Underground LNG (Liquefied Natural Gas) storage cavern freezing ring regulation and control system and cold energy utilization method

Abstract

The invention discloses an underground LNG (Liquefied Natural Gas) storage cavern freezing circle regulation and control system and a cold energy utilization method, the regulation and control system can regulate the size of a freezing circle formed by surrounding rock around an LNG storage cavern, and the regulation and control system comprises a main water inlet pipeline, a cooling unit, a main water outlet pipeline and a heat exchange unit. The cooling unit comprises a water injection drill hole and a water return drill hole, the water injection drill hole communicates with the water return drill hole, the main water inlet pipeline is used for conveying uncooled water to the water injection drill hole from the ground, and the main water outlet pipeline is used for outputting the cooling water output from the water return drill hole to the heat exchange unit. The water circulation system is established in the freezing ring of the underground LNG storage cavern, cold energy in the freezing ring of the underground LNG storage cavern is brought to the ground to be stored, and the economic benefits of the LNG storage cavern system are improved.

Description

Technical Field

[0001] This invention belongs to the field of underground LNG storage technology, and particularly relates to an underground LNG storage freeze zone control system and a method for utilizing cold energy. Background Technology

[0002] Liquefied natural gas (LNG) is a cryogenic liquid formed by cooling natural gas to its boiling point of approximately -162°C under normal pressure. This phase change process compresses the volume of natural gas to about 1 / 620th of its gaseous state. Compared to gaseous storage, storage space requirements are reduced by 99.8%; safe storage at normal pressure (tank pressure <30 kPa) is achieved; and transportation costs are reduced to 1 / 3 to 1 / 5 of pipeline transportation. Currently, large-scale natural gas storage mainly relies on two types of infrastructure: surface tanks and underground storage facilities.

[0003] Compared to surface tanks, underground liquefied natural gas (LNG) storage facilities leverage the natural barriers of geological formations, significantly increasing single-storage volume, effectively reducing unit storage costs, and enhancing overall safety. During operation, the low temperatures of underground LNG storage tanks cause surrounding groundwater to freeze, forming a freeze zone. The size of this freeze zone significantly impacts the storage's sealing performance and energy consumption. A freeze zone that is too small results in a poor seal, while a freeze zone that is too large indicates that some of the storage's cold energy is being consumed to maintain its size. However, current technology cannot effectively control the size of the freeze zone in underground LNG storage facilities, thus hindering the efficient utilization of cold energy. Summary of the Invention

[0004] (a) Technical problems to be solved

[0005] The present invention aims to solve the problems of uncontrollable size of the freeze zone in existing LNG storage facilities and ineffective utilization of the cold energy in the freeze zone.

[0006] (II) Technical Solution

[0007] This invention proposes an underground LNG storage freezing zone control system for regulating the size of the freezing zone formed by the surrounding rock around the LNG storage cavern in an LNG storage system. The underground LNG storage freezing zone control system includes a main water inlet pipe, a cooling unit, a main water outlet pipe, and a heat exchange unit. The cooling unit includes a water injection borehole and a water return borehole, with the water injection borehole and the water return borehole connected. The main water inlet pipe is used to transport uncooled water from the surface to the water injection borehole. The main water outlet pipe is used to output cooling water from the water return borehole to the heat exchange unit.

[0008] According to a preferred embodiment of the present invention, the underground LNG storage freeze zone control system further includes a surface water pump, which is used to inject water output from the heat exchange unit into the main inlet pipe.

[0009] According to a preferred embodiment of the present invention, the heat exchange unit includes a heat exchanger and a refrigerant storage tank connected to the heat exchanger; the heat exchanger is used to extract cold energy from the cooling water output from the return water borehole and output uncooled water; the refrigerant storage tank is used to store the collected cold energy.

[0010] According to a preferred embodiment of the present invention, the cooling unit further includes a temperature sensor for detecting the temperature of the cooling water flowing out of the return water borehole; the ground water pump further includes a water pump controller for controlling the output power of the ground water pump according to the temperature detected by the temperature sensor, so as to adjust the water circulation speed in the underground LNG storage freeze zone control system, thereby preventing the underground LNG storage freeze zone control system from freezing.

