A novel tri-generation energy system integrating solar energy and industrial waste heat

Global warming has forced researchers to find an alternative for fossil fuels and to enhance the energy efficiency of processes in industries. Waste heat recovery has a significant potential to reduce fossil fuel consumption and energy performance enhancement. The study cycle is a tri-generation system, heating, electrical power, that can capture carbon dioxide gas. The system works with the solar energy and waste heat of the cement plant. In this study, a model for a completely new system has been developed based on renewable energies. Thermodynamic analysis for the energy system is performed, and the system is based on the organic Rankine cycle, absorption chiller, solar energy, and waste heat recovery from the exhaust gases of the cement plant stacks. The results of the analysis showed that the energy and exergy efficiencies were calculated to be 35.78% and 12.77%, respectively, and the total exergy destruction was calculated 277327 kW. Also, the optimisation result with the direct algorithm method with the objective function of exergy efficiency improved both efficiencies. In this optimisation, the exergy efficiency reached 16.39% and energy efficiency was calculated 49.04%. The optimisation with the objective function of total exergy destruction decreased the value to 216813 kW, which was significantly reduced from the base state of the system; while energy and exergy efficiencies were calculated to be 54.61% and 13.85%, respectively. Cite this article as: Pourfarzad H, Saremia M, Reza M. Ganjali A novel tri-generation energy system integrating solar energy and indestrial waste heat. J Ther Eng 2021;7(5):1067–1078. Journal of Thermal Engineering Web page info: https://jten.yildiz.edu.tr DOI: 10.14744/jten.2021.0003


