Active solar thermal systems

A distinction must be drawn between passive
 means of utilising the thermal heat of the sun,
and those of a more ‘active’ nature.

Active systems take solar gain a step further
than passive solar.
They convert direct solar radiation into another
 form of energy.



Solar collectors preheat water using a closed circuit calorifier.
The emergence of Legionella has highlighted the need to store hot water at a temperature above 60 C which means that for most of the year in temperate climes active solar heating must be supplemented by some form of heating.
Active systems are able to deliver high quality energy. However, a penalty is incurred since energy is required to control and operate the system known as the ‘parasitic energy requirement’. A further distinction
is the difference between systems using the thermal heat of the sun, and systems, such as photovoltaic cells, which convert solar energy directly into electrical power.

For solar energy to realise its full potential it needs to be installed on a district basis and coupled with seasonal storage. One of the largest projects is at Friedrichshafen (Figure1).

The heat from 5600 m2 of solar collectors on the roofs of eight housing blocks containing 570 apartments is transported to a central heating unit or substation. It is then distributed to the apartments as required. The heated living area amounts to 39 500 m2.

Surplus summer heat is directed to the seasonal heat store which, in this case, is of the hot water variety capable of storing 12 000 m3. The scale of this storage facility is indicated by Figure 2.

The heat delivery of the system amounts to 1915 MWh/year and the solar fraction is 47 per cent. The month by month ratio between solar and fossil-based energy indicates that from April to November inclusive, solar energy accounts for almost total demand, being principally domestic hot water.
In places with high average temperatures and generous sunlight, active solar has considerable potential not just for heating water but also for electricity generation. This has particular relevance to less and least developed countries.