The drive to include renewable energy in projects has fuelled interest in the use of solar thermal energy for heating and hot-water services, writes Ian Dagley.
In the UK, however, solar radiation levels fluctuate widely and can range from less than 100Wh/m2 of collection area on a cloudy day to more than 1kWh/m2 on a sunny day.

Consequently, any solar heating system needs to be backed up by 100% auxiliary heating. This could be from other low-carbon heat sources such as biomass boilers or heat pumps, which will enhance the renewable element of the project. Or there may be other circumstances where it is more advantageous to combine solar heating with condensing boilers.

When solar heating is used for domestic hot water (DHW) the system has to be flexible and able to respond to rapid changes in conditions. On sunny days, there will often be sufficient solar energy to heat all of the water to the required temperature. And, when less solar energy is available, it may be used to pre-heat mains cold water before it is brought up to temperature by another heat source.

To a great extent, the optimum design will depend on the detail of the project. As a rule of thumb, the significant shifts in demand that are inherent in larger projects mean it is usually more cost effective to install solar equipment to pre-heat the DHW, and then top it up to the required temperature.

Not only does this keep the capital cost down, but it also reduces the frequency that the solar circuit goes into stagnation (periods when no energy is being removed from the collectors by the solar fluid) during the summer months. This in turn increases the system efficiency and specific energy produced by the collector loop.

It is also important to note that any stored volumes of pre-heated potable water should be kept to a minimum, as these may have to be regularly pasteurised as part of the anti-legionella regime.

Pasteurising large volumes of potable water is not only costly, it also has a dramatic effect on the solar yields and subsequent solar fractions attainable. In these circumstances, it is beneficial to store the solar energy in a thermal storage vessel and pre-heat the cold feed water through a suitable heat exchanger. This also has the benefit of allowing the solar energy to be safely stored at higher temperatures without the risk of scalding.

Piping a number of non-potable thermal storage vessels in series and using diverting valves to circulate the water in the vessels will enhance stratification within the vessels. This will promote solar gain and also allow biomass boilers to be fully integrated within the heating/DHW scheme and support the solar-powered system.

During summer months when solar energy is in abundance, another way to lessen the periods of stagnation is to increase the angle of inclination on the collectors. While this reduces the efficiency of the collectors in the summer, it increases the efficiency in the winter when the sun is lower in the sky, thus having a load levelling effect. Consequently, this is an ideal solution for low-grade heating such as underfloor systems and can also contribute to DHW demand in the summer.

Solar heating is also particularly good for heating swimming pools because the pool acts as a large heat sink (no need to buy thermal storage vessels), and the relatively low operating temperatures (typically 26-30°C) are easily achieved. These low temperatures result in high collector efficiencies and solar fractions. Additionally, most indoor pools have a large roof area, which is ideal for the solar collectors.

So, when considering renewables such as solar, the key is to decide which will work well together in a controllable fashion and which overall solution gives the best results for that project.

//Ian Dagley is sales and marketing director at Hoval//