Patent Application
20210055062
This invention relates generally to a system which transports thermal energy; more specifically, this invention relates to a system which cools a thermal reservoir by transporting a portion of its thermal energy to a heat sink without using a heat pump.
If a thermally conducting path is maintained between two thermal reservoirs, thermal energy will pass from the reservoir which has a higher temperature to the reservoir which has a lower temperature. The transfer process occurs spontaneously; that is, the process occurs without the driving influence of work done by an external system. The term “temperature-driven” is used herein to refer to a thermal energy transfer process that results from the temperature difference between two reservoirs.
The reverse of the process discussed in the previous paragraph is never observed. Thermal energy is never observed to pass spontaneously from a lower to a higher temperature reservoir. This fact is the basis for the second law of thermodynamics, formalized in 1850 by Rudolph Clausius: “Heat (thermal energy) will not pass from a colder body to a warmer body without some other change, connected therewith, occurring at the same time.” The phrase “other change” used in the Clausius statement refers to work that must be done by an external system. Unless some form of external work is done, the transfer of thermal energy from a lower to a higher temperature reservoir cannot occur. The term “work-driven” is used herein to refer to a thermal energy transfer process that is opposed by the temperature difference between two reservoirs, and therefore must be forced by an external system which does work. The term “external system” is used herein to refer to a system that has no component or elemental part in common with either of the two reservoirs exchanging energy. The word “work” is used herein to refer to the transfer of energy through the action of macroscopic forces. The term “external work” is used herein to refer to work done by an external system.
Modern cooling systems force the transfer of thermal energy from a lower to a higher temperature reservoir by making use of an external system which does work on a working fluid. The apparatus which performs the work is generally referred to as a heat pump. The most common type of heat pump currently in use today is the vapor-compression heat pump, which accepts energy—usually electrical energy—from an external source and uses that energy to force the cyclical compression and expansion of a working fluid. The working fluid accepts thermal energy from a reservoir which must be cooled and then rejects that energy to a separate reservoir, called a heat sink, which is generally at a higher temperature than the reservoir being cooled.
Work done by a heat pump enables an air conditioner to transfer thermal energy from the interior of a building to the warmer atmosphere outside of the building. Similarly, work done by a heat pump enables the transfer of thermal energy from the interior of a refrigerator to the warmer air outside of the refrigerator. In both of these examples, and in many other examples that could be listed, work is done by an external system in order to accomplish the transfer of thermal energy from a lower to a higher temperature reservoir. These are examples of work-driven thermal energy transfer processes. They are much less efficient than temperature-driven processes.
The electrical energy used by heat pumps in work-driven cooling applications is a large and growing percentage of global electricity consumption, accounting for roughly 20 percent of total electricity usage in most industrially developed countries. Electricity is the most versatile and the most generally useful form of energy consumed by modern industrial societies. It is problematic that so much of this valuable form of energy is consumed in cooling applications—applications that are nothing more than low-grade thermal energy transfer processes. The present specification addresses this problem by revealing a system which cools a thermal reservoir without using work-driven processes, using instead only highly efficient temperature-driven processes. Large scale deployment of systems of the type revealed herein would (1) greatly diminish the expense associated with most cooling operations and (2) significantly reduce requirements for electricity generation in many countries.
The current technology for cooling of temperature-critical reservoirs (residential, retail, and industrial structures; refrigerators; freezers; cooled process equipment; etc.) is based primarily on electrically driven heat pumps. Heat pumps do the work required to force the transfer of thermal energy from a lower to a higher temperature reservoir. These work-driven processes are relatively inefficient, with values for the coefficient of performance (COP) typically in the range of 2.0 to 4.0. (COP is a figure of merit related to the efficiency of a cooling or heating process. In this specification, COP is defined as the ratio of the quantity of thermal energy transferred during a cooling process to the quantity of energy consumed by an external device involved in the energy transfer process. For a work-driven process, a relatively large amount of energy is consumed by a heat pump that does work on a working fluid. For a temperature-driven process, a much smaller amount of energy is consumed by a mechanical pump that just drives coolant around a flow loop.
The object of this invention is to provide a system which cools temperature-critical reservoirs by using only temperature-driven thermal energy transfer processes.
