Patent Application
20240240868
Conventional air-based air conditioning systems enable cooling of a building interior. A heat pump system merely reverses the cycle to allow for both heating and cooling. Whereas an air-based heat pump exchanges heat with the ambient air, a ground-source heat pump (GSHP) relies on a “ground loop” to exchange heat between a fluid circulating in one or more pipes and the surrounding media, which in this case is the ground.
For GSHP and similar systems, refrigerant is not usually sent directly into the heat exchange loop. Rather, the refrigerant, whether gas or liquid, is routed through a liquid-to-liquid heat exchanger to heat or cool a liquid that is in turn circulated through a heat exchange loop. Simply put, a heat exchange loop serves the purpose of heating or cooling as needed to bring the “entering water temperature” (EWT) within the operating range of the heat pump. The “leaving water temperature” (LWT) differs from EWT to the extent that heat is exchanged as the fluid circulates through the loop. LWT is always between EWT and the surrounding media temperature.
In winter, a heat exchange loop circulating fluid such as a water-antifreeze mixture absorbs heat from the ground and transfers it to the heat pump via the liquid-to-liquid heat exchanger. The interior condenser then transfers heat to the building interior. In effect, the ground-based heat exchange loop acts as the evaporator. In summer, the function is reversed with the ground-based heat exchange loop fulfilling the role of condenser while heat is rejected from circulating fluid in a pipe to the surrounding media.
Equations are useful for estimating a) the projected rate per unit area at which heat is exchanged between fluid circulated in the pipes and the media surrounding the pipes; and b) the amount of surface area required to be in contact with the media in order to exchange a targeted amount of heat.
While heat exchange loops can be placed horizontally, for ground-based heat exchange loops in particular a vertical installation does a much better job of tapping into the “constant temperature zone” found 10-20 feet below the surface. The depth at which the temperature is approximately constant year-round depends on the ground materials properties and the latitude. Essentially, heat from the Sun is absorbed at the surface during summer days and diffuses downward. In winter, the ground still absorbs some energy from the Sun, but also loses heat to the cooler air. Roughly, at three diffusion lengths or more, such annual variations due to sunlight and air temperature variations have no effect. For example, with a diffusion length of 5 feet, ground temperature hardly varies throughout the year at a depth beginning at 15 feet and extending to several hundred feet.
Vertical GSHP loops today are typically formed by first drilling a borehole to 150-200 foot depth using apparatuses and methods typical of water well drilling. Next, one or more U-tube loops are placed into the borehole, followed by filling the remaining volume with thermal conductivity material. Roughly, a single U-tube loop installed in a 200 foot deep borehole can exchange energy equivalent to one ton (12,000 BTU) of heating/cooling. The average residential requirement of 3-5 tons can generally be met with 600-1,000 feet of borehole, perhaps divided into 3-5 separate boreholes, each populated with one or more loops. These are rough estimates and depend strongly on the ground properties.
Multiple U-tube loops are connected in serial/parallel combination before routing to the heat pump heat exchanger. One key advantage of connecting multiple pipe sections in parallel, one or two loops for each hole, is to minimize pressure drop along the pipes. Less pressure drop means that thinner pipe walls can be used, and also that lower pump horsepower is needed.
Parallel connections potentially create an issue with pressure balancing the multiple U-tube loops. One solution is to connect the pipe sections to an adjustable manifold including valves for evacuation and testing as well as for active balancing of flow rates and pressures. In addition, temperature sensors are installed near the manifolds to monitor at least EWT and LWT for each heat exchange loop.
Boreholes certainly must be large enough to accommodate one down pipe and one up pipe, with some spacing between the two pipes. Most typically, the borehole is about 6″ (150 mm) diameter. This is in part to allow adequate spacing for the two pipes, but in practice the hole must be large enough to accommodate the drill string with bit.
Once a borchole is formed and the drill string is removed, the next step is to place U-tube loops into the hole. Besides cost, U-tube pipe materials may be compared for thermal conductivity; lateral stress resistance; stress cracking resistance; corrosion resistance; chemical compatibility; thermal expansion and contraction; and pressure resistance for pipes with associated grout. Because of the relatively thin pipe walls and overall geometries, pipe thermal resistance is not usually the largest contributing factor to overall resistance of the loop to the surrounding ground. Therefore, U-tube pipe materials having low cost but relatively low thermal conductivity are often adopted.
Today, U-tubes are often formed of low-cost plastic pipe, especially HDPE or PEX-A pipe, which have been proven to meet lifetime requirements. HDPE PE4710 pipe is quite common, with standard pipe sizes of ¾″, 1″ and 1¼″. Typically, larger diameter pipe is associated with deeper, larger diameter boreholes.
A U-turn element is fitted to turn around flow from the down pipe to the up pipe. Custom, highly-engineered U-turn elements may be fusion bonded to plastic pipe ends. Optionally, a fully assembled plastic U-tube loop is factory tested at pressure to ensure no leaks. The plastic U-tube loop assembly is then wound onto a reel for distribution. PEX-A is an alternate material that has been successfully applied, and is compatible with both fusion bonding and ordinary compression fittings.
Due to the details of U-turn element design, the two U-tube pipes are necessarily somewhat spaced apart. For example, the lateral width taken up by a loop formed of ¾ inch diameter pipe might be 2.375″ center-to-center; compared to 3.25″ for 1″ diameter and 3.75″ for 1.25 inch diameter. Such lateral width is not an issue for conventional designs, but may be limiting for unconventional designs requiring smaller spacing.
One potential option with a U-tube formed of copper pipe is to simply bend the pipe into a U-shape to accomplish U-turn. Both Type K and Type L copper pipe is routinely bent to centerline radius of about 2.5 times pipe diameter, or about 1.5″ for pipe having outer diameter 0.625″. Spacing between bend centers should also be minimum 1.5 times the pipe outer diameter. With 2⅜″ diameter shell pipe formed of 16 gauge steel, the inner diameter is 2.24″. Therefore, forming a simple bend as an alternative to a U-turn element is only marginally possible with copper pipe and is not recommended.
For efficient heat exchange to the surrounding media, it is critical that the U-tube loops be in good thermal contact with the media. Any air gaps are deadly to the thermal path, and may lead to significant and perhaps permanent reduction in the heat exchange capability of the installation. Therefore, once the pipe sections are placed into the boreholes, common practice is to add thermally-conductive grout to displace air and fill the space between the pipes and the outer perimeter of the borehole.
Common practice is further to attach a “tremie pipe” to the U-tube while feeding into a borchole, such that the tremie pipe is simultaneously positioned in the hole. The tremie pipe, typically about 1″ to 1½″ diameter, is then used for adding grout. After grout is mixed to a relatively low viscosity, it is pumped at pressure through the tremie pipe and into the hole. The pressure is sufficient to ensure that the grout flows well around any air or water pockets or other obstructions such as cave-ins from the surrounding borehole walls. The tremie pipe is slowly withdrawn as the grout is dispensed, until the grout can be seen filling to the top of the borehole.
Grout serves multiple purposes of sealing the hole and minimizing thermal contact resistance between the pipe and the surrounding media. Grout flows to permanently fill any variations in diameter of the borehole, effectively creating a custom fit. With ground installation and most soil types, grout can be mostly sand and hydrated Bentonite, a relatively inexpensive type of clay that tends to swell a bit and “seal” the hole against water infiltration. However, a mixture of Portland cement and sand, with perhaps some additive amounts of Bentonite and a superplasticizer, is sometimes used. Cement-based grout is more expensive. In addition, cement-based grout is more rigid and may be susceptible to formation of gaps with the media as expansion and contraction occurs over time. Generally, Bentonite-based grouts form a flexible connection, and also allow for some ongoing exchange of moisture between the grout and the surrounding media.
It is important to understand that conventional grout undergoes irreversible chemical changes after being placed into the annular cylinder formed by the borehole and the U-tubes. Both Bentonite clay-based and cement-based grout undergo hydration, whereby water is incorporated into a semi-crystalline structure and converts into either a thixotropic or typical solid material. Temperature evolution due to heat of hydration occurs during the setting process, and the rate of heat release affects the ultimate mechanical strength of the grout.
There has been much emphasis on increasing the thermal conductivity of grout over the recent few decades. While historically grout thermal conductivity κ was limited to about 0.8 W/m−° K., today grout is readily available with thermal conductivity up to about 2.7 W/m−° K. Increased thermal conductivity has been achieved by filling the clay base with high thermal-conductivity materials such as graphite flakes or soot.
Due to convective resistance between flowing fluid and down pipe and up pipe sidewalls, heat exchange efficiency also depends on the speed of the circulating fluid. With heat exchange loops, fluid speed and other parameters are typically adjusted to maintain the difference (EWT−LWT) in the range of 3-15° F. (1.5-8° C.).
