RU2742156C1 - Evaporative circuit for ground source heat pump - Google Patents

Abstract

FIELD: obtaining or using geothermal heat.

SUBSTANCE: invention relates to the field of obtaining or using geothermal heat and can be used in ground circuits of geothermal heat pumps, in evaporative systems of geothermal heat pumps of direct heat exchange, in heating and cooling systems, including air conditioners, refrigeration plants using freons as a heat carrier. The evaporator circuit of the ground source heat pump is made of high temperature polyethylene PE-RT, and the collector tube diameter is 16 mm. The element that connects the inlet and outlet pipes with a system of collector pipes is a distribution header. Its central part is a cylindrical pipe segment plugged at one end. Three pipes are symmetrically connected with both sides of the cylindrical pipe segment. Each pipe is connected to a collector pipe. The connection of the inlet or outlet pipe with the open end of the distribution header and the connection of the distribution header pipe with the collector tube are made by means of socket welding. Disclosed is a method for the location of the earthen contour of a geothermal heat pump of direct heat exchange, in which the collector pipes are placed horizontally in the soil in 6 parallel planes in a trench 3 m deep below the freezing point of the soil at a distance of 0.5 m from each other, the end sections of the inlet and outlet pipes, connecting elements , the inlet and outlet pipes of the evaporation circuit of a geothermal heat pump with a system of collector pipes and the initial sections of the collector pipes are located in a caisson well.

EFFECT: invention is aimed at obtaining and using geothermal heat.

8 cl

Description

The invention relates to the field of obtaining or using geothermal heat, and can be used in ground circuits of geothermal heat pumps, in evaporative systems of geothermal heat pumps of direct heat exchange, in heating and cooling systems, including air conditioners, refrigeration units using freons as a heat carrier.

A heat pump is an installation in which the reverse Carnot cycle is carried out and which transfers thermal energy from a low-temperature environment (water, earth, ambient air) to a high-temperature heat transmitter due to energy consumption [1]

The principle of operation of a geothermal heat pump is that heat from low-potential sources along the earth circuit is transferred by a heat carrier to the installation, where in the evaporator, due to heat exchange, it is transferred to the refrigerant contained in the internal heat exchange circuit. From the internal heat exchange circuit, the heat in the condenser is transferred to the heating circuit [2].

A conventional ground source heat pump contains two external circuits with heat carriers and one internal one with refrigerant, two heat exchangers, a condenser and an evaporator, in which the heat carriers in the circuits exchange heat with a refrigerant. The compressor and relief valve increase and decrease the pressure, thereby changing the temperature of the refrigerant [3, 4].

Such a heat pump with three circuits, one of which is located inside the heat pump body, two heat exchangers, a condenser and an evaporator, in which the coolants in the circuits exchange heat with the refrigerant, and an antifreeze liquid circulates in the earth circuit - an aqueous solution of glycol, a mixture of water and antifreeze, called a glycolic heat pump.

The earthen loop is a long-distance pipeline with a coolant inside. The pipeline is most often closed, the movement of the coolant along it is provided by a pump.

As the ground loop of a conventional ground source heat pump, most heat pump manufacturers use high or low density polyethylene pipes with a diameter of 20-40 millimeters with circulation of an antifreeze liquid - an aqueous solution of glycol, a mixture of water and antifreeze such as propylene glycol, monopropylene glycol, denatured alcohol, methanol or the like.

Danfoss Swedish geothermal pumps http://www.geoteplo.com/danfoss-dhp_teplovoy_nasos_cena use three circuits, in the evaporating circuit the coolant is brine - antifreeze liquid (ethylene - glycol, ethanol). The material of the evaporating circuit is low pressure polyethylene. Known heating system company Danfoss Patent RU 2507453 C2, based on the operation of a three-circuit heat pump.

With a three-circuit principle of operation of a geothermal heat pump, losses occur during heat exchange due to the presence of two evaporator and condenser devices, the need to pump the coolant along the earth circuit with a pump. The design of such a heat pump is complex and cumbersome, which negatively affects the cost of manufacturing, installation and maintenance.

There are geothermal heat pumps for direct heat exchange (DX - direct exchange - "direct exchange").

In such heat pumps, in fact, the internal circuit carrying the refrigerant is simply extended into the soil, thereby directly exchanging heat with the earth. In this case, the heat carrier circulating underground is the heat pump freon itself. This heat pump has 2 circuits and no evaporator. The refrigerant evaporates directly in the earth circuit, therefore it can also be called an evaporative circuit.

