At first sight, the concept of waste heat appears easy to grasp, but is everything as easy as it seems? Have you also heard of “excess heat”, “surplus heat”, “recoverable heat”, “waste-to-energy”, “recovered heat”? Are these terms equivalent or do they cause a degree of confusion? Below you will find a non-exhaustive dictionary. In the ReUseHeat project we are targeting low temperature heat sources generated by urban activity.


In the revision proposal for the renewable energy sources directive,  ‘Waste heat or cold‘ is defined as “heat or cold which is generated as by-product in the industrial or tertiary sector which would be dissipated unused in air or water without access to a district heating or cooling system”.

The term ‘excess heat’ is widely used in research and innovation contexts (for example the Stratego and Heat Roadmap Europe projects), to refer to heat sources from both energy generation and industry or tertiary sectors.

Surplus heat’ is mainly used when talking about waste industrial heat, which can be recovered and reused in industrial processes or district heating or cooling networks. It has been used for example by the UK Government in assessing the potential of industrial waste heat.

Waste-to-energy or waste incineration plants burn household and similar waste that could not be prevented or recycled and generate energy in the form of electricity, steam or hot water. The hot water, depending on local infrastructure can be sent to nearby district heating or cooling networks.

Cogeneration (Combined Heat and Power or CHP) is the simultaneous production of electricity and heat, both of which are used. In other terms, the excess heat generated by the production of electricity can be reused in individual buildings, industrial facilities or cities served by district heating networks.

You may come across even more technical terms. Exergy is the energy that is available to be used. Energy is neither created nor destroyed during a process. Energy changes from one form to another. In contrast, exergy is always destroyed when a process is irreversible, for example loss of heat to the environment.

And the link to waste water? Waste water (or sewage water) can be a resource for energy generation, renewable energy, fertilizers and clean water. Waste water systems have therefore a major heat recovery potential which could be channelled through district energy networks.