A Sustainable model for the integration of solar and geothermal energy boosted with thermoelectric generators (TEGs) for electricity, cooling and desalination purpose
Introduction
Numerous countries have been considering to replace fossil fuels with highly reliable alternatives in order to approach sustainable development. Based on the comprehensive researches conducted in this area, examples of alternative renewable energies contain wind power, geothermal energy, solar energy, biomass energy, and tidal energy (Assareh and Biglari, 2015; Mosaffa et al., 2017). It is expected that by 2040, renewable energies will provide a large portion of our energy demand and electricity generation through these resources, which thus increase in comparison with that from fossil fuel resources. The leading countries in this field are Denmark, Germany, South Australia, and the United States, supplying a large portion of their energy demand from the production of renewable energies (Moharramian et al., 2019). Geothermal energy is known as clean and sustainable energy. It is also very economical to generate high electricity power with geothermal energy (Ghaebi et al., 2018).
According to what we have discussed about the importance of increasing the penetration of renewable energies, integrated energy systems have extensive attraction attention in previous decades. Combined heat and power (CHP) has proven itself as an energy system for the production of two useful outputs. In most cases, they are used for heating and power with just one prime mover. They have higher efficiency and lower emissions compared to single generation systems. As an example, if we make the benefit of the steam extraction of a large thermal power plant for heating purpose, the system can be a CHP system. Therefore, many research are being conducted to expand CHP systems to trigeneration and multi-generation systems (Ahmadi, 2013). Thus, having a system that produces electricity, heating, cooling, and even freshwater is of great importance.
Water is a critical aspect of daily life and is an essential element. We need water for drinking and sanitation, to produce food, to wash, to produce power and cool power plants, and to maintain our ecosystem. Because of all these activities, along with population growth, water demand has increased significantly during the last decade. There are already water problems, with 15% of the global population having insufficient access to clean water (Alkaisi et al., 2017). This value is 50% in southern Africa (Alkaisi et al., 2017). Freshwater is one of the basic needs of humanity. In recent years, since climate change and the rising global temperatures, the shortage of freshwater has been increasingly felt like a crisis all around the world.
As a consequence, desalination systems and similar technologies have been developed to tackle this problem. It should be noted that about 97% of the water on Earth is saline (A. Behzadi et al., 2019) and, therefore, using such technologies allow taking advantage of this vast source of saltwater. Recent estimates indicate that 40 percent of the world's population is facing a severe shortage of fresh water and will increase up to 60 percent by the year 2025 (Jones et al., 2019). In arid areas, this issue is even more problematic. Oceans and seas cover 70% of the Earth and about 97% of the water on the Earth is saltwater while only 3% is fresh-water (Subramani and Jacangelo, 2015). Thus, with population growth, increasing global temperatures, industrialization, increasing sea and ocean evaporation rates and greater human activities, a lack of fresh water is expected. Freshwater is one of the basic needs of humanity and other living entities. Due to climate change and the rising global temperatures, shortages of freshwater have been increasingly observed in recent years, in many parts of the world. As a consequence, desalination systems and similar technologies have been developed to tackle this problem. At the same time, solar energy is increasingly being applied; it is utilizable in electricity generation, heating, cooling, transportation and other applications. Solar energy use is very likely to increase in the future. Integrating solar energy with water desalination units can not only make fresh water production more environment friendly but also make it more beneficial. In addition, 66 percent of the world's population (4 billion) are currently in a state of severe freshwater shortage for at least one month per year (Jones et al., 2019). Over the last years, numerous studies have been conducted on converting saline to freshwater. An important option to address the crisis over water is seawater desalination, which can be classified into different types. The most commonly used and commercialized technologies are thermal and membrane desalination systems. Thermal processes utilize three main technologies: multi-effect desalination (MED), multi-stage flash (MSF) and vapor compression (VC). These technologies are based on the water phase change and, because of that, they need a heat source, which is usually provided by burning fossil fuels (Gude, 2016).
For desalination systems, integration with renewable energy resources, especially solar, can enhance sustainability. Alirahmi et al. (Alirahmi et al., 2020a) investigated and optimized an advanced energy system based on the integration of solar and geothermal energy for producing freshwater, hydrogen, electricity, hot water, and cooling. They considered six design parameters for system optimization and reported a total cost rate of 129.7 $/GJ and an exergy efficiency of 29.95%. Kianfard et al. (Kianfard et al., 2018) studied a thermal energy-based system for the production of freshwater and hydrogen, using two organic Rankine cycles (ORC). According to the economic analysis of this paper, 56% of the total investment costs were related to the reverse osmosis desalination unit, and the freshwater production cost was estimated to be 32.73 cents per m3.
