Dalla produzione di energia fino alla presa di casa la strada è lunga e piena di amare sorprese.
«Power plants – coal, natural gas, petroleum or nuclear – work on the same general principle. Energy-dense stuff is burned to release heat, which boils water into steam, which spins a turbine, which generates electricity. The thermodynamic limits of this process (“Damn that rising entropy!”) mean only two-thirds of the energy in the raw materials actually make it onto the grid in the form of electricity.»
«Energy lost in power plants: About 65%, or 22 quadrillion Btus in the U.S. in 2013»
«Step 2: Moving Electricity – Transmission and Distribution»
«First, electricity travels on long-distance, high-voltage transmission lines, often miles and miles across country. The voltage in these lines can be hundreds of thousands of volts. You don’t want to mess with these lines.»
«Why so much voltage? To answer this question, we need to review some high school physics, namely Ohm’s law. Ohm’s law describes how the amount of power in electricity and its characteristics – voltage, current and resistance – are related. It boils down to this: Losses scale with the square of a wire’s current. That square factor means a tiny jump in current can cause a big bump in losses. Keeping voltage high lets us keep current, and losses, low»
«High-voltage transmission lines are big, tall, expensive, and potentially dangerous so we only use them when electricity needs to travel long distances. At substations near your neighborhood, electricity is stepped down onto smaller, lower-voltage power lines – the kind on wooden poles.»
«transformers (the can-shaped things sitting on those poles) step the voltage down even more, to 120 volts, to make it safe to enter your house»
«Generally, smaller power lines mean bigger relative losses»
«Energy lost in transmission and distribution: About 6% – 2% in transmission and 4% in distribution – or 69 trillion Btus in the U.S. in 2013»
Attenzione! Questi sono i dati della linea ideale!
«Fun fact: Transmission and distribution losses tend to be lower in rural states like Wyoming and North Dakota. Why? Less densely populated states have more high-voltage, low-loss transmission lines and fewer lower-voltage, high-loss distribution lines. …. Transmission and distribution losses vary country to country as well. Some countries, like India, have losses pushing 30 percent. Often, this is due to electricity thieves»
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La dissipazione di energia lungo gli elettrodotti dipende dalla resistenza elettrica del conduttore (il cavo), dal quadrato della corrente elettrica trasportata, e dalla lunghezza del percorso. Più lungo il percorso, maggiore la dissipazione.
Noti i dati dei territori da servire ed avendo un dettagliato elenco delle richieste e della loro ubicazione, è possibile calcolare numero, sede e potenza delle centrali elettriche da costruire, in modo da ottimizzare i costi delle centrali con quelli indotti dalla dissipazione lungo gli elettrodotti.
Ma per costruire una centrale elettrica servono un elevato numero di permessi, tutti di natura politica. Ben si comprende come la rete di trasmissione non sarà mai quella tecnicamente più economica.
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Con la introduzione delle energie così dette rinnovabili, fotovoltaico ed eolico, il problema si complica a dismisura.
Non si è infatti “liberi” di poter costruire una centrale elettrica dove si voglia: occorre piazzare gli impianti ove la luce sia ottimale oppure i venti siano persistenti e non troppo veloci. In altri termini, le sedi di produzione sono obbligate.
Questo comporta necessariamente linee di trasmissione anche molto lunghe: in taluni casi si parla di migliaia di kilometri. La dissipazione in questi casi aumenta a dismisura.
Si pensi soltanto al trasferimento di energia dal Mecklenburg-Vorpommern al Bayern: sono circa 700 kilometri.
La dissipazione è enorme: oltre il cinquanta per cento.
Se adesso si facesse il conto esatto del rapporto benefici / costi tenendo conto anche delle dissipazioni sulla base di quanto corrente arriva alla fine alla spina di casa, i bilanci sarebbe invero poco entusiasmanti.
Ma un conto è parlare di economica ed un altro di politica.
Se la politica fosse logica non sarebbe politica e diverrebbe inutile.
Transmitting power over thousands of kilometres requires a new electricity infrastructure.
THE winds of the Oklahoma panhandle have a bad reputation. In the 1930s they whipped its over-tilled topsoil up into the billowing black blizzards of the Dust Bowl. The winds drove people, Steinbeck’s dispossessed, away from their livelihoods and west, to California.
