Abstract:
Methods and systems are disclosed for utilizing liquid desiccant air conditioning systems in connection with existing HVAC equipment to achieve reductions in electricity consumption.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from U.S. Provisional Patent Application No. 61/782,579 filed on Mar. 14, 2013 entitled METHODS AND SYSTEMS FOR LIQUID DESICCANT AIR CONDITIONING SYSTEM RETROFIT, 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 an optimized system configuration to retrofit 2- or 3-way liquid desiccant mass and heat exchangers that employ micro-porous membranes to separate the liquid desiccant from an air stream in large commercial and industrial buildings while at the same time modifying existing Heating Ventilation and Air Conditioning (HVAC) equipment to achieve a significant reduction in electricity consumption in the building. 
         [0003]    Desiccant dehumidification systems—both liquid and solid 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. (ASHRAE 2012 Handbook of HVAC Systems and Equipment, Chapter 24, p. 24.10). 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 coil. Desiccant dehumidification systems—both solid and liquid—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. 
         [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. A control system is used to properly balance the liquid desiccant 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]    In large stores, supermarkets, commercial and industrial buildings, energy is wasted because the existing unitary HVAC units serving the building do not adequately dehumidify the ventilation air that they provide to the building. This excess humidity winds up being condensed out of the air with excess energy usage from refrigeration and freezer equipment inside the building, which creates a load on that equipment resulting in a higher than necessary energy consumption. 
         [0006]    Older buildings typically have been designed with HVAC equipment that recirculates a large portion (80-90%) of the air from the space through its cooling coil. The equipment takes in approximately 10-20% of fresh outside air, which as discussed above requires dehumidification, which is not adequately done by this equipment. At time of construction and design, engineers will sometime add a desiccant system to create the necessary dehumidification, but such equipment is heavy, complex and expensive and is not retrofitable on buildings that were not originally designed to accommodate them. 
         [0007]    There thus remains a need to provide a retrofitable cooling system for buildings with high humidity loads, wherein the dehumidification of outside air can be accommodated at low capital and energy costs. 
       BRIEF SUMMARY 
       [0008]    Provided herein are methods and systems used for the efficient cooling and dehumidification of an air stream in a large commercial or industrial building using a liquid desiccant. In accordance with one or more embodiments, the liquid desiccant flows 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 or primarily horizontal 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 ideally 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 into the heat transfer fluid and a regenerator that rejects the latent and sensible heat from the heat transfer fluid to the environment. 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 refrigerant compressor is reversible to provide heated heat transfer fluid to the conditioner and cold heat transfer fluid to the regenerator, and the conditioned air is heated and humidified and the regenerated air is cooled and dehumidified. 
         [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 one or more embodiments, the water supplied to the indirect evaporator is seawater. In one or more embodiments, the water is waste water. In one or more embodiments, the indirect evaporator uses a membrane to inhibit or 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. 
         [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 combination the indirect evaporator and conditioner provide heated humidified air to the building space for winter heating conditions. 
         [0011]    In accordance with one or more embodiments, some number of Liquid Desiccant Air Conditioning systems (LDACs) is installed at existing large stores, supermarkets or other commercial or industrial buildings to replace a subset of the existing unitary heating ventilating and air conditioning (HVAC) recirculating rooftop units (RTUs) already present. In accordance with one or more embodiments, the new liquid desiccant air conditioning units are operated to provide heated or cooled 100% outside air ventilation to the conditioned space. In accordance with one or more embodiments, the remaining RTUs are modified in such a way that they no longer supply outside air to the space, but become 100% recirculating RTU&#39;s. In one or more embodiments, the modification is achieved by removing power to a damper motor. In one or more embodiments, the modification is achieved by removing a lever from a damper mechanism. In accordance with one or more embodiments, the remaining RTUs are modified to have a higher evaporator temperature so that moisture no longer condenses on the evaporator coils and the unit becomes more energy efficient. In one or more embodiments, the increase in evaporator temperature is achieved by replacing an expansion valve. In one or more embodiments, the increase in evaporator temperature is achieved by adding an APR valve such as the valve assembly supplied by Rawal Devices, Inc. of Woburn, Mass. In one or more embodiments, the increase in evaporator temperature is achieved by adding a hot-gas bypass system or some other convenient means of increasing the evaporator temperature. 
