Energy Storage - Methods - Efficiencies

Methods of Storing different types of Energy

Various methods of storing energy are compared here. I have come to realize that many people have no idea how much actual energy can be stored in a battery, or in water raised up, or in a hot water tank, or other methods of storing energy. So this presentation was composed!

As an introductory comment, we will mention from the discussion below that a standard car battery can contain around 80 Ampere-hours of stored electricity, at around 12 volts, which totals around 1 kiloWatt-hour of electricity. It might seem like a lot, but you buy 1 kWh of electricity for around a dime! It actually costs you about 15 cents due to Delivery costs and assorted taxes, but the point here is that the spectacular claims of batteries and electric vehicles, are VERY exaggerated! We can show why gasoline is so popular by noting that a single gallon of gasoline contains about 40 kWh of chemical energy in it. So we might start by noting that a single gallon of gasoline might be compared to FORTY car batteries regarding gross chemical energy storage! Somewhat different than most people think!

For comparison purposes, we will consider an amount of energy equal to one million Btus or 300 kWh. This is about the amount of heat energy needed to heat a medium-sized home in a northern climate for one cold winter day, or for about ten days of electricity for a house (NOT heated with electricity!).

First, let's consider the "normal" ways of storing energy (essentially all fossil-fuels):

Material, fuelenergy per unitnumber of units needed
Home heating oil140,000 Btu/gallon7 gallons (7 gal * $3/gal) ($21)
Natural gas1040 Btu/cubic foot950 cubic feet (1.5 cent/cu.ft. * 950) ($15)
Gasoline126,000 Btu/gallon8 gallons (8 gal * $3.10/gal) ($25)
Electric Heating
(not really storable)
3412 Btu/kWh300 kWh (300 kWh * 12 cents/kWh) ($36)

Notes: These are the raw amounts of (chemical) energy in each of these fuels. The devices used to convert the energy into some useful form are never perfectly efficient. For example, most modern vehicles are around 21% efficient at converting the energy stored in gasoline into actual movement (kinetic energy) of a vehicle. That sounds very low, and it is. But around 1970, the average vehicle only had around 15% overall thermal efficiency. The processes of internal combustion engines are such that the maximum theoretical thermal efficiency are around 29%. Why so low? Because great amounts of heat are wasted in hot and high pressure exhaust gases (about 40%), and additional great amounts of heat are wasted by the engine cooling system (about another 40%) to keeping it from melting down!

The efficiency of converting chemical energy into heat are generally far higher than in converting it into mechanical energy. House furnaces were long considered to have around 80% efficiency if gas-fueled and 70% if oil-fueled. There are high-efficiency (condensing) furnaces which now have around 95% to 97% overall efficiencies.

Nearly half of the electricity cost is often delivery fees and taxes.

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Other forms of storing energy:

This discussion has been meant to show that the several alternative ways of storing energy that companies seem to be aggressively promoting (like batteries) are fine if the needs are VERY small. As soon as you start considering significant amounts of energy, most get ridiculously expensive, and often impractical!

There actually MIGHT BE decent ways to store energy, probably a LOT of them, that simply no one has ever thought about! I will mention one (poor) possibility that seemed obvious to me. Say we BOUGHT that tank of (industrial) Hydrogen gas discussed above for $42. We get a high quality surplus weather balloon, for maybe $15. We release the 196 cubic feet of Hydrogen gas from the tank into the balloon. That DISPLACES 196 cubic feet of air, which weighs around 15 pounds. After accounting for the weight of the Hydrogen gas and the balloon itself, we probably have a Lifting Capability of around 12 pounds. So we attach a light plastic bag containing about 11 pounds of water in it (about 1.5 gallons). We then release it on a tether string, and let it rise! Eventually, it gets up to 10,000 feet altitude. Consider what this means! We have LIFTED 11 pounds of water up to 10,000 feet altitude, so we have given that water POTENTIAL ENERGY of 11 * 10,000 or 110,000 ft-pounds. (This energy did NOT come from the hydrogen gas, but from the sun in heating the atmosphere.) This example is a good one to show just how much energy is in a standard kilowatt-hour or ten cents of electricity! We had mentioned above that 778 ft-pounds is equal to 1 Btu. Therefore, in the 110,000 ft-pounds that we have given that raised water, we now have around 150 Btus of Potential energy. Since we have also noted that 3,412 Btus is equal to one kilowatt-hour, we can see that we have created about 150 / 3412 or around 0.04 kilowatt-hour, less than a penny's worth of stored energy!

