Exergy based optimization of a biomass and solar fuelled CCHP hybrid seawater desalination plant

Abstract

Keywords: Exergy; CCHP; Desalination; Optimization; Solar Energy; Biomass

Introduction

Increasing world population and demand for energy and potable water lead to burn more fossil fuels and its result is release a large amounts of greenhouse gases, particularly carbon dioxide. In order to reduce CO2 emission using renewable energy instead of fossil fuels is a main challenge for sustainable development. CCHP systems are a kind of cogeneration systems with three outputs and CCHP hybrid seawater desalination plant can be considered as a multigeneation system since they have more than three outputs (power, heating, cooling and fresh water). Issues like fossil fuel depletion and climate change amplify the advantages and significance of efficient multigeneration energy systems [1]. Dincer and Zamfirescu[2]showed that multigeneration systems based on renewable energies as clean and free alternatives of fossil fuels reduce fuel prices and emissions, compared to conventional systems such as cogeneration or trigeneration systems. Other studies show that exergy efficiency ofmultigeneration systems using an ORC with renewable energy source increase up to 10% and integration of two renewable energy sources such as biomass and solar energy can be beneficial with higher energy and exergy efficiency than a single renewable energy source [3,4,5]. Dincer and Zamfirescu [6] performed multigeneration renewable energy based system has better efficiency, sustainability and environment. Rubio-Mayaet et al. [7] designed a multigeneration system fuelled by natural gas, solar and gasified biomass and conclude that renewable energy is the source of the reduction of CO2 and environmental impact. Minciuc et al. [8] offered an approach for investigating of the multigeneration system and reported optimal energetic efficiency of the system. Ahmadi et al. [9] presented an exergy-based optimization of a multigeneration energy system to produce power, heating, cooling and domestic hot water. They find the best design parameters of the system considering exergy efficiency. Zamfirescu et al. [10] have examined the multigeneration system as a method of improving the exergy efficiency of the nuclear power system. Ratlamwala et al. [11] analyzed the performance of a novel integrated geothermalbased system for multigeneration, for producing cooling, heating, power generation, hot water and hydrogen. Ozlu and Dincer [12] developed a solar-wind hybrid multigeneration system and analyzed the energy and exergy efficiency of the system which were higher than equivalent single energy systems efficiencies. Sharifishourabi et al.[13] showed that a hybrid combination of renewable energy sources such as solar and wind energy can be a suitable alternative for power generation. Dube Kerme and Orfi [14] presented thermodynamic modeling of organic Rankine cycle (ORC) driven by parabolic trough solar collectors. Solar energy can be collected in many methods such as evacuated tube solar collectors (ETC) which is used in this study. Biomass is mainly derived from living or dead matter present on earth. Bagasse is a kind of biomass that is selected as one of the energy sources for the studied system and biomass fuels can be obtained from agricultural production wastes especially having This paper was recommended for publication in revised form by Regional Editor Kwok-wing Chau 1 Graduate Faculty of Environment, University of Tehran, Tehran, Iran *E-mail address: p.heidarnejad@ut.ac.ir

Journal of Thermal Engineering, Research Article, Vol. 3, No. 1, pp. 1034-1041, January, 2017 been related to sugarcane such as Bagasse. J. Werther et al.[15] studied the processes of different agricultural wastes combustion including sugarcane. L.A.B Cortez et al. [16] did Exergy analysis in order its heat to be used in bagasse combustion. A. Bhattacharja et al. [17] studied and researched on power generation process through the utilization of bagasse gas integrating system in terms of energy and exergy, but a CCHP hybrid desalination plant driven by solar energy and bagasse combustion has not been analyzed from the viewpoint of thermodynamics. In this paper, energy and exergy analysis of a CCHP hybrid seawater desalination plant driven by bagasse combustion and solar energy is investigated. Optimum point of system is carried out considering the exergy efficiency as an objective function.

System Description

Figure 1 shows the schematic of suggested CCHP hybrid desalination system which can be divided into Evacuated Tube Collector (ETC), biomass burner, Organic Rankine Cycle (ORC), absorption chiller, heater and Multi Effect Desalination system (MED). Outputs of the proposed system are electricity, heating, cooling and fresh water.

