Last week was a good week for CleanTech interest events in the Boston area. On Tuesday night there was the PEHub event at Bingham McCutchen’s office, which drew a great crowd, on Friday morning you could have listened to the Chairman of FERC speak at The Restructuring Roundtable at Foley Hoag’s downtown office, and on Friday evening the MIT Energy Club hosted a solar social which was good fun. That left the MIT Energy Initiative’s Thursday afternoon colloquium, featuring Tony Hayward, the CEO of BP, as “Most Infuriating Event of the Week”. It was enough to convince me to post my first blog entry.
Mr. Hayward’s 45 minute speech (prepared remarks available here), was mostly a position paper leading up to why shale-gas should be a cornerstone of US Energy Policy. And given BP’s foresight to heavily invest in shale-gas, he all but patted himself on the back. He also undermined, in one way or another, other policy tools that might deter support for shale-gas, littering his speech with the usual anti-renewable red herrings, including intermittency (I’ll deal with this in a later post), power density (I’m taking that on here), and existing track record and scalability (I’ll also deal with those in a later post).
Power Density 101 – Conventional Power is Good, Renewables are Bad
Power density is the power-generating capacity of a given area of land. Mr. Hayward for instance, during Q&A, compared a 10,000 bpd oil-well to a biofuel operation of comparable power production – he thinks you need to cultivate 100,000 acres of feedstock to generate the equivalent fuel. I don’t know much about biofuels, but on the subject of renewable power vs. conventional power, which he also alluded to, I think Solar compares well.
This issue also came to mind recently when I read Ecogeek’s comments on a paper (they attributed to The Nature Conservancy) which describes an impending land-use crisis from renewable energy development. The paper in question estimates that 73,000 square miles will be developed for renewable energy use in the next 20 years, which would make renewable energy the biggest land-use category over that period. For their paper they used the footprint of the power plant itself, and added allowances for mining and waste storage (supplemental data on power density calculations here).
On footprint alone, Solar, and other renewables, do look weak compared to conventional base-load power plants. But conventional power plants have severe second-order land-use impacts, that Solar and other renewables avoid.
Power Density 201 – Not So Fast…
Let’s take Vermont Yankee as an example, a 620 megawatt nuclear facility in Brattleboro, Vermont, which generates about 35% of Vermont’s energy. It sits on 120 acres next to the Connecticut River; on a crude measure of power density it achieves 5.3 megawatts per acre – pretty good compared to the accepted metric of 100 kilowatts (DC) / 75 kilowatts (AC) per acre that fixed-tilt crystalline Solar achieves. The 120 acres includes on-site waste storage and some administration buildings, but doesn’t include off-site activities like mining, manufacturing and fuel processing.
So initially Solar looks pretty bad, but things take a different turn when you look at the impacted land area outside the power plant’s “footprint” (you can check out Vermont Yankee’s footprint on pages 3-6 and 3-7 of this report). Outside of the footprint, nuclear facilities are surrounded by three zones of reduced but non-zero impact. The first zone, the Exclusion Zone (EZ), allows essentially no land-use or habitation, and in Vermont Yankee’s case it looks like an area of about 1,000 acres. The second is the Emergency Evacuation Zone (EEZ), a 5 mile radius, (approx. 75 square miles or 50,000 acres); this area is considered to be at a heightened risk of contamination in case of a mishap, and as long as the nuclear plant is commissioned it is overshadowed by fear of fallout. The final zone is the Emergency Planning Zone (EPZ), a fifty mile radius, and approx 75,000 square miles of impacted area. When Three-Mile Island had its near-meltdown, the Emergency Evacuation Area was called-into action; Chernobyl’s exclusion zone, the so-called” Zone of Alienation” is over 1,100 square miles (that’s 700,000 acres).
Now, skipping the off-site impacts, including extraction, manufacturing, waste management/recycling and transmission (all of which would probably look worse for nuclear than Solar), lets compare Solar’s power density against the Exclusion Zone and the Emergency Excavation Zone, land areas which are permanently and definably impacted by the presence of a neighboring nuclear power plant.
