Abstract:
A desiccant air conditioning system for treating an air stream entering a building space, including a conditioner configured to expose the air stream to a liquid desiccant such that the liquid desiccant dehumidifies the air stream in the warm weather operation mode and humidifies the air stream in the cold weather operation mode. The conditioner includes multiple plate structures arranged in a vertical orientation and spaced apart to permit the air stream to flow between the plate structures. Each plate structure includes a passage through which a heat transfer fluid can flow. Each plate structure also has at least one surface across which the liquid desiccant can flow. The system includes a regenerator connected to the conditioner for causing the liquid desiccant to desorb water in the warm weather operation mode and to absorb water in the cold weather operation mode from a return air stream.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from U.S. Provisional Patent Application No. 61/771,340 filed on Mar. 1, 2013 entitled METHODS FOR CONTROLLING 3-WAY HEAT EXCHANGERS IN DESICCANT CHILLERS, which is hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    The present application relates generally to the use of liquid desiccants to dehumidify and cool, or heat and humidify an air stream entering a space. More specifically, the application relates to the control systems required to operate 2 or 3 way liquid desiccant mass and heat exchangers employing micro-porous membranes to separate the liquid desiccant from an air stream. Such heat exchangers can use gravity induced pressures (siphoning) to keep the micro-porous membranes properly attached to the heat exchanger structure. The control systems for such 2 and 3-way heat exchangers are unique in that they have to ensure that the proper amount liquid desiccant is applied to the membrane structures without over pressurizing the fluid and without over- or under-concentrating the desiccant. Furthermore the control system needs to respond to demands for fresh air ventilation from the building and needs to adjust to outdoor air conditions, while maintaining a proper desiccant concentration and preventing desiccant crystallization or undue dilution. In addition the control system needs to be able to adjust the temperature and humidity of the air supplied to a space by reacting to signals from the space such as thermostats or humidistats. The control system also needs to monitor outside air conditions and properly protect the equipment in freezing conditions by lowering the desiccant concentration in such a way as to avoid crystallization. 
         [0003]    Liquid desiccants have been used parallel to conventional vapor compression HVAC equipment to help reduce humidity in spaces, particularly in spaces that require large amounts of outdoor air or that have large humidity loads inside the building space itself. Humid climates, such as for example Miami, Fla. require a lot of energy to properly treat (dehumidify and cool) the fresh air that is required for a space&#39;s occupant comfort. Conventional vapor compression systems have only a limited ability to dehumidify and tend to overcool the air, oftentimes requiring energy intensive reheat systems, which significantly increase the overall energy costs, because reheat adds an additional heat-load to the cooling system. Liquid desiccant systems have been used for many years and are generally quite efficient at removing moisture from the air stream. However, liquid desiccant systems generally use concentrated salt solutions such as ionic solutions of LiCl, LiBr or CaCl 2  and water. Such brines are strongly corrosive, even in small quantities, so numerous attempts have been made over the years to prevent desiccant carry-over to the air stream that is to be treated. In recent years efforts have begun to eliminate the risk of desiccant carry-over by employing micro-porous membranes to contain the desiccant. An example of such as membrane is the EZ2090 poly-propylene, microporous membrane manufactured by Celgard, LLC, 13800 South Lakes Drive Charlotte, N.C. 28273. The membrane is approximately 65% open area and has a typical thickness of about 20 μm. This type of membrane is structurally very uniform in pore size (100 nm) and is thin enough to not create a significant thermal barrier. However such super-hydrophobic membranes are typically hard to adhere to and are easily subject to damage. Several failure modes can occur: if the desiccant is pressurized the bonds between the membrane and its support structure can fail, or the membrane&#39;s pores can distort in such a way that they no longer are able to withstand the liquid pressure and break-through of the desiccant can occur. Furthermore if the desiccant crystallizes behind the membrane, the crystals can break through the membrane itself creating permanent damage to the membrane and causing desiccant leaks. And in addition the service life of these membranes is uncertain, leading to a need to detect membrane failure or degradation well before any leaks may even be apparent. 
         [0004]    Liquid desiccant systems generally have two separate functions. The conditioning side of the system provides conditioning of air to the required conditions, which are typically set using thermostats or humidistats. The regeneration side of the system provides a reconditioning function of the liquid desiccant so that it can be re-used on the conditioning side. Liquid desiccant is typically pumped between the two sides which implies that the control system also needs to ensure that the liquid desiccant is properly balanced between the two sides as conditions necessitate and that excess heat and moisture are properly dealt with without leading to over-concentrating or under-concentrating the desiccant. 
         [0005]    There thus remains a need for a control system that provides a cost efficient, manufacturable, and efficient method to control a liquid desiccant system in such a way as to maintain proper desiccant concentrations, fluid levels, react to space temperature and humidity requirements, react to space occupancy requirements and react to outdoor air conditions, while protecting the system against crystallization and other potentially damaging events. The control system furthermore needs to ensure that subsystems are balanced properly and that fluid levels are maintained at the right set-points. The control system also needs to warn against deterioration or outright failures of the liquid desiccant membrane system. 
