Fuelcell Based Cogeneration (Combined Heat and Power) Systems.
A bridge to a hydrogen-powered world.
If you study your gas and electricity bills you will see there is a big difference between the energy cost of gas and electricity. For example in March 2000 the price quoted by Powergen for gas was 1.235 pence / kWh. and for electricity 6.08 pence / kWh. the difference between the two being 4.845 pence / kWh.
If it were possible to generate electricity easily and quietly from natural gas at home then this difference in price would be available to pay the capital cost of the device to do so. Such a device could be a fuelcell running on natural gas.
The annual average electrical power demand for a small house is 1 to 2 kW per hour.
If say 0.5kW per hour of this electrical energy requirement was a base load to be provided by a fuelcell running on natural gas then the energy provided per year would be 0.5 x 365 x 24 = 4,380 kWh and the difference in cost would be 4,380 x 4.845 / 100 = £212 per year. If the fuelcell had a life of at least 5 years this would generate available capital of £1060 for a fuelcell based system with a power of 0.5 kW which is equal to £2120 / kW of fuelcell capacity. Over the next year or two in the USA domestic fuelcell systems are coming to market in this price range. The proposed fuelcells will last longer than 5 years and so should provide a cheaper source of electricity for the homeowner over the lifetime of the fuelcell.
At first impression it may not seem worthwhile to go to all the trouble of installing fuelcells in homes if it isn't going to save much money for the homeowner, but saving money is not the main attraction. It is the possibility of reducing CO2 output and conserving gas supplies that makes domestic fuelcells worth considering. The proposed fuelcells will be very efficient converters of gas into energy because they will be cogeneration units in which waste heat from the electricity generation part of the process is used to heat domestic hot water and for central heating of the home. Also the electricity is generated in the home and so there are none of the transmission losses associated with central generation.
It should be possible to achieve total energy conversion efficiencies that are 50% higher than the average efficiency achieved by gas fired power stations generating electricity centrally where the waste heat is lost and energy is lost in transmission. So from a given amount of gas 50% more energy is produced which means that the amount of CO2 produced per unit of energy is reduced by 33%. This assumes that the fuelcell runs all the time which might require reversible electricity metering so that surplus electricity can be fed to the local electricity grid
The 50% improvement in efficiency of energy transformation that could be achieved by domestic fuelcells is derived as follows:
The best combined cycle gas turbines (CCGTs) now used at gas-powered central electricity generating stations achieve an average energy conversion efficiency of 65% - assuming continuous operation. However, not all gas-fired generating plants are CCGTs and continuous running is not always available so assume an efficiency of 60%.
Domestic fuelcells in a cogeneration unit producing base load electricity (i.e. 24 hrs per day running to supply the basic needs of the house, and export surplus to the local grid) and domestic hot water and central heating will achieve an energy recovery of 85%.
Because there are no long distance electrical transmission losses, there is a further gain in conversion efficiency of, say, 6%.
The overall improvement in energy conversion efficiency is therefore:
If we now look at the bigger picture an interesting possibility emerges. Excluding Sizewell B the total generating capacity of the UK nuclear power stations is approximately 10 Gigawatts and all of this plant is going to close in the next ten to twenty years. This generating plant will have to be replaced and to continue to meet our need to reduce CO2 emissions it will have to be replaced with generating plant that emits no CO2.
The total generating capacity of all the UK gas fired power stations already built or in the process of being built is over 20 Gigawatts. If the natural gas these power stations burn was used in domestic fuelcells as described above then 50% more energy could be extracted from the gas without producing any more CO2. Now 50% of 20 Gigawatts is 10 Gigawatts, so we could replace all the electrical energy we currently get from nuclear power and gas-fired power stations by switching the gas from gas fired power stations to domestic fuelcells as the nuclear power stations close.
To achieve this relocation of generation capacity from centralised power stations to distributed domestic generation will require 20 Gigawatts of fuelcell capacity being installed.
The example given at the beginning of this section assumes a domestic baseload of 0.50 kW to be supplied by a fuelcell in each home.
Assuming 20 million homes, this gives a total installed fuelcell capacity of 10 Gigawatts. But it would be possible to accommodate more fuelcell capacity in homes if reversible electricity metering were introduced (i.e. the home owner received a credit at the retail price for all electricity returned to the grid) and if heat storage systems were incorporated in the home to absorb surplus heat for later use.
The domestic fuelcell capacity is only limited by the demand for heat in the home. The improved efficiency and energy saving given by the fuelcell is achieved by using the heat that would be wasted if the electricity generated by the fuelcell were generated at a remote power station.
It may be the case that, due to global warming, the demand for air conditioning in the UK will increase. Using absorption chillers, the heat from the domestic fuelcell could be used to drive a domestic air conditioning system which would absorb surplus fuelcell heat in summer. This approach is already used to achieve 85% energy recovery in centralised combined heat and power systems based on diesel engines or gas turbines.
The point we are making is that, with lateral thinking and imagination, we can move towards much more efficient energy conversion systems that will also establish the infrastructure we will need when natural gas runs out and we have to move to a hydrogen-powered world.
Just as the nuclear power stations will all be closed in the next 10 to 20 years, all the gas-fired power stations will be paid for and worn out during the same 20 year period. So, as the gas stations are closed, the gas becomes available for the domestic fuelcells and the extra 50% of energy utilisation makes up the energy supply losses from the closing nuclear stations. It could all be synchronised to effect an orderly change over a twenty year period. It does not matter that some of the 50% extra energy from the gas is made available as heat whereas the nuclear energy it replaces is in the form of electricity. The short fall in electricity could be made up using renewable electricity generation such as offshore wind powered generators and the extra heat available displaces other gas burning for domestic central heating.
In the longer term there is going to be a shortage of natural gas but the above change will not aggravate the shortage because no additional natural gas is required. The proposals will however help prepare us for when we do run out of natural gas because the gas supplied to the domestic fuelcells can be enriched with hydrogen made from renewable electricity. In due course areas of the country could be changed over to 100% hydrogen.
In this way the whole country would go over to hydrogen as the natural gas supplies were exhausted.
The changeover described here from today's nuclear and gas powered centralised electricity generating stations to decentralised domestic hydrogen-powered-fuelcell cogeneration systems could all be achieved over a 30 year period starting now in step with existing nuclear plant reaching the end of it's useful life and so avoid the waste of valuable capital assets. Also the proposed change over is in step with the likely run down in economic natural gas supplies for the UK and provides security of energy supplies without relying on nuclear power which could be phased out in an orderly manner.
To achieve the above scenario will require urgent development of fuelcells for domestic cogeneration applications. We can do this but it will require a greater UK commitment of funds for fuelcell research and development than is currently the case.
The UK market for 20 Gigawatts of domestic fuelcells would be worth £40 billion, the European market for domestic fuelcells will be worth £200 billion.
Web site Manager Jill Norris