CROSS-REFERENCE TO RELATED APPLICATION

[0001 ] This application claims priority to U.S. Provisional Application No. 62/077,335 filed on November 10, 2014, the entire contents of which are hereby incorporated herein by reference.

TECHNICAL FIELD

[0002] The present disclosure relates to HVAC (heating, ventilation and/or air conditioning) technology.

BACKGROUND

[0003] International Publication No. WO 201 1/161547 A2 describes a liquid-to-air membrane energy exchanger. The exchanger includes a housing having a front and a back. A plurality of panels forming desiccant channels extend from the front to the back of the housing. Air channels are formed between adjacent panels. The air channels are configured to direct an air stream in a direction from the front of the housing to the back of the housing. A desiccant inlet is provided in flow communication with the desiccant channels. A desiccant outlet is provided in flow communication with the desiccant channels. The desiccant channels are configured to channel desiccant from the desiccant inlet to the desiccant outlet in at least one of a counter-flow or cross- flow direction with respect to the direction of the air stream.

[0004] International Publication No. WO 2013/029148 A1 describes an energy exchange system for conditioning air in an enclosed structure. The energy exchange system includes a supply air flow path, an exhaust air flow path, an energy recovery device disposed within the supply and exhaust air flow paths, and a supply conditioning unit disposed within the supply air flow path. The supply conditioning unit may be downstream from the energy recovery device. A method of conditioning air includes introducing outside air as supply air into a supply air flow path, pre-conditioning the supply air with an energy recovery device, and fully-conditioning the supply air with a supply conditioning unit that is downstream from the energy recovery device. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] The drawings included herewith are for illustrating various examples of apparatuses and methods of the present disclosure and are not intended to limit the scope of what is taught in any way. In the drawings:

Figure 1 is a schematic view of a first example of an energy exchange system for conditioning air in an interior space of a structure;

Figure 2 is a schematic view of a second example of an energy exchange system for conditioning air in an interior space of a structure; and

Figure 3 is a detailed schematic view of an auxiliary loop of the systems of Figure 1 or Figure 2.

DETAILED DESCRIPTION

[0006] Various apparatuses or methods are described below to provide an example of an embodiment of each claimed invention. No embodiment described below limits any claimed invention and any claimed invention may cover apparatuses and methods that differ from those described below. The claimed inventions are not limited to apparatuses and methods having all of the features of any one apparatus or method described below or to features common to multiple or all of the apparatuses or methods described below. It is possible that an apparatus or method described below is not an embodiment of any claimed invention. Any invention disclosed in an apparatus or method described below that is not claimed in this document may be the subject matter of another protective instrument, for example, a continuing patent application, and the applicant(s), inventor(s) and/or owner(s) do not intend to abandon, disclaim or dedicate to the public any such invention by its disclosure in this document.

[0007] Semipermeable desiccant liquid-to-air membrane exchangers that transfer both heat and water vapor have the potential to provide better thermal comfort conditions for building occupants, reduce operating costs for HVAC systems, and/or be used for HVAC retrofit and restricted HVAC applications where current technologies cannot be used. These membrane exchangers may be designed, constructed and operated so that they will have high performance factors, while the construction costs are generally competitive with other exchanger technologies.

[0008] Nonetheless, under certain exchanger dewatering or desiccant liquid regenerating operating conditions (i.e. evaporation of water from a salt solution used as the desiccant liquid in the exchanger), these membrane exchangers may experience significant blockage of the membrane pores due to crystal growth at the air-liquid interface in each pore. When extensively distributed over the exchanger membrane surfaces, this phase change crystallization in the salt solution may cause the moisture transfer effectiveness of the exchanger to decline significantly. As well, it may reduce the sensible energy effectiveness, which may be a consequence of its coupling with latent effectiveness. For a given system employing membrane exchangers, these reductions in effectiveness may increase over time, even though the inlet conditions for the air and liquid may be steady.

[0009] This problem may be reduced by selecting a salt solution or mixture of salt solutions that is well suited to the operating conditions under which the exchanger will operate during each day and month of the year. This problem may also be reduced by selecting semipermeable membranes that are most suitable for integration into the design of heat and water vapor transfer exchangers. Furthermore, this problem may be reduced by using system designs and controls that are well suited to avoid crystallization problems. Moreover, this problem may be reduced by sizing and operating a regenerating exchanger at conditions where the salt solution does not, at any point in the exchanger or the system, approach the salt solution saturation conditions too closely for the design flux of heat and water vapor. That is, crystallization of the salt will generally occur at the air-liquid interface or meniscus in those regions of the exchanger and at adjacent membrane surface roughness imperfections where the liquid salt concentration is close to saturation conditions, the desiccant solution temperature is low, the temperature of the salt solution is much higher than the air temperature, and the flux of heat and water vapor out of the liquid interface is large.

[0010] Predicting the exact conditions for crystallization in a membrane exchanger may be difficult because there are uncertainties in fluid and crystallization properties, and there are many operating conditions and properties that must be known. As well, the water purity in the desiccant solution may be high to avoid hard water salt depositions on heated exchanger surfaces. Such deposits may also degrade the performance of exchangers. Extensive testing and data collection may be necessary to provide a complete set of operating conditions for each membrane and inlet fluid properties, and each of their uncertainties that will avoid crystallization and blockage of membrane exchangers used for moisture removal from salt solutions near saturation conditions. However, without these data, but using the teachings herein to avoid crystallization and the known properties of salt solutions for each phase of the solution at or near equilibrium, it may be possible to deduce: (1 ) the type of salt that should be used for the salt solution; (2) the type and characteristics of semipermeable membrane that should be used in each membrane exchanger; (3) the most effective configurations of the HVAC system and its operating conditions; and (4) the size (i.e. surface area) or capacity of each membrane exchanger that is most likely to avoid crystallization in each membrane exchanger.

