Saturday, 14 January 2017

Energy Capture, Conversion and Storage

This post is a part of the series An Acre of Sunshine.

Each time one form of energy is captured and converted into some other form, only some of the energy is converted to that new form, the rest of the energy is lost. This isn't to say that the energy disappears, as energy is neither created nor destroyed (see the 1st law of thermodynamics). Instead, I mean that only some of the energy goes to the targeted next step, and the rest goes somewhere less useful. Let's take the example of a typical car running on gasoline. The desired target for the energy from the gas is to move the car, but there are some unavoidable problems. In this case, the issues are such things as friction on the highway, which heats up the tires (converting some energy to heat), and wind resistance which slows the car by transferring some of the motion of the car into the air. The engine turns much of the energy of the gasoline into non-useful heat, doesn't burn all the gas perfectly well, and so on and so forth. For the example above of a gas-powered car, only about 20% of the total energy in the gasoline is converted into motion of the car. And this doesn't even include the fact that  energy is often converted many times between different forms before it is used for the motion that we want, with losses at each and every step. For that gasoline example, there are inefficiencies and losses in taking the oil out of the ground, refining it into gasoline and other products, as well as transport and storage losses.

Energy storage is also a key concern; What happens if there is available energy and motion now, but you don't want to use it until some time in the future? Well, you need to find some way to convert the energy into some stable form, and be able to also have some way to convert it again for the end use. Stored energy is 'potential energy', meaning that one has the ability to cause things to move at some point in the future. An easy way to imagine potential energy is with hydropower. Say that there is a river running through a valley. There is a lot of water motion that is there, that could potentially be harnessed and put to use. But what if the river runs dry every summer, and you wanted to be able to have a steady supply of power? You could then build a dam. Instead of the water moving now, the dam stops it and holds it still in a lake above the dam. All of that water has potential energy because gravity would like to pull it down, but the dam stops the water from moving. In this situation, much of the energy (motion) of the water in the river is now stored in the lake, and can be utilized whenever it is needed. The motion of the water, when it is released, is used to spin a turbine (basically a big wheel), and this spinning motion can be used to create electricity (though in the past it was for things like cutting wood or grinding wheat into flour). Other forms of energy storage act by the same underlying principle. They capture and freeze motion in place, setting up a situation where motion can be re-directed in the future.

A huge amount of the energy coming from the sun is captured and harnessed by living things, and passed between different living organisms via a food web - big fish eat small fish, small fish eat insects and other tiny creatures, the tiny creatures eat algae. Though the details are vastly different, the same basic principles of energy storage and conversion apply across all ecosystems. All living things store their energy largely in the form of proteins, fats, and carbohydrates (such as sugars). Each organism needs a steady supply of new energy inputs to metabolize, to grow, to reproduce. The base of this food web in most places is of course the process of plants converting energy from the sun into sugars and then on into other more complex forms. A good rule of thumb is that each level needs 10 times more of the level below to support it (i.e., an order of magnitude). So, 1 pound of large fish requires: 10 pounds of small fish, 100 pounds of tiny creatures, 1000 pounds of algae. In biology these are known as 'trophic levels'.

Animals and plants both stock up on energy reserves in times of plenty to prepare for the times of scarcity. This is both for the short-term as well as the long term. In the short term, plants need to have the energy to make it through the nights, as well as at least a few cloudy days. For animals, there are times when food isn't always available. Predators in particular may go relatively long periods between feeding. And of course there is the winter. Here in eastern Canada, the lead up to the long cold winter has animals putting on heavy layers of fat, and the trees building up stores of sugars through the summer and early fall, then dropping their leaves while sending most of their nutrients down into their roots.

Previous page: What is energy?
Next page: Measuring energy

No comments:

Post a Comment