Power, Fresh Water Generation and Heating
Table of Contents:
Reasonable water and electricity supply policies are of vital importance for the development of locations where there is inadequate water. Solar photovoltaic systems or diesel generator systems using reverse osmosis (RO) for fresh water generation are commonly used for a few kV to a few MW, capacity plants, for simultaneous generation of electricity and fresh water. For dispatchable (on demand) electricity and fresh water generation in isolated regions and on islands, diesel generator-based systems are used. With respect to CSP-based electricity and fresh water generation systems, steam Rankine cycle power systems (Palenzuela et al., 2015), organic Rankine cycle power systems (Astolfi et al., 2017), or supercritical carbon dioxide Brayton cycle power systems (Sliarau et ah, 2019) can be used for power generation, and reverse osmosis systems (El-Emam and Dincer, 2018) or thermal energy-driven desalination systems (Astolfi et ah, 2017) can be used for fresh water generation. Simplified schematics of a typical concentrated solar thermal energy-driven organic Rankine cycle-based electricity system with reverse osmosis-based desalination system and thermal energy-driven desalination systems are shown in Figures 3 and 4, respectively. A summary of previous works on concentrated solar power-based cogeneration systems using the ORC technology and desalination system (reverse osmosis or thermally driven) is given in Table 3.
Figure 3. Sunplified schematic of a typical concentrated solar thermal energy-driven orgamc Rankine cycle-based electricity
and reverse osmosis-based desalination system.
Toluene (Delgado-Torres et al., 2007), R134a (Karellas et al., 2011), isopentane (Bmno et al., 2008), MM (Li et al., 2013), and n-octane (El-Emam and Dincer, 2018) were proposed as promising organic working fluids for CSP-driven ORC systems with RO desalination. For concentrated solar thermal energy-driven ORC-systems with a thermal energy-driven desalination system, n-pentane (Astolfi et al.,
2017), toluene (Sharaf, 2012), and cyclopentane (Mathkor et al., 2015; Desai et al., 2019b) were proposed as promising organic working fluids.
The parabolic trough collector is the most widely-used CSP technology for ORC-based cogeneration systems (El-Emam and Dincer, 2018). Recently, a nanostmctured polymer foil-based concentrated solar collector technology was analyzed as a promising alternative compared to a РТС-based system for ORC power systems integrated with a multi-effect distillation (MED) desalination system (Desai et al., 2019b). The assumptions related to the solar irradiation, capital cost of the sub-systems and electricity consumption significantly influence the techno-economic performance of the cogeneration system. The concentrated solar thermal energy integrated MED-system is less expensive than a RO-based desalination system (Ghobeity et al., 2011; Sharan et al., 2019). Depending on seawater salinity, membrane configuration and efficiencies of components, the specific electricity consumption for reverse osmosis systems is about 3.5 kWh(/m3 to 5 kWhe/m3 (IRENA, 2012; Sharan et al., 2019). For multi-effect seawater distillation systems, the specific electricity consumption is about 1 kWhe/m3 to 1.5 kWly'm3 (Alfa Laval, 2018).
Power, Cooling and Heating
Conventional vapor compression refrigeration systems (VCRS) powered by electrical energy are widely used for cooling applications. Such systems can be powered by electrical energy produced by a concentrated solar thermal energy-based organic Rankine cycle power system. A low-grade thermal energy-driven vapor absorption refrigeration system (VARS) can also be integrated as a bottoming cycle to an organic Rankine cycle power system. A simplified schematic of a typical concentrated solar thermal energy-driven organic Rankine cycle-based vapor compression or absoiption refrigeration system is given in Figure 5. A summary of previous works on CSP-driven ORC-based cooling and/or heating systems is given in Table 4.
Figure 6. Sunplified schematic of a typical organic Rankine cycle-based electricity and cascaded vapor compression and absorption refrigeration system (Adopted from Patel et al., 2017).
