Combined ground-based heat exchange and thermal storage

Abstract

An integrated underground heat storage and heat transfer system is provided for CO2-based heating and cooling installations. The invention enables both energy storage and heat exchange in a single Underground Thermal Energy Storage (UTES) system. Excess heat produced during warm weather heats the ground to relatively high temperatures, which in cool weather serves as an artificial geothermal source. A geothermal heat pump (GHP) system reclaims this heat during cool weather. When the GHP is reversed in warm weather, the rejected heat is sent to a separate loop installed in cold earth.

Description

FIELD OF THE INVENTION

[0001] The present invention generally relates to the storage and retrieval of thermal energy, and in particular to the integrated use of heat exchange and ground storage as a means of managing thermal energy.BACKGROUND

[0002] Chilled data centers, laboratories and warehouses, and air-conditioned offices and residential buildings, are major consumers of energy in modern economies. The most common currently-used refrigerants are chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), which are potent greenhouse gases and deleterious to the atmospheric ozone layer. As these materials are phased out, the cooling and refrigeration industries will gradually transition to working fluids less environmental impact. Due to its safety and low cost, and high heat capacity when in the supercritical state, carbon dioxide is—in principle—an ideal refrigerant for future cooling applications, provided that the high pressures required for its liquefication can be economically managed. The use of CO2 as a refrigerant (R-744) in commercial freezer and refrigeration installations is already well-established.

[0003] The use of carbon dioxide as a refrigerant, heat transfer fluid, and working fluid in heat recovery and power cycles is, also an active area of research. Sec S. Glos et al., “Assessment of performance and costs of CO2 based next level geothermal power (NLGP) systems,” 3rd European Conference on Supercritical CO2 (sCO2) Power Systems 2019, pp. 49-58, doi.org / 10.17185 / duepublico / 48876; and M. Poerner, A. Rimpel, “Waste heat recovery,” in: Fundamentals and Applications of Supercritical Carbon Dioxide (sCO2) Based Power Cycles, K. Brun, P. Friedman, R. Dennis, Eds., Woodhead Publishing, 2017, pp. 255-267, doi.org / 10.1016 / B978-0-08-100804-1.00010-4.

[0004] Supercritical CO2, in particular, has a large heat capacity at pressures and temperatures close to the critical point, and can absorb or release large amounts of heat with minimal change in temperature, in addition to the changes in enthalpy associated with phase transitions. For this reason, supercritical and trans-critical carbon dioxide cycles are particularly desirable for refrigerators, heat pumps and power plants. Environmental heating and cooling normally employs water as the circulating heat transfer medium, and water / CO2 heat exchangers are common components of such systems.

[0005] Seasonal thermal energy storage (STES) is the storage of heat or cold for periods of up to several months. The thermal energy is collected whenever excess heat is available, and withdrawn whenever needed, adding considerably to the efficiency of heating and cooling in temperate climates having warm and cold seasons. In a similar manner, the natural cold of winter air can be “stored” for summertime air conditioning, by cooling the STES thermal reservoir to below room temperature.

[0006] One form of STES is Underground Thermal Energy Storage (UTES). UTES systems store energy by pumping heat into an underground reservoir of material, typically the ground itself, the water in an aquifer, or a soil or gravel-filled pit. For moderate temperature ranges, as is needed for interior environmental heating and cooling, the heat transfer medium typically used for this method of thermal energy storage is water, pumped through buried pipes or boreholes. Aquifers and water-filled underground caverns or pits may serve as large, natural storage spaces for thermal energy. Large amounts of thermal energy may transferred to these reservoirs by directly injecting or extracting hot or cold water from the aquifer or underground reservoir. For smaller installations, piping or tubing laid in soil-covered pits may be used to heat or cool the surrounding material, typically soil, sand, or gravel. Rock thermal energy storage (“rocks in a box”) employs the same concept, but using above-ground insulated tanks containing coarse gravel, crushed rock or concrete blocks as the thermal storage medium; these systems usually employ an inert gas or air as the heat transfer medium. These systems are most often used as “batteries” to store solar or wind energy generated in excess of immediate demand. Sec L. Amiri et al., “Progress on rock thermal energy storage (RTES): A state of the art review,”Energy Science &Engineering, 2:410-437 (2023), doi.org / 10.1002 / ese3.1478.

