Energies explorations

Energy Landscape

The role of energy in world affairs is hard to exaggerate, in part because of a close connection

between energy and wealth.People in wealthy countries use more energy. Does energy create

wealth, or does wealth lead to energy use? It is some of both. It takes energy to run a factory,

and wealthy people can afford air-conditioning. Economically developing nations are rapidly

increasing their energy consumption for both reasons.

The energy landscape is immensely complicated by the fact that fossil fuels, the primary source

of energy around the world, are the main culprits blamed for the human-caused (anthropogenic)

component of global warming. This conflict between wealth and warming is at the heart of the

energy debate. The energy landscape is also complicated by widespread misinformation. Despite

the beliefs of many people, the United States is not running out of fossil fuel, but only out of

conventional oil.

Moreover, the cost of energy is completely out of whack. Here is a dramatic example. The typical

cost of electricity purchased from a utility is 10cents per kilowatt-hour (lower in the southeastern

United States, higher in California). Now, instead of using that wall-plug power for a light-bulb,

suppose you buy undiscounted AAA batteries from a local fat-food store. I have priced them; they

sell for about $1.50 each. (Yes, I know this is expensive, but that was the actual price. They are

cheaper online.) How much more does the battery electricity cost compared to wall-plug electricity?

Five times as much? Ten times?

No. AAA battery electricity cost 10,000 times as much as the wall-plug electricity!

Here is the calculation : One AAA battery delivers about 1 ampere at 1.5 volts for about 1 hour.

That is 1.5 watt-hours; for a battery that cost $1.50, that is $1 per watt-hour. A kilowatt-hour (kWh)

is a thousand times more, so it will cost $1,000. That is 10,000 times more than the utility price of

10cents per kilowatt-hour. Yet we do buy such batteries, because they are portable when we need

a flashlight we really want it to work. Energy in batteries, which work during power outrages, is worth

the premium.

The public understanding of energy is so confused that it is difficult to make truly rational energy policy.

It will be your job, not only to understand energy yourself, but to explain and convince the public about

the relative costs and risks of fossil fuel, alternative energy, nuclear, and energy conservation. You must

be the country's energy instructor.

Even though energy is a commodity, you won't find it listed on the commodity pages of the Wall Street

Journal. You will find energy proxies there, coal, oil, and gas, primarily because of their value in delivering

energy. Yet the cost of these energy proxies depends critically on the form in which the energy is delivered,

as illustrated with the AAA battery example. for the same energy delivered, gasoline cost about 2.5 times

as much as retail natural gas, and about 7 times as much as wholesale gas! So why do we continue to use

gasoline? Why don't we switch to cheaper coal? The answer is something all future presidents and savvy

investors need to understand : because our automobile infrastructure (factories, filling stations, delivery

systems) was developed over the past 100 years to deliver gasoline, and this inherited infrastructure is too

extensive to change rapidly when the price of oil suddenly skyrockets. We are stuck with what economists

call an "inefficient market" in energy. The price discrepancy also suggests a big future for energy conversion,

and indeed there is a great deal of investment currently going into the required technologies.

Recycled Energy

==============

One of the most remarkable insights about energy is that there is a source that is typically

cheaper than cool, and yet often neglected by non-experts such as home owners. This

source is energy that is reused, recycled energy, conserved energy. An example is energy

used to heat a house that is trapped in the house, not allowed to escape out the windows

or through the walls. Keeping it in costs money, but more frequently than you might guess,

the cost of improved insulation is lower than the cost of a few years of extra energy.

Energy conservation was given a bad name by President Jimmy Carter when, during the oil

embargo of 1979, he declared the crisis to be the "moral equivalent of war." Among other things,

Carter urged US citizens to turn down their thermostats in winter, and to put on a sweater instead.

True, the hardship was minor; a sweater is no big deal. But many people felt that the quality of their

home life had to be lessened as a patriotic duty. Once the crisis was over the thermostats went

right up. Carter's call to nationalism had an inadvertent consequence : it convinced US citizens that

energy conservation meant enduring discomfort.

Perhaps colder homes in winter were necessary, the crisis was immediate, but an opportunity was

lost. President Carter could have told people to put a sweater on, temporarily, but let the US

government give you zero-interest loans to install insulation. Then you could turn up the thermostat

to whatever temperature you wanted. And the same trick works in summer; insulation to keep the

heat out lowers the cost of air-conditioning.

The cheapest form of energy is indeed energy that is not used. David Goldstein, one of the great

innovators in conservation, calls it "invisible energy". Amory Lovins, another great innovator, calls it

"negawatts". If you can make your heater or your refrigerator or your air-conditioner more efficient,

then you get the same benefit with less energy.

Unlike many other commodities, energy is not cheaply stored once it is produced. You can store

it in batteries, but batteries are expensive to manufacture and replace after their limited lifetimes.

