Indiana is the second biggest coal-consuming state in the US. It is also among America's top 10 producers of the fossil fuel.
So why has it just decided to ditch it?
The simple answer is cost. Having mined coal since 1830s, Indiana is abandoning the black stuff in favour of solar and wind because, says its major utility, the move will save consumers billions of dollars.
Indiana is not alone. Other US states, and other countries, are turning away from fossil fuels as the cost of producing renewable electricity continues to fall.
That’s encouraging news as the world battles to cut planet-warming carbon emissions.
However, cheap and clean energy is of no use unless it’s delivered to where it’s needed, day and night without fail.
This is where the power grid comes into play. Power grids match the amount of electricity being generated with the load, or the amount being consumed.
But today’s grid infrastructure, already under stress from extreme weather events such as hurricanes or wildfires, is ill-equipped to deal with renewables. The intermittent nature of solar or wind means these power sources can't be distributed easily.
This is why the world needs to develop a new transmission infrastructure for the 21st century.
Breaking the grid-lock
The current grid is based on a one-way system in which electricity flows from power plants to homes and businesses.
It relies on alternating current (AC) for both long distance transmission and local distribution.
AC has changed little since its development in the late 19th century, when it emerged victorious from the War of the Currents to become the standard for electricity grids worldwide. Using a transformer, AC can be easily converted to different voltages.
But its major shortcoming is that it loses power in transit. For a given voltage, an AC system has roughly twice the loss of a DC system, which can transmit energy more cheaply and efficiently over very long distances.
AC’s constraints become more obvious when it comes to distributing renewable energy. That's because solar, wind and hydroelectric power is usually generated far away from where the energy is used.
Consequently, AC grids’ renewable capacity is limited to just 15 per cent of their total power mix.
Raising the percentage could destabilise the grid and lead to regular blackouts.
This is where DC could make a difference.
In its modern form, ultra high-voltage direct current (UHVDC), is even more powerful. It can use voltages as high as 1,100kV, compared with the 1.5kV of traditional systems.
A UHVDC-based macro grid lets operators tap into different clean energy sources hundreds or thousands of miles away, day and night. It also allows utilities to switch between power sources depending on demand and the weather.
What's more, the grid can link unconnected AC transmission systems across different areas (see chart).
Almost a decade after becoming the first country to embrace the technology, China debuted the world’s longest and most-powerful UHVDC line in early 2019.
The new link stretches over 2,000 miles. It delivers 66 billion kilowatt/hour of electricity from the country’s far northwest – home to abundant solar and wind power – to the heavily populated east. The grid can meet the energy demands of some 50 million households.
The US, the world’s biggest energy consumer, has also become a DC convert.
The USD3 billion TransWest Express Project aims to install a UHVDC transmission line to bring wind power from Wyoming 730 miles to California - the US state with among the most ambitious carbon-reduction targets.
According to the US's National Renewable Energy Laboratory, the line will save USD1 billion per year for Californian consumers.
Research by the Earth System Research Laboratory found that such power grids, which make better use of wind power, could cut carbon emissions by as much as 80 per cent compared with 1990 levels.
Super and hyper grids
UHVDC also makes it easier to distribute power beyond borders. Renewable energy generated in sunny or windy regions can more easily be transported to countries where the sun doesn't shine and the wind doesn't blow.
That's why the technology is also central to Asia's proposed “Super Grid”. The project aims to link power transmission networks stretching over six countries -- China, Japan, South Korea, Russia and Mongolia.
The Super Grid should reduce the region's dependence on nuclear energy – whose use has become controversial since the 2011 earthquake and tsunami in Japan - while also helping it become energy self-sufficient.
Even more ambitious is China’s USD50 trillion, 30-year Global Energy Interconnection (GEI) initiative, championed by President Xi Jinping under his One Belt One Road programme.
GEI aims to use UHVDC as a backbone to connect existing and fragmented national gridlines in Asia, Europe and Africa by 2050 into a single super grid spanning 126,000 kilometres.
Such plans are ambitious, and will take time to bear fruit. But they could prove pivotal in hastening the world's transition to a carbon-free future.
