IEA Solar Heating and Cooling Programm - NachhaltigWirtschaften.at

IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask A Report, Date: November 2010
3 Control Strategy
In Figure 4 the control strategy of the solar cooling plant in Gröbming is illustrated. All the
temperatures and nomenclatures are linked to the hydraulic scheme (Figure 2). Only the
cooling cycle and the management of the solar cycle are described here.
Certainly the combination of the whole system, including the space heating, demands special
attention. Through the complex hydraulic layout of the system the control strategy of the
solar cooling plant had to be harmonized with all other components such as the domestic hot
water station and the pool heating. The main goal was to avoid a clocking behavior of the
chiller.
Figure 4: Control strategy of the solar cooling plant in Gröbming
Cooling cycle
To eliminate any control failure regarding the specific winter and summer behaviors the
change between winter and summer operation is done by the maintenance technician. If
summer mode is switched on cooling is basically enabled. The first continuous logical test is
checking if temperature T12 is higher than 70°C. A hysteresis of 7 K is defined to secure that
the chiller is not switched on and off too often. If the test turns out true another logical test for
the office room is carried out. A cooling request is stated if the room temperature (RT2)
exceeds 24°C and the chiller is switched on. In the next stage the chilled water supply
temperature has to be more than 3°C below the room temperature, then the distributor pump
is started and the control valve (Vb) is enabled to regulate the flow temperature at 17°C.
Solar cycle
Switching between summer and winter is done manually. Temperature T1 plus a margin of
5°C has to be above the lowest temperature of stora ge 1 (T8) to start the primary solar
pump. In summer this rpm-regulated pump (P1) is not controlled to any set temperature; it is
always running at maximum speed. In winter the primary solar pump is regulated to a set
temperature of 75°C. Because there are two solar co llector fields, T1 is averaged out of two
collector temperatures measured in those two fields. Analogically the secondary solar cycle
is turned on with T2. It is not regulated in summer but regulated to a set temperature of 70°C
in winter. To avoid too high pressure in the hot storage, the solar cycle is switched off if either
T1 or T2 is above 100°C.
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