Green Roofs Improve Solar Panel Efficiency

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Solar panels on green roofs, like these in New York City, can operate more efficiently because the surrounding air is cooled by plants.

Photo: Green Roof Technology

By Erin Weaver

Green roofs are known to extend roof life, conserve energy, and reduce stormwater runoff and air pollution; new studies show they can also boost the performance of solar panels. Plants reduce a roof’s contribution to the urban heat-island effect by lowering the surrounding air temperature through evaporation; this cooling can also make photovoltaic panels perform more efficiently. Plants also reduce airborne pollutants and dust particles, allowing the panels to absorb more sunlight. Studies vary in the efficiency boosts reported, from 3% to 16%. This is in addition to green roofs’ known effect of cooling indoor air, which reduces annual energy use by 6%, according to a recent French study. New designs for solar arrays allow water to drain evenly across the roof, and research suggests that the biodiversity created by including shade-tolerant plants below the panels can improve a green roof’s overall performance. A recent British study in Building and Environment found that broad-leafed plants such as lamb’s ear, which can thrive in partial shade, outperformed the traditional sedum in cooling the substrate and surrounding air.

Comments (6)

1 Green Roof Plants posted by Jorg Breuning on 12/02/2012 at 01:12 pm

Sedums are a large genus of flowering plants commonly known as stonecrops. They are found worldwide from tropical regions to the Polar Circle. There are more than 400 different Sedum species known and most of them survive in locations that have hardly soil. The survivability of Sedums under extreme conditions with a thin layer of soil and almost endless varieties makes them to the ideal, perennial groundcover for extensive green roofs.

The British study over less than two years with broad-leafed perennials like (mentioned Stachys byzantina, Bergenia cordifolia and Hedera hibernica) require deeper soil profiles. More greenroof growing media (soil) increases the costs of the green roof, the structure and the maintenance dramatically and so decrease the likelihood of having a green roof to start with. Comparing Sedums with the mentioned type of perennials is like comparing wild strawberries with apples.

The readers of this Solar/Green Roof (Sun-Root)article also must understand that the climate conditions in Great Britain are unique and generously supported with moderate temperatures by the golf stream all year around.

As a green roof expert with over 32 years of experience I can clearly say that this research is worthless and proves only common sense. If this study would have been done by experienced researches the plant selection would have considered broad-leafed, perennial plants that grow in the same conditions as the compared Sedums. In modern green roof technology there are multiple choices of herbaceous perennial plants that supplement the fundamental Sedum carpet.

2 Two Givens; Draw a Conclusion posted by James Kirby on 01/29/2013 at 05:48 pm

We know PV panels are more efficient as cell temperature drops (Given #1). We know vegetative rooftops stay cooler than "normal" roofs (Given #2).

The 'cooler' vegetative/green roof will help keep PVs cooler which means PVs are more efficient (i.e., produce more power under the same conditions).

Therefore, PVs over vegetative roofs will be more efficient. How can we not draw this conclusion?

3 Common sense and data posted by Paula Melton on 01/29/2013 at 08:47 pm

James, your conclusion certainly makes logical sense, but it remains a hypothesis until there's data to back it up. Also, the question is not so much whether the efficiency is increased, but how much it is increased, and the results vary considerably. The next step would be to determine why there is so much variation and then find strategies for maximizing the cooling effect.

4 Proven Technology posted by Jorg Breuning on 01/29/2013 at 09:41 pm

Dear Paula, Since you referred to common sense – the potential increase of efficiency depends on many factors like the exposure of the building, the size of the green roof, the surrounding environment, the geographical location and last but not least the weather (unpredictable every day).

Instead of putting millions into research to finally get (still) a range of results it is better to use our common sense that standing on soccer field is cooler than standing on a parking lot of a shopping mall.

Each technology (green roof and solar) have enough benefits to stand alone (what is already done) By combining them we generate a synergetic effect that is simply unmatched and doesn’t need to be improved – all without punching a hole I the waterproofing and all by reducing the overall costs for installation and maintenance.

Any change in the green roof system will super proportional increase these costs (you can’t improve nature without a higher input - Law of Maximum). Changes in the PV however are possible since this is man-made technology and solar panels only work profitable over 20-25 years (green roofs last 50+ years).

Investing the research $$ in more green roofs with solar would be common sense and would even lower the overall costs.

5 Conclusion vs Hypothesis posted by James Kirby on 01/31/2013 at 10:46 am

Paula, I'm certainly not going to argue terminology with you, but it seems you agree with my general conclusion. Correct, it is a matter of "how much more efficient" PVs are with vegetation below. And yes, there is some need to determine if the costs of the vegetative roof are justified by the increase in efficiency. Also, as you said in the article, the co-benefits of the vegetative roof are well known, similar to the co-benefits of renewable energy systems. However, both are very hard to include in a cost analysis. It all depends on how you set up the analysis model--longer roof life, less pollution, improved health and productivity would be appropriate parameters to include, in my opinion. Here's a link to some Northwestern University students performing analysis of PVs over vegetative roofs...and some efficiency improvement numbers! Thanks for the discussion! Jim

6 Nice article posted by Paula Melton on 01/31/2013 at 11:03 am

Cool research those students were doing. Thanks for the link!

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November 29, 2012