Almost everyone in New York City has weighed in on the new bike sharing program. Two of us here are members and, overall, we are big fans. There are a lot of discussions about the stations, the bikes, and the pricing structure, but no one is talking about the solar charging system in depth. On the surface, it looks shockingly inefficient.

Update: As customers, we are had trouble checking out bikes the last two days. Calls to customer support indicate that the solar panels aren’t generating enough power, effectively shutting down the stations.

solar pole for citibike station

At each station, there is a vertical pole mounted next to the kiosk that provides all the power to the station. The pole contains what looks like 64 x 3.4 Watt back contact solar cells made by SunPower (we think) for a total of 210+ Watts. These have a nice black even finish, and someone has done an impressive job mounting them on the pole. From a design standpoint, it looks quite cool, but it could mean that there is a lot of wasted power here.

solar panels citibike solar cells citibike

In a typical fixed installation, a small solar panel is mounted facing South (if you’re in the Northern hemisphere) and oriented at an angled roughly equal to the latitude of its location. This helps maximize the amount of power collected over the course of the year. Here’s an example from Treehugger.

muni meter credit treehugger

The kiosk for Citibike looks to be about the same setup as a muni-meter and we would guess the power consumption would be on par or slightly higher (to handle the bike locking and unlocking). Inside the kiosk is mostly air. The two other elements are paper feeder for the receipt and a “Power-Tec” gel deep-cycle battery. Its the size of your standard car battery, but we couldn’t find any specs for this brand online.

kiosk for citibike Battery inside Citibike Kiosk

With CitiBike’s solar pole, the cells wrap around the pole and some will face away from the sun. In the summer, when the sun is high in the sky, no cell gets a direct angle to the sun except at sunrise and sunset. Even in the winter, half the panels are always in the dark.

In addition, the black plastic surrounding the panels probably further diminishes efficiency. When we put a solar cell behind a window, we see up to a 50% drop in efficiency. We tested behind glazed windows, but there is going to be loss.

So either:

a) they simply overpowered the pole (240210 Watts of solar, about 16 14 times 15 Watts that you see on a parking station) to achieve a cool design look. Who cares if some of the cells never generate any power?

b) they’re putting this in a lot of locations with limited direct sunlight due to tall surrounding buildings (e.g. Midtown Manhattan), so they have to rely on ambient and reflected light. If this is the case, then having the cells point everywhere makes sense in these locations.

Our guess is that maybe the truth is somewhere in between. There is a practical aspect to the pole design, but they probably could have gotten a traditional style panel to work as well with less silicon… it just wouldn’t have looked as good.

Other thoughts on Citibike’s solar pole?

Update from a customer in Montreal. Here’s a picture of the BIXI bike share program that was a predecessor to the New York program. This one uses what to me looks like about 30-40 Watts of solar. It looks a bit cooler than the standard panel design.

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3 Responses

  1. JC in Nicaragua

    Isn’t it possible that the pole also powers its own rotation system? Certain cells may carry enough power to be drained/transferred during a full 360 degree turn takes place, every so often, it just occurred to me.

    Reply
  2. TR

    Compelling content gents. I’ve been wondering the same thing about these cells. There is likely very little voltage coming out of the panel and they have it hooked up to a 12v battery. Are they using a charge controller? Would be interested on the specs.

    Reply
  3. CZelov

    Fascinating design features. Do you know who is actually making the Solar Pole?

    Reply

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