INTRODUCTION
Over the past century, the cement industry has experienced significant advances in various processes to increase efficiency and reduce pollutant emissions. Global cement production has reached about 2.8 billion tons per year and is expected to reach 4 billion tons annually. The cement industry has been by challenged many problems such as increasing energy costs, global warming, the need to reduce greenhouse gas emissions, and the supply of raw materials [1]. On the other hand, the cement industry has a high energy concentration in comparison to the other industrial and producing operations and 2% of the world energy consumption.
The cost of energy because of the need for a large amount of thermal energy for the various sectors of cement products, such as calibration, drying and furnace operation, and power for the electric fan, mill and conveyor [2,3]. Studies express that about 50% of cement emissions occur during the conversion of limestone to calcium oxide. After the combustion of fuels, about 40% of the furnace gas emissions with the transport of about 5%, and electricity consumption in the production process is about 5% [4]. The effect of greenhouse gases on land heating was assessed by the earth heating potential unit which showed how much greenhouse gas caused global warming compared to the reference gas. For a 100-years interval, the potential for global warming of carbon dioxide gases, CH 4 , and N 2 O has been reported to be 1.25 and 298, respectively. Researches have indicated that carbon dioxide and methane concentrations have increased by 36% and 148% since 1750 [5]. The combustion of fossil fuels has accounted for about 75% of the increase in carbon dioxide emissions from human activities over the past 20 years. The main source of carbon dioxide emissions is the fossil fuel-based generation unit, accounting for about 32% of carbon dioxide emissions. Next, the largest source of carbon dioxide emissions is heating and cooling, which is about 33% of carbon dioxide emissions. About 65% of carbon dioxide emissions are associated with electricity generation and heating and cooling, both of which are directly related to human energy needs [5].
Distributed generation systems are used to optimize energy consumption, reduce losses due to transmission and distribution of electrical energy in the network, and reduce the pollution caused by the combustion of fossil fuels at large power plants [6]. Large-scale electric power generation and its transmission to consumers are a huge loss. On the other hand, large power plants have had low electrical efficiency due to production capacity. In addition, they have high costs for investment, installation, and commissioning as well as maintenance, and they increase the environmental pollutants. The set of these criteria and factors such as greater reliability, restructuring in the electricity industry led the countries of the world to the use of separate production processes [7]. Combined cooling, heating, and power generation cooling, along with new technologies is a way to solve global energy problems, including energy shortages, energy supply security, pollution control, and the energy economy.
Regarding the utilization of low-temperature sources, Organic Rankine Cycle (ORC) power systems have been proven to be one of the significant options. The organic working fluid in ORC power systems that has a lower boiling point than water grants this advantage. In an article, thermodynamic, economic, and environmental assessments were carried out for an ORC power system used for waste heat recovery [8]. Ahmed et al. [9] developed a design methodology for waste heat recovery using an ORC power system in cement plants. They claimed that the effectiveness of the ORC power system can be increased up to 95% by using R134a as a working fluid. Another study proposed an ORC power system for waste heat recovery of a cement factory in Sabzevar, Iran [10].
Al-Sulaiman et al. [11] analyzed and optimised thermo-economic for three different co-generation systems that used biomass, fuel cells, and solar panels, all of which are based on the ORC. They performed exergy modelling and economic analysis of the systems using the SPECO method. In this research, the application of thermo-economic analysis methods and optimisation of cogeneration systems have been investigated. Puig-Arnavat et al. [12], provided a model for design, optimisation, and simulation of a smallscale tri-generation system in Spain based on biomass gasification. They compared five different configurations, and all suggested configurations have a minimum efficiency of 27%, which is in line with the standard power generation in Spain. In a review study, Combined Cooling, Heating, and Power system (CCHP) was introduced as an instance of distributed generation systems to explore a variety of these systems. The history of the development of these systems has been analysed, and also the use of renewable energies, including wind and solar energy, is predicted to increase in the near future [13]. A small-scale tri-generation system based on fuel cells was developed by Wang et al. [14] to meet the needs of cooling, heating, and power of domestic applications. Another study proposed a micro-scale tri-generation system based on solar energy. They conducted their analysis based on two winter and summer modes. The results of the exergy analysis indicated that solar collectors and auxiliary boilers were two major sources of exergy destruction [15]. A hybrid ORC system driven by waste heat and solar energy was introduced by Bellos et al. [16] to maximise the electricity production of the system. In this study, several working fluids were examined. A thermo-economic assessment was performed for an ORC system that used solar energy, waste heat, and geothermal energy [17]. Moreover, particle swarm optimisation was performed to maximise heat recovery and net present value. Another integration of solar flat plate and waste heat recovery was exploited for hydrogen production [18]. A solar-assisted Kalina cycle integrated with waste heat recovery was developed [19]. The waste heat was provided by flue gas of a 500 MW subcritical coal-fired power plant. They established that this configuration can prevent 1008 tons of CO 2 emissions annually. For waste heat recovery in the cement industry, Júnior et al. [20] proposed the Kalina cycle for a cement plant in Brazil that was proved to be a favourable option due to the cost value regarding the Brazilian tariff market. A solar/biomassbased-multigeneration system was proposed and analysed by Ghasemi et al [21]. Moreover, they performed multiobjective optimisation to obtain the minimum product cost rate and maximum performance of the system. Another study developed an optimal model to meet the needs of a cement plant [22]. The system can provide the heating, water, and power requirements of a cement plant. Moreover, thermoeconomic and environmental analyses were performed for the energy hub. Biogas-based cogeneration was developed by Chakyrova [23] to perform thermoeconomic analysis on it. The specific exergy cost method was exploited and several environmental conditions were taking into account. In another study, energy, exergy, economic and environmental analyses were performed on a refrigeration system [24]. Two single-optimisation methods were performed considering exergy and economic performance of the system. Ahmadi et al. [25] performed a thermodynamic and environment assessment on the Ericsson engine. They also conducted a revolutionary-based multi-objective optimisation on the system. Integrating renewable energy with CO 2 capture has drawn interests among researchers. A novel solar-assisted post-combustion carbon capture system was proposed by Jordán et al. [26] in Mexico. Solar energy was exploited in this cogeneration system to mitigate the energy penalty imposed by the CO 2 capture process. An experimental study suggested using concentrated solar power for calcium looping that is one of post-combustion carbon capture technology [27]. Another study proposed the integration of solar energy and carbon capture and storage (CCS) systems to produce methanol [28]. Novotny et al. [29] compared waste heat recovery systems combined with different types of CCS combined including pre and post-combustion technologies. Another study was carried out by Jakobsen et al. [30] that different CCS system was compared for a cement plant in Norway. They performed a techno-economic analysis to investigate the impact of CCS on the cement production cost.
Hence, this article proposes a tri-generation energy system that takes The Abyek cement plant as a case study which is located in Qazvin, Iran. The goal of the energy system is to enhance the energy efficiency of the cement plant and to reduce the pollution of the exhaust gases from the stacks of the cement factory. And these can lead to energy consumption management of the cement plant finally.
Energy sources of the tri-generation energy system are solar energy and waste heat that is recovered from the flue gases of the cement plant. This energy system can produce power, hot water for residential purposes, and carbon dioxide. Another goal of the study is to low-temperature heat sources such as industrial heat loss and renewable energy sources. Energy and exergy analyses of the cycle are carried out and reported to evaluate the performance of the system in both energy and exergy views. Moreover, a single objective optimisation with the direct algorithm is performed for two objective functions such as exergy efficiency and exergy destruction separately.