       BRIEF SUMMARY 
       [0006]    Provided herein are methods and systems used for the efficient dehumidification of an air stream using a liquid desiccant. In accordance with one or more embodiments, the liquid desiccant is running down the face of a support plate as a falling film. In accordance with one or more embodiments, the desiccant is contained by a microporous membrane and the air stream is directed in a primarily vertical orientation over the surface of the membrane and whereby both latent and sensible heat are absorbed from the air stream into the liquid desiccant. In accordance with one or more embodiments, the support plate is filled with a heat transfer fluid that preferably flows in a direction counter to the air stream. In accordance with one or more embodiments, the system comprises a conditioner that removes latent and sensible heat through the liquid desiccant and a regenerator that removes the latent and sensible heat from the system. In accordance with one or more embodiments, the heat transfer fluid in the conditioner is cooled by a refrigerant compressor or an external source of cold heat transfer fluid. In accordance with one or more embodiments, the regenerator is heated by a refrigerant compressor or an external source of hot heat transfer fluid. In accordance with one or more embodiments, the cold heat transfer fluid can bypass the conditioner and the hot heat transfer fluid can bypass the regenerator thereby allowing independent control of supply air temperature and relative humidity. In accordance with one or more embodiments, the conditioner&#39;s cold heat transfer fluid is additionally directed through a cooling coil and the regenerator&#39;s hot heat transfer fluid is additionally directed through a heating coil. In accordance with one or more embodiments, the hot heat transfer fluid has an independent method or rejecting heat, such as through an additional coil or other appropriate heat transfer mechanism. In accordance with one or more embodiments, the system has multiple refrigerant loops or multiple heat transfer fluid loops to achieve similar effects for controlling air temperature on the conditioner and liquid desiccant concentration by controlling the regenerator temperature. In one or more embodiments, the heat transfer loops are serviced by separate pumps. In one or more embodiments, the heat transfer loops are services by a single shared pump. In one or more embodiments, the refrigerant loops are independent. In one or more embodiments, the refrigerant loops are coupled so that one refrigerant loop only handles half the temperature difference between the conditioner and the regenerator and the other refrigerant loop handles the remaining temperature difference, allowing each loop to function more efficiently. 
         [0007]    In accordance with one or more embodiments, a liquid desiccant system employs a heat transfer fluid on a conditioner side of the system and a similar heat transfer fluid loop on a regenerator side of the system wherein the heat transfer fluid can optionally be directed from the conditioner to the regenerator side of the system through a switching valve, thereby allowing heat to be transferred through the heat transfer fluid from the regenerator to the conditioner. The mode of operation is useful in case where the return air from the space that is directed through the regenerator is higher in temperature than the outside air temperature and the heat from the return air can be thus be used to heat the incoming supply air stream. 
         [0008]    In accordance with one or more embodiments, the refrigerant compressor system is reversible so that heat from the compressor is directed to the liquid desiccant conditioner and heat is removed by the refrigerant compressor from the regenerator thereby reversing the conditioner and regeneration functions. In accordance with one or more embodiments, the heat transfer fluid is reversed but no refrigerant compressor is utilized and external sources of cold and hot heat transfer fluids are utilized thereby allowing heat to be transferred from one side of the system to the opposite side of the system. In accordance with one or more embodiments, the external sources of cold and hot heat transfer fluid are idled while heat is transferred from one side to the other side of the system. 
         [0009]    In accordance with one or more embodiments, a liquid desiccant membrane system employs an indirect evaporator to generate a cold heat transfer fluid wherein the cold heat transfer fluid is used to cool a liquid desiccant conditioner. Furthermore in one or more embodiments, the indirect evaporator receives a portion of the air stream that was earlier treated by the conditioner. In accordance with one or more embodiments, the air stream between the conditioner and indirect evaporator is adjustable through some convenient means, for example through a set of adjustable louvers or through a fan with adjustable fan speed. In accordance with one or more embodiments, the heat transfer fluid between the conditioner and indirect evaporator is adjustable so that the air that is treated by the conditioner is also adjustable by regulating the heat transfer fluid quantity passing through the conditioner. In accordance with one or more embodiments, the indirect evaporator can be idled and the heat transfer fluid can be directed between the conditioner and a regenerator is such a fashion that heat from return air from a space is recovered in the regenerator and is directed to provide heating to air directed through the conditioner. 
         [0010]    In accordance with one or more embodiments, the indirect evaporator is used to provide heated, humidified air to a supply air stream to a space while a conditioner is simultaneously used to provide heated, humidified air to the same space. This allows the system to provide heated, humidified air to a space in winter conditions. The conditioner is heated and is desorbing water vapor from a desiccant and the indirect evaporator can be heated as well and is desorbing water vapor from liquid water. In one or more embodiments, the water is seawater. In one or more embodiments, the water is waste water. In one or more embodiments, the indirect evaporator uses a membrane to prevent carry-over of non-desirable elements from the seawater or waste water. In one or more embodiments, the water in the indirect evaporator is not cycled back to the top of the indirect evaporator such as would happen in a cooling tower, but between 20% and 80% of the water is evaporated and the remainder is discarded. 
         [0011]    In accordance with one or more embodiments, a liquid desiccant conditioner receives cold or warm water from an indirect evaporator. In one or more embodiments, the indirect evaporator has a reversible air stream. In one or more embodiments, the reversible air stream creates a humid exhaust air stream in summer conditions and creates a humid supply air stream to a space in winter conditions. In one or more embodiments, the humid summer air stream is discharged from the system and the cold water that is generated is used to chill the conditioner in summer conditions. In one or more embodiments, the humid winter air stream is used to humidify the supply air to a space in combination with a conditioner. In one or more embodiments, the air streams are variable by a variable speed fan. In one or more embodiments, the air streams are variable through a louver mechanism or some other suitable method. In one or more embodiments, the heat transfer fluid between the indirect evaporator and the conditioner can be directed through the regenerator as well, thereby absorbing heat from the return air from a space and delivering such heat to the supply air stream for that space. In one or more embodiments, the heat transfer fluid receives supplemental heat or cold from external sources. In one or more embodiments, such external sources are geothermal loops, solar water loops or heat loops from existing facilities such as Combined Heat and Power systems. 
         [0012]    In accordance with one or more embodiments, a conditioner receives an air stream that is pulled through the conditioner by a fan while a regenerator receives an air stream that is pulled through the regenerator by a second fan. In one or more embodiments, the air stream entering the conditioner comprises a mixture of outside air and return air. In one or more embodiments, the amount of return air is zero and the conditioner receives solely outside air. In one or more embodiments, the regenerator receives a mixture of outside air and return air from a space. In one or more embodiments, the amount of return air is zero and the regenerator receives only outside air. In one or more embodiments, louvers are used to allow some air from the regenerator side of the system to be passed to the conditioner side of the system. In one or more embodiments, the pressure in the conditioner is below the ambient pressure. In further embodiments the pressure in the regenerator is below the ambient pressure. 
