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The generation of cold and / or hot air or for industrial applications is generally carried out with a compression refrigeration systems, with the compressors driven by electric motors, or by absorption groups, such thermal power.
However, you can make another solution, where the compressor of the refrigeration cycle / heat pump is driven by an internal combustion engine or "prime mover", usually fueled by natural gas, so that in technical terms are also known as "heat pumps gas "(in the Anglo-Saxon terminology it comes to gas engine heat pumps, gas or heat pumps GEHP GHP)
The use of natural gas for air conditioning both summer and winter in a country like ours would in the first place to limit the increase in electricity consumption during the summer, and at the same time to balance the demand for gas, characterized by a highly seasonal. The increase in demand for gas during the summer season would enable a more rational management of the gas, with better use of reserves and a lower rate of energy consumption associated with the storage of the gas.
The benefits on the electricity grid would result in a decrease instead of the tips of summer electricity consumption, which is leading in recent years to significant problems distribuzionedell'energia electrical networks, especially in medium and low voltage.
Often in urban areas particularly critical, the user is denied the ability to increase the contractual commitments of power, thus making it mandatory solutions, which use natural gas for air conditioning.
The heat pumps driven by internal combustion engines, as well as the absorption machines direct flame, fall into this type of plant.
A refrigeration system / heat pump driven by a compression-combustion engine is then constituted by the following elements:
• a prime mover in gas (eg. reciprocating internal combustion engine, gas turbine, fuel cell;
• a system for the generation of cold-formed by a refrigeration system compression, optionally functioning also as heat pump;
• a system of heat recovery from the exhaust gases and / or from the cooling circuit of the engine first, with production of useful heat Supplementary in case of operation of the system for production of heat. In many cases, you can then get a production of domestic hot water (especially in spring and autumn and the summer months).
The connection between the first motor and the compressor can be direct, then mechanical type (dragging with mechanical joint), or indirect in the case where the heat engine shares an electric generator that powers an electric motor to drive the compressor of the refrigeration . In this case, and if it were produced in excess of that required to operate the heat pump and its auxiliaries, the electrical energy can also be exchanged with the network as in the cases cogeneration.
Besides the advantages linked to the use of a different energy source, the use of an internal combustion engine involves energy benefits, arising as already said by the possibility to exploit the heat recovery from the engine, which then functions as cogenerator. In particular, this allows heat:
• the production of domestic hot water;
• the suppression of the reverse cycle defrost the evaporator, which represent a limit of electric heat pumps since they involve a worsening of the energy performance and cause a discontinuous operation resulting in degradation of performance in terms of environmental performance and comfort;
• the maintenance of a power output in the heating practically unchanged even with very low external temperatures (-15 ° C and more), then a heating always guaranteed.
There is then the possibility of cogenerate, via an alternator connected to the combustion engine, part of the electricity needed to power of the indoor units and auxiliaries.
The heat pumps gas have already known widely in Asian markets, especially in Japan, also in the rest of the world are now well established for both civil and industrial applications. Technological advances have enabled us to achieve high levels of reliability, enabling machinery to operate for long periods without maintenance: in many cases manufacturers to ensure they arrive until 10.000 operating hours between service.
Energy benefits of heat pumps gas
A gas heat pump consists of a compressor, mechanically coupled (more rarely by electricity) to an endothermic engine; the circuit taken by the refrigerating fluid consists of a "four-way" reversal valve for the inversion of the cycle, of an exchanger positioned outside an exchanger positioned inside the rooms to be conditioned which alternatively act as evaporator or condenser according to the operating mode (heat pump or cooling).
The engine is cooled by a cooling circuit path from glycol and water, the water in output from the engine passes through a heat exchanger where it warms further taking heat from exhaust gases of the engine, which are then cooled before being expelled to the fireplace. The heat recovered from the engine can be usefully employed for the production of hot water by a suitable boiler.
In air conditioning systems with air handling units (AHU), the hot water produced by the thermal recovery of the engine can also be used to perform post-heating of the air after dehumidification.
In cases in which hot water is not required by the thermal loads, an auxiliary circuit with a suitable heat exchanger dissipates heat to the environment (as happens in car radiators).
The liquid accumulator allows to reduce the amount of refrigerant in the circuit when the heat demand is low, storing it until 30% of the total weight. Its function is useful especially during load transients: avoids the problems related to the achievement of liquid to the compressors, in addition to acting as a lung for the refrigerant gas decreases when the load demand.
