Friday, July 11, 2014

Is Storage the Solution?

In the energy discourse we hear often that once the challenge of grid scale energy storage is implemented, wind and solar energy will be able to fully replace fossil fuels for generating electricity.

Instead of just staying at this philosophical level, let's make our homework.

This exercise is an oversimplification to illustrate the challenges of relying on intermittent sources for all of our energy.

Let's take Germany as our working example.

According to the latest report by the IEA (International Energy Agency), Germany consumed 551 TWh during 2013*. To convert this amount into average power we perform the following calculation:

     551 TWh / 365 days / 24 hours = 0.63 TW = ~63 GW.

The actual energy consumed by this country fluctuates hour by hour and seasonally. Germany tends to consume more energy in winter than in summer. However, to simplify, we'll make our numbers with the average power consumed.

Let's consider a solar capacity factor (CF) of 12% for this country (a little generous).

Also (and again with the purpose of simplifying our homework) we'll consider that every day of the year is exactly the same and thus that the daily capacity factor is equal to the annual one.

Thus, the installed solar capacity we would need is (considering 90% efficiency in the battery / inverter system):

     63 GW / 0.15 CF / 0.90 efficiency = 583 GW 

     (Today Germany has ~36 GW of solar installed capacity).

At the very least, the batteries would need to store 12 hours of power and this translates into:

     63 GW x 12 = 756 GWh

According to the Guinness Book of World Records 2013 edition, the largest battery in the world** (pictured below) can store 36 MWh. Thus, we would need this many to store 756 GWh:

     # Batteries = (756 GWh x 1000) / 36 MWh = 21,000

According to the Energy Information Administration (EIA), by 2040 global solar electricity production will be 452 TWh. This would be 82% of the 551 TWh Germany consumed in 2013 (and would "leave" nothing for the rest of the world).***

To calculate the required investment we need to multiply the cost of the solar watt (including inverters and installation) by 583 billion (see installed capacity above). Additionally, the cost of the storage needs to be added. We are easily talking here of more than a trillion euros.

However, these "rosy" numbers won't pass muster in the real world. Why? Because in the real world we have sunlight variations between days and, even more important, seasonality. Germany, for example, tends to consume more energy during winter when there is less sunlight.

Thus the above amounts would have to increase significantly. In other words, in real life we would need considerably more than the 583 GW of installed solar capacity and at the same time the batteries would need to store not 12 hours of electricity, but full days or even weeks. The required investment would thus be much higher than in the simplified case we presented above.

Conclusion: storage for renewable energy is not a silver bullet and it is doubtful that a fully renewable economy (solar and wind) would ever make sense in financial or even environmental terms.

Feel free to add to the conversation on Twitter: @luisbaram

Thank you.


1. Sure, the renewable energy doesn't have to be 100% solar. We could have a combination of solar and wind. This would obviously make the exercise much more complicated but would make more sense in the real world. However, solar and wind are not fully complementary. Solar (obviously) operates only during the day. Wind is more random. See graph below for the first months of 2012 where we can see that solar / wind do complement themselves somewhat. We can see that in January there was little solar production but was a very good month for wind. Then, July was a very low month for wind but had substantial solar production. However February (a full month) was low on both. More storage would be required for compensating those long lulls. Additionally, February was the month with the highest consumption in the first 7 months of 2012. (If somebody can share the full year statistics, they are welcomed). Let's bear in mind that the randomness of wind can only be somewhat compensated by solar during the day, so for half of the year (nights) wind is by itself.

2. Another concern is the variability of wind, here we can see annual variations (solar is more stable although its CF is considerably lower than wind's in Germany):

Both graphs are from the last reference below.



** According to the Guinness Book of World Records, this battery is "larger than a soccer field."


German capacity factors for solar and wind:

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At 12:34 AM, Blogger Unknown said...

Let us have our goal to reduce CO2 by reducing dependence on fossil fuel generated energy and be SAFE by avoiding new nuclear energy. Think outside the box to reduce energy demand and harness from new sources such as from road traffic. That will provide energy for road lighting and reduce demand.
Solar panel energy should be used at local levels for homes, office buildings (Google and Apple examples), parking shades, etc. France has a km of solar panel rail tunnel that produces enough energy to take care of all energy needs for that line and train stations. New Zealand has integrated wind and thermal energy in the grid successfully. I visited in 2022. More on reducing fossil fuel for energy in my chapter 23 of 2012 Climate Change Mitigation Handbook.

At 4:05 AM, Blogger darkestkhan said...

@Waheed Uddin You clearly have a way to travel in time if you "visited" in 2022. On the other hand I would like to point out that France is actually almost exclusively nuclear.
And there is no more danger in new nuclear than in old one. (I actually don't see why someone would be against new nuclear) And when it comes to thinking outside the box - there are only two answers. Nuclear and enhanced geothermal. The second one is much more expensive than first one. And by far more experimental than even thorium.

At 7:08 PM, Blogger TheTracker said...

Let's think a bit more creatively about the storage problem.

You can use the electricity from renewable energy (or any other source) to turn carbon dioxide into methane (

Germany has an EXISTING storage capacity of 20,804 mcm of methane (1). A cubic meter of natural gas contains about 40MJ of energy (2). So Germany's existing storage capacity with natural gas, with no new infrastructure, is 2.08 * 10^9 * 4 * 10^7J, or 8.32 * 10^16J, which can also be expressed as 23,111 GWh.

You will lose at least 20% of that energy converting the gas back into electricity. You will also lose some more if, as I would suggest, you recapture the carbon dioxide when burning your stored gas, so as to turn it back into methane again. So let's say you lose 2/3 of the energy in the stored gas, leaving you with 7,700GWh -- ten times the amount you estimated we needed.

That's with no new storage infrastructure. No batteries. CCS technology and some synthetic gas plants. Nothing too hard.



At 9:42 AM, Blogger Bert said...

This comment has been removed by the author.

At 9:51 AM, Blogger Bert said...

Interesting topic and interesting approach! However, I don't think these calculations explain much on the need for storage, as renewable penetration has some huge drivers, conventional powerplants are suffering and storage is becoming cheaper fast.

Assessing the potential and need for storage can only be done by evaluating renewable penetration needs and analysing daily and yearly solar and wind patterns, all drivers for storage. Also, the interplay with (large-scale) interconnectors needs to be considered.

Here's an attempt to do just that, based on the future energy mix from the work of the GEA (Global Energy Assesment):

There's already a lot of hydro-power storage capacity deployed for the transmission grid, and on the distribution level storage can be competetive with deploying more distribution capacity.

Have you seen the latest price dive in storage technologies? Some of the demonstration projects now in place feature price levels of a couple of cents per kWh, meaning PV/Wind + storage will become or already is on grid-parity, depending on what level is considered of the energy system. Here's an open-source technology database on storage technologies and products for more information:

At 11:39 AM, Blogger Unknown said...

An eye-opening exercise, thank you very much for spreading some light in this subject... I was naively considering advancements in the storage technology (there in the "philosophical" level) as part of the solution !

Now I am clear that this is not, and it won't ever be, a mainstream solution. We have to look somewhere else for a "plan B".


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