A Novel SOFC Tri-generation System for Building Applications

Abstract

i AbstractIn response to the critical need to decarbonise the built environment, alternative methods for more effective energy utilisation need to be explored including trigeneration systems.Tri-generation is the simultaneous generation of electricity, heating and/or cooling from a single fuel source. Solid oxide fuel cell (SOFC) and liquid desiccant demonstrate many characteristics that make them an attractive option in the development of an efficient and effective tri-generation system. SOFCs have high operational electrical efficiencies and a thermal output in good agreement with the low temperature regeneration requirement of liquid desiccants.The aim of this thesis is to design, develop and test an efficient and effective proof of concept tri-generation system based on SOFC and liquid desiccant air conditioning technology for building applications. An extensive review of the literature shows that no previous work has been reported on such a system. The research has critically examined, both theoretically and experimentally, the novel tri-generation system concept.Simulations show tri-generation system efficiencies of up to 71% are achievable at a 1.5kW e capacity, which are encouraging values for a system of this size. An integration analysis, based on empirical data, provides good agreement with the simulations. At a 1.5kW e output, a tri-generation efficiency of 69% has been demonstrated. The inclusion of liquid desiccant air conditioning provides an efficiency increase of up to 15% compared to SOFC electrical operation only, demonstrating the merit of the novel tri-generation system in applications that require simultaneous electrical power, heating and dehumidification/cooling. An experimental system, using a micro-tubular SOFC shows the novel system can generate 150W of electrical power, 443W of heat or 279W of cooling. Instantaneous tri-generation system efficiency is low at around 25%. This is primarily due to the low capacity and poor performance of the micro-tubular SOFC. Although the performance is low, the experimental results demonstrate regeneration of a potassium formate desiccant solution using the thermal output from the microtubular SOFC in the first of its kind tri-generation system. The thesis has established that a clear operational advantage of the novel SOFC liquid desiccant Abstract f ii tri-generation system is the potential for nonsynchronous operation. The constant SOFC thermal output can be used to re-concentrate the desiccant solution as a form of thermal energy storage. Unlike thermal storage techniques based on sensible energy, a significant advantage of (chemical) thermal energy storage in the form of strong desiccant solution is that there are minimal losses over time.Using this nonsynchronous operating concept, the experimental system can generate an increased peak cooling output of up to 527W and a daily tri-generation efficiency of 38%.An economic assessment demonstrates questionable performance; however this is anticipated to improve with SOFC capital cost reduct...

Thermodynamics and economics of liquid desiccants for heating, ventilation and air-conditioning – An overview

“…The main capacity of LDAC systems is their ability in dealing with moisture removal. However, their ability to deal with sensible loads is fairly limited [40,41]. For this reason, the integration of liquid desiccant systems with systems able to control the supply air temperature has been crucial for the worldwide marketability of liquid desiccant systems.…”

“…The simultaneous production of power, heating and cooling obtainable by the combination of liquid desiccant technology with CHP systems is one of the most promising, particularly for applications where the humidity removal is fundamental [41]. Recent studies also showed the feasibility of the coupling of fuel cells with liquid desiccant technology [40,49]. Elmer [40] investigated the feasibility of using a Solid-Oxide Fuel Cell (SOFC) to produce electricity and drive the regeneration process of the liquid desiccant, while Kim et al [49] investigated the integration of the liquid desiccant system with a Proton Exchange Membrane Fuel Cell (PEMFC).…”

“…Recent studies also showed the feasibility of the coupling of fuel cells with liquid desiccant technology [40,49]. Elmer [40] investigated the feasibility of using a Solid-Oxide Fuel Cell (SOFC) to produce electricity and drive the regeneration process of the liquid desiccant, while Kim et al [49] investigated the integration of the liquid desiccant system with a Proton Exchange Membrane Fuel Cell (PEMFC). The efficiency of these fuel cells together with their thermal output makes them very promising for integration with liquid desiccant technology.…”

“…As aforementioned, the most interesting liquid desiccants for HVAC application are LiBr and LiCl for their higher dehumidification performance, CaCl2 for its lower cost, and HCO2K for its lower corrosion to metals and crystallization. Therefore in Figure 7, the equilibrium vapour pressures of the LiCl, LiBr, CaCl2 and HCO2K solutions are displayed, based on experimental regression models [19,20,40] which are more accurate and reliable than the calculation results by Cisternas-Lam model in Figure 6. In Conversely, the regeneration/desorption of the desiccant solution is the opposite process to the dehumidification.…”

“…The thermodynamic properties affecting this consumption are density, viscosity, and volume of the solution [18]. In Figure 12, the densities of LiCl, CaCl2, LiBr and HCO2K solutions are shown [19,20,40].…”