The term heat pump refers to those technologies that enable the transfer of heat from low to high temperature.
Such a heat transfer process requires the use of additional energy in the form of mechanical or thermal energy.
The heat recovered by the heat pump can come from natural or renewable sourcessuch as water or ambient air, or from industrial processes, in the form of residual thermal energy.
Operation of the heat pump by mechanical compression
In a heat pump the refrigerant receives heat in the evaporator, and gives it, together with the work of the compressor, into the condenser. In the evaporator, the heat pump receives heat from a medium or enclosure to be cooled.
This heat evaporates the refrigerant, which is then compressed by the compressor to a pressure that allows condensation at the temperature at which it is interested in producing the heat. The compressor is usually driven by an electric motor.
The merit of the heat pump lies in transferring as much heat as possible with minimal labor expenditure on the compressor.
The heat pump market in Spain
The penetration of air conditioning equipment in our country is still much lower than heating.
The heat pump has managed to reach all sectors, although it has not done so equitably.
While the residential domestic sector has 66% of the installations carried out (a total of 800,000 pumps) and the commercial sector with 33% (400,000 equipment), the complexity of the specific designs required in industrial processesmeans that only 1% of the facilities are located in this sector (7,500 equipment).
Types of heat pump
The main types of heat pumps for industrial applicationsare:
Heat pumps in closed compression cycle. The maximum temperature obtained by current refrigerant fluids is around 120ºC.
This is the most widespread type of pumps in the industry.
Mechanical steam recompression (MVR) systems. In these pumps the fluid that evolves is the process fluid itself in an open cycle (usually water).
These systems work with cold focus temperatures of 70ºC to 80ºC and give up heat at temperatures between 110ºC and 150ºC.
Simple effect absorption heat pumps. Current systems with water/lithium bromide reach an outlet temperature of 100ºC and a thermal jump of up to 65ºC, with a COP ranging between 1.2 and 1.4.
Double-acting absorption heat pumps. They are applied to fluids that have a waste heat and an intermediate temperature above that of the environment, but below that of usable.
Through the evaporator and the generator the fluid reaches a suitable temperature for use. In the absorber heat is transferred to the process.
With this system you can reach temperatures of up to 150ºC, with a temperature jump of 50ºC. The COP is between 0.45 and 0.48.
Bryton cycle reverse. With this cycle, the substances dissolved in various processes are recovered.
The air is cooled in expansion, and the dissolved substances are condensed and recovered. The expansion takes place in a turbine that drives a compressor.
Practical COP of a heat pump Electric HEAT PUMP With combustion engine Single effect absorption Double effect absorption COP 2.5-4 0.8-2 1-1.71,8-2,4
Compression heat pump with motor
Most existing heat pumps are of the type of mechanical compression of steamand can be driven by an electric motor or diesel or gas combustion.
Thermal vapor compression
It is also possible to replace the compressor with an ejector and perform thermal compression.
The advantages of the ejectors are their low cost and long operating life, practically indefinite.
An ejector is a device that uses the amount of motion and kinetic energy of a current of living or motor fluid to drag or compress a second fluid current.
Possible applications are limited by the fact that the products to be compressed are mixed with the motor steam.
Thevapors generated in the evaporation process are compressed by an ejector that uses high steam as the driving fluid. Compressed vapors at an intermediate pressure condense in a heat exchanger.
Absorption heat pump
This heat pump achieves the recovery of heat for its subsequent energy revaporization through an absorption cycle, the cycle being driven by thermal energy.
It takes advantage of the thermal energy that would be wasted in the capacitor, thus reducing its energy consumption.
From the functional point of view, there is no big difference between the thermocompressors and the absorption heat pump. Both technologies revalue thermal energy at low temperature by consuming energy at higher temperatures, to give up the sum of both as useful heat.
