COAXIAL BOREHOLE ENERGY EXCHANGE SYSTEM FOR STORING AND EXTRACTING UNDERGROUND COLD

FIELD OF THE INVENTION

The invention relates to using a drilled borehole as a coaxial underground heat exchanger to store cold in and extract cold from surrounding bedrock and overbnrrlpn materials.

BACKGROUND OF TI IE INVENTION

It is well-known that the earth's dwindling supply of finite fossil fuels is creating major initiatives and activity in the alternative energy industry. Considering the additional knowledge that fossil fuel usa^e is linked to significant increases in green house gases (GHG's) in the atmosphere, it is incumbent upon researchers to develop energy systems, which diminish this trend that strongly appears to be exacerbating global warming. Tt has been noted that gεoiheimal resoutces generally release only about 10% or less the amount of carbon dioxide produced by burning an energy- equivalent amount of fossil fuel. Hot dry rock is widely distributed around the globe and represents a vast energy reserve.

The main emphasis of exploiting this resource has been to harness the thermal and electrical energy producing ability. Extracting energy via the HDR process (US Pat. No-T ₁ TRn ₅ RiR, issued Jan.22, 1974), involves creating a closed liquid system comprised of an HDR reservoir and above- ground equipment. This equipment is linked to an injection well, and a possibly production well drilled into hot rock. The. method is made more efficient by employment of hydraulic fracturing to produce fractures to enhance water movement. Heat exchangers at the surface are used to recover the heat from pumped-up water for use in electricity generation or for direct thermal applications. An efficient HDR reservoir is one that can be continuously mined for heated water, presents minor leakage, operates without significant net consumption of water, and has essentially no venting of gaseous or saline fluids to the environment, Which may create acid rain The primary us>agc uT HDR reservoirs has been forecast as generating steam or to vaporize another working fluid to produce electric power. In addition, applications that require quantities of thermal energy, which vary in a periodic manner, demand, υr maximum demand may be, serviced.

Tt has been determined that drilling inlυ crystalline rock is difficult and thermal conductivities of hot, hard rocks are typically very low, while their specific heats are high, so that a relatively large amount of heat is available from a unit volume of hot rock. λ drawback is that this heat must be extracted from thr rock only through some free surface, such as the wnll of a borehole. Since heat is conducted to the surface quite slowly, a very large surface area is required if thermal energy is to be removed from the rock at an efficient rate. Various energy service companies have cited hydraulic fracturing (e.g. US Patent No. 3,786,858) as a useful method for fracturing crystalline rock to create greater surface area for thermal conductivity. Tn addition, hydrotheπnal resources are basically limited to areas of tectonic or volcanic activity, precisely the areas where human developments and subsequent energy demand should nr>( he nccurring.

The boundary conditions for Borehole Thermal Energy Storage arc tew, with the major boundary condition being connected to the temperature requirements. Operational limitations for coaxial heat exchangers suggest that soft sedimentary rocks arc excluded and crystalline rocks are desirable geological materials to exploit. Therefore, geologically suitable locations for coaxial boreholes include much of the eastern and western coasts of Canada and the United States, while the interior plains or large sedimentary basins in Canada and the United States are generally excluded from development of the coaxial borehole.

The major centres in Atlantic Canada and along the St. Lawrence Valley that are suitable for coaxial borehole development include Halifax, St. John's, Corner Brook, St John, Quebec City, Sherbrookc, Montreal, Ottawa, Oshawa, Toronto, Hamilton, London, Windsor-Detroit. On the west coast, Vancouver and Victoria-Seattle arc included »s having sukable geological regimes for the coaxial borehole. In addition, the technology can be applied across the globe in similar crystalline rock material.

Ground-source heat pumps (GSHP) move or transport heat like air-source heat pumps, but exchange heat wiih the earth rather ihan the atmosphere;. These OcυExuhaπgc systems arc efficient, environmentally sensitive, comfortable and economical. The key feature is that these systems use electricity to move heat, not to generate it by burning fuel or using electric resistance elements. GeoExchangc systems arc generally 2.5 to 4 times more efficient than resistance heating and water heating alone, and produce no combustion or indoor air pollutants. In addition, there is no weather-

related maintenance. By employing geoexchange, the average home in Canada could reduce green house gas (GHO) emissions by 2.5 to 5 tonnes annually.

There are two main types of GSHP systems: open and closed loop. The open loop system draws water from a well, lake or river or ocean and discharges it back to the source. Closed loop systems use a seated pipe buried in the ground that circulates on antifreeze solution. The pipes can be installed in horizontal or vertical loops. Water and antifreeze solution loop heat pumps used in GSHP applications arc available in sizes from 1.5 to 300 kW (0.5 to 60 tons). The costs range from $800 to $ 1000/ton for the common size ranges. Higher tonnage equipment tends to display tower costs per ton. Costs for the loop are in the range of $1000/ton for horizontal loops and $ 1500- $1700/ton for vertical loops.

The U SEPA has concluded that well-designed and properly installed high-efficiency Geoexchange heat pump systems produce less environmental harm than any other alternative technology available. On a full-fuel cycle basis, Geoexchange systems arc the most efficient technology available, with the lowest CO ₂ emissions for minimum greenhouse warming impact Accαrding tn the BC Sustainable Energy Association (BCSESA. 2005). GeoExchange heat pumps use mainly R- 22 in the compression cycle and have hermetically-sealed compressors, thus refrigerant leaks are rare. The Ground Heat Pump Consortium (GHPC) supports; contractor training, certification and International Ground Source Heat Pump Association (IGHPA) programs to assure that systems are designed, installed and operated to avoid potential environmental damage such as groundwater contamination. The GHPC also offers the following guidelines to customers choosing a GeoExchange heating and cooling system; ratings and certification, warrantees, sizing, design and installation to promote the proper use of the product

The Canadian government continues to work on the environmental issue of banning use of chlorofluorocarbons (CFCs), proven to be a leading cause of ozone depletion. Canada eliminated production and importation of CFCs in 1995. In the Jong term, CFC reclamation will become a cost of doing business throughout the supply chain (Industry Canada, 2005), thus cost savings are realized by substituting geoexchangc products.

The main function of a heat exchanger is to transfer thermal energy between two fluids. The two fluids are usually hot or cold water pumped in to the exchanger and cold or hot water pumped

out of the exchanger. To obtain a reasonable licaL exchange between the fluids inside and out3idc of the exchanger pipe, it is necessary to have the surface of heat exchange being as extensive as possible. The pipe is usually constituted by a tube centered in a borehole, positioned in the casing of the boicbole. A major consideration in achieving maximum efficiency in employing these borehole heat exchangers is to provide as large a surface area as possible for potential heat transfer.

