Effect of Switching Control Strategies on the Energy Performance of Electrochromic Windows

Abstract

This paper presents the results of a study investigating the energy performance of electrochromic windows under a variety of state-switching control strategies. The authors used the DOE-2.1E energy simulation program to analyze the annual cooling, lighting, and total electricity use and peak demand as a function of glazing type, size, and electrochromic control strategy. They simulated a prototypical commercial office building module located in the cooling-dominated location of Blythe, California. Control strategies analyzed were based on daylight illuminance, incident total solar radiation, and space cooling load. Their results show that when a daylighting strategy is used to reduce electrical lighting requirements, control algorithms based on daylight illuminance results in the best overall annual energy performance. If daylighting is not a design option, controls based on space cooling load yield the best performance through solar heat gain reduction. The performance of the incident total solar radiation control strategies varies as a function of the values of solar radiation which trigger the bleached and colored states of the electrochromic (setpoint range); for small to moderate window sizes which result in small to moderate solar gains, a large setpoint-range was best since it provides increased illuminance for daylighting without much cooling penalty; for larger window sizes, which provide adequate daylight, a small setpoint-range was best to reduce unwanted solar heat gains and the consequential increased cooling requirement. Of particular importance is the fact that reduction in peak electric demand was found to be independent of the type of control strategy used for electrochromic switching. This is because the electrochromics are generally in their most colored state under peak conditions, and the mechanism used for achieving such a state is not important.