1. Introduction
Drastic shifts toward renewable energy systems have been highlighted in recent decades with the recent advancements in technology and growing needs in the energy sector [
1]. In addition, the rise of carbon emissions and burdensome zero energy policies, alongside the depletion of fossil fuels, motivated researchers to focus on developing high-performance energy systems while investigating renewable fuels [
2,
3]. Multi-generational systems offer the potential to improve the efficiency of systems products and meet the desired energy demands by simultaneous production of power, heating, cooling, desalinated water, etc., efficiently. Improved performance and high efficiency, along with high flexibility for implementation in remote areas with no grid connection, are some benefits of multi-generational systems that have brought them into an advantageous position [
4,
5].
Numerous studies have been conducted to assess different scenarios for generating simultaneous products integrated with solar energy as a renewable energy source. CCHP systems can simultaneously satisfy a number of essential demands and be combined with other energy systems, such as different solar technologies and energy storage systems, to maintain a renewable and zero-carbon emission system. Hybrid CCHP systems also offer a reasonable degree of flexibility to be combined with internal combustion engines and different types of fuel cells. This can open new doors for designing and optimizing multi-generational energy systems based on energy demands, cost, and efficiency [
4].
The fluctuating nature of renewable energy sources is one of the major challenges in the way of improving the efficiency and cost of eco-friendly energy systems. To cope with this issue, energy storage systems expanded new horizons regarding the cost, reliability, and performance of energy systems, especially renewable and sustainable energy systems. Energy can be stored in different formats (mechanical, thermal, electrical) and can be used when needed. Various energy storage systems, such as thermal, mechanical, and electrical energy storage systems, are utilized in energy systems. Phase-change materials (PCMs), molten salt, and compressed air energy storage are widely explored in solar energy systems [
6] and can be exploited to solve the intermittency issues in energy systems, each of them offering interesting advantages. As a widely accepted technology, thermal energy storage (TES) systems can preserve heat either in sensible or latent forms. While each type offers special characteristics, latent heat energy systems are interesting due to their reliance on specific latent heat and higher energy storage potential per unit mass. This feature can substantially promote the utilization of latent heat thermal storage technology in energy systems and alleviate the cost and size. PCMs, as a type of TES, are widely used in multi-generational energy systems as TES systems to resolve the issue of intermittency of solar energy harnessing systems [
6].
Many researchers have put effort into proposing novel CCHP-integrated systems and enhancing their performance with parameter studies and optimization methods. Sadeghi et al. [
7] proposed a novel CCHP system alongside a compressed air system and an ERC system and optimized the system based on three objective functions, targeting cost, efficiency, and environment factors. The result showed a cost rate of 4817 USD/h and normalized CO
2 emissions of 157 kg CO
2 per megawatt hour. You et al. [
8] assessed a CCHP system and multi-effect desalination unit (MED) with a solid oxide fuel cell (SOFC) and micro gas turbine (MGT) by conventional and advanced exergo-economic analyses and revealed the largest exergy destruction in the burner, followed by the SOFC and multi-effect desalination system. Liu et al. [
9] proposed a CCHP system combined with LNG and flue gas heat recovery. Their optimization output showed a cost per unit exergy of 18.05 USD/GJ.
Solar-powered CCHP systems help unleash the potential of sustainable and renewable high-efficiency cogeneration systems to produce emission-free products. The thermodynamic assessment of solar-powered CCHP systems helps to accelerate the thermal performance of the system and has been widely investigated in recent literature. Peng et al. [
10] worked on a new combined solar collector and steam turbine CCHP system with a SOFC. In another study, Gholamian et al. [
11] developed a solar CCHP system with a double-effect chiller for recovering the waste heat to meet the residential needs of the city of Tehran in Iran and reported a maximum thermal efficiency of 34%. Ozturk et al. [
12] utilized rock beds as energy storage to maintain the system’s continuous operation and reported an exergetic efficiency of 37.3%. Song et al. [
13] introduced a CCHP energy system with solar energy, which, in addition to providing cooling, heating, and power generation in public buildings in a Chinese city, uses additional power to produce hydrogen in a closely located nearby hydrogenation facility; the optimal hydrogen from the system was 700 kg/day, and through optimization, the lowest unit energy cost obtained was 0.0615 USD/kWh. Assareh et al. [
14] performed a combined technical and economic analysis of a renewable multi-source CCHP system coupled with batteries and a hydrogen production subsystem to assess the system cycle cost and obtained maximum efficiency for key subsystems. Boyaghchi et al. [
15] optimized a solar micro-CCHP system integrated with ORC for domestic application needs. The proposed system’s thermal and exergy efficiency were 28% and 27% in summer, while in winter, these values were 4% and 13%, respectively. In a different study by the same author, Boyaghchi utilized a flat plate solar collector with water/CuO nanofluid and three different working fluids for the system with two scenarios and reported an exergy efficiency of 22.64–22.23% for R123, which is the best fluid from the thermodynamic viewpoint [
16]. Wang et al. [
17] worked on a parametric study on a solar-energy-driven CCHP system with transcritical CO
2 and reported thermal and exergetic efficiencies of 53% and 28.8%, respectively. In a different study by the same author, Wang et al. [
18] reported an exergetic efficiency of 24.6% for a solar-assisted CCHP system.
