Amongst the many site-based impacts of utility-scale solar facilities is the amount of terrain required to be graded, and how severe such grading needs to be. The classic solar facilities like those seen at Kramer Junction, California, required a completely clean scrape- grading the entire site to a 0% grade, rendering it essentially a parking lot without the asphalt. Newer facilities such as the infamous Ivanpah* SEGS purport to be an alternative, a kinder, gentler utility-scale solar facility, with far less grading than earlier designs. But does it matter?
For our brief analysis here, we’ll take a look at three different facilities, employing three different technologies (all links go to BLM’s Environmental Impact Statement [EIS] pages): the Genesis Solar Energy Project, the Ivanpah Solar Electric Generating System, and the Desert Sunlight Solar Farm. Genesis employs parabolic trough technology, the infamous Ivanpah (OK, I’m done for this post) utilizes concentrating power tower technology, and Desert Sunlight is a photovoltaic plant.
Parabolic troughs at Plataforma Solúcar in Sevilla province, Spain
Genesis Solar Energy Project
Genesis, a 250MW nameplate capacity facility sited on 1,950 acres of Public Land, is certainly the most heavy-handed of the three when it comes to grading. Parabolic troughs, by the very nature of their technology, require extremely flat land. They concentrate sunlight in trough-shaped mirrors, focusing on a central clear glass tube which is full of a thermal transfer medium, usually oil. This medium is then brought to a conventional heat engine, where (as with almost every other form of energy production known to man, except wind and photovoltaics) it is used to heat water, which boils into steam, which spins a turbine, thus generating electricity. The length of the troughs and tubes, and their rather sensitive alignment, means that almost perfectly flat ground is required for the facilities.
Water diversion scheme for Genesis Solar Power Project, as depicted in the EIS.
Genesis, built in the Chuckwalla Valley outside of Desert Center, CA, required substantial alteration of local hydrology. The entire 1,950 acre site was graded, resulting in 1,000,000 cubic yards of earth being moved, and substantially altering the function of 90 acres of ephemeral washes (90 acres of washes means literally hundreds or maybe even thousands of washes, given their linear nature and low acreage). Water from all washes crossing the site was diverted in massive engineered drainage channels, which send the water across the site and downslope in concentrated waterways. This causes downstream peak flow rates to increase dramatically, in some cases by as much as 300%, which increases downslope erosion, and potentially will “dry out” certain areas that are no longer receiving sheet flow. In order to attempt to slow down outgoing storm water, NextEra proposed to utilize hydraulic energy dissapators and downstream riprap splash pads. BLM and CEC were skeptical enough of the success of this plan that they required ongoing downstream monitoring for erosion and altered sediment loads, and revision of mitigation plans as needed.
There is some irony in the amount of time and effort spent discussing storm water diversion on this site in the EIS, given what happened in Summer of 2012. A powerful desert rainstorm, certainly not uncommon in the area, dropped 3.5” of water on the site in less than six hours, causing a massive flash flood. The project, which was still under construction at the time, experienced about $5 million in damages, as flood waters raced across the actual project site, breeching the flood channels and destroying solar panels. Afterwards, it was found that the channels silted up to the point where they could not accept water, hence the breech. According to NextEra, the channels and other flood control structures were not completely finished in their construction. But it was certainly a potent reminder that the desert’s hydrological patterns cannot be easily altered.
Ivanpah Solar Electric Generating System
An example of a concentrating solar power tower facility, Gemasolar, Sevilla province, Spain
More has been written about Ivanpah than I really care to reiterate here, so I’ll just give you a few links. It is a concentrating solar power tower facility, wherein sunlight is reflected from dispersed heliostats (mirrors on modular bases) to a central, tall tower, which contains a steam engine generating power just like at Genesis.
Ivanpah’s unique design means that its footprint is substantially different than that of Genesis. The heliostats’ sole function is to focus sunlight on the power tower, tracking the sun throughout the day to maximize the amount of reflected light. Given that there are 214,000 heliostats, each individual heliostat is relatively unimportant to the overall project. As such, rather than grading off the entire 3,500 acre area which is covered by heliostats, BrightSource is generally maintaining the hydrographic profile of the area underneath the heliostats, grading and diverting water only around the power blocks, which is where the power towers are located. Grading would result in the moving of about 250,000 cubic yards of fill, or one quarter as much as Genesis, spread over an area two times as large, which yields, by this particular measurement, a grading intensity one eighth as much as that of Genesis. Additionally, however, there were concentring rings of heliostat access roads graded, adding somewhat to the hydrological impacts of the project.
Detail of the heliostat access roads in concentric circles at Ivanpah. Photo courtesy Jamey Stillings.
