Heliostat

Intelligent sun-tracking mirror system  |  Daylight in the basement  |  Reflected sunlight and “rainbows”

Client

Westfield

Location

London, United Kingdom

Harnessing powerful technology to bring sunlight underground

LAPD’s challenge on this project was to reflect sunlight down into a 3- storey atrium in London.  Therefore we ensured that as long as the sun was out, it would be reflected on to the basement floor 20 metres below, even on the shortest day of the year.

To continually reflect sunlight requires an intelligent sun-tracking mirror, called a Heliostat.  These large mirrors continually reflect the sun towards an object or area, provided there is no cloud cover.

It was important to conduct investigations as to the feasibility of achieving uninterrupted sunlight throughout the day and year.  For the site in question LAPD contacted the Met Office, comparing sunlight hours in various Heliostat sites across Europe.

The next stage was to investigate any obstructions around the site in three ways:

Firstly to check all building elevations of the proposed site.  This would ensure the surrounding buildings would not obstruct the sun at its lowest elevation.

Following this, LAPD conducted site visits to view the existing surrounding buildings from the rooftop location. This ensured that none of the towers in the vicinity would have an effect on the possibility of sunlight reaching the site location.

Finally, checks with the local council established the locations proposed high-rise buildings at planning stage in the area.

The Feasibility Stage:

A heliostat needs a target in the form of a secondary mirror to direct light to areas inside the atrium.  This can be one single mirror or multiple mirrors to light multiple locations.  The need to capture all of the light from the primary mirror drives the sizing of the secondary array.  The more mirrors used the smaller they become to stay within the maximum size limit.

Instead of creating single patches of light on the basement floor LAPD wanted some of the mirrors to produce the effect of dappled light through trees.  Therefore, mirrors with multiple facets achieved this effect.

To ensure the convincing delivery of the dappled light effect, LAPD requested construction of mock up mirrors.  These allowed LAPD to vary the angles of every facet within the mirrors to create a footprint of light of a suitable size with a random spread of ‘dapple’.  We shone light from these test mirrors, at the correct offset, on to LAPD’s office building.  The square mirrors reflected shapes of light very similar to the dapple of sunlight through trees. Finally, the manufactured mirrors replicated these angles.

We arranged the secondary mirrors in such a way that they had 9 mirrors in total.  5 of these are faceted to reflect dappled light across different areas of the floor.

LAPD wanted the other mirrors to be flat because they had an alternative purpose within the atrium.

Secondary mirrors & tertiary reflectors

Secondary mirrors reflect their sunlight to tertiary mirrors or reflectors to create different effects. LAPD took inspiration from Steven Holl’s Philosphy department of New York University.  In that department, playful rainbows hit incidental surfaces of the spaces.

LAPD investigated various materials and experimented with forms.  In doing so, we established the best way to receive the direct beams of sunlight from the flat secondary mirrors.  So, reflection or refraction into colours within the atrium became a possibility.  The trialled materials also had to have UV stability.

LAPD’s chosen material was a thin film from 3M that reflected sunlight and rainbows in different directions.  The film was light weight and had a ‘shimmer’.  This effect was visible due to the motion occurring around the atrium space.

The tertiary reflectors are located at points where the sunlight is not interrupted. The atrium at the site in question had several escalators within it requiring a 3D model to be created to ensure that the primary and secondary mirrors could be located precisely, along with the tertiary reflectors within the atrium interior.

Designed by LAPD, the tertiary mirrors had to look beautiful with or without sunlight.  Although the location of the resulting rainbows and sunlight was dependent on the orientation of the film, in order for the elements to look intentional and architectural, LAPD opted to create static elements that were identical in each location.  The direction of the rainbows and peaks of light then became incidental, cast on to whatever surface they happened to hit first.

We designed the tertiary reflectors with a fixing hole on top of the frame.  The hole facilitates the replacement of the prismatic film after a fixed period of UV degradation.  We advised that this should take place every two years to keep the effect looking its best.  Maintenance employees use a pole, screwed into the hole, to pull the reflector up.  They then replace the film and secure it back in place.

Final Installation

The rainbow effects landed on the basement floor, the escalators and atrium white walls and were also projected into the general merchandising areas around the atrium.

The heliostat installation brought some life to the center of the store and added further visual interest to focal points around the atrium.

Ultimately there was sunlight in the basement on the shortest day of the year.