Tag Archives: hydrogen generators

How I size heat exchangers

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Heat exchange is a key part of most chemical process designs. Heat exchangers save money because they’re generally cheaper than heaters and the continuing cost of fuel or electricity to run the heaters. They also usually provide free, fast cooling for the product; often the product is made hot, and needs to be cooled. Hot products are usually undesirable. Free, fast cooling is good.

So how do you design a heat exchanger? A common design is to weld the right amount of tubes inside a shell, so it looks like the drawing below. The the hot fluid might be made to go through the tubes, and the cold in the shell, as shown, or the hot can flow through the shell. In either case, the flows are usually in the opposite direction so there is a hot end and a cold end as shown. In this essay, I’d like to discuss how I design our counter current heat exchangers beginning a common case (for us) where the two flows have the same thermal inertia, e.g. the same mass flow rates and the same heat capacities. That’s the situation with our hydrogen purifiers: impure hydrogen goes in cold, and is heated to 400°C for purification. Virtually all of this hot hydrogen exits the purifier in the “pure out” stream and needs to be cooled to room temperature or nearly.

Typical shell and tube heat exchanger design, Black Hills inc.

For our typical designs the hot flows in one direction, and an equal cold flow is opposite, I will show the temperature difference is constant all along the heat exchanger. As a first pass rule of thumb, I design so that this constant temperature difference is 30°C. That is ∆THX =~ 30°C at every point along the heat exchanger. More specifically, in our Mr Hydrogen® purifiers, the impure, feed hydrogen enters at 20°C typically, and is heated by the heat exchanger to 370°C. That is 30°C cooler than the final process temperature. The hydrogen must be heated this last 30°C with electricity. After purification, the hot, pure hydrogen, at 400°C, enters the heat exchanger leaving at 30°C above the input temperature, that is at 50°C. It’s hot, but not scalding. The last 30°C of cooling is done with air blown by a fan.

The power demand of the external heat source, the electric heater, is calculated as: Wheater = flow (mols/second)*heat capacity (J/°C – mol)* (∆Theater= ∆THX = 30°C).

The smaller the value of ∆THX, the less electric draw you need for steady state operation, but the more you have to pay for the heat exchanger. For small flows, I often use a higher value of ∆THX = 30°C, and for large flows smaller, but 30°C is a good place to start.

Now to size the heat exchanger. Because the flow rate of hot fluid (purified hydrogen) is virtually the same as for cold fluid (impure hydrogen), the heat capacity per mol of product coming out is the same as for mol of feed going in. Since enthalpy change equals heat capacity time temperature change, ∆H= Cp∆T, with effectiveCp the same for both fluids, and any rise in H in the cool fluid coming at the hot fluid, we can draw a temperature vs enthalpy diagram that will look like this:

The heat exchanger heats the feed from 20°C to 370°C. ∆T = 350°C. It also cools the product 350°C, that is from 400 to 50°C. In each case the enthalpy exchanged per mol of feed (or product is ∆H= Cp*∆T = 7*350 =2450 calories.

Since most heaters work in Watts, not calories, at some point it’s worthwhile to switch to Watts. 1 Cal = 4.174 J, 1 Cal/sec = 4.174 W. I tend to do calculations in mixed units (English and SI) because the heat capacity per mole of most things are simple numbers in English units. Cp (water) for example = 1 cal/g = 18 cal/mol. Cp (hydrogen) = 7 cal/mol. In SI units, the heat rate, WHX, is:

WHX = flow (mols/second)*heat capacity per mol (J/°C – mol)* ∆Tin-out (350°C).

The flow rate in mols per second is the flow rate in slpm divided by 22.4 x 60. Since the driving force for transfer is 30°C, the area of the heat exchanger is WHX times the resistance divided by ∆THX:

A = WHX * R / 30°C.

