The Energy Storage Problem

An extraterrestrial approaching Earth on the dark side would be astounded.Images of Earth at night from the Suomi National Polar-Orbiting Partnership satellite
What life form, she would think, is capable of concentrating so much energy? Surely, this planet contains some highly intelligent beings.  If she had approached the earth two hundred years ago, the view would have been quite different. Lighting then was by candles and whale oil lamps. Not much chance of seeing them from a few thousand miles.

The industrial revolution was made possible by harnessing the massive and readily available energy in fossil fuels. Without cheap, abundant energy, it never would have happened. For the first hundred years, the supply seemed infinite. Most people never thought about ever running out of oil or coal.

One man, a true visionary, was thinking about it. Thomas Edison, in a conversation with his friends Henry Ford and Harvey Firestone, not long before his death in 1931, had this to say about burning fossil fuels:

“We are like tenant farmers chopping down the fence around our house for fuel when we should be using Nature’s inexhaustible sources of energy — sun, wind and tide. … I’d put my money on the sun and solar energy. What a source of power! I hope we don’t have to wait until oil and coal run out before we tackle that.”

If Edison were alive today, he would probably be appalled at how little progress we have made since he uttered those words.  Now that the limits of our ever-expanding combustion of fossil fuels have become glaringly apparent, we are finally taking some tentative steps in the direction that Edison suggested over eighty years ago. So far, progress has been slow, hampered by economic and political forces that favor the status quo. 

The advance of technology has always been uneven, with periods of stagnation, followed by spectacular breakthroughs. The explosion of microelectronics is an example of such a breakthrough. But the development of renewable, nonpolluting energy sources that can compete with oil, coal and natural gas has been a tough nut to crack.  Those fossil fuel sources have been abundant and cheap…but they will not last much longer at the rate we are burning through them, and their growing scarcity and rising prices are a looming threat to the economies of nations that are not fortunate enough to have their own supply.

One of the problems that has proved most intractable is energy storage. Wind, solar and tidal energy are intermittent, but our energy needs are continuous. A coal-fired boiler making steam to drive a turbine can supply electricity 24/7, but a solar panel or a windmill cannot be relied upon to do that. So storage of electricity in huge quantities will be necessary in the coming era of renewable energy.

Rechargeable battery technology has seen steady improvement in capacity vs. size and weight, but the electrochemical processes in batteries result in limited charge/discharge rates and cycle life, and they are expensive.  Furthermore, the best performing ones today use lithium, a relatively rare element.  We desperately need a storage medium that is cheap, durable, and made from inexpensive and abundant raw materials.

Capacitors (hereafter called “caps” for brevity) are another kind of energy storage device. Until fairly recently, their ability to store electrical energy has been very limited, but recent developments in “supercapacitors” and “ultracapacitors” have made it practical to replace batteries with caps in some applications.

Caps store their charge electrostatically, instead of electrochemically as a battery does. The advantages of electrostatic store include faster and more efficient charge and discharge rates and virtually unlimited charge/discharge cycle life. Disadvantages are smaller storage capacity per unit weight and volume, higher cost, and a highly variable voltage during charge and discharge. Batteries tend to hold a more-or-less constant voltage until their charge is nearly exhausted.

There is a lot of research going on around the world on ultracapacitors at the moment. So far, progress has been modest, but there are hints of dramatic improvements.  A breakthrough in energy storage capacity would be a real game changer. While it would not immediately eliminate the need for fossil fuels, it would reduce the need for them, by enabling a massive conversion of the world’s transportation fleet to electric vehicles, powered by renewable, nonpolluting energy sources.  Moreover, it would provide a storage capability for solar and wind energy, reducing the need for fossil fuel-powered generation to back up those intermittent sources.

The energy stored in a cap is defined by a very simple equation:

E = ½CV2      C is capacitance (charge storage capacity).  V is voltage.

