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A Simple Guide to Building Photovoltaic Projects on Landfills and Other Waste Heaps

By Peggy Hock (1)

“It Seems Like a Good Idea…”

Many PV Developers get a sinking feeling of dread when considering what it will require to place their potential solar electric plants on waste dumps. Without doubt, there are great benefits from building renewable energy generation on closed capped landfills and other waste containment sites. These projects safely revitalize otherwise unusable property at lower (to no) land costs and without any potential economic dislocation associated with placing photovoltaic (PV) projects on working agricultural lands. They don’t have the same Environmental Impact Risks as PV projects planned on pristine undeveloped land. There is often enthusiastic public/stakeholder support for them as well. However, when designing, permitting and constructing these arrays, there are difficult challenges very specific to landfills and other closed waste sites which environmental engineers, EPCs and PV Project Developers must address. Here we will focus on many aspects of system design and constructability never encountered when planning “normal” ground arrays.

Keeping the Crust Intact is Non-Negotiable

Moving beyond the usual financial filters (2) which all PV Projects must pass, there are many additional design and regulatory/permitting complications which, in the end, can be simply summarized: The crucial concept one must always keep in the forefront is how any change to the site will affect the “cap” covering the waste. Construction and ongoing operation of the plant must never break, erode or otherwise impair the functioning integrity of the landfill final closure system (including any methane gas management system) already in place. That generalization will ultimately influence every design and construction aspect particular to these sites. The goal behind this moratorium is to protect the surrounding community/environment from harmful exposure (via air or, particularly, ground water dispersion).

Feel the Earth Move Under Your Feet?

Within that framework, there are filters Developers can use to better evaluate the challenges to project viability. (3)

Simply put, they must understand the following aspects of the land in question:

  • WASTE TYPE AND HISTORY? What are the contents under that crust? How much is there (i.e. volume in total and at different depths over the site)? What is the history/age of the waste in place and what have the responsible parties done to close it? For example, Municipal Solid Waste (“M.S.W”) Landfills contain organic material which decomposes and gasses off over time, causing the terrain to settle. The higher the organic waste volume and wetter the area– the faster, more dramatically it subsides. Other sites such as Construction & Demolition (“C. &D.”) landfills, Mining Residual heaps, Coal Combustion Residuals (or “Fly Ash Stacks”) and illegal toxic waste dumps (some “Brownfields”) do not have settlement issues. But whatever the type, all of them are capped.
  • HOW HAS IT BEEN CLOSED and CAPPED? In general, this will fall under two categories discussed below: (1) a multi-layer “prescriptive” a.k.a. “Subtitle D” Cap or (2) the thicker, unlined “Evapotranspiration” (“E.T.”) Alternate type cap. Either way, these covers cannot be removed or penetrated beyond ~ 6 inch depth. This means in every case: the PV will require ballasted racking.
  • WHAT IS THE TOPOGRAPHY SLOPE, [southern] ORIENTATION and AREA? Basic questions for any PV array, but add to that: how much will it settle and how will the slopes affect the ballast system design?
  • IS THERE SUFFICIENT ROAD ACCESS for installation of the PV arrays and ongoing O&M? Are there traffic restrictions (Air Quality Management) or, typically, restrictions on heavy equipment?
  • CAN THE ALTERED STORM WATER PATTERNS BE MANAGED?
  • IS ANY PROTECTED ENVIRONMENT or SPECIES IDENTIFIED? For instance, is there a migratory bird nesting area there which will periodically halt construction, for instance? Another example would be former dumps which were originally placed on what locals considered [unbuildable] “swamp land” but are areas we now call “wetlands” and have special environmental protections related to restoring them.
  • IS THERE AN ONSITE LANDFILL GAS SYSTEM in place? Note: accommodating that design and O&M will impact the PV array design but also may provide potential to leverage interconnection & off-taker agreements already in place.

A Brief Description of U.S. Final Cover Systems

  • A “Prescriptive” Cap

In general, the features of a conventional “Subtitle D” final protection barrier cover system on USA waste sites are shown in the illustration above and include the following layers added on top of a waste pile:

  1. First, a foundation Layer – usually soil—covers the trash to fill and grade the area and protect the liner.
  2. Then typically a geomembrane liner or a compacted clay layer .is spread over the site to entomb the waste mass in a water impermeable enclosure.
  3. A drainage layer (i.e. highly transmissive sands or gravels or a manufactured “Geonet”) is next added– especially in areas with heavy rainfall and steeper slopes. This is to prevent the sodden top layers of dirt from slipping off the impermeable barrier (a.k.a. a landslide).
  4. Next, typically 18 inches of soil is added as a “protection layer.”
  5. Finally, an “erosion layer” of soil – typically 6 inches of dirt of sufficient quality to support plant growth (grasses, etc., etc.) which the waste industry calls a “vegetative layer.”