System Description
As presented in Figure 1, the exhaust gas from the stacks of the cement plant comes into the vapour generator and increases the heat of the organic working fluid that is considered to be R123 in this energy system. The absorption chiller cycle uses solar energy to cool down the carbon dioxide gas required to be extracted. The flue gas from the vapor generator enters cooler 2 to reach a temperature of about 50 °C to provide the absorption conditions. The mono-ethanolamine (MEA) solvent is contacted to the cement plant flue gas in the absorber column. In the stripper column, the CO 2 is extracted from the CO 2 reach solution at high-temperature. The high-temperature of the stripper is provided by hot water. Finally, the MEA solvent is cooled and returned to the absorption column to continue the process as a closed cycle. The ABS2 and STP stand for absorber 2 and stripper, respectively. And HEX 2 denote heat exchanger 2.

Energy Analysis of The Tre-Generation System
Based on the conservation law of mass, and the first and second laws of thermodynamics equation are yield for each component of the tri-generation energy system. Therefore, each component of the system and the overall system is taken as a control volume, and the mass, energy, and exergy balances are written for them.

Energy Analysis of The PV/T Subsystem
The rate of useful energy absorbed by the water in the panel is determined from the following equations [31]: In which rd is defined as the ratio of hourly radiation to the daily radiation. And ω ss denotes the angle in the evening which can be obtain from equation No. 8. The coefficient of Collares Pereira and Rabl correlation [32]: Here, r is defined as the ratio of total hourly radiation (I) to total daily radiation (H). In the equations above, the angle in the evening is in degrees, which is calculated from the following equation [32]: In which, N represents the number of days.

Energy Analysis of The ORC System
The conservation laws of mass and energy for a turbine are defined as follows: The thermodynamic process in the adiabatic turbine is assumed. The turbine isentropic efficiency is expressed as follows: The heater supplies the required heating load with the turbine outlet flow, assuming no pressure loss occurs and the outlet flow is saturated vapour. The mass and energy balances equations in the heater are as follows: 11 12 11 ( ) In which, QHL is the heating load (kW). Assuming that the pumping process is adiabatic, the thermodynamic equations governing pump 2 are as below: Here, the v presents a specific volume of fluid (m 3 /kg), and η is,p 2 is isentropic efficiency of the pump. The equations for the pump 1, pump 3 and pump 4 are similar. Assuming that the pressure remains stable in the process of receiving the heat in the economizer and evaporator, the laws of conservation of mass and energy governing them are as described below.
  m m 5 8 = The electric generator outlet is defined as the ratio of the electrical power outlet to mechanical power: Electric power is obtained from the difference in turbine power and total consumption power of the pump. Also, the electric power is a function of the electric generator's efficiency:

Energy Analysis of The Absorption Chiller Subsystem
The cooling load of the cycle is provided by an evapourator, a component of the absorption chiller subsystem, and the equation is as follows:

Energy Analysis of Subsystem CCS
The amount of heat exchanged in the stripper, cooler and absorber are as follows:

