A Regenerator Driven by Warm Condenser Air

     Desiccants act as humidity pumps that move water vapor from air with high relative humidity to air with low relative humidity.  For vapor-compression air conditioners (VC-AC) operating in all but the driest climates, the process air leaving the evaporator will be close to 100% rh while the cooling air leaving the condenser will typically be less than 50% rh.  A desiccant--solid or liquid--alternately exposed to these two air streams will passively "pump" water vapor from the process stream to the cooling stream, significantly lowering the process-air relative humidity.  There can be a loss in total cooling since the exchanged desiccant will "dump" some heat from the condenser side to the evaporator side, but this heat "dump" can be less than 5%.  

     At least two companies—Munters and Mojave Energy Systems—offer a VC-AC that boosts its latent cooling using a desiccant that is regenerated with warm air leaving a condenser.  In both the Munters Humidity Control Unit and the Mojave ArctiDry, the desiccant dries the near-saturated air leaving the evaporator. For the Munters unit, water is absorbed as the evaporator air flows through a sector of a solid-desiccant rotor; and for the Mojave unit, water is absorbed as the evaporator air flows through a pad of contact media that is continuously flooded with liquid desiccant.  For the Munters unit, continuous absorption and desorption is maintained by revolving the solid-desiccant rotor between the evaporator and condenser air streams.  For the Mojave unit, the liquid desiccant absorber pad and regenerator pad share the same sump so, during operation, the concentration in the sump tends to a value that balances water absorption with water regeneration.

    Under two awards from the Department of Energy, AILR has designed, build and field operated fully functional prototypes of high latent VC-ACs that regenerate a liquid desiccant with warm condenser air.  The final reports for these awards can be downloaded using the links at the left.

     With lessons learned from the two DOD projects and other commercialization activities, AILR has a commercially ready design that has both a higher Moisture Removal Efficiency and potentially lower cost to manufacturer than competing high-latent air conditioners targeted at the market for Dedicated Outdoor Air System (DOAS).  Key features of AILR's LD-DOAS are:

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• A significant reduction in the volume of condenser cooling air, i.e., condenser cooling air is only 1.6X greater than process air,

• 100% utilization of condenser heat for regeneration; with more thermal energy available for regeneration, the desiccant carries a greater share of overall Moisture Removal and the vapor-compression circuit operates more efficiently with a higher evaporator temperature.

• Full filtering of both process air and condenser air; with both air streams filtered, the liquid desiccant stays cleaner and the O&M burden is reduced mostly to changing air filters

• Heat exchange between the warm, strong desiccant leaving the regeneration side and the cool, weak desiccant leaving the process side in an Interchange Heat Exchanger (IHX); with cooler strong desiccant delivered to the process side, heat "dump" is reduced  

 

    Perhaps the most important competitive advantage for AILR’s LD-DOAS is its significantly down-sized refrigeration circuit compared to a conventional DOAS.  To ensure we are making an apples-to-apples comparison, we've prepared a conceptual design for a conventional direct-expansion DOAS (DX-DOAS) with coils and a compressor that are scaled versions of the components in our LD-DOAS's refrigeration circuit.  The compressors for both DOASs are sized so that 55°F dewpoint air is delivered at the AHRI Standard 920 A Rating Point.  

     The component and performance comparison for the two DOASs are shown in the table at the right.  (Performance of the DX-DOAS in this table assumes that a separate coil downstream of the evaporator reheats the air to 70°F--although in many application reheating the make-up air is not required.)  As shown in the table, the LD-DOAS supplies dry, but warm make-up air to the building.  In many applications, warm but dry make-up is what the building needs since its central cooling system has sufficient capacity to provide the required sensible cooling.  With no need for the LDDOAS to sensibly cool the make-up down to 70°F, its refrigeration system is less than half the size of the overcool/reheat DX-DOAS.  With the savings in the refrigerant system costs plus the general reduction in size and weight of the entire DOAS, the cost to manufacture the LD-DOAS should be much lower than that of the competing DX-DOAS. 

     The combination of a reduction in first-cost AND a 30% improvement in ISMRE provides our LD-DOAS with a compelling competitive advantage over both entrenched technologies and "disruptive" technologies being developed by others.  This competitive advantage is further strengthened by U.S. Patent 10,655,870, which defines an operating envelope for desiccant flooding rates and air velocities that ensures zero carryover of desiccant droplets.  (Our Deep Dive into Zero Droplet Carryover provides more details.)

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