The use of different heat pump technologies in the industry brings the following ADVANTAGES:
The fundamental advantage of the heat pump is that it is able to supply more energy than it consumes. This contradiction with one of the most solid principles of thermodynamics is explained by the fact that the equipment recovers 'free' energy from the outside environment. The energy needed to drive the heat pump is about a third or less of the useful heat produced. For example, a heat pump can provide a local with 3kWh by absorbing only 1kWh from the grid. The remainder, 2kWh, is free of charge from the outside air. Consequently, the heat pump is potentially of great interest to the user, since the user pays for an amount less than that provided by the equipment to heat thepremises. The recovery of industrial waste heat by means of the heat pump can also represent a reduction in the demand for cooling water, in addition to representing a decrease in environmental thermal pollution. The heat pump is an effective technology in the production of heat, since it consumes less primary energy than conventional systems. Their greater energy efficiency reduces the consumption of fossil fuels and, as a consequence, reduces emissions of gases harmful to the environment (CO2, SO2 and NOx). The overall environmental impact of electric heat pumps depends a lot on the performance and type of fuel used in the generation of electric power. Theuse of heat pumps would reduceCO2 emissions by 6%. (which represent 1000 million tons ofCO2 in the residential and commercial sector, and 200 million tons of CO2 in the industrial sector), being able to reach 16% in the future thanks to new technological developments and a greater increase in electricity from renewable energies.
The fundamental BARRIERS that have slowed down the development and implementation of the heat pump in industrial processes can be specified in:
Widespread ignorance of the benefits that heat pumps can bring into processes. The need to study each process independently in order to adapt heat pump technology appropriately. The short experience in processes combined with heat pumps, as well as ignorance of its technology.Heat Pump Applications
The main justification for the use of the heat pump in industry is heat recovery.
The heat pump makes usable heat flows that would otherwise be dissipated without exploitation.
The heat obtained in the condenser of the heat pump can be used among other applications for:
- Heating, air conditioning and domestic hot water: they supply water for example to fan-coils, for the heating of premises and warehouses.
- Water heating: the demand for industrial water in the temperature range of 40ºC to 90ºC for laundry, cleaning and disinfection can be covered by heat pumps (mainly compression with mechanical motor).
- Drying of products: a highly developed application in Spain is the use of heat pumps in industrial dehumidification and drying at low and moderate temperatures.
The number of heat pumps installed in industrial applications is currently small compared to the number of technically and economically viable opportunities.
The possible applications of the heat pump in the industry are very diverse, and are summarized in the following tables:
Some industrial applications of the heat pump that cover very diverse industrial sectors and are currently used are:
Compression heat pump with electric motor
Wood drying. Malt drying. Drying of sausages. Bread industry; Industrial cultivation of marine species in fish farms.Mechanical heat recompression
Polypropylene distillation. Distillation of 1-butene.Thermal steam recompression
Absorption heat pump
Seawater desalination plant.Heat transformer
Renewable energies and their integration with heat pump systems
Renewable energies can be the source to supply the heat or electricity required by heat pumps, thus reducing dependence on fossil fuels.
The variety in the possibilities of combining heat pumps and renewable energies can be as wide as you want, as long as it is admitted to pay an ecological extra cost for the additional complexity in the design of the facilities.
There are prototype facilities that have successfully applied this fusion of technologies at a reasonable cost and using not excessively complex engineering.
Drying of sausages
The drying process consists of removing by evaporation the excess water contained in a product.
The process of drying sausages takes place in two parts: the burning and the drying or curing.
The heat pump used in this type of application is of the type of vapor compression with electric motor.
To see the interest of the heat pump in the drying of sausages we consider the case of a conventional plant with the following data:
In the heat pump system, the heating necessary in the commissioning still exists, eliminating thermal consumption when the installation is in regime.
The annual consumption of primary energy in theburning phaseis 7.8 toe, with primary energy savings of 37.1%.
In the drying process , the total annual consumption of primary energy is 39 toe, with a primary energy saving of 38.7% compared to the conventional system.
The main advantageof this type of heat pump application is that it does not represent a substantial increase in investment.
The consumption of electrical energy is not significantly altered, but the thermal consumption of the installation is practically eliminated.
The return on investment is made in less than a year in the event that the fuel used is diesel C.
