Design and tri-objective optimization of a hybrid efficient energy system for tri-generation, based on PEM fuel cell and MED using syngas as a fuel

Abstract

This article presents a novel solution to enhance the performance and cost-effectiveness of a biomass-based proton exchange membrane (PEM) fuel cell. It investigates the proposed configuration from energy, exergy, economic, and environmental aspects. The idea is conducted by the integration of the proposed system with a gasifier, a multi-effect desalination (MED) unit, and a series two-stage organic Rankine cycle (STORC) using various zeotropic mixtures for the use of waste heat for heat, power, and freshwater production. A comparative parametric study is carried out not only to evaluate the effect of main parameters on the performance of the proposed system but also to determine the best zeotropic mixture from different standpoints. In addition, the tri-objective optimization using a genetic algorithm approach is implemented to the system to ascertain the best operating condition to minimize the total cost rate and maximize the exergy efficiency and the produced fresh water simultaneously. The results of a comparative parametric study reveal the superiority of R601a-C₂Butene (75%–25%) among various STORC working fluids from thermodynamic and economic points of view. The results further show that the proposed integrated system has a considerably lower CO₂ index compared to the same system without the STORC unit. For the final tri-objective optimization point, while the minimum total cost rate is 64.91 $/h, the maximum exergy efficiency and produced freshwater are 23.43% and 162.86 m³/day, respectively. Furthermore, the scatter distribution of the main decision variable reveals that moisture content and current density are not a sensible variable, and their optimal points are distributed in the whole domain.

Introduction

These days the lack of freshwater, environmental pollution, a significant increase in energy uses, and reduction in fossil fuel resources are becoming the major concerns in societies (Gholamian et al., 2020). In this regard, researchers are working on designing a feasible and novel configuration of an energy system to apply a renewable-based resource, improve efficiency, reduce greenhouse effects, and decrease the total cost (Behzadi and Arabkoohsar, 2020). Biomass as a renewable and biological material with a net-zero carbon dioxide emission rate could be a suitable alternative for fossil fuels (Gholamian et al., 2020a). Because biomass comprises a low energy density, it should be converted into high enthalpy syngas via various methods, including gasification, which is an efficient thermochemical technique. Of all kinds of available energy resources, the low-temperature proton exchange membrane (PEM) fuel demonstrates the most significant promise for the early commercialization stage, attracting the most attention and investment in domestic application projects (Changizian et al., 2020). Exploiting waste heat of low-temperature PEM fuel cell via an organic rankine cycle (ORC) system as a passive method with a low cost can be a favorable option for a cost-effective and sustainable integrated energy system. The idea of using zeotropic mixtures as the ORC working fluid enhances the system performance from the quality of energy conversion due to the reduction of temperature mismatching in the evaporator and condenser and minimizing the irreversibility (Wang et al., 2019). Aside from electricity, freshwater is another first-order necessity in which our life highly depends on. Because only 3% of the water on the earth is fresh (Gleick, 1993), there should be particular attention to the integration of the energy system for the simultaneous generation of heat, electricity, and fresh water. Among different types of desalination systems, thermal ones, including multi-effect desalination (MED) and multi-stage flash (MSF), are highly promising and widespread in the world market because most of the countries enjoy waste heat from different industrial parts recovered for freshwater production (Mehrabadi and Boyaghchi, 2019). In comparison to MSF, MED systems have lower purchased costs and consume less energy; so, their integration with lower-temperature heat sources chiefly the renewable-based ones is economically beneficial and favorable.

Chen et al. (2019) investigated the waste heat recovery of a cogeneration system using a zeotropic mixture. They concluded that in comparison to the pure fluid, the use of zeotropic mixtures as working fluid results in a lower fluctuation along with the better performance of the evaporator. Performance evaluation of waste heat recovery of an internal combustion engine via a dual-loop ORC system using a zeotropic mixture was studied by Ge et al. (2018). They concluded that in comparison to the pure fluid, using a zeotropic mixture leads to a higher heat recovery ratio and lower irreversibility of condenser and evaporator. Kolahi et al. (2016) compared the use of pure fluid and zeotropic mixture as the ORC working fluid for exploiting the waste heat of a diesel engine from energy, exergy, and economic points of view. They showed that the use of a zeotropic mixture results in higher energy and exergy efficiencies and a lower payback period compared to the pure fluid. Lately, considering a zeotropic mixture of R1270/R600a as the working fluid of an ORC unit, the thermodynamic evaluation of a novel integrated system under various operating conditions was studied by Liu and Lin (2020). They revealed that the mass fraction of R1270 is a sensitive parameter and significantly affects the system indicators. Nguyen and Shabani (2020) studied the application of recovering the waste heat of PEM fuel cells for combined heating and cooling and combined heating and power systems. They found that among various low-temperature systems, the ORC unit has the best compatibility with PEM fuel cells. Li et al. (2019) investigated a geothermal-based multi-generation system integrated with an ORC unit exploiting the waste heat of PEM fuel cell. Their results indicated a major increase in net generated electricity as a result of implementing a PEM fuel cell. Thermodynamic and thermoeconomic evaluation of an integrated system consisting of ORC units recovering the waste heat recovery of a hydrogen-based PEM fuel was investigated by Marandi et al. (2019). They concluded that while the use of PEM fuel cell results in higher investment costs, the value of performance indicators, i.e., energy and exergy efficiencies, increase. The effect of significant decision parameters on the performance of a hybrid system comprising an ORC unit exploiting the waste heat of PEM fuel cells was studied by Sheshpoli et al. (Alijanpour sheshpoli et al., 2018). Guo et al. (2019) investigated a hybrid energy system comprising a PEM fuel cell and heat pump for the cogeneration of heat and power. It was shown that when the waste heat of PEM fuel is exploited, the value of power density increases by about 33.41%.

