Solar thermal systems convert solar radiation to heat energy. This energy is then transferred by a heat transfer medium to provide an energy input to a heating system.
Solar thermal systems in the UK are able to provide domestic water heating and, with larger systems, pool and space heating. Commercial applications with large solar arrays can provide energy for all types of heating systems. In nearly all cases, solar thermal systems are designed to be integrated with a conventional heating system as a back-up to accommodate the seasonal variation in solar energy.
Solar domestic hot water heating (SDHW) is the predominant form of solar thermal and has been fitted in UK homes since the 1970s. During 2004, over 4000 household systems were fitted and by 2006, there is expected to be 120,000 m2 of solar collectors across the UK. SDHW is increasingly specified alongside conventional fossil fuel heating systems to reduce emissions and conserve fossil fuel reserves. There are also benefits for energy security through the creation of a more diverse range of energy sources.
The performance of a solar thermal system is determined initially by the amount of solar energy converted into useful heat. This will initially depend on the intensity of the solar radiation reaching the solar collector, the area of the absorber surface of the solar collector, its efficiency, and its orientation and inclination.
A typical solar thermal system in the UK is used for pre-heating DHW and has a collector mounted on the roof of between 2 - 5 m2 area. This will provide between 40 and 50 per cent of annual hot water requirements assuming a target storage temperature of 60ºC (this fraction can be higher if the target temperature is lower).
Some solar heat can be used for space heating but since the coldest days coincide with the least solar energy, only a very marginal contribution of less than 10 % total annual space heating load can be expected.
Solar heat, where used for swimming pools, can maintain target temperatures in the summer but can not be expected to heat pools from cold or at the extremes of the swimming season.
Most of the UK has an annual irradiation (amount of solar radiation) between 900 -1100 kWh/m2 of ground area. The extreme north and south have levels of irradiation outside this range. Over two thirds of this energy occurs during the summer months. The efficiency of the solar collector indicates the proportion of the solar energy reaching the absorber surface that is converted into thermal energy. Modern glazed solar collectors are capable of achieving efficiency levels of around 70% whereas the complete system with storage would rarely exceed 35% efficiency. The best performance from a solar thermal collector is achieved when it faces due south and is inclined at the equivalent angle of latitude for the location. In practice, inclinations between 30° and 60° and orientations between SE and SW produce good results. Lower inclinations produce a better summer performance as the sun is higher in the sky and steeper inclinations produce an improved winter performance. East and west facing collectors receive over 70% of the energy of a South facing collector however there are further losses from reflection off the collector.
Solar energy is a renewable, zero carbon energy source and can be used to displace non-renewable or carbon-emitting fuels. For an average household, this can mean savings of 0.4.to 1.1 tonnes CO2 per annum (0.1 to 0.3 tonnes of carbon) where electricity is displaced. The figure is less for gas. This technology is not usually regarded as an alternative to other energy efficiency measures: it is usual to first reduce fuel use by improvements elsewhere in the dwelling before applying SDHW.
A correctly designed and fitted Solar DHW installation can be expected to produce between 350-400 kWh per year – and a peak power of 700 Watts – per m2 of net collector absorber area. The solar gains vary throughout the year and it is normal to design the system to use peak summer conditions to provide around 90% of the DHW requirements. Greater than this can result in overheating and lower system efficiencies although certain system designs can adequately cope with the extremes of conditions without releasing any fluids or steam to atmosphere.
Normally solar heat pre-heats a lower indirect coil in a twin coil DHW store. The top of the store is conventionally heated and maintained at 60°C. to reliably sterilise bacteria and provide end-user comfort. A separate pre-heat store can also be used providing the pre-heat store does not by-pass untreated water into the DHW distribution. It is essential to install enough storage for the solar heat to achieve reasonable performance and hold heat over from one day to the next. Recommended extra solar storage volumes are at least equal to 80% of the daily DHW usage.
Direct solar primary systems for DHW present significant difficulties in achieving reliability and durability due to the problems of limescale, sludge and scalding temperatures directly entering the DHW distribution. There are also problems of freezing blockages of safety vents and increased bacterial risks. Indirect solar systems with antifreeze are the predominant system form although a large heat exchanger area is required located at the coldest part of the secondary circuit. Direct solar systems for swimming pools however are possible due to the use anti-bacterial chemicals in the pool water.
