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SOLAR-POWERED ADSORPTION CHILLER WITH CPC COLLECTION SYSTEM: COLLECTOR DESIGN AND EXPERIMENTAL RESULTSManuel I. GonzálezPhysics Department, University of BurgosAvda. Cantabria s/nBurgos 09006, Spainmiglez@ubu.esLuis R. RodríguezPhysics Department, University of BurgosC/ Villadiego s/nBurgos 09001, SpainABSTRACTA new class of compound parabolic concentrator (CPC) useful in solar, solid sorption cooling is presented. The main feature of this class of CPC is that only a portion of its tubular receiver is exposed to sunlight. During the daily phase of the refrigeration cycle the non-exposed portion is covered with a thermal insulation, which is removed at the beginning of the nightly phase, in order to improve the natural cooling of the sorption bed.Geometric characteristics of the proposed CPC are given as a function of the concentration factor and the fraction of receiver area which is exposed to radiationA prototype of chiller using this type of collector and the activated carbon – methanol pair is described. The concentration factor is 1.41 and the receiver area is exactly half exposed to sunlight. The prototype was tested in Burgos (Spain) and the measured solar COP ranged from 0.078 to 0.096.1. INTRODUCTIONMany of the solar adsorption machines described in the literature are based on the flat-plate collector geometry [1]. Porous beds of some other prototypes are enclosed in tubular receivers, evacuated or non-evacuated. Very little attention in this field has been paid to concentrating collectors such as compound parabolic concentrators (CPC). An exception is Headley’s work [2], which reports COP’s of about 2% and the ability to produce ice even in overcast days.This work deals with the use of CPC collectors suitable for solar refrigerators. A new class of CPC concentrators is presented and some of their geometric characteristics are given. Section 3 describes a prototype of solar refrigerator based on CPC geometry, built and tested in Burgos (Spain); the working pair is methanol – activated carbon. Finally, section 4 shows typical experimental results obtained with this unit.2. CPC WITH PARTIALLY EXPOSED TUBULARRECEIVERA logical configuration of CPC collection system for adsorption machines consists of placing the porous bed inside the tubular, fully exposed to sunlight, receiver.H owever, this solution leads to a difficult compromise between the adsorbent mass and the geometric parameters of the concentrator, mainly the concentration factor.A suggested solution is what we call ‘CPC with partially exposed tubular receiver’, in which only a portion 2α of the angular perimeter of the tube is effectively exposed to solar radiation (Fig. 1).3 SOLAR COLLECTOR TECH NOLOGIES AND SYSTEMS917Fig. 1: A CPC with partially exposed tubular receiversuitable for adsorptive refrigerators. ζ is the acceptance half-angle. 2.1 The profile of the Proposed CPCThe general algorithm to calculate the profile of a CPC is explained in [3]. The basic idea is that every solar ray whose incidence angle is exactly the acceptance half-angle ζ should be tangent to the receiver after being reflected bythe sheet (see Fig. 2).Fig. 2: Geometric construction to determine the profile of aCPC with partially exposed tubular receiver.Although the details of the calculation will be omitted, a short explanation must be given. Let ϕ be the angle givingthe position of the point Q(ϕ) where the reflected ray reaches the receiver, and let P(ϕ) be the point of the reflective sheet where the reflection occurs. The distance a (ϕ) between P and Q, expressed in units of the receiver radius, is given by:ϕαϕ−=)(a (points between M and N) (1))sin(1)cos(22)(ζϕζϕπϕζαϕ++++−−+=a (points over N) (2) These two expressions permit an easy calculation of the sheet profile in Cartesian coordinates.In order to manufacture the CPC, the height and the length of the sheet must be known. Although (1) and (2) are exact, no attempt has been made to find closed expressions for both parameters. Instead, they have been computed numerically. Figures 3 and 4 show the results as a function of the angles ζ and α.Fig. 3: The height of a non-truncated CPC with partiallyexposed tubular receiver. The height is measured from the centre of the receiver to the top of the reflective sheet.Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement918Fig. 4: Total length of the reflective sheet vs. concentrationfactor for various values of the angle α.3. THE PROTOTYPEA prototype of solar chiller with a CPC as described in the previous section and using the methanol / activated carbon pair was built and tested in Burgos (Spain). The generator is an array of four CPC collector with tubular receivers oriented in the EW direction (see figures 5 and 6). The chosen concentration ratio is 1.41 (i.e. ζ = 45º) and half the receiver perimeter is exposed to sunlight (α = 90º). Total aperture area is 0.55 m 2; the porous bed contains 7.6 kg ofactivated carbon.Fig. 5: Schematic view of the prototype.Fig. 6: Front view of the prototype.The condenser is a cylindrical chamber crossed by an array of parallel water pipes; the condensation occurs inside the chamber. The total exchange area is 0.33 m 2. The cooling water is stored in a 100-litre tank annex to the unit (Fig. 7). The evaporator consists of a grid of vertical tubes, where the liquid methanol is stored as the condensation proceeds. Each tube is surrounded by a cylindrical reservoir containing the water to be chilled. Thermal insulation is provided by 8 cm thick polystyrene walls, including a cover. During the daily phase of the cycle the cover is removed (see Fig. 7). When the solar radiation ceases the cold box is covered and the bottom insulation of the generator isremoved.Fig. 7: Rear view of the prototype.3 SOLAR COLLECTOR TECH NOLOGIES AND SYSTEMS 919Measuring equipment includes a pyranometer; various thermocouple probes: ambient air, receiver surface, cold box; and three pressure meters: generator, top of the evaporator, bottom of the evaporator. The two latter allow knowing the amount of liquid methanol inside the evaporator and hence the concentration of methanol in the porous bed.4. EXPERIMENTAL TESTThis prototype was tested in the summer of 2005 in Burgos (Spain). Table 1 shows a summary of experimental resultsfor some selected cycles. As we can see, measured COP’s ranged from 0.078 to 0.096.TABLE 1: SUMMARY OF EXPERIMENTAL RESULTS DAY G M cond M ev COP 1 July 19.5 2.45 1.62 0.0965 July 27.2 2.96 1.98 0.0826 July 22.0 1.76 1.82 0.0937 July 26.6 2.05 1.84 0.0788 July 28.2 2.16 1.95 0.07811 July 27.5 2.91 2.25 0.09212 July 27.7 2.29 2.09 0.08513 July 27.4 2.15 20.5 0.08414 July 26.1 2.15 2.09 0.08915 July 25.9 2.27 2.04 0.088(G): daily irradiation (MJ·m-2); (M cond): condensed mass (kg m-2); (M ev): evaporated mass (kg·m-2).Referring to Table 1 it should be noted that both the condensed and the evaporated masses have been measured directly, since our experimental setup allows continuous record of methanol concentration in the porous bed, as explained before. The bed temperature was not measured but estimated with the help of the Dubinin – Astakhov equation, whose parameters were previously found in [4].Fig. 8 shows the time evolution of one of these cycles in a log P vs. estimated bed temperature diagram. As we can see, the bed temperature ranges between 37 ºC and 117 ºC –the latter, rather high value is possible thanks to the concentration provided by the CPC. The maximum and minimum methanol concentrations in the porous bed were 210 and 50 g/kg, respectively. The condensation temperature was 27 ºC at the beginning of the condensation stage and 33 ºC at its end. This increase is caused by the progressive heating of the water tank. The evaporation temperature drops from 15 ºC to -8 ºC as evaporation phase proceeds.Bed temperature (ºC)3456ln(P/mbar)Saturationtemperature(ºC)Fig. 8: The 12 July cycle in a (t, ln P) diagram. (Diagonal lines): isosters.The average gross cold production for the cycles shown in Table 1 was 2.2 MJ per m2 collection area. About 35 % of this quantity was extracted from the 9 kg per m2 chilled water, whose temperature dropped from 25 ºC to about 1 ºC in every single cycle.5. CONCLUSIONSThe CPC with partially exposed tubular receiver seems to be an interesting option as a collection system for solar adsorptive refrigerators. The characteristic half angles ζand α can be varied in various manners, but future work must be conducted to achieve the best compromise between daily collection efficiency, adsorbent mass per unit collection area and nightly cooling of the sorption bed.6. ACKNOWLEDGMENTSThe authors acknowledge the financial support provided by the Spanish Ministerio de Ciencia y Tecnología (Grant REN-2003-09684-C02-01).Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement 9207. REFERENCES(1) A. O. Dieng and R. Z. Wang. “Literature review onsolar adsorption technologies for ice-making and air-conditioning purposes and recent developments in solar technology”. Renewable and Sus tainable Energy Reviews 2001; 5: 313-42.(2) O. StC. Headley, A. F. Kothdiwala, and I. A. McDoom,“Charcoal–methanol adsorption refrigerator powered by a compound parabolic concentrating solar collector”.Solar Energy1994; 53(2): 191-7.(3) J. A. Duffie and W. A. Beckman, “Solar Engineering ofThermal Processes”, 3rd ed. John Wiley & Sons, New York 2006.(4) M. I. González, Thesis “Refrigeración Solar porAdsorción con Sistema de Captación CPC: Experimentos y Modelo”, University of Burgos (Spain), 2006.。