Last edited 19 Jul 2021

Renewable energy


[edit] Background

Our society on earth is heavily dependent on fossil fuels such as oil, gas and coal, and is likely to remain dependent on them for much of this century (Odell, 2009).

Every year we emit more than 20 billion tonnes of carbon into the atmosphere by burning fossil fuels, half of which is absorbed in the seas and by vegetation, and half of which remains in the atmosphere (Comby, 2008). The impact on human and natural systems is potentially irreparable (Schellnhuber et al, 2006). In addition, as fuels deplete and demand increases, so supplies become more vulnerable to disruption.

According to the International Energy Agency (IEA, 1999), 'the world is in the early stages of an inevitable transition to a sustainable energy system that will be largely dependent on renewable resources'. In 2009, US President Barack Obama said, “to truly transform our economy, protect our security, and save our planet from the ravages of climate change, we need to ultimately make clean, renewable energy the profitable kind of energy.”

In 2007, European Union (EU) countries committed to set a binding target that 20% of the EU's total energy supply should come from renewables by 2020 (European Union Committee, 2008). For the UK, the target is 15%, almost a seven-fold increase in the share of renewables in scarcely more than a decade (HM Government, 2009).

[edit] Renewable energy

Renewable energy is derived from sources which are naturally replenished or are practically inexhaustible. They are often described as 'clean', 'green' or 'sustainable' forms of energy because of their minimal environmental impact compared to fossil fuels.

However, there is some controversy about which forms of energy are genuinely renewable, as there are inevitable environmental consequences from any form of energy production and consumption:

The National Planning Policy Framework (NPPF) suggests that renewable and low-carbon energy: 'Includes energy for heating and cooling as well as generating electricity. Renewable energy covers those energy flows that occur naturally and repeatedly in the environment – from the wind, the fall of water, the movement of the oceans, from the sun and also from biomass and deep geothermal heat. Low carbon technologies are those that can help reduce emissions (compared to conventional use of fossil fuels).'

Very broadly the range of energy production techniques that have been described as 'renewable' includes:

[edit] Solar

[edit] Solar photovoltaics

Solar cells, or photovoltaic (PV) cells, convert sunlight directly into electricity. Photovoltaics gets its name from the process of converting light (photons) to electricity (voltage). See Solar photovoltaics for more information.

[edit] Solar thermal energy

The term 'solar thermal' is used to describe a system where the energy from the sun is harvested to be used for its heat. Small scale solar thermal collectors can be used for heating swimming pools or supplying building heating systems. Large scale solar thermal collectors use mirrors or lenses to focus solar radiation; allowing much higher temperatures to be generated.

See Solar thermal systems and Large scale solar thermal energy for more information.

NB: The use of the term 'solar thermal' is also associated with the integration of 'passive' heating and cooling technologies in buildings.

[edit] Geothermal energy

Geothermal energy is the second most abundant source of heat on earth after solar energy. It is the natural heat energy stored in the earth itself. Geothermal temperature increases with depth in the earth's crust. Using the technology available at present, it has been found that the average geothermal gradient is about 3°C per 100 m (Dincer et al., 2007).

Geothermal energy has been used on a commercial scale for over 100 years and more than 70 countries now exploit geothermal resources for electricity generation, space heating, hot water supply, cooling, industrial and agricultural uses. See Geothermal energy for more information.

[edit] Heat pumps

[edit] Ground source heat pumps.

The temperature of the surface layers of the ground remains fairly constant below a depth of approximately 7-10 m, at the mean ambient air temperature regardless of the time of year. Depending on the location and depth, this temperature is typically between 7°C and 13°C in the UK. This means that it can be used to as a heat source in the winter (and as a source of cooling in the summer).

A heat pump works by using the evaporation and condensing of a refrigerant to move heat from one place to another. An evaporator (analogous to the loop in the cold part of a fridge) takes heat from water in a ground loop; a condenser (analogous to the hot loop on the back of a fridge) gives up heat to a hot water tank which feeds a distribution system. See ground source heat pumps for more information.

[edit] Air-source heat pumps

Air-source heat pumps extract heat from the outside air in a similar way to ground source heat pumps. This can be used to supply heat to radiators, hot air systems or hot water systems.

[edit] Water source heat pumps

Water source heat pumps operate on a similar principal but use watercourses as a heat source.

[edit] Tidal power

[edit] Tidal range

Tidal range is the difference in height between high and low tide. Tidal barrages or tidal lagoons can capture the tide, and release it through turbines to generate electricity. Turbine blades can be of a size and speed that allow large fish to freely enter and exit without harm. However, there is concern that enclosing large areas can destroy tidal habitats.

The Swansea Bay tidal lagoon is calculated to save 216,000 tonnes of CO2 annually which is equivalent to taking 81,000 cars off the road. See tidal lagoons for more information. The UK's total theoretical tidal range resource is estimated to be between 25 and 30 GWs (12% of current UK electricity demand). (Ref. DECC Wave and tidal energy: part of the UK's energy mix)

[edit] Tidal stream

Tidal stream is the flow of water resulting from the continual ebb and flood of the tide. Tidal turbines can generate electricity from this flow in a similar way to wind turbines. This is particularly effective in areas where narrow channels or headlands increase the tidal flow.

