The term solar thermal is used to describe a system where the energy from the sun is harvested specifically to be used for its heat. (The other main UK solar technology applied to buildings being solar PV that is used to generate electricity). Using solar energy to heat water has been commercially available in Europe for 40 years and is a proven technology. The UK can offer a good climate for solar thermal solutions benefiting from around 60% of the solar energy that is received at the equator and similar amounts to that of other EU northern european states.

Solar thermal systems are rated in kWth (thermal kW). So, for example, the total solar thermal installations in the UK by mid 2010 was around 400,000 kWth.

The amount of solar radiation received (also known as 'solar insolation') is measured in kWh (kilowatt hours) over a particular time period. For example during a July day in Coventry there would be around 5kWh of solar insolation. The main application for solar thermal in the UK is domestic hot water heating although there are 'combisystems' that use non-potable thermal stores that are directly linked in with low temperature space heating (such as underfloor heating) and, in warmer climes, more technically challenging areas such as solar powered refrigeration may be used.

A recent Energy Savings Trust report on measured domestic installations indicates that properly installed and operated systems can provide 60% of domestic hot water energy for homes - month by month as shown in the example graph above. Typical savings from a well-installed and properly used system in a house are £55 per year when replacing gas heating and £80 per year when replacing electric immersion heating. This gives carbon savings of around 230kgCO2/year when replacing gas and 510kgCO2/year when replacing electric immersion heating.

The use of the term 'solar thermal' is also associated with the the integration of 'passive' heating and cooling technologies in buildings - although not included in this briefing the careful design of buildings to minimise energy use is the first essential stage of reducing carbon emissions and lifetime costs.

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 Initiative and the Green Deal and as a result the adoption of solar collector technologies is gaining pace in UK. Other European countries with climatic conditions that, in parts, are similar to the UK, for example Germany, have seen a steady growth in the installed area of collectors since the early 1990’s (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. 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 systems can maximise the capture of this power and translate 60% of it into useful energy for hot water systems. The UK is comparable with the European countries better known for their solar applications as shown in the graph. 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 a dynamic map.

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 required)
• Good quality collectors will have a life of 20 to 30 years
• Long term independence from fuel price inflation
• Total cost analysis predominantly based on known initial capital cost
• Low maintenance
• Potential for government subsidies
• Certainty of fuel supply
• They can enhance the environmental credibility of building

Assuring opportunity for government funding

All solar thermal installations of 45kWth capacity or less need to be certified under the Microgeneration Certification Scheme (MCS) to be eligible for financial assistance from the government as a safeguard against poor quality and inefficient installations with the hope of improving environmental benefits.

Both the technology and the company or person installing it needs to be certified under the MCS scheme (or equivalent). When applying for financial support, details of MCS certification will be required.

Renewable Heat Incentive - Solar thermal panels for commercial hot water installations up to 200 kW_th are eligible for support and those with a capacity of up to and including 45kWth will have to be MCS certified. For solar thermal installations larger than 45kWth, Ofgem will verify eligibility. Domestic installations are expected to be included in the RHI scheme later in 2014. (Information correct at April 2013)

Renewable Heat Premium Payment scheme is for households who install renewable heating including solar thermal systems. These are direct payments to the home owner of £300 subsidise the cost of installation and in return for the payments, participants are asked to provide some feedback on how the equipment works in practice and suppliers asked to provide a follow up service on any issues that are raised. The aim is to boost confidence in the technology and the information received aims to help Government, manufacturers, installers and consumers to better understand how to maximise performance of the various technologies.

Microgeneration Certification Scheme (MCS) - This is the scheme for the certification of microgeneration products and installers.

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 the equipment - the installer still needs to be MCS certified.

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 howver since the 2012 budget this provision is being phased out in recognition of the RHI.

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. The availability of solar energy can be modelled using tools such a RETScreen and fast assessments of solar radiation data can be obtained from this online EU resource (although designed for photvoltaics design the monthly data is very useful) and the daily insolation data for UK locations may be directly obtained from this solar map.

Experience from Germany where there is a very mature market show 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 never 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 the 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 can not 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 and cause evaporation inside the system. Long periods of stagnation may be a sign of over enthusiastic expectation of solar fraction, where solar collectors are too large (and uncontrollable) for the their particular application. (For more detail on the stages of stagnation see section 2 of this IEA report).

