IL282276A - A vertical heat exchanger equipped with geothermal ground of a multi-capsular structure - Google Patents

Description

VERTICAL GEOTHERMAL GROUND COUPLED HEAT EXCHANGER OF MULTI-CAPSULAR STRUCTURE FIELD OF THE INVENTION The present invention relates to a field of geothermal heating/cooling systems for individual houses, small and medium industrial enterprises, agriculture, greenhouses, cooling solar panels, etc. In particular, the present invention describes structures of a small vertical ground-coupled closed-loops heat exchanger with high effective heat energy transmission using the Earth source. The proposed structure ensures the minimum dependence of the transmission power on drought, daily and seasonal fluctuations in temperature and groundwater level, drastically simplifies and reduces the cost of installation works with application of general construction equipment.

BACKGROUND OF THE INVENTION Geothermal heating/cooling systems using heat ground source energy receive the worldwide application.

There are known problems of increasing the performance of an earth source heat exchanger by increasing heat transfer between the earth and the heat exchanger fluid. From the point of view of maximum heat transfer, vertical exchange geothermal systems are the most studied and most of the proposed structures relate to them. Patent US 7,370,488 proposes vertical geo- thermal ground coupled heat exchanging system providing "the transfer of heat energy using coaxial-flow heat exchanging structures installed in the earth for introducing turbulence into the flow of the aqueous-based heat transfer fluid flowing along the outer flow channel". This system requires significant energy consumption for the implementation of turbulent water flow through a large diameter channel.

Patent Application US20110308268 Al describes a vertical underground heat exchanger, which comprises an internal cylinder with low heat conductivity and external thin wall stainless steel cylinder with high heat conductivity coaxially installed in a bore. The lower ends of the cylinders have bottoms, the bottom of the internal cylinder has holes for water circulation. The gap between the cylinders is filled by sand. Water enters the outer cylinder, flows through the sand and exits into the inner cylinder. Water flow through sand has significant hydraulic resistance, requires energy consumption, which reduces the system efficiency.

Patent US 6,251,179 proposes vertical geothermal heat pump systems with high density polyethylene (HDPE) piping with circulating water or water/antifreeze liquid to use and with thermally conductive Grout 111 for boreholes filling. Grout 111 or analogous grouts were proposed for DX (direct exchanger) geothermal systems (patents US 7,856,839, US 7,938,904) with copper pipes. These grouts have relatively low thermal conductivity (less 2.8 W/(m*K)).

Patent 5,816,314 describes vertical geothermal system, in which the heat exchange unit has a heat exchange tube with a fluid supply section helically formed around a rigid hollow cylindrical core. Inside the core vertical return section of a tube is located. The core is filled with a thermal insulating material. The helically formed tube and related components are protected by a cylindrical outer casing made of a metal or other rigid and durable thermally conductive material. The space between the core and the casing is filled with a thermally conductive fill material, such as powdered metal or stone, concrete, or cement. The metal casing placed under ground requires corrosion protection of the exposed subterranean metal tubes. In addition, air gaps between the metal casing and the surrounding soil can occur in structures with a metal enclosure that reduces the actual heat conductivity and real effectiveness of the system.

Known vertical boreholes have depth 50 - 150 m and more with corresponding disadvantage of deep drillings, considerably complicated installation and maintenance. Deep vertical systems are considerably more expensive than horizontal geothermal heat exchangers.

Vertical deep drilling can provoke mixing between aquifers of different qualities and to be potential source of contamination. Before application of vertical geothermal system geological survey is required. For drilling of deep boreholes and mounting of exchanging system special equipment is required.

SUMMARY OF INVENTION The aim of the proposed invention is creating not expensive high productive vertical geothermal closed-loop ground-coupled heat exchanger, which effectively operates even in extreme weather conditions (heat, drought, etc.), simple in installation and using for installation commonly applied construction equipment. Also the aim of the proposed invention is facilitation on-site geothermal borehole assembly.

The proposed invention allows, on the one hand, to avoid the disadvantages of commonly used vertical exchangers (high cost, requirements of preliminary geological works, groundwater pollution hazard, requirements of special equipment for installation) and, on other hand, to avoid drawbacks of horizontal exchangers (the need for a large land area, dependence of productivity on climate conditions, seasons, weather, rainfall, etc.), to extend application of ground-coupled heat exchangers in arid and semi-arid regions and in areas with significant space constraints and to make geothermal energy the most convenient, widespread, safe, stable and weather-independent form of green energy.

This is achieved by using a vertical multi-capsular structure built on a number of small vertical boreholes with a new capsules construction.