Wang et al. [
19] investigated a multi-source CCHP system and reported an exergy efficiency of 22.4%. Compared with conventional CCHP system without solar combination, the hybrid system in the proposed system by Wang et al. was shown to save up to 11% of natural gas. Zarei et al. [
20] investigated a newly designed CCHP system powered by a photovoltaic thermal collector and ORC. The exergy efficiency of 10.70% was reported for R123 refrigerant with a payback duration of 6 years. The utilization of ORC in applications in which waste heat recovery struggles to heat water to its superheated state is shown to be promising [
21]. In another study by Wang et al. [
22], compound parabolic concentrated photovoltaic thermal (CPC-PVT) solar collectors were investigated as a solar energy source for CCHP systems. As a result, exergy efficiencies of 21.8% and 27.1% were reported for summer and winter in this system. Zhai et al. [
23] worked on another solar-based CCHP system for remote areas and showed that the exergy efficiency can increase to 15.2% from 12.5%. Perrone et al. [
24] conducted a comprehensive investigation on a micro-CCHP system that employed an internal combustion engine with the gasification of two types of wood biomass fuel (W1 and W2). The outcomes revealed that the efficiency of the micro-CCHP using W1 fuel was 51.2%, slightly surpassing the efficiency of W2 fuel, which was 50.3%. This research suggests that employing a micro-CCHP system utilizing biomass fuel provides a feasible solution with significant self-sustainability. Ghorbani et al. [
25] developed a comprehensive thermodynamic study using the 3E aspects and optimization of a new CCHP system with subsystems including ERC, ORC, and the Kalina cycle utilizing the geothermal (GTh) energy heat recovery as the main source of energy. Investigations showed that the thermal and exergetic efficiencies of the whole system are 23.04% and 26.55%, respectively. Assareh et al. [
26] investigated a poly-source renewable-based CCHP system with the main subsystems of PVT collectors and wind turbines, a FC, battery storage systems, and a heat pump with a technical and economic approach. The results of the system modeling showed that the largest share of electricity production belongs to the fuel cell subsystem and photovoltaic/thermal collectors with a power of 75 kW and 52 kW. Selected past studies on CCHP-integrated systems are summarized in
Table 1 with product information and corresponding exergy efficiency.
A review of recent investigations shows that integrated CCHP and solar energy systems suffer from low efficiency, and more work needs to be conducted to assess the influence of major parameters [
27,
28]. Moreover, not many studies have been conducted on solar-based CCHP systems integrated with hydrogen production.
The current study aims to conduct a comprehensive 3E analysis including energy, exergy, and exergo-economic analyses of an integrated energy system with solar energy for power, heating, cooling, and hydrogen generation simultaneously. The developed CCHP system is designed to perform independently and be powered solely by solar energy sources. In the meanwhile, the energy storage system integrated with the solar energy system allows the all-day operation of the system without daylight-hour dependency. To conduct this study, a parametric study is performed employing a thermodynamic analysis. The genetic algorithm is employed to optimize and find the system’s ideal design point. In the designed system, a gas turbine and PCM tank provide a continuous supply of energy, and an RC coupled with ERC maintains power, cooling, and heating production. Additionally, the electrolyzer utilizes the power produced by the RC to produce hydrogen as a valuable byproduct. Some of the major novelties in this work are as follows:
A standalone solar-driven multi-generational cycle with energy storage to produce clean and continuous cooling, heating, power, and hydrogen;
A comprehensive analysis of the developed system incorporating energy, exergy, and exergo-economic analysis techniques;
Assessment of PCM as a thermal storage medium to resolve the intermittency issues involved in the solar renewable energy system;
Utilizing a supercritical CO2 RC combined with a solar-powered gas turbine to produce power from two sources;
Parametric study of the high-performance multi-generational system to achieve high performance and lay down guidelines for follow-up studies on novel CCHP systems;
Multi-objective optimization of the present system using overall system cost rate and total system exergetic efficiency as two objectives.
