Weather-responsive shade control system
An automatic daylighting method adjusts a window covering to block direct sunlight from entering the room through a window when the exterior sky condition is a sunny sky state and, subject to blocking direct sunlight, provides a desired daylighting interior light illuminance level and, if possible, a desired interior solar heat gain through the window. To prevent window covering oscillation, a delay may be used when the sky condition changes from a sunny to overcast state. The covering control may be based on various factors including interior light illuminance entering the window, a room heating or to cooling mode, whether the room is occupied by people, whether occupants have manually operated an adjustable window covering, and the exterior sky condition. The method may also detect an interior temperature level, e.g., to determine a heating or cooling mode of the room.
1. A method for automatic daylighting, the method comprising:
detecting an interior light illuminance level primarily due to exterior light entering a window into a room;
determining whether the room is in a heating mode;
detecting whether the room is occupied by people;
detecting whether occupants have manually operated an adjustable window covering for the window in the room;
detecting an exterior sky condition;
calculating an adjustment of the adjustable window covering, such that the adjustable window covering is modulated to
i) block direct sunlight from entering the room through the window when the exterior sky condition is a sunny sky state and the room is in a cooling mode,
ii) subject to satisfying (i), provides a desired daylighting interior light illuminance level through the window that is within a target daylighting range and provides a desired interior solar heat gain through the window that is within a variable target heat gain range; and
automatically adjusting the adjustable window covering based on the calculated adjustment, wherein the adjusting is delayed when the estimated exterior sky condition changes from a sunny sky state to an overcast sky state.
2. The method of claim 1 wherein the calculated adjustment is calculated in dependence upon the detected interior light illuminance level, the exterior sky condition, the determined heating mode, whether the room is occupied by people, and whether occupants have manually operated the adjustable window covering.
3. The method of claim 1 further comprising detecting an interior temperature level to determine the heating mode.
4. The method of claim 1 wherein determining an estimated exterior sky condition comprises separately detecting four exterior light illuminance levels from four corresponding distinct directions, computing differentials between the four exterior light illuminance levels, and using logic to produce the estimated exterior sky condition.
5. The method of claim 1 wherein the adjustable window covering is an interior motorized bottom-up roller shade mounted on the window or skylight and having a shade cloth which allows diffuse light transmission but not specular light transmission.
6. The method of claim 1 wherein the adjustable window covering is a variable emissivity coating or electrochromic glass.