Here, R is the average resistance to heat transfer, m2*∆T/Watt. It equals the sum of all the resistances, essentially the sum of the resistance of the steel of the heat exchanger plus that of the two gas phases:

R= δm/km + h1+ h2

Here, δm is the thickness of the metal, km is the thermal conductivity of the metal, and h1 and h2 are the gas-phase heat transfer parameters in the feed and product flow respectively. You can often estimate these as δ1/k1 and δ2/k2 respectively, with k1 and k2 as the thermal conductivity of the feed and product, both hydrogen in my case. As for, δ, the effective gas-layer thickness, I generally estimate this as 1/3 the thickness of the flow channel, for example:

h1 = δ1/k1 = 1/3 D1/k1.

Because δ is smaller the smaller the diameter of the tubes, h is smaller too. Also small tubes tend to be cheaper than big ones, and more compact. I thus prefer to use small diameter tubes and small diameter gaps. in my heat exchangers, the tubes are often 1/4″ or bigger, but the gap sizes are targeted to 1/8″ or less. If the gap size gets too low, you get excessive pressure drops and non-uniform flow, so you have to check that the pressure drop isn’t too large. I tend to stick to normal tube sizes, and tweak the design a few times within those parameters, considering customer needs. Only after the numbers look good to my aesthetics, do I make the product. Aesthetics plays a role here: you have to have a sense of what a well-designed exchanger should look like.

The above calculations are fine for the simple case where ∆THX is constant. But what happens if it is not. Let’s say the feed is impure, so some hot product has to be vented, leaving les hot fluid in the heat exchanger than feed. I show this in the plot at right for the case of 14% impurities. Sine there is no phase change, the lines are still straight, but they are no longer parallel. Because more thermal mass enters than leaves, the hot gas is cooled completely, that is to 50°C, 30°C above room temperature, but the cool gas is heated at only 7/8 the rate that the hot gas is cooled. The hot gas gives off 2450 cal as before, but this is now only enough to heat the cold fluid by 2450/8 = 306.5°. The cool gas thus leave the heat exchanger at 20°C+ 306.8° = 326.5°C.

The simple way to size the heat exchanger now is to use an average value for ∆THX. In the diagram, ∆THX is seen to vary between 30°C at the entrance and and 97.5°C at the exit. As a conservative average, I’ll assume that ∆THX = 40°C, though 50 to 60°C might be more accurate. This results in a small heat exchanger design that’s 3/4 the size of before, and is still overdesigned by 25%. There is no great down-side to this overdesign. With over-design, the hot fluid leaves at a lower ∆THX, that is, at a temperature below 50°C. The cold fluid will be heated to a bit more than to the 326.5°C predicted, perhaps to 330°C. We save more energy, and waste a bit on materials cost. There is a “correct approach”, of course, and it involves the use of calculous. A = ∫dA = ∫R/∆THX dWHX using an analytic function for ∆THX as a function of WHX. Calculating this way takes lots of time for little benefit. My time is worth more than a few ounces of metal.

The only times that I do the correct analysis is with flame boilers, with major mismatches between the hot and cold flows, or when the government requires calculations. Otherwise, I make an H Vs T diagram and account for the fact that ∆T varies with H is by averaging. I doubt most people do any more than that. It’s not like ∆THX = 30°C is etched in stone somewhere, either, it’s a rule of thumb, nothing more. It’s there to make your life easier, not to be worshiped.

Robert Buxbaum June 3, 2024

Survey on hydrogen use

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My company makes hydrogen generators: devices that make ultra-pure hydrogen on demand from methanol and water using a membrane reactor. If you use hydrogen, please fill out the following survey. I need to know my customers needs better, e.g. so that I will know if I should add a compressor. Thanks.

Create your own user feedback survey

Robert Buxbaum, June 13, 2018

New REB hydrogen generator for car fueling, etc.

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One of my favorite invention ideas, one that I’ve tried to get the DoE to fund, is a membrane hydrogen generator where the waste gas is burnt to heat the reactor. The result should be exceptional efficiency, low-cost, low pollution, and less infrastructure needs. Having failed to interest the government, I’ve gone and built one on my own dime. That’s me on the left, with Shua Spirka, holding the new core module (reactor, boiler, purifier and purifier) sized for personal car fueling.

