How to Choose the Best Membrane for a Commercial Solar Power Installation

By Thomas J Taylor PhD 02-29-2016
QUICK SHARE
Tag Icon

At the end of 2014, there were 16,000 megawatts (MW) of installed solar power capacity in the U.S. Of that, around 4,750 MW were from commercial PV installations, which include low-slope rooftop installations (the remaining was from residential and utility installations). There is little doubt that solar power has become significant; in comparison, coal-fired power plants typically are between 400 to 600 MW each. Also, because efficiency is a factor, the most efficient U.S. coal-fired plant is right around 40% (the John W. Turk Plant in Arkansas).

Looking at past trends of solar installations as shown in the chart below, it's clear that we can expect continued rapid growth and falling solar costs that will help grow and expand the solar market.

chart

Although it varies depending on local circumstances, overall today, the cost of solar power has become competitive with traditional generation.

So, how do our roofing membrane choices play into this? There are several things to consider when selecting a membrane for a roof that might have a solar installation added.

Ease of Installation

Typical single-ply roofs have even surfaces that can be easily marked off and worked on during a solar installation. Wide and long sheets mean there are fewer seams and smooth surfaces mean that installers can rapidly get attachments and flashings installed.

Long-Term Roof Performance

No one wants to have to repair a roof that has a large solar array overburden. However, if the membrane is a multi-ply system, the job of even finding a leak gets exponentially harder. Solar arrays can't be "moved out of the way"—they are permanent and restrict access to the membrane. This again points to why single-ply membranes are the best choice.

However, membrane choice also comes down to the expected lifetime of the array versus that of the roof. Many studies have shown that solar arrays could be producing power well beyond 25 years. That makes it important to select a supplier and membrane type that can offer confidence in weathering resistance. EverGuard Extreme® is a good example of a membrane designed for long-term weather resistance, backed by an industry-leading warranty.

Solar Array Efficiency

The temperature of solar panels is a significant factor affecting how much electricity the panels produce. This is generally measured by the "temperature coefficient" of the solar panels, which is the percentage loss in efficiency per degree rise in temperature. So, as panels get hotter, they produce less power.

As listed on solar panel spec sheets, the power efficiency of an average panel is 16.21% at 111.2°F (45°C) and the "maximum power temperature coefficient" is -0.42% per °C. This means that the panel would lose 0.42% of its power output for every 1°C rise in rooftop temperature above 45°C (111.2°F). So, let's take a look at what that means for some typical roofs:

The GAF EverGuard Extreme® TPO membrane was designed to substantially reduce rooftop temperature. For instance, on a sunny day with an ambient air temperature of 89°F (32°C), the roof temperature measured on an EPDM, dark roof was 173°F (78°C), resulting in a 13.86% decrease in energy efficiency from the standard system; the roof temperature measured on an EverGuard Extreme® TPO roof was only 116°F (46.6°C), resulting in a decrease of only 0.67% in energy efficiency.

Of course, this is approximate because the air temperature around the panels might be slightly lower. However, it's clear that reflective membranes can result in more power output from solar arrays. This has actually been demonstrated in independent tests. So, typically we would expect the efficiency of a solar array to be about 13% higher when installed over a highly reflective membrane such as EverGuard Extreme® TPO, compared to a dark membrane with low reflectance.

Long-Term Membrane Reflectance

The reflectivity of a rooftop membrane is established through certifications by institutions such as the Cool Roof Rating Council (CRRC) or the ENERGY STAR® rating program administered by the Department of Energy and the Environmental Protection Agency. CRRC publishes searchable radiative data online. The reflectivity (Total Solar Reflectance or TSR) is the fraction of sunlight that a surface reflects and is measured on a scale of 0 to 1 (for example, a surface that reflects 55% of sunlight has a total solar reflectance of 0.55). According to the CRRC, the initial TSR for a typical dark EPDM membrane is a paltry 0.06 (or 3-year aged TSR of 0.07), while the TSR for the EverGuard Extreme® TPO roof is 0.835 (or 3-year aged TSR of 0.73)!

The following is a picture of a TPO roof with a solar array in New Jersey installed over 5 years ago.

solar1

The roof has never been cleaned, as can be seen by the dirt under the panels. But, where the membrane is fully exposed, rain has kept the membrane white and reflective. Clearly, based on their reflectivity, ease of maintenance, and longevity, white TPO membranes are the best choice for these applications.


Lead photo credit: Sanko Fukaya Factory Administrative Office

Don't miss another GAF RoofViews post!
LATEST UPDATES
With the labor pool stretched thin, finding time-saving solutions like installing a self-adhered roof is high on many contractors' to-do lists. Saving time doesn't mean jeopardizing quality: manufacturers continually work to deliver solutions that help increase efficiencies while still delivering a first-class installation.
Buying a home is the biggest financial investment many people will make. In a hot real estate market, homebuyers may be tempted to remove any conditions from their offer to purchase—including a roof inspection—so their bid wins the house.
A mansard roof is one of the more unique styles of roof, especially in a residential setting. Beyond its aesthetic, it offers a set of benefits that other roofs can't match—however, it also comes with some special considerations. On the right home, mansard roofs can provide both style and protection for you and your family.
Gambrel roofs, also commonly called barn roofs, are common across America. Brought over by the Dutch, some of the earliest gambrel roofs in America were found in buildings from the 16th century. Today, they fit in perfectly with the resurgence of the farmhouse style.
Since its inception a roof's primary purpose has been to shelter its inhabitants from the elements, but now the underutilized potential of roof surfaces is being realized. For roofs with large surface areas, the potential for large overburden installations, such as solar, vegetative roofing, or amenity decks can be exceptional. Even smaller roofs can have overburden that make a significant impact on the sustainability goals of a building including: increased energy efficiency, stormwater retention, energy generation, biohabitat restoration, food production, reduced urban heat island effect, and outdoor space.
Every roof is different, and that's especially true when comparing commercial vs residential roofing, also known as low-slope vs steep-slope roofing. Many roofing contractors start out installing residential roofs because they have a low cost of entry. The process requires minimal specialized equipment—most roofers are able to start out with a truck, a ladder, a nail gun, a compressor, and safety gear.
This blog contains information created by a variety of sources, including internal and third party writers. The opinions and views expressed do not necessarily represent those of GAF. The content is for informational purposes only. It is not intended to constitute financial, accounting, tax or legal advice. GAF does not guarantee the accuracy, reliability, and completeness of the information. In no event shall GAF be held responsible or liable for errors or omissions in the content or for the results, damages or losses caused by or in connection with the use of or reliance on the content.

Interested in sharing or republishing our content? We kindly ask you to adhere to our guidelines.