Written by Cathy Sheehan, Editor for EnergyWise News, EECA

A method to economise on the energy used by fruit coolstores has promising potential for the horticulture sector, but its inventor Robert Barnes finds it difficult to interest operators in investing in energy savings.

Robert Barnes’s three-pronged energy-efficient coolstore refrigeration is kind to fruit by avoiding sharp fluctuations in storage temperature.

Barnes, of Bay Refrigeration & Electrical (Tauranga) Ltd, developed the technique – which consists of advanced controls, a smart defrosting system, and “as-needed” fan operation – for the Zest 2000 Ltd coolstore on No. 2 Road, Te Puke. The store is 34 metres by 15.6 metres.

Fruit coolstores are typically unsophisticated in refrigeration design and control. Inside, they have a number of evaporators set out in a grid. They have only one controller for all the evaporators, usually in the centre of the building.

The coolstore has large doors at one end and the fruit is stacked in pallets, starting at the far end and gradually filling up towards the doors.

Each time the doors open, warm air rushes in and the refrigeration must work to remove the heat. It over-cools the fruit at the back unnecessarily to achieve the room’s required temperature.

The stores are used for kiwifruit six months of the year. It might take a week to fill a large store.

Zones

Barnes’s first energy savings idea was to segment the coolstore into a series of individually controlled zones.

He has a series of pairs of evaporators running the length of the coolstore. Each pair has its own controller.

Each evaporator operates according to demand. At least three zones have to need cooling before the compressor kicks in. The zone closest to the door operates the most frequently; the others cycle as required.

This avoids over-cooling the fruit at the back when it is necessary to cool the areas nearer the front of the room.

Gas defrost

Refrigeration coils continuously acquire a coating of ice and must be defrosted. In a typical coolstore, the cooling is switched off for 20 to 30 minutes, four times a day, so the coils can be defrosted with a burst of heat from electricity – in turn generating more heat for the refrigeration to remove once it is switched on again.

Barnes’s second energy-saving feature was to replace the electric defrost with the heat reclaimed from the refrigeration process, using it to warm the evaporator coils to defrost them.

The defrosting is controlled with solenoids and return valves. It takes only five to 10 minutes and occurs up to six times a day. Individual evaporators can be defrosted while the others are still cooling.

The frequency of defrosting depends on the cooling demand and the need to keep the evaporators ice-free, which keeps the air flowing freely and the evaporators running efficiently.

When the coolroom is closed and the product is down to temperature, the evaporator coils might need to be defrosted only twice a day.

Fans when needed

Barnes’s third energy saving idea was to avoid operating the air circulation fans unnecessarily. When the product is down to its desired temperature, the energy use of the fans becomes an important factor. Each of the 10 evaporator fans is using

1kW of electricity – generating 10 kW of heat altogether to be removed.

In “power-save” mode, the fan doesn’t switch on unless it’s required. Barnes says this can save 30-40% of the plant’s energy use.

The whole operation is monitored on-site and remotely through a locally made Intech SCADA system. Says Barnes: “It’s a great tool to iron out the bugs. It tells you the realities of what’s going on, using probes in the fruit.”

Workers on-site during the day can check what’s going on. Says Barnes: “Once it’s adjusted, no intervention is needed. If there’s a power failure, [the plant] automatically restarts.”

The Te Puke coolstore has been running with Barnes’s system for five years and won an energy efficiency award from EECA and IRHACE (Institution of Refrigeration, Heating and Air Conditioning Engineers) in 2002.

Many ideas, no time

Barnes began his career as an electrician for New Zealand Forest Products at Kinleith, where he did refrigeration, electrical and air conditioning work.

Now a sole business operator in Tauranga, he regrets he doesn’t have more time to research and develop energy efficient refrigeration.

He would like a student research project to refine the system, perhaps linking in the controls so the plant can respond to periods of high electricity prices and switch off without loss of quality for the fruit, but with a busy working day he lacks the resources. “If we were encouraged and had the funding we could make further innovations.”

Barnes can spot energy efficiency opportunities – such as using the right-sized pipes to carry refrigerants – from a mile off, but says energy consumption is not a big factor in the coolstore owners’ equation so it’s difficult to convince them of the benefits.

The costs of picking, packing, forklifts and transport all loom larger on the consciousness.

Says Barnes: “When you’re bidding for a job, it’s like a Dutch auction to get the lowest price. There’s no thought given to energy consumption.” He says his system added around 5% to the initial cost. “The more controls, the better the result, but the higher the cost.”

Barnes says it’s difficult to assess the precise energy savings from his system because of variables such as the amount of fruit going in, and the temperatures inside and out. “But we believe there’s big savings.”

An active coolstore can go to 800 kW energy demand; at rest the load diminishes to 200 kW. “If the door is opened it can use 100 kW more per day.”

He’d like to work out the number of kilowatt-hours required to cool a pallet of kiwifruit, and maintain it at that temperature, compared to a conventional coolstore. This would be a handy indicator to show operators just how much energy and money they could save.

Coolstores are essential for lengthening the selling season; and rising electricity prices could force operators to take a sharper look at energy efficiency. Says Barnes: “If it meant the difference between going broke and staying in business, they’d get serious.”

Upfront cost paramount

Barnes has also designed and built an energy-efficient dryer to dry macadamia nuts, bee pollen, echinacea and other Bay of Plenty produce. He said it would cost around $15,000 and draws 3 kW.

A local kiwifruit grower uses 80 domestic-scale dehydrators to dry samples as part of its kiwifruit testing process, which goes “flat-out for three months of the year”. Says Barnes: “We can do half their samples in this unit. It would take longer because it dries at a lower temperature but would give terrific energy savings.” However the kiwifruit grower prefers to persist with the 80 dehydrators, because of their lesser up-front cost.

Barnes believes people have a responsibility to look after their energy use.