         [0013]    In accordance with one or more embodiments, a conditioner receives an air stream that is pushed through the conditioner by a fan resulting in a pressure in the conditioner that is above the ambient pressure. In one or more embodiments, such positive pressure aids in ensuring that a membrane is held flat against a plate structure. In one or more embodiments, a regenerator receives an air stream that is pushed through the regenerator by a fan resulting in a pressure in the regenerator that is above ambient pressure. In one or more embodiments, such positive pressure aids in ensuring that a membrane is held flat against a plate structure. 
         [0014]    In accordance with one or more embodiments, a conditioner receives an air stream that is pushed through the conditioner by a fan resulting in a positive pressure in the conditioner that is above the ambient pressure. In one or more embodiments, a regenerator receives an air stream that is pulled through the regenerator by a fan resulting in a negative pressure in the regenerator compared to the ambient pressure. In one or more embodiments, the air stream entering the regenerator comprises a mixture of return air from a space and outside air that is being delivered to the regenerator from the conditioner air stream. 
         [0015]    In accordance with one or more embodiments, an air stream&#39;s lowest pressure point is connected through some suitable means such as through a hose or pipe to an air pocket above a desiccant reservoir in such a way as to ensure that the desiccant is flowing back from a conditioner or regenerator membrane module through a siphoning action and wherein the siphoning is enhanced by ensuring that the lowest pressure in the system exists above the desiccant in the reservoir. In one or more embodiments, such siphoning action ensures that a membrane is held in a flat position against a support plate structure. 
         [0016]    In accordance with one or more embodiments, an optical or other suitable sensor is used to monitor air bubbles that are leaving a liquid desiccant membrane structure. In one or more embodiments, the size and frequency of air bubbles is used as an indication of membrane porosity. In one or more embodiments, the size and frequency of air bubbles is used to predict membrane aging or failure. 
         [0017]    In accordance with one or more embodiments, a desiccant is monitored in a reservoir by observing the level of the desiccant in the reservoir. In one or more embodiments, the level is monitored after initial startup adjustments have been discarded. In one or more embodiments, the level of desiccant is used as an indication of desiccant concentration. In one or more embodiments, the desiccant concentration is also monitored through the humidity level in the air stream exiting a membrane conditioner or membrane regenerator. In one or more embodiments, a single reservoir is used and liquid desiccant is siphoning back from a conditioner and a regenerator through a heat exchanger. In one or more embodiments, the heat exchanger is located in the desiccant loop servicing the regenerator. In one or more embodiments, the regenerator temperature is adjusted based on the level of desiccant in the reservoir. 
         [0018]    In accordance with one or more embodiments, a conditioner receives a desiccant stream and employs siphoning to return the used desiccant to a reservoir. In one or more embodiments, a pump or similar device takes desiccant from the reservoir and pumps the desiccant through a valve and heat exchanger to a regenerator. In one or more embodiments, the valve can be switched so that the desiccant flows to the conditioner instead of flowing through the heat exchanger. In one or more embodiments, a regenerator receives a desiccant stream and employs siphoning to return the used desiccant to a reservoir. In one or more embodiments, a pump or similar device takes desiccant from a reservoir and pumps the desiccant through a heat exchanger and valve assembly to a conditioner. In one or more embodiments, the valve assembly can be switched to pump the desiccant to the regenerator instead of to the conditioner. In one or more embodiments, the heat exchanger can be bypassed. In one or more embodiments, the desiccant is used to recover latent and/or sensible heat from a return air stream and apply the latent heat to a supply air stream by bypassing the heat exchanger. In one or more embodiments, the regenerator is switched on solely when regenerator of desiccant is required. In one or more embodiments, the switching of the desiccant stream is used to control the desiccant concentration. 
         [0019]    In accordance with one or more embodiments, a membrane liquid desiccant plate module uses an air pressure tube to ensure that the lowest pressure in the air stream is applied to the air pocket above the liquid desiccant in a reservoir. In one or more embodiments, the liquid desiccant fluid loop uses an expansion volume near the top of the membrane plate module to ensure constant liquid desiccant flow to the membrane plate module. 
         [0020]    In accordance with one or more embodiments, a liquid desiccant membrane module is positioned above a sloped drain pan structure, wherein any liquid leaking from the membrane plate module is caught and directed towards a liquid sensor that sends a signal to a control system warning that a leak or failure in the system has occurred. In one or more embodiments, such a sensor detects the conductance of the fluid. In one or more embodiments, the conductance is an indication of which fluid is leaking from the membrane module. 
         [0021]    In no way is the description of the applications intended to limit the disclosure to these applications. Many construction variations can be envisioned to combine the various elements mentioned above each with its own advantages and disadvantages. The present disclosure in no way is limited to a particular set or combination of such elements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  illustrates a 3-way liquid desiccant air conditioning system using a chiller or external heating or cooling sources. 
           [0023]      FIG. 2A  shows a flexibly configurable membrane module that incorporates 3-way liquid desiccant plates. 
           [0024]      FIG. 2B  illustrates a concept of a single membrane plate in the liquid desiccant membrane module of  FIG. 2A . 
           [0025]      FIG. 3A  depicts the cooling fluid control system and chiller refrigerant circuit of a 3-way liquid desiccant system in cooling mode in accordance with one or more embodiments. 
           [0026]      FIG. 3B  shows the system of  FIG. 3A  with the cooling fluid flow connecting the return air and supply air of the building and the chiller in idle mode providing an energy recovery capability between the return air and the supply air in accordance with one or more embodiments. 
