A Novel SOFC Tri-generation System for Building Applications

Abstract

In response to the critical need to decarbonise the built environment, alternative methods for more effective energy utilisation need to be explored including tri-generation 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 1.5kWe 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 1.5kWe 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 micro-tubular 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 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 reductions. Environmental assessments establish that emission reductions of up to 51% compared to a base case system are possible, with the potential for zero carbon operation with the transition to a pure hydrogen fuel. The thesis presents the following general conclusions with respect to the novel SOFC liquid desiccant tri-generation system: (1) SOFC and liquid desiccant air conditioning are an effective technological pairing. High tri-generation efficiencies, particularly in hot and humid climates, are demonstrated; (2) appropriate matching of component capacity is necessary. Overall tri-generation system performance is more influenced by the SOFC component than the liquid desiccant; and (3) it is primarily the optimisation of the liquid desiccant component that facilitates effective tri-generation system integration and operation. The thesis proposes that future work should focus on improving the thermal agreement between the SOFC and liquid desiccant component, accompanied by field trial testing in a building context.