Positive energy - renewable sources / Matthew L. Wald

Developing alternatives to fossil fuels is an essential need in the fight against climate change and in securing energy sources. Here is a snapshot

by Matthew L. Wald

Illustrations: Don Foley

Renewable energy sources, such as ethanol or electricity from photovoltaic cells, currently provide less than 7% of the energy consumption of the United States. And if the hydroelectric power plants are subtracted from this, it is less than 4.5%. Renewable energy sources provide only about 3.5% of global electricity consumption, and even less when it comes to transportation fuel.

However, even though increasing the share of alternative sources in the US is necessary to control greenhouse gas emissions and reduce the export deficit and dependence on external energy suppliers, there are three problems. First, the question arises of how to utilize the energy of the sun, wind and agricultural crops in an economically profitable manner. Second, the energy must be transported from where it is easy to collect, such as the sunny desert or the windswept mountains, to places where it can be used. And finally, it should be converted into convenient forms. This is especially noticeable with electricity for driving vehicles: it must be charged on cars, buses and trucks, using batteries or perhaps as hydrogen.

In several respects, the field is developing very quickly. In a recent study funded by the United Nations, it was discovered that 148.4 billion dollars were invested in renewable energy worldwide in 2007, a 60% increase from 2006. However, the wind turbines and solar cells join the infrastructure that already includes coal-fired power plants that not only operate more hours every year than before, but also multiply.

Moreover, even though in recent years there has been a steep drop in the price of solar energy and especially in the price of wind energy, they are only profitable thanks to government subsidies or benefits. An average residential customer in the US pays about 11 cents per kilowatt hour (kWh) for electricity that comes from a combination of coal-fired power plants, gas-fired plants, nuclear energy and hydroelectric energy. But the renewable sources are much more expensive. All methods of energy production are of course handled by the "carrot and stick" method by the governments of the world - either to provide employment for the coal miners or to prove that atomic fission is not only intended for the production of bombs. But in many places the renewable energy sources have a prominent advantage: quotas. The demand for them is also increased by the increase in the prices of regular fuel, because with the increase in fuel prices, the profitability of alternative solutions also increases.

A carbon tax can also help. A tax of $10 on the emission of one ton of carbon dioxide will indeed increase the price of electricity from a coal-fired power plant by one cent per kWh, but this can bring about a huge change, because in terms of energy content, the scale of electricity production from coal is 70 times greater than electricity production from wind. The same is roughly true for oil and natural gas as well.

Here is an overview of the possible components of an energy production system in which renewable sources have a large share, and how all the methods fit together.

Thermo-solar energy

To produce thermosolar energy, the sun's rays are focused using a trough-shaped concave mirror, which follows the sun throughout the day, and heats an oil- or water-based liquid flowing through a black tube. The pipe winds for miles to a heat exchanger that creates steam to drive a turbine. The system can be built next to a natural gas powered power plant, so that the gas can be used to generate steam on cloudy days or at night. In the future, the liquid will be replaced with a liquid mixture of sodium salts that will allow work at higher temperatures without an increase in pressure.

A similar method uses a solar tower surrounded by a huge array of mirrors that can reach a radius of a kilometer. The mirrors heat a liquid, water or sodium salts, located at the top of the tower. The hot sodium salts can be stored in an insulated container that retains enough heat to run the turbine around the clock, or at least during peak consumption hours.

Condition: Concave mirror arrays are in commercial use. The efficiency of solar towers has been demonstrated.

Price: 19.9-28.1 cents per kWh (concave mirrors)

Advantages: Probably the most convenient solution for storing renewable energy

Disadvantages: requires a flat area, the energy source can be located far from existing infrastructure, causes damage to pristine desert environments, may need a water cooling system - a rare resource in the sunny desert

Ocean wave energy

Due to the environmental concerns caused by dams, it is impossible to continue developing hydroelectric power sources. But data from the US Department of Energy show that the northwest coast of the United States can generate 40 to 70 kilowatts per meter. However, the technology required to harness the energy of the ocean to produce electricity lags far behind the methods for utilizing geothermal energy, solar energy and wind energy, and it has been two hundred years since inventors have been trying to register a successful patent for utilizing wave energy.

One of the techniques is to build an air cell from steel or concrete. The cell is open to the ocean water at the bottom, but closed at the top. The rising and falling waves alternately raise and lower the air pressure in the upper part of the cell and thus it is possible to operate a turbine. The Wavegen company in Scotland, partially owned by the electricity giant Siemens, recently inaugurated a 100 kilowatt power plant based on this method. Another method utilizes the energy of rising and falling floats.