You can store energy by separating hydrogen from water (electrolysis), but the process is inefficient

and expensive.

Energy Security

==============

The two largest issues in the energy landscape are energy security and climate change.

The challenge is to address both of these in reasonable and balanced ways, and much

of that involves educating the public on the difference between effective policy and

feel-good policy. Some approaches, such as energy conservation, address both security

and climate change. Some, such as the conversion of coal to oil, are concerned primarily

with energy security. Some, such as large-scale adoption of solar power, are concerned

primarily with the dangers of climate change. Liberals tend to worry more about climate

change, and conservatives tend to worry about energy security, but as users, you will

need to address both.

To understand energy security, you must appreciate the enormity of our energy "flow",

the amount we use every day or every year. For example, here are the fossil fuel amounts

used by the average US citizen, including industrial use :

  Coal      : 18 pounds per day per person

  Oil        : 16 pounds per day per person

  Natural gas : 10 pounds per day per person

It is interesting that US citizens use similar amounts, by weight, of all three fossil fuels.

The most important fact of energy flow isn't the breakdown; it is the enormous size of the flow,

about 3,500 gigawatts. That is 3,500 large generating plants; it is 12 kilowatts per person.

Generating this much power take about 300 tons of fossil fuel every second. Three hundred tons

per second! Assume, for the purposes of illustration, that all this energy came solely from petroleum.

Then the amount of petroleum needed would be about one cubic mile per year.

This is worth pondering. The United States uses about a cubic mile of fossil fuel every year.

Visualize it. Any proposed alternative-energy sources must cope with this enormity.

Moreover, as far as energy is concerned, they lived a hand-to-mouth existence. They extract what

they need and use it almost immediately. They do put some aside, but only a little. In United States

they maintain a Strategic Petroleum Reserve consisting of oil pumped into geologic caverns created

in underground salt deposits in Texas and Louisiana. In many ways, the Strategic Petroleum Reserve

was a great investment. That oil was purchased at $20 per barrel, and the price of oil has skyrocketed

since. The caverns can hold up to 727 million barrels of oil, and are currently almost full. That sounds

like a lot, but they are big country. On the past decade they have been importing over 9 million barrels

of oil each day, so if the strategic reserves had to replace imports, the full reserves would be gone in

less than 2 months. But there is another issue : they have limited pumping capability. At present, they

can extract only 4.4 million barrels of oil per day from this reserves. So if they were suddenly cut off from

all imports, they would still have to cut their daily use enormously.

It is worth learning these approximate US numbers : 3,500 gigawatts, 300 tons per second, a cubic mile

of oil equivalent per year. They illustrate the huge size of the problem. One large coal or nuclear plant

produces 1 gigawatt, 1/3500 of the US need. The enormity of the flow limits the choices for significant

alternative fuels. If someone suggests, for example, that they use discarded cooking oil to address their

energy problems, you will think, "Do they really throw away 300 tons of cooking oil per second?

Do they generate a cubic mile per year?" The answers, of course, are no! and no! The amount

of available cooking oil is minuscule way.

There is a similarly important fact for energy supply : the rapid emergence and rising energy use of the

developing countries, particularly China and India. China is so hungry for liquid energy that it is the most

powerful driving force for increased oil prices. Whenever the margin of spare capacity, the amount of oil

that could be pumped around the world minus the amount that actually is pumped, drops below a few

percent, the price of oil soars. That is because companies that need oil sign contracts to buy it at

premium prices rather than taking the risk that they won't be able to get it. When this happens,

news media sometimes report that "speculators" are driving up the price, but that is a gross

oversimplification. It is the continuing growth of the economies of the developing world that keeps the

spare capacity low, and therefore the price of oil high. The 2007 --08 tripling of oil prices took place

when the margin of spare capacity dropped below 2%, thanks primarily to China's rapidly rising demand

(the Chinese economy has been growing at 10% per year for the last 20 years). Oil prices rose 10% in

response to the revolt in Libya because even the low production of that country (under 2 million barrels

per day) could affect the spare capacity.

The margin of spare capacity will increase significantly if they build factories to manufacture diesel fuel

and gasoline, synfuel, from coal and natural gas. Such synfuel factories could provide an extra measure

of supply security, in addition to that provided by the Strategic Petroleum Reserve. The spare capacity

will also increase if they succeed in exploiting their recently recognized shale oil reserves.

For every energy technology they have to be sensitive to the differences between the developed world

and the developing. Here is an example : the cost of producing solar cells is dropping so rapidly that,

is expected, in the near future the cost of the cells will be negligibly small. (There is an analogy in nuclear

power : the cost of raw uranium, only 0.2 cents per kilowatt-hour delivered, is virtually negligible).

Yet the cost of solar-cell power in the United States, if installation and maintenance are included,

may have trouble competing with natural gas. Natural gas is the primary enemy (economically speaking)

of US solar. But the conclusion is very different for China and the rest of the developing world, for the

simple reason that labour for installation and maintenance is cheaper than in the United States, and that

difference could make future solar cheaper in developing countries, enabling it to compare with natural gas.

