[Active solar power]
Introduction, How do active solar technologies work, Current Technology, How can you calculate energy production, Economics, Example, Social Implications, Discussions, Conclusions
Active solar technologies usually consist of a solar collecting device that is designed to capture the sun’s energy; it can be used to store or transfer heat energy in water or air. Unlike passive solar technologies there is usually a moving part of the system involved with active solar technologies hence the word – active – usually a pump type device
Solar energy availability in the UK is much greater than most people imagine. In Scotland each south-facing roof receives between 700 - 1100 kWh/m2 of solar radiation during a year. By using solar collectors it is possible to capture some of this solar radiation and reduce our consumption of fossil fuels like gas, coal and oil.
Usually hot water is produced by heating the cold mains water to the required temperature with a gas or oil fired boiler or an electric immersion heater. By slightly modifying this conventional system solar collectors can be introduced to the system. Solar collectors are securely attached to some convenient part of the house so that they face south, towards the sun and are tilted from the horizontal between 10° and 60°.
Sourced: www.solar-design.demon.co.uk
Solar Water Collectors are located outside the house and have to be designed to last for many years, it is important that the materials used in their construction are durable and compatible with the rest of the plumbing. The following types of collectors are most common in the UK:
In the UK most SWH systems are indirect systems whereby heat is transferred to the hot water tank from the collector circuit via a heat exchanger.
Flat Plate Collectors consist of a black absorber – dark colours attract most heat - contained in a weatherproof box (housing), insulated at the back and glazed at the front (transparent cover), with a series of pipes, containing the heat transfer medium, running through the box to the hot water tank. Both the insulation below the absorber and the glazing at the front reduce heat losses that occur due to convection.
A flat plate collector uses the well-known ‘greenhouse effect’. The energy from the sun reaching the earth is mainly in wavelength that we cannot see. The glazing material (glass or plastic) does not absorb these wavelengths and when the sun shines the heat is allowed to pass to the blackened absorber.
As the sun heat radiation passes through the collector cover it warms the absorber plate and air in the collector and is prevented from escaping directly back to the atmosphere. This rise in temperature is passed to heat transfer fluid in pipes, which then passes through the hot water tank and transfers heat to the water in the tank, heating water for washing, showers and baths
Sourced: www.solar-design.demon.co.uk
Evacuated Tube Collectors – By creating a vacuum between the glazing and the absorber plate, the losses in a collector, through convection and conduction, can be reduced similar to that of a thermos flask. In addition, this protects the absorber plate in the long term against corrosion or other environmental influences. However, the improvements in solar collection achieved by these techniques have to be weighed against the extra costs incurred.
Understand how a thermos flask works http://www.howstuffworks.com/thermos.htm
In this type of vacuum collector, rather than one large absorber plate there are individual absorber strips each located in an evacuated and pressure proof glass tube. The heat transfer fluid flows through the absorber directly rather than being heated by it. The heat pipe collector incorporates a special fluid, which begins to vaporize even at low temperatures. The steam in the individual heat pipes rises to warms up the carrier fluid in the collection tube by means of a heat exchanger.
The condensed liquid then flows back into the base of the heat pipe. A typical system consists of several single tubes connected together in a row
The pipes in the system must be correctly angled above horizontal to allow the vaporizing and condensing processes to occur. “There are two types of collector connection to the solar circulation system. Either the heat exchanger extends directly into the manifold ("wet connection") or it is connected to the manifold by a heat-conducting material ("dry connection"). A "dry connection" allows individual tubes to be exchanged without emptying the entire system of its fluid. Evacuated tubes offer the advantage that they work efficiently with high absorber temperatures and with low radiation. Higher temperatures also may be obtained for applications such as hot water heating, steam production, and air conditioning.” http://www.solarserver.de/wissen/sonnenkollektoren-e.html#vak
The Solar Energy from the sun travels to the earth via a vacuum in space. The solar energy falls on earth with varying intensity known as Radiant Flux Density (RFD), it is referred to as Irradiance or Insolation.
