Shall we have a discussion on efficiency numbers?

[Wayne 962]

I just posted a long thread on Reddit on this topic, but perhaps the brain trust here would be better suited for a discussion? The topic is our MIT Aztec Solar Car, whose restoration one can read about here: https://forums.pelicanparts.com/off-topic-discussions/1167039-after-10-years-so-were-getting-mit-aztec-solar-race-car-running-again.html

Here's the crux of the post I put up last night:

Steve sent me a note last night congratulating us on completing our restoration of MIT's Aztec - the solar car we raced back in 1993. I even bought a new domain name to celebrate: https://neverpluggedin.com We drove it several miles today, and it made it to a Redondo Beach car show and back! No problems at all!

Anyways, here's a recap of the efficiency discussion I had:

26 total miles is what the odometer shows - this is about 4-5% high based upon the calibrations that I did using a GPS Speedo today. So, I'll subtract 5%.

90% battery left currently (based upon battery gauge from BatteryEVO that came with the package). We’ve got 72V BatteryEvo Falcons installed (two of them) with a total capacity of 2.1 kWh each (total 4.2 kWh).

The solar panels are hooked up – it’s been left in the sun a couple of days, which is great, but skews the data a bit. We’re getting about 160W out of the panels in late afternoon here.

The terrain I’ve been testing it on is *very* hilly. I ride my bike on this similar terrain every day, and I do about 350 vertical feet or so each day. So, not a straight-line.

Based upon super-rough calcs right now, we’re getting let’s say 22 miles from .42 kWh or about 50 miles per kWh. Seems high.

Our original spec sheet from 1993 when we ran it – range of 150 miles on six die-hard deep cycle lead-acid batteries with 81 Amp-hrs and 5.8 Kw-hrs. That’s about 25 or so miles per kWh back then with the lead acid. I’ve lowered the weight considerably with the lithium-ion batteries (although lowered the total kWh capacity). But the lower weight should translate into higher performance per kWh. We also have the solar panels hooked up which skews things too.

So, to make a long story short, the answer is somewhere between 25-50 miles per kWh. Including whatever power I might have gotten from the solar panels.

The real test would be to disconnect the batteries, and then drive until empty on a race track. That’s what we did in 1993, but the car was so efficient, we rant out of time on the track, and they shut the track down without us running out our batteries. So, they had to estimate the remaining “range” based upon what we still had left. I think that whole race we ran about 700 miles or so.

I was at MIT last week pulling the archives on the car and found some interesting things - I've attached three images to this post.

It says here, back in 1993, we were recorded by the race officials at 42 watt hrs per mile, which translates into 24 miles per kWh during the race in 1993. That’s pretty much our current baseline before the lighter-weight batteries. We were even more efficient that the fastest car in the “racing” division (Dartmouth’s Sunvox).

So, to maybe answer the question of "how much distance can one get from the solar panels?" – if the panels output an average of 150W for about 6 hours, that would be 900 Whr in a day. Since we’re getting about 25-50 (roughly) miles per kWh, that translates into roughly about 35-45 miles per day from the sun. I’m not sure Aptera, being much heavier, will be able to top that. Our panels are crappy Chinese ones though – I’m not sure they are outputting as much as they could (I bought them mainly because they are the “correct” size for what we ran back in 1993 – maybe I will look at optimizing / upgrading them soon).

Another set of data points can come from Violet - the most recent carbon fiber car developed by the Sunswift team - https://www.borntoengineer.com/sunswift-solar-car They were able to achieve a "world record" (our car beat this handily, so I'm not sure what "world record" they are referring to?) of 5.5 kWh/100 km, which translates into 20-24 kWh of power consumption for 370 miles. That translates into 11.2 miles per kWh (we ran 24 with the lead acid batteries!).

Aptera's claim / goal is 10 miles per kWh. The Violet car was slightly better than this, and our car was significantly better than this. But Violet weighs a little more than 700 lbs, and Aztec (now with the lithium-ion batteries) is about 500+ lbs. Aptera is estimated to weight between 1,800 and 2,200 lbs. Most of that weight comes from (presumably) the larger battery - Violet has a 10-20 kWh battery, and I have 4.2 kWh of capacity now in Aztec. I estimate Aztec to have a range of about 150 - 200 miles (on flat terrain in a warm climate without headlamps on).

Our panels, when I've tested them, have been putting out about 160W - 10AM or so, pointing at the sun, but the sun is not super-directly overhead. I thought the power would be much higher - I am going to have to investigate further, perhaps I made a wiring error or something (they are all wired in series).

Anyways, there you have it. As we put more miles on the car, I'm happy to share efficiency data. I'm not too sure how relatable that would be to Aptera directly, but it's interesting to discuss.

[masraum]

Interesting stuff. So it seems that by your data/calculations your old car in its current configuration is better/more efficient than most of the other current vehicles (with 30 years of tech advancements). And your solar panels don't seem to be putting out what you think they should be.

Do I have that correct?