A heat pump of direct heat exchange is simpler in design, since it contains only two circuits, and the geothermal circuit itself serves as an evaporator, which reduces heat losses, increases efficiency by 12-20%, increases operational safety and ease of installation.

The pressure in the DX direct heat exchange circuit can reach up to 1.6 MPa. At this pressure, freon has increased fluidity. LDPE plastic pipes used in glycol geothermal circuits are highly permeable to gases and cannot be used in the evaporative system of a direct heat transfer heat pump. Therefore, the earth loop of a direct heat pump is usually made of copper or other metal and contains tubes with a diameter of 6-10 millimeters. The refrigerant circulates through a loop of copper pipe underground and exchanges heat with the ground. Such a soil contour is prone to accelerated soil corrosion and is expensive.

The evaporative circuit of a geothermal heat pump of direct heat exchange consists of the following parts: inlet and outlet piping, distribution manifolds, an earthen (ground) circuit, which may consist of a bundle or a set of collector tubes.

There are geothermal heat pumps of the Sundue brand “SDW-02 inv” on the Russian market http://sundue.ru/news/teplovoi-nasos-sdw-varianty-i-vozmozhnosti, they are glycol heat pumps and heat pumps of direct heat exchange.

The evaporator circuit, made of copper or other metal, has a reduced corrosion resistance compared to polyethylene material. Metal susceptibility to corrosion is one of the problems with direct heat transfer heat pumps. For example, KR101606417 "Method for preventing corrosion of ground heat exchange system using sacrificial anode" suggests using a sacrificial anode to prevent corrosion.

The main requirement for the location of the soil contour is that it must remove heat from such a volume of soil in order to ensure the production of the required amount of heat and at the same time have an optimal length without unnecessary connections.

The location of the earthen contour can be horizontal, below the freezing point of the soil by 30-50 cm, vertical, spiral, or their combination.

Tubes in horizontal loops can be connected in series so that only one long loop is present, or in series and in parallel so that multiple loops are parallel to use the same input / output channels.

When placed horizontally, the collector tubes can be placed in rings or twisting in horizontal trenches. For this arrangement, polyethylene tubing is the most suitable, which can be bent during installation. For example, the invention RU 2561840 "An underground circuit in a system of low-temperature energy and a method for its production" is known, in which the underground circuit contains a collector pipe system made in the form of coil turns with the formation of at least two annular pipes of different cross-sections.

The vertical arrangement of the geothermal circuit in the form of vertical probes requires less surface area, but the cost of drilling is high, for example, in [5] a vertical ground heat exchanger is described, which is two parallel pipes connected at the bottom.

Methods for vertical arrangement of the earthen contour are also described in RF patent No. 2359183, IPC F24J 3/08, publication date 06/20/2009, utility model patent Pvul34 303. publication date: 11/10/2013. CN206861689U invention "Ground - source heat pump ground low temperature radiation capillary network heating System".

Vertical hinges are also available in parallel or series configuration.

Copper pipes are more difficult to bend and lay in curved trenches or in a spiral manner. The copper connections in the manifold are more complex and less reliable than those of polyethylene pipes.

Known polyethylene of increased temperature resistance PE-RT, presented in the standards of the International Organization for Standardization (ISO) ISO 22391-5: 2009. used for hot and cold water supply. PE-RT is a further development of polyethylene with significantly improved high temperature durability [6]. This material has a unique molecular structure that provides sufficient durability at high temperatures without the need to crosslink the material.

Known invention CN201450952 "Ground temperature energy-saving constant-temperature greenhouse big shed", which describes a greenhouse heating system with a geothermal glycol heat pump, which includes heat exchange pipes PE-X, PE-RT or PD, which are evenly submerged under the soil surface in the greenhouse to a depth of 30 cm to 50 cm, and vertical fan coil units installed around the main body of the greenhouse. In the present invention, PE-RT polyethylene pipes are not used to take heat from the ground, but to maintain the temperature of the soil, and glycol is circulated in them.

The closest to this invention is the invention US 10345051 - "Horizontal ground heat exchanger of a heat pump", date 09.07.2019. In this invention, the ground heat exchanger of a heat pump has long pipes located in at least one layer, communicating in a fluid medium. with a friend, and at least two (2) feet apart. Shorter pipes can be placed between long pipes and connectors between adjacent pipes. Long pipes are made of materials with high thermal conductivity, such as aluminum, while short pipes and / or connectors can be made of flexible materials with low thermal conductivity. The heat exchanger is located at least twenty-four (24) inches from the ground.

The disadvantage of this invention: pipes made of aluminum are susceptible to corrosion, their joints are fragile and short-lived, they are difficult to bend.

