WO2022168098A1 - Systems and methods for compressing, storing, and expanding refrigerant in order to supply low-cost air conditioning - Google Patents

Abstract

An air conditioning system includes a compression unit; a plurality of high pressure condensing tanks; an expander for releasing the compressed refrigerant from the high pressure tanks while expanding the compressed refrigerant; an evaporator; low-pressure storage tanks for collecting discharged vapor from the evaporator; and a conduit for conveying the refrigerant vapor from the low-pressure storage tanks to an intake of the compression unit. The compression unit may include pairs of liquid-gas pistons, each pair having first and second cylinders having substantially equal volumes, a pressure equalizing valve arranged between the first and second cylinders of each pair, and a liquid pump. Pressurizing of gas through pumping of liquid through each respective pair of liquid gas pistons is performed with a constant time shift. An expander may capture work of expanding refrigerant for purposes of pumping of liquid in the compression unit.

Description

SYSTEMS AND METHODS FOR COMPRESSING, STORING, AND EXPANDING REFRIGERANT IN ORDER TO SUPPLY LOW-COST AIR CONDITIONING

Related Applications

This Application claims priority to U.S. Provisional Patent Application 63/145,606, filed February 4, 2021, entitled "System and Method for Energy Storage Using Compressed Coolant Fluid," and U.S. Provisional Patent Application No. 63/192,579, filed May 25, 2021, entitled "System and Method for Condensing Gas and for Energy Storage for Supplying Cooling and Air-Conditioning," the contents of both of which are incorporated by reference as if fully set forth herein.

Field of the Invention

The present Application relates to the field of sustainable energy systems. More specifically, the present application relates to systems and methods of supplying low-cost air conditioning by compressing and storing refrigerant during periods of low energy usage and of low cost of electric power and releasing the compressed refrigerant during periods of high energy usage and of high cost of electric power. The present application further relates to systems and methods of compressing refrigerant and expanding compressed refrigerant with high efficiency.

Background of the Invention

One major component of the cost of maintaining air conditioning systems, also known as chillers, is electricity consumption. Most demand for energy by chillers occurs during the daytime, when people are at work, requiring the cooling of large buildings. During daytime, electricity rates are high, thereby raising the cost of cooling. An estimated 3.3 billion room air-conditioning units will be installed in the world between today and 2050. Most of these units are inefficient, and will place a significant burden on electricity grid infrastructure and consumers, especially in developing countries. Drastic transformation of residential cooling technology through innovation can improve people's health, productivity, and well-being.

In addition, the planet is getting hotter. Already, 30 percent of the world's population is exposed to potentially dangerous heat conditions. By 2100, up to three-quarters could be at risk. Affordable cooling is becoming a global necessity, allowing for increased productivity, positive health outcomes, and accelerated economic development.

Accordingly, there is a need for midscale and large energy storage usable for straightening, or averaging, the electrical demand curve for HVAC (heat, ventilation, and air conditioning) systems.

One existing solution for such energy storage is ice storage. Ice storage energy systems use ice for thermal energy storage, enabling surplus wind energy and other such intermittent energy sources to be stored for chilling at a later time. However, in order to generate ice storage, it is necessary to use compressors at very low evaporating temperatures. Furthermore, ice storage requires a large volume of insulated tanks which are expensive to build and maintain over time. The cost of the building space used to store ice, instead of using the same space for offices or underground parking, for example, is also rather high.

The use of expanders to recover energy in a HVAC cycle is generally known.

One recently published survey reports on use of expanders to recover expansion power to improve the energy efficiency of vapor compression refrigeration systems. The types of expanders surveyed include reciprocating piston, rolling piston, rotary vane, scroll, and screw and turbine. Some of the reported expanders were implemented in transcritical CO2 refrigeration systems, and achieved improvements in the coefficient of performance (COP) of up to 30%. A. A. Murthy, A. Subiantoro, S. Norris and M. Fukuta: A review on expanders and their performance in vapour compression refrigeration systems. International Journal of Refrigeration, 106 (2019) 427 - 446.

Summary of the Invention

It is an object of the present disclosure to provide a stored energy air conditioning system that is economically viable. It is another object of the present invention to provide a stored energy air conditioning system in which it is possible to control the amount of power input into the air conditioning system and the timing of the input of this power. It is a further object of the present disclosure to provide a stored energy air conditioning system which is capable of storing cooling energy both at relatively low temperatures at which carbon dioxide is capable of being liquefied in a subcritical process, and at relatively high temperatures at which carbon dioxide may be compressed only in a transcritical process. It is a further object of the present disclosure to provide an air conditioning system that is capable of compressing the refrigerant with high efficiency, and of capturing work during expansion of the refrigerant for use during the compression.

The present disclosure achieves these objectives by including, within a cooling cycle, a plurality of high pressure condensing tanks and a plurality of low pressure storage tanks. The condensing tanks may store compressed refrigerant at high pressure for an extended period of time. The refrigerant is then available for delivery to an evaporator at a time of desired cooling. Following a cooling process in the evaporator, the refrigerant vapor is stored in the plurality of low pressure storage tanks until it is desired to begin the cooling process again.

Advantageously, the plurality of high pressure condensing tanks enables storage of the high pressure refrigerant during low-peak times. In addition, the low pressure storage tanks allow storage of the refrigerant during peak electricity demand, until nighttime when it is less expensive to compress the refrigerant.

The present disclosure further achieves these objects by disclosing highly efficient compression systems for gaseous refrigerants. The compression systems include one or more liquid-gas pistons arranged to operate in a reciprocal fashion for compressing gas therein. The compression systems may further include a water turbine for converting flow of water into electricity during operation of the liquid-gas pistons. In particularly advantageous embodiments, the compression systems include two pairs of liquid gas pistons, arranged to operate with a fixed time shift, so that a liquid pump may be in continuous operation.

The present disclosure further achieves these objectives by disclosing compression systems suitable for isothermal compression of the refrigerant. The isothermal compression may be accomplished in different ways depending on whether the compression is performed in a subcritical or transcritical fashion. . The transcritical compression is particularly suited to maximize thermal efficiency in hot areas that maintain a temperature of above 31°C at nighttime, wherein the subcritical compression is well-suited to maximize thermal efficiency in areas having a temperature below 31 °C at night.

The present disclosure further achieves these objectives by disclosing an expander for a cooling cycle that captures energy of the expanding gas in order to power the liquid piston pumps of the compression units. The expander system replaces a conventional expansion device in the vapor compression system, such as a valve, orifice or capillary tube, where the risk of water freezing inside is a potential problem, under the operating temperatures involved. While helping to overcome this practical problem, the expander is also thermodynamically superior and improves the overall cooling COP. One particular feature of the disclosed expander is its ability to recover the work of expansion, and use it to assist the water pumping in the compressor. This further increases the overall COP, without any moving mechanical parts, such as would be, for example, in a turbine-type expander.

Particularly advantageously, each of these improvements may be integrated in the same cooling system. Cooling systems that are predicated on compression units utilizing liquid-gas pistons are particularly well suited to having the compressed refrigerant stored for long periods of time. By contrast, traditional compressors lack the efficiency to compress sufficient quantities of refrigerant in a cost-effective manner. Similarly, such cooling systems are also well-suited for isothermal compression and the use of an expander to capture the work of expansion and use this work for pumping fluid in the compression unit. Similarly, a transcritical or subcritical cooling cycle may be used with the storage system, enabling performing

CO2 compression at low rate electricity prices and production of cooling at peak hours when electricity is at high rates, and thus providing financial savings for HVAC applications.

A cooling system including each of these components is expected to exhibit benefits in performance of at least 25%-50% compared to conventional cooling systems.