[001 1 ] For open surface or direct contact salt solution exchangers (i.e. with no membrane to separate direct contact between the salt solution and the air), cross-flow exchangers and some tower or liquid drain down exchanger beds, which may have significantly lower performance factors than counter flow exchangers, control of mal-distributed liquid flows may not be attempted. Low effectiveness factors may be experienced (e.g., 20 to 60% effectiveness) for direct contact exchangers, and lower limits for the heat and moisture fluxes may be required to avoid some of these low performance factors. That is, membrane exchangers may not only eliminate the risk of downstream salt drift in the air, they may have high performance factors (e.g. , 70 to 90% effectiveness) when they are well designed and operated at the most favorable conditions.

[0012] Nonetheless, this high performance may come with a risk of desiccant salt crystallization at the air-salt-solution interface within the membrane pores. Generally, these membrane exchangers are tested at selected operating conditions when the risk of super-saturation and crystallization may not be a problem; but they may not have been extensively tested when these risks are high. Generally, the risks will be greatest during the most extreme outdoor air operating conditions (i.e. when the HVAC system for buildings has greatest loads for heating in winter and cooling and dehumidification in summer). That is, just knowing what the performance factors are for these membrane salt solution coupled systems under some typical outdoor and indoor operating condition (e.g. , AHRI Standard 1060 test condition for air-to-air heat/energy exchangers) may not be enough; the HVAC system has to perform well at the typical most extreme outdoor and indoor conditions for each application (i.e. when the risk of crystallization is greatest). It is only when the operating conditions are at their extreme loading conditions that the load capacity of an HVAC system may be determined for any application.

[0013] Various salt solutions have somewhat similar thermodynamic properties and phase diagrams. Depending on the concentration of the salt, they may freeze at temperatures below 0 °C (e.g., LiCI solutions will not freeze above -50 °C for mass concentrations above 20% salt/solution), and they may have vapor pressures in air that will be less than atmospheric pressure unless its temperature is above boiling (100 °C). Between these temperatures, the equilibrium vapor pressures may be nearly, but not exactly, coincident with the relative humidity lines on a psychrometric chart for air and water vapor. Salt solutions at or near thermodynamic equilibrium will not experience nucleation and crystallization unless the salt solution saturation (i.e. S=C/C _S where C is mass concentration of salt in a unit mass of solution and the subscript "s" denotes the salt saturation condition) rises above 1 .0 [i.e. either due to the evaporation of water from an exposed meniscus (or air-liquid interface), or due to the cooling of the interface or other surfaces bounding the desiccant liquid (i.e. as on a heat exchanger surface)]. When the surrounding pressure is nearly constant, C _s may only be a function of temperature (i.e. C _S (T)), and C _s generally increases with liquid temperature, with the consequence that the relative humidity in any adjacent air at the same temperature at which crystallization may occur decreases with increasing ambient air temperature. At non-equilibrium conditions within membrane exchangers, one part of the membrane exchanger desiccant liquid may be at very different operating conditions than another part of the exchanger, and the solution phase diagrams and properties superimposed on a psychrometric chart may not be strictly correct (i.e. deviations from equilibrium conditions may be large).

[0014] Although nucleation sites for crystallization processes may occur anywhere within a saturated salt solution, but for small flow channels within membrane exchangers, preferred locations are generally at interface locations at air-liquid interfaces or meniscus and solid heat exchanger surfaces where there are finite fluxes of water vapor and heat out of the liquid (unless the solution is simultaneously heated). Once a nucleation site, for instance on the liquid side of a meniscus, is formed, it becomes the chemically preferred site for further crystal growth (i.e. the threshold chemical potential for super-saturation and formation of nucleation sites will be slightly higher than the super- saturation needed for continuous growth to the crystal at the site). That is, beyond a certain critical diameter of a spherical nucleated particle size (e.g. , 10 to 100 nm) in a pore, further crystal growth generally takes place with decreasing Gibbs function toward equilibrium (i.e. with decreasing solution temperature and increasing evaporation). Crystallization is a phase change process, so when it occurs, the local temperature of the solution and the crystal tends to decrease slightly and the surrounding concentration of the solution tends to decrease slightly. This decrease in the local temperature and concentration, surrounding the nucleated crystal, generally results in micro- diffusion processes of heat and salt surrounding each nucleated crystal. The crystals formed will generally not be pure salt; rather, they will be salt molecules plus hydrated water molecules (e.g. , for aqueous LiCI, LiCI:H ₂ 0 will form above 20 °C, and LiCI:2H ₂ 0 will form below 20 °C). This hydration of water will generally cause the crystal size to increase and alter the local or micro-concentration of salt more below 20 °C than above (i.e. larger crystals below 20 °C may be a consequence). Although the density of the crystallized salt may be slightly higher than the surrounding solution, surface tension forces tend to hold the nucleated crystals on the meniscus surface and on membrane solid surfaces. Thus, surface roughness on the solid surfaces may generally make preferred sites for nucleation to start.