Exergy Analysis of Tri-Generation System
Exergy is a property that defines the capability of producing the useful work of a system with a certain amount of energy in a given state. In exergy analysis, the first state of the system is given, the maximum amount of useful work can be obtained when the process is reversible and the final state is a dead state. The dead state defines as the temperature and pressure of the environment, kinetic and potential energy has considered negligible. The exergy of the system in the dead state equal to zero. Since exergy depends on the final state of the system, it can be expressed that exergy is the combined property of the system and the environment [33].
Exergy is transmitted in three ways: by work, by heat transfer, and by mass transfer to the control volume. The exergy balance can be written in different states of the system, in control volumes, and in the steady or dynamic states. Regarding the fact that most of the components used in this study are considered as control volume, the exergy balance for a control volume is given below [34]: In the equation above, Q J represents the heat transfer rate from the control volume boundary at its temperature that is presented by T J , Ẇ CV is work generated by the control volume. X 0 and X 1 are the specific exergies for inlet and outlet flows of the control volume (kJ/kg). Ẋ D defines the exergy destruction in kW, which is a result of irreversibility within the control volume and is obtained from the following equation [34]: In which T 0 is the dead state temperature (K), and Ṡ gen represents the entropy rate in kJ/Ks. The x 0 and x 1 values can include four types of exergy: physical exergy, chemical exergy, kinetic exergy, and potential exergy as follows [34]: Since this study, the velocity and height of flows are considered negligible, and it is assumed that these flows are unlikely to have a chemical reaction with the environment. As a result, the specific exergy of flow is transferred only by physical exergy and is written as follows: In which, the index 0 is related to the dead state. And h represents specific enthalpy (kj/kg), and the S is the specific entropy (kj/kg.K)

Exergy Analysis of Subsystem PV/T
The PV/T subsystem is considered as a control volume, the exergy balance for it can be expressed as follows: 14 14 13 13 0 In the equation above Ẋ S is the inlet exergy to the collector from the sun (kW), by considering the sun as a black body it can be expressed as below [35,36]: In which Ts is the surface temperature of the sun assumed to be 6,000 Kelvin [31].

ORC Subsystem Exergy Analysis
The ORC subsystem consists of components, all of which are considered as the control volume. There is not any heat dissipation from these components, therefore, the exergy loss is negligible. The exergy balance equations for each component of the tri-generation energy system are presented in Table 1.

Absorption Chiller Subsystem Exergy Analysis
The fuel and product exergy for this component is considered as follows considering the fact that the cooling load is provided by the evapourator in the absorption chiller subsystem: 14

CCS Subsystem Exergy Analysis
Fuel and product exergy for the components of the CCS subsystem are as follows:

System Performance Evaluation
In evaluating the performance of a system, a suitable criterion should be chosen, and the performance of the system would be measured accordingly. In the assessment of energy systems, the efficiency of the first law, which is defined as the useful tool that is power generated by the system to the overall input power to the system, is considered as one of the system evaluation criteria. The efficiency of the first law is expressed as follows:  (35) Given that the first law of thermodynamics deals only with the quantity of energy and its conservation, we need another means to examine the quality of energy and analyse it during the process and generation of entropy. This criterion, called the efficiency of the second law of thermodynamics, is defined as the ratio of the performance of a − system to its function in a reversible state. The efficiency of the second law of thermodynamics for the whole system is as follows:

RESULTS AND DISCUSSION
The analysis of the tri-generation system from the first law and second law (exergy) perspectives requires information such as system schematics and system design parameters ( Table 3). The modeling is performed in the Engineering Equation Solver (EES) software [37]. The above-mentioned equations are applied, and the thermodynamic performance of the whole system, including thermal efficiency, exergy efficiency, required panel surface, generated power, and heat are presented in Table 4.