         [0012]    In accordance with one or more embodiments, the new liquid desiccant air conditioning units provide all of the cooled, dehumidified outside air ventilation required by the building during the cooling season and warm humidified outside air ventilation during the heating season. The remaining existing unitary HVAC units have their outside air dampers shut so that they only provide heating or cooling of the indoor air. The benefit of this system retrofit that the new LDACs are more energy efficient and effective at dehumidifying the required ventilation air than the unitary HVAC units they replace. Another benefit of this system approach is that by the improved ability to reduce the space humidity in the building, the energy used by refrigeration and freezer units inside of the conditioned space is significantly reduced because they waste less energy having to condense humidity out of the air. Furthermore by modifying the remaining RTUs their energy consumption is also reduced. And lastly the advantage of replacing only a portion of the RTUs the cost of the upgrade is relatively minor since one can elect to replace mostly the oldest RTUs that are due for replacement anyway and payback periods are short because of the low upgrade cost and large energy savings. 
         [0013]    In accordance with one or more embodiments, a liquid desiccant air conditioning system is constructed of repeatable membrane module elements and membrane module support tubs. In one or more embodiments, the scalable membrane modules are sized so as to fit through a standard access hatch for a roof with an opening of about 2.5 ft×2.5 ft. In one or more embodiments, the repeatable module support tubs are arranged in a linear fashion in such a way that the module support tubs form a support structure and an air duct simultaneously. In one or more embodiments, the module support tubs are hollow. In one or more embodiments, the module support tubs have double walls so that they can hold a liquid. In one or more embodiments, the liquid is a liquid desiccant. In one or more embodiments, the liquid desiccant is stratified with higher concentrations near the bottom and lower concentrations near the top of the tub. In one or more embodiments, the tub bottom is sloped so as the conduct any spilled liquid to a single corner of the tub. In one or more embodiments, the corner is equipped with a sensor or detector that can detect if any liquid has collected in the corner. In one or more embodiments, such a sensor is a conductivity sensor. In one or more embodiments, the module support tubs have openings on both ends. In one or more embodiments, the two ends are used to provide two different air streams into a series of support tubs. In one or more embodiments, the air streams are a return air stream and an outside air air-stream. 
         [0014]    In accordance with one or more embodiments, a first series of membrane modules and module support tubs are arranged in a primarily linear fashion with a duct section that allows for a majority of air to be entered into a building and a portion of air to transported to a second series of membrane modules and module support tub sections. In one or more embodiments, the first series of modules and support tubs contain a membrane conditioner. In one or more embodiments, the membrane conditioner contains a desiccant behind the membrane. In one or more embodiments, the second series of modules contain a membrane conditioner. In one or more embodiments, the second conditioner contains water behind the membrane. 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 water is potable water. In one or more embodiments, the air flow in the second series of membrane modules and module support tubs is reversible. In one or more embodiments, the first series of membrane modules receives a hot heat transfer fluid in winter mode from a heat source and receives a cold heat transfer fluid in summer mode. In one or more embodiments, the second series of membrane modules supplies the cold heat transfer fluid to the first series of membrane module in cooling mode and receives a hot heat transfer fluid from a heat source in winter mode. In one or more embodiments, the first series and second series of modules receive hot heat transfer fluid from the same heat source in winter mode. 
         [0015]    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 
         [0016]      FIG. 1  illustrates an exemplary 3-way liquid desiccant air conditioning system using a chiller or external heating or cooling sources. 
           [0017]      FIG. 2  shows an exemplary flexibly configurable membrane module that incorporates 3-way liquid desiccant plates. 
           [0018]      FIG. 3  illustrates an exemplary single membrane plate in the liquid desiccant membrane module of  FIG. 2 . 
           [0019]      FIG. 4  shows an exemplary building roof layout showing existing rooftop units (RTUs) and RTUs that would be replaced as part of a retrofit. 
           [0020]      FIG. 5  shows a schematic aspect of an exemplary recirculating rooftop unit on a building space. 
           [0021]      FIG. 6  shows a schematic aspect of an exemplary modified recirculating rooftop unit assisted by a liquid desiccant dedicated outdoor air system. 
           [0022]      FIG. 7  depicts a psychrometric chart showing the processes of an exemplary recirculating rooftop unit as well as the liquid desiccant dedicated outdoor air system. 