Past researchers have determined that when using heat pumps in the heating and cooling r>f buildings, the efficiency of an air-to-air heat pump is high only when the outside ambient temperature is moderate. At ambient temperature extremes, the coefficient of performance of a heat pump falls drastically. At an ambient tpmppratπre of fl ⁰ C ₁ its operation continues only at an energy loss because the evaporator must be defrosted. Considering these factors, alternate heat sources, which will remain essentially constant despite fluctuating ambient air temperatures were developed. This lead to groundwater-to-air heat pumps, which operated at a higher coefficient of performance (ratio of energy out to energy in) year-round. Sources of water were originally, well-water or city water, which would quickly become exhausted if used as a heat source/sink. These drawbacks were then overcome with the advent of a closed water loop in which the earth itself serves as a major heat sink for air-conditioning in the summer and as a heat source in the winter.

Closed-loop systems have unfortunately proven to work at lesser than desired efficiency levels because only minor amounts of the available heat in the earth's crust can be abstracted with then existing heat exchangers. Thc3e heat exchangers typically comprised long sections of copper coil, with no enclosures surrounding (lie coils and no use of phase change materials. Copper has also become a rather expensive working material. Due to the low thermal conductivity and heat capacity of the earth, energy in near proximity nf these earth coils was rapidly dissipated, when the instantaneous heating demand of a heat pump system was engaged. Consequently, the temperature of the heat transfer fluid would continue to fall, and in a short time period, a second coil would have to be substituted for the exhausted coiJ. This would ultimately lead to malfunction and a required shut-down of the heat pump system. The coils displayed other drawbacks such as costly installation, a large area requirement, a need for custom-designed components and destruction of laws and ihey are only reliable for small receiving unit usage.

It was noted that when employing aquifers acl as heal sυuπ-cs _> υr these sources could be quickly exhausted if frequently or widely used. This lead to the development of a closed-loop

system (Leon ct al., US Patent No. 4,327,560), issued in 1982, in which heat was transferred from the earth to a conditioned space, wherein the heat transfer fluid was circulated in a substantially closed loop through an improved earth coil. A phase change material was in thermal contact with both the primary heat transfer fluid (mixture of water and antifreeze) and the surrounding soil. This approach employed the earth itself as a giant heat sink for air conditioning in the summer and as a heat source, from which large amounts of heat, warmer than the ambient air could be extracted in the winter. The next step would be. to employ earth materials and he.Hrork ;is an energy source and for energy storage to increase the potential energy available from a given point source.

Energy storage is an enabling technology for use in a variety of energy systems, from residential to commercial and from industrial to agricultural. By contributing to large-scale energy efficiencies; energy storage significantly reduces environmental impacts from energy activities, increases the potential uptake of some renewable energy technologies, increases the potential for sustainable energy development and subsequently contributes to enhanced energy security. Energy storage technologies overcome the temporal mismatch between energy supply and demand, and are especially useful for rencwabJc energy technologies. JEA Experts indicate that in terms of Thermal Energy Storage (TES) (IEA Annex 14, 2004), cooling is a first priority, followed by combined cooling and heating, and lastly heating. Thus, building cooling for human occupancy and process cooling of industrial or commercial products or processes are the main areas of interest in alternative energy production.

Borehole Thermal Energy Storage (BTES) is one type of Underground Thermal Energy Storage (UTES). Over the last 15 years Borehole Thermal Energy Storage (BTES) energy type systems have been used primarily for seasonal heat storage with heat pump technology. Recent International Energy Agency research has identified an opportunity to improve the thermal efficiency of boreholes tor cold storage. By specifically targeting cooling applications, which are experiencing dramatic growth rates, this technology could be used globally to help electric utilities meet capacity shortages through storage of "free" renewable cooling energy. North America has acutely IcJt the tightening of electricity supply and demand. This proposed technology provides a non-refrigerant, non-ozone depleting, renewable cooling alternative. Cost effective geo-heat transfer is considerably more difficult at low temperatures (below 4° C) for air conditioning. Generally, the larger the storage application is, the more cost effective the storage becomes. To double the capacity of the store only a smaller, incremental increase in the borehole is required.

To store cold only for direct cooling, using Borehole Thermal Energy Storage technology requires more efficient Borehole Heat txchaπgers (BHE) than the single or double U-pipcs that arc normally applied in these types of storage systems. Traditional borehole designs have used either a grouted or water-filled hoJe with a U-tnhe A more cost effective (low thermal resistance) coaxial (concentric) borehole heat exchanger suitable for cooling without chiller machines has been tested in Halifax, Nova Scotia, Canada. Field tests'wcrc conducted to determine whether a borehole's wall could be sealed (Cruickshanks et al. 2006) with bcntonite to make it water tiβht and suitable for a coaxial type borehole heat exchanger ( i.e. no groundwater flux).

Mogensen in Patent TJS 4,715.429. issued in 1987, has noted that several, suitably spaced drill holes are often used to recover or store heat in rock or loose soils. Also, to avoid negatively impacting groundwatcr and to permit the temperature to fall beneath the freezing point of water, it is generally preferred to recover the heat via a heat exchanger or heat collector, arranged in the drill hole, through which a heat transferring medium in the form of a liquid protected against freezing is permitted to flow. Many current conventional dcaigns employ a heat, exchanger in the form of U tube pipe in which both sides consist of polyethylene material. It has also been determined that a considerable amount of the temperature drop from the interior of the rock to the heat-carrying liquid occurs during heat transfer between the wall of the drill hole and the circulating liquid.

The temperature drop between the wall of the drill hole and the heating medium is comprised of three energy factors. The first factor is caused by the thermal resistance of the water between the wall of the drill hole and the exterior of the pipe. Negligible convection has been found in the water given the pipe and borehole dimensions and operating temperatures currently used. Thus, the heat transfer occurs strictly through conduction. It must be noted that water is a poor thermal conductor whose thermal conductance may be improved if heat recovery is continued until the water freezes in the hole. This factor remains prevalent if heat Is supplied to surrounding rock.

The second factor regarding temperature drop is caused by the thermal resistance posed by the pipe wall. Formulae arc available to calculate these values, which decrease with increasing pipe diameter and decreasing wall thickness. The third factor involves the heat transfer resistance between the inside of the pipe wall and the neat transporting medium. This factor mainly depends upon whether laminar or turbulent flow prevails within the heating medium, although pipe

dimensions and surface structure arc also notable influences. Noteworthy, is thai low heat transfer resistance could be achieved by increasing the flow rate.

