Liquid metal batteries

I've recently started a contract at work for Professor Donald Sadoway's lab at MIT working on liquid metal batteries. I can't describe the details of the project I'm working on, but the research going on in the lab is quite interesting. The idea is to solve the problem of energy storage that accompanies all of our promising renewable energy solutions, like wind and solar. When the wind stops blowing or a cloud obscures the sun, the electrical grid still needs to supply energy to the world. If you only have a small amount of renewable power attached to your grid, you can just ignore the problem, but around 20% penetration, you get into trouble. Our best solution right now is either firing up natural gas turbines to cover the peak loads or pumping water up a hill when we have extra capacity so it can run down through a generator when we need it back.

What we really need are massive, cheap, efficient batteries. The idea in the Sadoway lab is to make something like an aluminum refinery, but instead of just sinking huge amounts of electricity to extract aluminum, you set up a reversible reaction so you could get the electricity back later.

Go take a look at this awesome, enormous picture of one of these furnaces in an aluminum refinery so you know what I'm talking about. Look at the size of the guy in the picture, and then look at how many furnace chambers there are in the row. That's an industrial scale operation.

To make aluminum, you dig bauxite out of the ground and use heat and sodium hydroxide to extract the part that's aluminum. What you get out, unfortunately, is oxidized aluminum, known as alumina. This is because aluminum, in its elemental form, reacts with oxygen, and when it sits in the earth for eternity, there's plenty of air seeping around, so all of the aluminum bonds with oxygen.

Fortunately, we have electrochemistry on our side. The large smelting furnaces in the picture you looked at a moment ago are long steel troughs that are lined with carbon and filled with aluminum oxide. These form the two electrodes in a chemical reaction. When electricity is run through the carbon into the aluminum oxide, the oxygen releases from the aluminum and bonds to the carbon, creating carbon dioxide, which is then vented to the atmosphere to help keep the planet warm. During the reaction, the aluminum oxide in the center heats up and liquefies, while the outer crust remains solid, sort of like the liquid-filled gum of the 70's, Chewels. (You may also recall Freshen-up, "the gum that goes squirt.")

To turn this process into a battery, we need an electrode that doesn't turn into a gas, and we'd like both electrodes to be cheap and lightweight, relative to the amount of energy they can store. Sadoway's lab started with one magnesium electrode and one antimony electrode, with a salt electrolyte in between. (They have since moved on to better combinations that I'm not at liberty to describe.) If you heat the core of the battery up to 700 C, everything becomes liquid, and the resistance drops substantially. Most remarkably, the three materials separate by density — electrode, electrolyte, electrode — all in a stack.

What's so great about a liquid metal battery? They have several advantages, notably extremely low internal resistance and huge current capability. Aluminum refineries run at currents above 100,000 amps. For comparison, most household circuitbreakers blow at 15 amps. The low resistance of liquid metals means that the battery is likely to charge and discharge very efficiently.

At first, the fact that the battery needs to run at high temperature seems like a major disadvantage — if you have to dump a lot of energy into heating, that makes the battery less efficient. This is true, but what's not obvious is what happens to a furnace's thermal behavior as it grows in size. In general, hot objects cool off in proportion to their surface area, which grows proportional to the square of the size of an object, roughly speaking. The capacity of a battery, however, grows with its volume, which is proportional to the cube of its size. This means that as the battery becomes huge, the amount of heat loss per unit of capacity decreases, i.e. the volume overwhelms the surface area. It's this same property that allowed icehouses in pre-industrial times to store ice well into the summer. There's some hope that at the right scale, with the right insulation, the small inefficiency in charging and discharging the battery will suffice to keep the core in the molten state.

So that's what I'm working on recently. (I'm still working on a Linux board on the side, but it's kind of on the back burner for the next month or so.)

Estimating air changes per hour with a blower door

When I was trying to figure out how big a gas boiler we needed for our house a few months ago, I tried to account for both the insulation in our walls as well as the air leaks that let warm air escape as cold air sneaks in. I had read that an old, drafty house turns over its volume in air once per hour. That seemed high to me, but I was looking for a conservative estimate, so that's what I used in my calculations. Since then, I've been hoping to find a way to make a better estimate.

Solution: Colin's blower door

A friend of mine from Stanford, Colin Fay, runs a company with his dad, David Fay, called Energy Metrics. Last weekend, Colin and his nearly homonymic associate Cullen were kind enough to bring Colin's blower door over to our house to run a test to see how drafty our house is.

The basic idea of a blower door is this: you fill your front door with a curtain and a massive fan that forces air out of the house. While it's doing that, a small sensor measures the air pressure difference between the inside and outside of the house. There are a few different tests you can run, but the standard test that the fan controller runs is to automatically adjust the fan speed until the pressure inside is 50 Pa lower than outside. For comparison: 50 Pa is roughly equivalent to the pressure from a windspeed of 20 mph, but blowing at your house uniformly from all directions. Atmospheric pressure is around 100,000 Pa.

