Non-condensable gas vent for sorption heat pumps

Abstract

A thermally-driven sorption heat pump, comprising a desorber, a condenser, an evaporator, an absorber, and a passive vent including a permeable membrane comprising a porous rigid structure for supporting the permeable membrane. The permeable membrane passively venting non-condensable gas from the heat pump while blocking refrigerant and absorbent from diffusing through the permeable membrane.

Description

NON-CONDENSABLE GAS VENT FOR SORPTION HEAT PUMPSTECHNICAL FIELD

[0001] A non-condensable gas vent for sorption heat pumps.BACKGROUND

[0002] Thermally activated heat pumps (such as absorption or adsorption cycles, collectively sorption) can provide space or water heating and space cooling or refrigeration. Cycle efficiencies (coefficient of performance or COP) range from 0.5 to greater than 1.0 for cooling, and 1.2 to greater than 2.0 for heating. Sorption heat pumps also utilize natural refrigerants such as water or ammonia that are not harmful to the environment.

[0003] However, ammonia- water solution at temperatures required to achieve high heating efficiencies is mildly corrosive when in contact with carbon steel, a beneficial material for sorption heat pump construction. Corrosion inhibitors are commonly used in sorption systems to reduce the corrosion rate to acceptable levels with regard to long term heat exchanger reliability, however the small amount of non-condensable gas produced during the corrosion process can build up in the heat pump over time, reducing efficiency and / or causing reliability problems.SUMMARY

[0004] An example embodiment includes a thermally-driven sorption heat pump, comprising a desorber, a condenser, an evaporator, an absorber, and a passive vent including a permeable membrane comprising a porous rigid structure for supporting the permeable membrane, the permeable membrane passively venting non-condensable gas from the heat pump while blocking refrigerant and absorbent from diffusing through the permeable membrane.

[0005] In an aspect of the example embodiment, the permeable membrane comprises at least a partial chemical composition of palladium.

[0006] In an aspect of the example embodiment, the passive vent permeable membrane comprises a chemical composition of 20-25% silver, with a remainder of the chemical composition comprising palladium.

[0007] In an aspect of the example embodiment, the passive vent permeable membrane comprises a thickness between 10 and 30 microns.

[0008] In an aspect of the example embodiment, the thermally-driven heat pump comprises a liquid refrigerant accumulator, and the passive vent is coupled to the liquid refrigerant accumulator.

[0009] In an aspect of the example embodiment, the thermally-driven heat pump comprises a tank containing a mixture of the refrigerant and the absorbent between an outlet of the absorber and an inlet of the thermally-driven pump, and the passive vent is coupled to the tank.

[0010] In an aspect of the example embodiment, the passive vent is coupled to the absorber.

[0011] In an aspect of the example embodiment, the passive vent permeable membrane is sandwiched between 2 porous structures of the porous rigid structure made from steel, stainless steel or ceramic.

[0012] In an aspect of the example embodiment, a pore size of the porous rigid structures is between 10 and 50 microns.

[0013] In an aspect of the example embodiment, the passive vent permeable membrane comprises a chemical composition that allows hydrogen molecules to diffuse through the passive vent permeable membrane, while blocking the refrigerant and the absorbent from diffusing through the passive vent permeable membrane.

[0014] In an aspect of the example embodiment, the thermally-driven sorption heat pump comprises a liquid refrigerant accumulator, a tank containing a mixture of the refrigerant and the absorbent, and an absorber. The passive vent permeable membrane is coupled to a top portion above a liquid level of at least one of the liquid refrigerant accumulator, the tank and the absorber.

[0015] In an aspect of the example embodiment, the porous rigid structure includes a lower porous structure and an upper porous structure affixed together with the passive vent permeable membrane sandwiched therebetween.

[0016] In an aspect of the example embodiment, the thermally-driven sorption heat pump comprises a sealing member sealing the passive vent permeable membrane in between the lower porous structure and the upper porous structure.

[0017] In an aspect of the example embodiment, the passive vent is connected to the heat pump via a connection such that the passive vent is removable from the heat pump.

