Technical College for Engineering in Butzbach, Germany

Description of the application

In the low-energy building of the Technical College, demand for summer air conditioning arises due to high occupation rates and frequent use of computer equipment. The building is used throughout the summer season. Two existing ventilation systems (supply/return air with heat recovery, 1,250 m/h air volume flow rate each) are not sufficient to remove the sensible and latent cooling loads in summer. For this reason, a solar thermally driven chiller plant was added and supported in the frame of the German Solarthermie 2000plus funding programme. The ventilation units were extended by cooling coils, and additionally chilled ceilings and cooling panels were installed.

General description of the system

Two absorption chillers are installed and thermally driven with solar heat alone. The condensing boiler is used for space heating in winter only. Different operation modes are possible with the system:

  • only one chilled water temperature level, e.g. for the removal of sensible cooling loads, is required. In case both chillers are in operation, the driving circuits of the chillers are usually connected in parallel;
  • one chiller operates at low chilled water temperature level for supply air dehumidification and cooling, while the other provides chilled water at a higher temperature level for the chilled ceilings and cooling panels. In this mode, it is possible to connect the driving circuits of the chillers in series with the effect of an increased temperature difference in this circuit, resulting in a more favourable collector system operation.

The evacuated tube collector system contains pure water as collector fluid. A special freezing protection control runs the collector loop in short intervals at low ambient temperatures.
System concept and overall system control: Hindenburg Consulting (
Detailed planning: IGT GmbH

Central air-conditioning unit


closed cycle

Nominal capacity

2 x 10 kWcold

Type of closed system


Brand of chiller unit

SK Sonnenklima: Suninverse

Chilled water application

Supply air cooling and dehumidification, chilled ceilings and cooling panels


in supply air system

Heat rejection system

wet open

Solar thermal

Collector type

evacuated tube with CPC-mirror

Brand of collector

CPC Star azzurro

Collector area

60 m2 aperture

Tilt angle, orientation

30, south

Collector fluid


Typical operation temperature

80 C driving temperature for chiller operation


Heat storage

3 m3 water

Cold storage

1 m water

Auxiliary heating support

gas condensing boiler, 28 kW

Use of auxiliary heating system

for space heating only

Auxiliary chiller


System performance

Still in the evaluation phase. One of the chillers performs not as expected and has to be maintained, while the second chiller runs with a promising performance. The low driving temperature range of the chiller (minimum 55-60 C) allows a high exploitation of the solar heat.

System reliability and overall success of the installation

Beside problems with one of the chillers, the system runs reliable and has considerably increased the room comfort in the building. Solar autonomous cooling is an appropriate operation mode in this application.

General information

Type of building  College building
Location  Butzbach, Germany
Type of application  solar autonomous summer air conditioning
In operation since  2008
System operated by  Technical college
Air-conditioned area  335 m2
Chilling capacity  20 kW


Caption 1
Technical college building
Solar panels

Clicking an image opens the gallery view.

Specific information

Further specific information about the system are included in the filled in technical questionnaire.


289 K


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