A novel feature of the presently revealed system is an intermediate reservoir which serves as a thermal energy buffer (temporary thermal energy storage reservoir) between a temperature-critical reservoir, whose temperature is to be held within some pre-determined range, and a heat sink, whose temperature may vary with time over a broad range. Before the present system can be used in cooling applications, the intermediate reservoir must be cooled by transferring some of its thermal energy to a heat sink. This transfer is coordinated with naturally occurring temperature variations of the heat sink. The coordination involves operating the flow loop which transfers thermal energy between the intermediate reservoir and the heat sink only when the temperature of the heat sink is below the temperature of the intermediate reservoir. This assures efficient temperature-driven energy transfer between the two reservoirs. When the temperature difference between the two reservoirs is reversed or greatly reduced because of naturally occurring temperature variations of the heat sink, operation of the flow loop linking the two reservoirs is suspended. By utilizing this temporally coordinated, temperature-driven cooling operation, the intermediate reservoir can be brought to, and maintained at, a temperature below the lowest temperature desired for any part of the temperature-critical reservoir. The pre-cooled intermediate reservoir can thereby receive thermal energy from the temperature-critical reservoir by means of a temperature-driven process. The presence of an intermediate reservoir allows the present system to perform temperature-driven cooling operations even when the temperature of the heat sink is higher than the temperature desired for the temperature-critical reservoir.
The presently revealed system can function effectively in performing temperature-driven cooling operations only if the intermediate reservoir has a heat capacity that is large relative to the cooling requirements of the temperature-critical reservoir. Also, the intermediate reservoir must have relatively small naturally occurring temperature variations. When these two conditions are met, the temperature of the intermediate reservoir will remain fairly constant once it has been pre-cooled by the temperature-driven process described in the previous paragraph, changing noticeably only when thermal energy is added or removed by the present system when it is performing a cooling operation. When the present system is transferring thermal energy from a temperature-critical reservoir to the intermediate reservoir, the temperature of the intermediate reservoir tends to increase slowly. However, that increase is balanced over time as the present system removes thermal energy from the intermediate reservoir and transfers it to a heat sink. It is noted that cooling of a region within the temperature-critical reservoir may be required at any time, but the transfer of thermal energy from the intermediate reservoir to the heat sink occurs only when the temperature of the heat sink is less than the temperature of the intermediate reservoir. At times, the two operations are conducted simultaneously. At other times, the two operations are conducted singly.
When cooling of a temperature-critical reservoir is required, the pre-cooled intermediate reservoir can accept thermal energy from a temperature-critical reservoir by means of a temperature-driven process. The presence of the intermediate reservoir thereby enables a two-step process for transferring thermal energy from a temperature-critical reservoir to a final heat sink, with both steps using a highly efficient temperature-driven thermal energy transfer process. The temperature of the intermediate reservoir is always maintained at a low value, even though the temperature of the final heat sink may vary over a broad range, a range that may include temperatures well above the desired temperature of the temperature-critical reservoir.
The crucial factor for properly utilizing the presently revealed system's intermediate reservoir is timing. The intermediate reservoir is maintained at a low temperature by transferring its thermal energy to a heat sink only at times when the temperature of the heat sink is well below the temperature of the intermediate reservoir.
A preferred embodiment of the present invention involves using the atmosphere as a heat sink and a volume of subterranean earth as the intermediate reservoir, with the earth penetrated by the boreholes of a borehole heat exchanger. In this embodiment, during the winter, thermal energy is transferred from the (subterranean) intermediate reservoir to the colder gases of the atmosphere through a temperature-driven process, thereby lowering the overall temperature of the earth around the boreholes to a value below the lowest temperature required for the temperature-critical reservoir. This ensures that, when cooling is needed, thermal energy can be transferred from the temperature-critical reservoir to the intermediate reservoir by a high-efficiency temperature-driven thermal energy transfer process.
