(5) Residential cogeneration by solid polymer fuel cell
From the principle of taking out the power when the fuel is ignited, the ideal image of the author is that engines are constantly in operation wherever gas or kerosene is ignited in such places as the kitchen and the bathroom of each home. Honda Motor Co., Ltd. in Japan manufactured the residentical cogeneration unit of 1.8 MWe on the trial basis which is the smallest record in the world. The unit is of the single cylinder gas engine of 163cc in displacement, and designed to be 20,000 hours service life, and 6,000 hours in maintenance interval. Low NOx is achieved by the threeway catalyst, and the level of the noise and the vibration is extremely low. The power of high quality is obtained by rectifying the output from the generator once into the direct current through the inverter, and converting it again into the sinusoidal wave. The size of the unit is 480mm (width), 520mm (depth) and 1,000mm (height), and the unit has the capacity of supplying hot water of 4,100kcal/hr (4.8kWth) at 80℃, realizing the total efficiency of 80%. The first step is made to the image that the engines are in operation in the bathroom and the kitchen of each home.
However, a powerful rival appears now to the cogeneration using the heat engine such as the piston engines and the gas turbines. This is the fuel cell. According to Nippon Keizai Shimbun dated November 12, 1997, the then-Daimler-Benz announced in a fullpage announcement of the morning paper that they would establish the mass production system of fuel cell hybrid cars by 2004, giving an impact to those concerned with the energy. The tie-up partner of the fuel cell technology is Ballard Power Systems in Canada. The fuel cell stacks of 40kW and 54kW to be mounted on Benz cars were displayed at the 5th Environment Technology Exhibition (commonly known by the name "GLOBE 98"), an actual article comprising the PEM (Proton Exchange Membrane) forming its heart part and graphite plates was placed on the desk in an unprotected manner, and the participants took it in their hands to check the feeling of the membrane thickness. This was the exhibition sufficient for showing their confidence. Fig.5 shows the photos of the graphite plate and the polymer membrane at the exhibition. After the exhibition, the author paid a visit to their factory in Burnaby in the suburbs of Vancouver BC, which looks like a small factory of the venture business.
The unit cell comprises a polymer membrane (PEM) of about 0.2mm thick which is held between two graphite plates (Flow Field Plates) of 200mm in one side length and about 1mm in thickness. A small groove to constitute a flow passage in which hydrogen as the fuel and the air as the oxygen supply source flows is cut on the graphite plate. Both sides of the polymer membrane to separate the reactive gas from the water as the reaction product are coated with a platinum catalyst, and further held by thin positive and negative electrodes. The hydrogen molecules of the fuel are brought into contact with the catalyst layer and separated into the hydrogen ion (proton) and the electron, only the proton passes through the membrane and reaches the oxygen electrode, and again brought into contact with the catalyst layer, and forms the water molecule together with the oxygen molecule by obtaining the electron reaching through the external circuit. The generated water flows in the groove cut in the graphite plate, and the heat of the generated loss is taken out as hot water at 85℃. The voltage of 0.7V is generated by a set of unit cells, and the specified output is obtained by laminating these cells.
The target service life is 5,000 hours for cars, and 40,000 hours for power generation, respectively. The cost is 50 Canadian dollars/kW (5,000 yen/kW) for methanol cars including reformer, and 1,500 Canadian dollars/kW (150,000 yen/kW) for power generation by the natural gas fuel. The cost should be further reduced through the mass production like that of cars, which is also a large advantage for the cogeneration. This means that there is a possibility of diffusion of the residential cogeneration unit of 1kW class with the service life of 20,000 hours at the price of the household electric appliances of about 500 US$/kWe (60,000 yen/kWe). The fuel cell is free from any noise or vibration, and its exhaust gas is clean. Since its waste heat is hot water at 80℃, it can be defined as a hot water feeder "having an electric generator" to supply the power while making hot water in a bath tub. This means a first step to the ideal condition though it does not follow the theory that engines are in operation when the fuel is ignited.
