Gaining instead of losing - how to save heat rather than throw it away?


According to the last data 10,85 % of final electricity consumption in EU is intended for air cooling in non-residential buildings and 1,7 % in residential (Odyssee, Enerdata), differing from state to state depending on climate conditions and geographical position. IPCC estimates that global demand for residential air conditioning alone will rise from 300 TWh per year in the year 2000 to 4.000 TWh in 2050 and 10.000 TWh by 2100 (IPCC WGII 2014), with the majority of growth in developing countries. For the industry sector, there is a lack of statistical data for air cooling and process cooling energy consumption despite its magnitude and importance but at global level air conditioning and process cooling are estimated for about 17% of global electricity use. Process cooling is a service required in many industrial and service as food and beverage sectors, pharmaceutical, food retail, and data centres.

Space cooling in buildings and process cooling are according to official documents of the EC the most dynamically growing energy uses and the provision of cooling has become a vital service for modern society. According to GIZ ProKlima 2014a publication, globally the refrigeration and air conditioning sectors are responsible for just over 7% of GHG emissions when direct emission of refrigerants is combined with indirect emissions due to energy consumption.

Besides consuming electrical energy, air conditions outer units are heat producers. Basically, what air conditions do is removing heat from inner to outer space, but the question is how much heat they reject and can we recover it?

You may be tempted to think that this is all the heat available for recovery which is incorrect. Compression of the refrigerant in the compressor creates additional heat, so the total available heat is the heat removed from inner space plus heat of compression. Therefore, heat of compression is around 12,5 % of the related capacity which is suggestive. Nevertheless, during any kind of energy conversion an amount of energy is dispersed but why not use the usable?

The simplest way for using wasted heat from air conditioning units is heating water through water heat exchangers, even though the amount of available heat in limited because it would take extremely large heat exchangers to allow the water and the refrigerant to achieve the same final temperature. This kind of application, the refrigerant line leaving the compressor will be connected to a heat exchanger, so the hot refrigerant gases will flow from the compressor, through the heat exchanger, and then to the condenser. The water in heat exchanger is heated by the hot refrigerant gases, so heat is given up by the hotter fluid to the colder water circulating through the heat exchanger. At the output, according to the latest technologies we can obtain water at the temperature of 45°C which can be used for hot water supply through pipelines. Technologies have been considered by the universities and developed by equipment manufacturers.

Due to equipment costs, no energy is free. Still, we have the ability to utilize as much energy as possible, leaving nothing to waste, saving costs and maximizing efficiency in the process. 


  1. Odysee-Mure (Enerdata)
  2. European Comission, Communication from the Commission to the European Parliament, the Council, the European Economic and Social Committee and the Committee of the Regions on an EU Strategy for Heating and Cooling, Brussels, 2016
  3. Ronald E. Jarnagin; Heat Recovery From Air Conditioning Units, University of Florida
  4. Mitsubishi Electronic, Ingenious Use of Waste Heat from Air Conditioners, Hybrid heat recovery system

Authors: LL; ID  (EIHP, Energetski institut Hrvoje Požar)

Gaining instead of losing