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The generation of cold and / or hot air or for other applications can be made as low as absorption refrigeration systems, in which the useful effect is obtained thanks to the thermal energy supplied to an appropriate temperature level.
According to the type of machine can be distinguished:
Machines with direct flame (Or direct power supply), wherein the thermal source is constituted by a fuel (in most cases natural gas, LPG or alternatively, biomass or other fuels) that is burned directly in the machine.
Machines in which the heat source is a heat transfer fluid (water, thermal oil, hot gases, steam) supplied to the thermal level appropriate (machines Power indiretta)
In the case of indirect power machines may be further distinguished plants according to the origin of the thermal energy that feeds the machine:
thermal energy produced from waste heat (eg heat recovery from industrial processes) or renewable energy (eg solar thermal, geothermal heat);
thermal energy cogenerated by a prime mover (the heat transfer fluid may be water, in liquid phase or steam, diathermic oil or by hot fumes produced by the cogenerator)
The direct-fired absorption chillers, powered by natural gas (or LPG), are traditionally used in applications requiring high reliability and continuity of service in case of a power failure. The massive application of the models to direct flame in the summer air conditioning of buildings have the advantage of reducing the demands on the national grid, with benefits in terms of distribution costs and system reliability, while also leading to a greater use of the distribution network of gas even in the summer months.
There are also recent models of machines in configuration to direct flame heat pump, suitable for winter heating (and production of sanitary hot water), which achieve significantly higher yields of conventional gas-fired boilers, especially in conjunction with installations of heat distribution in low temperature and geothermal sources.
In addition to methane fuel and LPG models are also suitable for operation with biogas or biomass pellets.
The absorption chillers powered by a heat transfer fluid are the subject of renewed interest, because they allow to be fed with any type of heat source, including waste heat from industrial processes, renewable energy or waste heat from a CHP. In the latter case the absorption refrigerators allow you to extend the operation of the CHP also to the summer months, increasing its capacity utilization, profitability and overall performance (more properly talk of trigeneration plants).
Finally, these machines are an alternative to groups vapor compression, so the use of refrigerants based on halogenated hydrocarbons, with environmental benefits compared to the problems of impact on the greenhouse effect and typical disposal and recycling of refrigerants.
Principle of Operation
The absorption machines are refrigeration machines that exploit the solubility and the high affinity of the two substances, one of which acts as a coolant and the other by an absorbent, to achieve a cycle in which the introduction of energy occurs mainly in the form of heat .
The combinations refrigerant / absorbent that were strengthened commercially are two:
1) Lithium bromide and water: H2O - LiBr, where the water operates as a coolant;
2) Ammonia and water: NH3 - H2O, where the refrigerant is ammonia.
From the thermodynamic point of view the working cycle refrigerator with three heat sources: the cold source is located at the lower temperature and is constituted to be cooled by the environment (in the case of operation as a heat pump is the external environment), and the hot well , in which the heat is discharged, is the external environment and is located at an intermediate temperature (in the case of heat pump is the environment to be heated), and finally, through the source to the highest temperature is introduced in the heat input to the cycle (flame, heat transfer fluid).
The low pressure part of the machine is constituted by the evaporator and the absorber, thermally separate places but in communication between them. The evaporator is a solution rich in refrigerant, which evaporates, removing heat to the cold source (the useful effect if it is a refrigeration system), the evaporated refrigerant is "absorbed" by the rich absorbent solution present in the absorber, as the refrigerant in solution has a surface tension lower than the pressure in the evaporator. In this way it is made possible the evaporation of an additional amount of refrigerant, without increasing the pressure in the evaporator which would inhibit the absorption of further gas refrigerant. It is necessary to counteract the gradual dilution of the solution by the refrigerant, removing with a pump the mixture obtained in the absorber and treating it in a timely manner. The task of the pump, in addition to winning the load losses in the circuit, is to raise the pressure of the fluid up to the maximum value of the cycle, by sending it to a component named generator (or desorption). In the heat generator is supplied to the thermal level appropriate so as to evaporate the refrigerant present in the solution.
