As the world moves closer to fossil fuel depletion and global warming seems to be increasingly caused by carbon dioxide emissions, sources of green energy have become more and more important. But what are green energy technologies and how far have they gone towards adoption? What hold-ups are there in relinquishing fossil fuels? What can satisfy humanity's diverse and enormous requirements for energy? To even start to answer these questions, we need to know how energy is harvested, stored, and consumed.
Since Einstein we have known that energy is contained in all matter, and his famous formula represents an upper limit on how much energy can be harvested. And in a few cases, we have determined how it can be extracted, with varying efficiencies. Let's look at a few. The first method of energy extraction is from an exothermic (heat-producing) chemical reaction. This method is used in internal combustion, and it releases heat, measured in Joules. For example, when combusted with oxygen the following fuels release this much energy in kiloJoules per gram:
- Acetylene 11.8
- Ethanol 27.3
- Coal 17-21 (sub-bituminous) 29-33 (bituminous or anthracite)
- Kerosene (Petroleum) 43.1-46.2
- Methane 50.6
- Gasoline 51.6
- Hydrogen 120
The conversion of harvested heat into electricity or mechanical motion is quite a different matter.
Most people know that internal combustion engines are really controlled explosions. The addition of heat to a gas causes a significant change in its density. This change is implied by the law of ideal gases:
PV = nRT
Here, P is the pressure, V is the volume, n is the amount of the gas, R is a constant, and T is the temperature.
From this, we can see that when you increase the temperature, while keeping the amount of gas and the volume it is contained in constant, then you must increase the pressure proportionally. A massive increase in heat and a chemical reaction can create an explosion, which is a massive increase in volume. This is what happens in an internal combustion engine: the gas explodes, the massive increase in pressure drives a piston, and the motion of the pistons drives a cam shaft.
In modern cars, this is used to propel the car forwards, as work.
This can also be used to move a rotor and generate electricity through electromagnetic induction. Many power plants work this way, including the Moss Landing power plant in California.
Chemical reactions, to generate heat and drive turbines, generate about 65% of the world's electricity requirements today.
The second method of energy extraction is the harvesting of kinetic energy from matter that is already moving, such as water and air. This energy is almost always harvested using a turbine.
Have you seen the wind machines dotting the hillside near you? Those are wind turbines that harvest electricity from the wind itself, with very little effect on the environment. This is commonly called wind power, and is a renewable energy source, powered indirectly by the sun with the process of convection, and the turning of the earth, through the Coriolis force. Wind power currently generates about 1% of world electricity requirements, though the cumulative output is growing exponentially, suggesting in 20 years that an 8-fold increase will occur.
Another kind of kinetic energy that can be harvested is moving water. This is commonly called hydroelectric power. I'm sure you have noticed that water runs downhill. Here gravity itself is harvested because of the inexorable tendency of water to find a common level. Water, usually stored in a reservoir (fed by rain or snow melt) is stopped up at a dam. Some of the water is allowed to flow into a river, and in between the reservoir and the river is a turbine that runs a generator. About 1.5% of world electricity requirements are generated using hydroelectric power.
1700 times. The increase in pressure means steam can be used to drive a turbine to generate energy, in a technology known as steam turbines. This process is used in aircraft carriers to great effect: it can drive a steam catapult to propel planes from its deck, or it can be used to drive the main screws to propel the carrier through the water. This process is also extremely well-suited to energy generation via electromagnetic induction. About 90% of all electrical power in the US is generated using steam turbines both in conventional coal, natural gas, fossil-fuel, and also through nuclear power plants.
A third method of energy extraction is via the photoelectric effect. This is the method that solar cells employ to harvest energy directly from sunlight. The efficiency of this technique is determined by how many photons are required to generate a single electron. Modern solar power plants, however, usually employ a different method to harvest energy. The technique is known as solar collection. In this technique, mirrors and lenses concentrate the sunlight from a large area to a small area. Once in a small area, the sunlight can be used to heat water in a closed steam system, as in solar towers. This steam is then used to drive a steam turbine, which drives a generator, which generates electricity. Usually the mirror is shaped like a parabolic trough, and instead of heating water directly, it heats molten salt and then the molten salt is used as a heat source for the power generation system.
Solar power is considered to be renewable since it harvests energy directly from the sun, the effectively continuous free energy source.
The largest solar power station in existence, in the Mojave desert of California, generates 354 MW of power. In comparison, the Three Gorges Dam hydroelectric power station located on China's Yangtze river, generates 18.5 GW of power, and, when finished, is intended to generate a total of 22.5 GW of power. As of yet, solar power hasn't yet reached a generating level of even 0.1% of the world's electricity requirements.
A fourth method of energy extraction comes from the energy contained in matter itself. Nuclear power currently works by exploiting the chain-reaction properties of U-235, an isotope of Uranium. In a nuclear power plant, the runaway chain reaction is usually moderated by water and other slow-neutron absorbing substances, like graphite. Nuclear reactors generate heat in abundance. This heat is used to heat a second, insulated water cycle and generate steam, which then drives a turbine and a generator to make electricity. But it is also possible to use molten sodium, an excellent neutron absorber, to transfer the heat of the nuclear reaction, in a so-called liquid metal reactor.
