Water Desalination by Solar-Powered RO Systems

CHAPTER 3

Water Desalination by Solar-Powered RO Systems ˜ate‡, Agustı´n M. Delgado-Torres*, Lourdes Garcı´a-Rodrı´guez†, Baltasar Pen ‡ ‡ ´n Juan A. de la Fuente , Gustavo Melia *

Industrial Engineering Department, Higher School of Engineering and Technology, University of La Laguna (ULL), La Laguna, Spain, †Energetic Engineering Department, University of Seville, Seville, Spain, ‡ Water Department, The Canary Islands Institute of Technology (ITC), Gran Canaria Island, Spain

1 Introduction The increasing water demand worldwide has caused a strong development of desalination technologies and their use during the last years. However, desalination requires large quantities of energy supply, which is mostly provided from the combustion of fossil fuels with the consequent CO2 emissions. Moreover, desalination increases energy demand, which means a higher external dependence and economic expense in countries with low energy resources. Desalination by renewable energies solves those disadvantages; it is a free-pollution process that uses a local energy source. This chapter is about solar reverse osmosis (RO) desalination driven by solar thermals (ST) or photovoltaic (PV) energy systems. The general basic outline of the energy transformations and products of PV-RO and ST-RO systems are shown in Fig. 1. Although both combinations are solar RO technologies, there are significant differences in its technological development. In the case of ST-RO technology there is a great scarcity of experimental systems. On the other hand, PV-RO technology has a high degree of maturity and several tens of real units installed around the world exist. From the point of view of energy transformations and products, the main difference between PV-RO and ST-RO systems is that the latter would be able to produce heat at different temperature levels and hence would be useful in many different heat-driven applications: solar cooling with absorption machines, solar heating, thermal desalination process, preheating of RO feed-water and any other industrial process which needs thermal energy. ST-RO technology needs solar thermal collectors to convert solar energy into solar heat. Different values of energy conversion efficiency and heat production temperature are attainable depending on the collector technology used. In the case of medium (150–400°C) or high Current Trends and Future Developments on (Bio-) Membranes. https://doi.org/10.1016/B978-0-12-813545-7.00003-9 # 2019 Elsevier Inc. All rights reserved.

45

46

Chapter 3

Fig. 1 Basic outline of energy transformations and products of PV-RO and ST-RO systems. Energy storage systems—batteries and thermal energy storage—have been omitted for simplicity.

temperature systems (>400°C), overall solar to electric efficiencies can be higher than PV conversion. This feature and the poly-generation capability make ST-RO an interesting option against other solar desalination technologies (e.g., solar thermal distillation). Moreover, there are also several choices for the power cycle to carry out the thermo-mechanical conversion. In certain cases, this solar power technology is mature enough as in the case of concentrating solar power (CSP) with parabolic trough collectors (PTCs). In the case of photovoltaic (PV) energy coupled to RO desalination, this combination of autonomous desalination has already a significant testing period (Herold et al., 1998) and is the most popular option; about 32% of autonomous desalination systems are based on PV-powered RO units (Tzen, 2005). Both photovoltaic solar energy and RO technology have a high degree of maturity and a wide commercial network of manufacturers and suppliers.

2 Solar Thermal-Powered RO Desalination Systems 2.1 General Layout The layout of the general configuration of a solar thermal-powered RO desalination system (ST-RO system) is given in Fig. 2. Because the RO process is a work-driven desalination technology (mechanical or electrical), thermal energy delivered by the solar thermal system must first be converted to finally produce freshwater. Firstly, solar energy gathered by solar collectors is converted to solar heat, which can be transferred to a power conversion unit (PCU)

Water Desalination by Solar-Powered RO Systems 47

Fig. 2 General layout of a solar thermal-powered RO desalination system (ST-RO).

and/or to the thermal energy storage system. Conversion efficiency of this stage is lower than 100% because several radiation and thermal losses in the solar thermal system. Useful solar thermal energy is absorbed by the PCU together with thermal energy from auxiliary or backup system if it would be present. In general, the PCU is made up of a power cycle so thermal energy must be rejected to a cooling medium in accordance with second law statements. Finally, mechanical or electric power produced by the PCU is consumed by the RO plant, yielding a desalted water flow extracted from the feedwater flow. As with any other desalination system, a brine flow rejected from the RO plant exists. In systems like the one showed in Fig. 2 any kind of solar collector or solar energy gathering system could be used: solar ponds, stationary solar collectors (flat or evacuated), concentrating collectors (parabolic troughs, linear Fresnel reflectors (LFRs), parabolic dish) or central receiver systems (CRSs) (heliostat fields reflecting solar energy to a central receiver). Once the solar thermal technology is selected, its operation temperature can determine the PCU (or the power cycle) to be used: steam Rankine cycles, organic Rankine cycles (ORC), stirling engines, and even Brayton power cycles. Finally, feedwater could be brackish (TDS between 1000 and 15,000 ppm approx.) or seawater (TDS between 15,000 and 50,000 ppm approx.) and therefore the specific energy consumption (SEC) of the desalination plant could range approximately between 0.5 kWh/m3 (low salinity brackish water) to 5.5 kWh/m3 (high salinity seawater). The overall efficiency of the solar desalination plant will directly depend on photothermal conversion and thermomechanical conversion efficiencies and both values are affected by practical and theoretical limitations.

2.2 Conversion of the Solar Energy Resource: Solar Thermal Technologies 2.2.1 Solar to heat conversion Thermodynamics tells us that in energy transfer by heat not only its amount is important but also the temperature at which the heat can be produced or absorbed. Both the amount of heat and the temperature at which it is available define the usefulness of such quantity of energy.

48

Chapter 3

The International Standard 9488:1999 (ISO, 1999) defines a “solar thermal collector or solar collector as a device designed to absorb solar radiation and to transfer the thermal energy so produced to a fluid passing through it.” Therefore, the aim of any solar collector is convert the solar energy collected into heat in the most efficient and economical way possible. Conversion of solar energy into heat takes place in the absorber of the solar collector, so this is the key element of any solar thermal system. A schematic description of a simple flat plate solar collector is shown in Fig. 3. Basic thermal analysis of this design is also applicable to the case of more complex, e.g., solar concentrating collectors. In general, the solar collector can be equipped with a transparent cover to protect the absorber element and reduce their energy losses to the environment. In Fig. 3, the tubes inside which the heat transfer fluid of the collector flows (water, mixture of water and antifreezing, a thermal oil or air) are also displayed. In any solar collector there are two different characteristic surfaces of great importance to evaluate the collector’s performance: the aperture and the absorber surface. Corresponding areas of both surfaces have been projected to highlight the difference between them. The aperture of a solar collector is the opening through which unconcentrated solar radiation is admitted and the aperture area (Aa) is the maximum projected area through this unconcentrated radiation enters the collector. Absorber area is the maximum projected area of the absorber in the case of nonconcentrating solar collectors. Solar radiation (solar flux) arrives at a transparent

Fig. 3 Thermal and optical losses in a flat plate solar collector.

Water Desalination by Solar-Powered RO Systems 49 cover’s surface and most of it is transmitted to the space where the absorber surface is located because a small amount is reflected by the cover. Transmitted radiation reaches the absorber’s surface and a large amount is absorbed and converted into internal energy of the absorber’s material, leading to an increase of the absorber’s temperature (TA). If a fluid at a lower temperature passes through the absorber tube, heat transfer from the absorber to the fluid occurs and then useful thermal energy is produced as an increase of fluid’s temperature. According to the explanation given before, the energy balance of any solar thermal collector in steady-state conditions is as follows: 2 3 2 3 2 3 2 3 useful solar heat solar energy thermal energy solar energy 6 6 7 6 produced by 7 lost due to 7 6 7 6 incident on the 7 6 7 6 7 6 7¼6 7  4 lost due to 5  6 7 4 solar thermal 5 4 solar thermal 5 4 thermal 5 optical losses collector collector losses Q_ solar ¼ Aa  Ga  E_ opt  Q_ loss

(1)

where Ga is the solar irradiance on the aperture area (quotient of the radiant flux on the aperture surface to the aperture area), E˙opt is the solar power lost because optical effects, Q_ loss is the solar power lost as thermal losses, and Q_ solar is the instantaneous solar thermal energy produced by the solar collector. Energy losses associated with optical losses quantifies the solar energy which, entering the collector, is not finally absorbed by the absorber. Therefore reflected radiation by the cover and radiation not absorbed by the absorber in Fig. 3 represent the major part of optical losses. This term depends on the optical properties (reflectance, absorptance, transmittance) of the materials with which the radiation interacts on its way to the absorber and therefore also depends on the type of collector and its design. The fraction of solar energy which is eventually absorbed into the absorber is the zero-loss collector efficiency or optical efficiency of the solar collector:   Aa  Ga  E_ opt (2) η0 ¼ A a  Ga Energy losses associated with thermal losses quantifies the energy transfer from the surface of the absorber to the ambient, principally through the mechanisms of convection and radiation. This term depends on the area of the absorber surface (AA) and the difference between its temperature (TA) and the ambient temperature (Tamb). Thermal losses from the absorber can be modeled throughout complex models based, for example, on the finite volume element method. This approach is justified in the collector’s design stage. However, for practical calculations, the thermal losses term is modeled as linear dependent on the difference between average absorber temperature (T A ) and ambient temperature (Tamb):   (3) Q_ loss ¼ AA  UA  T A  Tamb

