Can Mapping Urban Albedo Help Control Urban Heat Island Effect?

 

Lucas Hamilton

Lucas Hamilton is Manager, Building Science Applications for CertainTeed Corporation

Urban temperatures are rising and it has a great deal to do with the types of materials we choose to construct our habitat. Historically, our construction materials have been great absorbers of infrared and near infrared solar radiation. As our urban centers have grown they have accumulated an excess potential for heat absorption which has put them out of balance compared to more rural areas. This is what is called the heat island effect.  The good news is that every urban surface exposed to the sun becomes a potential location to reverse this process and restore the balance.

While researching maps of Philadelphia (my home) for a previous blog on billing property owners for impervious surfaces that contribute to the rainwater run-off pouring into co-mingled storm/sewer systems, I came across the map used to identify these properties by the City (http://www.phila.gov/water/swmap). 

The interactive map shows the relative water permeability of surfaces delineating between general materials such as roofing, parking areas, roads, and open spaces. I started to think about how we could use similar technology to identify the albedo of the surfaces – a material’s natural ability to reflect or absorb radiant heat gain from solar radiation.

Some creative person (with a lot of time on their hands) should be able to use tools like Google Earth, identify the nature of the surfaces they see, and draw from a database of Solar Reflective Index (SRI) values to identify the potential targets for improvement. How can we influence global cooling?  By using technology that is available to identify the albedo of existing buildings. Once identified, municipalities can incentivize people to change to cool roofs or to living roofs where appropriate. The city could encourage the re-planting of native trees in unusable areas. There are all kinds of things each property could do to make a difference.

I would love to hear what other ideas may be out there to address this issue.  Any takers?

A Fistful of Pencils – Measuring Solar Radiation on a Building

Lucas Hamilton

Earlier this week during a webinar I conducted on working with solar radiation, I gave an example to help people visual how the energy of solar radiation strikes a building or object. 

In physics and mathematics we would picture this energy as a vector component. I know that is not clear to a majority of non-science or non-mathematics practitioners so I often use an example with a fistful of pencils to help people visualize exactly what this means.  This is a fun little exercise but is not meant to be a scientific determination of the impact of solar radiation on a surface. This is simply a way to visualize the invisible.

Imagine the sunlight or energy coming across space and beating down on the surface of the roof at a normal angle which is a mathematical term for a 90-degree or right angle. To understand the impact of the solar radiation on that roof, take a piece of paper and draw a 1-inch square. Take a fistful of pencils (as many as will fit comfortably in your hand) making sure all the tips are even and bring your fist straight down on the paper striking it within the square.  Then count the number of strikes within that box and if you imagine each one of those strikes as being a unit of energy it gives you some idea of the impact of solar radiation on your roof.

If you want to imagine how that same sunlight is striking your wall, picture the angle that your wall is from that sun – usually about a 45-degree angle give or take.  Take the pencils in your hand and while sliding them to keep them flat to the paper turn your hand to a  45-degree angle and strike them into a 1-inch square box, you can see the number of strikes and what that impact would be significantly less. So if you again imagine that the pencil points are units of energy, you can see that only a fraction of the energy hits your wall compared to the roof.

This can be done with any angle and it gives you a very general idea of the solar energy impact on a surface.  While this does not give you scientific data to help you determine where your peak power would occur, it is one method that can be used to help visualize the best angle for solar panels on a roof. 

There are some online tools that can help calculate the intensity of the solar radiation based on geographic location. One example of such a tool can be found at: http://www.kahl.net/solarch/.

Lucas Hamilton is Manager, Building Science Applications for CertainTeed Corporation