[0015] LiCI and LiBr salt solutions may be used for most HVAC applications having liquid salt-solution desiccants. LiCI solution and LiBr solution have relatively low saturation curves when superimposed on the psychrometric chart, and may co-exist with air at low relative humidity at equilibrium. That is, air properties anywhere above the LiCI or LiBr saturation curves on the psychrometric chart may be achieved or maintained in buildings using LiCI or LiBr solutions in HVAC systems. Other types of salt solutions that may also be used are mixtures of either LiCI or LiBr, and other salts such as MgCI ₂ , CaCI ₂ , NaCI, etc. However, eutectic mixtures of these other salts may differentially crystallize each of the salt species when they are cooled while the concentration is high, which may make them a poor choice when there is a risk of crystallization. Indeed, impurities in the desiccant liquid may potentially result in an increased risk of crystallization. Only a few climates have outdoor air properties that are not in the region for S>1 .0 for LiCI or LiBr (i.e. hot and dry and cold and dry), and virtually all indoor air properties in the comfort range for building occupants have S<1 .0. The salt solution conditions inside a semipermeable membrane exchanger may differ from the outdoor or indoor inlet air conditions because there will be heating and cooling, phase change due to evaporation or condensation of water, and perhaps the injection of water or a weak saline solution. At low fluxes for heat and water vapor transfer, the salt solution in exchangers may be close to equilibrium conditions but at high fluxes it may not, and this may cause both the region of hysteresis for crystallization and dissolution of existing crystals and the bounds of uncertainty for the onset or dissolution of crystallization to grow with the mass flux of water vapor and heat flux out of the solution.

[0016] Looking at typical hourly data for outdoor air conditions for a hot and dry city, such as Phoenix, AZ, for example, over one typical metrological year (TMY), using MgCI ₂ solutions exposed to the ambient air may result in crystallization over a large fraction of the range of outdoor air temperatures, because the minimum air humidity is well below the approximate saturation line (which is approximately 33% RH at room temperature, for example). However, the saturation line will not be similar to the curves for relative humidity on the psychrometric chart (i.e. the slope of the saturation concentration line, dCs/dT, will be significantly smaller than any of the corresponding concentration curves, dC/dT, at the same temperature, T, or some constant concentration curves, C, will intersect the saturation curve, Cs, as the temperature of the solution is decreased). This means that unless the solution saturation, S, is significantly less than 1 .0, a cooling process (with no water vapor transfer) in the liquid may cause some crystallization. This phenomenon may be seen in the phase diagram for an aqueous solution of LiCI, which shows that cooling an initially sub-saturated solution with C>0.3 (30%) will, at equilibrium, eventually cause crystallization. For example, cooling a LiCI solution from an initial temperature of 50 °C and concentration of 40% (i.e. saturation S=0.85) will, at equilibrium, initiate crystallization (i.e. S=1 .0) when the solution temperature drops below 0 °C. Below 0 °C and above -17.26 °C, the crystals formed in the solution will generally have a hydrated structure (LiCI:2H ₂ 0), or for each LiCI molecule there will be two hydrated water molecules.

[0017] Cold outdoor air that is heated without adding moisture will remain dry with the same moisture content as the outdoor air (i.e. the humidity ratio, W, will be constant without moisture transfer or phase change). This implies that just heating cool or cold outdoor air in a membrane exchanger will increase the risk of crystallization when it is exposed to a desiccant liquid at a higher temperature in a membrane exchanger, because the desiccant liquid will drop in temperature, and moisture transfer will increase the salt concentration, and so for the salt solution both the heat and moisture transfer will move the solution concentration, C(T), closer to the saturation concentration line, Cs(T). To avoid this problem, exhibited by measured data, the inlet solution concentration may be known to be sufficiently low (i.e. S<1 .0) at the exchanger inlet so that the solution will not come too close to the equilibrium saturation line anywhere within the exchanger, including the membrane pores [e.g. , C(T _in to T _0U t)<Cs(Tin to Tout) or S<1 .0 (inlet to outlet), including the liquid in the membrane pores]. Thus, knowing the conditions at which crystallization is a risk may guide the exchanger design process, and identify conditions which may have a small risk of decreased performance, so the exchanger will generally recover its full performance with a slight improvement in operating conditions provided hysteresis effects are relatively small.

[0018] Without exposure to air, confined salt solutions in a closed tube, vessel, or heat exchanger may only change their concentration by diffusion or convection, which for closed systems will not change their solution concentrations unless the saturation condition exceeds 1 .0, as it may when the solution is cooled (i.e. for most saline solutions near equilibrium conditions, S=1 .02 will tend to cause crystallization given sufficient time, and S=0.98 will tend to cause dissolution of existing crystals to occur, when they are near equilibrium conditions and hysteresis is small). For rapid changes in temperature, significant temperature gradients in the solution may accompany the heating or cooling of the solution. As a consequence, saturation conditions may exist in one part of the flow [e.g., near a heat exchanger (metal or membrane) surface while cooling the solution flow], but not elsewhere in the flow. That is, crystallization may occur on heat exchanger surfaces and within membrane pores. This crystallization, when extensive, may diminish the effectiveness of the exchanger. To avoid this problem, the heat exchanger may be designed with sufficient surface area so that the heat flux is not too large and the surface temperatures of the exchanger are not too low at any location in the exchanger. [0019] These principles, related to salt solutions, may be used to aid in the design and operation of air conditioning systems, particularly as it relates to the positioning or layout of components in the liquid flow loops, and especially for membrane exchangers used for regeneration or dewatering of the solution, and further, as noted above, for heat exchangers used which cool the solution flows. For air conditioning in summer in hot, humid climates, dewatering of the salt solution may be done at high temperatures and at a condition where high exchanger water vapor fluxes do not cause crystallization in the membrane pores, and significant surface area blockage, to achieve a high concentration salt solution in a regenerator exchanger, and then this high concentration salt solution may be cooled to a desired inlet temperature to both cool and dehumidify the supply air in the supply air conditioning exchanger. High fluxes of water vapor while the salt solution concentration is in close proximity to the equilibrium saturation concentration (i.e. 1 -S is small) during dewatering or regeneration of the salt solution may increase the risk of crystallization and blockage of the membrane pores. Since crystallization processes at non- equilibrium conditions may have significant uncertainties, spatially and temporally, and the micro surface geometry of the membrane may be a factor, predicting the effectiveness of exchangers in which this occurs may be difficult because, for a given set of operating conditions, there may not be a unique value. Thus, for any HVAC system with air conditioning, this unpredictable operating condition should be avoided for all typical weather conditions for each application.

