Figure 7 shows that a rising regeneration temperature causes a sharp increase in exergy destruction and a drop in the exergetic efficiency. Regeneration at lower temperatures is favourable, which is also a well known fact for rotary type DEC systems. At higher regeneration temperature the temperature differences during the sensible heating and cooling stages are high, therefore increasing irreversibilities of the heat exchange. The decrease in exergetic efficiency is further due to an increase in exergy losses as the air leaving the heat exchanger during desorption is of a temperature significantly higher than ambient (fig. 2), thus of significant exergy content.

An improved system configuration would maximize the exergetic product and minimize exergy losses, increasing the exergetic efficiency. This could theoretically be realized a) during adsorption by further cooling the dehumidified process air (state 2) by an additional indirect evaporative cooler and b) during desorption by exploiting the exergy of the air leaving the sorptive heat exchanger for the pre­heating of the regeneration air. Simulations were performed for an enhanced system configuration including both improvement measures. The heat exchanger for heat recovery was characterized by an efficiency of 80% and simulations were performed for a regeneration temperature of Treg=60°C. Due to the lower resulting process outlet temperature T2 during adsorption the exergetic product can be increased by 17%, whereas due to pre-heating of regeneration air the exergetic input is decreased by 9% compared with the simple configuration at equivalent regeneration temperature. Therefore, the exergetic efficiency rises from 0.18 to 0.23, which is an increase by 28%. To put these values in perspective, the thermal COP, calculated as the ratio of removed heat to heat input during desorption, increases from 0.99 for the simple to 1.05 for the improved configuration. Due to the high COP values a good energetic performance is expected, but the exergetic assessment characterized by comparatively low exergetic efficiencies suggests that the improvement potential is still high. Detailed analysis of the exergetic performance for the individual cycle stages and the sub-processes evaporative cooling and adsorption/desorption is within the scope of future work.

6 Conclusions

Simulation results from a dynamic model of a sorptive heat exchanger were presented in an exergetic framework. Results show that the exergetic efficiency reaches a maximum at specific combinations of ambient temperature and humidity ratio, suggesting that the heat exchanger performs best in a certain range of climatic conditions. The exergetic performance is highest at low regeneration temperature, thus underlining the system’s good applicability with low grade heat. Further simulations showed that the exergetic efficiency can be increased by an enhanced system configuration comprising heat recovery during desorption and an additional indirect evaporative cooler.

7 References

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[2] Schicktanz, M., Nunez, T. (2008): Modelling af an adsorption chiller for dynamic system simulation, International Sorption Heat Pump Conference, Seoul, Korea, September 2008, in press.

[3] Bosnjakovic, F. (1997): Technische Thermodynamik, Teil II, 6. Auflage, Steinkopf, Darmstadt.

[4] Szargut, J., Stryrylska, T. (1969): Die exergetische Analyse von Prozessen der feuchten Luft, Heizung, Lueftung, Haustechnik, 5, 173-178.

[5] Wepfer, W. J., Gaggioli, R. A., Obert, E. F.(1979): Proper evaluation of available energy for HVAC, ASHRAE Transactions, 667-677.

[6] Taufiq, B., Masjuki, H., Mahlia, T., Amalina, M., Faizul, M., Saidur, R. (2007): Exergy analysis of evaporative cooling for reducing energy use in a Malaysian building. Desalination, 209, 238-243.

[7] Chengqin, R., Ninping, L. Guanga, T. (2002): Principles of exergy analysis in HVAC and evaluation of evaporative cooling schemes. Building and Environment, 37, 1045-1055.

[8] Taufiq, B., Masjuki, H., Mahlia, T., Amalina, M., Faizul, M., Saidur, R. (2007): Exergy analysis of evaporative cooling for reducing energy use in a Malaysian building. Desalination, 209, 238-243.

[9] Kanoglu, M., Carpinlioglu, M., Yildirim, M. (2004): Energy and exergy analyses of an experimental open — cycle desiccant cooling system. Applied Thermal Engineering, 24, 919-932.

[10] Camargo, J., Ebinuma, C., Silveira, J. (2003): Thermoeconomic analysis of an evaporative desiccant air conditioning system. Applied Thermal Engineering, 23, 1537-1549.

8 Nomenclature and Subscripts

9 Acknowledgement

The project support of the German Federal Ministry of Economics and Technology and the support of the Reiner Lemoine Stiftung for the Ph. D. research of Constanze Bongs is gratefully acknowledged by the authors.