[0007] A geothermal heat pump (GHP) takes advantage of the relatively constant temperature of the ground to provide heating, cooling and hot water for residential and commercial buildings. Although electrical power is consumed in the operation of a compressor and pumps, the net energy output of a typical GHP is on the order of four times the input. GHP systems are mostly used to reject heat into the ground in the summer. While they are capable of collecting and concentrating heat in the winter, the amount collected from temperate ground is normally sufficient only for the provision of sanitary hot water.

[0008] In a typical ground-coupled system, a closed loop of plastic pipe is installed in the ground, and a water-antifreeze solution is circulated through the pipes to either collect heat from, or reject heat to, the ground. In some cases, an open loop system is employed, where well, river, or lake water is directly put through the heat exchanger and then discharged into a second well, or back into the river or lake.

[0009] Heat pumps are rated in units of Btu / hr, kW, or tons (the cooling power produced by a ton of ice, equal to 12,000 Btu / hr or 3.51 kW.) A unit for a typical one-family home would be rated at around three tons or 10.5 KW, while an installation for a large commercial building may have a capacity of 10-20 MW.

[0010] The above-described water / CO2 heat exchange systems and ground-coupled GHP and seasonal storage systems are currently separate installations, each requiring its own collection of pumps, piping, sensors, and controllers. Furthermore, the CO2-based systems generally reject a considerable amount of “waste” heat via rooftop gas coolers, with little appreciation for the fact that the user has paid for the energy that the coolers dissipate to the atmosphere. There is a need for a more integrated system that could carry out the functions of all of the above systems, with associated savings in energy efficiency and hardware and installation costs.BRIEF DESCRIPTION OF THE INVENTION

[0011] The invention provides a system for simultaneous heat exchange and seasonal heat storage for a carbon dioxide-based heating and cooling system for a facility, which combines an underground CO2 network, containing circulating liquid carbon dioxide, and two underground water networks containing circulating water. The first of the water networks is in thermal contact with the underground CO2 network, while the second is not in thermal contact with the underground CO2 network.

[0012] The facility may be a residence, apartment building, office building, or a mixed-use structure. Facilities with very high cooling needs, such as supermarkets, fish and meat wholesale and retail establishments, meat-packing, poultry-processing, and food-processing plants, and the like, often employ CO2 as a refrigerant (R-744) for cold rooms and freezers, and these facilities are particularly intended to benefit from the present invention.

[0013] The heating and cooling system within the facility employs liquid or supercritical CO2 as a working fluid. Excess heat generated during compression of the CO2 is stored for later recovery by routing the CO2 to an underground thermal energy storage (UTES) component. Recovery of the heat employs water circulating through the first water water network, which is positioned as a loop within the same UTES, with a geothermal heat pump (GHP) serving to move the recovered energy back into the building's heating and cooling systems. In this context, the term “water” should be understood to refer not only to water alone, but to water-miscible coolants such as alcohols and glycols, and mixtures thereof with water, and various brines, as are known in the art.

[0014] Ordinarily, the electrical energy expended during compression of CO2 is only partially stored as increased enthalpy of the working fluid, due to the cooling required between compression stages. Thermal compression systems, such as that disclosed in U.S. Pat. No. 11,774,186 (Oct. 3, 2023), the entire contents of which are incorporated herein by reference, likewise produce CO2 at temperatures in excess of what may be needed in warm weather. The present invention, rather than rejecting the heat of compression as waste heat, removes this energy to the UTES. Regulation of heat transfer to the UTES allows the temperature and pressure of the liquid or supercritical CO2 to be adjusted to those conditions where the heating and refrigeration cycles operate at maximal efficiency.

[0015] A separate, second water network is buried in cool ground, not in effective thermal contact with the CO2 network, and may be used for cooling in warm weather by the same heat pump running in the opposite direction.BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIG. 1 is a diagram showing the system of the invention, with arrows showing the fluid flows during warm-weather operation of the CO2 and water loops of the invention.

