The Degree of Completion of the Thesis Aim and Objectives
The preceding eight chapters have sought to achieve the thesis aim through the accomplishment of the seven thesis objectives as set out in Sect. 1.5. In this section the seven thesis objectives and the degree to which they have been accomplished are discussed, with particular reference to contribution to new knowledge. Following this, the achievement of the thesis aim is critically examined.
Degree of Achievement of the Thesis Objectives
Objective one of highlighting the current gap in the literature surrounding SOFC liquid desiccant tri-generation systems, particularly for building applications, has been achieved through an extensive and rigorous review presented in Chap. 2. The review has established that no previous work relating to a SOFC liquid desiccant tri-generation system has been reported.
Objective two of selecting an appropriate working fluid for the liquid desiccant air conditioning system has been achieved through the evaluation of lithium chloride, calcium chloride and potassium formate in Chap. 3. The three desiccant solutions have been evaluated with respect to their potential for application in the novel tri-generation system. Potassium formate, operating at a 0.65-0.7 solution mass concentration has been identified as a suitable desiccant solution due to its good dehumidification capacity, low temperature regeneration requirement, negligible environmental impact, low corrosiveness and cost.
Objective three of numerical evaluation of the liquid desiccant air conditioning system, with particular regards to its suitability for integration in a SOFC tri-generation system has been achieved in Chap. 3. The modelling has demonstrated at cooling capacities of less than 10 kW (a) excellent cooling and humidity control, and (b) effective use of low grade thermal energy with a low temperature regeneration requirement of less than 60 °C. It has been highlighted that a clear operational advantage of the novel SOFC liquid desiccant tri-generation system is the potential for nonsynchronous operation. Re-concentrating the desiccant solution is an effective means of storing the constant thermal output from the SOFC, and has the potential to bring about improvements to system performance. Whilst remaining within the boundaries of realistic operating values, COPth of 0.8 are attainable at a regenerator solution temperature of 40-50 °C, demonstrating the potential for effective SOFC tri-generation system integration.
Objective four of validating, theoretically, the feasibility of combining SOFC and liquid desiccant air conditioning technology into an efficient and effective trigeneration system has been accomplished in Chap. 4. It has been demonstrated that whilst working in a SOFC tri-generation system context, a potassium formate solution mass concentration of 0.65 is appropriate to facilitate balanced desiccant system operation. Based on realistic operating values, the novel 1.5 kWe tri-generation system can achieve an overall efficiency of 78.98 % in CHP mode and 70.07 % in tri-generation cooling mode. These are values competitive with other micro tri-generation systems presented in the literature. The inclusion of liquid desiccant air conditioning provides an efficiency increase of up to 24 % compared to SOFC electrical operation only. The theoretical tri-generation system analysis has demonstrated that compared to the SOFC CHP component there are more operational variables that may be controlled in the liquid desiccant air conditioning component. As a result, it is the desiccant system’s operation that should be optimised for successful tri-generation system integration.
The novel tri-generation system is comprised of two main components; the SOFC and liquid desiccant. Objective five of evaluating these two components using experimental data has been achieved. Chapter 5 presents a novel IDCS (integrated liquid desiccant air conditioning system). The aim of the IDCS was to overcome the often cited barriers of liquid desiccant application in tri-generation and building applications, and thus facilitate effective tri-generation system development. The IDCS demonstrates good dehumidification/cooling output, however issues of mass imbalance in the regenerator, desiccant solution leakage and poor controllability of operating variables meant the unit was deemed unsuitable for trigeneration system integration. As a result of these shortcomings a SDCS (separate liquid desiccant air conditioning system) was acquired and tested. Chapter 6 presents a detailed SDCS component evaluation. Results demonstrate good dehumidification ability, mass balance between the dehumidifier and regenerator, no desiccant solution leakage and a good control of operating variables. Testing of the SDCS within an environmental chamber simulates real life operation in a hot and humid climate. Using regenerator thermal input values typical of a SOFC CHP system, COPth values of up to 0.66 have been reported, demonstrating the potential for effective tri-generation system integration.
