37]: Ploss ðW=mÞ ¼ 0:141･Tabs þ ð6:48E−9Þ･Tabs 4
ð12Þ
The experimental results revealed that at the examined site, the maximum direct normal irradiation (DNI) from sun on the ground is relatively low and in most cases range from 350 to 500 W/m2. As shown in Fig. 3, the monthly measured DNI varies from as low as 65 kWh/m2 in January to about 125 kWh/m2 in June and the total yearly irradiation amounts to 1132 kWh/m2. The actual or effective energy incident on the collector's aperture is less than this maximum irradiation because all the mirrors will not be perpendicular to the impinging solar beam. As shown in Fig. 3, the monthly actual impinging solar irradiation flux varies from 32 to 107 kWh/m2 and around 13% to 57% of the direct solar irradiation is dissipated. The actual total yearly solar energy flux impinging on the solar collectors aperture is 835 kWh/m2 which represents 73.76% of the maximum impinging solar power. Fig. 3 also shows that the monthly thermal energy flux absorbed by oil varies from 21 kWh/m2 in January to 58.3 kWh/m2 in June. The total yearly thermal energy received by the oil is 414 kWh/m2 which represents 36.65 and 49.6% of the maximum solar power that would impinge onto the reflecting surface and the actual solar radiation striking the collector surface, respectively. Fig. 4 Shows the variation of the overall solar system efficiency (ηsy) with the year days. The temperature of the HTF leaving the receiver is initially set for the first two months at 340 °C. If the HTF outlet
Fig. 6. The daily variation of the optical efficiency(η0).
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Table 1 Performance characteristics of reported Fresnel solar collectors.
Collector reference condition External temperature Inflow temperature Outflow temperature No wind Direct normal irradiation Transversal angle θ_⊥ Longitudinal angle θ_// Optical efficiency at zero Thermal power output per unit area W/m2
The anticipated performance of the Fresnel collector evaluated in this study Optical efficiency at zero of Fresnel collector evaluated in this study Thermal power output per unit area W/m2 Percentage increase in thermal power output
Case 1 Novatec Solar [34]
Case 2 Novatec Solar [34]
Case 3 Industrial Solar GmbH reported [38]
40 °C 100 °C 300 °C No wind 900 W/m2 30° 10° 0.67 Novatec solar 537 W/ m2
40 °C 300 °C 500 °C No wind 900 W/m2 30° 10° 0.65 Novatec solar 525 W/m2
30 °C 160 °C 180 °C No wind 900 W/m2 30° 0° 0.663 Industrial solar 562 W/m2
0.73 Present study 620 W/m2 15.5%
0.73 Present study 567 W/m2 8.0%
0.73 Present study 632 W/m2 12.5%
temperature is below the set value, heated HTF oil is recycled and returned to the receiver tube. Otherwise it is passed to the steam generator. As a result of the prevailing site solar irradiance, it has been found that it is more appropriate to set the outlet temperature of the oil leaving the receiver at a relatively low value. It was first reduced from 340 °C to 250 °C and then finally to 225 °C. The overall system efficiency during the period when the outlet temperature is set to 225 °C, ranges from 30% to 63%. As shown in Fig. 5, the collector thermal efficiency which depends on climatic conditions such as solar insolation, ambient temperature, receiver temperature as well as heat losses ranges from 30 to 80%. When the receiver outlet temperature is set to 225 °C, the collector thermal efficiency improves and, in most cases, ranges from 60% to 80%. The optical efficiency at zero (η0 (eita zero) is an important design characteristic factor and is normally used to determine the energy yield of the solar collector field. The energy yield of the solar field (Pth) is the product of the primary reflector surface, the Direct Normal Irradiance (DNI) and the optical efficiency factor (η0), minus the heat losses from the receiver radiation (P). Pth ¼ A ðDNIeff η0 −Ploss Þ
ð13Þ
As shown in Fig. 6, the optical efficiency during the period when the receiver outlet temperature is set at 225 °C, ranges from 60% to 80% and with about 73% average optical efficiency. The optical performance of the linear Fresnel collector (LFC) tested in this study can be compared with commercial LFC developed by Novatec Solar [34] and Industrial Solar GmbH [38]. Novatec Solar has developed a patented solar reflector design based on Fresnel collector technology. The solar boiler uses parallel rows of flat glass mirrors to focus direct solar irradiation onto a linear receiver. Feed water is conveyed through an absorber tube to generate steam. The operating performance of the Novatec solar
collector for two different cases is shown in Table 1. In case 1, the LFC is operated at low evaporating temperature of 300 °C generating a thermal output of 537 W/m2, while in the second case it is operated at a high temperature of 500 °C, producing superheated steam using a vacuum receiver and with a thermal output of 525 W/m2. Under the same solar operating conditions the thermal power output per unit area expected to be produced by the modified Fresnel collector examined in this study, can be determined using the following relationship: Pth ¼ A ðDNI η0 K ⊥ðΘ ⊥Þ K ∥ðΘ ∥Þ−Ploss Þ
