Geothermal heating and cooling system

Abstract

The present invention relates to a geothermal heating and cooling system comprising a conduit comprising a biaxially oriented tube made by a process comprising: a) shaping a polymer composition comprising an ethylene-based polymer and / or a propylene-based polymer into a tube, and b) drawing the tube in the axial direction and in the circumferential direction to obtain a biaxially oriented tube.

Description

[0001] The present invention relates to geothermal heating and cooling systems.

[0002] Geothermal heating and cooling systems utilize the earth as a heat source and heat sink. Geothermal heating and cooling systems circulate a water-based solution through pipes containing tubes buried underground. High heat transfer efficiency of the tubes is important to reduce the length of the tubes to reduce installation costs.

[0003] In order to improve the heat transfer efficiency, the structure of the conduit in the geothermal heating and cooling system has been improved in the technical field. For example, WO2011 / 104607 discloses a conduit comprising two or more tubes twisted together for improving the heat transfer efficiency. WO2011 / 104607 mentions that the tubes useful in the present disclosure are plastic tubes, preferably polyethylene and cross-linked high-density polyethylene. Although the structure of the conduit is important for the heat transfer efficiency, another important factor is the material of the tube.

[0004] US2011 / 0011558 discloses a geothermal system for heating and cooling a building structure, the system comprising a plastic pipe embedded with heat transfer particles; and the elastic modulus of the pipe is less than 200,000 pounds force per square inch (psi).

[0005] Geothermal heating systems can take advantage of higher ground temperatures by installing the pipes deep underground, as ground temperatures rise with depth. After about 15 meters deep, the ground temperature rises by one degree every 30 meters. However, the pressure to which the pipe is subjected also increases with depth. Commonly used geothermal probe pipes are made of materials with an MRS value of 10MPa and a ratio of outer diameter (D) to wall thickness (W), commonly referred to as SDR, of about 11. This ensures that the bored heat exchanger tubes can withstand a pressure of 16 bar, thus reaching a usable depth of about 160 meters. For deeper geothermal probe pipes, considerable internal hydrostatic pressures are encountered. Based on a current practice of using tapered pipes, EP 2706308 suggests increasing the wall thickness of the pipe as the pipe goes deeper into the ground. This is a cumbersome method that limits the volume flow of the fluid if the inner diameter is reduced. If the outer diameter is gradually increased, it is difficult to coil and install such a pipe.

[0006] The type of ground is a determining factor in whether a geothermal heating and cooling system can be used in that ground for a long time. Once the pipes are installed in the ground, they are constantly subjected to pressure from the surrounding environment. The pressure from the surrounding environment varies depending on the type of ground. For example, rock in the surrounding soil creates a greater external point load than sand. Movement of the ground, such as earthquakes, further increases the external point loads.

[0007] Cracks appear on the outer surface of the pipe subjected to large external point loads, leading to pipe failure. Therefore, rock environments require pipes to have higher resistance to external point loads. Therefore, higher resistance to external point loads is required to allow installation in different types of environments.

[0008] It is an object of the present invention to provide a geothermal heating and cooling system in which the above-mentioned and / or other problems are solved.

[0009] Thus, the present invention provides a geothermal heating and cooling system comprising a conduit comprising a biaxially oriented tube made by a process comprising: a) forming a polymer composition into a tube and b) drawing the tube in an axial direction and in a circumferential direction to obtain the biaxially oriented tube.

[0010] The present inventors have recognized that the use of biaxially oriented tubing in geothermal heating and cooling systems provides various advantages.

[0011] Thermal conductivity through the tube wall is inversely proportional to the wall thickness of the tube, i.e., the smaller the wall thickness, the higher the thermal conductivity. However, lower wall thickness results in lower resistance to internal pressure of the tube. The inventors have realized that at the same wall thickness, biaxially oriented tubes have a higher resistance to internal pressure than conventional tubes, so for biaxially oriented tubes, reduced wall thickness can be used to withstand the same internal pressure. The reduction in wall thickness results in higher thermal conductivity. At the same outer diameter, reduced wall thickness also results in higher fluid capacity.

[0012] Higher resistance to pressure is particularly important in geothermal heating and cooling systems, where the pipe's resistance to internal pressure determines the depth to which the pipe can be installed. In order to install biaxially oriented pipe to the same depth as conventional pipe, the biaxially oriented pipe can be made to have a greater SDR than conventional pipe. This has a positive effect on thermal conductivity. In addition, the higher resistance to internal pressure of biaxially oriented pipe allows the biaxially oriented pipe to be installed deeper underground than conventional pipe with the same SDR. This has the advantage of taking advantage of the higher temperatures at greater depths.

[0013] Furthermore, the biaxially oriented pipe has great resistance to point loads that may be encountered in a borehole. Thus, the present invention provides a geothermal heating and cooling system that can effectively utilize geothermal heat even in environments where the pipe is subject to large external point loads (such as rock soil).

[0014] Geothermal Heating and Cooling Systems

[0015] Geothermal heating and cooling systems are well known per se, as described in US2011011558, in particular with reference to the accompanying drawings. The conduit of the system according to the invention comprises a closed loop comprising a portion made of biaxially oriented tubing. The system comprises suitable means for circulating a fluid in the conduit. The direction of circulation of the fluid depends on whether the system is used for heating or cooling. The conduit is connected to a heat pump or other fluid heat exchanger for heating or cooling a building or other structure. According to the invention, at least the portion of the conduit to be placed underground comprises a portion made of biaxially oriented tubing.