Each capsule is located in a small vertical borehole and comprises a flexible impermeable bag with a fill material and a conduit with heat transfer liquid. A number of capsules are connected to create a single geothermal ground coupled heat exchanger for providing the required system power.

The bags are made from flexible water impermeable polymer material. The bags are equipped by a load on the bag bottom.

Diameter of each impermeable bag exceeds diameter of the borehole for providing tight contact and pressing of the bag with fill material to the borehole wall without mechanical stressing the bags. Such structure of the exchanger provides, the first, good thermal conductivity between liquid flowing in the conduits and the earth surrounding the boreholes. The second, it prevents groundwater pollution. The third, as opposed to using metal pipes, the use of flexible polymeric materials for the bag prevents the occurrence of oxidative galvanic processes, and the device does not require cathodic protection.

In one of the preferable solutions the heat conductive filling material in the bags comprises mix of sand and carbon particles substantially saturated with water.

Inventors propose also ground coupled heat exchanger with small vertical boreholes and flexible water impermeable bag, in which the heat conductive filling material contains sand, water and carbon particles in a view of carbon flakes. Carbon flakes dramatically increase thermal conductivity of the fill material. Tests carried out by the Inventors showed that such composition provides thermal conductivity of the mix about of 4.5 - 5 W/m*K while commonly used groutings have this value about 2.5 - 2.8 W/m*K.

It is also proposed to use carbon particles in the form of carbon nanoparticles in the fill material. Contents of above mentioned carbon particles are in limits 1 - 6% of sand weight.

Application of the fill material with higher percentage of carbon particles is also possible.

Finally, the composition of the fill material should be defined as a compromise between the desired efficiency of the geothermal system and its cost.

Inventors propose a structure of the exchanger, in which pipes located in the bag and containing a heat transfer liquid are preliminary produced in the form of a spiral on the inlet way and the linear straight form on the return way. Also it is possible design with the straight pipe on the inlet way and the spiral pipe on the return way. U-shape design of the pipe also may be used, but in this case thermal insulation of one of the pipe branches is desirable.

Adjacent spiral turns of the above mentioned spiral are connected by flexible ropes or strips providing the required spiral pitch. This conduit structure allows to supply conduits in a view ready for installation. Also this allows transportation of the conduits in compact view and makes possible to provide the required spiral pitch in the mounted state.

For providing extension of the spiral in a borehole a load is fixed on the lower end of the conduit. U-shape conduits containing a heat transfer liquid also have a load at the bottom.

As Inventors propose, the conduits are made from polymer materials and heat transfer liquids are water or antifreeze solution. In one of the proposed structures for heat transfer increasing the liquid is water with addition of nano-scaled particles (nano-fluid). Content of the above mentioned carbon nano-scaled particles is up to 6% of water volume. It is possible the nano-scaled particles more 6 vol % The heat exchanger structure may be used also for direct exchanger. In this case conduits are made from copper tubing and the heat transfer liquid is refrigerant.

Inventors propose also method of installation of the described capsule. At the first stage the fill material is prepared. In the process of mixing of components (sand, water and carbon particles) the amount of water should be slightly higher than the value required for the saturation state.

At the second stage, the bag with its load is lowered into the borehole.

At the third stage the prepared conduit with its load is lowered into the bag. The bag and the conduit may be lowered simultaneously.

At the fourth stage the prepared filling material is poured into the bag with the conduit.

Upon completion of the filling process a part of water in the filling material, exceeding the amount of water required for saturation will rise to the top of the bag because its density is less than that of the sand and carbon particles. This water may be removed.

This method can also be supplemented with an additional preliminary step of a temporary casing method. Temporary casing method is used usually in the construction of buildings for foundation piles. (https://www.designingbuildings.co.uk/wiki/Bored piles#Temporary casing) "Designing Buildings Wiki". It applies a screw-joint temporary steel lining for the boreholes wall support during drilling.

The temporary steel lining may be useful also at installation of the geothermal ground coupled heat exchanger of multi-capsular structure. It makes more simple and reliable lowering the bag and the conduit and filling the fill material. The temporary steel lining is removed during filling of the bag by the fill material.

The conduits of the capsules are connected in a single multi-capsular geothermal ground couple heat exchanger.

BRIEF DESCRIPTION OF DRAWINGS Advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings: FIG. 1 is view of the scheme of a capsule of a ground coupled heat exchanger with a small borehole, spiral and linear pipes located in a bag with filling material.

Fig. 2 is view of the scheme of the test device for determination of thermal conductivity of the fills.

