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On the face of it, solar hot water systems are an attractive proposition, as they can provide up to 70% of the energy needed to heat water in the average home. In the context of energy conservation measures, solar hot water systems have a relatively long payback period so are only financially viable in the long term.

The issue for such systems in the UK is that they are most needed in the winter, when least solar energy is available. Ways of getting round this include increasing hot water storage capacity or increasing the area of solar collectors.

Despite these drawbacks, the legislative and economic environment is swinging the balance in favour of solar energy systems. Fuel prices are increasing and new Part L, which comes into force next April, will encourage the use of low and zero carbon energy supply systems, since they can make a substantial contribution to meeting the target carbon dioxide emission rates.

Specification options

Solar hot water systems are commonly categorised by solar collector type. The two most common are flat plate and evacuated tube solar collectors. BS EN 12976 is the British Standard for solar collectors and specifies durability, reliability and safety requirements for factory-made solar systems. Assurance is given for resistance to freezing, internal corrosion, thermal performance and pressure. Ancillary components, such as heat exchangers, pipework, circulation pumps, hot water storage, controls, and valves, are defined by reference to other European standards.

Flat plate solar collectors

These are flat panels or planks that can either be integrated into an existing roof or installed as a substitute for traditional roof coverings. Panels are assemblies ranging in size from 0.7 m2 to 8 m2 and are generally square or rectangular and set in an insulated box or frame. Inside is a combined aluminium and copper or all-copper absorber plate. The panel is covered by single or double glazing, typically toughened glass, and sometimes with enhanced surface coatings such as low reflection glass. Polycarbonate plastic is an alternative.

Service lives between 10 and 25 years can be expected. The glazing system may need replacing during the service life. Panels cost in the range of £200-£600/m2 for metal and glass assemblies. Plastic panels are generally cheaper.

Planks can be manufactured in lengths to span the roof and are typically of powder-coated aluminium hollow sections connected with hoses. Aluminium planks should give more than 60 years performance, provided the system is designed to avoid internal corrosion. The powder coating will need replacing at 20- to 30-year intervals, depending on the exposure level and rate of colour-fading to maintain absorption. (See cost table for whole life costs.)

Evacuated tube solar collectors

These are more efficient than flat plate collectors, as the water is circulated in an insulating vacuum, but they are also more expensive. Often installed on existing roofs as they are lighter, they comprise banks of glass tubes connected into a manifold. Tubes can be replaced in the event of the vacuum being lost. Service lives of between 20 and 35 years are quoted, and costs are in the range of £500 to £1200/m2.

Design and operational issues:

• Early integration with the overall building design, especially in terms of building orientation, aspect, insulation and backup heating system, is essential to maximise the benefits of solar hot water systems.
• Confirm that the panels or planks are designed to withstand wind and dead loads.
• Where used for potable water, ensure the requirements of the water bye-laws are met.
• Regularly check levels of protective inhibitors where used.
• Systems require inspection, maintenance and replacement of ancillary components during their service life.
• Use underfloor heating system in conjunction with solar hot water systems to maximise system efficiency.
• Solar hot water systems are ideal for outdoor swimming pools, as pools require large quantities of low temperature heat during summer when most solar radiation is available.
• The capital costs of plank or panel type of solar collector can be offset against the cost of traditional roof covering.
• Designing solar hot water systems as mini-district heating systems can reduce unit costs.
• Enhanced capital allowances or Clear Skies renewable energy grants may be available.
• A control panel with diagnostic systems will warn of system problems.