[0016] In some embodiments, the system is a vertical system. In the vertical system, the conduit includes a first vertically arranged tube (inflow tube) having a first outer diameter D1 and a first wall thickness W1 configured to transfer fluid downward, a second vertically arranged tube (return tube) having a second outer diameter D2 and a second wall thickness W2 configured to transfer fluid upward, and a U-shaped connecting tube connecting the lower ends of the first vertical tube and the second vertical tube. D1 / W1 may be greater than, equal to, or less than D2 / W2.

[0017] Preferably, at least the first vertically arranged tube is a biaxially oriented tube. In some embodiments, the first vertically arranged tube is a biaxially oriented tube and the second vertically arranged tube is a biaxially oriented tube. In some embodiments, the first vertically arranged tube is a biaxially oriented tube and the second vertically arranged tube is a uniaxially oriented tube.

[0018] It is desirable if the first vertically arranged tubes have a high thermal conductivity, since heat transfer between the fluid in the first vertically arranged tubes and the surrounding environment becomes efficient. The second vertically arranged tubes do not have to have a high thermal conductivity, since the temperature of the fluid delivered to the building should be maintained. Therefore, it is preferred that D1 / W1 is greater than D2 / W2.

[0019] In some embodiments, the system is a horizontal system.

[0020] Biaxially oriented tube

[0021] Preferably, the biaxially oriented tube has a substantially constant ratio of outer diameter to wall thickness, for example the ratio deviates by at most 5% in the longitudinal direction of the tube.

[0022] The biaxially oriented tube is composed of a biaxially oriented polymer composition made from a polymer composition comprising an ethylene-based polymer and / or a propylene-based polymer.

[0023] Preferably, the biaxially oriented tube has an MRS value of at least 10.0 MPa at 20°C over a period of 50 years, more preferably at least 12.0 MPa at 20°C over a period of 50 years, more preferably at least 14.0 MPa at 20°C over a period of 50 years, more preferably at least 16.0 MPa at 20°C over a period of 50 years.

[0024] Preferably, the biaxially oriented tube has an outer diameter of 25 mm to 250 mm, preferably 25 mm to 63 mm, more preferably 25 mm to 50 mm or 25 mm to 35 mm.

[0025] Preferably, the biaxially oriented tube has an outside diameter to wall thickness ratio (SDR) of 5.0 to 20, preferably 7.0 to 20.

[0026] In some preferred embodiments, at least a portion of the biaxially oriented tube is located at a depth of 100 to 160 meters and has an outer diameter to wall thickness ratio of 11 to 17.

[0027] In some preferred embodiments, at least a portion of the biaxially oriented tube is located at a depth of 160 to 256 meters and has an outer diameter to wall thickness ratio of 5 to 11.

[0028] Preferably, the biaxially oriented tube has an outer diameter of 25 mm to 250 mm, preferably 25 mm to 63 mm, more preferably 25 mm to 50 mm or 25 mm to 35 mm, and an outer diameter to wall thickness ratio (SDR) of 5.0 to 20, preferably 7.0 to 20, more preferably 11 to 20, and has a failure time of at least 500 hours, more preferably at least 1000 hours, preferably at least 2000 hours, more preferably at least 3000 hours, according to the accelerated point load test of PAS1075:2009, in particular according to the test instructions in Appendix A3 for point load testing of full wall tubes in an aqueous solution of NM5 at a hoop stress of 4 MPa and a temperature of 90°C.

[0029] Preferably, the biaxially oriented tube has an outer diameter of 25 to 35 mm, for example 32 mm and an outer diameter to wall thickness ratio (SDR) of 15 to 20, for example 17, and has a failure time of at least 500 hours, more preferably at least 1000 hours, preferably at least 2000 hours, more preferably at least 3000 hours, according to the accelerated point load test of PAS1075:2009, in particular according to the test instructions in Appendix A3 for point load testing of full wall tubes in an aqueous solution of NM5 at a hoop stress of 4 MPa and a temperature of 90°C.

[0030] Ethylene-based polymers

[0031] Preferably, the polymer composition comprises an ethylene-based polymer.

[0032] In some embodiments, the amount of ethylene-based polymer relative to the total amount of polymer in the polymer composition is at least 95 weight percent, at least 98 weight percent, at least 99 weight percent, or 100 weight percent.

[0033] In some embodiments, the amount of ethylene-based polymer is at least 90 wt%, at least 95 wt%, at least 98 wt%, at least 99 wt%, or 100 wt% relative to the polymer composition.

[0034] Preferably, the ethylene-based polymer comprises high density polyethylene (HDPE).

[0035] In some embodiments, the amount of HDPE is at least 95 wt%, at least 98 wt%, at least 99 wt%, or 100 wt% relative to the total amount of polymer in the polymer composition.

[0036] In some embodiments, the amount of HDPE is at least 90 wt%, at least 95 wt%, at least 98 wt%, at least 99 wt%, or 100 wt% relative to the polymer composition.