Status: The technology has been demonstrated, but is not yet ready for full production

Price: early to determine

Advantages: there is no need to transport electricity over long distances

Disadvantages: the high cost of buildings that will be resistant to strong waves

 

wind energy

The wind is the most promising and advanced renewable energy source, but perhaps also the most problematic. In 2007, more than 5,000 megawatts of wind turbines were installed across the US, an increase of 46%. But the energy contribution in terms of kWh is much smaller because even at a carefully chosen site, the wind, which does not blow all the time, produces only about 28% of the energy that could be produced if it blew around the clock. Also, the best wind blows at night, when consumption is low.

As technology develops, costs decrease and disruptions increase. The state-of-the-art wind turbines produce six megawatts, enough power to power several shopping centers at the same time. In such a large turbine, the blades reach a length of 65 meters, roughly the same as the wingspan of a Boeing 747. The new models are extremely efficient, and use about half of the wind energy blowing through them.

Condition: in commercial use and steep growth

Price: 6.1-8.4 cents per kWh (but the need for transmission may inflate the price)

Advantages: The largest energy production potential in the USA. No need for cooling water

Disadvantages: Production hours do not coincide with peak hours. The appearance of the turbines and power poles, as well as the noise they create, arouse objections among various parties. Danger to birds and bats. May cause interference with the aerial radar. The construction sites are far from settlements.

 

Geothermal energy

Unlike solar energy or solar energy, geothermal energy can be produced as needed. "The heat of the earth is always there. You can count on it," says Stephen Chu, director of the US Lawrence Berkeley National Laboratory, and the new Secretary of Energy in the Obama administration. Geothermal power plants usually operate around the clock. Not everywhere you can find an underground heat source, but in Hawaii a quarter of the electricity consumption is produced by this method, and in California, 6%. Many geothermal power plants utilize hot water that erupts from the ground itself, but according to supporters of the technology, large areas in the US contain "dry, hot rock" that can be utilized simply by injecting water into a deep well. In most methods, a heat exchanger is used to generate steam from clean water, to turn a turbine.

Condition: in limited commercial use

Price: 6.2-7.6 cents per kWh

Advantages: The supply is reliable enough to supply the basic demand

Disadvantages: Steam from an underground source may contain harmful components, which may damage the heat exchangers, and pollute the air in cases of leakage. The location is unpredictable and may be far from existing high voltage lines.

 

Solar energy: photovoltaic cells

 

In a photovoltaic cell, there are two layers, one on top of the other, of semi-conducting materials: one has an excess of electrons and the other has an excess of "holes". When the sunlight hits them, the excess electrons pass from one layer to another, thus creating an electric current. This phenomenon was discovered already 169 years ago, but scientists and engineers are still trying to improve the technology. The first cells were activated in the space program, and they are widely used in places where the electricity grid does not reach. And yet from an economic point of view they cannot compete with the use of mineral fuel, or even with other renewable sources that supply electricity to the grid. Photovoltaic cells can be added to new buildings, as roof tiles or as material for facade coating at lower costs.

 

Status: In commercial use, but not competitive in the network, except in cases where there are large quotas or subsidies

Price: 46.9-70.5 cents per kWh

Advantages: Cells can be deployed in urban areas where electricity consumption is high to save the cost of installing new supply lines. Peak hours of production correspond well with peak hours of consumption. No need for cooling water

Cons: Frankly, it's too expensive. The production volume is very small

 

Storage and transportation of energy from renewable sources

Inconsistent energy sources, such as solar and wind, require an energy storage and transport system. There are several candidates for the position.

Car manufacturers aim to develop a lithium-ion battery that will last 15 years and 5,000 charge cycles, much longer than the life of the lithium-ion batteries we are used to today in consumer products. The goal is to reach a price of 300 dollars per kWh in available storage, in a battery that can drive a car for a distance of 65 kilometers, assuming that the consumption will be about five kilometers per kWh. General Motors plans to launch a plug-in hybrid car in 2010. Ford wants to do this in five years.

fuel cells

 

Electricity from any source - sunlight, wind and even coal, can be used to break down water molecules into their components, hydrogen and oxygen, in a process of electrolysis. Then the hydrogen can be used to generate electricity using a fuel cell. However, the notable disadvantage of fuel cells is that each kilowatt costs thousands of dollars, and the efficiency of using the electric current to break down the water in electrolysis and get it back from the fuel cell is less than 50%, meaning that for every two kWh we invest in it, we only get one back.

 

stationary batteries

The VRB Power Systems company from Vancouver, Canada, sells "flow accumulators" that include electrolyte storage containers with a capacity of hundreds of liters. When operating the system in one direction, it absorbs energy on the order of megawatts per hour. When it is activated in the other direction, it emits it. The cost of storing a kWh ranges from 500 to 600 dollars, and the efficiency of the reversible process is 65% to 75%, so the battery loses 25% to 35% of the electricity stored in it. This system will raise the price of the kWh from solar sources by 50% or more.

