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Solar Thermal Photovoltaics
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Evacuated Tube vs Flat Panel Collectors Insulation Just about any solar collectors, including home made ones, work well when it is sunny and warm. It is in more marginal conditions, when it is overcast, windy or cold, that evacuated tube collectors become far better than flat panel designs (and occasionally we in the UK do experience less than perfect weather). Each evacuated tube is extremely well insulated due to its Thermos flask like design. The vacuum between the glass walls effectively eliminates losses via conduction and convection. Note that there is no comparison in the effectiveness of the insulation of a double glazed flat panel (not that most flat panel collectors are double glazed) and an evacuated tube. A vacuum cannot in practice be used with flat glazing because the atmospheric pressure would cause the glass to collapse and break. Radiated losses are reduced to an absolute minimum by the selective coating on the inner glass tube (to be fair, the best flat panel collectors use a similar selective coating, so do not lose much energy by re-radiation either). However, when there is a significant temperature difference between the solar circulating fluid and the ambient (eg on a cool day, or when the fluid is already warm because the water in the hot water cylinder is already warm), then there is a steep temperature gradient between the solar fluid and the outside air. In these circumstances, because of the inherently superior insulating properties, evacuated tube collectors will transfer a significantly higher proportion of the available heat energy to the solar fluid than will flat panel collectors. Passive Solar Tracking The performance of flat panel collectors reduces the further from solar noon (when the sun is at its highest point in the sky) the time is. Evacuated tube collectors do not suffer from this problem. Consider the following diagram:
This represents a side view of an evacuated tube collector (top) and a flat panel collector, at solar noon (left side) and, say, 4 hours before or after noon. The yellow lines represent the solar insolation. At solar noon, both panels receive the maximum insolation. However, later (or earlier) in the day, while the evacuated tube panel continues to receive the same maximum insolation, that received by the flat panel has reduced to, in this case 60% of the maximum, represented by the length of the blue line. The evacuated tube panel will continue to receive the same amount of insolation that it did at solar noon, and it will continue to do so until the sun is low enough for the tubes to shade each other. This is called passive solar tracking, and is due to the cylindrical, non directional nature of the Eco-nomical evacuated tube. Note that not all evacuated tubes exhibit this behaviour. Some types, with a single glass wall, have what amounts to a flat plate inside the tube, and cannot take advantage of passive solar tracking. Installation Evacuated tube collectors are easier to install, because the tubes can be lifted onto the roof one by one, avoiding the need for heavy and dangerous lifting. In contrast, almost all flat panel collectors cannot be disassembled, and must be lifted up to the roof in one (very heavy) piece. Conclusion In the past, the reason for choosing flat panel collectors rather than evacuated tube collectors was cost. The evacuated tube collectors were typically three or four times more expensive (yet many people still bought them, due to their superior properties!). Now, however, this is no longer the case, and Eco-nomical collectors are available at a cost which is less than flat panel collectors. To return to our FAQ page, click here.
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