University Hospital in Freiburg, Germany

Description of the application

During summer months, the supply air of a separate laboratory building at the University Hospital is cooled by means of a solar thermally assisted chilled water production. The chilled water temperature level is approx. 9C and is used for supply air cooling in two air handling unit of the laboratory building, equipped with heat recovery. Due to the requirements on the laboratory room air temperature, cooling demand may occur during summer also during night. Additional heat to guarantee the chiller operation is provided by the locally operated steam network of the Hospital. In winter, the solar thermal system assists the laboratory heating.

General description of the system

The system technology is a closed cycle chilled water system with an adsorption chiller. Two heat sources exist to operate the chiller:

  1. A solar thermal system, consisting of vacuum tube collectors (direct-flow type)
  2. Heat from the steam network of the Hospital is used via a condensate heat exchanger

A closed wet cooling tower is applied for heat rejection from the chiller. Additionally to the solar hot water storage of 6 m volume (consisting of three storages connected in series), a buffer of 2 m volume is used in the return flow of the driving heat in order to smooth the periodically temperature peaks in the hot water circuit, thus allowing a stable control of the collector system. During winter, the supply air heating is directly assisted by the solar collectors.

Central air-conditioning unit


closed cycle

Nominal capacity

70 kWcold

Type of closed system


Brand of chiller unit

Nishiyodo NAK 20/70

Chilled water application

supply air cooling



Heat rejection system

closed wet cooling tower

Solar thermal

Collector type

vacuum tubes

Brand of collector

Seido 2-16

Collector area

167 m2 aperture

Tilt angle, orientation

30 and 45, south

Collector fluid


Typical operation temperature

75 C driving temperature for chiller operation


Heat storage

6 m3 water

Cold storage

2 m2 water

Auxiliary heating support

condensating steam heat exchanger, driven by the Hospital steam network

Use of auxiliary heating system

Auxiliary driving source for chiller, auxiliary driving source for supply air heating in winter

Auxiliary chiller


System performance

The energy balance of 2003 stated an average thermal coefficient of performance (COP) of 0.42; the solar coverage of the total heat input (cooling and heating) was 28 %. The average COP is decreased through periods of part-laod operation during summer nights with low cooling demand. At clear sky summer days, a solar coverage of the driving heat input into the chiller of approx. 90 % may be achieved. The annual specific collector yield was 365 kWh/ma.

System reliability and overall success of the installation

Due to the complex hydraulic system, an optimised system control is difficult to achieve. Important improvements in the control strategy have been made by varying the cycle length (period between two adsorption cycles or regeneration cycles respectively) according to the part load status of the chiller. The overall reliability of the sytem is good, the laboratory users are satisfied with the system.

General information

Type of building  Laboratory building of the University Hospital
Location  Freiburg, Germany (48 North, 750 East)
In operation since  1999
System operated by  Energy Management of the University Hospital
Air-conditioned area  360 m2



Collector area
Site view
Collector area

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Specific information

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



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

Edo Wiemken