Potato and grain storage: can your connection keep up?

A normal farm connection can often carry a bulk store. The trouble starts when the fans, the cooling and the harvest work all peak at once. When they do, a heavier connection is the usual answer. A semi-off-grid system is the other one. Solar panels on the shed roof charge a battery by day, and the battery runs the night ventilation and cooling on top of the connection.

You know the rhythm. The shed is full and the crop has to come down a few degrees. The fans start when the outside air turns cold, often at night. Then a cold spell ends, the fridge has to take over, and the grading line runs during the day. One evening the fuses trip, or the network bill shows a peak you did not expect.

What makes it hard is that storage and solar power work in opposite hours. The panels on your big roof produce at noon, when the store should rest. The store needs power in the cold hours, when the panels give nothing. And a store that cannot ventilate in time loses weight and quality, which costs more than the electricity.

A farm in Tielt hit the connection limit when it wanted to grow: the next step was medium voltage. A semi-off-grid system with a 45 kVA inverter and 123.2 kWh of batteries now gives it more power, so a heavier connection was not needed. The farm expects to take only 5% of its yearly consumption from the grid.

Why does a potato, onion or grain store run mostly at night?

Because the cheapest cooling is outside air, and outside air is coldest at night. You can only cool a crop with air that is colder than the crop itself.

For potatoes, the Belgian farm energy platform Enerpedia gives the rules. After wound healing, the crop is cooled slowly, by 0.2 to 0.3 °C a day at most. The outside air should be about 2 °C colder than the potatoes. Enerpedia also advises to ventilate with humid air, and the most humid hours are at night. In spring, when the sun warms the shed, you wait until night to ventilate.

For grain, the French crop institute ARVALIS gives a clear rule. Start the fans as soon as the air is at least 7 °C colder than the grain. It calls summer nights the best hours, and recommends a thermostat so the fans start by themselves at any moment of the night.

Onions are the exception that proves the rule. After the first drying, Dutch advisers from DLV found that onions can be post-dried once a day in the warmest part of the day. That matches solar production much better.

How much electricity does a storage shed use?

It depends on the crop, the store and how long you keep it, and the spread is large. Measure your own store before you trust an average.

The British levy board AHDB found 0.09 to 0.27 kWh per tonne per day between potato stores of the same type. That is a threefold difference. For long-term storage, AHDB puts the total at up to 100 kWh per tonne. A Dutch practice network of 46 growers, led by Wageningen researchers, found an average of 1,300 kWh per hectare for potato storage. The differences between growers went up to a factor of five.

What does that mean for a store of 1,000 tonnes? Using the AHDB range, it needs 90 to 270 kWh a day. Most of it falls in the cold hours.

The mix of loads decides the peak, not only the kWh. Fans run for hours. A fridge adds its compressor and defrost cycles. At harvest, drying may add heaters and fans that run for days. And the grading line, the conveyors and the loading run in the daytime.

When does your farm connection become the limit?

When several of those loads fall in the same quarter-hour, usually around harvest. The average use of a store is modest. The peak is what trips the fuses.

• Drying at harvest. Grain that comes in slightly too wet needs drying ventilation. ARVALIS describes fans running nonstop for 2 to 6 days. They move up to 100 to 200 m³ of air per hour per m³ of grain.
• Cooling down. Enerpedia asks for a fan capacity of 100 m³ per hour for each m³ of potatoes. Less capacity means more running hours and more weight loss.
• Motor starts. Fans, compressors and conveyors draw several times their running current for a few seconds when they start.
• Everything else on the farm. The farmhouse, the workshop, the drying of another crop and, on mixed farms, the stables.

On a 3 x 230 V line, the margin is often gone first. Our article on a 3x230 V connection that is too weak explains why. For the options without a heavier connection, read more power without a grid upgrade. If Fluvius wants you on medium voltage, our article on a high-voltage cabin or a battery compares both routes. A fast-growing dairy farm in Lokeren chose solar panels, a wind turbine and a battery instead.

What does a storage peak cost on your bill?

In Flanders, the capacity tariff charges your network costs on your highest quarter-hour of each month. Fluvius then averages the last 12 monthly peaks. A quarter-hour at night counts just like one in the afternoon.

That makes a store different from a cold room. A potato or grain store runs for many months. Its peak can sit in most of your monthly peaks, not only in two harvest months. Our article on how long a battery keeps a cold room cold shows how a short harvest peak and a monthly peak weigh differently. For the full Flemish calculation, see how to calculate the capacity tariff for your business.

Can solar panels on the shed roof power a store that runs at night?

Not directly. The panels produce around noon, and the potato and grain fans want the cold hours. On most days the store is resting when the sun is strongest.

There are three ways to close that gap. Use the daytime loads you can move: grading, loading, onion post-drying and, on warm days, a fridge. AHDB notes that refrigerated potato stores can make good use of solar energy in the summer months. Store the midday surplus in a battery for the night fans. And keep the grid for what is left.

Many storage sheds have a big roof and a connection that limits injection. Our article on more solar panels than your connection allows explains that side. With a battery, more of the roof works for your own store.

What saves more: fan speed control or a bigger battery?

Start with the fans. Every kWh a fan does not use is a kWh no battery has to store.

Fan power drops much faster than air flow. The University of Wisconsin measured it in potato stores: at 50% speed, a fan gives 50% of the air for about 15% of the power. In their trial, speed control cut fan energy by 40% on average. Enerpedia gives the same message for EC fans: at 80% speed, they save 50% of the energy.

Speed control also softens the start of the motors, so it takes the edge off your peak. The catch, Wageningen researchers warn, is that running hours must not grow, or the crop loses more weight. A good storage computer that follows the weather does the rest.

When is a battery not the answer for your store?

When your connection has room to spare, or when the store only runs for a few weeks a year.

Think about a power cut too. A full bulk store holds its temperature longer than a small cold room. But a store that cannot ventilate during drying or wound healing is at risk. A battery system that can run as an island keeps the fans going. Our guide to an emergency power supply for your business compares the options.

What should you check before the next storage season?

Five questions, answered before the harvest.

What happens when you ask us to look at your store?

We start with what your store and your farm draw, and when.

1. Step 1: Analysis and sizing. We list the loads of the store and the farm. If data is missing, we can monitor your consumption for a short period first.
2. Step 2: Technical design. We choose the solar power, the battery capacity and the inverter power, and where the system stands on your site.
3. Step 3: Installation and commissioning.
4. Step 4: Monitoring and support. You follow the system in the app and the VRM portal.

Our semi-off-grid systems are built for SMEs and farms with a standard 3 x 400 V + N connection, up to 75.6 kVA. On sites with a larger connection, a business battery does the same job. Members of Boerenbond can also use the Boerenbond member quote request.