Solar industry on overheating alert
Mis-application of solar-thermal technology can result in potentially catastrophic overheating. Steve Addis of Lochinvar urges the industry to take an ‘active’ approach to tackling this threat.
Demand for solar thermal systems continued to grow in the face of the economic slowdown, and with the recovery the market will step up again. It is, therefore, 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 demand for hot water 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°C. At 200°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 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. 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.
Over the lifetime of an installation this can have a significant impact on its effectiveness and considerably lengthens payback. However, there is an alternative — active cooling.
Active cooling uses 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°C, but can be higher on high-pressure systems. It is set at 80°C on drain-back systems. Once the solar vessel has reached its set point, the collectors will quickly reach this overheat setting, 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.
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 l of pre-heat storage capacity per square metre 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. The investment in active cooling will, therefore, pay for itself many times over throughout the lifetime of the equipment.
Overheating is now a very serious consideration for consulting engineers and should be flagged up early in the design process. Active cooling is an excellent way of ensuring the solar thermal system continues to perform efficiently and effectively over a long period of time, so helping the industry meet its promises for reducing running costs and carbon emissions.
Steve Addis is sales manager for renewable products at Lochinvar.