The problem solved by the claimed invention: to create an evaporative circuit of a geothermal heat pump of direct heat exchange, not subject to corrosion, capable of withstanding high refrigerant pressures, having durability, flexibility, ease of installation and tightness, a high degree of heat exchange with the ground, with an optimal location of the earth circuit, having a modular structure for use in heat pumps of various capacities.

The task is solved by the fact that

The evaporative circuit of a geothermal heat pump of direct heat exchange, which has inlet and outlet pipes, a system of collector pipes and their connections, differs in that the material of the evaporator circuit, i.e. inlet and outlet pipes, connections and collector pipes are polyethylene of increased temperature resistance PE-RT, and the diameter of the collector pipe is 16 mm;

the connecting element of the inlet or outlet pipe of the evaporation circuit of a geothermal heat pump with a system of collector pipes is a distribution comb, the central part of which is a cylindrical section of pipe, plugged at one end, to the sides of which three pipes are symmetrically connected on both sides, each pipe is connected to collector tube, the connection of the inlet or outlet pipe with the open end of the distributor manifold and the connection of the manifold branch pipe with the collector tube are made by means of socket welding;

the number of collector pipes is 6, 12 or 18, depending on the power of the heat pump - 12 kW, 24 kW or 36 kW;

length of one collector tube 100 m;

the connecting element of the inlet and outlet pipes of the evaporation circuit of a geothermal heat pump with a system of collector pipes is a distribution manifold, the central part of which is a cylindrical pipe section, plugged at one end, to the sides of which three pipes are symmetrically connected on both sides, each pipe is connected with a collector tube, the connection of the inlet or outlet pipe with the open end of the distribution manifold and the connection of the branch pipe of the distribution manifold with the collector tube are made by the method of socket welding;

the connecting element is a single distribution comb, or a set of 2 or 3 connecting combs, depending on the power of the heat pump - 12 kW, 24 kW or 36 kW, connected to each other in series by the ends of the cylindrical parts, the connections are made by the method of socket welding;

the earthen contour of a geothermal heat pump of direct heat exchange is located so that the collector pipes are placed horizontally in the ground in 6 parallel planes in a trench 3 m deep below the freezing point of the soil at a distance of 0.5 m from each other, the end sections of the inlet and outlet pipes, the connecting elements of the inlet and the outlet pipe of the evaporation circuit of the geothermal heat pump with the collection pipe system and the initial portions of the collection pipes are located in the caisson well;

the earthen contour is located in 1, 2 or 3 separate trenches, depending on the heat pump power - 12 kW, 24 kW or 36 kW.

The diameter of the collector tubes is increased to ensure a high degree of heat exchange between the PE-RT evaporator circuit and soil, equivalent to that through copper or metal earth circuit.

The material for the manufacture of the evaporator circuit was selected on the basis of experimental studies. Tests have shown the possibility of creating an evaporation circuit of a geothermal heat pump of direct heat exchange from polyethylene of increased thermal resistance PE-RT. Let's call such an evaporative circuit of a geothermal heat pump of direct heat exchange, made of polyethylene of increased temperature resistance PE-RT, as DX-Plast.

The disadvantage of polyethylene pipes in comparison with copper pipes is the lower thermal conductivity of polyethylene. This problem was solved by increasing the diameter and surface of polyethylene pipes, which managed to overcome the low thermal conductivity of polyethylene. That is, with equal dimensions of the circuit, the same length, the efficiency of heat exchange of the freon refrigerant with the ground through a plastic earth circuit made of PE-RT of a larger diameter and through a copper circuit of a smaller diameter is the same. For example, to ensure the same heat transfer for a 12 kW heat pump, the copper pipes of the soil circuit have a diameter of 8 mm and a total length of 600 m, and the PE-RT pipes have a diameter of 16 mm, with the same length.

The creation of direct heat exchange geothermal heat pumps was based on the use of R-410a freon. The maximum pressure of R410a freon in the geothermal circuit of direct evaporation is 1.6 MPa, such a saturated high-pressure steam needs special working units and high-quality parts.

In the DX-Plast test evaporation circuit, the connecting elements of the inlet and outlet pipes with a system of collector pipes were made in the form of distribution combs, the central part of which is a cylindrical section of pipe, plugged at one end, to the sides of which three pipes are symmetrically connected on both sides ... Each branch pipe is connected to a collection tube. To ensure the strength of the connection of the inlet and outlet pipes with the cylinder of the central part and the branch pipes of the distribution manifold with the collector pipes of the evaporation circuit, the method of sleeve welding of polyethylene was used. In the sleeve welding method, special connecting elements are used that completely encircle the end part of the tubes. Distribution manifolds, collector pipes and inlet and outlet pipes for the evaporation circuit of the geothermal heating circuit are made of the same polyethylene and, when installed in the ground, have a minimum number of couplings, which in turn ensures the reliability and tightness of the entire product.