The system may be deployed as a central air conditioning module in factories, skyscrapers, buildings, private houses, mobile HVAC, electric vehicles, and mobile refrigeration.

According to a first aspect, an air conditioning system is disclosed. The air conditioning system includes a compression unit configured to compress a gaseous refrigerant; a plurality of high pressure condensing tanks connected to an outlet of the compression unit and configured to store the compressed refrigerant; an expander or expansion valve in fluid communication with an outlet of the plurality of high pressure condensing tanks, for releasing the compressed refrigerant from the plurality of high pressure condensing tanks while expanding a volume of the compressed refrigerant; an evaporator in fluid communication with an outlet of the expander or expansion valve for causing the refrigerant to absorb heat from a surrounding environment; a plurality of low-pressure storage tanks for collecting discharged refrigerant vapor from the evaporator; and a conduit for conveying the refrigerant vapor from the plurality of low-pressure storage tanks to an intake of the compression unit.

In another implementation according to the first aspect, the air conditioning system further includes a recuperator arranged between the plurality of high pressure condensing tanks and the expander or expansion valve, said recuperator configured to cool incoming condensed refrigerant with outgoing expanded refrigerant.

In another implementation according to the first aspect, the air conditioning system further includes a first heat exchange unit configured to receive heat from the refrigerant during compression thereof to thereby heat an external supply of water.

In another implementation according to the first aspect, the air conditioning system further includes a second heat exchange unit configured to be cooled by the refrigerant during expansion thereof to thereby cool an external supply of water.

In another implementation according to the first aspect, the refrigerant is carbon dioxide. Optionally, the carbon dioxide is liquid in the plurality of high- pressure condensing tanks and is gaseous in the plurality of low pressure storage tanks. Optionally, the carbon dioxide is a supercritical plasma in the plurality of high- pressure condensing tanks and is gaseous in the plurality of low-pressure storage tanks.

According to a second aspect, a compression unit for a cooling system is disclosed. The compression unit includes: at least two pairs of liquid-gas pistons, wherein each pair of liquid gas pistons comprises first and second cylinders having substantially equal volumes, and a combined volume of liquid substantially equivalent to a volume of one of the liquid-gas pistons; a pressure equalizing valve arranged between the first and second cylinders of each pair of liquid-gas pistons; a liquid pump in fluid communication with each of the four liquid-gas pistons; a gas intake valve for low-pressure gas and a gas outlet valve for high-pressure gas arranged at each liquid-gas piston; and a series of valves arranged between the liquid pump and the respective liquid-gas pistons, wherein the valves may be alternatively opened and closed such that pressure equalization among each pair of liquid gas pistons, and pressurizing of gas through pumping of liquid through each pair of liquid gas pistons, is performed in a constant time shift.

In another implementation according to the second aspect, the constant time shift corresponds to a time required to equalize between the pressures of the first and second cylinders of a pair of cylinders.

In another implementation according to the second aspect, each pair of liquid gas pistons is configured to compress gas in a series of alternating half-cycles, wherein in a first half cycle gas is compressed within a first cylinder through influx of liquid from the second cylinder, and in the second half-cycle gas is compressed within the second cylinder through influx of liquid from the first cylinder. Optionally, in each half-cycle, gas is first compressed through pressure equalization between the two liquid-gas pistons in a pair, and then is compressed further through pumping of fluid between the two liquid-gas pistons.

In another implementation according to the second aspect, the compression unit further includes a main tank having an inlet for receiving therein a gas at low pressure; an open liquid tank configured to deliver liquid to the first gas tank and thereby compress the gas within the main tank; and a valve assembly configured to switch between a first state, in which low-pressure gas passes through the main tank and is pressurized by the liquid-gas pistons, and a second state, in which the low- pressure gas is pressurized within the main tank by liquid from the open liquid tank. Optionally, the compression unit further includes a turbine arranged between the open liquid tank and the main tank, and configured to produce electricity from the flow of liquid from the main tank to the open liquid tank.

In another implementation according to the second aspect, an interior volume of each cylinder is comprised of a plurality of cooling tubes connected in parallel to each respective gas intake valve, gas outlet valve, and the liquid pump, each cooling tube including a plurality of cooling fins, and the compression unit further includes a cooling system for providing cooling fluid to an exterior of each of the cooling tubes between the cooling fins. Optionally, the cooling fluid is water.

In another implementation according to the second aspect, a cooling system includes the compression unit and an expander, said expander comprising a first cylinder, a U-shaped duct, and a second cylinder connected in series, wherein an outlet of the second cylinder is connected to the fluid pump of the compression unit and to a low-pressure fluid reservoir, and wherein, during expansion of refrigerant, the expanding refrigerant enters the first cylinder and displaces fluid through the duct and second cylinder to the fluid pump, and, wherein, following expansion of refrigerant, liquid from the low-pressure fluid reservoir enters the second cylinder to thereby evacuate expanded refrigerant from the expander into an evaporator and refill the expander with fluid.

In another implementation according to the second aspect, a cooling system includes the compression unit, a plurality of high pressure condensing tanks connected to an outlet of the compression unit and configured to store the compressed refrigerant; an expander or expansion valve in fluid communication with an outlet of the plurality of high pressure condensing tanks, for releasing the compressed refrigerant from the plurality of high pressure condensing tanks while expanding a volume of the compressed refrigerant; an evaporator in fluid communication with an outlet of the expander or expansion valve for causing the refrigerant to absorb heat from a surrounding environment; a plurality of low- pressure storage tanks for collecting discharged refrigerant vapor from the evaporator; and a conduit for conveying the refrigerant vapor from the plurality of low-pressure storage tanks to an intake of the compression unit. Optionally, the expander or expansion valve is the expander described above.

According to a third aspect, an expander is disclosed. The expander includes a first cylinder, a U-shaped duct, and a second cylinder connected in series. The first cylinder is connected to a source of compressed refrigerant and an evaporator. The second cylinder is connected to a fluid pump and to a low-pressure fluid reservoir. During expansion of refrigerant, compressed refrigerant enters the first cylinder from the source of compressed refrigerant, and displaces fluid from the first cylinder, duct, and second cylinder to the fluid pump, and, wherein, following expansion of refrigerant, liquid from the low-pressure fluid reservoir enters the second cylinder, duct, and first cylinder to thereby evacuate expanded refrigerant from the expander into the evaporator and refill the expander with fluid.

According to a fourth aspect, a method of air conditioning is disclosed. The method includes: compressing a gaseous refrigerant; storing the compressed refrigerant; expanding a volume of the compressed refrigerant with an expansion valve or expander; evaporating the refrigerant with an evaporator and thereby causing the refrigerant to absorb heat from a surrounding environment; collecting discharged refrigerant vapor from the evaporator; and conveying the discharged refrigerant vapor to an intake of the compressor, and repeating each of the previous steps.

In another implementation according to the fourth aspect, the storing step comprises storing the compressed refrigerant in a plurality of high-pressure storage tanks, and the collecting step comprises collecting the discharged vapor in a plurality of low-pressure storage tanks.

Optionally, the method further includes performing the compressing and storing steps at nighttime, and performing the expanding, evaporating, collecting, and conveying steps during daytime.

In another implementation according to the fourth aspect, the refrigerant is carbon dioxide, and further comprising performing the compressing step in a subcritical process. Optionally, the method further includes performing the compressing step substantially isothermally by using already liquefied carbon dioxide to cool the carbon dioxide during compression thereof.

In another implementation according to the fourth aspect, the refrigerant is carbon dioxide, and the method further includes performing the compressing step in a transcritical process.

In another implementation according to the fourth aspect, the method further includes performing the compressing step substantially isothermally by supplying a cooling fluid to an exterior of a cylinder used for compression of the carbon dioxide therein.

