Solar heating
Sunny side up Making the most of solar
Getting the best performance from solar heating is dependent on the design of the system and the way it’s controlled, says Lawrence Sheppard
The angle of inclination of flat plate collectors is important and this will vary from one project to another
THE BENEFITS of using a mix of conventional, renewable and other low carbon heat sources together are well established and this practice is becoming increas- ingly common. When this princi- ple is applied, however, it’s impor- tant to design the system so that minimum use is made of the fossil fuel heat sources.
This can be particularly chal- lenging with heating systems that include solar thermal, because of the variation in the UK’s solar irra- diation (from 100kWh/m2 cloudy day to 1,000kWh/m2
on a on a
sunny day). This variation means there always have to be back-up heat sources, which are frequently condensing boilers or, less often, heat pumps.
In some systems the renewable elements, such as solar thermal, are used to ‘top up’ the other heat sources. However, this imposes limitations on the renewable ele- ment and fails to gain maximum carbon and energy savings.
Pre-heating DHW When solar thermal heat sources are included in the system, an alter- native solution is to pre-heat the domestic hot water (DHW) using the solar energy, supplementing as required with other heat sources. In this arrangement a mild steel vessel is also incorporated into the
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system, solely for storing the water heated by the solar panels and pre-heating the DHW via a plate heat exchanger. A clear benefit of this arrangement is that it isolates the solar-heated water from the potable DHW supply so that there is no risk of Legionella. This means the water in the mild steel vessel does not require daily pasteurisa- tion, thereby reducing energy con- sumption. Other anti-Legionella measures for stored DHW will, of course, need to continue as usual. If high storage volumes are required, a number of non-potable thermal storage vessels can be piped in series, using diverting valves to circulate the water and enhance stratification. This will promote solar gain and also allow other renewable heat sources, such as biomass boilers or heat pumps, to be fully integrated with- in the scheme and support the solar thermal system.
A further benefit of this arrange- ment is that it makes use of the solar irradiation that is available even on cloudy days. Even if the hot water in the mild steel vessel only reaches 35°C this will help to reduce the energy consumption of the other heat sources. As long as the stored water is at least 6°C above the temperature of the incoming mains water, there is an opportunity to achieve a worth-
while heat transfer. On days when solar energy is plentiful, the water can be allowed to reach very high temperatures – up to 90°C – before the system pumps are switched off, without any risk of scalding.
Using the solar energy to pre- heat DHW also makes it very straightforward to retrofit solar panels to an existing heating sys- tem, thereby enhancing the busi- ness case for investing in this renewable heat source.
Types, numbers and angles In addition to the control aspects of such systems, the number and orientation of the solar panels – and the type of collector – can also make a big difference to the overall benefits delivered by solar thermal technology.
Historically, the choice of solar panel involved a trade-off between cost and efficiency as early flat plate collectors were lower cost than evacuated tube collectors but also less efficient. This situation has changed with advances in flat plate collector technology, which are now able to deliver high efficiencies whilst remaining competitive on price. Again, this will influence any return on investment calculations. In terms of the number of col- lectors included in the system, it is
generally better to have too few than too many, as the latter may generate more heat than can be stored. This may lead to stagnation, so that the system has to be turned off until the water cools, during which time any available solar irra- diation is not being utilised. The angle of inclination is also important and this will vary from one project to another. For most projects an angle of 45° facing south will provide the most effi- cient year-round performance. However, there may be cases where higher input is required at a particular time of the year. A lower angle from the horizontal, say 30 degrees, will provide higher effi- ciencies in the summer; while a higher angle from the horizontal, say 60 degrees, will improve effi- ciency in winter.
This latter point illustrates the importance of designing to meet the specific requirements of each project, in relation to the structure and location of the building as well as any engineering considerations. For that reason it makes sense to work with suppliers that have experience of designing and con- trolling mixed heat source systems that incorporate solar thermal.
• Lawrence Sheppard is the senior technical engineer for Hoval’s heating division
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