Fulfilment of the objective of SOFC component evaluation has been achieved and is presented in Chap. 7. The original, building-installed 1.5 kWe BlueGEN SOFC had a variety of technical problems, and as a result of commercial issues the SOFC could not be repaired. Electrical field trial data of the 1.5 kWe SOFC over an 8 months period shows stable operation during this period with electrical efficiency of 55-60 %, and availability for power generation of 91.7 %. Long term stable operation and a good thermal agreement between the 1.5 kWe SOFC output and required SDCS input indicate the potential for efficient and effective tri-generation system development. In response to the 1.5 kWe BlueGEN SOFC failure, a 250 We micro-tubular SOFC had to be acquired. Following issues of sulphur poisoning, experimental testing results show that the micro-tubular SOFC has an electrical output of 150 W, at a net electrical efficiency of 11.65 %. Operating with a 2 L min-1 water volumetric flow in the WHR circuit, flow temperatures of up to 65 °C are possible, with a thermal output of 450W recorded. The thermal output is low, but is sufficient to demonstrate the tri-generation system concept.
Chapter 7 has successfully achieved objective six of integrating SOFC and liquid desiccant air conditioning technology into a complete tri-generation system.
The developed system has been evaluated on an energetic, economic and environmental basis. The 1.5 kWe BlueGEN SOFC was not available for tri-generation system development. However, Sect. 7.2 presents a theoretical integration analysis based on collected empirical 1.5 kWe BlueGEN SOFC and SDCS data. At a 1.5 kWe output, a CHP efficiency of 81.6 % and a tri-generation efficiency of 68.9 % have been demonstrated, values competitive with other systems of this capacity reported in the literature and tri-generation system simulations presented in Chap. 4. The CHP and tri-generation system efficiency increases to 84 and 71.1 % respectively at a 2.0 kWe output, however, the electrical efficiency decreases from 60 to 56 %, resulting in lower cost and emission savings. The inclusion of liquid desiccant air conditioning provides an efficiency increase of 9-15 % 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. The novel tri-generation system concept is demonstrated experimentally using the micro-tubular SOFC. The experimental results demonstrate regeneration of the potassium formate solution using the thermal output from the micro-tubular SOFC in the first of its kind trigeneration system. The low thermal output from the micro-tubular SOFC means the moisture addition rate is low (0.11 g s-1). However, balanced operation with the dehumidifier is possible. The novel system can generate 150.4 W of electrical power, 442.6 W of heat output or 278.6 W 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. The SDCS COPth is 0.62, an encouraging value for a waste heat driven cooling system of this capacity. The inclusion of liquid desiccant air conditioning provides an efficiency increase of up to 13 % compared to SOFC electrical operation only.
The thesis has established that a clear operational advantage of the novel SOFC liquid desiccant tri-generation system is the potential for nonsynchronous operation. Using this concept the experimental system can generate an increased peak cooling output of up to 527 W and a daily tri-generation efficiency of 37.9 %. This is an encouraging value for a tri-generation system of this capacity and serves to demonstrate the novel tri-generation system operating in a building application. The micro-tubular SOFC was acquired at short notice to replace the 1.5 kWe BlueGEN SOFC. As seen in the low thermal output, it is not the ideal match for the developed SDCS. However, the novel tri-generation concept has been successfully demonstrated. SOFC and liquid desiccant air conditioning technology are a feasible pairing. Future work should look to improved optimisation and pairing of the components.
Chapter 8 presents a detailed economic and environmental assessment, comparing the 1.5 kWe BlueGEN tri-generation system and a base case system. The tri-generation system’s annual operating cost is significantly lower than the base case. However, NPC and SPBP analysis demonstrates that the novel system is currently uneconomical in a UK economic climate. This is primarily due to the high capital cost of the SOFC and the requirement of stack replacement. An SOFC capital cost of ?9000 or less is required for the tri-generation system to be feasible.
Government incubator support makes the tri-generation system economically viable. Use of the SDCS with ICE or SE technology would provide much better economic performance due to significantly lower system capital cost. The tri-generation system demonstrates good environmental performance. Emission reductions of up to 51 % compared to the base case system have been presented. The encouraging emission reductions are primarily due to the high electrical efficiency of the SOFC and the replacement of electrically derived cooling with waste heat driven cooling.
Objective seven of recommending future work related to the novel tri-generation system is presented in Sect. 9.3. However, the technical and commercial issues encountered with the building-installed SOFC in Sect. 7.2 highlight the real challenge of fuel cell deployment in the built environment. Reliability, durability and cost currently pose a great barrier to the wider use of fuel cell technology and demonstrate the need to focus future work on addressing these issues.