Sustainable Building Operation Using Simulation - Low Energy Cooling for Sustainable Buildings - page 207

of 130 ◦ C. If the heat recovery factor increases from the measured value of 0.30 to
0.60, both the cooling capacity and COP increase by a factor of 1.1 to 1.2. To achieve
this improvement it is necessary to optimize the heat transfer inside the GHX using
constructive steps to reach higher heat recovery factors.
Other important influences on the performance of the DACM are the heat transfer
coefficients UA and the heat losses Q ax and Q gx , which depend on the rich solution
mass flow m Sr ; that is, on the bubble pump performance.
A cooling capacity of 2.0 kW can be reached in the next prototypes to be built,
if the solution mass flow is increased by a factor of 1.8 and the evaporator wetting
factor reaches 1.0; that is, full evaporation takes place. Together with an increase of
heat transfer surface area by a factor of 1.5 for the absorber and 1.8 for the evap-
orator, the total heat transfer power UA is three times higher for both evaporator
and absorber. At a generator entry temperature of 120 ◦ C and a cooling water en-
try temperature of 27 ◦ C, the evaporator outlet temperature is 6 ◦ C with a COP of
nearly 0.5.
As the absorption chiller model is part of a more complex dynamic model including
varying meteorological conditions, there is a need to use dynamic system simulation
tools anyway. The simplification of the easy-to-use characteristic equation with con-
stant slopes is therefore not advantageous and the more exact quasi-stationary model
should be used.
6.1.2 Building Cooling Load Characteristics
To evaluate the energetic and economic performance of solar cooling systems under
varying conditions, different building cooling load files were produced with the simu-
lation tool TRNSYS. The methodology for choosing the building shell parameters is
as follows. For a given chiller power of 15 kW an adequate building size was selected,
for example a south-orientated office building with 425m 2 surface area and rectangu-
lar geometry. The orientation of the building was varied to study the influence of daily
fluctuations of external loads. The dimensions and window openings of the buildings
were adjusted, so that the given chiller power could keep the temperature levels below
a setpoint of 24 ◦ C for more than 90% of all operating hours.
The air exchange rates were held constant at 0.3 h − 1 for the office throughout
the year. This limited air exchange rate leads to cooling load files, which in some
cases contain cooling power demand during winter and transition periods for south
European locations. Only if the air exchange rate can be significantly increased either
by natural ventilation or by using a mechanical ventilation system can such a cool-
ing power demand be reduced by free cooling. In the buildings analysed in this
work, heat was always removed by a water-based distribution system, which was
fed by cold water from the cooling machines. At low ambient temperatures, the cool-
ing tower alone provides the required temperature levels for the cold distribution
system.