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This is perhaps the first—and certainly the most celebrated—of all flow charts, created by Charles Joseph Minard to depict the dwindling size of Napoleon's Grande Armée as it retreated from Moscow. Engineers now favor the design for the depiction of energy budgets.

Perhaps the most celebrated graphic image of all time was published in 1869 by Charles Joseph Minard, a French civil engineer. He traced the advance of Napoleon's army into Russia and its retreat from 1812 to 1813 by a sequence of thinning bands representing the total number of men. Four hundred twenty-two thousand soldiers crossed eastward into Russia, 100,000 reached Moscow, and 10,000 crossed the Neman River westward to Prussia, at which point the Grand Armée had lost 97.6 percent of its initial force.

A similar graphic technique was employed by a committee of Britain's Institution of Civil Engineers in its 1897–98 report on thermal efficiency in steam engines. The graphic illustrated the operation of the Louisville-Leavitt pumping steam engine by beginning with the combustion of coal on the boiler's grate, producing 193,708 kilojoules (183,600 British thermal units) per minute, continuing with the 149,976 kJ per minute that actually reached the engine, and ending with the effective work (brake horsepower) of 26,788 kJ per minute, for an overall efficiency of only 13.8 percent. Soon this representation became known as a Sankey diagram, after Matthew Henry Phineas Riall Sankey, the honorary chairman of the committee.

One of the most revealing uses of ­Sankey diagrams is to trace national energy flows, starting on the left with all primary energy inputs—all fossil fuels and biofuels, together with electricity generated from hydro, nuclear, wind, solar and geothermal sources—and ending on the right with actual energy services (industrial heat, kinetic and chemical energies, residential and commercial heating and ­air-conditioning, all forms of transportation). A set of these graphs is available for the United States for 1950, 1960, 1970, and then for every year from 1978 through 2019; they can be downloaded from twoLawrence Livermore National Laboratory websites. The latest Sankey diagram, for 2019, shows that the nation's useful energies (energy services) added up to 32.6 percent of the total primary energy input, a considerably poorer performance than in 1950, when the overall mean was 50.8 percent!

This Sankey diagram traces the U.S. energy budget as a system of rivers and tributaries. Note that electricity generation has the most diversified sources, transportation the least, and that for every joule that is used, more than two joules go to waste.

Two realities explain this retrogression. First, transportation has taken a larger share of the energy budget. The average efficiency of car engines has been improving since the mid-1970s, and flying has seen even more impressive efficiency gains per passenger-kilometer. However, rising car ownership, heavier vehicles, much more frequent flying, and longer distances traveled per year per capita explain the sector's higher share of final energy use (37 percent in 2019, 30 percent in 1950) and its slight efficiency drop from 26 percent to 21 percent during the past 70 years.

The second reality is the decline in the average conversion efficiency of residential and commercial energy use, from about 70 percent to 65 percent, as the gains from more efficient heating have been more than erased by the mass adoption of air-conditioning. Electricity for air-conditioning comes mostly from ­fossil-fuel-powered plants with their considerable inherent conversion losses: In 2019, the average efficiency of U.S. ­electricity-generating coal-fired plants was about 32 percent and that of the now-dominant gas-fired stations 44 percent.

The decline of average conversion efficiency has been much more pronounced in the industrial sector, from 70 percent to 49 percent, which is explained largely by the sector's ongoing electrification (which displaced former direct fuel uses) and by the expansion of ­electricity-intensive manufacturing. This is a common paradox that has accompanied improved design and higher efficiency of individual energy converters: Even as their specific performance gets better, the overall performance gets worse. The United States is now wasting significantly more energy than it did a lifetime ago. About two-thirds of the total primary input go directly into heating the universe without performing first any useful work, and only a third provides desired energy services, while in 1950 it was a 50/50 split. Another example of progressing by regressing.

This article appears in the February 2021 print issue as “Energy-Conversion Efficiency Is Falling."

Vaclav Smil writes Numbers Don’t Lie, IEEE Spectrum's column devoted to the quantitative analysis of the material world. Smil does interdisciplinary research focused primarily on energy, technical innovation, environmental and population change, food and nutrition, and on historical aspects of these developments. He has published 40 books and nearly 500 papers on these topics. He is a distinguished professor emeritus at the University of Manitoba and a Fellow of the Royal Society of Canada (Science Academy). In 2010 he was named by Foreign Policy as one of the top 100 global thinkers, in 2013 he was appointed as a Member of the Order of Canada, and in 2015 he received an OPEC Award for research on energy. He has also worked as a consultant for many U.S., EU and international institutions, has been an invited speaker in more than 400 conferences and workshops and has lectured at many universities in North America, Europe, and Asia (particularly in Japan).

