There is increasing talk of greenhouses being stacked on top of each other in the future in order to produce vegetables directly in the city. However, hardly anyone does the math to see if this is even possible or sensible.
You wouldn't need to build separate high-rise buildings for mushrooms, asparagus or chicory, because they also grow in dark cellars, which are available anyway. The top floor is of course great for growing plants. But you wouldn't need to erect a tower just for that - the existing roofs would suffice. This is about the idea of growing plants on several floors on top of each other on an industrial scale. I don't include green facades in this. Neither does another form of vertical farming, where plants that don't need much light are grown on top of each other on just one floor, with conveyor belts bringing each plant into the sun for an equal amount of time. This already works in Singapore ("Sky Greens" project). When the sky is overcast or in winter, there is not enough light for all the plants. This concept would therefore hardly be feasible in this form in Germany.
Artificial light
Visionary Dickson Despommier wants to make agriculture completely independent of sunlight. Artificial light is what makes the high-rise greenhouses he has in mind possible. Mirrors or the like would increase the amount of space required to the size of ordinary greenhouses. You can't increase the amount of sunlight, no matter how cleverly constructed, and plants need a lot of it. No lettuce grows at home at the kitchen table. In addition, the great technical effort involved speaks against gigantic mirror systems. Artificial lighting, on the other hand, requires an enormous amount of energy, as I calculate below, so I don't understand the benefits of greenhouses.
The calorific value of the finished food provides a lower limit for the amount of energy required. Wheat grains, for example, have around 14,000 kilojoules per kilogram. This energy must be supplied to the wheat plant via the lamps if it is to grow without sunlight. Ideally, this would be 3.9 kilowatt hours, which would cost 78 cents per kilogram. In practice, the plant only stores around 1 percent of the energy in the usable parts. The wheat from the "incubator" would therefore cost €78/kg - just for the exposure! Even if you could produce the electricity yourself at a tenth of the end customer price (2 cents/kWh), this would still be highly uneconomical, because the construction of the building and the highly efficient light sources are not free either. The indoor gardener does not seem to be able to compete with a farmer who uses the sun as a free source of energy.
Well, for energy-rich products, vertical farming with artificial light can be forgotten, but what about lettuce, for example? It is already being grown successfully in England without natural light. However, I don't read anywhere how much energy is needed for this. So I'll do the math again, this time from the other side: sunlight has 1000 watts per square meter, but only the red and green light is used by plants for photosynthesis. Efficient lamps therefore only need to produce these colors and can generate an intensity similar to sunlight with around 100 watts per square meter. Not all plants need this intensity. According to a brochure, lettuce grows at 10 watts per square meter. This is equivalent to an overcast sky, but still ten times more than the typical brightness in (illuminated) indoor spaces. Only at the beginning of its life does lettuce need 25 watts per square meter. After 3-8 weeks it is ready to harvest. With 16 hours of lighting per day, the lighting energy adds up to around 5 kWh/m². This energy can be saved, all other things being equal, in a single-storey greenhouse, as is currently standard.
Ecological benefits
What is the ecological benefit of the additional energy used? Reference is often made to the reduced land consumption of vertical farming. But energy is the biggest environmental problem. It is precisely this problem that we would exacerbate with cultivation in dark storeys and, in return, achieve relatively insignificant savings in other environmental factors (if agricultural land were actually returned to nature in return, which I doubt). Reduced transportation is not an argument for high-rise glass buildings, at least not for cities like Vienna, where greenhouses can also be located at ground level on the outskirts of the city. In general, transportation only accounts for around a quarter of the carbon footprint of fresh vegetables(see the example of broccoli). Despite transportation, tomatoes from Spain cause lessCO2 emissions than those from a domestic greenhouse if it has to be heated. Regionality is only a 3rd choice. The decisive factors are Type of production (preferably organic) and seasonal cultivation.
There is (fortunately) still no vertical farming with artificial light on a large scale. The fact that this concept would be uneconomical can be seen even without calculation simply from the fact that artificial lighting is not even used in Austrian greenhouses to extend the season for vegetables. Even more so, cultivation will not be worthwhile if almost all the light has to come from the power line.
Nevertheless, there are already empirical values that drastically demonstrate the extent of the energy required: For illegal drug production, hemp is grown with artificial lighting, isolated from the outside world. This requires an estimated 300-900kWh of electricity until a plant is ready for harvesting. At a price of €10 per gram of cannabis, it pays off. Ecologically, however, it is a disaster. It is estimated that 1 percent of all electricity consumption in the USA is due to clandestine hemp plantations. 1 percent! Just for a few hemp flowers! It's easy to imagine how bad it would look if all our vegetables came from an illuminated retort ...
Am I not enthusiastic enough or are greenhouses with LED lamps instead of sunlight really a crazy idea? Even if (in the distant future) the costs were to fall to 0, everyone would still set up decentralized growing machines at home. Why build your own towers?
About the author Mario SedlakBorn in Vienna in 1975, graduated in technical mathematics from the Vienna University of Technology in 2000, technical expert in the electricity industry since 2008.