To complete installation of the heat exchange loop heat, connections are made to the protruding pipes, which are connected to a manifold in series/parallel configurations and thence routed to the thermal work load.
Conventional analysis practice is to consider the thermal resistance of the media separately from the borchole thermal resistance (BTR). BTR is the sum of the components of resistance that are interior to the borehole wall, including a) convective resistance between the circulating fluid and the inner diameter of the pipes; b) conductive resistance of the pipe walls; c) conductive resistance of grout between the outer diameter of the pipes and the borehole wall; and d) potentially a contact resistance between the outer portion of the grout and the surrounding media. Partitioning of BTR might seem somewhat arbitrary, since energy is beneficially exchanged with the pipe and grout as well as the surrounding media. The key distinction is that each of the components of BTR can be directly influenced by design choices. Alternatively, with conventional design resistance of the media itself cannot be directly influenced. Also, media resistance as viewed from the borehole edge is not constant, but rather varies over time as thermal energy is charged and discharged. For accurate analysis, the thermal history of the surrounding media must be taken into account in determining media resistance.
With conventional ground loops it is well known that spacing of down pipe and up pipe results in significant variation of BTR, and further that it is difficult to ensure uniform spacing over the entire U-tube length. BTR repeatability from one installation to the next would be significantly improved by both fundamentally reducing nominal design BTR value and by improving uniformity and reproducibility of pipe spacing and other variables. Reduction of nominal BTR is beneficial, with more energy exchanged into the media in a given time period and for a given design length.
Primarily, BTR can be minimized by investing in grout having higher thermal conductivity, by increasing the spacing between the pipes composing the U-tubes, and by increasing the number of U-tubes in a single borehole. One partially successful approach is to place spring clips between the two pipes to separate them during insertion into the borchole and maintain the separation until grouting is completed and the pipes are locked into position.
If practical, BTR might also be reduced by increasing the thermal conductivity of materials from which U-tubes are constructed. There is some direct tradeoff between costs of forming the borchole and costs of the U-tube and other materials. If the U-tube is constructed from materials having higher thermal conductivity, heat exchange efficiency is increased and less borehole depth is required to meet exchange requirements. However, depending on details, total costs might be increased. There is a need for a design approach that optimizes material and installation costs along with heat exchange performance.
Besides reducing BTR, borchole formation costs might be directly reduced by improved practices. One potential cost reduction approach is to minimize the drilled borehole diameter. However, with conventional U-tube design it is difficult to distribute the grout and make effective thermal contact with the media if the borchole is smaller than about 4 inch diameter. Inventive U-tube design concepts enabling use of smaller diameters with potentially lower overall costs are demanded.
Overall, basic information on media thermal properties is crucial to designing a cost-effective heat exchange loop. In advance of fully committing to any installation, knowledge must be gained on average media parameters including 1) undisturbed media temperature; 2) media thermal conductivity κMEDIA and 3) media volumetric heat capacity (product of density p and specific heat capacity CP).
With regards to undisturbed ground temperature, maps are readily available to predict the temperature with reasonable accuracy. Temperature map contour lines tend to follow location latitude. Ground temperature data, primarily obtained from measurement of well water temperatures, has been codified in the ASHRAE Handbook. Included are annual average temperature, amplitude, and phase angle (number of days relative to January when the minimum ground temperature occurs). This enables simulations based on a simple harmonic model with annual period. Due to other unknowns, it is vitally important to assume that undisturbed ground temperature is known.
For ground installations, both thermal conductivity κGND and volumetric heat capacity (ρCP)GND are initially unknown and must be estimated. Although ground thermal conductivity broadly varies from 0.25-8 W/m·° K., in practice values are limited to 0.8-3.2 W/m·° K. For values below 0.8, a ground-source heat exchange loop is judged to not be economically practical. On the other hand, values above about 3.2 are generally associated with solid rock and again this is often not economically practical. Although thermal diffusivity a is initially unknown, it can be calculated once the other properties are known.
The conventional approach to analyzing vertical, cylindrical heat exchangers is based on the infinite line-source model. Valid a few hours after initiating operation, estimates based on the infinite line-source model conveniently rely on a logarithmic dependence of temperature vs time. This model is successful in many circumstances, but less successful when heat input is varied from ON to OFF.
Ground-based heat exchange loop installation typically relies on first forming a borehole using equipment and methods adopted from water-well drilling. A limitation with this approach is that the borehole may penetrate water-bearing fissures or conduits, potentially leading to contamination of underground water sources. For this reason, the practice is subject to regulations designed to prevent such contamination. Drillers must obtain licenses in advance and must report data obtained during the drilling process. There is a need for a system and method of installing ground loops carrying minimal risk of contaminating underground water sources, therefore avoiding the perceived need for regulation.
Conventional ground-based loop installations typically involve first forming a borchole having diameter 0.10-0.16 m, then inserting a heat exchange loop and filling the remaining volume between the loop and the borchole walls. Inserted heat exchange loops typically comprise two or more plastic pipes formed of materials such as HDPE or PEX having thermal conductivity of about 0.35-0.45 W/m−° K. Heat exchange performance measured in Watts per meter is limited by the low thermal conductivity of these materials. To compensate, total heat exchange loop length must be increased, leading to increased costs. There is a need to optimize costs and performance of ground-based loops, based on methods of inserting pipes into the ground and pipe material properties.
A hybrid geothermal heat pump system makes use of a regular air-source heat pump when ambient temperatures are above some cut-in point, for example 0° C. At the cut-in temperature, valves are switches to place a ground-based heat exchange loop in series with the outdoor air-based heat exchange system. Depending on the geographical location, cut-in of ground-based heat exchange loop might occur for only 2-20% of the operating life. This is a huge advantage, because the ground reservoir is only lightly charged with thermal energy and there is ample opportunity for recovery towards the undisturbed ground temperature. Due to increased complexity, costs must be managed very carefully for such a hybrid system.
It is also important to minimize disruption of the installation site. A large negative of current methods of drilling boreholes is that heavy equipment is required and water and mud wastes must be accommodated. In particular, there is a need for a convenient, low-cost way to install a ground-based heat exchange loop that can be used in conjunction with an air-based heat pump system.
In situations where direct push is feasible for positioning shell pipe within a media, there is opportunity to fundamentally reduce both total installation length and total costs.
There is also opportunity to ease environmental concerns by relying on a heat exchange design that is sealed, and incorporates corrosion-resistant materials.
The present invention is a system and method for heat exchange based on a U-tube loop inserted into a shell pipe, which in turn is immersed in a media. Circulation of a fluid through the U-tube loop results in exchange of thermal energy between the fluid and the surrounding media. The shell pipe, preferably having 0.03-0.10 meter outer diameter, is placed into the media to which heat will be exchanged. The method of direct push is optionally applied to position the shell pipe, using some combination of repetitively applied mechanical impact, hydraulics and pneumatics. Due to the requirement for high strength and resistance to impact forces, the shell pipe is necessarily constructed of metal, such as steel, which has high thermal conductivity. Once the shell pipe is in place, one or more heat exchange loops are placed internally followed by introduction of a thermally-conductive filler within the remaining volume. One enabling aspect of the invention is a U-turn element making a fluidic connection between a down pipe and an up pipe to form a loop meeting the constraint of fitting within a shell pipe. A second enabling aspect of the invention relates to the properties of the thermally-conductive filler and methods for introducing the filler into the available volume. With shell pipe, BTR is substantially reduced compared to conventional installations. First, the shell pipe material has high thermal conductivity. Second, due to much smaller required filler volume the filler thermal resistance is potentially decreased. Third, the sealed shell pipe provides superior environmental protection, thereby allowing use of more corrosion-susceptible materials such as copper for the loop itself.
Assuming that high thermal conductivity copper is used for the loop, there is further decrease in BTR. For a conventional U-tube, position within a borehole is a significant contributor to variation in BTR. With the shell pipe design, the possible range of variation of position of the U-tube pipes is limited. Overall, BTR with shell pipe design is both smaller in value and more precisely reproducible.
Due to the potential for contaminating ground water, regulations often require that boreholes be sealed, for example with clay-based grout. A beneficial aspect of the inventive scaled shell pipe is that potential for ground water contamination is essentially eliminated.
The shell pipe design is compatible with direct exchange (DX), where refrigerant circulated in a heat pump is routed through the U-tube loop that is contained by a shell pipe. Such a DX system can be installed in a smaller space and required ground heat exchange loop length is reduced.
