Energy from renewable sources, all except tidal power ultimately derived from solar energy (ie the sources of such energy are renewable from the point of view of the earth, but do not contradict the First Law of Thermodynamics!). Examples are the direct use of solar energy as heat (eg solar hot water services), arrays of photovoltaic cells which convert solar radiation directly to electricity; or the indirect use of wind power with wind-mills of various types, or hydroelectric systems which uses the head (pressure difference due to height) of stored water to operate a turbine generator. Firewood is often described as a renewable energy source, on the grounds that  ‘trees grow again’, but this ignores the ecological damage done in commercial forestry. After large-scale felling, trees may be replaced, but the complex eco-system does not return.