Introduction
The term 'solar thermal' (ST) is used to describe a system where the energy from the sun is harvested to be used for its heat. Solar thermal systems differ from solar photovoltaics which convert sunlight directly into electricity. The use of the term 'solar thermal' is also associated with the integration of 'passive' heating and cooling technologies in buildings.
The UK offers a good climate for solar thermal systems benefiting from around 60% of the solar energy that is received at the equator and similar amounts to other northern European states. The amount of solar radiation received (also known as 'solar insolation') is measured in kWh (kilowatt hours) over a particular time period. On a typical July day in Coventry there would be around 5 kWh of solar insolation.
Solar thermal systems are rated in kWth (thermal kW). The total of all solar thermal installations in the UK in mid-2010 was around 400,000 kWth.
The main application for solar thermal systems in the UK is domestic hot water heating although there are also 'combisystems' that use non-potable thermal stores directly linked with low temperature space heating (such as underfloor heating) and, in warmer climes, there are more technically challenging solar powered refrigeration systems.
A 2011 Energy Savings Trust report (Here comes the sun: a field trial of solar water heating systems) indicated that properly installed and operated systems can provide 60% of domestic hot water energy. Typical carbon savings from a well-installed and properly used system in a house amount to around 230 kgCO2/year when replacing gas and 510 kgCO2/year when replacing electric immersion heating.
The potential for solar heating in the UK
Solar thermal has had a great boost in recent times with the publicity around the Renewable Heat Incentive and the Green Deal, and as a result the adoption of solar collector technologies is gaining pace in the UK. Other European countries such as Germany, have seen a steady growth in the installed area of collectors since the early 1990s (so far around 10million kWth).
In the UK the average annual available solar irradiation varies between around 1,200 kWh/sq m on the south coast of England and up to 900 kWh/sq m in Scotland. Southern England has similar insolation to that in Holland, northern France and northern Germany. Solar data for the whole of the UK is available on the PVGIS Solar Map.
With only 55% of the sun’s light being visible, and much of the sunlight being diffuse there is potential for solar-powered water heating even on 'cloudy' days. A properly designed and installed solar thermal system can maximise the capture of this power and translate 60% of it into useful energy for hot water.
Solar systems have a number of positive attributes that are likely to promote greater use in the UK:
They provide no exhaust gases (there may be some related emissions from pumping energy if this is necessary).
Good quality collectors will have a life of 20 to 30 years.
They offer long-term independence from fuel price inflation.
Total cost analysis is largely based on the known, initial capital cost.
Low maintenance.
Potential for government subsidies.
Certainty of fuel supply.
They can improve the environmental credibility of building.
Government funding
All solar thermal installations of 45 kWth capacity or less need to be certified under the Microgeneration Certification Scheme (MCS or equivalent) to be eligible for financial assistance from the government. This provides a safeguard against poor quality and inefficient installations. Both the technology and the company or person installing the system needs to be certified under the MCS scheme. When applying for financial support, details of MCS certification will be required. For solar thermal installations larger than 45 kWth, Ofgem will verify eligibility.
The Solar Keymark is a quality label for solar thermal collectors and systems that fulfil minimum requirements according to specific European standards. It is recognised in the UK as equivalent to MCS for equipment, the installer still needs to be MCS certified.
Solar thermal panels for commercial hot water installations up to 200 kWth are eligible for the Renewable Heat Incentive (RHI). Domestic installations are expected to be included in the RHI scheme in 2014.
The Enhanced Capital Allowance (ECA) scheme enables businesses to claim 100% first-year capital allowance on investments in eligible solar thermal equipment against the taxable profits in the period of investment. However since the 2012 budget this provision is being phased out in recognition of the RHI.
NB The Renewable Heat Premium Payment scheme closed in May 2015.
Efficient solar thermal systems
The key for efficient system sizing is to meet as much of the annual domestic hot water requirement as is economically possible. This is known as the solar fraction and ranges from zero, for no solar energy use, to 1 to that indicates all the heat for the annual domestic water requirements is supplied by solar energy. The solar fraction of a particular system is dependent on many factors such as the load, the collector and storage sizes, the operation, and the climate.