In one application, a hybrid system makes use of a heat pump with refrigerant exchanging heat with air during normal temperature range, supplemented by a series ground heat exchange loop switched in to handle temperature extremes. Such application is directly analogous to the use of a cooling tower to augment a chiller system. Rather than being designed to handle 100% of the heating and cooling load, the series ground heat exchange loop may be designed to handle perhaps 20-40% of the load.
In a first embodiment, a first element of a shell pipe portion of a vertical heat exchange loop system is directly pushed into the media. Such shell pipe is sufficiently rigid to allow for direct push, and to minimize material costs has outer diameter less than about 110 mm and preferably about 60 mm. The bottom-most end of the shell pipe portion is closed by a conically-shaped head to prevent media from entering the pipe, wherein the head is sized to reduce friction during drive-in. With choice of steel as the example material meeting requirements, thermal conductivity of the shell pipe is greater than 5 W/m−° K. Once additional elements of the shell pipe are assembled and positioned as needed, a U-tube assembly is placed into the shell pipe. U-tube assembly is constructed of down pipe and up pipe formed from high thermal conductivity copper, with a custom U-turn element fitted to the bottom-most portion of the pipes. A slurry comprising a mixture of thermally-conductive particulates, water, and antifreeze is added into the volume between shell pipe and U-tube pipes to complete the thermal path between a fluid circulating through the U-tube assembly and the surrounding media. Optionally, a pilot hole is formed in the media prior to direct push of the shell pipe.
In a second embodiment, a first element of a shell pipe portion of a vertical heat exchange loop system is directly pushed into the media. Such shell pipe is sufficiently rigid to allow for direct push, and to minimize material costs has outer diameter less than about 110 mm and preferably about 60 mm. The bottom-most end of the shell pipe portion is closed by a conically-shaped head to prevent media from entering the pipe, wherein the head is sized to reduce friction during drive-in. With choice of steel as the example material meeting requirements, thermal conductivity of the shell pipe is greater than 5 W/m−° K. Once additional elements of the shell pipe are assembled and positioned as needed, a U-tube assembly is placed into the shell pipe. U-tube assembly is constructed of down pipe and up pipe formed from plastic material such as HDPE or PEX-A, with a custom U-turn element fitted to the bottom-most portion of the pipes. A slurry comprising a mixture of thermally-conductive particulates, water, and antifreeze is added into the volume between shell pipe and U-tube pipes to complete the thermal path between a fluid circulating through the U-tube assembly and the surrounding media. Optionally, a pilot hole is formed in the media prior to direct push of the shell pipe.
In a third embodiment, a first element of a shell pipe portion of a vertical heat exchange loop system is directly pushed into the media. Such shell pipe is sufficiently rigid to allow for direct push, and to minimize material costs has outer diameter less than about 110 mm and preferably about 60 mm. The bottom-most end of the shell pipe portion is closed by a conically-shaped head to prevent media from entering the pipe, wherein the head is sized to reduce friction during drive-in. With choice of steel as the example material meeting requirements, thermal conductivity of the shell pipe is greater than 5 W/m−° K. Once additional elements of the shell pipe are assembled and positioned as needed, a U-tube assembly is placed into the shell pipe. U-tube assembly is constructed of one of down pipe and up pipe formed from high thermal conductivity copper, and one of down pipe and up pipe formed from plastic material such as HDPE or PEX-A. A custom U-turn element is fitted to the bottom-most portion of the pipes. A slurry comprising a mixture of thermally-conductive particulates, water, and antifreeze is added into the volume between shell pipe and U-tube pipes to complete the thermal path between a fluid circulating through the U-tube assembly and the surrounding media. Optionally, a pilot hole is formed in the media prior to direct push of the shell pipe.
In a fourth embodiment, a first element of a shell pipe portion of a vertical heat exchange loop system is directly pushed into the media. Such shell pipe is sufficiently rigid to allow for direct push, and to minimize material costs has outer diameter less than about 110 mm and preferably about 60 mm. The bottom-most end of the shell pipe portion is closed by a conically-shaped head to prevent media from entering the pipe, wherein the head is sized to reduce friction during drive-in. With choice of steel as the example material meeting requirements, thermal conductivity of the shell pipe is greater than 5 W/m−° K. Once additional elements of the shell pipe are assembled and positioned as needed, two U-tube assemblies are placed into the shell pipe. U-tube assembly is constructed of down pipe and up pipe formed from high thermal conductivity copper, with a custom U-turn element fitted to the bottom-most portion of the pipes. A slurry comprising a mixture of thermally-conductive particulates, water, and antifreeze is added into the volume between shell pipe and U-tube pipes to complete the thermal path between a fluid circulating through the U-tube assembly and the surrounding media. Optionally, a pilot hole is formed in the media prior to direct push of the shell pipe.
In a fifth embodiment, a first element of a shell pipe portion of a vertical heat exchange loop system is directly pushed into the media. Such shell pipe is sufficiently rigid to allow for direct push, and to minimize material costs has outer diameter less than about 110 mm and preferably about 60 mm. The bottom-most end of the shell pipe portion is closed by a conically-shaped head to prevent media from entering the pipe, wherein the head is sized to reduce friction during drive-in. With choice of steel as the example material meeting requirements, thermal conductivity of the shell pipe is greater than 5 W/m−° K. Once additional elements of the shell pipe are assembled and positioned as needed, two U-tube assemblies are placed into the shell pipe. Each U-tube assembly is constructed of one of down pipe and up pipe formed from high thermal conductivity copper, and the other of down pipe and up pipe formed from plastic material such as HDPE or PEX-A. A custom U-turn element is fitted to the bottom-most portion of the pipes. A slurry comprising a mixture of thermally-conductive particulates, water, and antifreeze is added into the volume between shell pipe and U-tube pipes to complete the thermal path between a fluid circulating through the U-tube assembly and the surrounding media. Optionally, a pilot hole is formed in the media prior to direct push of the shell pipe.
In a sixth embodiment, a first element of a shell pipe portion of a vertical heat exchange loop system is directly pushed into the media. Such shell pipe is sufficiently rigid to allow for direct push, and to minimize material costs has outer diameter less than about 110 mm and preferably about 60 mm. The bottom-most end of the shell pipe portion is closed by a conically-shaped head to prevent media from entering the pipe, wherein the head is sized to reduce friction during drive-in. With choice of steel as the example material meeting requirements, thermal conductivity of the shell pipe is greater than 5 W/m−° K. Once additional elements of the shell pipe are assembled and positioned as needed, a U-tube assembly is placed into the shell pipe. U-tube assembly is constructed of down pipe and up pipe formed from high thermal conductivity copper, with a custom U-turn element fitted to the bottom-most portion of the pipes. A liquid comprising a mixture of water and antifreeze is added into the volume between shell pipe and U-tube pipes to complete the thermal path between a fluid circulating through the U-tube assembly and the surrounding media. Optionally, a pilot hole is formed in the media prior to direct push of the shell pipe.
In a seventh embodiment, a first element of a shell pipe portion of a vertical heat exchange loop system is directly pushed into the media. Such shell pipe is sufficiently rigid to allow for direct push, and to minimize material costs has outer diameter less than about 110 mm and preferably about 60 mm. The bottom-most end of the shell pipe portion is closed by a conically-shaped head to prevent media from entering the pipe, wherein the head is sized to reduce friction during drive-in. With choice of steel as the example material meeting requirements, thermal conductivity of the shell pipe is greater than 5 W/m−° K. Once additional elements of the shell pipe are assembled and positioned as needed, a U-tube assembly is placed into the shell pipe. U-tube assembly is constructed of down pipe and up pipe formed from plastic material such as HDPE or PEX-A, with a custom U-turn element fitted to the bottom-most portion of the pipes. A liquid comprising a mixture of water and antifreeze is added into the volume between shell pipe and U-tube pipes to complete the thermal path between a fluid circulating through the U-tube assembly and the surrounding media. Optionally, a pilot hole is formed in the media prior to direct push of the shell pipe.
In an eighth embodiment, a first element of a shell pipe portion of a vertical heat exchange loop system is directly pushed into the media. Such shell pipe is sufficiently rigid to allow for direct push, and to minimize material costs has outer diameter less than about 110 mm and preferably about 60 mm. The bottom-most end of the shell pipe portion is closed by a conically-shaped head to prevent media from entering the pipe, wherein the head is sized to reduce friction during drive-in. With choice of steel as the example material meeting requirements, thermal conductivity of the shell pipe is greater than 5 W/m−° K. Once additional elements of the shell pipe are assembled and positioned as needed, a U-tube assembly is placed into the shell pipe. U-tube assembly is constructed of one of down pipe and up pipe formed from high thermal conductivity copper, and one of down pipe and up pipe formed from plastic material such as HDPE or PEX-A. A custom U-turn element is fitted to the bottom-most portion of the pipes. A liquid comprising a mixture of water and antifreeze is added into the volume between shell pipe and U-tube pipes to complete the thermal path between a fluid circulating through the U-tube assembly and the surrounding media. Optionally, a pilot hole is formed in the media prior to direct push of the shell pipe.
