At the end of the last post you might remember that even an ambitious study to see how much Solar generation could integrate with the grid, they capped Solar at 25%. Why 25%? That is the subject of this post. Note that we are assuming there is no battery storage (or other type of energy storage) being used.
Again I remind you of the power of "squishing" -- if you don't know what I'm talking about take a look back at the post "Basic Grid Overview Interlude (Part 2.5)." The sun only shines from over our heads for a few hours in mid-day which creates bell shaped curve for solar power generation, all stuffed into 8 to 10 hours.
First let's look at the max amount of Solar generation that a grid could theoretically handle in the winter (without any batteries):
In this figure you can see that we managed to get 82 green dots squished into an 8 hour period. The 82 dots work out to 22% of the total generation needed for the day. Right around noon when the Solar is maxing out and the demand curve is in a saddle there is a "pinch point." If the Solar made any more power at this point then it would be higher than demand and cause a power outage. (Recall that generation and demand must be equal at all times.) We're assuming that in the graph above that the other power plants were able to be turned down to nearly zero, then turned right back on again. In reality, this really isn't possible. In California they are starting to run into this problem right now (it is known as the "Duck Curve" problem), and have just started trying to use some utility level energy storage to address the problem. The state most impacted right now is Hawaii, where they have had to suspend the installation of any further Solar for exactly this reason.
Next let's look at summer:
In this figure we see that the Summer Demand curve allows for more Solar generation than the Winter curve. In this graph there are 102 green dots spread over 10 hours, which works out to 27.5% of the generation for the day. Also note that the green dots are shifted over to peak at 1pm because of daylight savings time, which helps a little. Given that it wouldn't be possible to actually accommodate the full 27.5% in reality, we can see now that a Solar generation of 25% is really the limit of what one could practically achieve without batteries or some other strategy.
So how could the grid handle more than 25% Solar? You might think that batteries are the only option (and they are a great, albeit expensive, option) but there are a couple of others. Neither of these exist on any large scale for now, but let's take a look.
First there is a technique called "Load Response." If you remember the notion of "Demand Response" from the "What it will take" post, it is quite similar. Basically you can see that as Solar generation is peaking, one way to solve the problem would be to increase the amount of electricity that people are using (ie increase demand) in order to accommodate that solar energy. Normally utilities do not control demand, but there has been a demonstration program where a utility had control of customers' electric water heaters. When they needed more demand, they remotely turned on the water heaters, thereby allowing the use of more electricity when electrical output was going really high. Pretty cool idea. If and when the "internet of things" ever comes to fruition this idea could potentially expand quite a bit.
Next there is the idea of using long distance transmission lines to ship the extra Solar output from where the sun is shining to where it is not. We all know Pacific time is 3 hours behind Eastern time. So why not build some huge transmission lines from California / Nevada / Arizona all the way over to the East Coast? When it is 2pm in California it is 5pm on the East Coast, and at 5pm demand is starting to surge. Currently this is not done, given that the transmission lines would cost billions and cross many states (red tape anyone?). But a terrific idea.
Finally of course there is electricity storage, battery or otherwise (such as using excess electricity to pump water uphill, then running it back downhill to make electricity when needed, aka "pumped storage"). With enough batteries you could get rid of every Coal and Natural Gas power plant and power the entire country with 100% Solar and Wind. But at current battery prices that would cost billions and billions, maybe even trillions of dollars. At these costs, batteries will not be the whole solution, but certainly a big part of the solution. Yes, batteries are expensive...but what will be the cost of unchecked global warming?