In a next test both models, i. e. Type 307 (PD2) and Type 177, are fed with two weeks of real time measuring data of the solar cooling plant at Ebner Solartechnik. Since both types do not account for transient effects only stationary operation periods have been considered. Fig. 6 and Fig. 8 show the simulated cooling capacity (QE, sim) compared to the measured capacity at the plant (QE, Ebner).

Regarding these curves, large deviations are observed for Type 307 (PD2). Here, the simulated cooling capacity is always higher than the measured data at Ebner. The reason for this is different operating conditions for the chiller at Ebner compared to the chiller measured at ILK. Especially the cooling water mass flow rate at Ebner is only the half of the nominal mass flow rate (i. e. 5000 kg/h). In addition, the chilled water flow rate differs from the nominal value (2000 kg/h) and

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varies in a range of ~2200 to 2900 kg/h. Fig. 7 shows a decreasing deviation with increasing chilled water mass flow for Type 307 (PD2). This can be ascribed to an increasing heat transfer coefficient at the external side of the evaporator, which partially compensates the reduced heat transfer coefficient in the cooling water circuit.

pared to reality, the slope is calculated too low especially at low AtACE-values. Thus, the reduced cooling capacity at Ebner (having its physical cause in a reduced cooling water flow rate) is coincidentally calculated ‘correctly’ by the characteristic equation with variable parameters. As a consequence, due to the scatter in the measured data the variable parameters seem to be too uncertain for application at the lowest end of temperature lifts (e. g. AtACE < 13K).

On a first view the result is as expected: Better agreement of measured data to simulated values if variable parameters are used. Nevertheless, in this case the better agreement is just an artefact and even the method of characteristic equations has to be improved for non-nominal flow conditions.

3. Conclusion

Concluding the results it has been found that both TRNSYS-Types are able to predict laboratory test data correctly. For Type 107 based models care has to be taken when creating the performance data file. The opportunity to fill in a great many of supporting points describing the chillers’

performance easily entices the user to replicate each measuring point without taking the unavoidable measuring errors into account. Thus, if measured data are used in the performance file in combination with stepwise interpolation, numerical results are obtainable, were the physical background is questionable. Care has to be taken, that the same flow rates are used in the simulation as have been used to prepare the performance data file. Alternatively a further improvement of Type 107/307 would be necessary to consider mass flow deviations inside the performance data file by an enhanced look-up approach. Only apparently the simulation results with Type 177 show better agreement to the measurements of a solar cooling plant. However, also Type 177, which uses the method of characteristic equations, has to be improved for simulating non-nominal flow conditions.

Acknowledgements

The authors would like to thank Ebner Solartechnik for their collaboration and especially Stiftung Sudtiroler Sparkasse for the financial support.

Nomenclature

References

[1] Henning, H.-M.: Solar-Assisted Air-Conditioning in Building — A Handbook for Planners. Springer-Verlag/Wien, 2004.

[2] Witte et al: Modelling of a Solar Combi-Plus System for residential and small commercial applications. estec 2007 3rd European Solar Thermal Energy Conference, June 2007, Freiburg i. Br., Germany; pp. 444-445.

[3] University of Wisconsin-Madison: TRNSYS 16: A TRaNsient SYstem Simulation program — Volume 5 Mathematical reference; Type 107: Single Effect Hot Water Fired Absorption Chiller; Page 155-158. Solar Energy Laboratory, University of Wisconsin-Madison. 2004.

[4] H.-M. Hellmann, C. Schweigler, F. Ziegler : The characteristic equation of sorption chillers.

Proc. Int. Sorption Heat Pump Conf., Munich, 24.-26. March 1999; pp. 169-172.

[5] Albers, J.: TRNSYS Type107 Part load simulation of single staged absorption chillers in quasi steady states — Contribution to a design tool for solar assisted air conditioning systems developed in IEA TASK25 Subtask B, IEMB-Report 2-67/2002.

[6] H.-M. Hellmann, F. Ziegler: Simple absorption heat pump modules for system simulation programs, ASHRAE Transactions 105 (1), (1999)