Waste heat potential in production: How to calculate what pays off
In many manufacturing plants, waste heat is treated as wasted energy. Furnaces, air compressors, and exhaust air streams release heat, while gas or electricity is used elsewhere for heating, preheating, or hot water. It seems logical to connect the two, but only by calculating the waste heat potential can we determine what is truly worthwhile. This article explains how to assess waste heat recovery in the context of the overall system. Waste heat recovery in production is often misunderstood.
The next step is to calculate the waste heat potential. Whether waste heat recovery is worthwhile is not determined by the technology, but by assessing heat quantity, temperature, and availability over time. Anyone who knows these three variables can estimate the potential in a few steps and make an informed decision about where a more detailed analysis makes sense.
KEY FACTS: CALCULATE WASTE HEAT POTENTIAL INSTEAD OF ASSUMING
- Source and sink must match in terms of time, location, and temperature level.
- Typical waste heat sources include process gases from furnaces, exhaust air, air compressors, refrigeration systems, and warm wastewater.
- The potential can be roughly calculated from volume flow, temperature difference, and operating hours.
- The payback period is calculated by additional investment costs divided by annual savings, or in the case of subsidies, by the additional costs after deducting the subsidy amount.
- Not all waste heat is economically viable to use: a lack of a heat sink, mismatched timing, or long distances often prevent success.
WHERE HEAT IS LOST IN PRODUCTION
Waste heat is generated in almost every production process. The key is to determine which source provides heat at what temperature and in what form, and how it can be used:
For heat from ventilation and building systems, heat recovery is the right place to start.
Quantity, temperature, time: The three factors that decide
Whether a heat source is worth considering can be determined by checking three key factors.
Quantity and temperature of the source
How much air or exhaust gas flows per hour, and how hot is it? This determines the available heat output. Temperature fluctuations also matter: Exhaust gas that briefly reaches 700 °C but is usually at 300 °C requires a different design than a constant-temperature flow. The temperature level also determines what the heat can be used for. High temperatures are suitable for processes and preheating, while low temperatures are often only useful for heating or, with a heat pump, for domestic hot water.
The right heat sink
Having heat is not enough; there must be a suitable application for it: combustion air that needs preheating, hot water, space heating, or material preheating. The heat sink must require a similar amount of heat at a similar temperature. For space heating, the heat is usually only needed during the heating season, which affects operating hours.
Synchronicity and distance
Is the heat available when it is needed? If the compressor runs at night but the heat demand is during the day, supply and demand are not synchronized. The distance between the source and the sink also matters: long pipes mean higher investment costs and heat losses.
When waste heat recovery is not worth it
A reliable assessment can also be "no". Typical reasons:
- No suitable sink: There is no process that can utilize the heat at this temperature and quantity.
- Timing mismatch: Source and demand occur at different times; the cost of thermal storage may outweigh the potential savings.
- Small quantities or limited operating hours: The annual savings do not cover the investment.
- Complex media: Dust, oil, or condensate in the exhaust gas can increase the requirements for heat exchangers, cleaning, and minimum outlet temperatures.
- Losses before utilization: Uninsulated hot pipes and system components release some of the heat before it can be recovered. Up to approximately 250 °C, convection is the dominant factor; above that, it is radiation.
These limitations are part of an honest analysis. A calculation that shows early on that a project is not worthwhile saves planning and investment costs. Our article “The greatest efficiency potential in industry lies between systems” explains how efficiency losses occur at the interfaces between systems.
What data you need for a reliable initial assessment
With this information, a heat source can be reliably assessed. Much of this information is already available in most companies:
- Source: Exhaust volume flow (with units), operating hours per year and day, average, minimum, and maximum temperature, type of medium (dust, oil, moisture, corrosivity).
- Sink: Operating hours, temperature level, energy source, annual consumption, and costs.
- Connection: Distance between source and sink, and whether the heat supply is available at the same time.
- Energy costs: current prices for gas, electricity, or district heating.
These key figures are used to derive potential savings, investment costs, and payback periods. The approach follows the principles of THINK INFRANOMIC®:Understand the heat flows throughout the entire operation first, then choose the technology.
Conclusion: Calculate first, decide later
Waste heat recovery is worthwhile where the source and sink match in terms of quantity, temperature, and timing—and that can be calculated. Checking the potential against a few key figures allows you to make investment decisions based on facts rather than assumptions. And if the result shows that a measure is not worthwhile, that is also a valuable outcome.
FAQ
What are typical industrial waste heat sources?
Process gases from furnaces and smelting plants, exhaust and waste air, air compressors, refrigeration systems and recoolers, as well as warm wastewater and process media.
What temperature level is required for waste heat recovery?
That depends on the sink. High temperatures are suitable for processes and preheating. Low-temperature waste heat can be raised to a usable level using a heat pump.
When is waste heat recovery not worthwhile?
When there is no suitable sink, the heat supply and demand do not align in time, the quantities are small, or complex media make the technology too expensive.
Are there subsidies for waste heat recovery?
Subsidies can shorten the payback period, but they are subject to conditions, such as maximum payback periods. You should check the current status before making a decision.
