NREL has developed the novel concept of a desiccant enhanced evaporative air conditioner (DEVap) with the objective of combining the benefits of liquid desiccant and evaporative cooling technologies into an innovative "cooling core." Liquid desiccant technologies have extraordinary dehumidification potential, but require an efficient cooling sink. DEVap's thermodynamic potential overcomes many shortcomings of standard refrigeration-based direct expansion cooling. DEVap decouples cooling and dehumidification performance, which results in independent temperature and humidity control. The energy input is largely switched away from electricity to low-grade thermal energy that can be sourced from fuels such as natural gas, waste heat, solar, or biofuels.
Executive SummaryBuilding materials and furnishings play an important role in moderating relative humidity fluctuations. Accurately accounting for moisture buffering in building simulations is central in determining the need for, and the energy use from, controlling humidity. In building modeling, moisture buffering has typically either been ignored or has been lumped with the zone air using an effective moisture capacitance multiplier. Researchers have also used finite-difference models to simulate moisture transfer within materials, which are more physically realistic than the effective capacitance model, but require orders of magnitude more computation time.This study examines the effective moisture penetration depth (EMPD) model and its suitability for building simulations. The EMPD model is a compromise between the simple, inaccurate, effective capacitance approach and the complex, yet accurate, finite-difference approach. Two formulations of the EMPD model were examined, including the model used in the EnergyPlus building simulation software. We uncovered an error in the EMPD model in EnergyPlus, which was fixed with the release of EnergyPlus version 7.2. The EMPD model in earlier versions of EnergyPlus should not be used.Three simple building simulation cases were used to compare the two EMPD formulations, the effective capacitance model, and the finite-difference model. An analytical solution for the first case showed that the two EMPD formulations were not equal, but that both were improvements over the effective capacitance model. For the cases that more closely resemble real building loads, the improvement of the EMPD model over the effective capacitance model was small unless the EMPD model included two penetration depths: a surface layer for short-term humidity fluctuations, and a deep layer for longer term fluctuations. We are presently working to implement the dual-depth EMPD model in a future EnergyPlus version.vii
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