Sustainability Institute
working towards a sustainable future
Solar Energy
The Big Myth:
Evacuated tube solar panels are more efficient than flat plate collectors
Wrong!
Tests carried out on 160+ different solar panels by the internationally acclaimed research organisation Solartechnik Prufung Forschung (SPF) found that the gross efficiency of flat plate collectors was considerably higher than evacuated tube collectors.
The average gross efficiency of the 120+ flat plate collectors tested was about 70%, while the average efficiency of the 42 models of evacuated tubes was only 49%. In terms of range, the flat plates varied from 51% to 79% while the evacuated tubes varied from 31% to 62%. All but five of the flat plate collectors tested had a gross efficiency greater than 60%.
How is it that the manufacturers of evacuated tubes can claim their products are more efficient than flat plates?
Quite simple really - they compare only the area of the aperture (the bit the sun shines through) of the two types of panel. On average, the aperture of an evacuated tube captures solar radiation more efficiently than the flat plate. There is however considerable variation between different models. Much more significant however is the relationship between the aperture area and the gross dimensions of the panel.
Whereas the aperture of a flat plate collector may exceed 90% of the total area of the panel, on some evacuated tubes it is as low as 52%. As anyone who has looked at an evacuated tube solar collector will know, there is a lot of space between the tubes!
So contrary to the claims of the evacuated tube suppliers, you probably need a larger gross area of evacuated tube collectors to achieve the same heat output as can be achieved with a given area of flat plate collectors.
Not convinced?
Check out the SPF website: www.solarenergy.ch Click on the English language version, go to ‘test reports’ then ‘collectors’. There is a downloadable pdf for each collector tested. On page two of the pdf the gross efficiency of the collector is given. A figure of 0.408 for example, means the gross efficiency of the collector is 40.8%.
It should be noted that all these efficiency figures relate purely to the efficiency of the collector and not of the whole system. An indicative figure of the system efficiency can be obtained by dividing the gross efficiency of the collector by 2. The figure arrived at will only be approximate but will take into consideration distribution losses and unusable surpluses which may occur during periods of prolonged good sunny weather.
The SPF results also gave some clear indications as to the source countries of the best and worst performing panels. Of the flat plate collectors tested, models manufactured in Austria and Germany were among the most efficient.
Of the vacuum tube collectors, the best performers were from Switzerland, Northern Ireland and the United States.
Six of the ten worst performing evacuated tube collectors were manufactured in China. In addition, a further two of the worst ten performing evacuated tube collectors, nominally ‘manufactured’ by companies in Portugal and Italy, had identical dimensions and specifications to other evacuated tube collectors manufactured in China, and were presumably the same model.
Given that the collectors were tested in Switzerland, are these results valid for Ireland?
Certainly there are differences in the two climates. Switzerland receives more solar radiation than Ireland annually and particularly so during the winter months. During the summer the difference is less. Whether these factors alone make a lot of difference is debateable. Switzerland suffers much colder winter temperatures which one would have expected to disadvantage flat plate collectors. Conversely, in Ireland there are more days with very poor levels of solar radiation, and on these particular days the evacuated tubes may out-perform the flat plates. The sum contribution of solar radiation on these days however is only a few percent of the total annual solar radiation. The greater wind speeds experienced in Ireland are also a factor, as the flat plate collectors will lose heat faster than evacuated tubes collectors in windy conditions.
Different tests carried out by the UK Department Trade and Industry on 8 models of solar panels ( 2 evacuated tube and 6 flat plate) showed evacuated tubes coming out ahead of flat plates but the small sample suggests these results should be viewed with some caution. The two evacuated tube collectors tested are among the best of their type on the market and were certainly not representative of the whole sector. These tests also showed that the evacuated tube collectors performed better than the flat plates in winter, a fact which is often used as a selling point by the suppliers. In most situations however, solar heated domestic hot water is much more useful in the late spring, summer and early autumn when space heating requirments are minimal and there is no need for central heating systems and back boilers to be running. See www.dti.gov.uk/files/file16826.pdf.
The Conclusions?
While there may be specific circumstances in which the evacuated tubes may be the better option, there is clearly a need for further research into the performance of solar collectors before any clear conclusions can be drawn. In the meantime,the claims that evacuated tubes collectors are more efficient than flat plates should certainly be treated with suspicion.
Management of the Solar Resource
One 'fact' which should be treated with the proverbial pinch of salt is the oft repeated claim that solar panels will provide ' up to 60% of domestic hot water requirements'. But how can the supplier know that without first calculating what those annual requirements are? The short answer.... they can't. This figure is based on generously sized systems working at high levels of efficiency and should be looked upon as a best case scenario.
One rarely mentioned factor which has a huge influence of the efficiency of the system is the management of the resource by the building occupants. The energy aware household will try to match demand with supply. It makes sense to synchronise maximum hot water demand with periods of high output from the solar panel.
The 'energy oblivious' household, on the other hand, will unreasonably expect their solar panels to deliver hot water 24/7 and will invariably be disappointed when there is no discernable difference in conventional energy use.
Inspite of the fact such tests would be easy to carry out, very few studies on solar water heating have been done in real life situations...
Further Reading
Sunny Days: Demistifying Solar Heating. This article, which appears in Issue 2 of Sustainability magazine, examines the solar resource in great detail. (436KB)