Pipe collector for heat pump systems

Abstract

A collector for a heat pump installation includes a pipe to circulate a heat transfer liquid between a heat source and a heat pump. An inner surface of the pipe has an uneven surface structure that includes at least one of indentations or elevations extending helically in a longitudinal direction. The indentations or elevations are arranged to create a turbulent flow of the heat transfer liquid in the pipe.

Claims

1. A single pipe collector for a heat pump installation, comprising: a polymer pipe configured to circulate a heat transfer liquid between a geothermal heat source and a heat pump, the polymer pipe having a center axis that is exclusive to the polymer pipe relative to a center axis of another section of the polymer pipe, and wherein the polymer pipe includes, an inner surface that includes a plurality of separate polymer indentations or elevations extending continuously in a longitudinal direction of the polymer pipe, wherein, the polymer indentations or elevations are configured to induce turbulent flow in a heat transfer liquid flowing through the polymer pipe, each polymer indentation or elevation is spaced apart from adjacent indentations or elevations, the polymer indentations or elevations extend helically in continuously alternating rotational directions in the longitudinal direction of the polymer pipe, such that the polymer indentations or elevations continuously alternate between extending helically in a common first rotational direction and extending helically in a common second rotational direction, the second rotational direction is opposite to the first rotational direction, the polymer indentations or elevations continuously alternate between the common first rotational direction and the common second rotational direction at a common interval in the longitudinal direction, and the common interval is greater than 1 meter and less than 2 meters.

2. The single pipe collector according to claim 1, wherein the indentations or elevations are spaced apart substantially uniformly in the inner surface of the polymer pipe.

3. The single pipe collector according to claim 1, wherein the polymer pipe has a substantially uniform cross-sectional area.

4. A heat pump system comprising the single pipe collector according to claim 1.

5. The single pipe collector according to claim 1, wherein, at least a portion of the polymer pipe has a U-shape, such that the polymer pipe includes a bent portion of the polymer pipe between straight polymer pipe portions.

6. The single pipe collector according to claim 1, wherein, the polymer indentations or elevations continuously alternate between extending helically according to a first angle and extending helically according to indentations or elevations in the second pattern are oriented at a second angle.

7. The single pipe collector according to claim 6, wherein the first and second angles are substantially equal relative to an axis that crosses the inner surface of the polymer pipe.

8. The single pipe collector according to claim 1, wherein the plurality of separate polymer indentations or elevations include structures selected from a group consisting of protrusions and grooves.

9. A collector comprising: a polymer pipe in a non-coaxial configuration, the polymer pipe being configured to circulate a heat transfer liquid between a geothermal heat source and a heat pump, the polymer pipe including a first length and a second length coupled to the first length, and wherein the polymer pipe includes, an inner surface that includes a plurality of separate polymer indentations or elevations extending continuously in a longitudinal direction of the polymer pipe, wherein, the polymer indentations or elevations are configured to induce turbulent flow in a heat transfer liquid flowing through the polymer pipe, each polymer indentation or elevation is spaced apart from adjacent indentations or elevations, the polymer indentations or elevations extend helically in continuously alternating rotational directions in the longitudinal direction of the polymer pipe, such that the polymer indentations or elevations continuously alternate between extending helically in a common first rotational direction and extending helically in a common second rotational direction, the second rotational direction is opposite to the first rotational direction, the polymer indentations or elevations continuously alternate between the common first rotational direction and the common second rotational direction at a common interval in the longitudinal direction, and the common interval is greater than 1 meter and less than 2 meters.

10. The collector according to claim 9, wherein the indentations or elevations are spaced apart substantially uniformly in the inner surface of the polymer pipe.

11. The collector according to claim 9, wherein the polymer pipe has a substantially uniform cross-sectional area.

12. A heat pump system comprising the collector according to claim 9.

13. The collector according to claim 9, wherein, at least a portion of the polymer pipe has a U-shape, such that the polymer pipe includes a bent portion of the polymer pipe between straight polymer pipe portions.

14. The collector according to claim 9, wherein, the polymer indentations or elevations continuously alternate between extending helically according to a first angle and extending helically according to a second angle.

15. The collector according to claim 14, wherein the first and second angles are substantially equal relative to an axis that crosses the inner surface of the polymer pipe.

16. The collector according to claim 9, wherein the plurality of separate polymer indentations or elevations include structures selected from a group consisting of protrusions and grooves.

17. A single pipe collector for a heat pump installation, comprising: a polymer pipe configured to circulate a heat transfer liquid between a geothermal heat source and a heat pump, wherein the polymer pipe includes, an inner surface that includes a plurality of separate polymer indentations or elevations extending continuously in a longitudinal direction of the polymer pipe, wherein, the polymer indentations or elevations are configured to induce turbulent flow in a heat transfer liquid flowing through the polymer pipe, each polymer indentation or elevation is spaced apart from adjacent indentations or elevations, the polymer indentations or elevations extend helically in continuously alternating rotational directions in the longitudinal direction of the polymer pipe, such that the polymer indentations or elevations continuously alternate between extending helically in a common first rotational direction and extending helically in a common second rotational direction, the second rotational direction is opposite to the first rotational direction, the polymer indentations or elevations continuously alternate between the common first rotational direction and the common second rotational direction at a common interval in the longitudinal direction, and the common interval is greater than 1 meter and less than 2 meters.