[0017] FIG. 2 is a diagram showing the system of the invention, with arrows showing the fluid flows during cold-weather operation of the CO2 and water loops of the invention.DETAILED DESCRIPTION OF THE INVENTION

[0018] Most broadly, the invention provides at least two ground-based thermally-connected geothermal networks. These networks may be vertically or horizontally oriented, and parallel, intertwined, or otherwise configured, but in preferred embodiments will comprise a set of parallel, horizontally-laid-out loops of piping or tubing. One of the networks (the CO2 network) consists of one or more loops of circulating carbon dioxide, and another of the networks (the water network) consists of one or more loops of circulating water. This first water network is in effective thermal contact with the CO2 network, which is to say that heat transferred from the CO2 circulating through the CO2 network to the soil can be effectively recovered by transfer to the water circulating through the water network. Perpendicular distances between the piping components of the two networks may be, for example, on the order of 2-6 inches, or in large high-capacity systems may be as high as 12-18 inches. In alternative embodiments, the water-containing loop(s) may be replaced by a water reservoir, into which water is pumped and from which water is withdrawn. Accordingly, references to a “water network” in the description which follows should be understood to refer not only to water-filled loops of piping or tubing, but also to water reservoirs.

[0019] As noted above, a separate, second water network is buried in cool ground, not in effective thermal contact with the CO2 network.

[0020] The CO2 network is functionally connected to a CO2-based heating and cooling system, typically a system installed in a building for environmental or special-purpose (e.g., refrigerated rooms or freezers) temperature control. The water network is functionally connected one side of a water / water heat pump, and the second side of the water / water heat pump is functionally connected to the building's circulating water system.

[0021] The CO2 network installed in the ground is primarily a UTES receiver for discharging the waste heat generated during CO2 compression. In winter weather, the rejected heat can be used directly to heat the building, but in summer weather this heat is in excess of what is required, e.g., for hot water. In warm weather, the primary function of the UTES is to lower the return temperature to a desirable value, which is typically 25-30° C. for CO2 intended to be provided in the supercritical state at a pressure and temperature optimal for cooling or refrigeration. In the process, the UTES becomes a heat reservoir which can operate in seasonal storage / release mode. This avoids, or at least mitigates, the energy loss normally occasioned by the prior art method using rooftop gas coolers.

[0022] The invention employs a geothermal heat pump to transfer heat to and from the water networks, drawing heat from the first network during cool weather, and moving heat from the facility to the ground via the second water network.

[0023] FIG. 1 illustrates the operation of a representative embodiment of the invention. The facility 10 has a heating system 11 (sanitary hot water and environmental heating) and a cooling system 12. Carbon dioxide is compressed and heated by a 3-stage compressor system (13, 14, and 15, or alternatively by an inductive heater / compressor 16, into a non-gaseous fluid state, which may be a sub-critical liquid or a supercritical fluid. Preferably, the hot CO2 is cooled between stages by heat reclamation units 17 and 18, which are heat exchangers coupled to the hot water loop 21 feeding heating system 11. The hot fluid carbon dioxide exiting at 19 is preferably first passed through a heat exchanger 20, where heat generated during compression process is transferred to the hot water loop 21 supplying heating system 11. Hot CO2, exiting from the compression system and / or from the optional heat exchanger 20, is directed via valve 22 into the buried carbon dioxide network 25, where the remaining heat is transferred to the surrounding soil 26, or alternatively is directed to a gas cooler 23. Cooled CO2 from loop 25 and / or from gas cooler 23 is directed via valve 24 to an evaporator 31, where water arriving via line 33 from the facility chilled water system 12 is cooled, and returned to the system via line 32. The temperature of the fluid CO2 delivered to evaporator 31 can be optimized by operation of valves 22 and 24 and by variation of the fan speed within gas cooler 23. The valves 22 and 24 may be proportional valves if close control of the evaporator temperature is desired. The gaseous carbon dioxide exiting the evaporator 31 is returned via line 34 to the compressor system.

[0024] In an alternative embodiment, the fluid CO2 may be passed through a gas cooler placed in series with the CO2 network 25.

[0025] The geothermal heat pump 27 is switchable, via operation of valves 40-43, between a cool weather state and a warm weather state; wherein when in the cool weather state, the geothermal heat pump is configured to recover heat from the first underground water network and deliver the recovered heat to the facility, and when in the warm weather state, the geothermal heat pump is configured to remove heat from the facility and deliver the removed heat to the second underground water network.

