Commercial and industrial HVAC systems represent a large portion of U.S. energy consumption. According to the U.S. Environmental Protection Agency (EPA), buildings are responsible for 39 percent of total U.S. energy use and 68 percent of total U.S. electricity consumption. The EPA has predicted that energy and electricity demand in the United States will increase by 31 percent and 40 percent, respectively, within the next 25 years. The current economic situation and a worldwide desire to be green dictate a continued effort to reduce energy and utility consumption. This article will discuss how liquid-desiccant total-enthalpy-recovery systems can help fulfill those goals.

ENTHALPY-RECOVERY OPTIONS

The two basic types of air-to-air enthalpy-recovery systems are sensible heat and total enthalpy. Sensible-heat devices, such as heat wheels, heat pipes, flat-plate exchangers, and coil loops, transfer only temperature among ventilation and exhaust air streams. Total-enthalpy devices, such as enthalpy wheels and liquid-desiccant systems, transfer temperature and moisture among air streams.

However, enthalpy wheels cannot offer all of the benefits of liquid-desiccant total-enthalpy systems. Associated with rigid ductwork and seals that allow cross-contamination, enthalpy wheels experience frosting in cold climates. Not subject to cross contamination or frosting, liquid-desiccant total-enthalpy systems are known for flexible installation layouts and reducing the installed size of ancillary air-handling equipment.

Total-enthalpy devices can achieve greater energy savings than sensible-heat devices. In comfort-conditioning applications, total-enthalpy devices generally save about three times as much energy during summer and 20 percent more energy during winter.

Total-enthalpy-recovery systems utilize energy previously lost to the environment. Enthalpy wheels and liquid-desiccant systems can have a peak design of 65 percent of the total energy used. This amount is not altered much during off-peak conditions. The energy available plays the largest role in total savings.

Practical for facilities that require significant ventilation, such as office buildings, medical and research facilities, laboratories, and cleanrooms, liquid-desiccant total-enthalpy-recovery systems can save energy while improving indoor-air quality (IAQ) (Table 1). The systems exchange temperature and humidity among ventilation-air streams through a liquid-desiccant enthalpy-transfer solution, providing total enthalpy recovery. The air streams are scrubbed of bacteria, viruses, mold, and other airborne contaminants as they pass through the liquid desiccant, improving air quality. Impurities also are removed from exhaust streams, benefitting neighboring environments. One-hundred percent of the airborne contaminants captured in the desiccant solution are neutralized upon contact.

Additionally, the system design utilizes separate supply- and exhaust-air modules that are connected only by piping. Because ventilation and exhaust air are handled by separate modules, there is no cross-leakage or microbiological cross-contamination between exhaust air and ventilation air, which can occur with other technologies (see sidebar).

Liquid-desiccant systems offer a cost-effective means of satisfying strict IAQ standards, such as ANSI/ASHRAE Standard 62.1-2004, Ventilation for Acceptable Indoor Air Quality. The recent swine-flu outbreak served as a stark reminder of the need for microbiological protection. Contaminants from the environment can enter buildings through ventilation systems, which can discharge contaminants into the outdoor environment. Liquid-desiccant systems remove microorganisms from interior and exterior air via the desiccant spray's scrubbing effect.