Compressor heat recovery: up to 84% of the electricity can be reused
A screw compressor converts electricity into compressed air… and into heat. That heat accounts for 80 to 93% of the energy consumed, and 50 to 90% of that heat can be recovered to heat a building or a process. Method and calculation.
A compressor is, above all, a machine that turns electricity into heat
At full load, a 75 kW lubricated screw compressor draws around 75 kWh of electricity per hour, in fact a little more: 75 kW is the motor's rated output, and its losses come on top. Almost all of that energy ends up as heat: the heat of compression goes into the oil cooler and the aftercooler, and the compressed air leaves cooled. This is thermodynamics, not a fault in the machine.
On an air-cooled compressor, this heat is rejected to atmosphere through the coolers. On a water-cooled machine, it goes into the cooling loop. Either way, without a recovery system, it is pure loss.
The good news: according to the US Department of Energy's Sourcebook, the available heat amounts to 80 to 93% of the electricity drawn, and a properly designed recovery unit recovers 50 to 90% of it, i.e. 40 to 84% of the electricity drawn, at a usable temperature (typically 55 to 90 °C at the recovery unit outlet). It is one of the largest energy efficiency opportunities in industry, and still widely under-used.
If you have a compressor above 30 kW running more than 4,000 h/year and a heating need close by (domestic hot water, workshop heating, process preheating), heat recovery almost always deserves a study.
Where the heat is, and at what temperature
The heat of a lubricated screw compressor can be taken back at three points, at different temperature levels. Understanding them is how you choose the right recovery strategy.
| Lubricating oil (oil cooler) | 70 to 90 °C: the hottest and densest source |
|---|---|
| Compressed air (aftercooler) | 30 to 50 °C: usable for domestic hot water preheating only |
| Canopy ventilation air | 10 to 20 °C above ambient: direct workshop heating |
| Total recoverable energy (lubricated screw) | 40 to 84% of the electricity drawn (80 to 93% becomes heat, of which 50 to 90% is recoverable) |
| Total recoverable energy (oil-free screw) | Depends on the cooling (air or water): read it from the manufacturer's heat balance |
The main source is the oil: it carries most of the heat from the compression process. An exchanger can be fitted on this circuit, at the factory or afterwards.
Three technologies, three uses
- Air-to-air (canopy air ducted into the workshop): the simplest and cheapest of the three. A motorised damper diverts the cooling air into the workshop in winter. Ideal for heating an unheated space or offsetting heat loss.
- Air-to-water (heat exchanger on the oil circuit): produces hot water between 60 and 80 °C for domestic hot water, underfloor heating or process preheating. The most versatile, and the most common on high-power machines.
- Water-to-water (water-cooled compressor): direct integration into an industrial water network. The most expensive of the three to install, reserved for sites with a permanent hot water demand.
With a workshop to heat and a compressor > 30 kW, air-to-water recovery on the oil circuit is often the best balance between investment and versatility: it serves space heating as well as hot water.
Worked calculation on a 75 kW screw compressor
Take a 75 kW compressor running 4,500 h/year at an average load of 65%. The recoverable energy and the heating saving follow directly.
| Annual electrical energy absorbed | 75 × 4,500 × 0.65 = 219,375 kWh |
|---|---|
| Recoverable thermal energy (80%, the DOE worked example's assumption) | 175,500 kWh |
| Energy actually put to use (60% utilisation rate) | 105,300 kWh |
| Gas not burnt (boiler at 92% on LHV) | 105,300 ÷ 0.92 = 114,457 kWh LHV, i.e. ≈ 127,000 kWh HHV |
| To value at your own rate | ≈ 127,000 kWh HHV of gas per year, the usual unit on gas bills, to multiply by your price per kWh |
The utilisation rate is the critical parameter. If the heat demand is seasonal (workshop heating only, October to April), assume 50% at most. If the demand is permanent (process hot water), you can go up to 80-90%.
Three pitfalls to avoid
- Undersizing the heat exchanger: sizing it too tightly limits recovery to the peaks. Better to allow a 10-15% margin on the rated flow.
- Forgetting thermostatic control: without a bypass, the compressor can overheat in summer when the heating network is not calling for heat. Always provide an automatic bypass or a back-up cooler.
- Ignoring water quality: on sites with hard or aggressive water, the exchanger scales up or corrodes within 2-3 years. Plan a water softener or a stainless steel exchanger from the design stage.
Always ask your installer for the heat balance with quantified assumptions (operating hours, average load, utilisation rate). A sales proposal without a heat balance means an imaginary gain.
The carbon argument, often overlooked
Beyond the energy saving, heat recovery delivers a direct cut in CO₂ emissions. In the worked example above: 114,457 kWh LHV of gas not burnt, at the factor in the Base Carbone of ADEME, the French ecological transition agency (natural gas, 2022 average mix: 0.239 kg CO₂eq per kWh LHV), represent about 27 tonnes of CO₂eq avoided per year.
For a site certified to ISO 14001 or applying for CEE (Certificats d'Économies d'Énergie, France's energy savings certificates), this saving feeds directly into CSR targets.
CEE incentives fund part of the investment in compressor heat recovery, depending on the negotiated price per MWh cumac and on the quotation. Ask your installer to calculate operation sheet IND-UT-103 ("Système de récupération de chaleur sur un compresseur d'air", heat recovery on an air compressor): it changes the calculation.
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