[0037] In some embodiments, the ethylene-based polymer comprises additional polyethylene in addition to HDPE. The additional polyethylene may be, for example, a linear low-density polyethylene (LLDPE), a low-density polyethylene (LDPE), or a combination of LLDPE and LDPE. Preferably, the additional polyethylene is LLDPE or a combination of LLDPE and LDPE. More preferably, the additional polyethylene is LLDPE. If the additional polyethylene is a combination of LLDPE and LDPE, the weight ratio of LLDPE to LDPE may be, for example, at least 0.1, such as at least 0.2 or at least 0.3, and at most 10, such as at most 5 or at most 3. Preferably, the weight ratio of LLDPE to LDPE is at least 1, such as 2 to 10.

[0038] Preferably, the weight ratio of HDPE to additional polyethylene is greater than 1, preferably 1.2 to 5, such as 1.5 to 4 or 2 to 3. In some embodiments, the ethylene-based polymer contains substantially no additional polyethylene other than HDPE. The amount of HDPE in the polymer composition relative to the total ethylene-based polymer may be at least 95 wt%, at least 98 wt%, at least 99 wt%, or 100 wt%.

[0039] Preferably, the polymer composition has a melt flow rate of 0.1 to 4.0 g / 10 min, more preferably 0.1 to 1.0 g / 10 min, as determined according to ISO 1133-1:2011 (190° C. / 5 kg).

[0040] The production processes of HDPE, LLDPE and LDPE are summarized in Handbook of Polyethylene by Andrew Peacock (2000; Dekker; ISBN 0824795466), pages 43-66.

[0041] HDPE

[0042] Preferably, the HDPE is bimodal or multimodal (e.g. trimodal). Such HDPE has properties suitable for producing pipes. Furthermore, the bimodality or multimodality of the HDPE may allow such HDPE to be stretched at low stretch ratios without causing necking problems.

[0043] It is understood that a bimodal HDPE has a molecular weight distribution with two peaks corresponding to a first median and a second median at corresponding stages in the polymerization. Similarly it is understood that a multimodal HDPE has a molecular weight distribution with multiple peaks corresponding to a first median, a second median and one or more further medians at corresponding stages in the polymerization.

[0044] HDPE may be an ethylene homopolymer or may contain comonomers such as 1-butene or 1-hexene.

[0045] Preferably, the HDPE has a hardness of 940 to 960 kg / m 3 , more preferably 940 to 955 kg / m 3 density.

[0046] Preferably, the HDPE has a melt flow rate of 0.1 to 4.0 g / 10 min, more preferably 0.1 to 1.0 g / 10 min, determined according to ISO 1133-1:2011 (190° C. / 5 kg).

[0047] Preferably, the HDPE has a melt flow rate of 5 to 15 g / 10 min, more preferably 0.1 to 1.0 g / 10 min, determined according to ISO 1133-1:2011 (190°C / 21.6 kg).

[0048] In some embodiments, the composition includes a compound comprising HDPE and a colorant, wherein the compound has a carbon content of 947-965 kg / m 3 The colorant may be carbon black or a pigment having a color such as black, blue or orange. The amount of colorant is typically 1.0-5.0 wt %, more typically 2.0-2.5 wt %, relative to the compound containing HDPE and colorant, the remainder being typically HDPE.

[0049] HDPE can be produced by using a low-pressure polymerization process. For example, pipe materials of performance classes PE 80 and PE 100 are known, which are generally produced in cascade devices by so-called bimodal or multimodal processes. The production process of bimodal HDPE is summarized in "PE 100 Pipe systems" (Bromstrup ed.; Second edition, ISBN 3-8027-2728-2) pages 16-20. Suitable low-pressure processes are a combination of slurry cascades of stirred reactors, slurry cascades of loop reactors and different methods such as slurry loop gas phase reactors. It is also possible to use multimodal polyethylene, preferably trimodal polyethylene, as described in, for example, WO2007003530 as a high-density polyethylene pipe material.

[0050] The performance categories PE 80 and PE 100 are discussed in "PE 100 Pipe systems" (Bromstrup, ed.; 2nd edition, ISBN 3-8027-2728-2), pages 35-42. The quality test methods are described in "PE 100 Pipe systems", pages 51-62.

[0051] The production of bimodal high density polyethylene (HDPE) via a low pressure slurry process is described in "Bimodalpolyethylene-Interplay of catalyst and process" (Macromol.Symp.2001,163,135-143) by Alt et al. In a two-stage cascade process, the reactor can be continuously fed with a mixture of monomer, hydrogen, catalyst / cocatalyst and hexane recycled from the process. In the reactor, the polymerization of ethylene occurs as an exothermic reaction at a pressure between, for example, 0.5MPa (5 bar) and 1MPa (10 bar) and at a temperature between, for example, 75°C and 85°C. The heat of the polymerization reaction is removed by cooling water. The characteristics of polyethylene are determined in particular by the applied concentrations of the catalyst system and catalyst, comonomer and hydrogen. The concept of a two-stage cascade process is illustrated by Alt et al. at "Bimodal polyethylene-Interplay of catalyst andprocess" (Macromol.Symp.2001,163) on pages 137-138. The reactors are arranged in cascade with different conditions in each reactor, including low hydrogen content in the second reactor. This allows the production of HDPE with a bimodal molecular weight distribution and defined comonomer content in the polyethylene chain in each reactor.

