Vertical farming pushes the boundaries of a microclimate. The very best facilities can regulate lighting, humidity, temperature and nutrient levels to maximise the growth of thousands of plants simultaneously so that the yield is maximised and cost is reduced. Lighting has developed through developments in LEDs, and highly accurate sensors manage the nutrient application, but the cooling systems are almost identical to those which were first introduced at the start of the 20th Century by Willis Carrier.
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My colleagues Mehmet and Ben discussed more efficient energy use in their recent blog post, with an average of 28% of energy costs associated with the air management systems. Phase change materials, as a heat sink for cooling the atmosphere in vertical farming facilities, offer an opportunity to reduce costs and provide targeted cooling for individual rows or even plants, creating a nano-climate. Where micro climates are typically measured in square miles, sometimes even few square feet, this proposes controls which could be on the scale of a single plant.
In many facilities and situations, a standard vapor compression (VC) air conditioning system is the perfect solution. Lots of heat draw capacity and a fast response time mean that they can maintain comfort and respond quickly to changes in demand that might occur in small buildings. However, for predictable heat loads like vertical farming, the time of peak demand is known and often occurs during the period of highest overall power demand. This can be offset somewhat by inverting the diurnal pattern (turning on the lights during the night).
Phase change materials, which could be as simple as salt water, can provide thermal storage for cooling during the day, as I mentioned in a previous blog. Regions near the equator tend to have greater diurnal range and in some parts of the US, the diurnal temperature variation (difference in temperature between day and night) can be as high as 25ºC (45ºF). These colder temperatures at night offset the efficiency difference between drawing from a room and drawing from an ice bank.
So, there’s very little change in the overall energy demand required to extract the energy if it’s done at night into ice – and the energy was cheaper to buy. Then there are the benefits of the ice itself. A block of ice is similar to a VC system, but ice slurry can be pumped through a network of pipes in order to provide targeted cooling wherever the pipes travel. At 30% ice, a salt water solution has the cooling capacity many times that of a coolant oil and dozens of times better than air transport. To transfer this energy the brine can exist at a higher temperature than a standard VC cooling fluid, resulting in less unwanted dehumidification. If there is a major heat source such as a lighting bank or a pumping system, these can be cooled directly.
The heat pump dumps energy from the cold storage outside of the facility. The distribution system sends slurry throughout a network of tubing, so that cold can be allocated to different crops as required by either species or growth stage, as well as only cooling the growing regions of the facility.
And the benefit of a nano-climate; within a single facility there are often a multitude of plants at different stages of growth. By localising the air conditioning with slurry piping, each region can be provided with its optimum growth conditions, increasing yield while decreasing operational costs, all for the cost of installing a tank of slurry and conventional plumbing, where maintenance is simple and leakages innocuous.
Each plant or group of plants is provided a local supply of slurry for cooling as required. Any light source can be cooled as well by contacting the slurry transport system to the light source.
Moving slurry in efficient ways and maintaining it in a flowable state is not trivial; ice wants to agglomerate and ice crystals want to grow in diameter. With our experience in a range of slurry applications and a range of markets, together with knowledge in plant monitoring, dense sensor networks and automated systems, we are ready to bring our skill to bear on this challenge.