IL282276B2 - A vertical heat exchanger equipped with geothermal ground of a multi-capsular structure - Google Patents

Description

1 GEOTHERMAL GROUND COUPLED HEAT EXCHANGER 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 of solar panels, etc. In particular, the present invention describes structures of a 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.

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 geothermal 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 expenses for realizing turbulent water flow along a channel with large diameter.

Patent Application US20110308268 A1 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. 2 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 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.

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 placed. 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 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. Besides, in constructions with metal casing it is possible occurrence of air gaps between the metal casing and surrounding soil that reduces 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. 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.

Known horizontal geothermal system contains a special intermediate stratum between conduits and Earth, separated from surrounding soil by thin waterproof material from all sides, bottom, walls and top (IL 238,147, US 9,593,868). The stratum contains fill material consisting of sand with water content close to saturation, and said conduits with heat transfer liquid pass through fill material in this stratum. Sand with additional water content has increased thermal conductivity (about 2.6 – 2.8 W/(m*K)) in comparison with thermal conductivity of arid, semiarid and ordinary soil (in limits 0.5 – 1.5 W/(m*K)). 3 SUMMARY OF INVENTION The aim of the proposed invention is creating not expensive high productive 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 assembly.

The proposed bulk fill material (sand with crystalline carbon or graphite particles with saturated water content) allows reach the filling, having coefficient of thermal conductivity by 2-3 times higher than in commonly used filling.

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 application of proposed filling material in small vertical multi-capsular structure built on some number of small vertical boreholes with new capsules construction and also by applying capsular horizontal exchangers.

Each capsule comprises filling material and conduit with heat transfer flowing liquid. A number of capsules are connected to create a single geothermal ground coupled heat exchanger for providing the required system power.

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

Inventors propose also ground coupled heat exchanger in which the heat conductive fill material contains bulk composition comprising sand, water in volume about saturation and crystalline carbon particles in a view of carbon flakes. Carbon flakes dramatically increase thermal conductivity of the fill material.

For a rough estimate of the required quantity of carbon flakes Inventors use the mathematical formula, which determines the effect of the filler thermal conductivity on the 4 effectiveness of the capsule. The formula shows the ratio of the power transmitted by the capsule with the proposed filler and the power transmitted by the conduit in the traditional scheme (conduit in soil). wherein λ1 — thermal conductivity of the capsule filler, λ2—surrounding Earth thermal conductivity, α0 — coefficient of thermal transfer of liquid flow in pipes.

The formula serves as the basis for choosing the ratio of sand and carbon flakes weights in the capsule. Calculations show that the efficiency of the capsule increases with an increase of the filler thermal conductivity and reaches values 5 - 7. The formula shows that with further increasing thermal conductivity, the efficiency growth is slowing down.

Inventors tested bulk filler with particles of crystalline carbon. Tests show that at increase of the percentage of crystalline carbon particles in the filler the thermal conductivity of the filler increases and at a content of carbon particles about 6% by weight of sand, it reaches a value of W/m*K (while commonly used fill material have this value up to 2.5 - 2.8 W/m*K).

It is also proposed to use carbon particles in a view of carbon nanoparticles, for example, nanotubes, as the fill material.

Inventors propose a structure of the vertical exchanger, in which pipes, containing a heat transfer liquid, are located in the capsule with the heat conductive fill material consisting of mix of sand, substantially saturated with water and crystalline carbon or graphite particles, and they are preliminary produced in a form of spiral on the inlet way and 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 obligatory.

Adjacent spiral turns of the above mentioned spiral are connected by flexible ropes or strips providing the required step of the spiral pitch. These ropes allow maintain spiral pitch in the process of the capsule backfilling. 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 step of the spiral turns in the mounted state.

Keff 2 0 d0 2  ln 1.35 d1 d0                2 0 d0 1  ln d1 d0         0 d0 2   ln(1.35)  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.

In one of the proposed structures for heat transfer increasing the liquid is water with addition of nano-scaled particles (nano-fluid).

The heat exchanger may be made also as direct exchanger. In this case conduits in the capsule are is made from copper tubing and heat transfer liquid is refrigerant.

For increasing of effectiveness of the geothermal vertical capsule Inventors also propose structure, which decreases influence of ground layers close to the surface. In this case upper part of the capsule contains excessive water content (that provides low thermal conductivity between the conduit and ground layer close to the surface) and lower part of the capsule has saturated water content, Inventors propose also structure, wherein the conduits in the capsules contain two parts.

The upper part (for example, 0.3 – 0.5m from earth surface) of the conduit is straight pipe for providing low energy change between the conduit and ground layer close to the surface. Lower part has spiral form.

