(146b) System Analysis on Absorption Chiller Utilizing Intermediate Wasted Heat from Micro Gas Turbine and Solid Oxide Fuel Cell Hybrid System

A distributed power supply system will become very important in the near future, because it has no energy loss in electricity transportation and it can make wasted heat utilized. In such a system, a fuel cell is paid attention to as a core system. In this paper, 30kW class hybrid system including micro gas turbine (MGT) and solid oxide fuel cell (SOFC) system was treated as such a distributed power supply system. This system has very high performance for electricity supply compared with a large-scale power supply system. However, its energy efficiency still remains around 0.5 to 0.6. For the improvement of its energy efficiency, a highly efficient system of wasted heat utilization is required. In this paper, a system analysis has been performed for a multi-effect absorption chiller applied as a bottoming system of 30kW class MGT/SOFC hybrid system. A multi-effect absorption chiller can effectively provide cooling energy for building air-conditioning system or district cooling system from wasted heat. Then, it attracts many researchers working on energy saving systems with wasted heat utilization. MGT/SOFC hybrid system exhausts about 20kW heat from the bottom end of the system. However, its temperature is not high enough for a multi-effect absorption chiller in which a high-temperature generator requires a high-temperature heat source. Then, the cooling heat obtained from a multi-effect absorption chiller utilizing the terminal wasted heat is not so large in spite of its high coefficient of performance (COP), compared with that obtained from a single-effect absorption chiller. An intermediate wasted heat utilization (IWHU) system instead of a usual terminal wasted heat utilization (TWHU) system is suggested in this paper for lifting up the energy efficiency of the whole system. IWHU system utilizes the heat exhausted from SOFC, before given to MGT. There exits high temperature wasted heat with the temperature of about 1,000oC. On the contrary, such high temperature heat gives serious damage to MGT located in the downstream of SOFC. Then, air-cooling supply is required normally. If such high quality heat source is supplied to a high-temperature generator of a multi-effect absorption chiller, it cannot only make the energy efficiency of MGT/SOFC hybrid system high but also lead to prevent the heat damage to MGT. This is the concept of the present IWHU system. Energy and mass balances were considered in the present system analysis on whole IWHU system and single-, double- and triple-effect absorption chillers. The highest pressure of absorption chiller generator for each absorption chiller was kept at 4.0x105, 1.0x105 and 0.1x105 Pa, respectively. The wasted heat from IWHU system was supplied by steam but that from TWHU system was by heated air. The turbine inlet temperature (TIT) was kept at 900oC in the present analysis. In cases of IWHU system application, the additional heat was required to keep TIT when intercepted heat increases. In such cases, additional fuel was supplied to the combustor located in the upstream of MGT. From the analysis, energy and exergy efficiencies were obtained for the whole IWHU system including SOFC/MGT system. The results were compared with the performance of the TWHU system on SOFC/MGT system. The COPs of single-, double- and triple-effect absorption chillers were also obtained through the present system analysis. The results on multi-effect absorption chiller system utilizing the intermediate heat were compared with those of the multi-effect absorption chiller with direct fuel burning in the generator, which system is commonly used in the multi-effect absorption chiller. From the results, the suggested IWHU system is found to show the very high energy efficiency compared with that of TWHU system. When TWHU system was applied for a multi-effect absorption chiller, the utilized heat from the MGT/SOFC system is found to remain low because the temperature difference between the high temperature generator and the wasted heat becomes small. Then, the energy efficiency does not become high in spite of high COP of a multi-effect absorption chiller. On the other hand, the energy efficiency of a multi-effect absorption chiller increases when the IWHU system is applied and it increases with the number of heat utilization, because IWHU system can supply enough high temperature heat source to the high-temperature generator of a multi-effect absorption chiller. It causes high energy efficiency of the SOFC/MGT system. The IWHU system requires additional fuel as mentioned above when the intercepted heat increases. The fuel combustion may cause exergy loss from the system. Then, the comparison of the exergy efficiency of the present system with that of a multi-effect absorption chiller with fuel burning in the generator was performed. The exergy efficiency of IWHU system is also revealed to be higher than that of a multi-effect absorption chiller with fuel burning, because the wasted heat is effectively utilized in the IWHU system. However, the exergy efficiency decreases in the higher region of input energy, due to the additional fuel burning in the combustor located in the upstream of MGT. This paper concludes that an optimum condition exists for IWHU system to realize high energy efficiency and to keep high exergy efficiency.