Thunderstruck: The AC/DC Problem at the Heart of Renewable Energy

In the 1880s, two men argued about the best way to deliver electricity to people's homes. Edison wanted direct current. Tesla and Westinghouse wanted alternating current. It was, by all accounts, a magnificent row involving public demonstrations, deliberate elephant electrocutions, and the kind of professional bitterness that only engineers who are absolutely certain they are right can sustain over a decade.

AC won. We built a world around it. And then, roughly a century later, we started bolting wind turbines and solar panels to that world and assumed the matter was settled.

It isn't. It never was. We just stopped paying attention.

 

What a Wind Turbine Actually Makes

Yes, wind turbines generate AC. But not your friendly, well-behaved 50 Hz AC that your kettle expects. Wind turbines produce variable-frequency AC, and the frequency depends directly on how fast the blades are spinning, which depends on the wind.

When the wind gusts, the blades spin faster, the generator runs faster, and the frequency climbs. When the wind drops, it falls again. A large modern turbine might produce AC anywhere between roughly 10 and 50 Hz depending on conditions. Your kettle does not find this acceptable. Your grid, rather understandably, finds it even less acceptable.

Modern turbines solve this with power electronics. The variable-frequency AC from the generator is rectified to DC inside (or very close to) the turbine, and then inverted back to a proper 50 Hz AC for connection to the collection network that links all the turbines together before export to the grid.

Read that again. The turbine already converts to DC internally. And then, immediately, converts back to AC to travel through the cables to shore.

This is the moment where Angus Young would nod sagely, if Angus Young were an electrical engineer, which seems unlikely but would make for an extremely entertaining industry conference.

 

The Solar Irony Is Even Better

Solar panels produce DC. Not variable, not inconvenient, just plain direct current: exactly what every battery on the planet wants to receive. You would think, then, that connecting a solar farm to a battery storage system would be a reasonably elegant affair.

You would be wrong.

The standard approach, for most of the industry's history, has worked like this. The DC from the panels goes through an inverter to become AC. That AC travels through the collection network and through a transformer. At the far end, it is converted back to DC to charge the batteries. Three conversions. Panel to inverter (DC to AC). AC through cables and transformers. Inverter to battery (AC to DC). Every conversion loses energy.

 

 

The Collection Network: Where the Losses Hide

For offshore wind farms, the internal collection network, the cables connecting each turbine to the next and eventually to the offshore substation, is identified in the engineering literature as the single largest optimisable source of loss in the whole system. Not the export cable to shore. Not the onshore transformer. The internal wiring.

This is, when you think about it, a remarkable finding. We have spent enormous engineering effort optimising turbine blade aerodynamics, transformer efficiency, and export cable routing, while the biggest recoverable win was sitting in the collection network the whole time, quietly leaking energy into the surrounding seawater.

A DC collection network for offshore wind would eliminate the need for individual per-turbine AC-DC-AC conversion cycles, reduce resistive cable losses, and allow direct connection to DC battery storage. Research into this architecture is active. Deployment at scale, however, is still limited. Largely because the existing grid expects AC at the point of connection, and redesigning for storage-first rather than export-first requires rethinking what a wind farm is actually for.

 

The Question Nobody Is Asking

The energy storage debate is, right now, flourishing with impressive enthusiasm. Iron-air batteries, sand batteries, pumped hydro, compressed-air caverns, flywheel systems, gravity trains: we have no shortage of ideas about which vessel to use. What we are not asking, with anything like equivalent enthusiasm, is what happens to the electrons between leaving the turbine or panel and arriving at that vessel.

I have argued for some time that countries do not have the transmission infrastructure to move electricity on demand at the scale renewables require. The grid was built to push power from large central generators to dispersed consumers, and it does that reasonably well. What it does poorly is absorb large quantities of variable-source power and redistribute it dynamically in response to demand. What it essentially cannot do is replace itself fast enough to keep pace with how quickly the generation mix is changing.

The answer to this is local storage. Generate where the wind blows or the sun shines, store it there, distribute from storage rather than from the turbine. This is not a radical position; it is rapidly becoming engineering consensus.

But local storage only works if the path from generation to storage is efficient. If you are losing 9% of your generated power in the conversion chain between the panel and the battery, adding a better battery does not solve your problem. You have a leaky pipe. The vessel is not the issue.

 

The Fix Exists

DC-coupled solar-plus-storage systems are commercially available today and represent the correct architecture for any new-build installation where storage is part of the design. DC collection networks for offshore wind are the subject of active research and several demonstrator projects. The technology is not speculative. It is a matter of deployment choices and, frankly, of an industry asking itself a question it has been too busy arguing about batteries to notice.

The War of Currents ended, officially, in 1892 when Edison's company merged with Thomson-Houston to form General Electric and quietly began producing AC equipment. Tesla's AC had won, because the problem at the time was moving power over long distances to many dispersed consumers, and AC, with its ability to step voltage up and down through transformers, was the right tool for exactly that problem.

But the world AC was designed for is not the world we are building now.

We are building a world of distributed generation, local storage, and short-haul electrons. In that world, the question of AC or DC does not carry the same answer it had in 1892. The physics has not changed. The problem has. A transformer is a beautiful piece of engineering for moving power over hundreds of kilometres to a factory. It is considerably less beautiful as the third unnecessary conversion stage between a solar panel and the battery sitting twenty metres away.

 

 

The turbines are spinning. The electrons are there. The storage technologies are ready. The gap between them, that unglamorous first mile of cable, conversion stage, and architecture decision, is where we should be looking.

You're thunderstruck.

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