Patent Application
20230191312
This disclosure relates generally to systems that utilize electrochemical regeneration of a liquid desiccant.
Embodiments described herein are directed to a heat pump system using an electrodialysis apparatus. In one embodiment, a system includes a liquid desiccant regenerator configured to produce a first output stream from a first input stream. The first output stream has a higher concentration of a liquid desiccant than the first input stream. The regenerator also produces a second output stream from a second input stream. The second output stream has a lower concentration of the liquid desiccant than the second input stream. Regeneration of the liquid desiccant in the liquid desiccant regenerator decreases a temperature of the liquid desiccant regenerator. The system includes an air contactor coupled to the first output stream and exposing an input air stream to the first output stream. The first output stream absorbs water from the input air stream to form at least one diluted output desiccant stream. The at least one diluted output desiccant stream is circulated back into the liquid desiccant regenerator. A heat pump of the system is thermally coupled to move the heat from the first output stream to the liquid desiccant regenerator. The heat moved to the liquid desiccant regenerator increases an efficiency of the liquid desiccant regenerator.
Other embodiments are directed to a system that includes a liquid desiccant regenerator configured to produce a first output stream from a first input stream. The first output stream has a higher concentration of a liquid desiccant than the first input stream. The regenerator also produces a second output stream from a second input stream. The second output stream has a lower concentration of the liquid desiccant than the second input stream. The system includes an air contactor coupled to the first output stream and exposing an input air stream to the first output stream. The first output stream absorbs water from the input air stream to form at least one diluted output desiccant stream. The at least one diluted output desiccant stream is circulated back into the liquid desiccant regenerator. The system includes a vapor compression heat pump having a refrigerant loop between a condenser and an evaporator. The system also includes a fluid loop between the evaporator and the air contactor, the fluid loop thermally coupled to move the heat from the air contactor to the evaporator.
In another embodiment, a method involves producing a first output stream from a first input stream in a liquid desiccant regenerator, the first output stream having a higher concentration of a liquid desiccant than the first input stream. A second output stream is produced from a second input stream in the liquid desiccant regenerator, the second output stream having a lower concentration of the liquid desiccant than the second input stream. An input air stream is exposed to the first output stream in an air contactor. The first output stream absorbs water from the input air stream to form at least one diluted output desiccant stream. The at least one diluted output desiccant stream is recirculated back into the liquid desiccant regenerator. Heat is moved from the first output stream to the liquid desiccant regenerator via a heat pump.
The above summary is not intended to describe each disclosed embodiment or every implementation of the present disclosure. The figures and the detailed description below more particularly exemplify illustrative embodiments.
The discussion below refers to the following figures, wherein the same reference number may be used to identify the similar/same component in multiple figures. However, the use of a number to refer to a component in a given figure is not intended to limit the component in another figure labeled with the same number. The figures are not necessarily to scale.
The present disclosure relates to electrochemically regenerated liquid desiccant dehumidification systems. A liquid desiccant system may be used in, among other things, heating, ventilation, and air-conditioning (HVAC). Air conditioning is an energy intensive process and is responsible for nearly 10% of U.S. electricity consumption, with dehumidification accounting for more than half of the energy load in humid regions. The systems described herein provide an efficient, thermodynamic approach to dehumidification for air conditioning including a redox-assisted electrodialysis liquid desiccant regenerator that utilizes a heat pump.
Liquid desiccants (e.g., aqueous solutions of lithium chloride, LiCl and/or other salt such as NaCl, LiBr, and CaCl2) will absorb moisture from air across an air-to-liquid interface (e.g., a membrane interface), which decreases concentration of the desiccant solute, resulting in a diluted output stream of liquid desiccant. In order to regenerate the liquid desiccation system in a loop, the diluted liquid desiccants can be efficiently re-concentrated using a redox-assisted regenerator. This type of regenerator, referred to as a shuttle-promoted electrolyte removal (SUPER) cell, can increase or decrease concentrations of solutes in solutions through the use of ionic transport membranes and a redox shuttle.
