Daylight Harvesting : Basics of Tubular daylight devices (TDD)

We all are familiar with indoor spaces that are well lit having access to natural light.

We also are quite familiar with indoor spaces that do not have access to natural light like the interiors of a house or the underground attic.

Wouldn’t it be wonderful if we had access to natural light to these indoor spaces without major renovations?

Is it possible?

Yes, it is possible.

There are devices that enable access to natural light without making major building renovations. One of the devices used in such cases is the Tubular daylighting device(TDD).

It’s an advanced skylight alternative.

Want to know more about the TDD?

Let’s start.

Tubular Daylighting Devices

Tubular daylighting devices enable natural light to be piped into the building indoors from rooftops through sophisticated mirror tubes. It’s a great way to illuminate building indoors in an energy-conscious way.

"Of all the methods of innovative daylighting, the light pipe has had the most universal application. It is basically a method of roof lighting, which by means of association with reflective tubes, directs the light to the lower level."

• Derek Phillips, Author of the book Daylighting

Tubular daylighting devices are also known by different names such as Light tunnel, Sun tunnel, Sky tube, Solar light tube, Solar tube, Tubular skylight, Sun tube, Sun Tube Skylights and others.

Tubular Daylighting – How It Works?

The different parts of Tubular daylighting devices are:

• Roof light/ Dome
• Light Pipe / Light Tube
• Diffuser

Roof light/ Dome

The Roof light dome captures sunlight from the rooftop.

Roof lights are of two types:

• In-plane Rooflights: In this case, the roof light fits along with the roof and is usually used with metal roofing. This is helpful when we can use flat surfaces to capture sunlight.
• Out of plane Rooflights – These are usually mounted on the kerbs above the roof plane and can take the shape of a dome, pyramid etc.

Out of plane Rooflights fit on the kerb. The material used for Rooflight dome can be plastic or glass. The plastic roof dome is usually made of polycarbonate, a thermoplastic material that is UV resistant. It can be extruded into thin film sheets or molded into dome shape or the shape of the pyramid to trap light.

Advantages of using Polycarbonate material are:

• It’s lightweight
• Provides Increased Strength and flexibility
• Provides uniformity of Light without sacrificing light transmission

Light Tube / Light Pipe

It’s the cylinder with a metallic reflective surface allowing light to pass through to the other end of the tube for indoor illumination. The reflective surface of the cylinder enables to capture maximum uniform light without losing out on illuminance/intensity of the light.

The most efficient of the tubes are aluminum tubes. They make use of silver and silicon oxide that maximize the reflectivity as light travels through the cylinder. They have low diffusion quotient enabling direct light to be captured with minimal loss of light intensity. They are also resistant to changes in temperature and moisture in the tube.

Diffuser

A diffuser helps light to be diffused for uniform transmission.

When transmitting light two important considerations are Light transmission and Diffusion. When a clear material is used, direct light is allowed with maximum light transmission but introducing glare (spots of intense bright spots and shadows), hence light needs to be diffused for uniform transmission.

A good diffuser enables maximum light transmission with minimal light loss.

Solar Tube V/S Skylight

Skylights are fenestrations that provide the building indoors access to natural light using the roof or portion of a roof.

Solar tube offers significant advantages compared to Skylights:

• Solar Tubes help in a significant way to reduce heat gain or heat loss compared to skylights.
• It also helps avoid some of the negative outcomes such as significant glaring and dim lighting that could be caused by skylights.
• And it is much more cost-effective compared to skylights and works well for retrofit buildings

Solar Tube Installation: Amount Of Light Received From Light Tubes

There are a number of factors that could affect the light received by Light tube. These can be categorized into product and building specifications.

Building specifications could include areas like building design which includes Citing and Massing of buildings(i.e the way the building is positioned to sunlight), roof design, obstructions and so on

Product Specification can include areas like:

• Tube length
• Tube size
• Type of Suntube used – Rigid or Flexible Sun tubes

Rigid And Flexible Light Tubes

A rigid tube is used where there are no obstructions and light is made to pass vertically from the roof to the diffuser. The system is ideal when you want access to direct light with high intensity.

A flexible light tube brings in the flexibility to mold the light pipe in case of obstructions. This can slightly reduce the light intensity compared to a rigid tube. This works well in case of retrofit buildings.

Benefits Of Using Tubular Daylighting Devices

• Access to natural light – improves the wellbeing of the occupants
• Affordable with very less maintenance
• Green and environment-friendly
• Reduced Energy costs over the long-term

To summarize: Before considering going for tubular daylighting, understand the how tubular skylights are better compared to other devices. Does it provide the lighting to suit your needs? Look at what type of tubular daylighting to use. Consider a quality provider who meets your requirements.

What do you think are the other important things to consider for a TDD? Let us know in the comments!

The Author is a Solar Consultant and is passionate about Renewable Energy Technologies. https://www.lavancha.in