Tag Archives: energy

Land use nuclear vs wind and solar

An advantage of nuclear power over solar and wind is that it uses a lot less land, see graphic below. While I am doubtful that industrial gas causes global warming, I am not a fan of pollution, and that’s why I like nuclear power. Nuclear power adds no water or air pollution when it runs right, and removes a lot less land than wind and solar. Consider the newly approved Hinkley Point C (England), see graphic below. The site covers 430 acres, 1.74 km2, and is currently the home of Hinkley Point B, a nuclear plant slated for retirement. When Hinkley Point C is built on the same site, it will add 26 trillion Watt-hr/ year (3200 MW, 93% up time), about 7% of the total UK demand. Yet more power would be provided from these 430 acres if Hinkley B is not shut down.

Nuclear land use vs solar and wind; British Gov't. regarding their latest plant

Nuclear land use vs solar and wind; British Gov’t. regarding their latest plant

A solar farm to produce 26 trillion W-hr/year would require 130,000 acres, 526 km2. This area would suggest they get the equivalent of 1.36 hours per day of full sun on every m2, not unreasonable given the space for roads and energy storage, and how cloudy England is. Solar power requires a lot energy-storage since you only get full power in the daytime, when there are no clouds.

A wind farm requires even more land than solar, 250,000 acres, or somewhat more than 1000 km2. Wind farms require less storage but that the turbines be spaced at a distance. Storage options could include hydrogen, batteries, and pumped hydro.; I make the case that hydrogen is better. While wind-farm space can be dual use — allowing farming for example, 1000 square km, is still a lot of space to carve up with roads and turbines. It’s nearly the size of greater London; the tourist area, London city is only 2.9 km2.

All these power sources produce pollution during construction and decommissioning. But nuclear produces somewhat less as the plants are less massive in total, and work for more years without the need for major rebuilds. Hinkley C will generate about 30,000 kg/year of waste assuming 35 MW-days/kg, but the cost to bury it in salt domes should not be excessive. Salt domes are needed because Hinkley waste will generate 100 kW of after-heat, even 16 years out. Nuclear fusion, when it comes, should produce 1/10,000 as much after-heat, 100W, 1 year out, but fusion isn’t here yet.

There is also the problem of accidents. In the worst nuclear disaster, Chernobyl, only 31 people died as a direct result, and now (strange to say) the people downwind are healthier than the average up wind; it seems that small amounts of radiation may be good for you. By comparison, in Iowa alone there were 317 driving fatalities in 2013. And even wind and solar have accidents, e.g. people falling from wind-turbines.

Robert Buxbaum, January 22, 2014. I’m president of REB Research, a manufacturer of hydrogen generators and purifiers — mostly membrane reactor based. I also do contract research, mostly on hydrogen, and I write this blog. My PhD research was on nuclear fusion power. I’ve also written about conservation, e.g. curtains; insulation; paint your roof white.

My failed process for wood to green gasoline

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Most researchers publish the results of their successful projects, and ignore the rest. It’s an understandable failing given the cost and work to publish and the general sense that the project that flops indicated a loser – researcher. Still, it’s a shame, and I’d like to break from it here to describe a worthwhile project of mine that failed — turning wood into green gasoline. You may come to believe the project worthwhile too, and figure that you might learn from my story some pathways to avoid if you decide to try it. Besides I figure that it’s an interesting tale. All success stories are similar, I find; failure can come in many ways.

Failure can come from incorrect thinking – assumptions that are wrong. One basis of my thinking was the observation that gasoline, for the most part, was crude-oil that had been fluffed up with hydrogen. The density you buy weighs about 5.5 lb/gallon while crude oil weighs 9 lb/gallon. The difference is hydrogen. Perhaps wood too could be turned into gasoline if hydrogen were added. Another insight was that the structure of wood was the structure of a long chain -alcohol,  —(CHOH)-(CHOH)-(CHOH)—. My company had long experience breaking alcohols to make hydrogen. I figured we could do something similar with wood, fluffing up the wood by breaking the long-chain alcohols to short ones.

A possible first reaction step would be to break a C-O-C bond, a ketone bond, with hydrogen:

—(CHOH)-(CH2O)-(CHOH)— + H2 –>  —(CHOH)-CH2OH + CH2OH—

The next reaction step, I imagined was de-oxygenation:

—(CHOH)-CH2OH  +  H2 –>  —(CHOH)-CH3  + H2O

At this point, we are well on the way to making gasoline, or making a gasoline-relevant alcohol like C6H11-OH. The reactions I wanted were exothermic, meaning they would probably “go” — in actuality -∆G is the determinate of reaction favorability, but usually a -∆H and -∆G go together. Of course there are other reactions that I could have worried about –Ones that produce nasty goop. Among these:

–(CHOH)-(CH2O)-(CHOH)—  –> –(CO)-(C)-(CHOH)— + H2O +H2

I didn’t worry about these reactions because I figured I could outrun them using the right combination of a high hydrogen pressure, the right (low) temperature and the right catalyst. I may have been wrong. Then again, perhaps I picked the wrong catalyst – Fe2O3/ rust, or the wrong set of conditions. I picked Fe2O3 because it was cheap and active.

