Environmental Engineering Reference

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primary energy (Mendes
et al
., 1998). The exact value of the minimum solar fraction

required for energy saving depends not only on the performance of the thermal chiller,

but also on other components such as the cooling tower: a thermal cooling systemwith

an energy-efficient cooling tower performs better than a compression chiller at a solar

fraction of 40%; a low-efficiency cooling tower increases the required solar fraction

to 63%. These values were calculated for a thermal chiller COP of 0.7, a compressor

COP of 2.5 and an electricity consumption of the cooling tower between 0.02 and

0.08 kWh
el
per kWh of cold (Henning, 2004b).

Double effect absorption cycles have considerably higher COPs around 1.1-1.4, but

require significantly higher driving temperatures between 120 and 170
◦
C (Wardono

and Nelson 1996), so that the energetic and economic performance of the solar thermal

cooling system is not necessarily better (Grossmann, 2002).

Several authors have published detailed analyses of the absorption cycle perfor-

mance for different boundary conditions, which showed the very strong influence of

cooling water temperature, but also of chilled water and generator driving temperature

levels on the COP(Engler
et al
., 1997; Kim and Machielser, 2002). Most models re-

solve the internal steady-state energy and mass balances in the machines and derive the

internal temperature levels from the applied external temperatures and heat transfer

coefficients of the evaporator, absorber, generator and condenser heat exchangers. To

simplify the performance calculations, characteristic equations have been developed

(Ziegler, 1998),which are an exact solution of the internal energy balances for one

given design point and which are then used as a simple linear equation for different

boundary conditions. For larger deviations from design conditions or for absorption

chillers with thermally driven bubble pumps, one single equation does not reproduce

accurately the chiller performance (Albers and Ziegler, 2003). The disadvantages of

the characteristic equation can be easily overcome if dynamic simulation tools are

used for the performance analysis and internal enthalpies are calculated at each time

step - an approach which is chosen in this work using the simulation environment

INSEL (Schumacher, 1991). Dynamic models taking into account the thermal mass

of the chillers are also available and can be used for the detailed optimization of

control strategies such as machine start-up (Kim
et al
., 2003; Willers
et al
., 1999).

However, for an energetic analysis of annual system performance, steady-state models

are sufficiently accurate.

The available steady-state models of the absorption chillers provide a good basis for

planners to dimension the cooling system with its periphery such as fans and pumps,

but they do not give any hints for the dimensioning of the solar collectors or any

indication of the solar thermal contribution to total energy requirements. This annual

system performance depends on the details of the collector, storage and absorption

chiller dimensions and efficiencies for the varying control strategies and building load

conditions. An analysis of the primary energy savings for a given configuration with

renewable energy heat input requires a complete model of the cooling system coupled

to the building load with a time resolution of at least 1 hour. If stratified storage tanks

are modelled, the time resolution has to be even higher (10 minutes or less).

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