Environmental building products manufacturer Colt International provided the external shading development and also designed a new control system. It incorporates extruded aluminium hollow section solar shading fins, located on the roof of the gallery.
These are controlled via state-of-the-art Echelon Neuron chip technology. This allows the fins to track the rays of the sun, remaining closed on hot days, excluding sunlight from the gallery, and open on cooler days to maximise natural light.
The structural glazing over the new entrance in the quadrant area of the gallery is double glazed and suspended from a roof-mounted steel construction. The system incorporates a specially designed roof steelwork structure, which enables the glass to hang below the steel, and the shading above. A full-scale mock up was built, and eventually integrated into the building.
Motorised and controlled
The new and refurbished Tate Britain galleries have a striking double-glazed sloping roof. Internally, there are motorised black-out blinds which allow the gallery to conduct lectures with slide shows. The blinds are controlled by 230 V ac motors, driven in banks of up to five from each motor control node. There is no potentiometer feedback as the blinds are either fully open or fully closed, with no proportional opening.
The motors have internal limit switches which disconnect power when the blind has reached the end of its travel. Colt has installed a single blank Universal louvre screen for plant rooms, which includes weatherproof double bank angled slats at critical air intake points.
Louvres are also controlled by 230 V ac motors, driven in banks of up to five from each motor control node. There is a potentiometer per bank of five motors, giving positional feedback to the motor control node. The motors have internal limit switches that disconnect power when the actuator has reached the end of its travel.
A key part of the installation of the shading system was the control unit. Since art galleries rarely have large amounts to invest it was critical to get this right first time, and to leave the client a system which is flexible enough to be added to at a later date. As a result, the Tate has the first solar shading system to use Echelon building control technology.
The brief for the shading control system was clear: the louvre angle should be adjusted to provide the preset luminance level in the gallery according to internal lux sensors.
Also, there should be time and amplitude deadbands (preset) to avoid hunting and excessive movement – and to allow internal light levels to track changes in external lighting to a limited degree.
Appropriate responses
If there is insufficient light, the louvres should open to the maximum daylight position. Lux levels from each sensor are recorded in a log file which users can view. In order to achieve the brief, the Echelon control system contains three interrelated control loops. The inner control loop contains the louvre motor controller, and the networked internal lux sensors which sit behind the louvre and measure actual lux levels at that point. Upper and lower lux levels can be set by the user, such that the motor controller moves the louvre blade to achieve the desired levels.
Delays are incorporated with built-in hysteresis, to prevent hunting when lux level fluctuates as intermittent clouds pass overhead. The aim of the inner control loop is to ensure optimum lux levels within the building, corresponding to a specific bank of louvres.
Keeping track of the sun
The second control loop includes a sun tracking device. This contains a mathematical algorithm relating to sun position (azimuth and elevation) depending on date and time. It computes data based on the azimuth/elevation and the attitude of each louvre bank, distributing the information across the network. The computed data corresponds to a maximum sunscreening angle for each louvre bank.
Total control
If the angle is exceeded then direct sunlight on a sunny day will enter the building. The objective of this control loop is to limit the louvre angle to inhibit direct sunlight entering the gallery thereby damaging valuable artwork.
The third control loop distributes data on whether the sun is shining, or if the sky is overcast. The device contains three lux sensors positioned around the building. Thus, on a cloudy day, detected light levels are generally diffuse, and the reading from each sensor is more or less the same. However, during sunny periods, there will be a more marked difference in lux readings from each sensor as the sun tracks around the building.
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Building Sustainable Design