           [0027]      FIG. 3C  illustrates the system of  FIG. 3A  with the chiller in reverse mode supplying heat to the supply air and retrieving heat from the return air in accordance with one or more embodiments. 
           [0028]      FIG. 4A  shows the cooling fluid control circuit of a liquid desiccant membrane system that utilizes external cooling and heating sources in accordance with one or more embodiments. 
           [0029]      FIG. 4B  shows the system of  FIG. 4A  wherein the cooling fluid provides a sensible heat recovery connection between the return air and the supply air in accordance with one or more embodiments. 
           [0030]      FIG. 5A  shows a liquid desiccant air conditioning system utilizing an indirect evaporative cooling module in summer cooling mode in accordance with one or more embodiments. 
           [0031]      FIG. 5B  illustrates the system of  FIG. 5B  wherein the system is set up as a sensible heat recovery system in accordance with one or more embodiments. 
           [0032]      FIG. 5C  shows the system of  FIG. 5A  wherein the system&#39;s operation is reversed for winter heating operation in accordance with one or more embodiments. 
           [0033]      FIG. 6A  illustrates the water and refrigerant control diagram of a dual compressor system employing several control loops for water flows and heat rejection in accordance with one or more embodiments. 
           [0034]      FIG. 6B  shows a system employing two stacked refrigerant loops for more efficiently moving heat from the conditioner to the regenerator in accordance with one or more embodiments. 
           [0035]      FIG. 7A  shows an air flow diagram with a partial re-use of return air using a negative pressure housing compared to environmental pressure in accordance with one or more embodiments. 
           [0036]      FIG. 7B  shows an air flow diagram with a partial re-use of return air using a positive pressure housing compared to environmental pressure in accordance with one or more embodiments. 
           [0037]      FIG. 7C  shows an air flow diagram with a partial re-use of return air and a positive pressure supply air stream and a negative pressure return air stream wherein a portion of the outdoor air is used to increase flow through the regeneration module in accordance with one or more embodiments. 
           [0038]      FIG. 8A  illustrates a single tank control diagram for a desiccant flow in accordance with one or more embodiments. 
           [0039]      FIG. 8B  shows a simple decision schematic for controlling the liquid desiccant level in the system in accordance with one or more embodiments. 
           [0040]      FIG. 9A  shows a dual tank control diagram for a desiccant flow, wherein a portion of the desiccant is sent from a conditioner to a regenerator in accordance with one or more embodiments. 
           [0041]      FIG. 9B  shows the system of  FIG. 9A  wherein the desiccant is used in an isolation mode for conditioner and regenerator in accordance with one or more embodiments. 
           [0042]      FIG. 10A  illustrates the flow diagram of a negative air pressure liquid desiccant system with a desiccant spill sensor in accordance with one or more embodiments. 
           [0043]      FIG. 10B  shows the system of  FIG. 10A  with a positive air pressure liquid desiccant system in accordance with one or more embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0044]      FIG. 1  depicts a new type of liquid desiccant system as described in more detail in U.S. Patent Application Publication No. 2012/0125020 entitled METHODS AND SYSTEMS FOR DESICCANT AIR CONDITIONING USING PHOTOVOLTAIC-THERMAL (PVT) MODULES. A conditioner  10  comprises a set of plate structures  11  that are internally hollow. A cold heat transfer fluid is generated in cold source  12  and entered into the plates. Liquid desiccant solution at  14  is brought onto the outer surface of the plates  11  and runs down the outer surface of each of the plates  11 . The liquid desiccant runs behind a thin membrane that is located between the air flow and the surface of the plates  11 . Outside air  16  is now blown through the set of wavy plates  11 . The liquid desiccant on the surface of the plates attracts the water vapor in the air flow and the cooling water inside the plates  11  helps to inhibit the air temperature from rising. The treated air  18  is put into a building space. 
         [0045]    The liquid desiccant is collected at the bottom of the wavy plates at  20  and is transported through a heat exchanger  22  to the top of the regenerator  24  to point  26  where the liquid desiccant is distributed across the wavy plates of the regenerator. Return air or optionally outside air  28  is blown across the regenerator plate and water vapor is transported from the liquid desiccant into the leaving air stream  30 . An optional heat source  32  provides the driving force for the regeneration. The hot transfer fluid  34  from the heat source can be put inside the wavy plates of the regenerator similar to the cold heat transfer fluid on the conditioner. Again, the liquid desiccant is collected at the bottom of the wavy plates  27  without the need for either a collection pan or bath so that also on the regenerator the air can be vertical. An optional heat pump  36  can be used to provide cooling and heating of the liquid desiccant. It is also possible to connect a heat pump between the cold source  12  and the hot source  32 , which is thus pumping heat from the cooling fluids rather than the desiccant. 
         [0046]      FIG. 2A  describes a 3-way heat exchanger as described in more detail in U.S. patent application Ser. No. 13/915,199 filed on Jun. 11, 2013 entitled METHODS AND SYSTEMS FOR TURBULENT, CORROSION RESISTANT HEAT EXCHANGERS. A liquid desiccant enters the structure through ports  50  and is directed behind a series of membranes on plate structures  51  as described in  FIG. 1 . The liquid desiccant is collected and removed through ports  52 . A cooling or heating fluid is provided through ports  54  and runs counter to the air stream  56  inside the hollow plate structures, again as described in  FIG. 1  and in more detail in  FIG. 2B . The cooling or heating fluids exit through ports  58 . The treated air  60  is directed to a space in a building or is exhausted as the case may be. 
         [0047]      FIG. 2B  shows a schematic detail of one of the plates of  FIG. 1 . The air stream  251  flows counter to a cooling fluid stream  254 . Membranes  252  contain a liquid desiccant  253  that is falling along the wall  255  that contain a heat transfer fluid  254 . Water vapor  256  entrained in the air stream is able to transition the membrane  252  and is absorbed into the liquid desiccant  253 . The heat of condensation of water  258  that is released during the absorption is conducted through the wall  255  into the heat transfer fluid  254 . Sensible heat  257  from the air stream is also conducted through the membrane  252 , liquid desiccant  253  and wall  255  into the heat transfer fluid  254 . 
