PV turns old dumps into solar assets

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Click to Enlarge - Workers install the inverter unit that converts solar DC current into AC. The white strips dividing the black solar panels are wire chases that carry DC from the thin film panels to the inverter for conversion.

Aside from harvesting landfill gas, closed landfill acreage is largely wasteland. And where are these vast tracks of vacant land located? They are usually close to the metropolitan centers that filled them up or close to cities where large utilities are scrambling to meet renewable energy portfolio targets. This relative geography opens up large potential for the installation of plastic geomembrane caps covered with thin-film photovoltaics (PV). Very importantly, and unlike many solar or wind farms, this largely unproductive wasteland is usually sited next to power lines standing ready to deliver electricity into urban areas, which avoids building costly transmission infrastructure.

Traditional ground-mounted PV does not work for landfills because of prohibitions against drilling into soil caps to build the foundations necessary for steel racks to support conventional PV arrays, and the land is far too unstable to support other heavy structures like wind towers. After long term settlement these lands may serve as green space, golf courses or parklands, but usage is often problematic because of the underlying decomposition.

Exposed geomembrane caps are well established for closing landfills. A main advantage is that durable plastic covering is an environmentally safe and economical alternative that circumvents the high expense of buying, carting and spreading the required clay-content capping soils. Geomembranes are highly efficient in preventing rainwater from percolating through the waste and becoming troublesome leachate. And better than soil caps, they contain landfill gas for recovery while masking unpleasant odor.

Take the inherent benefits of modern geomembrane durability and combine it with the latest developments in thin-film solar technology, and many people are convinced that this is a hybrid solution with great potential. Combine a solar-geomembrane with gas recovery, and suddenly, what were once wastelands can become productive energy farms. At least, that’s the theory.

An expert appraisal of solar-geomembrane technology

Dr. Jean-Pierre Giroud has been a pioneer in the field of geosynthetics for 40 years and is recognized throughout the world as a leading expert in geosynthetics engineering. In 1977, he coined the words “geotextile” and “geomembrane” and was instrumental in the formation of the International Geosynthetics Society. In 2009, he was elected to the National Academy of Engineering. Following are his thoughts on solar-geomembrane technology, collected via interview by American Recycler writer Mike Breslin:

What is the significance solar-geomembrane technology for landfill operators?

Of course, solar-geomembrane technology can generate revenue for landfill operators. However, I would like to point out an additional benefit. Landfills are sometimes said to have bad reputation, which is undeserved in my opinion, because modern landfills built with state-of-the-art technology protect the environment very effectively. Nevertheless, we have to recognize that, in many circumstances, landfills are not well accepted by the public. The fact that, thanks to solar-geomembrane technology, landfills can generate clean energy can only contribute to improving landfill acceptance by the public.

Is there interest outside of the States for this technology?

I do not know if, at the present time, there is interest outside the United States in solar-geomembrane technology. However, I can say that, historically, the United States has been at the forefront of the use of geomembranes in landfills, and the domestic state of practice regarding geomembranes in landfills has inspired landfill owners, designers and regulators in many countries. Therefore, it can be expected that the United States solar-geomembrane technology will generate a lot of interest when it becomes better known abroad. Accordingly, it is important to describe the State’s solar-geomembrane technology in articles published in highly respected journals and magazines.

If you were advising a government agency, would you recommend they consider this technology?

Yes, I would. And I would recommend that research is encouraged by government agencies to develop the solar-geomembrane technology.

Are you optimistic about the widespread deployment?

The landfill industry is highly competitive. Therefore, I think that the growth of the solar-geomembrane technology will depend on economic considerations. Costs associated with the solar-geomembrane technology can be reduced if appropriate research and development effort is undertaken.

We should recognize that the technology is currently in its infancy. Progress can be made in at least two areas. First, the method used to attach the photovoltaic cells to the geomembrane and the method used to collect the electric power generated by the cells could probably be simplified. Second, inexpensive ways to secure the geomembrane against the risk of uplift by wind should be developed. Clearly, research and development work should be done by both the scientists who develop the photovoltaic cells and the civil engineers who design landfills. These are the conditions for the widespread deployment of the solar-geomembrane technology in the future.

Where do you think this technology will be in 10 years?

Predictions of this kind are generally incorrect. I can only say that the solar-geomembrane technology should be very successful in less than ten years if appropriate means are devoted to research and development.

And the theory is being deployed in a few large-scale commercial applications with early indications of success. United Solar Ovonic (known as Uni-Solar), a wholly owned subsidiary of Energy Conversion Devices, has been the pioneer in developing its amorphous silicon thin-film technology for landfill geomembranes. Energy Conversion Devices has been manufacturing thin-film panels for over 20 years and has approximately 250 megawatts in operation worldwide, primarily mounted on roofs.

No other solar manufacturer is doing geomembranes at this time, although others are interested because of the potential size of the global market.

