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Clean energy, all of the time? – why 24/7 carbon free energy is easier said than done

What does it mean for a company to commit to source 100% renewable electricity? How does an electricity grid get to net zero emissions? Given that wind and solar energy are intermittent, the answer to date has been something of a fudge.

Typically, the company can either buy a renewable tariff and let the supplier source green certificates – Renewable Energy Guarantees of Origin (REGOs) in the UK – to demonstrate that it has bought an equivalent volume of renewable power to meet the buyer’s consumption. Or the company can buy power and green certificates directly from a generator under a power purchase agreement (PPA).

Green certificates provide evidence that a certain volume of renewable electricity has been fed into the grid. They can be unbundled and traded separately to the underlying electrons. This allows buyers to demonstrate their ‘ownership’ of an amount of green power over an annual period, without needing to somehow track that power through the grid.

However, questions are increasingly being asked as to whether this passes muster. If a buyer is using green certificates from solar farms, how can these be applied to the power it’s consuming on a winter’s night? What about when the wind farm that is generating REGOs is becalmed?

Essentially, these buyers are still using power generated from fossil fuels, merely offsetting those emissions at some other time by buying green certificates from clean power that is being fed into the grid. Can such “100% clean powered” corporate claims stand up to scrutiny?

Companies seeking to make copper-bottomed green power commitments are therefore seeking 24/7 carbon-free energy (CFE) matching. This involves matching, on an hourly or half-hourly basis, green power as generated with the buyer’s consumption profile.

Both Microsoft and Google, for example, have committed to reach 100% 24/7 CFE by 2030. Meanwhile, Sustainable Energy for All and UN Energy have launched the 24/7 Carbon-free Energy Compact. More than 140 companies and government bodies have signed up, committing to moving, over time, towards sourcing clean energy 24/7.

However, reaching 100% CFE, 24 hours a day, seven days a week, is much harder than it sounds.

The problem with intermittent generation

According to our modelling, if a buyer with a flat baseload consumption profile enters into a PPA with various renewable energy technologies, it may only match its consumption with green power:

  • 28% of the time from a typical solar PV farm in England
  • 62% of the time from a typical onshore UK wind farm
  • 68% of the time from a best-in-class UK offshore wind farm.

The rest of the time, the buyer (via its supply arrangements) will be buying power from the grid, along with the associated emissions. Whilst Google has touted an achievement of 64% 24/7 CFE in 2022 (a target that would be quite easy to hit in the UK using offshore wind), reaching 100% CFE becomes an increasingly difficult challenge.

Although CFE matching can be slightly improved by contracting with a portfolio of different technologies, until a feasible storage solution can be found, getting to very high levels of CFE is very hard.

This is because renewable energy generation across different sites, but within the same geographic region, remains highly correlated. Solar farms along the same longitude all produce their power at the same time, with available sunlight varying dramatically from winter to summer. While wind speeds do vary, UK-wide weather systems can often becalm wind farms across the whole country.

This problem of temporal correlation can be seen in the impact of the massive build-out of wind capacity in the UK over the past six years. In January 2017, wind supplied 32.3% of UK power generation during the windiest half-hour period, contrasting to just 1.6% in the stillest half-hour, with a median contribution of 10.8%.

Five years on, in November 2022 the maximum contribution had doubled to 65.1% of UK power demand in a half-hour period, with the median tripling to 35.6%. Crucially, however, during the stillest period, the contribution of wind power was even lower than in January 2017, at just 1.4% of UK generation.

Wind share UK generation chart

Building more and more renewable energy capacity with correlated generation cannot solve the problem caused by periods of calm weather or cloudy skies. This means that the carbon emissions intensity of the UK’s overall grid is likely to stay stubbornly high – it fell rapidly from 505g CO2/kWh in 2012 to 168g CO2/kWh in 2020, but has yet to average below 150g CO2/kWh since.

This all means that companies seeking 24/7 CFE have a problem. So too do governments (or opposition parties) promising to completely decarbonise electricity supply.

How to solve 24/7 CFE

The good news is that there are a number of solutions that, technically, could solve the problem of getting to 100% CFE. The bad news is that, to date, none have been deployed anywhere near the scale required, and doing so is likely to be extremely expensive.

The solutions are:

Long-duration energy storage

Linking renewable energy to long-duration storage will enable excess renewable supply to better match demand. In practice, over the medium-term, this remains both unfeasibly expensive, and overly complex for corporates to contract. While battery technology is evolving rapidly, it is unlikely to be able to meet the needs of companies with 24/7 CFE commitments in the immediate future. Similarly, a range of long-term storage technologies are being developed – ranging from pumped hydro to compressed air storage – but few are currently viable at scale.


Use of hydrogen for energy generation

Some advocates of hydrogen suggest that it could be produced using excess renewable electricity, stored and then used to power gas turbines. There are several reasons why this is a bad idea, including the low round-trip efficiency of the process (at just 40%) and the greater value-add of using hydrogen to decarbonise hard-to-abate sectors such as steel, petrochemicals and heavy transport.


Carbon capture and storage

Most scenarios for reaching net zero anticipate the large scale deployment of carbon capture and storage (CCS) systems to tackle emissions that are otherwise impossible to avoid. But – despite being touted for decades by the fossil fuel sector as a key tool for solving climate change – barely any progress has been made deploying CCS at scale. Also, for a company claiming to source 100% renewables, relying on fossil fuel energy, even with the emissions captured, would stretch its environmental credibility.

The fact is, as companies debate whether green certificates (or REGOs) are an effective offset to their carbon emissions, reaching 24/7 CFE will mean chasing limited volumes of matching green power, until the solutions above reach scale and economic viability.