SOLAR-DRIVEN MULTI-EFFECT DISTILLATION OVERVIEW
Desalination technologies can be categorized into (i) nonmembrane-based technologies and (ii) membrane-based technologies, where among them, some technologies require thermal energy while others require electrical energy for operation. The multi-effect distillation (MED) technology is a nonmembrane-based technology mainly driven by thermal heat and a little amount of electricity to run the water pumps. The MED process mimics the natural water cycle, where freshwater is evaporated from seawater, leaving behind salts and other nonvolatile substances, and then the freshwater is collected and gets condensed. Similar to the natural water cycle, the MED process can be driven by solar energy. This article provides an overview of the solar-driven MED process, including its operation design and the appropriate integration with solar energy. In addition, the possibility of coupling a MED process with a concentrated solar power (CSP) plant for the cogeneration of clean electricity and freshwater is also discussed.
1. MULTI-EFFECT DISTILLATION OVERVIEW
The distillation process operates by heating feedwater in distillation units to generate water vapor, which is then collected and condensed for distillate production. This is described as a single-effect distillation. However, this single-effect distillation has a gained output ratio (GOR) of < 1. The GOR is defined as the ratio of the latent heat of distillate produced to the expended heating steam, which is given by
where mp is the total permeate produced, hfg is the latent heat of water, and Qin is the input heat rate. As a rule of thumb, the larger the GOR is, the higher the thermal energy recovery of the desalination process is. To further increase the GOR, the produced water vapor (from the first distillation effect) can be recycled to heat additional feedwater in another distillation effect. This takes advantage of the produced water vapor's latent heat and condenses it into liquid freshwater. This process could be repeated by combining multiple effects, described as multi-effect distillation. Figure 1 shows a schematic of a standard MED process.
Figure 1. Multi-effect distillation (Reprinted from El-Dessouky and Ettouney with permission from Elsevier, Copyright 2002)
In addition to having multiple distillation effects, a thermal vapor compressor (TVC) or a steam ejector could be used to further increase the thermal efficiency. The TVC is usually attached at the exit of the condenser to entrain some of the uncondensed permeate vapor from the last effect by compressing it using the motive steam to the desired temperature and pressure of the first distillation effect. In other words, the TVC recycles some of the last effect's uncondensed permeate vapor to partially operate the first distillation effect. This reduces the motive steam required to run the MED plant (i.e., less energy input) and improves the performance ratio by 20–50%. Table 1 summarizes the multi-effect distillation specifications with and without the thermal vapor compressor.
TABLE 1: Multi-effect distillation specifications
|Technology||Specific Energy Consumption||Feed Concentration|
|Unit Cost of Water
|Multi-effect distillation||1.5–3.5 kWe h /m3
60–110 kWth h /m3
|< 70,000||5,000–15,000||≈ 10||0.5–1.0|
|Thermal vapor compressor – MED||1.5–3.5 kWe h /m3
40–110 kWth h /m3
|< 70,000||10,000–30,000||≈ 10||0.8–3|
2. MULTI-EFFECT DISTILLATION MODELING
As mentioned previously, the MED process uses multiple distillation effects that evaporate the feedwater to produce the permeate vapor and (simultaneously) condense the previous effect permeate vapor to heat the feedwater. Each distillation effect consists of an evaporator and a condenser component. These components can be modeled by defining their overall heat transfer coefficients for evaporation and condensation on the outer and inner tube surfaces. A list of the common correlations for the heat transfer coefficients of the evaporator and condenser are shown in Tables 2 and 3, respectively. More information about those variables can be found in the Saldivia et al. (2019) study.
TABLE 2: Evaporator's common heat transfer coefficient correlations (Reprinted from Saldivia et al. with permission from Elsevier, Copyright 2019)
|Overall heat transfer coefficient
|Inside tube condensation
|Inside tube condensation
|Outside tube evaporation
|Outside tube evaporation
TABLE 3: Condenser's common heat transfer coefficient correlations (Reprinted from Saldivia et al. with permission from Elsevier, Copyright 2019)
|Overall heat transfer coefficient
|Inside tube heating
|Inside tube heating
|Outside tube condensation
|Outside tube condensation
3. SOLAR-DRIVEN MULTI-EFFECT DISTILLATION
As one of the most reliable and commonly used desalination technologies, the MED technology has the potential to be driven by solar energy since it can operate at low temperatures (e.g., 70°C). Therefore, MED could be integrated with flat plate solar collectors (FPCs), evacuated tube collectors (ETCs), or even parabolic trough collectors (PTCs) or linear Fresnel collectors (LFCs) for high-temperature operation. In addition, it could be integrated with photovoltaic panels that supply the necessary electric energy to operate the water pumps and vacuum. As such, there are various coupling techniques to drive a MED process using solar energy. In the former integration, the FPC, ETC, PTC, or LFC array can produce the necessary heat to generate the necessary motive steam to start the MED operation, as shown in Fig. 2.