[0020] Two exemplary types of HVAC systems may employ permeable membrane exchangers to transfer heat and water vapor between salt solutions and air flows: (1 ) passive energy recovery systems, which recover the available energy from exhaust air and is stored in a solid or liquid and then used to condition or partly condition supply air; and (2) active air-conditioning systems, which employ several devices or components and, through energy transfer from external sources, may be used to condition the inlet air to selected supply air conditions of flow, temperature and humidity. [0021 ] For example, and not intended to be limiting, a counter-cross-flow liquid-to-air membrane energy exchanger (LAMEE) arrangement in a passive run-around membrane energy exchanger (RAMEE) system, which conditions or partly conditions the supply air for a building space, is disclosed in Ge et al., "Analytical model based performance evaluation, sizing and coupling flow optimization of desiccant liquid run-around membrane energy exchanger systems", Energy and Buildings 62 (2013) 248-257, and the entire contents of which are hereby incorporated herein by reference. Also, exemplary construction details of LAMEEs are disclosed in International Publication Nos. WO 201 1/161547 A2 and WO 2013/029148 A1 , and the entire contents of each are hereby incorporated herein by reference.

[0022] Passive desiccant liquid energy exchange systems may contain only one supply and one exhaust LAMEE unit in its simplest configuration, but it could be designed to handle several exhaust and supply units. The RAMEE system is passive in the sense that, if these exchangers each have an inlet airflow, and a low power pump or pumps are used to circulate the desiccant liquid through these permeable membrane exchangers and the connecting tubing in a closed loop, heat and moisture may be exchanged between the exhaust and the supply airflows coupled by a closed loop desiccant liquid flow. That is, this passive system may not be entirely free of external energy inputs, and there may be a need for some external water injection to avoid crystallization in one of the solution flow exchangers during supply air humidification, but these auxiliary inputs may be relatively small compared to the mass of air circulated and the energy transfer rate between the two airflows [i.e. the coefficient of performance (COP) at design conditions may be high; e.g., COP>40 where COP is defined as the ratio of energy saving rate for the two air flows and the energy input rate for pumps, fans, and controls].

[0023] Using the physical principles described above, these passive systems may be most likely to experience crystallization when the supply inlet air is very dry (Wi<3 g/kg) and cold (Ti<0.0 °C), and the exhaust air conditions are dry and warm or typical (e.g., W ₃ <6 g/kg and T ₃ «20 °C), and this possibility may increase when the liquid mass circulation rate is low compared to the air mass flow rates (m _r *<1 .0 or Cr*<3.0), the exchanger at risk of crystallization (i.e. the exhaust exchanger) area is significantly smaller than the supply exchanger area, and the temperature of any liquid make-up water or salt solution is low (T _H 2o<5 °C). These conditions of operation are not atypical for cold or hot dry climates, and they may occur for any passive system operating in a wide range of climates if the volume of water in the loop decreases significantly (i.e. C increases close to CS or 1 -S is small), caused by a net rate of water vapor addition from the supply exchanger to the indoor supply air.

[0024] Outdoor air conditions will often be dry and cold for many cities, including much of North America. Hourly weather data may be ignored for those conditions when the RAMEE system is not in operation (i.e. when the economizer bypass is used for the airflows because the COP is low and will become less than 1 .0 when the outdoor air conditions are close to the thermal comfort conditions for the building occupants). For a significant fraction of time, the economizer cycle (i.e. the ventilation air flow may mostly bypass the HVAC system) may be used to reduce energy consumption for most climates, provided the HVAC system has an appropriate capacity and controls. Cold climates, e.g., Saskatoon and Chicago, may have a significant fraction of hours when the humidity ratios, W, are less than 3 g/kg (i.e. a high risk of crystallization region), and other areas may have a significant fraction of hours when 80 to 100% relative humidity for outdoor temperatures above the economizer operating region (e.g., 20 to 25 °C), which is another region of risk for crystallization for air conditioning systems because relatively large moisture content and mass flow rates must be removed from the ventilation air before it is supplied to the occupied space. Below the economizer temperature range region (e.g. , 15 to 20 °C), most cities have significant fractions of hours in the 80 to 100% relative humidity region.