[0052] Preferred examples of HDPE include bimodal PE 80, bimodal PE 100 and multimodal HDPE. PE 80 is a PE material having an MRS of 8 MPa (minimum required strength for water after 50 years at 20 degrees Celsius), and PE 100 is a PE material having an MRS of 10 MPa. Pipe classification is explained on page 35 of "PE 100 Pipe systems" (Bromstrup, ed.; 2nd edition, ISBN 3-8027-2728-2).

[0053] Preferably, the HDPE or a compound comprising HDPE and a colorant has one or more, preferably all, of the following properties:

[0054] A tensile modulus of 500 to 1400 MPa, preferably 700 to 1200 MPa (according to ISO 527-2);

[0055] Yield stress of 15 to 32 MPa, preferably 18 to 28 MPa (according to ISO 527-2);

[0056] Full notch creep test (FNCT): 100-20000 hours (according to ISO 16770, at 80°C / 4MPa).

[0057] It is noted that bimodal or multimodal HDPE is understood herein as a polyethylene produced in a reactor cascade, wherein a first type of polyethylene is produced in a first reactor and transferred to a subsequent reactor, and a second type of polyethylene is produced in the subsequent reactor in the presence of the first type of polyethylene. Thus, bimodal or multimodal HDPE does not include blends of polyethylenes of different types obtained independently.

[0058] LLDPE

[0059] The ethylene-based polymer may include LLDPE.

[0060] Technologies suitable for LLDPE production include gas phase fluidized bed polymerization, solution polymerization, polymer melt polymerization under very high ethylene pressure, and slurry polymerization.

[0061] LLDPE comprises ethylene and a C3-C10 α-olefin comonomer (ethylene-α-olefin copolymer). Suitable α-olefin comonomers include 1-butene, 1-hexene, 4-methylpentene and 1-octene.

[0062] The preferred comonomer is 1-hexene.

[0063] Preferably, the α-olefin comonomer is present in an amount of about 5 to about 20 weight percent of the ethylene-α-olefin copolymer, more preferably about 7 to about 15 weight percent of the ethylene-α-olefin copolymer.

[0064] Preferably, the LLDPE has a hardness of 900 to 940 kg / m 3 , more preferably 915 to 935 kg / m 3 , more preferably 920 to 935 kg / m 3 density.

[0065] Preferably, the LLDPE has a melt flow rate of 0.1 to 3.0 g / 10 min, more preferably 0.3 to 3.0 g / 10 min, measured according to ISO 1133-1:2011 (190° C. / 2.16 kg).

[0066] LDPE

[0067] The ethylene-based polymer may include LDPE.

[0068] LDPE can be produced using autoclave high pressure technology and by tubular reactor technology.

[0069] LDPE may be an ethylene homopolymer or may contain comonomers such as 1-butene or 1-hexene.

[0070] Preferably, the LDPE has a hardness of 916 to 940 kg / m 3 , more preferably 920 to 935 kg / m 3 Preferably, the LDPE has a melt flow rate of 0.1 to 3.0 g / 10 min, more preferably 0.3 to 3.0 g / 10 min, measured according to ISO 1133-1:2011 (190° C. / 2.16 kg).

[0071] Propylene-based polymers

[0072] In some embodiments, the polymer composition includes a propylene-based polymer.

[0073] In some embodiments, the amount of propylene-based polymer relative to the total amount of polymers in the polymer composition is at least 95 wt%, at least 98 wt%, at least 99 wt%, or 100 wt%.

[0074] In some embodiments, the amount of propylene-based polymer is at least 90 wt%, at least 95 wt%, at least 98 wt%, at least 99 wt%, or 100 wt% relative to the polymer composition.

[0075] The propylene-based polymer may comprise or may be a propylene copolymer selected from the group consisting of a random copolymer, a (multi)block copolymer and a heterophasic propylene copolymer and combinations thereof. The propylene-based polymer may further comprise a propylene homopolymer.

[0076] Non-heterophasic propylene copolymers

[0077] The polypropylene based polymer may comprise or may be a propylene copolymer including random copolymers and (multi)block copolymers.

[0078] The copolymer is preferably a random copolymer. The copolymer can be composed of at least 70% by weight of propylene monomer units and up to 30% by weight of ethylene and / or alpha-olefin monomer units based on the total weight of the copolymer. Preferably, the alpha-olefin is selected from the group of alpha-olefins with 4 to 10 carbon atoms, such as 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene or 1-octene. The propylene copolymer is preferably a propylene-ethylene copolymer.

[0079] The amount of ethylene and / or α-olefin monomer units in the propylene copolymer is preferably 1-15 wt%, more preferably 1-10 wt%, more preferably 1-6 wt%, more preferably 1-4 wt%, based on the total weight of the propylene copolymer. When the polypropylene-based polymer comprises a propylene α-olefin copolymer, the propylene copolymer is preferably a propylene-ethylene random copolymer, wherein the amount of ethylene monomer units is 1-15 wt%, more preferably 1-10 wt%, more preferably 1-6 wt%, more preferably 1-4 wt%, based on the total weight of the propylene copolymer.

[0080] Some preferred propylene copolymers may have an MFI of, for example, 0.1 to 10.0 dg / min, preferably 0.4 to 4.0 dg / min, more preferably 0.1 to 1.0 dg / min, measured according to ISO 1133-1:2011 (2.16 kg / 230°C).