Utrecht leads the world in using EVs for grid storage

The Dutch city of Utrecht is embracing vehicle-to-grid technology, an example of which is shown here—an EV connected to a bidirectional charger. The historic Rijn en Zon windmill provides a fitting background for this scene.

Hundreds of charging stations for electric vehicles dot Utrecht’s urban landscape in the Netherlands like little electric mushrooms. Unlike those you may have grown accustomed to seeing, many of these stations don’t just charge electric cars—they can also send power from vehicle batteries to the local utility grid for use by homes and businesses.

Debates over the feasibility and value of such vehicle-to-grid technology go back decades. Those arguments are not yet settled. But big automakers like Volkswagen, Nissan, and Hyundai have moved to produce the kinds of cars that can use such bidirectional chargers—alongside similar vehicle-to-home technology, whereby your car can power your house, say, during a blackout, as promoted by Ford with its new F-150 Lightning. Given the rapid uptake of electric vehicles, many people are thinking hard about how to make the best use of all that rolling battery power.

Utrecht, a largely bicycle-propelled city of 350,000 just south of Amsterdam, has become a proving ground for the bidirectional-charging techniques that have the rapt interest of automakers, engineers, city managers, and power utilities the world over. This initiative is taking place in an environment where everyday citizens want to travel without causing emissions and are increasingly aware of the value of renewables and energy security.

“We wanted to change,” says Eelco Eerenberg, one of Utrecht's deputy mayors and alderman for development, education, and public health. And part of the change involves extending the city’s EV-charging network. “We want to predict where we need to build the next electric charging station.”

So it’s a good moment to consider where vehicle-to-grid concepts first emerged and to see in Utrecht how far they’ve come.

It’s been 25 years since University of Delaware energy and environmental expert Willett Kempton and Green Mountain College energy economist Steve Letendre outlined what they saw as a “dawning interaction between electric-drive vehicles and the electric supply system.” This duo, alongside Timothy Lipman of the University of California, Berkeley, and Alec Brooks of AC Propulsion, laid the foundation for vehicle-to-grid power.

The inverter converts alternating current to direct current when charging the vehicle and back the other way when sending power into the grid. This is good for the grid. It’s yet to be shown clearly why that’s good for the driver.

Their initial idea was that garaged vehicles would have a two-way computer-controlled connection to the electric grid, which could receive power from the vehicle as well as provide power to it. Kempton and Letendre’s 1997 paper in the journal Transportation Research describes how battery power from EVs in people’s homes would feed the grid during a utility emergency or blackout. With on-street chargers, you wouldn’t even need the house.

Bidirectional charging uses an inverter about the size of a breadbasket, located either in a dedicated charging box or onboard the car. The inverter converts alternating current to direct current when charging the vehicle and back the other way when sending power into the grid. This is good for the grid. It’s yet to be shown clearly why that’s good for the driver.

This is a vexing question. Car owners can earn some money by giving a little energy back to the grid at opportune times, or can save on their power bills, or can indirectly subsidize operation of their cars this way. But from the time Kempton and Letendre outlined the concept, potential users also feared losing money, through battery wear and tear. That is, would cycling the battery more than necessary prematurely degrade the very heart of the car? Those lingering questions made it unclear whether vehicle-to-grid technologies would ever catch on.

Market watchers have seen a parade of “just about there” moments for vehicle-to-grid technology. In the United States in 2011, the University of Delaware and the New Jersey–based utility NRG Energy signed a technology-license deal for the first commercial deployment of vehicle-to-grid technology. Their research partnership ran for four years.

In recent years, there’s been an uptick in these pilot projects across Europe and the United States, as well as in China, Japan, and South Korea. In the United Kingdom, experiments are now taking place in suburban homes, using outside wall-mounted chargers metered to give credit to vehicle owners on their utility bills in exchange for uploading battery juice during peak hours. Other trials include commercial auto fleets, a set of utility vans in Copenhagen, two electric school buses in Illinois, and five in New York.

These pilot programs have remained just that, though—pilots. None evolved into a large-scale system. That could change soon. Concerns about battery wear and tear are abating. Last year, Heta Gandhi and Andrew White of the University of Rochestermodeled vehicle-to-grid economics and found battery-degradation costs to be minimal. Gandhi and White also noted that battery capital costs have gone down markedly over time, falling from well over US $1,000 per kilowatt-hour in 2010 to about $140 in 2020.

As vehicle-to-grid technology becomes feasible, Utrecht is one of the first places to fully embrace it.

The key force behind the changes taking place in this windswept Dutch city is not a global market trend or the maturity of the engineering solutions. It’s having motivated people who are also in the right place at the right time.

