Heat pump – for heating we take heat from the planet Earth

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In this Article: The History of the Heat Pump how the heat pump works and works; types of heat pumps; thermal energy from air, water and soil; at the end – the pros and cons of heat pumps.

Heat pump - for heating we take heat from the planet Earth

With the aim of defeating the winter cold, homeowners scour for energy and suitable heating boilers, envious of the lucky ones who have natural gas supply lines to their homes. Every winter, thousands of tons of wood, coal, oil products are burned in the stoves, megawatts of electricity are consumed for astronomical amounts that increase every year, and it seems that there is simply no other way out. Meanwhile, one constant source of thermal energy is always located near our homes, however, it is rather difficult for the population of the Earth to notice it in this capacity. But what if we use the heat of our planet to heat houses? And there is a suitable device for this – a ground source heat pump.

Heat pump history

The theoretical substantiation of the operation of such devices in 1824 was provided by the French physicist Sadi Carnot, who published his only work on steam engines, which described the thermodynamic cycle, which was mathematically and graphically confirmed 10 years later by the physicist Benoit Cliperon and called the “Carnot cycle”.

The first laboratory model of a heat pump was created by the English physicist William Thomson, Lord Kelvin in 1852, during his experiments in thermodynamics. By the way, the heat pump got its name from Lord Kelvin..

William Thomson, Baron KelvinWilliam Thomson, Baron Kelvin

The industrial heat pump model was built in 1856 by the Austrian mining engineer Peter von Rittinger, who used this device to evaporate brine and drain salt marshes to extract dry salt.

Peter Ritter von Rittinger Peter Ritter von Rittinger

However, the heat pump owes its use in heating houses to the American inventor Robert Webber, who experimented with a freezer in the late 40s of the last century. Robert noticed that the pipe leaving the freezer was hot and decided to use this heat for domestic needs by lengthening the pipe and passing it through the boiler with water. The inventor’s idea turned out to be successful – from that moment on, households had an abundance of hot water, while part of the heat was spent aimlessly, leaving the atmosphere. Webber could not accept this and added a coil to the outlet from the freezer, next to which he placed a fan, resulting in an installation for air heating of the house. After some time, the inventive American figured out that it was possible to extract heat literally from the ground under his feet and buried a system of copper pipes with freon circulating through them to a certain depth. The gas collected heat in the ground, delivered it to the house and gave it away, and then returned back to the underground heat collector. The heat pump created by Webber turned out to be so efficient that he completely transferred the heating of the house to this installation, abandoning traditional heating devices and energy sources..

The heat pump, invented by Robert Webber, for many years was considered more of an absurdity than a truly effective source of thermal energy – oil energy was in abundance, at quite reasonable prices. Interest in renewable heat sources grew in the early 70s, thanks to the 1973 oil embargo, during which the Gulf countries unanimously refused to supply oil to the United States and Europe. The shortage of petroleum products caused a sharp jump in energy prices – an urgent need to get out of the situation. Despite the subsequent lifting of the embargo in 1975 and the restoration of oil supplies, European and American manufacturers have come to grips with the development of their own models of geothermal heat pumps, the established demand for which has only been growing since then..

Design and principle of operation of the heat pump

As we sink into the earth’s crust, on the surface of which we live and whose thickness on land is about 50–80 km, its temperature rises – this is due to the proximity of the upper magma layer, the temperature of which is approximately 1300 ° C. At a depth of 3 meters or more, the soil temperature is positive at any time of the year; with each kilometer of depth, it rises by an average of 3–10 ° C. The rise in soil temperature with its depth depends not only on the climatic zone, but also on the geology of the soil, as well as endogenous activity in a given area of ​​the Earth. For example, in the southern part of the African continent, the temperature rise per kilometer of soil depth is 8 ° C, and in Oregon (USA), on the territory of which a fairly high endogenous activity is noted – 150 ° C for each kilometer of depth. However, for efficient operation of the heat pump, the external circuit supplying heat to it does not need to be buried hundreds of meters underground – any medium with a temperature of more than 0 ° C can be a source of heat energy..

The heat pump transfers heat energy from air, water or soil, increasing the temperature during the transfer to the required temperature due to the compression (compression) of the refrigerant. There are two main types of heat pumps – compression and sorption.