A bottoming VARS using lithium bromide-water (LiBr—H,0) as a refrigerant is limited to space cooling at a commercial level (Tassou et al., 2010). Vapor absoiption refrigeration systems with ammonia- water (NH,-H,0) are less advisable for food applications due to toxicity, flammability, low boiling point temperature difference of refrigerant and absorbent, low coefficient of performance and incompatibility with materials (Deng et al., 2011). Integrated systems based on adsorption cooling and liquid desiccant cooling technologies are still at the research and development phase (Jradi and Riffat, 2014).
Concentrated solar thermal energy-powered organic Rankine cycle systems integrated VARS, which works on thermal energy, are typically limited to space cooling. On the other hand, ORC-integrated VCRS, which works on electrical energy, can be used for refrigeration applications. Patel et al. (2017) proposed a concentrated solar thermal energy and biomass energy-powered ORC unit with a cascaded refrigeration system, as shown in Figure 6. In such a system, the electricity and heat duty requirements of the VCRS and VARS are fulfilled by the ORC-unit, combining the advantages of both systems. The cascaded system achieves low temperature (up to -20 °C) cooling and requires much lower electricity compared to the vapor compression refrigeration system (Patel et al., 2017).
The key aspects of designing CSP-integrated ORC-imit-based multi-generation systems optimally are briefly covered in this section.
Solar irradiation data
The duration and intensity of the solar irradiation affect the performance, capacity factor, and economic viability of the system significantly. The sizing and configuration of the system also depend on the solar irradiation. Local factors like fog and pollution level, dust/sand storms, wind speeds and their variations also need to be considered when selecting the place of installation.
Solar collector field and thermal energy storage
Concentrated solar collector type and size and thermal energy storage type and size need to be carefully selected, as both these systems har e major shares in the capital cost of the complete system. Optical efficiency and overall heat loss coefficients are crucial parameters for concentrated solar collector fields and improvements in these parameters often increase the solar field cost. The use of thermal energy storage facilitates delivery of the utilities according to the need by absorbing the variations due to fluctuations in the solar irradiation. For off-gr id locations, where there are no available central gr ids, integr ation of other energy sources, fossil fuel based or biomass based, may be needed to meet the utility demands.
Organic Rankine cycle power system and other sub-systems
The selection of cycle configuration, component types and designs, and working fluid is important for efficiently converting solar thermal energy into multiple products. All these parameters are dependent on the application type and maximum capacity requirement. The other products (cooling, heating, fresh water) should be selected based on the needs of the region. The primary need (electricity, cooling, heating or fresh water) of the place is of vital importance for successful implementation. The cooling and desalination systems can be thermal energy driven or electrical energy driven, and the selection depends on the techno-economic analysis. Integration of thermal energy driven systems with an organic Rankine cycle power system enables a high energy utilization factor and high overall system efficiency. However, in such a system, the net power output is lower compared to only power generating systems with a condensing turbine. Therefore, the selection of an ORC power system and other sub-systems should be done carefitlly.
The system design needs to be based on a detailed analysis of the part-load characteristics of the components of the multi-generation systems. Due to the mismatch between supply and demand, the major challenge is to provide a dilute and variable nature of solar energy input to the various demands. Moreover, it needs to be addressed that the actual system performance may differ from that of the design predictions due to the system inertia causing delays during the start-up and shut-down phases.
System configuration and control
For optimal system configuration, all of the aforementioned parameters need to be considered carefiilly. The process controls of the CSP-driven multi-generation systems based on the ORC technology should be designed as a subset of the overall plant control strategy. A proper system configuration and control provide desired products to the consumers cost-effectively and reliably.
Concentrated solar collector powered medium-scale dispatchable multi-generation energy systems with thermal energy storage are typically more costly than fossil fuel based and biomass based systems. However, factors like availability of fossil fuels and biomass as well as high carbon footprints for the former and high water footprints for the latter are the major drawbacks of these technologies.