 

  • Evapotranspiration Covers

Some states with arid climates (like California and Arizona) have adopted an alternative cover system which provides equivalent protection to a typical Subtitle D cover, but does not use a geomembrane liner or impermeable barrier— simply a lot more dirt. {See the diagram above.} This “E.T.” type of cover relies on a several-feet deep (usually at least 4 feet) earthen barrier composed of varying layers (with specific granularity and transmissivity) topped with a layer of native grasses or other vegetation.

 

“Whatever You Do, Don’t Smear the Frosting on the Cake!”

As mentioned earlier, with either cap described above, penetration of the final cover system is not permitted. At best, the 6 inches of erosion layer *might* be permitted to be scraped back, and replaced after installation is complete. Therefore, concrete foundation footing i.e. ballasted foundation systems are required to mount flat-plate PV panels on these sites. (4)

The ballast foundations must be stable, supporting the dead and live loads (such as snow) but must never destabilize the cap itself through excess weight nor create erosion. During construction and over time, erosion and water intrusion must be prevented. Racking systems on the ballast foundations must be able to accommodate some slope and also the projected continued settlement or subsidence of the site over time. One fantastic example of this is the 3MW PV array built by SunDurance at the New Jersey Meadowland’s A1 Kearny Landfill using Solar FlexRack mounting systems on concrete ballast foundations.

It’s important to source a racking system which can accommodate different ballast designs and a variety of posts as well as variable terrain. Solar FlexRack, for example, has a stainless steel Utility Rack which can accommodate as much as 20% (or 11.3 degrees) change of slope from east to west when mounted on a round-post. North to South variance can change from row to row and will depend on the slopes and berms of each site. Solar FlexRack arrays are engineered specifically for the unique parameters of each project. Moreover, their meticulous engineering team designs all their racks to meet the specific requirements if the module manufacturer, site loads, permitting codes, topography, ground-to-concrete friction coefficients, string sizes and many other variables. Solar FlexRack structural and geotechnical engineers work with the landfill project’s environmental engineers and construction engineers to arrive at the safest, best designed, most economical solution for the site… which is one of the reasons they have built such a strong reputation for suiting this type of project.

 

“Watch Out Where Those Huskies Go”

Another important consideration in terms of constructability is the restrictions on operating heavy equipment on these sites. There are often restrictions on the weight of the equipment and/or areas they will be allowed to traverse. This is especially important when planning the staging and placement of the ballast blocks (which often weigh several tons each) without the unrestricted use of cranes, etc., or—even more problematic– when planning to have concrete poured directly on site into pre-designed pans.

There is a corresponding relationship between the racking configurations (4×7 or 2×12 for example) with the number of posts and the corresponding number of concrete foundations– which in turn modifies the required weight and dimensions of the ballast blocks to be used.

With the case of Borrego Solar’s 2.26MW array on the Easthampton Oliver St. Landfill, three roads needed to be built for construction and maintenance access. On that site they installed 962 (3ft x 1ft x 11ft) concrete ballasts placed on crushed rock material extending 1 ft. beyond the blocks at a maximum slope of 6 degrees. These foundations formed the support for 481 (4×5 dual-post) Solar Flex Rack “Zip Racks” attached on round posts to the footings. These racks stably support 9,620 Yingli 235 watt solar panels at a 30 degree tilt, to withstand 100 MPH winds and 55 pounds per square foot snow loads for the next 30 years or more.

 

AMERESCO is currently constructing two 1.5MW PV arrays at the Sudbury Sand Hill MSW Landfill and the Acton MSW landfill in Massachusetts. There they will be using Solar FlexRack 4 x 7 “Zip Racks” each with three posts (round pipe) attached to three ballast blocks. (8 feet-5 inches x 4 feet x 18 inches thick), These arrays will be placed within the topsoil support layer i.e. the “erosion layer” of the final cover system on added gravel “leveling pads.” According to the Massachusetts Department of Environmental Protection,

Based on a geotechnical evaluation, the maximum acceptable slope for the panels and ballasts is 14 percent (8 degrees)… The footings will be placed at varying slopes as necessary due to the topography of the Landfill [in Acton].” (8)

“…Modules [at Sand Hill] will be placed on slopes with gradients of up to nine percent (9%). The application includes a stability analysis by AMEC. The Factor of Safety was determined to exceed the minimum acceptable (FS > 1.5). The module supporting system will not significantly alter the flow of storm water or snow melt to the existing storm water control basins at the Landfill.” (9)

These impressive systems are designed to withstand 100 MPH winds and 55 lbs. /sq. ft. snow loads at 25 degrees tilt angle.

