Investigation of the Solvis stratification inlet pipe for solar tanks

Since the 1960’ties the influence of the thermal stratification in hot water tanks on the thermal performance of solar heating systems has been studied intensively. It was found, that the thermal performance of a solar heating system is increasing for increasing thermal stratification in the hot water tank. The temperature of the storage water heated by the solar collector loop usually varies strongly during the day. In order to reach a good thermal stratification in the tank, different types of pipes, plates, diffusers and other devices have been investigated in the past (e.g. Loehrke, 1979). The aim pursued was to transport the heated water into the tank level of corresponding temperature. Flexible stratification pipes (manifolds) have been further developed for example by (Gari et al., 1982). Furthermore, a wide variety of non flexible tubes with either open holes and perforated vertical plates inside the pipes (Davidson, 1992) or openings in form of balls (e.g. Leibfried, 2000) or flaps (e.g. described in Krause, 2001) have entered the market during the recent years. In this paper an investigation of a stratification pipe with openings covered with flaps according to (Krause, 2001) is presented. The flaps are constructed with a soft material which allows the flap to close and open depending on the temperature and pressure differences inside and outside the pipe. Figure 1 shows schematic illustrations of the pipe. The total height of the pipe is 328 mm, the outer diameter 60 mm, and the flaps are located with a distance of 292 mm in vertical direction (distance between the centre of each opening). Fig. 1. Schematic illustrations of the investigated stratification inlet pipe. Preliminary laboratory tests by (Shah, 2002) with the same stratification pipe containing 5 openings showed that thermal stratification was well built up for a volume flow rate smaller than 8 l/min and larger than 4 l/min, regardless of the inlet temperature, the temperature level in the tank, and the thermal stratification in the tank. For volume flow rates larger than 8 l/min, however, the number of open flaps increased, so that water entered the tank at different levels instantaneously. For volume flow rates smaller than 4 l/min laboratory tests indicated that cold water could be sucked in through an opening in a low level due to low pressure differences. The cold water that entered the pipe through these openings from the bottom of the store mixed with the heated water that flew through the pipe and thereby induced mixing in the tank during charging. More detailed investigations of the flow structure close to the flaps of the stratification pipe are presented in the following for one set of operating conditions. Temperature measurements were carried out and an optical method called Particle Image Velocimetry (PIV) was used to visualize the flow around the flaps.