Basic energy units
The electrical and mechanical energy we use daily for heating is measured in many ways but I will ignore all those British Thermal Units, horsepower and therms, and deal instead in kilowatt-hours (kWh). To be scientifically correct all measurements of energy should be in Joules, Kilojoules, Megajoules and Gigajoules. But we are dealing with buildings, and the building trade has used kilowatt-hours since it went metric, so we will stick to them to avoid confusion. The advantage of the kilowatt-hour is that it is the unit on electricity bills so most people are familiar with it.
The kilowatt is a unit of power, rather than of energy, which means that kilowatts or watts measure the rate at which energy flows. A lkW electric fire can give out lkW or 1000W of heat energy. If it runs for an hour it will produce 1kWh or 1000Wh of heat energy and of course consume 1kWh or 1 unit of electricity. If you run the fire for a whole day it will consume 1 x24=24kWh, and so on. Similarly a 100W a boiler to make steam which drives a turbine which turns the generator which makes the electricity which lights the house that Jack or Jill built. Each step in this process is less than 100 per cent efficient and the result is that less than half of the energy in the coal or oil comes out in the form of electrical energy. Further losses are incurred in the transmission of the electricity because of heating effects in the wires of the National Grid. The final result is that the consumer receives about 27 per cent of the energy contained in the coal. This raises the question of the use of electricity as a fuel for space heating, as overall the efficiency is hardly better than that of a coal-burning open fire. The problem is one that will have to be dealt with on a national scale as most consumers are more concerned with their own bills than with national energy consumption.
The energy used to produce a fuel, for example the energy that is required to mine coal, clean it and transport it to the consumer, or the energy needed to extract oil from oil wells, transport it and refine it into heating oil, is termed ‘primary energy’. When politicians and others talk of ‘national energy demands’ or ‘energy consumption’, this is what they are referring to. All fuels need a certain amount of primary energy for their production, and in the UK the Building Research Establishment gives the efficiencies for the manufacture of various fuels as follows:
- electricity from fossil fuels 27%
- manufactured fuels (such as smokeless coal) 71%
- substitute natural gas made from coal (estimated) 79%
- oil 93%
- natural gas 94%
- coal and wood (estimated) 98%
Note: substitute natural gas or SNG will have to be made from coal when natural natural gas supplies cannot keep up with the demand.
To put it another way, to make 1kWh of electricity requires 3.73kWh of primary energy, but to make 1kWh of coal requires only 1.02kWh of primary energy.
When these efficiencies are combined with the efficiencies of conversion to useful heat in a building it turns out that overall, the most efficient way to heat your home is to burn coal in a closed stove or room-heater, which can be 69 per cent efficient in terms of primary energy. The least efficient is off-peak electric heating which has an efficiency of only 19 per cent in its use of primary energy. Other typical efficiencies of primary energy use are:
- wood-burning stove 59%
- natural gas central heating 57%
- oil central heating 56%
- coke-fired room-heater 46%
- on peak electric heating 27%
- coal in an open fire 20%
Net or useful energy
As a consumer you are more likely to be interested in the efficiency with which fuel is burnt in your house. If you heat a room with an open fire at 20 per cent efficiency you will burn £5 worth of coal or wood for every £1 worth of heat or 5kWh for every 1kWh. If you burn the same £5 worth of coal in a stove that is 70 per cent efficient you receive £3.50 worth of heat, or 3.5kWh.
To put it another way, you might calculate that your house would need 8000kWh to heat it over the whole winter. This is about the amount of energy contained in 1 tonne of coal. However, if you plan to heat with an open fire you will need 5 tonnes of coal to give out 8000kWh of useful heat energy, because the open fire is only 20 per cent efficient. The 5 tonnes of coal contains 40,000kWh (5×8000) of energy: this is the ‘net’ or delivered energy demand of the building. The 8000kWh you need to keep the place warm is the ‘useful’ energy demand. If the same house were heated with a coal burning room-heater which might be 70 per cent efficient, the net energy demand would be 11429kWh (8000kWh useful energy divided by 70 per cent or 0.7) which would be a little less than 1.5 tonnes of coal. For the same amount of useful energy the house with the open fire uses more than three times as much coal as the house with the more efficient heater.
The comparative study of the primary, net and useful energy consumption of various types of heating systems shows, in our opinion, that houses with electric central heating, even if fully insulated, use far more primary energy than a conventionally insulated house with gas or oil central heating; but, because the insulated house has a reduced useful energy demand, you pay no more for the heating than someone with a gas or oil system. If such all-electric houses were to become commonplace and replace other forms of heating the national primary energy consumption might increase although the demand of each house had been reduced.
Once a building has been properly insulated the best fuel to burn is one that requires little primary energy to produce it, such as wood, coal, oil or gas, leaving electricity to be used for lights and electrical appliances. In the long term coal or substitute natural gas are likely to become the most widely used fuels, so if you are planning a conventional heating system these are probably the fuels you should be considering.