[0011] According to a preferred embodiment of the present invention, the return water borehole and the injection water borehole are longitudinal boreholes that are vertically inserted into the freezing zone around each LNG storage chamber; and the cooling unit also includes a transverse borehole located below each LNG storage chamber, through which each injection water borehole communicates with the return water borehole.

[0012] According to a preferred embodiment of the present invention, for each LNG storage cavern, there are multiple longitudinal and transverse boreholes around it, and each borehole is arranged along its axial direction on the side and bottom of the LNG storage cavern 1 to form a row of boreholes.

[0013] According to a preferred embodiment of the present invention, the main water inlet pipe is connected to each water injection borehole through a pipe splitter, and each water return borehole is connected to the main water outlet pipe through a pipe manifold.

[0014] According to a preferred embodiment of the present invention, each transverse borehole is connected to each return water borehole via a tee pipe.

[0015] The second aspect of the present invention proposes a method for utilizing the cold energy of the frozen zone of an underground LNG storage facility. According to a preferred embodiment of the present invention, the method includes the following steps: establishing a water circulation system in the frozen zone of the underground LNG storage facility, and using the flow of water to bring the cold energy in the frozen zone of the underground LNG storage facility to the ground for storage.

[0016] According to a preferred embodiment of the present invention, the cold energy utilization method further includes: adjusting the flow rate of water in the water circulation system to prevent the water in the water circulation system from freezing.

[0017] (III) Beneficial Effects

[0018] This invention controls the size of the freezing zone and recovers and utilizes excess cold energy, thereby improving the economic efficiency of the overall storage system. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the overall main structure of an embodiment of the underground LNG storage freezing zone control system of the present invention.

[0020] Figure 2 Showing Figure 1 The illustrated embodiment shows a three-dimensional structure of an underground LNG storage freeze zone control system surrounding an LNG storage cavern.

[0021] Figure 3 yes Figure 2 The front view shows a more detailed view. Figure 2 Details of the structure.

[0022] Figure 4 This is a schematic diagram of the structure of a ground water pump in an embodiment of the underground LNG storage freeze zone control system of the present invention.

[0023] Figure 5 This is a schematic diagram of the structure of a heat exchange unit of the underground LNG storage freeze zone control system of the present invention. Detailed Implementation

[0024] To address the aforementioned technical problems, this invention proposes a method for regulating the frozen zone and utilizing cold energy in underground LNG storage facilities. This invention focuses on precisely regulating the size of the frozen zone within the surrounding rock of an underground LNG storage facility. This regulation method mainly comprises two key parts: First, during the construction phase, the dimensions of the storage chamber, the thickness of the insulation layer, and the connectivity of the surrounding rock fissure network are precisely determined. This ensures that after the storage facility is put into use, the size of the frozen zone meets the sealing requirements while avoiding excessive waste of cold energy. Second, a circulating water system is constructed around the storage facility to partially recover the cold energy consumed by the frozen zone during normal operation, while also appropriately regulating the size of the frozen zone during daily use of the storage facility.

[0025] Therefore, this invention provides some technical solutions for obtaining the most economical freezing zone thickness during construction:

[0026] Research has revealed that as the cavern expands, energy consumption is approximately directly proportional to its characteristic radius. This phenomenon arises because the internal boundaries of the storage facility, which provide cooling energy, are all isothermal boundaries, and the total length of these boundaries is proportional to the characteristic radius of the cavern. However, the cumulative energy consumption ratio decreases significantly as the cavern expands. From a storage efficiency perspective, the cumulative energy consumption ratio reflects the energy consumption per unit volume of liquefied natural gas (LNG) in the storage facility. Therefore, it can be inferred that expanding the cavern is an effective way to reduce storage facility operating costs. Thus, for tunnels with small initial areas, it is necessary to appropriately widen them. However, it should be noted that excessively large cavern dimensions will increase construction costs and reduce the stability of the physical structure. Therefore, this invention proposes widening the cavern to 12 meters to achieve the most economical effect.