[0081] In some preferred embodiments, the propylene-based polymer is or comprises a mixture of a propylene homopolymer and a propylene copolymer, such as a propylene-ethylene copolymer.

[0082] Heterophasic propylene copolymer

[0083] The polypropylene based polymer may comprise or may be a heterophasic propylene copolymer consisting of (a1) a propylene based matrix and (a2) dispersed ethylene-α-olefin copolymers, wherein the propylene based matrix consists of a propylene homopolymer and / or a propylene copolymer, the propylene copolymer consisting of at least 90 wt-% propylene monomer units and at most 10 wt-% ethylene and / or α-olefin monomer units, based on the total weight of the propylene based matrix, wherein the total amount of the propylene based matrix and the total amount of the dispersed ethylene-α-olefin copolymers in the heterophasic propylene copolymer sums up to 100 wt-%.

[0084] The heterophasic propylene copolymer of the composition of the present invention consists of a propylene-based matrix and a dispersed ethylene-α-olefin copolymer. The propylene-based matrix generally forms the continuous phase in the heterophasic propylene copolymer. As is well known in the art, the amount of the propylene-based matrix and the dispersed ethylene-α-olefin copolymer can be varied by 13 C-NMR determination.

[0085] The propylene-based matrix consists of a propylene homopolymer and / or a propylene copolymer consisting of at least 90 wt% of propylene monomer units and up to 10 wt% of comonomer units selected from ethylene monomer units and α-olefins having 4 to 10 carbon atoms, e.g. at least 95 wt% of propylene monomer units and up to 5 wt% of comonomer units, based on the total weight of the propylene-based matrix.

[0086] Preferably, the comonomer in the propylene copolymer of the propylene based matrix is ​​selected from ethylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene and 1-octene, and is preferably ethylene.

[0087] Preferably, the propylene-based matrix consists of a propylene homopolymer.Compared to the case where the propylene-based matrix is ​​a propylene-α-olefin copolymer, it is advantageous that the propylene-based matrix consists of a propylene homopolymer because higher rigidity is obtained.

[0088] Preferably, the propylene based matrix is ​​present in an amount of 60 to 98 wt%, such as up to 97 wt%, up to 96 wt%, up to 95 wt%, up to 93 wt% or up to 91 wt%, based on the total heterophasic propylene copolymer. Preferably, the propylene based matrix is ​​present in an amount of at least 70 wt%, more preferably at least 75 wt%, such as at least 80 wt%, at least 85 wt%, at least 87 wt% or at least 90 wt%, based on the total heterophasic propylene copolymer.

[0089] The propylene-based matrix is ​​preferably semi-crystalline, i.e. it is not 100% amorphous, nor 100% crystalline. For example, the propylene-based matrix is ​​at least 40% crystalline, such as at least 50%, such as at least 60% crystalline and / or such as up to 80% crystalline, such as up to 70% crystalline. For example, the propylene-based matrix has a crystallinity of 60 to 70%. For the purposes of the present invention, the crystallinity of the propylene-based matrix is ​​measured according to ISO 11357-1 and ISO 11357-3 of 1997 using differential scanning calorimetry (DSC), using a scan rate of 10°C / min, a sample of 5 mg, and the second heating curve using 207.1 J / g as a theoretical standard for 100% crystalline material.

[0090] In addition to the propylene-based matrix, the heterophasic propylene copolymer also includes dispersed ethylene-alpha-olefin copolymers. Dispersed ethylene-alpha-olefin copolymers are also referred to as "dispersed phase" in this article. Dispersed phase embeds the heterophasic propylene copolymer in a discontinuous form. The particle size of the dispersed phase is generally in the range of 0.05 to 2.0 microns, as can be measured by transmission electron microscopy (TEM). The amount of the dispersed ethylene-alpha-olefin copolymers in the heterophasic propylene copolymer can sometimes be referred to as RC in this article.

[0091] Preferably, the amount of ethylene monomer units in the ethylene-alpha-olefin copolymer is 5 to 65 wt%, such as at least 10 wt%, at least 20 wt%, at least 30 wt%, at least 40 wt% or at least 45 wt% and / or at most 60 wt%, at most 58 wt%, at most 55 wt% or at most 50 wt%. The amount of ethylene monomer units in the ethylene-alpha-olefin copolymer dispersed in the heterophasic propylene copolymer may sometimes be referred to herein as RCC2.

[0092] The α-olefin in the ethylene-α-olefin copolymer is preferably selected from the group of α-olefins with 3 to 8 carbon atoms. Examples of suitable α-olefins with 3 to 8 carbon atoms include, but are not limited to, propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene and 1-octene. More preferably, the α-olefin in the ethylene-α-olefin copolymer is selected from the group of α-olefins with 3 to 4 carbon atoms and any mixture thereof, more preferably the α-olefin is propylene, in which case the ethylene-α-olefin copolymer is an ethylene-propylene copolymer.

[0093] Preferably, the dispersed ethylene alpha-olefin copolymer is present in an amount of 2.0 to 40 wt%, such as at least 3.0 wt%, at least 4.0 wt%, at least 5.0 wt%, at least 7.0 wt% or at least 9.0 wt%, based on the total heterophasic propylene copolymer. Preferably, the dispersed ethylene alpha-olefin copolymer is present in an amount of up to 30 wt%, more preferably up to 25 wt%, such as up to 20 wt%, up to 15 wt%, up to 13 wt% or up to 10 wt%, based on the total heterophasic propylene copolymer.