Me and Shua and our new hydrogen generator core

Me and Shua and our new hydrogen generator core

The core just arrived from the shop last week, now we have to pumps and heat exchangers. As with our current products, the hydrogen is generated from methanol water, and extracted 99.99999% pure by diffusion through a metal membrane. This core fit in a heat transfer pot (see lower right) and the pot sits on a burner for the waste gas. Control is tricky, but I think I’ve got it. If it all works like it’s supposed to, the combination should be 80-90% energy-efficient, delivering about 75 slpm, 9 kg per day. That’s the same output as our largest current electrically heated generators, with a much lower infrastructure cost. The output should be enough to fuel one hydrogen-powered automobile per day, or keep a small fleet of plug-in, hydrogen-hybrids running continuously.

Hydrogen automobiles have a lot of advantages over Tesla-type electric automobiles. I’ll tell you how the thing works as soon as we set it up and test it. Right now, we’ve got other customers and other products to make.

Robert Buxbaum, February 18, 2016. If someone could supply a good hydrogen compressor, and a good fuel cell, that would be most welcome. Someone who can supply that will be able to ride in a really excellent cart of the future at this year’s July 4th parade.

Chemical engineers and boilers, ‘I do anything’

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One of the problems I run into trying to hire chemical engineers is that their background is so varied that they imagine they can do anything. Combine this with a willingness to try to do anything, and the job interview can go like this.

Me: You have a great resume. I suppose you know that our company is a leader in hydrogen engineering (in my case). Tell me, what do you see yourself doing at our company?

Engineer: I don’t know. I do anything and everything.

Me: That covers a lot of ground. Is there something that you do particularly well, or that you would particularly like to do here?

Engineer.: Anything, really.

Me: Do you see yourself making coffee?

Engineer: I could do that, but was thinking of something with more … responsibility.

Me: OK. Could you design and build a 5 kW, gas-fired boiler?

Engineer: Himm. How much coffee did you say you guys drink?

Current version of our H2 generators (simplified) and the combustion-heated modification I'm working on.

Current version of our H2 generators (simplified) and the combustion-heated modification I’m working on.

Not quite where I was going with that. The relevance of this joke is that I’m finally getting around to redesigning our hydrogen generators so that they are heated by waste-gas combustion instead of electricity. That was the plan originally, and it appears in almost all of my patents. But electricity is so easy to deal with and control that all REB generators have been heated this way, even the largest.

The current and revised processes are shown in the figure at right. Our general process is to make ultra pure hydrogen from methanol and water in one step by the following reaction:

CH3OH + H2O –> CO2  + 3 H2.

done in a membrane reactor (see advantages). My current thought is to make the first combustion heated hydrogen generator have an output about 2/3 as large as our largest. That is, to produce 100 scfh, or 50 slpm, or 6 kg of H2/ day. This could be advantageous for people trying to fuel fork lifts or a hybrid, fuel cell car; a car could easily carry 12 kg of hydrogen, allowing it to go an extra 300 miles.

The generator with this output will need a methanol-water feed rate of about 2/3 gal per hour (about 80¢/worth pre hour), and will need a heat rate of 2.5 to 3 kW. A key design issue is that I have to be sure not to extract too much energy value from the feed because, if there’s not enough energy in the waste gas, the fire could go out. That is, nearly pure CO2 doesn’t burn. Alternately, if there is too much flow to the flame or too much energy content, there might be over-heating. In order to avoid the flame going out, I have a pilot flame that turns off the flow if it goes out. I also plan to provide 30% or so of the reactor heat about 800 W, by burning non-wast gas, natural gas in this iteration. My plan is to use this flow to provide most of the temperature control, but to provide secondary control by (and safety) by venting some of the off-gas if the reactor gets hotter than a set limit. Early experiments suggest it should work.

The business side of this is still unknown. Perhaps this would provide military power or cabins in the woods. Perhaps ship-board auxiliary power or balloons, or hydrogen fueling stations, or perhaps it will be used for chemical applications. With luck, it’ll sell to someone who needs hydrogen.

Robert E. Buxbaum. December 4, 2015. By the way, hydrogen isn’t as flammable as you might think.

How do technology companies sell stuff?

