Sustainable Building Operation Using Simulation - Low Energy Cooling for Sustainable Buildings - page 203

Figure 6.1 Measured and simulated cooling power as a function of generator entry temperature for a
10 kW absorption chiller

and the generator enthalpy balance G E is needed:

Q G = G E s ( t − t min ) Q gx

+ (6.7)

Instead of using the external mean temperatures, the energy balance equations can
also be resolved as a function of the external entrance temperatures, so that changing
mass flow rates can be considered in the model.
For a given cold water temperature, the cooling power and the COP mainly de-
pend on the generator and the cooling water temperatures for the absorption and
condensation process. If the temperatures deviate significantly from the design point,
the variable enthalpy model reproduces the measured results much better than the
constant enthalpy equation. This is shown in Figure 6.1 for a new 10 kW LiBr/H 2 O
machine produced by the German company Phoenix, where the slopes for both evap-
orator cooling power and COP are better met at low and high generator temperatures.
The design generator entry temperature is 75 ◦ C at an absorber cooling water entry
temperature of 27 ◦ C and an evaporator temperature of 18 ◦ C.
At lower evaporator temperatures, the COP is lower and the generator temperature
has to be increased. This is shown for a 15 kW LiBr/H 2 O machine produced by the
German company EAWwith an absorber cooling water entry temperature of 27 ◦ C, an
evaporator entry temperature of 12 ◦ C and exit of 6 ◦ C (see Figure 6.2). The external
mass flow rates were 2m 3 h − 1 for the generator and evaporator and 5m 3 h − 1 for the
absorber/condenser cooling water circuit. For a 6 ◦ C evaporator exit temperature and
generator temperatures between 70 and 95 ◦ C the cooling power was then calculated
as a function of cooling water temperature (see Figure 6.3). Both the power and
COP drop with increasing cooling water temperature level. To compare performances