[0018] In an aspect of the example embodiment, the porous rigid structure in located inside a conduit extending from the thermally-driven sorption heat pump.

[0019] In an aspect of the example embodiment, the thermally-driven sorption heat pump comprises a structural member holding the passive vent in the conduit.

[0020] In an aspect of the example embodiment, the structural member is connected to the conduit via at least one of a press fit connection, threaded connection, welded connection or brazed connection.BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Fig. 1 depicts a single-effect absorption cycle, according to an example embodiment of the present disclosure.

[0022] Fig. 2 depicts a single-effect absorption cycle including a refrigerant reservoir, solution pump inlet tank, and possible locations for installing a non-condensable vent, according to an example embodiment of the present disclosure.

[0023] Fig. 3 depicts an example construction of a passive non-condensable vent for use on a sorption heat pump, according to an example embodiment of the present disclosure.DETAILED DESCRIPTION

[0024] It will be appreciated that the following description is intended to refer to specific examples of structure selected for illustration in the drawings and is not intended to define or limit this disclosure.

[0025] When ammonia- water solution reacts with carbon steel, iron-oxide (Fe3O4 or magnetite) and a small amount of hydrogen gas (H2) are formed. As the volume hydrogen gas, which is considered a non-condensable in a thermally activated heat pump system, builds up over an extended period of time, elevated pressures (above saturation) within the sorption heat pump are normally observed. The non-condensable gas also interferes with heat and mass transfer effectiveness, by forming a boundary layer as it flows through the system heat exchangers. The combination of cycle pressures above saturation and reduced heat and mass transfer rates result in lower heating or cooling efficiencies, and eventually reliability problems.

[0026] Therefore, most sorption heat pumps require that the non-condensable gases (mostly consisting of hydrogen) be bled from the system on a regular basis (at annual service for example). The non-condensable gases are normally bled via a service port installed at a location where the non-condensable gases can accumulate. The need to regularly bleed non-condensables from sorption heat pumps increases the annual service cost for the end-user, or if the gases are not bled, can eventually cause performance issues and require an expensive repair.

[0027] Large sorption systems for commercial or industrial applications often include automatic bleeding techniques using a solenoid or motor-operated valve that is connected to a line located on the sorption system where non-condensables collect (such as the top of a tank where the fluid exit is below the top of the tank). The bleed valve is opened for a short period of time on a regular basis, allowing non-condensables to exit the sorption system. The valve activation can simply be a timer (once per day for example) or can be triggered by a control system monitoring sorption system temperature(s) or pressure(s). If the sorption system utilizes ammonia as the refrigerant, the flow through the bleed valve is normally directed into a volume of water (where the ammonia is readily absorbed by the water) instead of directly venting to the atmosphere.

[0028] Each time the bleed valve opens, a small amount of refrigerant exits the sorption system along with the non-condensables. Automatic bleeding of non-condensables is problematic because it is wasteful for large sorptions systems, and detrimental for the operation of small residential or light commercial systems (such as a restaurant for example) that do not contain a large amount of excess refrigerant, where a small loss of refrigerant charge can cause performance or reliability problems. For sorption systems using ammonia as the refrigerant, incorporation of a water tank with freeze protection is difficult and expensive in residential and light commercial applications.

[0029] A simple and low-cost method of automatically and passively removing non- condensables for sorption heat pumps is desired so that the efficiency, performance and reliability of the heat pumps is not negatively impacted by the build-up of non-condensable gas over a period of time, and the need for regular service calls is avoided.

[0030] The disclosure describes a passive vent that allows non-condensables (e.g., hydrogen gas), to slowly escape a thermally-driven sorption heat pump system while blocking refrigerant and the absorbent from escaping (e.g., only hydrogen can escape). The passive vent utilizes a thin membrane (foil) material with a chemical composition that allows, for example, only hydrogen molecules to diffuse through the membrane. The membrane is sandwiched between two porous structures (typically metal or ceramic) that provide a physical structure for the membrane that can withstand the pressure inside the sorption heat pump (e.g., 250 - 400 psi), while allowing hydrogen molecules to exit the sorption system through the membrane.

