Jul
20
Rooftop Solar Is Better Than Thought
July 20, 2011 | 1 Comment
The quiet underside of rooftop solar now has some story to tell with hard research numbers in hand.
Jan Kleissl, a professor of environmental engineering at the UC San Diego Jacobs School of Engineering leading a team of researchers can show roof mounted solar panels aren’t just providing clean power; they are cooling your house, or your workplace during the day and holding in heat at night.
Kleissl and his team’s study “Effects of Solar Photovoltaic Panels on Roof Heat Transfer” has been published in the journal Solar Energy with its thought to be the first peer-reviewed measurements of the cooling benefits provided by solar photovoltaic panels.
Solar Cooling Powell Lab Roof Thermal and Photo. Click image for more info.
Using thermal imaging, the researchers determined that during the day, a building’s interior ceiling was 5º F cooler under solar panels than simply under an exposed roof. At night, the panels help hold heat in, reducing heating costs in the winter.
Kleissl explains as solar panels cover an increasing number of residential and commercial roofs, it becomes more important to consider their impact on buildings’ total energy costs. The team’s calculations show the amount saved on cooling the building amounted to getting a 5 percent discount on the solar panels’ price, over the panels’ lifetime. Put another way, for the building the researchers studied, savings in cooling costs amounted to selling 5 percent more solar energy to the grid than the panels are actually producing.
Data for the study was gathered over three days in April on the roof of the Powell Structural Systems Laboratory at the Jacobs School of Engineering in San Diego California with a thermal infrared camera. The building is equipped with tilted solar panels, solar panels that are flush with the roof, and panels do not cover some portions of the roof.
Anthony Dominguez, the graduate student lead on the project takes us through the logic. Rather than the sun beating down onto the roof, which causes heat to be pushed through the roof and inside the ceiling of the building, photovoltaic panels take the solar beating. Then much of the heat is removed by wind blowing between the panels and the roof. The benefits are greater if there is an open gap where air can circulate between the building and the solar panel, so tilted panels provide more cooling. Kleissl adds the more efficient the solar panels, the bigger the cooling effect.
For the building researchers analyzed, the panels reduced the amount of heat reaching the roof by about 38 percent.
Kleissl said he is confident, although the measurements took place over a limited period of time, his team developed a model that allows them to extrapolate their findings to predict cooling effects throughout the year. For example, in winter, the panels would keep the sun from heating up the building. But at night, they would also keep in whatever heat accumulated inside. For an area like San Diego, the two effects essentially cancel each other out, Kleissl said.
The idea for the study came about when Kleissl, Dominguez and a group of undergraduate students were preparing for an upcoming conference. They decided the undergraduates should take pictures of the Powell Laboratory roof with a thermal infrared camera. The data confirmed the team’s suspicion that the solar panels were indeed cooling the roof, and the building’s ceiling as well.
This is pretty good reality checking. Kleissl said, “There are more efficient ways to passively cool buildings, such as reflective roof membranes. But, if you are considering installing solar photovoltaic, depending on your roof thermal properties, you can expect a large reduction in the amount of energy you use to cool your residence or business.”
Just how much is quite a valuable answer. That awaits more funding, something an association of solar panel makers might want to get on quite soon. Kleissl notes if additional funding became available, Kleissl said his team could develop a calculator that people could use to predict the cooling effect on his or her own roof and in their own climate-specific area. To further increase the accuracy of their models, researchers also could compare two climate-controlled, identical buildings in the same neighborhood, one with solar panels, and the other without.
Obviously local savings calculations won’t be simple – solar orientation, roof angle, latitude, roof color and other variables are going to have affects – but knowing is a must when a product is still so far from economic mastery of the market for home power generation. That 38% early lab study heat reduction transferred to annual cooling load is going to have a major impact for locations that are more cooling need dominated.
There doesn’t seem to be any advantage or disadvantage for heating load in San Diego. Yet to assume that will be true anywhere is premature – and with the potential for great savings with a panel design with heating in mind – the lack of a marketing advantage can’t last long.
Seems obvious – if not thought about until now – those panels are a shade after all. Now let see where the product designers can take it. Perhaps the rooftop solar panel can be a much better deal than anyone thought.
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Some advanced architects are using screens–metal mesh–to cut heat gain into buildings, but still allow natural daylighting (thus also reducing power consumption). This is not for the rooftops, but the sides of buildings.
How long until screens are devised that also collect solar power?
Perhaps in the future, semi-translucent solar hanging screens can be devised, that cut heat gain into a building, let in some light, and also generate power.