Experience from Germany where there is a very mature market shows that systems are commonly oversized, having been based on assumed hot water consumption that is much higher than reality. Typically in summer the hot water usage was not reached and the expected solar insolation was exceeded. Combined with poor materials this led to overpriced, oversized systems that failed to meet expectations. Problems also arose in many cases due to poor integration with existing traditional hot water service systems. In the UK it should be reasonable to expect a solar fraction of 60%.
It is also important that when the system is designed it is able to deal with stagnation of water in the collector. This is the point at which the water system cannot accept all the heat from the collectors and so the heat from the sun may raise the temperature of the solar collectors well beyond 100°C, causing evaporation inside the system. Long periods of stagnation may be a sign of an over-estimation of solar fraction, where solar collectors are too large (and uncontrollable) for the their particular application.
Sizing solar thermal hot water systems
The method for sizing a solar thermal system is quite different to gas, oil or electric hot water systems. Conventional systems are sized based on peak hot water demand with additional capacity to provide potential for future expansion and safety margins.
A solar powered system would normally be sized so that it does not provide any more energy than is required to recharge the store of hot water in periods of low demand. This is normally a summer condition. A larger store may allow a greater solar fraction but an oversized store will mean that at times of low solar availability, stored water temperatures may be too low (at additional cost and space). The UK Building Regulations require that the store should be at least 80% of the daily hot water demand or 25 litres for every sq. m of collector area.
Practically, where systems are being installed in existing buildings, the capacity can be based on measurements of actual demand taken in periods of low consumption in summer. In new buildings they can be sized based on measured data in similar buildings.
There will almost certainly be a need to provide an auxiliary means of heating the water (normally from the main heating systems) for when demand cannot be met by solar collection. In this case, the solar hot water can be used as a means of preheating water that is subsequently fed into a separate continuous-flow water heater or a traditional hot water calorifier/cylinder. Purpose made cylinders are frequently used where a solar coil takes up the lower space in the cylinder and a traditional primary heating coil is in the top half.
Energy used in pumping can be significant, and so it is important to minimise pressure drops in systems through careful pipework design and by exploring the use of solar powered pumping.
The means to prevent legionella must be carefully thought through as poor scheduling of 'pasteurisation' cycles can reduce the opportunity to capture heat from the sun. The high temperature water can be circulated around the tank, but this will break up the temperature stratification (that would normally leave cooler water at the bottom) making the solar coil less effective.
Collecting solar heat
The selection of the type of collector will depend, amongst other things, on its temperature in operation and application. Collectors have several elements that combine together to ensure a consistent performance and longevity, including:
The geometry and type of the absorber.
The absorber coating.
The covering material and casing.
Basic operation is reliant on the 'green house effect'. Incident (high energy, short wavelength) solar radiation passes through the transparent or translucent surface of the solar collector and heats a metal or plastic surface. The glazed panels reduce the heat re-radiated back out and will also reduce convection of heat from the hot absorbing surface.
Unglazed (flat plate) solar collectors are used for low temperature water applications (such as swimming pools) where the loss of heat will not be as significant as with higher temperature panels.
The solar collector
There are two main types of solar thermal collectors currently used in the UK:
Flat plate collectors.
Evacuated tube collectors.
There are a number of variants of these in various materials, with the choice driven by the temperature of operation and the location and available area for mounting the panels.
Both types of absorber can reach high temperatures in operation and so must be properly constructed to maintain a long-lasting, consistent heat transfer between the absorbing surfaces and the tubes/channels through which the fluid flows that carries the heat to the hot water store. The collector must comply with BS EN 12975 - Thermal solar systems and components
Unglazed flat plate collectors
Unglazed panels are used where the required water temperature is no greater than 10K above the temperature of the air around it. This is suitable for swimming pool applications but not appropriate for domestic hot water systems.
Glazed flat plate collectors
Image: Flat plate collector
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