Inventors propose also structure, in which the capsules, containing the proposed heat conductive fill material, consisting of bulk mix of sand and carbon or graphite particles substantially saturated with water, are placed horizontally. Use capsules with thermal conductive fill materials in horizontal geothermal systems is significant step forward in the technology of geothermal systems.

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 capsule with filling material.

Fig. 2 is view of the scheme of the test device for determination of thermal conductivity of the fills. 6 Fig. 3 is view of the curve Dependence of the System Efficiency on the thermal conductivity of the filler.

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 water impermeable shell 6 of the capsule 1 with filling material 7.

As it’s shown on Fig.1, upper part of the conduit 4 (for example, 0.3 – 0.5m from earth surface) is straight pipe that provides low energy change between the upper part and ground surface.

Inventors also propose structure in which upper part of the capsule contains excessive water content and lower part of the capsule has saturated water content. Such structure also decreases influence of ground surface climate conditions.

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

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

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 capsule with filling material.

The capsule 1, containing the proposed heat conductive fill material, consisting of bulk mix of sand and carbon or graphite particles substantially saturated with water, can be installed 7 horizontally. Use capsules with thermal conductive fill materials in horizontal geothermal systems is significant step forward in the technology of geothermal systems.

Thermal conductivities of different compositions of the capsule fill 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 filling materials λ was calculated according to the following equation: λ = P*Δh/S*ΔT, where P, W – power applied to the bottom 24 of the container 22 Δh, m – distances between two the temperature sensors 26 - 27 and 27 - 28, S, m2 – the cross-section of the container, ΔT,˚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.

Studies have shown that an increase of the flakes percentage leads to a further increase in the thermal conductivity of the filler and, accordingly, the efficiency of the system. Fig. 3 shows dependence of the system Efficiency coefficient on the filler thermal conductivity. Also the graph shows that with further thermal conductivity increasing, growth of Efficiency is slowing down.

Thus, the Inventors have developed vertical and horizontal geothermal systems, filled with bulk materials and quite simple in execution. Systems have extremely for these systems high thermal conductivity and efficiency.

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 8 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, Vladimir Kominar

Claims (11)

1. Geothermal ground coupled heat exchanger, Comprising number of rigid or flexible water impermeable capsules in the multi capsular structure, have a conduit that located inside the capsule, wherein the capsule is filled with a heat conductive fill material, containing a bulk mix of sand and crystalline carbon or graphite particles more than 2% of sand weight substantially saturated with water, providing high thermal conductivity between the conduit with heat transfer liquid and the ground around the capsule.

2. Geothermal ground coupled heat exchanger according to claim 1, wherein the above mentioned crystalline carbon or graphite particles in fill material are carbon or graphite flakes.

3. Geothermal ground coupled heat exchanger according to claim 1, wherein the above mentioned crystalline carbon or graphite particles in the fill material are carbon or graphite nanoparticles.

4. Geothermal ground coupled heat exchanger according to claim 1, wherein the exchanger is vertical exchanger with the capsule containing a bulk mix of sand and crystalline carbon or graphite particles substantially saturated with water, and the conduit containing the heat transfer liquid are pre-formed as a spiral pipe in one direction and a straight pipe in the opposite direction and a load at the bottom.

5. Geothermal ground coupled heat exchanger of the multi-capsular structure according to claim 1, wherein the exchanger is vertical exchanger with the capsule containing a bulk mix of sand and crystalline carbon or graphite particles substantially saturated with water, and the conduits containing the heat transfer liquid are U-shaped.

6. Geothermal ground coupled heat exchanger of the multi-capsular structure according to claim 4, wherein adjacent turns of the preliminary formed spiral are connected by flexible ropes or strips providing the required spiral pitch in the process of the capsule filling by bulk materials mix. .

7. 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.

8. 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 is made of copper tubing and the heat transfer liquid is a refrigerant

9. Geothermal ground coupled heat exchanger of the multi-capsular structure according to claim 4, wherein upper part of the capsule contains excessive water content for providing low thermal conductivity between conduit and ground layer close to the surface, and saturated water content in the main lower part

10. Geothermal ground coupled heat exchanger of the multi-capsular structure according to claim 4, wherein the upper part of the conduit is straight pipe for providing low energy change between the conduit and ground layer close to the surface and the lower main part of the conduit is pre-formed as a spiral.

11. Geothermal ground coupled heat exchanger of the capsular structure according to claim 1, wherein the capsules, containing the heat conductive fill material, consisting of a bulk mix of sand and crystalline carbon or graphite particles substantially saturated with water, are horizontal capsule s . Inventors and Applicants Irina Loktev Vladimir Kominar