global irradiance [W mA2 ] global irradiation [J] evaporation latent heat for water [J kgA1 ] mass [kg] number [–] pressure [Pa] heat quantity [J] area [m2 ] temperature [K] time [s] d e fs h l L m N R S tl w day evaporator collector front side solar heat supply load liquid maximum net rough solar thermal losses (of the cabinet) water Indexes c condenser Guilleminot, 1986; Pralon Ferreira-Leite and Daguenet, 2000) using either a chemical reaction or adsorption, follow an alternative cycle of heating/cooling, also known as ‘intermittent’, the period of which corresponds to the alternation of day and night.
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 0 °C, 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 purpose of the present study is to obtain better performances than those reported above, with 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 ? at-plate solar collector containing the adsorbent. However, when referring to the work reported above, both the e? ciency 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 developing adsorptive solar refrigerators since 1999. The ? rst 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 days. We have highlighted the great in? uence of both external temperature and daily irradiation upon the daily coe? cient 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 ? uid are ? xed on the walls of a solid material. The adsorbed molecules undergo no chemical reaction but simply lose energy when being ?
xed: adsorption, the phase change from ? uid 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 u vs. A1=T ), Fig. 1. The cycle is explained in detail in (Buchter et al. , 2001). We can summarize it in four stages. C. Hildbrand et al. / Solar Energy 77 (2004) 311–318 313 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. ? 10 a. m. ) and step BD (10 a. m. ? 4 p. m. ); cooling period: step DF (4 p. m. ? 7 p. m. ) and step FA (7 p. m. ? 7 a. m. ). Step 1: isosteric heating ? A ! B?.
The system temperature and pressure increase due to solar irradiance. Step 2: desorption + condensation ? B ! D?. Desorption of the water steam contained in the silicagel; condensation of the water steam in the condenser; the water in the evaporator is drained through the valve. Step 3: isosteric cooling ? D ! F?. 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 ! A?. 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 SGa , Silgelac). Collector–adsorber. The solar collector (2 m2 , tilt angle of 30°) is double-glazed: a Te? ona ? lm 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. The tubes are layered with a material which presents high conductivity but low speci? c heat capacity (sheets of graphite: Papyexa , Le Carbone Lorraine). A central tube is made out of a grid (diameter 15 mm, mesh 1 mm, wire 0. 45 mm diameter). The ventilation dampers mentioned in the previous sections consist of a Fig. 2. Photograph and plan of an adsorptive solar refrigerator: solar collector–adsorber (1) with detail: glass cover (A), Te? ona ? lm (B), tube covered with selective surface (C) and internally layered with Papyexa , central tube for vapour
transport (D), silicagel bed (E), thermal insulation around the collector (F); ventilation dampers (2) closed (2a) and open (2b), condenser(3), cold cabinet (4), graduated tank (5), valve (6), evaporator and ice storage (7). mechanism that allows the thermal insulation to be opened on the rear side of the collector (50 mm glass ?bre), to provide e? cient cooling by natural convection during the night. Condenser. Eight parallel ? nned tubes make a condenser, and are cooled by natural convection of air. The total ? n area is 6. 9 m2 . Evaporator, ice storage and cold cabinet. The evaporator consists of three rings made of square tubes.
The total heat exchange area is 3. 4 m2 The evaporator contains 40 l of water which can be transformed into ice during the evaporation stage. The cold cabinet is chesttype and is well insulated (170 mm of expanded polystyrene) with an internal volume of 320 l. 314 C. Hildbrand et al. / Solar Energy 77 (2004) 311–318 Valve. 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 50°. 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 the 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 l) collects the condensed water. The level of liquid water is automatically measured by a level detector. 5. Meteorological conditions The series of measurements took place from July 25th to September 30th 2001 (68 days) in Yverdon-lesBains (altitude: 433 m, longitude: )6. 38°, latitude: 46. 47°). Fig. 3 shows the observed weather conditions (daily irradiation and mean external temperature).
This graph shows two di? erent periods: (1) From July 25th to the beginning of September: during this summer period, the mean external temperature is above 20 °C and the mean daily irradiation reaches 22 MJ/m2 . This ? ne weather period is interrupted between the 3rd and 9th August by less 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 °C 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 Labviewa program takes measurements and administers various commands (valve, dampers and load). A measurement is made every 30 s. For each day, a gross solar COPSR can be de? ned as the ratio of the heat extracted by evaporation of water to the solar heat supply, see equation (1). The ? rst one, Qe , is obtained by multiplying the mass of processed water, mL , by the enthalpy di? erence between the saturated vapour at Te and the saturated liquid at Tc The second one, Qh , is the product of the surface A of collector and the solar irradiation obtained by integrating the solar irradiance G from sunrise to sunset. This yields the following