Regarding performance, the highest values of COPSR (0. 10-0. 12) were obtained with the adsorption systems zeolite + water (Grenier et al. , 1988) and activated carbon + methanol (Boubakri et al. , 1992a,b; Pons and Grenier, 1987). As methanol can easily evaporate at temperatures below O oc, thus favouring the production of ice, the most environmentally friendly refrigerant must be water. Using water, ice can be produced within the evaporator, acting as a 'cold storage'. Both refrigerants, water or methanol, operate at below atmospheric pressure and therefore require vacuum technology. The main urpose of the present study is to obtain what is, technically speaking, a simple machine.
This aim seems reasonably achievable with an adsorptive machine, operated in a 100% solar-powered 24 h cycle with a flat-plate solar collector containing the adsorbent. However, when referring to the work reported above, both the efficiency of the solar collector and that of the adsorption thermodynamic cycle could be improved. These requirements were crucial to the design of the 'advanced' machine. The laboratory of solar energy of the Engineering school of the Canton de Vaud (EIVD, Yverdon-lesBains, Switzerland) has been eveloping adsorptive solar refrigerators since 1999. The first systems built used the adsorption pair of activated carbon + methanol.
For reasons of reliability and respect for the environment, this pair has been abandoned in favour of a silicagel + water pair. The prototype described and analyzed in this paper has been functioning since the summer of 2000 on the site of the EIVD. A thorough measurement system allows us to characterise it in a complete way. During the summer of 2001, a constant procedure of thermal load in the cold cabinet allowed us to observe the behaviour of the adsorption system over a continuous period of 68 ays. We have highlighted the great influence of both external temperature and daily irradiation upon the daily coefficient of performance (COPSR ). Previously, few articles were interested in the analysis of the storage. 2.
Description of adsorption and of the adsorption cooling cycle Adsorption, also known as physisorption, is the process by which molecules of a fluid are fixed on the walls of a solid material. The adsorbed molecules undergo no chemical reaction but simply lose energy when being fixed: adsorption, the phase change from fluid to adsorbate (adsorbed phase) is exothermic. Moreover this process is reversible. In the following, we will focus on adsorption systems mainly used in cooling (or heatpumping) machines: a pure refrigerant vapour that can easily be condensed at ambient temperature and a microporous adsorbent with a large adsorption capacity.
The main components of an adsorptive cooling machine are the adsorber (in the present case, the solar collector itself), the condenser, the evaporator and a throttling valve between the last two devices, see Fig. 2. An ideal cycle is presented in the D‚¬hring diagram (LnP vs. …I=T), Fig. 1. 2001). We can summarize it in four stages. C. Hildbrand et al. / solar Energy 77 (2004) 311-318 13 Fig. 1 . An ideal adsorption cooling cycle in the D‚¬ hring diau gram. Saturation liquid- vapour curve for the refrigerant (EC dashed line), isoster curves (thin lines), adsorption cycle (thick lines). Heating period: step AB (7 a. m. fl 10 a. m. ) and step BD (10 a. m. fl 4 p. m. ); cooling period: step DF (4 p. m. fl 7 p. m. ) and step FA(7 p. m. fl 7 a. m. ).
Step 1: isosteric heating ¶A ! BD. The system temperature and pressure increase due to solar irradiance. Step 2: desorption + condensation dB ! DD. Desorption of the water steam contained in the silicagel; condensation of the water steam in the ondenser; the water in the evaporator is drained through the valve. Step 3: isosteric cooling ?D ! FP. Decrease of the period of sunshine; cooling of the adsorber; decrease of the pressure and the temperature in the system. Step 4: adsorption + evaporation ¶F ! AD. Evaporation of water contained in the evaporator; cooling of the cold cabinet; production of ice in the evaporator; readsorption of water steam by the silicagel. 3.
Description of the machine tested in Yverdon-les-Bains, Switzerland Adsorptive pair. The refrigerant is water, and the adsorbent is a microporous silicagel (Actigel SG¤ , Silgelac). Collector-adsorber. The solar collector (2 m2 , tilt angle of 300) is double-glazed: a Teflon¤ film is installed between the glass and the adsorber itself. The adsorber consists of 12 parallel tubes (72. 5 mm in diameter) that contain the silicagel (78. 8 kg). The tubes are covered with an electrolytic selective layer (Chrome-black, Energie Solaire SA), which absorbs 95% of the visible solar radiation while presenting an emissivity of 0. 07 in the infrared wave-lengths.
A valve located between the graduated tank and the evaporator is needed on this machine. For control strategy reasons, this valve is electrically powered. 4. 5. Ventilation damper management Closing: when the irradiance goes above 100 W/m2 . Opening: at the end of the afternoon when the angle of the solar beam radiation incident upon the aperture plane of collector (angle of incidence) is above 500. 4. Measurements and operations The objective of the 2001 series of measurements was to obtain a high number of measurements continuously, in order to characterise he working of our adsorption machine. To do this, a system of measurement and a constant procedure of load has been established. 4. 1.
Measurements The temperature is measured (probes Pt100) in the silicagel of a central tube of the collector-adsorber (7 sensors), on two condenser tubes and three evaporator tubes; and the ambient air temperature is also measured. The vapour pressure is measured by a piezogauge in the collector-adsorber, in the condenser and in the evaporator. The global irradiance in the plane of the collector is recorded by a pyranometer. A graduated tank (6. 5 1) collects the condensed water. The level of liquid water is automatically measured by a level detector. The series of measurements took place from July 25th to September 30th 2001 (68 days) in Yverdon-lesBains (altitude: 433 m, longitude: )6. 380, latitude: 46. 470). Fig. 3 shows the observed weather conditions (daily irradiation and mean external temperature).
This graph shows two different periods: (1) From July 25th to the beginning of September: during this summer period, the mean external temperature is above 20 oc and the mean daily irradiation reaches 22 MJ/m2 . This fine weather period is interrupted between the 3rd and 9th August by ess favourable weather. (2) From the beginning of September to the end of the measurement: the mean external temperature and the daily irradiation are distinctly lower (13 oc and 13 MJ/m2 ). Furthermore, the conditions are very variable from one day to the next. 4. 2. Acquisition system and command 6. Performance of the tested unit A Labview¤ program takes measurements and administers various commands (valve, dampers and load). A measurement is made every 30 s.