[edit] Wave power

Wind blowing across the surface of water causes waves. Wave energy can be captured by a number of methods to power turbines that generate electricity.

Wave and tidal stream energy could meet up to 20% of the UK's current electricity demand. (Ref. DECC Wave and tidal energy: part of the UK's energy mix.)

[edit] Hydro electric

Moving water can be used to generate electricity at both large and small scales. The UK generates approximately 1.5% of its electricity is generated by this method, largely by creating dams to capture water and then discharging it through turbines. (Ref. DECC Harnessing hydroelectric power January 2013)

[edit] Wind

Wind energy can be converted into electricity by wind turbines. These can be onshore or offshore, large scale commercial wind farms or small domestic units. Wind energy generation in the UK is growing rapidly, however, wind turbines have proved controversial because of their impact on the landscape and the fact that they only generate electricity 70-85% of the time. (Ref. Renewable UK)

[edit] Biomass

Biomass is a generic term referring to organic materials that can be used as fuels. Biomass differs from fossil fuels because of the timescale required for replacement. While both take carbon out of the environment during their creation, before releasing it when used as a fuel, fossil fuels deplete faster than they are replaced and so are not sustainable whereas biomass can be replaced relatively quickly and so may be considered 'carbon neutral'. Solid bioenergy options include woodchips and pellets. Using these types of biomass fuel as a heating source is well established across Europe and the UK.

[edit] Waste

[edit] Combustion/incineration

Waste products can be incinerated at around 850°C and the energy recovered as electricity or heat.

[edit] Gasification and pyrolysis

Waste fuels can be heated with little or no oxygen present to produce gases which can be used to generate energy.

[edit] Anaerobic digestion/biogas

Biomass can be broken down by micro-organisms through a process of anaerobic digestion to produce methane-rich biogas that can be used as a fuel. The waste can also be used as a fertiliser.

[edit] Limitations of renewable energy

A key disadvantage of renewable energy at present lies in the rate at which it can be produced. Despite its successes, renewable energy production remains limited, partly because of the costs of the new technologies required and partly because their efficiency and productivity is partially dependent on variables such as the weather. A study conducted by The Renewable Energy Foundation revealed that the UK has missed its 2010 targets by a 'large margin' (ref. REF, 2011).

Renewable energy may also face the challenge of land constraint. For example, replacing crude oil-derived fuels by bio fuels would require between 1,000 and 10,000 times larger areas for crops than the land used by oil field infrastructures, and shifting from coal-fired to wind-generated electricity would require 10 to 100 times more space (Smil, 2006). Land issues apply to most renewable energies, along with direct or indirect impacts on natural habitats, the visual environment and loss of agricultural land.

These impacts can be seen in countries such as Brazil, Malaysia and Indonesia as producer countries of biofuels and Morocco, Libya, and Jordan where solar power plants are installed.

[edit] Nuclear energy

Although some sources claim that Uranium is inexhaustible with 4 billion tons dissolved in sea water, and that it can power the globe for 60,000-years at present rates (Comby, 2008; Fetter, 2009); there are concerns about the current state of available sources and the costs of processing that would be needed to extract uranium from sea water.

In addition, radioactive wastes are difficult and costly to dispose of and there is widespread concern about the diversion of nuclear materials to weapons production, as well as nuclear plants' vulnerability to attack. As a consequence, nuclear power may not be seen as a long-term option.

However, increasing C02 emission, rising demand and the limitations of renewable sources creates a dilemma. 'If you don't want nuclear, there are hard choices to be made on other issues' (Fitzgerald, 2005). For the time being at least, the UK government has chosen to support nuclear power as a low-carbon (rather than renewable) source of energy. This may provide some energy security whilst giving time for renewable energy technologies to be perfected.

[edit] Incentives

A number of incentives have been introduced by the UK government to encourage the take-up of renewable energy technologies.

[edit] Feed in tariff

The feed in tariff scheme enables consumers to receive payments from their energy supplier for renewable or low-carbon electricity that they generate, whether they use it themselves, or export surplus back to the grid.

[edit] Renewable heat incentive

The renewable heat incentive is a similar scheme to the feed in tariff scheme, but for heat generation. At present it is only available to the non-domestic sector, but the scheme is expected to extend to households in 2014 (ref. Increasing the use of low-carbon technologies).

[edit] Green deal

Under the green deal scheme, a 'green deal provider' finances the up-front costs of installing energy efficiency measures, and the consumer's energy supplier adds a 'green deal charge' to the consumer's bill. The range of energy efficiency measures that might qualify under the green deal include (amongst other measures):

See Green deal for more information.

[edit] Climate change levy

The climate change levy (CCL) is a tax chargeable on commercial energy suppliers. Electricity generated from renewable sources is exempt from the climate change levy.

[edit] Related articles on Designing Buildings Wiki

[edit] External references

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