Forschungsgruppe-NET - Hochschule Offenburg

Solar Heat

Solar Cooling at Festo AG & Co. KG in Esslingen

The solar thermal system consists of 290 vacuum-tube collectors with a total surface of 1330 m². 58 collectors have an aperture surface of 3 m² each (CPC30) and the other 232 have an aperture/ collector surface of 4.5 m² each (CPC45). The total aperture surface thus amounts to 1218 m². The collectors are installed on the sawtooth roof of a production and office building. Their azimuth is 18° West with an inclination of 30°.  The CPC 45 collectors consist of 21 vacuum tubes connected in series of three. One CPC30 and four CPC45 collectors are connected in series to form one collector line. In order to reach a homogenous heat distribution, the cold preheating feed is distributed to the collector lines via a central inlet. The water heated by solar irradiation is then fed from the individual collector lines into a reflow tank in the middle of the collector field. From there it is pumped into two buffer storage tanks (connected in series) with a capacity of 8500 liters each.

In summer, solar heat is pumped from the tanks via the heat distributor to the technology center and from there to the chillers. With three chillers (type MYCOM ADR-100) that have a capacity of 353 kW each, the solar thermal system at Festo AG & Co. KG is currently the largest solar cooling system worldwide. The cold that is generated cools down 26760 m² worth of office space as well as three atria of the technology center with a surface of 2790 m². Before the solar thermal system was set up, the chillers were operated by the waste heat of the compressors and by the heat from the gas boilers. With the solar thermal power station a third heat source has become available to the absorption chillers; it not only reduced the primary consumption of energy considerably but also the costs. 

The storage tanks in this system buffer the amount of heat for one work cycle of an adsorption chiller. During the charging and discharging processes, the buffer storage tanks are connected in series. In winter when there is no need for cooling, the generated heat is used at a low temperature level for the concrete core activation of the new office building. 

A special feature of this system is the water only system of the collector circuit. A frost protection circuit prevents the water from freezing in winter. This is why the heat exchanger between collector circuit and buffer storage circuit is obsolete as the heat capacity of water is higher than that of the water-glycol mixture. 

An adsorption chiller works as follows: similar to a compression chiller, a liquid (in this case water) is evaporated in order to generate cold. The evaporation enthalpy needed for this process is drawn from the coldwater circuit – which is to say it is cooling it down.

In order to makes sure that enough water enters the gas phase at a low temperature, the absorption chiller generates a vacuum. The evaporated chilling agent is deposited on the adsorbent (silica gel). The condensation heat thus released must then be discharged via a cooling tower.  

If the silica gel adsorbs (?) water, the system switches to the desorption cycle. Here, the silica gel is heated up to 55° - 90°C and the water, which is desorbed from the silica gel, returns to the liquid phase inside the condenser. 

The condenser has to be also cooled again to discharge the condensation heat. The condensed water is fed again in the evaporator, which ends the chilling cycle. To guarantee the continuous operation of the chillers, two sorption chambers are necessary, which are put into the adsorption and desorption cycle alternately.

Simplified Diagram of the Solar Thermal System at Festo AG, Esslingen


This table shows the most important specifications of the solar thermal system at Festo AG & Co. KG.

Project Data
Collector/Aperture surface 1330 m² / 1218 m²
Collector typeCPC Vacuum tubes
Buffer storage2 x 8500 ltrs
Heat exchangerNot applicable
Azimuth / InclinationSouth +18° / 30°
Guaranteed Solar Yield 500 MWh/a

Click on the pdf-file to receive detailed information on the specifications of the solar thermal system at Festo AG & Co. KG, Esslingen. The file also includes details of the collectors, the pipes, the carrier medium in the collector circuit and the heat exchanger between the different circuits.

Measurement Technique

The following diagram shows the system’s measurement and monitoring sensors. The monitoring sensors are essential for the functioning of the whole system; the measurement sensors are used for the extended functionality control of the solar thermal system

Measurement and Monitoring Points of the Solar Thermal system of Festo AG & Co. KG

DATAPOOL provides access to our data server

Via DATAPOOL you can access selected data of this solar thermal system. You can choose from various illustrations like line charts, carpet plots, or scatter plots.

The solar thermal system at Festo AG & Co KG, Esslingen can be found on the Datapool pages under Solar Thermal Systems as far as measurement data are available for this system. 

The individual measurement points of the data pool can be found in the diagram above or here

For a list of the measurement points, click here

toTopto top