PART I. INTRODUCTION AND SUMMARY
4. Detailed investigations
A well stratified storage tank improves the thermal performance of a solar heating system significantly. The benefit of stratified storage tanks for all solar heating systems has been studied intensively by numerous scientists, see e.g. (Lavan and Thompson 1977), (van Koppen et al. 1979), (Sharp and Loehrke 1979), (Furbo 1984), (Furbo and Mikkelsen 1987), (Furbo and Berg 1990), (Carlsson 1993), (Furbo 1995), (Jordan et al. 1999), (Andersen and Furbo 1999), (Jordan 2001), (Knudsen 2002), (Shah and Furbo 2003), (Furbo 2004), (Knudsen and Furbo 2004), (Furbo et al. 2004), (Furbo and Knudsen 2004), (Jordan and Furbo 2004). The question is how best to build up and maintain thermal stratification during all operation conditions in a simple and cost efficient way. This topic has also been studied intensively, see e.g. (Loehrke et al. 1979), (Gari and Loehrke 1982), (Abu-Hamdan et al. 1992), (Davidson et al.
1992), (Davidson and Adams 1994), (Davidson et al. 1994), (Essert 1995), (Krause and Kühl 2001), (Shah 2002), (Jordan and Vajen 2002). One way of enhancing thermal stratification is by using mantle tanks. Due to limitations in the heat transfer area between the mantle and the tank, mantle tanks are only suitable for tank volumes up to about 1 m3. Thermal stratification is also enhanced in tanks with several inlet possibilities with valves that open and close depending on the temperature level of the solar heated water and the temperature in the storage tank. Such systems require
heating system cold water hot water
advanced control systems. Finally, stratification inlet pipes are used to build up thermal stratification in storage tanks.
Experimental and theoretical investigations are carried out with marketed inlet stratification pipes but also new types are investigated.
In Figure 4.12 the investigated inlet stratification pipes are shown. In the left side of the figure, a rigid inlet stratification pipe with circular openings is depicted. The device is made of 2 mm thick synthetic material resistant to temperature and deformation and is built up by a variable number of “hats” connected by 3 thread rods forming an inner flow channel of 24 mm with an outer diameter of 100 mm. Free convection induced fluid flow can leave the stratification pipe through all circular openings which have a height of 18 mm. The device is patented and marketed by the German company Sailer GmbH & Co KG and was tested at ITW, Stuttgart University. In the middle of the figure, a two layer fabric inlet stratification pipe is shown. The investigated samples consist of pipes with an inner/outer diameter of 40 mm/70 mm and 25 mm/45mm. The pipes are closed at the top. The fabric pipe is invented during the Ph.D. project period and patented. In the right side of the figure, a rigid inlet stratification pipe with lockable openings acting as “non-return” valves is shown. The pipe is made of polypropylene (PP) with an outer diameter of 60 mm and a thickness of 3 mm (Krause 2001) and (Shah 2002). The distance between the centres of each opening is about 292 mm. The pipe is patented and marketed by the German company Solvis GmbH & Co KG.
Figure 4.12 Investigated inlet stratification pipes. Left: Rigid stratifier with circular openings. Middle: Fabric stratifier with two fabric layers. Right: Rigid stratifier with lockable openings acting as “non-return” valves.
The experimental investigations are carried out in laboratory tanks with forced volume flow rates in the range 2 l/min – 10 l/min both as heating tests and cooling tests: heating tests to simulate the thermal behavior of the stratifiers used for solar heat transfer to the tank and cooling tests to simulate the thermal behaviour of the stratifiers used for returning water from the space heating loop. The thermal behaviour is also investigated with natural convection flow.
Further, PIV measurements are carried out and compared to CFD calculations.
Figure 4.13 shows an example of PIV measurements with the rigid Solvis stratifier with openings without (left) and with (right) the “non-return” valves mounted in the openings during a heating test with identical operation conditions. The inlet temperature is relatively high compared to the tank temperatures. From the figure it can be seen that with the stratifier with “non-return” valves the lowest opening is closed preventing water from flowing into the stratifier while the water flows into the lowest opening of the stratifier without the “non-return” valves. The cold water entering the pipe mixes with the hot water in the pipe. Hence, the stratifier without
“non-return” valves acts more like a mixing device than a stratification device.
Figure 4.13 Measured velocity vectors for the stratifier without “non-return valves”
(left) and with “non-return valves” (right).
Whether water tries to flow into a stratification device or not is depending on the difference between the pressure in the stratification device and the pressure surrounding the stratification device in the same level. The pressure is described by the static pressure which is a function of the height and the temperature of the water placed above the level in question and the dynamic pressure which is a function of the velocity. The “non-return” valves prevent the inflow in spite of the pressure difference. If nothing physically prevents an inflow, the pressure difference, in the heating case, will create an inflow of colder water into the hot stratifier resulting in a temperature decrease for the water flowing upwards inside the pipe. This unwanted inflow can be registered by a fluctuating temperature inside the stratifier.
The fabric stratifier has a dynamic behaviour. The fabric stratifier eliminates the pressure difference by reducing the cross sectional area whereby the velocity and the dynamic pressure inside the pipe are increased until the pressure difference is eliminated. Therefore cold water will not enter into the fabric stratifier. The investigations revealed that thermal stratification can be build up in a nearly perfect way with a well performing stratifier.
The advantages of the fabric stratification pipe are:
x Water is not restricted to leave through fixed holes but can leave the pipe in all levels
x The diameter of the pipe can easily be adjusted to all volume flow rates x It is cheap to produce
The disadvantage of the fabric stratification pipe is that the long time durability has not yet been investigated. In order to make the fabric stratifier an attractive alternative to other stratifiers on the market, the long time durability must be demonstrated.
Therefore long time durability tests will be performed at the Technical University of Denmark in 2007.
Further details in Paper VI, Paper VII, Paper VIII and Paper IX.