Solar Surge

The price of solar cells is plummeting , as a result, interest in solar energy is surging.

Predictably, that in a decade or so, the cost of solar cells will be virtually negligible;

that is, it will not be a consideration when building a solar power plant, or when installing

solar power on your roof. That doesn't mean the total cost of solar power will be negligible;

you will still have to pay for installation and maintenance. And you will still need to have a

backup for rainy days.

Sunlight delivers about a kilowatt of power per square meter onto the surface of the Earth.

That is not hard to remember; think of it as ten 100-watt bulbs. Could we use solar power

to drive a car? If we had 2 square meters of solar cells on the auto roof and the sun was

directly overhead, 2 kilowatts would be incident. The best solar cells convert only 42% of

that energy to electricity, so the usable power would be 840 watts. That is 1.1 horsepower,

enough to compete with a real horse, but not to meet the needs of most consumers;

typical US autos cruise the freeway using 10--20 horsepower, and deliver 40--150 when

needed for acceleration.

On the other hand, with a large area, sunlight can add up. Over a square mile the sun delivers

2.6 gigawatts of power. Convert that at 42% efficiency, and you get over a gigawatt of electricity,

the same as the power from a large coal or nuclear plant. On average, however, you don't do that

well. Solar plants deliver less power when the sun is oblique, and none at all at night. Even in a

cloudless desert, the average solar power is only 25% of the peak, about 250 watts per square meter.

If you need the power primarily in the afternoon, when both air conditioners and factories are running,

then solar can be an excellent supplement.

The trick is to gather all this energy cheaply. One way is to focus it in the same way that you can use

a magnifying glass to start a fire. The idea goes back to Archimedes, who, according to legend,

used mirrors with sunlight to attack a Roman ship during the siege of Syracuse. For a solar plant,

the high temperature produced by this focusing of the sunlight is used to boil water, which in turn runs

a turbine. This approach is called solar thermal.

Photovoltaic Cells

==============

Solar cells, also called photovoltaic or PV cells, are thin wafers that absorb sunlight and

produce electricity directly. They use the physics discovery known as the photoelectric

effect, a phenomenon that was first explained by Albert Einstein and that earned him a

Nobel Prize. (No, he did not get it for his theory of relativity). In the photoelectric effect,

an incoming particle of light known as a photon knocks and electron away from the atoms

that is normally associated with, and it lands on a metal electrode. When the electron

moves from the electrode onto a wire, it is electricity, and it carries with it some of the

energy of the photon of light. The fraction of energy turned into electricity by a reasonably

priced cell is only 10%--15%, although it might reach as high as 20% in the near future.

It is 42% for expensive solar cells, and that percentage could also go up.

It is interesting to know that photovoltaic cells are truly quantum devices. The photoelectric

principle was one of the foundations of quantum mechanics; although Einstein is often thought

of as disliking quantum physics, he was indeed one of the key founders of this field.

Much recent interest in the solar-cell approach comes from the currently plunging cost of the cells.

One way to describe the expense is as "cost per installed watt". If the cell will produce one watt

of power at peak sunlight, and it costs $7 to produce, then we say the cost per installed watt is $7.

That was the actual cost just a few years ago, but the field is high-tech and very competitive,

in part because of the subsidies. In 2011 the cost dropped below $1 per installed watt; that

milestone was celebrated throughout the alternative-energy community, and the bottom is

not in sight. That is exciting.

It is also misleading. When we talk about the cost per installed watt for coal, nuclear, or

natural gas, we mean average power, delivered around the clock. But the convention in

solar is that we talk about the cost per installed peak watt. Recall that one watt at peak,

even if there are no clouds, is 1/4 watt on average when you include the varying angle

of the sun and its total absence at night. The energy in sunlight is lower still when it is

overcast; clouds reflect the light back to space. At a typical location, intermittent clouds

reduce the average solar energy by half, so on average a solar cell delivers only one-eighth

of the electric power that it can produce at its peak. Of course, electric power is often more

valuable during the day, when you might be running air conditioners or factory machines.

So the true value of solar depends on the time of day it is needed. Still, beware of fantastic

claims of low cost per watt.

Let's look in more detail at the cost. Assume the cell cost $1 per watt to buy and install.

That is for peak power. The average sunlight is one-eighth that, so on average, the cell

produces 1/8 watt. There are about 8,000 hours in a year, so that one cell will deliver

1,000 watt-hours, or about 1 kilowatt-hour of electricity, in a year. That is worth about

10cents to the consumer. Thus the return on the $1 investment is 10 cents per year, or

about 10% return, assuming no installation or maintenance costs. From this you need

to subtract the depreciation. If the cell lasts only 10 years, then you are losing value at

10% per year; in 10 years you have to replace the cell and put in another $1. The net

result : you get your money back, but with no profit. If the cell last for 20 years, then 

the effective return is 5% per year.