Sizing solar thermal hot water systems

The method needed to size 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 will be possibly too low and all 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 building 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. The actual capacity of the system is determined using practical experience

There will almost certainly be a need to provide an auxiliary means of heating the water (normally from the main heating systems) for when the demand can not be met by solar collection. 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 so minimising pressure drops in systems with thoughtful pipework design and exploring the use of solar powered pumping may be advantageous.

The means to prevent legionella in systems must be carefully thought through as poor scheduling of 'pasteurisation' cycles can reduce the opportunity to capture heat from the sun. The example shown here circulates high temperature water around the tank. This will unfurtunately also break up the temperature stratification (that would normally cooler water at the bottom) making the solar coil less effective.

Collecting solar heat

Collector technology has now matured so that it is a world away from early ‘diy collectors’ made of black painted domestic radiators! The selection of the type of collector will depend, amongst other things, on its temperature in operation and application as shown in the graph.

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 generally based 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 being used in the UK building services sector. These are flat plate and evacuated tube collectors. There are a number of variants of these in various materials - 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 would be used where the required water temperature is no greater than 10K above the temperature of the air around it and so very suitable for swimming pool applications but not appropriate for domestic hot water systems.

Glazed flat plate collectors

Most suitable for where the water temperature is between 10 and 50K above the surrounding air temperature so ideally suited to domestic hot water applications.

The typical construction of a glazed flat plate collector (shown on the right) comprises a light weight metal or polymer tray that contains a layer of insulation (normally glass fibre) to prevent heat loss via conduction through the rear of the collector. The 'absorber' is designed to maximise solar irradiation absorption and is likely to have selective surface coatings to maximise the solar gain and minimise re-radiation. For metal absorbers black chrome or nickel coatings are typically used.

A series of waterways bonded to the absorber carry the heat transfer fluid (water or glycol mix) through and away from the collector. The collector has a transparent glass or plastic cover across the whole 'aperture' with a low thermal expansion coefficient, and high transmission efficiency to maximise net incoming solar radiation and minimise convection losses. Transmission efficiencies for good quality products are over 90%, absorption efficiencies 95%, emissions (losses) 5% and maximum thermal efficiencies of around 78% over the year.

Copper is commonly used for the tubing in the collector and aluminium or copper for the absorber sheets (with stainless steel being used when aggressive mediums flow through the absorber as in direct fed swimming pool panels). Polymer and butyl rubber materials are used for applications where the system is designed to carry plain water that may freeze. Whatever tubing is used there must be a good thermal bond between the tubing and the absorber plate. Any connections to the panels together with the immediate fluid loops are likely to be made using mechanical joints rather than soft solder or even brazing as they are subject to high thermal stresses.

To provide good performance flat plate collectors have inclined mountings or are integrated into an appropriately pitched south facing roof between 30° and 40° to the horizontal. The influence of mounting angle on collector output is shown in the graph on the right.

Evacuated tube collectors

The evacuated tube works in the same way as a Thermos flask to reduce the convective and radiative heat loss from the collector back to the environment. They also frequently include some form of focusing reflective surface to provide less dependence on solar position. Since they are more effective per unit area than flat plate collectors they require a smaller installed area and are competitively priced.

The construction of an evacuated tube collector is entirely different to that of a glazed flat plate collector. There are two main types of evacuated tubes. Direct flow evacuated tube collectors, and heat pipe evacuated tubes.

The encompassing tubes themselves can be of a single skin or, more likely, a twin wall Dewar Tube ('Thermos flask’) made from borosilicate glass, a glass with high chemical and thermal shock resistance.

In a common application of this (known as the ‘Sydney’ tube, after its developer, Sydney University), the outer tube is transparent allowing solar radiation to readily pass through (90%+ transmissivity) and the inner tube is coated with a selective coating (eg aluminium based) that provides high solar absorption and minimal reflection.

If a tube develops a leak a silver coloured barium deposit inside the tube turns a white colour when it reacts with atmospheric oxygen.

Direct flow evacuated tube collectors are where the heat transfer fluid is pumped through a copper ‘U’ pipe in each tube and the U tube is bonded to a circular absorber that is slid in to the inside of the Sydney Tube.

