The performance of a thermosiphon solar domestic hot water system with hot water removal

Abstract

INTRODUCTION Thermosiphon (natural circulation) solar domestic hot water systems were investigated, both experimentally and numerically, by Close[l], Gupta and Garg[2], Ong[3], and Young and Bergquam[4]. Close used a 1.6m 2 (17.3ft 2) collector and a 113.6L (30gal) storage tank, while Ong utilized a 1.4m 2 (15ft 2) collector and a 106L (28gal) storage tank. These systems do not represent practical size systems although they do maintain the ratio between the storage tank volume and collector area, in English units, at about 2 which is a design rule of thumb[5]. Larger systems can have higher mass flow rates which can lead to transition or turbulent flow throughout the system. These higher mass flow rates increase the mixing in the storage tank and can complicate numerical modeling efforts. In the investigation of Young and Bergquam, a practical size thermosiphon system was tested, e.g. a 3.47 m 2 (37.4 ft 2) collector and 250L (66gal) storage tank§ Numerical models were presented in [4] for the collector, storage tank, and the thermosiphon mass flow rate. The relatively low velocities typical of natural circulation systems make the measurement of collector mass flow rates difficult. Consequently, only a few investigators have attempted to measure the flow rate in these systems. Even a small flow restriction placed in the flow to measure the flow rate can drastically reduce the flow and change the hydrodynamic behavior of the system. Ong[3] measured the mass flow rate by timing the passage of a single dye streak. A laser doppler anemometer was used by Morrison and Ranatunga[6] to measure mass flow rates of a scaled down laboratory system (an electric heater for the collector and a 66.7L (17.6gal) storage tank were used). The velocity profile was measured across a transparent test section and the resulting velocity profile integrated to obtain the mass flow rate. In all cases the flow was laminar. Accuracies of 2 per cent were reported in [6]. Although a laser doppler anemometer does not introduce any restrictions in the flow field its expense and complexity limit its use. An energy balance on the storage tank was used in [4] to calculate collector mass flow rates. The technique was verified by comparison to measurements taken on a pumped system using a turbine flow meter.