But we have ignored a major cost, that of the electronics that must be added to the

cell to make the produced electricity useful. Photocells deliver their power at only a

few volts, but most of our home electric devices, from light-bulbs to refrigerators, are

designed for 110 or 220-volt alternating-current electricity. The simplest solution is to

attach these cells in series, boosting the voltage to the sum. But then if one cell fails,

it brings down the whole array. A better and widely implemented solution is to use an

electronic device called an inverter that converts the low voltage to standard household

values. Add the cost of inverters, the cost of installation and maintenance, and the

optional cost of backup batteries, and most home and business rooftop installations

would show no profit if the government did not heavily subsidize them. Estimated

capital cost alone could amounts to 19.5cents per kilowatt-hour.

How can solar compete? 

Silicon

=======

Silicon crystals were the original solar cells, the ones that were used on the first

space missions, and still the ones widely used in the home market. Silicon is cheap;

it is a major ingredient in sand (silicon dioxide), but purifying it is a major expense.

Just a few years ago, most people were pessimistic about the future of silicon solar

cells, in part because growing large single crystals was expensive, and the cells were

not very efficient. In 2007, silicon solar cells cost about $5 per installed watt, but in

recent years that price has dropped dramatically. In 2010 the price dropped below $2,

and by 2011 it was below $1.

 There are two reasons for the rapid plunge in cost. The first is that it was possible;

that is, technology for cheap solar cells could be found. There is nothing fundamentally

expensive about silicon. Not all technologies can be made cheaper. The cost of computer

chips has dropped dramatically in the last few decades, but the cost of many other

technologies has not; for example, lead-acid batteries have stabilized in price. It turns

out that solar cells have followed the computer chip path. The other reason for the drop

in cost is that there was competition. Thanks in part to "renewable energy" legislation,

there has been a huge demand for carbon-free power. Given the existence of a market,

investors were willing to take risks on technologies that could compete.

The largest manufacturer of solar cells in the world is now Suntech Power in China; this

company produces cells with efficiencies of up to 15.7%. There are constant complaints

in the United States that Suntech's cells are being sold below cost in order to drive

US manufacturers out of business. Suntech is now producing more than 1 gigawatt

per year into perspective, recognize that because of nights and cloudy days, the

average power output of a year's worth of Chinese solar cells is not 1 gigawatt, but

only 1/8 gigawatt. China is installing about 50 gigawatts of coal power every year,

400 times greater than the added solar capacity. So although solar sounds big,

 it is way, way behind. It will take enormous growth for solar to really become a

substantial contributor even in the Chinese market, let alone the world market.

CdTe (Cadmium Telluride)

====================

Telluride has almost no commercial value, except that its compound with cadmium, CdTe,

has a superb ability to absorb sunlight and release electrons. A layer only 3 microns thick

(about one-tenth the thickness of a human hair) can produce electricity with an efficiency

of 15% or more. Moreover, CdTe can be deposited on thin sheets, yielding flexible solar

cells that don't have the crystalline fragility of silicon cells (which are typically 30 times thicker).

CdTe is used by First Solar, the largest solar-cell manufacturer in the United States. Already

First Solar is producing over 1 gigawatt of solar cells each year, and it is growing rapidly.

The company says its price per installed watt in 2012 dropped to 73 cents but it is hard to

know since it depends on how they amortize factory construction .

There is a serious concern that we will run out of tellurium. That worry is based on the fact

that only about 800 tons per year is produced, mostly as a by-product of copper mining.

It takes about 100 tons to make a gigawatt solar plant, so the world's supply will allow only

8 gigawatts per year. First Solar is soon going to reach 2 gigawatts per year, and in a few 

years, if its growth continues, it may be using the world"s entire yearly supply. Some experts

believe that the low production of tellurium simply reflects the fact that until recently there

was no market for it, and they think we will find abundant new sources of tellurium as the

demand for solar cells increases.

Another worry about CdTe is the cadmium is highly toxic. Proponents argue that the cadmium

is safely confined in the cell, but there is always danger of release in a fire, particularly if the

cells are installed on a roof. Studies of this hazard show that such release is not very likely,

but the public will undoubtedly continue to be concerned, particularly if the competitors

(CIGS and silicon manufacturers) keep raising this issue publicly.

CIGS (Copper Indium Gallium Selenide)

================================

The name of the four main constituent elements, copper, indium, gallium, selenium, are

too ponderous, so people refer to these cells by the acronym CIGS (pronounced "sigz").

Like CdTe, CIGS absorbs sunlight so readily that the cells can be made very thin. One form

of manufacture is amazing : small beads are deposited on a metal-coated glass or plastic

using a device that looks and behaves like an ink-jet printer. Once in place, the material is

"sintered" (treated with heat), causing the pellets to fuse. Finally, additional layers are

deposited and treated. In the end, the entire structure is only 3--4 microns thick, just as

with the CdTe cells.