The point here is NOT that that particular idea has any useful merit in large scale energy storage, but just that MANY such concepts actually do exist, which someone should carefully examine. No one does, because they have always simply assumed that oil and gas and nuclear and coal would last forever!

Peak Power Rating vs. Average Power Rating

You may drive a car which was advertised as having a 495 horsepower engine, and that may have even affected whether you bought that specific car. That engine rating can be called a PEAK POWER RATING, being the greatest amount of power that it is capable of producing. When creating that enormous amount of power, it is realistic to expect to get around one or two MPG gas mileage. But for AVERAGE driving on an Interstate Highway, your engine only produces around 40 horsepower, during which you may get 25 miles per gallon gas mileage. This AVERAGE situation is a far more accurate description of what YOU CAN ACTUALLY EXPECT, such as regarding gas mileage. Both situations are true, but they are extremely different. One is a situation which sounds very impressive, but which you will likely NEVER actually experience, except possibly rarely for a second or two at a stoplight! The other is a situation which you may experience every day of driving! IF you were only given ONE of the numbers, which would you consider more important to know?

Whenever electricity ratings are given for alternative energy devices, they seem to always be PEAK POWER RATINGS, meaning the greatest amount of electricity or power which can be created. That is entirely different than ratings for AVERAGE USAGE CONDITIONS, which would be realistic numbers of amounts of electricity or power which might NORMALLY be expected to be provided. The discussion and calculations included here will indicate that OFTEN the realistically expectable amounts of electricity or power is only around ONE-TENTH that of the PEAK POWER RATINGS. But no one bothers to mention this important fact! So advertising makes claims of spectacular performance numbers for photovoltaic solar-electric panels, and for solar roof panels, and for electric vehicles, and for Hybrid vehicles, and for windmill-electricity-generation, and even for FUTURE giant windmills and hydrogen as a fuel. They invariably state PEAK POWER RATINGS, like that 495 horsepower engine in the car, numbers that may be technically true but are extremely misleading.

Not Storage, but Photovoltaic Cells

We have heard from many dozens of people who have been promised by salespeople that the "photovoltaic cells" they buy for a few hundred or a few thousand dollars will make them "energy independent". That is a wild exaggeration just to sell their products! One square foot of such photovoltaic cells can intercept around 300 Btu of solar heat, at noon, on a perfectly sunny day. One third of that is more realistic for an average day nearly any climate. Then the photovoltaic cells that are available for tolerable prices are generally around 7% efficient at converting the solar energy into electricity, so we're down to around 7 Btu/hr worth of electricity, which is around 2 watts! This is therefore a realistic 2 watts of electricity for every square foot of actual PV cells.

(This 7% figure is for the most economical technology of solar cells, which is based on Cadmium Sulfide(CdS). There ARE higher efficient technologies which exist, such as those based on Gallium Arsinide(GaAs), but they are far more expensive and not within the price range of most people. There are even more expensive technologies that are based on silicon semiconductor technologies, which require a [metal] silicon ingot to be sliced so thin that sunlight can pass THROUGH it, which is extremely expensive to do! So higher efficiencies exist in solar cells, which are reported in media stories, but they are currently far too expensive for broad use. This all results in MOST commonly available solar cells being cadmium sulfide, and therefore around 7% efficient.)

So, if someone actually expected to get a usable 200 watts of electricity, they would certainly need at least 100 square feet of solar photovoltaic cells, which is a lot of money! And 200 watts for a few sunny hours is not very much electricity. No toasters or microwaves or televisions or computers! A few small lights and not much more.

The salespeople never present it this way! They say the 300 Btu/square foot/hour, sure, which they correctly say is around 100 watts of solar energy. They leave out those "details" which tend to ruin making a sale, and let the customer incorrectly believe that TWO square feet (duh, 2 * 100 = 200 watts, right?) is all they would need for the 200 watts of electricity. When the reality is around 100 square feet! How come it is not criminal to intentionally mislead customers so wildly?