In a ninth embodiment, a first element of a shell pipe portion of a vertical heat exchange loop system is directly pushed into the media. Such shell pipe is sufficiently rigid to allow for direct push, and to minimize material costs has outer diameter less than about 110 mm and preferably about 60 mm. The bottom-most end of the shell pipe portion is closed by a conically-shaped head to prevent media from entering the pipe, wherein the head is sized to reduce friction during drive-in. With choice of steel as the example material meeting requirements, thermal conductivity of the shell pipe is greater than 5 W/m−° K. Once additional elements of the shell pipe are assembled and positioned as needed, two U-tube assemblies are placed into the shell pipe. U-tube assembly is constructed of down pipe and up pipe formed from high thermal conductivity copper, with a custom U-turn element fitted to the bottom-most portion of the pipes. A liquid comprising a mixture of water and antifreeze is added into the volume between shell pipe and U-tube pipes to complete the thermal path between a fluid circulating through the U-tube assembly and the surrounding media. Optionally, a pilot hole is formed in the media prior to direct push of the shell pipe.
In a tenth embodiment, a first element of a shell pipe portion of a vertical heat exchange loop system is directly pushed into the media. Such shell pipe is sufficiently rigid to allow for direct push, and to minimize material costs has outer diameter less than about 110 mm and preferably about 60 mm. The bottom-most end of the shell pipe portion is closed by a conically-shaped head to prevent media from entering the pipe, wherein the head is sized to reduce friction during drive-in. With choice of steel as the example material meeting requirements, thermal conductivity of the shell pipe is greater than 5 W/m−° K. Once additional elements of the shell pipe are assembled and positioned as needed, two U-tube assemblies are placed into the shell pipe. Each U-tube assembly is constructed of one of down pipe and up pipe formed from high thermal conductivity copper, and the other of down pipe and up pipe formed from plastic material such as HDPE or PEX-A. A custom U-turn element is fitted to the bottom-most portion of the pipes. A liquid comprising a mixture of water and antifreeze is added into the volume between shell pipe and U-tube pipes to complete the thermal path between a fluid circulating through the U-tube assembly and the surrounding media. Optionally, a pilot hole is formed in the media prior to direct push of the shell pipe.
In an eleventh embodiment, a method for installing a heat exchange loop system comprises the steps of a) directly pushing a first element of an elongate shell pipe portion of a vertical heat exchange loop system into the ground, wherein said shell pipe is sufficiently rigid to allow for said direct push method, has thermal conductivity greater than about 5 W/m−° K, and has outer diameter less than 110 mm and preferably about 60 mm; b) attaching a second element to said elongate shell pipe and continuing direct push into said ground; c) continuing to add additional elements and directly pushing into said ground until a target total length of said elongate shell pipe is reached; d) inserting within said elongate shell pipe either one or two U-tube assemblies, each comprising a down pipe, an up pipe, and a U-turn element wherein fluid may be serially conducted from inlet of said down pipe through said U-turn element to outlet of said up pipe with minimal flow restriction; e) dispensing into the volume formed by inner radius of said elongate shell pipe and outer radii of said U-tube pipes a thermally-conductive filler comprising a mixture of particulates, water, and antifreeze to complete the thermal path between a fluid circulating through the U-tube assembly and the surrounding media, wherein said thermally-conductive filler has thermal conductivity greater than 0.4 W/m−° K; f) connecting the uppermost portion of said down pipe serially to a fluid pump and flow-through heat exchange apparatus and uppermost portion of said up pipe to complete a closed fluidic loop; g) positioning means for measurement and recording of temperature of both entering fluid temperature at uppermost portion of said down pipe and leaving fluid temperature at the uppermost portion of said up pipe; and h) activating said fluid pump and said flow-through heat exchange apparatus to exchange heat with the surrounding ground. Options are to form each individual U-tube assembly from either copper, HDPE or PEX-A materials. Yet another option is to mix materials, with one of down pipe and up pipe formed of copper and the other formed of plastic material, either HDPE or PEX-A.
In an twelfth embodiment, a method for installing a heat exchange loop system comprises the steps of a) drilling a pilot hole having diameter less than the diameter of a follow-on shell pipe; b) directly pushing a first element of an elongate shell pipe portion of a vertical heat exchange loop system into the ground, wherein said shell pipe is sufficiently rigid to allow for said direct push method, has thermal conductivity greater than about 5 W/m−° K, and has outer diameter less than 110 mm and preferably about 60 mm; c) attaching a second element to said elongate shell pipe and continuing direct push into said ground; d) continuing to add additional elements and directly pushing into said ground until a target total length of said elongate shell pipe is reached; e) inserting within said elongate shell pipe either one or two U-tube assemblies, each comprising a down pipe, an up pipe, and a U-turn element wherein fluid may be serially conducted from inlet of said down pipe through said U-turn element to outlet of said up pipe with minimal flow restriction; f) dispensing into the volume formed by inner radius of said elongate shell pipe and outer radii of said U-tube pipes a thermally-conductive filler comprising a mixture of particulates, water, and antifreeze to complete the thermal path between a fluid circulating through the U-tube assembly and the surrounding media, wherein said thermally-conductive filler has thermal conductivity greater than 0.4 W/m−° K; g) connecting the uppermost portion of said down pipe serially to a fluid pump and flow-through heat exchange apparatus and uppermost portion of said up pipe to complete a closed fluidic loop; h) positioning means for measurement and recording of temperature of both entering fluid temperature at uppermost portion of said down pipe and leaving fluid temperature at the uppermost portion of said up pipe; and i) activating said fluid pump and said flow-through heat exchange apparatus to exchange heat with the surrounding ground. Options are to form each individual U-tube assembly from either copper, HDPE or PEX-A materials. Yet another option is to mix materials, with one of down pipe and up pipe formed of copper and the other formed of plastic material, either HDPE or PEX-A.
In a thirteenth embodiment, an apparatus for exchanging thermal energy with a media is disclosed, wherein said apparatus comprises a) a shell pipe positioned vertically into the ground; b) either one or two U-tubes inserted into the shell pipe and configured to conduct fluid from an input side to a return side; c) thermally conductive filler comprised of water, anti-freeze and high thermal conductivity particulates occupying the volume formed by inner radius of said elongate shell pipe and outer radii of said U-tube pipes; d) a fluid pumping device; c) a heat exchange device located to introduce fluid to the input side of the U-tube and collect fluid from the return side of the U-tube; and f) two or more temperature sensors installed to detect at least the input side entering water temperature and return side leaving water temperature.
In a fourteenth embodiment, an apparatus for exchanging thermal energy with a media is disclosed, wherein said apparatus comprises a) a shell pipe positioned vertically into the ground; b) either one or two U-tubes inserted into the shell pipe and configured to conduct fluid from an input side to a return side; c) thermally conductive filler comprised of a water and anti-freeze mixture occupying the volume formed by inner radius of said elongate shell pipe and outer radii of said U-tube pipes; d) a fluid pumping device; c) a heat exchange device located to introduce fluid to the input side of the U-tube and collect fluid from the return side of the U-tube; and f) two or more temperature sensors installed to detect at least the input side entering water temperature and return side leaving water temperature.
In a fifteenth embodiment, a heat pump system comprises a) an indoor refrigerant-to-air heat exchanger; b) a compressor both increasing pressure and temperature and forcing circulation of refrigerant; c) an expansion valve for reducing refrigerant temperature; d) an outdoor refrigerant-to-air heat exchanger; e) a reversing valve to switch between heating and cooling modes; f) a ground-based heat exchanger, further comprising a shell pipe embedded in the ground, one or two U-tube assemblies inserted into said shell pipe, fluidic connections between the one or two U-tube assemblies and outdoor refrigerant-to-air heat exchanger, and expansion valve; and g) a plurality of valves selectively configurable to switch refrigerant flow from first direct path through said outdoor refrigerant-to-air heat exchanger to expansion valve to second longer path first through said outdoor refrigerant-to-air heat exchanger, next through said ground-based heat exchanger and then to expansion valve.
The present invention is a system and method for a heat exchange system immersed in a surrounding media, which relies on circulation of a fluid through a loop to exchange thermal energy between the fluid and the media. A shell pipe is placed into the media to which heat will be exchanged, optionally making use of the direct push method to insert the shell pipe into the media. Direct push involves using some combination of repetitively applied mechanical impact, hydraulics and pneumatics to force the shell pipe into the ground, much like an impact hammer screwdriver drives a screw.
