Sustainable Building Operation Using Simulation - Low Energy Cooling for Sustainable Buildings - page 200

Very few results of complete system simulations have been published. In the IEA
task 25 design methods have been evaluated. In the simplest approach a building load
file provides hourly values of cooling loads and the solar fraction is calculated from
the hourly produced collector energy at the given irradiance conditions. Excess energy
from the collector can be stored in available buffer volumes without considering
specific temperature levels. Different building types were compared for a range of
climatic conditions in Europe with cooling energy demands between 10 and 100
kW hm − 2 a − 1 . Collector surfaces between 0.2 and 0.3m 2 per square metre of
conditioned building space combined with 1-2 kWh of storage energy gave solar
fractions above 70% (Henning, 2004a).
System simulations for an 11 kW absorption chiller using the dynamic simulation
tool TRNSYS gave an optimum collector surface of only 15m 2 for a building with a
196m 2 surface and 90 kWhm − 2 annual cooling load, or less than 0.1m 2 per square
metre of building surface. A storage volume of 0.6m 3 was found to be optimum
(40 litres per square metre of collector), which at 20 K useful storage temperature
difference only corresponds to 14 kWh or 0.07 kWh per square metre of building
surface (Florides et al ., 2002). Another system simulation study (Atmaca and Yigit,
2003) considered a constant cooling load of 10.5 kW and a collector field of 50m 2 ;75
litres of storage volume per square metre of collector surface was found to be optimum.
Larger storage volumes were detrimental to performance. The main limitation of these
models is that the storage models are very simple and only balance energy flows, but
do not consider stratification of temperatures. This explains the performance decrease
with increasing buffer volume, if the whole buffer volume has to be heated up to
reach the required generator temperatures. Also, attempts have been made to relate
the installed collector surface to the installed nominal cooling power of the chillers in
real project installations. The surface areas varied between 0.5 and 5m 2 per kilowatt
of cooling power with an average of 2.5m 2 kW − 1 . In the present chapter it will be
shown that this rule of thumb is unsuitable for solar cooling system design and that
the required collector surface correlates much better with the annual cooling energy
than with the nominal cooling power.
To summarize the available solar cooling simulation literature, there are detailed
thermal chiller models available, which are mainly used for chiller optimization and
design and not for yearly system simulation. Solar thermal systems, on the other hand,
have been dynamically modelled mainly for heating applications.
In the following, three different chiller power ranges were analysed, which cover
the current absorption chiller market up to 100 kW. The lowest power chiller based
on NH 3 /H 2 O technology produces 2 kW in cooling power at a COP of 0.5 and is
currently in a prototype state of development at the University of Applied Sciences in
Stuttgart (Jakob et al ., 2003). Amedium-size LiBr/H 2 Omachine with 15 kW nominal
cooling power is now available on the German market (EAW, 2003) and there are
several manufacturers which produce machines around 100 kW cooling power. For
all three machines, theoretical models were developed based on hourly steady-state