Waste heat: reusing heat discharged by industry
Waste heat – thermal energy produced and then discharged to the atmosphere – can be reused and fed back into the industrial process.
A number of industrial processes generate heat that is not reused – waste heat. With industry accounting for more than 20% of energy consumption in France, several solutions are available to help manufacturing plants recover waste heat to save energy.
French industry discharges 16% (51 TWh/year) of the heat it produces to the atmosphere. Generally speaking, for example, a furnace uses only 20 to 40 % of the heat produced. The rest is lost as waste heat produced but unused, with several consequences: overconsumption of energy above and beyond the minimum amount required for production; an obligation to cool discharges; and the duty to comply with regulatory/environmental requirements applying to effluents. But for those who know how to put it to use, waste heat can be an asset.
Why recover waste heat?
Waste heat is more or less easy to recover depending on whether it is contained in gases, liquids or solids. It can be used directly or indirectly within the plant network or an external heating network (eco-industrial park) or else converted into another form of energy such as electricity.
To properly recover waste heat from intermittent processes, it is necessary to set up a system to store it. “Pasteurisation and sterilisation in food processing are good examples of the way waste heat can be put to use,” says Actemium energy efficiency engineer Paul Dède.
“Food conservation requires substantial heating and cooling. One waste recovery solution consists in setting up an intermediate heat storage system that heats the water for the following production cycle. This can save 20 to 30% of the natural gas that is invoiced.”
The recovery process depends on the type of heat discharged (temperature, flow rate, composition). The goal is for the temperature of the heat source to be as close as possible to that of the substance to be heated. “It doesn’t make sense to use waste heat at 500°C to heat water to 60°C because that wastes energy. We must always try to bring the energy contained in the source fluid down to a minimum level,” says the engineer.
How is waste heat recovered in industry ?
The waste energy recovery process is not new. “Some industrial sites took very good ideas on board from the start, even though energy was less expensive at the time they were built and the objectives at the time were different. But there is a lot of scope for further optimising energy efficiency in the manufacturing sector. Waste heat recovery is one way to achieve that“, says Paul Dède.
A waste heat recovery project generally starts with a systemic analysis to identify the potential energy savings in the process and utilities (boiler, air compressor, refrigeration unit, etc.). “To begin with, we propose solutions designed to reduce discharges of waste energy as far as possible by optimising equipment, operating parameters, and so on,” says Paul Dède. “We then draw up a comparison between the total energy discharged (hot water, condensation, furnace off-gases, etc.) and the energy required by the processes and utilities.”
This work uses the pinch method, which clarifies the scenarios and supports the choice of the best technical and economic scenario for recovering heat. “It helps us define the best trade-off between the heat discharged at Point A and the need for heat at Point B inside or outside the manufacturing plant.”
Transforming waste heat for re-use
If the heat emitted cannot be re-used as such, other solutions involving temperature level adaptation and/or energy conversion, either on or off site, are proposed.
“We can convert waste heat into another source of energy. For example, we installed Organic Rankine Cycle (ORC) machines at an ultimate waste disposal facility for Veolia Propreté. They use a waste heat source to convert heat into electricity. We recover at least 70% of the heat discharged by the facility,” says Actemium Bordeaux ProcessBusiness Unit Manager Gilles Saint-Germès.
The goal is obviously to propose the best waste heat recovery system for the client, depending on the issues to be addressed. “We carry out the project from start to finish, quantify the savings to be made, help the client seek financing, and so forth. We work closely with the equipment suppliers providing storage, heat exchangers and heat conversion systems to put together a customised solution,” says Paul Dède.
The maximum acceptable payback period for a heat recovery installation of this type in industry is generally three years. Subsidies (French Fonds Chaleur heat fund, ESCs, third-party financing, etc.) can provide an incentive to carry out such projects.