Employing short heat-conducting stretches in the drill hole water, while using a thin-walled, large diameter pipe, should produce a small temperature drop between the wall of the drill hole and the heat transfer fluid. The space between the pipe and drill hole wall should be as large as possible to enable conventional heat collectors to be fitted. In addition, pipe walls shr>nlH he sufficiently thick to provide satisfactory security against breakage. Previous designs required that clearance between pipe and drill-hole wall should be as large as possible to enable conventional heat collectors to be fitted and the pipe walls made thick, so as to provide security against rupture.

In addition, Mogensen (1987) sought to avoid these numerous drawbacks by employing a method, which produces more efficient heat transfer. Specifically, Mogensen lowered at least one heat-exchanging element into the drill hole, then expanded the heat-exchanging element in a radial direction, such that this element at least partially contacted the defining surface of the drill hole. The expansion was accomplished by inserting a separate spacer between the pipes or by having the popes form a spring arrangement in relation to each other.

λ single conventional U-pipe in a grouted borehole will typically have a thermal resistance that corresponds to a 5-6 ⁰ C temperature difference between the rock and the heat carrier fluid in the U-pipe. A double U-pipe will reduce this loss of temperature quality to 3-4 ⁰ C. As a major advancement, a coaxial pipe or tube will have an optimal thermal efficiency and cut the difference to 1-2 ⁰ C, since it allows the fluid to have direct contact with the entire borehole wall. Efficiency difference determinations by Hclistrom ct al. (1988) proved that die coaxial single tube borehole energy system was noticeably superior to; polyethylene U-tubc, copper U-tube, polyethylene double U-tube, and multiple co-axial tube energy systems.

The requirement for small δTs in cooling applications for UTES (underground thermal energy storage) systems is a prerequisite for direct cooling without heat pumps or chillers, and this is what made cooling with borehole storage uneconomical in the past. The coaxial borehole is a much more thermally efficient borehole heat exchanger than a conventional U-tube borehole. The coaxial borehole system has the ability to operate at small temperature differences (δT), which is

directly i elated to borehole thermal resistance (R _b ). IIcat exchanger efficiency is a function of contact area and the turbulent flow in the system. The larger the contact area and the more turbulent the flow, the more readily the approach temperature for the heat exchanger is achievable and the smaller the δT required, to meet the delivered temperature.

It is important to note that each lime » new φTES configuration is evaluated, other important parameters must also be considered as well. These include, but may not be limited to, the available land space, the thermal load to be stored and storage temperatures, the timing of cold availability, existing temperatures in the subsurface, building operating temperatures, and of course the thermal efficiency of the BHE. All are taken together in optimizing the final energy storage configuration. In addition, one should consider the availability of drilling rigs and operators who will be available and capable of drilling to the depth required, as this will improve the competitive nature of the tendering process and help ensure the success of the project. Thug, even though the thermal conductivity of the bedrock or soil is the single most important parameter required in dimensioning a RTRS system, it is but one of a number of parameters being considered in the overall BTES design process.

When considering decreasing the depth of the Borehole Heat Exchangers, one must then give more consideration to the effect of surface water recharge and subsequent temperature inputs over the summer months. The temperature inputs from the surface, which fluctuate widely throughout the year, arc generally not considered useful. Therefore, the preference is to have a lesser amount of BHE in this active zone. Stable temperatures, which tend to be at greater depths, are more easily managed from both a thermal and cost of drilling perspective. With more BHE in ϋic more stable temperature zone, operators arc better able to use the average mean temperature of the buieliule as a iefeience fui giυund tenipeiaturc. As a consequence to drilling the DHL to shallower depths, more well heads would be required to achieve the required storage volume. λ larger surface area would be required to accommodate the storage and more insulation would be needed to control surface water related temperature inputs from surface W3ter recharge.

The boundary conditions for Borehole Thermal Energy Storage are few, with the major condition being the temperature requirements. The system can be employed anywhere including in uπconsolidated (loose) rock formations or soils (overburden) through the use of a stiff liner to stabilize the wall of the concentric borehole. The coaxial BTES is the only system than can take advantage of the low temperature difference values (0.5C to 8C degrees). For coaxial heat

exchangers with an unlincd borehole, 30ft sedimentary rocks are excluded. Crystalline rocks are the most desirable geological materials. Therefore, geologically suitable locations for coaxial boreholes include much of the eastern and western coasts of Canada and the United States. The interior plains or large sedimentary basins in Canada and the United States are generally excluded from development of the coaxial borehole designs. Similar geologic regimes around the globe are also suitable for deployment of the coaxial borehole technology.

Borehole storage technology has also become more technically feasible due to the development of an Underground Thermal Energy Storage installation standard (CSA C448.3-02), which a I M> includes procedures for the Environmental Impact Assessment of ground 3torage projects. The Standard and the EIA procedures were outputs from Annex 8 IEA, 2005) and Annex 6 (IEA. 1996) of the Energy Storage Implementing Agreements (IEA), respectively. The Thermal Response Test, the CSA Standard and the ElA procedures provide contractors with a needed measure of due diligence regarding legal liabilities.

The two main types of applications for cooling in Canada, which together consume a significant proportion of total energy and impose large demands on energy production and distribution systems are; cooling of buildings for human occupancy (building cooling), and cooling of industrial or commercial products or processes (process cooling). The great majority of the cooiing equipment installed 10 handle these new demands use electricity purchased from utility grids. This factor has contributed to growth of summer peak demands such that in some locations like Southern Ontario, the summer peaks now rival the traditional winter peak demand for electricity. However, the short summer cooling period means that cooling systems and generating capacity to serve this load exhibit poor load factors (ratio of energy consumed to standby energy required).

Many new houses obtain cooling and most of their healing requirements from air-source heat pumps, with backup heat for the coldest periods of the year obtained from resistance heaters. The availability of summer cooling using these electric heat pumps has switched the traditional heating energy source from fossil fuels to electricity, with the associated impacts on utility's winter peak demands and resulting poor capacity factors caused by using resistance heating at low temperatures. Fn order to avoid the need for extra generating capacity, which is only required during short periods of the year, electric utilities are beginning to develop incentives for shifting cooling load to

off-peak periods ming cool storage.

Cooling Applications were previously classified into the general areas of building air conditioning and process cooling. Within building air conditioning, further segregation is based upon building function and size. Within process cooling, a further distinction is made between high- temperalure and tow-temperature cooling, and by size of cooling load. An appropriate opportunity for applying a cool storage system to meet a cooling load exists when: (a) a cool storage system is technically feasible for a given site, (b) a cool storage system would reduce electricity costs through demand and/or energy use charges significantly enough to pay for itself, (c) the economic benefit of the cool storage system would go to the party making the investment in installing the cool storage system, and (d) the appropriate representative of the investing parry is aware that cool storage is potentially a worthwhile investment.

