Saturday, February 22, 2014

Distributed energy storage

Tom Murphy thinks energy storage for a mostly solar- and wind-powered grid would be impractically large
Putting the pieces together, our national battery occupies a volume of 4.4 billion cubic meters, equivalent to a cube 1.6 km (one mile) on a side. The size in itself is not a problem: we’d naturally break up the battery and distribute it around the country. This battery would demand 5 trillion kg (5 billion tons) of lead.
 The figures he uses to get there:
Let’s also plan ahead and have all of our country’s energy needs met by this system: transportation, heating, industry, etc. The rate at which we currently use energy in all forms in the U.S. is 3 TW. If we transition everything to electricity, we can get by with 2 TW, assuming no growth in demand. Why? Because we currently use two-thirds of our energy supply (or 2 TW) to run heat engines, getting only about 0.6 TW out for useful purposes in the bargain. An electrical system could deliver this same 0.6 TW for only 1 TW of input, considering storage and transmission efficiencies.
Running a 2 TW electrified country for 7 days requires 336 billion kWh of storage.  We could also use nuclear power as a baseload to offset a significant portion of the need for storage—perhaps chopping the need in two. This post deals with the narrower topic of what it would take to implement a full-scale renewable-energy battery. Scale the result as you see fit.
This raises the question, if you have converted all transportation to run on electricity, how much of the storage requirement can be met by those batteries alone?

There are 254.4 million registered passenger vehicles in the United States. There are also several million commercial trucks and bus that are significantly larger than your average passenger vehicle. I'm going to count them as several "passenger-vehicle equivalents" and round that number to 300 million. Assume they are driven about 4 hours a day, and otherwise available to the grid, and give each one of Tesla's 85kWh batteries:

85 * (3*10^9) * (5/6) = 21.25 billion kWh.

So you could meet about 7% of the week's worth of power Mr. (Dr.?) Murphy estimates we might need (there's no point in being falsely precise in an exercise like this.) However the numbers look better if we make the scenario a little more realistic; for example, if we presume about 40% of the normal load will be baseload power -- about what a smart grid is thought to require to be stable. We can meet this through a combination of hydro, geothermal, nuclear, tidal, and/or space-based solar.

We should also suppose a dynamic pricing model for electricity, something the UK is already experimenting with. When solar are wind farms are idle, power costs more, reducing consumption. It seems likely that you could reduce energy consumption by a fair amount by this method -- 35% perhaps.

Now, in our nightmare scenario of unending darkness and perfectly becalmed winds, we have 35% of demand met by conservation, 40% by baseload power, leaving 25% to be covered by batteries. That's still more than we have, so let's add some hydroelectric storage.

In investigating the potential of hydroelectric storage -- no mean feat, when existing hydro storage plants tend to be rated by output (MW) not total storage (MW hours) I found myself right back with Tom Murphy:
The U.S. has 78 GW of hydroelectric capacity installed. In a year, these plants produce 272 TWh. Divide by 8766 hours in a year, and we find 0.031 TW (31 GW) of average power. This implies a 40% capacity factor.
In this post he is looking at hydroelectricity's potential as a power source, rather than as a form of storage, but let's borrow the numbers.
When we built things

We can suppose that most of the convenient sites where a large amount of water can flow abruptly downwards are occupied by these sites. We can further presume that if we can control the flow of water downwards, we can also, with the necessary infrastructure, pump the water upwards.

The potential of the dams for storage would reflect the amount of time we could run them at full capacity (78GW) instead of average capacity (31GW) presuming we were using intermittent renewables to "top off" the dam reservoir. Let's say, in the spirit of Fermi estimation, that that time is one week. That would give us a major boost to our storage capacity:

(78GW - 31GW) = 47GW
47GW * 24 hours = 1128 GW-h
1128 GW-h = 1,128,000,000 kW-h * 7 days
7.896 billion kW-h

So that puts us at 29.146 billion kW-h. Note that this is not a hard upper limit; reservoirs can be created artificially near the sea and energy stored via pumped sea water. But that's probably not necessary, because . . .

The Strategic Petroleum Reserve has a capacity of 727 million barrels (30.5 billion gallons, 115.4 billion liters). Let's fill that with biodiesel, which has a specific energy of about 35MJ/liter. That would provide a reserve of 115.4 billion * 35MJ = 4 trillion MJ. That's 1.1 trillion kW-h.

