Free lunch: an exploration

A friend linked to this story about a Turkish company that makes wind turbines to be installed between lanes of traffic, where they turn the moving air from passing vehicles into electricity. He wondered whether it would just slow down the buses.

My reply turned into something much too long for a Facebook comment, so I put it here.

It sounds like you’re asking a sort of First-Law question: since energy can’t magically come from nowhere, the energy turning these turbines has to come from somewhere. In other words, you’re asking if this is a free lunch, which is always a good question in the face of a proposed source of energy.

I suppose it’s possible that the turbines alter the airflow in the bus lanes in a way that makes it harder for the buses to push through the air. It that’s true, then yes, the bus engines are having to work harder to cover the same distance at the same speed.

I can also imagine it’s possible that the energy contained in the airflow away from the buses is just being dissipated as waste heat, and these turbines are capturing a piece of that and making it useful.

Or maybe there’s some drag on the buses, but not enough offset the energy generated by the turbines.

At any rate, it seems like it would be an interesting study for someone in aerodynamics (on the other hand, maybe someone who actually knows aerodynamics already knows the answer and would consider it a trivial question).

I was, however, taken aback by the video’s reference to “vertical access” turbines, because the term is “vertical AXIS,” which makes a whole lot more sense. Somewhere along the line in producing this video, there was a person who didn’t look at text, but just wrote down spoken language. Maybe it was the original reporter, in which case that person didn’t know enough about the technology to understand what was being described.

Along similar lines, it says, “1 turbine can create one kilowatt of electricity per hour.” This is a confused statement.

A kW is a unit of power, a rate at which energy is applied, something like “miles per hour”.

A kilowatt-hour (kWh) is an amount of energy, something like “mile.”

If someone said, “This car can travel up to sixty miles in an hour,” that statement makes sense.

If they said, “This car can go up to sixty miles per hour per hour,” you’d have a hard time knowing what they meant.

In this case, is the maximum instantaneous output 1 kW? Then you don’t need the “per hour” part, because it’s instantaneous.

More likely, the instantaneous maximum is something like 2 or 3 kW, so if the turbine were spinning at top speed for an hour, it would produce 2 or 3 kWh of electricity, but since its speed fluctuates, the most you can reasonably expect to produce in an hour is 1 kWh.

Now let’s do some back-of-the-envelope economics.

Electricity prices in the US are between $0.05 and $0.15 per kWh, depending on whether you’re talking wholesale or retail. One knock on wind power is that it’s unpredictable, but that’s maybe less the case here, since we’re not dependent on the weather to provide the wind, but are instead drawing on the relatively predictable flow of traffic. On the other hand, if there were a traffic jam, that would be just like a regular wind turbine stopping when the wind dies down.

So we can probably generously price the output from one of these turbines at $0.10 per kWh.

A turbine won’t produce 24 kWh per day, because the report says “up to a kilowatt.” I think we could generously say 10 kWh per day (all through the night there's probably minimal traffic, and at times during the day the traffic is either light or slow). At $0.10 per kWh, each turbine is producing revenue of about $1.00 per day, or $365 per year.

If you want it to pay off in 10 years with a 10% internal rate of return, the cost of the turbine can’t be more than $3,400. And I’m guessing that my estimate of the value of the electricity produced is slightly on the high side.

“But wait! There’s also a solar panel on top!”

You’re right, but that doesn’t really help.

We have to figure out what would be the cost of installing these solar panels without the turbines and compare that to the cost of putting them on top of the turbines as shown here.

Looked at one way, as far as the solar panels are concerned, the turbines are nothing more than very expensive posts.

But it’s probably fairer to say that the solar panels are cleverly piggy-backing on the wiring that was being installed anyway for the turbines.

Now we have to ask whether that “one kilowatt per hour” includes the output of the solar panel on top or not.

Case A: the “one kilowatt” is just from the turbine and the solar output is a bonus. Then we have to look at the cost of the solar panel itself and ask whether in that position, it produces enough power to justify that cost. This is where the “piggy-backing” comes in, and I can easily imagine that the panel is a good investment, once the wiring has been justified by the turbine, and the platform provided by the turbine is standing there just waiting to be used.

Case B: the “one kilowatt” is from the turbine plus the solar panel, in which case I’d want to know what portion of that electricity comes from the solar and what portion from the turbine. The greater the portion that comes from the solar panel, the more the turbine turns into an expensive post for a solar panel. At some point, we’d be better off putting in the wiring and then installing a continuous band of solar panels (rather than the individual spots atop each turbine, as here), and mounting them on the least expensive posts that will reliably do the job.

So to assess this, I’d have to know more about the cost of each post, the cost of installing the wiring, the relationship of solar output to wind output, and the appropriate value to assign the produced electricity.

But all in all, I suspect that this project doesn’t actually make sense.

PS – at first, I thought that some graffiti artist had “tagged” the turbine, but then I realized it was the identical image on each blade, so I was actually looking at the company’s logo.