The LDDX--An Integrated Liquid-Desiccant Direct-Expansion AC

     Willis Carrier's first commercial installation of a vapor-compression air conditioner in 1902 solved a humidity problem at a printing press.  Over 120 years later, Carrier's approach to humidity control—over cool air to condense water vapor—is still the most common one. 

     Desiccants offer a more efficient alternative to over-cooling.  As explained on our Liquid Desiccant Basics webpage, a desiccant acts as a humidity pump that moves water from air with a high relative humidity to air with a low relative humidity.   For a compressor-based AC with a direct-expansion evaporator, the cooled air leaving the evaporator has a much higher relative humidity than the warmed air leaving the condenser.  Alternately exposing a desiccant to these two air streams will pump water vapor in a direction that boosts the air conditioner's latent cooling (i.e., dehumidification) with only minimal energy penalties. 

     Latent cooling is the critical metric for Dedicated Outdoor Air Systems (DOAS).  As explained in more detail on the "Deep Dive" page, a liquid-desiccant direct-expansion air conditioner (LDDX) designed to be DOAS can have an Integrated Moisture Removal Efficiency (ISMRE) of 10 lb/kWh—far exceeding the 4 lb/kWh typical of conventional DX DOASs.  And, by avoiding overcooling, refrigeration components can be downsized, which could lead to an LDDX DOAS that was first-cost competitive with conventional units.

DEVap -- Combining Deep Drying with Desiccant and Dewpoint Evaporative Coolers

     Providing indoor comfort in hot, humid climates without the high electrical demand of a compressor is essential to a sustainable future. Liquid-desiccant air conditioners that are evaporatively cooled could help, but the help will be limited since outdoor wet-bulb temperatures are highest where cooling is most needed.

     Researchers at the National Renewable Energy Laboratory (NREL) have proven an alternative to a vapor-compression air conditioner that works at high outdoor wet-bulb temperatures.  In NREL's Desiccant Enhanced Evaporative Cooler (DEVap), the cooling process is split into two stages: a first-stage liquid-desiccant conditioner that rejects heat via outdoor-air evaporative cooling and a second-stage regenerative evaporative cooler in which evaporation is driven by air that has been deeply dried by the desiccant stage.

     As described in more detail in our Deep Dive, the key component of DEVap is an indirect evaporative cooler—first described in William Niehart's 1939 U.S. Patent No. 2,174,060—that can cool air to a temperature below the air's initial wet-bulb temperature--the lower limit being the air's dewpoint temperature.  With the first-stage liquid-desiccant conditioner providing very low dewpoint air, DEVap can produce the required cooling effect (typically 400 cfm per ton—or 6.7 Btu/lb) without a compressor-based refrigeration system.

     The Deep Dive link describes our patented design for a dewpoint indirect evaporative cooler (DIEC), explains how it overcomes critical limitations for commercially available DIECs and discusses an interesting synergy between the DIEC and another AILR patent-pending technology—the diffusion-gap electrically driven desiccant regenerator. 

Low Flow Technology and Suppression of Droplet Carryover

U.S. Patent 10,655,870 provides AILR’s LDDX with critical advantages over its competitors—the most important of which is its operation at desiccant flooding rates and air velocities that ensures zero carryover of desiccant droplets.  AILR’s presentation at the 2024 ASHRAE Winter Conference—Seminar 60 introduces a 3,600-cfm LDDOAS that will operate without desiccant droplet carryover on either the process side or regeneration side.  Our Deep Dive explains the carryover mechanism and shows videos of the desiccant flow on contact media as operating conditions go from a quiescent state to active carryover

EDDR - An Electrically-Driven Desiccant Regenerator

     Today, solar and wind can provide kilowatt-hours of electricity at lower costs than fossil-sourced and nuclear-sourced alternatives.  However, they do have an important limitation--in utility lingo, they are "non-dispatchable".  Energy storage can accommodate the mismatch between supply and demand, but the storage technology receiving the most attention—batteries—are expensive and resource-intensive. Solar and wind are meeting ever greater shares of the nation's demand for electricity, but continued expansion demands a low-cost means of energy storage .