One is Robin Berg, who started a company called We Drive Solar from his Utrecht home in 2016. It has evolved into a car-sharing fleet operator with 225 electric vehicles of various makes and models—mostly Renault Zoes, but also Tesla Model 3s, Hyundai Konas, and Hyundai Ioniq 5s. Drawing in partners along the way, Berg has plotted ways to bring bidirectional charging to the We Drive Solar fleet. His company now has 27 vehicles with bidirectional capabilities, with another 150 expected to be added in coming months.

In 2019, Willem-Alexander, king of the Netherlands, presided over the installation of a bidirectional charging station in Utrecht. Here the king [middle] is shown with Robin Berg [left], founder of We Drive Solar, and Jerôme Pannaud [right], Renault's general manager for Belgium, the Netherlands, and Luxembourg.Patrick van Katwijk/Getty Images

Amassing that fleet wasn’t easy. We Drive Solar’s two bidirectional Renault Zoes are prototypes, which Berg obtained by partnering with the French automaker. Production Zoes capable of bidirectional charging have yet to come out. Last April, Hyundai delivered 25 bidirectionally capable long-range Ioniq 5s to We Drive Solar. These are production cars with modified software, which Hyundai is making in small numbers. It plans to introduce the technology as standard in an upcoming model.

We Drive Solar’s 1,500 subscribers don’t have to worry about battery wear and tear—that’s the company’s problem, if it is one, and Berg doesn’t think it is. “We never go to the edges of the battery,” he says, meaning that the battery is never put into a charge state high or low enough to shorten its life materially.

We Drive Solar is not a free-flowing, pick-up-by-app-and-drop-where-you-want service. Cars have dedicated parking spots. Subscribers reserve their vehicles, pick them up and drop them off in the same place, and drive them wherever they like. On the day I visited Berg, two of his cars were headed as far as the Swiss Alps, and one was going to Norway. Berg wants his customers to view particular cars (and the associated parking spots) as theirs and to use the same vehicle regularly, gaining a sense of ownership for something they don’t own at all.

That Berg took the plunge into EV ride-sharing and, in particular, into power-networking technology like bidirectional charging, isn’t surprising. In the early 2000s, he started a local service provider called LomboXnet, installing line-of-sight Wi-Fi antennas on a church steeple and on the rooftop of one of the tallest hotels in town. When Internet traffic began to crowd his radio-based network, he rolled out fiber-optic cable.

In 2007, Berg landed a contract to install rooftop solar at a local school, with the idea to set up a microgrid. He now manages 10,000 schoolhouse rooftop panels across the city. A collection of power meters lines his hallway closet, and they monitor solar energy flowing, in part, to his company’s electric-car batteries—hence the company name, We Drive Solar.

Berg did not learn about bidirectional charging through Kempton or any of the other early champions of vehicle-to-grid technology. He heard about it because of the Fukushima nuclear-plant disaster a decade ago. He owned a Nissan Leaf at the time, and he read about how these cars supplied emergency power in the Fukushima region.

“Okay, this is interesting technology,” Berg recalls thinking. “Is there a way to scale it up here?” Nissan agreed to ship him a bidirectional charger, and Berg called Utrecht city planners, saying he wanted to install a cable for it. That led to more contacts, including at the company managing the local low-voltage grid, Stedin. After he installed his charger, Stedin engineers wanted to know why his meter sometimes ran backward. Later, Irene ten Dam at the Utrecht regional development agency got wind of his experiment and was intrigued, becoming an advocate for bidirectional charging.

Berg and the people working for the city who liked what he was doing attracted further partners, including Stedin, software developers, and a charging-station manufacturer. By 2019, Willem-Alexander, king of the Netherlands, was presiding over the installation of a bidirectional charging station in Utrecht. “With both the city and the grid operator, the great thing is, they are always looking for ways to scale up,” Berg says. They don’t just want to do a project and do a report on it, he says. They really want to get to the next step.

Those next steps are taking place at a quickening pace. Utrecht now has 800 bidirectional chargers designed and manufactured by the Dutch engineering firm NieuweWeme. The city will soon need many more.

The number of charging stations in Utrecht has risen sharply over the past decade.

“People are buying more and more electric cars,” says Eerenberg, the alderman. City officials noticed a surge in such purchases in recent years, only to hear complaints from Utrechters that they then had to go through a long application process to have a charger installed where they could use it. Eerenberg, a computer scientist by training, is still working to unwind these knots. He realizes that the city has to go faster if it is to meet the Dutch government’s mandate for all new cars to be zero-emission in eight years.

The amount of energy being used to charge EVs in Utrecht has skyrocketed in recent years.

Although similar mandates to put more zero-emission vehicles on the road in New York and California failed in the past, the pressure for vehicle electrification is higher now. And Utrecht city officials want to get ahead of demand for greener transportation solutions. This is a city that just built a central underground parking garage for 12,500 bicycles and spent years digging up a freeway that ran through the center of town, replacing it with a canal in the name of clean air and healthy urban living.