Borehole heat exchangers have been used for many years in solar heating applications and their characteristics are well-known. Using data obtained from heating systems with borehole heat exchangers, it is estimated that the (cooling) COP of the present system will be above 30, or roughly ten times the COP of conventional cooling systems that rely on work-driven thermal energy transfer processes. (The COP of the present system is defined as the ratio of the thermal energy transferred during a cooling operation to the work done by a mechanical pump in driving coolant around a flow loop. The present system does no work on a working fluid.) The system revealed in this specification could decrease the cost of electricity for most cooling applications to less than 10 per cent of the cost of electricity for conventional cooling systems that use heat pumps to force the transfer of thermal energy against a temperature difference.
This specification uses terms which have a technical meaning that may differ from the meaning assumed in everyday usage. The following paragraphs contain definitions and explanations of various terms and concepts with regard to the meaning intended herein.
The term “thermal energy” is used herein to refer to the kinetic energy associated with the rotations, vibrations and random translational velocities of atoms and molecules. For the purposes of this specification, the thermal energy associated with a mass of material is the same as its internal energy. Other forms of internal energy (nuclear, chemical, electrical, gravitational, etc.) are not are not considered in this specification.
The word “heat” is used herein to designate the transfer of thermal energy through the microscopic interactions (collisions) of atoms, molecule, and in some materials, electrons. Heat is thermal energy in transit from a higher to a lower temperature mass of material.
The word “work” is used herein to designate the transfer of energy through the action of macroscopic forces. In this specification, two types of work are considered. The term “thermodynamic work” is used herein to refer to the action of macroscopic forces in changing the thermodynamic state variables (pressure, temperature, specific volume, internal energy, etc.) of a fluid. The term “mechanical work” is used herein to refer to the action of macroscopic forces in driving the flow of fluid through a conduit. The term “working fluid” is used herein to refer to a fluid which transfers thermal energy as a result of processes that do thermodynamic work on the fluid.
The word “reservoir” and the term “thermal reservoir” are used interchangeably herein to refer to a mass of material (material body) which may be a solid, a liquid, a gas, or some combination thereof, with the mass of material being characterized by its temperature and the quantity of thermal energy it possesses. The temperature of a reservoir may change over time as it accepts thermal energy from, or transfers thermal energy to, other reservoirs.
The reservoirs discussed in this specification are not necessarily in a state of thermodynamic equilibrium. In particular, different regions within a reservoir could be at different temperatures. For instance, a residential structure is an example of a thermal reservoir, but different rooms within the structure, or different locations within a single room, could be at different temperatures. In this specification, reference to the temperature of a reservoir does not imply that the reservoir is at thermal equilibrium or that it can be characterized by a single temperature. Rather, the temperature of a reservoir refers to the temperature at some specific location within the reservoir or to a rough average of the temperatures at different locations within the reservoir. The word “temperature” and the term “overall temperature” are synonymous in the latter respect.
The term “temperature-critical reservoir” is used herein to refer to a reservoir whose temperature is to be brought to, and held within, some particular temperature range through the implementation of a cooling process. Examples of temperature-critical reservoirs would include residential, retail, or industrial structures, as well as refrigerators, freezers, industrial cooling units, etc. On a larger scale, examples of temperature-critical reservoirs would include an entire residential neighborhood or an industrial park or a large assembly of industrial cooling equipment, or combinations thereof. The invention revealed in this specification uses temperature-driven thermal energy transfer processes to provide cooling for temperature-critical reservoirs of any size or complexity.
The term “heat sink” is used herein to refer to the thermal reservoir which ultimately accepts the thermal energy removed from a temperature-critical reservoir during a cooling operation. Examples of heat sinks would include the atmosphere or a large body of water. The cooling system revealed in this specification conducts cooling operations by first removing thermal energy from a temperature-critical reservoir and then transferring that energy to a heat sink. However, the present system does not conduct the energy transfer directly. Instead, the thermal energy is transferred from a temperature-critical reservoir to a reservoir which temporarily stores the energy until it can be passed along to a heat sink by means of a temperature-driven process. The term “intermediate reservoir” is used herein to designate a reservoir which is thermally linked—by independent links—to (1) a temperature-critical reservoir and to (2) a heat sink, with the linkage being such that the intermediate reservoir can accept thermal energy from a temperature-critical reservoir, store that energy, and then transfer it to a heat sink by means of a temperature-driven process. The term “thermally linked” is used herein to refer to two reservoirs that are connected by a flow loop (defined later) which carries thermal energy from one reservoir to the other by means of the mass flow of a fluid. Thus, the term “thermal link,” as used herein, designates a flow loop which carries thermal energy from one reservoir to another by means of mass flow of a fluid.