I convinced myself that Fe2O3 was sufficiently specific to get the product to a good 5-6 carbon compounds for gasoline. Wood celluloses are composed of five and six-carbon ring structure, and the cost of wood is very low per energy. What could go wrong? I figured that starting with these 5-6 carbon ring structures, virtually guaranteed getting high octane products. With the low cost and all the heat energy of the wood, wood + H2 seemed like a winning way to store and transport energy. If i got 6 carbon alcohols and similar compounds they’d have high-octane and the right vapor pressures and the products should be soluble in ordinary gasoline.

And the price was right; gasoline was about $3.50/ gallon, while wood was essentially free.  Hydrogen isn’t that expensive, even using electrolysis, and membrane reactors (a major product of our company) had the potential to make it cheaper. Wood + Hydrogen seemed like the cheaper version of syngas: CO +H2, and rust is similar to normal Fischer Tropsch catalyst. My costs would be low, and I’d expected to get better conversion since I should get fewer low molecular weight products like methane, ethane and methanol. Everything fundamental looked like it was in my favor.

With all the fundamentals in place, I figured my only problem would be to design a reasonably cheap reactor. At first I considered a fluidized bed reactor, but decided on a packed bed reactor instead, 8″ long by 3/4″ OD. This was a tube, filled with wood chips and iron oxide as a catalyst. I introduced high pressure hydrogen via a 150 psi hydrogen + 5% He mix. I hoped to see gasoline and water come out the other end. (I had the hydrogen – helium mix left over from a previous experiment, and was paying rental; otherwise I would have used pure hydrogen). I used heat tape and a controller to keep the temperature near-constant.

Controlling the temperature was key, I thought, to my aim of avoiding dehydration and the formation of new carbon-carbon bonds. At too high a temperature, the cellulose molecules would combine and lose water to form a brown high molecular weight tar called bio-oil, as well as methane and char. Bio-oil is formed the same way you form caramel from sugar, and as with sugar, it’s nothing you’d want to put in a car. If I operated at too low a temperature (or with the wrong catalyst) the reaction would be too slow, and the capital costs would be excessive. I could keep the temperature in the right (Goldilocks) temperature, I thought with the right catalyst and the right (high) hydrogen pressure.

No matter how I did this, I knew that I’d get some carbon-carbon bond formation, and perhaps a little char, but so long as it wasn’t too much it should be manageable. I figured I could hydrogenate the tar and remove the char at the end of the process. Most of the gasoline energy would come from the trees, and not the hydrogen, and there would be little hydrogen wasted forming methane. Trees would always be cheap: they grow quickly, and are great at capturing solar energy. Many cities pay for disposal of their tree waste, so perhaps a city would pay us to take their wood chips. With cheap wood, the economics would be good so long as used all the hydrogen I put in, and got some reasonable fraction of energy from the wood. 

i began my reaction at 150°C with 50 psi hydrogen. At these conditions, I saw no reaction; I then raised the temperature to 200°C, then raised the pressure to 100 psi (still nothing) and then tried 250°C, still at 100psi. By now we were producing water but it was impossible to tell if we were hydrogenating the cellulose to gasoline, or dehydrating the cellulose to bio-oil.

As it turned out we were getting something worse that bio-oil: bio-oil gunk. Instead of the nasty brown liquid that’s made when wood is cooked to dehydration (water removal, caramelization), I got something that was nastier than I’d imagined possible. The wood molecules did not form nice chains but combined to form acidic, aromatic gunk (aromatic in both senses: benzine-type molecules and smelly) that still contained unreacted wood as a sort of press-board. The gunk was corrosive and reactive; it probably contained phenol, and seemed bent on reacting to form a phenolic glue. I found the gunk was insoluble in most everything: water, gasoline, oil, methanol (the only exception was ethanol). As best I can tell, you can not react this gunk with hydrogen to make gasoline as it is non-volatile, and almost impossible to get out of my clogged reactor. Perhaps a fluidized bed would be would be better, but I’m afraid it would form wood clumps even there. 

I plan to try again, perhaps using higher pressure hydrogen and perhaps a liquid hydrogen carrier, to get the hydrogen to the core of the wood and speed the catalysis of the desired products. The key is finding a carrier that is not too expensive or that can be easily recovered.

Robert E. Buxbaum, Dec 13, 2013. Here’s something on a visit to my lab, on adding hydrogen to automobile engines, and on the right way to do science. And here’s my calculation for how much wood a woodchuck chucks if a woodchuck could chuck wood, (100 lbs/ night) plus why woodchucks do not chuck wood like beavers.

yet another quantum joke

Why do you get more energy from a steak than from the same amount of hamburger?

 

Hamburger is steak in the ground state.

 

Is funny because….. it’s a pun on the word ground. Hamburger is ground-up meat, of course, but the reference to a ground state also relates to a basic discovery of quantum mechanics (QM): that all things exist in quantized energy states. The lowest of these is called the ground state, and you get less energy out of a process if you start with things at this ground state. Lasers, as an example, get their energy by electrons being made to drop to their ground state at the same time; you can’t get any energy from a laser if the electrons start in the ground state.