[0094] In the heterophasic propylene copolymer in the composition of the present invention the sum of the total weight of the propylene based matrix and the total weight of the dispersed ethylene α-olefin copolymer is 100 wt% of the heterophasic propylene copolymer.

[0095] Preferably, the heterophasic propylene copolymer has an MFI of 0.1 to 10.0 g / 10 min, more preferably 0.1 to 4.0 g / 10 min, especially 0.1 to 1.0 g / 10 min, determined according to ISO 1133-1:2011 (230°C / 2.16 kg).

[0096] Other components

[0097] The polymer composition may include components other than the ethylene-based polymer and the propylene-based polymer, such as additives and fillers. The strength of the additives may include nucleating agents; stabilizers, such as heat stabilizers, antioxidants, UV stabilizers; colorants, such as pigments and dyes; clarifying agents; surface tension modifiers; lubricants; flame retardants; mold release agents; flow improvers; plasticizers; antistatic agents; external elastomeric impact modifiers; foaming agents; and / or components that enhance the interfacial bonding between the polymer and the filler, such as maleated polyethylene. The amount of additives is typically 0 to 5.0% by weight, such as 1.0 to 3.0% by weight, relative to the total composition.

[0098] Examples of fillers include glass fibers, talc, mica, nanoclay. The amount of filler is typically 0 to 40 wt%, such as 5 to 30 wt% or 10 to 25 wt%, relative to the total polymer composition.

[0099] Thus, in some embodiments, the polymer composition further comprises 0 to 5.0 wt % of additives and 0 to 40 wt % of fillers.

[0100] The polymer composition may be obtained by melt mixing the ethylene-based polymer and / or the propylene-based polymer and optionally any other optional components.

[0101] Preferably, the total amount of ethylene-based polymer, propylene-based polymer, optional additives and optional fillers is 100 wt % relative to the total polymer composition.

[0102] Method steps

[0103] The biaxially oriented tube is made by a method comprising the steps of: a) forming a polymer composition into a tube and b) drawing the tube of step a) in the axial direction and in the circumferential direction to obtain the biaxially oriented tube. The method of making the tube can be carried out as a continuous process or a batch process. A continuous process is understood herein as a process in which the polymer composition is continuously fed for the tube manufacturing step a), while the stretching step b) is carried out continuously. The polymer composition can be formed into a tube (step a) by any known method, such as extrusion or injection molding. The biaxial elongation (step b) can be carried out by any known method. Preferably, the tube obtained by step a) is not crosslinked.

[0104] The terms "tube" and "tubing" are understood herein to mean hollow, elongated articles having a cross section of various shapes. The cross section may be, for example, circular, oval, square, rectangular or triangular. The term "diameter" is understood herein to mean the largest dimension of the cross section.

[0105] A method of forming a polymer composition into a tube and biaxially stretching the tube is described in US6325959.

[0106] A conventional plant for extruding plastic pipes comprises an extruder, a nozzle, a calibration device, a cooling device, a drawing device, and a device for cutting or coiling the pipe.

[0107] To achieve biaxial orientation, the plant may be supplemented downstream of the drawing device with: a device for controlling the temperature of the tube at a temperature suitable for biaxially orienting the tube, an orientation device, a calibration device, a cooling device, and a drawing device for supplying the biaxially oriented tube to a cutting device or a coiling device.

[0108] Biaxial orientation of the tube can be performed in various ways, such as mechanically with the aid of an internal mandrel, or with an internal pressurized fluid such as air or water or the like. Another method is to orient the tube with the aid of rollers, such as by arranging the tube on a mandrel and rotating the mandrel and tube relative to one or more pressure rollers engaging the tube, or via an internally arranged pressure roller that rotates relative to the tube relative to an externally arranged die or calibration device.

[0109] Furthermore, Morath et al., Biaxially oriented polypropylene pipes, Plastics, Rubber and Composites, 2006, Vol. 35, No. 10, pp. 447-454, describe a method for making biaxially oriented pipes from random propylene copolymers.

[0110] Conditions of step b)

[0111] The skilled person can select suitable conditions, such as the temperature of step b) to obtain a biaxially oriented tube.Step b) is performed at the stretching temperature, which results in the orientation of the ethylene-based polymer and the propylene-based polymer in the polymer composition.

[0112] The stretching temperature is selected based on the melting points of the ethylene-based polymer and the propylene-based polymer in the polymer composition.

[0113] Tm and Td

[0114] The melting point Tm of the polymer composition was determined by differential scanning calorimetry according to ASTM D3418. The DSC measurements were performed using a DSC TA Q20 and an Intracooler capable of reaching -90°C. The measurements were performed under a nitrogen flow to avoid degradation. The methodology followed was:

[0115] First heating: 10°C / min, -40°C to 230°C (hold at final temperature for 3 minutes)

[0116] Cooling: 10℃ / min, 230℃ to -40℃

[0117] Second heating: 10℃ / min, -40℃ to 230℃

[0118] The sample used was 3 to 5 mg

[0119] The melting point is the peak melting temperature observed during the second heating cycle.