The shell pipe must be compatible with the direct push method, and in particular must be formed of material having high strength; resistance to impact forces; low cost; and corrosion resistance to survive a long operating life embedded in a media. In fact, either galvanized or aluminized steel pipe is an excellent fit to the material requirements for the shell pipe.
Once the shell pipe is in place, one or two U-tube loops are positioned within the shell pipe, which is then filled with a thermally-conductive filler material that occupies the remaining volume and thermally connects the outer pipe diameters of the loops to the inner diameter of the shell pipe. Each loop comprises a down pipe and an up pipe, which are fluidically connected via a U-turn element. A U-turn element must fit easily within the shell pipe and route fluid flow between down pipe and up pipe of either one or two U-tubes. Double U-tubes, or two U-tubes, within a single shell pipe has an enhanced heat exchange rate relative to a single U-tube, and this arrangement is typically more cost-effective.
The invention enables reduced nominal BTR and improved consistency of BTR largely by use of a high thermal-conductivity, small-diameter shell pipe. Due to small diameter, the volume filled with grout is significantly reduced and the conductive resistance component due to grout is much lower compared to conventional systems. This means that the thermal resistance of the thermally-conductive filler between the U-tubes and the surrounding ground is substantially reduced since the path lengths are much shorter and the filler volume is much smaller. Optionally, the internal pipes comprising the U-tubes also have high thermal-conductivity.
Although the invention is generalized for any media, the ground at the surface of the Earth is one medium of special interest. This is due to the demand for ground-source heat pump (GSHP) systems for heating and cooling residential and commercial buildings. GSHP is also used for heating water. GSHP technology today relies on water-well drilling techniques to form a borehole. Use of such techniques is heavily regulated because a borehole may penetrate underground water pockets, flowing water, or even an aquifer. There is serious concern about potential for contamination. Typically, each drilled hole must be reported to authorities along with data on formation encountered during the drilling process. In addition, borehole sidewalls must be sealed using clay-based grout or equivalent, and all terminations at the surface must be carefully engineered to prevent contamination. The inventive shell pipe only minimally disturbs the site, and by design is sealed. Therefore, potential for ground water contamination either during or after insertion of the shell pipe and internal U-tubes is essentially eliminated.
Because the inventive shell pipe is sealed by design, there is freedom to form U-tubes from materials that are less corrosion resistant. Primarily, the U-tubes must be compatible with the filler material. This provides needed flexibility to incorporate alternate materials, with copper as one example.
Those schooled in the conventional GSHP design may develop concern that the inventive system is subject to two potential flaws. A first potential flaw is that bringing the up pipe and down pipe into close proximity might lead to reduced heat exchange effectiveness due to shunt effect; a second potential flaw is that the close proximity of the pipes will result in greater obstruction of the path for heat to flow between the U-tubes and the surrounding media. Anticipating these objections, detailed discussion follows below on both shunt effect and obstruction of adjacent pipes on thermal flow from a given pipe.
Heat circulating through one or more U-tubes is first exchanged with the BTR and then diffuses into the surrounding media. Essentially, there is a divider effect where a portion of the temperature is dropped across the BTR and the remainder across the ground resistance. With conventional design, BTR amounts to about 0.02-0.2 m−° K/W. BRT values on the lower end of the range generally correspond to having multiple pipe pairs deployed in a single borchole.
An important case to consider is ground as the media. Upon first heat exchange, ground resistance begins at zero but rapidly increases to a steady-state value when heat is continuously applied. For example, with κGND=1.2, RGND from the borehole edge approaches about 0.35 m−° K/W over 2 weeks. For 0.8<κGND<3.0, RGND at 2 weeks might range from about 0.16-0.48 m−° K/W. Therefore, between about 24-56% of the temperature is dropped across BTR depending on the unknown ground thermal properties. This variation highlights the importance of accurate knowledge of ground thermal properties.
In the limit with minimal filler (i.e. grout) volume, BTR can be reduced to the combination of pipe convection resistance and pipe conduction resistance. In such case, BTR might be reduced to about 0.01 m−° K/W, with only about 2-5% of the temperature dropped across BTR. Importantly, the same design changes that lead to minimization of BTR also result in near-perfect repeatability of BTR values.
Modeling of both BTR and media resistance is improved by developing an electrical analog method for both transient and steady-state heat flow analysis. Readily-available electrical analysis tools enable rapid, accurate simulations. In particular, SPICE (Simulation Program with Integrated Circuit Emphasis) modeling of equivalent electrical RC circuits is very effective. While not required by the invention, electrical analogy and SPICE simulation allow for more intuitive description of the problem and test results, and for comparisons based on rapid simulations of various designs. SPICE simulation is particularly well suited for reduced or variable duty cycle analysis. Both electrical analogy and SPICE simulation are discussed in more detail in co-pending application xxx.
It is challenging to extend SPICE analysis to multiple U-tubes. A complementary modeling approach is Finite element analysis (FEA), which has also been applied to both the inventive and conventional heat exchange loop designs. Using FEA, appropriate pipe geometries are first drawn and then thermal properties are defined for each region. FEA can be applied to any number of pipe geometries and materials. Therefore, comparing SPICE and FEA results is a good practice.
FEA results on Q/L typically agree to within about 5% of values obtained from SPICE simulations or other calculations. However, some initial adjustment on fluid thermal conductivity is required. Further, since convective resistance cannot be modeled using FEA, pipe thermal conductivity is adjusted to match the calculated sum of convection and conduction resistances.
A key advantage of FEA is that the analytical method is easily extended to multiple U-tubes. FEA results further make the case that shunt effect is of little importance in U-tube loop design, and in particular to the inventive design where pipes are constrained to be close to each other.
In
Compared to conventional borchole having 125-150 mm diameter, there is potential 6× volume reduction when using shell pipe. Both shell pipe length and radii are precisely known, and the same is true for U-tubes displacing volume as they are inserted into the shell pipe. Further, due to precisely known volume the amount of filler can be precisely metered such that fill is terminated near the top of the shell pipe and approximately at the edge of the constant temperature zone. The heat exchange rate of any U-tube above the point where fill is terminated is decreased to near zero, since air is a good thermal insulator.
A second advantage relates to the fact that the inventive shell pipe assembly comprises joints that are nominally leak-free. This means that the integrity of filler material can be maintained over lifetime.
In one preferred embodiment, a water-antifreeze mixture is used as thermally-conductive filler. Although thermal conductivity is lower compared to the conventional clay-based or cement-based filler, this is of little concern because with the inventive shell pipe and U-tube assembly the thermal path lengths are shorter and other contributions to BTR are substantially reduced compared to a conventional heat exchange system.
For example, with a steel shell pipe and copper U-tube, in response to decreasing filler thermal conductivity from 2.0 to 0.5 W/m−K, BTR increases from 0.049 to 0.072. With analysis at 4 weeks, the result is just a 6% decrease in Q/L. This may be tolerable, given the advantages of a precisely dispensed and highly stable filler material.
Few liquids have thermal conductivity higher than water at 0.6 W/m−° K. Thermal conductivity of anti-freeze liquids glycerol, methanol, ethylene glycol and propylene glycol are roughly about 0.2 W/m−° K. For each, a 20% water-anti-freeze mixture results in thermal conductivity of about 0.5 W/m−° K., which may be acceptable. Additionally, at 20% mix there is modest increase in viscosity compared to water with each of these liquids.
Glycerol, or glycerin which is the same as glycerol but having lower purity and lower cost, is a particularly interesting potential additive to water because it a) has thermal conductivity of 0.3 W/m−° K; b) has higher density than water; c) is completely miscible in water and indeed has an affinity for water; d) has boiling point of 290° C.; e) is non-toxic; f) is non-corrosive; g) acts as an anti-freeze agent when mixed with water; and h) is inexpensive and readily available. For example, a 20% mixture of glycerin with water has thermal conductivity ˜0.5 W/m−° K., freezes at about −5° C., and has at most double the viscosity of water alone.
With liquid media in thermal contact with a solid surface, thermal resistance may be increased due to presence of a boundary layer. The boundary layer effect is modest when the liquid media is not in motion. The boundary layer simply acts as an added thermal resistance. Regardless, BTR is often acceptable with use of a liquid as thermally-conductive filler.
An alternative embodiment based on a mixture of liquid and solid particulates avoids boundary layer concerns.
