Monday, January 5, 2015

Both Low EROEI and Low Power Density is a Serious Problem - Wind Addition

In my last post I talked about how having both low EROEI and low power density is a serious problem.  I used solar as an example.  In the comment section someone mentioned something about 80% of our power coming from wind, but wind isn't much better. It has a higher EROEI, but it's power density is terrible.  I'll explain below. 

If you haven't read the last article you might want to do so now.

First I'll start with the sources.

Source One - Catch 22 of energy storage.

EROEI for wind with storage is 3.9. 

Source Two - Sustainable Energy — without the hot air

The red stack (i.e. energy consumption) in figure 18.1 adds up to 195 kWh per day per person (page 103).

4,000 m^2 land per person in UK.  

Source Three - Rethinking wind power
Keith’s research has shown that the generating capacity of very large wind power installations (larger than 100 square kilometers) may peak at between 0.5 and 1 watts per square meter.
Now lets think about it a little.

Lets start by talking a bit about EROEI.   The comment that inspired this post gave me a link that says EROEI is meaningless.  I disagree.  EROEI is very important when it gets close to one.  I'll explain but first let start with this definition from Wikipedia for anyone unfamiliar with the term.
In physics, energy economics and ecological energetics, energy returned on energy invested (EROEI or ERoEI); or energy return on investment (EROI), is the ratio of the amount of usable energy acquired from a particular energy resource to the amount of energy expended to obtain that energy resource.

EROEI is important because it creates a multiplier effect for other quantities.  Other quantities include things like space, different material and man power.   The closer EROEI gets to one the closer the need for those other quantities get's to infinity.

For example:

Imagine that you had a some solar panels that had a EROEI of 2. One meter square of them produces let say 5 watts average. For simplicity's sake lets stick with only this one kind of power source for now.

If you wanted to get 5 watts from these panels you would need both the one meter squared, plus another half a meter squared to maintain the one meter squared, plus another quarter meter square to maintain the half meter square and so forth.  This goes on endlessly, and when you sum up the results you get the multiplier.


Thanks to Mark44 on physics forums for this!

Now lets talk about the multiplier

The multiplier works for all quantities not just area.

For example:

if it take 5 people to maintain some generation that produces 5 watts and the multiplier is 3 then that 5 watts really needs 15 people (5 X 3 =15).

Here is the formula you need to figgure out the multiplier for any given EROEIs.

Where X = EROEI
This doesn't only apply to a one power source systems.  Any power source that with EROEI close to one would require outrageous amounts of different resources in order to contribute significantly to our total energy supply.  

Now for Wind

The multiplier for wind isn't that bad, but the power density is crap.  At between 0.5 and 1 watts per square meter even if you covered the whole of the UK with wind turbines the yield would still kind of suck.

Here's the math

4,000 m^2 is space in uk for each person.

1.34 is the multiplier for wind


4000 / 1.34 ≃ 3,000

Around 3,000 is how much possible space for wind for each person when you minus the space for wind turbines needed to maintain the system.

So between...

           (3,000 * .5 * 24)/1,000   ≃ 36  kWh per day per person

           (3,000 * 1 *  24)/1,000   ≃ 72  kWh per day per person

So maybe you can get close to 80% if you cover the whole of the UK with wind farms (assuming the EROEI doesn't drop because of diminishing returns).  Do you think people can really cover so much of the UK with renewable energy?  What about space they need for other things like energy storage.  It's really hard for me to believe, and even if you could I think it would be pretty horrible.  

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