Tuesday, October 4, 2011

ALTERNATIVE ENERGY SOURCES - hydrogen, batteries, water tanks, compressed air, flywheels, etc.

Tom Murphy, Associate Professor of physics at University of California



This was recently posted by Tom Murphy. This is a guest post by Tom Murphy. Tom is an associate professor of physics
at the University of California, San Diego. This post previously appeared in
Tom's blog at Do the Math.

> Got Storage? How Hard Can It Be?
> Posted by Euan Mearns on October 3, 2011 - 10:03am
> Topic: Alternative energy
> Tags: batteries, compressed-air, energy storage, flywheels, fuel-cells,
> hydrogen [list all tags]
> This is a guest post by Tom Murphy. Tom is an associate professor of physics
> at the University of California, San Diego. This post previously appeared in
> Tom's blog at Do the Math.
> The recent city-wide power outage in San Diego made me appreciate my small
> off-grid photovoltaic system using four golf-cart batteries to store energy
> for use at night. Unlike most San Diegans, I did not immediately eat the ice
> cream in my freezer, which trucked along under stored solar energy just like
> it does every night. Energy storage becomes more important as we transition
> away from fossil fuels—already its own energy storage medium—to more
> intermittent sources. But besides batteries—which offer a limited number of
> cycles and for some types require monthly maintenance—what other non-fossil
> in-home energy storage alternatives might we consider, and how much energy
> might we expect to store in each case? We will look at gravitational
> storage, flywheels, compressed air, and hydrogen fuel cells as possible
> options. Some might even cost less than $100,000 to implement in your home.
> Setting the Scale
> We should first establish a meaningful scale and appropriate units for
> energy storage. Any household will use energy at a certain average rate, or
> power. The average American household (of which there are about 115 million)
> uses 30 kWh per day of electricity—equivalent to 1.25 kW average power.
> Additionally, the average American household uses 35 kWh of natural gas
> energy per day, generally for heating applications (natural gas is usually
> billed per Therm, which is 29.3 kWh).
> Substantial variation can exist in these numbers in any given house. For
> example, my wife and I use an average of 10 kWh/day in our house for
> electrical and natural gas energy combined, in roughly equal measure.
> Storage requirements will therefore vary according to usage. Conversely, the
> same storage will last longer in some houses compared to others. We will
> restrict our attention in this post to storing enough energy to cover
> electrical usage only.
> For off-grid applications, the rule of thumb is to have enough storage for
> three days of zero input. Not to be taken literally, this is effectively the
> same as 4 consecutive days at 25% of break-even, or 6 days at 50% of
> break-even—in each case running a deficit of 3 days. The spirit of this post
> is to establish storage independence within a home, rather than rely on
> external infrastructure to do it for you. To the extent that you want to
> rely on the outside world to provide for you, the targets established here
> can be scaled down accordingly. The ideas explored here are plausible
> options that might come to mind first in a “why couldn’t we just…” sense.
> Since we are trying to fit a storage solution into a home, let’s allocate a
> fixed volume for each of the solutions. A reasonable choice would be one
> bedroom-sized space. Let’s say it’s 3 m on a side, and 2.5 m tall (about 10
> by 10 feet, 8 feet tall). The total volume envelope is then 22.5 m³.
> Battery Reference
> Since we are comparing to batteries, let’s establish a reference case.
> Batteries are characterized by how many amp-hours they can generate. A 100
> A-h battery can put out 1 A of current for 100 hours, or 10 A for 10 hours,
> for instance (though the rating typically declines for high currents). The
> electrical power exerted by the battery is just the current times the
> voltage. So a 12 V battery putting out 2 A is delivering 12×2 = 24 W of
> power. If rated at 100 A-h, this battery would go for (100 A-h)/(2 A) = 50
> hours, producing (24 W)×(50 h) = 1200 W-h, or 1.2 kWh of energy. A
> rechargeable AA battery holds about 2300 mA-h (2.3 A-h) of charge, which at
> 1.3 V turns into 3 W-h of energy.
