Earth energy. Heat pumps. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Alternative energy sources

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A century and a half ago, British physicist William Thomson came up with a device called the "heat multiplier" based on the following physical phenomena:

• the substance expends energy during evaporation and gives off energy during condensation;
• The boiling point of a substance changes with pressure.

As a result, there appeared тепловой насос - a device for transferring thermal energy from a source with a lower temperature to a source with a higher temperature, in fact, it is a refrigerator with a source of a lower temperature in the external environment or an air conditioner that heats.

The principle of operation of a heat pump is based on the fact that the refrigerant evaporates in a chamber with low pressure and temperature and condenses in a chamber with high pressure and temperature, thus transferring energy (heat) from a cold body to a heated one, that is, in the direction in which spontaneous heat transfer is not possible.

As a low-potential source of thermal energy for heating a house, heat of natural origin (outdoor air; heat from ground, artesian and thermal waters; water from rivers, lakes, seas and other non-freezing natural reservoirs) can be used. Heat pumps are equipped with a control and automation system that maintains the specified operating mode of the heat pump.

The energy efficiency of the use of heat pumps depends on the temperature of the low-potential source and will be the higher, the higher the temperature it will have.

Heat pumps are not cheap equipment. The initial installation costs of these systems are slightly higher than those of conventional heating and air conditioning systems. However, if we consider operating costs. then the initial investment in geothermal heating, cooling and hot water supply will quickly pay off through energy savings. In addition, it must be taken into account that the operation of the heat pump does not require any additional communications, except for the household electrical network.

The performance of a heat pump (the ratio of the amount of heat transferred to the body to the work expended) is ideally equal to:

a \uXNUMXd T out / (T out - T in.),

where T out. and T in. - temperatures, respectively, at the outlet and at the inlet of the pump.

Heat pump device

The main elements of a heat pump are an evaporator connected by a pipeline, a compressor, a condenser and a flow regulator - a throttle, an expander or a vortex tube. Schematically, a heat pump can be represented as a system of three closed circuits: in the first, external, a heat emitter circulates (a heat carrier that collects the heat of the environment), in the second - a substance that evaporates, taking away the heat of the heat emitter, and condenses, giving off heat to the heat receiver, in the third - heat receiver (water in the heating and hot water supply systems of the building).

Fig.1. Scheme of heat pump operation: 1 - heat exchangers in wells (heat collection system); 2 - circuit of non-freezing coolant (antifreeze); 3 - heat pump; 4 - heating water circuit

The external circuit (collector) is a pipeline laid in the ground or in water (for example, polyethylene), in which an antifreeze liquid circulates. The source of low-grade heat can be soil, rock, lake, river, sea, and even the exit of warm air from the ventilation system of an industrial enterprise.

In the second circuit, where the refrigerant circulates, as in a domestic refrigerator, heat exchangers are built-in - an evaporator and a condenser, as well as devices that change the pressure of the refrigerant - a throttle spraying it in the liquid phase (narrow calibrated hole) and compressing it already in the gaseous state compressor.

Geothermal pump designs

During the operation of these systems, renewable heat from solar radiation is used, which is accumulated in the ground:

• pump with an open cycle (the coolant is supplied directly from the reservoir and, after passing through the cycle, is cooled back);
• closed cycle pump (the coolant is pumped through a closed circuit, which can be laid deep in the ground or along the bottom of a reservoir. This is a more environmentally friendly method than an open cycle);
• a pump with a horizontal heat exchanger (the closed circuit of the heat exchanger is laid horizontally in deep trenches);
• a pump with a vertical heat exchanger (a closed loop heat exchanger is installed vertically in prepared holes. It is used in heavy soil or when space is limited. A closed-loop heat pump is considered the most effective: the coolant is pumped through a closed loop, which can be laid deep in the ground or along the bottom of a reservoir ).

Heat pump duty cycle

The liquid refrigerant is forced through the throttle, its pressure drops, and it enters the evaporator, where it boils, taking away the heat supplied by the collector from the environment. Further, the gas into which the refrigerant has turned is sucked into the compressor, compressed and, heated, is pushed into the condenser. The condenser is the heat-releasing unit of the heat pump: here the heat is taken up by water in the heating circuit system. The gas is then cooled down and condensed to be again subjected to a vacuum in the expansion valve and returned to the evaporator. After that, the operating cycle starts over again.

Heat pump efficiency

During operation, the compressor consumes electricity. For every kilowatt-hour of electricity consumed, the heat pump generates 2,5-5 kilowatt-hours of thermal energy. The ratio of generated thermal energy and consumed electrical energy is called the transformation ratio (or heat conversion coefficient) and serves as an indicator of the efficiency of the heat pump. This value depends on the difference between the temperature levels in the evaporator and condenser: the greater the difference, the smaller this value.

For this reason, the heat pump should use as much of the low-grade heat source as possible without trying to cool it down too much. In fact, this increases the efficiency of the heat pump, since with a weak cooling of the heat source, there is no significant increase in the temperature difference. For this reason, heat pumps make sure that the mass of the low-temperature heat source is significantly larger than the mass being heated.