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As the owner of a technology company, REB Research, hydrogen generators and hydrogen purifiers, I spend a fair amount of time trying to sell my stuff, and wondering how other companies connect to potential customers and sell to them. Sales is perhaps the most important area of business success, the one that makes or breaks most businesses — but it was sadly ignored in my extensive college education. Business books are hardly better: they ignore the salesmen (and women); you’re left to imagine sales and profit came of themselves by the insight of the great leader. The great, successful internet companies are applauded for giving away services, and the failed interned companies are barely mentioned. And hardly any book mentions smaller manufacturing businesses, like mine.

So here are some sales thoughts: things I tried, things that worked, and didn’t. I started my company, REB Research, about 20 years ago as a professor at Michigan State University. I figured I knew more about hydrogen purifiers than most of my colleagues, and imagined this knowledge would bring me money (big mistake: I needed customers and profitable sales). My strategy was to publish papers on hydrogen and get some patents as a way to build credibility (worked reasonably well: I write well, do research well, and I’m reasonably inventive). Patents might have been a better strategy if I had not then allowed my patents to be re-written by lawyers. I built the company. while still a professor (a good idea, I think).

When I realized I needed sales, I decided to use trade fairs, conferences, and ads as the big companies did. Most of my budget went for ads in The Thomas Register of American Manufacturing, a fantastically large compendium of who did or sold what (it worked OK, but was since rendered obsolete by the internet). I bought $1500 worth of ads, and got 2 small lines plus a 1/8 page. That’s where I got my sales until the internet cam along. In retrospect, I suspect I should have bought more ads.

William Hamilton cartoon from the new-yorker. I sure wish I could make deals.

William Hamilton cartoon from the new-yorker.

My other big expense was trade fairs. Many big companies sold at trade fairs, events that are widely attended in my field. Sorry to say, I never found customers at these fairs, even when the fairs were dedicated to hydrogen, everyone who’d come by was was selling, and no one was buying, as best I could tell.Somehow, my bigger competitors (also at the fairs) seemed to get interest but I’m not sure if they got sales there. They seem to find sales somewhere, though. Is it me? Am I at the wrong fairs, or are fairs just a scam where no one wins but the organizers? I don’t know. Last month, I spent $2000 for a booth in Ann Arbor, MI, including $350 for inclusion into the promoter’s book and $400 for hand-out literature. As with previous events, few people came by and none showed anything like interest, I got no e-mail addresses and no sales. Some hungry students wandered the stalls for food and freebees, but there was not one person with money in his/her pocket and a relevant project to spend it on. I doubt anyone read the literature they took.

To date, virtually all of my sales have come from the internet. I got on the internet early, and that has helped my placement in Google. I’ve never bought a google ad, but this may change. Instead I was lucky. About 20 years ago, 1994?, I attended a conference at Tufts on membrane reactors, and stayed at a bed-and-breakfast. After the conference let out, the owner of the BnB suggested I visit something that was new at Harvard; a cyber cafe, the second one in the US. They had Macintosh computers and internet explorer a year before the company went public. I was hooked, went home, learned html, and wrote a web-site. I bought my domain name shortly thereafter.

The problem, I don’t know the next big thing. Twitter? Facebook? LinkedIn? I’m on 2 of these 3, and have gotten so sales from social media. I started a blog (you’re reading it), but I still wonder, why are the bigger companies selling more? The main difference I see is they attend a lot more product fairs than I do, have slicker web-sties (not very good ones, I think), and they do print advertising. Perhaps they match their fairs to their products better, or have a broader range of products. People need to see my products somewhere, but where? My latest idea: this week I bought HydrogenPurifier.com. Send me advice, or wish me luck.

Robert E. Buxbaum, flailing entrepreneur, September 10, 2014. Here’s a feedback form, the first time I’m adding one. 