From time to time, naturally occurring temperature variations of the heat sink may make it impossible to directly transfer thermal energy from a temperature-critical reservoir to a heat sink by a temperature-driven process. For example, in the summer, the temperature of the atmosphere is usually above the temperature desired for the interior of a residential structure, thus negating the possibility of a temperature-driven cooling operation. When this type of situation exists, the present system's intermediate reservoir can store the thermal energy removed from the temperature-critical reservoir for a period of time, releasing that energy only during time intervals when the temperature of the heat sink is below the temperature of the intermediate reservoir. This allows thermal energy to be transferred from the temperature-critical reservoir to the heat sink by means of two independent temperature-driven processes, even though the temperature of the heat sink may at times be above the temperature desired for the temperature-critical reservoir.
The intermediate reservoir may take any one of several forms, such as a body of water or a volume of earth; but its function is always the same: to serve as a thermal energy buffer (temporary thermal energy storage reservoir) between the temperature-critical reservoir and the heat sink. With the intermediate reservoir disposed in this manner, the present system has the ability to transfer thermal energy from a temperature-critical reservoir to a heat sink in two steps. The first step involves the transfer of thermal energy from a temperature-critical reservoir to the intermediate reservoir. The second step involves the transfer of thermal energy from the intermediate reservoir to a heat sink. The presence of the intermediate reservoir makes it possible to conduct both steps as spontaneous, highly efficient temperature-driven processes, regardless of temperature variations of the heat sink.
In this specification, two reservoirs are said to be in “thermal contact” if they can spontaneously exchange thermal energy, either because they are in direct physical contact or because they are joined by a thermally conducting path. The term “heat exchanger” is used herein to designate a passive device which facilitates the transfer of thermal energy from a higher to a lower temperature reservoir by providing a thermally conducting path between the two reservoirs. The heat exchangers in the presently revealed system are comprised of a length of closed conduit whose walls provide a thermally conducting path for transferring thermal energy between a thermal reservoir which is outside the conduit walls and a fluid (a moving thermal reservoir) which flows inside the conduit walls.
The term “flow loop” is used herein to refer to a continuous, closed passage formed by sections of conduit which physically join and flow-wise connect a mechanical pump, a set of valves, and two heat exchangers. The term “flow-wise connected” is used herein to refer to a physical interconnection of elements which allows a fluid (coolant) to flow through the connected elements. The term “coolant” is used herein to refer to a moving thermal reservoir (a mass of fluid) which transports thermal energy through a flow loop by means of mass flow. The term “mechanical pump” is used herein to refer to a device which does mechanical work in forcing the circulation of coolant around a flow loop. The term “valve” is used herein to refer to an inline device which may be opened or closed at different times, thereby allowing or blocking the flow of coolant so as to regulate, direct, and guide the flow of coolant as it passes around a flow loop. The present system's flow loops are the means by which thermal energy is transported from one thermal reservoir to another. Valves, which are components of a flow loop, direct the flow of coolant to various locations within the reservoirs that are involved in exchanging energy. Within the confines of a flow loop, thermal energy is transferred to or from a mass of coolant as it passes through a heat exchanger.
The temperature of any particular mass of coolant changes as the mass circulates around a flow loop. The temperature rises as the mass of coolant absorbs thermal energy from a warmer reservoir; the temperature falls as the mass rejects thermal energy to a cooler reservoir. The sequential process of absorbing and then rejecting thermal energy is the method by which flowing coolant transfers thermal energy from one thermal reservoir to another. It is noted that, in the present system, coolant is not a thermodynamic working fluid because its thermodynamic state variables do not change as the result of the macroscopic forces driving its flow. The mechanical pumps driving the flow of coolant around the present system's flow loops perform mechanical work, not thermodynamic work.
