Building pathology: Solar heating

Only now are solar heating systems beginning to approach the mature stage of product development. Failure rates are generally low, but they do exist. Problems can be categorised into three general groups: system issues such as deterioration, failure of components or faulty system design; integration issues between the solar heating system, building and supplementary heating systems; and installation and maintenance issues.

System placement issues

Unsuitable placement of solar collectors will lead to reduced system efficiency. Although there are optimum orientation and tilt angles for any given location, in practice, deviations of 45° from due south and 20° off the latitude for domestic hot water systems are acceptable. Collectors for combi-systems where the solar heating system provides both domestic hot water and space heating energy should be placed closer to the optimum tilt and orientation. Increasing the size of the solar collector may compensate for limited placement options.

Solar collector issues

At the heart of any solar heating system are solar collectors. Usually mounted on roofs, they have to withstand extreme weather conditions and exposure.

Failures may result from a variety of effects:

• Moisture may penetrate glazed collectors, whether as driving rain, capillarity or condensation effects
• Impact damage may result from human carelessness by, for example, dropping a tool during maintenance or from natural phenomena such as hailstones
• High wind speeds may induce high pressure fluctuations that can cause fatigue-related failure, especially if fixings are not sound. In some climates, snow loads and snow accumulation may cause problems
• Solar collectors can get very hot indeed, even up to 200ºC, with consequences such as thermal movement, high pressure within the system, and acceleration of the degradation processes. Thermal shock may occur in summer, for example, if a spell of clear sun is followed by a thunderstorm. Failures may manifest in distortion, swelling or rupture; leakage or loss of vacuum in evacuated tube collectors; cracking of glass; or, if a plastic cover is used, collapse or distortion.

Overheating may be managed by drainback technology, good emptying performance, inclusion of cooling devices in the collector loop or even incorporating a “discharge loop” to dissipate excess heat.

Expansion and contraction of pipework should be allowed for by expansion loops and flexible or movement-tolerant fixings.

Leaks at joins may be minimised by using sealant paste or jointing systems which are tolerant of high temperatures and pressures.

Overheating may be a greater risk with combi-systems as they generally have a larger solar collector area and so are over-dimensioned during summer

• Freeze thaw cycling can result in leakage, breakage or distortion. The risk of freeze thaw damage may be managed in several ways: using an anti-freeze heat transfer medium, designing a freeze-resistant solar collector or incorporating a drain-down protection mechanism which is activated when temperature approaches freezing point

• Corrosion of metals may be an issue in areas with high atmospheric concentrations of sulphates or salts associated with industrial and coastal zones

• Heat transfer liquid may degrade, especially where solar collectors are allowed to get very hot

• Insulation should be able to stand high temperatures. If exposed, it may need protection from birds.

Manufacturing standards

The European standard for solar collector durability, BS EN 12975-1, will give assurance of performance. However, not all performance criteria are addressed by this standard. Each part of the collector has its own set of failure modes.

For example, the long-term durability of absorber surfaces is a key performance requirement to achieve design heat absorption throughout the life of the collector. High temperatures, moisture, high humidity and pollution such as sulphur dioxide may degrade the absorber surface. ISO/DIS 12592 provides tests to assess absorber performance.

Other component parts and performance issues for which there are standards include:

• Long-term durability of transparent polymer covers – ISO/DIS 9495
• Elastomeric materials for absorbers connecting pipes and fittings – ISO 9808
• Preformed rubber seals and sealing compounds – ISO 9553
• Internal corrosion – ISO/TR 10217.

Installation and use issues

Almost any part of the solar heating system can be installed incorrectly. Installation issues can be minimised by using competent installers, specifying pre–assembled, factory-produced solar heating systems and commissioning prior to use.

Annual inspections and servicing will help to minimise failures once the system is installed and running.