02 August 2008

Solar Energy to Fuel Cell, welcome to your new lifestyle!

Update: Thank you, Victor! This article has been reworked with the section of my bad math now marked and after calculations done on the right math...

Fascinating stuff on the energy front with a MIT researchers David Nocera and Henry Dreyfus finding a new catalytic system for changing water into its constituent components: oxygen and hydrogen gas. This is, actually, excellent news, but trying to go beyond that to linking this with solar cells and other energy sources. This, by their diagram, is an electricity based system, not one depending on photons to do the work, as is seen in their diagram here:


So, this utilizes the efficiency of solar panels, currently hitting around 15% of incoming sunlight, to split water and hold the gases in storage until needed later. Not only do you become your own power plant, as Prof. Nocera points out, but you also become your own hazardous gas storage area, too! Hazardous?

The hydrogen actually isn't the bad part of this system, really, although it can be quite energetic which I will get to later, but it is the oxygen. Pure, life giving oxygen does something else which is known as 'oxidize' materials. If you light a candle you are oxidizing wax via the flame to get heat, thus changing hydrocarbons into various forms of carbon (dioxide, monoxide) and some water generated by liberated hydrogen and free oxygen in the air. Pure oxygen is damned dangerous stuff around an open flame or even spark source and creates lovely, high activity oxidation reactions when ignited. What the system is leaving out is that you, to get the energy on the cheap, will need to have excess energy unused during the day to have hydrogen and oxygen you use at night to get energy out.

Then there are the pumps... pumps? Well, if you have water pressure from a distribution facility (known by many as 'main water' or even 'city water') that is taken care of for you by the pumping facility and I won't consider it in this set-up. But if you get water from your own underground source, then pumping it up to the surface and pressurizing it is a non-zero energy input to the system. The pumps, here, would be those to pressurize hydrogen and oxygen into the tanks so you don't have huge low pressure tanks to put the stuff in which would take up more space than your home has. There are non-liquid storage systems for hydrogen, namely putting it into a reversible chemical reaction with a metallic sub-strate, but that gets you constant pressure hydrogen in which the substrate releases hydrogen as pressure drops, thus maintaining a constant pressure. Neat stuff, still needs a pump. Oxygen, in theory, you could vent out into the atmosphere and then just use an atmospheric intake for reversing the procedure later. Worth examining as it eliminates the really nasty part of the system from your house, although one hopes it doesn't become a 'ticking time bomb' of a gas jet ready to ignite next to your home if this option is taken. Probably would need some pre-mix with outside air via expanded tubing and air intakes to get some of the less oxidizing parts of the air in with it before exhaust. A mere technicality.

The first part of this is the kicker, and, really, getting a greater efficiency on the solar panel side with a cheap system would be great. Even Nanosolar has problems getting to that 15% and higher purity materials cost much, much more, but they make it up in low cost per square foot of production via printing techniques and presses. As I have looked at energy quite a few times and have a post up that does a bit of an overview, I will link to that and use some of the materials from previous posts to get some of the numbers. When looking at the electricity for vehicles, just to replace automotive gasoline (not diesel or aviation gas) I came up with 1.08E+12 kWh/yr, considering energy delivered to the vehicle equivalent after removing the inefficiencies of the internal combustion engine and considering in only an 85% transfer rate for storage in the vehicles (or 15% energy lost via transmission, storage, etc.). That would replace the gasoline used in 2002 I believe it was, which was 125 billion gallons. Electricity is a very good way to run a vehicle as it depends on much higher efficiency electric motors to deliver power rather than a relatively low efficiency internal combustion engine. The problem is: storing the energy.

Let me make a push for high temperature and capacity superconductors, as those would fit the bill *perfectly*. A loop of thousands or tens of thousands of strands of that sort of material would do the trick at, presumably, lower weight and near perfect efficiency. We aren't there, yet, although recent advances have gotten us to dry ice temps for superconductive materials. Ok, push given for near 100% efficiency!

[The following section is incorrect due to my poor math skills]

So lets start flipping the numbers around and see what this means on a personal scale. I will do the quick step here, but I do have links to back the numbers at the previous article:

1) Average insolation in the lower 48 is 4 kWh/sq. m./month. The tropics are pretty steady at 5 kWh/sqm/month while going northward sees higher seasonal variations. Basically, there is a lot of energy coming down through the atmosphere, although nothing like the pure stuff in the vacuum of space. If you want *real* energy, that is the place to go and to get outside the atmosphere. Diurnal frequency (variations for non-producing hours, taken into account with the above.