Description

DESCRIPTION OF THE DRAWINGS

(1) The present invention will now be described more in detail by examples of application, by reference to the accompanying drawings, without limiting the interpretation of the invention thereto, where

(2) FIG. 1 shows the principle for a single pipe collector in the shape of a conventional U-pipe collector,

(3) FIG. 2A shows, in a cross-section of a single pipe collector, helically indentations and/or elevations on the inward surface of the collector pipe, according to an embodiment of the present invention,

(4) FIG. 2B shows, in a longitudinal, perspective cross-section, a part of the collector shown in FIG. 2A,

(5) FIG. 2C schematically shows a stretched out pipe wall in a cross-section of the collector shown in FIGS. 2A-B, and

(6) FIG. 3 shows, in a longitudinal cross-section, a part of a collector, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

(7) FIG. 1 shows the principle for a conventional U-pipe collector. According to this principle, a continuous, sealed pipe 1 is arranged in a drilled hole 2. This is for instance carried out in such a way that a plurality of single pipes are assembled together to a continuous longitudinal pipe 1 of plastics, suitably polyethylene. Since the continuous pipe 1 forms a U-shaped curve 3 in the end towards the bottom 4 of the bore hole 2, where the pipe 1 for the heat transfer liquid that is conveyed down (see arrows in the figure) in the bore hole is connected to the pipe 1 for heat transfer liquid that is conveyed up (see arrows) and out of the bore hole, the system is called U-pipe collector. In other words, the continuous pipe 1 is bent in the middle such at it forms a U-shape. FIG. 1 only shows the principle. In reality, an above mentioned U-pipe collector system is all welded in order to fulfil the requirements for safety of operation. Hence, the return bend, that is the U-shaped lower portion or the curve 3, is therefore assembled in factory for operational security reasons. The upper part 5 of the collector system is usually terminated in a manhole at ground level 6, from where the collector pipes 1, 1, 1 are connected to a heat pump (not shown). During assembly of the collector system, the return bend 3 of the collector system is positioned above the bore hole 2, whereupon advance downwards is carried out in the bore hole. Accordingly, the U-pipe forms a single, continuous conduit for the heat transfer liquid in a circuit, in direction from the heat pump down in the bore hole and back up and out of the bore hole and further back to the heat pump in the one and the same pipe.

(8) A part of the single pipe collector for a geothermal heating system, according to an embodiment of the present invention, is shown in FIG. 2A in a cross-section T and in FIG. 2B in a longitudinal cross-section L. The collector comprises a pipe 12, suitably manufactured of polyethylene, intended for assembly in a drilled energy well, in which pipe a heat transfer liquid circulates in a closed cycle for conveyance of geothermal heat to a heat pump and return of the heat transfer liquid back to the energy well. The inward surface 14 of the pipe 12 has an uneven surface structure that comprises indentations and/or elevations 16. Although that a single pipe collector in the shape of a U-pipe collector is described with reference to the figures, such a single pipe collector is also applicable for sea heat systems and surface ground heat plants as well as three way collectors, within the scope of the present invention.

(9) According to a preferred embodiment, the single pipe collector according to the present invention is a continuous pipe 12 with a cross-sectional area that is essentially similar along the whole longitudinal direction L of the pipe.

(10) According to a preferred embodiment, the surface structure on the inward surface 14 of the single pipe collector is a grooved pattern, whereby the inward surface is designed with indentations 18 that suitably forms continuous grooves in the surface that extends essentially in the longitudinal direction L of the pipe. The grooves are evenly spread around the inner circumferential surface of the pipe, as seen in a cross-section T of the pipe. FIG. 2C shows in a cross-section a stretched out pipe wall of the collector pipe 12 where indentations in the shape of the grooves 18 are evident.

(11) FIG. 3 shows a part of a single pipe collector according to an embodiment of the present invention. The indentations and/or elevations 16 are extending helically in the longitudinal direction L of the pipe. The direction of the helical shape (see arrows in FIG. 3) can be altered at least at some portion in the longitudinal direction L of the pipe. The direction of the helical shape can be altered suitably at least every second meter, preferably every meter, in the longitudinal direction L of the pipe.

(12) According to the present invention, the grooves or the pattern, such as a surface structure of indentations and/or elevations, may be continuous or discontinuous in the longitudinal direction of the single pipe collector. As shown in the embodiment in FIG. 3, a flat portion on the inward surface of the pipe can be arranged as a temporary, but short transition between two helically portions.

(13) Usual dimensions for the collector pipes 12 according to the invention are within the range 25-63 mm in diameter. The height of the indentations and/or elevations 16, alternatively the grooves 18 or the grooving, can be varied, but can typically be within the range of 0.2-5 mm depending on the size of the pipes and the wall thickness, preferably 0.2-2 mm, for the most usual dimensions of the collector pipes 12.