A novel integration of the MED system with an ORC was recently studied by Aguilar-Jimenez et al. (Aguilar-Jiménez et al., 2020). They showed that the proposed hybrid system has a 22% higher efficiency than a standalone MED system. Marques et al. (2020) proposed and modeled a novel tri-generation system for hydrogen, power, and freshwater generation using EES software. The results revealed that exploiting the low-temperature heat source of different types of equipment of power plant via the MED unit leads to not only higher efficiency but also the production of extra freshwater of 980 kg/s, which is favorable. Considering the case of Kuwait, Abdulrahim and Chung, 2019 compared the performance of a novel plant for cogeneration of heat and freshwater with a reference plant. They showed since the number of MED units can have either a positive or negative effect on the hybrid system performance, it should be designed accurately. Performance assessment of an energy storage heat source driving a MED unit was carried out by Razmi et al. (2019), finding out that 62.5 kg/s potable water along with 80 MW power is produced in an arid climate.

Multi-objective optimization is a sharp cost reduction and energy enhancement tool to ascertain the best operating condition of energy systems, which is applied in various methods like genetic algorithm. Fang et al. (2019) investigated multi-criteria optimization and comparative performance assessment of an integrated energy system comprising an ORC unit using various zeotropic mixtures exploiting the waste heat of a diesel engine. Their results indicated that a considerable reduction in the heat transfer area of the evaporator is achieved as a result of using zeotropic mixtures. The results of optimization showed that the temperature of evaporation and condensation are sensitive parameters which should be kept at their highest and the lowest value, respectively. The idea of recovering the waste heat of a gas turbine system employing an ORC unit with a zeotropic mixture of cyclopentane and R365mfc to increase the exergy efficiency as well reduce the overall electricity cost was developed by Hou et al. (2018). They concluded that the optimal exergy efficiency and unit electricity cost 62.23% and 3.95 cents/kWh, respectively. A novel geothermal-based cogeneration system using various zeotropic mixtures was optimized by Han et al. (2020). They showed that the maximum exergy efficiency of 57.24% corresponds to Pentane/Butene (31/69). Considering the exergy efficiency and total cost rate as the conflictive objectives, multi-objective optimization of a hybrid system containing a PEM fuel cell and an ORC unit was performed by Khanmohammadi et al. (2020). Sadeghi et al. (2017) optimized a novel system comprising MED unit and ejector for multi-generational of cooling, electricity, and fresh water. They reported the refrigeration capacity of 120.4 kW and net produced electricity of 52.19 kW at the multi-criteria optimization point. Behzadi et al. (2020) applied multi-objective optimization of a cogeneration system containing a PEM fuel cell, thermoelectric generator (TEG), and a simple ORC unit. They demonstrated the thermodynamic, economic, and environmental aspects of their proposed novel system is highly influenced by the utilization factor and current density of the cell. Balafkandeh et al. (2019) optimized a novel biomass-based tri-generation system applying genetic algorithm approach via MATLAB software. They reported the optimum exergy efficiency and product unit cost of 47.09% and 5.43 $/GJ, respectively.

This work proposes a novel configuration of a biomass-based energy system comprising a gasification unit, PEM fuel cell, MED unit, and STORC using various zeotropic mixtures for multi-generational heat, power, and freshwater. A comparative investigation is carried out to find the best zeotropic mixture from energy, exergy, economic, and environmental aspects. Also, considering the performance and economic indicators, a tri-objective optimization using a genetic algorithm approach is implemented. In essence, the primary objectives and main novelties of this work are summarized as follow:

• To introduce a novel renewable-based energy system using feasible active and passive cost reduction and energy enhancement methods, including waste heat recovery.

• To utilize the multi-effect desalination system and series two-stage ORC unit to recover the waste heat of gasifier and biomass-based PEM fuel cell for fresh water and power production, respectively.

• To decrease the irreversibility of the evaporator and condenser of STORC by replacing a pure working fluid with zeotropic mixtures.

• To find the best mass fraction for each zeotropic mixture considering the exergy destruction minimization and temperature glide of evaporators maximization.

• To conduct a comparative parametric study to determine the best zeotropic mixture from various points of view.

• To investigate and compare the influence of waste heat recovery on environmental pollution using the best zeotropic mixture.

• To implement the tri-objective optimization to minimize the total cost rate and maximize the favorable objectives, containing overall exergy efficiency and generated potable water simultaneously.

• To examine the scatter distribution revealing the density of optimum points for each major decision parameters.