In
Examples of a redox shuttle solution include 1,1′-bis((3-trimethylammonio)propyl)ferrocene ([BTMAP-Fc]2+) and 1,1′-bis((3-trimethylammonio)propyl)ferrocenium ([BTMAP-Fc]3+), or 1,1′-bis((3-dimethylethylammonio)propyl)ferrocene ([BDMEAP-Fc]2+) and 1,1′-bis((3-dimethylethylammonio)propyl)ferrocenium ([BDMEAP-Fc]3+), which are highly stable ferrocene derivatives that have very rapid electrochemical kinetics and negligible membrane permeability, or ferrocyanide/ferricyanide ([Fe(CN)6]4-/[Fe(CN)6]3-). Additional details for example redox shuttle solutions can be found in commonly-owned U.S. Pat. Application 17/390,600, filed Jul. 30, 2021 (Attorney docket number 20210171US01/0600.382US01), which is hereby incorporated by reference in its entirety.
The redox shuttle 117 is circulated between the two electrodes 116, 118 as shown by redox shuttle loop 124. When an electrical potential is applied to each electrode 116, 118 by energy supply 123, the redox shuttle is oxidized at a first electrode (e.g., 116) and reduced at the opposite electrode (e.g., 118). The energy supply 123 may be any variety of direct current (DC) energy supply such as a battery, photovoltaic panel, galvanic cell, potentiostat, AC/DC power converter, etc., and the energy supply may be contained within the electrochemical cell 100 or be external and coupled to the cell 100. Thus, as the shuttle 117 circulates between the electrodes, the portions of the shuttle 117 are continuously alternating between the redox states. In other words, the electrical potential engenders faradaic reactions happening at the two different electrodes 116, 118 and the redox material undergoing the faradaic reactions is circulated from one electrode to the other and back again.
In certain embodiments, each electrode 116, 118 may contact separate redox-active solutions instead of the same redox shuttle solution 117 being flowed in a loop. The separate redox-active solutions may have the same redox-active electrolyte material or different redox-active electrolyte materials. When different redox-active solutions are used for the respective electrodes 116, 118, the energy supply may periodically reverse the potential supplied to the electrodes to restore the state of charge (e.g., the proportion of redox-active electrolyte material in each solution that is in the oxidized state compared to the reduced state) of each of the redox-active electrolyte material solutions.
Positioned between the electrodes 116, 118 are three ion exchange membranes, which alternate in the type of ion exchanged. For example, among three membranes, a center membrane 110 may be a cation exchange membrane flanked by second 112 and third 114 anion exchange membranes, as is shown in
The membranes 110, 112, 114 are ion-selective as well as water-permeable, are insoluble in organic solvents, and are inert (e.g., do not chemically change) in the reaction mixture and/or products. In certain embodiments, the membranes are reinforced with a polymer mesh integrated into the membrane itself and in other embodiments, the membranes are not reinforced. It will be understood that this can be extended to additional membranes, e.g., N membranes of alternating type that define respective N-1 channels or reservoirs.
A first stream 102 flows through the first chamber 106 of the electrochemical cell 100. The first stream 102 includes at least a solvent (water in this example) and a salt (LiCl in this example) dissolved in the solvent at a first salt concentration (about 35% by weight in this example) when it enters the first chamber 106. A second stream 122 flows through the second chamber 108 of the electrochemical cell 100. The second stream 122 has a second salt concentration (about 35% by weight) as it enters the first chamber. The second salt concentration is the same as the first salt concentration in this example, although could be different. During an operational mode of the electrochemical cell 100, an electrical potential is applied to the electrodes 116, 118 and the first and second streams 102, 122 are moved (e.g., pumped) through the first and second chambers 106, 108.
When an electrical potential is applied to the electrodes 116, 118, the redox shuttle 117 is oxidized at one electrode 116 and reduced at the other electrode 118, thereby driving salt ions 127 from the first stream 102 in the first chamber 106 into the second stream 122 in the second chamber 108. In particular, the redox shuttle 117 at the first electrode 116 accepts at least one ion 134 from the catalyst in the first chamber 106. The redox shuttle 117 at the second electrode 118 drives at least one ion 133 into the second stream 122 in the second chamber 108, and the charge is balanced by driving at least one ion 127, of opposite sign of charge to ions 133, 134, from the first stream 102 in the first channel 106 across the center membrane 110 into the second stream 122 in the second channel 108.