EXAMPLE

(14) Experiments were carried out in a heat pump system, with two heat pumps of 16 kW and 32 kW, respectively, for supply of hot water and heating in a building with 19 apartments each with an area of 40 m.sup.2. Four of the water filled bore holes, with a diameter of 140 mm and a depth of 260 mm, that constitutes the heat source of the system, were used in the experiment. Ethanol in a concentration of 20 percentage by volume in an aqueous solution, with a freezing point of 8 C. In the experiment, each of the four boreholes was equipped with four different types of single pipe collectors, respectively. The respective borehole design, dimension and arrangement are tabulated in. Table 1 below.

(15) TABLE-US-00001 TABLE I Horizontal deviation Active length at 260 m fr. Type of collektor No. of borehole initial position Dimension in mm 2 251.6 m 84.9 m PE40x3.7 3-pipe 4 254.5 m 64.1 m PE40x2.4 U-pipe 5 242.7 m 75.7 m PE40x2.4 U-pipe with spacers 6 245.7 m 96.9 m PE40x2.4 U-pipe with helical grooves on inner surface

(16) The mean undisturbed ground temperature was measured to 8.7 C., and the average ground thermal conductivity 3.75 W/m K. It should also be explained that single pipe collector no. 2 (BH2) in the Table, that is the three-pipe collector, is a variant of collector that comprises one pipe for conveyance of the heat transfer liquid down into the bore hole and two pipes that guides the heat transfer liquid back out of the bore hole and further to a heat pump. The single pipe collector no. 4 (BH4) is a conventional U-pipe collector. The single pipe collector no. 5 (BH5) is a conventional U-pipe collector provided with spacers, that are intended to keep the pipes apart in the bore hole such that they not will be in contact with each other. The single pipe collector no. 6 (BH6) is a U-pipe collector according to the present invention, that comprises indentations and/or elevations on the inward surface of the pipe in a helical extension along the longitudinal extension of the pipe. The helical shape is altered periodically along the longitudinal extension of the pipe.

(17) The flows in the respective collectors were checked. Each borehole heat exchanger was instrumented with thermocouples for temperature measurements at the bottom and outlet points, on the heat transfer liquid inwards of the collector. The total pressure drop in the collectors is also measured during the tests at the collector inlet and outlet lines using a pressure gauge. Temperatures have been measured at different flow conditions in the bore holes and when the conditions have stabilized after the heat pump start up. During a measuring period the fluid density, kinematic viscosity and heat capacity were calculated at the measured temperature. Hence, the Reynolds number, the friction factor and the pressure drop were calculated. Finally, the heat absorbed per meter by the heat transfer fluid was calculated for each collector, which was used in order to calculate the borehole thermal resistance for each of the collector as well. The temperature value for the borehole wall was measured by the aid of a fibre optical cable and was assumed to be constant and equal to 7.2 C. for the calculations in this experiment.

(18) With respect to heat extraction (kW), the best heat extraction performance is obtained in BH6 and the worst performance is in BH2. Nevertheless, it is not recommended to compare the collectors by looking at the extracted heat due to the fact that not all the measurements were taken at the same time, which could cause different inlet and groundwater temperatures at different measurement occasions. With respect to the thermal resistance, it was observed that BH6 had the lowest values of all the collectors (e.g. at an up flow of 1.8 m.sup.3/h, BH6 had about 0.16 K/(W/m) while BH2 had about 0.18, BH4 had about 0.23 and BH5 had about 0.22), with the exception for one measured value where BH5 were best. Hence, this means for one aspect that the single pipe collector according to the invention shows the best performance. The result for the pressure drop is evident from Table II below.

(19) TABLE-US-00002 TABLE II Pressure drop [KPa] for different flows [m.sup.3/h]: 1.5 1.8 2.5 experi- experi- experi- No mental estimat. mental estimat. mental estimat. 2 54.21 48.35 75.00 75.59 144.53 129.73 4 61.43 56.41 87.33 78.22 149.60 129.54 5 56.54 56.68 81.10 77.98 149.63 128.36 6 49.10 58.02 70.03 77.31 131.51 136.20

(20) To sum up, the results implies that the pipe dimensions have an important influence, the spacers (collector in BH5) contributes probably not to increased heat transmission and that a surface structure on the inside of the pipes improves the performance of the collectors. With the exception of BH6, it is generally observed that the calculated pressure drop is slightly lower than the experimental values. This is attributed to the fact that the accessories such as elbows, bends, bottom part of the collector, are not considered in the calculation. It is observed that the calculated values for BH6 are higher than the experimental ones, which unexpectedly shows that the real pressure drop in the single pipe collector with surface structure on its inward surface, in the shape of indentations and/or elevations according to the present invention, is in fact lower. BH6 has the lowest pressure drop of all the collectors of the BHEs, including BH4 and BH5 which are common U-pipe collectors with the same dimensions. This is surprising. The pressure drop analysis indicates that BH6 is the best option, since the required pumping power for the heat transfer fluid would be slightly lower for this collector.