Waste heat utilisation


Waste heat cannot be turned into anything useful? Take a look at all the possible ways of turning regular waste heat for heating, cooling and even electricity generation!

Image Copyright: Petersen

Waste heat utilisation

© Petersen

Large amounts of heat are required in industrial processes and manufacturing. Globally, around two-thirds of industrial final energy are consumed in this way. The greater part of this energy then escapes into the environment following direct use.

Utilisation of this waste heat offers huge potential. In Germany alone, companies could save approximately 5 billion euros annually in energy costs this way (see German Energy Agency, in German).

Waste heat can be used productively within the same process or elsewhere in the company. It is also possible to integrate energy consumers in the surrounding area and to generate electricity or cooling. Industrial operations that also use renewable energy to generate process heat can enjoy an additional boost in efficiency.

German companies are among the global leaders in the field of waste heat utilisation.

Various technological approaches can be taken to ensure an efficient use of waste heat, e.g. heat recovery, heat displacement, conversion into electricity and conversion for cooling processes.

A closer look: Waste heat recovery and heat displacement

Heat recovery or heat displacement are the most efficient and at the same time the simplest technological approaches to use waste heat in order to increase the overall energy efficiency and cost efficiency. Wenn die Abwärme aus einem Prozess oder aus der Umgebung wieder zurückzugewonnen wird, dann kann diese im selben Prozess, in derselben Anlage weiter genutzt werden.

Heat exchangers are often used for this purpose. Heat exchangers transfer the waste heat to a transport medium, which then forwards the heat to other units. Examples of direct use are combustion air preheating, or preheating and drying of the source materials, or preheating of water and evaporation.

If waste heat cannot be used directly, consideration should be given to using waste heat within the organisation at the highest possible temperature level in other processes, such as heating water or providing room heating. If the temperatures in the waste heat sink are too low, heat pumps can be used to increase the temperature level.

Another possibility is the sale of non-usable waste heat to third parties, such as neighbouring companies or a local or district heating network or to meet the heat requirements of public buildings and households. However, losses occur when this happens. Furthermore, transferring waste heat to third parties requires additional transport infrastructure such as local and district heating pipes, buffer storage etc.

A closer look: Waste heat conversion into electricity

For power generation from waste heat, several technically advanced and economical technologies comprising different steam processes are available.

There are different possible conversion processes which can be used to convert heat into electricity with a mechanical intermediate stage:

Steam processes: In steam processes, water vapour is used to drive a steam turbine that is coupled to an electricity generator. The electric efficiency of steam processes is specified at approximately 25 to 42% at a temperature level of 250 to 540°C.

Organic Rankine Cycle (ORC) processes: ORC processes have the same functional structure as steam processes. The main difference is that in ORC processes, organic liquids, which boil at lower temperatures or pressures than water, are used. Due to the lower operating temperatures, the efficiency of the ORC processes is lower than the efficiency of steam processes. The efficiency of ORC processes at temperature levels of 70 to 350°C is estimated to be 10 to 20%.

Kalina processes: Kalina processes do not use a single work medium, but rather a mixture of ammonia and water. With the evaporation of a single work medium, the temperature would remain constant. Due to the mixture of work mediums, the temperature can increase during evaporation and can better adapt to the temperature level of heat flows and return cooling flows. Because of larger heat exchanger surfaces and the necessary material separation of ammonia and water, Kalina processes are considered to be more complex than ORC processes.

Stirling processes: Stirling processes use the expansion and contraction of a working gas during the application or withdrawal of heat to drive a mechanical shaft. The shaft in turn is connected to an electricicity generator. Stirling motors are considered to be quiet and low-maintenance machines for waste heat use.

A closer look: Heat conversion into cooling and air conditioning

The demand for heating often fluctuates seasonally, with lower demand in the summer. Meanwhile, the options for heat production are either relatively constant over the course of the year, as in the case of geothermal energy for example, or even greater as in the case of solar heat in the summer. However, in the summer cooling requirements for buildings and processes are also higher. By implementing the appropriate technology, it is possible to exploit heat surpluses to cover cooling requirements, rather than leaving this energy unused. Its use is also possible in year-round applications such as industrial cooling.

Sorption chillers use this technology. In terms of their operating principle, they work in the same way as a conventional compression refrigeration unit such as a refrigerator. However, instead of using electricity to drive a compressor, these machines use heat for thermal compression. They use reversible bonding of two working substances for this purpose. With the supplied heat, the coolant can be expelled or desorbed from the other working substance again after providing cooling. It is then available for a new cycle.

In absorption chillers the coolant is mixed with a liquid sorbent, while adsorption chillers use a solid sorbent for mixing with the coolant. The operating temperature of adsorption chillers is generally lower than that of absorption chillers. Different cooling capacities can be provided, depending on the particular combination of coolant and receiving sorbent used.