To achieve working temperatures around 200°C with acceptable efficiency, the absorber area has to be further reduced. Two-dimensional concentrators create a focal line, in which the absorber is placed. These concentrators have the drawback that only direct radiation can be used. One approach to build these concentrators is the development of small and cost-effective parabolic trough systems.

Extensive research on a small parabolic trough collector was done by the institute AEE INTEC (Gleisdorf, Austria) for operating temperatures of 200°C [7]. Another example is the parabolic trough collector PTC 1800 from the company SOLITEM which was developed with research support of the DLR (Germany). Demonstration plants for solar air-conditioning with this collector were already installed in Turkey.

Currently other types of linear focussing collectors are under development [8]. The linear concentrating Fresnel collector of the PSE AG (Freiburg, Germany) is already available on the market and proved its feasibility in several demonstration plants.

3. Rough Comparison

The efficiency of a solar collector can be expressed by

where K (0) is the IAM (Incident Angle Modifier) of the collector, which expresses the ratio of the optical efficiency at a solar incident angle 0 to the optical efficiency at irradiance normal to the aperture of the collector. The IAM-characteristics of a collector type for all possible directions of incidence (3D-IAM) is a very important attribute, which highly influences the energy gain of a collector. Further information about the 3D-IAM can be found in the paper of Paolo Di Lauro et. al. within the EuroSun08 proceedings.

The collector parameter n0 describes the efficiency of the light conversion into heat without thermal losses. These are expressed by the factors a1 and a2. Tav is the working temperature and Tamb is the ambient temperature. For a detailed description see [5] and [6].

4. Conclusion

Within the IEA-SHC Task 33 SHIP alternatives to standard flat-plate and evacuated tube collectors were constructed or are still under development. From the rough comparison in figure 3 it turns out that for solar air-conditioning technologies that require working temperatures below 110 °C improved flat-plate collectors or collectors of the CPC-type can be a cost-effective alternative to evacuated tube collectors. The currently developed small parabolic trough collectors are predestinated to support double effect absorption

chillers or steam ejection chillers if the fraction of direct radiation is high and the specific collector costs are comparable to those of evacuated tubes.

To select the suitable collector technology for a specific system properly, the incident angle modifier, the weather conditions and the industrial load profiles or the characteristics of the cooling machine always have to be taken into account by a detailed simulation. Simulations can only be performed properly, when the optical behaviour of a collector for all directions of the incident radiation (3D-IAM) can be well approximated.

Besides the specific costs of the collector field also aspects like the resulting collector area and building integration have an influence on the decision for a certain collector type.

References

[1] Henning, Hans-Martin: Solar-assisted air-conditioning in buildings: A handbook for planners. Wien; New York: Springer-Verlag 2004

[2] Henning, Hans-Martin: Auslegung von solaren Klimatisierungssystemen.

In: 13. Symposium Thermische Solarenergie. Tagungsband.

Bad Staffelstein, 14.-16. Mai 2003, S. 253 — 258

[3] Deutsche Gesellschaft fur Sonnenenergie: DGS-Leitfaden Solarthermische Anlagen. 7. Aufl. Berlin: Landesverband Berlin Brandenburg 2006

[4] Rommel, Matthias: Medium Temperature Collectors for Solar Process Heat up to 250°C. In: 2nd European Solar Thermal Energy Conference estec. Proceedings. Freiburg, Germany, June 21.-22., 2005,

P. 167 — 172

[5] Duffie, J. A.; Beckman, W. A.: Solar Engineering of Thermal Processes.

3rd ed. Hoboken, New Jersey: John Wiley and Sons 2006

[6] Rabl, Ari: Active Solar Collectors and their Applications.

New York: Oxford University Press, Inc. 1985

[7] Jahnig, Dagmar: Development and Optimisation of a small-scale parabolic trough collector for production of process heat. Gleisdorf, Austria: AEE INTEC 2004. Available from: djaehnig@aee. at

[8] Weifi, Werner et. al.: Process Heat Collectors. IEA Task 33/IV: Solar Heat for Industrial Processes. Gleisdorf, Austria 2008. Internet: http://www. iea-shc. org/task33/publications/index. html

[9] Hefi, Stefan: Application of Medium Temperature Collectors for Solar Air-Conditioning. In: 2nd International Conference Solar Air-Conditioning. Tarragona, Spain, 18.-19. October 2007, Proceedings S. 118 — 123.

[1] Halogen lamp

2. Luxmeter

3. Flate plate collector

4. Temperature sensor 1-inlet

5. Temperature sensor 2-outlet

6. Temperature sensor 3-water tank

7. Hot water tank

8. Hot water tank overflow connection

9. Filler valve in primary circuit

10.Regulator valve for setting the volumetric flow rate

[2] Digital display

12. Air bleed valve

Fig. 1. The experimental stand and its components

Various incidence angles, measured using as reference the horizontal plane were investigated: 0o, 10o, 20o, 30o, 40o, 50o. For each measurement the light density, the flow rate, along with the inlet and outlet temperature were recorded after 15 minutes since the experiment was started. The thermal power and the system efficiency were calculated using the equations (1) and (2).