[0025] To avoid or reduce the risk of crystallization for a passive energy exchange system, the passive system may be designed: (1 ) to use relatively pure LiCI or LiBr salt solutions; (2) the selected semipermeable membrane materials used in the LAMEEs should not only have good permeability, heat transfer and strength properties, but also smooth fiber surfaces within the membrane at the air-liquid solution interface; and (3) the surface areas of the membrane in each LAMEE should be large enough to not result in excessive water vapor mass evaporation and heat transfer fluxes. When the passive system has a semipermeable membrane exchanger operating in a near steady state condition near the saturation curve (i.e. 1 -S is not too small), crystallization blockage may be avoided by: (i) injecting water or a low saturation salt into the exchanger solution upstream of the exchanger (e.g., into the storage tank where it may be fully mixed to avoid the risk of freezing the unmixed salt solution); (ii) increasing the rate of desiccant liquid flow or decreasing the air flow rate (i.e. increasing Cr*); (iii) increasing the inlet desiccant liquid temperature, T| _iq (i.e. so that the concentration difference between the solution and the saturation conditions [i.e. Cs(T)-C(T) or (1 -S)] increases); and (iv) designing the system exchanger surface area such that the exchanger with the risk of crystallization has a larger membrane area than the other exchanger (i.e. decrease the flux of heat and water vapor for the exchanger at most risk). These approaches to decrease the risk of crystallization, except (iv), involve external inputs, which generally makes the system less passive during the time durations of intervention or control to avoid crystallization.

[0026] For active energy exchange systems, which control the supply air inlet conditions for a building, or control the space air temperature and humidity conditions in situ, there may be various configurations of the system components. Referring now to Figure 1 , an example of an active energy exchange system is shown generally at reference numeral 100. The system 100 exchanges air between an interior space 102 and an outside 104 of a structure (not shown). In Figure 1 , dotted lines indicate desiccant liquid flow tubes extending between the numbered components, whereas air flow channels or ducts extending between the numbered components are illustrated to have a finite width. Both show arrow heads to indicate flow direction. [0027] In the example illustrated, first and second supply LAMEE units

106a, 106b are connected in parallel and are arranged to receive air from the outside 104. The supply LAMEE units 106a, 106b condition the outside air to form supply air, and deliver the supply air to the interior space 102. The supply LAMEE units 106a, 106b are shown to include desiccant liquid inlets 108a, 108b and desiccant liquid outlets 1 10a, 1 10b. An exhaust LAMEE unit 1 12 is arranged to receive air from the interior space 102 to form exhaust air, and deliver the exhaust air from the interior space 102 to the outside 104. The exhaust LAMEE unit 1 12 is shown to include a desiccant liquid inlet 1 14 and a desiccant liquid outlet 1 16.

[0028] An auxiliary loop 300 fluidly connects the desiccant liquid outlet 1 16 of the exhaust LAMEE unit 1 12 and the respective desiccant liquid inlets 108a, 108b of the supply LAMEE units 106a, 106b. In the example illustrated, the auxiliary loop 300 includes a regeneration LAMEE unit 302 for regenerating the desiccant liquid circulating in the system 100. Primary and secondary valves 306, 308 control flow of the desiccant liquid through the regeneration LAMEE unit 302. In a first mode, the valves 306, 308 are opened and closed, respectively, so that desiccant liquid is permitted to flow from the desiccant liquid outlet 1 16 of the exhaust LAMEE unit 1 12 to the desiccant liquid inlets 108a, 108b of the supply LAMEE units 106a 106b. In a second mode, the valves 306, 308 are closed and opened, respectively, so that the desiccant liquid is guided to flow through the regeneration LAMEE unit 302. In other modes, the primary and secondary valves 306, 308 may be controlled so that the regeneration LAMEE unit 302 receives only a portion of the flow of desiccant liquid.

[0029] In the example illustrated, the auxiliary loop 300 further includes an auxiliary dehumidifier mechanism 304 coupling the regeneration LAMEE unit 302 and the outside 104. The dehumidifier mechanism 304 serves to transfer moisture away from the regeneration LAMEE unit 302, to reduce the water content of the desiccant liquid. The dehumidifier mechanism 304 may be, for example, a desiccant-coated dehumidifier wheel. Exemplary dehumidifier wheels are commercially available from Munters AB, of Kista, Sweden.

[0030] It is evident from the hourly outdoor air TMY conditions for most cities that the outdoor air conditions will range over a wide variation for any city. However, a significant fraction of the hours will lie close to the condition when no further conditioning of the air will be necessary, so an economizer (not shown) may be used to bypass the LAMEE units so that outside air is mixed in directly to the interior space 102, while most of the time the ventilation air will need some conditioning using the system 100 to achieve the set-point ranges for the supply ventilation air temperature and humidity (i.e. T & W). It is also evident from weather data that often the outdoor humidity ratio for air will be less than about 10 g/kg when it will be likely that the regeneration LAMEE unit 302 will be used not at all, or under part load, because most of the dehumidification may be achieved using the exhaust LAMEE unit 1 12. Thus, at peak or near peak load conditions for summer, the ventilation air conditioning will only occur for a small fraction of time; however, during this time, the system 100 must still provide the required thermal comfort for the interior space 102. An advantage of desiccant systems is that they may be used to process ventilation air directly from the outdoor air conditions of T and W to the desired set-point T and W ranges (e.g., T ₂ =15 to 18 °C and W ₂ =6 to 8 g/kg in summer and T ₂ =18 to 21 °C and W ₂ =8 to 10 g/kg in winter), without extra or auxiliary heating or cooling, which generally occurs with conventional air conditioning systems and for passive desiccant systems. That is, the capacity of the system 100 at all operating conditions may be sized to be sufficient for ventilation air and occupant comfort.

