National Renewable Energy Laboratory

Distributed Thermal Energy Technology

Research and Development

Infrared imaging quickly scans a liquid desiccant prototype to evaluate air and desiccant flow uniformity.

Using analyses, field data, and detailed lab data, especially full-scale equipment data from the Advanced Thermal Conversion Laboratory (PDF 911 KB) (Download Acrobat Reader), we are identifying sensors, air-cleaners, and thermal conversion components and integration approaches that have the greatest likelihood of reaching project objectives. We also work with R&D partners offering the most complementary skill sets, including those with knowledge of key drivers in target markets and those developing prototypes for lab evaluation and beta units for field-testing.

NREL's strategic research objectives for this project include the following:

• Research and develop (R&D) thermal conversion systems that can convert waste heat from onsite prime movers to heating/cooling work, to more than double source energy utilization efficiency from approximately 30% to more than 70%,
• Assist industry in R&D of novel-packaged thermally activated technologies (TAT) systems that can achieve thermal coefficients of performance (COP) greater than 1.0 while improving humidity control and indoor air quality,
• Assist in the design, analysis, and evaluation of packaged, combined cooling, heating and power systems for buildings,
• R&D on advanced desiccant materials that enhance energy efficiency, durability, and performance in emerging applications,
• Develop accurate and fast air-quality sensors and low-concentration test protocols to support evaluation of the indoor environmental quality benefits of desiccant systems and other air-cleaners.

NREL's Advanced Thermal Conversion Laboratory measures the performance of full-scale desiccant dehumidifiers and other enhanced-efficiency HVAC/CHP components.

The air-conditioning industry has been battling to meet new economic, environmental, and regulatory challenges: improved ventilation-rate standards, upgraded indoor air quality, reduced levels of gaseous emissions, phase-out of chlorofluorocarbon (CFC) refrigerant, and lowering peak electric demand. At NREL, researchers are focusing on thermally driven air-conditioning and dehumidification technologies to meet these challenges.

Thermally driven air-conditioning technology has become a valuable tool in the industry's arsenal of space-conditioning options. These cooling units provide advantages over the more common vapor-compression units. For example, thermally driven air-conditioning systems don't require ozone-depleting refrigerants, and they can use natural gas, solar thermal energy, or waste heat, thus lowering peak electric demand. They are particularly effective at treating the large humidity loads resulting from ventilation air in much of the country.

TAT dehumidifier technology is based on desiccants, materials that naturally attract moisture from gases and liquids. The material becomes saturated as moisture is absorbed or collects on the surface; but when heated, the desiccant dries out (regenerates) and can be used again. Conventional solid desiccants include silica gel, activated alumina, lithium chloride salt, and molecular sieves. Titanium silicate, a class of material called "1M," and synthetic polymers are new solid desiccant materials designed to be more effective for cooling applications. Liquid desiccants include lithium chloride, lithium bromide, calcium chloride, and triethylene glycol solutions.

Desiccant dehumidifiers remove moisture from the air, which releases heat and raises the air-temperature. The air is then cooled by heat-rejection units and cooling devices, such as evaporative coolers or the cooling coils of a conventional air conditioner. In a stand-alone desiccant cooling system, air is first dried, then cooled by a heat exchanger and a set of evaporative coolers. This system is free of ozone-depleting CFC and hydrochlorofluorocarbon (HCFC) refrigerants. In most systems, a wheel containing desiccants continuously dehumidifies outside air entering the cooling unit. The desiccant is then regenerated by thermal energy supplied by natural gas, waste heat, or the sun. A desiccant system can also supplement a conventional air-conditioning system; the desiccant removes the humidity load while the air conditioner's evaporator lowers the temperature.

NREL is also developing its unique liquid desiccant dehumidifiers to provide dual-use, regenerative chembio filtration functionality. The U.S. Department of Defense R&D community has identified these features as important to the next generation of Collective Protection air filters. Because chemical and bio-aerosol threats may be launched through HVAC systems, air supply protection is critical for Homeland Security.