CIGS cells have the advantage over CdTe cells in that they don't contain any highly toxic

material, but they have the disadvantage that one essential ingredient, indium, is in very

high demand and short supply. Indium tin oxide is a transparent conductor of electricity,

and for that reason it is used on virtually every modern TV and computer and game display.

Some estimates say we will run out of indiumin a decade or two, even if we don't use it for

solar cells. But optimists argue that there is really a lot of indium available, if the demand

grows.

One of the stars of CIGS technology is a company called Nanosolar, with a large factory in

San Jose, California, currently producing over 640 megawatts of solar cells each year.

Nanosolar's current efficiencies are just over 10% (compared to 15% for silicon) but it has

achieved 20% efficiency in the lab. Efficient cells are particularly important for locations that

have limited area, such as rooftops. If you have plenty of room to spread things out

(for example, in the desert) than the key number is cost per watt.

Nanosolar and other thin-film companies are suffering from the sudden and surprising drop

in the cost of Chinese silicon cells, which also happen to have higher efficiency. US imports

of Chinese solar cells increased 15-fold between 2006 and 2010, primarily because of the

drop in prices. Some people think that China is heavily subsidizing its silicon solar industry

in order to grab market share and drive competitors out of business, and politicians in the

United States have called for action to protect their industry from such unfair competition.

CIGS got a bad name in 2011 because of the bankruptcy of Solyndra, a CIGS-based company

that had received over a half billion dollars in loan guarantees from the US government.

Solyndra blamed its bankruptcy on Chinese competition; the company said that the Chinese

solar cells were being subsidized (ignoring, in its argument, that it, too was subsidized).

However, a deeper reason may have been the complexity of the Solyndra design. Solyndra

put its CIGS cells inside a hollow glass cylinder (hence the name of the company). Although

Solyndra's web page claimed this would increase efficiency, it was not hard to show that it

actually decreased efficiency. Solyndra also claimed that the cylindrical design would help in

windy conditions and make the cells easier to install. When reviewed Solyndra technology

about a year before its bankruptcy , it was concluded that the wind resistance had no

significant benefit, and what value it did have could be easily matched with small innovations

applied to the standard flat-cell geometry. it is believe that the true reason Solyndra could

not compete with the Chinese competition was not because of subsidies or lower labour

costs in China, but because the Solyndra design was inherently more expensive to

manufacture and the cylindrical geometry lowered the average light intensity hitting the cells.

Multijunction Cells

===============

If cost doesn't matter, or if space is critical, you can use a multijunction cell, typically made

of gallium arsenide (GaAs), germanium, indium, and other metals and semiconductors.

These cells have multiple layers, one for each wavelength range in the solar spectrum, and

as a result they reach very high efficiencies, already 42%, and possibly higher in the future.

The number is their big selling point. Because they deliver more power per area and per

pound than anything else does, they are widely used in space. They are the solar cells used 

on the Mars rover. But they are very expensive, typically costing $500 for just one square

centimeter.

There is a trick, however, that allows the use of multijunction cells at low cost. Buy a small cell,

and use lenses and mirrors to concentrate the light onto that area by a factor of 500--1,000;

the result can be a good number of watts per dollar. This method, similar to the method used

in solar-thermal plants, is called concentrator PV. You prevent the cells from overheating, good

thermal conductors must be attached to the cells to conduct away the heat.

Multijunction cells have dropped in price to the point that concentrator PV is now a competitive

technology, and several firms have started to produce them. The disadvantage is that they

have to be carefully pointed at the sun, so they must move as the Earth rotates. If there is a

cloud between them and the sun, they don't work at all; the concentrator works only with pure,

unscattered sunlight. The manufacture of the pointing system and the reflectors has become the

major cost, not the price of the solar cells. The success of this approach depends on reducing

these expenses and on achieving low maintenance costs.

The major value of the concentrator PV approach is the high efficiency. If you have limited space

(for example, a rooftop), these cells can deliver 2--4 times the number of watts as its competitors.

Concentrator PV is still under development. SolFocus has raised over $170 million; it has two

products; a cell array that produces 6.1 kilowatts, and another that delivers 8.4 kilowatts.

A company named GreenVolts raised $39 million in late 2011 to build equipment to make a

1-megawatt system. It is also possible to design small systems appropriate for small area

such as rooftops; a start-up called Sun Synchrony uses small modules (just a few inches in size)

that aim themselves toward the sun automatically. If we take into account all of the inefficiencies,

these modules extract over 30% of the power in sunlight and convert it to electricity.

The solar-cell field is intensely competitive and developing fast. Prices are dropping so rapidly

that the winners are likely to be decided by criteria other than solar-cell price, including cost

of installation, cost of maintenance, cost of conversion to household voltages, lifetime of cells,

and efficiency.