[0120] When the polymer composition contains different propylene-based polymers or different ethylene-based polymers, more than one melting peak may be observed in the second heating cycle. In this case, the melting peak belonging to the propylene-based polymer or ethylene-based polymer present in the composition in the highest amount defines the Tm of the polymer composition. If more than one propylene-based polymer or ethylene-based polymer is present in the highest amount (e.g., a blend of 50 wt. % of the first propylene-based polymer and 50 wt. % of the second propylene-based polymer, or a blend of 40 wt. % of the first propylene-based polymer, 40 wt. % of the second propylene-based polymer and 20 wt. % of the third propylene-based polymer), the highest temperature among the temperatures of the melting peaks of the polymers present in the highest amount is defined as the Tm of the polymer composition.

[0121] Prior to stretching, the tubes of step a) are heated so that they have the desired stretching temperature. This can be done by soaking the tubes of step a) at the desired stretching temperature for a period sufficient to reach thermal equilibrium, for example 30 minutes. The temperature of the tubes is preferably controlled within ±1°C.

[0122] The stretching temperature may be selected to be below the melting point of the propylene-based polymer in the polymer composition.

[0123] The stretching temperature may be 1 to 30°C lower than the melting point of the polymer composition, for example 2 to 20°C or 3 to 10°C.

[0124] In some embodiments where the amount of propylene-based polymer is at least 95 wt% relative to the total amount of polymers in the polymer composition, step b) is conducted at a stretching temperature of 140 to 160 °C, preferably 145 to 155 °C.

[0125] When the polymer composition contains a propylene-based polymer including a heterophasic propylene copolymer, the stretching temperature may be selected to be equal to or higher than the melting point of the polymer composition.

[0126] For example, Td is equal to or higher than Tm. For example, Td≥Tm+0.1°C, Td≥Tm+0.3°C, Td≥Tm+0.5°C, Td≥Tm+1.0°C, Td≥Tm+2.0°C, Td≥Tm+3.0°C, Td≥Tm+5.0°C, Td≥Tm+8.0°C or Td≥Tm+10.0°C. Preferably, Tm+1.0°C≤Td≤Tm+15.0°C. More preferably, Tm+1.0°C≤Td≤Tm+10.0°C.

[0127] In some embodiments, Tm is 150 to 165°C and Td is 150 to 170°C, wherein Tm ≤ Td ≤ Tm + 15.0°C.

[0128] Stretch ratio

[0129] Typically, step b) is carried out with an axial stretch ratio higher than 1.0, for example from 1.1 to 5.0, and an average hoop stretch ratio higher than 1.0, for example from 1.1 to 3.0.

[0130] Preferably, the average hoop stretch ratio is from 1.1 to 2.0.

[0131] Preferably, the axial stretch ratio is from 1.1 to 4.0, such as from 1.1 to 3.6 or from 1.1 to 3.2. The axial stretch ratio is usually higher to obtain a biaxially oriented tube with a larger outer diameter.

[0132] The axial stretch ratio of the stretched tube is defined as the ratio of the cross-sectional area of ​​the starting isotropic tube to the cross-sectional area of ​​the biaxially oriented tube (i.e., the product), i.e.,

[0133]

[0134] OD stands for outer diameter and ID stands for inner diameter.

[0135] In the case of tube expansion, the hoop stretch ratio of the product varies from the inner wall to the outer wall. These stretch ratios are defined as:

[0136]

[0137] The average hoop stretch ratio can be defined as:

[0138]

[0139] in

[0140]

[0141] It is to be noted that the present invention relates to all possible combinations of features described herein, preferably in particular those combinations of features presented in the claims. Thus, it will be understood that all combinations of features relating to the composition according to the invention, all combinations of features relating to the method according to the invention, and all combinations of features relating to the composition according to the invention and to the method according to the invention are described herein.

[0142] It should also be noted that the terms "comprising", "including", "containing" do not exclude the presence of other elements. However, it is also understood that the description of a product / composition comprising certain components also discloses a product / composition consisting of these components. A product / composition consisting of these components may be advantageous because it provides a simpler and more economical method for preparing the product / composition. Similarly, it is also understood that the description of a method comprising certain steps also discloses a method consisting of these steps. A method consisting of these steps may be advantageous because it provides a simpler and more economical method.

[0143] When values ​​are mentioned for a lower limit and an upper limit for a parameter, it is understood that the range resulting from the combination of the value for the lower limit and the value for the upper limit is also disclosed.

[0144] The present invention is now illustrated with the aid of the following examples, but the invention is not limited thereto.

[0145] Material:

[0146] HDPE1: A continuous stirred tank reactor (CSTR) with a total volume of 20 L and an operating volume of 15 L was used to prepare a final density of 948 kg / m 3 , a trimodal HDPE with a melt index (21.6 kg, 190° C.) of 11.2 and a melt index (5 kg, 190° C.) of 0.48. The catalyst applied was a commercial Ziegler-Natta catalyst / cocatalyst system.

[0147] program:

[0148] HDPE1 was pelletized using a twin screw extruder. The processing temperature and screw profile were standard for polyethylene compounding.

[0149] These compounded pellets were used to produce thick tubular profiles with approximate dimensions of about 32 mm outer diameter and about 16 mm inner diameter. These thick tubes were stretched on an expansion conical mandrel with an outlet diameter of 32 mm and a half angle of 15 degrees at a temperature of 120° C. at a stretching speed of 100 mm / min. The axial stretch ratio was 3.2 and the average hoop stretch ratio was 1.25. A biaxially oriented tube with an outer diameter of 32 mm and a wall thickness of 2 mm was obtained.