           [0023]      FIG. 8  shows an implementation of an exemplary scalable liquid desiccant dedicated outdoor air system. 
           [0024]      FIG. 9A  shows a schematic diagram of the conditioner side of the system of  FIG. 8 . 
           [0025]      FIG. 9B  shows a schematic diagram of the regenerator side of the system of  FIG. 8 . 
           [0026]      FIG. 10  shows how the system of  FIG. 8  can be expanded to increase the system&#39;s air flow and cooling capacity. 
           [0027]      FIG. 11  shows an alternate embodiment of the system of  FIG. 8  wherein the chiller has been replaced by an indirect evaporative cooling system. 
           [0028]      FIG. 12  shows a detail of the membrane mass and heat exchanger tub support structure of  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION 
       [0029]      FIG. 1  depicts a new type of liquid desiccant system as described in more detail in U.S. Patent Application Publication No. US 20120125020, which is incorporated by reference herein. A conditioner  101  comprises a set of plate structures that are internally hollow. A cold heat transfer fluid is generated in cold source  107  and entered into the plates. Liquid desiccant solution at  114  is brought onto the outer surface of the plates and runs down the outer surface of each of the plates. The liquid desiccant runs behind a thin membrane that is located between the air flow and the surface of the plates. Outside air  103  is now blown through the set of wavy plates. The liquid desiccant on the surface of the plates attracts the water vapor in the air flow and the cooling water inside the plates helps to inhibit the air temperature from rising. The treated air  104  is put into a building space. 
         [0030]    The liquid desiccant is collected at the bottom of the wavy plates at  111  and is transported through a heat exchanger  113  to the top of the regenerator  102  to point  115  where the liquid desiccant is distributed across the wavy plates of the regenerator. Return air or optionally outside air  105  is blown across the regenerator plate and water vapor is transported from the liquid desiccant into the leaving air stream  106 . An optional heat source  108  provides the driving force for the regeneration. The hot transfer fluid  110  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  102  without the need for either a collection pan or bath so that also on the regenerator the air flow can be horizontal or vertical. An optional heat pump  116  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  107  and the hot source  108 , which is thus pumping heat from the cooling fluids rather than the desiccant. 
         [0031]      FIG. 2  describes a 3-way heat exchanger as described in further detail in U.S. patent application Ser. No. 13/915,199 filed on Jun. 11, 2013, Ser. No. 13/915,222 filed on Jun. 11, 2013, and Ser. No. 13/915,262 filed on Jun. 11, 2013, which are all incorporated by reference herein. A liquid desiccant enters the structure through ports  304  and is directed behind a series of membranes as described in  FIG. 1 . The liquid desiccant is collected and removed through ports  305 . A cooling or heating fluid is provided through ports  306  and runs counter to the air stream  301  inside the hollow plate structures, again as described in  FIG. 1  and in more detail in  FIG. 3 . The cooling or heating fluids exit through ports  307 . The treated air  302  is directed to a space in a building or is exhausted as the case may be. 
         [0032]      FIG. 3  describes a 3-way heat exchanger as described in more detail in U.S. Provisional Patent Application Ser. No. 61/771,340 filed on Mar. 1, 2013, which is incorporated by reference herein. 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 . 
         [0033]      FIG. 4  shows an example of a commercial or industrial building  403  rooftop. Some of the existing Roof Top Units (RTUs)  401  are kept in place and modified to provide only sensible cooling and are further modified to no longer accept outside air. A few (typically 1 in 3 to 1 in 5) of the unitary rooftop units  402  are to be replaced with new liquid desiccant air conditioning (LDAC) dedicated outdoor air units (DOAS). The replacement units  402  are chosen based on age of the equipment that they are replacing as well as based on the air distribution requirements to ensure that fresh air is distributed evenly in the space. 