Technical feasibility is an issue in many potential applications of cool storage. This relates to the fact that most cool storage systems rely on a circulating liquid to charge and discharge the storage. Two potential cooling storage applications that do not normally use a circulating liquid are:(a) air conditioning of single-family residences and separately-metcrcd apartments, and (b) air-conditioning of .small commercial huildings. Current cool storage systems are most easily applied to cooling systems using a liquid circulating medium, which are pertinent to a large electric customer who can save a large amount of money on demand and time-of-use electricity rales by reducing total amounts of purchased energy required for cooling. These are large building and process cooling applications.

Considering the economics of cool storage systems in the remaining technically-feasible applications, they are best suited for those which operate under large demand charges, those which operate under both Time-Differentiated Rates" and demand charges, and those which have large cooling loads that peak simultaneously with other electrical loads. Short-term cool storage systems should provide a reduction in electrical costs large enough to provide a reasonable payback in these situations. For longer-term storage systems, even larger cooling loads are required to be shifted in order to provide acceptable return on tho higher capital investment costs.

For instance, due to the utility rate structure, nearly all commercial and institutional buildings tn Ontario which have a central heating and cooling plant using water distribution will find some form of cool storage economic. Some Ontario utilities currently do noi charge for peak demands set during

the off-peak part of the day, end typical rates for off-peak electricity are less than half that of or\-peak. Similarly, Ontario industries which use electrically-produced process cooling during peek periods will find thai cool storage systems provide attractive payback periods. Those that get "free" process cooling through water or air cooling systems will not find the economics as attractive

In other provinces in Canada, the minimum size, variability, and phasing of the peak cooling loads with other electrical demands will determine the economics of cool storage. Applications in provinces which have high demand charges will represent better cool storage economics. Seasonal cool storage will also be more attractive in other provinces because the building cooling season is shorter and exhibits more peaks than in Ontario. From the viewpoint of process cooling applications, many industries in the Prairie Provinces already use groundwater to meet process cooling needs.

There is also a distinction to be made between existing buildings and new buildings. In a new building, the opportunity exists to further enhance the economics of cool storage by designing the systems to better match the temperature ranges. In some instances, this can result in lower first costs for thermal storage, thus eliminating the problem of owner resistance to capital investment Thus, the stock of large buildings in Canada arc prime candidates for cool storage systems.

The applicability of cool storage for process cooling is more related to the specific process. The owner of the process equipment is nearly always the same party that pays for the energy consumed, however questions of reliability and maintainability arc more important in process cooling, where failure may mean loss of production. The extra cooling backup capacity of the storage system may be a significant benefit.

DESCRIPTION OF PRIOR ART

Applicable Canadian Patents -

CAl 187480 May 1985 Binet eUl.

Applicable USA and Overseas Patents -

US4, 157,730 June 1979 Despois et al.

US4,248,305 February 1981 Scarborough et al.

US4,327,J<50 May 1982 Leon

US4.375.831 March 1983 Downing Jr.

I JS4,SO7,9?5 April 1985 Schaetzle et al.

US4.538.418 September 1985 Lawrence et al.

US-4,633.676 January 1987 Dittoll

US4644750 February 1987 Lockett

US4.741 388 May 1988 KuroiwB

US4867229 September 1989 Mogenseπ

US4,936,J I 0 June 1990 Kuckens

US5,467,812 November 1995 Dean el al.

US5,65 J .265 Jury 1997 Oreπier

U95,678.626 October 1997 Gillcs

US5,937,934 August 1999 Hildebrand

US6.450.2.A7 September 7.002 Raff

US6,688,129 February 2004 Ace

US7, 124,594 October 2006 McRdI

US6,994, 156 February 2006 K.opko

GB 1326458 September 1973 Aladiev

GB2058334 April 1981 Feisi

JP56053388 May 1981 Sebasuchiyan

BES93570 November 1982 Boiickacrt

JP6016214J August 1985 Matsuό

SE860 I 086 September 1986 Soron

JP5039986 February 1993 Hamada

DE4423702 January 1996 Wetzel-Horst

JPl 0300266 November 1998 Ytunamulυ

JP10274444 October 1998 Kurojwa

JP1 1 1 42076 May 1999 lnada

JP1 1 182943 July 1999 Uchikawa

CN2432495 May 200! Li

CN 1292478 April 2001 Zhang

JP2003 I 30471 May 2003 Morita et al

JP2003 I30494 May 2003 Aiga

JP2003240358 August 2003 Matsumoto

JP2003262430 September 2003 Ikeuchi

JP2003302108 October 2003 Suzawa

JP2003301434 October 2003 Suzawa

JP2003307352 October 2003 Suzawa

JP2O033O73S3 November 2003 Suzawa

CN 1542357 November 2004 Ma

JP2005233527 September 2005 Endo

JP2005003272 January 200S Sasaki

JP2005048972 February 2005 Saeki

SE860 I 086 September 2006 Soron

JP2006071258 March 2006 Morita

Several prior art devices have utilized the earth as a source of heat and as a heat sink. One example is the geothermal heat pump having the working fluid from the pump flow through tubes that are buried several feet below the ground. The heat pump can act as either a heater or an air

conditioner, thus the fluid flowing through the pipes υaca the surrounding earth as both a heat source and heat sink.

US Pat. No. 5,738,16-1 (Hildcbrand), issued in 1998, discloses a system for energy exchange between the earth and an energy exchanger. The device effects energy exchange between earth soil and an energy exchanger. The device is comprised of a soil exchanger and supply and return flow conduits for connecting the soil exchanger with the energy exchanger. The soil heat exchanger includes a thermo-insulated supply pipe arranged in a bore well drilled in the ground, with a pump provided at the end of the flow duct, and a shroud pipe surrounding the flow duct and the pump. The system also includes lateral inlet openings and a return flow pipe. A section of the shroud functions as a thermo-pipe and the system can reach a depth of 800 meters. A thermopile is designated as a thermal insulated section formed with a correspondingly greater wall thickness.

This prior ait does not disclose a system for using the ground as a heat sink without a compressor.

Storage and retrieval of underground energy, both heat and cold from aquifers or groundwater circulating sygt*rns, some involving dual wells, ht>3 been the subject of several inventions. Some employ a form of heat exchange either in the ground or above ground to provide air conditioning.

