Renewable energy is energy that is collected from renewable resources that are naturally replenished on a human timescale. It includes sources such as sunlight, wind, rain, tides, waves, and geothermal heat. Renewable energy stands in contrast to fossil fuels, which are being used far more quickly than they are being replenished. Although most renewable energy sources are sustainable, some are not. For example, some biomass sources are considered unsustainable at current rates of exploitation.
Renewable energy often provides energy in four important areas: electricity generation, air and water heating/cooling, transportation, and rural (off-grid) energy services. About 20% of humans’ global energy consumption is renewables, including almost 30% of electricity. About 8% of energy consumption is traditional biomass, but this is declining. Over 4% of energy consumption is heat energy from modern renewables, such as solar water heating, and over 6% electricity.
Globally there are over 10 million jobs associated with the renewable energy industries, with solar photovoltaics being the largest renewable employer. Renewable energy systems are rapidly becoming more efficient and cheaper and their share of total energy consumption is increasing, with a large majority of worldwide newly installed electricity capacity being renewable. In most countries, photovoltaic solar or onshore wind are the cheapest new-build electricity
Photovoltaics (PV) is the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry. The photovoltaic effect is commercially utilized for electricity generation and as photosensors.
A photovoltaic system employs solar modules, each comprising a number of solar cells, which generate electrical power. PV installations may be ground-mounted, rooftop-mounted, wall-mounted or floating. The mount may be fixed or use a solar tracker to follow the sun across the sky.
A heat pump is a system used to heat or cool an enclosed space or domestic water by transferring thermal energy from a cooler space to a warmer space using the refrigeration cycle, moving heat in the opposite direction in which heat transfer would take place without the application of external power. When used to cool a building, a heat pump works like an air conditioner by transferring heat from inside the building to the outdoors. When used to heat a building, the heat pump operates in reverse: heat is transferred into the building from the outdoors. Common heat pump types are air source heat pumps, ground source heat pumps, water source heat pumps and exhaust air heat pumps. Heat pumps are also often used in district heating systems.
The efficiency of a heat pump is expressed as a coefficient of performance (COP), or seasonal coefficient of performance (SCOP). The higher the number, the more efficient a heat pump is and the less energy it consumes. When used for space heating these devices are typically much more energy efficient than simple electrical resistance heaters. Heat pumps have a smaller carbon footprint than heating systems burning fossil fuels such as natural gas, but those powered by hydrogen are also low-carbon and may become competitors.
Air Source Heat Pump
Air source heat pumps are used to move heat between two heat exchangers, one outside the building which is fitted with fins through which air is forced using a fan and the other which either directly heats the air inside the building or heats water which is then circulated around the building through heat emitters which release the heat to the building. These devices can also operate in a cooling mode where they extract heat via the internal heat exchanger and eject it into the ambient air using the external heat exchanger. They are normally also used to heat water for washing which is stored in a domestic hot water tank.
Air source heat pumps are relatively easy and inexpensive to install and have therefore historically been the most widely used heat pump type. In mild weather, COP may be around 4.0, while at temperatures below around 0 °C (32 °F) an air-source heat pump may still achieve a COP of 2.5. The average COP over seasonal variation is typically 2.5–2.8, with exceptional models able to exceed this in mild climates
| Air Source Heat Pump
Ground Source Heat Pump
A geothermal heat pump (North American English) or ground-source heat pump (British English) draws heat from the soil or from groundwater which remains at a relatively constant temperature all year round below a depth of about 30 feet (9.1 m). A well maintained geothermal heat pump will typically have a COP of 4.0 at the beginning of the heating season and a seasonal COP of around 3.0 as heat is drawn from the ground. Geothermal heat pumps are more expensive to install due to the need for the drilling of boreholes for vertical placement of heat exchanger piping or the digging of trenches for horizontal placement of the piping that carries the heat exchange fluid (water with a little antifreeze).
A geothermal heat pump can also be used to cool buildings during hot days, thereby transferring heat from the dwelling back into the soil via the ground loop. Solar thermal collectors or piping placed within the tarmac of a parking lot can also be used to replenish the heat underground
| Ground Source Heat Pump
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