      The single largest summer load served by the country's utility grids is air conditioning.  But battery storage is not the only storage option for cooling loads—storing "cooling effect" can be equally effective.

     Although ice storage is one option for storing "cooling effect", it is not the "breakthrough" technology needed to broadly serve HVAC loads with wind and solar.  Storing "cooling effect" as concentrated liquid desiccant could be the answer.

     Two technologies must prove themselves commercially ready if the preceding "could be the answer" is to be rewritten "are the answer".  The first is a means to provide indoor comfort for large segments of the HVAC market using liquid desiccants.  As described in our Deep Dive into DEVap, evaporatively-cooled liquid-desiccant air conditioners could replace high-demand compressor-base air conditioners in many HVAC markets.  A number of liquid desiccants (e.g., potassium acetate) cost less than $1 per kg.  When combined with DEVap, storing "cooling effect" as concentrated liquid desiccant will be a less expensive, more energy efficient than ice storage.  

    The second technology—and the one explored in the following Deep Dive link—is an electrically driven means to produce concentrated desiccant.  As explained in the Deep-Dive link, AILR's patent-pending EDDR is a compact design that achieves exceptional efficiency while recovering all water released by the desiccant as mineral-free condensate. 

A Wicking-Fin Heat and Mass Exchanger

     Liquid desiccants have been successfully used to produce dry air for a surprisingly long time.  Dr. Russell Bichowsky, working for the Frigidaire Division of General Motors, first used solutions of lithium chloride to dry air in the 1930s.  ​What is also surprising is that the dominant technology for industrial drying with liquid desiccants has changed little in almost a century of use. 

     Similar to Dr. Bichowsky's early work, most industrial applications of liquid desiccants (as well as many recent efforts to commercialize an HVAC product) first chill the desiccant and then spray or drip the desiccant  onto a packed bed of contact media.  Air is dried and cooled as it flows through the contact media.

    This time-tested approach to drying air is simple and it works--but it has an important limitation.  As the desiccant absorbs water it becomes both weaker and its temperature rises—the later effect due to the heat released by the desiccant's water absorption.  The most direct approach to sustaining high absorption rates throughout a packed-bed condition is to operate at high flooding rates so that temperature and concentration changes are small.  However, limiting the desiccant's temperature rise turns out to be the more important than limiting its concentration change.

     As an example, consider a 24"-deep packed-bed absorber flooded at 5 gpm/ft2  (200 lpm/m2) with 59°F (15°C),  40% lithium chloride.  Outdoor air at 95°F (35°C) and 0.014 lb/lb humidity ratio, flowing counter to the downward flowing desiccant will leave the packed bed at 61.5°F (16.4°C) and 0.00202 lb/lb humidity ratio.  Desiccant will flow off the packed bed both warmer and weaker: 72.7°F (22.6°C) and 39.8%.  The air is deeply dried, but clearly the flooding rate is much more than is needed to limit the loss of drying due to a weaker desiccant, i.e., desiccant has warmed 13.7°F, but decreased only 0.2% in concentration.

    We first reported on the performance gains captured by an absorber that continually (or periodically) cools the desiccant as it absorbs water vapor in a 1992 ASHRAE paper*.  The Deep Dive link below describes a wicking-fin heat and mass exchanger—our approach to internally cooling a liquid desiccant absorber or internally heating a liquid desiccant regenerator.

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(*) Lowenstein and Gabruk, "The Effect of Absorber Design on the Performance of a Liquid Desiccant Air Conditioner," ASHRAE Paper No. AN-92-3-3, 1992.