The difference between a heat pump and fuel heat sources is that in order to work, in addition to energy for the compressor, it also needs a source of low-grade heat, while in traditional heat sources the heat generated depends solely on the calorific value of the fuel.

The problem of binding a heat pump to a low-potential heat source with a large mass can be solved by introducing a mass transfer system into the heat pump, for example, a water pumping system. This is how the central heating system in Stockholm works.

Conditional efficiency of heat pumps

The heat pump is able, using high-potential energy sources, to "pump" into the room (as a percentage of the spent) from 200% to 600% of low-grade thermal energy. There is no violation of the law of conservation of energy in this, since the environment is cooled.

Theoretically, the use of heat pumps for space heating is more efficient than gas boilers. Modern steam and gas turbine plants at power plants have an efficiency slightly lower than the efficiency of gas boilers. As a result, with the transition of the electric power industry to modern equipment and the use of heat pumps, gas savings can be obtained up to 3-5 times in comparison with gas boilers. In reality, the overhead costs of transmission, conversion and distribution of electricity (ie grid services) have to be taken into account. As a result, the sale price of electricity is 3-5 times higher than its cost, which negates the use of generally progressive technology. In this regard, it is advisable to use electricity from alternative sources (wave, wind, solar power plants) or combine the generation of electricity from gas with its use here on site, to produce heat in a heat pump.

Recommendations for the operation of the heat pump

• When using grant energy as a source of heat, the pipeline in which antifreeze circulates is buried in the ground 30-50 cm below the level of soil freezing in the given region. The minimum recommended distance between collector pipes is 0,8 1 m. Special soil preparation is not required. But it is desirable to use a site with wet soil, but if it is dry, the contour must be made longer. The approximate value of the thermal power per l m of the pipeline is 20-30 watts. Thus, for the installation of a heat pump with a capacity of 10 kW, an earth circuit 350-450 m long is required, for laying which a plot of land of about 400 m2 (20x20 m) will be required. When properly calculated, the contour does not affect green spaces.
• If there is no free area for laying the collector or rock is used as a heat source, the pipeline is lowered into the well. It is not necessary to use one deep hole - several shallow, cheaper ones can be drilled to get the total estimated depth. Sometimes foundation piles are used as wells.
• Approximately 1-50 watts of thermal energy per 60 linear meter of a well. Thus, to install a heat pump with a capacity of 10 kW, a well with a depth of 170 m is required.
• The refrigerant is supplied directly to the earth type source, which ensures the high efficiency of the geothermal heating system. The evaporator is installed in a pound horizontally below the freezing depth or in wells with a diameter of 40-60 mm, drilled vertically or at a slope to a depth of 15-30 m. Thanks to this engineering solution, the heat exchange circuit is built on an area of ​​​​only a few square meters, does not require the installation of an intermediate heat exchanger and additional costs for the operation of the circulation pump.
• When using a nearby reservoir as a source of heat, the circuit is laid on the bottom. This option is considered to be ideal not too long external circuit high ambient temperature (water temperature in the reservoir in winter is always positive; high energy conversion efficiency of heat pumps).
• The approximate value of thermal power per 1 m of the pipeline is 30 watts. Thus, to install a heat pump with a capacity of 10 kW, it is necessary to lay a 300 m long circuit in the lake. To prevent the pipeline from floating up, for 1 running. m is installed about 5 kg of cargo
• To obtain heat from warm air (for example, from the exhaust of a ventilation system), a special model of a heat pump with an air heat exchanger is used. Heat from the air for heating and hot water systems can also be collected in industrial plants.
• If the heat from the external circuit is still not enough for heating in severe frosts, it is practiced to operate the pump in tandem with an additional heat generator (in such cases, they talk about using a bivalent heating scheme). When the outside temperature drops below the calculated level (bivalency temperature), the second heat generator is switched on - most often a small electric heater.

Advantages and disadvantages of a heat pump

First of all, the advantages of heat pumps include economy: to transfer 0,2 kWh of thermal energy to the heating system, the installation needs to spend only 0,35-50 kWh of electricity. Since the conversion of thermal energy into electrical energy at large power plants occurs with an efficiency of up to XNUMX%, the efficiency of fuel use when using heat pumps increases. Simplified requirements for ventilation systems and increases the level of fire safety. All systems operate using closed loops and require virtually no operating costs, other than the cost of electricity required to operate the equipment.

Another advantage of heat pumps is the ability to switch from heating mode in winter to air conditioning mode in summer: just instead of radiators, fan coils are connected to an external collector.

The heat pump is reliable, its operation is controlled by automation. During operation, the system does not require special maintenance, possible manipulations do not require special skills and are described in the instructions.

An important feature of the system is its purely individual nature for each consumer, which consists in the optimal choice of a stable source of low-potential energy, calculation of the conversion coefficient, payback, and other things.

The heat pump is compact (its module does not exceed the size of a conventional refrigerator) and is almost silent.

The disadvantages of heat pumps used for heating include the high cost of installed equipment.

See other articles Section Alternative energy sources.

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