In another implementation according to the fourth aspect, the expanding step comprises using an expander to capture work of the expanding refrigerant to thereby deliver fluid to a fluid pump used to compress the refrigerant during the compressing step.

In another implementation according to the fourth aspect, the method further includes releasing at least a portion of the compressed refrigerant through a turbine to thereby generate electricity.

In another implementation according to the fourth aspect, the method further includes performing the expanding, evaporating, and collecting steps without investment of electricity.

Brief Description of the Drawings

FIG. 1 is a schematic illustration of components of a prior art HVAC system;

FIG. 2 is a schematic illustration of components of an air conditioning system having refrigerant storage tanks, according to embodiments of the present disclosure;

FIG. 3 is a schematic diagram illustrating flow of refrigerant during operation of an air conditioning system such as that disclosed in FIG. 2, according to embodiments of the present disclosure;

FIG. 4 is a schematic illustration of a compression unit for compressing refrigerant, according to embodiments of the present disclosure; FIG. 5 is a schematic illustration of a second embodiment of a compression unit, according to embodiments of the present disclosure;

FIG. 6A is a schematic illustration of the compression unit of FIG. 5 in operation, according to embodiments of the present disclosure;

FIG. 6B is a schematic illustration of a time shift during operation of the compression unit of FIG. 5, according to embodiments of the present disclosure;

FIG. 7A illustrates a transcritical compression cycle in an adiabatic process, according to embodiments of the present disclosure;

FIG. 7B illustrates a transcritical compression cycle in an isothermal process, according to embodiments of the present disclosure;

FIG. 8 illustrates a cylinder equipped with cooling fluid for cooling in a transcritical process, according to embodiments of the present disclosure;

FIG. 9 illustrates a cross section view of the cylinder of FIG. 8, according to embodiments of the present disclosure;

FIGS. 10A-10D schematically illustrate flow of gas and liquid in an expander in order to generate work for use during compression, according to embodiments of the present disclosure;

FIG. 11 is an exemplary implementation of the air conditioning system of any of the preceding embodiments in an apartment building, according to embodiments of the present disclosure; and

FIG. 12 is an exemplary implementation of the air conditioning system of any of the preceding embodiments an electric vehicle, according to embodiments of the present disclosure; FIG. 13A is a schematic diagram illustrating the flow of refrigerant through an air conditioning system of any of the preceding embodiments during a cooling process, according to embodiments of the present disclosure; and

FIG. 13B is a schematic diagram illustrating the flow of refrigerant through the air conditioning system of any of the preceding embodiments during a heating process, according to embodiments of the present disclosure.

Detailed Description of the Invention

The present Application relates to the field of sustainable energy systems. More specifically, the present application relates to systems and methods of supplying low cost air conditioning by compressing and storing refrigerant during periods of low energy usage and of low cost of electric power and releasing the compressed refrigerant during periods of high energy usage and of high cost of electric power. The present application further relates to systems and methods of compressing refrigerant and expanding compressed refrigerant with high efficiency.

Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not necessarily limited in its application to the details of construction and the arrangement of the components and/or methods set forth in the following description and/or illustrated in the drawings and/or the Examples. The invention is capable of other embodiments or of being practiced or carried out in various ways.

As used in the present disclosure, the term "transcritical compression" refers to compression of a gas from a subcritical state to a supercritical state, which is a state in which the gas is at a high pressure and at a temperature that is higher than the critical point of the gas. As a result, the compressed gas forms a condensed plasma but does not liquefy. The term "subcritical compression" refers to compression of the gas when the gas remains at a temperature that is lower than the critical point of the gas. As a result, the gas liquefies. The critical point of carbon dioxide is 73.8 bar and 31.0 °C.

As used in the present disclosure, an "adiabatic process" is a process in which compression is performed without transfer of heat between a compression unit and an outside environment. When a gaseous refrigerant is compressed in an adiabatic process, the temperature of the refrigerant increases. As used in the present disclosure, an "isothermal process" is a process in which compression is performed while maintaining a temperature of the refrigerant constant. In order to achieve an isothermal compression process for a gas, it is necessary to supply outside cooling to the compression unit, to compensate for the inherent increase in temperature of a gas resulting from an increase in pressure.

High Pressure Refrigerant Storage in a Cooling Cycle

Referring now to FIG. 1, a conventional prior art air conditioning system 1 is illustrated. In system 1, a gaseous refrigerant is compressed by compressor 2, and is passed in stage 3 to condenser 4. At condenser 4, the gaseous refrigerant is converted to a liquid at high pressure. In stage 5, the liquid refrigerant exits the condenser at expansion valve 6. The expansion valve 6 causes an abrupt pressure drop of the liquid refrigerant. In stage 7, the refrigerant reaches evaporator 8 when its pressure has been reduced, dissipating its heat content and making the refrigerant much cooler than the ambient temperature. This causes the refrigerant to absorb heat from the warm air and reach its boiling point rapidly. The refrigerant then vaporizes in step 9, absorbing the maximum amount of heat, and is returned to the compressor 2 to repeat the cycle. This process is in widespread use, although it is not particularly cost-efficient.

FIG. 2 schematically illustrates main components of a stored energy air conditioning system 10, according to embodiments of the present disclosure. System 10 includes a compression unit 12 for compressing 13 a refrigerant. Embodiments of compression unit 12 will be described in depth further herein. Compression unit 12 may be operated at night time, or when grid electricity costs less, or when a renewable energy source is available. The compressed refrigerant enters high pressure storage tank 14, which is also referred to herein as a condensing tank. The compressed refrigerant is passed 15 through an expander or expansion valve 16, which enables fast pressure reduction of the refrigerant, thereby allowing expansion or change of state of the refrigerant. The refrigerant then passes through an evaporator 18, which causes the refrigerant to absorb heat by vaporizing the refrigerant, as discussed above in connection with FIG. 1. The evaporated refrigerant then passes in stage 19 to a low pressure tank 20, where the discharged refrigerant vapors are collected. The cycle is then restarted as the discharged vapors are forwarded in stage 11 to an intake of the compression unit 12. Then, during night time or during availability of a renewable source of energy, the compression unit 12 re-compresses the collected refrigerant vapors, to commence a new cooling cycle. Unlike prior art system 1, system 10 does not employ a compressor in the cooling path, which is activated on-demand during peak hours. Instead, system 10 uses the condensing tank 14 to store compressed refrigerant at high pressure. Relatedly, in system 10, it is not necessary to expend electricity at every time when cooling is desired. Rather, electricity may be expended at night time or off-peak hours, or from a renewable energy source, and cooling is provided at a later time, when electricity is more expensive and/or when the electric grid is not functional.

High pressure storage tank 14 may be underground, or underwater, or in another suitable low-cost area. Exemplary locations for installation of the high pressure storage tanks are disclosed in FIGS. 11 and 12 herein.

In a typical embodiment, the refrigerant used is carbon dioxide. One advantage of the use of carbon dioxide is that relatively low volume tanks may be used for storage of the refrigerant in the gas phase, due to the relatively low ratio of the gaseous volume (V_g) to the liquid volume (Vf). Another advantage of the use of carbon dioxide is that it is more environmentally friendly than other types of refrigerants.

A sample calculation for the size of the low pressure storage tanks is as follows. In order to power a 700 kW cooling system, the mass flow rate of carbon dioxide, at 4 °C, is required to be approximately 4 kg/s. This calculation is based on the equation Q = M * Hf_g, where Q is the cooling capacity, m is the mass of carbon dioxide, and Hf_g is the latent heat of vaporization, which is approximately 220 kJ/kg at 4°C. In order to deliver a mass flow rate of 4 kg/s for four hours, it is necessary to store 57,600 kg of carbon dioxide. To deliver the same cooling for 8 hours, it is necessary to store twice as much carbon dioxide, or 115,200 kg of carbon dioxide. In addition, it is known that the V_g/Vf ratio of carbon dioxide is approximately 10. Thus, it is necessary to provide a volume of storage for the low-pressure carbon dioxide that is approximately 10 times as large as the volume of storage of liquid carbon dioxide.