 

More on the Subject of Snow…

Many of these projects are going forward in very snowy areas. Snow presents design challenges because of the added downward live loads, the concentrated storm water management (when clumped in piles and melted) and also to the electrical generation of the PV plant itself. It is useful to note that when considering different racking systems, some designs are better than others. “Many racking systems are designed so the long side of the PV panel rests on the horizontal rails creating a channel in between the panels that will drain rain (slowly) but will act as a dam for snow as it slides off the panels effectively blocking all drainage of the melting snow. ..Snow continues to accumulate and starts to cover the PV panels. Energy production is drastically reduced. Another risk [from this design] is that, under certain conditions, the trapped slush may freeze, expand and cause damage in the form of micro-fractures to the panels.” (10)

Better racking designs for snowy areas are systems where the modules are attached to the vertical rails which are in turn mounted on top of the horizontals rails. This design creates a gap between the panels to allow the snow to shed freely. This has been proven in side-by-side comparisons in Canada.

 

One Final Note about Installation Costs

Projects built on MSW landfills in particular are typically required to pay the established prevailing wage rates for any labor performed on site. This is especially true if the Municipality itself will own the project, or if the Muni will receive lease payments (even only $1.00/acre/year) under the terms of a Power Purchase Agreement or other contractual arrangement. As with all PV projects, every aspect of the construction costs must be analyzed but–especially on “Public Works” projects- it may make the utmost sense to use local factory cast ballasts (not poured) and to use simple, elegant racking systems in which a large portion of the assembly normally done in the field is pre-assembled in the factory. Very significant cost savings can be achieved this way. Please let us know how we can help.

 

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FOOTNOTES:

1National Sales Director, Solar FlexRack, peggy.hock@solarflexrack.com. who has been actively involved since 2007 in successfully developing PV projects on landfills and other waste impoundments.

2 In addition to Federal & State tax credits and PV Solar incentives which vary locally …see http://www.dsireusa.org/solar … there may be additional funding sources for these projects available. Check the following sites for more information regarding contaminated lands redevelopment incentives including grants, loans, bonds, etc.,etc. /http://www.epa.gov/oswercpa/incentives.htm Also, Brownfield tax incentives for cleanup and

3Some informative sources the reader may refer to for this information can be found at the following sites:

  • US EPA’s “Renewable Energy Interactive Mapping Tool” http://www.epa.gov/oswercpa/mapping_tool.htm and US EPA “Repowering America’s Lands” OSWER program http://www.epa.gov/oswercpa/
  • NRDC – DoD’s Renewable Energy And Defense Geospatial Database [“READ-Database”] http://www.nrdc.org/energy/readgdb.asp
  • EPA’s excellent “Solar Decision Tree” http://www.epa.gov/oswercpa/docs/solar_decision_tree.pdf
  • Environmental Stakeholders Listing : http://www.eco-usa.net/orgs/index.shtml
  • Landfill Methane Outreach Program (LMOP) Database http://www.epa.gov/lmop/documents/xls/opprjslmopdata.xls
  • Check State databases of RCRA, CERCLA and of solid waste sites e.g. the site for California: http://www.ciwmb.ca.gov/SWIS/Search.aspx
  • NAVFAC developed the Naval Installation Restoration Information Solution (NIRIS), system for maintaining all environmental remediation site data (including geographic information system data), documents, and records. A Web-based tutorial on the development and functions of NIRIS is available on line www.ert2.org/NIRIS/tool.aspx
  • Local press!

4 The author strongly recommends PV Developers and EPCs work with local Environmental Engineers familiar with their site to help them with this design (& permitting) process.

5 Photo of the A1 Kearny Landfill Courtesy of SunDurance For more details, please see http://www.sunduranceenergy.com/projects/view/njmc_landfill and the Solar Power Worls Feb. 28, 2013 Article by Stephen Bushong http://www.solarpowerworldonline.com/2013/02/racking-and-mounting-system-provides-flexibility-at-landfill-installation/

6 Photo of the ballast foundations at the Oliver Street Landfill in Easthampton, MA, courtesy of Borrego Solar

7Oliver Street Landfill, Easthampton, MA. Courtesy of Borrego Solar

8 MassDEP permit approval http://www.mass.gov/eea/docs/dep/energy/solar/pcup-sudbury.pdf

9 MassDEP permit approval h http://www.mass.gov/dep/energy/solar/pcup_acton.doc

10 Sasha Honsl, Director of Canada Operations, Solar Flexrack