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NREL's core capabilities in the thermal sciences, along with its work in solar heating and cooling for the past 20 years, provide the background for its current R&D efforts to advance the next generation of thermally powered HVAC technologies. NREL's efforts leverage synergistic phenomena identified from a focus on psychrometric cycles and materials. Advances in the interrelated physics of water, energy, and human comfort feed the evolution of novel, optimized HVAC solutions.
Thermally driven cooling technology is required to achieve the target 70% or better source energy efficiency in integrated cooling, heating, and power systems. Desiccant and absorption components can be powered by waste heat from a local onsite producer of electricity such as a fuel cell, micro-turbine, internal combustion engine, or other prime mover. In addition, high-performance desiccant systems can improve the energy performance of comfort cooling systems. Conventional air-conditioning systems must lower the air temperature below its dew point to dehumidify. This chilled air must then be heated to bring it back to a comfortable level, consuming extra energy and increasing peak energy demands. Compounding the problem, these systems generally become less efficient as the cooling coil temperature is lowered to meet dehumidification requirements. Ventilation air is particularly challenging to conventional systems because of its relatively high humidity content. The inability of conventional cooling equipment to control the humidity loads imposed by ventilation air at a reasonable cost leads to decreased ventilation rates, buildup of indoor air contaminants, and sick-building syndrome.
The addition of desiccant components to an HVAC system directly removes the water vapor (latent heat) from the air, overcoming inherent dehumidification limitations of cooling coils. Desiccants enable independent control of temperature and humidity, improving HVAC system efficiency by freeing direct expansion cooling components to run at more efficient operating points. They can provide free reheat and also control humidity in the shoulder seasons when cooling coils are ineffective and allow indoor moisture problems to persist. Desiccant cooling systems can be integrated with new evaporative heat rejection components that do not use chlorofluorocarbon (CFC) and hydrochlorofluorocarbon (HCFC) refrigerants, which are currently banned and restricted. The Energy Policy Act of 1992 identifies thermally activated cooling technologies as needed parts of DOE Distributed Energy research and development programs.
The use of desiccants and other thermally regenerated sorbents in buildings can also improve, and in some cases, eliminate those airborne particles and gases that cause sick-building syndrome, dust mites, and overall discomfort in building environments. Accurate, portable, low-cost sensors are required to further develop and evaluate the indoor environmental quality benefits of desiccant and other sorbent-based air-treatment systems.
Incorporating desiccant components as an integral part of onsite power, heating, and cooling equipment will save energy and improve the performance of the entire energy delivery system. A comprehensive development program is required to ensure the reliability and performance of desiccant components in future integrated thermally activated technologies (TAT)/combined heating and power (CHP) systems.
Liquid Desiccants
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 Liquid desiccant R&D has produced this packaged rooftop air conditioner, ready for beta testing. |
A new generation of liquid-desiccant air conditioners is coming to the U.S. market that is designed to compete in packaged rooftop cooling applications. Advanced liquid-to-air contactors have been developed that contain the desiccant without mist eliminators. This is a critical advance that eliminates the maintenance traditionally associated with liquid systems, making possible their broad application as packaged systems. These contactors further improve on the industrial state-of-the-art in that they can be mass manufactured, provide the same dehumidification at a fraction of the pressure drop, and incorporate independent cooling and dehumidification in one air-conditioning component to simplify and reduce the cost of the system.
Liquid desiccant systems have a number of features that make them uniquely promising. In the context of waste-heat recycling, they can make effective use of low-temperature waste streams that other technologies can't, like the cooling water from engine generator sets and proton-exchange membrane (PEM) fuel cells. Their air-conditioning and regeneration functions can be physically separated, so the humidity-absorbing solution can be regenerated at any onsite heat source and conveniently piped throughout a building to where dehumidification is needed. Liquid desiccant can also be regenerated when heat is available and stored until there is a call for dehumidification, a critical ability needed to maximize CHP efficiency. Storing cooling potential as chemical energy in this way also minimizes losses relative to thermal storage approaches. Finally, the new contactor design opens the door to staged regeneration techniques that are predicted to double the energy efficiency of existing desiccant systems, raising the thermal Coefficient of Performance to more than 1.2, exceeding project goals.
This thermally activated cooling and dehumidification technology offers new opportunities for commercializing waste heat and other systems powered by renewable energy. Early applications for the liquid-desiccant air conditioner will be as a thermally activated cooling system for processing ventilation air in humid climates. In the future, adding solar thermal collectors can reduce ownership costs. In rooftop installations, the collectors can be located near the air conditioner to simplify installation and reduce costs. Energy storage by concentrated desiccant rather than hot water is especially valuable to these applications.
Polymer Desiccant Membranes
NREL has leveraged advances in PEM fuel-cell membrane research to produce a desiccant membrane capable of transferring moisture nearly as well as metals transfer heat. These membranes retain the moisture transport abilities of their fuel-cell counterparts but don't require their unique electrical properties, so they are more straightforward to produce. NREL and its industry partners are optimizing quasi-counterflow plate heat exchangers that recover more than 75% of the heat and moisture from waste energy flows. The resulting product, which is available in commercial HVAC sizes, is a drop-in replacement for existing heat-only recovery devices and performs as well as rotary exchangers, but without the complexity of moving parts.
This novel polymer is unique in its ability to attract, transport, and reject water vapor while remaining impermeable to other gas molecules. As such, it enables several intriguing applications that were previously impossible, ranging from hyper-efficient air conditioners and cooling towers to chemical and biological agent protective cloths. One synergistic application arising from NREL's expertise in moisture control is the containment of liquid desiccant. These membranes show great promise in devices that utilize the powerful drying ability of ionic salt solutions, while resisting their corrosive effects and completely eliminating the possibility of their entrainment into the dehumidified air stream.
Staged Evaporative Heat Rejection
Highly effective heat rejection is crucial to the ultimate energy efficiency and success of desiccant TAT. These heat exchangers must collect the heat that evolves during the desiccant drying process and are ideally implemented as indirect evaporative coolers that cool desiccated air toward some wetbulb temperature. Ninety percent effective indirect evaporative heat rejection has long been an elusive goal for energy-efficient desiccant cooling. Recent advances in staged evaporative coolers have resulted in products that routinely achieve 90%-100% effectiveness. NREL has demonstrated consistent 120% effectiveness at its Advanced Thermal Conversion Lab and is evaluating the long-term durability of this unconventional evaporative configuration. The commercial cooling product won an R&D100 award as one of the top 100 new technologies in 2004. The two technologies are perfectly complementary; desiccants enable the use of evaporative cooling in any climate, and the new staged coolers supply desiccant-dried process air at cooling-coil temperatures. The unique staged approach employs indirect evaporative cooling to cool process air with dewpoint temperature rather than wetbulb as its lower limit without ever reintroducing humidity to the process air.
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