At 1,000 acres, the Exclusion Zone achieves 650 kilowatts per acre. But the result for the Emergency Evacuation Zone is distinctly worse, only 11 kilowatts per acre. In comparison, fixed-tilt photovoltaic solar in Southern Vermont, tilted at 40 degrees, achieves a power density of about 75 kilowatts per acre. A 620 megawatt Solar plant requires only 13 square miles, versus ~2 square miles for the Nuclear Exclusion Zone and ~75 square miles for the Emergency Evacuation Zone. By the latter comparison, Solar has six times the power density as the nuclear plant.
While typically power density is the measure in question, you could make an argument that this analysis should be extended to reflect capacity factor, accounting for Solar’s “intermittency”. Intermittency is another red herring, but let’s look at it that way, a measure you might call Energy Density. Nuclear power plants typically have 90-95% capacity factors, while Solar is typically 20-25% (and more like 18% in southern Vermont). Even if you give nuclear the benefit of the doubt on that one, you’re still looking at Solar, in Vermont, having about one and a half times the Energy Density of a recently upgraded nuclear power plant. Even a thin-film Solar power plant, with it’s decreased efficiency compared to crystalline silicon, would still have energy density equal to the nuclear plant.
And nothing against Vermont Yankee – pretty much any nuclear plant in the US is going to look bad on this comparison; as long as you count the impacted neighboring land. Coal plants are going to look even worse, when you consider that their particulate pollution isn’t contained to a convenient 5-mile radius.
Power Density, you might say, is in the eye of the beholder, but there’s plenty to say in favor of renewables. Ultimately it might depend on how you feel about living or working in the shadow of a thirty-seven year old nuclear power plant. And we should remember that all power plants have some land-use impact, and you have to pick your poison.
November 4, 2009
1. The harsh realities of energy, prepared remarks by Tony Hayward, delivered at MIT on 29 October, 2009; http://www.bp.com/genericarticle.do?categoryId=98&contentId=7057586
2. Energy Sprawl or Energy Efficiency: Climate Policy Impacts on Natural Habitat for the United States of America; Robert I. McDonald, Joseph Fargione, Joe Kiesecker, William M. Miller, Jimmie Powell; http://www.plosone.org/article/info:doi/10.1371/journal.pone.0006802
 For liquid fuels, energy density is the key metric, and it is computed differently: it is the energy contained in a given volume, typically measured in mmbTu per gallon in the US.
 For simplicity I’ll refer to “Solar” – apologies to the solar thermal folks.
 For ease of comparison I’ll state all power ratings in Alternating Current (AC); photovoltaic solar is more commonly rated in Direct Current (DC) and suffers approx. 25% loss when inverted to AC
 That’s a good figure for an economically optimized ground-mounted installation. Roof mounted installations tend to have shallower tilts, and hence considerably more power density. Roof mounted installations also avoid consuming land, so from a land-use perspective have even higher power densities. Here are some rough figures to show that a density of 75,000 watts AC is reasonable:
Assumption 1: say, 180W module, 1,310 mm x 990 mm, 13.5% nominal efficiency
Assumption 2: tilted at 40 degrees, with 6.5 ft row spacing to eliminate shading… ~1,000 modules per acre = 180,000 watts (DC)
Assumption 3: allow 30% loss for difficult grades, shading setbacks, access roads and the like… = 130,000 watts (DC)
Assumption 4: allow 25% inversion loss, = 97,500 watts (AC)
 As a quick primer for those of you who aren’t engineers, power is a unit of capacity (like speed, measured in miles per hour) and energy is a unit of power over time (like how many miles you’ve driven in the past two hours). As nuclear power plants typically run about 90-95% of the time, and Solar runs about 20-25% of the time, the distinction is materiel.
 Thin-film solar panels use a lower-efficiency photovoltaic materiel and are heavier; they have the advantage of being cheaper per watt, but are 8-11% efficient, versus the 14-17% of crystilline silicon.