Sustainable Building Operation Using Simulation - Low Energy Cooling for Sustainable Buildings - page 201

energy balance equations and validated against experimental data in the case of the
2 kW machine or against manufacturers' data for the 15 and 100 kW machines.
The heating input power to the thermal chillers is provided by solar thermal col-
lectors, which are connected via an external heat exchanger to a buffer storage vol-
ume. The required temperature level for the chillers is taken from the storage tanks
and series-connected auxiliary heaters, which depends on the control strategy chosen
(fixed or variable temperature levels). The cold produced is obtained from the thermal
chiller model and used to cover the building's cooling load with the option of including
a cold storage tank.
The building construction (insulation standards, orientation, glazing fraction, size,
etc.) was chosen so that a given chiller power is sufficient to maintain room temper-
ature levels at a given setpoint of 24 ◦ C for at least 90% of all occupation hours. To
evaluate the influence of the time-dependent building cooling loads on the solar frac-
tion, building load files were calculated with a predominance of internal loads through
people or equipment or with dominating external loads through glazed facades.
For the economic analysis, a market survey of thermal chillers up to 200 kW cool-
ing power and for solar thermal collector systems was carried out. The annuity was
calculated for different system combinations and cooling energy costs were obtained.

6.1.1 Component and System Models
Absorption Chiller Models

Steady-state absorption chiller models are based on the internal mass and energy
balances in all components, which depend on the solution pump flow rate and on the
heat transfer between external and internal temperature levels. Several problems are
associated with a single characteristic equation which calculates all internal enthalpies
only for the design conditions: if bubble pumps are used, as in diffusion-absorption
machines or in some single effect absorbers, the solution flow rate strongly depends on
the generator temperature. Also, if the external temperature levels differ significantly
from design conditions, the internal temperature levels change and consequently the
enthalpies. In the current work, a comparison was carried out between the constant
characteristic equation and a quasi-dynamic characteristic equation based on changing
internal enthalpies and - if necessary - changing solution flow rates.
The characteristic equation is based on a double temperature difference t
between the mean external generator and absorber temperatures t G and t A on the one
hand and the external condenser and evaporator temperatures t C and t E on the other
hand:

t = ( t G − t A ) − ( t C − t E ) · B (6.1)

The constant B is the Duhring factor determined by Equation 6.2, which is the ratio
of the slope of the isosteres of the pure refrigerant to the solution. It is determined by