Hot-water generation in a carbon conscious age
Published:  04 February, 2009
David Pepper

David Pepper considers the potential implications of underestimating the importance of selecting the right water-heating equipment to work with solar thermal if the potential of renewable energy is not to be largely negated.

Building-services engineers are now faced with endless clamouring for renewable technology from customers, who, in turn, are attempting to meet the needs and requests of the commercial end user.

For generating hot water, the most commonly used or considered technology appears to be solar thermal — with many good reasons. It is relatively easy to install in conjunction with a traditional system, and as hot water supply can be a significant proportion of a building’s energy use it is good sense to make use of what is, as many people regard it, a truly renewable energy source. It is also highly visible, which may be another reason why the UK has seen a substantial increase in the use of solar thermal hot water systems in the recent past. Companies and organisations may derive satisfaction from being ‘seen to be green’.

The use of solar systems in providing energy for building hot water supplies should continue to be encouraged, but it is important to remember that at best a solar installation will provide only 25 to 30% of annual hot water consumption for commercial properties such as a hotel, school or leisure centre.

The traditional and primary source of hot-water generation therefore needs equally as much, if not greater, consideration than the solar proportion. In the rush to be green, it doesn’t appear that this is always the case. When one considers the media coverage given to solar and other renewable techno­logies, building owners and oper­ators could be forgiven for think­ing that they alone are the answer to their future energy requirements and the quest to conserve fuel and reduce carbon emissions. Building-services and heating engineer professionals may have a difficult task convincing them that the selection of the traditional water heater is of equal, if not greater importance.

In an era where traditional boilers and water heating are not considered to be trendy, it is still the case that direct gas-fired water heaters offer an excellent way of providing a reliable, economical and efficient supply of hot water. Such products have been successfully installed in many applications for more than 30 years, and most manufacturers have models that utilise condensing technology, which further improves operating efficiency while also reducing carbon emissions. They can, of course, be installed as part of a package which includes solar thermal.

There is no doubt that there have been what could be described as ‘token’ renewable installations during the past couple of years. Many of the benefits of a solar-thermal system may be negated if it is installed in with an inefficient primary hot water generation source.

The usual principle of using solar energy to supply hot water is that the collector array will be connected via pipework to a pre-heat storage cylinder. Even in winter, temperatures of up to 30°C can often be achieved — with much higher temperatures in summer. Fuel is saved, as the primary water heating source burns less fuel when heating pre-heated water rather than cold mains water, which is typically at 10°C.

Solar Heater

In domestic situations, the use of twin-coil cylinders is commonplace. The lower coil is heated by solar gain, and the upper coil is the indirect heating system supplied via the heating boiler. Cost considerations and space restrictions are probably the main reasons why twin-coil cylinders are utilised, but this is not the best method capturing maximum solar gain, as the upper coil will in part reduce some of the hot water generated via the solar gain in the lower coil.

Sadly, such systems are occasionally used in commercial applications. In addition to reducing the amount of solar gain, the primary method of hot water generation is indirect and less efficient than a direct gas-fired system. The less-efficient primary water heater system will reduce and, possibly, eliminate the fuel-conserving, cost-saving and carbon-reducing benefits of the solar installation.

This may lead to disappointment for the end user when the system does not deliver all that was hoped for.

Many local authorities now require a proportion of future energy to be generated on new buildings via renewable sources before planning approval will be granted. How will it reflect on those authorities if the forecasted renewable contributions and carbon emission reductions are not achieved? The best solution is surely to pay attention to the primary hot-water source and not just the trendy renewable proportion.

The drive to reduce carbon emissions and save our precious resources is important, and renewable systems undoubtedly can go a long way to helping us achieve this. However, we are doing everyone a huge disservice by not giving due consideration to the primary water heating source which is still such an important part of the building energy usage.


David Pepper is managing director of Lochinvar Ltd.



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