Technology

Daylight to illuminate building interiors

Lankan scientist invests low-cost lighting unit:

A Sri Lankan scientist has invented a low-cost lighting unit to illuminate building interiors more efficiently using freely available daylight.

Senior lecturer, Department of Physics of the University of Kelaniya N. W. K. Jayatissa has developed this unit for the Ensave Lanka company. It has been designed to reduce the heat component of solar radiation substantially so that the building interior does not get heated up by the increased cooling load due to lighting.

In addition the system entirely eliminates the Ultra Violet (UV) component of the solar spectrum, thus only the filtered visible spectrum is taken into the interior of the building.

Environment friendly

The performance of the 26 cm diameter unit has been tested and a maximum of 168 Lux was measured at the table-top height inside the building with the daylight. The diameter of the illuminated area is about 5-6 m of which the intensity variation is minimal within the inner region.

The lighting unit designed to be environment friendly, low-cost and energy saving in its operation is to be installed on the roof of the building. The unit transfers outside sunlight into the building without the heat component and the UV component of the solar radiation. The unit has a cylindrical shape. It is 80cms high with a diameter of 26cms.

The transparent window at the top of the unit has a hemispherical shape to increase the effective area of the solar radiation input into the interior of the building. The filtered radiation is sent through a light diffuser at the bottom of the unit before the light is finally radiated into the building.

The unit is installed vertically on the roof of the building, through which the sunlight is allowed to fall in via the convex glass window at the top. The vertical height of the unit can be varied depending on the height of the ceiling because the lower light diffuser level should be more or less at the same level of the ceiling while the top glass window is on the upper roof level.

Filter characteristics

It is expected to cut down more than 90% of the UV and Infra Read (IR) components of the incident solar radiation falling into the unit with the filter. The interior surface of the cylinder is highly reflective so that the light absorption inside will be minimised while transferring the daylight into the interior of the building. The diffuser at the bottom spreads out the light evenly down into the interior space of the building irrespective of the angle of the incident daylight to the top of the unit.

Filter characteristics were studied using a monochromatic light source for different frequencies and the spectrum with the filter was compared with the light obtained without the filter. Results of the test showed that the visible light region is not reduced very much by the filter.

However, there is a substantial reduction of the intensity of the solar radiation with wavelengths beyond those of the red band of the visible spectrum, where the heat component of the solar radiation exists with infrared wavelengths.

Jayatissa said that this unit can be used to illuminate the building interiors only during the daytime and is operated from natural solar radiation with no additional power being required at all.

A possible glare of light due to direct solar radiation is prevented by the light diffuser used at the bottom of the unit. Also the incoming daylight through the unit is evenly dispersed to light a large area with a lux level which is enough to work around table-top height inside the building. Variation of the lux level around table-top height with horizontal distance was measured for several different orientations of the sun with and without cloudy environments and proved viable.

As the light radiated to the interior of the building through the unit is virtually free from ultra violet radiation and infrared heat radiation and only consist of visual spectrum for the interior illumination. this type of light source is environment friendly and more suitable for human health.

Advantages

Jayatissa said that there are several advantages of daylight illumination of buildings compared to artificial light. Firstly, this creates a pleasant and appealing environment. Secondly it has excellent colour rendering and makes the interior natural. Thirdly, the unit will save energy significantly.

A maximum light intensity of about 170 Lux can be obtained around table-top height inside the building. This is well beyond the average lux level required for a working environment except for a few cases where the higher light intensity level is demanded.

Inside lux level is reduced up to 40 Lux when the clouds cover the sun blocking direct solar radiation to the lighting unit.

We propose a light-intensity compensation under these circumstances by using a low-power consuming LED lighting system parallel with the lighting unit, which is automatically operated with the decreasing Lux level due to the clouds so as to have a Lux level enough for the working environment inside the building. By this the required Lux level for the user can be maintained using a daylight compensator.

This unit operates with a required number of LEDs to maintain the set- point Lux level. A cluster with 300 jumbo white LEDs is expected to consume only about 10W. Since the solar radiation of the infrared region is filtered out from the filter, the daylight creeping through the unit into the building is virtually free of heat resulting in less temperature inside the building compared to other normal conditions with electric bulbs. The low-temperature conditions will require less energy to air condition the interior of buildings.

A series of these lighting units can be installed on the roofs of large buildings (ex: garment factories, warehouses) to illuminate the interior to substantially reduce the electricity bill by removing fluorescent bulbs to save energy.

Jayatissa said that the Energy Conservation Fund under the Ministry of Energy has assisted the project financially. Senior Lecturer of the Department of Physics, University of Kelaniya M. A. Punyasena assisted to test the unit.

GW

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