Today, the panhandle’s steady winds are a force for creation, not destruction. Wind turbines can generate electricity from them at rock-bottom prices. Unfortunately, the local electrical grid does not serve enough people to match this potential supply. The towns and cities which could use it are far away.
So Oklahoma’s wind electricity is to be exported. Later this year, lawsuits permitting, work will begin on a special cable, 1,100km (700 miles) long, between the panhandle and the western tip of Tennessee. There, it will connect with the Tennessee Valley Authority and its 9m electricity customers. The Plains and Eastern Line, as it is to be known, will carry 4,000MW. That is almost enough electricity to power Greater London. It will do so using direct current (DC), rather than the alternating current (AC) that electricity grids usually employ. And it will run at a higher voltage than such grids use—600,000 volts, rather than 400,000.
This long-distance ultra-high-voltage direct-current (UHVDC) connector will be the first of its kind in America. But the problem it helps with is pressing everywhere. Fossil fuels can be carried to power stations far from mines and wells, if necessary, but where wind, solar and hydroelectric power are generated is not negotiable. And even though fossil fuels can be moved, doing so is not desirable. Coal, in particular, is costly to transport. It is better to burn it at the pithead and transport the electricity thus generated instead.
Transmitting power over thousands of kilometres, though, requires a different sort of technology from the AC now used to transmit it tens or hundreds of kilometres through local grids. And in China, Europe and Brazil, as well as in Oklahoma, a new kind of electrical infrastructure is being built to do this. Some refer to the results as DC “supergrids”.
AC’s ubiquity dates from the so-called “war of the currents” that accompanied electrification in the 1880s and 1890s. When electricity flows down a line as AC, energy travels as a wave. When it flows as direct current, there is no oscillation. Both work well, but the deciding factor in AC’s favour in the 19th century was the transformer. This allows AC voltages to be increased after generation, for more efficient transmission over longish distances, and then decreased again at the other end of the line, to supply customers’ homes and businesses. At the time, direct current had had no such breakthrough.
When one eventually came, in the 1920s, in the form of the mercury arc valve, AC was entrenched. Even the solid-state thyristor, a cousin of the transistor invented in the 1950s, offered no great advantages over the tens or hundreds of kilometres that power grids tended to span. Some high-voltage DC lines were built, such as that under the English Channel, linking Britain and France. But these were justified by special circumstances. In the case of the Channel link, for example, running an AC line through water creates electromagnetic interactions that dissipate a lot of power.
Over transcontinental distances the balance of advantage shifts. As voltages go up, to push the current farther, AC employs (and thus wastes) an ever-increasing amount of energy in the task of squeezing its alternations through the line. Direct current does not have this problem. Long-distance DC electrical lines are also cheaper to build. In particular, the footprint of their pylons is smaller, because each DC cable can carry far more power than an equivalent AC cable. Admittedly, thyristors are expensive—the thyristor-packed converter stations that raise and lower the voltage of the Plains and Eastern line will cost about $1bn, which is two-fifths of the project’s total bill. But the ultra-high voltages required for transcontinental transmission are still best achieved with direct current.
For all the excitement surrounding the Plains and Eastern Line, however, America is a Johnny-come-lately to the world of UHVDC. Asian countries are way ahead—China in particular. As the map on the previous page shows, the construction of UHVDC lines is booming there. That boom is driven by geography. Three-quarters of China’s coal is in the far north and north-west of the country. Four-fifths of its hydroelectric power is in the south-west. Most of the country’s people, though, are in the east, 2,000km or more from these sources of energy.
China’s use of UHVDC began in 2010, with the completion of an 800,000-volt line from Xiangjiaba dam, in Yunnan province, to Shanghai. This has a capacity of 6,400MW (equivalent to the average power consumption of Romania). The Jinping-Sunan line, completed in 2013, carries 7,200MW from hydroelectric plants on the Yalong river in Sichuan province to Jiangsu province on the coast. The largest connector under construction, the Changji-Guquan link, will carry 12,000MW (half the average power use of Spain) over 3,400km, from the coal- and wind-rich region of Xinjiang, in the far north-west, to Anhui province in the east. This journey is so long that it requires 1.1m volts to push the current to its destination.
China’s UHVDC boom has been so successful that State Grid, the country’s monopolistic electricity utility, which is behind it, has started building elsewhere. In 2015 State Grid won a contract to build a 2,500km line in Brazil, from the Belo Monte hydropower plant on the Xingu River, a tributary of the Amazon, to Rio de Janeiro.