Patent No. JP2000027177 (Sakai and Mikota), issued in 2000, describes forming an artificial aquifer underground to prevent groundwater flow designed with continuous walls reaching an impermeable layer forming an aquifer and an underground warni layer and an underground cold layer at the aquifer. In winter time groundwater pumped-up through a hot water well is utilized as a thermal source and when cooled is returned to the well. In summertime, grourutwater pumped-up through a cold well is utilized as a cooling source for facilities, then when warmed it is also reinjected.

Patent No. CN 1542357 (Ma), issued in 2004. relates a groundwater energy-storage system in which, return air from an air-conditioned room is twice cooled or heated in Me first return air heat exchanger with cool or heat from the energy storing underground water system and the secondary

return air heat exchanger with cool or heat from the refrigerators. Fresh air from outdoors is also mixed with return air in the fresh air heat exchanger before being fed into the system.

Patent No. JP2005233527 (Endo and Mari). issued in 2005, describes groundwater being used as a source of cold heat in summer and hot heat in winter to an underground space near the groundwater source. The method is designed to save power by installing an air-conditioner and heat exchanger in an underground space to accept grovndwater via piping, such that the groundwater is used as a cold source in the summer and as a heat source in the winter.

The abovementioned disclosures are functional, but noticeably more complex, often more expensive and less efficient than the new single coaxial borehole invention. They emphasize extracting heat from underground water sources, which is less efficient that extraction of energy from bedrock and there is no provision for energy storage.

The next area of development has involved various approaches for storing heat underground and circulating heated water from underground earth, rock, gravel or other sources through boreholes to the surface, often through a below or above ground heat exchanger and/or heat pump, for use in heating and cooling applications.

Patent No. CN 1074018 (Xing), issued in 1993, describes heating water to 40-60C using solar energy or a heat exchanger, then pumping the water to an underground stratum using a reversible submersible pump. The heated water may be pumped-up for industrial heating applications. Cold water may be pumped-up in summer as a resource for air-conditioning.

Potent JP10274444 (Kuroiwa), issued in 1998, describes a. method of storing a large amount of intermittent, natural energy for long-term usage. The energy is supplied continuously as a heat source for hot water supplies and in air-conditioning applications and for refrigeration/cold storage. The system employs an underground heat exchanger, a high temperature heat source heat accumulating body and a low-temperature heat source heat reservoir, and a heat medium circulation line for connecting the underground heat exchanger and the heat source heat reservoirs with an improved heat conductive material around the underground heat exchanger. The heat reservoir continuously exchanges heat with the ground and intermittently exchanges heat with the heat

exchanger, thus forming high temperature thermal storage from Spring to Autumn and low temperature thermal storage from Autumn to Spring.

Paicnt. JP20032fi2430 (Ikeuchϊ), issued in 2003, describes a heat pump having inlet and outlet pipes, provided to take out underground heat at high efficiency of heat exchange, by improving upon an underground U-tube type heat exchanger. The heat purnp is thermally coupled to the heat exchanger of a heat using facility above ground. Underground heat is transferred through the heat exchanger of the ground facility, while being held at medium temperatures, without being affected by the high or low temperature heat carrier flowing down the inlet pipes, which results in a greatly increased efficiency of heat exchange.

Patent No. JP2005048972 (Saeki), issued in 2005, describes an underground heat utilising air-conditioning system, which can recover the heat collecting and heat releasing capabilities of an underground heat exchanger. The method couples an underground heat exchanger and a heat pump with the space to be air-conditioned and reveracs the operation to cooling/heating by coupling the heat pump and heat exchanger with the atmosphere.

The proceeding energy exchange system disclosures require use of heat pumps, separate heat exchangers inside or outside the borehole and employment of U-tubes. All these approaches are less efficient and more costly approaches than the invention.

The following disclosures relate to coaxial collectors mostly comprised of two pipes in a borehole for the purpose of withdrawing heat and passing it through a heat exchanger and in some cases a heat pump, for various surface applications.

Patent JP I 1 182942 (Uchikawa), issued in 1999, describes a heat transfer pipe for ground heat exchange with a heat medium. The heat medium passes from ground surface through two adjoining pipes and returns via two other adjoining pipes. The pipes are enclosed by a borehole.

Patent US6,450,247 (Raff), issued in 2002, describes a well drilled deep into the ground and encased and sealed at the bottom to prevent water loss and to provide heat storage. Heat conduction occurs through the casing m contact with the surrounding earth. A pipe attached to a pump at its end is placed in the well to draw cold water from the well into a heat exchanger, where it absorbs heat

and cools the air to cool 21 domicile. Exchanged water is returned to the well. Heat accumulated during summer cooling months is dissipated through heat pipes in winter.

Parent .IP2002013828 (Sakai), issued in 2002, describes using an underground heat exchanger designed to improve thermal conductivity compared to conventional coaxial systems. It consists of an inner cylinder and outer cylinder for support and provision of a casing function. The finned outer cylinder is installed in a hole excavated in the ground. A heating medium is caused to pass through a space between the outer and inner cylinders downwards and then through the inner cylinder upwards. Heat exchange occurs between the heat medium and the ground, while the lower end of the outer cylinder is closed.

Patent JP2003307353 (Suzawa), issued in 2003, discloses a device for storing underground heat and utilizing heat on the earths surface. It consists of an inner cylinder that is coaxially inserted and fitted inside an outer cylinder. The lower end of the outer cylinder is closed to form a heat exchanger with passage between die two tubes. λn outside borehole is drilled into deep rotk and the heat exchanger is inserted into the vertical hole. Silica sand is placed between the vertical hole and the heat exchanger. The upper end of the inner cylinder is connected to an inlet or outlet of the heat application.

These approaches are useful, but require antifreeze and an above or below ground heat exchanger to provide cold energy for cooling applications and their efficiencies are lower fh;in the invention. The invention provides easier and less expensive installation than these approaches since by using the borehole as a heat exchange surface it does not require a separate heat exchanger and only requires one additional pipe, to efficiently move fluid in and out of the borehole. Fn addition, since the invention only moves cold fluid, heat pumps are also not required. Also, the invention employs a less expensive fan coil system to produce heat from cold energy rather than a heat pump.

Patent No. JPl 1 142076 (Inada), issued in 1999, discloses simultaneous storage of cold heat and heat in one underground heat storage region by forming a cavity suiruuπdcd by a wall face in underground base rock and partitioning the cavity into two sections by a heat shielding partition wall. A heat shiclding.cαver prevents thermal diffusion from the cavity to the longitudinal hole. This disclosure is not particularly reliable at prohibiting energy losses and it would be difficult to maintain cold at the consistent levels achieved by the new coaxial borehole design.

