Comparison of the control strategies

To compare both control strategies TRNSYS-simulations of two different places with different irradiation conditions have been carried out.

Firstly, a cloudy summer day in Berlin using a 34 m2 flat plate collector field inclined 30° in direction south separated from the chiller by a counter flow heat exchanger has been simulated. Results are presented in Figures 4 and 5.

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Fig. 4. External temperatures for a cloudy day in Berlin.

In Figure 4 we see the inlet temperatures of hot and cooling water and the outlet temperature of the generated chilled water temperature for the conventional and the new control. As described before the conventional control cuts driving temperature peaks in order to ensure a minimum chilled water temperature. Cooling water temperature is constant at 27°C. The new control can use the whole solar offer. It does not cut any peaks. Cooling water temperature is adapted depending on driving temperature but limited to a minimum value of 23°C. At lower values electric energy consumption of the cooling tower fan would increase too much as it rises with the third power of the fan speed. The chilled water temperature can not be kept at the required level of 15°C for the whole working period but it is better approached as in case of the conventional control strategy.

The advantage of the new control strategy in respect of meeting the required cooling load even at cloudy days with moderate irradiation is better seen in Figure 5. Assuming a cooling load of 10 kW we see that the new control strategy can provide the full load nearly an hour earlier than the conventional. Small drops in driving temperature can be compensated completely, bigger ones better than in the case of the conventional control.

Total cooling energy supplied could be increased by 13% to 72 kWh compared to 64 kWh with the conventional control strategy (see Table 1). Total water consumption increases by 12% but specific water consumption drops slightly. Specific fan power consumption is about 11% higher. 8 kWh more cooling energy are generated by an additional fan power consumption of only 1.3 Wh. This corresponds to a COP of more than 6,000. That means that the small additional fan power consumption is well justified.

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Fig. 5. Cooling capacity provided by conventional and new control strategy.

Table 1. Comparison of cooling tower power and water consumption for Berlin, 25th August 2003.

Conventional control

New control

Difference [%]

Total cooling energy supplied [kWh]

64

72

13

Total fan power consumption [kWh]

0.75

0.93

25

Specific fan power consumption [kWh/kWh]

0.012

0.013

11

Total water consumption [kg]

171

191

12

Specific water consumption [kg/kWh]

2.7

2.7

-1

Secondly, a sunny day in Iran has been simulated. In this case higher driving temperatures up to 115°C have been available. In Figure 6, the temperatures of the external water circuits are presented.

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Fig. 6. External temperatures for a sunny day in Iran.

Again, after start-up and before shut-down when irradiation is low the new control strategy has the advantage to meet the required chilled water temperature or to meet it better than the conventional. Shortly after start-up a cooling capacity of around 9 kW can be supplied, already. The full cooling capacity is reached 40 minutes earlier than in the case of the conventional control strategy. In the evening the full cooling capacity can be supplied for 20 minutes longer.

Figure 7 shows the comparison of cooling tower fan power consumption for both control strategies.

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Fig. 7. Comparison of cooling tower fan power consumption.

The advantage of the new control strategy regarding parasitic electricity consumption is here clearly to be seen. At peak irradiation at 2 pm the conventional control needs nearly five times more fan power (270 W compared to 56 W). Table 2 gives an overview about fan power and water consumption for the complete day. Total fan power consumption can be reduced by 29% even though 3% more cooling energy is supplied.

Table 2. Comparison of cooling tower power and water consumption for Iran, 22th July 2004.

Conventional control

New control

Difference [%]

Total cooling energy supplied [kWh]

97

99.5

3

Total fan power consumption [kWh]

2.2

1.6

-29

Specific fan power consumption [kWh/kWh]

0.023

0.016

-30

Total water consumption [kg]

320

309

-4

Specific water consumption [kg/kWh]

3.3

3.1

-6

In this simulation both control targets — energy and water saving and meeting of the required chilled water temperatures — are combined. If the main target of the control strategy is to minimise electricity consumption and not to meet exactly the chilled water temperature a lot more energy can be saved. In the morning it is certainly often advantageous to provide the full cooling capacity earlier. But if we assume that in the evening after 5 pm full cooling load is not required anymore we can limit cooling water temperature to 27°C as in the case of the conventional control strategy. Thus, specific fan power consumption can be reduced by 50%!

2. Conclusion

The new control strategy produces the expected result: stable chilled water outlet temperature by adjusting the cooling water temperature. It provides the opportunity to operate a solar cooling system with low electricity consumption when irradiation is high enough to cover the cooling load. Simulations showed that at a sunny day in Iran up to 50 % of the fan power consumption can be saved compared to the conventional control strategy. On the other hand, a high cooling capacity can be provided even if the irradiation is not yet sufficient as e. g. in morning hours. At a cloudy day in Berlin full cooling capacity can be supplied nearly an hour earlier compared to the conventional strategy. This could also be a possibility to dispose of a short-time storage tank.

References

[1] P. Kohlenbach (2005). “Solar cooling with absorption chillers: Control strategies and transient chiller performance“, Dissertation, Technische Universitat Berlin, Germany.

[2] C. Schweigler, A. Costa, M. Hogenauer-Lego, M. Harm, F. Ziegler (2001). “Absorptionskaltwassersatz zur solaren Klimatisierung mit 10 kW Kalteleistung“, Tagungsbericht der Deutschen Kalte-Klima — Tagung 2001 Ulm, Deutscher Kalte — und Klimatechnischer Verein, Stuttgart, Germany.

[3] A. Kuhn, F. Ziegler (2005). “Operational results of a 10 kW absorption chiller and adaptation of the characteristic equation”, Proceedings of the 1st International Conference Solar Air Conditioning, 6/7th October 2005, Bad Staffelstein, Germany.

[4] V. Claufi, A. Kuhn, C. Schweigler (2007). “Field testing of a compact 10 kW water/LiBr absorption chiller“, Proceedings of the 2nd International Conference Solar Air Conditioning, 18/19th October 2007, Tarragona, Spain.

[5] E. Wiegand, P. Kohlenbach, A. Kuhn, S. Petersen, F. Ziegler (2005). “Entwicklung eines offenen Nasskuhlturmes kleiner Leistungsklasse“, KI Luft — und Kaltetechnik 10/2005, S. 413-415.

[6] J. Albers (2002). “TRNSYS Type 107.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 Final Report, Forschungsbericht IEMB Nr. 2-67/2002.

[7] S. A. Klein et al.(2006). “TRNSYS Manual for TRNSYS 16”, Solar Energy Laboratory, University of Wisconsin-Madison, USA.

[8] V. Claufi, A. Kuhn, F. Ziegler (2007). “A new control strategy for solar driven absorption chillers”, Proceedings of the 2nd International Conference Solar Air Conditioning, 18/19th October 2007, Tarragona, Spain.