Figure 2. Solar thermal-driven multi-effect distillation configuration (Reprinted from Saldivia et al. with permission from Elsevier, Copyright 2019)
In this configuration, the thermal heat from the solar array is delivered to the MED plant via a heat transfer fluid (HTF) medium (e.g., therminol, sodium, or solar salts). Generally, these HTFs have a very high specific heat, allowing them to “catch” and “release” solar heat very quickly. However, some of these HTF mediums (such as sodium) cannot be near water because mixing them can cause a hazardous exothermic reaction. An alternative would be using a solar steam generation array, where water is used as the HTF that gets heated directly to the required motive steam conditions before entering the MED plant's vapor generator. This will eliminate the risk of mixing the other HTFs with water and keep the plant simple by only handling water through its pipes.
4. CONCENTRATED SOLAR POWER–MULTI-EFFECT DISTILLATION
Another alternative to drive the MED process with solar energy is integrating it with a concentrated solar power plant. In this configuration, both clean electricity and freshwater are produced with relatively the same capital equipment. Figure 3 shows a typical configuration of a CSP layout with five different possible locations, where energy could be extracted to run the MED process. Initially, solar irradiation is concentrated in the solar array via PTCs, a central receiver (i.e., a solar tower), or LFCs to heat the HTF. In the case of having a thermal storage facility on site, some of the HTF is directed to the thermal storage tanks to store some of its heat to be used later to extend the CSP plant's operation time and improve its dispatchability, whereas the rest of the HTF enters the heat recovery steam generator to generate steam that gets expanded in the steam turbine in the conventional steam Rankine cycle.
Figure 3. CSP-desalination configuration (Reprinted from Omar et al. with permission from Elsevier, Copyright 2019)
As shown in Fig. 3, there are five energy coupling points (Integration Points 1–5) in the CSP plant that a MED process could extract energy from to operate, as follows:
Waste heat recycling
Intermediate pressure steam from the turbine
Electrical energy output to drive MED pumps
Thermal energy storage
Solar field array
It is estimated that a conventional large-scale CSP plant can release ∼ 30% of its energy as waste heat to the environment. As such, this energy could be redirected to run a MED process (i.e., Integration Point 1). A considerable number of studies are available that analyze the possibility of recycling the waste heat, which is usually dumped by a condenser—whether it is an air-cooled condenser, wet-cooled condenser, or once-through cooling—to run a MED process. The MED process can partially (or even fully) replace the condenser, wherein the MED could directly condense the turbine's exhaust steam, acting as a condenser itself. This method can make a robust economic argument by trading one piece of capital equipment with another, but the exhaust steam temperature and pressure must nominally match well with the requirement of the MED process.
For instance, the exhaust steam temperature of a conventional CSP plant is ∼ 35°C, which is lower than the desired top brine temperature of a MED process (typically, 60–90°C). Thus, the steam should not be fully expanded in the turbine, meaning that the steam exhaust temperature could go up to the desired operating temperature of the MED process, but that will be at the expense of reducing the thermal efficiency CSP of the plant. This reduction in thermal efficiency will directly affect the electricity generated, which then raises the question: “Is it worth it to sacrifice electricity generated to produce freshwater?” In addition to that, this coupling method results in operation inflexibility, in which the CSP plant cannot operate without the MED and vice versa.
To mitigate the abovementioned drawbacks, one method is to use a highly advanced power block that can release high-temperature exhaust at nominal conditions. One power block that can do this is the supercritical CO2 cycle. This cycle could achieve an exhaust temperature of up to 120°C (Omar et al., 2021), which is sufficient to operate a MED process.