50

Chapter 3

where UA is the heat loss coefficient of the solar collector. UA gives the power lost by the collector per unit of absorber area and per every kelvin of difference between absorber’s and ambient temperature. This expression shows that thermal losses of any solar collector grow with the absorber’s temperature and surface area. In Eqs. (2), (3), thermal energy produced by the solar collector is:   (4) Q_ solar ¼ η0  Aa  Ga  AA  UA  T A  Tamb Therefore, if optical properties of the solar collector and its aperture area are fixed, the higher the absorber’s temperature and its area, the lower the thermal energy produced. Qualitative analysis of Eqs. (1), (4) is shown in Fig. 4. In general, optical losses do not depend on the absorber’s temperature, and energy lost by this mechanism in most of solar collectors is around 20%–35% of solar energy incident (a value of 25% was selected in Fig. 4). If the ambient temperature is fixed, the solar collector’s output decreases when absorber’s temperature increases because of thermal losses to the ambient temperature Tamb < TA. Finally, the fraction of captured solar energy that is finally produced as useful solar heat is the collector efficiency and can be expressed in terms of solar irradiance on the collector’s aperture, optical efficiency, absorber’s temperature, ambient temperature, and absorber’s heat loss coefficient through its instantaneous efficiency curve:     AA UA  T A  Tamb Q_ solar ¼ η0  (5) ηSTC ¼ Aa  Ga Aa Ga

Fig. 4 Qualitative description of optical losses (Eopt), thermal losses (Qloss) and solar thermal energy (or energy rate) output (Qsolar) of a solar collector as a function of the temperature difference (absorber  ambient) for different aperture (Aa) to absorber area (AA) ratios.

Water Desalination by Solar-Powered RO Systems 51

Fig. 5 Qualitative description of zero-loss efficiency (optical efficiency) and collector’s efficiency as a function of the temperature difference (absorber  ambient) for different aperture (Aa) to absorber area (AA) ratios.

Therefore, if the optical properties of the solar collector and its aperture area are fixed, the higher the absorber’s temperature and its area, the lower its efficiency (see Fig. 5). Solid curves for thermal losses and output heat in Figs. 4 and 5, respectively, correspond to the situation in which aperture area and absorber’s area are very similar. This would be the case of the flat plate collector (FPC) of Fig. 3. Dashed curves will be used to explain the effect of the concentration in solar concentrating collector’s performance. In general, the use of Eq. (5) to compute the efficiency of a solar collector needs the absorber’s surface local temperature, which could be difficult to measure. Therefore, it is convenient to use the average heating temperature of the heat transfer fluid of the collector Tm instead of TA because its measure is straightforward. In addition, the heat loss coefficient can be expressed per unit of aperture area, and temperature dependence is usually considered. With these additional changes, Eq. (5) can be replaced by Eq. (6): ηSTC ¼

ðTm  Tamb Þ ðTm  Tamb Þ2 Q_ solar ¼ η0  a1  a2 A a  Ga Ga Ga

(6)

where a1 is the heat loss coefficient and a2 its temperature dependence. Readers interested in standard procedures to obtain the performance of solar thermal collectors are referred to the International Standard 9806:2014 (ISO, 2014) about test methods for fluid heating collectors.

52

Chapter 3

2.2.2 Stationary solar thermal collectors The main performance characteristics of solar thermal collectors are connected with the temperature at which thermal energy can be produced. Except for specific advanced or prototype designs, flat-plate collectors (FPC) (see Fig. 3), compound parabolic concentrators collectors (CPC) and evacuated tube collectors (ETC) can produce heat at temperatures typically not higher than 150°C: FPC < 100°C, ETC < 120°C and CPC < 100–150°C (Horta, 2016). In general, FPC and CPC collectors are flat-glazed collectors where a transparent cover protects the absorber from adverse meteorological conditions (e.g., hail) and reduce thermal losses to the ambient. In ETC the absorber is placed inside an evacuated glass tube so heat losses by convection are minimized. The design and principle of operation of these collectors does not require continuous solar tracking, but its tilt and orientation can be optimized depending on the load’s profile. 2.2.3 Rising the conversion efficiency and value of the solar heat: Solar concentration technologies When higher temperatures of heat production are demanded, heat losses from the absorber’s surface must be reduced. The way to do this is reducing the absorber’s area in order to increase the solar flux on its surface and hence its temperature (see Fig. 6). Reflectors, lenses or other optical elements are used in solar concentrating collectors to redirect and concentrate the solar radiation passing through the aperture onto the absorber’s surface. From Eq. (5), when an absorber’s area AA is reduced for a fixed aperture area Aa, thermal losses from the absorber are also reduced (see Figs. 4 and 5). However, this absorber’s area reduction implies the use of solar tracking in order to extend the heat production throughout the day. Two types of concentrating collectors exist: line-focus and

Fig. 6 Theoretical view of a solar concentrating system.

Water Desalination by Solar-Powered RO Systems 53

Fig. 7 Parabolic trough collector.

point-focus collectors. Line-focus collectors are single axis tracking collectors where concentrated solar radiation is focused on a tubular absorber. PTCs (see Fig. 7) and LFRs belong to this category. Point-focus collectors are double-axis tracking collectors where concentrated solar radiation is focused to a point. Parabolic-dish collectors (PDCs) belong to this category. CRSs are a third type of solar concentration system that do not exactly fit the standard solar collector’s definition. In a CRS, a field of double-axis tracking mirrors—called heliostats—which reflect and redirect the solar radiation onto the absorber’s surface is used. However, reflecting surfaces and the absorber are not assembled in the same structure. To sum up, in currently available solar thermal collector technologies the higher the temperature to which heat is produced the higher the heat losses from the absorber or, in other words, the lower the collector efficiency. To maintain high or acceptable collector efficiency values at medium (150–400°C) or high temperatures (>400°C), concentrating collectors (linefocus or point-focus collectors) or CRSs must be used, which necessarily implies the use of solar tracking. For more information on this topic, the following open references are recommended to the reader: Weiss and Rommel (2008), Giovannetti and Horta (2016), and Horta (2016). Finally, in Romero and Gonza´lez-Aguiar (2014), short descriptions of every solar concentration technology and their integration into solar thermal power plants are available.

54

Chapter 3

2.3 Power Cycles for Solar Energy Conversion Once the solar thermal energy is available, it has to be converted into mechanical work or more usually electricity to be consumed by the RO system (see Figs. 1 and 2). Because the system needs to operate on a continuous basis and because economic reasons use of power cycles is suited. Most used in solar thermal applications are the Rankine cycle with organic substances and water as working fluid and Stirling engines. It is also developing the use of gas Brayton— type power cycles, preferably using the carbon dioxide as working gas (Amelio et al., 2016; AlSulaiman and Atif, 2015). Each of the above cycles has advantages and disadvantages compared to the rest, but it can be said that which cycle has to be used depends, in general, on two factors: the temperature of the heat source and also the unitary nominal power of the PCU. For example, Stirling engines of net power output between 10 and 25 kW are usually used with PDC and steam Rankine cycles of net power output between 10 and 20 MW with CRS. Both systems are high-temperature point focusing systems (>400°C), but the net power output levels determine the PCU to be used. In general, power cycles are characterized because their thermal efficiency increases with the temperature of the heat source from which the heat is absorbed. This thermal efficiency is defined as the fraction of absorbed heat converted into useful work: ηCycle ¼

jWnet j jQout j ¼1 Qin Qin

(7)

To avoid entering into the details of each of the cycles, this behavior can be observed easily if the Carnot power cycle is taken as an example. This theoretical cycle would represent the ideal cycle from the thermodynamic point of view (reversible cycle) and its thermal efficiency would be the maximum attainable in the thermo-mechanical conversion between two given temperature heat reservoirs (TH  hot source temperature, TC  cold source temperature). ηCarnot ¼

TC jWnet j ¼1 Qin TH

(8)

At first approximation, the overall performance of the whole cycle (the solar power cycle) can be estimated by the following expression: η¼

jWnet j jWnet j ¼ η Esolar Qsolar STC

(9)

where ηSTC is the efficiency of the solar collector. If, for simplicity, it is assumed that all the thermal energy produced by the solar thermal system is absorbed by the power cycle the overall efficiency is equal to the product of efficiencies: η¼

jWnet j jWnet j jWnet j ¼  ηSTC ¼  ηSTC ¼ ηcycle  ηSTC Esolar Qsolar Qin

(10)

Water Desalination by Solar-Powered RO Systems 55

Fig. 8 Efficiency of several line-focus solar collectors, Carnot power cycle, and solar Carnot power cycle as a function of average heating temperature.

To illustrate this point, Fig. 8 displays the following: •

• •

Collector efficiency (ηSTC) (Eq. 6) as a function of the average heating temperature of the heat transfer fluid (Tm). Efficiency curves are shown within the recommended or typical temperature interval and correspond to real collector models of small and large PTCs and LFRs. Thermal efficiency of the Carnot power cycle (ηCarnot) (Eq. 8) operating between a cold source at TC ¼ 300 K and a hot source at TH ¼ (Tm  10 K). The product of ηSTC and ηCarnot, that is, the efficiency of the solar power cycle with every solar collector model and the theoretical power cycle with the highest thermal efficiency. Therefore, the resulting curve gives an idea of the maximum values of the solar-tomechanical or electricity conversion efficiency with every kind of single-axis tracking collectors as a function of its operating temperature.

