BSEE SOLAR ENERGY
Solar thermal technology can deliver signicant energy benets, as long as the design is right. Lawrence Sheppard, Senior Technical Engineer for Hoval’s Heang Division, explains the benets of using solar energy to preheat domesc hot water.
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GETTING UPFRONT with solar thermal technology
Solar thermal systems illustrate this point very well. In the UK solar irradiation can range from less than 100Wh/m2 of collection area on a cloudy day to over 1000Wh/m2 on a sunny day. Consequently, solar thermal systems require back-up from other heat sources to account for this variation.
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These back-up sources are very often in the form of heat pumps to retain a complete renewable theme or alternatively, a gas fired condensing or standard boiler – though other heat sources may also be included in the mix. In these circumstances it is the control strategy that will determine the extent of energy and carbon saving benefits that are achieved. For example, many such integrated systems have been designed to use renewables as a supplementary ‘top up’ source to the traditional heating technologies. Inevitably, this imposes limitations on the use of the renewable element and, in most cases, is not the optimum configuration.
A better solution when solar thermal heat sources are included in the system is to use the solar energy as the pre- heat source for domestic hot water (DHW) and then supplement with heat from other sources. In such a pre-heat configuration, the system is also fitted with a mild steel vessel that is used solely to store the water heated by the solar panels. This water is then used to pre- heat the DHW via a plate heat exchanger, with heat from other sources being used to achieve the required DHW temperature, if required.
In situations where high storage volumes are required, a number of non-potable thermal storage vessels can be piped in series. Stratification within the vessels can then be enhanced by using diverting valves to circulate the water. This will promote solar gain and also allow other renewable heat sources, such as biomass boilers or heat pumps, to be fully integrated within the scheme and support the solar thermal system.
This arrangement brings a number of benefits. For example, as the solar-heated water is isolated from the potable DHW supply there is no Legionella risk, so the water in the mild steel vessel does not have to be subjected to daily pasteurisation. Clearly the water stored in the DHW vessel(s) will still be subject to anti-Legionella measures but this would be happening anyway.
uWhen it comes to solar thermal installaons there are no o the shelf soluons, says Hoval.
Such an arrangement also means that even on days when solar irradiation levels are low and, perhaps, the hot water in the vessel only reaches 35°C, this heat energy can be utilised to reduce the use of more expensive, less environmentally- friendly heat sources. Essentially, as long as the stored water
s renewable and other low carbon heat sources are increasingly included in heating and hot water systems, the design and control of the system is vital in minimising the use of fossil fuel heat sources in the system.
is at least 6°C above the temperature of the incoming mains water, there is an opportunity to achieve a worthwhile heat transfer. On days when solar energy is plentiful, the temperature of 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. A further benefit of this pre-heat arrangement is that it makes it very straightforward to retrofit solar panels to an existing heating system, thereby increasing the financial viability of projects that seek to extend the use of this renewable heat source. Given that the UK’s existing building stock makes such a large contribution to energy consumption and carbon emissions, this is an important consideration.
Designing for opmum performance
Clearly when designing such systems it is important to take account of the types of solar panels, their number and their orientation.
In the past, when choosing the type of solar panel for a project, there has been a trade-off between cost and efficiency. This was because early designs of flat plate collectors were less efficient than evacuated tube collectors but the cost was significantly less.
More recently, flat plate technology has advanced to deliver high efficiencies while retaining competitive prices. As a result of this, return on investment calculations have changed considerably.
‘ In the past,
when choosing the type of solar panel for a project, there has been a trade o between cost and eciency. This was because early designs of at plate collectors were less ecient than evacuated tube collectors but the cost was signicantly less.
Whilst specifying exactly the right number of solar panels is clearly ideal, in general it is better to have too few than too many. Too many panels in the system may generate more heat than can be stored in the vessel(s) and this will lead to system stagnation. If stagnation occurs the system will have to be turned off until the water cools and during this time the available solar irradiation is not being collected. The angle of inclination of the solar panels is also important and needs to take account of the specific requirements of each project. In most situations an angle of 45 degrees, facing south, will give the most efficient performance all year round. A lower angle from the horizontal, say 30 degrees, will provide higher efficiencies in the summer, while a higher angle, say 60 degrees, from the horizontal will improve efficiency in winter.
The key point here is that each project will have its own particular requirements, relating to the structure and location of the building as well as engineering considerations. Because of this, there are no ‘off-the-shelf’ solutions and the optimum configuration and control strategy will only be achieved through tailored designs. To that end, it makes sense to work with suppliers that can add value to a project through their specialist expertise.
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www.hoval.co.uk
28 BUILDING SERVICES & ENVIRONMENTAL ENGINEER MARCH 2016
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