As a heat source in the high-temperature generator is possible to use the heat generated by a combustion (machine direct flame), or alternatively the heat coming from another source, for example that cogenerated by a prime mover, which is transferred to the fluid the generator by means of a heat exchanger and a suitable heat transfer fluid (water, steam, thermal oil, smoke) and it is called indirect feed equipment.
The evaporated refrigerant is first condensed, releasing heat to the source to an intermediate temperature (the external environment in the case of the refrigeration cycle) and then expanded through a throttling valve in the evaporator to return and resume the cycle.
The rich absorbent solution produced by the generator is also rolled back to the absorber and restore the proper concentration of the solution. In practice requires two different pressures: a higher for the group generator-condenser, so that the fluid has to condense at a temperature such as to transfer heat to the outside environment (internal environment if it works in heat pump) , and a lower group in the absorber-evaporator, in order to remove heat by evaporation from the internal source (the external environment for the heat pump).
To improve the performance of the machine there is then a regenerative heat exchanger placed between the concentrated solution in output from the generator (rich absorbent) and diluted from the absorber: is obtained in this way, the preheating of the liquid to be sent to the generator, with evident saving of heat introduced, while obtaining the cooling of the solution in the absorber entrance, with a consequent decrease of the heat to be removed.
In the absorber must in fact provide a cooling circuit (typically the same that cools the condenser) to remove the heat of condensation of the coolant, equal to that of evaporation (useful effect in the case of refrigeration system) plus the heat of mixing and l ' Any cooling of the mixture that comes from the generator.
Even for absorption machines defining a efficiency, called COP, equal to the ratio between useful effect and energy expenditure incurred in the operation of the machine
In a machine with simple effect, the COP is approximately 0.6-0.75 that rises to about 1,1 1,35-in machines with double effect. The latter are characterized by the adoption of two cycles like the one just considered places "cascade" so that the heat of condensation of the cycle above constitutes the input energy to the generator of the underlying cycle, thus realizing a double-effect cycle.
The relatively low values of COP must not be misleading, since it is considered that these machines require thermal energy at relatively low temperatures, and then to a power source much less valuable, from the thermodynamic point of view, the electrical energy used by traditional vapor compression cycles.
The advantages offered by the absorption machines, in addition to the possibility of using waste heat and not requiring the use of fluids harmful to the ozone, are surely the high reliability resulting from the presence of very few moving parts, the high life useful (in some cases even higher than 20 years for power machines indirect, when subjected to proper maintenance), low noise and vibration, reduced demand for electricity and the good performances obtained at partial loads.
They are also free from the problems inherent in the choice of a lubricant compatible with the system, unlike the groups to compression. The low demand for electricity makes it interesting to use, despite the low values of COP achieved in operation as a refrigerator, even when using a direct flame, in all those cases in which there exists the need for continuity of service, even in failure of the electricity supply. For these applications it is sufficient to provide a small uninterruptible power supply or an emergency generator for the production of electricity required by the auxiliary in the absence of the network. Thanks to the very low demand for electricity, in addition, the machine is certainly suitable for applications in places where the electricity network is far away or absent, with solutions of the type described above, or in all those cases in which the connection to the network and its electrical substation to operate a group compression would be somewhat problematic or expensive.
With regard to energy efficiency is to be noted that the low values of COP are to be put in relation with the "value" modest thermodynamic energy used by the machine. As mentioned the heat to the generator in a machine to direct flame can be provided, as well as combustion of natural gas, fuels of little value or of renewable origin (eg. biogas or pellets); in power machines indirect heat to a relatively low level heat may come from solar energy, or even result from waste heat from industrial processes (eg flue gas) or from a CHP. In the case of solar energy systems (solar cooling) the absorption refrigerators allow you to use the excess heat typically available during the summer months, which would otherwise be dissipated, especially in cases where the solar system is designed with the function of integration to heating invernale7. Given the uncertainty and discontinuity of solar energy, the system needs a combination boiler and back-up, usually in series downstream of the solar collectors. In the case of cogeneration (or trigeneration) is to the advantage of the absorption energy and economic nature, because in general these machines allow you to increase the utilization factor of the system. It is important to remember the fact that it is possible to operate the machine, with the right devices, in heat pump mode, so you achieve high yields during the winter season. In this way, the machines direct flame fed by natural gas become energetically interesting. Finally, from the point of view of the electricity network, as already said, there is an advantage, related to the use of absorption machines in the field of air conditioning: play action for lightening of electricity distribution networks, more and more congested especially during summer because of the tips
of demand for air conditioning. A massive use of air conditioning gas will also lead to "flatten" the curve of the annual consumption of natural gas.