> As detailed in the Nation Sized Battery post, large lead-acid batteries
> occupy 13 liters (0.013 m³) of volume, and 25 kg of mass per kWh of storage.
> Practically speaking, we would not fill 100% of a bedroom’s volume with
> batteries, because we need physical access for maintenance/replacement. If
> we filled 10 m³ of the available volume with actual battery, we would end up
> with about 750 kWh of storage, and a mass of 19 tons (better reinforce the
> floor!). This bank would provide about 25 days of storage for the average
> American electricity demand.
> The lesson is that batteries are a can-do solution in terms of fitting into
> a house envelope. A more practical few-days of storage is that much easier.
> Although at approximately $150 per kWh of storage, three days of storage for
> the average American house will cost about $15,000 to cover electricity
> demand, with the cost recurring every five years or so. But let’s not worry
> our pretty little heads over mere economic concerns at the moment.
> Since lead-acid batteries require monthly equalization and topping-off with
> water, have a limited number of deep cycles (500–1000 typically), and do not
> tolerate extended periods at low charge, it would be nice to identify better
> options.
> Gravitational Storage
> Hydroelectric dams and pumped storage solutions rely on the gravitational
> energy stored in an elevated mass. What could we do in a home environment?
> Could we get much out of our personal pumped storage tank on-site?
> Let’s start small by considering the 3 W-h of energy stored in a AA battery,
> as computed above. One kWh of energy is 3.6×106 J of energy, so our AA
> battery stores 10,800 J of energy. A mass of m kilograms, hoisted h meters
> high against gravity at g˜10 m/s² corresponds to E = mgh Joules of energy.
> If we were willing to hoist a mass 3 m high, how much mass would we need to
> replace the AA battery? Have a guess? The answer is 360 kg, or about 800 lb.
> A battery the size of your pinky finger beats the proverbial 800 lb gorilla
> lifted onto your roof!
> The lesson is that gravitational storage is incredibly weak. A volume of
> water the size of our bedroom raised even 10 m above our home in a
> precarious threat to the neighbors would store 0.625 kWh. That’s enough for
> 30 minutes of typical household electricity consumption. You’ll forgive me
> if I ignore efficiency losses. It’s not even worth the effort. It’s over.
> Flywheel Storage
> Let’s put a massive spinning disk in our energy-storage “bedroom.” These
> might end up being popular in Malibu, as the gyroscopic stability inherent
> in the spun-up system could be very handy in a mudslide—keeping the house
> level on its way down the hill, mimicking the surfers it’s been watching all
> these years.
> The kinetic energy stored in the rotation of a cylindrical-shaped solid disk
> is ¼mv², where m is the mass of the spinning cylinder and v is the velocity
> at the outer edge. For a fixed mass, it is better to put as much of the mass
> as possible on the outer edge, in a hollow cylinder (supported by spokes,
> for instance), which can deliver a factor of two more energy per mass. But
> in the case where space, not mass, is the constraint, the solid disk has
> more mass than the hollow version would, making it a net win to just go
> solid.
> How big do we make this thing? Let’s give it a diameter of 2.5 m and a
> height of 2 m (need room for mounting, and surrounding container/structure)
> yielding a 10 m³ volume. At the density of steel—about 8× that of water—we
> get 80 tons (now even more important to reinforce that floor!).
> How fast do we spin it up? Let’s pick the speed of sound—345 m/s—and see
> where that puts us. Go big, or go home! We get 2.4 GJ, or about 650 kWh of
> energy stored in this scary flywheel. That’s somewhat comparable to a
> similar volume of lead-acid batteries (though four times as massive). We
> would want to evacuate the air around the spinning disk or we will suffer a
> drain rate of something like 1 kW (consuming 24 kWh/day just to keep it
> spinning; the room would also get warm-ish).
> The acceleration at the outer radius is about 10,000 times that of gravity,
> and it turns out the geometry and speed we picked indeed approaches the
> yield strength of steel. Structural weaknesses then risk breakup, which
> would dump the unwelcome energy equivalent of half a ton of TNT in your
