The performance of two innovative collector-loop heat exchangers used in pumped circulation solar water heaters was investigated experimentally and numerically, and TRNSYS simulation models were developed for evaluating long-term performance. The heat exchangers evaluated are a narrow gap mantle on a vertical tank and a falling film system in a standard gas hot water tank. The falling film system is based on minor changes to a mass produced tank and does not require the addition of a special purpose heat exchanger. The heat exchangers were assessed for a range of operating conditions to quantify the overall heat exchanger UAhx value and the effect of thermal stratification in the tank. Flow visualisation experiments and CFD modelling were also performed to provide detailed understanding of the operation characteristics of the heat exchangers. For each heat exchanger, the measured and computed collector loop side heat transfer coefficients were correlated by developing new Nusselt number versus Reynolds and Prandtl number functions. New heat transfer correlations were developed for both types of heat exchanger and have been implemented into full solar water heater simulation models in TRNSYS. Predictions of tank stratification conditions in each heat exchanger were in good agreement with experimental data. Compared to a standard direct circulation system, the annual performance of the mantle system and falling film system are 8% and 18% less respectively. The decrease in system performance is due to the heat exchanger penalty and reduced thermal stratification in the storage tank, however they have the added features of freeze protection and suitability for hard water areas. They are also cheaper to manufacture than convectional heat exchange systems. The annual performance of a falling film solar water heater was shown to be affected by de-stratification if an in-tank electric booster was used. However, improved performance can be achieved if an advanced switching controller is used to modulate the flow rate such that the collector return temperature is always higher than the temperature in the top of the tank.
| Identifer | oai:union.ndltd.org:ADTP/232577 |
| Date | January 2007 |
| Creators | Soo Too, Yen Chean, Mechanical & Manufacturing Engineering, Faculty of Engineering, UNSW |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://unsworks.unsw.edu.au/copyright, http://unsworks.unsw.edu.au/copyright |
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