The total energy of a thing can be thought of as having a kinetic and a potential energy part. The potential energy is usually higher the more an item moves from its ideal (lowest potential point). The kinetic energies of though tends to get lower when more space is available because, from Heisenberg uncertainty, ∆l•∆v=h. That is, the more space there is, the less uncertainty of speed, and thus the less kinetic energy other things being equal. The ground energy state is the lowest sum of potential and kinetic energy, and thus all things occupy a cloud of some size, even at the ground state. Without this size, the world would cease to exist. Atoms would radiate energy, and shrink until they vanished.

In grad school, I got into understanding thermodynamics, transport phenomena, and quantum mechanics, particularly involving hydrogen. This lead to my hydrogen production and purification inventions, what my company sells.

Click here for a quantum cartoon on waves and particles, an old Heisenberg joke, or a joke about how many quantum mechanicians it takes to change a lightbulb.

R. E. Buxbaum, July 16, 2013. I once claimed that the unseen process that maintains existence could be called God; this did not go well with the religious.

 

What’s the quality of your home insulation

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By Dr. Robert E. Buxbaum, June 3, 2013

It’s common to have companies call during dinner offering to blow extra insulation into the walls and attic of your home. Those who’ve added this insulation find a small decrease in their heating and cooling bills, but generally wonder if they got their money’s worth, or perhaps if they need yet-more insulation to get the full benefit. Here’s a simple approach to comparing your home heat bill to the ideal your home can reasonably reach.

The rate of heat transfer through a wall, Qw, is proportional to the temperature difference, ∆T, to the area, A, and to the average thermal conductivity of the wall, k; it is inversely proportional to the wall thickness, ∂;

Qw = ∆T A k /∂.

For home insulation, we re-write this as Qw = ∆T A/Rw where Rw is the thermal resistance of the wall, measured (in the US) as °F/BTU/hr-ft2. Rw = ∂/k.

Lets assume that your home’s outer wall thickness is nominally 6″ thick (0.5 foot). With the best available insulation, perfectly applied, the heat loss will be somewhat higher than if the space was filled with still air, k=.024 BTU/ft. hr°F, a result based on molecular dynamics. For a 6″ wall, the R value, will always be less than .5/.024 = 20.8 °F/BTU/hr-ft2.. It will be much less if there are holes or air infiltration, but for practical construction with joists and sills, an Rw value of 15 or 16 is probably about as good as you’ll get with 6″ walls.

To show you how to evaluate your home, I’ll now calculate the R value of my walls based on the size of my ranch-style home (in Michigan) and our heat bills. I’ll first do this in a simplified calculation, ignoring windows, and will then repeat the calculation including the windows. Windows are found to be very important. I strongly suggest window curtains to save heat and air conditioning,

The outer wall of my home is 190 feet long, and extends about 11 feet above ground to the roof. Multiplying these dimensions gives an outer wall area of 2090 ft2. I could now add the roof area, 1750 ft2 (it’s the same as the area of the house), but since the roof is more heavily insulated than the walls, I’ll estimate that it behaves like 1410 ft2 of normal wall. I calculate there are 3500 ft2 of effective above-ground area for heat loss. This is the area that companies keep offering to insulate.

Between December 2011 and February 2012, our home was about 72°F inside, and the outside temperature was about 28°F. Thus, the average temperature difference between the inside and outside was about 45°F; I estimate the rate of heat loss from the above-ground part of my house, Qu = 3500 * 45/R = 157,500/Rw.

Our house has a basement too, something that no one has yet offered to insulate. While the below-ground temperature gradient is smaller, it’s less-well insulated. Our basement walls are cinderblock covered with 2″ of styrofoam plus wall-board. Our basement floor is even less well insulated: it’s just cement poured on pea-gravel. I estimate the below-ground R value is no more than 1/2 of whatever the above ground value is; thus, for calculating QB, I’ll assume a resistance of Rw/2.

The below-ground area equals the square footage of our house, 1750 ft2 but the walls extend down only about 5 feet below ground. The basement walls are thus 950 ft2 in area (5 x 190 = 950). Adding the 1750 ft2 floor area, we find a total below-ground area of 2700 ft2.

The temperature difference between the basement and the wet dirt is only about 25°F in the winter. Assuming the thermal resistance is Rw/2, I estimate the rate of heat loss from the basement, QB = 2700*25*(2/Rw) = 135,000/Rw. It appears that nearly as much heat leaves through the basement as above ground!

Between December and February 2012, our home used an average of 597 cubic feet of gas per day or 25497 BTU/hour (heat value = 1025 BTU/ ft3). QU+ QB = 292,500/Rw. Ignoring windows, I estimate Rw of my home = 292,500/25497 = 11.47.

