Jeff House considers the different types of solar collector currently on the market
More efficient building fabric and air tightness measures have resulted in space heating loads that are gradually reducing and, in many instances, the production of domestic hot water is the predominant plant loading.

This trend has led to the popularity of decentralised systems, using direct gasfired water heaters, to negate the typical seasonal inefficiency of large boiler plant operating during summer months to heat calorifiers. This arrangement lends itself particularly well to the integration of low carbon technologies, a prime example being the use of solar thermal energy to pre-heat the mains cold feed prior to entering into a water heater.

When considering the application of solar thermal energy, the role of the collector array is crucial to performance and longevity of system components. There is no standard solution, which means that each application will require a considered design that deals with the particular challenges posed at the location.

Glazed flat plate: Glazed flat plate collectors are the most familiar design having been on the market for many years. This collector type must be installed at an inclination between 20 and 45 degrees to ensure adequate performance. With this in mind they are suited to installation on a pitched roof or with specific angled mounting frames for flat roof applications. In this situation spacing of collector rows must be considered to avoid inter-row shading. As a guide, the ratio between stored water volume and the area of the glazed flat plate collector array should be 50 litres per square metre.

Direct flow evacuated tube: Evacuated tube collectors employ vacuum sealed collector tubes thereby vastly reducing thermal losses, making them a most efficient method of generating solar hot water, even in wet and windy conditions. The collector comprises a number of tubes inserted into a manifold. The solar transfer fluid flows through each tube, hence direct flow.

Collectors are designed to allow the rotation of each tube to meet the desired inclination, which is both flexible and space efficient and makes it possible to lay the collector flat on a flat roof or even vertically on a building façade.

Heat pipe: The heat pipe collector is similar in appearance to direct flow evacuated tubes, with the heat pipe collector using a dry pocket connection and condenser bulb principle. This dry pocket design enables individual tubes to be replaced without draining down the solar system, offering lifetime repair and maintenance savings.

Each pipe contains a small volume of evaporator fluid and in order to allow the heat pipe evaporation cycle to operate the collectors must be installed with a minimum inclination of 20 degrees.

In leading edge models, the heat pipes contain a device which, in the event of low hot water demand and continued collector heat gain, prevents the condensed content of the pipe from leaving the condenser bulb. This feature provides protection from overheating and associated unwelcome consequences. This design makes such heat pipes ideally suited to buildings with low summertime hot water demand or irregular demand patterns such as schools and sports facilities.

Solar thermal solutions can be based upon a fully filled pressurised arrangement or on a drain back system.

If a drain back system is being considered, several points should be taken into account. Typically, only employed on smaller schemes, the design of a drain back system is such that in periods of no hot water demand the content of the collector array drains back into a vessel thereby leaving the collectors full of air. The air content of the system can cause issues, especially the passing of air rich hot water though the system circulating pump.

Due to the hydraulic design of a drain back system it is typically only possible to employ glazed flat plate collectors, which may limit application flexibility. The water content of a drain back system must be accurately calculated and measured upon system filling; too much water will reduce system efficiency, too little will lead to poor circulation.

//The author is applications manager at Baxi Commercial Division //