In [13] the COP is considered as key figure to characterise the energy performance of a refrigeration machine. For thermal driven refrigeration machines, the COP, which indicates the required heat input for the cold production, can be defined as follows:

Qlow Heat flux extracted at low temperature level

(eq. 2) COP = &— = .

Qhigh Driving heat flux to cooling equipment

Based on the measurement data of the years 2004 to 2006, the COP depending on the daily generated cold work by the absorption chiller is given in the diagram of figure 5.

COP [-1

The COP of the absorption chiller increases with increasing amount of daily generated cold work. The absorption chiller reaches its nominal COP, when it generates more than 200 kWh cold work per day. Below a daily amount of 200 kWh/d cold work, part load is governing the operating time with timing operation of the absorption chiller. The total cold work generated by the absorption chiller in the observed five year period is 31,365 kWhth.

Besides the operation of the absorption chiller, some cold work was generated by the operation mode free cooling. Free cooling does not only occur in spring and autumn, but also under German weather conditions in summer time. The diagrams in figure 6 established the amount of cold work generated by the absorption chiller and the operation mode free cooling in relation for the months May to September for the years 2005 and 2006.

The operation mode of free cooling ranges from 1.2 up to 70 % in some months. In the years 2002 to 2007, 25% of the cold demand of the solar cooling plant was covered by free cooling, which means 10,499 kWhth. These results show the extensive potential of the operation mode free cooling. The total supplied cold work of the solar cooling plant is 41,864 kWhth.

In 2003 leaks in the re-cooling cycle were found. The reason of the leakages was corrosion of the welding seam of the steel pipes, although a re-cooling water treatment was running. A refurbishment of the re-cooling loop was carried out in the year 2004. The pipelines of the re-cooling cycle were pickled by hydrochloric acid to remove corrosion products, afterwards the pipelines were rinsed and existing leakages repaired. Figure 7 shows two photos of the inner wall of the pipelines before and after the treatment.

To avoid further corrosion of the pipelines, the re-cooling water treatment was adjusted and a regularly water analysis and maintenance have been done to ensure the quality of the re-cooling water. Since the refurbishment, which took three weeks, the re-cooling cycle is operating well.

3. Conclusion

This study presents the operational experiences of the solar cooling plant of the Fraunhofer Institute in Oberhausen, Germany, which has operated for space cooling since 2002. The plant includes a single­effect lithium bromide-water absorption chiller with a cold capacity of 58 kWth, vacuum tube collector with an aperture area of 108 m2, a hot water storage with the capacity of 6.8 m3, a cold water storage with the capacity of 1.5 m3 and a 134 kWth cooling tower. The solar cooling plant is integrated into the supply infrastructure of the institute, so that solar surplus heat can be supplied to the heating system

and vice versa. Furthermore the operational concept of the cooling plant enables the operation mode free cooling. In the observed time period from 2002 to 2007, the total cold work provided by the solar cooling plant is 41,864 kWhth. Free cooling covers 25 % of the cold demand. The average solar collector efficiency is 0.28 and the COP of the absorption chiller varies from 0.37 to 0.81. In the five year period, 60 % of the driving heat of the absorption chiller is produced by the solar collector field.

Acknowledgments

The author Ahmed Hamza H. Ali wishes to acknowledge the Alexander von Humboldt Foundation, Germany, for the fellowship grant during this work. The authors acknowledge the funding of the German Ministry of Economy within the project “Study on Measurements of Optimization of Operation of Solar-thermal Driven Plants for Cold Generation (FKZ 0327406D). The project is furthermore integrated within the IEA Task 38.

References

[1] K4RES-H., (2006). Solar Assisted Cooling-State of the Art, Key Issues for Renewable Heat in Europe, Solar Assisted Cooling — WP3, Task 3.5, Contract EIE/04/204/S07.38607, 21p.

[2] Balaras, C. A., Grossman, G., Henning, H. M., Ferreira, C. A.I., Podesser, E., Wang, L. and Wiemken, E. (2007). Solar air conditioning in Europe—an overview. Renewable and Sustainable Energy Reviews, Vol.11, pp.299-314.

[3] Ward D. S. and. Lof G. O. G., (1975). Design and construction of a residential solar heating and cooling system. Solar Energy, Vol. 17, No.1, pp. 13-20.

[4] Ward D. S., Lof G. O. G. and Uesaki T., (1978). Cooling subsystem design in CSU solar house III Solar Energy, Vol. 20, No. 2, pp. 119-126.

[5] Ward, D. S., Weiss, T. A. and Lof G. O. G., (1976). Preliminary performance of CSU Solar House I heating and cooling system. Solar Energy, Vol. 18, No. 6, pp. 541-548.

[6] Bong, T. Y., Ng, K. C., and Tay, A. O., (1987). Performance study of a solar powered air conditioning system. Solar Energy, Vol. 39, No. 3, pp. 173-182.

[7] Al-Karaghouli, A., Abood, I., Al-Hamdani, N. I., (1991). The solar energy research center building thermal performance evaluation during the summer season. Energy Conversion and Management, Vol. 32, pp. 409-417.

[8] Yeung, M. R., Yuen, P. K., Dunn, A., and Cornish, L. S., (1992). Performance of a solar powered air conditioning system in Hong Kong. Solar Energy, Vol. 48, No. 5, pp. 309-319.

[9] Best, R., Ortega, N., (1999). Solar refrigeration and cooling, Renewable Energy, Vol. 16, pp. 685- 690.

[10] Syed, A., Izquierdo, M., Rodriguez, P., Maidment, G., Missenden, J., Lecuona, A. and Tozer, R., (2005).

A novel experimental investigation of a solar cooling system in Madrid, Int. J. Refrigeration, Vol. 28, pp. 859-871.

[11] Pongtornkulpanich, A., Thepa, S., Amornkitbamrung, M., and Butcher C. (2007). Experience with fully operational solar-driven 10-ton LiBr/H2O single-effect absorption cooling system in Thailand. Renew Energy, doi:10.1016/j. renene.2007.09.022.

[12] Ahmed Hamza H. Ali, Peter Noeres and Clemens Pollerberg. (2008). Performance assessment of an integrated free cooling and solar powered single-effect lithium bromide-water absorption chiller. Solar Energy. doi:10.1016/j. solener.2008.04.011.

[13] Henning, H. M. (Ed.) (2007). Solar-Assisted Air-Conditioning in Buildings, A Handbook for Planners. ISBN 978-3-211-73095-9. Second Ed., Springer Wien New York.