When the fan reaches a steady state, air is whistling in through all the gaps around your windows, doors and foundation, and you can tell where the problems are. For us, the largest draft was coming under the basement door. The next worst were the gaps between the sashes in our larger, older double-hung windows. In real life, I suspect that the gap under the basement door is not so bad-- the thermal gradient keeps the colder, denser air sunk down in the basement. I didn't realize it at the time, but most of the draft was probably coming down through our vestigial chimney.

Colin's blower door, a Retrotec 2000 with a DM-2 Mark II controller, pulled air through our house at 3900-4000 ft³ per minute to generate a pressure difference of 50 Pa. Our house has a volume of around 18000 ft³, so with the fan blowing, we were replacing all the air in our house every 4.5 minutes, or 13.3 times per hour.

Assembling the blower door frame

The blower door from the inside

Once you know how drafty your house is with a fan pulling the heavens through it, you need to scale that to match the typical conditions for your house. As a rough rule of thumb: just divide by 20. With the fan, we had 13.3 air changes per hour, so that's about 0.7 air changes per hour without the fan.

But if you want to ascend to the peak of Mount Energygeek, and you're willing to do it unashamedly, you can use the empirical corrections of Max Sherman of the Energy Performance of Buildings Group at Lawrence Berkeley National Lab, who completed his thesis on modeling building air infiltration in 1980, when oil rolled down like waters and righteousness like acid rain. You look up correction factors for climate (~18 for Boston), building height (0.7 for our house), wind shielding (1) and leakiness (1), multiply them together, and you've got a better correction factor than the rough guess of 20. For our house, we end up with 13.3/(18 x 0.7 x 1 x 1) = 1.06 air changes per hour.

With that knowledge, you can calculate the power required to offset the drafts cooling or heating your house. Our house, nominally a 1900 ft² Victorian, has an internal volume of 18000 ft³, or 510 m³, so when it's 0 C outside, we're heating about 1.06 x 510 m³ of air per hour by around 20 C. The heat capacity of air is around 1200 J/m³C. That means we need to pour in 1200 J/m³C x 540 m³ x 20 C every 3600 seconds. By my calculation, that's about 3.6 kW.

The conductive heat loss model I developed for our house a few months ago when we were installing the boiler predicts that the conductive heat loss at the same temperature will be around 18 kW, so we lose about 1/6th of our heat from air infiltration.

Colin suggested we could reduce our draftiness by around 2x before we'd have to worry about the effects of too little fresh air (farts, basically). He suggested picking up a tube of transparent silicone caulk in the fall to fill the gap between the window sashes, as that's where our worst leaks are. In the spring, when it's time to open the windows again, the silicone peels off.

After seeing fellow energy geek Holly's sexy basement windows last weekend, I think I might look into replacing those as well.

Measuring insulation with an IR camera

I got my hands on a thermal imaging camera for a few hours recently and took a look around the house to see what I could learn. The camera detects infrared radiation, which is proportional to surface temperature. When you're inside a house in the winter, the poorly insulated bits look blue, because they're colder. From the outside, the poorly insulated parts look red, because they're hotter than the surroundings. (This is assuming you have the camera set to adjust the spectrum to cover the temperatures in the field of view. The camera I was using, a Flir i60, could either adjust automatically or stay fixed so you could compare temperatures across multiple pictures.)

I learned some interesting stuff. The gables of our attic appear to be completely uninsulated. There is one stud bay missing insulation next to our front stairs. I hadn't noticed it, but once I knew to check, I could feel the temperature change with my hand.

Our windows are generally our most poorly insulating surface, but in the picture below, you can see the panels in the front door are a close second. (The large box of fire is a radiator.)

I was thinking I should insulate the garage roof, but the thermal camera revealed that the door was leaking more heat.

I had noticed that our cats preferred lounging in certains parts of our kitchen floor, but I hadn't noticed the large cold stripe down the middle.

Here, you can see that there are two rooms we aren't heating-- their windows are blue.

Hard Times athletics

(See my previous post for the background to this post.)

There aren't many activities in our modern society where it makes economic sense for me to engage in the kind of physical labor that will keep me healthy, but there are a few. From what I can tell, they tend to cluster at the margins of fossil fuel consumption. I'll explain what I mean by that shortly.

The first good example I've come up with involves the woodstove we had installed recently. (Barry John Chimney did a great job, by the way.) We can buy a cord of wood for around $250, but I can collect pallets from around Somerville pretty easily as well. We get some pallets at work, but I can also pick up pallets around Davis Square; I guess they're a waste stream diffuse and intermittent enough that the only collectors are amateurs such as myself.

