It has been estimated that around 30 per cent of the UK’s CO2 emissions are the result of energy used in the home. Most is used for space heating, followed by water heating, cooking, lighting and comfort cooling.
Since domestic central heating has become more common, homes have been heated to higher temperatures; a very marked trend was found between 1970 and 2001, when the preferred living temperature rose by an average of 6 degrees Celsius. Surveys have also found that almost everybody now responds to a drop in temperature by turning up the thermostat; whereas before houses had central heating, occupants were more likely to put on extra clothes. Energy use for space heating therefore seems unlikely to drop; so it will become increasingly important to use renewable and non-polluting energy sources.
Combined Heat and Power (CHP) plants represent one of the most energy-efficient and cost-effective methods of heating homes. CHPs generate electricity, often from biofuels, and recover the thermal heat left over from the process; this heat is then used to maintain a supply of hot water. However, the water can only be distributed to households within a short radius of the plant, as it will lose its heat if it travels far. Each year more CHPs are constructed, but at present still only a small proportion of UK householders have access to this type of energy.
For the majority of UK homes, the energy choices for space heating systems are still predominately fossil fuel: gas, oil and electricity. ‘Green’ electricity, generated at power stations fuelled by renewable energy, is widely available but is more expensive. Solid fuel heat sources such as log burners are another option; these can be used to heat a single room, or can be linked to a boiler to provide hot water as well, which can be then piped around the house. Logs are a low carbon energy provided they are from a renewable forest.
A minority of homes use electricity generated by domestic micro hydro plants, wind turbines or solar panels.
There are various ways of producing a cooling effect in a room. Some consume no energy at all and some consume a great deal.
The most energy-efficient cooling is achieved when a building has passive solar energy features such as solar chimneys. A solar, or thermal chimney is a vertical ventilation shaft painted black and running from ground level to roof. Sunlight heats the chimney and the air inside it; as the air becomes hot, it rises; and as the air inside the chimney flows upwards, air from inside the building is sucked in to take its place, creating air currents and enhancing natural ventilation. But only a small minority of houses are equipped with such a feature.
The simplest way of cooling a room is to open windows to increase air circulation. If natural ventilation itself is not sufficient, a small electric fan can be used to stimulate air flow. In humid environments such as kitchens, extractor fans also reduce internal humidity by exchanging the hot, moisture-laden air from the kitchen with dryer, cooler air drawn in from outside.
The most energy-hungry means of cooling a building is by installing an electrical air conditioning system.
Air conditioners use a refrigerant to move thermal energy from the inside of a building to the outside. The refrigerant is put through an evaporation cycle where it absorbs heat from the air inside the building, and a condensing cycle where it loses the heat to the air outside. Systems can use a lot of energy. Pumps are needed to circulate the refrigerant; a compressor unit is used to compress the refrigerant at the beginning of the cycle; and fans are installed to create air flow inside the building and often also to help disperse the heat from the external condenser unit into the outside environment. A separate issue is that many of the refrigerants used in the past have now been discovered to contribute to the depletion of the ozone layer. Modern systems, however, are more environmentally-friendly.
Balancing Domestic Comfort and Energy Use
Over the last 50 years, the amount of thermal energy our homes waste has reduced considerably because we have more double glazing and better insulation. But we have counteracted this by becoming accustomed to higher levels of comfort in our living environment; technology has made it possible to keep the insides of our houses at a constant temperature throughout the year, at the flick of a switch. Reductions in CO2 emissions from domestic heating and cooling could be achieved without affecting our comfort levels by increasing the number of homes able to generate electricity from renewable energy sources, and also by building more eco-friendly houses where heating and cooling are achieved through passive solar power.