[0027] As a key structure controlling the temperature field of the surrounding rock, the thickness of the insulation layer significantly affects the freezing zone and the temperature of the surrounding rock. Increasing the insulation layer thickness can effectively reduce the area of ​​the freezing zone; conversely, decreasing the insulation layer thickness will significantly lower the interface temperature between the surrounding rock and the concrete lining. Without an insulation layer, the lowest temperature in the surrounding rock area will drop to -138℃. Under these extreme conditions, even a single freeze-thaw cycle can significantly impact the rock strength, potentially drastically shortening the service life of the storage facility. Therefore, the installation of an insulation layer is essential. However, considering the requirements for the thickness of the freezing zone, a precise numerical analysis is needed to determine the appropriate value for the insulation layer thickness.

[0028] Numerical simulation results show that the equivalent thickness of the freezing zone decreases significantly with increasing insulation layer thickness. Without insulation, the surrounding rock temperature changes rapidly, leading to significant damage from cold shock. Furthermore, increasing insulation thickness reduces the freezing zone's expansion rate and significantly prolongs its formation time. From an energy consumption perspective, storage facilities without insulation consume extremely high power in the initial operational phase, necessitating high-power natural gas liquefaction equipment. Insulated storage facilities, however, consume less power and experience a slower rate of change. Increasing insulation thickness reduces the cumulative energy consumption ratio, but this effect diminishes with excessive thickness. Moreover, considering the influence of groundwater flow, the freezing zone thickness will further decrease. To ensure the rapid formation and expansion of the freezing zone to a greater thickness, this invention sets the insulation layer thickness at approximately 50 mm.

[0029] Thermal convection between groundwater and surrounding rock can alter the temperature field distribution. As the pressure gradient increases, the area of ​​the frozen zone decreases accordingly. Seepage within fissures reduces the uniformity of the frozen zone. Overall, the frozen zone is mainly concentrated in areas with lower groundwater flow velocities on the left and right sides of the reservoir, while the frozen area above and below the reservoir is relatively small. Because downstream groundwater is pre-cooled by the reservoir as it flows upstream, freezing is more likely to occur downstream, resulting in a larger frozen zone area on the downstream side. Locally, fissures affect the smoothness of the frozen edge. Simultaneously, the equivalent thickness of the frozen zone gradually decreases with increasing groundwater pressure gradient. When the groundwater pressure gradient is large, the temperature field distribution of the surrounding rock stabilizes more quickly. Similar to models with good connectivity, the reservoir's energy consumption also increases with increasing groundwater flow velocity. In the data calculation model, the difference between the equivalent thickness of the frozen zone in a fractured network with poor connectivity and that in a fractured network with good connectivity is defined as DEC; the difference between the cumulative energy consumption ratio of the fractured network model with good connectivity and that of the fractured network model with poor connectivity is defined as DCC. These two indicators, DEC and DCC, reflect the degree to which the connectivity of the fractured network affects the characteristics of the frozen zone. Overall, both DEC and DCC increase with increasing groundwater flow velocity, indicating that an increase in pressure gradient will highlight the importance of fractured network connectivity. Therefore, this invention proposes that for reservoir formations with complex hydrological conditions, engineering methods should be adopted to reduce the connectivity of the fractured network. For reservoirs with high groundwater flow velocities, grouting can be performed on the fractured network of the surrounding rock to save on natural gas liquefaction costs, reduce the requirements for geological conditions for reservoir construction, and enhance the economic value of underground LNG storage facilities.

[0030] This invention also proposes to achieve efficient cold energy collection using a circulating water system. Specifically, a complete circulating water system is constructed around the LNG storage facility. This system includes main water supply and drainage pipes and numerous smaller branch pipes distributed within the cryosphere. All pipes are interconnected, forming an orderly network to ensure stable water circulation. Based on the water circulation principle, the system can recover excess cold energy within the cryosphere and transport it to the surface.

[0031] Cold energy is collected through a circulating water system. A circulating water system is established around the storage facility, including the main water pipeline and a large number of small pipes distributed inside the frozen zone. The excess cold energy in the frozen zone is collected by water circulation. On the one hand, the size of the frozen zone can be controlled to a certain extent, and on the other hand, the cold energy in the frozen zone can be collected and utilized for a secondary purpose.

[0032] Specifically, the underground LNG storage freeze zone control system consists of a circulating water system, including:

[0033] A surface water pump is used to make water flow in underground pipes.