As expected, the solar collector’s efficiency decreases as the operation temperature increases but power cycle’s efficiency (Carnot power cycle in this case) increases continuously. The net result of these two trends is that the efficiency of the solar to mechanical conversion process increases as temperature at which heat is absorbed increases. For this reason, from the

56

Chapter 3 Table 1 Examples of solar thermal to mechanical conversion systems (commercial, experimental or design proposals)

Solar Thermal Technology FPC

CPC ETC Small PTC

Small LFR

Large PTC

Large LFR Parabolic dish Solar tower

Examples Small scale solar ORC unit with a glazed FPC. Working fluid of the ORC flows inside the absorber of the collector so this would be a direct vapor generation (DVG) system (Marion et al., 2014). Design study of small scale solar ORC with CPC (Antonelli et al., 2015). Small solar ORC experimental system with ETC (Baral et al., 2015). Laboratory prototype. Solar ORC of 2 kWe nominal electric power tested with two different small PTCs (1.54 and 1.8 m aperture width). Water as heat transfer fluid in the solar collector (Taccani et al., 2016). IRESEN 1 MW CSP-ORC Plant. LFR Soltigua with mineral oil. Solar field inlet/ outlet temperatures: 180°C/300°C. Power block: 1 MWe gross power output ORC unit (NREL, 2017). Pilot scale solar ORC plant of 50 kWe nominal electric power with 7.3 m aperture width PTCs (Chambers et al., 2014). Andasol-1 commercial concentrating solar power plant of 50 MWe net nominal power (NREL, 2017). Puerto Errado 2. Commercial concentrating solar power plant of 30 MWe net nominal power with Novatec Solar linear Fresnel reflector (NREL, 2017). Eurodish systems. Dish-Stirling systems with a net nominal power of 10 kWe (Mancini et al., 2003). Commercial concentrating power plant PS10 (NREL, 2017). Design studies about integration in solar power systems (Amelio et al., 2016; AlSulaiman and Atif, 2015).

FPC, flat plate collector; ORC, organic Rankine cycle; CPC, compound parabolic concentrator; ETC, evacuated tube collector; PTC, parabolic trough collector; LFR, linear Fresnel concentrator.

efficiency point of view, solar concentration technologies are preferred to carry out this conversion. If a real power cycle (e.g., ORC) was considered, a maximum in η could be found depending on the efficiency performance of the solar collector. General information about this topic can be found in Delgado-Torres and Garcı´a-Rodrı´guez (2012). On the other hand, the results shown in Fig. 8 give an idea about maximum values that can be attained theoretically with these systems. Several examples of real solar power cycles are given in Table 1.

2.4 Solar-Thermal Powered RO Systems In previous sections, it has been shown that ST-RO technology can also be framed in a polygeneration scheme and solar power cycle’s technology is a mature technology in certain cases. If the RO’s SEC decreasing due to the use of new energy recovery devices (ERDs) (pressure exchangers) and high permeability membranes is also taken into account, it can be concluded that ST-RO is a promising technology. However, when the specific literature about ST-RO systems is reviewed, a great scarcity of experimental, pilot, and demonstration facilities is

Water Desalination by Solar-Powered RO Systems 57 found (Delgado-Torres and Garcı´a-Rodrı´guez, 2007a, 2012; Delgado-Torres, 2009; Ghermandi and Messalem, 2009; Shalaby, 2017; Tauha Ali et al., 2011). In fact, from the knowledge of the authors, only five ST-RO experimental systems have been erected and operated in the last four decades. In this section, ST-RO research activities, design proposals and experimental facilities have been grouped into small-, medium-, and large-scale systems. The experimental systems belong to the category of low-scale systems so they will be explained in the next section. For readers interested in details of design proposals about low-scale and medium-scale ST-RO systems following review papers are recommended: Delgado-Torres and Garcı´a-Rodrı´guez, (2007a, 2012), Delgado-Torres (2009), Ghermandi and Messalem (2009), and Shalaby (2017). Several designs of low and medium scale systems not included in the aforementioned are explained in next sections. Finally, recent activities in the design of large-scale concentrated solar power plants coupled with desalination units are reviewed. 2.4.1 General assessment For a quick and simple assessment of ST-RO systems (see Fig. 2) at the design point, Fig. 9 is supplied in this section. This figure can also be useful to make comparisons against other solar desalination technologies like PV-RO and solar thermal distillation. SEC of the RO plant is placed in the x-axis so for RO plants with similar technology (e.g., pressure exchangers as ERD and high permeability membranes) feedwater type and salinity are fixed indirectly. Cubic meters of fresh water per day produced by the RO plant per unit of consumed power are given in the right y-axis. Therefore, for given values of SEC and consumed power of the RO plant the output capacity of the plant is easily calculable. Finally, in the left y-axis the solar energy consumed per cubic meter of fresh water produced is given as a function of RO’s SEC for different values of the solar-to-electric or mechanical efficiency conversion (η). Using all this information, an estimation of the required aperture area to cover the RO energy consumption at design point can easily be given if the solar resource at the design point (solar irradiation or solar irradiance) is known. As an example, aperture area values for efficiencies (η) attainable with solar power cycles at design point operating with stationary and concentrating solar collectors are given in Table 2 for a fixed value of SEC and different RO size levels. Solar irradiation values correspond to solar irradiance values of 1000 and 850 W/m2 along a period of an hour on to the aperture of the solar system. These values of solar irradiance are commonly used for systems with stationary and concentrating solar collectors. 2.4.2 Low-scale solar thermal RO systems The first historical designs of STRO experimental systems date from the late 1970s and early 1980s. They were low-scale brackish water RO systems because high specific SWRO energy consumptions in those days: 12 kWh/m3 without energy recovery and 8 kWh/m3 with Pelton

58

Chapter 3

Fig. 9 Solar energy consumption per cubic meter of fresh water for different values of the solar to mechanical or electric conversion and nominal output capacity of the RO plant per kW of energy consumed both as a function of the RO’s specific energy consumption. Table 2 Examples of low, medium and large scale ST-RO assessment at the design point Aperture Area at Design Point (m2) (No Thermal Storage)

RO Power Consumption (kW)

SEC (kWh/m3)

RO Plant Output Capacity (m3/day)

Stationary Solar Collectors Ha 5 1.0 kWh/m2 η 5 8%

10 (low scale) 250 (medium scale) 2000 (large scale)

3.0 3.0 3.0

80 2000 16,000

34.7 868.1 6944

Concentrating Solar Collectors Ha 5 0.85 kWh/m2 η 5 20%/24% 16.3/13.6 408.5/340.4 3268/2723

Water Desalination by Solar-Powered RO Systems 59 turbines or integrated turbo-pump systems (Libert and Maurel, 1981). Stationary solar collectors were used in the solar systems and ORC units were the dominant PCU options. To the best knowledge of the authors, the system erected in Cadarache (France) in 1978 (Maurel, 1979) is the first technical reference about a ST-RO desalination system. The system was based on a SOFRETES 2.5 kW-output solar heat engine with R114 as working fluid of the ORC. The solar field of FPCs had a total aperture area of 228 m2. This solar cycle was coupled to a 2.5 m3/h brackish water RO unit with a Pelton turbine as ERD. As can be observed in Fig. 10, power output from the solar motor was consumed by the high pressure’s pump of the RO unit and by the brackish water’s intake pump. Feedwater of the RO unit was used as the cooling medium of the solar heat engine. With feedwater at 2 g/L and 50% of conversion, the SEC was 0.67 kWh/m3 (Maurel, 1979). Information about past and present experiences about experimental STRO systems is given in Table 3. Several low-scale design proposals not covered by the review papers quoted at the beginning of this section are the following: Attia (2012) proposes a STRO with a parabolic dish concentrator and direct mechanical coupling with a RO unit. High pressure steam is generated within a spherical tank placed on the focus of the dish concentrator. Steam pressure is transferred to the feedwater of the RO unit with ENERGIE SOLAIRE (Collecteurs plans)

MOTEUR SOLAIRE

Evaporateur Stock nébulaire

Condenseur

EAU SAUMATRE

EAU DEMINERALISEE

POMPE D OSMOSE PRETRAITEMENT

OSMOSEURS

TURBINE PELTON POMPE D’ EXHAURE

Puits ou forage

EAU SAUMATRE

Fig. 10 Layout of the first ST-RO system (Maurel, 1979).

SAUMURE

60

Chapter 3 Table 3 Documented ST-RO experimental systems

Location (year)

Solar Thermal System

Cadarache (France) (1978)

228 m2 of flat plate collectors (selective) and thermal energy storage

El Hamrawin (Egypt) (1981)

384 m2 of flat plate collectors

El Paso, Texas (EEUU) ˜os, Los Ban California (EEUU) Near Marathon village, Atenas (Greece) (2006)

Power Conversion Unit

System Description

n.a.

2.5 m3/h brackish water RO unit with 2.5 kW organic Rankine unit a Pelton turbine as energy recovery manufactured by the system. Specific energy consumption company SOFRETES of 0.67 kwh/m3 for with feedwater at with R114 as working 2 g/L and 50% conversion fluid; brackish water as (Maurel, 1979). cooling medium 9 m3/h brackish water RO unit, 6 h/ 10 kW organic Rankine unit day operation. Specific energy manufactured by the consumption of 1 kwh/m3 with company SOFRETES feedwater at 3 g/L. Daily production of with R11 as working 130 L/m2 of solar collectors (Libert fluid and Maurel, 1981). Organic Rankine cycle n.a. (Garcı´a-Rodrı´guez, 2007)

n.a.