2) Average household energy use. Now this one gets a bevy of conflicting numbers as many divide up the energy into different types, like Underwriters Laboratories does and I will harvest a couple of numbers from them -

Annual household lighting use: 2,100 kilowatts/hour (kwh)

Annual household electricity use: 10,660 kwh / household

Of which the second is the one needed. Now this is across all families, all places, all times, etc. and isn't the cyclic energy used per day with seasonal variations. But as a starting point it gives an idea of the amount of energy the system needs to capture overall - 10,660 kWh/yr. or, divide by 12, and I will round up a bit to the easier to work with 890 kWh/month.

3) Total insolation area that gets 890 kWh/mo is 223 square meters.

4) Actual area to capture that 890 kWh/mo given a 15% conversion rate of solar cells is:

223: X as 15:100

22,3000 = 15 X

X = 1500 sq. m. (rounding generously up from 1486).

Now if you go for a much steeper price material that doubles the efficiency, you get half the area necessary (going from 15 X to 30 X) brings that down to 750 sq. m. which is something doable and you have probably seen those nice array of high efficiency cells on homes and such. Nanomaterials, however, weigh far less (even with mounting you are mounting something the weight of aluminum foil) so you could do up your home in Nanosolar. I really don't have a problem with that and if you have any yard space with, say, 15 degrees to 45 degrees clearance for viewing the sky, that would help a lot in putting up such a structure.

That is to power your home. And there are seasonal variations in the non-tropics, so you will want a bit more material for winter use as you will get less overall sunlight than the average say a 40m x 40m area (120 ft x 120 ft). The great part of Nanosolar is, in theory, it replaces your siding, gets built into roofing materials, and generally becomes thin, paste-up films you can use to put over just about everything that gets sunlight. And cheap enough to replace when damaged (in theory once they get a few more production facilities going and such).

But Prof. Nocera wants you to power your car by this system, too... an all electric or plug-in hybrid vehicle, no doubt. Time to run the numbers on cars, to see just what it is you are doing with those poor things. Lets take your nice 40 mpg non-hybrid vehicle... diesel or gasoline, both have the same energy density, so fuel doesn't matter. Average driving per car is... well, the numbers vary all over the place, but they center between 12,500 miles/year to 15,000 miles/year, with spare drivers like myself, when actively driving, having cut below that minimum, but not by much. I will use the former as the economy is changing to an online one and using economical delivery services to bring goods to your door instead of you going out and getting them.

Your average car delivers 20% of the energy from the engine to actual motion based energy for the vehicle, and the best that steel engines can do is 37% based on melting points of steel and so forth. There are many other ways to make engines, and some are trying to get into that lovely 50%+ territory, but they are also far more sophisticated than your basic 4-stroke engine. So it is time to start running the numbers:

1) Average distance driven is 12,500 miles divided by 40 miles/gallon - 312.5 gallons of gasoline. Hmmm... call it 315 gal. for utility's sake. Round numbers are easier to work with and as the works of mankind are less than efficient at most things, I round up to capture inefficiency. And obey the laws of thermodynamics.

2) Delivered energy, and lets assume with good aerodynamics, low rolling resistance tires, and a better than average efficiency engine that your car gets 25% efficiency in converting gasoline to motive power. That is the energy equivalent *delivered* of, again rounding up from 78.75 gallons, 80 gallons of gas or diesel.

3) Per gallon of gasoline US is 36.6 kWh (from onlinecoversion).

4) Total delivered energy to your car, per month is amount used ( 2 ) multiplied by its conversion factor (3) or 2,928 kWh/mo.

Or over 3 times what your home uses.

Time to buy up some spare property as you have a car to run!

What, more than one car?

Say, just where *is* this 'extra energy' that Prof. Nocera is talking about coming from, anyways? Even with super-efficiency, low drag, low slung, low friction vehicles, and say you cut that amount in *half* you still need more property to run your car. Your car's energy budget is actually much higher than that of your HOME.

Ok, now that you have expanded your property (neighbors? you don't need no steenkin' neighbors! you are being GREEN) and are going to see about what you have to do about the good Prof. Nocera's system. It is, as stated previously, a fuel cell which is technology that has been around for... well, conceptually the 19th century if memory serves but reliably since the 1950's and NASA's need for a compact energy source it can charge up and then have discharge as needed. As a form of battery, what fuel cells do is add energy in (electricity, by and large) and split up water into hydrogen and oxygen. They are extremely efficient at putting the two back together and getting three by-products on the catalyst material: electricity, heat, water. The 'round trip efficiency', that is the efficiency of electrical energy to crack water and then to put it back together again and get electricity is between 30% and 50%. Using a fuel cell that operates efficiently at a high temperature does get higher efficiency (up to 90%) but those operate around 1500 degrees F (800 C), which puts it into the furnace category of heat containment.