The present system has two flow loops: a primary flow loop and a secondary flow loop. The primary flow loop provides a thermal link between a temperature-critical reservoir and the intermediate reservoir. The secondary flow loop provides a thermal link between the intermediate reservoir and a heat sink. Primary loop coolant circulates through the primary flow loop; secondary loop coolant circulates through the secondary flow loop. The bodies of primary and secondary loop coolant are flow-wise isolated, which means that they are never allowed to mix. Operation of the primary flow loop (by activation of the primary flow loop's mechanical pump) transfers thermal energy from a temperature-critical reservoir to the intermediate reservoir. The valves in the primary flow loop, referred to herein as “primary loop valves,” can be opened and closed in different combinations so as to change the geometrical configuration of the primary flow loop. This reconfiguring can be done so as to allow primary loop coolant to accept thermal energy from selected regions of the temperature-critical reservoir and then reject that thermal energy to selected regions of the intermediate reservoir. Operation of the secondary flow loop (by activation of the secondary flow loop's mechanical pump) transfers thermal energy from the intermediate reservoir to a heat sink. The valves in the secondary flow loop, referred to herein as “secondary loop valves,” can be opened and closed in different combinations so as to change the geometrical configuration of the secondary flow loop, thereby allowing secondary loop coolant to accept thermal energy from selected regions of the intermediate reservoir and then reject that energy to the heat sink.
There are four heat exchangers in the presently revealed cooling system. Two of the heat exchangers are heat-accepting heat exchangers; that is, they accept thermal energy from a reservoir outside of the conduit walls and transfer that thermal energy to coolant flowing inside the conduit walls. The other two heat exchangers are heat-rejecting heat exchangers; that is, they transfer thermal energy from a coolant flowing inside the conduit walls to a reservoir outside of the conduit walls. Each flow loop has two heat exchangers, one of which is a heat-accepting heat exchanger and the other of which is a heat-rejecting heat exchanger. Since the heat exchangers discussed in this specification are passive devices, the transfer of thermal energy through their walls is always a spontaneous, temperature-driven process. Operation of the present system must be conducted so that the desired direction of thermal energy flow is favored by the temperature differences maintained between the temperature-critical reservoir and the intermediate reservoir, and between the intermediate reservoir and the heat sink.
One of the present system's four heat exchangers is referred to herein as the “primary loop heat-accepting heat exchanger.” This heat exchanger is a component of the primary flow loop and it is disposed so as to be in direct thermal contact with a temperature-critical reservoir. Its function is to cool different regions of the temperature-critical reservoir by facilitating the transfer of thermal energy from those regions to primary loop coolant flowing through the conduit comprising the primary loop heat-accepting heat exchanger. It is noted that the temperature-critical reservoir may have several separate regions which have different cooling requirements. In this situation, each region would, of necessity, be serviced by its own individual segment of the primary loop heat-accepting heat exchanger. The individual segments of the primary loop heat-accepting heat exchanger would be connected in parallel. Valves connected in series with the individual segments can be opened or closed in order allow or block coolant flow through any segment, depending on whether or not cooling is required for the region of the temperature-critical reservoir which is in thermal contact with that particular heat exchanger segment.
Primary loop coolant carries (by mass flow) thermal energy that has been removed from various regions within the temperature-critical reservoir to the other primary loop heat exchanger, which is referred to herein as the “primary loop heat-rejecting heat exchanger.” The primary loop heat-rejecting heat exchanger is disposed so as to be in thermal contact with the intermediate reservoir. Its function is to facilitate the transfer of thermal energy from the primary loop coolant to the intermediate reservoir. It is seen from the preceding discussion that the overall function of the primary flow loop is to cool a temperature-critical reservoir by transferring some of its thermal energy to the intermediate reservoir. This is the first step of the two-step process revealed herein for cooling a temperature-critical reservoir by means of temperature-driven processes. It is noted that this first step (temperature-driven thermal energy transfer from the temperature-critical reservoir to the intermediate reservoir) is possible only if the temperature of the intermediate reservoir at the location of the primary loop heat-rejecting heat exchanger is maintained at a temperature below the temperature desired for the temperature-critical reservoir. If this condition is fulfilled, valves associated with various segments of the primary loop heat-accepting heat exchanger can be opened in order to provide cooling as needed to specific regions of the temperature-critical reservoir.
