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.

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.

Description

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 CO.sub.2 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.