Source; rredc.nrel.gov/solar/pubs/ shining/page12_fig.html
This Solar Irradiance hits the surface of the earth in two forms, beam (Gb) and diffuse (Gd). The beam component comes directly as irradiance from the sun, while the diffuse component reaches the earth indirectly and is scattered or reflected from the atmosphere or cloud cover.
In this way the total irradiance on a surface is G = GB + Gd (beam and diffuse)
A basic solar collector consists of an absorber / collector plate which absorbs solar energy, creating heat. In order to maximise the quantity of solar energy collected the absorber / collector plate should have a
high (a) absorptance value – which dictates how good the collector is at absorbing
a very low (r) value – so that the heat is not reflected from the collector
zero (t) transmission value – allowing full irradiance to fall on surface
Materials used in solar collectors have three characteristics when used in relation to solar energy. These three characteristics are;
Transmission (t), Absorption (a) and Reflectance (r)
For a given material the total of these characteristics should equal 1.
(t + a + r = 1)
The power absorbed into the collector (Qp) is given by:
Qp = GA(tc +ap)
Qp = absorbed power (W)
G = total irradiance (W/m2)
A = area of solar collector (m2)
tc = transmission factor of cover
AP = absorptance factor of collector plate
The power loss from the collector (QL) is given by:
QL = UA(Tc - Ta)
QL = power loss from collector (W)
U = collector U value (W/m2K)
A = area of solar collector (m2)
Tc = temperature of collector plate (K)
TA = ambient air temperature (K)
Useful power supplied by collector (W)
Qs = GA(tc AP) - UA(Tc - TA)
Some people may install a solar water heating system to help conserve the world’s supply of fossil fuels. Most people, however, will be more concerned with the money that the system could help them save in the future. The cost of a solar water heater consists of:
• Cost of the components
• Cost of paying the installer to fit it
These costs represent an initial investment in the system. The savings made each year in conventional heating fuels will repay your initial investment and the time taken to get your money back is often called the ‘payback’ period. The payback time for a SWH system can depend on a variety of factors, firstly whether it is retrofit or new build and the value of the energy it is displacing. However, even the most favourable conditions the minimum payback time can be around eight or ten years. Payback periods could be reduced if system costs were lowered, or if the cost of conventional fuels was increased. However solar water heating systems provide a significant energy contribution over a long period (typically 20 years or more).
A solar water heating system for a typical family of two adults and two children might have 3-4m² of collector and hot water cylinder capacity of 150 litres.
For families with smaller or larger hot water needs these amounts can be adjusted accordingly.
Cost and Performance data for a Typical SWH system
Typical System area 3-4 m2
Typical system price
-retrofit including VAT(£2 000 - £6 000)
-DIY including VAT or new build(£1 000 - £2 500)
“Such a system would require a installation time of typically 1-2 days and would have an estimated lifetime of 25 years. Annual pump running costs are estimated at £10 / year”
A typical system of 3-4 m² would collect up to 1000 – 1500 kWh annually
Figures from New and Renewable Energy: Prospects in the UK for the 21st Century: supporting analysis, ETSU, March 1999.
In typical UK conditions such a system will collect up to about 350kWh/year per m² of installed collector depending on the amount of water used. Of course, if you don’t use much water, you can’t save much energy, so solar water heaters are most economically in places with large hot water demands, such as schools and hospitals. Currently 80% of UK dwellings use gas or oil for their heating and hot water, which is less expensive than electricity. Therefore, for the majority of dwellings, the payback is significantly longer than the minimum. In Germany system costs are higher but the cost of conventional fuels is also greater.
Until April 2000, purchasers of solar water heating systems had to pay VAT at the rate of 17.5, this has now been reduced to 5% for professionally installed systems. Whether or not this reduction of VAT has given a significant boost to growth to the SWH in Scotland is unknown. Any future legislation on energy saving measures on renewable energy products should include provisions to reduce further or remove VAT on solar water heating systems, including DIY systems. Similarly the introduction of any grants would require careful planning, as grant funding is a politically sensitive issue. In several countries grants have been used successfully to stimulate the growth of the market for solar water heating technology. This use of grants within long-term solar water heating market strategy has been adopted with success within Denmark and The Netherlands. In these countries after the grants were introduced, they were phased out gradually, with price reductions made possible by market growth sustaining a significant solar water heating market without the need for continuing grants.