Thermoelectric generators (TEG) are a type of energy generator used to convert heat directly into electrical energy, and come to be more and more popular in recent years due to their low operating and maintenance costs, lack of noise and environment-friendly characteristics (Abbasi and Pourrahmani, 2020). Numbers of studies have been focused on the application of thermoelectric generators, some of which are reviewed as follows. Habibolahi et al. (Habibollahzade et al., 2018) studied an advanced energy system composed of PTC parts, thermoelectric generator, Rankine cycle, and PEM electrolysis. In this study, the effect of using a thermoelectric generator was examined. At their proposed optimum point, the overall efficiency and purchase equipment cost rate were calculated, and the quantitative results were provided. Their analyses indicated that using a thermoelectric generator replaced by a condenser an effective method to improve the performance of the system. Behzadi et al. (Amirmohammad Behzadi et al., 2019) analyzed and assessed and optimized an energy system for the sake of comparison between the TEG and condenser. They concluded that the use of the thermoelectric generator increases efficiency and reduces costs.
Today, considering the environmental and economic issues, the development of renewable-based energy systems is considerably growing. Some of the recent investigations in this context are as follows. Moharamian et al. (Moharramian et al., 2019) analyzed the exergoeconomic and energy analysis of a cycle combining solar thermal energy (Photovoltaic energy conversion) and biomass fuel, as well as a Brayton cycle for electricity and hydrogen generation. In this study, compressor pressure ratio increment results in an increase in the exergoeconomic factor, the discharge rate of CO2, and the destruction rate of exergy while reducing the exergy efficiency and the exergy destruction costs. In addition, an enhancement in the area of the photovoltaic system's area resulted in a reduction in energy efficiency and, therefore, an increase in the cost of production. Kahraman et al. (Kahraman et al., 2019) analyzed thermo-economically and thermodynamically an advanced energy system based on geothermal energy. They studied the parametric study of the variation of reference air temperature on the exergy efficiency of the power plant. They also looked at effective parameters of the ORC system. The exergy destruction rate and the potentials of the various components of the cycle were obtained by applying thermodynamics laws. Given the fact that in geothermal power plants, ambient temperature carries out a vital role in the efficiency of these units, this study carried out thermal and economic analyses based on the different ambient temperatures. Keshavarzzadeh et al. (Keshavarzzadeh et al., 2019) modeled, analyzed, and applied several optimization techniques to a solar-based advanced energy system. They showed the Pareto curve of each optimization and compared the results. Their considered system consisted of a heating load, cooling load, and solar collectors, as well as an ORC with a LiBr-H2O type absorption chiller. Energy analysis was considered for the simulation and investigation of the proposed system. The system can fulfill cooling, electricity, and heating needs at the same time. Additionally, an optimization study was performed where techno-economic objectives satisfying some realistic constraints along with proper design parameters were considered. For optimization of this system, the IBEA, SPEA, and MPSO algorithms were used in addition to the NSGA-II algorithm.
Safari and Dincer developed an advanced system powered by biomass fuel for desalination, electrification, and heating purposes driven by biogas from anerobic digestion of wastewater. Electricity was generated via a Brayton cycle integrated with an ORC working with 245fa, and freshwater was produced through a six-stage parallel/cross desalination system. Moreover, electrolysis was considered for electrochemical hydrogen production. Their calculations showed that energy efficiency and second law efficiency can reach to 63.6% and 40%, respectively. Bivola et al. (Briola et al., 2019) assessed the performance of a biomass-geothermal hybrid power plant and proposed a new configuration of the hybrid biomass-geothermal power plant. The ORC obtains the thermal energy supplied by the biomass heat source by way of the geothermal mediated fluid. Seasonal variations in the temperature of the environment, geothermal fluid, and biomass energy rates were considered in this study, such that the results showed that these changes had significant effects on the values of cycle output power and thermodynamic efficiency.