Do-it-yourself heat pump for home heating The basic structure of a compression heat pump: 1 – ground; 2 – brine circulation; 3 – circulation pump; 4 – evaporator; 5 – compressor; 6 – capacitor; 7 – heating system; 8 – refrigerant; 9 – choke

Despite the confusing name, compression heat pumps are not heating devices, but refrigeration devices, as they operate on the same principle as any refrigerator or air conditioner. The difference between a heat pump and refrigeration units well-known to us is that, as a rule, two circuits are required for its operation – an internal one, in which the refrigerant circulates, and an external one, with a coolant circulation..

During the operation of this device, the refrigerant in the internal circuit goes through the following stages:

  • the cooled refrigerant in a liquid state enters the evaporator through the capillary opening. Under the influence of a rapid decrease in pressure, the refrigerant evaporates and turns into a gaseous state. Moving along the curved tubes of the evaporator and contacting in the process of movement with a gaseous or liquid heat carrier, the refrigerant receives low-temperature thermal energy from it, after which it enters the compressor;
  • in the compressor chamber, the refrigerant is compressed, while its pressure rises sharply, which causes an increase in the temperature of the refrigerant;
  • From the compressor, the hot refrigerant follows the circuit into the condenser coil, which acts as a heat exchanger – here the refrigerant gives off heat (about 80–130 ° C) to the coolant circulating in the heating circuit of the house. Having lost most of the thermal energy, the refrigerant returns to a liquid state;
  • when passing through an expansion valve (capillary) – it is located in the internal circuit of the heat pump, following the heat exchanger – the residual pressure in the refrigerant decreases, after which it enters the evaporator. From this moment, the working cycle repeats again.

Air heat pump device Working principle of an air heat pump

Thus, the internal structure of a heat pump consists of a capillary (expansion valve), an evaporator, a compressor and a condenser. The operation of the compressor is controlled by an electronic thermostat that cuts off the power supply to the compressor and thereby stops the process of generating heat when the set air temperature in the house is reached. When the temperature drops below a certain level, the thermostat automatically turns on the compressor.

Freons R-134a or R-600a circulate as a refrigerant in the internal circuit of the heat pump – the first is based on tetrafluoroethane, the second is based on isobutane. Both of these refrigerants are safe for the Earth’s ozone layer and environmentally friendly. Compression heat pumps can be driven by an electric motor or an internal combustion engine.

Sorption heat pumps use absorption – a physicochemical process during which a gas or liquid increases in volume due to another liquid under the influence of temperature and pressure.

Schematic diagram of an absorption heat pump Schematic diagram of an absorption heat pump: 1 – heated water; 2 – cooled water; 3 – heating steam; 4 – heated water; 5 – evaporator; 6 – generator; 7 – capacitor; 8 – non-condensable gases; 9 – vacuum pump; 10 – heating steam condensate; 11 – solution heat exchanger; 12 – gas separator; 13 – absorber; 14 – mortar pump; 15 – coolant pump

The absorption heat pumps are equipped with a natural gas thermal compressor. In their circuit there is a refrigerant (usually ammonia), which evaporates at low temperature and pressure, while absorbing thermal energy from the environment surrounding the circulation circuit. In the vapor state, the refrigerant enters the heat exchanger-absorber, where, in the presence of a solvent (usually water), it is absorbed and heat is transferred to the solvent. The solvent is supplied by a thermosyphon that circulates through the pressure difference between the refrigerant and the solvent, or a low energy pump in high capacity installations.

As a result of combining the refrigerant and solvent, which boiling points are different, the heat supplied by the refrigerant causes both of them to evaporate. The refrigerant in the vapor state, having a high temperature and pressure, enters the condenser along the circuit, turns into a liquid state and gives off heat to the heat exchanger of the heating network. After passing through the expansion valve, the refrigerant goes into its original thermodynamic state, in the same way the solvent returns to its original state.

The advantages of absorption heat pumps are the ability to operate from any source of thermal energy and the complete absence of moving elements, i.e. noiselessness. Disadvantages – less power, compared to compression units, high cost, due to the complexity of the design and the need to use corrosion-resistant materials that are difficult to process.