         [0034]      FIG. 5  shows a schematic diagram of a typical RTU  401  installed on a building  403 . The RTU will have between 10 and 25% outside air  503  and provide about 300-400 Cubic Feet per Minute (CFM) of total air flow per ton of cooling capacity. A typical 10 ton RTU will thus provide about 3,000 to 4,000 CFM of total air  505  with 300 to 1,000 CFM of outside air mixed in. It is well known that the outside ventilation air can represent more than 60% of the humidity load in grocery stores. (ASHRAE 2012 Handbook of HVAC Systems and Equipment, Chapter 24, p. 24.10). The air  505  supplied to the space is nearly 100% saturated unless some form of reheating is employed. Reheating however adds a significant heat load to the cooling coil since a lot of air  501  that is returned to the RTU  401  is internally directed  504  to the cooling coil and a small portion  502  is exhausted, typically about the same amount as the RTU took in. The evaporator coil  506  is providing the primary cooling function of the mixed air stream  503  and  504 . Compressor  507  is providing a refrigerant  508  and rejecting its heat to condenser  509 . A typical condenser will have some 800 CFM of outside air  510  per ton of cooling, or about 8,000 CFM for a 10 ton unit. An expansion valve  511  provides the cold liquid refrigerant to the evaporator coil  506 . 
         [0035]      FIG. 6  shows how the RTU  401  of  FIGS. 4 and 500  can be modified and supplemented by a liquid desiccant dedicated outdoor air system  402 . The RTU  401  has been modified to no longer provide or take in outside air. As a result only return air  501  from the building is recirculated  601  through the evaporator coil  506 . Alternatively, the RTU is modified to reduce the intake of outside air. The evaporator temperature has also been increased from a normal 40 F to about 50-60 F. There are several ways that this can be accomplished: one can replace the expansion valve  511  with a different valve  610  set for a higher evaporator temperature. Other ways to increase the evaporator temperature is for example to provide an APR bypass valve made by Rawal Devices, Inc., Woburn, Mass. 01888-0058. By increasing evaporator temperature the cooling load of the remaining RTU is reduced and the system operates more efficiently. 
         [0036]    One of the RTUs is replaced with a liquid desiccant system  402  as discussed earlier. The main liquid desiccant system components are the conditioner  603 —which can be like component  101  in  FIG. 1  and the regenerator  606 —which can be like component  102  in  FIG. 1 . An optional compressor  609  pumps heat from the conditioner to the regenerator using refrigerant  608  and using expansion valve  610 . Outside air  605  is brought through the conditioner  606  and supplied  607  at a lower temperature and humidity than the space requires. Return air  602  enters the regenerator  606  where it picks up heat and moisture after which is exhausted  604 . (In case return air is unavailable for the liquid desiccant system  402 , the air stream  602  can comprise outside air.) The liquid desiccant system is sized in such a way that it provides all of the outside air as was previously provided by the recirculating RTU&#39;s  401 . Since the LDAC is providing dry, cool air, the space itself is much drier which will reduce the load on refrigeration equipment and freezers in the space. 
         [0037]      FIG. 7  shows a psychrometric chart of the processes involved in the recirculating RTU and the liquid desiccant system. A conventional RTU takes in 10-25% of outside air (“OA”) and mixes that air with return air from the building (“RA”). The resulting mixed air (“MA”) point is determined by the amount of outside and return air that is combined. The cooling coil  506  subsequently takes the mixed air (“MA”) and cools it to the saturation line where water vapor condenses out and ultimately supplies air to the space at low temperature but near saturation level (“COIL”). This air needs to be heated by the building somehow, which on hot sunny days can be naturally done, but on cloudy days or mid temperature days may not occur unless an additional reheat system is employed. In supermarkets, grocery stores and the like, the refrigeration cases and freezers can provide an additional cooling effect, indicated by the arrow (“FR”) from the return air position (“RA”). As can be seen in the figure, the additional sensible cooling provided by the freezers and refrigerators and a lack of reheat, result in a space that is too cold and too humid, with relative humidity exceeding 70%. Furthermore, water sprayers in vegetable sections and short cycling of the RTU&#39;s make this situation even worse. 
         [0038]    The liquid desiccant air conditioning system of  FIG. 6  however, also takes in outside air (“OA”) and produces cooler, dry air (“DA”) to the space. The additional cooling by the remaining RTUs and freezers and refrigerators (“RTU”) results in a much smaller increase in relative humidity which can be avoided by simply not operating the RTU&#39;s unless necessary. 
