Solar Thermal Heat Utilisation - Renewable Energy: Technology, Economics and Environment - page 153

DHW system with closed forced circulation. Fig 4.13 shows a complete layout of
a solar thermal system with closed forced circulation to support domestic hot wa-
ter production. The main criterion for dimensioning the system is the demand for
domestic hot water. Demand values for average scenarios are available according
to Table 4.5. According to these values, the solar system should cover the entire
demand for domestic hot water 70 to 90 % of the time in summer. The volume of
the storage is approximately 1.5 to 2.5 times as big as the rated demand for one
day. For a household with four people with a daily demand of 50 l per person, a
collector area of approximately 7 to 8 m 2 non-selectively coated or 5 to 6 m 2 se-
lectively coated are to be installed, using standard flat-plate collectors. Addition-
ally, a storage with a volume between 250 and 500 l is required /4-1/, /4-2/, /4-3/.
If the dimensioning parameters are maintained, approximately 50 to 65 % of the
domestic hot water supply can be covered by solar energy.

Deventing (manual) Thermostatic

mixing valve

Temperature sensor

Hot water

Storage

Heat from
boiler

auxiliary

Control

Safety

valve

Temperature
sensor

One
way
valve

Manometer

Thermo-
meter

Pump

Emptying
valve

Closing valves

Cold water

Emptying/filling valves Expansion device

Fig. 4.13 Solar thermal forced flow-through system with flat-plate collector to support the
domestic hot water supply (see /4-2/)

Table 4.5 Standard values for the domestic water demand /4-14/

Domestic hot water
l/(Person d)

Utilisable heat
in MJ/(Person d)

High demand
Medium demand
Low demand

70 - 115
50 - 70
35 - 50

10.44 - 16.70
7.31 - 10.44
5.22 - 7.31

In order to achieve a higher solar fractional saving of around 70 %, a non-
selectively coated collector area of approximately 15 to 18 m 2 or a selectively
coated collector area of 10 to 12 m 2 would be required in the described case. The
storage volume would have to be around 600 l. During the summer, however,
collector standstills can occur on a regular basis.