The installation and maintenance costs are likely to be lower in a developing country such as

China where wages are low. For that reason, the truly rapid breakout of solar power will be

in such countries. That is good news for people concerned about global warming, since most

of the future greenhouse gases will come from developing countries. If we are going to limit

such emissions, we need an energy technology that does not produce carbon dioxide and

that the developing world can afford. Solar could be that technology. But keep in mind the

huge gap between the solar cell production rate in China (1-gigawatt peak of cells

manufactured each year, equal to 1/8-gigawatt average) and the introduction of new coal

plants (over 50-gigawatt average per year).

Energy Storage

=============

Can you save solar energy for a rainy day?

Or just for a dark minute, if a cloud passes over your home solar cells?

The answer is yes; There are almost as many ways to store energy as there are to generate it,

including batteries; compressed air, flywheels, ultra-capacitors, and even hydrogen fuel generated

from water by electrolysis.

Batteries :

---------------

Batteries are little chemical laboratories that use fuel to separate electrons from their atoms.

Let the electrons return through a wire, and you have electricity. Pump energy into a lead-acid

automobile battery by charging it up, and when you draw the power out you will get 80%--90%

of it back. That is an amazing efficiency, and it is the reason that many solar cells for the home

have lead-acid backup batteries included in the price. Four car batteries weighting 250 pounds

in total can store 5 kilowatt-hours of electricity, enough to run a small home for 5 hours. That

efficiency is amazing, but the downside is that the energy stored is very small compared to the

energy available in ordinary fuels. With gasoline, 250 pounds provides 1,320 kilowatt-hours of

heat energy, 263 time more than car batteries. With a generator that turns that into electricity

at only 20% efficiency, gasoline still provides about 50 times the electric energy of an

equal-weight battery.

Even though their energy density is low, batteries are an efficient way to store energy,

particularly if you have plenty of space. For solar and wind farms, the common lead-acid

battery is not the obvious choice; a strong contender is the sodium-sulfur battery. The

largest sodium-sulfur battery yet installed is located in Presidio, Texas, and is named Bob.

Actually, Bob is not used as backup for solar and wind, but as an emergency measure in case

(or rather, when) the single power line connecting Presidio to the US power grid fails. Bob can

provide 4 megawatts, enough to supply power for 4,000 homes, for 8 hours. But sodium-sulfur

batteries are also being used for solar and wind backup, and for power "leveling", keeping

the line power constant in the face of generator variations. Duke Energy plans to install a

36-megawatt sodium-sulfur battery (made by a start-up named Xtreme Power) at its

153-megawatt Notrees Windpower Project in Texas.

The advantage of sodium-sulfur over other batteries is the price per charge-discharge cycle.

Current sodium-sulfur batteries can be recharged 4,500 times (for 80% discharge), versus only

500 for typically lead-acid and lithium-ion batteries(although laboratory models do better).

That is a 9-fold advantage. Lithium-ion batteries will never be used for large-scale energy

storage; they are too expensive. Lithium cost 40 times more per pound than sodium, and

10 times more atom,, a more relevant measure for batteries. With the 9-fold recharge

advantage and the 10-fold cost-per-atom advantage, sodium-sulfur has a 90-fold

advantage over lithium-ion.

 Why, then, don't we use sodium-sulfur batteries in our cell phones and tablet computers?

The catch is that these batteries require liquid sodium and thus work only at high temperatures,

typically about 660'F (350'C), hotter than your home oven. Such temperatures are not a serious

problem for commercial applications. Sumitomo Electric Industries has announced that it hopes

to reduce the operating temperatures to lower than 212'F (100'C), the boiling point of water,

to make the batteries usable in buildings and maybe even large vehicles, such as buses, but

even then, not in laptops.

The Physics and Chemistry of Batteries

=============================

Batteries take advantage of the unusual properties of two kinds of materials : metals and

electrolytes. Metals conduct electrons; that is why we make wires out of metals. More

mysterious are the electrolytes, materials that don't conduct electrons but do allow atoms

to flow through them. In a lead-acid auto battery, lead and its compounds act as the metal,

and the acid-water mixture is the electrolyte. The key trick is to have positively charged atoms

(called positive ions) drift through the electrolyte and then become chemically stuck on the

other side. They attract the electrons they left behind, but since the electrons can't move

through the electrolyte, they have to take a roundabout way through a wire that you provide.

While the electrons are flowing through the wire, you can extract their energy.

