INNOVATIONS IN SOLAR WATER HEATING

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1 INNOVATIONS IN SOLAR WATER HEATING I m writing this article in the bleak midwinter, when sunshine is feeble and scarce. The contribution of solar to water heating in midwinter is small about 15% of that contributed in midsummer (though a higher percentage is claimed for vacuum tube collectors). But of course, selfbuild is about the long term, and solar water heating requires consideration at any time. In carbon-saving terms, solar water heating is very effective. And in money-saving terms, as the price of energy increases, as it surely will, solar water heating will become increasingly cost effective. We saw last month that for a new build the preferred method is to integrate one or more flat plate collectors into the roof covering. The technology of flat plate collectors is fairly mature, but still evolving. Most absorbers now have selective surfaces (originally developed fifty years ago). Like black paint, a selective surface absorbs most of the solar energy falling on it, but, unlike black paint, it emits only a small amount of long-wave infra-red radiation so heat losses from the selective surface by radiation are small. In this article, we look at some of the more recent innovations that have been developed in the UK for solar water heating. Solartwin system In conventional solar systems, two methods are commonly used to prevent bursts due to freezing in cold weather: Antifreeze Added to the solar fluid (water). Drainback In cold weather the collectors are automatically emptied of solar fluid (plain water). The pump stops running and the solar fluid drains back into a tank. The Solartwin system has a different approach: it uses freeze tolerant pipework made of silicon rubber. In cold weather the solar fluid (plain water) is allowed to freeze, but no damage is done. Another distinctive feature of the system is that the pump is powered by a small PV panel. The stronger the sunshine and hence the faster that heat is captured, the faster the variable speed pump can pump the heated solar fluid away to the cylinder. (But in weak sunshine it is desirable to pump the fluid slowly through the collector, so that the fluid s temperature can be raised by a significant amount. Varying the pump s speed is said to be more efficient than the on/off pumping of conventional systems.) In 2001, the Department of Trade and Industry (DTI) published a report comparing eight solar systems (one of which was Solartwin). The results showed that, amongst other things, the average mains power used by the pumps equalled 7.7% of the output. By avoiding the use of mains power, the Solartwin system effectively made a further 20% cut (approximately) in carbon emissions compared to the other systems. However, pumps have become more efficient since the DTI tests. Last month, I mentioned the BRE tests of Viridian systems in In these tests, average pump energy was less than 5% of output. Nonetheless, it remains a pleasing feature of the INNOVATIONS IN SOLAR WATER HEATING 1 MARCH 2009.

2 Solartwin system that no mains power at all is required. (Of course, conventional solar systems can also be independent of mains power if the circulation can be arranged to be thermosiphonic.) Another feature of the Solartwin system is that it can be easily retrofitted there is usually no need to change the existing hot water cylinder. However, some precautions are required in hard water areas to prevent scaling. Solartwin suggest chemical methods, or in the case of very hard water the use of an indirect system and in this case a solar cylinder with two heating coils would be required. If you are intending to retrofit a solar system, then Solartwin is an easy way to do it it can even be done on a DIY basis. The panel is double glazed, but with polycarbonate, so it is not too heavy 31 kg. Another distinctive feature of the collector is that it does not have a selective surface. In an overheating situation (eg, when the occupants are away on holiday in midsummer), the non-selective black surface allows excess heat to be radiated away, thus avoiding boiling. The cost of the basic system is about 2,200 plus VAT. The collector has a gross area of 3.2 m 2 (aperture area, 2.8 m 2 ). In the DTI report, the annual output of the Solartwin system was given as 1,001 kwh (for the climate at Kew, near London). So per square metre of collector (gross area), the annual output was 313 kwh/m 2 (1001 / 3.1). Willis Solasyphon Another innovative product eminently suitable for retrofitting is the Willis Solasyphon. This is a sort of mini, pre-warm cylinder, and it is easily plumbed in between the collector (to be supplied by you) and the existing cylinder. The diameter of the Solasyphon is only 10 cm, so it will usually fit into the same cupboard as the existing hot water cylinder. As in the Solartwin system, solar heated water is added to the top of the cylinder water, and so, when the sun starts to shine, solar hot water soon becomes available, though initially in small quantities. As the Solasyphon uses an indirect circuit (with antifreeze), it can be used in hard water areas. The price of a Solasyphon is 275 plus VAT. The way the device works is both simple and ingenious, and is based on siphonic circulation between the Solasyphon and the cylinder. To find out more, go to the Willis website. (See Further Info.) Solex collectors Solex collectors are unlike any others and are particularly suitable for new build. The collectors are not ready-made. Rather, you build your own into the roof. The method is based on a black rubber absorber strip with built in waterways. The strip is supplied as a roll, either 21 cm wide (with 6 waterways) or 26 cm wide (with 8 waterways) to suit the batten spacing of your roof. You make your own collector by laying the strip in the gaps between successive battens. If the collector is to be, say, 3m wide, the strip runs along a gap between battens for three metres. It is then turned up under the batten above it into the next higher gap, and it runs back along this gap for three metres. The absorber strip is very flexible, so it can be easily looped up underneath a batten. The result is that one continuous absorber strip can be run along successively higher gaps between battens, in our case forming a collector three metres wide and, say, a couple of metres up the slope of the roof. A copper manifold fits the waterways at one end (on the bottom row), and another manifold fits the waterways at the other end (on the top row). So the solar fluid will eventually INNOVATIONS IN SOLAR WATER HEATING 2 MARCH 2009.