Solar primary designs can be quite varied but predominately use fully-filled, indirect sealed circuits using special high temperature, non-toxic antifreeze. A partly-filled, indirect sealed circuit can also be used by allowing the fluid to drainback from the collector once the pump switches off. In all cases, pumped circulation is controlled using temperature sensors for best efficiency and safety. Many components in solar primary systems must be capable of containing very high temperatures above 150°C. and are sourced from specialised suppliers. Solar space heating applications must be designed to use low emitter temperatures (sub-40°C.) if reasonable performance is to be expected.
| Absorber | Component of collector which absorbs solar radiation |
| Aperture area | Opening through which unconcentrated solar radiation is admitted |
| Auxiliary heat source | Heat other than solar to supplement the output of the solar energy system |
| Closed loop | A closed circuit where the fluid is recirculated |
| Cold feed | Start of the incoming water supply to a system |
| Collector | Absorbs solar radiation and transfers thermal energy to fluid passing through it |
| Collector efficiency | Ratio of energy removed by transfer fluid to incident solar radiation |
| Collector loop | Circuit that includes the collector, pump, pipes and exchanger for transferring heat |
| Combi boilers | Boilers that provide DHW and space heating |
| Combi stores | Combination of DHW and space heating storage |
| Combined storage | A single vessel that includes heat sources other than solar |
| Commissioning engineer | Responsible person for declaring a fitted system is fit for purpose and safe |
| Conduction | Movement of heat in solid |
| Convection | Movement of heat in a fluid |
| DHW | Domestic hot water consumed in the dwelling |
| Differential temperature controller | (DTC) Compares two temperatures which may vary independently |
| Direct system | Where the heated water that is to be consumed passes through the collector |
| Downstream | Direction with the movement of water |
| Drainback | A system that automatically fills and refills the primary fluid into the collector |
| Draindown | Where a direct system drains the primary contents to waste |
| DTC | Differential Thermostat Control – switches on equipment by comparing two varying temperatures |
| ELV (Extra Low Voltage) | Electricity that is sub 50 V AC or ripple-free120 V DC |
| Evacuated collector | Where the space between the absorber and cover is evacuated |
| Flat plate collector | Where the absorber surface is essentially planar |
| Flow | The part of a circuit which is hot from the heat source |
| Fully –Filled | A system normally above atmospheric pressure with all air removed |
| Gross collector area | Maximum projected area of complete collector excluding any integral mounting brackets and pipework |
| Indirect system | Where a heat transfer fluid other than the consumed water passes through the collector |
| Installer | Assembles and fits components into a system. Equivalent to a manufacturer where the assembly is customised |
| Interlock | A means of control wiring which prevents a device from operating unnecessarily |
| Irradiation | Electromagnetic energy incident per unit area |
| Load | Amount of energy or power required from a system |
| Manufacturer | Produces components and assembly kits |
| Net absorber area | Maximum projected area of an area reached by solar radiation |
| Open vented system | Where there is contact by the heat transfer fluid to the atmosphere via a vent pipe and cistern |
| Parasitic losses | Energy consumed by pumps, fans and controls during operation |
| Peak power | The maximum rate of energy flow |
| Pre-heat vessel | A contained body of fluid which accepts heat prior to heating by an auxiliary heat source |
| Return | The part of the circuit that is cool to the heat source |
| SDHW | Solar Domestic Hot Water |
| Sealed system | (Closed and unvented). Where the system is sealed from the atmosphere |
| Selective | A surface whose optical properties are wave-length dependent |
| Solar fraction | Energy supplied by the solar system divided by the total system load |
| Stagnation | Status of a collector or system when no heat being removed by the transfer fluid |
| Standing losses | Energy consumed by equipment continuously irrespective of operation |
| Stratification | Natural layering of fluids of different densities |
| Switching point | Where a control creates an event based on a varying parameter |
| Thermosyphon system | Utilises only density changes to achieve circulation |
| TS | Previously known as ENV – Pre-standard |
| Upstream | Direction against the movement of water |
| Useful energy | The solar energy, after losses, that reaches the appliances |
| Utilisation factor | Factor of time delay and the quantity of surplus |
Water Supply (Water Fittings) Regulations 1999 www.wras.org.uk Applies to all fittings in contact with the utilities' supply of water. Responsibility of compliance lies with the householder and installer, although certain approved contractors can issue certificates of compliance which provide immunity from prosecution to the householder. All solar water heating work is normally notifiable to the water utility and permission must be awaited for ten days, except when installed in extensions or if alterations of water systems in existing houses. Water (over 15 litres) must not be stored in domestic premises above 100ºC, while avoidance of water contamination and undue consumption is mandatory. There are requirements for protection from freezing, use of certified fittings, methods of pipe fixing, safety devices on water stores plus extra requirements for non-metallic fittings. Note that different rules apply in Scotland and Northern Ireland.
