Michael Crowley, Contributor
June 18, 2013 | 4 Comments
San Diego, CA -- How strange that the development of many solar array fields has largely become an exercise in massive excavation, grading and stormwater management. This is ironic when we consider that such projects carry the “sustainable” tag. Yet there is a better way. Such terrains can be developed with minimal disturbance to the site footprint, within an existing regulatory framework and with less impact to sensitive environmental features.
A module elevation site development technique can be used as a primary tool for determining finished grades. This approach emphasises an engineering design that focuses on environmental harmony, which allows developers to better control development and cost schedules while minimising mitigation, conveyances and land banks. Several solar developers have taken this path, with demonstrated results.
Solar arrays have historically been large grading and storm water management projects because of the client's criteria for installing the fixed or movable support racks of the array. Developers typically grade sites to a profile such as 2 percent maximum east, west and south slopes and a 1 percent maximum north slope. On sites with existing slopes of 5 percent or more, the biggest line item is earthmoving, which, of course, requires pre-construction notifications, permitting, costly studies and significant mitigation efforts, and the co-ordination of electrical, structural, geotechnical, environmental and civil engineers.
One solar energy developer decided to take a different perspective as it considered the development of a 9.7 ha site in North Carolina, U.S., that is topographically diverse and on which slopes are greater than 10 percent. The site had one drainage way and two jurisdictional wetland areas that had to remain undisturbed. Initially the developer looked to achieve a grading profile to match the industry standard.
Early calculations estimated the need to move nearly 38,000 m³ of material, which came to more than US$500,000 in estimated earthmoving and excavation costs.
Subsequently the client relaxed grading specifications to 3–5 percent maximum east, west and south slopes and 1 percent maximum north. The new specifications cut the grading estimates to nearly 19,000 m³ and halved the earthwork costs. But, even with relaxed grading criteria, the cost for grading and earthwork exceeded the project budget.
Instead the developer's engineer recommended that the team take a module elevation plan approach that focuses on the orientation of the arrays and the array post heights instead of the grading.
Typical slopes for an array allow for a 5 percent-plus maximum slope for south, east and west facing arrays and a less than 1 percent downhill slope for north facing arrays. In fact some manufacturers allow slopes of up to 9 percent and, in special cases, up to 15 percent for south, east and west facing arrays. The less than 1 percent north is preferred to avoid panel shading, but can be mitigated with sometimes costly increases in support post height.
Support posts higher than the conventional optimal 1.83 metres would likely require considerably more in materials, which would exceed the cost of grading. Post heights lower than 1.22 metres would create a clearance issue between the panels and the ground.
In a module elevation approach, the engineer divides the site into blocks of arrays. For instance, the site in North Carolina was divided into 18 array blocks. For each block, the engineer elevated the array 1.22–1.83 metres above grade and assigned elevations to the four corners and intermediate points. Areas where the array plane was greater than 1.83 metres above existing grade were designated for fill, while those areas that showed less than 1.22 metres above existing grade were designated for cut. Subsequently the site grading plan became a series of localised cuts and fills and a much smaller grading footprint.
A southwestern U.S. solar developer derived similar benefits from the module elevation approach when it sought to develop a site that included a number of jurisdictional washes (dry creeks or stream beds that temporarily or seasonally fill and flow after sufficient rain).
The typical approach would have been to submit a pre-construction notification and apply for a nationwide or individual permit through the U.S. Army Corps of Engineers. The developer would have had to fill the washes and construct the site to an appropriate grading profile.
Depending on the required permit, an additional year or more would have been added to the project schedule, which would have put the project at risk of missing the customer's commercial operation date. In this case, due to the need for a Section 404 (B) (1) Alternatives Analysis under an individual permit scenario, there was a high likelihood that the project would not be approved at this site. Even if approved, the study alone would have exceeded $150,000, not including mitigation measures both on and off site.
The approximately 20 ha site in southern Arizona would have required the sitework contractor to import over 76,000 m³ of material with an estimated cost of nearly $1.5 million to meet the typical grading profile and fill the washes. The foundations in this development ranged from 0.61–2.4 metres. Engineers would have had to design the array to span several washes, which would have exceeded the maximum 3.01 metres of centre span between foundations. Ultimately the cost of grading and earthwork made the site undevelopable. Instead the engineer stepped back and created a design tool to analyse the site and avoid the jurisdictional washes.
Using a predecessor to the now-validated module elevation plan method, each string of arrays was profiled to view the existing grade and array plain slope. By focusing on the slope of the array plain and iteratively comparing each array to the next successive string, the engineers were able to minimise grading requirements by varying the height of the solar array supports so that there was no impact to the jurisdictional washes and the topography of the site was left largely unchanged. The site was developed with a site preparation cost, including minimal grading, of less than $200,000.
The project included buffer areas and native plantings which, while protecting the integrity of the ephemeral waterways, minimised impact on the local habitat. The project could have been designed and permitted conventionally using techniques of mass grading, installation of storm water best management practices and a much greater footprint. However, applying the module elevation approach to sitework allowed the team to deliver a solution that worked within the existing regulatory framework with a minimal footprint. There were fewer permits to obtain and far less impact on sensitive environmental features on the property.
The site development strategy using the module elevation plan took the regulatory effort out of the equation, resulting in a project with a lower site development cost and fewer permitting obstacles.
Developers exert considerable effort to screen sites in an effort to avoid potential encumbrances and minimise regulatory involvement. Permits and inconsistencies from one jurisdiction to the next often contribute to a high degree of risk and often compromise project viability.
While permits ensure environmental sustainability and protect natural resources, they also take considerable time and cost to meet. The developer must engage consultants to perform the requisite studies and applications, and allow time for the agency having jurisdiction to review them.
Typically the biggest threat to any renewable resource project is permitting and the possibility that the reviewing agency will not give the green light, a growing concern considering that increasingly stringent regulations will make it even more difficult to advance development without regulatory involvement. Had the solar site development projects in North Carolina and Arizona moved forward with conventional mass excavation, sitework and grading, the developer would have been subject to a lengthy permitting process.
The module elevation approach emphasises finding a solution with minimal impact on the environment that allows developers to better control development and cost schedules while minimising mitigation, conveyances and land banks.
Optimising Site Selection
The module elevation approach is also an effective site-selection tool for solar developers looking to gain knowledge about potential site development costs before becoming financially committed to a site.
Traditionally, true site development costs were known only after the civil site design was completed. But by using GIS topography (often available for free from public domain websites) and applying a module elevation plan analysis, the developer can gain valuable cost information early in the site selection process, improving financial forecasting and decision-making well before the civil site design process begins.
In addition, an engineer can combine the module elevation plan technique with other desktop constraints-analysis services to provide the developer with a very complete early picture of the constructability, permitability and extent to which the proposed development will impact on jurisdictional areas, minimising or eliminating possible permitting nightmares later.
Focusing on array orientations and the heights of support posts – instead of the grading specifications – through the module elevation approach offers developers an opportunity to minimise regulatory concerns while delivering more affordable, timely and sustainable solar projects with a much smaller grading footprint.
Michael A Crowley, PE is a programme manager with architecture, engineering and science consultancy Kleinfelder.
Lead image: Solar array support post heights are 1.22–1.83 metres on average to optimise material costs and issues surrounding array rotation clearance. Via Kleinfelder