In another paper, Hashemian and Noorpoor (Hashemian and Noorpoor, 2019) simulated an advanced energy system integrating solar and biomass energy designed to generate electricity, hydrogen, cooling load, heating load, and freshwater. The system consists of a steam Rankine cycle (SRC), a multi-effect desalination, a dual-effect absorption chiller, an electrolyzer, and PTCs. Four design parameters were considered for the optimization process, and finally, the product cost and the exergy efficiency were calculated to be 0.71 $/s and 16.53%, respectively. Atiz et al. (Atiz et al., 2019) examined the exergy, energy, and power generation of an integrated system using EES software. The system consisted of a low-grade geothermal resource, a solar collector with 100 m2 area, and an ORC. This cycle was investigated for three cities with different geothermal sources temperatures. Finally, the maximum production of electrical power was calculated at 19.46 kW, and the lowest was calculated at 0.6168 kW. Additionally, the results showed that n-butane was the best fluid to use in the ORC.
In the current research, a novel integrated energy system to produce four useful commodities are proposed. This proposed system operates with solar and geothermal energy. Since the high temperature is required, parabolic through solar collectors are considered. This system can provide electricity, cooling, desalinated water, and hot water. A comparison of system performance in the case of using a thermoelectric generator with the case of the condenser is also studied. Subsequent to comparing the two cases, the better system in terms of performance was selected for optimization and then optimized by the NSGAII multi-objective algorithm. The series of optimum points was presented on the Pareto frontier.
Here is a summary of the main goals and innovations of this research:
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To see the effect of the replacement of the condenser with a thermoelectric generator for the advanced integrated system
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Hourly analysis of the system for four different days a year, to observe system performance and freshwater production rate in various seasons (for Shiraz)
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A techno-economic analysis is carried out for the system to see the performance when TEG is used.
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Multi-objective optimization to define the optimum values with respect to the objective functions.
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Selection of the optimum endpoint via the TOPSIS decision-making criterion.
Section snippets
Description of the Considered system
Fig. 1 represents a layout of the considered integrated energy system. A geothermal well, a set of solar parabolic trough collectors, a single-effect absorption chiller, a SRC, and reverse osmosis (RO) desalination unit are the main system components. The system performance for two cases of the Rankine cycle with a condenser and with a thermoelectric generator are investigated. The geothermal fluid (point 1) first enters the solar collectors, and after its temperature increases, enters an
Thermodynamic Modeling
In this work, the energy balance and mass balance equations are applied for the simulation of the system. The main components of the system are treated as a control volume, and for a control volume with i input and o output, the mass, and energy conservation are given in Eq.1 and Eq.2 (Pratama et al., 2020).
The assumptions made for energy analysis in this study listed as follows (Razmi et al., 2019b, 2019a):
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The system designed under the steady-state
Validation
To ensure the modeling accuracy, the main components of the proposed system have been validated with experimental results, and validation of subsystem components has been performed separately. In Table 4, the results of the RO unit are compared with the work of Nafey et al. (Nafey and Sharaf, 2010), which can be concluded that due to the small errors obtained for each component, the modeling is acceptable. Additionally, Fig. 2 shows the comparison between the results of the current study and (
Results and discussion
This work aims to study the system performance with different fluids. Different features and costs are considered the significant advantages of thermal oils. Two main factors are affecting the exergy efficiency: one is the oil pressure drop in the PTC and oil heat-transfer coefficients in the receiver tubes (Moya, 2016). Fig. 3 illustrates the exergy efficiencies for various geothermal fluids. Since a higher amount of power can be produced by using a thermoelectric generator, thus the second
Optimization
In this research, the multi-objective optimization algorithm was applied for parameter optimization. The considered objective functions for optimization were exergy efficiency (must be maximized) and total cost rate (must be minimized). It is worth noting that in multi-objective optimization, objective functions must be contrary to each other such that increasing one objective function leads to reducing the other function (Keshavarzzadeh et al., 2020). Moreover, there is no one specific optimum
Conclusion
In the current research, a new integrated system based on solar and geothermal energy is investigated. The products of this system were freshwater, electricity, cooling, and hot water. For selecting the geothermal fluid, four different fluids were investigated, which, among them, the syltherm800 exhibited better performance. The results indicate that the most exergy destruction rate occurs in the solar collector and the absorption chiller. The parametric study was conducted to compare the
CRediT authorship contribution statement
Ehsanolah Assareh: Methodology, Software, Investigation, Supervision. Seyed Mojtaba Alirahmi: Conceptualization, Data curation, Writing - original draft. Pouria Ahmadi: Visualization, Supervision, Writing - review & editing, Validation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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