Facades and Summer Performance of Buildings - Low Energy Cooling for Sustainable Buildings - page 53

the surroundings or may be used for solar thermal cooling systems. The fa¸ade is
ventilated without interruption the whole year long (8760 h).
Case PV4: This case represents a more realistic control strategy. The preheated
fa¸ade air is led into the building only during winter days between 15 September and
15 May. During the night (from 18:00 to 07:00) the fan is switched off and the flow
velocity within the fa¸ade gap is reduced from 0.4 to 0.1m s − 1 , which simulates free
convection. The volume flow through the fa¸ade causes an air exchange of 0.7 h − 1 .
The building supply air temperature corresponds either to the gap outlet temperature
or to the ambient temperature depending on the operation of the fan. In the summer
period, the fan works only during the daytime but the hot fa¸ade air is not connected
with the building and is usable for additional solar cooling systems.

In the following section the complete thermal and electrical energy balance of the
fa¸ade is evaluated, including pressure drops from forced ventilation systems. The
parameters for the study are summarized in Table 2.5.

2.4.1 Thermal and Electrical Energy Balance of the Fa¸ade

The thermal energy balance for heating and cooling, the electrical energy produced
by the PV system as well as the necessary power of the fan to ventilate the double
fa¸ade are considered in this section. The average electrical efficiency and the solar
irradiance transmission through the glazed part of the PV modules are 10%. To cal-
culate the electrical power P el of the fan for ventilating only the double fa¸ade, the
pressure drop p within the fa¸ade must be estimated over the sum of the single flow

Table 2.5 Gap size and flow velocities of the ventilated PV fa¸ades considered

Flow velocity / m s − 1 Air change / h − 1 Case Gap width / m

PV0a 0.10 0.40 0.7

PV0b 1.00 0.7

PV0c 1.50 0.7

PV1a 0.05 0.81 0.7

PV1b 1.16 1.0

PV1c 2.31 2.0

PV2a 0.10 0.40 0.7

PV2b 0.58 1.0

PV2c 1.16 2.0

PV3a 0.20 0.20 0.7

PV3b 0.29 1.0

PV3c 0.58 2.0

PV4 0.10 0.40 0.7