We now add the windows. Our house has 230 ft2 of windows, most covered by curtains and/or plastic. Because of the curtains and plastic, they would have an R value of 3 except that black-body radiation tends to be very significant. I estimate our windows have an R value of 1.5; the heat loss through the windows is thus QW= 230*45/1.5 = 6900 BTU/hr, about 27% of the total. The R value for our walls is now re-estimated to be 292,500/(25497-6900) = 15.7; this is about as good as I can expect given the fixed thickness of our walls and the fact that I can not easily get an insulation conductivity lower than still air. I thus find that there will be little or no benefit to adding more above-ground wall insulation to my house.

To save heat energy, I might want to coat our windows in partially reflective plastic or draw the curtains to follow the sun. Also, since nearly half the heat left from the basement, I may want to lay a thicker carpet, or lay a reflective under-layer (a space blanket) beneath the carpet.

To improve on the above estimate, I could consider our furnace efficiency; it is perhaps only 85-90% efficient, with still-warm air leaving up the chimney. There is also some heat lost through the door being opened, and through hot water being poured down the drain. As a first guess, these heat losses are balanced by the heat added by electric usage, by the body-heat of people in the house, and by solar radiation that entered through the windows (not much for Michigan in winter). I still see no reason to add more above-ground insulation. Now that I’ve analyzed my home, it’s time for you to analyze yours.

My steam-operated, high pressure pump

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Here’s a miniature version of a duplex pump that we made 2-3 years ago at REB Research as a way to pump fuel into hydrogen generators for use with fuel cells. The design is from the 1800s. It was used on tank locomotives and steamboats to pump water into the boiler using only the pressure in the boiler itself. This seems like magic, but isn’t. There is no rotation, but linear motion in a steam piston of larger diameter pushes a liquid pump piston with a smaller diameter. Each piston travels the same distance, but there is more volume in the steam cylinder. The work from the steam piston is greater: W = ∫PdV; energy is conserved, and the liquid is pumped to higher pressure than the driving steam (neat!).

The following is a still photo. Click on the YouTube link to see the steam pump in action. It has over 4000 views!

Mini duplex pump. Provides high pressure water from steam power. Amini version of a classic of the 1800s Coffee cup and pen shown for scale.

Mini duplex pump. Provides high pressure water from steam power. A mini version of a classic of the 1800s Coffee cup and pen shown for scale.

You can get the bronze casting and the plans for this pump from Stanley co (England). Any talented machinist should be able to do the rest. I hired an Amish craftsman in Ohio. Maurice Perlman did the final fit work in our shop.

Our standard line of hydrogen generators still use electricity to pump the methanol-water. Even our latest generators are meant for nom-mobile applications where electricity is awfully convenient and cheap. This pump was intended for a future customer who would need to generate hydrogen to make electricity for remote and mobile applications. Even our non-mobile hydrogen is a better way to power cars than batteries, but making it mobile has advantages. Another advance would be to heat the reactors by burning the waste gas (I’ve been working on that too, and have filed a patent). Sometimes you have to build things ahead of finding a customer — and this pump was awfully cool.

Camless valves and the Fiat-500

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One of my favorite automobile engine ideas is the use of camless, electronic valves. It’s an idea whose advantages have been known for 100 years or more, and it’s finally going to be used on a mainstream, commercial car — on this year’s Fiat 500s. Fiat is not going entirely camless, but the plan is to replace the cams on the air intake valves with solenoids. A normal car engine uses cams and lifters to operate the poppet valves used to control the air intake and exhaust. Replacing these cams and lifters saves some weight, and allows the Fiat-500 to operate more efficiently at low power by allowing the engine to use less combustion energy to suck vacuum. The Fiat 500 semi-camless technology is called Multiair: it’s licensed from Valeo (France), and appeared as an option on the 2010 Alfa Romeo.

How this saves mpg is as follows: at low power (idling etc.), the air intake of a normal car engine is restricted creating a fairly high vacuum. The vacuum restriction requires energy to draw and reduces the efficiency of the engine by decreasing the effective compression ratio. It’s needed to insure that the car does not produce too much NOx when idling. In a previous post, I showed that the rate of energy wasted by drawing this vacuum was the vacuum pressure times the engine volume and the rpm rate; I also mentioned some classic ways to reduce this loss (exhaust recycle and adding water).

Valeo’s/Fiat’s semi-camless design does nothing to increase the effective compression ratio at low power, but it reduces the amount of power lost to vacuum by allowing the intake air pressure to be higher, even at low power demand. A computer reduces the amount of air entering the engine by reducing the amount of time that the intake valve is open. The higher air pressure means there is less vacuum penalty, both when the valve is open even when the valve is closed. On the Alfa Romeo, the 1.4 liter Multiair engine option got 8% better gas mileage (39 mpg vs 36 mpg) and 10% more power (168 hp vs 153 hp) than the 1.4 liter cam-driven engine.

David Bowes shows off his latest camless engines at NAMES, April 2013.

David Bowes shows off his latest camless engines at NAMES, April 2013.