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Steam-Generating Evacuated-Tube Solar Collectors

    In June 2013, U.S. Patent No. 8,459,250 issued to AIL Research for an innovative solar thermal collector that directly produced atmospheric-pressure steam within dewar-type evacuated solar tubes.  In field tests of arrays as large as eight 60-tube panels, AILR has field demonstrated very high solar efficiencies in low-cost installations that address important limitations in other solar thermal technologies, e.g., steam-generating solar collectors do not require a "dump" radiator to protect against stagnation; the produced steam flows to the load with no need to circulate a high-temperature heat transfer fluid.

     Unfortunately for AILR (but fortunately for the planet) the wholesale price for silicon PV panels has fallen from $5 per watt when we first started to work on the steam-generating solar collector in 2010 to under $0.5 per watt today.  While we project 20-year levelized cost for thermal energy delivered by a large array of our collectors to be a very attractive $0.015 per kWh in a sunny southwest location, competing against PV is a challenge.

      The Deep Dive link below is a more in-depth technical explanation of our steam-generating solar collectors.  While we've discontinued development and have no plans to commercialize this technology, we'd welcome the chance to help others who have ideas that might improve the commercial prospects for the technology.

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Desalting with Diffusion Gap Distillation

     Large, commercial, thermal desalination plants achieve high efficiencies by reclaiming the heat released when water vapor condenses to evaporate additional water.  In these plants, the processes of evaporation >> condensation >> heat recovery >> additional evaporation are done within vacuum vessels with heat transfer across expensive cupronickel or titanium heat exchangers.         AILR’s Diffusion-Gap Distillation (DGD) duplicates the thermodynamics of a desalination plant that uses Multi-Stage Flash evaporation.  However, instead of passing the hot brine through stages of every decreasing pressure, the DGD process operates at one pressure, conveniently chosen to be ambient.  The hot brine again passes through regions of every decreasing water vapor pressure, but total pressure stays constant.  With all components operating at ambient pressure, the DGD process requires no pressure vessels and heat exchangers can be made from inexpensive plastics. 

   As described in our U.S. Patent No. 9,770,673, the novel feature through which DGD achieves high performance is to locate hot, evaporating surfaces that are wetted with brine very close to cooled, condensing surfaces, the gaps between the surfaces being less than 4 mm (about 1/6th inch). This close positioning of evaporating and condensing surfaces produces a very high flux of water vapor across the gap even when the evaporating surface is only a few degrees warmer than the condensing surface.  (There are parasitic losses for radiation and sensible heat transfer across the gap, but with vapor pressure increasing exponentially with temperature, latent heat transfer dominates.)  DGD's high water fluxes driven by small differences in temperature with the heat released by the condensing water vapor returned to the feed brine leads to Gain Output Ratios that approach 10.

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A Liquid Desiccant Air Conditioner Driven by Solar Thermal Energy

With both cooling loads for HVAC and solar resources peaking in the summer, solar-driven air conditioners would appear to be a natural solution to the summer peak demands plaguing many electric utilities.  Unfortunately, the solution is not quite as "natural" as it first appears--the demand for air conditioning peaks in the late afternoon, but solar resources peak midday.  The compact, low-cost storage of "cooling effect" provided by liquid desiccants can reconcile the temporal mismatch between solar supply and cooling demand.  The Deep Dive link below provide information on our field tests of solar-driven, liquid-desiccant air conditioners.

Radiant Cooling Panels

   Radiant cooling is an underappreciated technology that could play a major role in addressing "extreme heat" in outdoor settings.  Almost every day this past summer, news headlines reported record-breaking heat and humidity (the "misery index") around the planet.  While it isn't practical to cool and dehumidify the "great outdoors", AILR's patent-pending radiant cooling panels provide relief at a low cost and low energy demand.  The Deep Dive link below describes the unique features of our panels that give them an advantage over the competition.

     Although still coming together as an independent venture, AILR is working with Princeton University to launch Cleary-Cool—a technology company dedicated to radiant cooling.