In the UK, the Energy Saving Trust provided grants for investments in condensing boilers, radiator controls and other energy efficiency measures. When discussing grants to provide incentives to purchasers of solar water heating systems, there are different options to consider:
Other issues to consider are the administrative implications of any new grant, the assurance of the quality of the systems that attract grants and the fairness of the grants "playing field". The introduction of grants is usually a short-term stimulus. In order to avoid damaging market swings when grants are subsequently withdrawn, grants, if used, should form part of a longer-term market strategy.
“The accessible resource for solar domestic hot water systems by 2025 is estimated to be 12 TWh/year (1TWh = 1,000,000,000 kWh) based on 50% of the housing stock being suitable for a SWH system and that each system provides 1200 kWh/year”
‘New and Renewable Energy: Prospects in the UK for the 21st Century:
ETSU for DTI, March 1999, Supporting analysis’, ETSU
Awareness of SWH technology among the general public is low. In Scotland, and the UK, many people are of the wrong opinion that the UK does not receive sufficient solar energy for SWH systems to work effectively. This general lack of awareness therefore has to be overcome by the sales and marketing efforts of the SWH industry, which is currently responsible for the high costs of unit being sold. Increased awareness about the effectiveness of SWH technology could not only lead to higher sales volumes, but also to a reduction in system prices due to manufacturing costs.
Some potential users perceive that there are installation difficulties, which is an issue that needs to be addressed. Apart from the installation of the collector, the installation of SWH system is essentially no more difficult than the installation of any central heating system. The lack of awareness of the technology, the low cost of conventional fuels and the high cost of the systems have all contributed to the failure of the industry to expand quickly.
Many potential customers were put off SWH due to a bad perception of the technology from the installation of unreliable systems in the 1970s. Similarly a report by the Department of Energy published in the early 1980’s, did not portray SWH favourably as a renewable energy source of the future. At the time the Government also cut back its research and development for renewable energy sources and concentrated on nuclear energy.
The solar heating industry in the UK is growing with people realising the potential as a energy saving device. The UK has developed a strong manufacturing base for solar heating equipment, supported by the manufacture of ancillary equipment, testing, training and consultancy services for both the domestic and export market, although the majority of systems are exported. It is estimated that around 20 companies in the UK have solar hot water as a significant part of their business with a further 30 companies having SWH as part of their overall activities.
Most systems fitted to existing buildings do not need to apply for planning permission; exceptions are in conservation areas, areas of outstanding natural beauty, national parks and listed buildings. Similarly planning permission may be required for residential flats, local planning policy allocations and developments that have had the removal of development rights.
Planners would normally consider the appearance and visual impact of the system – how much area it takes up in relation to the roof, its impact on the surrounding area including visibility from road.
More information regarding the regulatory framework and planning guidelines can be found at http://www.itpower.co.uk/swhmarket.
In terms of wider use of solar water heating in Scotland there is a lack of positive influence by potential promoters of SWH such as architects and other building professionals, house builders/developers, social housing developers, plumbing contractors etc.
This is largely due to a lack of awareness and an insufficient availability of information regarding SWH technology among these professionals. Wider circulation of information could increase both professional and public awareness to make the technology mainstream.
As part of an energy efficient system, a solar water heater will save fuel, help to achieve reductions in CO² emissions, and as more use is made of this free source of natural energy, our exposure and that of future generations to dangerous technologies, such as nuclear power, can be reduced.
Several factors may reduce the costs of solar water heating systems and lower cost of the technology itself:
Solar water heating has environmental benefits, which should be made clear in publicity and information material. Many people do not see SWH as modern or relevant to them. In our society, it is very important whether something is seen as ‘in’ or fashionable or not. SWH is a way for the general public to ‘do their bit for the environment’; it is not just for enthusiasts.
The Solar energy reaching the ground (average over day and year) in Glasgow is 2.38 kW hours/day
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