2. Materials and Methods
2.1. System Description
The developed system integrates different subsystems and components to generate continuous power, heating, cooling, and hydrogen. The solar energy system is integrated with a solar-powered gas turbine and an energy storage system to maintain continuous power supply. This is accomplished by converting solar energy from the sun into the thermal energy of working fluid, feeding into the system. The heliostat solar energy subsystem consists of two main parts: a solar field full of mirrors that concentrates the sun’s radiation at a particular point to maintain a large amount of solar energy, and a receiver that is placed at the upper part of a tower and receives highly concentrated solar energy. The solar heliostat field is deployed to concentrate the solar rays on the receiver. In state 1, the compressor compresses ambient air and then heats up the air in state 2 by entering the solar tower. There is a circulation of fluid in the tower to absorb the heating and channel it into the system in states 2 and 3. Some popular working fluids in solar towers are molten salt and compressed air [
29]. Compressed air is considered to direct the thermal energy from the solar subsystem into the energy storage system, solar-powered gas turbine, and other parts of the system. Another main component in the first subsystem is a PCM tank, acting as a TES system to store energy as heat and to provide a continuous energy supply when the sun is down or in cloudy weather in states 3a and 3b. In the first subsystem, compressed air flow heated by the sun enters a turbine in stage 4 and generates the required electricity in a solar-powered gas turbine.
The second part of the system is an RC integrated with an ERC system. This subsystem produces simultaneous power for hydrogen production, heating, and cooling. An ERC system combined with an RC is used in this system to meet the purpose of cooling, and the power output is maintained by the RC. The desired heating load is also delivered by the various number of heat exchanges.
In the RC, the liquid fluid is compressed to a higher pressure by the pump in state 8 after being cooled in state 7 in the condenser. The fluid stream is then pre-heated in the heat exchanger and recovers thermal energy by passing through the gas heater in state 9. The expanding stream in the turbine generates power in the following steps (states 10 to 11). At the turbine outlets, the expanded fluid stream is then divided into two streams, one low supercritical pressure to reject heat in the water heater for generating the required heating in state 12 and one high supercritical pressure to run the ERC for cooling purposes in state 11.
In the ERC, gas cooler fluid reaches a low temperature in state 13 after rejecting the heat in the heat exchanger. The fluid stream in state 14 which is the primary line mixes with the secondary flow (18) and undergoes the diffusing process in the ejector. The mixed stream (15) then passes through a separator and is divided into the liquid (16) and vapor (19). The liquid creates a cooling effect in the evaporator by throttling process in state 17, and the vapor is channeled back into the condenser to complete the cycle. Finally, to complete the whole cycle, the compressed fluid stream (20) joins the stream (21) before cooling down to liquid in the condenser.
The working fluid also serves as a heating context in heat exchanger 2. Finally, the last subsystem is deployed to use the output power from the turbine to produce hydrogen with an electrolyzer and store it as a byproduct. The system description of each part is elaborated in what follows. The studied system in this paper is presented schematically in
Figure 1. The entire system can be viewed as comprising three main parts: the solar molten-salt-driven cycle, which supplies the input energy; the CCHP cycle; and the hydrogen production component. The flow streams and primary components are clearly indicated. The details of each stream are also presented in
Table A1.
2.2. Energy and Exergy
Mass and energy conservation are utilized to achieve the thermophysical properties of every single state for technical modeling of the system. The mentioned formulas are presented as follows [
30]:
where
,
,
, and
represent heat, work, mass, and specific enthalpy, respectively. The exergy balance equation is also utilized to perform exergy analysis as follows [
30]:
where
,
and
denote exergy destruction rate, heat-related exergy, and work-related exergy, respectively. Also, the
sign represents the specific exergy and is defined as
, while
and
represent physical and chemical specific exergies [
30].
The exergy destruction equation for each piece of equipment as well as the input data and basic assumptions for the thermodynamic analysis of this study are presented in
Table A2 and
Table A3, respectively.
Ambient properties are assumed to be and for pressure and temperature, respectively.
The energy and exergy efficiency of the system is calculated as follows:
indicates the exergy coming from the sun as presented in the following Equation (6) [
31]. As the system is solely based on solar energy, the exergy input is completely coming from the sun.
where
is the heat transfer from the sun as presented in the Equation (A1),
is the surface temperature from the sun (5000 K), and
is the environment temperature [
32].
2.3. Exergo-Economic Assessment
An exergo-economic assessment is conducted to determine the new aspect of system design. In order to analyze each piece of equipment based on the exergy, economically, and calculate the input and output costs, the general cost equation is formulated as follows [
33]:
where
,
,
,
,
,
,
and
indicate the cost rate, cost per unit exergy, capital and O&M cost rates, purchased equipment cost (PEC) for the
component, capital recovery factor, O&M factor, annual plant operating hours, nominal interest rate, and facility lifetime, respectively. The subscripts
and
refer to streams of exiting, entering, heat, and work. The economic input parameters for the exergo-economic analysis are provided in
Table A4.
