The performance of thermosiphon solar water heaters has been studied extensively, both experimentally and analytically, by numerous researchers. Among the first studies were those of Close (1962) and Gupta and Garg (1968), who developed one of the first models for the thermal performance of a natural circulation solar water heater with no load. They represented solar radiation and ambient temperature by Fourier series and were able to predict a day's performance in a manner that agreed substantially with experiments.
Ong performed two studies (1974; 1976) to evaluate the thermal performance of a solar water heater. He instrumented a relatively small system with five thermocouples on the bottom surface of the water tubes and six thermocouples on the bottom surface of the collector plate. A total of six thermocouples were inserted into the storage tank and a dye tracer mass flowmeter was employed. Ong's studies appear to be the first detailed ones on a thermosiphonic system.
Morrison and Braun (1985) studied the modeling and operation characteristics of thermosiphon solar water heaters with vertical or horizontal storage tanks. They found that the system performance is maximized when the daily collector volume flow is approximately equal to the daily load flow, and the system with horizontal tank did not perform as well as that with a vertical one. This model has also been adopted by the TRNSYS simulation program (see Chapter 11, Section 11.5.1). According to this model, a thermosiphon system consisting of a flat-plate collector and a stratified tank is assumed to operate at a steady state. The system is divided into N segments normal to the flow direction, and the Bernoulli's equation for incompressible flow is applied to each segment. For steady-state conditions the sum of pressure changes around the loop is 0; that is,
Pi = density of any node calculated as a function of local temperature (kg/m3). fhi = friction head drop through an element (m). Hi = vertical height of the element (m).
The collector thermal performance can be modeled by dividing it into Nc equally sized nodes. The temperature at the midpoint of any collector mode k is given by
where mt = thermosiphonic flow rate (kg/s). Ac = collector area (m2).
The collector parameter, F' UL, is calculated from the collector test data for FrUl at test flow rate mT by
1 _ FrUlAc
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