Wednesday, June 25, 2008

Algae Biodiesel vs. Solar Panels

Which is the more efficient source of energy for sustainable transportation, biofuels or solar panels?

To find out, lets compare the amount of land that is needed to produce the fuel to drive a car 12,775 miles a year (35 miles a day) either as biodiesel to run a diesel-hybrid car or for solar panels to run an electric car.

The most efficient producer of biodiesel by far is algae which can produce 15,000 gallons of biodiesel an acre. Converted to metric we get 37,000 gallons per hectare (2.47 acres per hectare) or 3.7 gallons per square meter (10,000 square meters per hectare).

The VW hybrid TDI Golf concept car gets 70 mpg. To drive 12,775 miles would take 182.5 gallons. At 3.7 gallons per square meter, this would take 49.3 square meters of land to grow algae.

Solar panels generate 263 kWh per square meter a year. A Tesla Roadster uses .2kWh/mile. To drive 12,775 miles would take 9.7 square meters of solar panels.

The solar panel fueled electric car takes a little less that 1/5 the amount of land to run as a algae biodiesel powered car. It is by far the more efficient source of energy for sustainable transportation.

Why is the electric car system so much more efficient? I believe that most of the difference has to do with how much efficient an electric motor is compared to an internal combustion engine (90% efficiency vs 25%). The solar panels also collect slightly more energy from the sun (18% vs 15%), and not all the energy the algae collects is converted into oil (not sure what % of energy is converted to oil).

It should also be noted that the yield for algae is a theoretical one that has not yet been matched in the field. Other estimates put it at 1800 gallons per acre, which is much less that the 15,000 value, but also substantially larger than the #2 producer: palm oil which comes in at 500 gallons per acre. It is also larger than any source of ethanol. It should also be mentioned that biodiesel stores more energy per gallon (130,000 BTU vs. 84,000 BTU) and a diesel engine is more efficient than a gasoline engine, so a gallon of biodiesel can propel a car much further than a gallon of ethanol.

This analysis also doesn't take into account the economics of the two systems. Nor does it take into account that biodiesel allows for quicker refueling times, greater range between refueling and can more easily be integrated into the current transportation system. When those are taken into account biodiesel is much more competitive, especially in the short term.


Rebelfish said...

The problem, as you touched on, is energy storage. For one, the "round trip efficiency" (energy in-out) of batteries isn't 100%. I dono what it is, but I've heard between 50% and 90%. And then there's the weight. If we used the extra-light Li Ion batteries, a semi could go half of its normal range if it converted half of its cargo capacity into batteries.

Rebelfish said...

Ahh, I figured out how to post the spreadsheet with the data.

Fat Knowledge said...


Thanks for the numbers.

I agree with you that there will still be places where electric battery powered transportation will not make sense. Number one on that list for me is air travel. For these something like biodiesel will be needed.

But, for personal transportation (which I believe uses something like 60% of oil consumption in US), I think battery powered vehicles makes sense.

If I had to bet which would technology would have the greater impact in reducing American consumption of oil over the next 20 years, I would go with batteries and solar power over biofuels.

Rebelfish said...

If the car always went its average of 35 mpd, it would work great. But if it was taken on some big trip, then you've got to worry about fast recharge or battery swap or something. I've seen arguments that a pure electric car is great for a family's second car, one that would just be used for commuting or errands. Of course if we all lived in places where we could walk/train/bike to work, we might not need that second car anyway.

Fat Knowledge said...


I agree completely. You raise a good point that most houses have 2 cars, and 1 car is mainly used for short distance commuting. Instead of converting both cars to plug-in hybrids, why not leave one as a conventional car and make the other a completely electric battery powered car? Seems like the second family car is a good place to start the adoption of completely electric vehicles.

Stretch the plug-in hybrid over two vehicles so that you don't need to put an internal combustion engine, battery and an electric motor in both. I wonder if that would give you similar gasoline consumption reductions while making the cost of the combined vehicles cheaper.

Anonymous said...

Has anyone tried to buy a battery recently and noticed how the price has gone up?

It is because batteries contain some relatively scarce minerals, and the price of those is shooting up. Simply, they are as finite or more finite that petroleum.

So forget "solutions" based heavily on battery-technology

Fat Knowledge said...


Actually I haven't noticed how batteries have been going up lately. But, in the longer term the trend has been unmistakably for lower prices.

As for the scarce minerals, yes lithium is in demand right now. But, there is a fair amount of lithium on the planet, and unlike petroleum, lithium from a dead battery can be recycled to build a new battery. Longer term, I would think that nano technology using carbon to build batteries will be available, and carbon isn't scarce at all.

So, feel free to bet against batteries, but I am quite confident that battery powered transportation will be prevalent in 10-15 years.

Anonymous said...

I'd prefer preheated straight vegoil diesel than the chemical challenges of biodiesel for now. But electricity is the puristic ultimate solution, when cheap +30% efficient solar carpet (or maybe solar jel paint) + cheap battery/capacitor/hub-motor technology will be achieved.

Then, no worries whether the grid power comes from coal, nuclear or whatever. Every house can have its own cheap electricity source, enough to supply home utilities, heating/cooling & a car, plus excess to the grid.

Anonymous said...

There will be autonomous quick-charge stations widespread worldwide generating & storing from renewables like solar & wind power.

Just for fun: Year 2015. The driver approaches at QC point, astonished reading this notice: "We're temporarily out of power due to a gray & calm day, but you may try our fresh algae fuel if you have a range extender engine" :)

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Anonymous said...

then there's the weight. If we used the extra-light Li Ion batteries, a semi could go half of its normal range if it converted half of its cargo capacity into do you make solar panels

Casey said...

I dont like hoping for a new technology to be developed (i.e. solar panels and batteries). Biodiesel could be used today and it supports farmers!

Ted Lemon said...

Algae can be grown in ponds. The per square meter measurement is meaningless without considering the cost of a square meter of collector. Given the other factors here, a mere 4x difference in the size of the collector is probably swamped by the higher cost of *building* a silicon collector.

Anthony said...

It would be nice to see an updated comparison, considering the announcement that Joule Unlimited's biofuel is able to produce more than 20,000 gallons per acre per year.


Eric said...

Hydrogen fuel cells that use ethonol will trump both solar and bio-fueled internal combustion engines. The hydrogen fuel cell is at least 50% to 60% thermally efficient at converting fuel to electricity. The battery car and hybrid car are a stepping stone to the fuel cell.

Daniel Tan said...

The maximum efficiency of conversion of solar energy to biodiesel is around 4.5% - see

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