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Flexible geomembrane and solar PV technology for solar-powered landfills

Flexible geomembrane and solar PV technology for solar-powered landfills

Landfills are a highly engineered environment, designed to safely contain decomposing waste and methane while keeping out the elements. Energy and waste-disposal companies recognize that landfills are not just burial sites for garbage, but reactors where gases produced by decomposing waste can be used for energy production.

At the Tessman Road Landfill in San Antonio, Texas, Republic Services Inc. has incorporated a biogas-to-energy recovery system into one of its 213 landfills. The project is taking energy recovery to a new level, including installation of a geomembrane landfill cover that includes flexible solar technology that will help power a sustainable energy park. By combining a first-of-its-kind solar technology with an existing biogas-to-energy system, the company is turning the landfill into a sustainable energy park. This green energy venture covers portions of the closed areas of active landfills with flexible, laminate-type photovoltaic (PV) solar collection strips. The flexible solar laminates, which capture the sun�s rays for conversion into electricity, are adhered directly to the synthetic, green-colored geomembrane used to cover and close landfill cells as they reach capacity. Unlike traditional rigid solar panels, which are bulky and frequently cost-prohibitive to install, this system uses flexible, nonreflective collection strips less than 0.25in. thick. The flexible solar strips can be configured to maximize the hours of sunlight exposure throughout the year, depending upon a landfill�s design and site contours.For this demonstration project, the company is partnering with CPS Energy, Greater San Antonio�s electric and natural gas provider, to deploy 5.6 acres of the 680-acre landfill with the solar energy cover, attaching more than 1,000 flexible solar strips to the landfill�s south-facing sideslope. The new solar cover will complement the landfill�s existing biogas-to-energy system, in operation since 2002. The system collects and processes biogas, which is produced naturally at the landfill through the decomposition of waste. The solar strips, which have flexible photovoltaic silicon cells that convert sunlight directly into electricity, will complement the amount of renewable energy provided by the landfill. With more than 300 days of sunlight per year in San Antonio, Republic estimates that the energy produced by the two fully-operational systems, will continuously create about 9 megawatts of power-enough to power 5,500 area homes.

Spectral Power Cap has developed a first-of-its-kind 45-acre landfill cover combining flexible geomembrane and solar photovoltaic (PV) technology into a dual-purpose system to close the landfill and generate solar energy. The geomembrane is made of thermoplastic polyolefin, similar to the material used on commercial white roofs. It contours to the shape of the landfill and can flex over time, maintaining a snug fit. The innovation is the integrated solar panels. About 7,000 flexible 144-watt PV panels are factory bonded to the geomembrane, shipped to the landfill, unrolled on site, and welded together into a solid cover. The PV panels are Teflon-coated, durable enough to walk on, and connected by a million feet of wire to four inverters that sends the solar energy onto the grid. All in all, the Spectral Power Cap combines four 250-kilowatt arrays covering 10 acres into a total operating capacity of one megawatt, enough to power 224 homes. Best of all, the system makes money for the landfill operators through an agreement with Georgia Power to sell the energy into the wholesale electricity market. The Conley geomembrane is the largest of its kind, much larger with more generating capacity than two similar installations in New York and Texas, and its success could lead to many more systems across the country. Since a lot of these landfills are built in urban settings, and are close to transmission lines, this type of system can be built across the country.

The Hickory Ridge Landfill Solar Energy Cover uses approximately 7000 solar panels to generate more than 1 MW of renewable electricity. Republic Services, who used the EGSC system, has also used this technology to perform partial closure at its Tessman Road Landfill. The 35 acre closure at Hickory Ridge converts the landfill into a solar park, transforming a liability into a revenue stream with the following potential benefits:
• Landfill post-closure care cost savings
• Solar incentives and rebates for project construction
• Solar renewable energy credits
• Sale of renewable power
• Carbon cap and trade credits
• Positive image of sustainability and energy independence
The Hickory Ridge Landfill closure represents a milestone in the solid waste industry because it replaces the prescribed Subtitle D closure cap with an alternative cap system, which provides a number of environmental and economic benefits. The transformation of a landfill that has reached its design capacity into a commercial sized solar energy facility is an extension of the �solar moment� in the solid waste industry, realized earlier in 2009 with HDR Engineering Inc.�s design of Republic Services Tessman Road Landfill Solar Energy Cover. This project represented the first design and installation of a solar landfill capping system, integrating an exposed geomembrane cap design and modern photovoltaic technology with a landfill closure. The Hickory Ridge Landfill in Georgia capped the majority of a 35 acre landfill with the same type of alternative exposed geomembrane solar capping system as that at Tessman Road Landfill. This system allows a closed landfill to generate revenue while eliminating the ongoing maintenance costs of mowing and soil replacement. With this technology, long term care has a new positive economic and sustainable component that may change the way landfill closures are approached in the future. The EGSC was engineered to meet all EPA landfill closure requirements, while providing a stable surface on which to mount an array of thin, flexible photovoltaic laminates for large-scale renewable electricity generation. For the geomembrane portion of the system, HDR used a 60-mil reinforced TPO (thermoplastic polyolefin) roofing material with a long history of successful application and performance characteristics, including UV resistance, seam strength, chemical and puncture resistance and interface friction. The Hickory Ridge solar energy cover caps three tiers of southerly-facing landfill side slope and crown. There are benches, or relatively flat areas, separating the tiers. The panel layout design includes 580 sub-arrays made up 12 panels each. The solar panel area of the closure is approximately 10 acres. The solar panels are laid out to allow access to landfill utilities such as landfill gas collection wells, while also designing it for cost effective wiring and efficient electrical operations. The entire array of panels and their accompanying infrastructure are installed on the exposed geomembrane to produce year-round renewable electricity during the 30-year post closure long-term care period and beyond. Exposed geomembrane caps are designed to outperform traditional landfill closure designs with greater environmental protection, at less than half the material cost of a conventional Subtitle D prescribed landfill closure. As per Mark Roberts, senior project manager for HDR, when comparing a solar energy cover to a traditional closure, what appears to be a missing component - the lack of topsoil or vegetative support above the geomembrane - is actually design strength. The exposed geomembrane anchors directly into the landfill whereas a traditional Subtitle D closure drapes the geomembrane atop the landfill, holding it in place with soil layers that shift and erode over time. The solar energy cover system takes advantage of the strength and flexibility of the geomembrane material to provide a final cover that is engineered to encapsulate the waste mass. A traditional cover system uses soil to act both as a ballast for the underlying geomembrane and also as a material to support the overlying vegetative growth. Furthermore, at many landfills weather conditions can make it difficult to consistently maintain the vegetative cover, leading to an overall loss of top soil materials and organic nutrients. Conversely, a solar energy cover is designed for both long-term outdoor exposure and to withstand specific weather events. The solar energy cover is anchored directly into the landfill with a series of horizontal and vertical anchors. These strengthen the overall liner system by limiting the stresses and strains the material encounters during a storm. Veneer type slope failures resulting from saturated soil conditions are a critical consideration inherent in conventional landfill closure systems. Saturated cover soil conditions can occur for a number of reasons such as changes in flow due to differential settlement, erosion, and clogging of the drainage layer. With a solar energy cover system, there are no soil or geosynthetic layers that can slip, slide and pull away from the liner in the event of saturated soil conditions. The design of the solar energy cover creates an easily maintained, durable and stable surface that conforms to landfill surface variations with long-term reliability for both energy generation and environmental protection. In conclusion, a solar energy cover creates a new source of renewable energy, helping communities pave the road to energy independence with creative land re-use and potential for widespread application on many other types of brown fields.
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