Heat recovery using high-temperature heat pumps is key to integrating heating and cooling requirements in industrial processes, transforming waste heat into a resource to reduce consumption and emissions. However, before sizing new systems, it is essential to eliminate inefficiencies and optimise the existing production line.
In food processing plants, cooling and heating are often produced simultaneously. Linking these two requirements can transform wasted heat into a process resource, reducing energy consumption, costs and emissions. This topic is at the heart of the review “A Review of Energy-Efficient Technologies and Decarbonisation Solutions for Process Heat in the Food Industry”, by François Faraldo and Paul Byrne, published in 2024 in Energies, and revisited and updated this year by the following papers: ‘Heat Pump Integration in Food Industry Pasteurisation’, by Giovanni Paolo Carlo Tancredi and Giuseppe Vignali, focusing on pasteurisation, and ‘High-temperature heat pumps: market overview, state of the art, and application potential’, by Cordin Arpagaus on high-temperature heat pumps.
The three publications demonstrate that decarbonisation does not depend on a single machine, but on an integrated approach to the plant. According to Faraldo and Byrne, approximately 62 per cent of the energy used in food processing is utilised as heat and 38 per cent as electricity, largely for cooling. In many plants, therefore, the heat rejected by refrigeration systems coexists with boilers that consume fuel to heat water, air or process fluids. The food industry, in fact, combines refrigeration and heating requirements.
Raw materials and products must be cooled or stored at controlled temperatures, whilst many processing operations require hot water, steam or heated air. The applications considered by Faraldo and Byrne range roughly from –50 to 300°C and include refrigeration, pasteurisation, sterilisation, evaporation, drying, cooking and frying. This results in a network in which heat sources and loads at different temperatures coexist. Before replacing a generator, the authors recommend drawing up an energy flow map, recording temperatures, power ratings, flow rates and operating times, to determine which sources can supply the loads directly and which require the temperature to be raised.
Condensation heat as a resource
A refrigeration system does not eliminate heat, but transfers it to the outside, adding the electrical energy absorbed by the compressor. The condenser therefore dissipates a greater quantity of thermal energy than the load removed from the product or the environment alone. This heat, which is often discharged to the outside, can be used to preheat wash water or boiler feed water, heat rooms and floors, or power low-temperature processes. The review therefore suggests systematically evaluating the heat exchangers on refrigeration units.
Direct recovery is generally the simplest solution with the shortest payback times, provided that the source and the load are compatible in terms of temperature, power, duration, simultaneity and distance. A heat stream that is available when the load is inactive, or is located a long way from the point of consumption, may prove to be uneconomical. The fundamental distinction is between heat that can be reused immediately and heat that is available at an insufficient temperature. In the first case, a single heat exchanger may suffice: hot water from the condenser, for example, can be used for the initial rinse or to preheat the wash water.
When the source does not reach the temperature required by the process, a heat pump can be used, which transfers the recovered energy to a higher thermal level using electricity. A substantial proportion of the useful heat therefore comes from the thermal source. Efficiency is expressed by the COP, the ratio of heat produced to electricity consumed; a COP of 3 indicates that every unit of electricity consumed yields three units of thermal energy.
Performance depends primarily on the temperature difference: the closer the source is to the required temperature, the higher the COP tends to be. The technologies considered by Faraldo and Byrne achieve COPs typically ranging from 2.5 to 4, with flow temperatures between 120 and 160°C. To produce heat above 90°C, sources with temperatures between approximately 60 and 100°C are generally preferable; with cooler sources, the cost-effectiveness must be assessed more carefully.
Applications in the food industry
The food industry offers favourable conditions for high-temperature heat pumps, as many processes operate at lower temperature levels than those in heavy industry; examples include washing and sanitisation, pasteurisation, evaporation, concentration, drying, air heating and low-pressure steam generation. Electrification reduces the direct use of fossil fuels, but the climate benefit depends on the electricity mix and the correct matching of the heat source to the load. The review by Tancredi and Vignali, based on 67 studies on pasteurisation, reports COPs ranging from 1.6 to 5.8 and a growing focus on natural refrigerants.
