It was one of those “good news / bad news” stories, as reported by James Bruggers over at the C-J:
- Good news: The downtown Convention Center is getting a two-year renovation that will make it more inviting and up-to-date AND add 55,000 square feet of exhibit space. (Cool!)
- More good news: One goal of the plans is to upgrade the facility to the point that it can qualify for a Silver LEED rating. (Excellent!)
- And then the bad news: There are no plans for roof-top solar, a vegetative green roof, or even painting the roof a light color. (Whaa..?)
This stuck me as a massive missed opportunity. Why would you spend $180 million in a city that says it is trying to be serious about sustainability, and not do something with the largest roof in all of downtown?
So I decided to do some research, to see just how big a miss this really was. Here’s what I found out.
Yeah, It’s a Big Roof
I decided to see if there was a way to estimate how much electricity could be generated if you put solar panels on the roof of the Convention Center. To start with, I had to know just how big a surface was available. That part was easy — just go to Google Maps and measure. Roughly speaking, our Convention Center is 400 x 860 feet, or about 344,000 square feet of roof space.
So, how many panels can you put up there, and how much electricity could they generate? I spent a couple hours looking for reliable information on solar panel output and sizes. Turns out most of the panels are roughly 39″ by 65″, or 2,935 square inches, or about 20 square feet. Throw that against the square feet of the roof, and you’ve got about 17,000 panels if you just laid them out flat. (Of course, they don’t normally get installed that way; more on that in a moment.)
That Internet Thing Comes In Handy
When I tried to calculate how much power that many panels would generate, though, I was stuck. Why? Because solar generation is dependent on your location and your weather, and I wasn’t sure how to calculate the effect of either. Fortunately, we have this thing called the Internet, with all sorts of handy-dandy calculators and such.
First, I discovered that Weather Underground has a solar power calculator, that takes your location and tells you how much solar-powered electricity you can generate per month, based on the average sunlight at your location. It was okay, but you had to provide a lot of details that I wasn’t sure about.
Then I found a solar power calculator run by the Department of Energy, and it was very cool! It was easy to use, and even let you draw your installation on a Google map. It then calculated everything else, including sunlight, panel angle, savings based on your cost of electricity, and so on.
The back-of-the-napkin result from the DOE calculator? About 6.5 million kilowatthours (kwh) generated a year. Since LG&E charges about ten cents a kilowatt-hour, this would save about $650,000 a year in electricity costs. Installation costs? Harder to find current data, but it looked to be between $2 and $3 per watt installed, so between $12 million and $18 million. A 20- to 30-year payback wasn’t looking as cheerful as I thought.
The next day, I decided to get a reality check on my figures, so I called Dan Hofmann with Regenen Solar. I explained what I was researching, and Dan graciously agreed to run a hypothetical Convention Center install through his estimating spreadsheet.
He started with roughly the same roof area that I had started with, but he had the advantage of knowing how much space would be needed for the racks, and how much tilt, and how much space to leave between rows for walking and wiring, and so on. As a result, his hypothetical installation had less panels and produced less electricity than mine — but it also cost less.
- In that space, you could install about 12,785 panels at a 10 degree angle.
- This would cost about $8.7 million, before any credits were applied.
- It would produce about 4.25 million kilowatthours.
- And it would save about $425,000 a year.
If you assume a flat price for electricity, that’s roughly a 20-year payback. However, if you factor in a 5% price increase per year, the payback drops to 14 years.
But remember, solar panels are warrantied for 25 years, and they keep producing electricity even after they’re paid for. In other words, after the payback period, you are actually making money on your investment. And frankly, even though they are only warrantied for 25 years, they could last much longer.
Just to be fair, though, let’s run out a 25-year calculation. In 25 years, at a flat rate of $425,000 a year, you would save $10,625,000 on electricity. In other words, enough to pay for the solar panels AND make about $2 million in profit.
If, though, you figure a 5% annual increase in electricity costs, in 25 years you save $20,284,000 — enough to pay for the panels TWICE, with millions left over.
Uhm, “penny wise and pound foolish,” anyone?
The Closing Questions
I realize there are other factors involved, including the weight on the roof. I also freely admit this is a set of rough estimations, and that actual installation might cost more (or less — note the comment about credits).
Even so, I am left with two obvious questions:
If you’re already going to spend $180 million on the project, why wouldn’t you add less than 5% to your project costs and install the one thing that actually makes money over time?
And if Louisville is supposed to be this progressive city, all concerned about sustainability, why is sustainable energy not the FIRST thing in the budget for a public building?
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Agree? Disagree? Got a better idea? Post it in the comments!