Clay-based materials mixed with water, often simply termed “mud”, are predominantly used as thermally-conductive filler in conventional ground-based heat exchange installations. Thermal conductivity of mud ranges from about 0.6 W/m−° K to 3.0 W/m−° K, depending on inclusion of higher thermal-conductivity additives. Recent trends are to increase thermal conductivity of mud with a few percent of high thermal conductivity additives. Example additives in the form of fine aggregates include a) graphite, either flake or expanded; b) aluminum shavings; c) aluminum oxide (alumina); d) silicon carbide; and e) limestone. For example, estimated thermal conductivities are 800, 200, 120 and 30 W/m−° K for graphite, aluminum, silicon carbide and aluminum oxide respectively.
Additive particulates immersed in a lower thermal conductivity media tend to act as thermal shorting paths. However, theoretical analysis shows that the lower thermal conductivity of the base media surrounding the additive particulates tends to dominate. This is especially true, for example with powders having porosity of 10-50%, where the pores are filled with air. Dominance of the base surrounding media thermal conductivity limits the degree of effect available from high-conductivity additives. For example, if the base surrounding media is a mixture of glycerin and water, the beginning thermal conductivity is about 0.5 W/m−° K. With particulate additives amounting to 10% or less by volume, it is difficult to obtain overall thermal conductivity more than about 2.0 W/m−° K. Above roughly 10% additive, viscosity increases to the extent that typical piston/plunger style “mud pumps” are unable to dispense the mixture in a conventional borehole.
Another preferred embodiment depends on having a large percentage of high thermal conductivity particulates along with some liquid thermally tying particulates to one another and to the pipe surfaces. Adjacent individual particulates tend to make limited 3-point contact with each other, resulting in highly resistive paths. Thermally-conductive liquid acts to bridge gaps between individual particulates, lowering thermal resistance. Again, air filling the pores between particulates must be avoided. For these reasons, potential loss of liquid over the operating life of the heat exchange system is a concern. Of course, shell pipe joints must be leak-tight. There is also potential for evaporation of water from the top of the column of filler material in the shell pipe over long time periods. This concern is optionally addressed by capping the filler column with 1-10 cm3 of oil or other lighter-than-water liquids. Oil has density below that of water and naturally floats on water. At the temperatures typically associated with the ground, mineral oils tend to have evaporation rates much lower than water. Semi-synthetic and fully-synthetic lubricants having greatly reduced evaporation rate compared to mineral oil are available. Due to the small volume required to cap the filler column, high cost of synthetic lubricants or alternate liquids is of little concern. In a preferred embodiment, a glycerin-water mixture is used as the liquid portion of thermally-conductive filler and the filler is capped with an oil or lubricant having very low evaporation rate.
The inventive method makes use of fine particulates having high thermal conductivity. Particulates filling the volume must jointly completely bridge the space between inner radius of shell pipe and outer radii of down pipe and up pipe. The additive particulates “cut through” any liquid-solid interface boundary layers to independently make thermal contact with pipe surfaces. Additive loading of 10-90% by volume results in acceptably low thermal resistance.
To accomplish this, in one inventive method a slurry is formed by stirring a small amount of liquid into a matrix of such fine particulates. For example, a 20% glycerin-water mixture is stirred into a particulate powder. This slurry is then pumped into the shell pipe using a tremie pipe, similar to conventional methods but making use of a centrifugal slurry pump designed for pumping highly abrasive particulates while avoiding both clogging of suction and discharge lines and settling of particulates.
It is critically important to eliminate trapped air in preparing and introducing slurry to the shell pipe. Thermal conductivity of air is orders of magnitude lower than liquids, and any included air is deadly to the final average filler thermal conductivity. Similar to conventional practice with grout, in one inventive method a particulate-free water-antifreeze mixture is first pumped through a tremie pipe to displace air from the bottom-most portion of the shell pipe and to progressively force air out of the shell pipe. Optionally, once air is displaced for a sufficient length of the shell pipe, the pump feed is switched to a slurry containing a high percentage of high-conductivity particulates.
Optionally, the slurry is first mixed in a vacuum-tight chamber which is then connected to a vacuum pump to pull out dissolved air. Once the slurry is free of dissolved gases, it is fed into the tremie pipe to both mix with and partially displace the particulate-free water-antifreeze mixture. The result is a thermally-conductive filler between the shell pipe and the U-tube assembly comprising a liquid-solid mixture having overall thermal conductivity of at least 1.0 W/m−° K, with minimal influence of liquid boundary layers.
The additive particulates “cut through” any liquid-solid interface boundary layers to independently make thermal contact with pipe surfaces. Additive loading of 10-90% by volume results in acceptably low thermal resistance.
An initial, very simple schematic based on prior art is illustrated in
With the prior art circuit as a starting point, the circuit is radically modified to develop SPICE models representing the heat exchange system. While in the prior art a shunt resistance is included to represent diffusion of heat energy from the fluid in down pipe to the fluid in up pipe, with the SPICE model shunt resistance is not found to be necessary.
SPICE modeling begins by drawing the equivalent circuit schematic. A graphic tool is available for schematic construction. A netlist, extracted from this schematic, summarizes the individual nodes, their interconnections, and the circuit values. Note that a SPICE model can be exercised using the netlist by itself, although the schematic cannot be easily back-generated from the netlist.
The circuit shown in
Inclusion of the SPST switch and controlling pulsed voltage source sets the stage for varying duty cycle during transient analysis. This is important for the most detailed analyses.
Neither the prior art circuit of
Direct push of a shell pipe obviously relies on progressive assembly of multiple pipe sections to develop the full design length. For example, with commercially available steel pipe sourced in 21 foot lengths, as the first length is pushed mostly into the media a next length must be connected. It is important that once the connection is made, the joint only minimally disrupts the propagation of mechanical waves through the pipe. Any back-and-forth sliding of the pipe joints will absorb energy and reduce direct push effectiveness. Therefore, some specific requirements must be met.
First, there are multiple reasons to use the smallest possible pipe wall thickness. Pipes having thicker walls obviously have more mass per unit length. This correlates to both higher costs and greater weight. With direct tradeoff of material costs vs. costs of drilling or otherwise forming a borehole, the cost function is quite sensitive to pipe wall thickness. In addition, weight is a concern when handling long sections of pipe. Simpler tools and equipment can be used with lighter pipe.
The method of joining shell pipe sections is also a concern. The obvious approach of threaded joints is in fact deadly to cost consideration, since threading absolutely requires thicker pipe walls. Further, threaded connections are subject to shear stresses and may rip out with repetitive impact force. A key advantage of thinner pipe material is that the topmost pipe end can easily be expanded such that a next section fits neatly within the first pipe, overlapping by perhaps 2-4 inches. This approach delivers the required tight fit, but likely allows for back-and-forth sliding upon application of impact forces. A method of locking the joint in place is needed.
One available solution for locking pipes in place is to simply weld the two pipes together at the joint. The welded joint will only minimally absorb impact energy, allowing mechanical waves to progress. Welding is recommended in a preferred embodiment.
Other solutions include application of epoxy or other adhesives to the joint, although drying time may be limiting.
Once the shell pipe is in place, the next step is to insert one or more U-tubes. U-tubes may be formed of either flexible or rigid pipe. With flexible pipes, a single length of rolled pipe is feasible. Alternatively, with rigid pipes multiple sections must be joined as insertion progresses.
U-tubes formed of soft copper pipe fit the category of flexible pipe. Single rolls having length of 50-100 feet or more are commercially available. It is anticipated that manufacturers will respond to demand to supply soft copper lengths of 100-500 feet as needed. The weight of ½″ soft copper pipe is 0.25 pounds per foot (wall thickness 0.035″). For a 200 foot shell pipe length and 400 feet of copper pipe in a U-tube, total weight of 100 pounds is easily manageable. Even with ¾″ soft copper pipe weighing 0.305 pounds per foot (same wall thickness), the U-tube can be readily handled.
For U-tubes formed of rigid copper pipe (for example Type L), 21 feet is again a standard length that is available. Insertion requires that a length of pipe be lowered into the shell pipe and safely held in place while a coupling and another section of pipe are soldered or otherwise joined in place. Since couplings are a bit larger diameter, joints on side-by-side pipes are offset by a few inches. Soldering is an inexpensive, highly reliable process for leak-proof joining of copper pipe. Since the weight of ½″ Type L copper pipe is 0.285 pounds per foot (wall thickness 0.035″), the U-tube is again manageable.
Of course, assembled plastic U-tubes provided on reels having lengths of 500 feet are standard today. Both HDPE and PEX-A are examples of plastic pipe that is readily available and have been proven to be reliable over long installed times. Equipment and methods for inserting such U-tubes into a borchole are readily available and easily adapted to use with a shell pipe.