A battery powered car uses about one Kilowatt-Hour (kWh) of electrical energy to travel 4 miles. Small battery-powered cars (EV’s) currently available have an energy storage capacity of 16-24 kWh, giving them a range of less than 100 miles. Many people would be uncomfortable with a range of less than 150 miles, even for a car dedicated to urban commuting and errand-running. Moreover, the batteries cost $10K or more, and weigh hundreds of pounds. The battery in the Nissan Leaf weighs 600 pounds, and occupies a lot of space in the vehicle. The typical manufacturer’s warranty for these batteries is 12 years or 100,000 miles, but nobody really knows if they will last that long. Battery life is highly dependent on charge/discharge rates and ambient temperature. The high summer temperatures in the Southwest might shorten battery life. Furthermore, battery capacity is greatly reduced by low temperature, making these cars marginal propositions in regions with cold winters.  Caps do not suffer from temperature or cycle-life problems.

How much energy storage do we need in an EV?  About 40 kWh to get a range of 150 miles.

Look again at that equation:

E = ½CV2

It is obvious that voltage is critical. Energy storage goes up linearly with capacitance (C), but it goes up exponentially with voltage. Doubling the voltage increases the stored energy by a factor of four!

The physical characteristics of a cap make it difficult to boost the voltage rating without increasing leakage (self discharge) to unacceptable levels. That’s the main reason why caps haven’t been able to compete with batteries yet, and that is where current research is concentrated.

The bottom line is: We need to cram more capacitance into a smaller volume or charge it to a higher voltage…or some combination of the two.  Of course, cost is still an issue too.

For example, a 1200 farad cap charged to 500 volts would provide:
E = ½ × (1200) × (500×500) / (3.6 × 106 ) = 41.7 kWh
Dividing by (3.6 × 106 ) converts the energy units (joules) to kWh.
At 4 miles per kWh, that would give a range of 167 miles.
How big would such a capacitor be? How much would it weigh? How much would it cost?

Don’t try to buy one today.  Ultracapacitors with capacitance (C) values up to 5000 farads are currently available, but they are typically limited to around 2.7 volts.  At 2.7 volts the energy storage in a cap is about 1 watt-hour per 1000 farads.  In an EV application, where 40 kWh of energy is required, a total of 40 million farads would be required!  This is a truly mind-boggling number.  It would weigh tons, and require a good-sized truck to carry it.

Increasing the voltage from 2.7 to 10 volts would shrink the required C to around 3 million farads, still a daunting number, but if it were combined with improvements in C per unit volume and weight, it could lead to a practical energy storage device for an EV.
Powering EV drive motors and accessories like A/C and headlights with a 10 volt supply would require wires the size of your arm, but caps, like batteries, can be combined in strings to produce any voltage required.  For example a string of 50 caps, each charged to 10 volts can produce 500 volts.  Multiple strings can be connected in parallel to increase the total energy capacity.

When caps are combined in a string, the C value of the string goes down by a factor of N where N is the number of caps in the string.  A string of fifty 5000 farad caps would have a total C value of 5000 / 50 = 100 farads.  But no energy is lost.  If you work the energy equation for a 100 farad cap at 500 volts, you will see that its energy content is exactly the same as fifty 5000 farad caps at 10 volts.  If that were not the case, the principle of Conservation of Energy would be violated. 

How about size and weight?  A typical 5000 farad cap that you can buy today is about 2 x 2 x 6 inches, 24 cubic inches, and weighs around 1.5 pounds.  At 10 volts, we would need around 600 caps to make 40 kWh.  If they were arranged in 12 strings of 50 caps, they would be exactly equivalent to the 1200 farad, 500 volt cap in the example above, but they would occupy a volume of about 8 cubic feet, and weigh 900 pounds, much too big and heavy for a small car. 

If the voltage could be raised to 20 volts, only one-fourth as many caps would be required.  Each string would need only 25 caps to make 500 volts, and its C value of the string would be 200 farads (5000 / 25),  so only six strings would be needed to make 1200 farads, a total of 150 caps.  Assuming they were the same size and weight as the 10 volt caps, the package would shrink to 2 cubic feet and 225 pounds…reasonable values for a small EV.  Leaving the voltage at 10 volts, but packing four times as many farads into the same size and weight would accomplish the same thing.  Many combinations of increased voltage and/or denser packaging could work. This is what is needed to make a practical EV car powered by caps.

Although the required improvements sound daunting, several recent announcements have suggested that researchers are on a path to achieve these goals.  It can’t come too soon.  With China and other emerging industrial economies using more and more fossil fuels, the time remaining to make a smooth transition to renewable energy sources without major economic upheaval is dwindling rapidly.

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