The «best» type of solar collector

It doesn’t exist a best type of solar collector. The best depends on climate, on the intended uses and is closely related with the price issue.

Choice depends on...

Choosing a type of thermal collector - flat-unglazed, flat-glazed, evacuated tubes... - depends on the intended uses (swimming pools, hot-water, solar radiant heating...), prices and climate...

Prices

As we pass from 1) unglazed solar collectors to 2) glazed solar collectors and on to 3) evacuated tubes panels the prices increase.

We can use evacuated tube solar panels in swimming pools heating, but it would be too expensive. Evacuated tubes panels are more efficient (30% or so) than unglazed panels, but these last collectors are much cheaper, and with some extra unglazed surface we can reach the same effectiveness of evacuated-tube collectors at a lower price.

Similarly, we can also use evacuated tube panels in domestic water heating, but in some climates the difference of efficiency doesn’t justify the difference of price.

Summary on the residential uses of the different types of thermal solar panels

Since choice depends greatly on 1) prices, 2) temperatures and 3) climate and intended uses, it is possible to draw some rough conclusions about the "best" solar collector:

Solar Unglazed collectors

They are used in swimming pools and simple space heating (solar air heaters, designed just to heat a room or a small part of the house).

Unglazed panels are used in solar applications demanding low temperatures.

They have a wide range of specific applications in sunny climates, where they can substitute more expensive panels.

Structure of un Unglazed Solar Collector Image credit: EERE

Solar Air-Flat glazed panels

They are mainly used in solar space heating, and may be tied to forced air systems. They have some advantages over solar liquid-flat panels (they produce heat earlier and later in the day than liquid systems, they do not freeze...) but they are also less efficient than liquid collectors, and aren’t as flexible in their applications.

Solar Liquid-flat glazed panels

These panels can be used in solar domestic water heating systems and central heating solutions (integrated in systems involving forced air systems and radiant heating). They are a flexible, tested and a competitive option, mainly in moderate climates.

Evacuated tube panels

They share most of the applications of flat glazed panels, mainly the more demanding applications (those that demand higher temperatures); they are the most powerful solution for more demanding applications and colder climates.

A very probable scenario is an increasing use of evacuated tube panels technology in cold and cloudy climates or even in moderate ones.

Solar Electric panels

The production of solar electricity demand a special type of collectors: the PV solar panels. See Solar Electricity: Photovoltaic PV Systems