Exergy Destruction of System Components
In this section, the exergy destruction and the ratio of exergy destruction of each component to the total exergy destruction of each component, and the results are presented in Table 5. As presented in the table below, CCS, solar panel, and chiller are the largest sources of exergy destruction. While other components have less contribution to the overall exergy destruction.
In the CCS system, the most destruction occurs in the absorber and stripper towers due to the high-temperature difference between the inlet flow, the temperature of absorption and extraction. In the absorption tower, the difference in the temperature of the inlet gas and the absorption, temperature is the cause of the destruction. And in the stripper tower, the temperature difference between the flow containing CO 2 and MEA and the outlet flow is the reason. The available solutions to reduce this destructions are the use of adsorbents with a high absorption rates and low exhalation temperatures. Thus, in the heat exchangers and chiller evapourators, due to the high-temperature difference between the inflow and the flows, the amount of exergy destruction is significant. Figure 2, shows the effect of turbine inlet pressure on energy and exergy efficiencies of the system. As shown in Figure 2, it decreases with increasing energy efficiency, and this decrease is due to an increase in the input energy of pump 2. Exergy efficiency is initially increased and then decreased, the increase is because of increasing the outlet exergy of the system in comparison to the inlet exergy. And, similarly, in the reduction trend, it is justified that the inlet exergy of the system has increased. Figure 3 illustrates the effect of turbine inlet temperature on energy and exergy efficiencies of the energy system. As shown in Figure 3, the energy efficiency decreases with increasing temperature from 80 to 180 °C. And this decrement is because of higher energy input than energy output.  However, for the exergy efficiency of the turbine input temperature, the outlet exergy of the system increases in terms of power or heat, which overcomes the increase of exergy inputs. Figure 4 demonstrates the effect of the mass flow rate of the fluid in the solar subsystem on the energy and exergy efficiencies of the whole cycle. As shown in Figure 4, the energy efficiency decreases with increasing mass flow rate in the solar cycle which points out that increasing the mass flow rate of the fluid in the solar cycle has been reduced the number of system products compared to the energy input of the system. The exergy efficiency has the same behaviour and is justified in the same way. Figure 5 shows the effect of the temperature of the exhaust gases from the cement plant on the energy and exergy efficiencies of the proposed system. As illustrated in Figure 5, the energy efficiency increases with increasing mass flow rate of the fluid in the solar cycle which points out the fact that this rise in temperature has led to a drastic rise in the system's products compared to the input energy of the system. The exergy efficiency trend is similar to the energy efficiency trend and is justified similarly. The effect of this parameter on the overall efficiency of the system is very high, which reminds us of the effect of the amount of sunlight, which is one of the energy sources in this cycle.

Optimisation
In this section, single-objective optimisation is performed and the exergy efficiency of the system and the exergy destruction of the system components are separately considered as objective functions. The objective functions are maximising exergy efficiency and minimising exergy destruction. Six independent variables are selected by sensitivity analyses to optimise the performance. The variables are such as the ambient temperature, the temperature of cement plant flue gas, turbine input temperature, turbine    input pressure, the flow rate of solar subsystem working fluid, and temperature difference of the heater. The optimisation results for each objective function are presented in Tables 6 and 7.

CONCLUSION
In this study, a model for a completely novel system has been developed based on renewable energies. Thermodynamic analysis and single objective optimization are performed for a tri-generation system that produces power, heating and extracts carbon dioxide from flue gases of the cement plant in Qazvin, Iran. The results of all sections are as follows: • The energy and exergy efficiencies were calculated to be 35.78% and 12.77%, respectively, and the total exergy destruction was calculated 277327 kW. • From the exergy destruction of each component it was concluded that the solar panels, CCS, and absorption chiller destroy most of the exergy entering the system, and are considered to be the largest source of exergy destruction. • In the sensitivity analysis, the energy efficiency remained constant with increasing the temperature difference, and the exergy efficiency decreased. The increase in the mass flow rate of working fluid in the solar subsystem reduced the energy efficiency and exergy of the cycle, and the increase in radiation was accompanied by increasing the efficiencies. Increasing the turbine inlet pressure reduced the energy efficiency and the exergy efficiency was increased. And the increasing the flow temperature to the turbine also reduced the energy efficiency. Changes in ambient temperature significantly increased the energy efficiency of the cycle. The energy and exergy efficiencies increased significantly with increasing the temperature of the exhaust gas from the cement plant. • The result of optimisation using the direct algorithm in EES software with the objective function of exergy efficiency yielded a great deal in this efficiency so that the exergy efficiency reached 16.39% and the energy efficiency was calculated to be 49.04%. The optimisation with the total exergy destruction objective function reduced this value to 216813kW, which differs greatly from the base state of the system, while the energy and exergy efficiencies are calculated to be 54.61% and 13.85%, respectively.

Dead state condition Ap
Panel Area x Total specific exergy Figure 5. Effect of the exhaust gases temperature on energy and exergy efficienciesexergy efficiencie.

DATA AVAILABILITY STATEMENT
No new data were created in this study. The published publication includes all graphics collected or developed during the study.

CONFLICT OF INTEREST
The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

ETHICS
There are no ethical issues with the publication of this manuscript.