To ensure ease of installation and control, the inlet and outlet pipes, distribution manifolds and the initial sections of the collector pipes of the evaporator circuit were placed in a caisson well.

The service life of the geothermal evaporation circuit made of PE-RT, due to the increased corrosion resistance, is significantly more than 50 years, because polyethylene does not decompose in the ground, for a copper circuit - 30 years.

Tubing made of PE-RT can be bent and laid in an optimal way in a sinuous contour to maximize soil heat removal and reduce the capital cost of the earth contour.

The selected polymer is frost-resistant and does not lose flexibility at low temperatures (up to 26 degrees Celsius below zero). To test, pipe sections made of PE-RT, after being kept for 24 hours in a freezer at 26 degrees Celsius below zero, were subjected to bending failure. As a result of the tests, PE-RT pipes were bent with a break, but did not lose tightness.

The efficiency tests of the DX-Plast evaporation circuit were carried out on real facilities. Two 12 kW “SDW-02 inv” heat pumps with inverter compressors fueled with R-410A freon were installed in two residential properties. The heat pumps were installed on properties of the same size and layout, with underfloor heating systems. The air temperature in the premises was maintained the same - 22 degrees Celsius above zero. The first heat pump was a conventional, three-circuit, and contained a low-pressure polyethylene glycol earthen loop, the other was a direct exchange heat pump and contained a PE-RT evaporation loop.

The evaporation circuit of the second direct heat exchange geothermal pump, DX-Plast, consisted of inlet and outlet pipes, two distribution manifolds (outlet and collecting) and collector pipes. The number of collector tubes suitable for each of the combs was 6, the length of one collector tube was 100 m. The diameter of the collector tubes was 16 mm.

The inlet and outlet pipes, distribution manifolds and the initial sections of the header pipes have been placed in the caisson well for ease of installation and control.

The collector pipes were located horizontally in the ground in trenches 3 meters deep from the freezing point of the ground in 6 parallel planes at a distance of 0.5 m from each other. The manifolds were connected to the inlet and outlet pipes and the collector pipes by means of socket welding.

Ground loops of geothermal heat pumps - low pressure polyethylene glycol and DX-Plast were located at a distance of 250-300 meters from each other, with the same geology. Comparative performance monitoring was carried out monthly during the heating season.

The purpose of the tests was

1) checking the consequences of long-term exposure to freon, namely difluoromethane and pentafluoroethane, on the material of the walls of the DX-Plast contour;

2) the tightness of the DX-Plast circuit connections, namely, the connections of the inlet and outlet pipes with the manifolds, and the joints of the manifolds with the manifold pipes, produced by the method of socket welding;

3) vapor permeability of DX-Plast walls;

4) the efficiency of heat exchange of the refrigerant with the ground through the walls of the collector pipes made of PE-RT.

The tests were carried out during the heating season - 8 months. The check showed, after the completion of the tests, the absence of the effect of freon on the wall material, the tightness of the circuit joints made by the method of socket welding, the impermeability of the DX-Plast walls for freon vapors, the efficiency of heat exchange of the refrigerant with the soil through the walls of the collector pipes of increased diameter made of PE-RT.

The efficiency of heat exchange between the refrigerant and the ground through the walls of the collector pipes made of PE-RT was confirmed as a result of a comparative analysis of the amount of electricity consumed by heat pumps at the end of the heating period. For SDW-02 inv with a glycol earthen loop, the power consumption was 6363 kW. For SDW-02 inv with DX-Plast contour, the consumption was 5481 kW. The average annual economic effect of a geothermal heat pump of direct heat exchange with the DX-Plast circuit was 16.5%.

In two years of field testing, 14 prototypes of DX-Plast direct heat transfer evaporator circuits were installed, including the test circuit. The reliability and tightness of the DX-Plast earth contour was monitored. Not a single DX-Plast evaporator circuit required refueling of R410a freon during the indicated period. Visual inspection of collectors, pipes, welded joints did not reveal defects or visible changes in the DX-Plast contour. All tests showed the tightness of the joints made by the method of socket welding, the impermeability of the walls of the circuit and the connections for freon, the absence of the effect of freon on the walls of the circuit and the joint, the flexibility of the evaporating circuit of direct heat exchange DX-Plast, the efficiency of heat exchange of the refrigerant with the soil through the walls.

