It's not just space that's one of the primary factors driving the interest but also the advantage of being able to produce fresh produce such as leafy greens year-round. This enables extreme weather events to be avoided – an important element, given the increasing concerns over global warming and the environmental impact of intensive farming. 

Vertical farming is also applauded for using 90% less water – estimates do vary but are at least around that level – than conventional techniques because it can be recycled and recirculated. 

The technology has also been touted as a potential means of cutting retailers' food waste, and, because the growing systems can be located nearer to the consumer, is claimed to potentially reduce the carbon footprint. 

Perhaps more importantly, from a commercial perspective, the technology provides greater yields because the growers can control the nutrient inputs, which it is claimed, in turn, provides fresher and better quality produce, as well as improved nutrition for the consumer. 

However, a major drawback for many vertical farming operators revolves around energy use and cost, because most of the systems employ LED lighting, although tech firms are investing in NPD and more efficient devices. 

There's also the aspect of raising the capital to fund the required technology, research and development and to make it commercially viable. 

Vertical farming is also currently limited to fresh leafy vegetables, soft fruits and herbs. That said, it's not regarded by those in the industry as a method destined to replace arable farming. 

So, can urban farming become mainstream?

"I think it definitely will. I think it will go into niche markets to start with; microgreens, leafy greens and then the higher-value foods. So there's definitely a market now for high-value nutrition foods coming through," Fraser Black, the CEO of CHAP, tells just-food.

CHAP, which is part of the UK's Agri-Tech strategy, and Innovate UK, a government-backed innovation agency, have teamed up to open a centre called The Innovation Hub for Controlled Environment Agriculture (IHCEA), based in Dundee, Scotland. 

Black continues: "The consumer view of what they eat and how they eat it is definitely helping fuel this whole area. It's not going to be mainstream arable, it's not going to be cereals, but, as we drive down the price and improve varieties, it will become more mainstream. I think it's going to be a companion rather than a competitor to existing farming.

"A lot of the technology, we suspect, will start to be driven not by salads and leafy greens but by the cannabis boom that's happening in the States and in other places. A lot of the technology that will be used in that will transfer over into the consumable-food market as well. And clearly then driving down the cost.”

Research from US-based MarketsandMarkets.com suggests the global vertical-farming sector could reach almost $6bn in value over the next two years, from around $1.5bn in 2016.

There are a myriad of firms already involved around the globe, including in the US, Europe and Asia. Infarm, based in Berlin, has drawn venture capital for its vertical-farming technology targeted at retail and foodservice channels. 

UK retailer Marks and Spencer recently installed systems from Infarm for fresh herbs – basil, mint, parsley and coriander – in seven stores across London, joining other European retailers such as Edeka and Intermache using the company's technology.

Announcing the development last September, M&S said: "Each in-store farm unit uses 95% less water and 75% less fertiliser than traditional soil-based agriculture and is capable of producing the equivalent of 400 square meters of farmland, resulting in a more sustainable use of natural resources and ensuring zero-pesticide use."

Infarm is seeking to expand its own services across the whole food chain as innovation in the sector accelerates.

"We are sensitive to the fact that the Infarm approach to vertical farming is only one way to tackle the urgent agricultural and ecological challenges of our time," says Osnat Michaeli, co-founder and head of marketing. "Solving these challenges will require innovation and collaboration across the entire industry to generate a lasting impact. 

"Over the next several years we are building a farming-as-a-service model that will be capable of delivering the technology, processes and expertise to enable an entire ecosystem of partners – from distribution centres to food manufacturers, from schools to hospitals and beyond – to participate in making cities truly self-sufficient in their food production."

Agricool in Paris has also secured funding from investors, including Danone Manifesto Ventures, the investment arm of the French dairy giant. The company started out growing strawberries in an apartment building in the capital before switching to cooltainers, and is now also present in Dubai, another major food importer.

Guillaume Fourdinier, the founder and CEO of Agricool, agrees with others in the vertical-farming sector that it will take five years before the market becomes more mainstream. However, as an example of the growth potential, he tells just-food that Agricool's systems are currently in five supermarkets in Paris, which will increase to 25 in the next three months.

Nevertheless, Fourdinier emphasises the financial cost of getting an urban farming business going and the efforts that go into R&D. Fourdinier says Agricool has raised €30m in funding and "are only just starting".

