1. Solar-Thermal Energy Technologies
In recent years, the research and development of solar technologies have not only enhanced the efficiencies of diverse systems but have also made significant strides in improving thermal energy storage and distribution, as well as the overall design, monitoring, control, and integration of these systems. Diverse solar technologies offer versatile applications, capitalizing on one of the most popular renewable energy sources.
Solar thermal energy is practical in diverse applications such as water and space heating, cooking, drying, desalination, distillation, industrial processes, thermal power generation, cooling and air conditioning, thermal energy storage, thermal hybrid systems, and steam generation. Notably, thermal power generation has garnered substantial attention from researchers in recent decades due to the environmental impact, particularly the contribution to global warming, associated with fossil fuels commonly used in thermoelectric power plants (often relying on natural gas or coal).
According to the 2021 Statistical Review of World Energy
[1], coal and natural gas dominated power generation, contributing 36% and 22.9% to total production, respectively. While renewable energy sources, including wind and solar, have made strides, their combined contribution to global electricity production was reported at 10.2% for the same year. Some experts suggest directing research efforts toward expanding solar power applications, particularly in electricity generation, to reduce dependency on conventional grids
[2].
2. Organic Rankine Cycles (ORCs)
Regarding power production using alternative energy sources instead of fossil fuels, a very popular system that has gained the attention of researchers in that field is the organic Rankine cycle (ORC) since it offers several advantages over the conventional Rankine cycle, the main one being that it can use low and medium-temperature heat sources which could not be used by a conventional Rankine cycle since it requires higher operation temperatures
[3]. Thus, the ORC is interesting, mainly when used for waste heat recovery, low-temperature heat sources, and applications where water as a working fluid is not feasible. Other key advantages of the ORC compared to the traditional Rankine cycle are described next:
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A wider range of working fluids. Such flexibility enables the optimization of the cycle for specific heat source temperatures. Recent research on organic fluids has addressed issues like their thermodynamic performance
[4], actual greenhouse effect
[5][6], the use of zeotropic mixtures as working fluids
[7][8][9][10][11], and the effect of using super dry working fluids on system performance
[12][13].
-
A more compact design. ORCs require lower operating pressures and, thus, smaller equipment sizes than conventional Rankine cycles. This advantage can lead to more compact and cheaper system designs. This advantage is particularly convenient for limited-space applications.
-
Customizable systems: ORCs can be customized and optimized for specific applications and heat source conditions. This flexibility allows engineers to design systems that match the requirements of the particular energy source
[3][14][15][16] and heat sink, maximizing efficiency
[3][14][15][16][17][18][19][20].
-
Modular and Scalable: ORC systems are often modular and can be scaled to suit different power output requirements. This characteristic makes them suitable for various applications, from small-scale distributed power generation to large industrial processes.
-
Decentralized Power Generation: ORCs are suitable for decentralized power generation, enabling the utilization of local heat sources to produce electricity. This can improve energy efficiency and reduce transmission losses.
-
Combined Heat and Power (CHP) Systems: ORCs can be integrated into combined heat and power systems, allowing the simultaneous generation of electricity and useful heat and increasing the overall energy utilization efficiency. New researchs on combined heat and power systems have utilized ORC, taking advantage of several heat sources
[21][22][23][24][25].
It is important to note that these cycles also come with their challenges and considerations, such as the choice of the working fluid, thermodynamic cycle design, equipment compatibility, and safety concerns associated with some organic fluids; despite this, their versatility and advantages make of them one of the main options to consider for power production when there are low or medium temperature heat sources.
3. Absorption Cooling Systems (ACS)
On the other hand, it is well known that vapor compression cooling systems are the most utilized systems worldwide for cooling purposes. However, they have direct and indirect negative impacts on the environment due to direct emissions of some refrigerants with Ozone Depletion Potential (ODP) and their use of electricity produced mainly by fossil fuels, indirectly contributing to global warming
[26]. Moreover, according to some prospects
[27], the cooling demand in the next decades will increase so that only the contribution of cooling systems to global warming will surpass the limit aimed for in the Paris Agreement. Thus, it is necessary to focus on actions aiming to create efficient and reliable alternative cooling systems capable of satisfying increasing cooling demand in the future. An alternative to conventional compression systems is absorption systems, which have gained attention in the last decades since they can be operated with a heat source, reducing (or even avoiding) the dependence on electricity to produce a cooling effect. In general, the research on absorption cooling systems seeks an improvement in efficiency, sustainability, and applicability. To achieve that, recent research has focused on topics such as the following:
-
Advanced absorbent materials: Researchers are investigating new materials to enhance vapor capture and release efficiency in absorption cycles. These new materials could lead to more efficient and lower-energy consumption cooling systems. Some of these absorbents are ionic liquids
[28][29][30], although other fluids have been studied
[31].
