There generally two kinds of collectors, the flat-plate and the evacuated tubes. Flat-Plate systems comprise an insulated, weatherproofed box containing a dark absorber plate under one or more transparent or translucent covers. Heat transfer fluids pass through pipes located below the absorber plate. This collector, although inferior in many ways to evacuated tube collectors, is still the most common type of collector in many countries.
 
Evacuated-tube collectors are made up of rows of parallel, transparent glass tubes. Each tube consists of a glass outer and glass inner tube. The inner tube is covered with a selective coating that absorbs solar energy well but inhibits radiative heat loss. The air is withdrawn ("evacuated") from the space between the two glass tubes to form a vacuum, which eliminates conductive and convective heat loss. Evacuated tube collectors are the most efficient, reliable and cost effective collectors commercially available today.

The heated fluid is then stored in a tank similar to a conventional gas or electric water tank, and an electric pump is used to circulate the fluid through the collectors. Sensors in the tank and in the collector measure the temperature in order to suspend the circulation if the water in the collector drops below the temperature in the tank. If the solar radiation is not sufficient, the system is supplemented with other heat sources such as oil furnaces, natural gas, electrical heating, or biofuel.

There are two main distribution systems, the open loop system and the closed loop system.

Open-Loop Active Systems
Open-loop active systems are inexpensive, efficient, and simple options that circulate household water through the collectors. They are primarily used in regions that do not experience subzero temperatures, but are not appropriate if the water is hard or acidic due to scale and corrosion quickly disables the system. They may also be installed in mild climates that experience occasion subzero temperatures, but freeze protection must be implemented. 
Re-circulation systems are a specific type of open-loop system used with flat plate collectors that do provide freeze protection. They use the system pump to circulate warm water from storage tanks through collectors and exposed piping when temperatures approach freezing. Re-circulation systems are suitable where mild subzero temperatures occur no more than once or twice a year. 

Closed-Loop Active Systems
These systems pump heat-transfer fluids (usually a glycol-water antifreeze mixture) through the collector in a closed-loop system without connection to household water. Heat exchangers transfer the heat from the fluid to the household water that is stored in tanks. Double-walled heat exchangers prevent contamination of household water. Closed-loop systems are popular in areas subject to extended subzero temperatures because they offer good freeze protection. However, the systems are more expensive to purchase and install and the glycol-water antifreeze mixture must be checked each year and changed every 3 to 10 years, depending on glycol quality and system temperatures. 

Demonstration Projects:
Improving efficiency of urban energy supply systems, France - Grenoble
Leamington House – Highly efficient heating system for improving thermal performance in Portsmouth, UK
A new "tune up" of innovative technologies, Italy - Brescia
Introduction of local district heating in spain, Spain - Vilanova Ila Geltru
Energy efficient technology to control costs of modern architecture, Finland - Kuopio
High-tech preservation of architectural heritage, Greece - Volos
Renewable energy supply to a "Green Region", Italy - Abruzzo
Poland, Rawicz: Rawicz Hospital – Use of solar technology in public buildings

Technology Producers
www.batec.dk
www.dansksolenergi.dk
www.arcon.dk
www.izen.be
www.clipsol.com
www.viessmann.com