[0031 ] In the example illustrated, the system 100 further includes tanks 1 18, 120 and pumps 122, 124a, 124b. A water source 126 is shown connected to the tank 1 18, to selectively increase water content of the desiccant liquid being supplied to the inlets 108a, 108b. When the system 100 is used to condition the air in the interior space 202 in situ so that room air is recirculated, the supply LAMEE units 106a, 106b may be placed at selected locations for each room or zone or region within the structure, and the air may be locally circulated through each of the supply LAMEE units 106a, 106b to achieve the desired comfort conditions for the occupants. The conditions of the desiccant liquid in the system 100 may be predetermined and set at selected mechanical rooms near external walls of the structure, so that large ducts and large flow rates of air in the structure may be generally avoided [i.e. ducts may be used for ventilation, and perhaps exhaust air only, and desiccant tubing may be used in a closed loop (or loops) to condition the air in situ]. In the mechanical rooms, outside air or direct heat may be used to heat or cool, and dewater or inject water, as needed for the desiccant liquid. The conditioned desiccant liquid will be pumped to each room LAMEE. The return desiccant liquid flow goes to the mechanical room for conditioning again in a closed loop.

[0032] The system 100 may be used to maintain in situ occupant comfort conditioning (i.e. both T and W are controlled by the operating conditions for the supply LAMEE units 106a, 106b), and may have many types of applications for buildings new and retrofit, and for applications where cross contamination between exhaust and supply air is not permitted.

[0033] Conventional HVAC systems may operate at part load conditions for most of the time for building applications in most cities (i.e. the peak load conditions will only occur for a small fraction of the hours per year, but this condition must be satisfied otherwise the inlet air conditions in the building spaces will become uncomfortable for occupants). For these conventional systems, control of comfort conditions in humid summer conditions is often a problem because, as the cooling load decreases, the on-off cycle time durations shorten because much of the condensed cooling coil water gets recycled back into the supply air. This short-cycle dehumidifying problem may be avoided with the system 100 because it will generally not exhibit any short cycles. For many structures, the supply air ventilation flow rate may differ from the exhaust flow rate. For a small fraction of hours per year, an economizer cycle may be used to bypass the supply air ventilation flow between the outside 104 and the supply LAMEE units 106a, 106b, and the exhaust flow between the interior space 102 and the exhaust LAMEE unit 1 12, because the outdoor air is close to comfort conditions. The system 100 may modulate the fraction of ventilation and exhaust air bypass to optimize these operating costs.

[0034] Although the system 100 has two supply LAMEE units 106a, 106b, one exhaust LAMEE unit 1 12, and one dewatering or regeneration LAMEE unit 302, other arrangements with more of these units may be implemented in other examples. In some examples, the auxiliary loop may include two or more regeneration LAMEE units, connected in parallel. In some examples, the auxiliary loop may include two or more exhaust LAMEE units, connected in parallel to one another. Various configurations are possible.

[0035] Referring to Figure 2, another example of an active energy exchange system is shown generally at reference numeral 200. Reference numerals are repeated in Figures 1 and 2 and in this description to indicate corresponding or analogous elements or steps. The system 200 includes a dehumidifier mechanism 228 arranged between the supply LAMEE unit 206 and the exhaust LAMEE unit 212. The dehumidifier mechanism 228 serves to exchange moisture from the air from the interior space 202 received by the exhaust LAMEE unit 212, and air from the outside 204 received by the supply LAMEE unit 206. The dehumidifier mechanism 228 may be, for example, a desiccant-coated dehumidifier wheel.

[0036] Referring now to Figure 3, further details of the auxiliary loop 300, which may be implemented in the system 100, the system 200, or another system, are illustrated. The regeneration LAMEE unit 302 is shown to include a desiccant liquid inlet 336 and a desiccant liquid outlet 338. The inlet 336 is fluidly connected to the outlet 1 16 of the exhaust LAMEE unit 1 12 (Figure 1 ), whereas the outlet 338 is fluidly connected to the inlets 108a, 108b of the supply LAMEE units 106a, 106b.

[0037] In the example illustrated, the auxiliary loop 300 further includes heat exchangers 310, 312, 314, 316, 318, 320, 322, and a heat pump 324. The heat exchangers 310, 312 may be used to increase the temperature of the desiccant liquid, before it is delivered to the regeneration LAMEE unit 302. The heat exchangers 314, 316, 318 may be used to decrease the temperature of the desiccant liquid, before it is delivered to the supply LAMEE units 106a, 106b. A third valve 326 is shown connecting the heat exchangers 310, 316, and may facilitate a tube-in-tube transfer of heat between the flow of desiccant liquid downstream of the regeneration LAMEE unit 302 and the flow of desiccant liquid upstream of the regeneration LAMEE unit 302. Reference numeral 328 indicates a waste heat sink coupled to the heat pump 324, and a power input, indicated at reference numeral 332, may also be connected to the heat pump 324. To provide heat to the air in the auxiliary dehumidifier mechanism 304, in addition to the heat exchanger 314, reference numeral 330 indicates a waste heat source, which may be derived from within the structure, e.g., from lights, electric motors, and solar energy. A low-temperature heater 334 is also shown connected to the auxiliary dehumidifier mechanism 304, and may be configured to heat the air flow up to, e.g., 100 °C before entering the auxiliary dehumidifier mechanism 304.

