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Renewable energy is the term given to energy sources that are constantly being replenished, mainly originating from solar energy, but the term is also applied to a variety of non-fossil fuel based sources.
Fossil fuels are drawn from finite stocks laid down over millions of years, mainly originating in the conversion of solar energy to complex organic molecules by photosynthesis. Once extracted and burned these stocks are not replaced (at least not in any timescale useful to humankind). Fossil fuels also release large quantities of the greenhouse gas carbon dioxide (CO2), although how much depends on the fuel. The use of renewable energy thus slows down the rate of fossil fuel depletion, and reduces the energy related impact on atmospheric warming. Most renewable energy is derived from solar energy: solar photovoltaics, solar thermal, hydropower, wind, waves and biofuels. Other forms exist: tidal energy comes from the gravitational effects of both moon and sun, whereas geothermal energy is drawn from the internal heat of the Earth. However the term is often applied to energy from solid waste and from landfill gas. These last two are not strictly renewable as much of the calorific value of the waste stream is due to the high plastics content, and landfill gas is actually a finite resource for a given landfill site. Nuclear energy, although devoid of CO2 emissions, is not a form of renewable energy. Although renewable sources have been used for centuries, modern renewable energy technologies are able to deliver higher conversion efficiencies, and in many cases have resulted in scientific and engineering breakthroughs that enhance energy delivery systems. Renewables also present opportunities for using new and emerging conversion technologies such as fuel cells, which use hydrogen as a fuel to generate electricity. However, there is a downside. Renewable energy sources are either diffuse and intermittent in nature, or have significantly lower energy content than conventional fuels; but perhaps more importantly the conversion technologies are more expensive than fossil fuel-based technologies. The UK does have a 15% overall target. It also has a 10% target for renewables in transport energy consumption in 2020. The exact way in which the 15% target is met is up to the UK, so long as the 10% transport sub-target is met. A number of mechanisms have been introduced to help with overcoming these obstacles including subsidies, obligations and regulations. This article introduces those renewable technologies most suitable to small scale, building type applications.
BUILDING INTEGRATED PHOTOVOLTAICS
The total annual amount of solar radiation entering the Earth™s atmosphere is more or less constant about 3.8 x1024 J (approximately 20,000 times humankind™s annual fossil fuel demand). Much of this drives the atmospheric heat engine and hydrological cycle. On average the UK receives about 3.2 GJ/m2 (on a horizontal plane), although its availability varies greatly throughout the year. A horizontal plane in the south of England in summer can receive up to 1 kW/m2 at peak times, but this falls to 200 W/m2 on a dull winter™s day. Photovoltaic technology is able to generate electricity throughout the year (as long as there is daylight), but the output of a module depends on the solar intensity. The photovoltaic (PV) effect has been known since the middle of the 19th century, and practical applications grew out of the US space programme. PV cells generate electricity directly from sunlight using semi-conductor materials. Materials used are either silicon based (mono- or poly-crystalline, or amorphous), or else heavy metal compounds such as cadmium telluride (CdTe) or copper indium diselenide (CIS) that are deposited as thin films. The most commonly used commercial cells are poly-crystalline, with conversion efficiencies of around 15 per cent. PV may be the best cost option for remote applications where there is no electricity grid, but in Europe the main impact of this technology is likely to be in building integrated photovoltaic (BIPV) modules. There are a number of advantages. Buildings already have large surface areas available for cladding with PV modules, obviating plant space or cost of land; and the costs of PV cladding can be offset by the cost of alternative types of cladding. In buildings used during daylight hours (offices, schools, hospitals etc) this offers CO2 free electricity generation at the point of use, where and when it is needed, reducing system transmission and distribution losses. In domestic applications it is possible to sell (export) to the grid during the day, and import electricity at night; a typical detached house roof area can have outputs of 3 - 5 kW giving a zero net electricity balance. A number of such systems already exist in the UK, but the economics depend on being able to sell the PV electricity to the local grid for a reasonable price; currently only a few electricity suppliers give parity with sale and purchase of electricity (i.e. buy at the same price they sell to the customer), but the practice is growing. A PV installation is a relatively simple system, with low maintenance costs. PV modules produce direct current, and therefore an AC inverter is required (unless the building systems are all converted to DC). Inverters have low losses and are relatively compact. Some PV systems employ a number of small inverters for groups of modules (so-called string inverters), which can further reduce space requirements. Once installed there is little maintenance, apart from occasional cleaning of the PV array. In stand-alone applications it is necessary to have a back up, usually in the form of a battery pack with a charge controller. (A possible option is to use the PV electricity to electrolyse water, store the hydrogen, and use this in a fuel cell to generate electricity when it is needed - a few such systems exist but are currently extremely expensive.)