[0031] Membranes containing palladium are permeable to hydrogen molecules, where it is possible to separate out just hydrogen from mixed gases. Palladium-metal membranes are useful for producing pure hydrogen for fuel cells and the manufacturing of semiconductors and LEDs.Typically, palladium (Pd) is mixed with a metal such as copper (Cu), silver (Ag), or gold (Au) to form a thin membrane (or foil) with a thickness ranging from 5 to 25 microns.

[0032] The permeability of a palladium-metal membrane to hydrogen is a function of the membrane temperature, differential pressure across the membrane, the thickness of the membrane, and the chemical composition. Permeability increases with increasing temperature and pressure, with high permeability rates achieved at temperatures ranging from 400 - 750°F and decreases with decreasing temperature and pressure.

[0033] For ammonia-water sorption heat pumps, a beneficial metal to alloy with palladium is silver, due to copper not being chemically compatible with ammonia, and the cost of gold being high, with a composition percentage of 10 to 40%. A silver composition percentage of 20 to 25% is beneficial, with the remainder palladium. In one example, thickness of the membrane may be between 10 and 30 microns. In one example, the thickness of the membrane for structural integrity within a sorption heat pump may be 15 microns or higher.

[0034] In ammonia-water sorption heat pumps, non-condensable hydrogen gas flows with the working fluids (e.g., ammonia the refrigerant and water the absorbent) throughout the heat pump, and typically collects in high concentrations at the top of tanks or heat exchangers where the flow velocity is zero or low. These locations include, but are not limited to, the refrigerant accumulator tank at the exit of the condenser, the solution (e.g., a mixture of ammonia and water in liquid form) tank at the inlet of the solution pump, or the top of the absorber heat exchanger if it is of the counter-flow arrangement. In a counter-flow absorber arrangement, the solution enters the absorber near the top and ammonia refrigerant vapor enters at the heat exchanger mid-point or below, which means the top of the absorber is a location with zero or near-zero vapor flow and velocity.

[0035] The locations where high concentrations of hydrogen gas collect are suitable locations for the passive membrane vent to be installed. Of these locations, the solution pump inlet tank and the top of the absorber heat exchanger operate at the low-pressure side of the cycle (e.g., typically less than 100 psia), while the refrigerant accumulator tank operates at the high-pressure side of the cycle (e.g., typically 250 to 400 psia). The temperature at the top of the absorber is the highest (e.g., typically 130 to 200°F), while the temperature of the solution pump inlet tank and refrigerant accumulator is lower (e.g., typically 90 to 140°F). A beneficial location for the vent in anammonia-water heat pump is the top of the ammonia accumulator due to the higher operating pressure and reasonable temperature, although the other two cited locations will also work.

[0036] The surface area of membrane required is a function of the permeability of the membrane selected at the temperature and pressure at the installation location, and the estimated hydrogen gas generation rate of the heat pump. For example, an ammonia-water heat pump with a heating capacity of 80,000 btu / hr, the typical Pd-Ag membrane surface area is approximately 0.1 square inches.

[0037] Fig. 1 depicts a single-effect absorption heat pump cycle 100, including desorber 101, condenser 103, evaporator 106, absorber 108, and solution pump 110. Heat energy Qa is input into desorber 101, causing the refrigerant to boil out of a strong solution (e.g., mixture of refrigerant and absorbent) at high temperature and pressure. The refrigerant vapor flows through conduit 102 to the condenser, where heat energy Qb is removed from the refrigerant causing it to condense into a liquid. The heated weak solution (e.g., mixture of absorbent and a small amount of refrigerant) exits the desorber through conduit 111 and expansion device 112 to the absorber 108. The weak solution is reduced from high pressure to low pressure in expansion device 112. The liquid refrigerant exiting the condenser 103 flows through conduit 104 and pressure expansion device 105 to the evaporator 106, where the refrigerant is reduced from high pressure to low pressure in expansion device 105. Heat energy input Qc causes the liquid refrigerant to evaporate into a vapor in evaporator 106. The low pressure refrigerant vapor flows through conduit 107 to absorber 108. In the absorber, the refrigerant vapor is absorbed back into the weak solution, which is an exothermic process that produces heat flow Qd. The resulting strong solution flows through conduit 109 and solution pump 120 to the desorber, where the solution pump 110 increases the pressure of the strong solution from low to high pressure.