FIG. 3 depicts an exemplary flow diagram for a refrigerant of an air conditioning system 300, according to embodiments of the present disclosure. Air conditioning system 300 is similar in most respects to air conditioning system 10, except that it includes additional components, as set forth herein.

System 300 includes a compressing section 310, a cooling section 320, a high pressure storage section 330, and a low pressure storage section 340. Compressing section 310 and cooling section 320 may mutually operate in a closed cycle, bypassing storage sections 330 and 340, as in a conventional air conditioning system. Compressing section 310 and cooling section 320 may also operate with high pressure storage section 330 and low pressure storage section 340 disposed between the compressing section 310 and cooling section 320, as described in connection with system 10.

Compressing section 310 may receive low pressure refrigerant at point A. For example, the refrigerant may be carbon dioxide. In exemplary embodiments, the low pressure carbon dioxide entering the compressing section 310 is gaseous, at 23°C and 38.6 bar.

The refrigerant is then conveyed to a compression unit 312. In one example, compression unit 312 raises the pressure of the carbon dioxide to 82 bar at 93.1 degrees centigrade. Under these pressure and temperature conditions, the carbon dioxide is a supercritical plasma. The refrigerant exits the compression unit 312 at point B.

Optionally, the carbon dioxide passes from the compressing unit 312 to heat exchange unit 314. The heat exchange unit 314 is configured to heat city supply water received at point Cl via pump 316, for providing hot water at point C2, at 60 °C. The carbon dioxide exiting heat exchange unit 314 may be at 81.5 bar and 83.9 °C, which is still a supercritical plasma.

From heat exchange unit 314, the carbon dioxide may pass through condenser unit 318, which is cooled by a flow of ambient air propelled by fan 311. The condenser unit 318 may provide carbon dioxide at 81 bar and 28 °C at point D, which is the exit from compressing section 310. Under these pressure and temperature conditions, the carbon dioxide is liquid.

In alternative embodiments, the carbon dioxide is pressurized directly to conditions in which it is liquid, in a subcritical process, without cooling the carbon dioxide through heat transfer to a water system or through a separate condenser unit.

From point D, the compressed carbon dioxide may be provided directly to cooling section 320, or may be stored in high pressure storage section 330, as may fit the desired mode of operation. Storage section 330 may include a plurality of high pressure condensing tanks, as described above in connection with system 10 in FIG. 2. For example, system 300 may be used during hours when electricity is cheaper for compressing carbon dioxide, and store the compressed carbon dioxide for use during hours when electricity is more expensive. Valve 332 may be used to alternatively direct the compressed carbon dioxide to high pressure storage section 330 or to cooling section 320.

Optionally, at the entrance of the cooling section 320, the compressed carbon dioxide passes point D' and enters recuperator unit 322. Recuperator unit 322 is used to maximize the cooling power of the carbon dioxide, by transferring heat to the refrigerant in a different stage of the cooling section 320, and thereby decreasing the temperature of the provided carbon dioxide from 28 °C to about 22°C.

The compressed carbon dioxide exits the recuperator unit 322 and passes to expansion valve 324. At expansion valve 324, the pressure of the carbon dioxide drops to 38.7 bar, and the temperature drops to 4 degrees centigrade. Optionally, instead of an expansion valve, cooling section 320 may incorporate an expander such as the expander described further herein.

Optionally, the cooled carbon dioxide passes through heat exchanger 326. Heat exchanger 326 may cool down service liquid (for example, water) provided at 12 °C at point DI by water pump 328, down to 7°C and 2.7 bar. The service liquid may be used, for example, to provide cooling during an isothermal compression process, as will be described further herein. The carbon dioxide is then ready to be used in water/air heat exchangers for providing air conditioning to rooms, at point D2.

To complete the cycle, the carbon dioxide leaves the heat exchanger, and passes again through the recuperator unit 322. At this second pass in the recuperator unit 322, the carbon dioxide is heated by the refrigerant in the earlier stage of the cooling section 320, thereby providing more pre-cooling to the refrigerant before the expansion stage, as set forth above. The refrigerant leaves the cooling section 310 at 23 °C and 38.1 bar, at point Al.

The carbon dioxide refrigerant is then rerouted back to the compression section 310, to close the cooling cycle. The carbon dioxide may be stored in low- pressure tank 340 for later use, or may be provided directly to the compression section 310, as may be selected using selector valve 342. Compression Units Including Paired Liquid-Gas Pistons

FIG. 4 depicts components of a compression unit 400, which may take the place of pump 12 in system 10 or of compression unit 312 in system 300. Compression unit 400 includes a main tank 406 (T-l) connected via a fluid pump 408 to an open fluid reservoir 409. In exemplary embodiments, the fluid is water. Carbon dioxide is received from an evaporator at inlet 402 of the compression unit 400, which corresponds to point A of the compression cycle of FIG. 3. The carbon dioxide is fed via valve 404 (V-l) to tank 406. The received carbon dioxide may be accumulated in tank 406 at a constant pressure, which may be the pressure at which it is received from the cooling cycle (e.g., 38 bar).

In order to enable ongoing accumulation of carbon dioxide in tank 406, water pump 408 is operated to compensate for the changing volume of incoming or outgoing carbon dioxide. Water pump 408 provides water into tank 406 from open water tank 409, which is at atmospheric pressure, or releases water from tank 406 back to tank 409. When water is released from tank 406 back to water tank 409, water pump 408 may operate as a turbine and produce electricity.

The compressing process in compression unit 400 is achieved by reciprocatively operating cylinders 416 (T-2) and 418 (T-3) as dual liquid-gas pistons (LGPs). Throughout the description of FIG. 4, the terms "cylinder" and "liquid-gas piston" are used interchangeably. The total amount of liquid (e.g. water) existing in liquid-gas pistons 416 and 418 at any given stage of the compression operation is constant, and the sum of liquid in both tanks may be equal to substantially the volume of one of liquid-gas pistons 416 and 418. Liquid-gas pistons 416, 418 are connected both via pressure equalizing valve 420 and via pump 428, whose functions will be described in further detail herein.

The liquid-gas pistons and accompanying valves are illustrated with various symbols thereon to indicate the status of different pumps, valves, and cylinders at different times. Symbol 401 indicates the initial water level within a cylinder, and symbol 403 refers to the final water level of a cylinder. Symbol 405, marked by a rectangle with its four corners truncated and diagonal hatching, indicates a valve that is open from the beginning of the compression stage. Symbol 407, a rectangle with horizontal hatching, indicates a valve that is opened at some point during the compression stage.

Liquid-gas pistons 416, 418 are used to compress carbon dioxide in a series of two half cycles. Gaseous carbon dioxide enters compression unit 400 at a pressure and temperature typical to that at the exit from a cooling unit such as unit 320 of FIG. 3, e.g., a pressure of 38 bar and a temperature of 25 °C. During the first half-cycle, gaseous CO2 is delivered to cylinder 416 via valve 410 and valve 412 at a suction pressure of approximately 38 bar. Valve 412 is a passive pressure valve. At this time, the remaining valves in unit 400, such as valves 412A, 414, 414A, 422, and 424 are closed. Once the CO2 occupies the entire displacement volume in cylinder 416, high-pressure water from adjacent twin cylinder 418 flows to cylinder 416 through pressure equalization valve 420. Water flowing through the pressure equalization valve 420 pushes gaseous CO2 (starting at 38 bar) upward until the pressure in the two cylinders 416, 418 equalizes. During the pressure equalization process, the water level rises to line 417 in cylinder 416, and lowers to line 419 in cylinder 418.