A driving force in shaping these changes is Matthijs Kok, the city’s energy-transition manager. He took me on a tour—by bicycle, naturally—of Utrecht’s new green infrastructure, pointing to some recent additions, like a stationary battery designed to store solar energy from the many panels slated for installation at a local public housing development.

This map of Utrecht shows the city’s EV-charging infrastructure. Orange dots are the locations of existing charging stations; red dots denote charging stations under development. Green dots are possible sites for future charging stations.

“This is why we all do it,” Kok says, stepping away from his propped-up bike and pointing to a brick shed that houses a 400-kilowatt transformer. These transformers are the final link in the chain that runs from the power-generating plant to high-tension wires to medium-voltage substations to low-voltage transformers to people’s kitchens.

There are thousands of these transformers in a typical city. But if too many electric cars in one area need charging, transformers like this can easily become overloaded. Bidirectional charging promises to ease such problems.

Kok works with others in city government to compile data and create maps, dividing the city into neighborhoods. Each one is annotated with data on population, types of households, vehicles, and other data. Together with a contracted data-science group, and with input from ordinary citizens, they developed a policy-driven algorithm to help pick the best locations for new charging stations. The city also included incentives for deploying bidirectional chargers in its 10-year contracts with vehicle charge-station operators. So, in these chargers went.

Experts expect bidirectional charging to work particularly well for vehicles that are part of a fleet whose movements are predictable. In such cases, an operator can readily program when to charge and discharge a car’s battery.

We Drive Solar earns credit by sending battery power from its fleet to the local grid during times of peak demand and charges the cars’ batteries back up during off-peak hours. If it does that well, drivers don’t lose any range they might need when they pick up their cars. And these daily energy trades help to keep prices down for subscribers.

Encouraging car-sharing schemes like We Drive Solar appeals to Utrecht officials because of the struggle with parking—a chronic ailment common to most growing cities. A huge construction site near the Utrecht city center will soon add 10,000 new apartments. Additional housing is welcome, but 10,000 additional cars would not be. Planners want the ratio to be more like one car for every 10 households—and the amount of dedicated public parking in the new neighborhoods will reflect that goal.

Some of the cars available from We Drive Solar, including these Hyundai Ioniq 5s, are capable of bidirectional charging.We Drive Solar

Projections for the large-scale electrification of transportation in Europe are daunting. According to a Eurelectric/Deloitte report, there could be 50 million to 70 million electric vehicles in Europe by 2030, requiring several million new charging points, bidirectional or otherwise. Power-distribution grids will need hundreds of billions of euros in investment to support these new stations .

The morning before Eerenberg sat down with me at city hall to explain Utrecht’s charge-station planning algorithm, war broke out in Ukraine. Energy prices now strain many households to the breaking point. Gasoline has reached $6 a gallon (if not more) in some places in the United States. In Germany in mid-June, the driver of a modest VW Golf had to pay about €100 (more than $100) to fill the tank. In the U.K., utility bills shot up on average by more than 50 percent on the first of April.

The war upended energy policies across the European continent and around the world, focusing people’s attention on energy independence and security, and reinforcing policies already in motion, such as the creation of emission-free zones in city centers and the replacement of conventional cars with electric ones. How best to bring about the needed changes is often unclear, but modeling can help.

Nico Brinkel, who is working on his doctorate in Wilfried van Sark’s photovoltaics-integration lab at Utrecht University, focuses his models at the local level. In his calculations, he figures that, in and around Utrecht, low-voltage grid reinforcements cost about €17,000 per transformer and about €100,000 per kilometer of replacement cable. “If we are moving to a fully electrical system, if we’re adding a lot of wind energy, a lot of solar, a lot of heat pumps, a lot of electric vehicles…,” his voice trails off. “Our grid was not designed for this.”

But the electrical infrastructure will have to keep up. One of Brinkel’s studies suggests that if a good fraction of the EV chargers are bidirectional, such costs could be spread out in a more manageable way. “Ideally, I think it would be best if all of the new chargers were bidirectional,” he says. “The extra costs are not that high.”

Berg doesn’t need convincing. He has been thinking about what bidirectional charging offers the whole of the Netherlands. He figures that 1.5 million EVs with bidirectional capabilities—in a country of 8 million cars—would balance the national grid. “You could do anything with renewable energy then,” he says.

Seeing that his country is starting with just hundreds of cars capable of bidirectional charging, 1.5 million is a big number. But one day, the Dutch might actually get there.

This article appears in the August 2022 print issue as “A Road Test for Vehicle-to-Grid Tech.”