         [0048]      FIG. 3A  illustrates a simplified control schematic for the fluid paths of  FIG. 1  in a summer cooling mode arrangement, wherein a heat pump  317  is connected between the cold cooling fluid entering a liquid desiccant membrane conditioner  301  and the hot heating fluid entering a liquid desiccant membrane regenerator  312 . The conditioner and regenerator are membrane modules similar to the membrane module depicted in  FIG. 2A  and have plates similar to the concept in  FIG. 2B . The 3-way conditioner  301  receives an air stream  319  that is to be treated in the 3-way conditioner module. The 3-way conditioner also receives a concentrated desiccant stream  320  and a diluted desiccant stream  321  leaves the conditioner module. For simplicity, the liquid desiccant flow diagrams have been omitted from the figure and will be shown separately in later figures. A heat transfer fluid  302  which is commonly water, water/glycol or some other suitable heat transfer fluid, enters the 3-way module and removes the latent and sensible heat that has been removed from the air stream. Controlling the flow rate and pressure of the heat transfer fluid is critical to the performance of the 3-way module as is described in U.S. patent application Ser. No. 13/915,199. A circulating pump  307  is chosen to provide high fluid flow with low head pressure. The module&#39;s plates (shown in  FIGS. 1 and 2A ) have large surface areas and operate best under slightly negative pressure as compared to the ambient air pressure. The flow is set up in such a way that the heat transfer fluid  302  undergoes a siphoning effect to drain the fluid from the conditioner module  301 . Using a siphoning effect makes a marked improvement on the flatness of the module plates since the liquid pressure is not pushing the plates apart. This siphoning effect is achieved by letting the heat transfer fluid  302  fall into a fluid collection tank  305 . Temperature sensors  303  located in the heat transfer fluid before and after the 3-way module and the flow sensor  309 , allow one to measure in the thermal load captured in the heat transfer fluid. Pressure relief valve  311  is normally open and ensures that the heat transfer fluid is not pressurized which could damage the plate system. Service valves  306  and  308  are normally only used during service events. A liquid to refrigerant heat exchanger  310   a  allows the thermal load to be transferred from the heat transfer fluid to a refrigeration loop  316 . A bypass valve  304   a  allows a portion of the low temperature heat transfer fluid to bypass the 3-way conditioner. This has the effect as to lower the flow rate through the 3-way conditioner and as a result the conditioner will operate at higher temperatures. This in turn allows one to control the temperature of the supply air to the space. One could also use a variable flow of the liquid pump  307 , which will change the flow rate through the heat exchanger  310   a . An optional post-cooling coil element  327  ensures that the treated air temperature supplied to the space is very close to the heat transfer fluid temperature. 
         [0049]    A refrigerant compressor/heat pump  317  compresses a refrigerant moving in a circuit  316 . The heat of compression is rejected into a refrigerant heat exchanger  310   b , collected into an optional refrigerant receiver  318  and expanded in an expansion valve  315  after which it is directed to the refrigerant heat exchanger  310   a , where the refrigerant picks up heat from the 3-way conditioner and is returned to the compressor  317 . As can be seen in the figure, the liquid circuit  313  around the regenerator  312  is very similar to that around the conditioner  301 . Again, the siphoning method is employed to circulate the heat transfer fluid through the regenerator module  312 . However, there are two considerations that are different in the regenerator. First, it is oftentimes not possible to receive the same amount of return air  322  from a space as is supplied to that space  319 . In other words, air flows  319  and  322  are not balanced and can sometimes vary by more than 50%. This is so that the space remains positively pressurized compared to the surrounding environment to prevent moisture infiltration into the building. Second, the compressor itself adds an additional heat load that needs to be removed. This means that one has to either add additional air to the return air from the building, or one has to have another way of rejecting the heat from the system. Fan-coil  326  utilizes an independent radiator coil and can be used to achieve the additional cooling that is required. It should be understood that other heat rejection mechanism besides a fan coil could be employed such as a cooling tower, ground source heat dump etc. Optional diverter valve  325  can be employed to bypass the fan coil if desired. An optional pre-heating coil  328  is used to preheat the air entering the regenerator. It should be clear that the return air  322  could be mixed with outdoor air or could even be solely outdoor air. 
         [0050]    The desiccant loop (details of which will be shown in later figures) provides diluted desiccant to the regenerator module  312  through port  323 . Concentrated desiccant is removed at port  324  and directed back to the conditioner module to be reused. Control of the air temperature and thus the regeneration effect is again achieved through an optional diverter valve  304   b  similar to valve  304   a  in the conditioner circuit. The control system is thus able to control both the conditioner and regenerator air temperatures independently and without pressurizing the membrane plate module plates. 
         [0051]    Also in  FIG. 3A  is shown a diverter valve  314 . This valve is normally separating the conditioner and regenerator circuits. But in certain conditions the outside air needs little if any cooling. In  FIG. 3B  the diverter valve  314  has been opened to allow the conditioner and regenerator circuits to be connected creating an energy recovery mode. This allows the sensible heat from the return air  322  to be coupled to the incoming air  319  essentially providing a sensible energy recovery mechanism. In this operating mode the compressor  317  would normally be idled. 
         [0052]      FIG. 3C  shows how the system operates in winter heating mode. The compressor  317  is now operating in a reversed direction (for ease of the figure the refrigerant is shown flowing in the opposite direction—in actuality a 4-way reversible refrigerant circuit would most likely be employed). Diverter valve  314  is again closed so that the conditioner and regenerator are thermally isolated. The heat is essentially pumped from the return air  322  (which can be mixed with outdoor air) into the supply air  319 . The advantage that such an arrangement has is that the heat transfer (properly protected for freezing) and the liquid desiccant membrane modules are able to operate a much lower temperatures than conventional coils since none of the materials are sensitive to freezing conditions, including the liquid desiccant as long as its concentration is maintained between 15 and 35% in the case of Lithium Chloride. 