At this point, the compression process is taken over by pump 428. Specifically, following the pressure equalization process, pump suction valves 426 and 430 are opened and the water level in the cylinder 416 continues to rise through the effort of pump 428, transferring water from cylinder 418, and thereby compressing the CO2 to (for example) 82 bar. Simultaneously, carbon dioxide from tank 406 enters the freeing volume in LGP 418 via valves 410 and 414. At the end of this half-cycle, the volume of carbon dioxide in LGP 416 has reached its minimum, and the pressure has increased to the desired pressure, for example, 82 bar, as discussed in connection with FIG. 3.

Pump 428 (P-2) is configured to supply water from one cylinder 416, 418 to the other, one at a time. Optionally, pump 428 is a centrifugal pump. In other embodiments, pump 428 may be a different type of pump such as a positive displacement piston pump. Pump 428 may operate periodically at high capacity (when carbon dioxide gas is compressed by water) and may be turned to an idle stage during the rest of the cycle (when no compression is carried out). It is assumed that ramping from an idle stage to high capacity takes a very short time, such as approximately a second.

The phase of the rising water column in a cylinder, for example cylinder 416 or 418, simulates the operation of a piston in an identical cylinder of a reciprocating compressor. This model may provide a fair estimation of energy saving that may be expected in the "liquid piston" scheme as compared to the use of a reciprocating compressor for CO2 compression.

Once CO2 reaches the target pressure level, e.g., 82 bar, gas evacuation (nonreturn) valve 412A is opened and the rising water column continues to compress the CO2 in isobaric fashion until the water level reaches the top water level, as indicated by symbol 403. The compressed gas in LGP 416 is then released to the compressed carbon dioxide line at point 432, which corresponds to point B in the system 300 of FIG. 6. CO2 evacuation valve 412A then closes, and a small quantity of compressed CO2 remains in the top section of cylinder 416. This concludes the compression (working stroke) of CO2 by cylinder 416. This process repeats itself at a certain frequency, with the two cylinders 416, 418 working interchangeably.

In sum, during the first half-cycle, cylinder 416 serves as a compression chamber and cylinder 418 (initially filled by water up to the maximum level) supplies water for compression.

At this point a second half-cycle is commenced, in which the roles of LGP 416 and LGP 418 are switched. During the second half of the cycle, compression takes place in cylinder 418, and cylinder 416, which was filled by water in the prior compression phase, supplies water to cylinder 418 in a similar fashion (first through connecting valve 420 until pressure equalization, and then through the water pump 428). Pump 428 is operated to provide liquid from LGP 416 to LGP 418 via valves 422 and 424, while valve 414 functions as a passive pressure valve. Valves 414 and 414A are pressure relief valves functioning at different directions and pressures. The rising liquid in LGP 418 raises the pressure of the carbon dioxide in LGP 418 from an initial pressure (e.g., 38 bar) to a final pressure (e.g., 81 bar), at which stage the pressurized gas may be released to the line towards point 432 via gas evacuation valve 414A.

The compression that is achieved using the twin liquid-gas pistoncompressors 416, 418 provides an improved energetic efficiency over other types of compressors that are required to operate in a large range of pressures through the compressing cycle.

Optionally, CO2 may be compressed in one continuous phase without using cylinders 416, 418, by using pump 408 to convey water into tank 406, turning tank 406 into a liquid-gas piston, compressing the CO2 in tank 406, and providing the compressed gas onward via line 432. To do so, it is required that tank 406 will withstand the highest pressure in the cycle (e.g., 80 bar). This option may cause a higher cost of installation, but the system is simpler.

To summarize, compression unit 400 may work in two modes of operation (normally at night, or generally speaking, at low electricity cost times): 1. With multiple half-cycles (strokes) of compression in cylinders 416, 418, which serve as "small pistons"; or 2. With one or a few "long strokes" of compression in tank 406 which serves as a "big piston."

Compression unit 400 may be used as part of a cooling system, and in particular may serve in place of the pump 12 of FIG. 2. Alternatively, compression unit 400 may be used in any system in which compression of gas is desired, for example an energy storage system. In addition, although, in the above-described implementation, the gas is carbon dioxide and the liquid is water, other gases and liquids may be used in compression unit 400 without departing from the scope of the present disclosure.

In the illustrated embodiment, compression unit 400 has only one pair of liquid-gas pistons 416, 418. In alternative embodiments, there are additional liquidgas pistons. Additional liquid-gas pistons enable the pump to work continuously. In addition, the compression unit 400 is scalable, in that the unit 400 may incorporate even more pairs of cooperative liquid-gas pistons 416, 418. Use of more than one pair may provide easy and simple scale-up of the system 400 as well as improved, better averaged and more fluent operation of the pumping unit 428. An example of a compression unit with more than one pair of liquid-gas pistons is illustrated in FIGS. 5, 6A, and 6B.

FIG. 5 illustrates a second embodiment of a compression unit 500, according to embodiments of the present disclosure. Compression unit 500 may be used for both subcritical and transcritical compression, although additional cooling components are required for transcritical compression, as will be explained further herein. A main difference between compression unit 500 and compression unit 400 - 1 - is that compression unit 500 includes two pairs of cylinders, namely cylinders 510a and 510b, and cylinders 510c and 510d, instead of one pair of cylinders. Although, in the illustrated embodiments, compression unit 500 does not include a main storage tank of carbon dioxide and an open water storage tank, these components are omitted solely for simplicity, and compression unit 500 may be integrated with such an apparatus.

Compression unit 500 includes four cylinders 510a-d. Each respective cylinder 510a-d is connected to inlet line 502 for low-pressure carbon dioxide, via gas inlet valves 504a-d, and to gas outlet valves 508a-d, which output compressed carbon dioxide to outlet line 508. At the bottom, each cylinder 510a-d is connected to a water pump discharge line 522 via water inlet valves 526a-d, and a water pump suction line via water outlet valves 528a-d. Water pump 520 pumps water from the pump suction line 520 to pump discharge line 522. In addition, cylinders 510a and 510b are connected to each other with piping 521a and valve 523a, and cylinders 510c and 510d are connected to each other with piping 521b and valve 523b.

In exemplary embodiments, the internal diameter of each cylinder is 715 mm, and the height of each cylinder is 2.5 meters.

In operation, the two pairs of cylinders 510a-510b and 510c-510d operate in parallel with a small and constant time shift. The time shift is required by each pair of cylinders for equalizing pressure before suction of low-pressure gas and compression of the gas. The time shift is achieved through selective opening and closing of water intake valves 526 and water outlet valves 528, as required in order to achieve the effects described herein. During the equalization process, one cylinder of each pair, which had residual compressed gas therein from a previous compression cycle, admits a "fresh" portion of uncompressed gas via a suction valve 504. In the other cylinder of the pair, a water admission valve 526 opens. The water piston in that cylinder, driven by the pump 520, compresses the newly admitted gas.

FIG. 6A illustrates the different processes of pressure equalization and compression taking place in different pairs of cylinders 510. Liquid from cylinder 510a is discharged from cylinder 510a into pump suction line 524 and into water tank 525. From water tank 525, the water is pumped via pump 520 into cylinder 510b. The result of this pumping action is to increase the pressure of the carbon dioxide to a maximum desired pressure, in this case 68 bar. Simultaneously, liquid flows in a pressure-equalizing manner from cylinder 510d to cylinder 510c via pipe 521b and valve 523b. In the illustrated embodiment, the effect of the pressure equalization is to raise the pressure in cylinder 510c from 38 bar to 40.4 bar, and to lower the pressure in cylinder 510d from 68 bar (as it had been following a previous compression cycle) to 40.4 bar.

