Drainback systems (DBS) were developed in the Netherlands in the 1980's, due to governmental regulations regarding potable water quality. The crucial attribute of drainback systems is the draining of the working fluid from the collector - always when the solar circuit is not in operation. The draining ensures a protection against both, overheating and freezing of the circulating fluid in the solar loop. Small installation failures can disturb the desired filling/draining processes. This can lead not only to improperly operation, but even damage the DBS. Therefore the knowledge on behavior of the DBS during the filling and draining is essential for ensuring the proper operation.
An experimental setup was developed in order to investigate the filling and draining processes of the drainback system. Two types of drainback systems were considered: system with drain-back reservoir and heat storage as drain-back reservoir. The first type has a separate, so-called drain-back reservoir which is located on the solar flow circle. The collector is hydraulically connected to the heat storage through an internal heat exchanger coil. In the second system type, the heat storage itself is the drain-back reservoir. The measurement equipment comprised magnetic-inductive flow meters, manometers and a data acquisition system. For additional visual assessments, almost all hydraulic components (tubes, collector's absorber, heat storage, drain-back reservoir) were made transparent and colored water was used as working fluid in the solar loop. Based on the dimensions of a real collector with a meander absorber its “imitator” was made of PVC pipes. The 100 liter heat storage and 25 liter drain-back reservoir were made of PP-H plastic.
Different experiments were carried out and evaluated. The processes of filling and draining are described in detail in this paper. Results of this investigation are e.g.:
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A siphon is a crucial effect for both filling and draining processes
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A minimal hydrostatic height difference less than 10 mm between flow and return sides is sufficient for draining
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The largest share of the heat carrier drains over the solar return pipe, caused by siphoned draining
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Horizontal piping without slope does not disturb the draining process
Explanations of these processes will be presented in the paper.
