Broadly speaking, solar panels can be broken into two categories: thermal panels and photovoltaic panels. Thermal panels are intended for hot-water heating, pool heating and air-space heating. Photovoltaic panels are used for power generation.
Solar thermal panels - the type of panels we are going to deal with in this page - can be broken in three types:
1) solar liquid collectors (glazed and glazed),
2) evacuated-tube collectors and
3) solar air collectors.
As we pass from 1) unglazed collectors to 2) glazed collectors to 3) evacuated-tube collectors there is a price increase.
We can install tube solar panels in swimming pools, but that's not cost effective. Tube solar panels are more efficient (30% or so) than unglazed flat panels but these are much cheaper.
Similarly, we can also use tube solar panels in domestic water heating, but it may not be cost-effective in hot or moderate climates.
Summing up: choice depends primarily on 1) prices, 2) intended temperatures 3) climate and 4) intended uses
Solar Liquid Collectors
Common flat-plate panels use a liquid (water, glycol…) as the heating fluid.
They comprise a net of flow tubes (where water or other heating fluid flows), a dark color absorber plate (to absorb the sunlight heat) and a glazing surface. See image at right.
Unglazed Flat Solar Panels: they are used in applications requiring low temperatures, in sunny climates: swimming pools, home space heating.
Flat Glazed Solar Panels: Flat glazed panels have the same uses of unglazed panels, especially residential water heating; their efficiency doesn't deteriorate significantly over time.
Solar Air Collectors
Solar air collectors are less common than solar liquid collectors.
As their name suggests, they use air as the heating medium, instead of water or other liquid fluid. Their absorber plates are often metal sheets.
They can produce heat earlier and later in the day, and do not freeze. However, since air is a bad conductor, they aren't efficient.
They are mostly used in air-space heating solutions.
Uses of Tube Solar Collectors
Tube solar collectors share most of the uses of solar liquid collectors.
Choosing tube solar collectors or solar liquid collectors is a question of price, climate and type of application. Tube solar collectors are mostly used in colder climates, in applications requiring relatively high temperatures.