In the plans for reduced grading, BrightSource purports to adhere to the principles of Low Impact Design (LID). LID is a set of principles intended to guide development such that it minimizes its impact on water resources, for instance by promoting natural flow regimes, by promoting groundwater recharge, and other virtuous impacts. And indeed, compared to a wholesale grading of all 3,500 acres, they have minimized the impact of their design. But the CEC/BLM staff who prepared the EIS are skeptical of the project’s success in this regard: “Even with these LID methods employed, project development would likely have effects that result in reduced storm water infiltration and increased runoff,”. And indeed, the EIS reveals substantial changes to the hydrology during peak flow events: a 10-year storm event would see a 3% increase in peak flow volume after construction, with a 16% increase in maximum water velocity; while a 100-year storm event would see a 4.5% increase in peak flow volume, with a very significant 44% increase in maximum water velocity.
Desert Sunlight Solar Farm
Desert Sunlight is a 550MW nameplate capacity photovoltaic plant, sited on 4,144 acres of Public Lands, also in the Chuckwalla Valley, in the same vicinity as Solar Genesis. Photovoltaics, not requiring a heat transfer medium or other centralized energy production facilities, are much more flexible in their deployment. They can be mounted on steep rooftops, undulating terrain, or even insanely steep slopes. As a result, Desert Sunlight utilized what might be referred to as a selective grading system.
Grading schematic for Desert Sunlight, from the EIS. Blue shaded areas outlined in dark solid black are Type 1; those outlined in red are Type 2; and the remainder of the area within the hashed-line project boundary is Type 3. Areas outlined in green are proposed stormwater retention basins.
Type 1 grading, traditional cut-and-fill leveling off of the ground, occurred on 31% of the site. They claim that they are using an “isolated cut/fill and roll” grading method (Type 2), which they also refer to as “micrograding,” on about 9% of the site; these areas retain their basic hydrographic form. And on the remainder of the site, they used a novel type of grading they call “disc and roll,” (Type 3) wherein conventional farming equipment is used to mulch vegetation and compact the mulch and churned up soils into a uniformly flat surface. These non-conventional grading plans mean that they reduced their cut-and-fill amount from 1,350,000 cubic yards to 755,000 cubic yards, an almost 45% reduction. (It should be noted that some amount of searching reveals no previous instances of “disc and roll” grading in any previous environmental review documents for any project of any kind; nor is there any mention of this sort of grading via google searches.)
The exact relationship between these “lighter impact” grading techniques and the flow of water across the project site is unclear from the EIS. The majority of the heavy Type 1 grading occurs at the northwest corner of the project site, the upslope side, where a number of ephemeral washes come into the site. These washes actually flow through the site, but a series of retention basins is to be built in various locations across the site, to slow down incoming water and reduce flow volumes and speeds. However, behind those retention basins lies the area most heavily graded, which is meant to only support sheet flow, not concentrated flow as occurs in a wash. Downslope from this area is the portion of the site that is lightly graded, where presumably the natural drainage structure will remain intact. The question then, is will the retention basins be enough to stop water from concentrating in the washes which flow across the site? The modeling in the EIS shows nominal increases to peak flow volume and velocity, less than was revealed in the Ivanpah EIS. But, as was revealed in the Genesis flood, the desert can behave in unexpected ways. Additionally, the analysis as to adverse impacts to downstream riparian communities is far less rigorous in the Desert Sunlight EIS, as they seem to anticipate no downstream impacts. This seems highly unlikely, given the still significant alteration of flow regimes that the Desert Sunlight grading plan entails.
The Real Question
In my mind, the real question is: so what? I’ve been analyzing EIS’s like those referenced above for months, trying to determine if there is a significant difference in the level of on-the-ground impacts between them. Sure, Genesis is a completely clean scrape, leaving nothing of the native flora or fauna or habitat intact. But is Ivanpah any better? Is it any better to leave the bottom 18″ of plants on the ground, in a vain attempt to maintain current hydrological flow patterns? Or is that simply paying lip service to maintaining ecological function in a site that will be horrifically degraded for centuries to come? Will Desert Sunlight’s “selective grading” yield a site that is better able to recover in fifty years, when those photovoltaic panels are so pathetically obsolete that they aren’t even worth recovering for scrap? I’m not even really sure how to answer this question yet (I’m just not there yet in my research). I’ve heard opinions that Ivanpah might have a substantially smaller long-term ecological footprint, as compared with a clean scrape like Genesis. I’ve also heard opinions that it’s really not even worth examining if one is “less bad” than another, in terms of impact, because ultimately it’s like comparing a gut shot to a head shot- they’ll both kill you. So, in parting, I’ll leave you with a video posted by this blog’s illustrious host, Chris Clarke. It shows Ivanpah’s “light on the land” grading in action.
*I’ve decided that from now on, I’ll always refer to Ivanpah as infamous. Or perhaps ignominious.