KC-03-05-1 — An Hourly Building Simulation Tool to Evaluate Hybrid Desiccant System Configuration Options

This paper develops a building simulation tool to evaluate hybrid desiccant systems. The software tool is used to evaluate the impact of various desiccant equipment configurations and control approaches on annual operating costs and the ability to control space humidity levels. The simulation model is unique in that it properly accounts for many of the building/ HVAC system interactions that must be considered to properly predict space humidity levels. The software tool is used to assess the impact that alternative equipment configurations would have had on the desiccant system installed at a test school (Henderson et al. 2002). The simulation results predicted that the desiccant system as installed had an annual operating cost premium of 61%, compared to a base cooling unit without humidity control. If the system had been installed to recirculate air during unoccupied periods, humidity control in summer and on weekends would have been greatly improved with only a modest increase in operating costs. The results also show that configuring the same desiccant unit to operate in the recirculation mode year-round (instead of pretreating ventilation air) would have significantly reduced operating costs, primarily due to lower process fan power. This somewhat surprising result shows that fan power issues can sometimes overshadow the thermal performance of the desiccant wheel in terms of overall operating costs. The operating cost benefit of common desiccant system components, such as the regeneration-side evaporative cooler, was also shown to be modest, bringing into question the cost-effectiveness of this component. Overall, these results show that configuration and control issues can have a big impact on the cost-effectiveness of desiccant technologies. Manufacturers must consider these details in order to minimize equipment costs and operating costs so that desiccant technology can meet its full potential.