Most species of algae contain oil that can be refined to make motor fuel. For ten years or more, entrepreneurs have been looking at ways of growing algae quickly, harvesting the product and then crushing it to create ‘green crude’ oil. What is probably the first commercial scale algae production plant has just opened in New Mexico. Does it look as though algae-to-fuel will be commercially viable? It is certainly a vastly better biofuel than corn ethanol but doesn’t yet appear to compete with solar PV as a source of low-carbon motor fuel. (I’m classing electricity as a fuel for cars). Using photosynthesis, almost all algae capture CO2 from the air as their source of growth. They produce oil and when this oil is burnt, the CO2 returns to the air. Motor fuel made from algae can thus claim to be close to carbon neutral. Most algae grow best in warmth and strong sunlight, meaning that the product is potentially well suited to sunny deserts where the land has few alternative uses.

The Sapphire Energy bio-refinery in Columbus, New Mexico is an extraordinary new venture that demonstrates the viability of commercial algae cultivation. Make no mistake, this seems to be a huge technical success. Huge ponds 200 metres long grow different species of algae depending on the time of year. The refinery extends over 120 hectares (300 acres). All the processing is done on site. It doesn’t use potable water.

By most metrics, Sapphire’s plant isn’t yet competitive with solar PV. And since most places (hot deserts) that are suited to algae are also suited to PV, the long-term future of algae refineries isn't immediately clear. Nevertheless, my quick calculations suggest that the plant produces about five watts of power for each square metre of space. (The numbers to support this assertion are appended at the bottom – if you find a mistake, please tell me). By contrast, a big solar farm in sunny New Mexico may achieve 15 watts/sq metre, about three times greater.

(Is there a logic to this difference? Yes, PV cells turn about 15% of the energy falling on them into electricity, although not all space is used because of the gaps between rows in solar farms. Photosynthesis is much less efficient, averaging less than 2% in most circumstances. So PV will always tend to be more efficient in terms of energy conversion per unit area.)

Although algae cannot easily compete with PV at generating power, they are far better than corn ethanol, a petrol substitute made from corn cobs. My calculations suggest that land growing corn/maize produces about 0.2 watts per square metre, less than a twentieth of the figure for algae. So the mad and immoral policy of mandating that almost half the US corn crop is converted into motor fuel is clearly an extremely inefficient way of generating biofuels. Algae production is a much, much better tool for the decarbonisation of oil.

So does it matter that PV is better at converting light into fuel than algae? In the US, with its huge resources of unused desert, probably not. Sapphire has produced some estimates of what its new bio-refinery will produce and my quick calculations suggest that the entire crude oil need of the country could be grown on about 3% of the area of the continental US. This is a huge expanse of land, but slightly less than the approximately 3.5% given over to growing corn.

But more important than the land taken up by algae is the capital and operating costs of a biorefinery. The company’s press releases suggest that the cost of constructing the algae farm exceeded £135m. Dividing this by the energy value of the output of oil suggests a capital cost of about $2.40 for every yearly kilowatt hour. In the same sun-drenched location, PV would cost around a third as much. Of course, the cost of the refinery reflects that it is an ambitious prototype. Perhaps the cost will halve by the time the tenth bio-refinery is constructed? But it will still be more expensive than PV today.

The much higher operating costs of the algae farm also weight the economics in favour of PV, which needs no permanent workforce once it is constructed. But all these disadvantages are comprehensively outweighed, you might say, by the fact that Sapphire Energy delivers liquid energy, able to be poured into cars and planes as a direct replacement for refined oils. Algae may well turn out to be the most efficient way of generating low-carbon liquid fuels outside those areas lucky enough to be able to grow sugar cane. It is difficult to see any other way of replacing aviation fuel at reasonable cost to people and to planet.

Nevertheless, the better overall performance of PV should cause us to hesitate before backing algae for petrol replacement for land-based vehicles. And there’s one crushing final argument in favour of using electricity for powering cars. The energy coming from a PV panel flows into the grid and is extracted to charge a battery in an electric car. The car then uses this electricity with about 80% efficiency (the ratio of useful power delivered to the wheels as a percentage of the energy value of the electricity taken from the grid). By contrast, even efficient modern internal combustion engines only deliver about 25% conversion. Not only are algae plants probably more costly, far more space-using and have higher operating costs, they also produce a fuel with a third the value of electricity when converted to the energy of motion. Fantastic achievements Sapphire Energy, but we still should be pushing for electric cars.

Energy performances per square metre – back of the envelope numbers drawn from Sapphire Energy’s press releases

Expected eventual oil output per day – 100 barrels

Energy value barrel of oil - 1,600 kWh

Daily energy value of algae oil – 160,000 kWh or 160 MWh

Yearly energy ouput – 58,400 MWh

Space used – 120 hectares

Energy output per hectare – about 487 MWh

Square metres per hectare – 10,000

Annual output per sq metre – about 48.7 kWh

Hourly output per sq metre – about 5.5 watts compared with substantially less than 1 for other biofuels.