By Alec Stewart, Vaughn Tooze and David Wills

Recently there has been a lot of focus on food; what has been left on our supermarket shelves and fear of running out due to panic buying. But, virus aside, is there an answer to producing field grown fruit and veg 365 days of the year regardless of seasons and location? Could this help reduce waste, reduce field and transportation emissions and produce a better-quality product with a longer shelf life?

Vertical farming answers all these questions and allows the growing of plants indoors, under fully controlled conditions, in multiple stacked layers and without solar light. It’s been estimated that the market size of this industry was worth $2.23 billion in 2018 and predicted to grow to over $12 billion by 2026. It could also answer food shortages in some of the most densely populated areas of the world.

It has been quoted by one leaf vegetable supplier, that one acre of vertical farming could yield the equivalent crops of 200 acres of conventional farming. It appears that there are many advantages to this and being able to feed more people with less space seems a no brainer. But how does it work in reality?

Unlike large scale single layer ‘glasshouse’ production, which relies on both sunlight for light and solar gain and natural ventilation, vertical farming with stacked layers makes use of inter-level LED lighting combined with mechanical ventilation/climate control to achieve the optimum growing conditions 24-7-365.

The practice is better suited to some crops more than others and uses either hydroponics, where the crop roots are submerged in water and nutrients, aeroponics, where the roots are exposed and sprayed with a mist containing water and nutrients, or natural/synthetic ‘soil’ trays which are automatically watered with nutrients.

We need to consider how plants grow with school-level biology. Recall the simple experiment with the Petri dish; the cotton wool ball soaked in water and the cress seeds? The school experiment generally involved placing one dish in the dark and one in the sunlight and comparing the rate of growth and the impact of photosynthesis on the plant and phototropism, the plant growth towards the light. Glucose made in photosynthesis is then moved to cells in the phloem vessels for respiration. It is the transpiration rate (the movement of water through the plant) that is critical when designing ventilation and cooling solutions for vertical farms. This should mimic natural day and night conditions, matched to the plants natural evolved growth cycles.

With a basic understanding of plant biology and a realisation of the importance of transpiration for growth and crop yield, it is clear humidity also plays a major part in plant growth. Plants breathe through tiny openings on the underside of their leaves called stomata. Plants open and close their stomata under certain conditions. For example, if heat becomes excessive and causes a plant to start losing more water than it can take-up through its roots, the plant will close its stomata to slow down the water loss. Unfortunately, by closing the stomata and slowing evaporation the plant has also slowed down its cooling mechanism, causing the plant to ‘cook’ itself. As a plant transpires, the surrounding air can become saturated by humidity, if not adequately engineered through HVAC solutions. .When the relative humidity of the space is too high or there is a lack of air circulation, the plant cannot make the water evaporate or draw nutrients from the hydroponic trays, should this occur for a prolonged period the plant will rot.

Then there is artificial lighting. Lighting design is based on the latest LED technology with output wavelengths tuned to provide optimum growing conditions for the plants. The advances in lighting technology are increasing luminaire efficacies, however lighting loads of approximately 55W/m2 are still seen in such installations. This represents a significant electrical load, and of course, a heat load to be removed from the space (particularly when LED drivers are also within the conditioned space).

Did we mention air velocity? Air direction and velocity can have a positive influence on crop growth. Air velocity can increase the supply of carbon dioxide to the plants resulting in greater photosynthesis rates. However, if the air velocity is too great there can be a mechanical impact on the plants which could negatively affect them as they at different stages in their growth cycle.

There is no doubt this approach requires the best scientific and engineerable ‘know-how’, so what could be the catch?

Within vertical farms there will be many millions of plants and seedlings all at various stages in their growth cycle, all densely packed in to growing trays, potentially stacked ten or more trays vertically. The environment must remain within a relatively narrow temperature and humidity range for the plants to grow at their optimum and the air speed equally has to be controlled. At the same time, the ventilation systems must absorb and remove a potentially enormous volume of moisture from the space so as not to impede the transpiration process through the plants. All of this takes a vast amount of energy to control.

Whilst we can provide low energy designs to our customers which include; variable air volume solutions, full fresh air and recirculation, desiccant dryers, glycol dosed chillers to super-cool air to remove moisture, considered air-flow and conducting computational fluid dynamics (CFD) studies to demonstrate the optimum solutions, we have to ask ourselves; is this enough?

What suggestions can we make to the industry to help bring the energy consumption of vertical farming down? We think optimised geographic location should be considered. There are several industries outside of farming that are producing waste energy, but if an indoor vertical farm was nearby, then this energy could be harvested and used, rather than wasted. Renewable or waste energy will almost certainly be at the forefront of the facilities in coming years.

You would never have imagined that growing something as simple as a leafy salad or vegetable indoors could be so challenging!

We would like to hear your thoughts on this, please leave a comment.

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Alec Stewart, Building Services Engineering, Electrical Engineering, Vaughn Tooze

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