-
Cycle Efficiency Enhancement: Different techniques to optimize and enhance absorption cycles are being explored to reduce heat losses and improve mass transfer during absorption and desorption processes
[32].
-
Renewable Energy Integration: Researchers are exploring ways to integrate absorption systems with renewable energy sources such as solar or geothermal energy to make cooling systems more sustainable and self-sufficient
[33][34][35][36][37].
-
Hybrid Systems: Absorption systems integrated with other cooling technologies, such as mechanical compression systems, have been investigated to achieve optimal performance and higher energy efficiency
[38][39].
4. Integrated Cooling and Power Systems
According to the report “The Cooling Imperative: Forecasting the Size and Source of Future Cooling Demand”
[27], it was estimated that 470 million people in poor rural areas lack access to safe food and medicines due to inadequate electricity and refrigeration. In this context, hybrid systems for the simultaneous production of power and cooling could be an effective way to satisfy these two elemental needs at once. For this purpose, integrating organic Rankine cycles and absorption cooling systems is very attractive because of the advantages previously described for every system. Moreover, hybrid systems driven by solar energy for power and cooling could satisfy these basic needs where no grid access is available. In such cases, its energetic and economic performance could reach attractive values because of its high potential for an off-grid operation.
Research on hybrid systems for power and cooling has been intensified in the last few years. Most of this research is the result of modifications to basic power cycles (i.e., Rankine and Kalina cycles) and their integration into a cooling cycle, which, for convenience, turns out to be the absorption cycle. From that point, some cycles are continuously modified to get more complex systems to produce more than two energy effects. Some representative examples of the research on integrated systems are briefly described in the next paragraphs.
One of the most known cycles for simultaneous cooling and power production is the Goswami cycle
[40], which combines absorption and a Rankine cycle with power production as the primary goal. The working fluid in this cycle is the ammonia–water mixture, which, according to the authors, is ideally suited for solar thermal power using low-cost solar concentrating collectors. The Goswami cycle has been extensively studied
[41][42][43][44] because of its high versatility for using different heat sources, including those at low and medium temperatures. Some studies have been performed based on the Goswami cycle. Hasan and Goswami analyzed the Goswami cycle operated with solar energy from the second law perspective
[45]. The analysis considered optimizing the operating conditions to get maximum exergy efficiency. Heat source temperatures from 47 to 187 °C were considered in the analysis. The authors found that increasing the heat source temperature does not necessarily increase the energy or exergy efficiencies. However, it was proved that the heat source temperature affects the fractions of power and refrigeration. Another study
[46] theoretically analyzed several configurations for simultaneous cooling and power production using absorption systems with working fluids based on ammonia. This research considered different configurations based on the Goswami and single-effect absorption cycles coupled to several solar collectors (evacuated tube, parabolic trough, and linear Fresnel). This research found an optimum heat source temperature for each configuration depending on the solar technology, the evacuated tube solar collector being the most suitable for single-effect configurations since the temperature requirement is low. However, for applications requiring a higher heat source temperature, as the Goswami cycle does, the authors found that the parabolic trough collector could be a good option.
Other studies for the simultaneous production of power and cooling have been focused on the integration of organic Rankine cycles (ORC) and cooling systems such as vapor compression refrigeration cycles (VCRC), ejector refrigeration cycles (ERC), and absorption cooling systems (ACS).
Regarding the studies integrating an ORC and a VCRC, Alshammari et al.
[47] modeled and experimentally validated a new single-rotor expander-compressor device in a combined VCRC and ORC. The system was analyzed at a driven temperature of 90 °C, evaporator temperatures in the VCRC between −20 °C and −5 °C, and rotor speed (500–3000 rpm). The maximum cooling effect, heat-to-cooling efficiency, and exergy efficiency achieved were 5.38 kW, 56%, and 63%, respectively, at evaporator temperatures of 62.75 °C for the ORC and −5 °C for the VCRC. Kim
[48] modeled a combined power generation and cooling system comprising an ORC and a VCRC using R245fa, R114, R600, R142b, R152a, and R1234yf. The results showed that the thermal efficiency of the combined ORC–VCC system was almost twice that of the basic ORC system. The results showed that the R245fa exhibited the highest thermal efficiency of 25%, which was 29% higher than the 19.4% achieved using R1234yf under the same operating conditions. Grauberger et al.