         [0053]      FIG. 4A  illustrates a summer cooling arrangement in a flow diagram similar to that of  FIG. 3A  however without the use of a refrigeration compressor. Instead, an external cold fluid source  402  is provided using a heat exchanger  401 . The external cold fluid source can be any convenient source of cold fluid, such as a geothermal source, a cooling tower, an indirect evaporative cooler or centralized chilled water or chilled brine loop. Similarly  FIG. 4A  illustrates a hot fluid source  404  that uses heat exchanger  403  to heat the regenerator hot water loop. Again such a hot fluid source can be any convenient hot fluid source such as from a steam loop, solar hot water, a gas furnace or a waste heat source. With the same control valves  304   a  and  304   b  the system is able to control the amount of heat removed from the supply air and added to the return air. In some instances it is possible to eliminate the heat exchangers  401  and  403  and to run the cold or hot fluid directly through the conditioner  301  and/or regenerator  312 . This is possible if the external cold or hot fluids are compatible with the conditioner and/or regenerator modules. This can simplify the system while making the system also slightly more energy efficient. 
         [0054]    Similar to the situation described in  FIG. 3B , it is again possible to recover heat from the return air  322  by using the diverter valve  314 , as is shown in  FIG. 4B . As in  FIG. 3B , the hot and cold fluid sources are most likely not operating in this condition so that heat is simply transferred from the return air  322  to the supply air  319 . 
         [0055]      FIG. 5A  shows an alternate summer cooling mode arrangement wherein a portion (typically 20-40%) of the treated air  319  is diverted through a set of louvers  502  into a side air stream  501  that enters a 3-way evaporator module  505 . The evaporator module  505  receives a water stream  504  that is to be evaporated and has a leaving residual water stream  503 . The water stream  504  can be potable water, sea water or grey water. The evaporator module  505  can be constructed very similar to the conditioner and regenerator modules and can also employ membranes. Particularly when the evaporator module  505  is evaporating seawater or grey water, a membrane will ensure that none of the salts and other materials entrained in the water become air borne. The advantage of using seawater or grey water is that this water is relatively inexpensive in many cases, rather than potable water. Off course seawater and grey water contain many minerals and ionic salts. Therefore the evaporator is set up to evaporate only a portion of the water supply, typically between 50 and 80%. The evaporator is set up as a “once-through” system meaning that the residual water stream  503  is discarded. This is unlike a cooling tower where the cooling water makes many passes through the system. However in cooling towers such passes eventually lead to mineral build up and residue that needs to the be “blown down”, i.e., removed. The evaporator in this system does not require a blow down operation since the residues are carried away by the residual water stream  503 . 
         [0056]    Similar to the conditioner and regenerator modules  301  and  312 , the evaporator module  505  receives a stream of heat transfer fluid  508 . The transfer fluid enters the evaporator module and the evaporation in the module results in a strong cooling effect on the heat transfer fluid. The temperature drop in the cooling fluid can be measured by temperature sensor  507  in the heat transfer fluid  509  that is leaving the evaporator  505 . The cooled heat transfer fluid  509  enters the conditioner module, where it absorbs the heat of the incoming air stream  319 . As can be seen in the figure, both the conditioner  319  and the evaporator  505  have a counter flow arrangement of their primary fluids (heat transfer fluid and air) thus resulting in a more efficient transfer of heat. Louvers  502  are used to vary the amount of air that is diverted to the evaporator. The exhaust air stream  506  of the evaporator module  505  carries off the excess evaporated water. 
         [0057]      FIG. 5B  illustrates the system from  FIG. 5A  in an energy recovery mode, with the diverter valve  314  set up to connect the fluid paths between the conditioner  302  and regenerator  313 . As before this setup allows for recovery of heat from the return air  322  to be applied to the incoming air  319 . In this situation it is also better to bypass the evaporator  505 , although one could simply not supply water  504  to the evaporator module and also close louvers  502  so not air is diverted to the evaporator module. 
         [0058]      FIG. 5C  now illustrates the system from  FIG. 5A  in a winter heating mode wherein the air flow  506  through the evaporator has been reversed so that it mixes with the air stream  319  from the conditioner. Also in this figure, the heat exchanger  401  and heat transfer fluid  402  are used to supply heat energy to the evaporator and conditioner modules. This heat can come from any convenient source such as a gas fired water heater, a waste heat source or a solar heat source. The advantage of this arrangement is that the system is now able to both heat (through the evaporator and the conditioner) and humidify (through the evaporator) the supply air. In this arrangement it is typically not advisable to supply liquid desiccant  320  to the conditioner module unless the liquid desiccant is able to pick up moisture from somewhere else, e.g., from the return air  322  or unless water is added to the liquid desiccant on a periodic basis. But even then, one has to carefully monitor the liquid desiccant to ensure that the liquid desiccant does not become overly concentrated. 
         [0059]      FIG. 6A  illustrates a system similar to that of  FIG. 3A , wherein there are now two independent refrigerant circuits. An additional compressor heat pump  606  supplies refrigerant to a heat exchanger  605 , after which it is received in a refrigerant receiver  607 , expanded through a valve  610  and entered into another heat exchanger  604 . The system also employs a secondary heat transfer fluid loop  601  by using fluid pump  602 , flow measurement device  603  and the aforementioned heat exchanger  604 . On the regenerator circuit a second heat transfer loop  609  is created and a further flow measurement instrument  608  is employed. It is worth noting that in the heat transfer loops on the conditioner side  2  circulating pumps  307  and  602  are used, whereas on the regenerator a single circulating pump  307  is used. This is for illustrative purposes only to show that many combinations of heat transfer flows and refrigerant flows could be employed. 