Fiat used a similar technology in the 1970s with variable valve timing (VVT), but that involved heavy cams and levers, and proved to be unreliable. In the US, some fine engineers had been working on solenoids, e.g. David Bowes, pictured above with one of his solenoidal engines (he’s a sometime manufacturer for REB Research). Dave has built engines with many cycles that would be impractical without solenoids, and has done particularly nice work reducing the electric use of the solenoid.

Durability may be a problem here too, as there is no other obvious reason that Fiat has not gone completely camless, and has not put a solenoid-controlled valve on the exhaust too. One likely reason Fiat didn’t do this is that solenoidal valves tend to be unreliable at the higher temperatures found in exhaust. If so, perhaps they are unreliable on the intake too. A car operated at 1000-4000 rpm will see on the order of 100,000,000 cycles in 25,000 miles. No solenoid we’ve used has lasted that many cycles, even at low temperatures, but most customers expect their cars to go more than 25,000 miles without needing major engine service.

We use solenoidal pumps in our hydrogen generators too, but increase the operating live by operating the solenoid at only 50 cycles/minute — maximum, rather than 1000- 4000. This should allow our products to work for 10 years at least without needing major service. Performance car customers may be willing to stand for more-frequent service, but the company can’t expect ordinary customers to go back to the days where Fiat stood for “Fix It Again Tony.”

Nuclear Power: the elephant of clean energy

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As someone who heads a hydrogen energy company, REB Research, I regularly have to tip toe about nuclear power, a rather large elephant among the clean energy options. While hydrogen energy looks better than battery energy in terms of cost and energy density, neither are really energy sources; they are ways to transport energy or store it. Among non-fossil sources (sources where you don’t pollute the air massively) there is solar and wind: basically non-reliable, low density, high cost and quite polluting when you include the damage done making the devices.

Compared to these, I’m happy to report that the methanol used to make hydrogen in our membrane reactors can come from trees (anti-polluting), even tree farming isn’t all that energy dense. And then there’s uranium: plentiful, cheap and incredibly energy dense. I try to ignore how energy dense uranium is, but the cartoon below shows how hard that is to do sometimes. Nuclear power is reliable too, and energy dense; a small plant will produce between 500 and 1000 MW of power; your home uses perhaps 2 kW. You need logarithmic graph paper just to compare nuclear power to most anything else (including hydrogen):

log_scale

A tiny amount of uranium-oxide, the size of a pencil will provide as much power as hundreds of train cars full of coal. After transportation, the coal sells for about $80/ton; the sells for about $25/lb: far cheaper than the train loads of coal (there are 100-110 tons of coal to a train-car load). What’s more, while essentially all of the coal in a train car ends up in the air after it’s burnt, the waste uranium generally does not go into the air we breathe. The coal fumes are toxic, containing carcinogens, carbon monoxide, mercury, vanadium and arsenic; they are often radioactive too. All this is avoided with nuclear power unless there is a bad accident, and bad accidents are far rarer with nuclear power than, for example, with natural gas. Since Germany started shutting nuclear plants and replacing them with coal, it appears they are making all of Europe sicker).

It is true that the cost to build a nuclear plant is higher than to build a coal or gas plant, but it does not have to be: it wasn’t that way in the early days of nuclear power, nor is this true of military reactors that power our (USA) submarines and major warships. Commercial nuclear reactors cost a lot largely because of the time-cost for neighborhood approval (and they don’t always get approval). Batteries used for battery power get no safety review generally though there were two battery explosions on the Dreamliner alone, and natural gas has been known to level towns. Nuclear reactors can blow up too, as Chernobyl showed (and to a lesser extent Fukushima), but almost any design is better than Chernobyl.

The biggest worry people have with nuclear, and the biggest objection it seems to me, is escaped radiation. In a future post, I plan to go into the reality of the risk in more detail, but the worry is far worse than the reality, or far worse than the reality of other dangers (we all die of something eventually). The predicted death rate from the three-mile island accident is basically nil; Fukushima has provided little health damage (not that it’s a big comfort). Further, bizarre as this seems the thyroid cancer rate in Belarus in the wind-path of the Chernobyl plant is actually slightly lower than in the US (7 per 100,000 in Belarus compared to over 9 per 100,000 in the USA). This is clearly a statistical fluke; it’s caused, I believe, by the tendency for Russians to die of other things before they can get thyroid cancer, but it suggests that the health risks of even the worst nuclear accidents are not as bad as you might think. (BTW, Our company makes hydrogen extractors that make accidents less likely)

The biggest real radiation worry (in my opinion) is where to put the waste. Ever since President Carter closed off the option of reprocessing used fuel for re-use there has been no way to permanently get rid of waste. Further, ever since President Obama closed the Yucca Mountain burial repository there have been no satisfactory place to put the radioactive waste. Having waste sitting around above ground all over the US is a really bad option because the stuff is quite toxic. Just as the energy content of nuclear fuel is higher than most fuels, the energy content of the waste is higher. Burying it deep below a mountain in an area were no-one is likely to live seems like a good solution: sort of like putting the uranium back where it came from. And reprocessing for re-use seems like an even better solution since this gets rid of the waste permanently.

