The desired temperature range of the material to be heated is the most important factor in choosing the correct type of collector. An uncovered absorber is certainly not suitable for producing process heat. The amount of radiation on that spot, exposure to storms, and the amount of space must all be carefully considered when planning a solar array.

Vacuum tubes are in thermos bottles. You can pour in the hot coffee and it will stay hot for quite some time in the winter. This is because of the perfect insulation of a vacuum.

Then add a selective surface coating to reduce the black body emissivity. Then add a phase change heat pipe. Then add an insulated manifold with the heat tube condenser inserted. Flow water across the condensers and the laws of thermodynamics take effect-----the higher energy level ( heat radiation ) will move to the lower level of the heat transfer fluid. The heat transfer fluid again gives off its higher energy level to the potable water at the heat exchanger and voila---solar domestic water heating

Flat plate collector, vacuum tube collectors, batch tank collectors and coiled black garden hoses all do this. But, in cold climates only the vacuum tube heat pipe technology will do it with any efficiency or cost benefit.

It can be shown that a collector to ambient differential temperature of about 50 degrees Celsius is the point at which the evacuated tube collectors become a better economic value than the glazed flat plate alternatives. From 27 to 47 deg Celsius the evacuated tube collectors requires a lesser footprint than the glazed flat plate for the same energy produced.

For all applications requiring differential temperatures below 47 deg Celsius (eg Florida in the wintertime ) the flat plate glazed collectors may be the better value as they can be used in open loop configuration where the potable water is pumped directly through the collectors. The coiled black garden hose will also work out in such a warm climate. Vacuum tube collector applications requiring average differential temperatures of ~50 deg Celsius through below 50 degrees Celsius may require only 1/3 to ½ the surface area of the glazed flat plate collectors.

Another nice fact about the vacuum tube heat pipe system is the ease of installation. While they must be handled gingerly, they to not require a small elephant to hoist them up onto steep second story roofs. A homeowner can install a system working alone and save the contractor installation costs. Which must be factored into the cost efficiency of the device over its useful life.

The specific costs of collectors are also important. Evacuated-tube collectors are substantially more expensive (at 511,29 - 1278,23 Euro /m² collector surface) than flat-plate collectors (153,34 to 613,55 Euro /m²) or even plastic absorbers (25,60 to 102,26 Euro /m²). However, a good collector does not guarantee a good solar system. Rather, all components should be of high quality and similar capacity and strength.