Different than the heavy crystalline-silicon solar panels with aluminum frames and glass surfaces that must be bolted down, these thin-film panels are less than a quarter-inch thick and adhesive-applied to geomembranes. Built on a flexible, stainless steel substrate, the layers of photovoltaics are encapsulated in polymers.

While having lower energy output per square foot than conventional panels, thin films cost less to manufacture and are much lighter weight. Despite requiring a larger footprint than conventional panels for equivalent electric production, Uni-Solar claims that the payback time on investment is better than conventional panels.

In late 2007, the Malagrotta landfill outside of Rome, Italy became the world’s first to use thin-film solar to cover a landfill, but the panels were not affixed to a geomembrane. Rather, a 226,000 square foot sloping area of a closed portion was graded and covered with a thin concrete cap. On top, a two-foot high metal roof was built and covered with solar panels – essentially a roofed-over landfill. Installed by SolarIntegrated and using Uni-Solar panels, the nearly one megawatt system produces approximately 1,421,000 kWh per year.

The next evolution in the technology happened last spring when Republic Services became the first in the world to inaugurate a solar-geomembrane cap at the Tessman Road Landfill in San Antonio, Texas. This 134.4 kW DC system also uses Uni-Solar collection strips, but they are glued directly to a geomembrane made by Firestone and cover 5.6 acres of the 680 acre site.

“The system has been working great. But keep in mind it’s only been online since April. We’re averaging about 750 kWh per day. It’s not a big system, but very reliable and low maintenance. We have done no maintenance since we installed it. We get quite a bit of rain so it washes down the panels for us,” said Tony Walker, Republic’s project manager, who also pioneered the concept.

This geomembrane was installed by the traditional method of anchor-trenching the perimeter edges, and Tessman Road was specified to withstand winds up to 90 mph. The Firestone material was selected for its durability. It’s a 60-mil polyolefin thermoplastic that has been used in roofing, is warranted for 20 years and green to look like a lawn from a distance.

Large sections of 60’x60’ geomembrane were used to mount the flexible solar panels. Trench depressions between sections were used to carry wires in weather resistant chases to inverters that convert solar DC to AC, at which point the power goes to CPS Energy.

Each solar panel affixed to the geomembrane measures 18’ long, 15” wide and has a power output rating of 128 watts. 1,050 panels were installed on graded slopes for a total of 134.4 kW DC output. “It’s interesting that the modules start producing early in the morning and go to late evening. They work in very low light and we will probably be over our estimate in projected kilowatt hours by the one year anniversary date,” Walker said. Like other types of solar panels, Republic’s are performance warranted – 92 percent of minimum power output for 10 years, 84 percent for 20 years and 80 percent for 25 years.

Republic reported only one outage of less than 24 hours since the system became operational. That was caused by a major electrical storm. Lightning struck the inverter unit, but as the unit was designed to handle lightning, it only required changing a blown fuse.

Walker, who is based in Phoenix, is able to remotely monitor the Tessman Road system on a web-based monitoring system by Fat Spaniel, a provider of monitoring and control systems to the solar industry. In addition to monitoring real-time energy production, the system archives daily, monthly and annual production.

Republic’s monitoring is for company tracking only, but many solar generators publish information on websites to show the public how much renewable energy is being generated and the amount of emissions being prevented by the system.

“We’ve had several world experts on geomembranes visit our site, people who advise the EPA. All endorsed it highly. We’ve had other solid waste companies visit us that are interested in the concept. We also have a lot of solar manufacturers calling and touting that they have the same type of product, but don’t have the manufacturing capability that we need. It will be interesting to see how the competition develops,” Walker noted.

Meanwhile, solar geomembrane capping has been capturing the interest of landfill operators across the country and from several foreign countries. Republic was recently awarded a permit to construct a solar geomembrane project in Illinois of 140 kW that will generate approximately 180,000 kWh per year. “There’s not as much sun as in Texas, but we want to test this system there because it’s a different climate, colder with ice and snow. We want to see how the geomembranes and solar cells perform under those conditions,” said Walker. “The State of Illinois has voiced interest in this type of solar for the redevelopment of brownfields.”

Republic is also applying for a permit for a one megawatt system for a 48-acre closed landfill just outside downtown Atlanta. “That will be a good project because of the magnitude. When you fly over, it will bring wide public exposure to the technology,” said Walker. “Municipal landfills and project developers around the country, particularly in Florida, New York, New Jersey, Massachusetts and California have inquired. The market potential is huge.”

The flexible solar landfill cover approach is just that: flexible. By helping prevent erosion, sediment buildup and the growth of vegetation, it makes inspections easier and more thorough. To accommodate settling, sections of the solar-geomembrane can be removed, the ground remediated and then retrenched.

There is a lot of government and media hype about the “new green economy” and how it will create millions of new green jobs. However, many people do not know that the vast majority of conventional solar panels and wind turbines installed in the United States are manufactured overseas. Uni-Solar’s thin-film panels are manufactured domestically at four factories in Michigan, and they have broken ground on a fifth factory there. It’s just one more reason why flexible solar systems make sense.