Organic Rankine cycle

n.a. (Garcı´a-Rodrı´guez, 2007)

RO unit of 0.3 m3/h fresh water Organic Rankine cycle 54 evacuated tube collectors. Gross area with R134a as working production capacity with a reported of 216 m2. Only 22 of fluid; 2.5 kW expander specific energy consumption about type: scroll 2.3 kWh/m3. Mechanical energy them were connected compressor in reverse delivered by the ORC unit was directly to the ORC unit. operation used for driving the high pressure pump of a small reverse osmosis desalination unit, the feed pump of the ORC, the cooling water pump and the circulating pump. Low experimental values of the solar ORC efficiency were measured (1.5%) because low values of the expander efficiency (Manolakos et al., 2009).

a movable piston in a pressure tank. A dynamic thermal analysis of the proposed system is presented to determine the productivity and efficiency of the design. With 4.75 kg working fluid charge within the spherical tank, daily fresh water productivities of 55, 200, and 1800 L/m2 of aperture are obtained with feedwater salinities of 42,000, 22,000, and 2500 ppm, respectively. Ibarra et al. (2014) analyze the performance of a STRO system at part load operation with a numerical model of the proposed solar ORC cycle. The solar thermal system has a PTC-solar field with a total aperture area of 166.05 m2 and a water thermal energy storage (stratified tank) with a total capacity of 10 m3. The ORC unit has an internal heat exchanger (regenerator) and a scroll expander as its mechanical energy production device. R245fa is considered as the

Water Desalination by Solar-Powered RO Systems 61 working fluid of the unit and a nominal net power of 5 kWe is assumed. With this configuration of the solar power cycle, the study of the performance by simulation was carried out and the coupling with a RO unit was assessed taking a SEC value of 4 kWh/m3. Global efficiency calculated using the model was always below 7% and a stable water production of around 1.2 m3/h was obtained thanks to the thermal energy storage use. 2.4.3 Medium-scale solar thermal RO systems In medium-scale systems, power demanded levels generally fit with ORC unitary commercial nominal powers. These systems would be too small for using a conventional steam Rankine cycle for driving the RO unit, and too large to be coupled to e.g., a dish-Stirling system. On the other hand, medium temperature solar concentrating collectors would be the recommended option because their typical operation temperature levels match up with typical heat source temperatures of ORC units in this nominal power level. Li et al. (2013) propose a STRO with a supercritical regenerative ORC driven by a PTC solar field without thermal energy storage. Because no storage is used, the ORC produces variable net power because the variable incident solar radiation. A 40 m3/h single stage SWRO plant is simulated yielding a SEC of 2.32 kWh/m3 when feedwater is preheated with the heat rejected by the ORC unit. Power demanded by the RO plant is around 100 kW and the maximum net power production of the ORC is around 200 kW because two operation modes of the system are considered: electricity only and water-electricity cogeneration mode. In the cogeneration mode, the RO unit consumes 100 kW and the remaining power would be fed to the electric grid. In the electricity only mode, net electric power produced by the systems ranges from 0 to 200 kW. Optimization of the system shows that global system efficiency is >18%–20% for a wide range of solar irradiance values. 2.4.4 Case study: Preliminary design of medium-scale SWRO desalination plant driven by a double cascade solar ORC This section provides the information of a case study of a medium-scale STRO system. Firstly, a 2500 m3/d SWRO plant is designed in order to assess the power consumption. The analysis performed considers that the power required is supplied by an ORC (two different options are analyzed), heated by a solar field (PTCs). Based on the analysis of the ORCs selected, the aperture area is calculated for a given design point (without thermal energy storage). The power consumption is calculated for every technology and then the aperture area required at the same design point. Since those systems are driven by the same solar collectors, and the operation temperatures required are quite similar, all the systems considered can achieve a similar period of operation throughout the year in the same location. Therefore, the comparison at the design point should be representative.

62

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The solar ORC is selected from those proposed by Delgado-Torres and Garcı´a-Rodrı´guez (2007b) and Delgado-Torres et al. (2007). The sizing is adapted to drive a 2500 m3/d SWRO plant. Two different options are considered: (1) A solar ORC with regeneration and superheating. The solar ORC produces enough electricity to supply the electricity consumption of the overall solar desalination system. (2) A double cascade ORC. The top temperature ORC provides the high-pressure pump (HPP) of the RO system with the required power consumption. Besides that, the rest of electrical consumptions—mainly due to the BOoster pump (BOP), the ERD, and the seawater intake pump—are satisfied by the electrical energy generated by the ORC at lower temperatures (see Fig. 11). A general scheme of the RO process is depicted in Figs. 12 and 13. The HPP processes the same flow that the permeate production. Pressure exchangers (ERDs) process the same flow of brine and seawater. A BOP in series with the ERD is necessary to increase the high-pressure of the seawater (D/E) up to working pressure. Table 4 shows the main features of the selected membranes (see Fig. 13).

Fig. 11 Basic scheme of the solar thermal cascade organic Rankine cycle-driven seawater reverse osmosis desalination plant.

Water Desalination by Solar-Powered RO Systems 63

Fig. 12 SWRO plant diagram using an isobaric pressure exchanger device.

Fig. 13 FILMTEC membranes assembly in the hybrid interstage configuration proposed. Table 4 FILMTEC seawater reverse osmosis elements selection for a hybrid interstage design (FILMTEC, 2009) Type of Membranes SW30HRLE-400i SW30ULE-400i

Permeate Flow Nominal Active Rate (Flux) (m3/d) Surface Area, Am (m2) 28.4 41.6

37.2 37.2

Max. Pressure (MPa)

Stabilized Salt Rejection (%) (minimum)

8.27 8.27

99.75 99.70 (99.55)

The solar ORC is composed of the following: •

A solar thermal collector field of PTCs with a total aperture area, ASF. The input/output temperatures of the solar field are suitable for heating the ORC. The solar field is operated with synthetic oil. This is the conventional layout of a PTC solar field. The heated oil coming from the heat storage drives the boiler in which the working substance of the ORC is evaporated.

The solar collectors analyzed are two different PTC models: LS3 and IND3. A given design point is considered in order to permit the technology comparison. In order to compare different technologies, a design point is set. The selected design point is direct normal irradiance, Gb,N, of 850 W/m2 and incidence angle 0 degrees.

64 •

Chapter 3 An ORC system driven by the PTC solar field. In order to permit the technology comparison, the energy production of the solar ORC fits the energy requirement of a 2500 m3/d SWRO desalination plant with optimized design. The whole RO energy consumption defines the requirement of the solar ORC. Auxiliary energy requirement of the solar field are not considered. The main parameters are as follows for two configurations analyzed: • A solar ORC based on a simple cycle with regeneration and superheating. This configuration produces electricity for supplying the energy consumption both, main and auxiliary. The corresponding condensation pressure is 9 kPa. In order to avoid this low pressure, the second configuration is also considered. • A double cascade ORC, which uses hexamethyldisiloxane (MM) as working fluid in the topping cycle. It generates 250 kW of raw power with regenerator effectiveness of 0.8 and condensation temperature 115°C. The heat rejected from the topping cycle is absorbed by the bottoming cycle with condensation temperature at 35°C and regenerator effectiveness of 0.8. The bottoming cycle working substance is isopentane. The condensation pressure is above atmospheric pressure in both cycles.

Tables 5 and 6 show the main parameters of the preliminary design of the SWRO system. In addition, useful information related to the selected solar ORC is given in Tables 7–10. 2.4.5 Large-scale solar thermal RO systems Several designs of large-scale solar thermal power plants coupled with RO systems have been proposed and studied up to now. These have been named by several authors as Concentrated Solar Power + Desalination (CSP + D) plants, so this will be the name used in this section. These designs were initially proposed in a cogeneration scheme (electricity plus desalted water) to be coupled with thermal desalination plants instead of RO systems (Trieb et al., 2002). Readers interested in large-scale solar thermal desalination plants—the majority of them without net electricity output—can find information in, for example, Mittleman et al. (2007). However, due to the energy efficiency of the RO process, this technology has been also considered in the last decade joined with multieffect desalination (MED) technology (Trieb and M€ uller-Steinhagen, 2008). In this section, a brief review of research on CSP + D plant designs in which RO technology has been considered is given. With respect to solar thermal technology, most of the design proposals use PTC technology in the solar field. Moser et al. (2013) analyze four configurations of CSP + D plants, three of them using RO as desalination technology in a cogeneration of electricity and desalted water scheme. The study was carried out for Middle East and North Africa (MENA) conditions, and for a fixed values of desalination plant capacity (100,000 m3/day), net power production (90 MWe) and hours of thermal energy storage (7.5 h). One advantage of a CSP + D design with a RO plant over the design with a thermal desalination plant (multistage desalination (MSF) or MED) is that the CSP plant does not have to be located close to the desalination unit. Solar electricity can

Water Desalination by Solar-Powered RO Systems 65 Table 5 Main design parameters of the SWRO plant with 2500 m3/d of nominal capacity Lines of process of the desalination subsystems Seawater intake RO pretreatment

Beach-wells Antiscaling dosing and filtration with sand and cartridge filters See Fig. 1 A brine pipe drain by direct drainage to sea Chemical cleaning pump and tank

RO process Brine discharge Clean-in-place system

Seawater and brine Seawater characteristics

38,170 mg/L TDS (East Atlantic seawater beachwell), temperature tsw ¼ 22°C, pH 7.2, density: ρsw ¼ 1.03 kg/L, kinematic viscosity: νsw ¼ 1.25  106 m2/s Brine density: ρb ¼ 1.05 kg/L, brine kinematic viscosity: νb ¼ 1.80  106 m2/s

Blow-down

Design parameters of the RO subsystems Fouling factor Seawater intake pump

0.85 Submerged centrifugal pump Hydraulic performance, ηIP ¼ 0.74 Seawater pressure required (from intake to high pressure pump inlet), pIN ¼ 850 kPa R ¼ 40% Multistage ring section pump Hydraulic performance: ηHHP ¼ 0.74 Centrifugal pump Hydraulic performance: ηBOP ¼ 0.77 VDF performance, ηVDF ¼ 0.98 Motor (M) performance, ηM ¼ 0.95 PX-260 model Brine volumetric flow, 50–59 m3/h Low-pressure site differential, pLP ¼ 70 kPa High-pressure site differential, pHP ¼ 110 kPa Lubrication flow—leakage rate: 1.3% Number of PX-260, 3 Energy recovery efficiency, ηERD ¼ 0.95 PX-260 energy savings, 286.5 kW

Recovery rate HPP (high pressure pump) BOP (BOoster pump) VFD (variable frequency drive) ERD (energy recovery device)

Pipeline

Volumetric Flow (m3/d)

Pressure (MPa)

Pipe Dimension (mm)

Pipeline design Feedwater intake (A) Product (I) Blow-down (H) High-pressure pump outlet (C) Energy recovery device inlet (G) BOoster pump inlet (D)

6250 2481 3770 2500

0.25 0.07 0.20 5.17

250 160 250 154 (6 in.)

