Overview:
Flat plate collectors perform very well in warm sunny conditions, as the ambient temperatures are high, but the summer is not when hot water requirements are the highest. During cold months when the ambient temperatures are low is when hot water demand is highest and it takes the most energy to heat the water, and so when solar gain is of most value - evacuated tube greatly outperform flat plates under such conditions due to the superior insulation properties. Wind therefore also has much less effect on evacuated tubes than on flat plates.
The delta-t values presented in the tables show the system operating temperatures (collector - ambient). In the morning when the water in the bottom of the tank is cold the delta-t will be low, but as the tank heats it will quickly rise. As the water in the tank gets hotter the performance of the flat plate will drop off, particularly if it is cold outside, so while performance in the morning might be good, it drops off throughout the afternoon.
Solar radiation levels also have an effect. Flat plates perform very well in strong sun, but the performance drops quickly in the moderate to low sunlight levels common in the cooler months - evacuated tubes maintain much higher efficiency under such conditions. The result is more heat when you need it most!
The comparison is made between a flat plate collector and Apricus collector with a very similar sized aperture area.
