The trick is again in the efficiency. The usual (widespread) solution so far consists of a solar cell and a classical (compressor-)refrigerator - and it doesn't matter whether it is a silicon semiconductor solar cell or a thermo-solar cell with a steam engine and a power generator. The efficiency will be better if we do without the multiple energy conversion, and go directly from the thermo-solar collector to the absorber refrigerator, which is a technology that supplies the cooling units with thermal energy rather than electricity. It sounds surprising for the layman, for the expert it is known: Heat can be used to power a refrigerator. It is a special refrigerator, called 'absorber-refrigerator".
Such devices have a design dependant operating temperature, which we need to supply sufficiently. How large the temperature interval is, in which the supply temperature may vary in order to work efficiently, is part of the development work.
In the case of a small temperature interval, we need either solar position tracking of the
thermosolar collectors, or larger area solar collector cells from which thermal energy is
dissipated to supply other users than the absorber-refrigerator during the period of intense midday
In the case of a large temperature interval, we have the advantage that during the phase of strong heat in the outdoor space (midday), there is also more power available for the refrigerator at the same time, than during the time of lower heat in the outdoor space (evening time). This is a self regulating system.
In principle, absorber refrigerators can be ordered ready on the Internet, but there they are not really cheap, because they are usually offered as camping accessories. However, we can use this existing technology and develop or dimension our own devices in such a way, that they optimally match the temperature supplied by the thermo-solar cells, and can be manufactured cheaply in poor sunny countries.
Our price advantages compared to the classical compressor-refrigerators has the reason, that we do not need any mechanical moving parts.
Regarding the operating costs, our absorber-refrigerator is interesting only in connection with the thermo-solar cells, because we save the losses by the steam engine and the current generator. Alone these two components only achieve an efficiency of η ≈ ⅔ · 47% · 92% ≈ 28%, this is not really much. The losses in the compressor are also added, so you end up with an efficiency of less than 20% for the compressor-refrigerator (at about 15% ... 18%).
If you have only electricity but no heat energy available in cold countries like Europe, you cannot avoid these losses of the compressor-refrigerator. In sunny countries the technology of absorber-refrigerators are useful.
Let us come back again to the efficiency: Because of our thermo-solar cells we can avoid the losses of the steam engine and the electric generator, but we have to calculate an efficiency of about 25 ... 30 % on the part of the absorber refrigerator against (which we can not avoid), which is still a little bit more favorable, than the efficiency of the compressor-refrigerator.
In figures: Operating cost savings ≈ (27-17)/17 % ≈ 59%.
This is the namely energy saving for the absorber-refrigerator, assuming that one operates it on the thermo-solar cell.
Another advantage is that if you operate the absorber refrigerator on a thermo-solar cell, you will have a strong cooling power during the day when the outside temperature is hot and a less strong cooling power at night when the outside temperature is moderate, because the thermo-solar cell cools according to the solar radiation. The system works by itself always the strongest when the most power is needed.
For optimal dimensioning of the refrigerator, the price sum for refrigerator walls plus cooling unit plus thermo-solar power supply has to be minimized. The following parameters play important roles:
- the thermal conductivity coefficient (λ) of the insulation material
- the material thickness (wall thickness of the refrigerator)
- the purchase price of the refrigeration unit
- the dimensioning of the matching thermo-solar cell (surface, pump, oil,...)
- depending on the outside temperature and on the inside temperature of the refrigerator.
Summary: Our refrigerators cost a little less than half the price of classic refrigerators.