Friday, September 20, 2019

Replacing coal with wind and solar power

I am often reminded by my friends that renewable power like wind and solar are making tremendous strides—their deployment is rising exponentially, and costs are coming down. As a result, my friends claim, greenhouse gas emissions in the US are declining. They note that renewable sources are a cheaper alternative than coal power in many parts of the world. We should therefore close down the coal plants and replace them with wind and solar farms. For backup these installations should use batteries, whose costs, I am told, are also plummeting. Indeed, Mr. Bloomberg has pledged $500 million to hasten such a transition.

I wish I could share their positive outlook. Deployment of renewable power technologies has been increasing, but even after twenty years of this exponential growth, in 2018 they provided a mere 3% of global energy, while fossil fuels accounted for over 80%. Wind and solar simply do not scale. Here’s a graphic from the BP Statistical Review of World Energy, 2019. It shows the global primary energy consumption various sources in MTOE, metric tons of oil equivalent. Yes, you can see the increase in renewables, but the even larger increases in consumption of fossil fuels has led to emission of ever-increasing amounts of carbon dioxide.

There has indeed been a decline in the CO2 emissions in the US electricity sector. This decline, though, is largely a result of switching from coal to natural gas, and not due to the rise renewables. Wind and solar contributed only a small fraction. Here are the data from the US Department of Energy’s Energy Information Administration that illustrate the point.

The falling price of wind and solar power that the proponents point too does not reflect their true costs.  Policies such as Renewable Portfolio Standards, subsidies, and alternate revenue streams such as curtailment allowances hide the costs. Wind and solar installations must come with the disclaimer, “Batteries Not Included.” If one includes the cost of storage and other systems for managing their intermittency, the cost of wind and solar would be considerably higher.

Consider closing a 1-GW coal power plant, say the Bruce Mansfield in Pennsylvania, and replacing it with renewables. First, to get the same number of GWh of electricity over a year, you will have to install about 3 GW of wind or solar facilities to account for their reduced capacity factors. Installation costs are often reported in $/W, and so for starters we have allow for the higher installed capacity to get the same amount energy.

Next, you will also have to provide some storage to cover for days that wind might not blow or the clouds obscure the sun. Currently, natural gas plants are used to provide backup power because natural gas in cheap—thanks to fracking—and they can ramp up quickly. But natural gas is a fossil fuel, and we do not want that; instead we want to put in batteries for backup. If we choose to provide storage for just 100 hours, a tad over four days; that would mean installing battery storage capacity of 100 GWh. How much lithium would that require? According to Tahil, theoretically you could store 1 kWh of energy from 73 g Li in lithium ion batteries. Note that g/kWh are the same as Tons/GWh. Thus, theoretically, you would need 73 Tons for storing 1 GWh of electrical energy.

In practice, the amount required is often 3 to 4 times higher because of several factors: discharge rate, irreversible losses, reaction kinetics, etc. Tahil discusses these issues in the paper and concludes by suggesting a requirement of 320 g Li per kWh of storage. In other words, shutting down just a 1-GW coal plant and replacing it with renewables and providing only 100 hours of storage would require 32,000 Tons of lithium. To put that amount in perspective, note that in 2018 the global production of lithium was 62,000 Tons.

In other words, about half the world’s lithium supply would go for backing up renewables to replace one coal plant!  Sure, we could expand the production of lithium, but how soon could we scale it up to get millions of tons per year to replace all the coal power? We do not have the luxury of time. As Greta Thunberg and children all over the world implore us, we must take action to combat climate change. Let's do right by them and not make the problem worse than it already is.

Time to get real and embrace nuclear power!