Patent No. US6450247 (Raff), issued in 2002, describes an air conditioning system utilizing earth cooling employing a well drilled deep into ihc ground and filled with water. The wεil is cased and sealed to prevent loss of water. A pipe p'accd in the well draws cold water from within the well into a heat exchanger, where it cools the air, which in turn cools a domicile. Water that has released cold is returned to the well. Heat that is accumulated during summer months is dissipated during winter months. Heat pipes extending outwards from the top of the well contain a substance to absorb heat that evaporates at the end in the well and condenses to release heat at the opposite end. This device requires heat pipes, a heat exchanger above ground and a heat absorbing substance to provide cold water, which greatly increases the expense and complexity of the system as compared to the new invention.

Patent No. JF2005003272 (Sasaki), issued in 2005, describes a rock underground storage space, such as an underground quarry site, linked to the surface by digging a horizontal or vertical well in the peripheral rock. Heat transfer U-shapcd pipes are connected in scries or parallel in the well to accumulated cold heal in Il it luck. λ licaL pump arid heal exchanger arc maintained in Lhc storage space. The spatial temperature of the underground storage space is controlled by the rock and heat pump. This device exhibits higher installation and operational costs by employing a heat pump, and U-shaped tubes, which are also less efficient than a coaxial borehole system, and this approach does not take advantage of storage duration to provide cold energy at peak demand times.

Patent JP200fi084097 (Hamahiro), i^rnrd in 7006. discloses a device composed of a U-tube type storage pipe and air circulating pipe installed in the water storage pipe buried in the ground to at least 5 meters. This device uses a less efficient U-tube system and due to the minimal depth of exposure is less efficient than deeply placed borehole energy exchangers.

Patent CN 1546926 (Gao), issued in 20U6, describes a method lor using one underground heat exchanger system to produce alternate heating and cooling. The heat exchanger is comprised of an outer pipe and a spiral core pipe in the outer pipe. The heat exchange media enters the underground from die υuicr pipe and flows out from the inner pipe during cooling. Heat exchange efficiency is upgraded. This disclosure exhibits the extra cost of two pipes as a heat exchanger in a borehole compared to the new coaxial borehole design and there is no element of storage to better meet the cooling demand of peak periods. The invention is less costly to install and operate and is more efficient than this approach due to employing the borehole as the heat exchange medium.

None of the prior inventions mentioned have explain their consideration of energy transfer surfaces and the influence on thermal efficiencies, which is the basis of cost-effective cold energy storage

SUMMARY OF THE INVENTION

The invention pertains to a coaxial heat exchanger including casing inio bedrock and an inlet pipe that passes through the center of a borehole cap. A pipe centered in the borehole and attached to the inside of the borehole cap has grooves along its length and extends to near the bottom of the borehole. An outlet pipe perforates the side of the borehole near the top of the borehole and is aUuulitd lυ ilie bυrehyle close Iu lhc borehole cap.

OBJECTS OF THE INVENTION

The main object of the invention is to improve the thermal efficiency of borehole heat exchanger systems through reduction of borehole thermal resistance (R _b ) by permitting water to have direct contact with the entire borehole wall.

A second object of the invention is to achieve reduction in pumping energy by the use of larger contact areas, which decreases the flow rates for the same amount of energy transferred.

A third object of the invention is to achieve a reduclioπ in energy charging time. This is accomplished by providing for the even distribution of warm water through distribution holes along the length of the coaxial heat exchanger. This approach changes the shape of the thermal plume from a teardrop shape along a portion of the heat exchanger, commonly found with conventional botcholii heal exchangers, tυ υπc that is more uniform along the entire length of the coaxial borehole heat exchanger.

λ fourth object of the invention i3 to employ a thermal response test (TRTl to determine thermal conductivity of the bedrock by employing major subsurface parameters such as; ground and/or groundwater temperatures, bedrock structure, groundwater flow and groundwater levels.

DETAILED DESCRfPTION OF THE PREFFERED EMBODIMENTS

The current technology underlying the invention is an application of seasonal Underground Thermal Energy Storage (UTES) technology, storing renewable cold energy from sυrfar.p water, lake, river or ocean water, or cooling tower water for air conditioning. The coaxial energy storage system utilizes a new coaxial borehole heat exchanger design that dramatically reduces the required ςiT-e of the borehole field and enables cold storage for direct cooling, (i.e. without the use of heat pumps or chillers). Drilling costs of the borehole portion of energy storage systems may be reduced by 2/3 ^rd<c with the use of the coaxial energy storage and cold transfer system.

The invention is the breakthrough in borehole design required for cooling applications- This new coaxial borehole system offers three significant advantages as it (J ) rednres the borehole thermal resistance, (2) increases the effective thermal transfer surface area, resulting in increased thermal flow rates over current designs, and (3) produces higher volumetric flow rates, resulting in higher thermal transfer rates with minimum pumping losses. Collectively, the overall design increases borehole efficiency by 300 %.

Effective heat transfer is considerably more difficult at low temperatures (below 4° C), and direct cooling is not practical with current U-tube borehole designs. This is due to the high frictional heat gains by the working fluid during cold charging and cold discharging. This new coaxial energy storage design allows for direct charging and discharging of cold energy for air conditioning with low frictional heat gains and without the use of supplemental heat pumps or chillers.

Other advantages of this new coaxial energy storage approach arc: (a) with this new heal, exchanger design being 300% more efficient than current designs, the surface area required for the storage is significantly reduced, therefore, energy storage systems can fit in a confined space and arc more adaptable in serving existing buildings, (b) water without antifreeze can be used as the heat carrier, such that the systems capacity is increased and the pumping costs decreased, (c) smaller diameter boreholes can be drilled to greater depths, which will reduce drilling costs and (d) the. boreholes ran be sealed with bentonite grout to make them water tight and suitable for coaxial type BHEs (i.e. eliminates groundwater flux to move cold water of site). In addition, the adoption of this technology will directly reduce greenhouse gas emissions from energy use and air polluting

emissions from refrigerants. The technology^ primary focus is reducing energy consumption and its associated greenhouse gas emissions. The improved system could also be considered an enabling technology for application of other renewable energy sources such as cold harvested from lakes and ambient air.

Under the auspices of the Panel on Energy Research and Development (PERD) and under the International Energy Agency-Implementing Agreement on Energy Storage, Environment Canada with technical assistance from High Performance Energy Systems of Halifax Nova Scotia, Canada have led the development and use of tools and procedures for implementing and optimizing Underground Thermal Energy Storage systems generally. This has included developing and testing of the high performance coaxial borehole heat exchanger.