ð14Þ
Using the same operating conditions reported by Novatec and shown in Table 1, the performance characteristics of the collector system tested in this study K_⊥(Θ_⊥),K_∥(Θ_∥ and P loss are determined using Eqs. (9), (10) and (12) respectively. For each case as shown in Table 1, the thermal power output expected to be produced using the tested collector with optical efficiency (η_0) value of 0.73, is then obtained using Eq. (14). For case 1, the calculation results revealed that the thermal power expected to be generated by the tested collector is 620 W/m2 which is about 15.5% higher than that yielded by the Novatec solar collector. On the other hand, when the Novatec solar collector is operated at the conditions as shown in Case 2, the expected thermal yield is around 8.0% higher than that generated by the Novatec solar collector. Industrial Solar GmbH also reported [38] the operating conditions of linear solar Fresnel collector LF-11 as shown in case 3 of Table 1 for generating 562 W/m2 of process heat. Under the same operating conditions, it has been determined that the modified Fresnel collector generates thermal output of 632 W/m2, which is about 12.5% higher than that generated by the Industrial Solar Fresnel collector. It has thus been confirmed that the modified Fresnel collector tested in this study yields relatively high thermal power output per unit area as compared to the reported performance of industrial Fresnel solar collectors. This can be attributed to its high optical efficiency and low thermal loss. Industrial Fresnel solar collectors which are slightly curved can focus sunlight on receiver, but the focal point varies depending on mirror location and sun angle. However the modified Fresnel collector system
Table 2 Cost data.
Fig. 7. Configuration of solar assisted thermal desalination plant.
Specific CAPEX solar field US$/m2
279 US$/m2
CAPEX MED desalination plant CAPEX Back-up boiler OPEX MED without energy OPEX Solar field OPEX boiler
$ 10,000,000 $600,000 $0.28/(m3 distillate) $5.4/(m2 year) $0.1/(m3 distillate)
76
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Fig. 8. Impact of fossil fuel cost on water production cost without storage.
examined in this study has the unique capability of controlling the curvature of each mirror together with sun tracking to focus all reflected rays on the receiver. 4.2. Potential of coupling the solar collecting system with a commercial desalination plant The solar field performance test results are utilized to assess the economic feasibility of a commercial solar assisted thermal desalination plant operating under the same specific climatic conditions. A small capacity MED-TVC standalone desalination plant as normally operated at relatively low top brine temperature (TBT) of 65 °C and water production capacity of 1 MIGD (4546 m3/d) is selected. The MED-TVC distiller will be integrated with the modified Fresnel CSP solar collector and combined with a fossil fuel run boiler to supply part of its thermal energy requirements. When the supply of the solar energy is interrupted, thermal energy requirements of the MED TVC desalination plant is supplied by the fossil fuel backup boiler to maintain constant heat supply and 97% plant availability. The MED-TVC pumping and electrical energy requirement which is around 2.0 kWh/m3 is supplied from the national grid as shown in Fig. 7. Based on an MED-TVC distiller with a performance ratio of 9 kg/ 2326 kJ, the specific energy requirement will be 258.44 kJ/kg. The total thermal energy requirements of 4546 m3/d (1 MIGD) distiller will be 13.6 MWth. The monthly average thermal energy absorbed by oil from the tested Fresnel CSP system under the specific site conditions with annual DNI of 1132 kWh/m2 and transferred to the steam generator as obtained from Fig. 3, is 40 kWh/m2 month. The average sunshine period at the tested site to maintain this value is around 5 h. Subsequently, the solar full load hours are 1770 h/year with a 20.8% solar share without thermal energy storage. The remaining 79.2% of the MED thermal requirements is covered by fossil fuel run boiler. The thermal energy flux that will be supplied by the solar field and gained by generated steam is 254 W/m2 assuming 90% thermal efficiency. Accordingly, the total Fresnel minor area required with a solar multiple of 1.0 and without thermal energy storage to supply the MED thermal energy requirements of 13.6 MWth is 55,737 m2. The levelized cost of water (LCOW) is determined from the following relationship:
CAPEX is total capital expenditure of solar field ,MED plant and backup fossil fuel boiler, OPEX is total annual operational and maintenance expenditure of solar field, MED plant and fossil fuel boiler as well as the annual energy cost of fossil fuel. WC = MED annual water production CRF is capital recovery factor (CRF) and determined from: CRF ¼
zð1 þ zÞn ð1 þ zÞn −1
