Solar refrigeration adsorption LESBAT: Catherine Hildbrand, Olivier Cherbuin Julien Mayor. Project leader: Philippe Dind
The purpose of this article is to introduce the principle of solar cooling adsorption which simply allows for the cold with solar radiation.
The Laboratory of Solar Energy and physics Bâtiment1 (LESBAT) of the High School of Engineering and Management of Vaud (HEIG-VD) in Switzerland is active in the field of renewable energy for more than two decades.
The development of solar adsorption refrigerator is one of the main axes applied research in recent years.
These systems were originally designed for cold production in non-electrified areas of the countries of the Sahel where refrigeration is sought for storage of medical and food products.
Based on work by various research teams in 80 years we have made 1999 a demonstration prototype, and various laboratory prototypes to achieve a solar sorption cooler whose construction is feasible in small series in the country in development.
We accompanied our development by technology transfer with the Ouagadougou Ecological Center.
In this article, we present the operating principle of solar refrigerators adsorption, examples of constructive developments and technology transfer to Burkina Faso.
Funding for this experimental development was mainly provided by the Foundation Gebert RÜF (Basel - Switzerland).
Why solar cooling?
An alternative and clean solution for developing countries to store medicines and foods.
Solar cooling is an alternative to the following:
- absorption refrigeration (not adsorption) which also transforms in the cold hot but requires higher temperatures whose combustion,
- the vector by a thermodynamic refrigeration refrigerant circuit including the pump is powered by a generator,
- solar-electric refrigeration which poses serious storage problem of electrical energy (batteries whose life is very low ...)
None of these solutions 3 perfectly meets the needs of developing countries because they consume hydrocarbons (rare and expensive in Africa) and reject the CO2 or indirectly pollute share the technology used (batteries, refrigerants ...).
Solar cooling by adsorption using the direct conversion of solar thermal energy in cold, do not have such disadvantages.
How does the solar adsorption refrigeration?
Adsorption is the phenomenon that occurs when striking a balance between a gas and a solid.
The concentration of the gas molecules is always greater near the surface of the solid in the gas phase. Thus, in any solid, the surface atoms are subject to uncompensated attraction forces, perpendicular to the surface. The balance of these forces is partially recovered by the adsorption of gas molecules.
It is frequently spoken of torque when one refers to the adsorption. The pair consists of an adsorbent (solid) and an adsorbate (making liquid refrigerant office).
The body used as adsorbents are generally body which have a very large exterior surface. We can cite activated aluminas, activated carbons, silica gels and zeolites. The choice of the adsorbent will depend on the desired temperature at the evaporator and temperature available at the hot source (collector).
The other part of the pair (the adsorbate) must meet two essential conditions: possess a large latent heat of evaporation and consist of small molecules, easily absorbable. Fluids propertied these characteristics include water, ammonia, methanol and carbon dioxide. In addition to these two conditions "physical" characters, it is necessary to consider the nature of the fluid and its dangerousness (flammability, toxicity, ..).
Our applications have successively used the active coal-methanol couples, silica gel-water and water-zeolite. The use of these pairs requires an average vacuum level (10-3 mbar) to reduce to a minimum the presence of noncondensable gases which could obstruct the passage of the vapors of adsorbate. The cycle works intermittently, because the cold is produced only during the night.
Refrigerators as designed in our developments include four main elements:
- The sensor-adsorber: part containing the adsorbent is heated by the solar radiation, with the result
desorption of the adsorbate.
- The condenser: This element is used to condense the vapors desorbed adsorbate in the adsorber sensor.
- The refrigerating chamber: this element is the useful part of the refrigerator, composed of the insulated enclosure and evaporator including the adsorbate in liquid and solid form.
- Autonomous valve: Body to separate the high pressure part of the low pressure. This body developed LESBAT is automatic and has no electronic (patented).
The solar refrigerator operates on an adsorption heat cycle. The cycle
can be divided into four distinct phases as explained below. We also provide in this table the cycle shown in the Clausius-Clapeyron plot (Ln (P) (pressure) versus (-1 / T) (temperature)).
Advantages / Disadvantages of air conditioning adsorption
- Times (at least) 12h production between hot and cold production (this could be quite easily solvable by buffer batteries)
- Need to have a cold source (at night) to produce the "cold" adsorption cycle
- COP widely (1 / 10 approximately) below the compressor refrigerating machines (but is it really important since the solar source is quasiement unlimited?)
- No energy source other than the sun (or other source of heat, as heat loss) is required.
- No moving parts (except the shutter), maintenance is much easier.
- Ability to use refrigerants as simple as water!
- COP equivalent see slightly higher than the electro-solar cooling (solar panels powering a mechanical refrigeration circuit), the disadvantages less (wear and pollution batteries, maintenance ...)!
Regarding the cost of manufacturing a solar sorption cooler constructed according to the description given in this article, it is very difficult to articulate figures.
The cost of a prototype developed in Switzerland or in France has nothing to do with the cost of a refrigerator built a few dozen copies a year in Burkina Faso with qualified labor in the country (goal referred initially). This price would drop even more in case of series production to thousands of copies a year in an automated line here or in Africa.
A market study 2002 in Burkina Faso by the CEAS-BF showed that more than a hundred solar refrigerators adsorption of 300 liters could be passed this year and for three years with NGOs, hospitals, hotels and Burkinabe agricultural cooperatives provided that the sale price does not exceed 750'000 FCFA (about € 1'250).
The challenge is twofold: to meet the market demand after production start in a craft and pace to produce without exceeding the sales price mentioned above. Current market conditions and the price of labor Burkinabe make us think that the CEASBF is able to meet this challenge.
Experiences in the field of solar adsorption refrigeration within the LESBAT helped to understand all the difficulties inherent in this technique.
So that the latter can propagate without suffering failures that may discredit, it is essential to support the transmission of knowledge to developing countries through the creation or strengthening of actual skills on site, as we have tried to do as part of our collaboration with the CEAS-BF.
Constructive changes in our solar refrigerator systems aimed constructive simplification while maintaining performance. This objective is achieved. The result of the project is to develop small coolers. This will define the optimum size of the room taking into account technological limitations of our system.
Solar cooling adsorption is part of the range of solutions to environmental problems created by our excessive consumption of fossil fuels.
Furthermore, it should improve the quality of vied'un many people, especially the Sahel countries, giving them access to the conservation of medicines, vaccines and food.
- LESBAT Site
- Download the article in .pdf format LESBAT
- General and operating principle
- Optimization of the solar collector
- presentation panel solar cooling: Operating
- presentation of the solar air panel: African collaboration