The most common waste heat streams in industry are cooling water, effluent, condensate, moisture, and condenser heat from refrigeration plants. Because of the fluctuation in waste heat supply, it can be necessary to use large storage tanks for accumulation to ensure stable operation of the heat pump. Some common applications can be summarized as follows
• Space heating: Heat pumps can utilize conventional heat sources for heating of greenhouses and industrial buildings, or they can recover industrial waste heat that could not be used directly, and provide a low- to medium temperature heat that can be utilized internally or externally for space heating. Mainly electric closed-cycle compression heat pumps are used.
• Process water heating and cooling: In many industries, warm process water in the temperature range from 40-90°C is needed particularly for washing, sanitation and cleaning purposes. Heat pumps offer good potential for such applications and may be a part of an integrated system that provides both cooling and heating. Although electric closed-cycle compression heat pumps are mainly installed, some absorption heat pumps and heat transformers may find application.
• Steam production: In industrial processes, vast amounts of low-, medium- and highpressure steam in the temperature range from 100-200°C are consumed. In the market, at present, the high-temperature heat pumps can produce steam up to 300°C. In this regard, open and semi-open MVR systems, closed-cycle compression heat pumps, cascade systems and some heat transformers are employed.
• Drying and dehumidification process: Heat pumps are used extensively in industrial dehumidification and drying processes at low and moderate temperatures (maximum 100°C). The main applications are drying of pulp and paper, various food products, wood and lumber. Drying of temperature-sensitive products is also interesting. Heat pump dryers generally have high coefficient of performance (with COP of 5 to 7). and often improve the quality of the dried products as compared with traditional drying methods. Because the drying is executed in a closed system, odors from the drying of food products etc. are reduced. Both closed-cycle compression heat pumps and MVR systems are used.
• Evaporation, distillation and concentration processes: Evaporation, distillation and concentration are energy-intensive processes, and most heat pumps are installed in these processes in the chemical and food industries. In evaporation processes the residue is the main product, while the vapor (distillate) is the main product in distillation processes. Most systems are open or semi-open MVRs, but closed-cycle compression heat pumps are also applied. Small temperature lifts result in high performance with COPs ranging from 6 to 30.
In addition to the above mentioned processes, there are some significant applications of heat pumps, covering a very wide range, from connected loads of a few watts for the thermoelectric heating/cooling units in the food industry, to loads of several megawatts for large vapor compression plants in industry, including:
• Small, mass-produced, hot water heaters, sometimes combined with refrigerators and with connected loads between 200 and 800 W.
• Heating heat pumps for individual rooms, single-family houses, smaller office buildings, restaurants and similar projects. Package heat pumps (in closed casings) are also available as split units with an indoor and an outdoor section for installation in the open. Mass-produced, sometimes on a large scale, these have a heat output often with supplementary heating (electric, liquefied gas, warm water) up to about 120 kW, and connected loads from 2 to 30 kW.
• Heat pumps for heating and heat recovery for large air-conditioning plants in office blocks, department stores and similar projects. Appropriately adapted mass-produced chilled water units as well as systems individually assembled from the usual components for large refrigeration plants are used. Heat output is up to more than 1200 kW and the connected load is between 20 and 400 kW. If the heat pump is also used for cooling in summer, it is often better to use it to recover heat from the extract air in winter than to use an additional recuperative heat exchanger.
• Heating-cooling heat pumps for cooling and heating of rooms, objects of mass flows. The main task of these plants, also determining the control, is usually for either cooling or heating, not both, since the other effect is an additional gain which is not available during non-operational periods of the system and can only be supplied by a store, e.g. a hot water boiler.
• Waste heat utilization heat pumps for utilizing or reusing discharged heat which cannot be reused immediately because of its low temperature. This is so, for example, in drying processes in which the waste heat contained in the extracted water vapor is used for heating the drying air, or in laundries where practically all the applied heat energy is discharged with the waste water and can be recovered by a heat pump. The plants are controlled by heat demand, often combined with the storing of waste heat.