Peculiarities of motor driven systems is to possess a "sub evaporator", which provides heat the cooling circuit of the engine. In this way the heat of the engine can be partially used in the sub evaporator, allowing to raise the temperature of evaporation of the refrigerant, with consequent decrease in the work of compression and increase in the COP of the heat pump.
In general the amount of heat recovery from the engine used in the evaporator is inversely proportional to the external temperature, whereby the decrease of the outdoor temperature increases the fraction of the heat from the engine, thus allowing the system to:
1) operate at nearly constant evaporation temperature;
2) counteract the decrease of the thermal power produced during the colder winter;
3) avoid periodic defrost cycles of the evaporator.
For these reasons, the performance of heat pumps gas are not very sensitive to changes in the outside temperature. In contrast, the typical behavior of conventional electric heat pumps air leads to a drastic reduction in heat output and COP of the machine just in time for the colder days, both due to the decrease in the evaporation temperature, both the periodic cycles defrosting required for the evaporator.
Even in winter, then it is possible to exploit the cooling system and heat recovery from the flue gases of the engine for the production of sanitary hot water.
With more complex circuits, there is also the possibility to realize particular system configurations that allow the simultaneous operation in heating and cooling, with some indoor units operating in the heating mode and in cooling mode other. In practice, some internal heat exchangers to the premises while others act as capacitors from evaporators.
To carry out an energy assessment of a heat pump driven by internal combustion engine, in different set-ups operating in summer and winter, you need to compare its performance with those of conventional machines that typically perform the same tasks. In other words, the energy performance of heat pumps winter gas must be compared with those of gas boilers for heating (or alternatively with that of electric heat pumps), while in the summer should be compared with refrigeration units based on traditional machines Electrical compression. We should always remember that having a consistent comparison must consider the different energy value of natural gas and electricity, which is produced in power mostly from fossil fuel-fired power plants.
The correct comparison expected to refer in both cases the primary energy consumed. We define a factor called the coefficient of utilization of the fuel, CUC (in English often called fuel utilization efficiency, FUE) to indicate precisely the efficiency of fuel use, referring then to the primary source of energy: where QU is the useful effect produced by the machine, respectively warmth in winter and cooling energy in summer, while QF is the energy of the fuel used to achieve that effect.
CUC = QU / QF
You want to emphasize the substantial difference with the COP of the refrigeration machinery electric compression, in which the denominator is represented by electric power and not by primary energy (energy of the fuel).
In operation in cooling only, the gas machines have lower efficiency of electric drive systems, this is due to the substantially reduced size of the motor, which involves mechanical efficiencies lower than the large electricity generation systems. However, if you include the production of domestic hot water (DHW), achieves considerable energy benefits even for modest values of mechanical efficiency.
In general it can be concluded that the heat pump driven by internal combustion engine result in cooling mode for CUC values higher than the reference situation, where it is recovered a certain fraction of the available heat from the engine for DHW production.
During heating operation, there is a value of CUC always above with respect to boilers reference (average annual return equal to 95%). In order to compare the production of heat with that of an equivalent electric heat pump air with average COP equal to 3 (corresponding to a flow temperature to users of 55 ° C), already for relatively low values of mechanical efficiency performance is achieved energy better than the electric solution.
Furthermore, considering the additional useful effect given by the production of domestic hot water (DHW), achieves considerable energy benefits in all cases.
Some manufacturers of heat pumps gas cogeneration offer models. In this case it is foreseen the additional production of a certain electric power (usually in small quantities, typically a few kW), to compensate for the consumption of the plant auxiliaries such as pumps and fans, or to provide electricity to the user, in relation to the size of the electric generator.
In addition to ensuring the functioning of the system in case of power outage (island operation), cogeneration of electricity allows a further economic returns.
With the gas heat pumps it is possible to envisage configurations with multiple external units. The use of multiple external drives involves the division of the load, an operation which allows to reach higher yields at partial loads. A further benefit of using more external drives is the ability to perform maintenance on one of the drives without turning off the system, because the other units can continue to operate.
Other utilities deemed attractive for these applications are the hotels, sports centers, hospitals, nursing homes, and in general all loads with high consumption of energy-efficient hot water that make the car even during the summer months. Other successful applications can be represented by some industrial users, whose production process is characterized by simultaneous application of heat and cold (for example, the processes of drying, drying or dehumidifying, a few cycles of processing in the food industry, etc.).