Among the cases examined is a Norwegian dairy plant where the integration of refrigeration, high-temperature heat pumps and thermal storage achieved an overall COP of 4.1, recovering over 95 per cent of the available heat. Compared with conventional systems, energy consumption was reduced by 37.9 per cent and emissions, in the most favourable scenario, by 91.7 per cent. Arpagaus highlights that the most advanced technologies can provide process heat up to approximately 200°C and produce steam. In the European demonstration projects analysed, COPs range roughly between 2.5 and 4.5. The data confirm the shift towards industrial application, whilst reiterating the importance of the temperature difference and system integration.
The case studies cited in the review show that efficiency depends first and foremost on process optimisation. In the dairy sector, energy consumption is concentrated in pasteurisation, evaporation and drying, and the solutions suggested by the authors include reducing the volume of water to be evaporated, heat recovery, steam recompression and the use of heat pumps. A study cited by Faraldo and Byrne estimates energy savings of up to 65 per cent with a centralised heat pump system and 56 per cent with decentralised configurations, with a reduction in emissions that can approach 80 per cent.
These results relate to specific conditions but are indicative of the potential for integrating heat recovery and electrification. Another case concerns the production of frozen chips, where the highest energy consumption can stem from frying (up to 65 per cent) or from freezing (up to 46.3 per cent). Heat from the exhaust streams can be used to preheat other processes, whilst the removal of surface moisture reduces the energy requirements of subsequent stages. Both cases confirm that heat recovery units, heat pumps and storage systems should only be sized after reducing energy demand and eliminating avoidable inefficiencies.
The role of natural refrigerants
Replacing fluids with a high global warming potential with ammonia, CO₂, propane and isobutane can reduce direct emissions caused by leaks. CO₂ is suitable for integrated systems that produce both cooling and useful heat, whilst ammonia remains widely used due to its thermodynamic properties. However, the choice must also take into account specific constraints, such as the toxicity of ammonia, the high pressures associated with CO₂ and the flammability of hydrocarbons, as well as the charge, the location of the equipment, maintenance and process continuity.
Cost-effectiveness and hierarchy of measures
The question for the industrial user is not whether the heat pump works, but when it is cost-effective. Faraldo and Byrne use the levised cost of heat (LCOH), which takes into account the investment, operating and maintenance costs, incentives, service life and the thermal energy produced. Cost-effectiveness depends on the ratio between gas and electricity prices, as well as the temperature and stability of the heat source.
The project is more attractive with continuous demand, many annual operating hours, a modest temperature difference, proximity between the heat source and the load, and favourable or self-generated electricity. An oversized or underutilised system, on the other hand, may prove to be uneconomical. One of the clearest findings of the review is that direct heat recovery generally offers the shortest payback periods. The sequence should begin with reducing demand, continue with compatible heat recovery and, , utilise a heat pump for flows at insufficient temperatures. Renewables and conventional generators can cover residual loads, peaks and emergencies.
Thermal storage and simultaneity
It is also necessary to manage situations where the heat source and the load are not available at the same time. Condensation heat may be abundant whilst compressors are running, whereas hot water is mainly required during washing cycles. Thermal storage separates heat production from its use. Faraldo and Byrne distinguish between sensible, latent and thermochemical storage systems.
The former, often consisting of water tanks, are the simplest solution; latent systems use phase-change materials, whilst thermochemical systems allow for longer storage periods but are more complex. For storage lasting a few hours or a few days, sensible or latent solutions are often more appropriate. Storage enables the system to cover washing cycles and production peaks, reduce the heat pump’s output and the use of boilers, shift electricity consumption to cheaper hours and make the most of photovoltaic energy, thereby stabilising the system’s operation.
Efficiency is measured against actual data
A heat pump with a high nominal COP is useless if the heat source is unstable or the storage tank is undersized. The same applies to heat recovery units, which are ineffective without a compatible and synchronised load. Rather than the choice of an individual unit, the economic viability of the project hinges on the collection of field data: sensors and metering systems must map actual flow rates, temperatures and operating hours, moving away from estimates based on rated values.
There is no universal, turnkey solution. The technological mix must be tailored to the specific production line, combining direct recovery, storage and natural refrigerants. From a plant engineering perspective, this requires a shift in logical perspective: the refrigeration system ceases to be an isolated cost centre and becomes the plant’s primary heat source. Before allocating a budget for the purchase of new generators, the first step is to measure how many thermal kilowatts the condenser is still dissipating into the atmosphere when idling.
