Due to the conditions on the site some degrading of the collector reflectivity could occur due to the effect of dust exhausted by a close chimney. This effect is presented in figure 14 and figure 15 for a system with varying number of collectors, a 2 m3 storage vessel and mass flow in the HX of 3000 kg/hr.

In systems with a small collector field the effect of dust has bigger influence on the solar fraction as well as on the overall efficiency. Because a bigger amount of heat is dumped due to overheating in systems with bigger collector fields only part of the radiation lost by the effect of dust is “missing” in the system, these losses reduce the total system efficiency with about 1% .While for systems with smaller solar fields, which dump less energy are therefore more affected by the effect of dust, and these losses reduce the total system efficiency with about 5% . With respect to this effect the collector field should be designed in an appropriate way to gain the expected heat and frequent cleaning measurements should be done for the collectors to ensure high fraction of cleanliness.

6 Conclusion

The preliminary development of a solar assisted refrigeration system has been object of a simulation study with the aim to characterise the main components. The innovative concept is meant for process cooling of received fresh milk in a dairy factory in the city of Marrakech, Morocco. A portion of the process has been selected and the daily cooling load profile has been characterized: cooling power and the duration of the cooling process. The system components’ selection and system configuration was carried out to ensure the highest solar energy use considering the given cooling load profile. Moreover a parametric study on the main system components has been worked out through simulations.

Regarding the collectors’ field area, its increase resulted in a growth of both, the solar fraction and the dumped energy. The latter is due to defocusing of the collectors when the input energy exceeds the demand; which can be significant for large collectors’ areas. As a result for the given application 18 modules of parabolic through collectors were selected, in order to guarantee enough thermal energy with an acceptable amount of dumped energy.

The optimum size of the PCM storage, for the selected collector area, is 2 m3. Considering the flow rate, a middle value of 3000 kg/h was selected as the flow rate for the cold side of the system, aiming at a compromise between increase in solar fraction and electricity costs. Three percent of the solar fraction depends on the cleanliness of the collector field, and this was taken into consideration during the sizing process as well as for the planned frequent maintenance of the system. As result of this study, the solar refrigeration system, which will be installed at the diary in Marrakech (October 2008) will consist of: 18 modules of parabolic through collectors, 2m3 PCM storage, with cold water flow rate of 3000 kg/h.

References

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[2] M. Motta, M. Aprile, H. Henning — High efficient solar assisted sorption system for air-conditioning of buildings; — Proc. of Eight international symposium gleisdorf solar, 6th — 8th September 2006, Gleisdorf, Austria.

[3] Duffie, J. A., and W. A. Beckman; Solar Engineering of Thermal Processes; J. Wiley and Sons, 1991

[4] Tess Documentation Library, chapter 10.Solar Library Technical Reference, type 536: LINEAR PARABOLIC CONCENTRATING SOLAR COLLECTOR. pp536.1-5

[5] Solar Energy Laboratory, University of Wisconsin-Madison, USA et. al.; TRNSYS 16 — a Transient System Simulation program; http://sel. me. wisc. edu/trnsys/user-resources/index. html (08.08.2008)

[6] Eams, I. W. and Adref, K. T. (2002) Freezing and melting of water in spherical enclosures of the type used in thermal (ice) storage systems. Applied Thermal Engineering. 22, pp. 733-745.

[7] M. Aprile, Simplified models implementation, MEDiterranean food and agro Industry applications of Solar COoling technologies (MEDISCO). STREP — FP6 — EU contract Project no. 032559.

[8] Aprile, M. (2006) — Simulation study of an innovative solar absorption cooling system — Master Thesis “Dalarna University” 2006.