Hydrogen cars and buses are better than Tesla

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Hydrogen fueled cars and buses are as clean to drive as battery vehicles and have better range and faster fueling times. Cost-wise, a hydrogen fuel tank is far cheaper and lighter than an equivalent battery and lasts far longer. Hydrogen is likely safer because the tanks do not carry their oxidant in them. And the price of hydrogen is relatively low, about that of gasoline on a per-mile basis: far lower than batteries when the cost of battery wear-out is included. Both Presidents Clinton and Bush preferred hydrogen over batteries, but the current administration favors batteries. Perhaps history will show them correct, but I think otherwise. Currently, there is not a hydrogen bus, car, or boat making runs at Disney’s Experimental Community of Tomorrow (EPCOT), nor is there an electric bus car or boat. I suspect it’s a mistake, at least convening the lack of a hydrogen vehicle. 

The best hydrogen vehicles on the road have more range than the best electric vehicle, and fuel faster. The hydrogen powered, Honda Clarity debuted in 2008. It has a 270 mile range and takes 3-5 minutes to fuel with hydrogen at 350 atm, 5150 psi. By contrast, the Tesla S-sedan that debuted in 2012 claims only a 208 mile range for its standard, 60kWh configuration (the EPA claims: 190 miles) and requires three hours to charge using their fastest charger, 20 kW.

What limits the range of battery vehicles is that the stacks are very heavy and expensive. Despite using modern lithium-ion technology, Tesla’s 60 kWh battery weighs 1050 lbs including internal cooling, and adds another 250 lbs to the car for extra structural support. The Clarity fuel system weighs a lot less. The hydrogen cylinders weigh 150 lb and require a fuel cell stack (30 lb) and a smaller lithium-ion battery for start-up (90 lb). The net effect is that the Clarity weighs 3582 lbs vs 4647 lbs for the Tesla S. This extra weight of the Tesla seems to hurt its mileage by about 10%. The Tesla gets about 3.3 mi/kWh or 0.19 mile/lb of battery versus 60 miles/kg of hydrogen for the Clarity suggesting  3.6 mi/kWh at typical efficiencies. 

High pressure hydrogen tanks are smaller than batteries and cheaper per unit range. The higher the pressure the smaller the tank. The current Clarity fuels with 350 atm, 5,150 psi hydrogen, and the next generation (shown below) will use higher pressure to save space. But even with 335 atm hydrogen (5000 psi) a Clarity could fuel a 270 mile range with four, 8″ diameter tanks (ID), 4′ long. I don’t know how Honda makes its hydrogen tanks, but suitable tanks might be made from 0.065″ Maranging (aged) stainless steel (UTS = 350,000 psi, density 8 g/cc), surrounded by 0.1″ of aramid fiber (UTS = 250,000 psi, density = 1.6 g/cc). With this construction, each tank would weigh 14.0 kg (30.5 lbs) empty, and hold 11,400 standard liters, 1.14 kg (2.5 lb) of hydrogen at pressure. These tanks could cost $1500 total; the 270 mile range is 40% more Than the Tesla S at about 1/10 the cost of current Tesla S batteries The current price of a replacement Tesla battery pack is $12,000, subsidized by DoE; without the subsidy, the likely price would be $40,000.

Next generation Honda fuel cell vehicle prototype at the 2014 Detroit Auto Show.

Next generation Honda fuel cell vehicle prototype at the 2014 Detroit Auto Show.

Currently hydrogen is more expensive than electricity per energy value, but my company has technology to make it cheaply and more cleanly than electricity. My company, REB Research makes hydrogen generators that produce ultra pure hydrogen by steam reforming wow alcohol in a membrane reactor. A standard generator, suitable to a small fueling station outputs 9.5 kg of hydrogen per day, consuming 69 gal of methanol-water. At 80¢/gal for methanol-water, and 12¢/kWh for electricity, the output hydrogen costs $2.50/kg. A car owner who drove 120,000 miles would spend $5,000 on hydrogen fuel. For that distance, a Tesla owner would spend only $4400 on electricity, but would have to spend another $12,000 to replace the battery. Tesla batteries have a 120,000 mile life, and the range decreases with age. 