3721

4.98

203 (8 in.)

3750

4.87

154 (6 in.) (Continued)

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Table 5

Main design parameters of the SWRO plant with 2500 m3/d of nominal capacity—Cont’d

Pipeline

Volumetric Flow (m3/d)

Pressure (MPa)

Pipe Dimension (mm)

3750 6201

5.17 5.14

154 (6 in.) 203 (8 in.)

BOoster pump outlet (E) RO rack inlet (F)

Design parameters of the RO rack Membrane SW30HRLE400i (HRLE: high rejection-low energy) Membrane SW30ULE400i (ULE: ultra low energy) Pressure vessels

Permeate flow rate (flux): 28.4 m3/d Nominal active surface area: Am ¼ 37.2 m2 Maximum pressure: 8.27 MPa Stabilized salt rejection: 99.75% Permeate flow rate (flux): 41.6 m3/d Nominal active surface area: Am ¼ 37.2 m2 Maximum pressure: 8.27 MPa Stabilized salt rejection: 99.70% (99.55% minimum) 28 pressure vessels PV configuration: 2 SW30HRLE-400i + 5 SW30ULE-400i Total number of membrane elements: 196 Total active area: Am ¼ 7283.36 m2 Average flux: qv P =Am , 14.3 (l/(m2 h)

Table 6 Power consumption of the SWRO plant designed (2500 m3/d of nominal capacity) HPP, high pressure pump BOP, BOoster pump Intake pump Total auxiliary consumption Main consumption Total power consumption RO process specific energy consumption Total specific energy consumption of the RO plant

194.4 kW 16.5 kW 88.4 kW 104.9 kW 194.4 kW 299.3 kW 2.14 kWh/m3 2.99 kWh/m3

Table 7 Case 1: Main design parameters of the solar power subsystem Solar irradiance at design point Solar collector Organic Rankine cycle

Solar field

850 W/m2 with normal incidence Parabolic trough collector LS3 Working fluid: Hexamethyldisiloxano Maximum temperature: 290°C Evaporation temperature: 235°C Vapor turbine performance, ηVT: 0.75 Thermal power rejected: 975.2 kW Rankine cycle performance: ηR ¼ 24.6% Efficiency of the mechanic to electric conversion: 0.95 Collector area: A ¼ 2099 m2 Production per unitary aperture area: 0.148 kWe/m2

Adapted from Pen˜ate, B., Garcı´a-Rodrı´guez, L., 2012. Seawater reverse osmosis desalination driven by a solar organic Rankine cycle: design and technology assessment for medium capacity range. Desalination 284, 86–91.

Water Desalination by Solar-Powered RO Systems 67 Table 8 Case 2: Main design parameters of the solar power subsystem Solar irradiance at design point Solar collector Organic Rankine cycle

Solar field

850 W/m2 with normal incidence Parabolic trough collector IND300 Working fluid: Hexamethyldisiloxano Maximum temperature: 262°C Evaporation temperature: 235°C Vapor turbine performance, ηVT: 0.75 Thermal power rejected: 1009 kW Rankine cycle performance: ηR ¼ 24% Efficiency of the mechanic to electric conversion: 0.95 Collector area: A ¼ 2576 m2 Production per unitary aperture area: 0.121 kWe/m2

Adapted from Pen˜ate, B., Garcı´a-Rodrı´guez, L., 2012. Seawater reverse osmosis desalination driven by a solar organic Rankine cycle: design and technology assessment for medium capacity range. Desalination 284, 86–91.

Table 9 Case 3: Main design parameters of the solar power subsystem Solar irradiance at design point Solar collector Top organic Rankine cycle

Bottom organic Rankine cycle

Solar field

850 W/m2 with normal incidence Parabolic trough collector LS3 Working fluid: Hexamethyldisiloxane Maximum temperature: 336°C Evaporation temperature: 235°C Vapor turbine performance, ηVT: 0.75 Thermal power rejected: 1299 kW Rankine cycle performance: ηR ¼ 15.3% Mechanical power output: 250 kW Working fluid: Isopentane Maximum temperature: 145°C Vapor turbine performance, ηVT: 0.60/0.75 Thermal power rejected: 1157/1125 kW Rankine cycle performance: ηR ¼ 11/13% Electrical power output: 140/170 kW Collector area: A ¼ 2540 m2 Total performance: η ¼ 16.6/18.1%

Adapted from Pen˜ate, B., Garcı´a-Rodrı´guez, L., 2012. Seawater reverse osmosis desalination driven by a solar organic Rankine cycle: design and technology assessment for medium capacity range. Desalination 284, 86–91.

be produced inland—where the solar direct irradiation is usually better than in the coastal areas—and the SWRO plant can be located on the coast. This is an inherent advantage of electricity-powered desalination technologies. When the CSP plant is not located on the coast, an air cooling system instead of a wet-cooling system could be used and, in any case, when the CSP plant is located beside the RO plant on the coast, the feedwater (seawater) can act as the wet-cooling media of the CSP. Within the field of CSP + D plants, extensive work is being done from the Spanish center Plataforma Solar de Almerı´a (PSA-CIEMAT). Studies have also been focused in the MENA

68

Chapter 3 Table 10 Case 4: Main design parameters of the solar power subsystem

Solar irradiance at design point Solar collector Top organic Rankine cycle

Bottom organic Rankine cycle

Solar field

850 W/m2 with normal incidence Parabolic trough collector IND300 Working fluid: Hexamethyldisiloxano Maximum temperature: 280°C Evaporation temperature: 235°C Vapor turbine performance, ηVT: 0.75 Thermal power rejected: 1537 kW Rankine cycle performance: ηR ¼ 13.4% Mechanical power output: 250 kW Working fluid: Isopentane Maximum temperature: 130°C Vapor turbine performance, ηVT: 0.60/0.75 Thermal power rejected: 1374/1336 kW Rankine cycle performance: ηR ¼ 11/13% Electrical power output: 142/173 kW Collector area: A ¼ 3543 m2 Total performance: η ¼ 13/14%

Adapted from Pen˜ate, B., Garcı´a-Rodrı´guez, L., 2012. Seawater reverse osmosis desalination driven by a solar organic Rankine cycle: design and technology assessment for medium capacity range. Desalination 284, 86–91.

region because of the high level of solar resources (direct normal irradiation, DNI) and due to its population growth and related water demand increase. MED and RO technologies are considered as suitable technologies and detailed analysis are presented for different coupling configurations between the parabolic-trough CSP plant and the desalination system. Palenzuela et al. (2011a) present a thermodynamic evaluation of four different configurations, one of them with a CSP + RO scheme where the power output from the CSP plant is totally consumed by the RO plant. A CSP plant with a net power production of 50 MWe and a thermal storage capacity of 24 h is designed. At the design day the output capacity of the seawater RO plant would be 48,498 m3/day if the system were located at Abu Dhabi (1925 kWh/m2 of yearly DNI and RO’s specific consumption of 5.6 kWh/m3 is considered because the high salinity of the feedwater). Similar analysis is presented by Palenzuela et al. (2011b). In this case, the CSP + D plant is located in Almerı´a (1990 kWh/m2 of yearly DNI and RO’s specific consumption of 4.0 kWh/m3) and the output capacity of the desalination plant is fixed at 14,400 m3/day and net power production is 50 MWe so the CSP + RO plant act as a cogeneration plant (electricity plus water). Results show that the CSP + RO configuration yields the best thermodynamic efficiency and the lowest values of levelized electricity and water costs, but with small differences regarding the CSP + low temperature MED configuration (Palenzuela et al., 2011b). How the cooling of a CSP plant is made has great effect in the own water consumption of the power plant. When the integration of a MED or RO plant is analyzed, important design features can be studied. For example, in the case of a CSP + MED configuration, the desalination plant could be

