Condenser drying could reduce cost of onion storage – The Vegetable Farmer
Onions have unique requirements in terms of conditioning and storage regimes, meaning that designing, building and operating onion stores presents individual challenges for management and energy use ...
The UK produces around 450,000 tonnes of bulb onions each year, and with some stores beings used for two crops, estimates suggest that there are around 350,000 of dedicated storage capacity in the country. The production is fairly static at c. 9,500 ha grown each year, but with many stores now 30 or 40 years old, a number of growers are looking to replace old and inefficient infrastructure, as well as the odd store being built to provide new capacity.
“Demand for stores can fluctuate depending on wholesale crop prices,” comments Adam Fryer, Commercial Director at Farm Electronics Ltd. “We usually carry out two or three onion storage projects a year, as well as carrying out store refurbishment in terms of new components ranging from controllers and electrical panels to fans and refrigeration.
“Onion storage is not like the storage regime for any other crops. It requires significantly more air volume at a higher back pressure than potatoes, so onion storage is positively ventilated, however there can be different store layouts.
“Usually, onion storage is split into four stages ranging from using gas burners to increase the temperature within the store along with high air volumes through to longer term storage bringing the crop down to 0.5C for several months. Research is now being done to look at the effects of higher temperatures at long terms storage.”
The four stages are generally recognised as drying, curing, cooling and refrigeration, although crops may also be conditioned after storage before grading or packing. “The big difference between onions and other crops is the tremendous amount of energy required,” comments Andy Richardson, Managing Director of Allium & Brassica Agronomy. “Crops are usually dried using electrically driven fans and large propane burners for four weeks, so the current high energy prices are a serious concern. Once dried the crop is usually cooled using ambient air and then refrigerated to 0 to 1 C for long term storage.”
Well designed and managed stores can keep spring drilled onion crops harvested in September in good condition until the following July. Adam points out that some growers also use the same stores to store onion sets, so stores are utilised for longer.
Andy estimates that around two-thirds of the stores in the UK are bulk stores but points out that many of these are older structures, “Most new builds are box stores, which have the advantage of more precise traceability,” he adds.
“In our experience there is a fair balance between box and bulk onion stores,” comments Adam. “It usually depends on what the farm is already geared up for in terms of loading and unloading the store and where the onions are destined to go.
There are advantages and disadvantages to both systems. “Usually, you can get a better air distribution through the crop when it is in boxes, it is easier to monitor, and the quality is better. However, the rising cost of timber has a significant impact on investment if it is a new store and requires new boxes.
“On the other hand, if loading in bulk you need to consider blade damage when unloading the onions from the store and the potential for compression damage depending on harvest conditions and storage equipment setup. Box storage does have limitations when it comes to store design, as there needs to be a long duct with short box runs as you can easily loose pressure when positively ventilating through long rows of boxes.”
He adds that another major consideration between bulk and box stores is the warming of the crop at the end of the long-term storage period. “In bulk it is difficult as you get condensation on the adjoining crop, which is still cold, whereas in a box store you simply take out the onion boxes you want and warm them in a separate building,” explains Adam. “Usually, this building has a duct with a small fan and a burner to just bring them up to temperature gently.”
Because of the amount of air movement required, onion stores are positively ventilated, but there are different ways to achieve the necessary air movement. Pushing air through the crop is the most common method of ventilation but there are also some systems that use suction. Both systems have advantages and challenges, but in practice both systems work well as long as they are well designed and not overloaded.
“Obviously bulk is all positively ventilated, but there are a couple of different ways to positively ventilation box onion stores,” explains Adam.
“One option is to have a long duct down one wall and blow down short box rows, or alternatively you can use air bags which sit between every two rows of boxes with a timber wall with long vertical doors under the airbags where the air is returned.
“Although the initial investment in the second system is slightly higher, and you lose some floorspace of the store, this is very much the best way to ventilate a box onion store as it keeps all the equipment along with the duct at the gable end of the building and you can stack the boxes in longer rows. You can also easily partition off rows if you have a part loaded store.”
This air movement is a key part of the skin curing stage. “Drying and curing is all about the flow of air and accurately maintaining its temperature and humidity, using a combination of louvres and heaters,” explains Andy, adding that the use of computer control has greatly simplified the
management of new stores.
“A good onion store is all about airflow,” he stresses. “Poor airflow and loading, and slow drying, often exacerbate the incidence of storage diseases such as Fusarium basal rot, neck rot and bacterial rots.”
“The equipment in the store has a massive impact during the skin curing stage of the storage process,” says Adam.
“The ventilation has to be able to offer the high air volumes the crop requires as well as creating a high temperature which takes into account the air volumes from the fans and the desired temperature lift above ambient.
There are many factors that go into the calculations to get these figures, including tonnage, building insulation type and size, how fast will the customer load the store and many others.
“Store design and equipment can have a massive impact upon the quality of the product when being unloaded as well as having a more uniform quality throughout the store. There are many ways onions rot in store and most problems are related to not getting the wound caused by topping the onions properly dried down in the initial storage drying and curing phase.
“There can also be issues with pressure spots in bulk stores and neck rots, and these are usually a result of having the incorrect temperature and/or humidity in the store. There are many components that make up an onion store and they all have to work in harmony with the controller so the conditions within the store are correct.”
Current record energy prices have focused growers’ minds on the energy costs of onion storage. Andy Richardson says that for long term storage crops, the cost of gas and electricity is likely to account for around a third of the total production cost for the 2021 season, and that this figure is likely to more than double in 2022. “The industry needs to look at more efficient ways of drying such as condenser drying systems if we are to maintain profitability in the current climate,” he warns.
“Condenser drying is a major new development in terms of onion storage,” adds Adam. “In essence it replaces the gas burners with a heater coil with the refrigeration circuit. This is quite a revelation in terms of onion storage as you remove the requirement for gas burners and the constant refuelling of these and instead create the heat using the refrigeration equipment which is already required for long terms storage.
“One of the other major benefits is that it makes the storage inside the building totally independent of the outside conditions. This means you are not affected by hot wet days where you ideally want to bring in the ambient air however the humidity is too high.
“With the condenser drying solution, you can recirculate the air freezing out the moisture in the air and heating it afterwards creating hot dry air. This means you can have a more consistent curing and wound healing through the storage process and not turn equipment on and off throughout these stages.”
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