Heat pipe evacuated tubes consist of a heat pipe inside an evacuated tube. The pipe, which is a sealed copper pipe, is then attached to a heat transfer fin that fills the tube (this is the absorber plate). Protruding from the top of each tube is a metal tip attached to the sealed pipe. The tubes are mounted, the metal tips at the top, into a heat exchanger or manifold assembly. Water, or glycol mix, flows through the manifold and picks up the heat from the tubes. The copper at the tip of the heat tube can reach well over 150 degrees so heating water to 90 °C on hot days and to 60°C even in the winter.

A self limiting capability makes the heat pipe collector very tolerant to extreme temperatures a A large number of variations of the absorber shape are available including those with integral reflectors.

As with flat plate collectors evacuated tubes collect global insolation (be direct and diffuse) however, their efficiency is higher than flat plates at low incidence angles so they can be more effective over a longer period in the day and when the sun is low in the sky. They can be fixed practically flat on the roof or vertically on a façade. Even if the location is not quite directly facing due South the tubes can be adjusted to maximise solar irradiance.

The completed evacuated tube collectors typically comprise a manifold and a series of glass tubes (typically 20 of 30) connected in parallel.

Whilst transmission efficiencies, absorption efficiencies and emissions are comparable to those offered by glazed flat plate collectors, the thermal efficiency is higher as a result of the presence of the vacuum with values being typically 83%. The actual increase in efficiency at ‘normal’ operating temperatures as would be used with a hot water system are likely to be somewhat less.

The increased efficiency (particularly at higher temperature) will lead to higher stagnation temperatures – this means that the materials associated with an evacuated tube installation must be rated at an appropriately high temperature.

Evacuated tubes are not so sensitive to positioning as flat plates but are more challenging to integrate seamlessly into the fabric of a building.

Solar thermal installations

As with any H&V installation specialist knowledge and expertise is required to design, install and maintain solar thermal systems. This web briefing is not a design guide however B&ES's 'Solar Heating Design and Installation Guide' provides that level of detail ( and the system diagrams below are taken from that guide).

For commercial installations on flat roofs it is likely to be cost effective to provide an appropriately designed framework so that there is correct collector tilt and azimuth (orientation). As collectors may be used as part of the fabric it is advantageous to integrate their needs (tilt, orientation, fixing and access) early on in the design process. Flat plate collectors are likely require a higher tilt angle than evacuated tubes.

The solar thermal system

The typical components of an indirect solar thermal system are shown in the diagram on the right. This diagram and others on this page are simplified and do not show the full controls for efficiency and safety needs however it is essential to note that no means of isolation should be placed between the collector and the safety relief valve.

The water supplying the domestic hot water outlets is potable but the primary water from the solar collectors may be a glycol mix or non-potable water in indirect systems but for direct systems (that heat the domestic hot water directly in the collectors) it must be potable.

In many stores, to maximise the heat capacity, and to make water available for space heating, the water drawn off from the solar cylinder can be at a temperature that is too high to pass directly to hot water outlets. Some mixing arrangement, or a secondary lower temperature will be used to ensure safe temperatures.

The design of the systems and their installation should specifically prevent the following

• Scalding risk from steam or hot water - failsafe control is required to keep temperatures safe at water outlets
• Freezing of fluids where it might cause damage or block pipes and safety valves.
• Accumulation of solids or bacteria - some water treatment may be needed - particularly in direct systems
• Legionella bacteria developing within the consumed drinking or shower water - the system operation must meet the requirements of HSE Code of Practice L8 for the control of Legionella. This will normally mean a loss in overall seasonal efficiency as auxiliary heating will be required and must be maintained in use.
• Degrading of water quality due to contact with materials and fittings during stagnation - the oversizing of systems will make the chances of stagnation more likely
• Backflow or thermo-siphoning of heated water into a cold water cistern - hence the check valve shown in the system above
• Disturbance of stratification in the solar storage vessel during normal operation - this will be disturbed in 'sterilisation' cycles
• Loss of dedicated solar storage capacity - systems that are designed to operate with storage will stagnate if they are run without the storage being available.
• Loss of liquid from system through overflow - any liquid that is lost has to be replaced that will affect the composition of the anti freeze additives and will also allow more oxygen and solids into the system. The expansion vessel should be large enough so that any stagnation does not cause fluid loss through the safety device. In closed systems a heat dump mechanism - typically a radiator - can be used to dissipate heat when the hot water store can accept no more heat.