I should mention that nuclear power-derived electricity is a wonderful way to generate electricity or hydrogen for clean transportation. Further, the heat of hot springs comes from nuclear power. The healing waters that people flock to for their health is laced with isotopes (and it’s still healthy). For now, though I’ll stay in the hydrogen generator business and will ignore the clean elephant in the room. Fortunately there’s hardly any elephant poop, only lots and lots of coal and solar poop.

 

Hydrogen versus Battery Power

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There are two major green energy choices that people are considering to power small-to-medium size, mobile applications like cars and next generation, drone airplanes: rechargeable, lithium-ion batteries and hydrogen /fuel cells. Neither choice is an energy source as such, but rather a clean energy carrier. That is, batteries and fuel cells are ways to store and concentrate energy from other sources, like solar or nuclear plants for use on the mobile platform.

Of these two, rechargeable batteries are the more familiar: they are used in computers, cell phones, automobiles, and the ill-fated, Boeing Dreamliner. Fuel cells are less familiar but not totally new: they are used to power most submarines and spy-planes, and find public use in the occasional, ‘educational’ toy. Fuel cells provided electricity for the last 30 years of space missions, and continue to power the international space station when the station is in the dark of night (about half the time). Batteries have low energy density (energy per mass or volume) but charging them is cheap and easy. Home electricity costs about 12¢/kWhr and is available in every home and shop. A cheap transformer and rectifier is all you needed to turn the alternating current electricity into DC to recharge a battery virtually anywhere. If not for the cost and weight of the batteries, the time to charge the battery (usually and hour or two), batteries would be the obvious option.

Two obvious problems with batteries are the low speed of charge and the annoyance of having to change the battery every 500 charges or so. If one runs an EV battery 3/4 of the way down and charges it every week, the battery will last 8 years. Further, battery charging takes 1-2 hours. These numbers are acceptable if you use the car only occasionally, but they get more annoying the more you use the car. By contrast, the tanks used to hold gasoline or hydrogen fill in a matter of minutes and last for decades or many thousands of fill-cycles.

Another problem with batteries is range. The weight-energy density of batteries is about 1/20 that of gasoline and about 1/10 that of hydrogen, and this affects range. While gasoline stores about 2.5 kWhr/kg including the weight of the gas tank, current Li-Ion batteries store far less than this, about 0.15 kWhr/kg. The energy density of hydrogen gas is nearly that of gasoline when the efficiency effect is included. A 100 kg of hydrogen tank at 10,000 psi will hold 8 kg of hydrogen, or enough to travel about 350 miles in a fuel-cell car. This is about as far as a gasoline car goes carrying 60 kg of tank + gasoline. This seems acceptable for long range and short-range travel, while the travel range with eVs is more limited, and will likely remain that way, see below.

The volumetric energy density of compressed hydrogen/ fuel cell systems is higher than for any battery scenario. And hydrogen tanks are far cheaper than batteries. From Battery University. http://batteryuniversity.com/learn/article/will_the_fuel_cell_have_a_second_life

The volumetric energy density of compressed hydrogen/ fuel cell systems is higher than for any battery scenario. And hydrogen tanks are far cheaper than batteries. From Battery University. http://batteryuniversity.com/learn/article/will_the_fuel_cell_have_a_second_life

Cost is perhaps the least understood problem with batteries. While electricity is cheap (cheaper than gasoline) battery power is expensive because of the high cost and limited life of batteries. Lithium-Ion batteries cost about $2000/kWhr, and give an effective 500 charge/discharge cycles; their physical life can be extended by not fully charging them, but it’s the same 500 cycles. The effective cost of the battery is thus $4/kWhr (The battery university site calculates $24/kWhr, but that seems overly pessimistic). Combined with the cost of electricity, and the losses in charging, the net cost of Li-Ion battery power is about $4.18/kWhr, several times the price of gasoline, even including the low efficiency of gasoline engines.

Hydrogen prices are much lower than battery prices, and nearly as low as gasoline, when you add in the effect of the high efficiency fuel cell engine. Hydrogen can be made on-site and compressed to 10,000 psi for less cost than gasoline, and certainly less cost than battery power. If one makes hydrogen by electrolysis of water, the cost is approximately 24¢/kWhr including the cost of the electrolysis unit.While the hydrogen tank is more expensive than a gasoline tank, it is much cheaper than a battery because the technology is simpler. Fuel cells are expensive though, and only about 50% efficient. As a result, the as-used cost of electrolysis hydrogen in a fuel cell car is about 48¢/kWhr. That’s far cheaper than battery power, but still not cheap enough to encourage the sale of FC vehicles with the current technology.

My company, REB Research provides another option for hydrogen generation: The use of a membrane reactor to make it from cheap, easy to transport liquids like methanol. Our technology can be used to make hydrogen either at the station or on-board the car. The cost of hydrogen made this way is far cheaper than from electrolysis because most of the energy comes from the methanol, and this energy is cheaper than electricity.