Water Desalination by Solar-Powered RO Systems 69 driven by the heat rejected by the power cycle. Palenzuela et al. (2013) assess different cooling alternatives for a CSP + D system located on the coast with a yearly DNI representative of a Mediterranean location. Two cooling options were considered when RO technology acts as the desalination technology: wet cooling and dry cooling. In CSP + MED coupling, the desalination plant acts as the condenser of the steam Rankine cycle. The best values of overall efficiency and levelized cost of electricity were obtained for wet cooling CSP + RO configuration, and lower levelized cost of water values than CSP + MED values were found for all CSP + RO configurations. However, these conclusions should not be extrapolated to other locations because the SEC of the RO process is highly dependent on the feedwater salinity (Palenzuela et al., 2013). Blanco et al. (2013) perform a detailed preliminary analysis of a case study of CSP + D configurations in Port Safaga (Egypt). Information about solar resource assessment from satellite images for site selection is given. For the case study, net power of 50 MWe for the CSP plant and output capacity of 35,000 m3/day were considered for all configurations analyzed: two of them with MED technology and the third with RO technology (SEC of 5.5 kWh/m3 in this case). For the location selected, a CSP + MED configuration with the desalination plant acting as cooling unit exhibits the highest energy efficiency, followed by the CSP + RO option. Finally, Palenzuela et al. (2015a,b) present the analysis of the four CSP + D configurations and study the effect of different cooling alternatives for the cases where the MED plant is not used as the cooling unit of the CSP plant: two configurations with an MED plant driven by steam extracted from the turbine and the CSP + RO configuration. Two different locations—Abu Dhabi and Almerı´a—with different ambient conditions and feedwater salinities were considered, and operational data of the steam power cycle of the commercial Andasol-1 CSP plant were also assumed. From the results of the techno-economic analysis, it is concluded that for the Arabian Gulf area, the recommended configuration would be the CSP + MED option with a low-temperature MED unit with thermo-compression. For a Mediterranean location like Almerı´a, the authors consider the CSP + RO configuration to be the more realistic one. Detailed analysis of this study and extended information can be found in Palenzuela et al. (2015c). Olwig et al. (2012) perform a technical and economical evaluation of CSP + RO and CSP + MED systems with a freshwater capacity production of 24,000 m3/day of water with a target salinity of 200 ppm. Two different locations with important differences in yearly DNI values were selected for the evaluation: 2461 kWh/m2 in Aqaba (Jordan) and 1984 kWh/m2 in Ashdod (Israel). The desalination plant is located by the sea, while the CSP plant is inland, 5 km from the sea. The CSP plant with PTCs in its solar field and a gross electrical power of 42 MWe is modeled as a solar only-driven plant and 0 h (no storage) up to 12 h of thermal storage are considered. In the CSP + RO configuration, an ultrafiltration (UF) pretreatment step and a two-pass RO process is considered in the desalination system. RO feedwater is not preheated by the heat rejected from the steam power cycle, and the RO plant operates at full load conditions consuming electricity from the local grid with a SEC between 3.0 and 3.2 kWh/m3.

70

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Designs of CSP + D systems have also been proposed for producing agricultural water where its scarcity makes the sector’s activity unsustainable. Weiner et al. (2015) consider a CSP + D hybrid design (MED + RO) to produce a permeate flow of 7600 m3/day with a salinity of 500 ppm from brackish feedwater in California. Power delivered by the CSP plant is consumed by the RO system and auxiliary components of the MED system. The main thermal energy consumed by the latter comes from a steam turbine extraction. With regard to the desalination system, a two-stage configuration is considered for the RO and its brine is taken by the MED plant as feedwater. Finally, the mixing of RO and MEDs permeates is carried out. An example of a fuel-source and desalination technology hybrid design proposal can be found in Iaquaniello et al. (2014). Within the framework of the MATS research project, a technoeconomical analysis of high capacity RO-MED seawater desalination system (20,592 m3/day) is carried out. Power demanded by the RO plant and auxiliaries is produced by a natural gas-fueled open cycle gas turbine and the steam turbine of the CSP plant. The MED plant acts as the cooling unit of the steam power cycle, and exhaust gases from the gas turbine are used as a heat source for the backup system of the CSP plant. Regarding the hybrid desalination system, MED brine reject is mixed with seawater from the intake yielding a RO feedwater flow with a salinity of 36,000 ppm. Alegria-Casimiro (2015) studies the modeling of CSP + D plants and its performance through several simulation tools and new models developed, particularly in the case of the CSP + MED option. One of the configurations modeled is a CSP + RO cogeneration plant (electricity plus water) located at Trapani (Italy) with a net power capacity of 110 MWe and freshwater output of 36,000 m3/day. In this case, different configurations for the cooling system of the steam power plant were also considered: wet cooling, dry cooling, and once-trough seawater cooling. The CSP plant uses parabolic troughs and a 13 h molten salts thermal storage system.

3 PV-RO Desalination: State of the Art and Case Study 3.1 Generalities The coupling of desalination technologies with RE sources has involved a large number of publications describing generalities and presenting the state of the art of these technologies (Hanafi, 1994; Bellesiotis and Delyannis, 2000; Garcı´a-Rodrı´guez, 2002; Delyannis, 2003; Kalogirou, 2005; Mathioulakis et al., 2007; Eltawil et al., 2009; Al-Karaghouli and Kazmerski, 2013). In all these references, specific mentions are made of PV-powered RO systems. In historical terms, the use of desalination plants driven by renewable energy sources (RES) is a technique that has been implemented for more than thee decades; a PV-powered RO plant was first investigated on a commercial scale in Saudi Arabia in 1981 when a 3.2 m3/d SWRO desalination plant coupled to an 8 kWp (kWatt peak) PV system was installed in Jeddah (Boesch, 1982).

Water Desalination by Solar-Powered RO Systems 71 In the 1980s, not long after the start of commercial markets for both RO desalination and PV power generation, the first projects combining them to use RE for desalination emerged, generally with public financial support. Several reports were published on design and implementation of these plants and on the experiences of their operation. The Desalination Guide Using Renewable Energies (1998), edited by the Centre for Renewable Energy Sources (CRES) in Greece on behalf of the European Commission, presented the first comprehensive review and comparison with other combinations of RE with desalination. An update of this information was published by Garcı´a-Rodrı´guez (2003), including a list of 20 PV-RO plants. This paper presented a valuable and extensive collection of references on all possible combinations of RE with desalination processes published up to 2003. At the same time, Tzen and Morris (2003) reviewed the status of technologies for desalination and decentralized power supply in regard to the most promising couplings such as PV-RO. Five years later, Tzen et al. (2008) made a worldwide count of small-scale desalination systems (up to the capacity of 50 m3/day) and found 32 systems combining PV-RO. However, most of these systems were installed for research and demonstration purposes and operated under noncommercial conditions. PV-RO systems have been implemented in different regions, e.g., remote areas of the Libyan dessert, isolated areas of Jordan, Tunisia, and Morocco, and outlying areas in Australia. When considering commercial photovoltaics for connection to an RO system, PV-RO has previously been regarded as not being a cost-competitive solution when compared with conventionally powered desalination. However, the decline in PV costs over the last few years has changed this outlook. The distance to the national electric grid at which PV energy is competitive with conventional energy depends on the RO plant capacity, and on the salt concentration of the feed (Carvalho et al., 2013; Davis, 2013; Fthenakis et al., 2015; Shatat et al., 2013). Since the pioneering studies of solar-powered RO desalination in the late 1970s, the technical feasibility of this technology has been tested in a relatively large number of experimental units. Most of the research was conducted in regions or countries where the conditions for solar-driven desalination are the most favorable, i.e., intense solar radiation and severe water scarcity. These include the MENA region (Fthenakis et al., 2015; Ghermandi and Messalem, 2009), the southernmost part of Europe, and Australia. The global spread of these desalination plants is shown in Fig. 14; they are all sited in areas where the yearly average solar irradiance on a horizontal surface is significantly higher than the worldwide average (Bourouni, 2012). PV-RO has been found to be the technology used most often in these regions (Almaktoof et al., 2015). Most systems in operation were designed to function autonomously for small-scale desalination plants located in remote areas where freshwater resources are scarce and connection to the local grid power is unavailable. Although several full-scale plants are in operation in Saudi Arabia (Alawaji et al., 1995), the US Virgin Islands (Headley, 1997), the Maldives (Kanzari, 2005),

72

Chapter 3

Fig. 14 Solar-driven RO desalination systems: geographical distribution and type in Mediterranean and MENA countries, and worldwide (Ghermandi and Messalem, 2009).

Australia (Harrison et al., 1996), Mexico (Kunczynski, 2003), and Tunisia (Castellano and Ramirez, 2005), most of the experimental systems in Fig. 14 are demonstration or prototype units. System capacities range from <0.1 m3/d for prototype units up to 75.7 m3/d for full-scale systems. Several design studies have investigated the technical and economical feasibilities of medium and large-size desalination units, but to date, no experimental study of large-scale solar-driven RO desalination plants has been done. Despite the many technological improvements of recent years, however, the conversion efficiencies of PV modules remain low, rarely exceeding 15%–16% (Goetzberger et al., 2003). In addition to such low efficiencies, the retail price for PV modules stood at 4.70 € and 4.83 US$ in 2008 per Wp in the European and US markets, respectively (Solarbuzz, 2008). At the time of writing, the Wp can be between 0.6 and 2 €, making the solar sub-unit cost a key factor in the economic feasibility of PV-RO desalination. Ghermandi and Messalem (2009) investigated the current developments in the field of solarpowered RO desalination on the basis of the analysis of 79 experimental and design units worldwide. They concluded that PV-RO desalination is ready for commercial implementation. Although no standard design approach has been developed, the technical feasibility of different design concepts was demonstrated in a relatively large number of case studies. Battery-less systems that couple the PV modules directly to variable speed direct current (DC) pump motors seemed to have the highest potential for energy-efficient and cost-effective small-scale PV-RO desalination. Some concern was expressed about the long-term performance and reliability of such systems.

