Research PaperThermoeconomic analysis and multi objective optimization of a molten carbonate fuel cell – Supercritical carbon dioxide – Organic Rankin cycle integrated power system using liquefied natural gas as heat sink
Introduction
Molten carbonate fuel cell (MCFC) is considered as one of the most interesting and promising alternatives for fossil fuel powered plants. These fuel cells commonly operate at the temperature range of 600–700 °C and are classified as high temperature fuel cells [1]. The high temperature exhaust from the MCFCs can be utilized in bottoming cycles for efficiency improvement. There are a lot of research works in the literature in this regard.
Rashidi et al. [2] investigated the performance of a MCFC-simple gas turbine hybrid system. They predicted an energy and exergy efficiency of 57.4% and 56.2%, respectively for the system. Applying the first and second laws of thermodynamics, Emam et al. [3] determined the exergy destruction in each component of a combined MCFC-regenerative gas turbine hybrid system and reported an overall energy and exergy efficiency of 42.89% and 37.75%, respectively for their system. Among the cycles which can be coupled with the MCFC, the super critical carbon dioxide (SCO2) cycle shows promising features because of a better temperature matching between the MCFC exhaust gas and the SCO2 cycle input heat source. Bae et al. [4] studied various SCO2 cycle layouts for molten carbonate fuel cell application. Sanchez et al. [5] proposed a number of MCFC–SCO2 turbine hybrid systems and applied different control strategies for part load operation on them. In a similar work Sanchez et al. [6] performed a comparison between molten carbonate fuel cell based hybrid systems using air and SCO2 cycles. They concluded that the reducing compression work in the SCO2 cycle has a significant effect on the net power increment.
Furthermore, because of its small size and higher operating temperature range, this cycle can be used as one of the candidates for nuclear power production [7]. Because of the special properties of carbon dioxide at its near critical point the supercritical CO2 cycle operating in a closed-loop Brayton cycle has an efficiency equivalent or higher than the supercritical or superheated steam cycles at similar temperatures [8]. Some of the characteristics of CO2 as working fluid are as follows:
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It causes Less damage to the environment and has an ozone depletion potential (ODP) of zero and a global warming potential (GWP) of 1 (calculated over 100 years).
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It is abundant, non-flammable, non- toxic, cheap and compatible with the materials and lubricants.
Using organic Rankine cycles (ORC) for utilizing waste heat from toping cycles has been practiced by some investigators. However, depending on the temperature of energy source for organic Rankine cycle the type of working fluid used changes. In fact the ORCs are ideal cycles for utilizing low temperature energy sources. Akbari and Mahmoudi [9] proposed a combined supercritical CO2 recompression Brayton/organic Rankine cycle in which the waste heat from the SCO2 cycle is utilized by an organic Rankine cycle for generating electricity. Yari et al. [10] studied a combined cycle in which a transcritical CO2 cycle is employed to utilize the waste heat from the pre-cooler of a recompression SCO2 Brayton cycle.
On the other hand, based on the Kyoto protocol, the environmental related properties of working fluids including atmospheric life time (ALT), ozone depletion potential (ODP), global warming potential (GWP) are important criteria in selecting the working fluid. Many researchers have studied the effects of utilizing ORC for heat recovering, power producing and reducing the environmental impact of topping cycles. Tchanche et al. [11] evaluated the thermodynamic and environmental performance of a low-temperature solar organic Rankine cycle with heat storage system using twenty different working fluids. They concluded that the R134a was the most suitable working fluid. Aneke et al. [12] performed a thermodynamic analysis for an ORC considering R245fa and R134a as working fluids. The results showed that the R245fa performs better than the R134a. Considering six different working fluids and focusing on the amount of thermal efficiency, irreversibility losses and environmental effects, Hu et al. [13] studied the performance of an ORC. They reported that R245fa is the most useful and appropriate working fluid for ORC.
The followings are some characteristics of the R245fa [14]:
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The condensing pressure of R245fa is higher than the atmospheric pressure so that the equipments are more compact and the leakage of non-condensable gases into the system is avoided.
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R245fa is a safe and non-flammable working fluid.
