Sustainable Building Operation Using Simulation - Low Energy Cooling for Sustainable Buildings - page 216

Figure 6.17 Influence of storage tank insulation and heat exchanger size on solar fraction and specific
collector yield

2 percentage points for a typical insulation thickness of 10 cm compared with an ideal
loss-free storage tank (see Figure 6.17). This corresponds to an additional auxiliary
heating demand of 340 kWh or 9% more. Doubling the storage volume increases
the solar fraction by 1 percentage point, which corresponds to a reduction of auxiliary
heating energy of 200 kWh or 5% less. The larger the storage tank, the more important
the good insulation quality. The specific collector energy delivered to the storage tank
even drops if the insulation quality improves, as the storage tank is generally hotter.
However, the energy delivered from the storage tank to the absorption chiller increases,
so that in total the solar fraction improves.
The influence of the solar circuit heat exchanger was analysed by varying the trans-
ferred power UA per degree of temperature difference between the primary and sec-
ondary circuit. The heat exchanger is usually dimensioned for the maximum power
of the solar collector field. At a mean operating design temperature for the collectors
of 85 ◦ C and an ambient air design temperature of 32 ◦ C the efficiency of the vacuum
tube collectors chosen here is 67.5%, that is, the collectors produce a maximum of
675Wm − 2 at full irradiance. For the given surface area of 31m 2 and a set temper-
ature difference across the heat exchanger of 3 K, this results in a transfer power of
7kWK − 1 . A reduction in transferred power from the solar circuit heat exchanger by
50% does not reduce the solar fraction at all.

6.1.4 Influence of Dynamic Building Cooling Loads

If a given cooling machine designed to cover the maximum load is used for different
cooling load profiles, the influence of the specific load distribution and annual cooling
energy demand can be clearly seen. The boundary conditions for the control were set