With any rigid pipe assembly, it is important to prevent premature dropping of the assembly into the borehole or shell pipe. One embodiment of the invention involves first drilling a borehole having diameter larger than that of the shell pipe, then assembling a shell pipe into the borchole and filling with conductive material. In such case, it is necessary to safely suspend the already-assembled portion of the shell pipe as each new section is added. This necessitates use of a rope or chain having sufficient working load limit to handle the pipe weight. Such rope or chain must be attached to the bottom of the shell pipe. In this embodiment, the shell pipe must be closed but a conical point is not required. Therefore, the shell pipe can be closed using a custom-designed cap that accommodates the rope or chain being used. If a rope is used, the design allows for removal of the rope after lowering into the borehole. If a metallic chain is used, the thermally-conductive chain might be left in place. The standard 2⅜″ steel pipe weighs 1.61 pounds per foot. Therefore, a 200 foot length weighs 320 pounds. A winch or equivalent is required to manage the shell pipe as it is assembled into the borchole.
As a U-tube is assembled into an already positioned shell pipe, the U-tube must be safely supported. A rope or chain is used and attached to the top of the U-turn element. After a first length is joined to the U-turn element, the assembly is lowered until the topmost portion of the first length is just above the media. A second length is next joined and the assembly is lowered. This process is repeated until the completed U-tube assembly is lowered into the shell pipe. Again, if a rope is used, the design allows for removal of the rope after lowering into the borchole. If a metallic chain is used, the thermally-conductive chain might be left in place.
Design of the U-turn element is important for either single or double U-tubes included in a shell pipe. Specifically for embodiments including HDPE U-tubes, a key concern is how to form the U-turn element in the limited space allowed by a shell pipe. For a single U-tube comprising two copper pipes, a simple design based on solder connection is adequate.
Another embodiment comprises two copper pipes and two plastic pipes within 2⅜″ outer diameter Shell pipe 120. Either HDPE or PEX-A qualify as materials for the plastic pipes. In one example embodiment, down pipes might be formed of HDPE while up pipes are formed of copper. This has the advantage of enhancing the heat exchange rate from the up pipe.
When feasible, bending pipe to form a U-shape is preferable to inclusion of a custom U-turn element, requiring less labor and resulting in reduced chance of a fluid leak. A custom casting is another alternative to a machined U-bend element. Such a casting may be less expensive in high volume production and may also have less flow restriction compared to a machined U-bend element.
Tremie pipe 206 is shown in
In one or more preferred embodiments, use of direct push technology is one aspect of the invention. Direct push is a method of inserting a pipe predominantly vertically into the media, although angles up to 45 degrees are possible. Overall, there is potential to reduce costs and improve performance by applying the direct push technology to insert pipes with less or even no drilling.
Similar to driving a nail or screw into wood, friction is largely overcome by repeatedly applying impact force to the top end of the pipe. A percussive hammer is used to apply force, assisted by the force due to the weight of the pipe itself. A drive shoe optionally is temporarily positioned over the top end of the pipe to better distribute the force and to minimize damage to the pipe end.
Pipe direct push technology has been developed and commonly used for obtaining soil samples, and typically involves some combination of hydraulically-applied force and percussive methods. A conically-shaped head is often attached to a long shank portion, with the point of the head leading into the media. The total length of the shank is incrementally increased by adding pipe sections as penetration progresses.
Friction for the pipe head and shank are considered separately. Direct push technology relies on first minimizing the force required to incrementally advance the pipe head into the media, and second on minimizing friction along the pipe shank. When the shank diameter is less than the head diameter, a first approximation is that shank friction is zero in media that are subject to compression. In “caving” type formations such as sand or gravel, shank friction may build up over time and as overall length increases.
Vibration of the media adjacent to the pipe head and shank can act to reduce friction, enabling further penetration. Vibrations are initiated along the pipe by periodic application of impact force. Pressure waves, launched in response to applied impact force, travel up and down the pipe. With some art, the pressure waves can be made to reinforce constructively at the natural resonance frequency. In such case, conditions are set for adjacent media to remain in constant motion, with greatly reduced contact with the shank leading to friction reduction.
Direct push is most applicable for use in unconsolidated formations. When encountering smaller rocks, progress may be maintained as the pipe head either breaks up the rock or pushes the rock out of the path.
To enable penetration, the media immediately beneath the head must be rearranged, compressed or pushed to the side. Just like a nail, an alternative is to reduce friction at the head by forming a pilot hole in advance. In formation types subject to caving, such as sand or gravel, removal of material from the path taken by the pipe can act to minimize cave-in or collapse as the pipe progresses. This in turn reduces friction along the shank of the pipe.
As mentioned above, the extent of penetration by direct push is further enhanced by exciting the length of pipe at one or more resonant frequencies. Unfortunately, resonant frequency changes as additional pipe sections are added. Furthermore, damping is complicated by the fact that some portion of the pipe is already engaged with the media, while several feet are not yet engaged. In one embodiment, complicated equipment is required to dynamically adjust excitation frequency to meet changing conditions. In another embodiment, active sensing is used to provide feedback on vibration amplitude. Regardless of the complexity, resonant excitation is of great advantage and leads to broader opportunity for application of direct push.
The total required U-tube length is simulated for a specific project. If feasible, it is desirable to install a single U-tube or pair of U-tubes having the required length. However, often this total length must be divided into two or more U-tube loops. Most typically, conventional U-tubes for ground-based heat exchange systems extend 200-400 feet below the ground surface. For example, with a requirement to exchange heat from a 3 ton AC system, the total required U-tube length might be 1,200 feet. One option is to divide this length into 4 U-tubes each having 300 foot length.
Specifically with direct push, exceeding single U-tube length of about 100 feet is difficult in some ground formations. In such case, the total length may be divided into 4-20 individual U-tubes. This is not necessary a problem, since overall installed costs are dominated by material and labor costs, which tend to be proportional to overall length. Two inefficiencies result from having multiple, separate U-tubes. First, the manifold enabling series-parallel connections of various U-tubes becomes more complex as the number of U-tubes increases, perhaps leading to increased costs. Second, each U-tube is less effective at exchanging heat above the constant temperature zone, which begins at 10-15 feet depth. In the extreme, these first few feet of U-tube might be considered as having zero contribution to heat exchange. If a single, 1,200 foot long U-tube is applied, to compensate for the ineffective portion the total length must be increased to 1,220 feet. The resulting inefficiency amounts to 1.7 percent. On the other hand, if 12 U-tubes, each 100 foot long are applied to meet the same initial 1,200 foot requirement, to compensate for the ineffective portions the total length must be increased to 1,440 feet. Alternatively, the number of U-tubes might be increased from 12 to 14. The resulting inefficiency amounts to about 20 percent.
Depending on specific knowledge of local soil conditions, it might be deemed necessary to execute tests prior to committing to a project. One important tactic is to perform any tests in such a manner that expenditures are not wasted, but can be directly applied to the project. With this in mind, it is helpful to make the project more modular. Instead of 4 deep boreholes, a design for 8-9 boreholes both means that each hole is less deep, but also that the cost of a first borchole is reduced.
A single borehole can be formed, populated with heat exchange loop, grouted or otherwise filled, and tested to extract needed information on media thermal properties. If economic analysis based on media parameters proves disappointing, the single installation might simply be abandoned. Forearmed with data from other sites nearby, the more likely outcome is that media parameters prove favorable, and the single U-tube becomes the first of the several to be installed. Importantly, test data obtained from a first U-tube enables design optimization, thereby leading to cost reduction while simultaneously managing overall financial risk.
Obstruction with Multiple Pipes
A single U-tube can be analyzed as two separate, individual pipes each radiating heat in the radial direction. Calculation is first made on the down pipe in the absence of a second pipe. With this first pipe treated as being in the center of the heat exchange system, calculation is straightforward for the four resistance components of a) pipe convection; b) pipe conduction; c) filler annular ring conduction; and d) surrounding media conduction.
Two important adjustments are made when considering a second, adjacent pipe with fluid flowing in the opposite direction. First, the average temperature of the up pipe (returning to heat exchanger) is necessarily lower in cooling mode, higher in heating mode. Some heat has already been exchanged from the down pipe before the fluid begins return in up pipe. Second, each pipe obstructs the other pipe to some extent.
The temperature at the bottom of a U-tube is the average of the EWT and LWT, assuming linear temperature drop along the pipe. Therefore, the temperature of the down pipe and up pipe averaged along the length is expressed as:
With these average temperatures, less heat is exchanged with the media in the up pipe compared to the down pipe.
For a single U-tube, obstruction is conveniently approximated as a multiplicative factor F:
Factor F is simply obtained by evaluating the angles involved.