The result of the electrical potential being applied to the electrodes is that the first stream 102 has a reduced concentration of salt (e.g., below a 1% threshold concentration) during the operation mode when exiting the first chamber 106 and the second stream 122 increases in concentration of salt when exiting the second chamber 108. The outputs of the first and second chambers 106, 108 can be further processed by subsequent stages of a similar SUPER cell to achieve similar or increased levels of desalinization and salinization. Such a system may be used with various other salts, such as water-soluble ionic salts. Example cations that can be present in the salts include, but are not limited to, hydronium, lithium, sodium, potassium, magnesium, calcium, aluminum, zinc, and iron. Example anions that can be present in the salts include, but are not limited to, chloride, bromide, iodide, halide oxyanions, sulfur oxyanions, phosphorous oxyanions, and nitrogen oxyanions.
As noted above, the SUPER cell 100 can be used to regenerate a liquid desiccant stream that flows through a liquid-to-air heat and mass exchanger, including a direct contactor, packed bed air contactor, or liquid-to-air membrane energy exchanger (LAMEE) 130, which is shown in
As seen in
The absorption of water into the liquid desiccant results in an increase in the temperature of air 142, 144 flowing through the LAMEE 130, which is a well-known thermodynamic phenomenon when water condenses from a gas to a liquid. This increase in temperature can be reduced or eliminated by heat absorbing/accepting element 136 (e.g., heat exchanger) that absorbs heat energy 135 from the LAMEE 130. Heat can also influence energy consumption of the SUPER cell 100. For example, running the SUPER at higher temperatures can make its operation more efficient, e.g., by lowering the electrical resistance of the membranes/electrodes/solutions and increasing the electrochemical kinetics. Therefore, a heat emitting/rejecting element 138 (e.g., heat exchanger) can supply heat 139 to the SUPER cell 100 and/or any of its internal flows. For instance, applying heat to specific components or fluid streams in the SUPER can be used to induce a temperature gradient inside the SUPER to encourage favorable phenomenon (like resistance) and discourage unfavorable phenomenon (like water osmosis). In a SUPER design with multiple stages, heat can be applied to specific stages to promote favorable performance. Inputting heat to SUPER cell 100 can also compensate for the endothermic effects due to regeneration of the liquid desiccant.
Specific subsets of components of the SUPER cell 100 can be heated via the heat rejecting element 138 using conventional heat transfer elements, such as heat conductive structures, vapor chamber heat pipes, convective transfer from heat sinks, etc. In one embodiment, heat from element 138 can be applied to one or more of the membranes 110, 112, 114 to lower electrical resistance. In another embodiment, heat from element 138 can be applied to one or more of the electrodes 116, 118 to lower electrical resistance. In another embodiment, the stream 143 input to the SUPER cell 100 may be heated before or after entering the cell. For example, heating the concentrated stream 122 while keeping the dilute stream 102 relatively cooler can reduce water osmosis across the center membrane 110. In other embodiments, the redox shuttle loop 124 may be heated.
The desiccant flow rate through the LAMEE 130 can also affect temperatures and system energy consumption. A high flow rate of liquid desiccant has low concentration change between the input stream 131 and the output stream 143. This may require more energy to reconcentrate the input stream 131 via the SUPER cell, as regeneration requires much more energy at higher concentrations. For example, the graph in
The heat absorbing element 136 and heat emitting element 138 may be thermally coupled to a same heat pump or two different heat pumps. Generally, a heat pump is a system that utilizes a heat transfer medium (e.g., gas, liquid, or solid) to move heat in a direction opposite that of spontaneous heat transfer. Well-known heat pump systems include vapor-compression (VC) cycle machines used in refrigerators and air-conditioning. A working fluid (e.g., refrigerant such as R-134A, R-407C, etc.) is compressed and condensed in a condenser. The compression and condensation cause a rise in fluid temperature which results in heat transfer to the outside air (OA) or other heat sink. The cooled working fluid is sent from the condenser to an expansion valve where it evaporates into an evaporator. The evaporation absorbs heat and the working fluid is sent back to the compressor to complete the cycle. This flow path of the working fluid is also referred to herein as a refrigerant loop.