There are many electrolytes to choose from (salt water works, as well as the flesh of potatoes)

and lots of materials for the metals. The real trick with batteries is figuring out how to make them

rechargeable. To recharge a battery, you use a generator to force the electrons to return to their

original side; when there, their negative charge will atract the positive ions to break away from

the compounds that they stuck to and drift back through the electrolyte. That is a great idea, but

the difficulties are in the details. The ions must go back to the electrode and attach themselves in

a benign way, ideally back to the same configuration that they had before they left. They often

don't; a persistent problem with rechargeable batteries is that the returning ions tend to form

long finger-like structures called dendrites. If the dendrites grow with each recharge cycle, they

may eventually make the battery unusable. Typical rechargeable batteries fail after a few hundred

recharges. Part of interest in sodium-sulfur batteries is their ability to be recharged thousands of

times without failure.

Bottled Wind : Compressed-Air Energy Storage (CAES)

==========================================

Air can be readily compressed to several hundred times atmospheric pressure. That gives it

an energy storage capacity per volume comparable to that of batteries. The energy is readily

retrieved by running the air through a turbine (that is, a fancy fan). A compressed-air vehicle

in a gold mine; in that confined space, with poor ventilation, using gasoline is inadvisable,

and compressed air are considered safer than lead-acid batteries, which, of course, contain

sulfuric acid. Moreover, unlike batteries, (which last only 500 to a few thousand cycles),

compressed air tanks can be used and reused virtually indefinitely. You force the air in using

a pump, often a piston or a turbine. This takes energy, typically from an electric motor, and

that energy is the energy you are storing.

One problem is the weight of the tank. It is a remarkable engineering fact that, regardless of

size, a steel tank holding compressed air will weight about 20 times more than the air itself;

a modern fiber-composite tank will weigh 5 times as much as the air. So you don't get a

weight advantage by using one large tank versus several smaller ones. The reason for this

surprising result is that a larger tank requires thicker walls to hold itself together against the

force of the compressed air.

Another problem with compressed-air energy storage (CAES) is that the air heats up when

you compress it. That can be a big factor; if the heat can't flow away, then when you pressurize

to 200 atmospheres (a typical value), the gas temperature will rise to nearly 1,370'C (2,500'F)!

On the other hand, if you let that heat escape (for example, you let the tank return to room

temperature), you lose a good fraction of the energy you put in. You can get that back if you

release the pressurized gas slowly enough, since as the gas expands it cools, and it will absorb

heat from the surrounding environment.

Currently, not many CAES systems are in operation. There is one in Huntorf, Germany, and

another in McIntosh, Alabama; neither uses a metal tank. The Alabama facility puts compressed

air in a cavern that was hollowed out of a salt dome (a solid underground salt deposit) by flushing

water through it. The cavity is 900 feet long and 238 feet wide. A new plant planned for an

abandoned limestone mine near Norton, Ohio, will be able to deliver 2.7 gigawatts. Other CAES

projects have been designed for California, New Jersey, and New York. In the advanced designs,

the heat generated by compression is removed and stored, and then used to heat the gas again

when it is expanded to run in a turbine. Calculations show that with such an advanced system,

we may be able to recover as much as 80% of the energy pumped underground, comparable to

the recovery you get from a battery. This kind of "adiabatic" CAES project is also planned to begin

in Stassfurt, Germany, in 2013.

Although manufactured tanks can be used for small vehicles , for city-scale CAES the cost of the

 tank is too high. The cavities have to be geologic. According to the Department of Energy, suitable

sites exist over much of the Midwest, where the wind is also abundant.

Many people are optimistic about compressed-air energy storage. The Electric Power Research Institute

(EPRI) predicts that CAES will be an important part of our energy future. The ultimate fate of CAES may

be determined by the competition of natural gas, and by whether there is a financial incentive to reduce

carbon emissions. 

Flywheels

=========

Spin a wheel using a motor, and you have stored kinetic energy in the rotational motion.

Make the wheel heavy and spin it fast, and it can store a lot of energy; such wheel is called

a flywheel. If you are clever, you use a motor that can act in reverse, that is, become a

generator. Spin it up, and then detach motor power leads from the energy source and attach

them to a light-bulb. The energy of the spinning flywheel will now cause the motor to generate

electricity; the bulb will light and the rotation will slow as the kinetic energy is converted to

electrical.

A flywheel is not only a way to store energy, but it is a useful way to condition energy delivery,

to even out the fluctuations. A flywheel can deliver its energy very rapidly when called upon. At

the Lawrence Berkeley National Laboratory, the atom smasher called the Bevatron had multi-ton

flywheels to smooth out the power it took from the grid. The Bevatron needed energy for only a

brief time every 6 seconds, and without the flywheel its draw would have made the lights of

Berkeley dim every 6 seconds. The flywheel took in energy when the Bevatron didn't need it, and

delivered it as a supplement when the Bevatron hit its maximum need.

A fascinating anecdote about the Bevatron flywheels was told to by Nobel Laureate Luis Alvarez,

who had participated in their design. These huge flywheels, about 10 tons each, were carefully

oriented so that if they ever broke loose (an event that was not expected, but they were situated

near the Hayward Fault), then after they smashed their way through the walls of the Bevatron

building they would roll away from the city of Berkeley and up over the hills toward the reservoir.