[0038] For an ammonia-water absorption heat pump cycle, the refrigerant is ammonia and the absorbent is water. The high-side pressure typically ranges from 200 to 400 psia, and the low pressure typically ranges from 0 to 100 psia. Heat energy Qa into the desorber 101 can be from, but not limited to, the combustion of a fossil fuels (e.g., natural gas, propane, fuel oil, bio gas, etc.), the combustion of hydrogen, solar or waste energy streams. In one example, energy output Qb of condenser 103 can be used, but is not limited to, heat a building, potable water, swimming pool, industrial processes or for melting snow and ice. The energy input Qc to evaporator 106 can be from the ambient air, geothermal, water (e.g., lake or pond), or a waste heat stream.

[0039] Fig. 2 depicts a single-effect absorption heat pump cycle 200, with similar components (e.g., desorber, condenser, evaporator, absorber, solution pump and expansion devices) as in Fig. 1, as well as added components (e.g., accumulators, non-condensable permeable vents) as compared to Fig. 1. High pressure liquid refrigerant exiting condenser 203 enters refrigerant accumulator 213 through conduit 204. The liquid refrigerant partially fills the accumulator, creating a liquid-to-vapor interface 214. The top of accumulator 213 is a region of low velocity where non-condensable gases such as hydrogen accumulates and is one possible location for the installation of a non-condensable permeable vent 215. Liquid refrigerant exits accumulator 213 through conduit 216 and expansion device 205 and enters evaporator 206 which is connected to absorber 208 through conduit 207. Strong solution exiting absorber 208 enters the solution pump inlet tank 217 through conduit 209. The solution partially fills solution pump inlet tank 217, creating a liquid-to-vapor interface 218. The top of solution pump tank 217 is a region of low velocity where non-condensable gases such as hydrogen accumulates and is another possible location for the installation of a non-condensable permeable vent 219. The top of absorber 208 is another location where non-condensable gases such as hydrogen accumulate and is a third possible location for the installation of a non-condensable permeable vent 221. Strong solution exits solution pump inlet tank 217 and travels to desorber 201 via conduit 209 and solution pump 210 and conduit 220. It is noted that desorber 201 is connected to absorber 208 via conduit 211 and expansion device 212.

[0040] Fig. 3 depicts an example construction of a passive non-condensable gas vent 300 for use on a sorption heat pump. Thin membrane 323 is sandwiched between two porous structures including lower porous structure 324 and upper porous structure 325 inside an outer conduit 322. Sealing members 327 and 328 provide a pressure seal on either side of membrane 323, ensuring that the flow of all pressurized gases or liquids 331 contained in the sorption heat pump must pass through membrane 323. The outer conduit 322 is connected to a heat exchanger or tank 330 of the sorption heat pump via a connecting means 329. Physical structure 326 is used to prevent the pressurized gases 331 from the sorption heat pump from pushing the membrane 323, porous structures 324 and 325, and sealing members 327 and 328 out of conduit 322.

[0041] For an ammonia-water absorption heat pump cycle, membrane 323 is made from palladium or a mixture of palladium and silver, and selectively only allows hydrogen molecules contained in pressurized gas mixture 331 contained inside the heat pump to pass through. Sealingmembers 327 and 328 may include, but not limited to, O-rings or a gasket made of a material compatible with ammonia such as EPDM or Teflon. Porous structures 324 and 325 can constructed from metal (e.g., stainless steel) or ceramic, with pore opening size large enough to allow hydrogen gas to pass through, but not large enough to allow membrane 323 to extrude into the porous material. The pore size of porous structures 324 and 325 may be on the order of 20 microns. Physical structure 326 can be connected to conduit 322 via a press fit, threads, weld, or braze. The connection 329 between outer conduit 322 and the tank 330 can be made by welding, brazing or threading.