[0034] The main water inlet pipeline originates from the surface water pump and extends deep into the underground LNG storage area, near the freeze zone. It then passes above the storage facility. This pipeline is used to transport uncooled water from the surface to the cooling unit.

[0035] The cooling unit consists of multiple smaller-diameter boreholes and temperature sensors at the ends of the main water pipes, which are vertically and horizontally led out from the main water supply pipes. The water pipes are distributed around the reservoir, and the temperature sensors detect the temperature at the end of the water flow and feed the temperature back to the ground water pump to adjust the flow rate in real time, so as to ensure that the water temperature at the end of the water pipes is always close to 0 degrees and does not freeze.

[0036] The main water outlet pipe connects the end of the small pipes in the cooling unit to a main water outlet pipe, which connects the cooling unit to the ground surface, allowing the cooled water, which is close to zero degrees, to be directed to the ground surface.

[0037] The heat exchange unit extracts the cold energy contained in the cooling water supplied in the outlet pipe and sends the water back into the inlet pipe.

[0038] The underground LNG storage freeze zone control system can provide the following two functions: (1) It can use the water circulation system to collect the cold energy dissipated during the formation and maintenance of the freeze zone in the underground LNG storage, thereby reducing the overall energy consumption of the entire storage system. (2) It can control the flow rate and velocity of the water circulation system to adjust the efficiency of cold energy collection and control the size of the freeze zone to a certain extent. This allows the freeze zone to be manually adjusted even after the storage is built and put into normal use.

[0039] This system has significant dual application value. First, it allows for a certain degree of artificial control over the size of the frozen zone, ensuring it remains in a reasonable state. Second, it enables the secondary collection and utilization of cold energy within the frozen zone, improving energy efficiency and achieving effective resource allocation.

[0040] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0041] Figure 1 This is a schematic diagram of the overall main structure of an embodiment of the underground LNG storage freeze zone control system of the present invention. Figure 1 As shown, an LNG storage system typically has multiple LNG storage chambers 1, and each LNG storage chamber 1 has an insulation layer 2 installed on its inner side. When the LNG storage system is in operation, a freezing zone 3 will form around the surrounding rock of each chamber, which is represented by a dashed line in the figure.

[0042] In summary, the underground LNG storage freeze zone control system of this invention mainly comprises five parts: a surface water pump, a main inlet pipe, a cooling unit, a main outlet pipe, and a heat exchange unit. Figure 1 The ground water pump section is not shown (it will be described below), only the main water inlet pipe 20 and the main water outlet pipe 40 are shown, as well as the cooling unit consisting of longitudinal boreholes 301a, 301b and transverse boreholes 302.

[0043] like Figure 1 As shown, longitudinal boreholes are vertically inserted into the freezing zone 3 surrounding each LNG storage chamber 1, while transverse boreholes 302 are located below each LNG storage chamber 1 and communicate with each longitudinal borehole 302. At least a portion of the longitudinal boreholes serve as water injection boreholes 301a, and the remaining longitudinal boreholes serve as water return boreholes 301b. Water injection boreholes 301a are connected to the main inlet pipe 20, while water return boreholes 301b are connected to the main outlet pipe 40. Thus, water flows from the main inlet pipe 20 through the water injection boreholes 301a and the transverse boreholes 302, and then flows out of the main outlet pipe 40 through the water return boreholes 301b.

[0044] As a preferred method, longitudinal boreholes 301a and 301b are evenly arranged in the freezing zone 3, so that the number of longitudinal boreholes around each LNG storage chamber 1 is approximately equal.

[0045] Figure 2 This image shows a three-dimensional structure of an underground LNG storage freeze zone control system surrounding an LNG storage cavern, according to the above-described embodiment of the present invention. Figure 2 What is displayed is Figure 1 The leftmost chamber is 1, and the other chambers have similar structures. Figure 2 For a 3D image, from Figure 2 As can be seen, the LNG storage cavern 1 is generally elongated, with a certain depth along its axis, and its cross-section perpendicular to the axial direction is roughly rectangular with an arched upper section. For this LNG storage cavern 1, there is one main water inlet pipe 20 and one main water outlet pipe 40, but there are multiple water injection boreholes 301a, water return boreholes 301b, and transverse boreholes 302, which form a row of boreholes. Each borehole is arranged along the axial direction of the LNG storage cavern 1 on the side and bottom of the LNG storage cavern 1.