Water Desalination by Solar-Powered RO Systems 73

3.2 PV-RO Experiences As mentioned above, the PV-RO combination is the most applied autonomous desalination concept. Table 11 presents a list of the installed systems of this technology. Numerous PV-RO plants of small to medium capacity (0.5–50 m3/d) have been built in different locations around the world. A small PV-RO desalination plant with an average daily drinking water production of 0.8–3 m3/d was constructed at the test facilities of the Canary Islands Institute of Technology (ITC) in Pozo Izquierdo on the island of Gran Canaria (Herold and Neskakis, 2001; Banat et al., 2012). In Saudi Arabia, a PV-RO brackish water desalination plant was installed (Fthenakis et al., 2015; Hasnain and Alajlan, 1998), and it was connected to a solar still with a delivery capacity of 5 m3/d. The feedwater for the still was the blowdown from the RO unit (10 m3/d). Riffel and Carvalho (2009) presented the concept of a small-scale battery-less PV-RO desalination plant for stand-alone applications, specifically for treating brackish water in equatorial areas. This plant is capable of operating under varying flow and pressure conditions. Recognizing the importance of energy efficiency when using PV (Kunczynski, 2003) three commercially available small-scale ERDs were tested in a PV-seawater RO plant (19 m3/d). The plant achieved an energy consumption of 2.6 kWh/m3 and, with the three systems together, accumulated over 70,000 h of operation running entirely on solar energy. Helal et al. (2008) performed a study on the economic feasibility of PV-RO desalination for a production capacity of 20 m3/d, for remote areas in the United Arab Emirates. They compared a fully solar-driven RO plant (22 kWp) against one fully diesel-driven plant (10 kW) and one diesel-assisted PV system (11 kWp), where all plants produced the same total annual quantity of water. No backup batteries were used, and in the hybrid system the annual contribution of the PV source covered about one-third of the electricity demand. All RO systems were equipped with energy recovery from the concentrate. The conclusions from this study are quite remarkable, as the study was carried out based on conditions in a country known to have access to very cheap fossil fuel resources, leaving no chance for competition by RE in the minds of energy experts (Cipollina et al., 2009): •

• •

Optimal design selection depends primarily on the cost of primary energy and on the cost of solar panels. The solar-driven plant configuration becomes most favorable at panels costs of below 6 €/Wp. For small-capacity RO plants in remote areas, the labor cost becomes a significant cost fraction of the water cost: around 0.75 €/m3. For the input data used in this study, results showed that the fully solar-driven alternative is very competitive, having a specific water cost of 5.5 €/m3. This cost could be reduced via incentives to encourage the use of solar panels, such as reduction of interest on capital expenditure, exemption from taxes, and reduction of land cost.

74

Table 11 Overview of PV-RO systems installed worldwide

Abu Dhabia Agricultural Univ. Athensa AitBenhssaine villageb Aqabaa Baja California Sura Brounsville, Texasb Chaniaa Chbeika Cretea Coite-Pedreirasa ´n del Oroa Concepcio CRESTa Dohaa El Hamraweina Florida St. Lucieb Fredericksteda Gillen Borea Gizaa Hammam Lifa Hassi-khebia Heelat Ar Rakaha Hiroshimab Hotel in Fethiye regionb INETI Portugalb Denver, ITNa Java, Cituis Westa Jeddaha Ksar Ghile`nea Kulhudhuffushia Kuwaita Lampedusaa Las Barrancas, Mexicob Lavrio, Attikib

Photovoltaic Capacity (kWp)

Battery Back-Up System

Country

Year

Pump Drive

Production (m3/day)

Cost (US$ m3)

ARE GRC MOR JOR MEX USA GRC MAR BRA MEX BGR QAT EGY USA VIR AUS EGY TUN DZA OMN JAP TUR POR USA IDN SAU TUN

2008 2006 2008 2005 2005 1987 2004 1998 2000 1978 2001 1984 1986 1995 1986 1996 1980 2003 1987 1999 1987 2008 2000 2003 1981 1981 2005

45,000 30,000 – 4000 4000 – 40,000 40,000 1200 3000 32,800 35,000 4400 – 4400 1600 1600 2800 3500 1010 – – – 1600 1600 42,800 3500

11.25 0.85 4.80 16.80 25.00 – 31.20 26.30 1.10 2.50 1.54 11.20 19.84 2.70 19.84 4.16 7.00 0.59 2.59 3.25 – 6.00 0.05–0.10 0.54 24.50 8.00 10.50

No No – Yes Yes – Yes Yes Yes Yes No No Yes – Yes Yes Yes No Yes Yes – Yes – No Yes Yes Yes

AC DC – AC AC – AC AC DC/AC DC AC AC AC – AC AC AC DC AC AC – AC – DC DC DC AC

20.00 0.35 8.00 58.00 11.50 36.00 12.00 12.00 6.00 0.71 1.45 5.70 53.00 0.60 75.70 1.20 6.00 0.05 0.85 5.00 20.00 2.00 0.10–0.50 1.50 12.00 3.22 7.00

7.30 9.80 – 9.80 9.80 – 8.30 35.90 12.80 12.80 3.00 3.00 11.60 – 11.60 11.60 11.60 11.60 10.00 6.50 – – – 6.50 6.50 6.50 6.50

MDV KWT ITA MEX GRC

2005 2005 1990 1982 2001

2500 8000 8000 – 36,000

0.30 0.30 100.00 – 3.96 (+900 W WT)

No Yes Yes – Yes

DC DC AC – –

1.00 1.00 40.00 24.00 3.12

6.50 6.50 10.60 – –

Chapter 3

Location

Feedwater Salinity (mg/L)

a

ITA PRT ITA MOR ERI USA MOR MOR MOR MOR AUS CYP EGY AUS ESP JOR SAU ITA AUS AUS KUW IND UK ESP JOR GRC BHR CAN JOR USA AUS AUS SAU

1991 2000 1989 2008 2002 2003 2008 2008 2008 2008 2003 2005 2002 2008 2000 2000 1994 1984 1982 1982 1988 1986 2002 1988 1988 2000 1994 1983 2007 1999 1982 2003 –

8000 2549 – – 40,000 3480 2900 2900 2900 – 3480 3480 2000 5300 35,500 3400 5700 5700 5700 5000 – 5000 – 3360 400 400 35,000 33,000 7000 7000 7000 3500 –

63.00 0.10 – 3.90 2.40 0.54 4.00 2.50 4.00 4.00 0.06 10.00 1.10 0.60 4.80 32.00 10.08 65.00 1.20 0.12 – 0.45 2.40 23.50 0.07 1968.00 0.11 4.80 1.10 1.10 6,0.00 0.26 –

Yes No – – No No – – – – No Yes Yes No Yes Yes Yes Yes Yes No – No – Yes No Yes Yes No Yes No No No –

AC DC – – AC DC – – – – DC AC AC DC AC AC AC DC/AC DC DC – DC – DC DC DC DC DC AC AC AC DC –

International Desalination Association (IDA) and Global Water Intelligence (GWI), Desalination Yearbook (2008–2009). ProDes Project (2008–2010).

b

13.70 0.02 5.00 8.00 3.90 1.28 8.00 4.00 8.00 8.00 0.05 50.40 1.00 1.10 1.24 45.00 5.70 12.00 0.55 0.40 45.00 1.00 3.00 8.09 0.10 1000.00 0.20 0.86 3.60 3.60 3.60 0.50 20.00

10.60 10.60 – – 10.60 3.60 – – – – 3.60 2.30 3.70 3.70 9.60 9.60 9.60 9.60 9.60 9.30 – 9.30 – 2.50 2.50 2.80 2.80 9.00 9.00 9.00 9.00 9.00 –

Water Desalination by Solar-Powered RO Systems 75

Lipari Islanda Lisbon, INETIa Marett islandb Msaim villageb Massawaa Mesquite, ITNa Municipality of Amelloub Municipality of Tangarfab Municipality of Tazekrab Municipality of Sidi Ahmedb Murdoch Univ.a Nicosiaa NRC, Cairoa Pine Hilla Pozo Izquierdoa Qatar Villagea Sadou, Riyadha San Nicola, Tremitia SERIWA, Pertha Solarflowa Sulaibiyab Tanote, Thar Deserta U.K.b Univ. Of Almerı´aa Univ. De Ammana Univ. De Atenasa Univ. De Bahraina Vancouvera Varios Lugaresa VARI-ROa Wanoo Roadhousea White Cliffs Yanbu, Saudi Arabiab

76

Chapter 3

The ITC started to test this combination in 1998 (Subiela et al., 2009). The long experience of ITC in this field has allowed passing from the laboratory to the real world, offering a solution to water supply in remote inland areas several years ago. The milestone of this work was an international patent of PV-RO with batteries called DESSOL. Thanks to this knowledge, a brackish water system, with a capacity of 2.08 m3/h, was installed in the Tunisian village of Ksar Ghile`ne, and commissioned in May 2006. The desalination plant has successfully produced >15 million liters of freshwater in >8000 h of operation during the last 7 years of operation (Pen˜ate et al., 2015). More recently, and thanks to the EU initiative MEDA-Water Program, six units were installed and commissioned in Morocco along 2008 (ADIRA project, 2003–2008): two by the Moroccan NGO FM21 and the other four by the ITC. The experience has demonstrated the useful role of autonomous desalination to solve the water supply situation in rural areas (Banat et al., 2007). It is not possible to reduce the cost of desalination using solar energy to a comparable range with conventional desalination at least in the near future. Currently, solar energy desalination will probably find only remote location applications where there is no electrical grid connection. For this reason, ITC is currently working in the optimization of its DESSOL system, with the aim of obtaining a small-scale PV-SWRO desalination system intended to supply drinkable water to small populations isolated from the grid, and located near coastal regions with medium/high solar irradiation. All the efforts are aimed at reducing the costs of such systems. This and other studies related to RE desalination will be carried out within the DESALINATION LIVING LAB which is a R&D&I platform promoted by DESAL + Project cofinanced by the Interreg MAC Programme 2014–2020 (MAC/1.1a/094).