The proof of concept for this new coaxial borehole design was tested in October 2004. It was demonstrated that the introduction of the Coaxial Energy Storage technology could leverage the BTES concept by improving the thermal efficiency and expand the applications for direct cooling. This new application enables economic storage of cold energy in the ground during the winter season to be used for direct cooling during the summer season.

A field test to determine the in-situ thermal properties of boreholes and rock formations (Thermal Response Test (TRT)) was developed in collaboration with Annex 8 (2005) of the Energy Storage Implementing Agreement. A TRT test is an on-site testing procedure performed to determine thermal conductivity (K/(W/m)) of the bedrock, by accounting for all subsurface parameters, including but not limited to, ground and/or groundwater temperatures, bedrock structure, gmundwater flow and grnnndwater levels. The Thermal Response Test was instrumental in development of this new coaxial borehole heat exchanger, in that the thermal efficiency of this and other borehole heat exchangers can be measured and compared. Therefore we now know empirically that the coaxial heat exchanger is feasible and potentially far superior to the standard U- tube heat exchanger. In addition, the TRT significantly reduces the potential for over-designing BTES systems by providing an in-situ thermal conductivity value, the mosi singly important value for determining the number, depth and spacing of boreholes in an underground thermal storage system.

Coaxial BTES Icdmϋlϋgy provides an economic opportunity to better match energy supply and demand and will enable use of renewable energy for cooling. The use of renewable cooling energy available from lakes, rivers, seawater, and winter air can significantly reduce the cost of utilities. The development of the coaxial borehole is the conversion efficiency breakthrough allowing for the economical implementation of cold thermal storage systems, and more cost effective heat storage applications as well. This is considered a major advancement over the existing geoexchange products in the marketplace. This technology can initially be considered as an advanced form of geoexchangc system and a product that can replace conventional HVAC systems.

Conventional U-tube drilling nςes a \ S7 mm (6 inch) diameter borehole, whereas this new coaxial energy storage borehole system uses a M 5 mm (4.5 inch diameter) to 152mm borehole. In drilling a 1 15 mm diameter borehole as compared to a 152 mm diameter borehole there is a 25 % savings in fuel due to less drilling resistance. It takes about 2,000 litres of diesel fuel to drill a J 52 mm diameter borehole to a depth of 150 m. by using a 1 15 mm borehole fuel savings of 500 litres per borehole (or 25 %) can be achieved. In addition, there would be 42 % less drilling consumables (water and foam for example) used and 42 % less drill cuttings by volume produced. The exact amount of fuel savings and consumables used depends on the local conditions.

The coaxial borehole approach is a much more thermally efficient borehole heat exchanger than a conventional U-tube borehole. Therefore, further savings with respect to drilling costs are achieved by the simple fact that fewer boreholes are required. More specifically, the cost savings for BTES cooling is based upon the ability of the system to operate at small temperature differences (δT), which is directly related to borehole thermal resistance (R _h ).

The invention can store cold energy and operate at temperatures ranging from approximately 0 5 "C to R ⁰ C. (AT nf 7.5 ⁰ C), which i<? an ideal range for 'direct ¹ (i.e. no heat pump) cooling. A conventional U-tube system would have to store and operate at temperature ranges below freezing to accomplish the same task, due to higher borehole thermal resistances (R _h ). Therefore, to provide the same level of cooling, conventional U-tube type BTES designs require heat pumps and antifreeze protected ground loops, whereas the new BTES approach eliminates the need for heat pumps and antifreeze.

Moreover, llic issue of operating temperature differences (δT) in direct BTES heating and cooling applications, and how this relates to drilling lengths, requires further explanation. An inefficient BHF. gives up at least 5 to 6 ⁰ C in the process of transferring energy to and from the borehole store. λ heat gain of 5 to 6 ⁰ C in a storage 3ystem operating with a δT of 7.5 ⁰ C would eliminate the direct cooling potential of lhc BTES system.

Jn the case of high δT's, as in a high temperature BTIiS heat store (e.g. the Drake Landing thermal solar project in Okotoks, AB, Canada) the typical operating range is from 40 ⁰ C to about 80 ⁰ C. Thi3 gives me to α δT of about 40 ⁰ C in 'direct' heating mode. Adding 5 to 6 ⁰ C to a δT of about 40 ⁰ C would noc adversely affect the operational functioning of the thermal store as adjustments can easily be made at these temperature levels. Therefore, the thermal resistance of a borehole heat exchanger is less critical in heating applications with large δT's, and conversely is far more critical in cooling applications with small δT's. Based on field measurements, the coaxial system's R _b (<0.005 K/ (W/m) (Cruickshanks et al., 2006) is much lower than the U-tube κ _h (0.2 K/ CWhn) (HellstrOm ct al., J 988) allowing it to handle even peak cooling loads.

Again, regarding drilling length, another issue with U-tube heat exchangers, especially in cooling applications, is the very small heat transfer contact area on the borehole wall. Essentially, the heat transfer fluid circulating in the U-tube has only a tangential contact area with the borehole wall at best, if properly installed. However, the coaxial/concentric BHE provides full contact of the heat transfer fluid with the borehole wall in the annular space between the centre tube and the υuUside borehole wall, thereby reducing tile borehole thermal resistance (R _h ) of the borehole compared to the conventional U-tube configuration. This greatly increases the thermal energy transfer to and from the rock store, subsequently reducing the total drilling length required.

The Earth Energy Designer (EED) modeling software, an industry standard, employed for illustrative purposes to conduct sensitivity analyses, has shown that thermal efficiency can decrease or increase the amount of drilling required. EED accepts measured R _b values or it can calculate Rt values based on standard installation practices. The calculated R _b values of 0.2 K/(W/m) for the U- tube system and 0.02 KV(VvVm) for the concentric system were used. Since the in-silu (measured) R _h value of 0.005 K/(W/m) is an order of magnitude less than the calculated value, EED tends to penalize the concentric system in terms of drilling length required. Even with this factor considered, the new BTES cooling system would require significantly less total drilling length as compared to a U-iube type system.

