Low-Flow Liquid Desiccant Air-Conditioning: Demonstrated Performance and Cost Implications

Abstract

Cooling loads must be dramatically reduced when designing net zero energy buildings or other highly efficient facilities. Advances in this area have focused primarily on reducing a building's sensible cooling loads by improving the envelope, integrating properly sized daylighting systems, reducing unwanted solar heat gains, reducing internal heat gains, and specifying cooling equipment with high nominal efficiencies. As sensible loads decrease, however, latent loads remain relatively constant, and thus become a greater fraction of the overall cooling requirement in highly efficient building designs, particularly in humid climates. This shift toward low sensible heat ratio (SHR) systems is a challenge for conventional heating, ventilating, and airconditioning (HVAC) systems. Electrically driven vapor compression systems typically dehumidify by first overcooling air below the dew-point temperature and then reheating it to an appropriate supply temperature, which requires additional energy. Another dehumidification strategy incorporates solid desiccant rotors that remove water from air more efficiently than vapor compression; however, these systems are large and increase fan energy consumption due to the increased airside pressure drop of solid desiccant rotors. A third dehumidification strategy involves high-flow liquid desiccant systems. These systems require high-maintenance mist eliminators to protect the air distribution system from corrosive desiccant droplet carryover. These are commonly used in industrial applications but rarely in commercial buildings because of the high maintenance cost. Low-flow liquid desiccant airconditioning (LDAC) technology provides an alternative solution with several potential advantages over previous dehumidification systems: This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. Acronyms and Abbreviations AHU air handling unit ASH anti-sweat heater ASHRAE American Society of Heating, Refrigerating and AirConditioning Engineers CaCl 2 calcium chloride cfm cubic feet per minute CHP combined heat and power COP coefficient of performance DB dry bulb temperature DOE U.S. Department of Energy DP dew point temperature DX direct expansion hp horsepower HR humidity ratio HVAC heating, ventilation, and air conditioning LDAC liquid desiccant airconditioning LHR latent heat ratio LiCl lithium chloride MCBD mean coincident dry bulb temperature MRC moisture removal capacity NREL National Renewable Energy Laboratory OA outdoor air RH relative humidity RSHI regeneration specific heat input SHR sensible heat ratio TMY3 Typical Meteorological Year 3 x This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. Nomenclature Product or process airstream Air that leaves the conditioner and will eventually be introduced into the building as supply air. The product air may go through other equipment or processes before it is introduced into the conditioned space. Regeneration speci...

An overview of solutions for airborne viral transmission reduction related to HVAC systems including liquid desiccant air-scrubbing

“…The liquid desiccant technology is usually driven by low-grade heat sources, such as industrial waste heat [152], solar [153] and geothermal energy [154], to energy-efficiently control moisture and temperature, particularly for processes where moisture control and moisture removal is required. It has therefore found application in buildings with high latent loads (such as gyms and swimming pools [155]), in buildings where moisture control is essential for the conservation of goods (such as art galleries, libraries, museums and archives [156]), in hospitals and healthcare facilities [157], in refrigeration and cold rooms [158], in greenhouses [159], etc.…”

An overview of solutions for airborne viral transmission reduction related to HVAC systems including liquid desiccant air-scrubbing

“…The liquid desiccant technology is usually driven by low-grade heat sources, such as industrial waste heat [152], solar [153] and geothermal energy [154], to energy-efficiently control moisture and temperature, particularly for processes where moisture control and moisture removal is required. It has therefore found application in buildings with high latent loads (such as gyms and swimming pools [155]), in buildings where moisture control is essential for the conservation of goods (such as art galleries, libraries, museums and archives [156]), in hospitals and healthcare facilities [157], in refrigeration and cold rooms [158], in greenhouses [159], etc.…”

Thermodynamics and economics of liquid desiccants for heating, ventilation and air-conditioning – An overview

“…(2) Buildings with a high latent load, such as pools, have been identified as a very promising application for LDAC technology [144,148]. In this case, the pool water can be used to remove the heat of absorption resulting from the dehumidification process with beneficial effect in terms of reducing heating water pool consumption [149].…”

“…The system has been proved to perform well in some demonstrated application cases, providing cooling for the refrigerant space or reducing the condensation-related problem, such as frost. Moreover, LDAC cooling systems do not require defrost system, resulting in an economic saving [146,149]. 5Greenhouses require a control on the relative humidity for the optimal growing condition of plants, the relative humidity required for greenhouse application is higher than residential air-conditioning.…”