[0046] In this embodiment, the main inlet pipe 20 is connected to each water injection borehole 301a via a pipe splitter 303, while each return water borehole 301b is connected to the main outlet pipe 40 via a pipe manifold 401. Since the transverse boreholes 302 also need to be connected to other return water boreholes 301b in other LNG storage chambers 1, each transverse borehole 302 is connected to each return water borehole 301b via a tee pipe 305.

[0047] Figure 3 yes Figure 2 The front view shows a more detailed view. Figure 2 Details of the structure. For example... Figure 3 As shown, in addition to water injection hole 301a, water outlet hole 301b, horizontal hole 302, pipe diverter 303 and pipe manifold 401, the cooling unit also includes a temperature sensor 304 (PT100 type in this embodiment).

[0048] Temperature sensor 304 is installed at the end of the main outlet pipe 40, and the wiring of temperature sensor 304 is laid through a conduit to the water pump controller 102 of the ground water pump. Thus, the water pump controller 102 can make the ground water pump adjust the water circulation speed in real time according to the water temperature to prevent freezing of the underground LNG storage freezing zone control system.

[0049] In this embodiment, the pipe diverter 303, pipe manifold 401, temperature sensor 304, and tee pipe 305 are all arranged within artificial passages 306 distributed along the axial direction of the storage cavern 1 around the perimeter of the storage cavern 1. Horizontal boreholes 302 and longitudinal boreholes 301a and 301b are all constructed within the artificial passages 306.

[0050] In this embodiment, the main outlet pipe 40 extends vertically to the ground surface, and its outlet end is connected to the inlet of the heat exchange unit via a flange.

[0051] Figure 4 This is a schematic diagram of a surface water pump according to an embodiment of the underground LNG storage freeze zone control system of the present invention. The surface water pump is located above ground, as shown below... Figure 4 As shown, it includes a variable frequency centrifugal water pump 101, a water pump controller 102, a water pump inlet 101a, and a water pump outlet 101b. The water pump inlet 101a is connected to the return water pipe 504 of the heat exchange unit via a flange, and the water pump outlet 101b is welded to the inlet end 201a of the main water inlet pipe 20. Furthermore, the water pump controller 102 is connected to the temperature sensor 304 of the cooling unit via wiring, and controls the power of the water pump 101 based on the temperature obtained from the temperature sensor 304, thereby controlling the water flow rate in the main water inlet pipe 20 and the main water outlet pipe 40.

[0052] In this embodiment, the main water inlet pipeline includes a horizontal water inlet pipeline 201 and a vertical water inlet pipeline 202. The horizontal water inlet pipeline 201 is constructed of a double-layer insulated steel pipe (DN200). The inlet end 201a of the horizontal water inlet pipeline 201 is welded to the water pump outlet 101b. The vertical water inlet pipeline 202 extends underground to the vicinity of the underground LNG storage facility, and its end is connected to the water injection borehole 301a of the cooling unit.

[0053] Figure 5This is a schematic diagram of the structure of a heat exchange unit in an embodiment of the underground LNG storage freeze zone control system of the present invention. Figure 5 As shown, the heat exchange unit includes a heat exchanger 501 and a refrigerant storage tank 502. The heat exchanger 501 can be a plate heat exchanger. The primary side inlet 501a is connected to the end 402a of the main outlet pipe 40 to introduce low-temperature water into the heat exchanger. The primary side outlet 501b is connected to a return water pipe 504, which is connected via a flange to the water pump inlet 101a of the ground water pump to reintroduce high-temperature water into the underground LNG storage freeze zone control system. The secondary side refrigerant pipe 505 is connected to the refrigerant storage tank 502 to store the collected cold energy.

[0054] This system has two significant functions:

[0055] Firstly, the underground LNG storage freeze zone control system of this invention relies on a water circulation system to partially recover the cold energy dissipated during the formation and maintenance of the freeze zone in the underground LNG storage. During the operation of the underground LNG storage, the outward dissipation of cold energy is unavoidable in order to build and maintain the necessary freeze zone. The water circulation system of this invention can effectively collect the cold energy that would otherwise be wasted. The core objective of cold energy recovery is to reduce the total energy consumption of the entire storage system. In the daily operation of the storage, energy consumption is a significant cost factor. Through cold energy recovery, the additional energy consumption of the system to maintain the low-temperature environment can be reduced, achieving energy conservation and consumption reduction, and improving the energy utilization efficiency of the storage system.