The analysis showed that a U-tube BTES system comprising 1(52 BU x 150 m deep X B in spacing would take at least 10 yean to achieve suitable cooling temperatures, whereas a concentric BTES system of the same depth and BH spacing achieves the same cooling temperatures in less than 3 years. Note that these timeframej are strictly illustrative examples. There would also be a need for manv more boreholes, with the U-tube approach, as merely drilling deeper would actually cause cooling capacity to deteriorate due to the effect of the geothermal gradient (i.e. 15 °C/km or 1.5 °C/100 m) There should be no signifiant energy losses from the cold store, only moderate anticipated energy losses. A preliminary calculation indicates that the losses will be in the order of 14 % for a cold storage with a depth of 150 m x 64 m ² square. It is standard procedure to apply an insulating cap to reduce thermal losses. Since this is a renewable energy application with the cold source coming from the natural environment, make-up cold is readily available.

The radiogenic heat flow in Megurna bedrock is very low (1.6 μ W/m ³ ) and will not adversely add io neat gain in the cold store. In terms Of the geothermal gradient, the high value of 15°C/km is used in calculations for the cold store, instead of the lower value of 13 °C/km. In addition, all of the information provided here is predicated on the use of conservative data and assumptions, from average storage temperature availability, using the depth of the cold source from prior research, and even including the thermal performance of the boreholes.

No thermal properties have been assumed in testing the new coaxial borehole system, but rather, at least one Thermal Response Test (TRT) was conducted for each test conducted, depending on the size of test (thermal loads, physical footprint) and the type or geology. A TRT system and procedures have been developed in concert with Environment Canada under the auspices of an International Energy Agency, Implementing Agreement on Energy Conservation through Energy Storage (IEA-I A-ECES). This effort was carried out under Annex 8 - 'Implementing Underground Thermal Energy Storage Systems', of the Energy Storage agreement. In hard rock, there is reliable geological stability. Smaller diameter boreholes are more stable than larger diameter boreholes because less rock pressure is released. In unconsolidated (loose) rock formations or soils (overburden), a stiff finer would be used to stabilize the wall of the concentric BHE.

The major improvements attributable to the invention include; increased efficiency of cold energy transfer and storage, savings in materials, construction and operational costs, and optimization of ihe energy transfer process for borehole energy exchange. This is achieved by the novel approach of employing the borehole as the heat exchanger, thus requiring no individual heat

exchanger above ground with affixed pipes. In addition, the invention requires no heating pipes and no heat pump. As a general rule, the energy exchanger referred to herein, will be constructed for dissipation of energy (i.e. or cooling purposes). However, the energy exchanger may also be used for producing heat when attached to a fan coil, such that the installation can be used for heating.

As required, detailed embodiments of the present invention are disclosed herein, however, it is understood thai the disclosed embodiments arc merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting.

With reference to the drawings and, in particular, with reference to Fig1 and Fig.2, the borehole heat exchanger system comprises; an inlet pipe (1 ) from the application to the 90 degree angle fitting (2) a borehole casing cap (3) to which the inlet pipe (1) and fitting (2) are attached, and which caps the borehole (4), a steel casing (5) which fits inside the borehole (4) and tightly abuts the borehole casing cap (3), a central tube (6) fabricated of steel, or other suitable material, with designed steel mesh (7) on the outer surface, industrial bentonite filler (8) infilling the borehole (1), spacers (9) between the borehole (4) and ventral tube ( 6), heat exchange surfaces (10) as part of the borehole surface (4) and central tube surface (6), welded nipple attachment (1 1) between borehole steel casing (5) and outlet pipe (12).

Referring to Fig. 1 and said parts (1 ). (2),(3) and (6), the 50 to 75mm inlet pipe (1) connects to a right angled fitting (2) on the inlet end, while the fitting (2) connects to a nipple projecting from the borehole casing cap (3). The fitting (2) permits heated inlet water to enter the borehole (4) through the borehole casing cap (3) and into the central pipe (6). The inlet pipe (1), right angled fitting (2) and nipple projecting from the borehole casing cap (3) may be threaded or welded together, or a combination of threading and welding. Referring to Fig. 1 and said parts (3) and (5), the central pipe (5) is welded to the borehole casing cap (3) at the inlet to the central pipe (6) Io prevent leakage of water, which could reduce pressure in the system.

Referring to Fig.1 and said parts (4), (5) and (6), the borehole casing (5) is placed within the borehole (4) and the borehole casing (5) thickness is dependent upon the integrity of the overburden and bedrock. The borehole casing (5) extends to within 150mm of the bottom of the borehole (4), thus leaving a space at the bottom of the scaled borehole (4) that permits the inlet water to flow

from the central pipe (6) to strike the bottom of the borehole (4) and return upwards along all sides of the central pipe (6) and the total cross sectional area of the borehole casing (5).

Referring to Fig.1 and said parts (6) and (7), the steel mesh (7) welded onto the surface of the central steel pipe (6) at pre-determined intervals along and around the length of the pipe creates turbulent flow in the length of the borehole (4) between the central pipe (6) and the borehole casing (5). This increase in turbulent flow throughout the borehole (4) system leads to lowering the resistance to energy transfer, which noticeably increases the efficiency of the coaxial borehole system.

Referring to Fig.1 and said parts (10a,b) and (4), as the heat exchange surfaces of the borehole (10a) and central pipe ( 10b), the effective thermal transfer surface area is dramatically increased over conventional borehole heat exchangers by using the new design described by the invention. The resulting increased thermal flow rates greatly reduce the thermal borehole resistance (R _b ) of the borehole (4) compared to the conventional U -tube configured systems.

Referring to Fig.1 and said parts ( 10a,b) and ( 12), the energy exchange from the incoming heated water to the cold borehole surface (10a) and cold central pipe surface (10b) provides the cold energy, which eventually exits the borehole (4) through the exit pipe (12).

Referring to Fig.1 and said parts (5) and (12), the exit pipe of dimension 50 to 75 mm is welded or threaded to a nipple that projects from the borehole casing (5) and is the of similar dimension as the outlet pipe ( 12) to permit the outlet pipe ( 12) to be threaded or welded to the projection from borehole casing (5). This connection permits the heat transfer fluid (water) to exit the borehole (4) and borehole casing (S) and flow to the desired application.

It is to be understood that the foregoing description and specific embodiments are merely illustrative of the best mode of the invention and the principles thereof, and that various modifications and additions may be made to the apparatus by those skilled in the an, without departure from the spirit and scope of this invention.

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EXAMPLE OF USAGE. OF THE INVENTION

An example of employing the invention is to provide periodic cooling to an apartment building.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig.l is a frontal plan view of the coaxial borehoJe design and connecting elements of the heat exchanger system. The diagram indicates that heated water from the source flows down the center of Uic borehole and returns along the sides of the borehole to the outlet and then to the application.

Fig. 2 is a plan view (cross-section) of the coaxiaJ borehole design displaying the dimensions of the borehole, borehole casing and central pipe.