Pie in the sky hopes


The idea that vertical farming in urban tower blocks is an answer to the challenge of feeding the world fails on many practical fronts.


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A recent documentary showcased urban agriculture, hailing it as the solution to feeding Earth’s ever-growing population. The featured urban farmer, whose fish tanks provided fertiliser for a greenhouse bursting with lush vegetables, argued that the potential savings in food miles spelled the end of large rural farms.

Elsewhere a vast New York rooftop served as market garden to appreciative local residents. And a Columbia University professor joyfully opined that one day every city would include 30-storey city buildings devoted to plants rather than people.

But this incurable engineer was flummoxed. Not a word was uttered about production costs, sourcing the electricity for artificial sunlight on 30 ceilings, or life-cycle carbon dioxide emissions. It was a classic case of “never letting the facts get in the way of a good story”.

Take costs. Traditionally farming land is inherited, leaving labour, water and fertiliser as the biggest outlays. Not so for urban farms. In my hometown of Melbourne Australia, it costs about $1,500 per square metre to erect a multistorey building shell (and that’s without lights, plumbing or fittings).

That equates to $15 million per hectare, outrageously high compared to about $5,000 a hectare for farmland 100 km from the city centre. The urban farmer would have to pay rent in excess of $1 million per year per hectare. Even the most intensive greenhouse farming with world’s best yields would not produce enough revenue to justify that.

Then there is the wanton waste of solar energy. In open fields, the light for photosynthesis comes directly from the sun. In a 30-storey building, apart from the rooftop, blue and red LED ceiling lights would have to irradiate plants with their favourite wavelengths. Let’s suppose the electricity for the lights comes from solar panels. Solar panels are about 20% efficient, while the best commercial LED lights are about 15% efficient. The combined efficiency is only 3%.

Although sunlight is boundless and free, converting it into artificial light is not.

What that means is that to take sunlight, convert it into electricity, then convert it into artificial sunlight, you need to cover 33 hectares of rural land with solar panels for every hectare of indoor growing space, in a process that squanders a massive 97% of the starting sunlight. Clearly sunlight can be used to much greater advantage in the field than inside an urban vertical farm.

Although sunlight is boundless and free, converting it into artificial light is not. Solar panels, the land they occupy, the transmission systems and the artificial lighting are expensive. Farmers have long understood the inefficiency of artificial lighting because it impacts their bottom line. Apart from extremely high value crops such as marijuana, indoor crops do not cover their electricity costs.

Some vertical farming enthusiasts have suggested using skylights and light tubes to pipe the sunlight down from the rooftop to lower levels. Sigh. The reality is that plants on a rooftop need all the sunlight they can get. If you tried to split the sunlight across 30 interior levels, the crop yield on each level would be woeful.

Last, let us address food miles – the distance food is transported from production to consumer. When it comes to reducing carbon dioxide emissions, food miles are only one component of the total life-cycle emissions. When you take into consideration the emissions associated with constructing the building, pumping potable water to the upper floors, transporting fertilisers to each level, removing waste and finally taking the food to market by elevator and small trucks, it is not at all certain that urban agriculture adds up to an ecological advantage.

Growing backyard vegetables is a rewarding experience for many homeowners, but urban agriculture is unlikely to feed the planet. I pin my hopes on rural farms, high technology and economies of scale.

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