What Prof. Nocera has done is work on the front end cracking/catalyst portion, to get a better system for actually getting water to split without really noxious chemistry involved. What has to be factored in is the energy lost in splitting the water will not be returned due to heat loss and other inefficiencies in actually performing that task. Electrolysis of water can be, theoretically, very efficient, up to 50 to 80% efficient getting the molecule to split apart although more realistic numbers come in at the 30-45% range. Oh, and your efficiency goes down using air to do the work, so pure oxygen is prefered because that is another 10% or so loss, so you do want to keep the pure stuff around... All of that is part of the loss in the 'round trip efficiency': getting water to split up via electrolysis, pressurizing, utilizing oxygen vs air. Plants, as in the green, leafy kind, actually don't produce electricity, but form longer chain hydrocarbons that come in the form of carbohydrates as they much prefer to actually have a chemical diet than electrical one.

They do have a catalyst, called chlorophyll, but it is doing something a bit more difficult, which is why Prof. Nocera was so certain of the outcome. Actually, if you grew photosensitive bacteria in sunlight you can duplicate that... like algae. Although it is possible to eat some kinds of algae, we generally prefer electricity as an energy source for our homes and vehicles. That higher amount via the ceramic high temp system uses those temps on both cracking and rejoining to facilitate both, but is still not in wide use and has support problems due to the need for its operating temps and output constancy. Thus that round-trip efficiency is one of 30-50%, give or take a bit on the high end. So now we can start to see how fuel cells, operating at highest rated efficiency (not theoretical) start to look as storage systems.

1) Daily energy your vehicle needs on average - 2,928 kWh/month or, divide by thirty and round a bit, 100 kWh/day.

2) Using 45% efficiency of the entire fuel cell for round trip, means that you will need 222 kWh/day and I will round down, at this point, to 220 kWh/day or 120 kWh MORE per day just to run the fuel cell. Lead acid batteries get you 90% efficiency, just so you know.

3) Total energy via fuel cell per month: car - night charging - 6,600 kWh; the rest of your home - 890 kWh. That is, yes, 7,490 kWh.

Want an efficient lifestyle?

Plug in your car and sleep during the day, and then use the inefficient energy at night and get on 'the graveyard shift'.

Then you will be living a GREEN lifestyle by avoiding the sunlight.

No wonder Bruce Wayne gets such high marks: he is leading an energy efficient life.

[end section]

Now lets try that from the top!

The parts that can be salvaged are from the home daily use section and effeciency amounts, but my incorrect reading of my own, damned post means I need to re-do the math.

Insolation - 4 kWh/sq.m./day!

Avg. Household Use section still the same - 890 kWh/month

So, average insolation - 30 x 4 = 120 kWh/sq.m./month

Effective useful insolation using 15% efficiency per sq.m. - 18 kWh/sq.m./month from solar cells.

Thus, 18 divided into 890 gets you 49 sq.m. collection area for the average home. Yes, that number did seem wacky above and should have been an in-process tip-off... grrr...

Call it 110 sq.m. for your average home, average days, average insolation (you do get more insolation in the summer than winter, when you need it).

Your car's motive needs (absent waste, and yes I expect this is lower once you change drive train to batteries and such, but for current state of the art vehicles) - approx 3,000 kWh/mo.

Amount of solar cell space you need for your car - 18 divided into 3,000 - 167 sq.m.

Space needed for your home and car, on average - 216 sq. m. on average.

Or about 15 m per side of collection area if the energy was going directly into your vehicle, which it isn't.

That 3,000 kWh/mo has to factor in the 45% round trip efficiency of the fuel cell system - which gets you to the 6,600 kWh/mo using the fuel cell as storage. Or 366 sq. m. of colletion area, which is just a bit over 19 m per side, just for sizing.

Somehow lead acid batteries begin to look really good here... and you still wind up with the Batman lifestyle if you want a lower collection area.

The variables are: efficiency of the solar cells (and I am giving them a lot here), efficiency of the fuel cell (in theory it can get up to 80%, which is still not the 90% of lead acid batteries), and just how 'average' you live. Unless you have a different need for the hydrogen and oxygen, my bet is that you won't want a fuel cell unless it is damned cheap compared to a lead acid battery system. And a good way is found to store the gases.


Unknown said...

There seems to be an error in calculation. The average insolation is 4kWh/sq meter/per day, not per month.
From the source given in the article.

A Jacksonian said...

Need to take a look at that...Thanks for the catch!