It might be different if PV technology improved the current 7% conversion efficiency of solar to electricity. Or if such panels got a LOT less expensive and a lot more reliable and durable. But for a few hours of collecting 200 watts in any 24 hour day, it seems truly wasteful to invest thousands of dollars! Unless you simply want to be able to brag that you bought GREEN!

But the specific point of this Storage discussion is that the few hundred watt-hours of that PV electricity need to be stored for several hours until needed in the evening. Or worse, if the day is without any sun, maybe an entire day later. The electrical equipment and batteries usually sold cannot store enough energy to really use! Virtually all such people are faced with either conceding that they will NOT be able to be off-the-power-grid or they will have to go back (to the same dealer) to buy lots more expensive stuff!

YOU know lots of people who have mentioned wanting to go "off-the-grid", and they have been convinced by some PV salesperson or promotional literature that a few small solar panels is all they will need. No, it isn't, BY FAR!

Unless they install something like $100,000 of solar equipment, like that well-publicized house in the Northeast, where they actually often have enough electricity (but STILL have to sometimes buy conventional electricity), you are never going to hear such friends actually confirm that they accomplished that goal. Their lives may turn out to be more like Lincoln growing up in that log cabin with one wall missing, a very, very primitive existence!

We hate to see people get taken advantage of like that by salespeople, just because they do not understand subjects like those discussed here. We are providing this info so fewer people might get taken advantage of in a lop-sided conversation with such a salesperson. Unfortunately, that salesperson invariably gets a commission on whatever he/she sells, and so remarkably impressive claims are always made. And if a person only considered a noon situation on a perfectly sunny day and everything else was laboratory perfect, yes, such equipment CAN produce outputs that keep them from being sued! But not enough for "real life" situations where people actually NEED the electricity!

There are a few other methods of storing energy, such as phase-change salts and other exotic things, but they tend to be even worse than the ones described above, and also more expensive.

HG 3a

An entirely new concept has occurred to me in early 2007! This is a good one!

If you own a house in a temperate climate like Chicago, then you pay at least $1,500 every winter to heat it. Worse, you are spending that money to buy fossil-fuels, which are making the Global Warming problem worse. You buy around 80 million Btus worth of heat each winter.

In your good-sized yard, grass and weeds grow and trees have leaves, and you mow and rake and bag everything up to have it all taken away. Wherever that material goes, it will decompose, and in that process it will release heat energy. A LOT of heat energy! Farm studies have shown that each acre of land can produce around 95 to 125 pounds of organic matter (glucose) each summer day, and other studies have shown that an acre of grasslands and trees can produce around 17,000 pounds in an entire year. We know that each pound of [dry] organic matter contains and releases around 9,000 Btus of energy when it decomposes.

You can see that, if you leave that grass and those leaves on the one-acre yard, and they do not blow away, they necessarily give off more than 150 million Btus of heat energy during the months of decomposition. You have never noticed it because it is so slow and so spread out.

I have discovered a way to collect and use that heat to entirely heat your home. NO heating bills! NO fossil fuels used! Absolutely "carbon-neutral". It is a high-tech version of doing the Composting that farmers have done for centuries!

You can both contribute to saving the world and also save yourself a lot of money! You only need a few hundred dollars of common construction materials to build the things you need to accomplish this!

I have provided free complete directions on how you can make such systems. I actually have provided two different sets of instructions, one for a small version which is designed to eliminate the cost of heating domestic how water. Alternative GREEN Water Heater HeatGreen - A Simple and Non-Fossil-Fueled Water Heater, HG3a (biodecomposition) (March 2007).

The other is a larger version, which is designed to entirely heat your entire house, with NO cost for any bought fuel, no Global Warming consequences, and a new independence from utility companies! Alternative GREEN Furnace with no Fire HeatGreen - A Simple, Non-Fossil-Fueled Home Heating Furnace.

This presentation was first placed on the Internet in Jan. 2005.

A different web-page provides a lower-tech approach to using Savonius rotors to produce massive amounts of electricity for a house that aspires to be self-sufficient! Self-Sufficiency - Many Suggestions

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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago