New Roofing Technologies for the Eco-Home
Filed Under: Green Technology on October 18, 2009
There are a lot of exciting developments in new tech for the rooftop of your home. In July, I talked about a new solar panel roofing tile that mimics the Spanish tile styling and generates electricity as a photovoltaic panel. Turns out, those tiles from SRS are just the tip of the iceberg when it comes to cool new technology you can put on top of your house.
Low-Tech and Cheap - The cheapest, low-tech way is to paint the roof a lighter or darker color, depending on your area on the planet. Most rooftops are dark in color and this absorbs heat. In hot areas, that is obviously not the way to go as you want to reflect and dissipate heat. Many homes in states like Georgia, Arizona, etc. can save up to 40% of their A/C bills just by painting the rooftop white.
A simple, illustrated cartoon at WorldChanging.com explains the idea fairly well, though it is focused mainly on hot parts of the planet. Like Florida and LA. It doesn’t talk about places that are heated from below the rooftop by hot air, like Washington D.C. nor does it mention cooler climates like where I live.
In D.C., I think the best solution is obvious: put a cork in it. For the rest of us who live in more northern climes, it turns out that darker tiles aren’t actually the answer. The answer is green. Either literally or figuratively. In a literal sense, green, living and growing rooftops are the answer in every location, but require a fair amount of maintenance and could be dangerous on the average house. So we look at green as a color instead.
Our home here in Wyoming has darker green roof tiles. The roof had to be replaced when we moved in and the contractor gave us a choice according to our first requirement, which was a Class 4 tile (most resistant to wind and hail). The colors available weren’t many, but this green was among them. So that’s what we chose. Guess what? It also lowers the heating and cooling bills annually by about 15%. How?
Color, man, color. In this part of the world, we have relatively mild (sub-100°F) summers and harsh, but not deadly winters (sub-zero). So for that situation, halfway between dark and light is best. The best color for that? Green. It absorbs heat in the summer, but reflects a large portion of it. In the winter time, it still absorbs that heat, but doesn’t tend to give off or lose as much. So it’s a half-way between answer.
High-Tech Color - If that halfway answer isn’t good enough, why not opt for color-changing roofing tiles instead? MIT researchers have developed roof tiles that can change color based on temperature. They go all the way to white when hot and down to black when very cold and shades of gray in-between.
They reflect from 30 to 80% of sunlight, depending on their color. So when it’s hot out and they’ve turned white, they’re reflecting 80% of the sun’s heat and as it cools, they’ll decrease their reflection as they deepen their gray until they hit black and are reflecting only about 30%.
The tiles are made of a common commercial polymer which is in a water solution and sandwiched between flexible plastic tiles. The polymer acts as an anti-freeze and also can be tailored to react at different temperatures. As it gets colder, the polymer and water condense, creating a white background and they expand, going clear, with warmer temperatures. The bottom layer of plastic sandwich is black.
Work at MIT is now progressing on a cheaper version that can be retro-fitted to existing dark or black rooftops. They’re aiming for a paint-like material than can be clear-coated onto an existing rooftop.
Colored PV Cells - Neither of those covers power generation, however. While the Spanish tile replacements I talked about in July 
are PV cells, they are still limited in how they can be used. What about solar panel tiles that can be any color you want and don’t require direct sunlight to make electricity?
Well, it’s no longer fantasy-land. A research team in Jerusalem is working on a special mixture of nanoparticulate metals mixed with fluorescent dyes that make colored panels that look similar to plexi-glass. The company developing them, called GreenSun Energy, is putting these glass-like tiles onto solar panels and is using 80% less silicon to do it.
The panels are then capable of generating power in both direct and indirect sunlight by diffusing the light towards the panel’s edges, where the silicon is. Currently, these panels achieve a 12% efficiency rate with a $2.10/W cost compared to conventional panels that average $4.54/W. Most of that cost difference is in the silicon used. The panels also suffer less loss from heat than do conventional solar panels.
The company is working to increase the panels’ efficiency to 20% and bring the cost per watt to under $1. With weather coating, the panels are more weather resistant than plexiglass and are equivalent to the stronger materials normally used as glass covers for panels.
The colors can also vary according to aesthetics and type of light normally hitting the panels.
All of these cool things are currently being done (or can be done) to improve how our houses and their rooftops affect the planet and our footprint on it.