[0150] The sample "isotropic tube" was made from HDPE1 by extrusion into a tube with an outer diameter of 32 mm and a wall thickness of 2.9 mm without a drawing step.

[0151] Internal pressure test

[0152] The biaxially oriented tubes and the isotropic tubes were internally pressure tested according to ISO 9080 at 20°C under the pressures shown in Table 1. The results are shown in Table 1.

[0153] Table 1

[0154] sample Stress(MPa) Failure time (h) Failure Mode Isotropic tube 17 immediately Ductility Biaxially oriented tube 17 Under test >25210 - Biaxially oriented tube 19 12100 Ductility Biaxially oriented tube 20 8050 Ductility

[0155] Accelerated Point Load Test (PLT+)

[0156] Accelerated point load tests are performed on biaxially oriented and isotropic pipes in accordance with PAS1075:2009, in particular the test instructions for point load tests on full wall pipes in Annex A3. The test is performed in an aqueous solution of NM5 at a hoop stress of 4 MPa and a temperature of 90°C.

[0157] sample Failure time (h) Isotropic tube <400 Biaxially oriented tube >5975

Claims

1. A geothermal heating and cooling system comprising a conduit comprising a biaxially oriented tube, the biaxially oriented tube being made by a process comprising: a) forming a polymer composition comprising an ethylene-based polymer and / or a propylene-based polymer into a tube and b) drawing the tube in an axial direction and in a circumferential direction to obtain the biaxially oriented tube.

2. The system of claim 1, wherein the biaxially oriented tube has an outer diameter of 25 to 250 mm and / or an outer diameter to wall thickness ratio (SDR) of 5.0 to 20.

3. The system of any of the preceding claims, wherein the biaxially oriented tube has an MRS value of at least 10.0 MPa at 20°C over a period of 50 years, more preferably at least 12.0 MPa at 20°C over a period of 50 years, more preferably at least 14.0 MPa at 20°C over a period of 50 years, and at least 16.0 MPa at 20°C over a period of 50 years.

4. A system according to any of the preceding claims, wherein the biaxially oriented tube has an outer diameter of 25 mm to 63 mm and an outer diameter to wall thickness ratio (SDR) of 11 to 20, and has a failure time of at least 500 hours, more preferably at least 1000 hours, preferably at least 2000 hours, more preferably at least 3000 hours, according to PAS1075:2009 Annex A3 at a hoop stress of 4 MPa and a temperature of 90°C.

5. A system according to any one of the preceding claims, wherein the biaxially oriented tube has an outer diameter of 25 to 35 mm, such as 32 mm, and an outer diameter to wall thickness ratio (SDR) of 15 to 20, such as 17, and has a failure time of at least 500 hours, more preferably at least 1000 hours, preferably at least 2000 hours, more preferably at least 3000 hours, according to PAS1075:2009 Annex A3 at a hoop stress of 4 MPa and a temperature of 90°C.

6. The system of any one of claims 1-6, wherein the system is a vertical system.

7. A system according to claim 7, wherein the conduit includes a first vertically arranged tube having a first outer diameter D1 and a first wall thickness W1 configured to transfer fluid downward, a second vertically arranged tube having a second outer diameter D2 and a second wall thickness W2 configured to transfer fluid upward, and a U-shaped connecting tube connecting the lower ends of the first vertical tube and the second vertical tube, wherein at least the first vertically arranged tube is a biaxially oriented tube.

8. The system of claim 8, wherein D1 / W1 is greater than D2 / W2.

9. The system according to claim 8 or 9, wherein D1 / W1 is 13 to 20 and D2 / W2 is 7 to 13.

10. The system according to any one of claims 1-6, wherein the system is a horizontal system.

11. The system of any one of the preceding claims, wherein the ethylene-based polymer comprises a high density polyethylene (HDPE), preferably having a density of 940 to 960 kg / m 3 and / or a melt flow rate of 0.1 to 4.0 g / 10 min, more preferably 0.1 to 1.0 g / 10 min, measured at 190° C. and 5 kg according to ISO 1133-1:2011, preferably Wherein the HDPE is bimodal or multimodal HDPE, and / or The ethylene-based polymer further comprises linear low density polyethylene (LLDPE) and / or low density polyethylene polymer (LDPE).

12. The system of any one of claims 1-12, wherein the amount of the ethylene-based polymer relative to the total amount of polymers in the polymer composition is at least 95 wt%, at least 98 wt%, at least 99 wt%, or 100 wt%.

13. The system of any one of claims 1-12, wherein the amount of the propylene-based polymer relative to the total amount of polymers in the polymer composition is at least 95 wt%, at least 98 wt%, at least 99 wt%, or 100 wt%.

14. System according to any one of the preceding claims, wherein the tube obtained by step a) is not cross-linked.

15. Use of a biaxially oriented tube as a conduit for geothermal heating and cooling systems, wherein the biaxially oriented tube is made by a process comprising: a) forming a polymer composition comprising a polyolefin into a tube, and b) drawing the tube in an axial direction and in a circumferential direction to obtain the biaxially oriented tube.

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