Absorption heat pump unit Absorption heat pump unit

Adsorption heat pumps use solid materials such as silica gel, activated carbon or zeolite. During the first working stage, called the desorption phase, heat energy is supplied to the heat exchanger chamber, which is coated from the inside with a sorbent, from, for example, a gas burner. Heating causes vaporization of the refrigerant (water), the resulting steam is delivered to the second heat exchanger, which in the first phase gives off the heat obtained during the condensation of steam to the heating system. Complete drying of the sorbent and completion of water condensation in the second heat exchanger completes the first stage of work – the supply of thermal energy to the chamber of the first heat exchanger stops. At the second stage, the condensed water heat exchanger becomes an evaporator, delivering heat energy from the external environment to the refrigerant. As a result of the pressure ratio reaching 0.6 kPa, upon contact of heat from the external environment, the refrigerant evaporates – water vapor flows back into the first heat exchanger, where it is adsorbed into the sorbent. The heat that the steam gives off during the adsorption process is transferred to the heating system, after which the cycle is repeated. It should be noted that adsorption heat pumps are not suitable for domestic use – they are intended only for large buildings (from 400 m2), less powerful models are still under development.

Types of heat collectors for heat pumps

Sources of thermal energy for heat pumps can be different – geothermal (closed and open type), air, using secondary heat. Let’s consider each of these sources in more detail..

Ground source heat pumps consume thermal energy from the ground or groundwater and are divided into two types – closed and open. Closed heat sources are subdivided into:

  • Horizontal, while the collector collecting heat is located in rings or zigzags in trenches with a depth of 1.3 meters or more (below the freezing depth). This method of placing the heat collector circuit is effective for a small land area.

Geothermal heating with horizontal heat collector Geothermal heating with horizontal heat collector

  • Vertical, i.e. the collector of the heat collector is placed in vertical wells immersed in the ground to a depth of 200 m.This method of placing the collector is resorted to in cases where it is not possible to lay the contour horizontally or there is a threat of disturbing the landscape.

Geothermal heating with vertical heat collector Geothermal heating with vertical heat collector

  • Water, while the collector of the circuit is located in a zigzag or annular manner at the bottom of the reservoir, below the level of its freezing. Compared to drilling wells, this method is the cheapest, but it depends on the depth and total volume of water in the reservoir, depending on the region..

In open-type heat pumps, water is used for heat exchange, which, after passing through the heat pump, is discharged back into the ground. It is possible to use this method only if the water is chemically pure and if the use of groundwater in this role is permissible from the point of view of the law.

Open type geothermal heating Open type geothermal heating

In air circuits, respectively, air is used as a source of thermal energy.

Heating by air heat pump Heating by air heat pump

Secondary (derivative) heat sources are used, as a rule, at enterprises, the operating cycle of which is associated with the production of third-party (parasitic) heat energy that requires additional utilization.

The first models of heat pumps were completely similar to the design described above, invented by Robert Webber – copper pipes of the circuit, acting simultaneously as external and internal, with the refrigerant circulating in them, were immersed in the ground. The evaporator in such a design was located underground at a depth exceeding the freezing depth or in angled or vertical wells drilled at an angle (diameter from 40 to 60 mm) to a depth of 15 to 30 m. The direct exchange circuit (it received this name) allows it to be placed on small area and when using small diameter pipes, do without an intermediate heat exchanger. Direct exchange does not require forced pumping of the coolant, since there is no need for a circulation pump, then less electricity is spent. In addition, a heat pump with a direct exchange loop can be effectively used even at low temperatures – any object emits heat if its temperature is above absolute zero (-273.15 ° C), and the refrigerant can evaporate at temperatures down to -40 ° C. Disadvantages of such a circuit: large refrigerant requirements; high cost of copper pipes; reliable connection of copper sections is possible only by soldering, otherwise refrigerant leakage cannot be avoided; the need for cathodic protection in acidic soils.

