Friday, April 25, 2014

Solar Photo-voltaic: Let's Get Real

This exercise is a simplification in which we try to explain why solar photo-voltaic (PV) has difficulty in living up to its hype.

Let's consider an isolated country that consumes on average 40 GW of electricity.

To simplify things, let's consider they consume this amount of power 24 hours a day. In real life, consumption has peaks and valleys.

They plan to install enough solar PV to supply 100% of the electrical energy of the country at peak solar production.

Again, to simplify, let's consider a perfectly cloudless day during the spring or fall equinox. The output would look like this:

So, from midnight to 6 AM, another energy source would supply 100% of the electricity.
Then again, from 6 PM until midnight, another energy source would supply 100% of the electricity.
Then, from 6 AM until 6 PM solar would provide continually variable production and will reach 100% of this country's energy needs at local noon. In other words, at local noon, solar PV would be producing 40 GW of power. (8)

What would happen if this country decided to go above 100% solar at peak production (as an isolated country, they couldn't "dump" the excess production into another country. Also, we are not considering storage that could be an article in itself).

Then, they would have to curtail (disconnect) solar capacity at peak production hours. This is how the graph would look (100% peak vs. 125% peak comparison):

As we may see, there is not too much sense in taking the PV capacity above the peak requirements.

Now, how would the production of the "other" sources (usually fossil fuels) look to compensate for the variable nature of PV. Here we can see it:

In other words, from midnight to 6 am, and from 6 pm until midnight, the other source would supply 100% of the electricity. Then from 6 am to 6 pm it would have to continually adjust its output to compensate for the PV production. 

If the Earth were a perfectly cloudless planet, this sort of arrangement would allow PV to provide close to one third the energy requirements of a country. What would be the carbon intensity of such electricity? Here we calculate it:

According to the table referenced below, solar PV has a carbon intensity of 46 grams per kWh, and let's say the rest of the electricity is produced by natural gas (469 grams per kWh), thus the combined carbon intensity would be:

     46 x 0.33 + 469 x 0.67 = 329 grams per kWh.

However, in real life the Earth is not cloudless and thus the actual annual capacity factor of solar PV is closer to 15%. If we re-calculate with this more realistic number, we get:

     46 x 0.15 + 469 x 0.85 = 406 grams per kWh.

If a component of coal is used in the "other" energy then the emissions would rise even higher. 

Again, this article is a simplification, but the point is to explain in simple terms why solar PV is not living up to its hype.

Thank you.


1. In real life, clouds reduce the output of the solar panels.
2. Seasonality also greatly impacts power generation: winter days are shorter and possibly cloudier.
3. The "other" power plants need to be idled, modulated, shut down, restarted and this causes inefficiencies in the system and additional emissions.
4. From a purely operational point of view, "nothing would happen" if all solar capacity were disconnected.
5. Yes, a solar + fossil fuels system produces less emissions than a purely fossil fuel one, but at the cost of duplicated investment.
6. Yes, excess solar energy could be "dumped" into another country, but if that country also installed significant solar capacity, this wouldn't be an option anymore. 
7. The other option is storage but currently this (expensive) technology hasn't been widely deployed. Also, storage would add to the emissions per kWh (once life-cycle emissions are taken into consideration).
8. "Local noon" doesn't happen at the same time in all the country, so the curve would be a little bit flattened. 


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At 11:49 PM, Blogger .?. said...

Nicely explained. Will possibly reuse it and hope to translate it into German. With your approval of course.


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