 

compressed air

The Alabama State Energy Cooperative opened a compressed air energy storage plant in 1991. The facility uses the excess electricity generated by coal-fired power plants at night to compress air into a hollow salt dome at a pressure of more than 70 atmospheres. When electricity consumption increases during the day, the compressed air is transferred to a combustion turbine fed with natural gas. The turbine usually has to compress the air it uses, and even the most efficient generator on the market today uses more than 6,000 kilojoules extracted from natural gas to produce one kWh. Compressed air storage, on the other hand, reduces gas use by a third.

Conductivity

Distributed power sources can be less problematic if they feed into a larger power grid. An area where there are 100 solar and wind facilities producing electricity separately can always enjoy some kind of average production, but the electricity grid that exists in the US today is not able to handle large-scale electricity transmission over huge distances. A possible solution, proposed by the US Department of Energy in 2008, is the construction of a "backbone" of high voltage lines, similar to the interstate highway network. This new network of high voltage lines will stretch over 30,000 km, using electricity poles that are 40 meters high, and cost 1.6 million dollars per km. The grid voltage will be extremely high, 765,000 volts, to reduce transmission losses. Although this is not a new technology, the system needs two things that the US is unable to offer at the moment: a national commitment to unify the electricity grid in all of North America, and about 60 billion dollars to finance the project.

Transportation fuel from renewable sources

 

There are three main ways to produce liquid fuel for transportation from renewable sources. The first is the use of vegetable oil, usually soybean oil or palm oil, in diesel engines. According to US law, the oil must be converted into an ester-type chemical compound in order to use it. The conversion process is simple, but it can only be done on a limited scale, and the whole matter is controversial, because growing plants for fuel comes at the expense of growing food.

It is also possible to turn sugar into alcohol with the help of yeast, but although this process is also simple, it can only be done on a limited scale. This is also where the competition arises between the gas station and the supermarket for field products.

However, it must be remembered that huge amounts of sugar are found in cellulose (cellulose) of plants, or in inedible parts of grain, such as straw and corn stalks. This cellulose contains normal sugar, which has six carbon atoms, but also sugars with five carbon atoms, which yeast have difficulty digesting.

Several experimental plants are now engaged in trying to decompose these sugars, with the help of acid, steam, or both. Another possibility is the use of enzymes from genetically engineered bacteria or fungi. There are methods that turn the sugars into liquid fuel with the help of catalysts, and others use yeast for this purpose, usually after genetic engineering. Some break down the cellulose molecules into gaseous fuel consisting of carbon monoxide and hydrogen, which can be turned into hydrocarbons or other solvents and liquids such as ethanol. The raw material for these reactions can come from pruning, tree bark or pine cones, paper and plastic, household waste and agricultural waste.

Although all of these methods have been shown to be applicable at the laboratory level or on an experimental scale, we have not yet reached commercial production capacity. However, thanks to government encouragement and quotas, many proposals in the field are starting to emerge.

Status: Stubborn struggle to achieve commercial production

Price: unknown. It is difficult to set a target price, because gas and diesel prices are so fluid

Advantages: the total emissions from several biofuels are equal to zero. Reduces the need to import fuel

Disadvantages: Some biofuels cause food prices to rise. To produce fuel from corn, huge amounts of mineral fuel must be burned, so the energy advantage is small. Most biofuels contain less chemical energy, so they are less economical in terms of fuel consumption.

Ice storage

The Ice Energy company from California sells equipment that produces blocks of ice at night in a volume of about 2,000 liters in the basement of the building. It is easier to make ice at night because, in most cases, the outside air temperature, where the compressor releases the heat, is lower at night. The ice that forms at night is used to cool the building during the day. This is how energy produced at night, such as wind energy, is used to perform work required during the day.

 

Matthew L. Wald has been a New York Times energy reporter since 1979. He has written articles on oil refining, power generation, electric and hybrid cars, and air pollution. Vald currently lives in Washington, D.C., and is engaged in research on traffic safety and other issues. This is his fourth article in Scientific American.

And more on the subject

The US Department of Energy website: A scenario for the supply of 20% of US energy needs by wind energy by 2030:www1.eere.energy.gov/windandhydro

A hybrid power plant planned to be built in California, which will generate solar energy when the sun shines and energy from gas at other times: www.inlandenergy.com

Information regarding energy storage using compressed air:

www.solarfeeds.com/index.php?option=com_contentview=articleid=3256:compressedair-energy-storage-further-alongcatid=80:80Itemid=173

Facts about cellulosic ethanol on the website of the association for fuel from renewable sources:

www.ethanolrfa.org/resource/cellulosic

 

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