The extraction of heat from the air is most suitable for hot climates, since at sub-zero temperatures its efficiency will seriously decrease, which will require additional heating sources. The advantage of air heat pumps is that there is no need for expensive drilling of wells, since the external circuit with an evaporator and a fan is located in an area not far from the house. By the way, any monoblock or split air conditioning system is a representative of a single-circuit air heat pump. The cost of an air heat pump with a capacity of, for example, 24 kW is about 163,000 rubles.

Air source heat pump Air source heat pump

Thermal energy from the reservoir is extracted by laying a circuit made of plastic pipes on the bottom of a river or lake. Laying depth from 2 meters, pipes are pressed to the bottom with a load at the rate of 5 kg per meter of length. About 30 W of thermal energy is extracted from each running meter of such a circuit, i.e., for a 10 kW heat pump, a circuit with a total length of 300 m will be needed.Advantages of such a circuit are relatively low cost and ease of installation, disadvantages – in severe frosts, it is impossible to obtain thermal energy.

Laying the heat pump circuit in a reservoir Laying the heat pump circuit in a reservoir

To extract heat from the ground, a PVC pipe loop is placed in a pit, dug to a depth exceeding the freezing depth by at least half a meter. The distance between the pipes should be about 1.5 m, the coolant circulating in them is antifreeze (usually water brine). The effective operation of the soil contour is directly related to the moisture content of the soil at the point of its placement – if the soil is sandy, that is, not capable of holding water, then the length of the contour must be approximately doubled. A heat pump can extract an average of 30 to 60 W of thermal energy from a running meter of the soil contour, depending on the climatic zone and the type of soil. A 10 kW heat pump will require a 400 meter circuit laid on a 400 m2 plot2. The cost of a heat pump with a soil circuit is about 500,000 rubles.

Laying a horizontal heat pump circuit Laying the horizontal contour in the ground

Recovering heat from the rock will require either the laying of wells with a diameter of 168 to 324 mm to a depth of 100 meters, or several wells of shallower depth. A contour is lowered into each well, consisting of two plastic pipes connected at the lowest point by a metal U-shaped pipe that acts as a weight. Antifreeze circulates through the pipes – only a 30% solution of ethyl alcohol, since in the event of a leak it will not harm the environment. The well with the contour installed in it will eventually fill with groundwater, which will supply heat to the coolant. Each meter of such a well will give about 50 W of thermal energy, i.e., for a heat pump with a power of 10 kW, it will be necessary to drill 170 m of a well. To obtain more heat energy, it is not profitable to drill a well deeper than 200 m – it is better to make several smaller wells at a distance of 15–20 m between them. The larger the borehole diameter, the shallower it needs to be drilled, while at the same time a greater intake of thermal energy is achieved – about 600 W per running meter.

Geothermal probe Installation of a geothermal probe

Compared to the contours placed in the ground or a reservoir, the contour in the well takes up a minimum of space on the site, the well itself can be made in any type of soil, including rock. The heat transfer from the well circuit will be stable at any time of the year and in any weather. However, the payback of such a heat pump will take several decades, since its installation will cost the homeowner more than a million rubles..

At the end

The advantage of heat pumps is their high efficiency, since these units consume no more than 350 watts of electricity per hour to obtain one kilowatt of heat energy per hour. For comparison, the efficiency of power plants that generate electricity by burning fuel does not exceed 50%. The heat pump system works in automatic mode, the operating costs during its use are extremely low – only electricity is needed to operate the compressor and pumps. The overall dimensions of the heat pump unit are approximately equal to those of a household refrigerator, the noise level during operation also coincides with the same parameter of a household refrigeration unit..

Heat pump Heat pump “brine-water”

You can use a heat pump both to obtain heat energy and to remove it – by switching the operation of the circuits to cooling, while the thermal energy from the premises of the house will be removed through the external circuit into the ground, water or air.

The only drawback of a heat pump-based heating system is its high cost. In Europe, as well as in the USA and Japan, heat pump installations are quite common – in Sweden there are more than half a million, and in Japan and the USA (especially in Oregon) – several million. The popularity of heat pumps in these countries is due to their support from government programs in the form of subsidies and compensation to homeowners who have installed such installations..

There is no doubt that in the near future heat pumps will cease to be something outlandish in Russia as well, given the annual increase in prices for natural gas, which today is the only competitor for heat pumps in terms of financial costs for generating heat.

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