Building air conditioning is the single largest electrical load at many Department of Defense (DOD) bases and installations creating both large energy bills and high peak demands that stress electrical infrastructure. Other problems may arise when conventional compressor-based cooling systems struggle to control indoor humidity. In addition to creating an uncomfortable work environment that undermines productivity, high indoor humidity promotes mold and mildew growth that increases both the morbidity of personnel and maintenance costs. These problems are most severe in humid climates where inadequate latent cooling can lead building managers to restrict ventilation to minimal levels that further compromise both the comfort and health of the building’s occupants.
The objective of the demonstration was to prove the ability of a novel dehumidification technology to efficiently control indoor humidity without overcooling the process air. The demonstration provided the first test of the novel technology in a real-world environment where its Operating and Maintenance (O&M) characteristics can be assessed.
The Liquid Desiccant Direct Expansion AC (LDDX) integrated a liquid desiccant circuit (LD) into a conventional compressor-based direct expansion (DX) air conditioner (AC) to produce a cooling system that can supply dry air with a dewpoint that was lower than its evaporator temperature. This allowed the LDDX to serve large latent loads without overcooling the process air so that moisture was condensed.
Two LDDX technologies were demonstrated. The first technology modified the refrigerant circuit of a conventional DX AC so that the plate-fin evaporator and condenser were replaced by the Wicking Fin Heat and Mass eXchangers (WFHMX) shown in Figure S1. As shown in this figure, the WFHMX operated with the films of LD that flow over both the refrigerant tubes and wicking fins in direct contact with the process air. For a WFHMX evaporator, the LD simultaneously cooled and dried the process air so that the air supplied to the building was very dry, i.e., its relative humidity (rh) would typically be in the 45% –60% range as opposed to near 100% for a conventional DX AC.
The second LDDX technology relied on a fundamental property of all desiccants: the amount of water that they absorb depends only on the rh of their environment. Referring to Figure S2, the process air left the evaporator of a conventional DX refrigeration circuit (Point A) at close to 100% rh, while the cooling air left the condenser at a rh that was typically less than 50%. The two desiccant-wetted pads of contact media—one behind the evaporator and one behind the condenser—which exchange desiccant will “pump” water from the high rh side to the low rh side. The process air leaving the absorber pad was supplied to the building at a rh typically between 50% and 70%.
The project reported here had a 51-month period of performance that began in April 2013. Two prototype LDDXs were built, tested in the lab and then installed on DOD buildings: a 3-ton prototype, which used wicking-fin technology, Liquid Desiccant Direct Expansion AC with WFHMX (LDDX-WF), was installed at Picatinny Arsenal and operated for almost the entire 2015 cooling season, and a 5-ton prototype, which used adiabatic desiccant-wetted pads, Liquid Desiccant Direct Expansion AC with AHMX (LDDX-Ad), was installed at Fort Belvoir and operated for part of the 2015 cooling season and the entire 2016 cooling season.
The LDDX-WF prototype met its performance objectives to supply dry air and to modulate the Sensible Heat Ratio (SHR) of the delivered cooling: at the Air Conditioning, Heating and Refrigeration Institute (AHRI) rating conditions for packaged ACs, the LDDX-WF supplied air at a 46.5oF dewpoint and it modulated its SHR between 0.28 and 0.5. However, the as-built prototype’s Energy Efficiency Ratio (EER) of 9.3 was below the performance objective for efficiency of 11.0. A LDDX-WF with 1.5 X larger coils was projected to have a 12.0 EER.
The LDDX-Ad prototype also met its performance objectives to supply dry air and to modulate its SHR: at AHRI rating conditions it supplied air at 50oF dewpoint and modulated its SHR between 0.40 and 0.78. It also met its performance objective for efficiency by achieving an EER of 11.46 (versus the performance goal of 11.0).
The LDDX-WF prototype and the LDDX-Ad prototype (after the replacement of the desorber) both operated with no problems throughout their field test periods. Performance of both prototypes was stable, with the LDDX-Ad delivering air between 42% and 70% rh and the LDDX-WF delivering air between 35% and 52% rh. (There was a two-day period in the middle of the field test when the rh of air delivered by the LDDX-WF increased to between 60% and 70%. Researchers speculate that this anomalous operation was caused by a temporary partial blockage in the flow of desiccant, but were not able to confirm this.)
Packaged roof-top or ground-mounted, compressor-based ACs that use either the LDDX-WF or LDDX-Ad technology can be energy efficient alternatives to conventional DX ACs for indoor humidity control. In applications where conventional ACs would provide too much sensible cooling when meeting the latent load, the LDDX can save the energy expended for “overcooling.”
Field operation of the LDDX-Ad did uncover a compatibility problem between the LD and the contact media used in the desorber pad. After about six weeks, field of operation, the contact media weakened and the desorber pad collapsed. A suitable, alternative contact media was identified through lab exposure tests, and the LDDX-Ad was refitted with a new desorber pad at the start of the 2016 cooling season.
The LDDX can also address costly maintenance caused by indoor humidity. Despite the best efforts at Heating, Ventilation and Air Conditioning (HVAC) design, indoor humidity can sometimes reach levels that promote the growth of mold and mildew. A packaged LDDX may be a retrofittable solution to the problem.
Finally, the most important, early driver for the adoption of the LDDX by DOD may be the need to control corrosion by storing material in drier environments.