The scenario illustrates the potential problem with all energy storage systems (not just flywheels) :

safety.

Modern flywheels don't look at all like those giant wheels at the Bevatron; they look more

like tubes, similar to the centrifuges used for uranium enrichment. They are tall and narrow

because it comes down to some basic physics and material science. Energy is stored in the

velocity of motion of the flywheel material (typically high-strength steel or a carbon-fiber

composite). For optimum use, such material should be in the form of a hoop, so that most

of the material is moving at the top speed. (A wheel has slower velocity close to the hub).

The speed at which the hoop rotates is limited by the strength of the hoop. A calculation

shows that the maximum velocity of the hoop is independent of the hoop radius. ( This is a

good exercise for an undergraduate physics major). The result is that you can use space

more effectively by using small hoops and placing them close to each other. Of course, the

hoops can also be stacked, making cylinders.

The 2,500-pound carbon-fiber composites in Beacon Power "Smart Energy" flywheels twirl

around at 1,500 miles per hour. That is Mach 2! To reduce supersonic friction with air, the

chamber holding the flywheel is pumped to a high vacuum. Each cylinder can stored

25 kilowatt-hours of energy. Beacon Power recently installed an array of 200 of these

flywheels in Stephentown, New York, capable of storing 5 megawatt-hours of energy.

The flywheels are designed to deliver 20 megawatts, and that means they can run for

1/4 hour = 15 minutes. That doesn't seem like a lot, and it isn't. These flywheels are

being used for the traditional purpose : power regulation. They help keep the power

of the local grid at constant frequency, despite a rapidly changing load.

Will flywheels ever be used for large-scale energy storage, for example, for a wind or

solar farm? The kinetic energy at 1,500mph is about 30 watt-hours per pound,

comparable to that of a lithium-ion battery. That makes them seem attractive. But only

a third of the weight of the system is in the spinning flywheel; the rest is in the vacuum

vessel. And most of the space in the flywheel structure is empty. The current flywheels

(including the vacuum vessel) are about 10 feet tall and 6 feet in diameter. That means

they store only 2.6 watt-hours per liter. In contrast, a lead-acid battery holds 40 watt-hours

per liter. Beacon Power's current system can deliver energy for bout $1.30 per kilowatt-hour,

very expensive compared to the average US wall-plug price of 10cents. The design is very

sophisticated and the price is unlikely to come down very much. As a result, flywheels will

continue to be used for power conditioning, but not for large-scale energy storage wind or

solar farms.

Super-capacitors

=============

A capacitor is a set of two metal surfaces separated by an electric insulator.

Put positive electric charge on one plate, negative on the other, and the

combination can store energy for a long time, much longer than batteries can.

Add more electric charge on the plate and you store more energy, but you also

raise the voltage. Keep adding charge, and eventually the increasing voltage

will cause electric breakdown, a spark that could permanently damage the

capacitor. The trick for storage energy in capacitors is making the insulator very

thin, so that you can have lots of energy per unit volume while keeping the

voltage low. Unlike batteries, capacitors don't depend on chemical reactions,

so they can release their energy extremely quickly, and they don't degrade with

use and time, at least not as rapidly as rechargeable batteries do.

Just in the last few decades there has been an astonishing development in capacitors.

The new high-energy-density capacitors are called, naturally, super-capacitors, or

sometimes ultra-capacitors. They are also sometimes called EDLCs, for "electric

double-layer capacitors," but that is a much more boring name. Super-capacitors can

store as much as 14 watt-hours per pound, about a third the energy of a same-weight

lithium-ion batteries, but they currently cost over 3 times as much. That is a nine-fold

disadvantage in energy stored per dollar.

The main value will probably be for use in combination with an ordinary battery. Battery

lifetime is significantly hurt when batteries have to provide short, intense bursts of energy,

but this is precisely what a super-capacitor can do easily. Because they can be charged so

quickly, super-capacitors can be used to improve the efficiency of regenerative breaking;

they absorb the energy and then can transfer it at a more leisurely and efficient pace to the

battery. Super-capacitors will not be major contributors to our large energy storage needs.

Hydrogen and Fuel Cells

Fuel cells have a romance about them, perhaps born in the space program when the public first became

aware of them. You will see a lot of fantastic claims about their future potential, a lot of optimism bias.

They do have true value, and important applications, but they will not be a general replacement for electric

batteries or generators.

A fuel cells is basically a battery that doesn't have to be recharged. Instead, you simply replace the

chemicals that provide the energy. For a hydrogen fuel cell, you pump in hydrogen and air, and out

comes electricity. But for energy storage, you have to also operate them in reverse, to produce the fuel.

Unfortunately, this process has low efficiency, typically only 25%. Compare that to the 80%--90% that

batteries provide. Fuel cells may find a role as a substitute for a turbine in primary energy generation.