Tuesday, January 6, 2015

What It Takes To Kill A Watt

From space, the view at night gives a good indication of how
much electricity we humans use. With the exceptions of
North Korea and Antarctica, you can’t find many places on
Earth that don’t glow at night. But even at that, it would
take 20 years for us to produce the same amount of energy that
shines on the Earth from the Sun in one day.

Christmas has just passed and everyone has new phones, PlaySatations, drones and other gadgets. Unless your phone is nuclear or solar, it also came with a recharging cord because these tool/toys work on electricity, the very lifeblood of modern society. To illustrate - the total amount of electricity generated in the U.S. in 2012 was 4,047,765,000,000 kilowatts.

For something so important to commerce, entertainment,medicine – just about anything you can name – the average person knows shockingly little about how it comes to them. Let’s take some time to acquaint ourselves with something called the National Grid of electrical generation and distribution. Let’s consider power generation this week.

There are a couple of ways to directly generate electricity from a potential energy source. Solar photovoltaic cells turn solar radiation directly into electricity.  These technologies have been around for several decades. However, even though sunlight has many wavelengths of light, most photovoltaic cells only harvest a fraction of them. Historically, this has made solar energy inefficient to use.

New cells use several different semiconducting materials, each absorbing a different wavelength of light. According to 2014 research, these new cells achieve an efficiency of 44%, meaning that 44% of the energy that strikes the surface is converted to electricity. This is pretty good compared to single junction cells that have about a 25% efficiency.

Fuel cells generate electricity by moving hydrogen ions across a catalyst. The metal catalyst strips electrons from the hydrogen and these electrons form an electrical current. The resulting positive hydrogen ions combine with oxygen on another catalyst to produce water as the only by product. The problem is that the materials (exchange membranes or electrolytes) to move the positive hydrogens to the oxygen are every expensive.

Fuel cells produce water as their only byproduct. When the
catalyst strips hydrogen of its electron, this flows as
current, and is then replaced by an electron from oxygen.
Most cells differ only by the electrolyte used. Upps now has
a fuel cell on the market to power your smart phone for a
week. I wonder how you don’t end up with a soggy phone.
Batteries use electrochemical reactions to liberate electrons (electricity). Primary batteries only produce electricity, while secondary batteries can reverse their chemical reactions and store electrons if an electrical charge is applied – these are rechargeable batteries. In most cases, batteries are used on small scales, although we will see in a couple of weeks how they can be used on a very large scale.

These technologies excepted, most electricity is produced from the turning of generator turbines. The turbines spin a coil of wire in a magnetic field. This produces electricity – thank you very much Michael Faraday. The key is how we generate the force that turns the turbine blades that then spin the wire coil. In most cases, we burn some sort of fuel to turn the blades, but in a few cases turbine blades can be turned directly.

The wind turbine is an old technology use to harness the power of air moving from areas of high atmospheric pressure to areas of low pressure (wind). The Europeans used wind power to grind grain into flour (hence the name wind mill) but they have been around since the time of the Greeks. American and Australian farmers use them to pump water, but today we mostly use wind turbines to generate electricity.

Hydroelectric generators use the power of gravity and water under pressure to turn turbine blades. By constructing a dam between an area of high elevation and one of lower elevation, water is put under pressure. Allowing a controlled volume to pass through a turbine will spin it and the associated generator will produce electricity.

The Three Gorges Dam and Hydroelectric Plant in China
(top) was built to be world’s largest hydroelectric generator
system. However, the Itaipu Dam on the Brazil/Paraguay
border (bottom, all generators first online in 1991)
generated significantly more electricity in 2013 and 2014.
Ocean waves can also be used to generate electricity. Whether on the shoreline, near shore or offshore, wave power use the movement of water to turn turbines. There are several different mechanisms to capture the wave motion and transfer it to a turbine, but designing an efficient mechanism has proven to be harder than originally thought. It may be another 30 years before wave power matches the wind power we generate today.

These direct "energy to turbine" mechanisms are helpful, and are becoming larger players in the electricity game (along with solar power and fuel cells). But the burning of fossil fuels - natural gas/petroleum/coal - is still the main way we turn turbine blades turn.

Fossil fuels are non-renewable natural resources and their importance in energy can’t be underestimated, both from the standpoints of how helpful and efficient they are, but also in how dirty they are and how they are running out. However, they aren’t the most important natural resource for the generation of electricity. Can you guess what is?

Believe it or not, water is the resource used in greatest amount so that you can charge your iPhone.  Name an energy generation technique that doesn’t use water – you can’t. Water cools power plants that burn fossils fuels; it cools nuclear reactors too. It is used to water the fields that grow plants for biodiesel. Water is heated for almost all generator turbines. Water is even used to extract petroleum from the ground – ever heard of fracking? A research report in 2008 stated that the U.S. alone uses 500 billion liters (132 billion gallons) of fresh water each day just to produce electricity. 

Frack - expletive, energy process, or jacket? All three, actually.
In dressage, the frack is the dress coat worn in some
competitions. In energy circles, it refers to hydraulic fracturing,
where water, chemicals and sand are injected into shale in order
to fracture it and release trapped crude oil or natural gas. Some
people are against both and use the expletive to show
their displeasure.
Water in liquid form maintains a certain volume, but water in vapor form – steam – takes up a lot more room. The volume expansion of water when boiled creates force; it can make a whistle blow on your coffee pot or it can be used to turn the blades on a generator turbine. So the key to making most electricity is turning water into steam. In truth, it's the kinetic energy of motion from escaping steam that is used to turn a magnet across a coil of copper wire.

How do we produce steam to turn the blades of a generator turbine? Nuclear fission is one way. Decay of naturally radioactive materials in a water bath generates a lot of steam. Other ways include solar thermal energy, where the energy of the sun is directed by mirrors at a large mass of water that then boils, or by burning biomass (wood, sawdust, biodiesel, trash, methane).

It's beyond this discussion to explain how a spinning magnet across a coil produces moving electrons, so you can read about it here. The point is that whatever means is used to spin the turbine, its shaft is attached to the magnet that spins.

In most power stations, one magnet passes three coils on each revolution, so three alternating current lines are produced. This is the three phase generation that will become important when we next week talk about the electrical lines coming to your house.

contributed by Mark E. Lasbury, MS, MSEd, PhD
As Many Exceptions As Rules

Sheng, X., Bower, C., Bonafede, S., Wilson, J., Fisher, B., Meitl, M., Yuen, H., Wang, S., Shen, L., Banks, A., Corcoran, C., Nuzzo, R., Burroughs, S., & Rogers, J. (2014). Printing-based assembly of quadruple-junction four-terminal microscale solar cells and their use in high-efficiency modules Nature Materials, 13 (6), 593-598 DOI: 10.1038/nmat3946

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