Book - School of Science and Technology

700 Combined heat and power (CHP)weight,relatively low operating temperatures (around 90 C),quick start-up at full powercapacity and quick adjustment to variable power demands. These advantages make thistechnology very attractive to the automotive industry as well as for CHP and distributedpower applications.The main disadvantages of this technology include high costs and difficulties withmaintenance.Phosphoric acid fuel cells (PAFC)Technically well developed,PAFCs are the first commercially available type of fuel cell.PAFC are a candidate for use in a wide range of applications. However,their relativelyhigh operating temperature (around 200 C) may favour CHP and distributed power/uninterruptible power supply applications. Electrical efficiency is typically around 40%.Alkaline fuel cells (AFC)AFCs are relatively simple,operate at low temperatures (around 70 C) and have beenextensively used in the space industry. Commercial development of this technology is wellunderway,oriented towards transport applications. This type of cell can achieve powerefficiencies of up to 70%.Molten carbonate fuel cellsThis type of cell uses a molten alkali metal carbonate as the electrolyte and operate at hightemperatures (around 650 C). Therefore,they are a candidate for large-scale stationarypower and CHP applications. Demonstration projects in the order of 2 MW e (electricaloutput) have already been carried out. Advantages of operating at such high temperaturesinclude the wide range of fuels that can be used and higher overall system efficienciescompared to PAFC systems.Solid oxide fuel cellsThis type of fuel cell uses solid,non-porous metal oxide electrolytes and operates at hightemperatures (around 1000 C). They have similar advantages and uses to molten carbonatecells. Power generation efficiencies could reach 60% for this type of cell.Commercial systemsMost commercial fuel cells systems will not use hydrogen and oxygen gas directly.Reformers will use a variety of hydrocarbon fuels to provide hydrogen rich gas to thefuel cell,while oxygen will be taken directly from air. This process will produce somecarbon emissions,but emission levels will be very small compared to even the bestcombustion process.As with all other CHP installations,an alternator to convert the direct current outputfrom the fuel cell to alternating current power,and heat recovery equipment,are required.In theory any size of system can be provided: fuel cells have small outputs andare combining to give the required output. Current commercial CHP/power developmentsare aimed at the range of several hundred kilowatts to several megawattselectrical output with electrical efficiencies around 40% and heat to power ratios ofaround 1:1.Micro-turbinesStirling engines are predominantly being developed for the domestic market. Their moresimple mechanics is expected to lead to better and cheaper reliability at these smalloutputs compared to conventional internal combustion engines.