Another of the present system's heat exchangers is referred to herein as the “secondary loop heat-accepting heat exchanger.” This heat exchanger is a component of the secondary flow loop and it is disposed so as to be in thermal contact with the intermediate reservoir. Its function is to facilitate the transfer of thermal energy from the intermediate reservoir to secondary loop coolant flowing through the conduit that comprises the secondary loop heat-accepting heat exchanger. The secondary loop coolant carries thermal energy (by mass flow) from the intermediate reservoir to the other secondary loop heat exchanger, referred to herein as the “secondary loop heat-rejecting heat exchanger.”
The secondary loop heat-rejecting heat exchanger is a component of the secondary flow loop and it is disposed so as to be in thermal contact with a heat sink. Its function is to facilitate the transfer of thermal energy from the secondary loop coolant to the heat sink. It is seen from the preceding discussion that the overall function of the secondary flow loop is to transfer thermal energy from the intermediate reservoir to the heat sink. This is the second step of the two-step process revealed herein for cooling a temperature-critical reservoir by temperature-driven processes. It is noted that this second step (temperature-driven thermal energy transfer from the intermediate reservoir to the het sink) is possible only if the temperature of the heat sink at the location of the secondary loop heat-rejecting heat exchanger is less than the temperature of the intermediate reservoir at the location of the secondary loop heat-accepting heat exchanger. A control system, which will be discussed later, allows the secondary flow loop to operate only when this condition exists. This ensures efficient, temperature-driven transfer of thermal energy between the intermediate reservoir and the heat sink.
From this discussion it can be seen that two different processes occur within the intermediate reservoir. One process is that the intermediate reservoir accepts thermal energy into a region that has previously been cooled by passing thermal energy along to a heat sink. The other process is that the intermediate reservoir rejects thermal energy from a region that has previously been warmed by accepting thermal energy from a temperature-critical reservoir. These two processes must occur at the same location within the intermediate reservoir, and therefore they cannot occur at the same time (without significant and unnecessary duplication of hardware). This problem is solved by having a multiplicity of heat exchangers disposed at various locations within the intermediate reservoir. Each heat exchanger that is in thermal contact with the intermediate reservoir must at times serve as a primary loop heat-rejecting heat exchanger, and at other times serve as a secondary loop heat-accepting heat exchanger. Therefore, each of the intermediate reservoir's heat exchangers must be connected to valves which can be opened or closed so as to connect the heat exchanger to either the primary or the secondary flow loop. At any given time, only two of the heat exchangers that are in contact with the intermediate reservoir will be active; the others will be flow-wise isolated (by closed valves) from both the primary and secondary flow loops. The flow-wise isolated heat exchangers are called “dormant heat exchangers.” For example, if a heat exchanger has cooled a region of the intermediate reservoir to a low temperature while it was connected to the secondary flow loop, that heat exchanger can be isolated from the secondary flow loop (by closing associated secondary loop valves) and a different heat exchanger can then be connected into the secondary flow loop so as to cool a different region of the intermediate reservoir. The heat exchanger which has cooled the region of the intermediate reservoir remains dormant until it is needed for accepting thermal energy from the temperature-critical reservoir. Then its primary loop valves are opened and it becomes part of the primary flow loop.
Regarding the transfer of thermal energy to and from the intermediate reservoir, it is noted that the temperature profile of the intermediate reservoir changes during operation of the present system. Changing the location of the primary loop heat-rejecting heat exchanger and the secondary loop heat-accepting heat exchanger (by opening or closing certain primary and secondary loop valves) maintains system performance at a high level. The primary loop heat-rejecting heat exchanger and the secondary loop heat-accepting heat exchanger are always at different locations within the reservoir.