The systems in the UK are likely to be either a fully pumped or drain back configuration. In climates where freezing is unlikely syphonic systems may be used but there application is unlikely in the UK.

Fully pumped systems

This is the dominant type of system that is used in residential through to large commercial and industrial applications. They are generally designed so that the exposed components will not tolerate freezing liquids and so must circulate water including glycol antifreeze in a closed loop separated from the potable domestic hot water using a coil to exchange heat in the cylinder.

Some systems are available using materials and pipework that can accept the freezing water - these can use potable water in a direct system avoiding the need for a domestic hot water heat exchanger. Since the water is being continuously refreshed there may be some water treatment needed to prevent an accumulation of scale or solids.

Temperature sensors compare the temperature in the collector and the store and if heat is available and needed the pump will switch on - typically when the bottom of the store is 6K - 10K cooler than the collector.

When heat is available but the store is already at design temperature there is a risk of stagnation - some systems will have an additional circuit to reject this heat. Stagnation occurs when the solar system doesn't have anywhere to put the heat that is being collected from the sun. In such conditions flat plate systems can reach temperatures of over 170°C and the evacuated tubes over 200°C. At these temperatures in indirect systems the propylene glycol becomes more acidic and the effectiveness of the glycol to act as an antifreeze will degrade. Closed systems have to be designed to accommodate some boiling (and expansion) using an expansion vessel or it will release fluid through the safety valve, which can be hazardous and cause a maintenance problem.

When it is expected that systems will be left unattended or unused during periods of high solar insolation appropriate provision for controlling the potentially damaging build up of heat must be included.

The check valve prevents reverse gravity circulation cooling the water store. This may otherwise occur when the pump is not operating and the collector contains colder, more dense fluid as would be typical at night.

Any pumps in such systems are required by UK Building Regs to consume fewer than 50W or less than 2% of the peak thermal power of collector, whichever is higher.

Drainback system

The drainback system uses a differential temperature pump controller to compare the temperature in the collector and the store - if heat is available and needed the pump will switch on. The pump draws liquid from the bottom of the drainback vessel and pumps it up through the top of the solar collectors to return by gravity to the drainback vessel. When the system is not in use or when there is no call for heat the pump switches off and the water drains from the collectors into the drainback vessel.

Draining back prevents unwanted reverse circulation and overheating and risks from stagnation but is practically limited to residential sized applications.

The claimed benefits of a drain back system are ....

• Gravity is fail-proof and maintenance free
• Water, (or a glycol mixture) may be used in the collector loop
• System is not damaged if the pump fails
• System cannot reverse thermosyphon at night
• Collector plates are likely to last longer than in a pressurized glycol system

However there are also disadvantages ....

• Collector(s) and all piping must be above and slope downwards towards the reservoir
• Larger piping and insulation must be used
• Large or relatively large pumping requirements especially if the design involves multistory buildings
• System and pump controls cost approx 10% of solar savings - PV powered pumps are not strong enough
• Components cost about 10-15% more than a glycol system
• Systems can be noisy - like a coffee percolator

A drainback system is more likely to be applied to a residential system.

Thermal stores and combi-systems

A thermal store will be needed to make solar heating practical. For small residential applications this frequently shares the function of the hot water cylinder. With indirect systems an second coil will be in the lower part of the cylinder to exchange the heat from the solar collectors into the potable domestic hot water.

Larger non-domestic applications will commonly use an additional storage vessel that will act as a preheated feed to the traditional hot water heating system.

In the case of a swimming pool installation the pool itself acts as the thermal store.

Purpose designed thermal stores are becoming increasingly common where the heat from solar panel is used to indirectly preheat water in the store that is then heated directly by the auxiliary heating device (eg a gas boiler). This water then provides the (non-potable) primary heating water and a coil is immersed in the cylinder to instantaneously provide heat for domestic hot water. This general layout is sketched to the left and a cutway of a thermal store is shown above. These systems are known as 'combisystems' (and more can be read about them on this EU project webpage).