[0038] In the first mode, by closing the secondary valve 308 and opening the primary valve 306, the desiccant liquid bypasses most of the auxiliary loop 300. In the second mode, intended for peak summer conditions, by closing the primary valve 306 and opening the secondary valve 308, desiccant liquid is guided by the auxiliary loop 300 through the regeneration LAMEE unit 302. This may allow for control of the operating condition of the energy exchange system 100 (Figure 1 ; or energy exchange system 200 shown in Figure 2) to be at or near favorable conditions of air temperature and humidity (e.g., T| _iq >45 °C, W| _iq >6 g/kg), with moderately high fluxes for heat and moisture from the regeneration LAMEE unit 302, and the desiccant liquid is moved away from the solution equilibrium saturation curve or line. This regeneration process may help to reduce or eliminate the risk of membrane creep caused by pressure differences across the membranes at elevated operating temperatures (e.g., 45<T| _iq <50 to 55 °C). As well, very low supply air outlet conditions may be achieved (e.g., T _air <10 to 12 °C, W _air <6 to 7 g/kg), thereby minimizing the required mass flow rate of supply air. [0039] The heat exchangers 310, 312, 314, 316, 318, 320, 322 may each be liquid-to-liquid heat exchangers designed to meet the peak load and achieve a high effectiveness (e.g., ε>80-90%), and the heat pump 324 may be a liquid-to-liquid heat pump having a COP>5, because the auxiliary loop 300 may have a small or moderate temperature range. Also, at part load operating conditions, the full capacity of the regeneration LAMEE unit 302 may not be used because the valves 306, 308 may be coordinated to restrict flow in the auxiliary loop 300.

[0040] Downstream of the regeneration LAMEE unit 302, and upstream of the tank 1 18, temperature of the desiccant liquid may be reduced using the heat exchangers 314, 316, 318 so that a desired inlet temperature to the supply LAMEE units 106a, 106b is met from the flows in the auxiliary loop 300, controlled via the primary valve 306, while throughout the loop the solution saturation, S, is less than 1 , and upstream of the tank 1 18 the condition is such that (1 -S)«1 (i.e. the final solution saturation is close to but less than 1 everywhere within the heat exchangers 314, 316, 318 where cooling of the solution occurs).

[0041 ] During summer operations, when there is a risk of crystallization in the salt solution within the semipermeable membranes, the regeneration LAMEE unit 302 will be utilized, and these outdoor conditions may extend over a wide range of psychrometric chart conditions for most cities. For part load conditions, defined to be the case for summer operations when the energy removal rate is not near its peak value and the flow rate at the regeneration LAMEE unit 302 is less than its peak value, the system 100 may require controls for the air flow through each of the LAMEE units and flow of the desiccant liquid, and, at times when an economizer may be used, because the outdoor air temperatures and humidity are close to the supply air set point, then the supply and exhaust air flow may bypass the entire system. The heat pump 324 may be used to condition the desiccant liquid's temperature to a desired set point, with the heat exchanger 318 being the last component of the auxiliary loop 300, which may be used to precisely control the temperature of the desiccant liquid for delivery to the inlets 108a, 108b. The regeneration LAMEE unit 302 may also be used to condition the desiccant liquid's solution concentration to a desired set point, for delivery to the inlets 108a, 108b of the supply LAMEE units 106a, 106b. Thus, crystallization conditions may be avoided in both the exhaust LAMEE unit 1 12 and the regeneration LAMEE unit 302.

[0042] During winter heating conditions, drying of the supply air in summer may change to adding moisture to the supply air, as well as heating it. The supply LAMEE units 106a, 106b may achieve both these functions, but may risk crystallization within its membranes unless the desiccant liquid's solution concentration at the inlets 108a, 108b is sufficiently low, because water will be transferred from the desiccant liquid to the supply air. To avoid this, the water source 126 may be used to selectively add water to the tank 1 18. As well, the heat pump 324 may be used to preheat the desiccant liquid, via the heat exchanger 322 upstream of the inlets 108a, 108b. That is, the auxiliary loop 300, required for summer operations, may not be generally used, except as a backup. Without any waste heat, heat may be directly added to the solution upstream of the inlets 108a, 108b. The heat exchanger 320 is shown coupled to the heat pump 324, which may have the waste heat energy sink 328 with a significant potential for heat removal, including, e.g. , ground water, river water, lake water, etc. The heat exchanger 322 may be an auxiliary heat source, e.g., provided by a boiler and fired by natural gas. During summer operations, the heat exchangers 320, 322 may be bypassed, e.g., by closing the primary valve 306.

[0043] The inlet and internal solution temperature may exceed occupant comfort temperatures (i.e. 45 °C or higher) for the solution regeneration process in the regeneration LAMEE unit 302 during the highest summer load conditions for outdoor air, so that the solution saturation ratio, S, will not be too close to 1 (e.g., 1 -S>0 everywhere in the regeneration LAMEE unit 302), and so the risk of crystallization during regeneration of the solution will be avoided even though the salt solution concentration will increase for solution flow through the regeneration LAMEE unit 302. To avoid any significant cooling of the desiccant liquid in the regeneration LAMEE unit 302, heat may be supplied to the regeneration LAMEE unit 302 via the air supplied from the auxiliary dehumidifier mechanism 304 using the heat exchanger 314, the waste heat source 330, and/or the heater 334, while the outlet air relative humidity is very low (e.g., less than 10%).

[0044] The systems described herein may avoid many uncertainties that are inherent in other active systems, which dewater the desiccant liquid at lower temperatures, and at or near the saturation line. In the systems described herein, the desiccant liquid may be monitored for temperature and concentration prior to the supply LAMEE units, with proper controls, so that the desiccant liquid does not approach super-saturation anywhere in the loop or in the LAMEE units.