         [0039]      FIG. 8  illustrates an embodiment of a liquid desiccant air conditioning system (LDAC)  402  that is able to provide cool, dry air to a space from 100% outside air. Several of the components in the system of  FIG. 8  have been identified in  FIG. 6 . The conditioner modules  603  (of which there are 4 in this example), contain the membrane plate structures as were shown in  FIGS. 1-3 . Similarly the regenerator modules  606  (of which there are also 4 in this example) have similar construction to the conditioner modules. Outside air  605  is entering the conditioner section through louvers  802 . The outside air is then transported through optional internal ducts  806 , downward through the conditioner modules  603  and then through the tub modules  803  to exit the system as supply air  607 . The return air from the building (not shown) receives some additional outside air  805  through louvers  807 . This air is then transported through regenerator modules  606  and regenerator duct modules  812  and eventually exhausted out of the system (not shown). A power interface module  801  and an integral chiller/heat pump system  609  provide electrical facilities and hot and cold water for the regenerator and conditioner modules respectively. As shown in the figure, the system has 4 conditioner and 4 regenerator modules, mounted 2 at a time on tub supports  803 . The size of the modules was chosen such that they are able to fit through standard roof access hatches. As can be seen from the figure, it would be very easy to add additional tub modules  803  and membrane conditioner or regenerator modules  603  and  606 . The right side of the system of  FIG. 808  is terminated by removable end plate  800  for the conditioner tub modules, removable end plate  810  for the conditioner duct  806 , and removable end plate  809  for the regenerator duct. Cold water supply and cold water return and hot water supply and hot water return are shown as item  811  in the figure. The louver  807  is mounted to the last tub module and is easily removed and mounted on a different tub module. Also shown is one of the desiccant pumps  813  which will be discussed in more detail under  FIG. 900  and  FIG. 12 . The whole system is mounted on a modular support frame  804 . 
         [0040]      FIG. 9A  illustrates the conditioner side of the system of  FIG. 8 . As mentioned before, outside air  605  enters the system through louvers  802 . Fan  901  brings the air through ducts  806 . Conditioner membrane modules  603  cool and dehumidify the air stream which is transported through tubs  803  into the supply air stream  607 . End plates  808  and  810  terminate the system. Water lines for cold water supply and return  811  bring cold water to the individual conditioner modules  603 . For clarity only one of the water lines  904  is shown, the other modules  603  receive cooling water in a similar fashion. The desiccant pump  813  receives liquid desiccant from the tub modules  803 . The pump distributes the liquid desiccant to the conditioner modules  603  through supply lines  905 . For clarity the desiccant supply lines for two of the conditioner modules are shown in the figure and the remainder has been omitted. As can be seen in the figure, the desiccant drains out of the conditioner modules back into the tub modules  803 . 
         [0041]      FIG. 9B  shows—similarly to FIG.  9 A—the main components of the regenerator side of the system of  FIG. 8 . Return air  602  from the building is directed through the tub modules  803  and through the regenerator modules  606 . The regenerator ducts  812  conduct the air streams back through fan  902  and louver  903  where the hot, humid air  604  is exhausted. Since in buildings the amount of available return air can be less than the amount of air that is supplied to the building (supply air  607  is larger than return air  602 ) and additional outside air stream  805  can be mixed in through louver  807 . This helps to ensure that the system has adequate air supply for the regenerator modules. Similar to the conditioner side, desiccant pump  908  provides liquid desiccant to the regenerator modules  606  through supply lines  907 . Hot water  906  is also supplied to the regenerator modules. For clarity only some of the water and desiccant lines have been shown. 
         [0042]    It is also possible to reverse the direction of the chiller  609  in a winter operating mode so that the conditioner  603  receives hot heat transfer fluid and the regenerator  606  receives cold heat transfer fluid. In this mode the conditioner will desorb water vapor and humidify and heat the supply air stream  607  and the regenerator will absorb heat and water vapor from the return air stream  602  from the space. In effect the system will recover heat and moisture from the return air stream  602  in this mode. 
         [0043]      FIG. 10  show the system of  FIG. 8  with an additional section  1001  comprising 4 conditioner and 4 regenerator modules have been inserted in the system of  FIG. 8 . The chiller  1002  as well as the fans and water pumps (not shown) now need to be sized to accommodate for the increase in air flow and cooling load of the system. It will be apparent that one can continue to increase the air flow and cooling capacity of the system by continuing to add membrane models and other components, at least until the air flow capacity of the ducts and tubs is exceeded. 
