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.