Heat Rejection

Kühlturm im Praxistest
© Fraunhofer ISE
Cooling tower in a field test.
© Fraunhofer ISE
Section of a sample heat exchanger.

Heat rejection, the dissipation of unused heat – in most cases to the environment, plays an important role in numerous HVAC applications and industrial processes. Each year, the German industry alone generates approx. 700-800 TWh of waste heat. For waste heat rejection, it is estimated that, in addition to dry and hybrid heat rejection units, more than 30,000 evaporative cooling systems are in use. This technology has the advantage that the units are relatively compact, energy efficient, and allow cooling water temperatures below ambient. The disadvantages are the expense of the required water treatment and compliance with hygiene requirements. Dry coolers, on the other hand, consume no water and require little maintenance. However, they have a higher power consumption and can only cool the waste heat down to ambient temperature. In addition, there are also mixed variants such as hybrid coolers or heat rejection systems with adiabatic pre-cooling of air. These variants aim to combine the advantages of both basic technologies.

In the system operation, the aim is to reject heat to the targeted temperature level with the least possible auxiliary energy and low maintenance requirements. This reduces power consumption, saves costs and protects the environment. However, Fraunhofer ISE has found in operational analyses that the heat rejection units in use often consume significantly more electricity than necessary. The reasons for this may be faulty planning and poor system dimensioning or also non-existent or unfavorable operating controls, especially under partial load conditions.  Contamination and corrosion additionally reduce efficiency and lead to hygiene problems.

Wahrscheinlichkeitsverteilung
© Fraunhofer ISE
Probability distribution of predicted daily energy consumption compared to the measured value.

System designers often focus on the investment costs and not enough on the life cycle costs of the recirculating chillers. In addition, automated continuous monitoring of heat rejection units in operation is only implemented in a few fields of application up to now (e.g. in large processing plants such as petroleum refineries). In other fields of application, operation is monitored not at all or only based on rules set down by the operators according to their expertise. 

The energy efficiency of system operation is an important focus for Fraunhofer ISE. Based on our extensive R&D expertise in this area, Fraunhofer ISE offers comprehensive research and development services to manufacturers, planners and operators of heat rejection units. Our services include the analysis and optimization of the unit design, layout and operation.

Our R&D Activities: 

  • Development of novel heat rejection processes, production and testing of demonstrators
  • Optimization of heat rejection units through the combined analysis of heat transfer and pressure drop
  • Evaluation of heat rejection concepts (dry/wet/hybrid) based on simulations, laboratory or field measurements
  • Analysis and evaluation of the influence of various contaminants on heat transfer and pressure drop
  • Methods of automated operation monitoring and optimization
  • Acoustic evaluation of heat rejection systems, analysis and optimization of vibration behavior of components

Fraunhofer ISE has extensive expertise along the entire value chain of heat rejection systems. For the development of innovative heat exchanger structures, their integration into the overall device up to its optimal design and operation, we have extensive simulation-based and experimental methods at our disposal.

More information on this research topic:#

Research project "SolaRück"

Efficient Heat Exchange for Solar Thermally Driven Refrigeration Generation

Research project "WCS-energy"

Online Monitoring of Energetic Performance of Wet Cooling Towers

Research project "MinWaterCSP"

Online Monitoring of Energetic Performance of Wet Cooling Towers