[0056] Secondly, the underground LNG storage freeze zone control system of this invention can precisely control key parameters such as flow rate and velocity of the water circulation system to adjust the cold energy collection efficiency. In the water circulation system, changes in flow rate and velocity directly affect the cold energy transfer and collection process. To improve cold energy collection efficiency, the water circulation flow rate and velocity can be appropriately increased to accelerate cold energy transfer and collection; conversely, the flow rate and velocity can be decreased.

[0057] More importantly, by adjusting the cold energy collection efficiency, the size of the frozen zone can be controlled within a certain range. After the storage system is built and put into operation, the size of the frozen zone may need to be adjusted due to changes in actual conditions. This system can flexibly adjust the parameters of the water circulation system according to actual needs, realizing artificial intervention in the size of the frozen zone and ensuring the safe, stable, and efficient operation of the underground LNG storage facility.

[0058] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A freezing zone control system for underground LNG storage facilities, used to adjust the size of the freezing zone formed by the surrounding rock around the LNG storage cavern of the LNG storage system, characterized in that: The underground LNG storage freezing zone control system includes a main water inlet pipe, a cooling unit, a main water outlet pipe, and a heat exchange unit. The cooling unit includes a water injection hole and a water return hole, and the water injection hole and the water return hole are connected. The main water inlet pipe is used to transport uncooled water from the ground to the water injection borehole; The main outlet pipe is used to output cooling water from the return water borehole to the heat exchange unit.

2. The underground LNG storage freeze zone control system according to claim 1, characterized in that, It also includes a ground water pump, which is used to inject water output from the heat exchange unit into the main water inlet pipe.

3. The underground LNG storage freeze zone control system according to claim 2, characterized in that: The heat exchange unit includes a heat exchanger and a refrigerant storage tank connected to the heat exchanger. The heat exchanger is used to extract the cold energy from the cooling water output from the return water borehole and output uncooled water. The refrigerant storage tank is used to store the collected cold energy.

4. The underground LNG storage freeze zone control system according to claim 2, characterized in that: The cooling unit also includes a temperature sensor for detecting the temperature of the cooling water flowing out of the return water borehole; The ground water pump also includes a water pump controller, which controls the output power of the ground water pump according to the temperature detected by the temperature sensor, so as to adjust the water circulation speed in the underground LNG storage freezing zone control system, thereby preventing the underground LNG storage freezing zone control system from freezing.

5. The underground LNG storage freeze zone control system according to any one of claims 1 to 4, characterized in that: The return water boreholes and injection water boreholes are longitudinal boreholes, inserted vertically into the freeze zone surrounding each LNG storage chamber; and... The cooling unit also includes a transverse borehole located below each LNG storage chamber, through which each water injection borehole is connected to the return water borehole.

6. The underground LNG storage freeze zone control system according to claim 5, characterized in that: For any one of the LNG storage caverns, there are multiple longitudinal and transverse boreholes around it. These boreholes are arranged along the axial direction on the side and bottom of the LNG storage cavern 1, forming a row of boreholes.

7. The underground LNG storage freeze zone control system according to claim 6, characterized in that: The main inlet pipe is connected to each injection borehole via a pipe distributor. Each return water borehole is connected to the main outlet water pipe through a pipe manifold.

8. The underground LNG storage freeze zone control system according to claim 7, characterized in that: Each horizontal borehole is connected to each return water borehole via a tee pipe.

9. A method for utilizing the cold energy of the frozen zone of an underground LNG storage facility, characterized in that, Includes the following steps: A water circulation system is established within the frozen zone of the underground LNG storage facility. Through the flow of water, the cold energy within the frozen zone is carried to the surface for storage.

10. The method for utilizing cold energy in the freeze zone of an underground LNG storage facility according to claim 9, characterized in that, Also includes: Adjust the water flow rate in the water circulation system to prevent the water in the system from freezing.

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