Many regions around the world are riddled with tunnels that are no longer in use. Coalmines whose reserves have been depleted; civic air defence infrastructure from more troubled times; many are currently lying abandoned, and are often underneath land which cannot be used for cultivation, such as cities. Researchers from Nottingham University are about to embark on a research project to determine how such tunnels could be reused as underground farms, both providing space to grow crops and forming a sink to absorb carbon dioxide.
Civil engineers around the world have in recent years become interested in underground building techniques, trying to solve the problem of increasing populations by building downwards rather than upwards. However, most of the effort has looked at residential, retail or commercial space rather than considering agriculture.
Prof Saffa Riffat, chair in sustainable energy at Nottingham's faculty of engineering, is working with research fellow Prof Yijun Yuan, a specialist in mining engineering and sustainable energy, on the two-to-three year project, which will focus on opportunities in the UK and China.
"In the UK there are over 1,500 redundant coal mines, and in China, there are over 12,000 abandoned coal mines (0.6 million m3), 7.2 billion m3 of tunnels and about one billion m3 of civic air defence tunnels," said Prof Riffat.
Such facilities would of course need artificial lighting, and also a source of nutrients for the plants. The researchers envisage using hydroponic or even aeroponic systems, growing the plants in nutrient-rich water or a mist environment, and lighting the tunnels using LEDs selected to provide the optimum light wavelengths to meet the plants’ needs for photosynthesis in the absence of sunlight. Harvesting would be fully automated, using advanced robotic systems which could also handle planting.
Groundwater could be used directly in these farms or water could be condensed from ambient air (coalmines in particular are often humid environments). The researchers are also designing advanced control systems including sensors and remote controls to monitor the health of the crops and adjust conditions to optimise growth and yields. These, along with the necessary pumps and lighting, would be powered by renewable energy or run on an off-peak electricity tariff.
One major advantage of growing crops in such artificial conditions is that they are no longer dependent on natural variations of the weather cycle or seasonality, allowing them to be produced all year round. It would also potentially allow them to be grown in arid or water-deficient conditions, as well as in cold climates and regions which do not receive regular or reliable sunlight.
“Many crops are now being grown in greenhouses. Although this provides a controllable growth environment, greenhouses are heavy energy consumers,” said Prof Riffat. “Vertical farms are a relatively recent adaptation of the traditional greenhouse and are suitable for use in cities, as their tall glass structures provide high crop yields on a small land area.
“However, vertical farming systems are expensive to manufacture and install, and require a large amount of water and energy for heating and cooling. They are also vulnerable to extreme weather conditions, wars and terrorism."
The project will also investigate connecting the existing redundant tunnels with new networks, which could be drilled using automated equipment or made by controlled explosion.
The Nottingham team has obtained a patent for the techniques, including the construction of deep vertical shafts to house farms, which could then be connected to existing tunnel networks.