DETAILED DESCRIPTION OF DRAWINGS A variant of a principal scheme of a capsule of the proposed Geothermal Ground Coupled Heat Exchanger of multi-capsular structure is shown on Fig. 1. The capsule structure 1 is located in ground 2 in a small vertical borehole 3. The conduit containing a heat transfer flowing liquid has spiral form on one way 4 and straight form on the opposite way 5. The conduit 4 is formed in spiral during its producing.

Heat transfer liquid is water or antifreeze solution. For heat transfer increasing the liquid may additionally contain nano-scaled particles (nano-fluid).

The conduits 4, 5 are located in the flexible water impermeable bag 6 with filling material 7.

Heat conductive filling material 7 contains sand, water (up to saturation) and carbon particles, preferably in a form of carbon flakes. Fill material may contain carbon nanoparticles.

The bag 6 is made from two or more layers of polymer water impermeable film and has diameter exceeding diameter of the borehole. To place the bag in the well a load 10 is fixed at the bag bottom.

For providing extension of the spiral during installation, the conduit 4 is provided with the load 8, which is fixed on the lower end of the conduit.

Adjacent spiral turns are connected by flexible ropes or strips 9 providing the required step of the spiral. This allows transportation of the conduit in compact form and provides the required step of the spiral turns in the capsule.

Thermal conductivities of different compositions of the bag filling materials comprising sand, water and carbon particles were tested experimentally. Scheme of the experimental bench is shown on Fig. 2. A test device 20 consists of a heating plate 21 with uniform temperature, a container 22 with thermal insulated walls 23 and a high thermal conductive bottom 24, a fill material 25 and three temperature sensors 26, 27, 28. The thermal sensors are located on different distances from the bottom.

As carbon particles Inventors used carbon flakes of dimensions 75 mkm and 200 mkm. The tests were performed at the following levels of carbon flakes content: 0%, 2%, 4% and 6% of the sand weight. Water content was close to saturation. Heating of the device was continued up to steady state. Thermal conductivity of the filing materials X was calculated according to the following equation: X = P*Ah/S*AT, where P, W - power applied to the bottom 24 of the container 22 Ah, m - distances between two the temperature sensors 26 - 27 and 27 - 28, S, m2 - the cross-section of the container, AT,°C - temperature difference between the adjacent temperature sensors.

Value of the thermal conductivity of mix of sand and water was 2.7 W/m*K. Thermal conductivity for the same conditions is increased with carbon flakes content.

Application of carbon flakes particles 200 mkm gave higher thermal conductivity than flakes particles 75 mkm. The best result was obtained at application of 6% particles 200 mkm.

Maximum achieved thermal conductivity value in these conditions was about 5 W/m*K.

Described structure of a capsule may be used also in direct exchangers. In this case the conduits are made from copper tubing and heat transfer liquid is refrigerant. The copper tubes may be performed as spiral in one direction and straight in the opposite direction, and they are placed in the described above bag with filling material.

Inventors also propose a method of installation of the described capsule. This method includes the following steps: - Mixing of the filling material 7 (sand, water and carbon particles), wherein the amount of water added to the mixture slightly exceeds the amount required for saturation. It provides fluidity of the mixture during filling.

- Lowering the bag 6 with the load 10 into the borehole to the bottom, - Lowering the conduit 4, 5 with the flexible ropes or strips 9 and its load 8 into the bag 6 up to the bottom, - Filling the bag 6 with the prepared filling material 7, - Removing excess water If for boreholes drilling temporary casing method is used, when the capsule is installed in a borehole with a temporary steel lining, the method of capsule installation includes the following steps: - Mixing the filling material 7 (sand, water and carbon particles), - Lowering the bag 6 with the load 10 into the borehole with a temporary steel lining to the bottom, - Lowering the conduit 4, 5 with the flexible ropes or strips 9 and its load 8 into the bag 6 up to the bottom, - Removing the temporary steel lining of the borehole during filling the bag with the prepared filling material 7, - Removing excess water The described Capsular Structure and the proposed manufacturing method for ground coupled geothermal exchangers use factory-produced ready components: the bag and the conduits with the loads that considerably facilitate the system installation.

High efficiency, weak dependence from weather and natural disasters, stability and reliability, simplicity and cheapness of installation and availability of the applied equipment makes Multi-Capsular Structures of geothermal systems especially promising. This opens the way to widespread use of the proposed geothermal system for individual and commercial buildings, farms etc., that is about 70% of US energy consumption (Building Energy Data Book 2011, U.S. Department of Energy https://ieer.org/wp/wp-content/uploads/2012/03/DOE-2011 - Buildings-Energy-DataBook-BEDB.pdf p.29/286).