In the next phase, the compressed gas is removed from cylinder 510b, with some residual compressed gas being permitted to remain, and pressure is allowed to equalize between cylinders 510b and 510a. In addition, the pump 520 operates to compress the gas in cylinder 510c, through infusion of water into cylinder 510d. The process continues in a cycle, with equalization and pumping taking place in each pair of cylinders 510. As a result, the pump 520 is always in operation, and operates at maximum efficiency. FIG. 6B graphically illustrates the time shift between the two pairs of cylinders. In the example of FIG. 6B, the pressure equalization time is 5 seconds, and the water pumping time for each half cycle is 25 seconds. Graph 550 illustrates the flow of water through the pump 520 and each of the cylinders 510 over time. Line 551 depicts a measure of the cumulative water flow; line 552 depicts a measure of water flow through a first pair of cylinders, and line 553 depicts a measure of a water flow through a second pair of cylinders. The half-cycles are provided in lengths of 30 seconds. At the beginning of a half-cycle, water is pumped through the pair of cylinders indicated by line 552, while pressure is equalizing in the pair of cylinders indicated by line 553. After 5 seconds, the pressure equalization is complete, and water is also pumped through the second pair of cylinders. After 25 seconds, the first pair of cylinders is permitted to equalize their pressures, and after 30 seconds, the second pair of cylinders is permitted to equalize their pressures. Isothermal Compression

In preferred embodiments, the compression process described above is performed in an isothermal fashion.

The benefits of isothermal compression in a vapor compression cycle may be realized regardless of the type of compression system that is used, and regardless of whether the vapor compression cycle includes storage of compressed gas. In exemplary embodiments, the isothermal compression described herein is performed in a vapor compression cycle having four main components: an evaporator, condenser, compressor (which may be any of the compression units discussed above), and an expansion device (which may be the expander described infra). The vapor compression cycle may further include a suction side heat exchanger (SSHX) that recuperates heat from the condensate in order to superheat refrigerant at an outlet of the evaporator.

Isothermal compression is advantageous both for subcritical compression and for transcritical compression. As discussed above, it is typically advantageous to use subcritical compression for temperature conditions below 31°C and to use transcritical compression in temperature conditions above 31 °C. A transcritical compression process differs from the subcritical compression processes in various ways. In particular, the condenser may be an air-cooled condenser, which, in exemplary embodiments, "condenses" the gas at a temperature of 50°C. The term condensation is used in a non-specific way, as, in a transcritical system, condensation does not really occur, and is instead substituted by transcritical gas cooling.

FIGS. 7A and 7B demonstrate the energy advantages of isothermal compression in a transcritical process. Each of FIGS. 7A and 7B depict a pressureenthalpy diagram for transcritical compression cycle of a refrigerant. The X-axis of each graph 700 is enthalpy (kJ/kg), and the y-axis is pressure (MPa). The continuous curved lines represent temperature isotherms. In both FIG. 7A and FIG. 7B, the critical point of carbon dioxide is indicated with reference numeral 710. Carbon dioxide that is within bell-shaped region 721 is liquid, while carbon dioxide that is in region 722 above the bell is gaseous.

In FIG. 7A, the compression process is adiabatic. Points 701-709 represent different steps in the adiabatic vapor compression cycle. Dry saturated refrigerant leaves the evaporator at 5 °C and 39.69 bar (state 701). The refrigerant is superheated to 46 °C at the same pressure (state 702) in the SSHX. The superheated gas is compressed adiabatically in the compressor (states 703 and 704) to the heat rejection pressure of 100 bar. Heat rejection to the ambient (state 705) takes place at constant pressure, followed by further cooling in the SSHX at 41.74 °C (state 706). Then, the supercritical fluid expands in an isenthalpic expansion device (state 707) down to the evaporator pressure. Alternatively, the expansion is performed with an expander, either with isentropic expansion (state 708), or a more realistic adiabatic expansion with 0.85 isentropic efficiency (state 709).

In the isothermal process of FIG. 7B, similar processes are indicated at similar locations on the pressure-enthalpy graph, except that the reference numerals 711- 719 are used in place of the reference numerals 701-709. The main difference between the isothermal process of FIG. 7B and the adiabatic process of FIG. 7A is that the process of FIG. 7B is performed nearly isothermally. The transition from state 712 to 713 involves an increase in 4°C, and the transition from state 713 to 714 is isothermal.

As can be seen from a comparison of FIG. 7A with 7B, in the adiabatic process, the compression increases the enthalpy of the gas. More enthalpy is introduced into the system in order to compress the gas, and more enthalpy is released during the expansion of the gas. In the isothermal process, however, the compression of the gas reduces the enthalpy. Less enthalpy is introduced during the compression process and less enthalpy is released during the cooling. The reduction in enthalpy is associated with more efficient heating and cooling. Referring now to FIG. 8 and FIG. 9, as discussed above, in order to achieve an isothermal compression process, it is necessary to supply external cooling. As shown in FIG. 8, each cylinder 510 is modified to include a plurality of thin cooling tubes 512. Each cooling tube 512 may be a copper tube having a diameter of one inch. Gaseous carbon dioxide flows from the evaporator into combined low-pressure conduit 505, and from low-pressure conduit 505 into each of the cooling tubes 512. Water coming from the water discharge line 526 enters discharge conduit 527, from which the water enters each of the cooling tubes 512 at the bottom. The water exits the cooling tubes 512 through valve 528 to the water pump suction line, as discussed above in connection with FIG. 5. A separate water cooling system is also provided. The water cooling system includes a water cooling pump 530, which pumps cooling water into gaps 534 between each of the cooling tubes 512.

FIG. 9 illustrates a cross-section of each of the cooling tubes 512. Each cooling tube includes a plurality of solid heat transfer fin plates 513. The gaps 534 through which cooling water is pumped extend between the fin plates 513. Optionally, gaps 534 are closed tubes which are configured to fit between the fin plates 513. Continuous circulation of the cooling fluid around the fin plates 513 during the isothermal compression cools the cooling tubes 512 and helps achieve the isothermal compression.

In alternative embodiments, instead of the cooling fluid being water, the cooling fluid may be air. In such embodiments, air gaps are formed in between the cooling tubes 512, and cool air is blown therebetween during the isothermal compression. In still other embodiments, the cooling may be achieved through spraying of water directly within the cooling tubes 512 or within the cylinders 510, instead of external to the cooling tubes 512.

In an exemplary embodiment, for cooling of a 200 TR system in a transcritical manner, the power consumption of the compressor pump, considering its internal efficiency, electrical motor efficiency and the variable speed driver sums up at 256 kWe. This power consumption is compared to the estimated power of 352 kWe needed by a conventional compressor and represents therefore a saving of about 27% in the electrical consumption of the system.

Recapture of Energy from Expansion of Refrigerant for Liquid Pump

Referring now to FIGS. 10A-10D, an additional improvement resulting in energy savings is an expander 560. ,A classical expansion valve such as valve 6 of FIG. 1 outputs only expanded gas, without capturing the energy resulting from the expansion of the gas. By contrast, in expander 560, the output of the expansion valve is captured as work that is delivered to the pump of the compression unit.

Before proceeding to describe the characteristics of the expander 560, it should be noted that the vapor compression cycle, utilizing the compression units as described above, does not function at steady state, but rather intermittently, due to the nature of the compressor. As discussed above, the compression unit utilizes liquid pistons to compress gas directly with pumped water in dedicated columns. These liquid pistons may operate in half-cycles, with the direction of liquid flow through a pump changing in between half cycles. In addition, in embodiments with multiple pairs of liquid pistons, such as those described in FIG. 6A and 6B, the amount of fluid pumped by the liquid pump changes throughout the cycle, depending on the phase of the cycle at any given time. Accordingly, in order to be best adapted to provide compression work to the compression units, the expander must also be configured for intermittent capture of energy output. The expander 560 described below is particularly adapted for intermittent capture of work of the expanding refrigerant.

