As described in our Deep Dive link, DEVap is one of the more promising technologies for dramatically reducing energy use for air conditioning.  However, DEVap can only fulfill its potential if the desiccant that provides DEVap's cooling can be efficiently regenerated.

      One potential source of concentrated desiccant for DEVap is the electrically driven desiccant regenerator (EDDR) patented by NREL.  As shown in first figure on the right, the NREL EDDR operates with a stream of outdoor air (OA) first flowing through a heat pump’s condenser and then its evaporator.  The surfaces of the condenser are wetted by a weak desiccant that releases water as it is heated.  The released water vapor is collected by the air flowing through the condenser.  By flowing this warm, humid air through the evaporator, the temperature lift for the heat pump is reduced—which increases its COP—and a fraction of the released water can be collected as condensate (with the balance of the water exhausted to ambient).

     The critical metric for an EDDR is its Moisture Removal Efficiency (MRE)—the amount of water separated from the weak desiccant (kg or lb) per unit of electricity that drives the process (kWh).  Since MRE is a strong function of the desiccant’s final concentration, this parameter must be specified when comparing technologies.

     As described in the NREL patent, the EDDR shown in the first figure at the right will have an MRE of 5.8 kg/kWh when regenerating lithium chloride to 38% concentration.       In addition to improving MRE, perhaps by as much as a factor of 1.75, our proposed EDDR will address the following weaknesses in the NREL technology:

• The NREL EDDR is an open loop system with a power demand for fans to drive the flow of ambient air. With the volume of regeneration air being an appreciable fraction (perhaps 1/3rd) of the volume of air processed by the liquid-desiccant air conditioner, the EDDR’s fan power will be a significant penalty (that is not included in the preceding MRE).

• The air exhausted from the EDDR will almost always be more humid than the entering air. With net water leaving the system via the exhaust air, the amount of mineral-free condensate that drains off the evaporator will be significantly less than the amount separated from the desiccant (75% or less).

• The ambient air used for regeneration will have particulates that create a maintenance burden either for air filter replacement or desiccant filter replacement.

• The need for a forced flow of air will increase the size and cost for a packaged EDDR.

Our technical innovation is an unconventional configuration of a conventional heat pump in which the heat pump’s high temperature condenser and low temperature evaporator are a nested array of alternating hot and cold surfaces with small intervening air gaps.  Films of desiccant on the hot surfaces release water vapor, that then diffuses across the air gaps and condenses on the cold surfaces. Our EDDR does not need a fan-forced flow of air to move water from its condenser to its evaporator—the water simply diffuses across a small air gap.

     As shown in the simple one-gap configuration presented in the second figure on the right, the heat pump has a planar evaporator and planar condenser, both oriented vertically and positioned with a small, ambient-pressure air gap between them.  Weak desiccant is delivered to the upper edge of the condenser and flows down its inboard face as a thin film. The compressor-driven refrigerant flow maintains a temperature difference between the condenser and evaporator that then creates a water-vapor pressure gradient that drives the diffusion of water vapor across the gap.  The water vapor condenses as a film of mineral-free water on the cooled evaporator and concentrated desiccant flows off the condenser*. In this configuration, the technical innovation is referred to as Diffusion-Gap Thermal Distillation (DGTD).

     An important aspect of the DGTD EDDR is the ambient-pressure air gap that separates the evaporator and condenser.  Most thermally driven separation processes with salt solutions (e.g., MSF and MED for desalination) operate under vacuum so that the temperature differentials that drive the distillation are minimized.  Any non-condensable gases that “leak” into the vacuum vessel are purged.

     The DGTD EDDR intentionally operates with a non-condensable gas in the gap to avoid the cost, weight and complexity associated with vacuum vessels and their purge systems.  The temperature differential for driving the distillation is kept low by the close proximity of the high and low temperature surfaces (e.g., gaps as small as 3 mm are be practical). 

    If we ignore temperature and concentration gradients within the flowing desiccant film and possible buoyancy-driven recirculation, a thermophysical model of the heat transfer across the gap of a DGTD EDDR is fairly simple with three mechanisms dominating: (1) radiation, (2) heat conduction (sensible heat transfer) and (3) mass diffusion with accompanying evaporation/condensation at the gap boundaries (latent heat transfer). The first two mechanisms—radiation and heat conduction—are driven by a temperature differential, and the third—mass diffusion—is driven by a differential in water-vapor concentration.  This later differential depends on the temperature and concentration of the desiccant on the hot boundary and the temperature of the condensing water on the cold boundary.

     This simple model suggests ways to improve the MRE of a DGTD EDDR.  For reference, the heat flux across a 3 mm gap with a film of lithium chloride at 36.5% concentration and 94°C on the hot side and a condensing surface at 60°C will be 2,460 W/m2 under the assumptions that (1) the desiccant-wetted and water-wetted boundaries have an emissivity of one, and (2) the transport properties of the air/water-vapor mixture in the gap (both thermal conductivity and water vapor diffusivity) are the same as those for ambient air. This total heat flux of 2,460 W/m2 is 73% latent transport (i.e., water diffusion), 14% sensible heat conduction and 13% radiation. 

     We have developed a computer model of a one-gap EDDR within the framework of Engineering Equation Solver (EES).  The model includes a simple representation of the refrigerant circuit that operates with any of the refrigerants in the EES library, heat exchangers with minimal refrigerant-side pressure drops, specified subcooling at the condenser exit, specified superheating at the evaporator exit, and a constant isentropic efficiency for the compressor (typically set at 0.72).  Desiccant properties for solutions of lithium chloride over a range of concentrations and temperature are incorporated as EES library functions.

     Under conservative assumptions (i.e., emissivity for the desiccant-wetted surface is one and transport properties for the air/water-vapor mixture in the gap are the same as those for ambient air) our EES model predicts the DGTD EDDR delivers 38% lithium chloride at an MRE of 6.4 kg/kWh—an approximately 10% improvement over the NREL EDDR.  However, perhaps more important than the efficiency gain is the DGTD EDDR's operation without an open-loop air flow, which allows for a much more compact, lower cost, and lower maintenance EDDR.  

     As described in our Deep Dive into DEVap, the liquid-desiccant first stage of a 2,500-cfm DOAS DEVap absorbs 125 lb/h of water on a summer design-day (95/78°F DB/WB).  The second-stage Dewpoint Indirect Evaporative Cooler (DIEC) sensibly cools the warm, dry air leaving the desiccant stage to 75°F by evaporating 77 lb/h of water. Although water use and water production will change as weather conditions change, the design-day has one of highest DIEC evaporation rates projected for a humid climate like Miami, FL.  With the DGTD EDDR producing 1.6X the minimal water needed by the DIEC on a design-day, the DGTD EDDR is projected to produce more than enough mineral-free water to meet the DIEC's need on all days requiring cooling.      A secure supply of mineral-free water greatly simplifies the design and operation of the DIEC.  As explained in the Deep Dive into DEVap, high water flooding rates can degrade the counter-flow temperature gradient within the DIEC.  With mineral scale no longer a problem, water flooding rates can be reduced to levels that do not affect the essential temperature gradient.

     In October 2023, AIL Research filed a PCT patent application that claims the general concept of the DGTD EDDR.  The EDDR is in an early stage of development.  We've developed considerable know-how in our work on a DGTD process for desalination—know-how that solves problems with fluid delivery and collection for closely spaced, multi-plate heat and mass exchangers that must isolate a condensate flow from a salt flow.  This know-how will be applied to the EDDR, but a fully functional prototype is still a work in progress.

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