Uranium is about 40 times more commonly occurring naturally than silver, and so it is hard to prevent technologically advanced nations from acquiring it. For instance, in Israel, the sands of the Negev desert contain trace amounts of Uranium. The separation of U-238, the isotope of Uranium that makes up 99.3% of naturally-occurring Uranium, from U-235 (that makes up the rest) is complicated.
|The Diablo Canyon nuclear power plant|
Source: PG&E via Power Plants Around the World
Nuclear power presently generates considerably more energy than solar power. For instance, the Diablo canyon nuclear power plant in California generates 2.4 GW (7.5x the largest solar power plant) and makes up about 20% of the Northern California power grid.
However, nuclear power has some serious drawbacks. The disposal and storage of radioactive waste (particularly spent nuclear fuel rods) presents problems that, while they can be solved, are nonetheless controversial. The Östhammar facility in Sweden shows promise for treating this problem with the proper respect and care required for 100,000-year storage systems. The site was chosen partly because the rock at the waste-storage level is relatively free of fractures. After the rock is excavated, two tons of spent fuel is stored in 25-ton copper canisters. Each copper canister is then welded shut using a special robotic welder and robotically deposited in an individual tunnel in the repository. Then, bentonite clay is injected into the tunnel, mixed with water, to expand into place. This forms a watertight barrier that is essentially earthquake-proof. The storage repository is scheduled to open in 2025.
Portable Energy Storage
Once energy is harvested and converted to electricity, it may be stored for later use and carried around. You can consider fuel to be a portable energy storage system also, although usually fuel needs to be combusted and this makes it normally unsuitable for battery usage. But even this axiom is being challenged by the fuel cell.
Portable energy may be stored in several ways. The first is a battery, which generates electricity through electrochemistry. The second is a capacitor, which stores energy in its electric field. The third is a fuel cell, which, similar to a battery, uses an electrochemical reaction to generate electricity.
We grade portable energy storage systems on:
- capacity, which is the amount of electric charge they can store
- charge time, the amount of time required to return the device to full or substantial charge
- discharge rate, the maximum amount of constant current the device can produce
- energy density, a measure of how much energy the device will produce by weight
Each characteristic is useful for different uses. For instance, in an electric car, the energy required to start the engine and move the car from a standing start is related to the discharge rate. Also, a device must be light in relation to the amount of energy it contains, and thus energy density must be high for an electric car battery.
On Hydrogen Fuel Cells as an Energy Source
The energy density of hydrogen gas is the highest of all chemical sources, in excess of 120 kiloJoules per gram. The energy density of a lithium-ion battery, in contrast, is only about 0.7 kiloJoules per gram. Of course, nuclear material such as U-238 has an energy density of 20 gigaJoules per gram. Antimatter contains a theoretical maximum of 180 teraJoules per gram. Presently, nuclear material and antimatter are unsuitable for portable energy storage systems due to the weight of a nuclear reactor and the general unavailability of antimatter.
All above considerations point to hydrogen fuel cells as the most likely successor for portable energy systems. Per weight, hydrogen fuel cells are about three times the energy density of gasoline. The volume of hydrogen, even stored as a compressed gas, far outstrips that of gasoline for comparable amounts of energy generation. This makes hydrogen use a bulky problem.
Also, most all the world's hydrogen production emits CO2, since it uses the steam methane reforming process. So some improvement is needed to cut down on its carbon footprint.
The characteristics of a capacitor are short charge time and fast discharge rate, really the opposite of a typical battery. This is why several companies are trying to merge the two technologies to get the best of both.
Supercapacitors are a new technology which promises to replace the battery as we know it. One valuable attribute of supercapacitors is the apparent ability to charge and discharge thousands of times, making the device stable enough to outlive the device it is intended to power. The main problem with super- and ultra capacitors (battery-capacitor hybrids) is the energy density. The capacitance of these devices is directly proportional to the electrode surface area. The use of materials like activated charcoal (with its unbelievably large surface area) have increased the energy density into the usable domain. The promise of nanotechnology, such as nanotube carbon filaments, also can lead to high surface-area solutions and still greater energy density.
How Energy is Consumed
The amount of electricity used per year by humanity is in excess of 20 petaWatt hours per year. The US uses about 20% of that, and China uses another 20%. This doesn't include the energy produced by internal combustion engines, or by burning coal, wood, and kerosene for heating. When all energy consumption is added up, total annual consumption is 474 exaJoules.
It is interesting that world energy consumption decreased 1.1% in 2009, due mainly to economic downturn in North America. But that trend doesn't seem to be a continuing story, since in 2010 world energy consumption grew about 5%. In particular, China's energy consumption did not decrease in 2009, and consequently it is now the world's largest energy consumer, at about 18% of global energy consumption.
In part 2, we will drill down farther into how energy is consumed, and discuss what we can do to cut down on energy consumption, and what is already happening in that regard. We will also discuss the thorny issues surrounding fossil fuel usage. Also, the carbon footprint of energy production and consumption will be discussed. Which energy sources have the smallest carbon footprint? It's not obvious at all.