Referring now to FIG. 10A, expander 560 includes two vertical cylinders 561, 563, interconnected at their tops by a U-shaped duct 562. Each of the cylinders has an aperture at the bottom sealed by a valve 571 or 572. Each aperture may serve as a liquid inlet or outlet, or be closed off.

FIGS. 10A-10D illustrate the different stages of operation of expander 560. In FIG. 10A, the entire expander 560 is filled with water 581, and all apertures are closed. Expansion begins by opening the two valves 571, 572. As shown in FIG. 10B, condensate 582 at 70 bar enters the bottom of cylinder 561 from the condenser, moves through the entire volume of the expander 560, and exits from the bottom of cylinder 563. As the pressure decreases, part of the entering liquid condensate 582 evaporates and forms a gas volume 583 above the liquid volume. Water is pushed out from the bottom of cylinder 563. The pushing out of this water thus utilizes work of the expander 560 to assist the water pump of the compressor unit. Specifically, water at high pressure from the expander may be connected to the pump inlet of the compression unit, and thus provide additional compression power to help the pumps in the compression unit work and reduce electrical consumption.

In the state of FIG. 10B, expanded carbon dioxide has pushed water 583 out of the first cylinder 561 and part of the U-shaped duct. The pressure is reduced from 70 bar to 50 bar. In the state of FIG. 10C, expanded carbon dioxide gas 582 has filled the top of cylinder 561, U-shaped duct 562, and the top of cylinder 563, and has pushed out nearly all of the water 583 from cylinder 563. The pressure of the carbon dioxide has been reduced to 40 bar.

With the expansion process now complete, and the entire expander 560 at 40 bar, the apertures at both cylinders 561, 563 are closed, and reopened with the direction of flow reversed and with new connection points. The opposite flow is illustrated in FIG. 10D. Water from a low-pressure (40 bar) reservoir is introduced into the bottom of cylinder 563, and pushes the gas and remaining liquid carbon dioxide into the evaporator, thereby continuing the cooling cycle while simultaneously re-priming the expander 560. This entire process takes place at 40 bar. Once complete, the expander system 560 is again full of water, all valves 571, 572 are closed, and the process may be repeated starting again with the state of FIG. 10A. Optionally, the process of FIG. 10D may be divided into two parts, with carbon dioxide liquid first being pushed into the evaporator inlet, and subsequently carbon dioxide gas being pushed into the evaporator outlet. The main objective of the process of FIG. 10D is to clear out all carbon dioxide of both cylinders from the expander 560, refill the expander 560 with water, and prepare the expander560 for the next series of stages.

Although the function of a single expander system 560 has been illustrated here, for the sake of continuous function, two expander systems 560 may be functioning in parallel, with the stages depicted in FIGS. 10A-10C occurring in one expander system 560, while the stage depicted in FIG. 10D takes place in the other expander system. This duplicate functioning mirrors the duplicate functioning of the pairs of liquid piston cylinders in the compression unit, as discussed above in connection with FIGS. 5, 6A, and 6B.

While the foregoing description has focused on the energetic benefits of capturing work of expansion with the expander, the expander 560 may provide other benefits as well. For example, the expander 560 obviates the problem of water freezing, which is common in expansion valves, by providing a moderate expansion process that ensures that water in the carbon dioxide cannot reach freezing temperatures.

Calculations indicate that capturing the work of the expander 560 for liquid compression reduces the power consumption of a liquid pump by about 70 kWe (for a 200 TR system). Calculations show that the water pump power consumption will be further reduced to about 186 kWe, which is about 47% lower than the conventional compressor requirement of 352 kWe.

To best use the work produced by the expander 560, the pumped water that it produces must be available when the compressor needs it. A control system (not shown) may be configured between the expander 560 and the compression unit to ensure that they are operated in sync.

Table 1 below summarizes total energetic measurements for vapor compression cycles according to embodiments of the present disclosure, considering the alternatives of transcritical and subcritical processes, adiabatic and isothermal compression, different temperature and pressure values, and the use of expander

560: Table 1 - Energetic Efficiency of Vapor Compression Cycles

While the data speaks for itself, a few observations are presented. First, the expander provides modest energetic benefits even without use of the work of the expanding gas. These energetic benefits may relate at least in part to the prevention of freezing which is a common problem in expansion valves. In addition, using the expander work can additionally increase the cooling cycle efficiency and give a coefficient of performance (COP) as high as 10.2. As can be seen, the COP was greater in all cases for isothermal compression vs. adiabatic compression, and still greater when the expander work was utilized, as discussed above. This analytical calculation is a basis for a practical implementation that can give increase cooling efficiency at least 20% compared to standard air conditioners.

Implementations and Benefits

FIG. 11 depicts an implementation of any of the embodiments described above as a central air conditioning system in an apartment building. Each of the units in the apartment building is connected to central air conditioning system 110. The operative parts of the system, including pump 112, high pressure tank 114, and low pressure tanks 120, are all underground. There is a single high-pressure tank 114 and three low pressure tanks 120. The low pressure tanks have a volume about 10 times as large as the high pressure tank 114, in order to compensate for the different volume requirements of the different tanks, as discussed.

FIG. 12 depicts an embodiment of system for providing air conditioning in mass transportation vehicles. Bus 210 includes a plurality of tanks 214 at an undercarriage thereof, and an air conditioning system 216. When the bus 210 is an electric vehicle, the electric power that is used to charge the battery of the bus may simultaneously be used to compress the air into the high pressure storage tanks. Then, during running of the bus 210, the air conditioning system may run without any additional expenditure of energy.

The systems described herein are cost-efficient and may supply cooling and electricity during an electrical shutdown by using the in-system electricity generation to power critical electrical equipment, while using stored compressed refrigerant for cooling the air. As described above, the system may also enable storage of compressed refrigerant during low-cost times for electricity consumption and use of the stored compressed refrigerant at other times. It also allows exerting control on cooling accuracy, and allows extra capacity without purchasing new AC systems. The disclosed systems are module are may be installed above ground or underground.

The described systems may also be deployed efficiently in factories, in skyscrapers, in buildings, as well as in private homes. The system is also beneficial for industrial cooling, in which required temperatures are far below 0 °C. One option is to cool water with glycol to temperatures as low as -40 °C.

In addition, the same systems may be used both for providing cooling and for providing heating. FIGS. 13A and 13B schematically depict a dual function cooling/heating cycle and heating cycle structures. Specifically, the system may be used as a heat pump, which is simply a reverse air conditioner, by which heat is rejected into the heated room and the system provides cooling into the ambient environment, which is colder than room temperature. In FIG. 14A, the solid line indicates the heat path of the system when used for cooling, and the dashed line indicates the heat path of the system when used for heating. Other advantages that may be achieved during operation of the disclosed systems, as discussed above, include: 1) storing the compressed gas at low price and using or selling the stored gas at peak demand (time of use tariffs). 2) saving energy during the condensation stage due to isothermal compression. 3) generating electricity during operation of system 10, in addition to the production of cooling. 4) using materials that are environmentally friendly such as carbon dioxide and water, as opposed to less environmentally friendly refrigerants. 5) generation of cooling during periods when electricity is unavailable (for example during a blackout).