> house. We would need to slow to a speed of 250 m/s at the outer rim to
> provide an adequate material safety factor, resulting in 250 kWh of storage.
> Another safety concern: if the flywheel comes off its support, it could
> barrel through the neighborhood, popping through houses like they weren’t
> even there. Not ideal in earthquake country.
> Obviously, we can afford to scale things down a bit, since our first cut
> provided three weeks of storage capacity. The same cylinder spinning at 125
> m/s (275 m.p.h.) at the edge gives about 90 kWh of storage, and may be
> somewhat more tolerable from a safety point of view. Scaling down the
> size/mass in addition to velocity begins to result in a less useful storage
> solution for the average house. If you’re going to go through the effort,
> expense, and sacrifice of space for a scary flywheel, you’d better feel like
> it provides enough energy storage to be worthwhile.
> A recent $53 million flywheel storage facility in Pennsylvania uses 200
> large units storing 25 kWh each, working out to $10,000 per kWh of storage
> capacity. Each unit’s vacuum chamber looks to be about 1.5 m in diameter and
> 3–4 m tall. If we raise the ceiling and squeeze four into our bedroom, we
> could get 3 days of electricity storage for the typical American house for a
> cool million bucks. But the frictional losses—while painstakingly
> minimized—likely preclude these units from being useful over periods of
> days.
> Compressed Air
> We could store energy in something akin to a spring by compressing air. A
> high-quality tank can store air at 200 atmospheres of pressure. If we make a
> big bedroom-sized tank in cylindrical form similar to the flywheel
> dimensions, it has a volume of 10 m³. The steel walls would have to be about
> 6 cm thick to withstand the stress, so that the tank would have a mass of
> approximately 12 tons. You did reinforce the floor, right?
> We want to take a volume of air, V0, 200 times larger than our tank volume
> at atmospheric pressure (P0 = 105 Pa) and compress it to fit in the tank
> (adding two tons of mass!). If done slowly enough to maintain approximately
> constant temperature (several hours), the energy required is P0V0ln(P/P0),
> where ln() is the natural logarithm function. For our volume, this turns
> into 1 GJ, or almost 300 kWh—enough for 10 days of typical American
> electricity use. So we could get away with a smaller tank or simply charge
> it to a less extreme pressure.
> The efficiency for compressing the air and later turning a turbine for
> electricity generation may be less than what one might find for a flywheel.
> The storage itself is not the hard part. I could go out today and get some
> lab-sized cylinders (~50 liters), which could store 1.5 kWh each—about like
> a golf-cart battery, although heavier and bulkier. But I would have a very
> difficult time arranging an efficient pumping and extraction/turbine system.
> If not for that, I would find compressed air to be an attractive system
> compared to batteries: minimal maintenance; no apparent cycle limitations,
> reasonably low-tech, and perfectly tolerant of remaining at low charge
> indefinitely.
> Laboratories that frequently use compressed gas cylinders have strict safety
> protocols to prevent explosions from structural rupture due to mishandling.
> If houses across the land had high pressure vessels in various states of
> neglect/corrosion, we’d get the occasional boom. I might worry about having
> a gun in the house. But I’m guessing that house fires would still represent
> a bigger net threat.
> Hydrogen Fuel Cell
> What about electrolysis of water into hydrogen for later use either in fuel
> cells or combustion engines? I’ll ignore the combustion option, as the heat
> engine efficiency would be abysmal compared to the other storage options on
> the table. Batteries, gravitational storage, and flywheels can achieve
> better than 80% round-trip efficiency. Compressed air is harder for me to
> evaluate, lacking adequate knowledge on compression/extraction devices built
> for efficiency.
> Electrolysis for the production of hydrogen tends to range between 50–70%
> efficient. Then the fuel cell converts the stored energy back into
> electricity at 40–60% efficiency for a round-trip efficiency of 20–40%. If
> you happen to want some of the waste heat, then you might boost the
> efficiency estimate (true for any of these storage methods, actually). But