Before I can burn the pallets, I have to cut them up. I'm not strong enough to break them into 13" lengths without the help of steel, at least stone, tools. Cutting them up with only a handsaw is possible, but grueling. A better combination for casualties of the modern workplace like me is to use a circular saw with the blade set to cut just shy of the full depth of the cross planks. If you cut all the way through, the planks sag and bind the blade. Once the planks are 95% cut, you can stomp on them, and then cut up the remaining stringers wih a handsaw. Between the two types of sawing, the stomping, and the lugging of pallets, it's a fair bit of work.

The pattern I've noticed is that I can substitute labor for fossil fuels at margins of our consumption. Heating our house entirely with wood would take a lot of effort; I've spent enough time with a splitting maul (10 hours, maybe) to know that I don't want to do it all winter. But, dragging home some pallets and cutting them up piecemeal in the basement is pretty satisfying.

One of the other large fossil fuel sinks in our lives is commuting. When I worked out in Lexington, I commuted 22 miles a day on a bike, rain or shine, all winter long. But, as we've moved and I've changed jobs, I switched to biking 16 miles per day, then 5 miles on foot. (Last year, we moved just a few blocks from my office, so I had to take up running, but you won't hear me complaining about that.)

I've been casting about for a name for these activities, and Hard Times Athletics is the best I can do.

(If you called it Fake Athletics for the Hard Times, or FAHT, you could say, "Have you been FAHTing in the basement again? It smells terrible down there." In Boston, that's funny.)

Below are some snapshots of my Hard Times gymnasium. Further suggestions of new Hard Times Athletics events are welcome in the comments.

The input

The tools of transformation

The output

Efficiency, exercise, and the modern condition

Shortly after we bought our house, with its 500 ft² lawn, one of my colleagues was kind enough to give me a vintage reel mower. It weighs 48 pounds. It was manufactured shortly after 1918, as dated by the patent on the cast handle, which I believe refers to this patent. It's very similar, but not quite identical, to this mower.

A modern reel mower of similar dimensions, like the Brill Razorcut, weighs around half as much. While I haven't actually used one, I imagine that it is substantially easier to push than my lovely behemoth. This brings us to the question of what I'm really trying to accomplish by mowing the lawn. Would it be better or worse if mowing the lawn were easier?

Here are my lawn-mowing goals: I want the grass to be uniformly short across the extents of the lawn. I want to avoid loud machinery, burning petrochemicals, and smelling like gas. I definitely want to avoid cutting my feet or otherwise hurting myself. If it's an unpleasant task, I'd like it to be quick.

At the same time that I'm looking for the best solution to that problem, I'm also trying to solve a larger problem, one endemic to the modern condition. As an educated American engineer, I spend most of my waking hours sitting at a desk, either manipulating a computer, talking to other engineers, or using a pencil. Perhaps 20% of my working time is spent on light physical work-- fabricating, assembling, or adjusting equipment. A very small fraction of my time-- less than 1%-- involves heavy work, like moving equipment or using hand tools. I see no reason that these divisions would change in future, unless I were to do still less manual labor.

Unfortunately, as a human, I die early if I don't live vigorously enough. We don't know the exact trade-off yet, but the data from the Framingham Heart Study, a longitudinal study of exercise and heart disease started in 1948, suggests that spending around 1 year of your life exercising (split into 30-minute daily stints) will extend your life by around 3.5 years, particularly if you're a good demographic match for Framingham, MA. Given that I grew up in a suburban Massachusetts town near route 495 much like Framingham, and I now live outside Boston, it's likely a great predictor for me.

My estimate of 1 year of exercise for 4 years more life is based on an analysis by Jonker, et al of the Framingham data in the journal Diabetes Care, put out by the American Diabetes Association. 2.25 hours of exercise per week, or 117 hours per year, puts you in their "high activity" group. That's about 8775 hours over a 75 year lifetime; there are 8760 hours in a year.

Since I don't have a time machine yet, I'm faced with the task of figuring out how to invest 2.25 hours per week in exercise, knowing that I'm likely to get a 4x return on the investment. (Maybe it's only a 3x return if I count the time I spend before or after exercise, like putting on sneakers or taking a shower. Still that's a huge return, especially given that it's paid in life, rather than in American dollars, which cannot be redeemed for life.)

So then the question is what goals I can accomplish using the 2.5 hours of vigorous activity I'd like to insert into my week. There are a few obvious candidates: shoveling snow in the winter and mowing the lawn in the summer. Strangely, due to the twisted incentives of the modern condition, I'd rather (or might as well) mow the lawn with a gargantuan, cast iron lawn mower than a light, nimble piece of German engineering. Our neighbors have been snowblowing our sidewalk as a friendly gesture, and I've found myself thinking, "But . . . you're taking away the only 30 minutes I've got in the winter where engaging myself in physical labor actually makes sense, and you're doing it with a two-stroke gas engine."

I am, however, not without recourse. I'm not lunkheaded enough to ask my neighbors not to snowblow our sidewalk, but this idea comes close. It's late, so the details will be saved for the next post.