For a bus or truck at EPCOT, the advantages of hydrogen grow fast. A typical bus is expected to travel much further than 120,000 miles, and is expected to operate for 18 hour shifts in stop-go operation getting perhaps 1/4 the miles/kWh of a sedan. The charge time and range advantages of hydrogen build up fast. it’s common to build a hydrogen bus with five 20 foot x 8″ tanks. Fueled at 5000 psi., such buses will have a range of 420 miles between fill-ups, and a total tank weight and cost of about 600 lbs and $4000 respectively. By comparison, the range for an electric bus is unlikely to exceed 300 miles, and even this will require a 6000 lb., 360 kWh lithium-ion battery that takes 4.5 hours to charge assuming an 80 kW charger (200 Amps at 400 V for example). That’s excessive compared to 10-20 minutes for fueling with hydrogen.

While my hydrogen generators are not cheap: for the one above, about $500,000 including the cost of a compressor, the cost of an 80 kW DC is similar if you include the cost to run a 200 Amp, 400 V power line. Tesla has shown there are a lot of people who value clean, futuristic transport if that comes with comfort and style. A hydrogen car can meet that handily, and can provide the extra comforts of longer range and faster refueling.

Robert E. Buxbaum, February 12, 2014 (Lincoln’s birthday). Here’s an essay on Lincoln’s Gettysburg address, on the safety of batteries, and on battery cost vs hydrogen. My company, REB Research makes hydrogen generators and purifiers; we also consult.

Thermodynamics of hydrogen generation

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Perhaps the simplest way to make hydrogen is by electrolysis: you run some current through water with a little sulfuric acid or KOH added, and for every two electrons transferred, you get a molecule of hydrogen from one electrode and half a molecule of oxygen from the other.

2 OH- –> 2e- + 1/2 O2 +H2O

2H2O + 2e- –>  H2 + 2OH-

The ratio between amps, seconds and mols of electrons (or hydrogen) is called the Faraday constant, F = 96500; 96500 amp-seconds transfers a mol of electrons. For hydrogen production, you need 2 mols of electrons for each mol of hydrogen, n= 2, so

it = 2F where and i is the current in amps, and t is the time in seconds and n is the number electrons per molecule of desired product. For hydrogen, t = 96500*2/i; in general, t = Fn/i.

96500 is a large number, and it takes a fair amount of time to make any substantial amount of hydrogen by electrolysis. At 1 amp, it takes 96500*2 = 193000 seconds, 2 days, to generate one mol of hydrogen (that’s 2 grams H2 or 22.4 liters, enough to fill a garment bag). We can reduce the time by using a higher current, but there are limits. At 25 amps, the maximum current of you can carry with house wiring it takes 2.14 hours to generate 2 grams. (You’ll have to rectify your electricity to DC or you’ll get a nasty H2 /O2 mix called Brown’s gas, While normal H2 isn’t that dangerous, Browns gas is a mix of H2 and O2 and is quite explosive. Here’s an essay I wrote on separating Browns gas).

Electrolysis takes a fair amount of electric energy too; the minimum energy needed to make hydrogen at a given temperature and pressure is called the reversible energy, or the Gibbs free energy ∆G of the reaction. ∆G = ∆H -T∆S, that is, ∆G equals the heat of hydrogen production ∆H – minus an entropy effect, T∆S. Since energy is the product of voltage current and time, Vit = ∆G, where ∆G is the Gibbs free energy measured in Joules and V,i, and t are measured Volts, Amps, and seconds respectively.

Since it = nF, we can rewrite the relationship as: V =∆G/nF for a process that has no energy losses, a reversible process. This is the form found in most thermodynamics textbooks; the value of V calculated this way is the minimum voltage to generate hydrogen, and the maximum voltage you could get in a fuel cell putting water back together.

To calculate this voltage, and the power requirements to make hydrogen, we use the Gibbs free energy for water formation found in Wikipedia, copied below (in my day, we used the CRC Handbook of Chemistry and Physics or a table in out P-chem book). You’ll notice that there are two different values for ∆G depending on whether the water is a gas or a liquid, and you’ll notice a small zero at the upper right (∆G°). This shows that the values are for an imaginary standard state: 20°C and 1 atm pressure. You can’t get 1 atm steam at 20°C, it’s an extrapolation; behavior at typical temperatures, 40°C and above is similar but not identical. I’ll leave it to a reader to send this voltage as a comment.