The term “borehole heat exchanger” is used herein to refer to an array of holes bored into the earth, with a U-shaped conduit inserted into each hole. The holes are typically backfilled with grout or other thermally conducting material so as to establish a thermally conducting path between the U-shaped conduits and the earth around the boreholes. The individual U-shaped conduits within the boreholes are usually connected in a series/parallel flow arrangement and a set of valves is installed in the connecting lines. The valves can be used to divide a borehole heat exchanger into a multiplicity of separate heat exchangers, each of which is situated at a different location within the intermediate reservoir. At any given time, one of these heat exchangers serves as the primary loop heat-rejecting heat exchanger, and one serves as the secondary loop heat-accepting heat exchanger. The other heat exchangers are flow-wise isolated from both the primary and secondary flow loops. Selection of which heat exchangers are active and which are dormant is made by opening and closing valves that control the flow of coolant through the various heat exchangers. It is noted that a heat exchanger may be comprised of several boreholes with interconnected U-shaped conduits,
The earth around the boreholes can serve as an effective intermediate reservoir because of its large heat capacity and immunity to seasonal variations in temperature. In the preferred embodiment of the present invention, a borehole heat exchanger is divided into multiple smaller heat exchangers, with one of the heat exchangers being flow-wise connected to the primary flow loop so as to serve as a primary loop heat-rejecting heat exchanger, and with one of the heart exchangers being flow-wise connected to the secondary flow loop so as to serve as the secondary loop heat-accepting heat exchanger. The remaining heat exchangers are dormant. As the temperature profile of the earth within the borehole array changes, the valves may be opened or closed so as to change the location of the regions where thermal energy is deposited or removed. Thus, a borehole heat exchanger can serve as a very efficient and versatile heat exchanger and the earth around the boreholes can serve as an effective intermediate reservoir.
It is clear from the preceding discussion that temperatures within the various reservoirs must be measured and the results of the temperature measurements must be used to control the operation of flow loop hardware (valves and pumps).
The present system includes a set of temperature sensors, with the individual sensors deployed at various locations within the temperature-critical reservoir, the intermediate reservoir, the heat sink, and the primary and secondary flow loops. The present system also includes a control system which responds to signals from the temperature sensors by activating the system's pumps and by opening or closing valves in such a way that a temperature-driven process is used to maintain the temperature of the of the system's intermediate reservoir at a level below the lowest temperatures desired for the temperature-critical reservoir. When this condition exists, the present system can efficiently cool a temperature-critical reservoir by means of a temperature-driven process that transfers thermal energy from the intermediate reservoir to the heat sink.
The following points are emphasized regarding the hardware of the present system. First, only one heat exchanger is in thermal contact with the temperature-critical reservoir. That heat exchanger, the primary loop heat-accepting heat exchanger, has several segments connected in parallel. Each segment is capable of accepting thermal energy from a specific region of the temperature-critical reservoir. A valve in series with each segment may be opened when cooling is required for the region around the segment, or closed when the region around the segment is as cool as desired. Second, there are several heat exchangers in thermal contact with the intermediate reservoir. Each heat exchanger has a primary loop valve and a secondary loop valve at each of its ends. Opening the two primary loop valves (and closing the two secondary loop valves) connects a heat exchanger into the primary flow loop and the heat exchanger becomes the primary loop heat-rejecting heat exchanger. Opening the two secondary loop valves (and closing the two primary loop valves) connects the heat exchanger into the secondary flow loop and the heat exchanger becomes the secondary loop heat-accepting heat exchanger. Each heat exchanger that is in thermal contact with the intermediate reservoir will sometimes serve as the primary loop heat-rejecting heat exchanger and each will sometimes serve as the secondary loop heat-accepting heat exchanger, depending on the (temperature) status of the regions around the various heat exchangers. At any given time, there is one primary loop heat-rejecting heat exchanger and one secondary loop heat-accepting heat exchanger. The remaining heat exchangers are dormant until they are needed as part of the primary flow loop for cooling the temperature-critical reservoir or as part of the secondary flow loop for transferring thermal energy to the heat sink.
Operation of the present system is intended to cool temperature-critical reservoir 101 by transferring some of its thermal energy to heat sink 102. Intermediate reservoir 103 allows this energy transfer process to take place in two steps, with one of the steps carried out by operating primary flow loop 104, and with the other step carried out by operating secondary flow loop 105. When the system is properly operated, both steps involve highly efficient temperature-driven thermal energy transfer processes.
Primary flow loop 104 includes and flow-wise links primary loop pump 106, primary loop valves 107, primary loop heat-accepting heat exchanger 108, and primary loop heat-rejecting heat exchanger 109. Secondary flow loop 105 includes and flow-wise links secondary loop pump 110, secondary loop valves 111, secondary loop heat-accepting heat exchanger 112, and secondary loop heat-rejecting heat exchanger 113.