When the power of the heat pump is increased by 2 or 3 times (24 or 36 kW), a connection by the method of socket welding of two or three distribution combs is used, the number of collector pipes increases by two or three times, respectively. With a constant length of one collector tube of 100 m, the total length of the earth loop increases by 2 or 3 times. Collector pipes are located in 2 or 3 earthen trenches 3 m deep from the freezing point of the soil, 6 pipes in one trench in parallel planes at a distance of 0.5 m from each other.

The use of an evaporative circuit of a ground source heat pump, made of PE-RT, allows to increase its service life, increase its tightness and flexibility. It does not corrode, can withstand high refrigerant pressures, has a high degree of heat exchange with the ground, has an optimal location of the earth circuit, and has a modular structure for use in heat pumps of various capacities.

The evaporative circuit, made of PE-RT, can also be used in other heating and cooling systems, including air conditioners, refrigeration units that use freons as a heat carrier.

Literature:

1. Polytechnic dictionary, edition 3. M .: Soviet encyclopedia, 1989.

2. Gasho E.G., Kozlov S.Α., Puzakov B.C., Razorenov R.N., Sveshnikov N.I., Stepanova M.V. Heat pumps in modern industry and municipal infrastructure. Informational and methodical publication. - M .: Publishing house "Pero", 2016.

3. RF patent №2116586.

4. RF patent No. 2655087.

5. Vasiliev, G.P. Monograph: Heat and cold supply of buildings and structures using low-potential thermal energy of the surface layers of the earth / G.P. Vasiliev. ISBN: 5-94691-202-X. - M .: Publishing house "Border" - 2006. - 173 p.

6. A. Yu. Chermyanin. PE-RT - New Class of Heat Resistant Polyethylene, https://www.abok.ru/for_spec/articles.php?nid=6431

Claims (8)

1. Evaporation circuit of a geothermal heat pump of direct heat exchange, having inlet and outlet pipes, a system of collector pipes and their connections, characterized in that the material of the entire evaporator circuit is polyethylene of high temperature resistance - PE-RT, and the diameter of the collector pipe is 16 mm. 2. Evaporation circuit of a geothermal heat pump according to claim 1, characterized in that the connecting element of the inlet or outlet pipe of the evaporation circuit of the geothermal heat pump with a system of collector pipes is a distribution comb, the central part of which is a cylindrical pipe segment, plugged at one end, to to the sides of which three branch pipes are symmetrically connected on both sides, each branch pipe is connected to the collector pipe, the connection of the inlet or outlet pipe with the open end of the distribution manifold and the connection of the distribution manifold branch pipe to the collector pipe are made by the method of socket welding. 3. Evaporation circuit of a geothermal heat pump according to claim 1, characterized in that the number of collector pipes is 6, 12 or 18, depending on the power of the heat pump, 12 kW, 24 kW or 36 kW. 4. Evaporation circuit of a geothermal heat pump according to claim 1, characterized in that the length of one collector tube is 100 m. 5. Connecting element of the inlet and outlet pipes of the evaporation circuit of a geothermal heat pump with a system of collector pipes, characterized in that it is a distribution manifold, the central part of which is a cylindrical pipe section, plugged at one end, to the sides of which they are symmetrically connected on both sides three branch pipes, each branch pipe is connected to the collector pipe, the connection of the inlet or outlet pipe with the open end of the distribution manifold and the connection of the distribution manifold branch pipe to the collector pipe are made by the method of socket welding. 6. The connecting element according to claim 5, characterized in that it is a single distributor comb or a set of 2 or 3 connecting combs, depending on the power of the heat pump - 12 kW, 24 kW or 36 kW, connected to each other in series by the ends of the cylindrical parts, connections are made by the method of socket welding. 7. A method of positioning the earthen contour of a geothermal heat pump for direct heat exchange, in which the collector tubes are placed horizontally, characterized in that the collector tubes are placed horizontally in the ground in 6 parallel planes in a trench 3 m deep below the freezing point of the soil at a distance of 0.5 m from each other, the end sections of the inlet and outlet pipes, the connecting elements of the inlet and outlet pipes of the evaporation circuit of a geothermal heat pump with a system of collector pipes and the initial sections of the collector pipes are located in a caisson well. 8. The method according to claim 7, characterized in that the earthen contour is located in 1, 2 or 3 separate trenches, depending on the power of the heat pump - 12 kW, 24 kW or 36 kW.