China’s neighbour India is following suit—though its lines are being built by European and American companies, namely ABB, Siemens and General Electric. The 1,700km North-East Agra link carries hydroelectric power from Assam to Uttar Pradesh, one of the country’s most densely populated areas. When finished, and operating at peak capacity, it will transmit 6,000MW. At existing levels of demand, that is enough for 90m Indians. The country’s other line, also 6,000MW, carries electricity 1,400km from coal-fired power stations near Champa, in Chhattisgarh, to Kurukshetra, in Haryana, passing Delhi on the way.
Valuable though they are, transcontinental links like those in China, Brazil and India are not the only use for UHVDC. Electricity is not described as a “current” for nothing. It does behave quite a lot like a fluid—including fanning out through multiple channels if given the chance. This tendency to fan out is another reason it is hard to corral power over long distances through AC grids—for, being grids, they are made of multiple, interconnected lines. Despite UHVDC connectors being referred to as supergrids, they are rarely actual networks. Rather, they tend to be point-to-point links, from which fanning out is impossible. Some utilities are therefore looking at them to move power over relatively short distances, as well as longer ones.
One such is 50Hertz, which operates the grid in north-east Germany. Almost half the power it ships comes from renewable sources, particularly wind. The firm would like to send much of this to Germany’s populous south, and on into Austria, but any extra power it puts into its own grid ends up spreading into the neighbouring Polish and Czech grids—to the annoyance of everyone.
50Hertz is getting around this with a new UHVDC line, commissioned in partnership with Germany’s other grid operators. This line, SuedOstLink, will plug into the Meitingen substation in Bavaria, replacing the power from decommissioned south-German nuclear plants. And Boris Schucht, 50Hertz’s boss, has bigger plans than that. He says that within ten years UHVDC will stretch from the north of Sweden down to Bavaria. After this, he foresees the development of a true UHVDC grid in Europe—one in which the lines actually interconnect with each other.
That will require new technology—special circuit-breakers to isolate faulty cables, and new switch gear—to manage flows of current that are not simply running from A to B. But, if it can be achieved, it would make the use of renewable-energy sources much easier. When the wind blows strongly in Germany, but there is little demand for the electricity thus produced (at night, for instance), UHVDC lines could send it to Scandinavian hydroelectric plants, to pump water uphill above the turbines. That will store the electricity as potential energy, ready to be released when needed. Just as sources of renewable energy are often inconveniently located, so, too are the best energy-storage facilities. UHVDC permits generators and stores to be wired together, creating a network of renewable resources and hydroelectric “batteries”.
In Asia, something similar may emerge on a grander scale. State Grid plans to have 23 point-to-point UHVDC links operating by 2030. But it wants to go bigger. In March 2016 it signed a memorandum of understanding with a Russian firm, Rosseti, a Japanese one, SoftBank, and a Korean one, KEPCO, agreeing to the long-term development of an Asian supergrid designed to move electricity from windswept Siberia to the megalopolis of Seoul.
This project is reminiscent of a failed European one, Desertec, that had similar goals. But Desertec started from the top down, with the grand vision of exporting the Sahara’s near-limitless solar-power supply to Europe. Today’s ideas for Asian and European supergrids are driven by the real needs of grid operators.
Such projects—which are transnational as well as transcontinental—carry risks beyond the merely technological. To outsource a significant proportion of your electricity generation to a neighbour is to invest huge trust in that neighbour’s political stability and good faith. The lack of such trust was, indeed, one reason Desertec failed. But if trust can be established, the benefits would be great. Earth’s wind-blasted and sun-scorched deserts can, if suitably wired up, provide humanity with a lot of clean, cheap power. The technology to do so is there. Whether the political will exists is the question.
How much energy is lost along the way as electricity travels from a power plant to the plug in your home? This question comes from Jim Barlow, a Wyoming architect, through our IE Questions project.
To find the answer, we need to break it out step by step: first turning raw materials into electricity, next moving that electricity to your neighborhood, and finally sending that electricity through the walls of your home to your outlet.