In our membrane reactors methanol-water (65-75% Methanol), is compressed to 350 psi, heated to 350°C, and reacted to produce hydrogen that is purified as it is made. CH3OH + H2O –> 3H2 + CO2, with the hydrogen extracted through a membrane within the reactor.

The hydrogen can be compressed to 10,000 psi and stored in a tank on board an automobile or airplane, or one can choose to run this process on-board the vehicle and generate it from liquid fuel as-needed. On-board generation provides a saving of weight, cost, and safety since you can carry methanol-water easily in a cheap tank at low pressure. The energy density of methanol-water is about 1/2 that of gasoline, but the fuel cell is about twice as efficient as a gasoline engine making the overall volumetric energy density about the same. Not including the fuel cell, the cost of energy made this way is somewhat lower than the cost of gasoline, about 25¢/kWhr; since methanol is cheaper than gasoline on a per-energy basis. Methanol is made from natural gas, coal, or trees — non-imported, low cost sources. And, best yet, trees are renewable.

Heat conduction in insulating blankets, aerogels, space shuttle tiles, etc.

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A lot about heat conduction in insulating blankets can be explained by the ordinary motion of gas molecules. That’s because the thermal conductivity of air (or any likely gas) is much lower than that of glass, alumina, or any likely solid material used for the structure of the blanket. At any temperature, the average kinetic energy of an air molecule is 1/2kT in any direction, or 3/2kT altogether; where k is Boltzman’s constant, and T is absolute temperature, °K. Since kinetic energy equals 1/2 mv2, you find that the average velocity in the x direction must be v = √kT/m = √RT/M. Here m is the mass of the gas molecule in kg, M is the molecular weight also in kg (0.029 kg/mol for air), R is the gas constant 8.29J/mol°C, and v is the molecular velocity in the x direction, in meters/sec. From this equation, you will find that v is quite large under normal circumstances, about 290 m/s (650 mph) for air molecules at ordinary temperatures of 22°C or 295 K. That is, air molecules travel in any fixed direction at roughly the speed of sound, Mach 1 (the average speed including all directions is about √3 as fast, or about 1130 mph).

The distance a molecule will go before hitting another one is a function of the cross-sectional areas of the molecules and their densities in space. Dividing the volume of a mol of gas, 0.0224 m3/mol at “normal conditions” by the number of molecules in the mol (6.02 x10^23) gives an effective volume per molecule at this normal condition: .0224 m3/6.0210^23 = 3.72 x10^-26 m3/molecule at normal temperatures and pressures. Dividing this volume by the molecular cross-section area for collisions (about 1.6 x 10^-19 m2 for air based on an effective diameter of 4.5 Angstroms) gives a free-motion distance of about 0.23×10^-6 m or 0.23µ for air molecules at standard conditions. This distance is small, to be sure, but it is 1000 times the molecular diameter, more or less, and as a result air behaves nearly as an “ideal gas”, one composed of point masses under normal conditions (and most conditions you run into). The distance the molecule travels to or from a given surface will be smaller, 1/√3 of this on average, or about 1.35×10^-7m. This distance will be important when we come to estimate heat transfer rates at the end of this post.

 

Molecular motion of an air molecule (oxygen or nitrogen) as part of heat transfer process; this shows how some of the dimensions work.

Molecular motion of an air molecule (oxygen or nitrogen) as part of heat transfer process; this shows how some of the dimensions work.

The number of molecules hitting per square meter per second is most easily calculated from the transfer of momentum. The pressure at the surface equals the rate of change of momentum of the molecules bouncing off. At atmospheric pressure 103,000 Pa = 103,000 Newtons/m2, the number of molecules bouncing off per second is half this pressure divided by the mass of each molecule times the velocity in the surface direction. The contact rate is thus found to be (1/2) x 103,000 Pa x 6.02^23 molecule/mol /(290 m/s. x .029 kg/mol) = 36,900 x 10^23 molecules/m2sec.

The thermal conductivity is merely this number times the heat capacity transfer per molecule times the distance of the transfer. I will now calculate the heat capacity per molecule from statistical mechanics because I’m used to doing things this way; other people might look up the heat capacity per mol and divide by 6.02 x10^23: For any gas, the heat capacity that derives from kinetic energy is k/2 per molecule in each direction, as mentioned above. Combining the three directions, that’s 3k/2. Air molecules look like dumbbells, though, so they have two rotations that contribute another k/2 of heat capacity each, and they have a vibration that contributes k. We begin with an approximate value for k = 2 cal/mol of molecules per °C; it’s actually 1.987 but I round up to include some electronic effects. Based on this, we calculate the heat capacity of air to be 7 cal/mol°C at constant volume or 1.16 x10^-23 cal/molecule°C. The amount of energy that can transfer to the hot (or cold) wall is this heat capacity times the temperature difference that molecules carry between the wall and their first collision with other gases. The temperature difference carried by air molecules at standard conditions is only 1.35 x10-7 times the temperature difference per meter because the molecules only go that far before colliding with another molecule (remember, I said this number would be important). The thermal conductivity for stagnant air per meter is thus calculated by multiplying the number of molecules times that hit per m2 per second, the distance the molecule travels in meters, and the effective heat capacity per molecule. This would be 36,900 x 10^23  molecules/m2sec x 1.35 x10-7m x 1.16 x10^-23 cal/molecule°C = 0.00578 cal/ms°C or .0241 W/m°C. This value is (pretty exactly) the thermal conductivity of dry air that you find by experiment.

