Conclusions and Achievement of the Thesis Aim
This section concludes the chapter, with particular respect to the achievement of the thesis aim: the design, development and testing development of an efficient and effective SOFC liquid desiccant tri-generation system.
This chapter has successfully presented the evaluation, based on experimental data, of a first of its kind SOFC liquid desiccant tri-generation system. Two SOFC liquid desiccant tri-generation systems have been presented. First, Sect. 7.2 presented a theoretical integration analysis, based on collected empirical SOFC CHP and liquid desiccant data. Second, an experimental tri-generation system has been introduced. Section 7.3 provides a summary of this novel experimental tri-generation system and Sect. 7.4 presents the results and analysis from the tri-generation system evaluation.
Technical and commercial issues have meant the 1.5 kWe building-installed (BlueGEN) SOFC CHP system was not available for tri-generation system integration. However, using collected empirical SOFC and SDCS data, a theoretical tri-generation system integration analysis has been completed. The tri-generation system performance has been evaluated at a 1.5 and 2.0 kWe capacity. The highest tri-generation efficiency of 71.1 % is achieved at a 2.0 kWe capacity; however the electrical efficiency is lower than the 1.5 kWe case. As a result, the 1.5 kWe case produces the greatest cost and emission savings. The encouraging performance is primarily due to the high electrical efficiency of the SOFC and the reasonable COPth of the liquid desiccant system. The inclusion of liquid desiccant air conditioning technology provides an efficiency increase of 9-15 % compared to SOFC electrical operation only. The performance of the novel tri-generation system is competitive with other systems of this capacity reported in the literature, and in good agreement with simulations presented in Chap. 4. The technical and commercial issues encountered with the 1.5 kWe building-installed BlueGEN SOFC highlight the real challenge of fuel cell deployment in the built environment. Reliability, durability and cost currently pose a great barrier to their wider use, and demonstrate the need to focus future work on addressing these issues. However, the SDCS shows significant potential for integration with other CHP prime mover technologies such as ICE or SE. Due to the greater technological maturity of ICE and SE, the reliability of the complete tri-generation system can be expected to be improved.
Following the failure of the 1.5 kWe SOFC, a 250 We micro-tubular SOFC had to be acquired. The novel tri-generation system concept has been proven experimentally using the micro-tubular SOFC and SDCS. The experimental results demonstrate regeneration of the potassium formate solution at a 0.65-0.7 solution mass concentration using the thermal output from the SOFC in the first of its kind tri-generation system. 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 approximately 25 %. This is primarily due to the low capacity and poor performance of the micro-tubular SOFC. Insufficient WHR means only 450 W of thermal energy is available for regeneration purposes, and thus the cooling output is low. However, it has been suggested that if these issues are addressed, the novel system can provide higher overall efficiency.
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 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 trigeneration system operating in a building application. Compared to a base case scenario, the novel tri-generation system generates a cost and emission reduction of 56 and 42 % respectively, demonstrating the potential of the novel tri-generation system in applications that require simultaneous electrical power, heating and dehumidification/cooling.
The difference in performance seen between the two tri-generation systems presented in this chapter demonstrates the significance of (a) the performance of the SOFC component and (b) the requirement of optimal paring of components in the development of an efficient and effective tri-generation system. 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. Both tri-generation system analyses presented have considered balanced liquid desiccant system operation, demonstrating the strength and rigour of the work presented.
Based upon the experimental work presented in this chapter, three conclusions are provided with respect to the design, development and testing of an efficient and effective tri-generation system based on SOFC and liquid desiccant air conditioning technology for building applications.
- (1) SOFC and liquid desiccant is an effective technological paring. The inclusion of liquid desiccant can bring significant improvement to system performance, particularly in applications requiring simultaneous electrical power, heating and dehumidification/cooling.
- (2) Overall tri-generation system performance is more influenced by the SOFC component than the liquid desiccant. Appropriate matching of component capacity is necessary.
- (3) The novel tri-generation system concept has been demonstrated experimentally. Future work needs to focus on improving the current unreliability and sensitivity of fuel cell technology.
The aim of the thesis is to design, develop and test an efficient and effective trigeneration system based on SOFC and liquid desiccant air conditioning technology. This chapter has demonstrated a clear contribution to new knowledge with the development and evaluation of two SOFC liquid desiccant tri-generation systems and as a result it is proposed that the thesis aim has been completed.
Next, Chap. 8 provides an economic and environmental assessment of the novel SOFC liquid desiccant tri-generation system.