Renewables: introduction

Renewable energy technologies are those that derive energy from natural forces continuously at work in the earth's environment, and which are not depleted by being used. Most renewable energy is derived from the sun either directly (through solar power) or indirectly from wind, waves, water flow or from plants and animals (e.g. from straw, wood or dung). Other sources include geothermal and tidal energy and energy from waste.

There are two basic ways of delivering useful energy to housing, i.e. heat and electricity. Heat is usually either stored as hot water or fed directly into emitters (radiators or other heating system) or delivered as warm air. Renewable technologies that generate electricity can either operate as 'stand-alone' systems (off-grid) or as 'grid-connected' systems. Stand alone systems require an array of batteries to store the electricity for use either with low voltage DC loads or via an inverter for mains voltage AC loads. A well-designed system would normally incorporate a mechanism for 'dumping' surplus energy (for example by heating water) should the system generate more electricity than needed to charge the batteries. A back-up generator may also be incorporated into the system to charge the battery array in periods of low output.

Grid-connected systems do not require a battery array and convert all the power to mains voltage and frequency via an inverter. Surplus power can be sold to the grid and thus offset the cost of purchasing power when demand exceeds that being generated by the renewable technology.

Incorporating renewable technologies will alter the balance of fuel use in different ways. Technologies that generate electricity only will reduce electricity consumption but will not affect the consumption of gas or oil for heating. Solar water will reduce the costs of heating water but will not affect electricity use (unless water is heated by electricity). Other technologies such as micro CHP may affect both. The more established renewable technologies along with typical changes to running costs and carbon emissions are summarised in the table below. These figures should be compared with the effect of replacing the boiler with a more efficient one (see line shaded pale blue).

1. Assumes all the heat required will be produced by the mCHP system, but in reality a good proportion might need to come from a secondary gas boiler, in which case the amount of electricity generated would be much less. Also assumes all electricity is used on site (or exported at same price paid for imported electricity). Hence, cost savings quoted will be maximum possible, and probably less than quoted.
2. Assumes half heat comes from mCHP and half from an 88% efficient gas boiler
3. Assumes output is 60% heat, 30% electricity and that the electricity produced is worth 3p/kWh (taken from SAP 2005).

Note: 2005 stock average insulation levels are assumed (U-values: Wall - 1.2, Roof 0.44, Floor 0.68, Windows 3.5) and where applicable, the renewable technology replaces a 72% efficient gas boiler.

A number of technologies regarded as renewable are also employed as the main heating system.

Typical running costs and carbon emissions are given in the table below.

Technical Guidance Available

CE69 'Renewable energy sources for homes in urban environments'
CE70 'Renewable energy sources for rural in urban environments'