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Maintaining the Temperature Stratification in the Tank

Hot water returning from the collector should enter the storage tank about one-third of the way down from the top. This may not be the hottest water collected during the day, because isolation and outside ambient temperatures vary during the day. This water will disturb the water at the very top of the storage tank. Water for use is taken from the very top of the tank, which is where the water is hottest. When hot water is drawn from the tank, it is replaced by new cold water, which should enter at the bottom. Water circulating to the solar collector should be drawn from the bottom of the tank on the grounds of efficiency. Always supply the collector with the coolest water available. The cooler a solar collector runs, the less heat it loses to the surrounding environment.

The Brazed Plate Heat Exchanger is designed for a solar panel with a maximum heat load of 600 W/m2. The statement 600 W/m2 is obtained from an average value of the solar radiation and the degree of effectiveness of the collector. Here the heat load of the collector is 1000 W/m2, with an assumed effectiveness of 0.6.

The most common heat exchanger for solar applications is the BL50C. Different types of BL50Cs are discussed in the tables relating to Figures 6.22 and 6.23. (Please note that BL50C*21/2P means a two-pass BL50C with 21 plates.) The principle of a two-pass Brazed Plate Heat Exchanger is shown in Figure 6.21.

The tank has a volume of 1000 to 4000 liters, and the water it contains does not change, i.e. it is a closed circuit. There are different temperature layers in the tank, ranging from 15 degrees at the bottom to as high as 70 degrees in the top.

It is therefore possible to choose a layer at the desired temperature from which to take the water.

High Flow System (Circulation load: 30 l/m)

Temperature program:

Ethylene glycol (40%)-Water 56 -> 36℃

Water 50- 30℃

Low Flow System (Circulation load: 15 l/m)

Temperature program:

Ethylene glycol (40%)-Water 76 -36℃

Water 70 - 30℃

Tap Water System

Temperature program:

Water 60 -15℃

Water 52- 10℃

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