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R245fa is classified as a dry fluid with a positive-slope saturated vapor line in the T-s diagram so that it expands in the superheat region [15].
There are different types of energy sources to produce biomass fuel for electrochemical reaction in fuel cells. Palm oil industry, as traditional energy source, is commonly utilized to produce biomass fuel in areas where other energy resources are not accessible due their availability or cost [16]. Advanced generation biofuel production from lignocellulosic material (LCM) was investigated by Wilkinson et al. [17]. Sekoai et al. [18] studied the feasibility of using biohydrogen production as a potential energy fuel in South Africa. An integrated system of biological H2 production including three different steps was proposed by Gavrisheva et al. [19]. Based on the photosynthesis process, as one of the most efficient and eco-friendly pathways of energy conversion, Voloshin et al. [20] introduced photobioelectrochemical cells.
The concept of using liquid natural gas (LNG) stream as a heat sink and also as a fuel supplier has been introduced in Refs. [21], [22], [23], [24]. In addition to its high quality chemical energy, the LNG has a cooling capacity of 840 kJ/kg at a temperature of 113 K. When the cold energy is used for power generation, it is estimated that one ton of LNG will produce about 240 kW h of electrical energy [25], [26]. Based on the information provided above about the MCFC, SCO2 cycle, ORC (with R245fa as working fluid) and the cooling effect of LNG, a new system configuration is proposed for producing power. The system is a combination of the above mentioned cycles and to our knowledge it has not been investigated yet. The present work is an attempt to fulfill this lack of information and it is hoped that the obtained results are useful for thermal designers.
Section snippets
System description and assumptions
Fig. 1 shows a schematic diagram of the MCFC-SCO2-ORC system combined with a LNG tank, the associated T-s diagram and the details of the reactions taking place in the MCFC. The MCFC cycle consists of a number of mixing units, heat exchangers, feed pumps, a HRSG, a catalytic burner and the basic parts of the MCFC. The bottoming cycle is a SCO2 cycle coupled with an ORC. The SCO2 cycle consists of a turbine, a compressor, two heat exchangers and an evaporator. The ORC involves a turbine, a pump,
MCFC modeling
Oxygen is reacted with CO2 at the cathode and the two electrons coming from anode (via an external electrical circuit) produce CO32− (see Fig. 1c). In the electrolyte, carbonate ions migrate from the cathode to the anode where hydrogen is oxidized to produce water, CO2 and two electrons.
The half reactions in the anode and cathode, the sum of which produces water, are presented by Eqs. (1a), (1b) as follows:
To make the reactions possible, the supplies of H2 to
Energy analysis
Neglecting the kinetic and potential energies, the conservation of mass and energy for a control volume is expressed as:where are the mass flow rate, heat transfer rate and power, respectively.
First law efficiency
The first law efficiency for an energy converting system is defined as follows:where is the net power produced by the system and is expressed as follows:In Eq. (16) is the
Optimization
Multi-objective optimization method is used in engineering problems in which several conflicting objectives are to be dealt with. The technique can be based on a genetic algorithm and is used for determining a solution depending on how one criterion is valued against another. A genetic algorithm, first presented by Holland [33], mimics streamlined principles of biological evolutionary processes and detects an optimal solution by means of an iterative method [34]. As a primary population in the
Results and disscution
Fig. 6 demonstrates the effects on the exergy efficiency and product unit cost of the LNG tank temperature, T30. It can be seen as the LNG tank temperature decreases the exergy efficiency increases and product unit cost decreases, simultaneously. In fact using the cryogenic cooling effect of LNG leads to a reduction in the ORC condenser temperature. Therefore, the LNG tank temperature has a great influence on the output power and exergy efficiency of the system as it can be accounted as an
Conclusion
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In the present work, a combined system including a molten carbonate fuel cell - a super critical carbon dioxide and an organic Rankine cycle is proposed and analyzed from the viewpoints of thermodynamics and economics. The system utilizes a LNG tank as heat sink. Three cases are identified in optimizing the system performance: in the first one the product unit cost is minimized (economic optimization case: cp = 0.039 ), in the second one the efficiency is maximized (exergy optimization
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