Another method is to extract intermediate pressure steam from the turbine and mix it with the exhaust steam using a thermal vapor compressor (i.e., Integration Point 2). This would boost the exhaust energy to the desired operating condition for the MED process. Otherwise, to avoid the dependency problem between the two systems mentioned previously, the intermediate pressure steam could be directly used as the heat source for the MED process.
Another simple coupling method is to use the generated electricity (i.e., Integration Point 3) from the turbine to operate the MED electrical components, such as feed and brine pumps, vacuum pumps, control systems, etc. In addition, this electricity could become the energy input of a heat pump to supply the necessary heat for a MED process. However, this is not a common approach as (technically) low-grade heat (from the solar source) is converted into high-quality electricity and then back to low-grade heat, which is not desirable due to the inefficiencies of energy conversion.
A fourth coupling method is going upstream to the thermal storage loop and using some of the stored heat (i.e., Integration Point 4) to drive the MED process without affecting the power block, hence not reducing its thermal efficiency or electricity generated. In addition, this method benefits from tapping an upstream energy flow prior to all the thermal losses that occur throughout the CSP plant. However, this approach would reduce the plant's operational time, resulting in a lower CSP capacity factor and reducing the annual electricity generated.
Finally, the fifth coupling point is as far upstream as possible by integrating the MED plant with the solar array (i.e., Integration Point 5). This is a relatively straightforward method as it imitates the abovementioned conventional solar-driven MED process (see Fig. 2) with the slight difference of having another heat load—the power block—other than the MED plant for the cogeneration of electricity and freshwater. Nevertheless, this method would require a larger solar field size to simultaneously adjust the heat required by the MED and the power block.
5. CONCLUDING REMARKS
There is a strong inverse correlation between solar resources and freshwater resources. As such, water scarcity sites usually coincide with high solar resources. If these sites are utilized effectively using solar energy collecting devices, it would positively impact the environment and diversify the energy supply matrix that could be used for freshwater production. Thus, it is critically important to explore different methods of using solar energy as the driving force to operate a desalination process, such as the well-developed multi-effect distillation technology. Solar-driven MED is an attractive alternative to overcome freshwater scarcity sustainably. Conventional solar collectors (e.g., flat plate collectors, evacuated solar tubes, or PTCs) could provide the necessary heat to run the MED plant. Alternatively, the MED process could be coupled with a concentrated solar power plant at five different integration points. This integration offers complimentary benefits to achieving clean energy and freshwater that can play a more significant role in green-terraform arid lands in the future.
El-Dessouky, H.T. and Ettouney H.M. (2002) Fundamentals of Salt Water Desalination, Amsterdam: Elsevier.
Omar, A. Nashed, A., Li, Q., Leslie, G., and Taylor, R.A. (2019) Pathways for Integrated Concentrated Solar Power – Desalination: A Critical Review, Renew. Sustain. Energy Rev., 119: 109609.
Omar, A., Saldivia, D., Li, Q., Barraza, R., and Taylor, R.A. (2021) Techno-Economic Optimization of Coupling a Cascaded MED System to a CSP-sCO2 Power Plant, Energy Convers. Manag., 247: 114725.
Saldivia, D., Rosales, C., Barraza, R., and Cornejo, L. (2019) Computational Analysis for a Multi-Effect Distillation (MED) Plant Driven by Solar Energy in Chile, Renew. Energy, 132: 206–220.
- El-Dessouky, H.T. and Ettouney H.M. (2002) Fundamentals of Salt Water Desalination, Amsterdam: Elsevier.
- Omar, A. Nashed, A., Li, Q., Leslie, G., and Taylor, R.A. (2019) Pathways for Integrated Concentrated Solar Power – Desalination: A Critical Review, Renew. Sustain. Energy Rev., 119: 109609.
- Omar, A., Saldivia, D., Li, Q., Barraza, R., and Taylor, R.A. (2021) Techno-Economic Optimization of Coupling a Cascaded MED System to a CSP-sCO2 Power Plant, Energy Convers. Manag., 247: 114725.
- Saldivia, D., Rosales, C., Barraza, R., and Cornejo, L. (2019) Computational Analysis for a Multi-Effect Distillation (MED) Plant Driven by Solar Energy in Chile, Renew. Energy, 132: 206–220.