For the above example, factor F=0.952. Therefore, each pipe “has access to” only 95.2% of the surrounding media. In the limit of the two pipes actually touching, S=d and obstruction angle 136 equals 57.3 degrees, or 16% of a full circle. Turning this around, in the limit of maximum separation of the two pipes, heat exchange effectiveness increases by at most 16%. In a practical case of increasing separation from 1 inch to 3 inches, heat exchange effectiveness increases by 13% (Factor 0.95 vs 0.84). Obviously, there is rapidly diminishing return in further separation.
To summarize for a single U-tube in a borehole, the total heat exchange effectiveness of two pipes having the same outer diameter is estimated as:
Much research has been published relating to the improvements found in separating the down pipe and up pipe as far as possible in the borehole. With the above analysis, the potential improvement with such separation is rapidly estimated. With EWT=15° C. and LWT=12.5° C., and pipe diameter d=1.05″, & increases from 0.916 to 0.958 as spacing S increases from 2″ to the maximum allowable 4″ in a 6″ diameter borehole. This 4.6% improvement is modest but real.
Analysis can be extended to estimate obstruction with two or more pipe pairs in a single borehole. Parallel piping is assumed, with at any vertical position each down pipe is at a common first temperature, and each up pipe is at a second temperature. With the small diameter of the inventive shell pipe it is possible to fit two U-tube loops, but three or more is not possible. Without proof, the equation estimating effectiveness of two loops is given as:
Consider a shell pipe having internal diameter of 2.24″ (0.057 m) and populated with single U-tube constructed of copper pipe having outer diameter 0.625″ (0.016 m); spacing of 1.5″ (0.039 m); and EWT=15° C. and LWT=12.5° C. Calculated Obstruction Factor=0.93. For the same U-tube construction and two pairs, Obstruction Factor=0.833. Therefore, heat exchange with two pairs is increased by 79% over a single pair. Since material and labor costs of installing the shell pipe is constant, this is usually a good tradeoff.
It is not feasible to include two pipe pairs having this outer diameter. However, it is possible to include mixed pipe pairs where, for example, the down pipes are HDPE while the up pipes are copper. In such case, the U-turn element is appropriately engineered.
Heat flowing directly between the U-tube down pipe and up pipe portions is termed the “shunt effect”. Shunt effect has received attention in research literature, given concern over potential to reduce the opportunity to exchange heat with the media. The inventive design employs a small diameter shell pipe, which forces close spacing of down pipes and up pipes. However, shunt effect is largely ignored in practice with the inventive design since:
It is worth noting that heat exchange loops of the coaxial design historically preceded the U-tube design, and with coaxial design shunt effect directly results in reduced performance. In sharp contrast to the coaxial design, with the single U-tube both down pipes and up pipes exchange heat with the surrounding media. Because heat is continuously exchanged as the fluid moves through the down pipes, the average fluid temperature is roughly the fluid temperature at the vertical midpoint of the pipes. The average fluid temperature in the up pipes is necessarily closer to the ground temperature.
For the typical case LWT0 is 70-90% of EWT0, where the subscript “0” indicates that specified temperature is relative to the temperature of the surrounding media. Therefore, the entire length of the U-tube heat exchange loop is effective at exchanging heat and simplifying approximations can be made. The simplest approximation, that the entire loop is at the average temperature ΔTAVG=(EWT0+LWT0)/2, works reasonably well. Next in increasing complexity is the assumption that the down pipe portion is at one temperature, while the up pipe portion is at a second temperature. For example, in Equation 1 substitute EWT0 and LWT0 for EWT and LWT.
Regardless of the total U-tube length, shunt effect occurs largely in the upper half of a U-tube. This is because the pipe temperatures are much closer to each other in the bottom half, with lower driving force for shunt effect. Temperature difference between pipes in the top half of the heat exchange loop is ˜3× that of the bottom half. In addition, with any shunting that occurs in the bottom half of a U-tube there is ample opportunity for shunted heat to be exchanged with the surrounding media before the fluid is returned to the system exchanger. In fact, this “feed forward” of heat actually benefits overall heat exchange.
Referring back to
In fact, with low duty-cycle operation shunt effect tends to be even more beneficial, since it acts in the direction of balancing heat distribution between down pipe and up pipe. Such balancing makes for a more effective heat exchanger.
Looking at shunt effect in yet more detail, it is obvious that the calculations supporting
One of the primary payoffs from modeling and simulation of media-based heat exchange systems is development of an estimate of required U-tube length. In fact, determining the average heat exchange rate Q/L leads naturally to a decision on length. The three methods of calculating Q/L are: a) SPICE simulation; b) FEA; and c) closed-form calculations.
Comparison of the estimates developed using the three different methods are shown in
In the second part of the analysis, a shell pipe having 2⅜″ outer diameter is used. The inner diameter is 2.24″. For HDPE pipe, convective heat transfer coefficient h is set at 1,500 by policy, whereas for Copper pipe the value is set at 4,400. From the third part of the analysis, the same shell pipe and Copper pipe is used. However, the time is varied from 0.5 to 4.0 weeks. The closed-form equations are expected to be valid after about 48 hours of 100% duty cycle.
There is reasonable agreement between estimates obtained by the three different methods. Again, key strengths with SPICE analysis are that less than 100% duty cycle can easily be handled and that voltages representing temperatures at every circuit node can be retained from a given simulation and used as starting inputs in a subsequent simulation. FEA has the advantage of being able to handle multiple pipes with asymmetrical configurations.
From the first part results, it is obvious that there are various tradeoffs when changing both pipe diameter and pipe material. Smaller pipe diameter, with spacing held constant, results in less obstruction effect but increased pipe and grout resistances and pump energy. However, for a borchole with a single U-tube the net effect is a 17.5% increase in Q/L based on 0.625″ diameter copper versus 1.05″ diameter HDPE. With use of copper, there is some small risk that the filler material will have corrosive interaction with copper pipe over operating life. However, this risk can be minimized by proper choice of filler material and additives specifically to reduce copper corrosion.
From the second part results with shell pipe, the single U-tube based on 0.625″ diameter copper retains a 7% increase in Q/L compared to the conventional HDPE U-tube in a borehole, even though the smaller outer diameter of the shell pipe is somewhat disadvantageous. In an example embodiment of a mixed U-tube, with one 0.625″ copper pipe and one 1.05″ HDPE pipe, results are comparable to the conventional HDPE-based U-tube in a borchole. Using a double U-tube in the shell pipe results in only a 6% increase in Q/L, and may not be economically favorable. With the shell pipe, conditions are much more controlled and reproducible compared to an open borehole, and corrosion risk associated with copper pipe in this environment is negligible given proper choice of filler material with additives.
From the third part with time varied, calculations show that Q/L is 28% larger at 0.5 week vs. 4 weeks of continuous heat application. This highlights the importance of the increase in the effective ground resistance as “charging” occurs. In fact, for reduced duty cycle, discharging also occurs and the ground resistance recovers to some extent. This illustrates the potential advantage of using a hybrid heat pump system with direct exchange system ground loop acting as a backup for an air-source heat pump.
The illustrated ground-source heat pump can operate with almost the same efficiency as a ground-source heat pump during all but extreme weather conditions. When air temperature drops to below zero, which occurs for perhaps 2-20% of the heating season depending on local climate, the DX ground-based system can be switched into service. With these operating conditions, the ground loop is idle for substantial time periods, allowing for thermal recovery. The result of thermal recovery is lower effective ground resistance and associated higher Q/L.
SPICE simulations using average ground thermal parameters show that Q/L increases significantly as duty cycle is lowered. At 20% duty cycle for 4 weeks, Q/L increases about 20% compared to 100% duty cycle for 4 weeks. SPICE output charts of voltage vs. time show that the reason for this increase is that the circuit largely discharges, or recovers, during the OFF time period. When a media-based heat exchange system is applied as a supplemental source, it can be expected that duty cycle will be well less than 50%. For the specific cases of 25% and 50% duty cycle, SPICE modeling was executed to compare Q/L with different periods. The baseline case is 4 weeks at 25% duty cycle (1 week ON, 3 weeks OFF). With nominal media parameters and the practical case of 12 hours ON time, Q/L is 43% higher compared to baseline. To further demonstrate the recovery effects, simulations show that output is undistinguishable between 25% duty cycle and 96 hour period and 50% duty cycle and 48 hour period (ON time is the same.)
This effect furthers the advantage of using a ground-source heat exchange loop as supplement to an air-based heat pump. For an overall comparison, a loop design length of 1,200 feet meeting 100% of capacity requirements and 100% duty cycle over 4 weeks compares to a length of only 233 feet for supplemental use providing 25% duty cycle and 24 hour periods over 4 weeks. The instant invention is well suited to meet requirements for a DX ground-source heat exchange loop used in conjunction with an air-source heat pump
| Number | Date | Country | |
|---|---|---|---|
| 63438621 | Jan 2023 | US |
