Vertical versus horizontal tank configurations

Because the operation of the thermosiphon system depends on the stratification of the water in the storage tank, vertical tanks are more effective. It is also preferable to have the auxiliary heater as high as possible in the storage tank, as shown in Figure 5.1, to heat only the top of the tank with auxiliary energy when this is needed. This is important for three reasons:

1. It improves stratification.

2. Tank heat losses are increased linearly with the storage temperature.

3. As shown in Chapter 4, the collector operates at higher efficiency at a lower collector inlet temperature.

To reduce the overall height of the unit, however, horizontal tanks are frequently used. The performance of horizontal tank thermosiphon systems is influenced by the conduction between the high-temperature auxiliary zone in the top of the tank and the solar zone and by mixing of the flow injection points (Morrison and Braun, 1985). The performance of these systems can be improved by using separate solar and auxiliary tanks or by separating the auxiliary and preheat zones with an insulated baffle, as shown in Figure 5.3. A disadvantage of the two tank systems or segmented tanks is that the solar input cannot heat the auxiliary zone until there is a demand.

Thermal stratification in shallow horizontal tanks also depends on the degree of mixing at the load, make-up water, and collector inlets to the tank. The load should be drawn from the highest possible point, whereas the make-up water flow should enter the tank through a distribution pipe or a diffuser so that it is introduced into the bottom of the tank without disturbing the temperature stratification or mixing the top auxiliary zone with the solar zone. The collector return flow to the tank also should enter through a flow distributor so that it can move to its thermal equilibrium position without mixing with intermediary fluid layers. Because the collector return is usually hot, many manufacturers make a small bend at the inlet pipe, facing upward.

Generally, the penalty associated with horizontal tanks is that the shallow tank depth degrades stratification because of conduction through the walls of the tank and water. Additionally, for in-tank auxiliary systems conduction between the auxiliary and solar zones influences the solar performance. For horizontal tanks with diameters greater than 500 mm, there is only a relatively small performance loss relative to a vertical tank, and the above effects increase significantly for smaller tank diameters (Morrison, 2001).

From collector i Cold water IN

Solar zone

Auxiliary zone

=> Hot water OUT


FIGURE 5.3 Configuration of a segmented tank with an insulating baffle.

To collector

FIGURE 5.3 Configuration of a segmented tank with an insulating baffle.

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