Two terms, energy density and specific energy are commonly used here and in many sites that are linked or as sources. Email messages make clear that not every reader is on to just what these mean or how and why the terms are of such significance.

Fossil fuels are so popular because of the energy density and specific energy. They pack a huge amount of chemical energy at low weight and in a small volume. Then of course one needs the reaction machinery, engines, transmissions and all the gear to get the energy released in the reaction into power such as motion and heat. Most of the weight and volume is used in the reaction and motion machinery instead of the storage. Centuries of tradition and the manufacturing, the tooling, the products are deeply ingrained parts of life.

The newer forms of stored energy are much different, both in the energy density and specific energy.

If you’re building a rocket to go into orbit the task is to lift the minimum weight to the highest speed as quickly and efficiently as possible. The premium is the weight or mass as the more energy in a given weight the less overall weight needed to design a successful rocket. So density applies to the ratio of energy to a unit’s weight. It’s very important to things where weight is involved, such as motion when things will move faster and slower. If it moves, energy density is a primary concern.

The other mater is the energy that can be held in a volume of space. Most commonly and here it’s used to consider the attributes of energy storage by volume. In the rocket example the weight of the tank and the machinery to burn the fuel are more significant than the volume used. In a very short time the air resistance to be overcome drops off quickly. Thus rockets can be huge. Specific energy can be thought of the energy in a volume or size.

Other uses have other demands. Something carried would need greater energy density and higher specific energy. Just how the two aspects are engineered depends on the item and how it’s used. Volume might be much more important than weight or the reverse. Somewhere each product engineer will have to choose. Weight vs. volume. How does the choice affect the happiness and satisfaction of the consumer?

Say the mobility device choice is a motorcycle. That would put a big premium on the volume as motorcycles have obvious limits to the volume that can be displaced as they move through the air. Weight is also an issue, so carrying a giant tank of hydrogen gas is simply out of the question even before considering the tank’s weight.

As mobility devices get larger the volume can grow, as more volume is available. A battery that’s much larger would make more sense so allowing more and likely more economical choices in specific energy. Weight also has a role, and density would have more meaning in the total range rather than raw weight practicality.

Finding the best choices can be pre-calculated with experience guiding consumers to more practical choices matching personal needs and preferences. What isn’t known is how each of us will measure the values that are used to make calculations. Where I might be satisfied with high volume or a low specific energy with a low energy density with more weight in a large long range vehicle you may need something much more compact with less range requiring better specific energy and energy density in a smaller vehicle.

This leads to the truth – each of us will need a better understanding of our needs. For some a compressed air power vehicle would do, as it would be cheaper, require far less maintenance, and give very rapid low cost recharge for frequent short-term uses. Others will need less use frequency and much longer ranges at high speed in some comfort – which points to very high density and tolerance for low specific energy – a candidate for continued liquid fuel use.

Energy Density Specific Energy Graph.  Click image for more.

Energy Density Specific Energy Graph. Click image for more.

Today the storage is actually limited to the very best. Diesel is a very dense, high specific energy store of energy. The efficiency is not so bad that the technology can be expected to be with us a very long time. Gasoline isn’t far behind.

Batteries and capacitors have a long way to go to catch the value of a liquid fuel. The very best batteries today are getting to 120-watt hours per kilogram. To compete with gasoline or diesel batteries would have to get well past 500-watt or 600-watt hours per kilogram and solve the time to recharge matter. That may be quite a way off. But when it comes, there will be a switch – more battery energy storage vehicles will sell than gasoline fuel storage vehicles.

What’s the best? That depends on our needs and preferences. A vehicle that gets us 10 to 30 miles with an easy recharge or refill for just pennies rather than dollars could see a huge number of sales as people realize that such an investment could free up considerable income for other uses. One might find that utility companies might realize that overnight recharge would justify a large input of capital to get such vehicles into our hands. Just hat one of several possible points depends on the automotive producers to sell not just consumers but the suppliers of the energy itself.

Consumers don’t give much thought to the full implications of the purchase. For some it’s little more than the monthly payment with as much status as they can get. For others a purchase needs to fill exacting demands. What matters is that manufacturers get a little smarter about selling the range of products not just to the buyer but about the things that will need bought over the life of the purchase.

That opens many opportunities to those with an open mind to filling their needs. A firm concept of just what would be a good purchase might surprise many. The cost of energy is a primary issue and the coming technologies will make having a grasp of how energy density and specific energy can help get a better product or save a lot of money will get more important over time.

A quick look at the Wikipedia page will show just how extreme the range is from nuclear or hydrogen to the batteries. Keep in mind, the storage plus the apparatus to use the energy or fuel makes up the total consideration. When using either measure, specific energy or the energy density, it’s the whole thing that matters when in use.


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