Where is Z discount rate and (n) amortization period in years. The levelized cost of water (LCOW) is calculated based on discount rate of 5%/y, 30 years amortization period, 0.0751 capital recovery factor, 97% plant availability and $0.04/kWh electricity cost. A sensitivity analysis is carried when fuel cost is varied between $10 to $120 oil per barrel equivalent. The capital expenditure (CAPEX), operational and maintenance expenditure (OPEX) of solar field, MED plant and backup fossil fuel boiler are estimated and shown in Table 2. Fig. 8 shows the impact of variation of fuel cost on the levelized cost of water (LCOW) for two different scenarios. In the first scenario as shown by the dotted line, the MED-TVC thermal requirements are solely supplied by back-up fossil fuel run boiler. The LCOW ranges from $1.43/ m3 to $7.1/m3 as the fossil fuel price is increased from $10/barrel oil equivalent (bbl) to $120/bbl. In the second scenario, part of the MEDTVC system thermal energy requirement is supplied by the Fresnel solar collector, (which is not oversized) with a solar multiple of 1.0 and the remaining part is provided by a fossil fuel run boiler. The solar
0 B B LCOW B BUS @ m3Þ¼
Where
X
CAPEX CRF þ OPEX
ð16Þ
ð15Þ
WC
Fig. 9. Impact of fossil fuel cost on water production cost without storage.
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levelized cost of water of the two cases will be equal is yielded at a fuel cost of $92/bbl. The high value of the breakeven fuel cost is attributed to a low total yearly irradiance of 1132 kWh/m2 experienced at the specific test site. When the tested Fresnel solar collection system is operated at a location of a relatively high annual DNI level of 1937 kWh/ m2, the fuel breakeven cost at this location falls to $52/bbl. The economic study also revealed that combining a Fresnel solar collection system with an MED thermal desalination plant under the specific climatic conditions is more cost effective when operated without thermal energy storage.
References Fig. 10. Impact of fossil fuel cost on water production cost with 8 h thermal energy storage.
full load hours are 1770 h/year and fossil fuel load hours are 6727.2 h/ year. The LCOW varies from $2.24/m3 to $6.8/m3 for the same range of fuel cost. The breakeven fuel cost whereby the LCOW of the two scenarios will be equal is yielded at a fuel cost of $92/bbl. The obtained high breakeven fuel cost is attributed to the low total yearly irradiance of 1132/m2, the level which is exhibited at the specific test site. When the tested Fresnel solar collection system will be operated at a location with a relatively high annual DNI of 1938 kWh/m2, the fuel breakeven cost at this location reduces to $52/bbl as shown in Fig. 9. The cost effectiveness of integrating thermal energy storage (TES) with an oversized Fresnel solar field is also examined. An eight hour storage system is assumed to be integrated with the solar collection system. The solar collection area has to be selected to generate the required thermal energy to operate the MED TVC desalination plant for a 13 h period at the rated capacity. A fossil fuel run boiler will supply the additional heat to operate the desalination plant continuously. The required oversized solar collection area, which corresponds to a solar multiple of 2.6, is 144,917 m2 when the annual DNI is 1132 kWh/m2. Fig. 10 shows a comparison between a solar assisted desalination plant equipped with an eight hour thermal energy storage (TES) system with a100% fossil fuel operated desalination plant assuming that the specific capital cost of the thermal energy storage system is $35/(kWhth ). The solar full load hours are 4603 h/year and fossil fuel load hours are 3895 h/year In this case, 54% of the thermal energy requirements of the 1 MIGD MED desalination plant will be supplied by the solar system and the remaining 46% by fossil fuel. The breakeven fuel cost is found to be $104/bbl. Meanwhile the breakeven fuel cost without TES system as shown above is $92/bbl at annual DNI of 1132 kWh/m2. It can be concluded that combining the Fresnel solar collection system with MED thermal desalination plant under the specific climatic conditions considered in this study is more cost effective when operated without thermal energy storage. 5. Conclusions Extensive pilot plant tests covering one year period were carried out to assess the optical and thermal performances of an innovative configuration of a Fresnel CSP collector under Saudi Arabia climatic conditions. Performance comparison of the modified Fresnel collector tested in this study with those of industrial Fresnel solar collectors reported in literature reveals that it yields a relatively high thermal power output per unit area. This can be attributed to its relatively high optical efficiency and low thermal loss. The cost effectiveness when the modified Fresnel solar collection system with a solar multiple of 1.0 is combined with a commercial 1 MIGD MED-TVC thermal desalination plant is compared with one completely run by fossil fuel. The breakeven fuel cost whereby the
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