The primary objective of this project was to demonstrate the capabilities of a new high-performance, liquid-desiccant DOAS to enhance cooling efficiency and comfort in humid climates while substantially reducing electric peak-demand. This was the first solar-powered demonstration of the technology. The goal of the project was to quantify energy and water consumption, solar energy utilization, and cost savings relative to DX air conditioners. The LDAC system installed at Tyndall was a pre-commercial technology and given that it was the first solar-powered demonstration, a fundamental objective of the demonstration was to evaluate the performance of the system and use the lessons learned to develop design/manufacturing guidance for future commercial LDAC systems.
At the end of 2011, the LDAC technology was licensed to Munters, one of the largest HVAC manufacturers in the United States, and the first demonstration of a commercial LDAC system is being evaluated on the Coral Reef Fitness and Sports Center on Andersen AFB in Guam.
Desiccants reverse the paradigm of standard DX AC by first dehumidifying and then sensibly cooling the outside airstream to meet a given cooling load. Desiccant at any given temperature has a water-vapor pressure equilibrium that is roughly in line with constant relative humidity (RH) lines on a psychrometric chart. If the free surface of the desiccant is kept at a constant temperature, the ambient air will be driven to the dehumidification potential line. If used with an evaporative heat sink at temperatures between 55°F and 85°F, the air can be significantly dehumidified, and dew points less than 32°F are easily achieved. At this point, the air can be sensibly cooled to the proper supply temperature. This type of desiccant AC system decouples sensible and latent cooling by controlling each independently.
During the dehumidification process, the liquid desiccant (about 43% salt concentration by weight in a water solution) absorbs water vapor in an exothermic reaction. The heat released by the desiccant is carried away by a heat sink, usually cooled water from a cooling tower. As water vapor is absorbed from the ambient air, it dilutes the liquid desiccant, and decreases its vapor pressure and its ability to absorb additional water vapor. Lower concentrations of desiccant come into equilibrium at higher ambient air RH levels. Dehumidification can be controlled by the desiccant concentration supplied to the device. The outlet humidity level of the processed ambient air can be controlled by the desiccant concentration and/or the flow of highly concentrated desiccant. The latter allows the highly concentrated desiccant to quickly be diluted and thus “act” as a weaker desiccant solution in the device.
In general, the LDAC system did not perform as well as expected due to design, installation, and operation issues. Consequently the project’s focus was necessarily changed to focus on discovery of technical issues with this new emerging technology. Many of the issues arose because the installation had many unique features including the following:
- Due to initial budgetary constraints, the LDAC relied solely on solar heat, with no natural gas backup to ensure that the unit operated throughout the cooling season. A properly designed system that uses solar heat will have backup. Due to this, the system did not achieve peak-cooling capacity for significant hours of operation. Because the system largely has static electrical power draw, this resulted in a low average EER.
- The solar field design and LDAC system design were not tightly coordinated by the prime installation contractor (Regenesys). This resulted in a design that did not consider the frequency and duration of stagnation periods for the solar field. The collector design was not designed to withstand more than about two stagnations per year. Furthermore, the collector system was not initially designed to withstand the massive volume of steam from these collectors when stagnation occurred. The solar field required significant redesign. The end result was workable for the demonstration, despite being problematic and suboptimal in operation.
- The demonstration was the first to create a split system where the conditioner and regenerator were contained in separate packages and separated by a distance of approximately 120-foot (ft). This technical challenge resulted in a suboptimal pumping design because of the necessary pump size to transfer desiccant this distance. Future designs should reduce the distance from the regenerator and conditioner.
- This demonstration was the first to have 10 hours of desiccant storage using calcium chloride (CaCl2). Tuning the storage to achieve optimal efficiency was required. The desiccant charge and the tank’s low and high levels have significant impact on efficiency, capacity, and solar utilization. These variables were tuned as the demonstration progressed.
- This demonstration required the placement of the conditioner unit about 100 feet from the outdoor intake to the building. This required significant fan power to move the air from the mechanical yard to the building. Future designs and applications should consider the duct length and reduce the duct run from the conditioner to the outdoor air intake as much as possible.
The demonstration did not treat 100% of the outdoor air, thus limiting the benefit to energy savings from offset cooling. In order to offset the reheat for such an installation, a system should be designed to ensure that the LDAC meets a significant portion of the latent load. Typically, the LDAC can meet 100% of a building’s latent load if designed to treat 100% of the outdoor air.