CREJ - Office Properties Quarterly - September 2017
The top complaints from tenants of nearly every office building is that the space is too hot or too cold, and window glare is too intense, in which case shades are used that end up blocking high rent views. Trying to regulate the temperature and glare inside office buildings is a constant battle, especially in a semi-arid climate like Denver with wide variations in temperature and very intense sunshine. Even on cold days, the cooling system is working hard to counteract multiple heat sources, including lights, body heat and equipment. However, the largest-single source of heat gain inside a building with an abundance of windows typically is heat from the windows. To combat these issues, often an insulated, double-pane window with a static tinted low-e coating (not a film) is used to reduce solar heat gain by about 60 to 70 percent and reduce glare by about 40 percent. This performance is fixed, regardless of the changes in outside heat and light levels. A new window system, which uses a dynamic (auto-adjusted) tinted electrochromic glass coating (not a film) in a double-panel window unit, provides even greater reduction in solar heat gain and glare because the tint level is automatically adjusted electronically as the sun moves in order to provide up to 90 percent reduction in heat gain and up to 99 percent reduction in glare. Each window is connected to a small central computer in the building, which is custom programmed to change the tint level as the sun moves. A light sensor on the roof makes corrections so the tint level is not too dark on a cloudy day. We conducted a building energy study to compare the energy reduction capabilities of windows with static tinted low-e glass versus dynamic tinted electrochromic glass with its control system. To demonstrate the potential energy reduction benefits for a typical office building, a study site was prepared with two identical south-facing perimeter offices. One was installed with the static tint while the other had the dynamic tint system. Both offices were located on the second floor of an office building and were built with identical room dimensions, ceiling lights, furniture and HVAC systems. They were adjacent south-facing perimeter offices and received the same level of sun exposure. To maintain a controlled environment, both rooms were unoccupied during the duration of the monitoring period. Both rooms were tied to a building automation software platform used to control and calculate energy consumption. The lighting and HVAC occupancy schedule stayed active from 7 a.m. to 7 p.m. on weekdays. The proprietary intelligence control package of the dynamic tinted electrochromic glass system was implemented into the demo room starting October 2012. The intelligence package used geometrical solar penetration, radiated energy and real-time environmental condition monitoring to automatically change the tint state of the glass for optimal solar control and comfort, without window blinds/shades. Prior to installing the dynamic tinted electrochromic glass system in demo room B, it was necessary to ensure that both rooms were receiving identical solar radiation exposure and room performance (HVAC, insulation and lighting). Therefore, both rooms were initially fitted with the same static tinted low-e glass and monitored for two weeks. The sensors and controls were calibrated and tuned to identical parameters. The resulting data showed there was less than a 2 percent difference between the two rooms. After 12 months of data collected, it was found that the dynamic tinted electrochromic glass system resulted in 39 percent total energy savings. Figure 1 shows the average total energy use from October 2012 to September 2013. Under glare conditions, which typically relate to high radiation, the electrochromic glass system transitions to the fully tinted state, blocking 90 percent of the solar heat entering the space, resulting in significant cooling savings. In its fully tinted state, demo room B required slightly more artificial lighting to maintain desired light levels. However, the additional energy required for lighting was negligible compared to the total cooling energy saved. On the weekends, the electrochromic system resulted in an 85 percent cooling savings, as shown in Figure 2 on Page 31, which shows the total energy used during one of the summer weeks in August. Results reveal significant savings during the weekend due to the weekend cooling setback set point. On weekdays, the cooling set point is 73 degrees Fahrenheit, meaning cool air will be supplied to the room once it detects a temperature of 73° F or higher. It is common to raise the setback temperature on weekends due to building vacancy and, therefore, there is no need to cool the area as attentively. Hence during the weekend, the setback temperature is set to 82° F. The electrochromic glass system’s solar heat gain coefficient keeps the temperature inside the space so cool that it hardly goes above 82° F during the day, requiring minimal cooling, as opposed to low-e glass. When examining the impact of this new technology on glare, it was discovered that low-e glass with static tinting reduces glare by about 40 percent, which proved insufficient during times of low sun angle and blinds/ shades were required to block the glare. The glare problem occurs only during times of low sun angle, which occurs throughout the day in the morning on the east side, mid-day on the south side during the winter and the afternoon on the west side. The dynamic glass system automatically adjusts the tint level starting in the morning and throughout the day. In its highest tint level, the dynamic glass system reduced glare by 99 percent, which is the level required to eliminate the use of window blinds or shades.