3.3 Case Study of a PV-RO System Working With Constant Production Capacity (With Batteries) vs. Variable (Battery-Less) Existing demonstrations of PV-RO desalination generally employ lead-acid batteries, which allow the equipment to operate at constant flow. In practice, however, batteries are notoriously problematic, especially in hot climates. The system tested in this study operates at variable flow, enabling it to make efficient use of the naturally varying solar resource, without the need of batteries. A comparative study based on hours of operation was carried out. The operation of a conventional DESSOL was compared with a modified DESSOL that was battery-less (Melia´n, 2016): • •

Desalination plant with a nominal production capacity of 2 m3/h. Desalination plant of variable production between 1 and 2 m3/h. To simplify the variable operation modes of the plant, this was operated in six operating modes ranging from 1 to 2 m3/h in intervals of 0.2 m3/h.

Water Desalination by Solar-Powered RO Systems 77 A DESSOL system plant working with a constant nominal production rate, with battery capacity to work with an average annual autonomy of 6.5 h/d, was compared to a DESSOL desalination plant working in a variable production regime with battery capacity to work only with a minimum autonomy of 0.5 h for flushing and the 24/365 control. The data were estimated for similar conditions of solar radiation (the area has an annual average solar radiation of 6.55 kWh/m2 daily). The estimated PV field power required was 19 kWp for the system with batteries, while for the battery-less system it was 9 kWp. The variable operating regime of a standard DESSOL plant was adapted to the electric generation of a variable photovoltaic field, improving its energy efficiency, by achieving more operation hours, higher productivity, and lower cost of energy storage. According to studies carried out by the ITC at Pozo Izquierdo testing facilities in Gran Canaria, for a PV-RO desalination plant working on a constant regime with a batteries back-up system (C10 ¼ 3000 Ah), an average daily operating time of 9 h/d in summer and 5 h/d in winter was achieved, with an annual average of 6.5 h/d, obtaining a total of 2372 h of operation per year with an estimated water production of 4.744 m3/y. For a plant working in a variable operating regime, an average daily operating time of 8.8 h/day in summer and 8 h/day in winter was achieved, with an annual average of 8.4 h/day, resulting a total of 3066 h of operation per year with an estimated water production of 4913 m3/y. Therefore, with this type of installation, it has been possible to increase the annual operating time, which entails a lower cost in the m3 produced with respect to the same installation with energy accumulation, since the investment in this one is smaller (Table 12).

3.4 Conclusions and Future Trends The current state of the existing technological options for powering the RO (RO) desalination with solar energy shows great differences between the development level of solar thermalpowered RO (STRO) systems and photovoltaic-driven RO desalination (PV-RO). The scarcity of STRO experimental systems is the main feature of this option; several documented examples exist only in the low-scale level. On the medium and large-scale levels, interesting design proposals and studies exist. The solar ORC is the most promising technology for medium RO capacity ranges whereas CSP plants with PTCs delivering fresh water and electricity could be a promising option in high solar resource locations. In general, in the case of solar thermal-driven RO technology, poligeneration schemes where electricity and/or cooling loads are also present could be interesting situations for the technological development of STRO. Photovoltaic technology can be directly connected to RO desalination processes. Many small PV-based desalination systems have been demonstrated throughout the world, especially in

78

Chapter 3 Table 12 PROS and CONS of both systems

PV-RO With Constant Production Capacity (Batteries) Advantages

• Lower-scale desa-

Disadvantages

Advantages

Disadvantages

• Higher investment

• Lower investment

• Higher desalination plant

cost in PV panels (50% more than in the battery-less system) Increased space requirement

cost in PV panels and batteries (for 24/365 control)

• Higher investment

• Lower investment

• Does not guarantee stable

cost in electrical installation

water production, which could generate short-term production uncertainty

lination plant dimension

•

• Guarantees stable water production

• Lower water storage capacity required

• Cloudy day’s •

operation Water supply in emergency situations (during the night)

PV-RO With Variable Production Capacity (Battery-Less)

cost in battery capacity (89%)

• Higher investment cost in electrical installation (20%)

• Higher maintenance costs (batteries/PV)

• Lower maintenance costs (batteries/PV)

• Less space requirement

investment: due to variability of operation a higher scale desalination plant dimension is required and devices (membranes, valves, control) for guaranteeing a daily production

• Needs greater water storage capacity

• Lack of understanding of the behavior and life of RO membranes operating under variable conditions during several years

remote areas and islands. The main issue of PV desalination is the cost of PV cells and batteries for energy storage, which are still high. Careful maintenance and operation of battery systems are also necessary. PV-powered RO desalination is mature for commercial implementation; the technical feasibility of different design concepts has been demonstrated in a large number of case studies. State-of-the-art, battery-less systems that couple the PV modules directly to variable speed DC pump motors seem to have the highest potential for energy-efficient and cost-effective smallscale PV-RO desalination. However, the long-term performance and reliability of such systems has not yet been sufficiently tested. The combination of solar power with additional power sources may be beneficial in both smalland large-scale desalination. In small-scale systems, PV panels may combine favorably with

Water Desalination by Solar-Powered RO Systems 79 wind turbines, achieving lower overall costs where the complementary aspects of the two renewable sources can be exploited. Photovoltaic systems are currently economical for low power installations. For autonomous systems, the cost of energy storage is the biggest constraint. Minimizing the cost of storage and reducing its capacity are the main reasons for the combination of wind and photovoltaic systems. Mohamed and Papadakis (2004) worked out the comparison of the water production cost with an autonomous PV system to that of a hybrid wind-PV system. They identified a cost of 5.21 €/m3 for a hybrid system, while the cost for the PV system was 6.64 €/m3. As a conclusion, the combination PV-RO tends, in stand-alone applications, to be a system as compact as possible to respond to domestic and urban uses on a small scale. Energy storage is a key issue, and makes it possible to have hybrid systems with small wind-generation systems or diesel generation groups. Isolated systems with high production capacities would be specific cases encouraged by the surface availability for solar PV panels, high solar radiation availability, or lack of other renewable energy resources. For large installations, PV may be seen as a support system for desalination connected to the grid or as a microgrid system combined with other energy sources in parallel (e.g., wind, biomass).

List of Acronyms BOP CIEMAT CPC CRS CSP DC DNI DVG ERD ETC FPC HPP HRLE ITC LFR MED MENA MM MSF ORC PCU PDC PSA PTC PV

BOoster pump Centro de Investigaciones Energeticas, Medioambientales y Tecnolo´gicas compound parabolic concentrator central receiver system concentrated solar power direct current direct normal irradiation direct vapor generation energy recovery device evacuated tube collector flat plate collector high pressure pump high rejection-low energy Canary Islands Institute of Technology linear Fresnel reflector multieffect desalination Middle East and North Africa hexamethyldisiloxane multistage desalination organic Rankine cycle power conversion unit parabolic-dish collector Plataforma Solar de Almerı´a parabolic trough collector photovoltaic

80

Chapter 3

RE RES RO SEC ST SW TDS UF ULE VFD

renewable energy renewable energy sources reverse osmosis specific energy consumption solar thermal seawater total dissolved solids ultrafiltration ultra low energy variable frequency drive

List of Symbols Am ASF Aa AA a1 a2 D Esolar Eopt E_ opt Ga Gb, N Ha L Q_ solar Q_ loss Qin Qout Qloss Qsolar TA TA Tamb Tm TC TH UA W Wnet η0 ηSTC ηCycle ηCarnot η

nominal active surface area (m2) aperture’s area of the solar field (m2) aperture area of the solar collector (m2) Absorber’s surface area (m2) heat loss coefficient (W/m2 K) temperature dependence of heat loss coefficient (W/m2 K2) absorber’s outer diameter (m) solar energy gathered by a solar collector or solar thermal system (J) solar energy lost because optical effects (J) solar power lost because optical effects (W) solar irradiance on the aperture area (W/m2) direct normal solar irradiance (W/m2) solar irradiation on the aperture area (J/m2) collector length (m) instantaneous solar thermal energy produced by the solar collector (W) solar power lost as thermal losses (W) heat absorbed by a thermodynamic power cycle (J) heat rejected by a thermodynamic power cycle (J) solar energy lost as thermal losses (J) solar thermal energy delivered by a solar collector or solar thermal system (J) average absober’s temperature (K) absorber’s temperature (local) (K) ambient temperature (K) average heating temperature of the heat transfer fluid of the collector (K) cold source temperature (K) hot source temperature (K) global heat loss coefficient of the solar collector (W/m2 K) collector width (m) net mechanical work delivered by a thermodynamic power cycle (J) zero-loss collector efficiency or optical efficiency of the solar collector efficiency of solar thermal collector thermal efficiency of a thermodynamic power cycle Carnot cycle’s thermal efficiency overall efficiency of a solar power cycle

Water Desalination by Solar-Powered RO Systems 81

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Further Reading Pen˜ate, B., Garcı´a-Rodrı´guez, L., 2012. Seawater reverse osmosis desalination driven by a solar organic Rankine cycle: design and technology assessment for medium capacity range. Desalination 284, 86–91.