[0045] The systems described herein may employ lithium chloride (LiCI) or lithium bromide (LiBr) as the desiccant liquid, and without significant impurities. In some examples, laboratory grade LiCI and LiBr may be utilized, with, e.g. , approximately 99.3% purity by weight (or the impurities will be less than 0.7%). In some examples, industrial grade LiCI and LiBr may be utilized, with, e.g., approximately 98% purity by weight (or less than 2% impurities). Another factor to be considered for aqueous solutions is the purity of the water used in the desiccant solution. Generally, high quality reverse osmosis (RO) water may be used instead of local tap water, because tap water may have impurities that crystallize out on heat exchanger and LAMEE surfaces, causing the performance of each to decrease until such time that these surfaces are cleaned with a solvent (e.g., mild acid).

[0046] The desiccant liquid may be conveyed by pumps in a closed loop or loops between the LAMEE units and other components. In some examples, the semipermeable membrane material implemented in the LAMEE units may have homogeneously distributed micro-pores (e.g., 1 to 5 μιη) of generally uniform sizes, with a relatively small standard deviation of diameter (e.g., less than 1 or 2 μιη), and smooth internal surface structures (e.g. , surface roughness less than 100 nm), which helps to reduce the surface nucleation sites for the nucleation and growth of crystals. Examples of commercially available semipermeable membranes are QL822™ (General Electric, of Schenectady, NY, USA), and EZ2090™ (Celgard LLC, of Charlotte, NC, USA).

[0047] The LAMEE units' heat and moisture transfer membrane surface areas, which transmit both heat and moisture to or from the air and solution flows through the semipermeable membranes, may each be selected or designed for the building application such that they will satisfy various system operating conditions, regardless of the weather and specified building conditions for air temperature and humidity, and while avoiding blockage of the membrane surface areas due to crystallization. Generally, this requirement may mean that the membrane surface areas of the exhaust LAMEE unit(s) plus the regeneration LAMEE unit(s) will be larger than that for the supply LAMEE unit(s).

[0048] The components used to control the systems described herein for summer supply air dehumidifying and cooling may differ from those used in winter for heating and humidifying air; i.e. the auxiliary loop 300 will not normally be used for winter operations. The layout of the components in the systems may take several different forms, including, for example: (1 ) the components may be housed in a single HVAC unit, which is installed with electrical, water, natural gas and ducting connections, with the internal components sized for each particular application, and such a unit may be installed on roof tops or within a building envelope and used primarily for ventilation air conditioning with the exhaust and supply air discharge vents and supply air intakes in relatively close proximity; (2) a plurality of supply LAMEE units may be distributed throughout the conditioned space of a building, each with separate controls and connecting tubing to condition the internal building air to comfort conditions for the local occupants served by each unit, with separate fans to circulate the local air in situ for occupant comfort control; (3) a plurality of exhaust LAMEE units may be distributed on the building space exterior perimeter, and may be used to reclaim the exhaust air energy in situ from several different parts of the building before it exits the building into the ambient outdoor air; and (4) there may be various combinations of the foregoing.

[0049] As mentioned above, during cold weather operating conditions, part or most of the outdoor ventilation air may be brought through the supply LAMEE unit, which will result in heating to form supply air to a set-point temperature and humidification level. This may necessitate the injection of water, e.g., by the water source 126 upstream of the supply LAMEE units 106a, 106b. To avoid crystallization within the supply air LAMEE during this operating condition, the saturation ratio S should be significantly less than 1 everywhere in this supply LAMEE unit (e.g., 1 >S).

[0050] Accordingly, crystallization avoidance within heat exchangers and semipermeable membrane exchangers for heat and moisture transfer for control of building occupant comfort and humidity conditions is a factor to be considered for HVAC system operations when salt solutions are used as the desiccant liquid. Effective designs may use relatively high purity LiCI or LiBr salt solutions as coupling desiccant liquids, to avoid or reduce the risk of salt crystallization for a wide range of operating conditions. Suitable semipermeable membranes may have good water vapor transfer and heat transfer resistance properties, good stress and strain and creep properties, and high liquid breakthrough transfer resistance, with relatively smooth internal pore surfaces and generally uniform micro-pore sizes to lessen crystallization nucleation. Effective designs may also provide for larger heat and water vapor transfer surface areas for desiccant liquid cooling for regeneration/dewatering than for solution heating, air dehumidifying or processes with decreasing solution concentration. Solution regeneration and dewatering may be done with the desiccant liquid at elevated temperatures (e.g., above 45 °C), and while the temperature of the solution is not decreasing due to a heat flux out of the solution. (The limiting temperature of the solution at which this regeneration or dewatering of the solution is carried out may be dictated by properties of the semipermeable membrane and the purity of the water used in the LAMEE units.) For high cooling load conditions, the solution regeneration and dewatering may be done when the saturation state of the solution is not near 1 .0 [i.e. (1 -S) does not approach 0]. Again, for high cooling load conditions, the low temperature and high saturation condition required for the inlet of the supply LAMEE unit may be achieved by using a cooling only heat transfer process for the solution [i.e. so that (1 -S) may be moved by mostly cooling close to 0].

[0051 ] The systems described herein may also include features for improved energy savings that will lead to high values of COP, including, for example: economizer cycle with part load provisions for bypassing air; heat recovery for the desiccant liquid heating and cooling processes; waste heat recovery from any valuable heat source when it is economically feasible; heat recovery from the operation of dehumidifier mechanisms; moderate to low temperature change for the operation of the heat pump (allowing a high COP); and provisions for bypassing the desiccant liquid from the auxiliary loop during part cooling load conditions.

[0052] While the above description provides examples of one or more apparatuses or methods, it will be appreciated that other apparatuses or methods may be within the scope of the accompanying claims.