Claims

Claims What is claimed is:

1. An air conditioning system, comprising: a compression unit configured to compress a gaseous refrigerant; a plurality of high pressure condensing tanks connected to an outlet of the compression unit and configured to store the compressed refrigerant; an expander or expansion valve in fluid communication with an outlet of the plurality of high pressure condensing tanks, for releasing the compressed refrigerant from the plurality of high pressure condensing tanks while expanding a volume of the compressed refrigerant; an evaporator in fluid communication with an outlet of the expander or expansion valve for causing the refrigerant to absorb heat from a surrounding environment; a plurality of low-pressure storage tanks for collecting discharged refrigerant vapor from the evaporator; and a conduit for conveying the refrigerant vapor from the plurality of low- pressure storage tanks to an intake of the compression unit.

2. The air conditioning system of claim 1, further comprising a recuperator arranged between the plurality of high pressure condensing tanks and the expander or expansion valve, said recuperator configured to cool incoming condensed refrigerant with outgoing expanded refrigerant.

3. The air conditioning system of claim 1, further comprising a first heat exchange unit configured to receive heat from the refrigerant during compression thereof to thereby heat an external supply of water.

4. The air conditioning system of claim 1, further comprising a second heat exchange unit configured to be cooled by the refrigerant during expansion thereof to thereby cool an external supply of water.

5. The air conditioning system of claim 1, wherein the refrigerant is carbon dioxide.

6. The air conditioning system of claim 5, wherein the carbon dioxide is liquid in the plurality of high-pressure condensing tanks and is gaseous in the plurality of low pressure storage tanks.

7. The air conditioning system of claim 5, wherein the carbon dioxide is a supercritical plasma in the plurality of high-pressure condensing tanks and is gaseous in the plurality of low-pressure storage tanks.

8. A compression unit for a cooling system, comprising: at least two pairs of liquid-gas pistons, wherein each pair of liquid gas pistons comprises first and second cylinders having substantially equal volumes, and a combined volume of liquid substantially equivalent to a volume of one of the liquidgas pistons; a pressure equalizing valve arranged between the first and second cylinders of each pair of liquid-gas pistons; a liquid pump in fluid communication with each of the four liquid-gas pistons; a gas intake valve for low-pressure gas and a gas outlet valve for high- pressure gas arranged at each liquid-gas piston; and a series of valves arranged between the liquid pump and the respective liquid-gas pistons, wherein the valves may be alternatively opened and closed such that pressure equalization among each pair of liquid gas pistons, and pressurizing of gas through pumping of liquid through each pair of liquid gas pistons, is performed in a constant time shift.

9. The compression unit of claim 8, wherein the constant time shift corresponds to a time required to equalize between the pressures of the first and second cylinders of a pair of cylinders.

10. The compression unit of claim 8, wherein each pair of liquid gas pistons is configured to compress gas in a series of alternating half-cycles, wherein in a first half cycle gas is compressed within a first cylinder through influx of liquid from the second cylinder, and in the second half-cycle gas is compressed within the second cylinder through influx of liquid from the first cylinder.

11. The compression unit of claim 10, wherein, in each half-cycle, gas is first compressed through pressure equalization between the two liquid-gas pistons in a pair, and then is compressed further through pumping of fluid between the two liquid-gas pistons.

12. The compression unit of claim 8, further comprising: a main tank having an inlet for receiving therein a gas at low pressure; an open liquid tank configured to deliver liquid to the first gas tank and thereby compress the gas within the main tank; and a valve assembly configured to switch between a first state, in which low- pressure gas passes through the main tank and is pressurized by the liquid-gas pistons, and a second state, in which the low-pressure gas is pressurized within the main tank by liquid from the open liquid tank.

13. The compression unit of claim 12, further comprising a turbine arranged between the open liquid tank and the main tank, and configured to produce electricity from the flow of liquid from the main tank to the open liquid tank.

14. The compression unit of claim 8, wherein an interior volume of each cylinder is comprised of a plurality of cooling tubes connected in parallel to each respective gas intake valve, gas outlet valve, and the liquid pump, , each cooling tube including a plurality of cooling fins, and further comprising a cooling system for providing cooling fluid to an exterior of each of the cooling tubes between the cooling fins.

15. The compression unit of claim 14, wherein the cooling fluid is water.

16. A cooling system comprising the compression unit of claim 8 and an expander, said expander comprising a first cylinder, a U-shaped duct, and a second cylinder connected in series, wherein an outlet of the second cylinder is connected to the fluid pump of the compression unit and to a low-pressure fluid reservoir, and wherein, during expansion of refrigerant, the expanding refrigerant enters the first cylinder and displaces fluid through the duct and second cylinder to the fluid pump, and, wherein, following expansion of refrigerant, liquid from the low-pressure fluid reservoir enters the second cylinder to thereby evacuate expanded refrigerant from the expander into an evaporator and refill the expander with fluid.

17. A cooling system comprising the compression unit of claim 8, a plurality of high pressure condensing tanks connected to an outlet of the compression unit and configured to store the compressed refrigerant; an expander or expansion valve in fluid communication with an outlet of the plurality of high pressure condensing tanks, for releasing the compressed refrigerant from the plurality of high pressure condensing tanks while expanding a volume of the compressed refrigerant; an evaporator in fluid communication with an outlet of the expander or expansion valve for causing the refrigerant to absorb heat from a surrounding environment; a plurality of low-pressure storage tanks for collecting discharged refrigerant vapor from the evaporator; and a conduit for conveying the refrigerant vapor from the plurality of low-pressure storage tanks to an intake of the compression unit.

18. The cooling system of claim 17, wherein the expander or expansion valve is the expander of claim 16.

19. An expander, comprising: a first cylinder, a U-shaped duct, and a second cylinder connected in series, wherein the first cylinder is connected to a source of compressed refrigerant and an evaporator; wherein the second cylinder is connected to a fluid pump and to a low- pressure fluid reservoir; and wherein, during expansion of refrigerant, compressed refrigerant enters the first cylinder from the source of compressed refrigerant, and displaces fluid from the first cylinder, duct, and second cylinder to the fluid pump, and, wherein, following expansion of refrigerant, liquid from the low-pressure fluid reservoir enters the second cylinder, duct, and first cylinder to thereby evacuate expanded refrigerant from the expander into the evaporator and refill the expander with fluid.

20. A method of air conditioning, comprising: compressing a gaseous refrigerant; storing the compressed refrigerant; expanding a volume of the compressed refrigerant with an expansion valve or expander; evaporating the refrigerant with an evaporator and thereby causing the refrigerant to absorb heat from a surrounding environment; collecting discharged refrigerant vapor from the evaporator; and conveying the discharged refrigerant vapor to an intake of the compressor, and repeating each of the previous steps.

21. The method of claim 20, wherein the storing step comprises storing the compressed refrigerant in a plurality of high-pressure storage tanks, and the collecting step comprises collecting the discharged vapor in a plurality of low- pressure storage tanks.

22. The method of claim 20, further comprising performing the compressing and storing steps at nighttime, and performing the expanding, evaporating, collecting, and conveying steps during daytime.

23. The method of claim 20, wherein the refrigerant is carbon dioxide, and further comprising performing the compressing step in a subcritical process.

24. The method of claim 20, wherein the refrigerant is carbon dioxide, and further comprising performing the compressing step in a transcritical process.

25. The method of claim 20, further comprising performing the compressing step substantially isothermally by supplying a cooling fluid to an exterior of a cylinder used for compression of the carbon dioxide therein.

26. The method of claim 20, wherein the expanding step comprises using an expander to capture work of the expanding refrigerant to thereby deliver fluid to a fluid pump used to compress the refrigerant during the compressing step.

27. The method of claim 20, further comprising releasing at least a portion of the compressed refrigerant through a turbine to thereby generate electricity.

28. The method of claim 20, further comprising performing the expanding, evaporating, and collecting steps without investment of electricity.