The Pressure Equipment Regulations (PED) 1999 www.eurodyn.com Applies where the equipment could hold pressures in excess of 0.5 bar above atmospheric under any foreseeable circumstances. Applies to the manufacturer (where placed on the market as a functional assembly) or else the commissioning engineer. Requires equipment to be safe and either to a sound engineering practice or CE-marked to a higher safety requirement. Where an attached system can overheat according to BS EN 12976, the collector is categorised as a steam generator.
The Building Regulations (L1) 2001 www.odpm.gov.uk Applies to the conservation of heat and power in all new build, extensions and existing dwellings. Particularly affects replacement of hot water storage vessels and accompanying controls. All solar water heating work normally involving additional hot water storage is therefore notifiable to a building control officer unless a competent person can self-certify and issue a commissioning certificate. Requires minimum levels for insulation, time and temperature controls for hot water storage. Also, water storage vessels to have a minimum performance. A commissioning certificate is required to be left. Note that different rules apply in Scotland and Northern Ireland.
The Building Regulations (G3) 2000 www.odpm.gov.uk Applies to hygiene in buildings, particularly unvented hot water storage. Requires only competent operatives should install such certified equipment along with associated safety equipment.
The Building Regulations (P) 2005 www.odpm.gov.uk Applies to any electrical work that involves adding new circuits to dwellings and any work in special locations such as kitchens, bathrooms, swimming pools and photovoltaic power. A competent person must do the work and notify to a building control officer unless the person can self-certify. There are certain exceptions; it is permitted to add or replace outlets to existing circuits in non-special locations. All work should comply with, and further details may be found, in BS 7671.
Control of Substances Hazardous to Health Regulations (COSHH) 1994 www.hse.gov.uk Require employers to assess the risks from hazardous substances and take appropriate precautions. This requires precautions to reduce legionella poisoning.
| BS 7431:1991 | Method for assessing solar water heaters. Elastomeric materials for absorbers, connecting pipes and fittings |
| BS 6785:1986 | Code of practice for solar heating systems for swimming pools |
| TS 12977-3:2001 | Performance characterisation of stores for solar heating systems |
| TS 12977-2:2001 | Thermal solar systems and components. Custom built systems. Test methods |
| TS 12977-1:2001 | Thermal solar systems and components. Custom built systems. General requirements |
| BS EN ISO 9488:2000 | Solar energy. Vocabulary |
| BS EN 12976-2:2001 | Thermal solar systems and components. Factory made systems. Test methods |
| BS EN 12976-1:2001 | Thermal solar systems and components. Factory made systems. General requirements |
| BS EN 12975-2:2001 | Thermal solar systems and components. Solar collectors. Test methods |
| BS EN 12975-1:2000 | Thermal solar systems and components. Solar collectors. General requirements |
| BS 8000 | Workmanship on building sites |
| BS 7206 | Specification for unvented hot water storage units and packages |
| BS 7671 | Requirements for electrical installations |
| BS 1566 | Copper indirect cylinders for domestic purposes |
| BS 4814 | Specifications for expansion vessels using an internal diagraph for sealed hot water heating systems |
| BS 7074 | Application, selection and installation of expansion vessels and ancillary equipment for sealed water systems |
| BS 5422 | Methods of specifying thermal insulation materials on pipes, ductwork and equipment in the temperature range of –40ºC to 700ºC. |
| BS 5449, BS EN 12831 BS EN 12828 | Specification of forced circulation hot water central heating systems for domestic premises |
| BS 6701 | Telecommunications equipment and telecommunications cabling |
| BS 5970 | Code of practice for thermal insulation of pipes and equipment |
| BS 6700 | Specification and design, installation, testing and maintenance of services supplying water for domestic uses within buildings and their curtilages |
| BS 6920 | Suitability of non-metallic products for use in contact with water intended for human consumption with regard to their effect on the quality of the water |
No formal competence scheme for solar primary systems but more conventional schemes are available for hot water storage and electrical.
Solar Trade Association
International Solar Energy Society – UK section
BPEC solar water heating course and assessment EST Best Practice for solar water heating installers
Tapping the sun Centre of Alternative Technology
CIBSE Domestic Building Services Panel - Design Guide for solar water heating