Passive Cooling Circuit on the Basis of Radiative Cooling
Background
There is an ever-increasing demand for cooling within production plants and office buildings. Currently typical cooling systems are compression chillers which run on R134a (a greenhouse gas) and achieve a coefficient of performance (COP) of approximately 3. To save primary energy, new cooling concepts with higher COPs and more environmentally-friendly refrigerants have to be developed.
Passive cooling by way of radiative cooling
ZAE Bayern’s experimental building in Würzburg has a water cooling circuit (primary circuit) to cool apparatuses and laboratories by way of a radiant-cooling ceiling system. The building used to be cooled with a conventional compression chiller with a correspondingly high primary energy consumption.
This project tested the concept of regenerative (passive) cooling by way of radiative cooling: Rainwater is collected on the roof of the building and fed into a cistern. From the cistern the water is pumped through a heat exchanger which absorbs the waste heat from the building’s cooling circuit (P). The warmed water is fed back into the cistern. The warmed water in the cistern is pumped over the building roof at night. As the water runs down the roof it is cooled by way of radiation, convection and evaporation. The water collects in the roof guttering and is fed back into the cistern where it is once again used to cool the building.
Principle of the regenerative (passive) cooling system
Results
The system was installed and put into operation within the framework of the project in 2000. Measurements and simulations were carried out and know-how gained; in principle the system has been proven to work.
Typical values
- building with water cooling circuit: supply water temperature: 18°C, cooling load approx. 3 kW (in summer)
- rainwater in cistern (40 m³) as thermal mass, temperature: approx. 13 – 18°C
- recooling at night, water runs over the building roof (150 m² corrugated steel sheet)), flow rate approx. 3.6 m³/h
- cooling performance (in summer):
- 7 – 20 kW equiv. 50 – 150 W/m² at night (i.e. approx. 8 hours a day)
- 3 – 7 kW equiv. 20 – 50 W/m² constant cooling performance (24-hour-average )
- COP (performance factor):
- 10 – 30 roof cooling without secondary circuit
- 5 – 15 roof cooling with secondary circuit
Outlook
Seen hydraulically, the cooling circuit is an open system; the water from the cistern flows openly over the roof and is cooled via radiation, convection and evaporation.
The follow-up project Metal Solar Roof Collectors in a Closed Cooling Circuit for Passive Cooling on the Basis of Convection and Radiation (likewise funded by the German Environmental Foundation, DBU) tested closed systems which avoid evaporation and therefore also water loss.
Project partners
Project duration: September 1999 – December 2002
This project was funded by the German Environmental Foundation (DBU) under the funding code AZ 14890.
Contact
Dr. Dietrich Büttner
Tel.: ++49-(0)931/70564-41
e-mail: buettner@zae.uni-wuerzburg.de
Pilot system in ZAE Bayern's experimental building in Würzburg