DOE EERE Research Reports

Thermally Activated Heat Pumps

Desiccant Dehumidification Systems

Performance Evaluations

Performance Characteristics for a Desiccant System at Two Extreme Ambient Conditions
January 2002
Edward A. Vineyard
James R. Sand
David J. Durfee
Oak Ridge National Laboratory
Oak Ridge, Tennessee 37831

Abstract: The purpose of this study is to investigate the performance of a thermally regenerated desiccant system at two extreme ambient conditions (high sensible and high latent load) and to report the results using quantitative measures, such as latent capacity and latent coefficient of performance (COP), that better describe the efficiency of the moisture removal process. For this study, latent COP is defined as the latent capacity divided by the total energy input of the process. In addition to testing at two different inlet air conditions, several operating parameters, such as desiccant wheel speed, regeneration temperature, and face velocity, were varied to quantify their impact on system performance. The results will be used to improve the understanding of the operation of desiccant systems and to optimize their performance by changing certain operating parameters or improving components.

Desiccant space conditioning system designs are typically specified for a 95°F (35°C) dry-bulb temperature and 75°F (23.9°C) wet-bulb temperature outdoor ambient condition (condition 1 in ARI Standard 940-98). However, most systems operate at this condition less than 2% of the year. A more meaningful indication of desiccant system performance is shown by testing at an ambient condition that reflects a higher latent load and is experienced for a greater number of hours, such as condition 2 in ARI Standard 940-98, which specifies an 80°F (26.7°C) dry-bulb temperature and a 75°F (23.9°C) wet-bulb temperature. In order to evaluate the performance of a desiccant system against conventional vapor compression equipment, this ambient condition would also give a better comparison since the efficiency for air-conditioning equipment is determined at 82°F (27.8°C).

Thermally regenerated desiccant systems, which are used to reduce the moisture (latent load) of the conditioned air in buildings, are specified on the basis of grain depression (grains of water removed per pound of dry air) for a given face velocity at a specified dry bulb and humidity ratio. While grain depression gives an indication of the performance of the system, it does not adequately describe the efficiency of the moisture removal process. Several operating parameters, such as desiccant wheel speed, face velocity, regeneration temperature, wheel thickness, sector angles, and desiccant loading, affect the ability of the desiccant dehumidification system to remove moisture. There are so many design parameters that influence the operation of a desiccant system that it is difficult to quantify the impact from the interactions on system performance. Thus, this study seeks to quantify the impact of varying some of the most important operating design parameters individually so as to isolate the changes and report the results using terms, such as capacity and COP, that are common to the air-conditioning industry.

For both inlet air conditions, the latent capacity increased significantly as desiccant wheel speed, regeneration temperature, and face velocity increased. The results also indicate that latent capacity increased significantly more for the 80°F(26.7°C) dry-bulb temperature, 75°F (23.9°C) wet-bulb temperature condition as these variables increased. This suggests that the moisture removal of the desiccant system is much improved over that indicated by testing at design condition 1. The results for the latent COP were much different from those for the latent capacity. As desiccant wheel speed increased, the latent COP increased. However, as the regeneration temperature increased, the latent COP showed a slight decrease. The latent COP was quite sensitive to face velocity and showed a maximum at a particular face velocity that best matched the other system operating/design conditions. Latent COPs for the 80°F (26.7°C) dry-bulb temperature, 75°F(23.9°C) wet-bulb temperature condition were slightly less than those for the higher temperature condition. This result is most likely caused by the increased energy that must be supplied on the regeneration side of the wheel to heat the air due to the lower entering temperature of the outside air.

Keywords: desiccant dehumidification, latent capacity, latent COP, desiccant loading, regeneration temperature, desiccant wheel speed

American Society of Heating, Refrigerating, and Air-Conditioning Engineers

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ASHRAE Transactions 2003
Vol. 108, Pt. 1
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