The correlations utilized to assess the equipment purchasing cost for each piece of equipment are listed in
Table A5. It is noteworthy to mention that cost functions related to previous years should be adjusted based on inflation by the following equation [
33]:
where subscripts C and R denote the current and reference years.
The average costs based on fuel units (
), products (
) and the cost of destroyed exergy (
) for the
component are obtained by the following relations [
33]:
With the help of the above correlations, it is possible to calculate the relative cost difference (
) and the exergo-economic factor (
) for the
component, which are two essential parameters in an exergo-economic analysis.
represents the increase in the cost of each exergy unit between fuel and product, and
shows the importance of the purchased equipment costs relative to the exergy costs [
33]:
Finally, the ratio of the exergy destruction of kth piece of equipment to the total exergy destruction and the total cost rate are determined by the following equations:
2.4. Multi-Objective Optimization
Optimization algorithms are a set of methods and techniques used to find the best possible solution to a given problem. These algorithms help researchers and engineers optimize thermodynamic systems by simultaneously considering multiple objectives, including cost, efficiency, reliability, and environmental impact. In this study, the non-dominated sorting genetic algorithm (NSGA) is utilized. This algorithm applies genetic operators, non-dominated sorting, and a Pareto frontier to find optimal compromise solutions when faced with conflicting objectives. This algorithm utilizes factors such as selection, cross-over, and mutation to create new data in each iteration. Then, NSGA employs a non-dominance sorting technique to identify and rank points based on their dominance in multiple objectives. The non-dominance sorting technique is chosen because it effectively handles the complexity of multi-objective optimization problems by categorizing solutions into different levels of non-domination. This allows the algorithm to maintain a diverse set of potential solutions across the Pareto frontier, ensuring that the trade-offs between different objectives are clearly represented. It also aids in preserving solution diversity and avoiding premature convergence to suboptimal solutions, which is crucial for finding a well-distributed set of optimal solutions in complex optimization landscapes. Finally, after finite iterations, the NSGA converges towards a set of data called the Pareto frontier. The best Pareto solution will be found by applying the Linear Programming Technique for the Multidimensional Analysis of Preference (LINMAP) multicriteria decision-making method after the Pareto frontier has been evaluated.
A Pareto front provides a set of optimal solutions that decision-makers choose based on their preferences and priorities. Cost rate and exergy efficiency are selected as two aims in this study. The selected input data with their range and logic are provided in
Table A6 of
Appendix D.
4. Conclusions
In this study, a standalone solar-based poly-generation cycle is proposed to produce clean and continuous cooling, heating, power, and hydrogen. Compressed air is selected to deliver energy from the sun into the PCM tank and resolve the intermittency issue of solar energy as an energy storage subsystem. Moreover, a supercritical CO2 RC is coupled with an ERC and electrolyzer to produce the required heating, cooling, and power. Thermodynamic modeling and exergo-economic analysis of the designed system are performed. Furthermore, to perform a comprehensive system analysis, a thorough parametric study is conducted to analyze the effects of critical input parameters on the different numbers of outputs. The selected parameters are the influence of the RC turbine bleed fraction, mirror number, solar GT pressure, RC turbine inlet pressure, and condenser pressure.
The influence of input parameters on the main efficiency evaluation outputs such as energetic and exergetic efficiencies, destroyed exergy, system products, cost rate, and cost per exergy is investigated. Considering two objectives, namely overall system exergetic efficiency and overall system cost rate, the optimization of a CCHP plant integrated with the hydrogen production unit is accomplished using the NSGA-II algorithm. The significant findings that emerge from this study are as follows:
The proposed system operates with promising values of 40.61% and 33.50% total energy and exergy efficiency, respectively. This finding presents the very high exergy efficiency and the energy efficiency of a standalone solar-powered CCHP plant with a hydrogen byproduct.
The system produces heating and cooling loads of 13.92 and 206 MW and a net power generation of 11.70 MW with a hydrogen production rate of 12.95 g/s simultaneously and continuously, solely dependent on the sun.
The initial findings of the thermo-economic investigation showed that the cost rate and product cost are 2875.74 USD/h and 25.65 USD/GJ, respectively.
The largest impacts on the overall destroyed exergy rate and PEC rate are exhibited by the heliostat mirrors, and they are 20 MW and 834.99 USD/h, respectively. Also, the highest cost rate of exergy destruction, 287.33 USD/h, belongs to heat exchanger 1.
The outcomes yielded from the multi-objective optimization of the designed system indicate that an optimal solution from the Pareto front achieves the highest accessible exergy efficiency of 34.17% and the lowest possible total system cost rate of USD 1263.35 per hour.
A comparison between the proposed system and previous solar-powered CCHP systems has proven that the present study offers substantially high exergy efficiency. This underlines the excellent design and enhanced performance of the developed system in this study.