Figure 1. Schematic of CCHP hybrid desalination system The suggested system is based on two renewable energy sources, solar energy which the solar radiation is collected through ETC, bagasse which is combusted in a biomass burner. Hot combustion gases pass through the vapor generator and enter to a single-effect absorption cycle which uses LiBr-H2O to provide cooling. In the ORC the heat released from biomass burner is transferred to R123 and causes to make a superheated vapor for producing power in ORC turbine. The wasted energy of turbine is used by a heater to provide heating. In Heat Exchanger2 the remaining heat is transferred to sea water to preheat it and R123 enters to Vapor Generator to complete the cycle. Last purpose of this system is producing fresh water by a MED which contains four effects. Because the sufficient and low cost heating can be provided in this system, MED is best option compare to any other types of desalination plant. Seawater enters to first effect after preheating and a part of it is converted to fresh water. In each effect pure water is produced at lower pressure and lower temperature than the previous effect. The produced vapor of each effect feeds as the heating steam for the next effect and so on. Feed water passes through preheater and enters to ETC then steam which is produced in ETC enters to ejector. Ejector by increasing inlet steam pressure provides an appropriate pressure for the first effect of MED entrance. Data have been utilized in Khuzestan province, Iran. This province is among the most prone areas in Iran in exploitation of solar energy

Journal of Thermal Engineering, Research Article, Vol. 3, No. 1, pp. 1034-1041, January, 2017 having had over 300 sunny days [18] and there are 86588 hectares under cultivation of which about 6536976 tons has been attained from 9 large sugar production factories [19].

Thermodynamic Analysis

Thermodynamic analysis including energy and exergy is carried out using following assumptions:    

The reference-environment state has a temperature T0 = 298 K and a pressure P0 = 100 KPa. The changes in kinetic and potential energy and exergy terms are negligible. HHV for bagasse is 16793 KJ/Kg The higher heating value (HHV) is the primary contributor to the chemical exergy of a biomass fuel and obtained from bellow equation[20]: ch

e f  1.00  1.04 HHV First and second thermodynamic laws have been to each component as below [21]: .

The exergy of a substance is often divided into four components. Two common ones are physical and chemical exergy. The two others, kinetic and potential exergy, are assumed to be negligible here:

Bagasse combustion equation based on wet is as following [17]: [0.268 C + 0.239 H2 + 0.099 O2 + 0.394 H2O + Ash] + [1.075 N2 + 0.289 O2 + 0.013Ar] → [0.268 CO2 +0.633 H2O + 1.075 N2 + 0.013 Ar] (6)

Results And Discussion

Thermodynamic modeling is implemented through Engineering Equation Software (EES) [22] using input data which is shown in Table 1, and the result of thermodynamic modeling is presented in Table 2. The system is proposed for locating in Ahwaz, Iran since it has sufficient sources of bagasse and solar radiation. Average daily solar radiation is extracted from NASA internet site [23].

Journal of Thermal Engineering, Research Article, Vol. 3, No. 1, pp. 1034-1041, January, 2017 Table 1. Input data for thermodynamic modeling Parameters Turbine inlet temperature Turbine inlet pressure Turbine pressure ratio Heat exchanger temperature difference Temperature difference of each effect of MED Evaporation Temperature Condenser temperature difference

Table 2. Result of thermodynamic modeling for proposed system

Journal of Thermal Engineering, Research Article, Vol. 3, No. 1, pp. 1034-1041, January, 2017 Table 2. Result of thermodynamic modeling for proposed system (cont.)

Journal of Thermal Engineering, Research Article, Vol. 3, No. 1, pp. 1034-1041, January, 2017 Based on these data, the performance of the system is evaluated and is presented in Table 3. The mentioned exergy balance is applied for each component of the system and the exergy destruction rate and exergy efficiency system components are shown in Table 4. Table 3. Performance of the system Parameters Thermal efficiency Exergy efficiency Cooling load Heating load Power Fresh water