[49] designed and evaluated an experimental ORC–VCRC of 300 kWth using novel heat integration strategies. The system uses a turbo-compressor sharing a single shaft. The system operated with waste heat at 91 °C and generated chilled water at 7 °C. The thermal efficiency of the Rankine cycle (accounting for pump work) was 7.7%, and the COP of the VCRC was 5.25. Nasir et al.
[50] analyzed a biomass-powered combined cooling, heating, and power system based on ORC and VCRC for small-scale developing and underdeveloped communities. The ORC operated with xylene and could deliver 100 kW of electricity. Meanwhile, isobutane was used in the VCRC. The system delivered as much as 30 kW of cooling and 528 kW of heating at various combinations of parameters.
Concerning the integration of an ORC with an ERC, Gorhbani et al.
[51] proposed a cogeneration system to produce cooling, heating, and power from geothermal energy. The cogeneration system comprises a Kalina cycle (KC), an ERC, and an ORC. The authors conducted energy, exergy, and exergoeconomic analyses with a multi-objective optimization. Thermal efficiency, exergy efficiency, total investment cost, total exergy destruction rate, net power production capacity, and cycle cooling capacity at optimal conditions were 23.04%, 26.55%, 45,944.5 $/yr, 226 kW, 75.17 kW, and 111.6 kW, respectively. Tao et al.
[52] analyzed and optimized a combined power and refrigeration system based on ORC and ERC. The evaporation temperature was as low as −60 °C. The system was compared with others reported in the literature, finding that under the same operating conditions, the system’s net power was increased by 12.52 kW, the thermal efficiency was increased by 4.27%, and the energy efficiency was increased by 2.57%. The optimum system thermal efficiency, the exergy efficiency, the sum unit cost of products, and the sum unit environmental products were 15.01%, 43.18%, 45.5 USD/MWh, and 5122.6 MPTS/MWh, respectively. Chowdhury and Mokheimer
[53] analyzed the performance of a combined power and cooling cycle consisting of an ORC and an ejector absorption refrigeration cycle. The ORC was modeled with different working fluids, while the refrigeration cycle operated with the ammonia–lithium nitrate mixture. Parabolic trough collectors drove the whole system. The modeling results showed that the PTC accounted for 60–80% of the total exergy destruction. The highest energy utilization factor was 25.31% at a fluid inlet temperature of 550 K, using toluene as a working fluid in the ORC. The highest exergy efficiency was 17%.
Regarding the systems integrating an ORC and an ACS, Liu et al.
[54] analyzed a system integrated by two ORCs (one of them of multiple stages), a compressed air energy storage, and an ACS to produce heating, cooling, and power simultaneously. The optimization was performed using
Aspen Plus software to maximize the round-trip efficiency and minimize the total investment cost per output power. The results showed that the system has the advantages of a high efficiency of 68.38% and a low cost of 0.1984 $/kWh. Sharifishourabi and Chadegani
[55] studied a system for the production of hydrogen, cooling, hot water, and power through the integration of an organic cycle, a triple-effect absorption cooling system, a dehumidifier, and an electrolyzer using a compound parabolic trough solar collector. In the proposed system, the power output was used to activate the electrolyzer and produce hydrogen, while the organic fluid at the turbine outlet was used to activate the cooling system. The performance parameters achieved by the system were 0.39, 1.34, and 14.4% for the energy utilization factor, the COP of the cooling system, and energy efficiency, respectively. Anvari et al.
[56] proposed and analyzed a trigeneration system consisting of a gas-turbine cycle, a heat-recovery steam generator, and an absorption cycle operating with the H
2O–LiBr to produce a combined cooling, heating, and power. The proposed configuration capacity can generate a power of 30 MW, 40 MW of heating, and 2 MW of cooling. The authors found that the combustion chamber had the highest contribution to the overall exergy destruction and that nearly 29% of the total irreversibility in the cycle was endogenous-avoidable. From the second-law perspective, Pashapour et al.
[57] analyzed a polygeneration system for heating, cooling, and power. The proposed system integrated a gas turbine, an ORC, and an ACS. The system uses the heat lost from a gas turbine to drive the organic cycle and, at the same time, to produce warm water. Geothermal heat is used in a reheater in the organic cycle to improve the power produced (achieving an increase of 29.4% regarding the non-reheating cycle) and then to drive the absorption cycle for cooling production. It was found that a maximum exergy efficiency of 50.65% was achieved. Jiménez et al.