I did all that math, though I already knew the thermal conductivity of air from experiment for a few reasons: to show off the sort of stuff you can do with simple statistical mechanics; to build up skills in case I ever need to know the thermal conductivity of deuterium or iodine gas, or mixtures; and finally, to be able to understand the effects of pressure, temperature and (mainly insulator) geometry — something I might need to design a piece of equipment with, for example, lower thermal heat losses. I find, from my calculation that we should not expect much change in thermal conductivity with gas pressure at near normal conditions; to first order, changes in pressure will change the distance the molecule travels to exactly the same extent that it changes the number of molecules that hit the surface per second. At very low pressures or very small distances, lower pressures will translate to lower conductivity, but for normal-ish pressures and geometries, changes in gas pressure should not affect thermal conductivity — and does not.

I’d predict that temperature would have a larger effect on thermal conductivity, but still not an order-of magnitude large effect. Increasing the temperature increases the distance between collisions in proportion to the absolute temperature, but decreases the number of collisions by the square-root of T since the molecules move faster at high temperature. As a result, increasing T has a √T positive effect on thermal conductivity.

Because neither temperature nor pressure has much effect, you might expect that the thermal conductivity of all air-filed insulating blankets at all normal-ish conditions is more-or-less that of standing air (air without circulation). That is what you find, for the most part; the same 0.024 W/m°C thermal conductivity with standing air, with high-tech, NASA fiber blankets on the space shuttle and with the cheapest styrofoam cups. Wool felt has a thermal conductivity of 0.042 W/m°C, about twice that of air, a not-surprising result given that wool felt is about 1/2 wool and 1/2 air.

Now we can start to understand the most recent class of insulating blankets, those with very fine fibers, or thin layers of fiber (or aluminum or gold). When these are separated by less than 0.2µ you finally decrease the thermal conductivity at room temperature below that for air. These layers decrease the distance traveled between gas collisions, but still leave the same number of collisions with the hot or cold wall; as a result, the smaller the gap below .2µ the lower the thermal conductivity. This happens in aerogels and some space blankets that have very small silica fibers, less than .1µ apart (<100 nm). Aerogels can have much lower thermal conductivities than 0.024 W/m°C, even when filled with air at standard conditions.

In outer space you get lower thermal conductivity without high-tech aerogels because the free path is very long. At these pressures virtually every molecule hits a fiber before it hits another molecule; for even a rough blanket with distant fibers, the fibers bleak up the path of the molecules significantly. Thus, the fibers of the space shuttle (about 10 µ apart) provide far lower thermal conductivity in outer space than on earth. You can get the same benefit in the lab if you put a high vacuum of say 10^-7 atm between glass walls that are 9 mm apart. Without the walls, the air molecules could travel 1.3 µ/10^-7 = 13m before colliding with each other. Since the walls of a typical Dewar are about 0.009 m apart (9 mm) the heat conduction of the Dewar is thus 1/150 (0.7%) as high as for a normal air layer 9mm thick; there is no thermal conductivity of Dewar flasks and vacuum bottles as such, since the amount of heat conducted is independent of gap-distance. Pretty spiffy. I use this knowledge to help with the thermal insulation of some of our hydrogen generators and hydrogen purifiers.

There is another effect that I should mention: black body heat transfer. In many cases black body radiation dominates: it is the reason the tiles are white (or black) and not clear; it is the reason Dewar flasks are mirrored (a mirrored surface provides less black body heat transfer). This post is already too long to do black body radiation justice here, but treat it in more detail in another post.

RE. Buxbaum

Small hydrogen generators for cooling dynamo generators

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A majority of the electricity used in the US comes from rotating dynamos. Power is provided to the dynamos by a turbine or IC engine and the dynamo turns this power into electricity by moving a rotating coil (a rotor) through a non-rotating magnetic field provided by magnets or a non-rotating coil (a stator). While it is easy to cool the magnets or stator, cooling the rotor is challenging as there is no possibility to connect it cooling water or heat transfer paste. One of the more common options is hydrogen gas.

It is common to fill the space between the rotor and the stator with hydrogen gas. Heat transfers from the rotor to the stator or to the walls of the dynamo through the circulating hydrogen. Hydrogen has the lowest density of any gas, and the highest thermal conductivity of any gas. The low density is important because it reduces the power drag (wind drag) on the rotor. The high heat transfer coefficient helps cool the rotor so that it does not burn out at high power draw.

Hydrogen is typically provided to the dynamo by a small hydrogen generator or hydrogen bottle. While we have never sold a hydrogen generator to this market, I strongly believe that our membrane reactor hydrogen generators would be competitive; the cost of hydrogen is lower than that of bottled gas; it is far more convenient and safe; and the hydrogen is purer than from electrolysis.