3 flow through the collector in a meander pattern from the bottom manifold to the top one. To prevent heat loss, the absorber strip is laid on insulation board 2½ cm thick. Extra space is required between the battens and the roofing membrane to accommodate both the insulation board and the absorber strip (where the strip loops under the battens). For this reason counter-battens are necessary, fixed along the top of the rafters. To keep out the rain while allowing light to reach the absorber strip, the area of the collector is covered with toughened glass slates (for a slate roof) or clear polycarbonate tiles (for a roof tiled with tiles of a matching profile). The system integrates well with the Nu-Lok system for covering roofs, but it is equally applicable to conventional slates and to some tiles. The drainback method is used to prevent overheating (boiling). In hard water areas, use an indirect system. At present, to combat the problem of freezing, Solex recommend that antifreeze is used though since a drainback tank is already required to combat boiling, I think the drainback method might also be used to combat freezing, making the use of antifreeze unnecessary. As might be expected, such a Solex collector does not perform as well per unit area as the best flat plate collectors. Nonetheless, an annual solar yield of more than 400 kwh per square metre is claimed. It is a low cost system, so it is cheap enough to increase the area of the collector to compensate for its small underperformance. (The cost of absorber strip plus glass slates is about 115 per square metre, plus VAT.) The appearance of the built-in collector is neat. Nuaire s Sunwarm system The Sunwarm system overcomes the problem of freezing by using a solar fluid that remains fluid in cold weather without any antifreeze. A fluid is not necessarily a liquid. It may be a gas, and in the case of the Sunwarm system the solar fluid is simply air. Air is pumped through the Sunwarm collector to a heat exchanger, where the heat of the warmed air is transferred to water. This warmed water is then pumped to the cylinder, using either a direct or an indirect circuit, as in a conventional solar system. But there is more to this system than just the production of hot water. The Sunwarm system also incorporates ventilation. The basic Sunwarm system is based on Nuaire s simple and popular PIV ventilation system (PIV Positive Input Ventilation). The more sophisticated Sunwarm Plus system is an HRV system (HRV Heat Recovery Ventilation), and this includes the extraction of air from wet rooms. Via a heat exchanger, the heat of the extracted air warms the fresh incoming air. Remember that in the airtight houses of tomorrow, HRV is going to become increasingly common. (See my article about Passive Houses in last October s SelfBuild & Design. For more about PIV and HRV systems see my articles in the March 2007 and May 2007 editions of the magazine.) In summertime, the heat from the solar collectors is used to produce hot water, as described above. But in wintertime, the warmish air from the collector is used to warm the incoming fresh air of the ventilation system. This is making good use of the weak warmth of winter sunshine. A rise in temperature to, say, 25ºC is obtained fairly easily, and this is useful for warming incoming fresh air. (Whereas a rise to, say, 40ºC for producing hot water is difficult to obtain in wintertime.) INNOVATIONS IN SOLAR WATER HEATING 3 MARCH 2009.

4 Nuaire claim exceptionally high overall efficiencies for their systems. They say the Sunwarm system for an average house, with two solar collectors, has potential annual savings of 2,500 2,850 kwh. But some of this saving is due to the use of the PIV ventilation system. Take this factor out, and the saving due to the solar collectors is something like 2,300 kwh. The gross area of a single collector is 2.57 m 2, so if the annual solar yield of the Sunwarm system were, say, 2,300 kwh, then the annual yield per square metre of collector would be 447 kwh a very creditable figure. DTI tests of solar water heating systems To round off this mini-series of articles about solar water heating, I d like to mention two tests conducted for the DTI a few years ago. The report of the first test was published in 2001 as Side by side testing of eight solar water heating systems. The systems included two with vacuum tube collectors, and six with flat plate collectors (one of which was the Solartwin collector). Analysing the results shows that the average annual output of the systems was 344 kwh/m 2 (based on gross areas of collectors, and for a climate at Kew). This is well down on the 500+ kwh/m 2 of the best solar yields given by the Swiss SPF Institute see my article about Solar Collectors two months ago. And the SPF figures showed that the yields per square metre (gross area) of the best flat plate collector and the best vacuum tube collector were nearly the same. In contrast, in the DTI test the two tube collectors gave distinctly better solar yields than the flat plate ones. (However, they used more mains electricity, which spoilt their performance when carbon emissions are considered.) For new build, the neatness of a built-in flat plate collector must make this type of collector the clear choice. The following year, the DTI published a follow-up report, Further testing of solar water heating systems. Amongst other results this showed that the use of a pre-heat solar cylinder is likely to give a small improvement in performance overall, compared to using just one cylinder. The improvement may be only small, but it does enable solar yields to be maintained no matter at what time of day the hot water is used. Fitting a pre-heat cylinder is likely to be easier if are doing a new build rather than if you are retrofitting a solar system. FURTHER INFO: Side by side testing of eight solar water heating systems. Further testing of solar water heating systems. DTI reports, published in 2001 and Free downloads via: Solar Trade Association Find an installer or supplier near you. Solar Keymark Certified solar collectors (mainly Continental). Solartwin INNOVATIONS IN SOLAR WATER HEATING 4 MARCH 2009.

5 Willis Solasyphon willis-renewables.com. Solex Nu-Lok Innovative roof covering system. Nuaire Words: Copyright article by Robert Matthews in SelfBuild & Design magazine, March, INNOVATIONS IN SOLAR WATER HEATING 5 MARCH 2009.