         [0060]      FIG. 6B  shows a system similar to that of  FIG. 3A  where the single refrigerant loop is now replaced by two stacked refrigerant loops. In the figure heat exchanger  310   a  exchanges heat with the first refrigerant loop  651   a . The first compressor  652   a  compresses the refrigerant that has been evaporated in the heat exchanger  310   a  and moves it to a condenser/heat exchanger  655 , where the heat generated by the compressor is removed and the cooled refrigerant is received in the optional liquid receiver  654   a . An expansion valve  653   a  expands the liquid refrigerant so it can absorb heat in the heat exchanger  310   a . The second refrigerant loop  651   b  absorbs heat from the first refrigerant loop in the condenser/heat exchanger  655 . The gaseous refrigerant is compressed by the second compressor  652   b  and heat is released in the heat exchanger  310   b . The liquid refrigerant is then received in optional liquid receiver  654   b  and expanded by expansion valve  653   b  where it is returned to the heat exchanger  655 . 
         [0061]      FIG. 7A  illustrates a representative example of how air streams in a membrane liquid desiccant air conditioning system can be implemented. The membrane conditioner  301  and the membrane regenerator  312  are the same as those from  FIG. 3A . Outside air  702  enters the system through an adjustable set of louvers  701 . The air is optionally mixed internally to the system with a secondary air stream  706 . The combined air stream enters the membrane module  301 . The air stream is pulled through the membrane module  301  by fan  703  and is supplied to the space as a supply air stream  704 . The secondary air stream  706  can be regulated by a second set of louvers  705 . The secondary air stream  706  can be a combination of two air streams  707  and  708 , wherein air stream  707  is a stream of air that is returned from the space to the air conditioning system and the air stream  708  is outside air that can be controlled by a third set of louvers  709 . The air mixture consisting of streams  707  and  708  is also pulled through the regenerator  312  by the fan  710  and is exhausted through a fourth set of louvers  711  into an exhaust air stream  712 . The advantage of the arrangement of  FIG. 7A  is that the entire system experiences a negative air pressure compared to the ambient air outside the system&#39;s housing—indicated by the boundary  713 . The negative pressure is provided by the fans  703  and  710 . Negative air pressure in the housing helps keep tight seals on door and access panels since the outside air helps maintain a force on those seals. However, the negative air pressure also has a disadvantage in that it can inhibit the siphoning of the desiccant in the membrane panel ( FIG. 2A ) and can even lead to the thin membranes being pulled into the air gaps ( FIG. 2B ). 
         [0062]      FIG. 7B  illustrates an alternate embodiment of an arrangement where fans have been placed in such a way as to create a positive internal pressure. A fan  714  is used to provide positive pressure above the conditioner module  301 . Again the air stream  702  is mixed with the air stream  706  and the combined air stream enters the conditioner  301 . The conditioned air stream  704  is now supplied to the space. A return air fan  715  is used to bring return air  707  back from the space and a second fan  716  is needed to provide additional outside air. There is a need for this fan because in many situations the amount of available return air is much less than the amount of air supplied to the space so additional air has to be provided to the regenerator. The arrangement of  FIG. 7B  therefore necessitates the use of 3 fans and 4 louvers. 
         [0063]      FIG. 7C  shows a hybrid embodiment wherein the conditioner is using a positive pressure similar to  FIG. 7A  but wherein the regenerator is under negative pressure similar to  FIG. 7B . The main difference is that the air stream  717  is now reversed in direction compared to the mixed air stream  706  in  FIGS. 7A and 7B . This allows a single fan  713  to supply outside air to both the conditioner  301  and the regenerator  312 . The return air stream  707  is now mixed with the outside air stream  717  so that ample air is supplied to the regenerator. The fan  710  is pulling air through the regenerator  312  resulting in a slightly negative pressure in the regenerator. The advantage of this embodiment is that the system only requires 2 fans and 2 sets of louvers. A slight disadvantage is that the regenerator experiences negative pressures and is thus less able to siphon and has a higher risk of the membrane being pulled into the air gap. 
         [0064]      FIG. 8A  shows the schematic of the liquid desiccant flow circuit. Air enthalpy sensors  801  employed before and after the conditioner and regenerator modules give a simultaneous measurement of air temperature and humidity. The before and after enthalpy measurements can be used to indirectly determine the concentration of the liquid desiccant. A lower exiting humidity indicates a higher desiccant concentration. The liquid desiccant is taken from a reservoir  805  by pump  804  at an appropriately low level because the desiccant will stratify in the reservoir. Typically the desiccant will be about 3-4% less concentrated near the top of the reservoir compared to the bottom of the reservoir. The pump  804  brings the desiccant to the supply port  320  near the top of the conditioners. The desiccant flows behind the membranes and exits the module through port  321 . The desiccant is then pulled by a siphoning force into the reservoir  805  while passing a sensor  808  and a flow sensor  809 . The sensor  808  can be used to determine the amount of air bubbles that are formed in the liquid desiccant going through the drain port  321 . This sensor can be used to determine if the membrane properties are changing: the membrane lets a small amount of air through as well as water vapor. This air forms bubbles in the exit liquid desiccant stream. A change in membrane pore size for example due to degradation of the membrane material will lead to an increase in bubble frequency and bubble sizes all other conditions being equal. The sensor  808  can thus be used to predict membrane failure or degradation well before a catastrophic failure happens. The flow sensor  809  is used to ensure that the proper amount of desiccant is returning to the reservoir  805 . A failure in the membrane module would result in little or no desiccant returning and thus the system can be stopped. It would also be possible to integrate the sensors  808  and  809  into a single sensor embodying both functions or, e.g., for sensor  808  to register that no more air bubbles are passing as an indication of stopped flow. 
