Solar energy


Photovoltaic: Electricity from sunlight

There are two practically important variation using electricity from PV (photovoltaic) cells:

A grid connected system should sell all the electricity to the grid. The electricity from renewable sources sold to the grid earns more than you pay for electricity from the grid because the use of renewable energy is politically wanted and supported.
Under these conditions it does not make sense to consume the PV electricity at home.

Depending on the contract conditions a grid injecting system pays back in 6 years and gives interest rates around 10%. The exact solar radiation conditions should be checked on each individual case.

To sell electricity to the grid a grid-connection and an injection licence is needed. Although grid injection is law all over Europe it is almost impossible to have a licence in Portugal.

In case of a power failure from the public grid the PV electricity produced on the roof can normally not be used to supply your house.
Grid injection makes sense because it results in a good load distribution because small amounts of current are transported over short distances. A large number of small systems have a higher reliability of supply than a few big ones because failure or shading of one among many has a much smaller effect than failure in one of few.
There is also a political advantage if consumers of energy are also producers and therefore not totally depending an energy companies.

Off-grid-systems use their own home-produced electricity.
The independence given by this kind of system includes some inherent disadvantages:
To bridge the night and short periods of bad weather batteries are needed. Batteries can be environmentally harmful, are expensive and need maintenance.
To have enough energy in winter a solar peak performance needs to be installed that causes overproduction in summer. This overproduction is normally unused and therefore worsens the economic efficiency.

An off-grid-system can make economic sense if there is no connection to the grid, a grid connection is very expensive if the annual price rise in electricity cost is over 20%.

Tracking systems for off-grid-systems are normally not advantageous because the output of a tracked system is big in summer while the demand is bigger in winter.

A Solar driven water-pump is a special case that does not need a battery; it represents an ideal combination of supply and demand because the more the sun shines the more water is needed.
On the other hand the special pumps needed are very expensive.
The installation and design of a PV system should be based on a detailed analysis of the consumption to avoid misfit.

Buying solar modules and deciding between mono- and poly crystalline cells is not a matter of efficiency! It’s the price per kilowatt multiplied by the guaranteed lifetime that matters because you want to buy energy not solar cells.

PV Module



Solar thermal: warm water, heating, cooking & baking, sauna

The extraction of heat from sunlight is based on the very simple principle of a (black) tube in the sun: in a tube with a big heat-absorbing surface water is heated. The thermal energy is then transmitted via a heat exchanger to a well insulated storage tank where it is collected. Although the basic principle seems to be much simpler than the physical basics of photovoltaic the design of a solar thermal system is much more complicated.

Solar heating of water is without any doubt the most economic version of using solar energy.

The most simple system is the thermo siphon system where the warm liquid in the collector rises to the storage that is located above because the warmer the liquid gets the lighter it becomes. The principle is genius and simple because it works without a pump or regulator as soon as the collector is warmer than the storage.
But the storage has to be located above the collector. Mostly it is simply a horizontal, cylindrical storage on top of the collector which is often seen, very ugly and inefficient.
A vertical storage is much more efficient than a horizontal one because the water can stratify according to its temperature. The solar- heat-exchanger as well as the back-up-heater can be located where they belong.
Very bad horizontal storages are heated completely during the night by the electric back up so that the collector is just decoration.
Never use a system where the water runs directly through the collector because it will soon block the collector with calcium and destroy it in cold nights because water in a collector freezes already at plus 2 degrees centigrade on a clear night.

Warm water systems should be heated above 60 degrees regularly and have constant water exchange to avoid the legionaires disease. The best solution is an external heat exchanger for the warm water.


If a solar system is used to support the indoor heating, then wall- or under-floor-heating with big surfaces help to use the solar heat much more efficiently. The optimisation of the inclination and orientation of the collectors will also increase the efficiency of the system.

The basic precondition for successful use of a solar system for heating is good thermal insulation of the building. A well designed combination of insulation and solar system can cover 90% of the heating demand!
600 litres of water can be heated from 20 to 70 degrees centigrade by 12 square meters of collector in one day. This stores about 32 kWh which is the equivalent to 4 electric radiators with 2 kW each for 4 hours. For a well insulated house this is enough even in winter!

Choosing the type of collector depends on the purpose: a simple absorber will do best for a pool in summer while a vacuum tube collector is ideal at low temperatures on sunny days. The flat collector normally is the best compromise in terms of price to performance.
The type of collector that is ideal for you has to be checked individually.

The solar sauna is a very pleasant speciality. In southern countries a well insulated winter-garden with south facing and inclined windows will heat up to 60 degrees centigrade – using absorber elements the temperature can rise to 90 degrees. A solar sauna is also ideal for drying clothes and heating the house in winter.
For baking an insulated box painted black on the inside with an inclined window (similar to a sauna) heats to about 160 degrees (depending on the quality).

A parabolic mirror with a surface of 1,5 square meters concentrating the sunlight on a black pot cooks faster than a gas or electric fireplace.