Liquid H2O formation ∆G° = -237.14
Gaseous H2O formation ∆G° = -228.61

The reversible voltage for creating liquid water in a reversible fuel cell is found to be -237,140/(2 x 96,500) = -1.23V. We find that 1.23 Volts is about the minimum voltage you need to do electrolysis at 0°C because you need liquid water to carry the current; -1.18 V is about the maximum voltage you can get in a fuel cell because they operate at higher temperature with oxygen pressures significantly below 1 atm. (typically). The minus sign is kept for accounting; it differentiates the power out case (fuel cells) from power in (electrolysis). It is typical to find that fuel cells operate at lower voltages, between about .5V and 1.0V depending on the fuel cell and the power load.

Most electrolysis is done at voltages above about 1.48 V. Just as fuel cells always give off heat (they are exothermic), electrolysis will absorb heat if run reversibly. That is, electrolysis can act as a refrigerator if run reversibly. but electrolysis is not a very good refrigerator (the refrigerator ability is tied up in the entropy term mentioned above). To do electrolysis at reasonably fast rates, people give up on refrigeration (sucking heat from the environment) and provide all the entropy needed for electrolysis in the electricity they supply. This is to say, they operate at V’ were nFV’ ≥ ∆H, the enthalpy of water formation. Since ∆H is greater than ∆G, V’ the voltage for electrolysis is higher than V. Based on the enthalpy of liquid water formation,  −285.8 kJ/mol we find V’ = 1.48 V at zero degrees. The figure below shows that, for any reasonably fast rate of hydrogen production, operation must be at 1.48V or above.

Electrolyzer performance; C-Pt catalyst on a thin, nafion membrane

Electrolyzer performance; C-Pt catalyst on a thin, nafion membrane

If you figure out the energy that this voltage and amperage represents (shown below) you’re likely to come to a conclusion I came to several years ago: that it’s far better to generate large amounts of hydrogen chemically, ideally from membrane reactors like my company makes.

The electric power to make each 2 grams of hydrogen at 1.5 volts is 1.5 V x 193000 Amp-s = 289,500 J = .080 kWh’s, or 0.9¢ at current rates, but filling a car takes 20 kg, or 10,000 times as much. That’s 800 kW-hr, or $90 at current rates. The electricity is twice as expensive as current gasoline and the infrastructure cost is staggering too: a station that fuels ten cars per hour would require 8 MW, far more power than any normal distributor could provide.

By contrast, methanol costs about 2/3 as much as gasoline, and it’s easy to deliver many giga-joules of methanol energy to a gas station by truck. Our company’s membrane reactor hydrogen generators would convert methanol-water to hydrogen efficiently by the reaction CH3OH + H2O –> 3H2 + CO2. This is not to say that electrolysis isn’t worthwhile for lower demand applications: see, e.g.: gas chromatography, and electric generator cooling. Here’s how membrane reactors work.

R. E. Buxbaum July 1, 2013; Those who want to show off, should post the temperature and pressure corrections to my calculations for the reversible voltage of typical fuel cells and electrolysis.

New hydrogen generator from REB Research

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Here’s the new, latest version of our Me150 hydrogen generator with our wonder-secretary, Libby, shown for scale. It’s smaller and prettier than the previous version shown at left (previous version of Me150, not of secretary). Hydrogen output is 99.9999% pure, 9.5 kg/day, 75 slpm, 150 scfh H2; it generates hydrogen from methanol reforming in a membrane reactor. Pricing is $150,000. Uses about 7 gal of methanol-water ($6 worth) per kg of H2 (380 ft3). Can be used to fill weather balloons, cool electric dynamos, or provide hydrogen fuel for 2-10 fuel cell cars.

New REB Research hydrogen generator 150 scfh of 99.9999% H2 from methanol reforming

New REB Research hydrogen generator 150 scfh of 99.9999% pure H2 from methanol-water reforming against metal membranes.

Dr. Robert E. Buxbaum