Flat plate collectors are the most commonly used arrangement for domestic systems and as such will form the largest section of this information. These collectors consist of airtight boxes with a glass, or other transparent material cover. Several designs on the arrangement of the internal tubing of flat plate collectors exist as shown in figure 1. Traditional collectors, like the Serpatine and Parallel tube examples above, consist of a number of copper tubes, known as risers are orientated vertically with respect to the collector and placed in thermal contact with a dark coloured, metal absorbing plate. At the top and bottom of the metal absorbing plate, thicker copper pipes, known as headers, assist in the removal of heated water and the arrival of colder water for heating.
Insulation is placed between the absorbing plate and the external wall to prevent heat losses.
Whilst the principles of operation for flat plate collectors are fairly consistent, significant improvements in the design of systems, particularly absorber plates has occurred. Flooded plate collectors are similar to their tubed cousins, except that two metal absorbing plates are sandwiched together, allowing the water to flow through the whole plate. The increased thermal contact results in significant improvements in the efficiency of the system. In recent years, much research has been conducted on selective surfaces, which has seen significant improvements in the efficiency of solar water heaters. According to Planck's Law, the wavelength of a radiation emitted from a surface is proportional to the temperature of the surface.