[58] analyzed the coupling of an organic cycle and an absorption cooling system using different organic fluids. The authors found that power output is, at best, one-quarter of the cooling output for a wide range of operative conditions. This is because the expansion of the organic fluid in the power cycle is limited by the need to obtain a fluid at a high temperature at the outlet of the expander to activate the double-effect absorption cycle. The energy utilization factor and the exergy efficiency varied between 0.62 and 0.76, and 0.14 and 0.35, respectively. Grosu et al.
[59] integrated an ORC using R245fa and an H
2O–LiBr ACS driven by solar energy. It was designed to supply electricity and air conditioning to a building; however, this research mainly focused on the organic and absorption cycles, leaving aside the solar system details. The authors recommend adding a recovery heat exchanger at the inlet of the condenser in the ORC and including a solution heat exchanger in the absorption cycle to improve the efficiency of the integrated system.
Additionally, Gupta et al.
[60] reviewed a solar ORC and its polygeneration applications. The authors reported 160 references to systems related to the topic, most of which analyzed systems for the production of electricity and heating, electricity, heating and cooling, electricity, heating, and freshwater, among other applications. Regarding the systems studied for electricity and cooling, in almost all cases, the systems were integrated using an ORC and a VCRC, while just a couple of them used absorption systems; however, in both cases, the absorption systems were not used for cooling production but for heating or fresh water.
From the literature reviewed, it is clear that there have been many studies of ORC driven with solar energy
[60] for a wide range of applications. Also, many studies have been reported for the simultaneous production of power and cooling, but most of these studies integrate a VCRC into an ORC
[47][49][50]. Also, many systems have used an ERC for cooling purposes
[51][52][53]. Only a few studies have integrated an ACS for cooling production
[54][55][56][57], and in fact, just the system analyzed by Grosu et al.
[59] was driven by solar energy. Thus, it is clear that there is a lack of studies integrating ORC and ACS for the simultaneous production of power and cooling driven by solar energy. Additionally, it was observed that, in all the cases in which an ACS was integrated into an ORC, the ACS operated with the H
2O–LiBr mixture, thus limiting their applications just for air conditioning without the possibility of producing cooling under 0 °C. Therefore, the present study proposes and analyzes the theoretical performance of an integrated organic Rankine cycle/single-effect absorption cooling system (ORC–ACS) driven by a solar system composed of a commercial Parabolic Trough Solar Collector (PTSC) coupled to thermal storage. For that purpose, the analysis considered the solar conditions available in Temixco, Morelos, Mexico. The solar analysis was carried out using the NREL System Advisor Model (SAM 2022.11.21 version) software
[61], whose output values for thermal load and temperature (delivered by the solar system) characterize the thermal input to the integrated cooling and power system. Thus, the cooling and power production will be assessed under the specified conditions.
As for the working fluids selection, the proposed fluids for the system are benzene, toluene, cyclohexane, and R123 for the ORC and the ammonia–water (NH
3–H
2O) mixture for the ACS. The choice of the organic fluids was based on previous reports
[4][62][63][64], where the best efficiencies for the organic Rankine cycles operated in similar conditions to those of the present research were obtained with benzene, toluene, and cyclohexane. As for the selection of R123, it was influenced by several recommendations in the literature
[65][66][67][68], mainly considering the thermodynamic and environmental performances achieved. The organic fluids chosen are dry fluids suitable for the ORC operation. Some relevant properties of these fluids are presented in
Table 1.
As for the NH3–H2O mixture, it is a reliable and proven fluid that makes it possible to produce a cooling effect at the temperatures required for refrigeration applications.
About the characteristics that make this system different from others found in the literature, researchers can count on the following:
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A deeper analysis of the dynamic behavior of the solar collector for a particular location, including the sizing of the thermal storage.
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A higher capacity for cooling over power production when compared to the Goswami systems, which prioritize power over cooling.
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A benefit of the power production as a result of the lower activation temperatures for the single-effect absorption cycle, regarding those required for double or triple-effect absorption cycles.
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A higher versatility of the proposed system to produce cooling suitable for refrigeration or air-conditioning applications due to the use of NH3–H2O Instead of H2O–LiBr as other studies in the literature.
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A wider range of operating conditions is due to the higher heat source temperatures provided by the PTSC in comparison to other solar technologies proposed in similar studies.
Moreover, the analysis and presentation of the results in this research make it possible to assess the operation of the integrated configuration (ORC–ACS) with heat sources different than solar, such as industrial waste heat or geothermal energy, significantly expanding its potential applications and uses.