SOLAR WATER HEATING
Solar energy can be captured as heat using thermal collectors. The two main types of collectors used in building applications are flat-plates and vacuum (or evacuated) tubes. Flat-plate collectors are simpler and cheaper, but have lower efficiencies and operate at lower temperatures than vacuum tubes. The most common application of both types is for domestic hot water heating. Other applications include hot water services in schools, hotels and hospitals, etc, and for heating swimming pools; vacuum tubes may also be used to service higher temperature processes. Solar thermal collectors are rarely able to contribute to space heating in winter due to short daylight hours and low solar radiation levels. A domestic hot water system typically comprises a number of solar collectors, a dedicated pump and a storage tank. There are a number of possible system configurations, but a particularly space efficient solution uses a tall, double coil storage cylinder. The solar collectors are connected to the heat exchanger coil at the bottom of the cylinder, and the boiler (or supplementary heating) is connected to the coil at the top. During periods of high sunshine the solar system will heat the entire cylinder - with flat plates, temperatures of 70oC are not unusual - and in winter the boiler is used to heat just the upper part of the cylinder. Even on dull days, or in winter, stratification in the tank means that the solar system can preheat the water at the bottom of the cylinder. Evacuated tube collectors, because of their reduced losses, will have a longer period of operation than flat plates. Sizing can be a complex business, but as a rule of thumb a typical household requires about 4 m2 of collector area, with storage capacities of around 20 - 40 litres per person. Self-installation is cheaper, but there are VAT reductions on professionally installed systems. Typical domestic systems can save between 1000 and 2000kWh per year (perhaps 50 per cent of domestic hot water demand). In climates with more extreme seasonal variation (e.g. Scandinavia) large solar thermal collector arrays have been used to store summer heat in underground stores, which is then extracted by heat pumps in the winter. Such seasonal storage systems are unlikely to be economic in temperate climates such as the UK.
Wind energy is not normally considered as a building application, but there is a growing tendency to consider wind turbines for urban situations. The output of a wind turbine is calculated from: 0.6Cp AV3 Here V is the upstream wind velocity, A is the swept area of the blades and Cp is the power coefficient (usually in the region of 0.4). In practice there is a velocity at which the power output does not increase further - the so-called rated wind speed; however, wind turbines will clearly produce much more energy in windier sites. The windiest sites are on the uplands or out at sea, which are usually remote from grid connections (for the uplands there is also the issue of visual intrusion). However, if located near an urban population the wind resource may be less, but connection to the system is cheaper and the environmental impact less intrusive. There are now a few such urban wind turbines. They take up relatively little land area, and can generate significant quantities of electricity. A 1 MW turbine in Swaffham, Norfolk, has a rotor diameter of over 60 m, and stands 60 m high at the hub, and stands next to a supermarket; a second larger turbine is being planned. Not all turbines are this big. Sizes range from a few kW up to 2 to 3 MW.