The sizing of the store is as critical as the collectors in ensuring that the maximum benefit is gained from the system. As solar systems become more commonly used dedicated solar stores (separated from the potable hot water system) are being applied in the UK. The size of the associated solar storage tank will depend on the type and frequency of hws use in the building. Its role is to provide a store for solar heat that is not immediately used by the building. Information from Germany has established that a storage requires approximately 50 litres per sq m collector area for large scale installations with reasonable constant weekday use and 60-70 litres per sq m collector area for those where there are consumption free weekdays.

Issues of store sizing and design are discussed here

Factors to consider prior to installation

There is a comprehensive list of factors in the Energy Savings Trust's Solar water heating systems – guidance for professionals, conventional indirect models that include .....

• Occupant’s DHW usage pattern - whether predicted or measured this is a key piece of information
• Shading - The seasonal influence of trees and local buildings.
• Collector fixing slope and orientation - the limitations of the particular roof, wall or ground fixing
• Collector fixing area, structure and covering - the methods that may be used to affix the collector
• Access to collector location - ensuring safe access for installation and servicing
• DHW heat sources - to confirm that auxiliary heating is able to coexist and work in conjunction with solar thermal
• Pre-heat storage locations - ensure that there is opportunity to integrate appropriate pre-heat facilities (if required)
• Secondary water pressure and quality

Books to buy or borrow

B&ES's 'Solar Heating Design and Installation Guide' – Written with a whole bunch of other institutions this is probably the most complete guide to small scale solar thermal - a great read and provides essential knowledge for designers and installers

CIBSE Knowledge Series 15 – 'Capturing solar energy' - This is a more general introduction to the whole area of solar (of which solar thermal is just part) but it is in a very readable format and just in a 50 page document.

The B&ES document - SMG 2000 - Standard Maintenance Specification for Services in Buildings provides advice on the maintenance needs of such systems.

ASHRAE Systems Handbook 2008, Chapter 36:

ASHRAE Applications Handbook 2007 – Chapter 33: Solar Energy Use

An excellent resource for detailed insight into solar collectors and system design (from a German perspective but in English) - 'Solar Thermal Systems - Successful Planning and Construction' - Felix Peuser et al.

Web Sites and freely downloadable resources

Department of Energy & Climate Change - Solar Thermal Water Heating

An excellent set of web pages that go through solar thermal clearly and with practical detail is Pennsylvania's Solar Refit site

US Department of Energy resource - Active Solar Heating

Natural Resources Canada - This includes a great free downloadable spreadsheet tool, RETScreen to determine the benefits of using solar thermal in just about any global location

US based Whole Building Design Guide provides detailed descriptions on solar thermal technologies

See how much solar energy is likely to be hitting your site - Solar Electricity Handbook or to see where the sun is shining from the SunAngle calculator will help you out

Energy Savings Trust - A downloadable document describing systems as well as providing links to relevant government documents - Solar water heating systems – guidance for professionals, conventional indirect models (2006 edition)

ASHRAE Solar Heating Guidelines - this is a fantastic and wide ranging free downloadable set of books that not only covers the technology but also gives extensive feedback on the practical installation and operation issues

The Carbon Trust document Solar thermal technology - A guide to equipment eligible for Enhanced Capital Allowances provides some explanations of technologies as well as giving useful information on the relevant ECA scheme

A well written Canadian publication has much that is relevant to the solar water heating in the UK - Solar Water Heating - A Buyer’s Guide

Standards and Regulations

Microgeneration Installation Standard: MIS 3001 (Available at no cost) Requirements for Contractors Undertaking The Supply, Design, Installation, Set to Work Commissioning and Handover of Solar Heating Microgeneration Systems Issue 2.0 DECC 2009

BS5918:1989 Code of practice for solar heating systems for domestic hot water

BS EN 12975:2006 Thermal solar systems and components

Legionnaires' disease. The control of legionella bacteria in water systems.(Available at no cost) Approved Code of Practice and guidance L8 includes the requirements on hot water storage

See the extensive lists in the Energy Savings Trust Solar water heating systems – guidance for professionals, conventional indirect models (2006 edition) for extensive references (Available at no cost)

Although its labeled 'Domestic' the DCLG Domestic Building Services Compliance Guide 2010 Edition includes requirements for collector installations up to 20 sq m and solar heated water storage of less than 440 litres (but only indirect systems)