Step 1: Making Electricity
Power plants – coal, natural gas, petroleum or nuclear – work on the same general principle. Energy-dense stuff is burned to release heat, which boils water into steam, which spins a turbine, which generates electricity. The thermodynamic limits of this process (“Damn that rising entropy!”) mean only two-thirds of the energy in the raw materials actually make it onto the grid in the form of electricity.
Energy lost in power plants: About 65%, or 22 quadrillion Btus in the U.S. in 2013
Step 2: Moving Electricity – Transmission and Distribution
Most of us don’t live right next to a power plant. So we somehow have to get electricity to our homes. This sounds like a job for powerlines.
First, electricity travels on long-distance, high-voltage transmission lines, often miles and miles across country. The voltage in these lines can be hundreds of thousands of volts. You don’t want to mess with these lines.
Why so much voltage? To answer this question, we need to review some high school physics, namely Ohm’s law. Ohm’s law describes how the amount of power in electricity and its characteristics – voltage, current and resistance – are related. It boils down to this: Losses scale with the square of a wire’s current. That square factor means a tiny jump in current can cause a big bump in losses. Keeping voltage high lets us keep current, and losses, low. (For history nerds: This is why AC won the battle of the currents. Thanks, George Westinghouse.)
When that electricity is lost, where does it go? Heat. Electrons moving back and forth crash into each other, and those collisions warm up power lines and the air around them.
You can actually hear those losses: That crackling sound when you stand under a transmission tower is lost electricity. You can see the losses, too: Notice how power lines sag in the middle? Some of that’s gravity. But the rest are electrical losses. Heat, like the kind from lost electricity, makes metal power lines expand. When they do, they sag. Powerlines are saggier, and leakier, on hot days.
High-voltage transmission lines are big, tall, expensive, and potentially dangerous so we only use them when electricity needs to travel long distances. At substations near your neighborhood, electricity is stepped down onto smaller, lower-voltage power lines – the kind on wooden poles. Now we’re talking tens of thousands of volts. Next, transformers (the can-shaped things sitting on those poles) step the voltage down even more, to 120 volts, to make it safe to enter your house.
Generally, smaller power lines mean bigger relative losses. So even though electricity may travel much farther on high-voltage transmission lines – dozens or hundreds of miles – losses are low, around two percent. And though your electricity may travel a few miles or less on low-voltage distribution lines, losses are high, around four percent.
Energy lost in transmission and distribution: About 6% – 2% in transmission and 4% in distribution – or 69 trillion Btus in the U.S. in 2013
Transmission and distribution losses vary country to country as well. Some countries, like India, have losses pushing 30 percent. Often, this is due to electricity thieves.
Step 3: Using Electricity Inside Your Home
Utility companies meticulously measure losses from the power plant to your meter. They have to, because every bit they lose eats into their bottom line. But once you’ve purchased electricity and it enters your home, we lose track of the losses.
Your house, and the wires inside your walls, are kind of a black box, and figuring how much electricity gets lost – electricity that you’ve already paid for – is tricky. If you want to find out how much electricity gets lost in your home you’ll either need to estimate it using a circuit diagram of your house or measure it by putting meters on all of your appliances. Are you an energy wonk attempting this? Let us know, we’d love to hear from you!
Energy lost in the wiring inside your walls: We don’t know! It could be negligible, or it could be another few percent.
The Future Of Transmission and Distribution Losses
Grid engineers are working on technologies like superconducting materials that could essentially reduce electricity transmission and distribution losses to zero. But for now, the cost of these technologies is much higher than the money lost by utility companies through their existing hot, leaky power lines.
A more economical solution to reduce transmission and distribution losses is to change how and when we use power. Losses aren’t a constant quantity. They change every instant based on things like the weather and power consumption. When demand is high, like when we’re all running our ACs on hot summer days, losses are higher. When demand is low, like in the middle of the night, losses are lower. Utilities are experimentingwith ways to spread out electricity use more evenly to minimize losses.
The same principle applies to your house, which is basically your own personal grid. You can reduce losses in your home by spreading out your electricity use evenly throughout the day, instead of running all your appliances at once.
Adding Up The Losses
– Generating electricity, we lost 22 quadrillion Btu from coal, natural gas, nuclear and petroleum power plants in 2013 in the U.S. – that’s more than the energy in all the gasoline we use in a given year.
– Moving electricity from plants to homes and businesses on the transmission and distribution grid, we lost 69 trillion Btu in 2013 – that’s about how much energy Americans use drying our clothes every year.