The number of average productivity of 1000kWh/m2 may confuse some people, as evacuated tube solar water heaters normally claim only around 500kWh/m2 of productivity.
This is only due to a difference of computational method. The productivity of an evacuated tube solar collector is computed according to the whole opening space within the collector frame. However, this space contains a lot of gaps that do not receive sunlight in most of the time. These gaps include the space between two adjacent tubes, as well as space within an evacuated tube that does not receive light.
A quick computation often leads to about 55% the effective collecting space within the computed area for the evacuated tube solar collector.
On the other hand, because of the tracking, a parabolic trough collector is always oriented so that its surface is almost maximally exposed to sunlight, in case of a tilted installation. As my computation uses the net collecting surface for my collectors, there is amost a one-to-two difference between the surface computations of the two methods.
One could argue that the evaculated tube collector also receives diffuse light, that the concentrating collector cannot do. However, the former has a much greater thermal loss ratio, so the extra heat collection is more or less lost.
Quoted from Power From the Sun, where the comparison is done with low-performance concentrators during 1970s: It was found that the amount of diffuse energy collected by flat-plate collectors in most regions of the contiguous United States was not sufficient to compensate for the tracking capability of the troughs. Also that the typically higher optical efficiency of the flat-plate collector compensates only partially for the higher thermal efficiency of the concentrators. Over a full year?s operation, the north-south trough orientation and the latitude-tilt flat-plate orientations provided the most energy.
In fact, evacuated tube collectors normally claim a thermal loss ratio about 1.5W/m2.K (which will degrade after a few years due to helium permeation). As the effective collecting surface is only about the half, this is equivalent to a loss ratio of 3W/m2.K for concentrating collectors. The ratio for the latter is less than 1W/m2.K (commercial products will be around 0.5W/m2.K). This difference is very important, especially during cold seasons.
Therefore as a rule of thumb, one m2 of parabolic trough corresponds to 2m2 of evacuated tube collector. It should be noted that the parabolic trough usually needs extra space around it when installed. Therefore at the end, the land use will be equivalent between the two types of collectors.
As for flat plate collectors, they are well known to have a lower efficiency.
Another important difference is that the parabolic trough collector has a much less internal heat capacity for the fluid circulating in it. This heat capacity is about 0.1L/m2, while it is usually several liters for non-concentrating collectors. The greater internal heat capacity leads to important heat loss when the system turns on at every morning, especially if the temperature difference is important between the water in the collector and the environment. This kind of loss is negligeable for a parabolic trough, therefore it can efficiently heat water to high temperature during cold season.
See also an efficiency comparison with big parabolic trough.
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