Figure 2, shows the amount of exergy destruction for some components of system which has the considerable share in total exergy destruction of the system in comparison to others. As it is shown, biomass burner (21539KW), ETC (9829KW) and vapor generator (741.7 KW)are the major sources of exergy destruction rate. In the biomass burner, the irreversibility is because of occurring combustion in it and combustion is one of the sources of irreversibility in a process. In the ETC, the irreversibility created is due to large temperature difference between solar heat and fluid in the tubes. The main reason of irreversibility in vapor generator is related to the stream-tostream heat transfer. Since the amount of irreversibility of remaining components is negligible, their exergy destruction is not considered here. 25000 20000 15000 10000 5000 0

Components Figure 2. Some selected components exergy destruction rate (kW) Three independent variables including turbine inlet temperature, turbine inlet pressure and Heat Exchanger1 temperature difference are varied to investigate their effects on system total exergy efficiency and is illustrated in Figure 3, 4,5.

Journal of Thermal Engineering, Research Article, Vol. 3, No. 1, pp. 1034-1041, January, 2017

Figure 3. Effect of turbine inlet temperature on exergy efficiency Figure 3 demonstrates the effect of turbine inlet temperature on exergy efficiency of the system, by increasing turbine inlet temperature between 110 C and 140 C exergy efficiency increases about 27%. The effect of variation of turbine inlet pressure on exergy efficiency of the system is shown in Figure 4,as turbine inlet pressure varies between 650 kPa and 800 kPa, exergy efficiency of the system decreases about 5%.This decrement is due to decrement of amount of electricity and heating power produced by multigeneration system.

Figure 4. Effect of turbine inlet pressure on exergy efficiency

Figure 5 represent the effect of Heat Exchanger1 temperature difference on the exergy efficiency of the system, as it can be observed by increasing heat exchanger1 temperature difference between 5 and 15, exergy efficiency of the system decreases because of increment in total exergy destruction of the system. 7.6 7.5 7.4 7.3 7.2 7.1 7 5

Figure 5. Effect of temperature difference of heat exchanger1 on exergy efficiency 1040

Journal of Thermal Engineering, Research Article, Vol. 3, No. 1, pp. 1034-1041, January, 2017

Optimization

Optimization is carried out to determine the best design parameters of the system and different objectives can be defined for this purpose. For this purpose genetic algorithm (GA) through EES software is used for optimization of CCHP hybrid desalination plant. The selected decision variables are turbine inlet temperature (T[22]), turbine inlet pressure (P[22]) and Temperature difference of Heat Exchanger1(ΔTHE1). Table 4 shows the base case and optimal case values of the decision variables and objective functions. Table 4. Exergy destruction rate and exergy efficiency of some system components Component

Journal of Thermal Engineering, Research Article, Vol. 3, No. 1, pp. 1034-1041, January, 2017 Table 5. Base case and optimal case values of the decision variables and objective functions Parameters Exergy efficiency (%) Turbine inlet pressure (kPa) Turbine inlet temperature (oC) Temperature difference of Heat Exchanger.1 (oC)

Conclusion

The present work proposes a biomass and solar based CCHP hybrid desalination system which produces power, cooling, heating and fresh water. Energy and exergy analysis is applied to identify the components with high exergy destruction rate and calculate the exergy efficiency of the system. Results show that biomass burner, evacuated tube solar collectors and vapor generator are the major sources of exergy destruction thus it is necessary to have better design for these components to minimize the system exergy destruction. By increasing turbine inlet temperature, decreasing Turbine inlet pressure and decreasing Temperature difference of Heat Exchanger.1 exergetic performance of the system improves. The exergy efficiency is 7%.Optimization is carried out through EES software using GA and results show that exergy efficiency in optimum case improves 21% in comparison to base case value.

Nomenclature

Acoll Solar collector area, m2 Ex Specific exergy, kJ/kg  Exergy, kW Ex Gt Total instantaneous radiation, W/m2 H Specific enthalpy, kJ/kg In Inlet

Mass flow rate, kg/s Outlet Pressure, bar Heat transfer rate, kW

Gas constant, kJ/kgK Specific entropy, kJ/kgK Temperature, oC

Abbreviations

Number of molecules of gas k molecules Ambient Chemical Destruction Higher Heating Value Physical Turbine Exergy efficiency Combined Cooling, Heating and Power 1042

Journal of Thermal Engineering, Research Article, Vol. 3, No. 1, pp. 1034-1041, January, 2017 EES ORC

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