Other types of heat pump systems include vapor absorption systems where a liquid refrigerant evaporates in a low partial pressure environment, absorbing heat from its surrounding. The vapor is then absorbed in another liquid, which is then heated to cause the refrigerant to evaporate out again. One advantage to absorption systems is that they can be built using no moving parts, other than the refrigerant itself. Other heat pumps, such as ground source heat pumps, utilize a constant temperature source (e.g., the earth) transfer heat to or from the ground using a working fluid, and may not need to rely on phase changes of the working fluid. Solids can be used as a heat pump media, such as in thermoelectric cooling devices.
The embodiments described herein improve the performance of electrochemically regenerated liquid desiccant dehumidifiers by use of a secondary heat pump. Generally, as the term is used herein, a primary heat pump moves heat between a heating/cooling target (e.g., forced air in an HVAC system or a water stream) and a thermal sink (e.g., the ground or atmosphere). A secondary heat pump includes additional heat exchangers in the primary heat pump path to heat or cool other components in the system. Many dehumidification systems use either a primary heat pump (as in the case of VC cycle air-conditioning) or a secondary heat pump (as in the case of a thermally regenerated desiccant wheel). The heat pump can be used to increase the relative humidity of air by cooling it, or to cool the air after dehumidification, compensating for heating caused by the dehumidification. In some embodiments, an electrochemically regenerated liquid desiccant system directly couples with the heat rejection (hot side) of the secondary heat pump, the heat accepting (cold side) of the secondary heat pump, or both.
Before discussing the heat pump aspects in greater detail, it will be understood that a liquid desiccant system as shown in
In
The output stream 308 is fed back into a salinization channel 300a of the first SUPER cell 300 via a fluid junction 310 (e.g., T-junction or manifold), where it is regenerated to the input concentration. The other output stream 309 is fed into a desalinization channel 300b of the first SUPER cell 300, where it is desalinized to around 10% concentration. This lower concentration solution is divided at fluid junction 312, which sends a first stream 314 through a desalinization channel 302b of the second SUPER cell 302, resulting in a discharge stream 315 of low concentration, e.g., <1%. A second stream 316 of the lower concentration solution from junction 312 is sent into a salinization channel 302a of the second SUPER cell 302, where it comes out as an increased concentration stream 318 and is rejoined with LAMEE exit stream 308 at junction 310.
In this example the SUPER cell 302 forms a first stage, and the SUPER cell 300 forms a second stage. The subsequent, second stage produces an output stream having a concentration (30% in this example) of the liquid desiccant higher than the corresponding output stream of previous, first stage output. The corresponding output of the first stage is 20% in this example. Pumps 320, 322 are shown driving the flows of liquid desiccant, although the number and location of pumps can vary from what is shown here. Generally, one pump may be used for each SUPER cell that is used in a different stage of processing. Other pumps (not shown) may be used to drive the redox shuttle in the SUPER cells 300, 302.
As with the arrangement shown in
In
The hot side heat exchanger 402 of the secondary heat pump will offset some of the cooling effect in the SUPER stack 400. The SUPER stack 400 experiences reduced electrical and ionic resistance at higher temperatures leading to reduced losses at higher temperature. The hot side heat exchanger 402 also lowers the hot side temperature of the vapor compression loop before it reaches the condenser 404, which has direct thermodynamic benefits for the primary heat pump. The thermal contact between the SUPER stack 400 and the hot side heat exchanger 402 can be achieved via direct integration (e.g., heat conduction to solid components of the stack), indirect integration (e.g., via conduction and/or convective heat transfer to fluid pumped into the stack) or some combination thereof. This can be accomplished using direct contact to the refrigerant (which is pumped by the compressor 407) or with a third fluid loop (not shown).
