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Why Coal Ash Impoundments and Landfills Make Great Solar Sites

Doug Kilmer | September 25, 2019

Renewable energy is continuing to grow at a rapid clip in the United States, with more than 10 GW of solar photovoltaic capacity added to the grid in 2018.  The continued growth of solar provides an opportunity for power generators and developers to utilize existing assets – namely closed-in-place (CIP) coal combustion residual (CCR) units (coal ash ponds and landfills) – to site solar facilities.

According to the Environmental Protection Agency, there are more than 310 CCR landfills and 735 CCR ponds in the United States. Some of them are closed and hundreds more will be closing over the next several years, presenting the opportunity to site solar facilities on these otherwise underutilized pieces of land.

So why do coal ash units make such good locations for utility-scale solar farms?

First off, the size is right. Coal ash landfills and ponds average 120 and 50 acres, respectively, with many much larger. Assuming five acres per MW of solar, these areas are generally suitable for distributed generation scale solar farms of at least 10 MW. Some may be able to accommodate arrays of 100 MW or more.

Secondly, interconnection is already in place, whether the facility is continuing to generate power or if it has been shut down but has an active switchyard/substation. This is normally one of the most challenging aspects of siting distributed generation and utility-scale solar.

Finally, permitting can prove easier since the sites are already zoned for industrial use. That often makes it easier to build community support for the project than if building on a greenfield or similar space in a different part of town.

Factors to Consider

When considering a CCR unit as a potential site for distributed and utility-scale solar power generation, it’s important to account for the following:

Regulatory Matters

Closure compliance is a must for both the unit owner and the solar developer . The closure must comply with federal (40 CFR 257, Subpart D) and state CCR regulations, as well as with other unit-specific requirements (such as landfill requirements), including cap requirements, post-closure care (if applicable), and ancillary plans (SWPPP, NPDES, etc.). It is especially important for a third-party developer to demonstrate this compliance to secure financing and limit future liability.

Engineering and Design

  • Closure status: Units that have already been closed in place must be evaluated to ascertain cap thickness, materials, slope, construction, etc., to ensure the solar array design, especially the racking and cabling systems, can be accommodated. For units yet to be closed, the final cap design can be modified to accommodate the selected solar design, while still meeting the CCR technical design requirements.
  • CCR characteristics: The heterogeneity of the CCR material should be reviewed, especially for new CIP approaches, because it can affect the load-bearing capacity of the impoundment. More robust CIP activities can also decrease the chances that significant settlement of the cover system will occur.
  • Slope characteristics: Most current racking systems are designed for a maximum 15 percent slope, but new designs are coming out quickly as the solar industry continues to mature. Designing cover systems with gentler slopes and constructing the slopes to be more robust and less prone to slippage can provide increased area for solar development, thereby maximizing the potential of each site.
  • Permitting and risk management issues: Because CCR sites are already developed, it reduces the risk of impacting sensitive environmental and ecological systems. In addition, much of the information about site conditions has probably already been gathered. This can reduce the time required to obtain permits and make the project more attractive to government and community stakeholders compared with building in an undeveloped area, such as an agricultural field.
  • Stormwater management: For sites already closed, careful consideration must be given to existing stormwater conveyances and the potential impact the solar array will have on stormwater controls. It is paramount that the array not increase the chance of erosion, which could lead to cap breaches and increase maintenance costs. For newly planned CIP sites, the stormwater controls to accommodate the solar array can be designed directly into the cover system.

Construction and Cap Maintenance

  • Construction methods and equipment: Protection of the cover system is extremely important during construction of the solar array, which is likely to be the activity with the greatest potential to damage a cap.
    • Over sizing equipment pads to reduce bearing weight is a good practice, as well as placing transformers off the cap altogether.
    • Trenching for stringer lines – if they’re not to be placed in cable trays – should be done with toothless buckets, and hand excavation should be used to locate geomembrane layers.
    • Low pressure equipment should be used, and equipment and materials should be deployed from off the cap (for example, using a concrete pumper to pour ballasts in place).
  • Inspections and communications: As with any project, unforeseen conditions and challenges will arise during construction of both the cover system and solar array. Routine construction inspections and stakeholder communication is key to managing these occurrences quickly and efficiently.
  • Vegetation and wildlife management: Larger impoundments and landfills repurposed for solar power generation may also be an opportunity for habitat enhancement. Pollinator habitats can be designed into peripheral areas, and areas between the panels may be usable as pastureland (i.e., agrivoltaics).


CCR units can provide readily available solar sites with the added benefit of utilizing existing infrastructure to reduce costs.  They are also ideal for repurposing industrial land in lieu of greenfield development. For closed CCR units, a thorough engineering evaluation should be done on the existing cap to determine the feasibility of siting a solar array.  For CCR units yet to be closed, the final cap can be designed to minimize costs and maximize the PV array, while complying with state and federal CCR requirements.

Learn more about TRC’s comprehensive renewable energy development solutions and our proven engineering, construction and environmental strategies for coal ash management.  

And don’t miss our Southeast Solar Conference in Atlanta October 16-17. This event brings together landowners, developers and industry leaders for dynamic discussions and professional networking to advance solar energy generation in new areas of the Southeast.

Doug Kilmer, P.G.

Doug Kilmer, P.G. is Vice President of Renewables and Hydrogen at TRC. He has 30 years of professional experience representing stakeholders’ technical, logistical and financial interests. He is an expert in site characterization, energy siting and permitting, legacy site and remedial risk management, and Brownfield redevelopment. Doug’s expertise includes technical strategy development, regulatory interaction, stakeholder management, peer review, work plan preparation and implementation, and contractor supervision. He has led project teams providing siting, due diligence, development, construction and compliance on more than 10 GW of ground mount and rooftop solar PV, wind, energy storage, hydrogen and biomass/biogas developments.

One of Doug’s specialties is supporting development of environmentally distressed properties, also known as Brightfields (e.g., brownfields, Superfund Sites, landfills and coal ash ponds) for solar power generation, hydrogen and power storage across the U.S. His work includes incentives structuring and management. Doug has spoken on this topic nationally and is currently engaged on such projects in multiple states. Contact Doug at

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