[0042] It will be appreciated by those skilled in the art that the preceding examples are exemplary and not limiting. It is intended that all permutations, enhancements, equivalents, and improvements thereto are apparent to those skilled in the art upon a reading of the specification and a study of the drawings are included within the true spirit and scope of the present disclosure. It is therefore intended that the following appended claims include all such modifications, permutations, and equivalents as fall within the true spirit and scope of these teachings.

Claims

CLAIMS1. A thermally-driven sorption heat pump, comprising: a desorber; a condenser; an evaporator; an absorber; and a passive vent including a permeable membrane comprising a porous rigid structure for supporting the permeable membrane, the permeable membrane passively venting non-condensable gas from the heat pump while blocking refrigerant and absorbent from diffusing through the permeable membrane.

2. The thermally-driven sorption heat pump of claim 1, wherein the permeable membrane comprises at least a partial chemical composition of palladium.

3. The thermally-driven sorption heat pump of claim 2, wherein the passive vent permeable membrane comprises a chemical composition of 20-25% silver, with a remainder of the chemical composition comprising palladium.

4. The thermally-driven sorption heat pump of claim 1, wherein the passive vent permeable membrane comprises a thickness between 10 and 30 microns.

5. The thermally-driven sorption heat pump of claim 1, wherein the thermally-driven heat pump comprises a liquid refrigerant accumulator, and the passive vent is coupled to the liquid refrigerant accumulator.

6. The thermally-driven sorption heat pump of claim 1, wherein the thermally-driven heat pump comprises a tank containing a mixture of the refrigerant and the absorbent between an outlet of the absorber and an inlet of the thermally-driven pump, and the passive vent is coupled to the tank.

7. The thermally-driven sorption heat pump of claim 1, wherein the passive vent is coupled to the absorber.

8. The thermally-driven sorption heat pump of claim 1, wherein the passive vent permeable membrane is sandwiched between 2 porous structures of the porous rigid structure made from steel, stainless steel or ceramic.

9. The thermally-driven sorption heat pump of claim 1, wherein a pore size of the porous rigid structures is between 10 and 50 microns.

10. The thermally-driven sorption heat pump of claim 1, wherein the passive vent permeable membrane comprises a chemical composition that allows hydrogen molecules to diffuse through the passive vent permeable membrane, while blocking the refrigerant and the absorbent from diffusing through the passive vent permeable membrane.

11. The thermally-driven sorption heat pump of claim 1, further comprising: a liquid refrigerant accumulator; a tank containing a mixture of the refrigerant and the absorbent; and an absorber, wherein the passive vent permeable membrane is coupled to a top portion above a liquid level of at least one of the liquid refrigerant accumulator, the tank and the absorber.

12. The thermally-driven sorption heat pump of claim 1, wherein the porous rigid structure includes a lower porous structure and an upper porous structure affixed together with the passive vent permeable membrane sandwiched therebetween.

13. The thermally-driven sorption heat pump of claim 12, further comprising: a sealing member sealing the passive vent permeable membrane in between the lower porous structure and the upper porous structure.

14. The thermally-driven sorption heat pump of claim 1, wherein the passive vent is connected to the heat pump via a connection such that the passive vent is removable from the heat pump.

15. The thermally-driven sorption heat pump of claim 1, wherein the porous rigid structure in located inside a conduit extending from the thermally-driven sorption heat pump.

16. The thermally-driven sorption heat pump of claim 15, further comprising: a structural member holding the passive vent in the conduit.

17. The thermally-driven sorption heat pump of claim 16, wherein the structural member is connected to the conduit via at least one of a press fit connection, threaded connection, welded connection or brazed connection.

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