Dealing with overheating issues will become increasingly urgent as the solar heating market grows this year, says Steve Addis
2013 promises to be a significant year in the development of the solar thermal market. The launch of the Green Deal means consumers and commercial building owners can now borrow the cost of an installation upfront and use the energy savings to repay the loan.

The Renewable Heat Incentive (RHI) is now well established in the commercial sector and is due to be rolled out to domestic users later this year. Take up has been sluggish, but there were signs towards the end of 2012 that the commercial RHI was starting to overcome some of its teething problems. The scheme operator reported an 80 per cent jump in approvals for the final quarter of the year and, although biomass continued to account for the lion's share, solar thermal now accounts for nearly 5 per cent of the total installed capacity generated by the scheme - and that proportion is growing.

Raising concerns

However, as demand increases, it becomes increasingly important that users are not left disappointed. Potential problems with incorrect sizing and commissioning of systems have already raised concerns as more suppliers have become involved in the market - and one particularly serious consequence is overheating.

Overheating will be a problem if there is insufficient hot water demand and the system has, therefore, been wrongly designed and specified. This will lead to significant pressure build up in the solar collectors that can develop into a serious safety issue and have a detrimental impact on collector efficiency.

Without regular demand for hot water, the temperature in a solar thermal system will rapidly increase and has been known to reach over 280 deg C. At 200 deg C the glycol turns into a tarry sludge that will block the system and, in many cases, require a full system replacement with all the associated cost that entails.

A number of system suppliers recommend the 'drain back method' as a possible solution. This involves the collectors being emptied of their fluid when the temperature reaches a certain point. However, drain back can only be used with less efficient flat plate collectors, and another drawback is that the system will not refill until it has cooled down considerably. This means the collectors are out of action for many hours and are not, therefore, providing renewable hot water.

This can have a significant impact on its effectiveness and considerably lengthens payback over the lifetime of an installation.

However, there is an alternative: 'active cooling'. This is a method of using fan convectors to keep the glycol in the system cool and maintain collectors at their optimal temperature without the need for the fluid to be drained. With active cooling, the systems will continue capturing energy from the sun continuously, but without the risk of overheating if hot water demand drops for any reason.

All solar systems also have a built-in overheat function designed to prevent the system circulating steam as the flow temperature rises. This function is set at 120 deg C, but can be higher on high pressure systems. It is set at 80 deg C on drain back systems.

Once the solar vessel has reached its set point, the collectors will quickly reach this overheat setting and so the system will shut down. If this occurs by midday, as often happens during the summer months, the system will not be able to provide any more input to the solar vessel even if it is fully discharged and ready to accept more heat. The system will only start up again in the evening once the collectors have cooled down so wasting many hours of solar opportunity.

With active cooling, this cannot happen as the system will always keep the glycol temperature below the overheat setting, thus increasing the solar potential for the system as well as providing protection from stagnation problems.

Active cooling can meet both safety and performance aspirations and will, therefore, become a far more common component of solar thermal systems in the UK. In fact, it would be foolhardy in the extreme to install a system without some method of cooling unless there is an absolute guarantee of sufficient hot water demand in the building.

Improving energy performance

By keeping the systems working at their most effective temperature, active cooling improves the energy performance of the whole installation. Also, by avoiding repeated emptying and refilling, this method will lengthen the operating life of the collectors through reduced wear and tear.

The active approach has the added benefit of improved installation flexibility. It is usually recommended that a solar system has 70 litres of pre-heat storage capacity per sq m of collector. This allows the collectors to be maintained at a safe temperature if there is not enough hot water demand during the day to draw heat away from the solar array.

However, plant room space restrictions often make it difficult to provide this amount of pre-heat storage. With active collector cooling, the pre-heat storage capacity can be reduced, which will also provide a saving on capital costs.

Many system designers have underestimated the risk of overheating in the UK as they did not think solar arrays would reach particularly high temperatures in our less than tropical climate. It is now a very serious consideration for heating engineers and active cooling is the best way to reassure them and their clients that a solar thermal system will continue to perform efficiently and safely long into the future.

// The author is sales manager for Renewable Products at Lochinvar //