Solar Landfills: the Future?

Landfill solar energy covers are now starting to be taken more seriously following the installation at a site in Texas, US. A new project in Georgia now aims to go one step further and convert a 35-acre landfill into one of the world’s largest solar covers. Mark Roberts looks at the advancements in geomembranes and cost and practical considerations for landfill owners. A Georgia, U.S. based landfill site has been transformed from an operating landfill that has reached capacity into a commercial scale, solar energy generating facility. A key part of this development is the use of an Exposed Geomembrane Solar Cover (EGSC) system. This technology combines an enhanced final cover anchoring system and thin film photovoltaic solar panel attached to a geomembrane. The result? An integrated final landfill cover system that allows a landfill owner to close a landfill but generate renewable electrical power. 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 in San Antonio, Texas. 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. Landfill secondary use 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 (HDR) design of Republic Services Tessman Road Landfill Solar Energy Cover in San Antonio, Texas. This Tessman Road Landfill Solar Energy Cover 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. Facing the right direction: geomembrane sidelopes on which the solar panels are mounted have an angle of approximately 18 degrees from horizontal Approximately 10 of the sites’ 35 acres are covered by solar panels 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. Long-term performance 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 sideslope 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. Engineered environmental controls 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. 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 a 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 brownfields. Mark Roberts, P.E., is a senior project manager for HDR.e-mail: More Waste Management World Articles Waste Management World Issue Archives