By contrast, the absorption machine has some disadvantages arising primarily from higher unit cost than the fridge compression (see par. Later), the relatively low values of COP achieved and, for many models, the necessary presence of towers for the evaporative cooling of the absorber. In case of use of a solution water-lithium bromide as the working fluid, in fact, it is necessary to keep under control the temperature of the absorber, to avoid phenomena of crystallization and precipitation of lithium bromide, which would lead to clog the pipes and heat exchangers, leading to scadimenti of performance or to the block of the machine. The cooling tower is usually also cools the condenser, and in relation to typical COP values of these machines the potential of the tower is always significantly greater than the cooling capacity to be removed. Finally, in models with water-lithium bromide, you can reach very low values of pressure in the evaporator (next to 0.009 bar), with a consequent risk of ingress of air in the system, which would lead to a rapid deterioration in the performance and all 'onset of corrosion. The problem is solved at plant level giving appropriate care to the seals of the machine.
Particular applications of absorption chillers
Absorbers powered by solar energy (solar cooling)
The level reached by thermal solar collectors may be sufficient to power a car absorption. You can then make a solar cooling system. In which the heat transfer fluid of the solar plant, usually water, feeds the generator of an absorption machine. The system will then be fastened an auxiliary boiler or a heat pump, to integrate the solar source, when not enough.
Pairing with geothermal heat pump
Solar thermal systems can be combined with a geothermal heat pump, both in compression and absorption, thus exploiting some interesting synergies. The solar thermal system is to minimize the cost related to geothermal wells: solar collectors, usually of the plan, work to supplement the energy supplied to the evaporator of the heat pump. During the winter, the solar panels is required the production of water at modest temperatures (of the order of 10 ° C), for which they maintain high efficiency. In summer produce domestic hot water or can feed the absorption machine, if present, with acceptable yields due to high values of solar radiation. In a solar thermal collector efficiency is in fact affected by the presence of poor sunlight and high operating temperatures. Obviously, given the uncertainty of solar energy, there should be a supplementary boiler required to cover the tips of the heat demand on very hot / cold and lack of sun, respectively, in winter / summer
Absorbers fueled biomass
Biomass plants that generally employ wood-fired boilers, wood chip and pellets, can be combined with absorption machines, both in direct-fired plant configurations, either by indirect supply with a heat transfer fluid. The most frequent form is the one with biomass boiler and absorption machine powered indirect water or thermal oil.
Peripheral production of cold in district heating systems
A district heating network provides transport of heat from the user, where it feeds an absorber for the local production of chilled water. Compared to the centralized solution for district heating and cooling system combined with network 4 tubes, the production of cold device is advantageous in terms of energy and often cheaper. The total cost of refrigeration equipment tends to increase compared to a centralized solution, but you'll save on the cost of the network is to be buried as a network rather than 2 4 tubes also results in a significant energy savings because the thermal losses on the network of the cold weigh significantly, considering the T content with which the network operates in the cold (usually with 6 7 ° C-in-flow and 12 13 ° C in return). Consider also that the thermal losses of the network of hot water would still be present in summer, even in the case of a network to 4 tubes, for the production of sanitary hot water, for which the solution with decentralized production of cold does not alter practically losses the network of the heat, while eliminating the high energy losses along the distribution network of the cold.
Absorbers driven by waste heat
The heat source may be constituted by water, diathermic oil, steam or hot fumes made available from industrial processes. Compared to other applications mentioned so far, it should be noted that the use of absorbers supplied with waste heat that would otherwise be lost, can easily achieve significant energy savings in industry, directly contributing to the containment of emissions and ensuring CO2 normally a short time payback. The thermal energy available as cascame heat from an industrial process is recovered with special heat exchangers and transferred to the absorber by means of suitable heat transfer fluid, usually hot water or steam or pressurized.