Literature Review Conclusions and Impact on Trigeneration System Development

An extensive and thorough review of literature has been presented, covering five key topics: fuel cell technology in Sect. 2.2, desiccant air conditioning in Sect. 2.3, combined heat and power in Sect. 2.4, tri-generation systems in Sect. 2.5 and trigeneration assessment methods in Sect. 2.6.

The aim of the review has been to (1) highlight the current gap regarding a SOFC liquid desiccant tri-generation system, (2) demonstrate the feasibility of the development of such a system, and (3) provide a comprehensive background study on the state of the art of the technology. All literature has been selected based on its appropriateness and suitability in addressing the central aim of the thesis; the design, development and testing of an efficient and effective SOFC liquid desiccant tri-generation system. The main literature review conclusions are summarised as follows.

• • No work has been reported on an SOFC liquid desiccant tri-generation system. Literature found regarding SOFC tri-generation is predominately simulation or larger commercial applications utilising VAS, with little work on experimental based domestic building studies. Furthermore, there is little work reported on fuel cell tri-generation in domestic built environment applications. Work that has been completed is again simulation based. The current highlighted gaps are believed to be a result of three key factors (1) cost, (2) system size/complexity and (3) relative infancy of fuel cell/heat driven cooling technology, particularly at a domestic scale.
• • The low electrical requirement and high COP_th makes stand-alone evaporative based liquid desiccant systems an attractive option for effective waste heat utilisation in tri-generation system applications. The use of membrane based contactors eliminates the issues of desiccant carry over. Potassium formate used as a working fluid demonstrates good dehumidification capacity, particularly in high humidity applications. There is limited published and available literature within the public domain regarding the specific physical characterisation of the potassium formate solution in the required operating range for liquid desiccant air conditioning applications.
• • Opportunities that the literature review have highlighted are; the high operating electrical efficiency and stable operation of SOFC technology, coupled with the effective use of low-grade thermal energy seen in stand-alone liquid desiccant air conditioning demonstrates the potential for the first of its kind SOFC liquid desiccant tri-generation system. The outlet water temperature in a SOFC

CHP system (60 °C) is in good thermal agreement with the required desiccant solution regeneration temperatures (45-50 °C). Confirming the feasibility of integrating the two technologies into an efficient and effective tri-generation system.

• Challenges that the literature review has highlighted are; system integration, optimisation and energy utilisation is not an issue in theoretical or laboratory based tri-generation systems, thus high system efficiencies have been reported. However, in real world building applications, effective energy utilisation and low COPth in the heat driven cooling cycle can have a marked impact on trigeneration system performance and thus feasibility.

Further to the general literature review conclusions provided above. Specific conclusions relevant to the design, development and testing of the novel system are presented below.

• • Technical constraints will mean that the SOFC will run continuously. The balance between energy supply and energy demand needs careful and thoughtful consideration to ensure high system performance. To ensure high tri-generation system performance, there needs to be a constant electrical and thermal energy demand, or an effective way of storing it. Electricity can be imported/exported to the grid. However, the issue of heat and coolth will be more problematic. This issue will be even greater outside the hours of building inhabitation and in spring/autumn when air conditioning will not be required.
• • Due to the issues other researches have encountered whilst coupling off the shelf components into a tri-generation system, careful consideration as to the thermal pairing of the SOFC CHP and liquid desiccant systems is required.
• • The developed tri-generation system should have the option to be modified once built in order to facilitate any changes required to improve its performance i.e. inclusion of thermal energy storage, heat exchangers etc.

The literature review has highlighted that there is a clear need for research in the field of SOFC liquid desiccant tri-generation and domestic scale fuel cell tri-generation systems. This is due to the patent operational advantages the system can deliver in terms of energy efficiency, operating costs and energy security. This thesis aims to address the highlighted research gaps through theoretical and experimental means.