The Invention has been described in an illustrative manner, and it is to be understood that the terminology, which has been used, is intended to be in the nature of words of description rather than of limitation. Clearly, many modifications and variations of the present invention are possible in light of the above teachings. Accordingly, it is to be understood that the invention can practiced otherwise than specifically described.

Inventors and Applicants Irina Loktev, —______V Vladimir Kominar —5

Claims (16)

1. Geothermal ground coupled heat exchanger of multi-capsular structure, comprising number of small boreholes, containing conduits with heat transfer liquid flowing therein, and each conduit is placed in a small borehole, and the conduits are interconnected, wherein each of the above-mentioned boreholes additionally contains a capsule, composed of a flexible water impermeable bag with a diameter exceeding the borehole diameter, and the flexible bag is provided by a load on the bag bottom, and each conduit is located inside the bag, and the bag is filled with a heat conductive fill material, containing a mix of sand and carbon or graphite particles substantially saturated with water, producing tight pressure of the bag to the wall of the borehole for providing high thermal conductivity between the conduit with heat transfer liquid and the ground around the borehole and for preventing groundwater pollution.

2. Geothermal ground coupled heat exchanger of the multi-capsular structure according to claim 1, wherein the water impermeable bag consists of at least two layers of a waterproof polymer film, providing reliability in preventing water leakage and the absence contact between ground water and the fill material.

3. Geothermal ground coupled heat exchanger of the multi-capsular structure according to claim 1, wherein the above mentioned carbon or graphite particles in fill material are carbon or graphite flakes.

4. Geothermal ground coupled heat exchanger of the multi-capsular structure according to claim 1, wherein the above mentioned carbon or graphite particles in the fill material are carbon or graphite nanoparticles.

5. Geothermal ground coupled heat exchanger of the multi-capsular structure according to claim 1, wherein weight of the above mentioned carbon or graphite particles is in the range of 1 - 6% of the sand weight.

6. Geothermal ground coupled heat exchanger of the multi-capsular structure according to claim 1, wherein weight of the above mentioned carbon particles is more than 6% of the sand weight.

7. Geothermal ground coupled heat exchanger of the multi-capsular structure according to claim 1, wherein the conduits containing the heat transfer liquid are pre-formed as a spiral pipe in one direction and a straight pipe in the opposite direction and have a load at the bottom.

8. Geothermal ground coupled heat exchanger of the multi-capsular structure according to claim 1, 2 wherein the conduits containing the heat transfer liquid are U-shaped and have a load at the bottom..

9. Geothermal ground coupled heat exchanger of multi-capsular structure according to claim 7, wherein adjacent turns of the preliminary formed spiral are connected by flexible ropes or strips providing the required spiral pitch.

10. Geothermal ground coupled heat exchanger of multi-capsular structure according to claim 1, wherein the conduits are made from polymer materials and the heat transfer liquids are water or antifreeze solution

11. Geothermal ground coupled heat exchanger of the multi-capsular structure according to claim 1, wherein the conduits are made from polymer materials and the heat transfer liquid is a nano- fluid.

12. Geothermal ground coupled heat exchanger of the multi-capsular structure according to claim 11, wherein heat transmitting nano-fluid in the conduits contains nano-scaled particles up to 6% by volume.

13. Geothermal ground coupled heat exchanger of the multi-capsular structure according to claim 11, wherein heat transmitting nano-fluid in the conduits contains more than 6% nano-scaled particles by volume.

14. Geothermal ground coupled heat exchanger of the multi-capsular structure according to claim 1, wherein the heat exchanger is a direct heat exchanger with the multi-capsular structure and conduits are made of copper tubing and the heat transfer liquid is a refrigerant

15. Method of manufacturing a capsule of a geothermal ground coupled heat exchanger of the multi- capsular structure, comprising the following steps of: mixing the fill material with a water content exceeding saturation, lowering the bag with the load into the borehole to the bottom, lowering the conduit with the flexible ropes or strips and its load into the bag up to the bottom, filling the bag with the prepared fill material, removing the excess water, connecting the conduits of the capsules into a single multi-capsular system.

16. Method of manufacturing a capsule of the geothermal ground coupled heat exchanger of the multi-capsular structure, when the capsule is installed in a borehole with a temporary steel lining, which comprises mixing the filling material with a water content exceeding saturation, lowering 3 the bag with the load into the borehole with a temporary steel lining to the bottom, lowering the conduit with the flexible ropes or strips and its load into the bag up to the bottom, filling the bag with the prepared fill material and removing the temporary steel lining of the borehole during the filling process, connecting the conduits of the capsules into a single multi-capsular system. Inventors and Applicants Irina Loktev Vladimir Kominar