         [0065]    Again in  FIG. 8A , a second pump  806  pulls dilute liquid desiccant at a higher level from the reservoir. The diluted desiccant will be higher in the reservoir since the desiccant will stratify if one is careful not to disturb the desiccant too much. The dilute desiccant is then pumped through a heat exchanger  807  to the top of the regenerator module supply port  323 . The regenerator re-concentrates the desiccant and it exits the regenerator at port  324 . The concentrated desiccant then passes the other side of the heat exchanger  807 , and passes a set of sensors  808  and  809  similar to those used on the conditioner exit. The desiccant is then brought back to the reservoir into the stratified desiccant at a level approximately equal to the concentration of the desiccant exiting the regenerator. 
         [0066]    The reservoir  805  is also equipped with a level sensor  803 . The level sensor can be used to determine the level of desiccant in the reservoir but is also an indication of the average concentration desiccant in the reservoir. Since the system is charged with a fixed amount of desiccant and the desiccant only absorbs and desorbs water vapor, the level can be used to determine the average concentration in the reservoir. 
         [0067]      FIG. 8B  illustrates a simple decision tree for monitoring the desiccant level in a liquid desiccant system. The control system starts the desiccant pumps and waits a few minutes for the system to reach a stable state. If after the initial startup period the desiccant level is rising (which indicates that more water vapor is removed from the air then is removed in the regenerator then the system can correct by increasing the regeneration temperature, for example by closing the bypass valve  304   b  in  FIG. 3A  or by closing the bypass loop valve  325  also in  FIG. 3A . 
         [0068]      FIG. 9A  shows a liquid desiccant control system wherein two reservoirs  805  and  902  are employed. The addition of the second reservoir  902  can be necessary if the conditioner and regenerator air not in near proximity to each other. Since the desiccant siphoning is desirable having a reservoir near or underneath the conditioner and regenerator is sometimes a necessity. A 4-way valve  901  can also added to the system. The addition of a 4-way valve allows the liquid desiccant to be sent from the conditioner reservoir  805  to the regenerator module  312 . The liquid desiccant is now able to pick up water vapor from the return air stream  322 . The regenerator is not heated by the heat transfer fluid in this operating mode. The diluted liquid desiccant is now directed back through the heat exchanger  807  and to the conditioner module  301 . The conditioner module is not being cooled by the heat transfer fluid. It is actually possible to heat the conditioner module and cool the regenerator which makes them function opposite from their normal operation. In this fashion it is possible to add heat and humidity to the outside air  319  and recover heat and humidity from the return air. It is worthwhile noting that if one wants to recover heat as well as humidity, the heat exchanger  807  can be bypassed. The second reservoir  902  has a second level sensor  903 . The monitoring schematic of  FIG. 8B  can still be employed by simply adding the two level signals together and using the combined level as the level to be monitored. 
         [0069]      FIG. 9B  illustrates the flow diagram of the liquid desiccants if the 4-way valve  901  is set to an isolated position. In this situation no desiccant is moved between the two sides and each side is independent of the other side. This operating mode can be useful if very little dehumidification needs to be obtained in the conditioner. The regenerator could effectively be idled in that case. 
         [0070]      FIG. 10A  illustrates a set of membrane plates  1007  mounted in a housing  1003 . The supply air  1001  is pulled through the membrane plates  1007  by the fan  1002 . This arrangement results in a negative pressure around the membrane plates compared to the ambient outside the housing  1003  as was discussed earlier. In order to maintain a proper pressure balance above the liquid desiccant reservoir  805 , a small tube or hose  1006  is connecting the low pressure area  1010  to the top of the reservoir  805 . Furthermore a small, vertical hose  1009  is employed near the top port  320  of the membrane module wherein a small amount of desiccant  1008  is present. The desiccant level  1008  can be maintained at an even height resulting in a controlled supply of desiccant to the membrane plates  1007 . An overflow tube  1015  ensures that if the level of desiccant in the vertical hose  1009  rises too high—and thus too much desiccant pressure is applied on the membranes—excess desiccant is drained back to the reservoir  805 , thereby bypassing the membrane plates  1007  and thereby avoiding potential membrane damage. 
         [0071]    Again referring to  FIG. 10A , the bottom of the housing  1003  is slightly sloped towards a corner  1004  wherein a conductivity sensor  1005  is mounted. The conductivity sensor can detect any amount of liquid that may have fallen from the membrane plates  1007  and is thus able to detect any problems or leaks in the membrane plates. 
         [0072]      FIG. 10B  shows a system similar to that of  10 A except that the fan  1012  is now located on the opposite side of the membrane plates  1007 . The air stream  1013  is now pushed through the plates  1007  resulting in a positive pressure in the housing  1003 . A small tube or hose  1014  is now used to connect the low pressure area  1011  to the air at the top of the reservoir  805 . The connection between the low pressure point and the reservoir allows for the largest pressure difference between the liquid desiccant behind the membrane and the air, resulting in good siphoning performance. Although not shown, an overflow tube similar to tube  1015  in  FIG. 10A  can be provided to ensure that if the level of desiccant in the overflow tube rises too high—and thus too much desiccant pressure is applied on the membranes—excess desiccant is drained back to the reservoir  805 , thereby bypassing the membrane plates  1007  and thereby avoiding potential membrane damage. Having thus described several illustrative embodiments, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to form a part of this disclosure, and are intended to be within the spirit and scope of this disclosure. While some examples presented herein involve specific combinations of functions or structural elements, it should be understood that those functions and elements may be combined in other ways according to the present disclosure to accomplish the same or different objectives. In particular, acts, elements, and features discussed in connection with one embodiment are not intended to be excluded from similar or other roles in other embodiments. Additionally, elements and components described herein may be further divided into additional components or joined together to form fewer components for performing the same functions. Accordingly, the foregoing description and attached drawings are by way of example only, and are not intended to be limiting.