Now we have about 16 weeks of stored energy based upon the assumptions above (40% baseload power, 35% drop in consumption secondary to dynamic pricing) or almost four weeks based on Dr Murphy's pessimistic scenario (no adaptive drop in consumption, absolutely no baseload power, not even the 10% of our electrical supply currently provided by hydroelectric dams.

That's almost excessive, but we can trim it down by using imported sugarcane ethanol from Brazil, to give us one of the cleanest biofuels in the world (remember, we are not using this for everyday consumption, but rather as an emergency reserve, so we can acquire it gradually over time.) Ethanol is a little better than half as energy dense as biodiesel, which still gives us a nice margin under either set of assumptions.

So there you have it. Do we need billions of tons of lead to acquire the infrastructure to store a week's worth of energy? No, in fact we have it already!

Tuesday, February 4, 2014

David Keith vs the apocalypse

David Appell highlights this passage from David Keith's new book:
Imagine how effectively the world might collaborate if we discovered a massive asteroid inbound for a 2050 impact. But, this is not what we face. The claim that climate change threatens an imminent catastrophe is an attempt to play a trump car of (seemingly) objective science in order to avoid debate about the trade-offs at the heart of climate policy and about the role that values play in driving each of our personal judgements of the moral weight we accord to competing interests. But climate change is one of many problems, so there is no substitute for realistic assessment of our risks and open debate about the trade-offs between them, for we cannot avoid all risk or solve all problems.

-- David Keith, A Case for Climate Engineering
There's no question that there is a strain of apocalyptic thinking in environmentalism (is there any ideology that does not have some element of this?)  But as so often when you try to ascribe motives to people, Keith does not have this quite right.

Yes, some people picture climate change as an "imminent catastrophe." And depending on your definition of "catastrophe" or "imminent" you could be right by your own lights. But I don't think the appeal of the end-of-the-world thinking is that it short-circuits a debate about "values." The idea that we are having a conflict about values is just wrong, wrong, wrong. So what is the appeal?

In the first place you have to acknowledge the general appeal of extremism in political debates. This has been apparent for thousands of years, ever since there was a need to appeal to large groups of unrelated people using abstract ideas. Go back to the Greece of Thucydides, 2,500 years ago, and you find oligarchs swearing that democracy will lead to economic collapse, famine, incest, and cannibalism, and democrats swearing that oligarchs will crush the life out of the people, steal their money, rape their women, and so forth.

Extremism makes it easy to talk to low-information voters, which has always been a prime need of all political activists. Keith views environmental extremists as cunningly evading a serious and rational weighing of the relative risks and benefits of alternate courses of action. In reality, of course, insofar as shouting calamity from the rooftops is strategic at all, the strategy is to compete with other prophets shouting a different kind of doom from other rooftops.

An Assyrian clay tablet dating to around 2800 B.C. bears the inscription: “Our Earth is degenerate in these later days; there are signs that the world is speedily coming to an end; bribery and corruption are common; children no longer obey their parents; every man wants to write a book and the end of the world is evidently approaching.”[1]

Then, too, there is the even longer tradition of prophecies of doom, predicting an apocalypse that will consume the wicked. This goes right back to the Hebrew Bible and even before that. What appeals about these stories is not that they change the narrative of values -- prophets will have that conversation with you all day, every day, firmly in the belief that they are getting the best of the exchange. It is to reconcile their certainty that their values are correct and that they are in harmony with the universe with all the evidence around them that they have little power and the world is not ordered as it should be. This is the appeal of apocalypse stories to the devout, be they Christians, Muslims, extreme environmentalists, or libertarians (for libertarians, of course, the part of the Beast is played by fiat currency.)

The people who think the world is ending, imminently, should not be confused with the people who think the world is on a bad path, and needs to get on the right one, imminently. The two groups have plenty in common. They don't like large parts of the society we live in. They may dress funny. They may be strident and intolerant of gradualism. But while the former are in retreat from reality, the latter most decidedly are not. They are the Quaker abolitionists smuggling slaves to freedom, the women's suffragettes being dragged off to jail, the marchers on Selma and Washington braving the fire hoses and the dogs. And they are also, very proudly, the scientist arrested fighting mountaintop coal mining and the activist laying down in front of trucks to stop pipelines.

There is the judicious weighing of risks by armchair intellectuals, and there is action by those who have weighed the risks to those that cannot defend or protect themselves and chose to act, according to their own moral code. The latter may or may not succeed in affecting significant change; the former never will.