"It's a lot of effort and money and investment to make it profitable," he says. "We are basically inventing a new industry. This is something that will change the next 100 years of agriculture but it will take time, for sure."

In the UK, Saturn Bioponics, located in the city of Birmingham, uses hydroponic technology to grow fruit and leafy salad vegetables such as strawberries, lettuce, pak choi and herbs. 

The company uses natural light, growing produce in greenhouses. It has been backed by Innovate UK through project financing and grants, and is also assisting Shockingly Fresh in Scotland. 

Saturn chief executive Alex Fisher says the company is engaged in projects not only in the UK, but also in France, Romania and Japan. He argues the main stumbling block in getting vertical farming off the ground is finance because of the cost of technology and R&D, before a venture can get to the commercialisation stage and start delivering profits.

"We do see it going mainstream," Fisher tells just-food. "It's all about money in the end. It's been very much about de-risking it technically, proving it financially and then getting people to adopt it on a large scale. And that's been our journey over nearly ten years. Probably, you will see the market exploding on our end in about five years." 

Like Black at CHAP, Fisher says urban farming was never intended to compete with the well-established agrarian sector, which is much larger by comparison. 

Agricool's Fourdinier points out the technology isn't suitable for crops such as cereals and potatoes, which tend to have a longer shelf-life, particularly as the sector is geared toward providing the freshest produce in closer proximity to the consumer.

Fisher adds when asked to quantify the size of the market: "You are talking many tens of billions or even hundreds of billions of dollars per annum in farm-gate crop values. Your lettuce, strawberries, herbs – it's huge – obviously that's a drop in the ocean for agriculture overall but it's still very significant values. As we move into other areas that number will expand."

Saturn is also exploring aromatic products, botanical-type products, brassica and other soft fruits.

Black says the main driver behind vertical farming is commercial, although Agricool's Fourdinier tags on the environmental aspect, too. Both believe China and India will embrace the technology to feed their growing populations, especially with vast numbers relocating to the big cities from rural areas. 

And, with the mistrust among Chinese consumers over the food they eat, greens and herbs grown in indoor and controlled environments might be one solution to quell concerns over pollution, adds Fourdinier. 

"I think in the next five years you are going to see some major breakthroughs. You've already got big companies investing in this. You've also got an investment community eager to get involved," says Black. 

"There's huge interest in making this work and being able to have this combination of what is fresh produce 365 days a year. It has to be in a fashion that is cost-effective and provides value to the consumer, and we are definitely heading in that direction.

"You are going to see the US grow, partly because of the growth in cannabis and the use of medicinal and recreational cannabis. I think you will see the rest of the world follow quickly behind it because of the convenience factor."

Looking at the UK's vertical-farming market, Saturn's Fisher says it's a hard nut to crack when farmers generally in the country are struggling. If they are to venture into the sector, the Government needs to improve access by offering grants or other incentives because farmers are already too highly leveraged with debt to take on the extra capital.

"If what you are looking for is for the farmer to progress, you need to take risk out of the adoption of new technologies. That's a huge thing," he says. "Access to finance in general is very difficult for farmers. So it's very hard for them to borrow money for new tech and stuff like that."

"We should note that with Brexit approaching, vertical farming significantly increases the UK season and will reduce reliance on imported leafy veg from mainland Europe in the early spring and late summer 'shoulder months'," he tells just-food. "If tariffs come in, this could make vertical farms even more compelling.

"The UK alone imports over £1bn of green leafy veg per year, providing a very significant market for systems that can compete with traditional crops on price whilst providing a much better quality crop closer to the point of supply."  

More broadly, Fourdinier highlights the benefits to the consumer in terms of the nutritional content of foods grown in a controlled environment in close proximity, and says they need educating to fully understand the advantages.

He adds, for example, that 50% of the vitamins in fruit are lost after five days storage, arguing vertical farming is all about being "fresher, the nutritional value and taste, and also consumers know where their food is coming from".

While the market is limited in terms of product selection, Fourdinier says it's natural to start off with the "easy ones" as other vegetables tend to be more complex, and the nascent industry would need to develop the appropriate technologies. However, he sees tomatoes, peppers and courgettes coming through.

"We have to invent a lot of new technologies to make it profitable and you have to be productive to become mass market," Fourdinier explains. "It's the same as electric cars. We think we will be able to show that it's really profitable to grow indoors and that you get more sustainable food by doing so in large cities."