Activating primary loop pump 106 drives primary loop coolant around primary flow loop 104, thereby transferring thermal energy from temperature-critical reservoir 101 to intermediate reservoir 103. Individual primary loop valves 107 are opened or closed so as to direct primary loop coolant through specific segments of primary loop heat-accepting heat exchanger 108, thereby allowing primary loop coolant to accept thermal energy from specific regions of temperature-critical reservoir 101. Also, opening a pair of primary loop valves 107 at opposite ends of a heat exchanger that is in thermal contact with the intermediate reservoir allows primary loop coolant to be directed through one of the multiplicity of heat exchangers that are in thermal contact with the intermediate reservoir. This heat exchanger, which is selectable by choosing which pair of primary loop valves to open, becomes primary loop heat-rejecting heat exchanger 108 when it is flow-wise connected to the primary flow loop.
Activating secondary loop pump 110 drives secondary loop coolant around secondary flow loop 105, thereby transferring thermal energy from intermediate reservoir 103 to heat sink 102. Individual secondary loop valves 111 are opened or closed in order to select one specific heat exchanger as the secondary loop heat-accepting-heat exchanger 112. Secondary flow loop 105 is only operated when the temperature of intermediate reservoir 103 at the location of the selected secondary loop heat-accepting heat exchanger is greater than the temperature of heat sink 102. This ensures that the energy is transferred from the intermediate reservoir to the heat sink by a temperature-driven process. It is noted that there is only one heat exchanger in thermal contact with the heat sink. That heat sink, the secondary loop heat-rejecting heat exchanger, may have several parallel segments.
The control system which coordinates the operation of the present system's valves and pumps is not shown, nor are the temperature sensors which provide temperature information to the control system.
It is noted that, before the present system can be used in cooling operations, some regions of the intermediate reservoir 103 must be pre-cooled to a temperature below the lowest temperature desired for any part of temperature-critical reservoir 101. Like all other transfers of thermal energy from intermediate reservoir 103 to heat sink 102, the pre-cooling operations are accomplished by operating secondary flow loop 105 only when the naturally occurring temperature variations of heat sink 102 cause it to be at a lower temperature than one or more regions of intermediate reservoir 103. After one or more regions of intermediate reservoir 103 have been pre-cooled, temperature-critical reservoir 101 can be cooled by operating primary flow loop 104 whenever cooling operations are necessary. (Secondary flow loop 105 is operated only when the temperature of heat sink 102 is below the temperature of one or more regions of intermediate reservoir 103.) On average, intermediate reservoir 103 is maintained at a relatively low temperature by balancing, over time, the input of thermal energy from primary flow loop 104 with the removal of thermal energy by the secondary flow loop 105.
The efficiency of the cooling system revealed in this specification is dependent in part on the nature of the interface between the heat sink and the wall of the conduit comprising the secondary loop heat-rejecting heat exchanger. In nearly all cooling operations, the atmosphere serves as the ultimate heat sink for accepting the thermal energy that has been removed from a temperature-critical reservoir during a cooling operation. One possible embodiment of the present system is that the walls of the conduit comprising the secondary loop heat-rejecting heat exchanger are in direct thermal contact with the atmosphere. Thermal energy would then be transferred through the walls of the heat exchanger, passing from the secondary loop coolant to the atmospheric gases outside of the walls of the secondary loop heat-rejecting heat exchanger.
It is likely that some cooling applications will require that a temperature-critical reservoir be cooled below the lowest temperature which can be achieved by using only the temperature-driven thermal energy transfer processes of the present system. For those applications, the present system could be joined with a conventional vapor-compression heat pump, thereby forming a cooling system which could efficiently cool a temperature-critical reservoir to very low temperatures. In that situation, the primary loop coolant of the present system could be used to cool the condenser of the vapor-compression heat pump. This would significantly decrease the amount of energy consumed by the heat pump's compressor and improve the overall efficiency of the cooling process, while at the same time making lower temperatures available for the temperature-critical reservoir.
