Revealed: a store’s hidden losses
Revealed: a store’s hidden losses
Energy losses are the unseen costs of a potato store, and are likely to impact on growers’ profitability more as electricity prices rise. The British Potato Council has commissioned energy experts FEC Services to find out how the industry could make an estimated £11 million saving.
If your irrigation line sprang a leak, chances are you’d fix it – there’s no point in pumping the water if it’s not going to where it’s needed. You’d be likely to renew a washer in a leaky tap, or fix a water leak on the farm. But what about fixing an energy loss from your potato store?
Potato growers spend anything up to £10 per tonne on post-harvest energy costs. Over the last two years the price of power has risen by as much as 60%. But there are few growers who have yet identified how much energy costs them, let alone started to think about how they could save money.
“The stock view is that you can’t do anything about it,” says Tim Pratt of farm energy specialists FEC Services. “The fact is you can. What’s more you’d be surprised just how much you can save.”
With colleague Andrew Kneeshaw, Mr Pratt has been working with the Potato Council to help growers identify their energy costs and what simple steps and quick fixes can be made to save money. This started with a comprehensive review of energy usage in British potato stores. And this has flagged up a big reason for growers to take action: savings of up to £11 million could be made by the industry, and that’s at today’s, still rising energy prices.
“Potato growers don’t do anything about it because, unlike a dripping tap, you can’t see energy losses. You know it’s costing you through your quarterly electricity bills, but even then few potato stores have separate meters, so few growers can quantify the cost. Rest assured, though – in our experience potential savings can add up to a decent payback in a remarkably short period of time.”
Energy used for cooling stores dwarfs all other energy uses in an average potato enterprise, advises Mr Pratt. So this is where growers should start to look for savings. Using an infra-red camera highlights potential energy losses, but it doesn’t take much to identify a few in-store howlers.
“Growers should pay particular attention to doors and louvres – these are a common point of air leaks. Fitting brush seals or rubber flaps can be a low-cost way to make a dramatic improvement.”
Controlling air leakage is critical to whether your store is economical or not, Mr Pratt explains. “If you look at your domestic fridge, you’ll notice it’s almost hermetically-sealed. But many British potato stores bear a closer resemblance to a colander – stand in one with the doors closed on a windy day and you can actually see the leaks and feel the draught. That’s warm air coming in to replace the cold air you’ve just spent money chilling.”
If you pay a little attention to sealing these leaks, you can get payback in a season or two, notes Mr Pratt. But air leakage is just one of three heat sources in a potato store: 70% of refrigeration cost is associated with heat coming through the structure. The final source is respiration of the crop, although this is fairly minimal once temperature pull-down is complete.
“Solar gain through a store roof can be significant. On a sunny day the temperature of a roof can be 40-50°C. If your insulation is not adequate you can be fighting an uphill battle to keep your crop cool.”
While re-insulating your entire roof may not be cost-effective, identifying and solving hot spots can be worthwhile. This is where a thermal-imaging camera comes in useful as it reveals hot spots that can be easily remedied.
Longer term, growers should always consider running costs when buying new store equipment, Mr Pratt advises. “It’s odd that we all take fuel consumption into account when buying a car, but not energy use when buying store equipment. It takes eight to ten years for a car to consume its purchase value in fuel while an inefficient fan, for example, will consume its cost in electricity in just six or seven months. But typically the main priority for a grower when choosing a replacement model is ‘does the fitter have it in the back of his van?’ or ‘do the bolt holes match up?’.”
Variable speed drives can bring big savings here. “Fans are involved in three distinct tasks: drying, temperature pull-down and a holding job. It’s daft to think that the same fan speed will do the most economical job for all three. That’s like thinking the best way to drive around a town is in bursts at 80mph and then periods at a standstill.”
In fact if you drop the fan speed by 50%, you reduce its energy use by around 75%. This is an example of how the figures speak for themselves, Mr Pratt points out, and why growers should look into how they can make energy savings. Such measures should be adopted in a complete and considered way, he notes – Slowing down crop ventilation fans will extend their running time and, without humidification, is likely to add to dehydration. On a fridge, the coils would freeze up if some of the fridge duty can’t be cut back – so it is important to get specialist help if you are looking to make these kinds of changes.
“But many energy saving measures deliver additional benefits, such as less weight loss through dehydration and fewer condensation events that can cause diseases to develop. In many cases these are worth far more than the energy saving.”
There may also be additional drivers for change on the horizon. Awareness over the past year of climate change and the carbon cost of products has risen remarkably. Walkers, for example, has worked with the Carbon Trust recently to calculate the carbon cost of producing a packet of crisps from farm to disposal. The on-farm element represents 44% of the 75g carbon footprint calculated (http://www.walkerscarbonfootprint.co.uk/).
“It may be a while before your customer asks you for a carbon cost per tonne of produce. But demonstrating an environmental benefit for your potatoes could become an important marketing tool of the future, and it will be the most sustainable potato enterprises that will benefit most. So you shouldn’t ignore the value of paring down your carbon footprint.”
- Further reading: Energy status report: GB potato storage
- Install a separate electricity meter – check the reading regularly and see if you can relate peaks to particular activities in store.
- Check your electricity prices – The competition is fierce, so look around for the most competitive provider. Make the best use of off-peak usage.
- Stop the leaks – Doors, louvres and joints in the store structure are common areas where warm air can come in, displacing air you’ve chilled. Brush seals, thermal quilts or just a bit of insulation stuffed into gaps can help seal your store.
- Eliminate hot spots – harder to identify, but often easy to remedy with extra insulation. See panel for more detail.
- Upgrade equipment – consider the running cost of fans and fridges when you replace them. Systems such as variable speed drives add control and can save significantly on energy.
A thermal-imaging camera illustrates common areas in a potato store where structural heat gain and air leaks can be costing you money. The pictures below show both interior and exterior shots, with the different colours showing the temperature gradient – colder areas shown in blue and warmer areas in red. If taken from the inside, the bulk of the structure is blue, while the problem spot shows up as red or orange. Outside the colour gradient reverses, with the problem spot showing as blue or green.
You don’t need a thermal-imaging camera to show up the howlers, however – the best test is to stand in the store for a few minutes on a sunny day with the lights out. It can be surprising how the sunlight outside shows up the hidden gaps.
Not the easiest areas to seal because by their very nature they are there to provide air flow but good quality, well maintained louvres are worth the cost. Make sure they are well-fitted and shut tight when closed. In this image the right hand side of the louvre is 7oC compared to 12oC at the left hand side. The cause can be as simple as dirt/debris or a dislodged brush seal.
Taken from the inside, this shot shows a hot spot along the gap between the door and the store structure. The main body of the door is 10oC compared to 16oC at the hot spot. In this case there was an obvious gap between the door and the frame. However, the absence of a chink of light does not guarantee low heat loss. At the very least all doors should have close fitting brush seals even if they have to be replaced every year.
Here a steel beam (15°C) is providing a ‘hot bridge’ to the warmer conditions outside. This is a common mistake when insulating existing buildings. Once identified it’s a relatively simple job to insulate and remedy.
This is where most structural heat gain could be taking place, especially on a sunny day. Here the insulation looks fairly sound, although hot spots (16oC) show up at panel joints. This highlights the attention to detail that is required when carrying out any work.
This shot, taken from outside the store on a cloudy day in May, shows the cladding is 17oC on the right hand side but only 13oC where it is in the shadow of a stack of boxes. This demonstrates the impact solar gain can have on the most unlikely days. Light colours on the outside of buildings help to reflect solar radiation, although this can conflict with planning requirements, so the value of good insulation is absolutely key.