It is anticipated that in some cases the SUPER cell stack 400 cannot absorb all the heat from the secondary heat pump. In other embodiments, the hot side of the heat pump can be coupled (directly or indirectly) with a secondary air contactor, such as humidifying LAMEE 408 which desorbs water from a liquid desiccant to an airflow (not shown) resulting in a lowering of temperature of the liquid desiccant. In other embodiments, the outlet fluid 410 of the SUPER cell stack, which has a very low concentration solution outlet stream, can be reused in the system by being fed into the humidifying LAMEE 408, which outputs a more concentrated liquid desiccant stream 411. Using the hot side of the heat pump, this outlet fluid 410 could be regenerated even if the ambient humidity levels were at 100%. This provides unique benefit to the regenerator that the SUPER cell stack 400 would no longer need a drain, in that the outlet fluid 410 would be reconcentrated by the humidifying LAMEE 408 and be fed back into the SUPER cell stack 400. In cases of sub-100% humidity, the heat pump would benefit from evaporative cooling lowering the temperature span of the secondary heat pump and increasing its efficiency.
In another embodiment, the dehumidifying LAMEE 401 is brought into thermal contact with the cold side of the heat pump, as indicated by heat exchangers 412 before the expansion valve 405 and the heat exchanger 414 after the expansion valve 405. The amount of cooling provided could be adjusted, e.g., by utilizing optional variable bypass valves 415, 416, which in this example, regulate the flow through heat exchanger 414. A similar valve arrangement could be used to regulate the flow through heat exchangers 402, 412 or any other component of the secondary heat pump. The valves could be linked to a system controller that monitors system temperatures and automatically adjusts the valves to maintain one or more desired operating points.
Cooling of the LAMEE input flow would allow the SUPER cell stack to operate at lower salt concentrations due to increased RH at lower temperatures of the LAMEE 401. Lowering salt concentrations would increase the efficiency of the SUPER cell stack 400. Additionally, the secondary heat pump can directly control outlet temperature in the LAMEE air contactor negating the need for a separate heat exchanger, and possibly reducing overall system costs. The illustrated evaporator 406 provides sensible cooling for the primary airflow, e.g., before or after passing through the LAMEE 401.
In
In some desiccant technologies, the heat pump is used to accept the latent heat of condensation from the dehumidifier. This is shown in
Alternatively, the evaporator can be placed downstream of the dehumidifier. This is shown in
To operate at the highest possible evaporator temperature and minimize desiccant system size, a pre-evaporator cooling to the dehumidifier can be used. In
The evaporator 606 no longer needs to cool below the target outlet temperature but a significant amount of moisture may be removed on the evaporator 606 at no additional energy cost. The dehumidifier 601 can still be reduced in size due to condensing on the pre-evaporator. If the liquid desiccant regenerator 600 is a SUPER stack, then the dilute solution can be further diluted by the evaporator condensate, as shown in
Note that in the embodiments disclosed above, a heat pump is shown in with an evaporator that absorbs heat and a condenser that outputs heat. In any of these embodiments, the evaporator can be replaced with a chilled solution heat exchanger and/or the condenser can be replaced with a heated solution heat exchanger. The chilled and heated solutions may include water, water/glycol solution, saline solution, etc. The heat pump may still include a vapor compression system with an evaporator and condenser, but heat transfer is effected through the circulation of the solution through the system rather than direct contact with the evaporator and condenser.
In
In summary, systems and methods are described that can reduce energy consumption in electrochemically regenerated dehumidification and air conditioning systems, extend system performance, and enable co-located sensible heating and cooling with separate control. In one embodiment, a secondary heat pump is used to adjust the operating conditions of an electrochemically regenerated liquid desiccant system. An electrochemically regenerated liquid desiccant dehumidifier has at least one air contactor for dehumidifying air where a secondary heat pump system is utilized to control the water vapor absorption temperature, the regeneration temperature or a combination of both. The regeneration temperature can be controlled directly in the regenerator, in one or more air contactors used for humidification, or a combination of both.
Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein. The use of numerical ranges by endpoints includes all numbers within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5) and any range within that range. All descriptions of solute concentrations by percentage are meant to describe percentage by weight unless otherwise indicated.
The foregoing description has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Many modifications and variations are possible in light of the above teachings. Any or all features of the disclosed embodiments can be applied individually or in any combination and are not meant to be limiting, but purely illustrative. It is intended that the scope of the invention be limited not with this detailed description, but rather, determined by the claims appended hereto.
