What are Solar Collectors?

The collector converts sun radiation to heat, using the “greenhouse effect”
Sun radiation in the visible range (what we humans see as colors and the blend of colors called white) penetrates through a transparent window (usually glass) and hits a black absorbing sheet (or black coated surface) at the rear side of the collector. The absorbing material radiates back heat waves in the infrared wavelength (what we humans sense as heat)
The front window is opaque to infrared and the heat is trapped inside the collector.

Next step – the heat is transferred to A fluid, either to water (in mild areas)or to an anti-freeze liquid (like glycol) in areas where temperatures might drop bellow freezing point (32 degrees). The liquid flows through a closed loop thin meandered pipe.
Bottom line – we managed to convert sun radiation to hot water.

A picture of vacuum tubes collectors on a UK roof attributed to thingermejig

The Two Types of Collectors

The one seen mostly has a planar construction. This type is usually called flat plate collector.

The evacuated tube collector is the one seen less, typical to the northern regions where sun radiation is not so abundant. It consists of circular shaped tubes

Cost of Solar Collectors

Flat plate collectors can cost around 700-900$ for 20 square feet size and 1100-1300$ for 40 square feet size (shipping cost and installation cost not included)

Evacuated tube can run way above 2000$. I saw lower prices from new comers. However, I usually recommend to go on a mature product line.

Federal energy tax credits as well as state level (in some of the states) are available

Life Span of Solar Thermal Collectors

The life expectancy of collectors is long and 10 years is not unusuall. For open systems the maintenance is low, for closed system you might need to add glycol (same as in a car cooling liquid)