[0026] Thus, in cool weather, the geothermal heat pump 27 recovers heat from the UTES by employing the water circulating in the first underground water network 30 via valves 40 and 41, while in warm weather, the geothermal heat pump is reversed, rejecting heat delivered by the chilled water system of the facility into the water circulating in the second underground water network 37 via valves 42 and 43. The second underground water network 37 is buried in soil in a separate ground 38, which is not heated by the CO2 network 25, in order to ensure that the soil of ground 38 is an efficient heat sink.

[0027] Heat pump 27 operates in cool weather by collecting heat from network 30 and delivering it via lines 28 to the heating system 11. In warm weather, heat pump 27 operates in the reverse direction, collecting rejected heat from cooling system 12 via lines 29 and delivering it to network 37.

[0028] When the demand for heat is low, the enthalpy of the compressed CO2 will be sufficient for all heating needs-typically, the provision of hot water will constitute most or all of the demand. Under these conditions, the heat generated during compression is not fully utilized, and the UTES functions as a heat sink, storing the excess heat for later use during periods of cold weather.

[0029] When demand for heat is high, e.g. in cool weather, the geothermal heat pump runs in reverse, transferring heat from the ground to the facility's heating and / or hot water systems. In this mode, the invention effectively provides hot soil, as would a deep-drilled geothermal system, but at a convenient and inexpensive depth. Furthermore, the hot soil provided by the UTES of the invention is at a considerably higher temperature than the ground accessed by typical geothermal systems. The efficiency of the heat pump in the system of the invention thereby approaches that obtained when exploiting the naturally hot ground in a geothermally-active environment, such as an area of hot springs or geysers.

[0030] In warm weather, a heat pump, preferably the same heat pump described above, operates in the reverse direction, absorbing heat rejected into the facility cooling system and transferring it into the second water network, which is buried in cool ground and / or exposed to a natural source of cool water.

[0031] The operations of the heat pumps in concert with the facility heating and cooling systems is readily implemented with commercially available controls and hardware, since such uses are already known and implemented. Likewise, the use of CO2 as a working fluid in cooling and refrigeration systems is readily implemented with known equipment and controls. The increased efficiency of the system of the invention derives primarily from the integration of the hot fluid CO2 network into the UTES, thereby storing for later use the excess heat that, in prior art systems, is rejected to the atmosphere via rooftop gas cooler / condenser units.

Claims

1. A system for simultaneous heat exchange and seasonal heat storage for a carbon dioxide-based heating and cooling system for a facility, comprising:(a) an underground CO2 network containing circulating liquid carbon dioxide;(b) a first underground water network containing circulating water, in thermal contact with the underground CO2 network;(c) a second underground water network containing circulating water, not in thermal contact with the underground CO2 network; and(d) a geothermal heat pump switchable between a cool weather state and a warm weather state; wherein(i) when in the cool weather state, the geothermal heat pump is configured to recover heat from the first underground water network and deliver the recovered heat to the facility, and(ii) when in the warm weather state, the geothermal heat pump is configured to remove heat from the facility and deliver the removed heat to the second underground water network;wherein(e) the carbon dioxide is compressed and heated by a compressor system into a non-gaseous fluid state;(f) the fluid carbon dioxide is passed through a heat exchanger, where heat is transferred to a hot water system of the facility;(g) the fluid carbon dioxide exiting the heat exchanger is directed into the underground CO2 network, where heat is transferred to the surrounding soil;(h) the fluid carbon dioxide exiting the buried loop is directed to an evaporator, where the fluid CO2 is cooled by conversion to the gaseous state and absorbs heat collected by a chilled water system of the facility; and(i) the gaseous carbon dioxide exiting the evaporator is supplied to the compressor system at step (e).

2. The system according to claim 1, wherein the compressor system is a three-stage mechanical compression system.

3. The system according to claim 1, wherein the compressor system is a thermal compression system.

4. The system according to claim 1, wherein the second underground water network is in thermal contact with an underground aquifer.

5. The system according to claim 2, wherein the second underground water network is in thermal contact with an underground aquifer.

6. The system according to claim 3, wherein the second underground water network is in thermal contact with an underground aquifer.

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