PVGU windows – reducing glare and generating power

An important factor when working towards zero emission buildings is the usage of natural daylight as a replacement for artificial light sources. However, the goal is not to use as much day light as possible, but to use the right amount and the right type.

Obviously all natural light comes from the sun. But the light may travel many different paths before entering the room. Let’s look at the following figure: [1]


As we can see, this figure separate between three main light components. Sky component (SC) is light coming directly from the sky vault. External reflected component (ERC) is light reflected from an external component, such as the ground, a building wall, etc. And the internal reflected component (IRC) is light reflected from an internal surface.

Some of these components are more desirable than others. Most of you have probably experienced glare and discomfort due to direct sunlight. This is because of the extreme brightness of direct sunlight.  


Direct sunlight causes glare. [2]

Direct sunlight often has a negative influence on thermal comfort as well. During warm weather it may cause discomfort and overheating. Possible consequences is excessive use of air-condition or people closing shades completely to block all incoming sunlight.

So how do we avoid the discomfort from direct sunlight while still utilizing the benefits from natural daylight? The most common solution is probably to install blinds. Blinds reflect direct sunlight as illustrated in the figure below. The sunlight is reflected to the roof so that the daylight reaches further into the room, providing homogeneous illumination as illustrated in the figure below. [1]


Blinds used to reflect direct sunlight in the Paul V. Galvin Library at IIT:



However there is a large number of shading systems available on the market, blinds being just one of them. Most available systems are summarized in the brilliant report “A guide to dayligthting and solar shading systems at high latitude” wich can be found here: http://www.sintef.no/home/Publications/Publication/?pubid=SINTEF+S21271

One of the most innovative shading solutions is a combined shading and energy generating window. Pythagoras Solar delivers PVGU windows that have horizontal solar cells between the two glass panes.  This solution provides shading from direct sunlight as well as it generates power. Since the solar cells are placed horizontally the effect on visual properties will not be changed significantly. The pictures below illustrate the visual properties of a laser cut panel, which is assumed to be similar to a PVGU window. [3]


This unique design provides four main benefits:

  • Resistance to glare from direct sunlight
  • Better distribution of daylight
  • Power generated from excess heat
  • Reduced cooling costs

All these factors contribute to higher value and reduced payback time. Pythagoras Solar lists a 3-5 years payback time on the investment [4]. Obviously this number is to be taken with a pinch of salt, but the technology is indeed promising and has been highly praised by several environmental organizations.  

The product is still fairly new to the marked and the construction industry is not known for adapting new technology quickly. It will be interesting follow the development and see whether owners see the value in such an investment.

Technical metrics and specification can be found on Pythagoras Solar’s website http://www.pythagoras-solar.com. However, data and reports from full size projects are hard to obtain since the technology is new to the market. I will keep searching for reports and feedback from projects who implemented the technology, and hopefully I’ll come back to this in later blog posts. Stay tuned!

[1] Arnesen, H. Kolås, T and Barbara, M. (2011) A guide to dayligthting and solar shading systems at high latitude. Online: http://www.sintef.no/home/Publications/Publication/?pubid=SINTEF+S21271

[2] Autodesk Education (2011) Light Distribution & Glare. Online: http://sustainabilityworkshop.autodesk.com/buildings/light-distribution-glare

[3] Soltran (2013) Laser Cut Panel. Online: http://www.solartran.com.au/lasercutpanel.htm

[4] Pythagoras Solar (2012) Solar Windows. Online: http://www.pythagoras-solar.com/wp-content/uploads/2012/04/product-brochure.pdf


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