         [0044]      FIG. 11  shows a schematic diagram of an alternate embodiment of the linkable system of  FIG. 6 . The chiller section  609  from  FIG. 6  and  FIG. 8  has now been omitted in favor of an indirect evaporative cooling section  1111 . The supply air  607  is partially diverted (typically between 0 and 30%) as air stream  1105  into duct  1101  and louver  1102  to enter tub  1103 . The air stream now moves upward through membrane modules  1106 . But unlike the conditioner modules and regenerator modules, these evaporator membrane modules have water rather than desiccant behind their membranes. Since the air stream  1105  is very dry, a large cooling effect can be obtained in the membrane modules  1106  by evaporating the water behind the membranes. This in turn results in the heat transfer fluid  1109  becoming substantially cooled. This cold heat transfer fluid  1109  can then be used to remove heat from the original membrane modules  603 . The warmer heat transfer fluid  1110  is circulating back to the indirect evaporative cooling section  1111  from the conditioner modules  603 . Since the evaporator modules  1106  evaporate water, there needs to be a constant supply of water  1113 . This water can be clean drinking water, in which case the membranes on the evaporator modules  1106  are not absolutely necessary. In that case also, the remaining water can be drained  1115  from the membrane modules  1106  into the tub  1103  and removed from the tub by pump  1112  to be re-used at the top of the evaporative modules  1106 . Attention has to be paid to ensure that like a conventional cooling tower, there is no buildup of scale and other contaminants. There are several methods common in the industry that can be employed to deal with scale issues, for example blow-down systems or ultrasonic precipitation. 
         [0045]    However, the use of membrane in evaporator modules  1106  also enables the use of seawater or waste water: the membranes will contain any salt particles or other contamination. In this case, the intent is to evaporate only a portion (typically around 50% or less) of the water supplied by supply  1113 . The concentrated remaining water is then drained through line  1114  and disposed of in an appropriate drainage system. The pump  1112  can now be omitted and no scaling or blow-down system is required. However membrane fouling may become an issue and can be dealt with using flushing and a proper pre-filtration system. The exhaust air stream  1108  leaving the evaporator modules  1106  is warm and near saturation and is pulled through the system by fan  1107 . It should be clear from the figure that when one adds additional conditioner modules  603  there should also be additional evaporator modules  1106 . This can be easily accomplished by removing cover  1104  and adding an additional section  1111 . Fan  1107  would also have to be sized larger and moved to the added section. 
         [0046]    It is also possible to reverse the air stream  1105  while at the same time providing hot heat transfer fluid to the conditioner blocks  603 . In this winter heating mode, the conditioner will desorb water vapor into the air stream  1105  and the conditioners  603  will combine to supply warm, moist air to the space  607 . 
         [0047]      FIG. 12  illustrates a detail cross section of the membrane module support tub  803  and part of the desiccant distribution system connected to it. The tub  803  is constructed as a hollow shell structure with walls  1205  and  1206 . The inner area  1201  functions as a liquid desiccant storage tank. This is beneficial since it eliminates the need for a separate tank and since the volume is located directly below the membrane modules, syphoning of desiccant into the tank structure is enhanced. Furthermore the tank structure allows for stratification of the desiccant wherein the higher concentration of desiccant can be found near the bottom of the tub and the lower concentration can be found near the top. The inner bottom  1202  of the tub  803  is slowed in such a way that any leaks from the membrane modules above will drain into a single corner where a detector or sensor can be located to indicate that a leak has occurred. Furthermore bottom has a lip  1208  constructed in such a way that the air stream cannot transport any droplets that may fall from the membrane modules. The membrane modules physically sit on the support plate  1203  which has designed in rail features  1209  to allow for a tight air seal between the membrane modules and the tub structure. Since the tub  803  contains the desiccant for the system, pump  908  pulls desiccant from the lower port on the tub, pumps it to the top of the membrane module  603  wherefrom it drains by gravity back through drain  1204  into the top port of the tub. A secondary port  1207  allows for dilute desiccant to be removed and pumped to the regenerator modules which are set up in a similar fashion except that the regenerator pumps from the top port to the top of membrane modules  606  and removes concentrated desiccant from the bottom port back to the conditioner. The air duct  1210  can also be seen in the figure. 
         [0048]    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.