The program also includes control algorithms for electrochromic coatings. In these simulations one energy optimization, and three daylighting control strategies were employed. The energy optimization mode, here referred to as “Energy”, sets the window in the dark state when cooling is needed and in the bleached state when heating is needed. This strategy minimizes the energy use, but may not be ideal from a daylighting perspective. The two daylighting controls, here referred to as “Dayl.100-400” and “Dayl.250-400” respectively, set the window in the dark state if the solar radiation exceeds 100 (250) W/m2 and sets it in the bleached state when solar radiation is below 400 W/m2. For radiation levels between the two set points the glazing is regulated to intermediate values proportional to the radiation level. In these simulations the “Dayl.100-400” and “Dayl.250-400” control strategies where used during office hours (7am to 6pm) and for the rest of the time the window was regulated according to the energy optimization control algorithm. A refinement was made to the “Dayl.100-400” control strategy by isolating a lunch hour (12noon to 1pm), during which the window was operated according to the “Energy” control strategy. It is assumed that the user is absent during this time, and therefore not affected by unacceptable lighting conditions. This refined control strategy will be referred to as “Dayl. f0“0”!400 ”.

4. Results and Discussion

Results of the energy simulations are shown in figures 3-6. The simulations were performed for a relatively well insulated building of medium thermal mass and with a balance temperature of 8 degrees Celsius. The control strategies described above where applied to the solid state electrochromic window (SS light/dark), and the two solar control (Soft SC and Abs. SC) windows in table 1 were used as references. Solar energy transmittance and thermal data for the simulated windows are presented in table 2.

Fig. 3 shows the cooling energy balance per square meter glazed area window oriented towards the south in Stockholm, Brussels, and Rome. The negative values indicate the need to remove energy from the building, i. e. cooling, in order to maintain the indoor temperature set point. As expected

the overall cooling need is much higher in Rome than in Stockholm and Brussels. It can be noted that all three electrochromic control strategies performed better than the static solar control coatings, and that the best cooling energy performance was achieved with the electrochromic “Energy” control strategy. However, this may result in unacceptable visual comfort levels for the user. It is also likely that low daylight levels will increase the electric lighting energy consumption, which is not accounted for here. The difference between the absorbing and soft solar control coating can be contributed to the lower g-value of the absorbing coating. The U-value of the low-e coating is significantly better, but when studying cooling load the g-value is of greater importance.

Fig. 4 Heating energy balance for a one square
meter window positioned on a south facing facade
in Stockholm, Brussels, and Rome.

The heating energy balance per square meter glazed area positioned on a south facing faqade in Stockholm, Brussels, and Rome is shown in Fig. 4. The “Energy” control of the electrochromic window and the soft solar control window outperform all other windows in terms of heating energy consumption. However, the high visible transmittances of these coatings, combined with the absence of other solar shading strategies and a south facing faqade, may result in visually uncomfortable conditions. From a heating perspective the “Dayl.250-400” is a slightly better control strategy than the “Dayl. i00-400”, but as previously mentioned the opposite is true for the cooling energy balance. The net result is that the “Dayl.100-400” control strategy is better in all locations except Stockholm, where the two control strategies perform equally well. In mixed climates with both cooling and heating needs it could be meaningful to use different control strategies during different parts of the year. For example, if the “Dayl.250-400” control strategy was used from November through April, and the “Dayl.100-400” for the rest of the year an additional 13 kWh/year, could be saved for each square meter window area on a south facing faqade in Stockholm. The absorbing solar control window is outperformed by all other windows due to its high U-value and low g-value.

Fig. 6 shows the total energy balance for the “Dayl.100-400” and the refined “Dayl. ” control strategy versus eight faqade orientations of a building located in Stockholm. Utilizing the “Dayl. f0“0K;400 ” control strategy, which employs energy efficient switching algorithms between 12noon and 1pm, can save up to 32 kWh/year for a south facing window. Considering the position

of the sun during this time it is not surprising that the biggest savings are achieved for south and southwest facing facades. In Stockholm the most significant energy reduction is due to increased utilization of passive heating during the winter. Similar results where obtained for Brussels and Rome, but the energy savings were to a greater extent attributed to reduced cooling need.