Biofuels is a general term that covers a range of fuel types. These include wood from biomass crops (e.g. willow, poplar or miscanthus), agricultural and forestry residues, bio-diesel, ethanol and methanol, and biogas from anaerobic digestion processes (including landfill gas, farm slurries and sewage treatment works). When replacement crops are continually being grown, the amount of carbon released during their combustion is continually being absorbed from the atmosphere (i.e. carbon neutral). In general these fuels are used locally to areas of production for the generation of electricity, however solid fuels (wood chips) and bio-diesel may have an increasing application in buildings. Bio-fuels require storage, but therefore do not suffer from problems of intermittent operation discussed later. Wood chips can either be burned in solid fuel boilers for central heating applications (several modern solid fuel boilers exist), or for raising steam for power generation, although this latter option is not an efficient use of the fuel. A more attractive option is to convert the solid feedstock into a gas, which can then be used in a as turbine combined heat and power (CHP) plant. Gasification technology has recently developed to such an extent that biomass fed gas turbines are commercially available. The low density and calorific value of wood chips (about 19 MJ/kg) means that large storage volumes are required, and there must be suitable access for the fuel delivery vehicles. Methane gas is generated from natural microbial action on organic wastes. Landfill sites produce large quantities - so-called landfill gas - which can be recovered to be used in gas engines for electricity and heat production. The process is also exploited by putting wastes (usually farm slurries) into anaerobic digestion tanks, which result in both the production of both methane and a solid residue that can safely be used as fertiliser. Gas produced from anaerobic digestion is cleaner, and has a higher calorific value, than the products of gasification. Another potential fuel is bio-diesel, derived from oil-seed crops. Bio-diesel can be used on its own, or mixed with conventional fuel to run diesel engine CHP. The calorific value is again low (about 22 MJ/litre against 39 MJ/litre for conventional fuel), but the density is higher than wood chips and storage volume requirements are less. The choice of technology will depend on relative costs, fuel availability, and heat and power demand patterns. Gasification technology has higher capital cost with a relatively low cost of feedstock fuel - bio-diesel on the other hand is expensive, but can be used in conventional and robust engine technology. The heat to power ratios also differ for the two technologies, with gas turbines operating at about 3 units of heat for every unit of electricity, and diesel engines having equal measures of each. CHP will be the topic of a future module.
RENEWABLES AND INTERMITTENCY
A major problem with renewable electricity - particularly solar and wind - is the intermittency and uncertainty of outputs. The electricity trading arrangements in England and Wales militate against uncertain output, and this may make electricity distribution companies reluctant to accept variable power generators onto their system. The major generators do not like to be treated as a means of back-up, or peak load following. This has led to a fall in the prices paid to renewables, although government measures such as the Renewables Obligation have attempted to address this. Generally, local building integrated electricity generation is not subject to the operation of the bulk electricity market, but there is no doubt that the operation of the market ultimately has an impact on the viability of small scale renewables. If intermittency is to be avoided then suitable storage systems need to be employed. For electricity this usually means batteries (although using fuel cells with hydrogen storage may be a future option), or perhaps pumped water storage for large systems. This storage adds high costs to systems already at a capital cost disadvantage to conventional power generation.
BENEFITS OF RENEWABLES
Renewables provide clean, CO2 free (or, in the case of biomass, neutral) energy for use as both heat and electricity. They can reduce dependency on foreign fuel imports, and provide diversity and increased security in the energy supply mix. Generation close to the point of use can also reduce transmission and distribution losses. Renewable energy does not attract climate change levy, helping to redress some of the cost imbalance with fossil fuels. Another measure designed to help is the Renewables Obligation, which requires electricity suppliers to purchase a proportion of their electricity from renewables. This has led to a new market in Renewable Obligation Certificates; suppliers with an excess of certificates can sell to those without enough renewable capacity. Suppliers with a shortfall in renewable capacity must either purchase such certificates or pay a penalty over and above the market price of electricity. This has led to a significant increase in the price paid for renewable electricity.
Resource - InsideENERGY