Mind your language when you’re talking about renewables

Presenting complex ideas in simple terms can be counterproductive when comparing different ways of keeping the lights on.

Complicated subjects need to be simplified so that the public can understand them, and energy policy is probably the most complicated area of public policy currently needing urgent attention. Unfortunately, the simplification has gone too far. Politicians, academics and policymakers don’t understand the complexities but are confident that they do.

Consider the language used in both the national and, often, the technical press. Referring to a new ‘solar farm’, this is a typical example: “The plant will have a peak output of 48MW and will provide enough energy to power 11,000 homes.”

To an engineer, this is nonsense. We need to take it apart piece by piece. First, the umbrella term solar covers several methods of extracting electrical and thermal energy from the Sun. What is referred to here is photovoltaic electrical generation. The 48MW is the declared maximum capacity based on laboratory figures and is unlikely to ever be achieved. The orientation and inclination will need to coincide with the position of the Sun in a clear sky. This will not coincide with the January evening maximum demand of the ‘homes’ cited.

Using energy and power as the noun and verb for some vague electrical capacity gives us no clue as to what the 11,000 homes represents. It is not an SI unit. Is it the maximum demand of a home multiplied by 11,000, or the maximum demand of 11,000 homes? These are very different figures and the first is unlikely as it is so pessimistically high. The second is not linear as the demand of 11,000 homes is not 11 times that of 1,000. Neither are useful means of comparison.

It is more likely that the measure of a home is energy use, measured in kWh/year. The number of homes is drawn from the annual energy output of the PV facility – but this energy output is rarely stated, only the maximum power capacity.

There are ways of assessing these mysterious measures. At the time of writing, the trade body RenewableUK relates UK wind generation of 34,777,422MWh/ year to a ‘homes powered equivalent’ of 8,451,378. This equates to about 4,115kWh per home. Industry regulator Ofgem defines ‘typical domestic consumption’ as 3,100kWh/year.

Either way, whether maximum demand in kW or energy consumption in kWh/year is the unit  defining a ‘home’, it seriously misrepresents the usefulness of a generator whose output is greatest in the middle of a June day and zero on a winter evening.

Professor David MacKay, who sadly died earlier this year, was among the most prominent scientists using their public voice to expose the lazy use of rhetoric. He pointed out, among many other anomalies, that a home does not actually represent much in the way of national power consumption as it ignores domestic heating, power used in industry, at work, for leisure, shopping and travelling between them. How many ‘homes’ equal one air-conditioned supermarket?

He would have been amused by the claim made earlier this year by renewable energy investment company Low Carbon that in the past 12 months it had “added 12 solar parks to its portfolio amounting to 157.7MW and saving in excess of 67,600 tonnes of CO2, the equivalent of 21,400 fewer cars on the road. The solar parks generate enough energy to power up to 47,478 homes.”

In studying various claims, MacKay judged that the current conversion rate was 1.5 tonnes of CO2 per car taken off the road. Here the rate is over three tonnes.

On 4 July each year, the solar industry celebrates Solar Independence Day to illustrate how PV is progressing in its takeover of fossil-fuel generation. The over-​simplification continues as Good Energy claimed last year that, “...today over 15 per cent of the UK’s electricity is produced by solar.” And the Solar Trade Association this year: “The solar industry estimates the country now has almost 12GW of solar panels, on homes, offices, warehouses, schools and other buildings and in solar farms – enough to power the equivalent of 3.8 million homes.”

The belief that energy and power are the noun and verb for the same thing is sincerely held among those responsible for energy policy, but as engineers, and as institutions, we have a duty to challenge and correct sloppy use of made-up units. If this makes it too difficult to understand, it is still preferable to a confident evangelical belief that more PV and wind means less fossil fuel. That is too dangerous to persist. *

Dr Steve Parlour IEng MIET is an electrical engineer and independent researcher in energy policy


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