> in a straight-up apples-to-apples comparison, the hydrogen method is a very
> lossy storage option. If it were dirt cheap and low-tech, I might be more
> excited about its potential, despite the poor efficiency. But since the
> opposite is true, I’m not revved up over hydrogen storage.
> I spent some time searching for a hydrogen fuel cell that I could buy today
> with a rating in the 10 kW range (appropriate for a home). I saw some
> production models achieving efficiencies ranging from 40–53%, but never a
> price tag. If you have to submit a query to learn the price, you probably
> can’t afford it…
> Other Ideas?
> Have I exhausted all the possibilities? Certainly not. I picked obvious and
> representative techniques spanning gravitational, kinetic, spring force (in
> air), and chemical storage. These are the ideas that come to mind for me,
> each with some reasonable footprint in the panoply of relevant small-scale
> “solutions” often discussed. I stayed away from thermal storage because the
> round-trip efficiency will make hydrogen fuel cells look fabulous. I also
> stayed away from fossil fuels (gas generators, storing natural gas at home)
> for the obvious reason that we don’t generally need storage as long as we
> have a reliable supply of fossil fuels.
> A short digression to contrast the miraculous energy density in fossil
> fuels: our 3 days of electricity storage at 30 kWh/day requires just 12
> gallons of gasoline (1.6 cubic feet; 45 liters) burned in a 20% efficient
> generator (it seems like the other 80% is noise!). The Earth’s battery—a
> one-time gift to us—turns out to be vastly superior to any of these other
> “solutions” in terms of energy density and long-term storage, measured in
> millions of years. It will be sorely missed when it’s gone.
> They’re All Hard
> With the exception of the feeble gravitational storage example, each of the
> ideas presented here are technically challenging, expensive, and sometimes
> dangerous. I am left thinking that batteries look pretty good for home
> storage. And they already perform a key function in my household. But even
> the cheapest lead-acid solution is still expensive, high-maintenance, and
> requires replacement every few years. For $150, you get 1 kWh of storage.
> 500 cycles means 500 kWh of service (get about 1000 cycles if
> half-discharge, but still roughly the same total energy service). This comes
> to about $0.30/kWh, which makes it an expensive source of electricity. Even
> so, lead-acid is the most economic storage medium of choice for off-grid
> households, and loads better than no storage at all!
> For short-term outages, we might get by with storage for critical functions
> only, like refrigeration and cooking. In a renewable-energy future, where
> storage must fill a larger role, the solutions are not obvious. As explained
> in the post on a nation-sized battery, we can’t simply scale up our trusty
> lead-acid, lithium, or nickel-based batteries to satisfy our current demands
> in a fully-renewable energy scenario because of resource limitations.
> Large-scale pumped storage, subterranean (or underwater) compressed air, and
> sodium-sulfur batteries may become important players. But these don’t help
> the independent spirit who wants personal storage, and by now is perhaps
> feeling—well—powerless.
> The Easy Path: Ratchet Down
> Many of the difficulties explored here become immediately easier with a
> simple reduction of scale. Because my household only uses 5 kWh of
> electricity per day, the 7 kWh of lead-acid storage I have in the form of
> four golf-cart batteries is enough to provide a meaningful service.
> For me, the lesson is that adequate storage appears at first-blush to border
> on impossible under the current profile of consumption in the U.S. But cut
> consumption down by a factor of five or so, and I become optimistic. Such
> deep cuts are not impossible: I can personally still participate in a
> western lifestyle at a fifth of the energy cost at home. It’s a choice, and
> I’m happy with mine.


AJ. Baxter said...

Excellent insight on energy storage and the reality of our consumption and technology limitations.

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They are truly an incredible cost effective renewable energy plant with a 50+ year life span.

Aaron Carson
Global Green Energy, Inc.

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