Office building of S.O.L.I.D., Graz, Austria

Description of the application

The office building of SOLID was renovated in 2004 and the solar cooling device was installed in the year 2008. The façade of the office rooms are south and west oriented. To reduce the solar gains external shading devices are installed at each glazing. Because of internal gains and ventilation via windows, active cooling is indispensable. The solar cooling equipment is installed in a so called “Cooling Cabin” placed under a pergola which is situated in front of the office building as a nice main entrance. The solar collectors are installed at the roof of this pergola. The hybrid cooling tower is placed at the flat roof of the office building. The cooling load of the office rooms is taken out via ceiling cooling elements.

General description of the system

A closed absorption cycle for generating cooling energy is employed. Autonomously solar thermal generated heat by high temperature flat plate collectors is used to regenerate the process. Within the absorption cooling machine water is used as refrigerant and lithium bromide is used as solvent. The cold water generated by the absorption cooling machine is used to cover internal and external heat gains and also the heat income caused by window-ventilation in the office rooms. In winter the solar collectors are in assistance to the space heating. In summer and winter the solar generated heat is stored in one buffer storage and all energy demand is taken out of it. The additional heat from the district heating is not stored in the tank but directly carried to the space heating system. A special application is the usage of the hybrid cooling tower for direct cooling via the ceiling cooling elements.

Central air-conditioning unit


closed cycle

Nominal capacity

17.5 kWcold, 21 kW peak

Type of closed system


Brand of chiller unit

Yazaki WFC SC5

Chilled water application

cooled ceilings elements



Heat rejection system

hybrid cooling tower

Solar thermal

Collector type

high temperature flat plate

Brand of collector

ÖkoTech Gluatmugl HT

Collector area

60 m2 gross

Tilt angle, orientation

15°, south

Collector fluid


Typical operation temperature

88 °C driving temperature for chiller operation


Heat storage

1 x 2,000 l

Cold storage

1 x 200 l

Auxiliary heating support

district heating

Use of auxiliary heating system

only for heating in winter

Auxiliary chiller


System performance

The nominal coefficient of performance of the absorption cooling machine is 0.7. The efficiency of the high temperature flat plate collector with integrated teflon foil according to the data sheet is 58% at 88 °C collector temperature and 30 °C air temperature with an irradiation of 800 W/m². The thermal COP of the chiller could be monitored during first test periods with about 0.6. The electrical COP of the overall system was not perfectly satisfying in the first test periods. Main reasons for that most likely are very high flow rates in the cold distribution loop (indicated by temperature differences of less than 2 K) but also in generator and cooling tower loop causing high electricity consumption but obviously also high thermal COP. Also free cooling mode was in operation during times when it should not be on causing quite high electricity consumption with unexpected low cooling effect. Further monitoring and optimization of the overall control strategy will be done during the following years.

System reliability and overall success of the installation

The solar cooling installation was working reliable the first year and the concept of that pre-installed Cooling Cabin was a successful example of easy installation and nice architectural integration to existing buildings.

General information

Type of building  Office building
Location  Graz, Austria
In operation since  2008
System operated by  SOLID
Air-conditioned area  435 m2
Cooling load  21 kW


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For more information on the project please feel free to contact:

Alexander Thür