A tank of ice slurry puts cooling capacity in the bank for a dairy farm so it can be stored off-peak and used on-peak. The accompanying heat exchanger provides hot water for cleaning up almost free.

Invensys engineer Warwick Bines in front of the tank which stores cooling energy, with the refrigeration, pumping and heat exchange equipment (far right) beneath.

A Te Awamutu dairy farmer is trialling a dairy farm milk refrigeration system for his 220-cow herd which if adopted throughout New Zealand would see enormous electrical demand shifted to off-peak periods.

The system developed by Invensys New Zealand, in co-operation with EA Technology in the UK, combines ice formation technology with a smart controller and a telecommunications link with the power supplier, in a package which will provide the farmer with low-cost refrigeration and almost free hot water for cleaning the milking equipment and sheds.

Ice storage refrigeration has been used before, but Invensys is wrapping it up with heat recovery, a controller and telecommunications. 

The system, called iStorm, will respond to changes in the network's overall demand for electricity, favouring operation at times of low demand and minimising use times of high demand. This will translate to lower costs for the farmer. 

The farmer, who is not being named because of commercial sensitivity, says the system offers "Free hot water, better use of energy and better quality milk." It addresses twoof the three major uses of electricity on a dairy farm: refrigeration and hot water (the other is irrigation).

Invensys estimates that 110 MW of electricity demand used to cool the 82 million litres produced every day during peak milk supply  equivalent to one-quarter of the capacity of the Clyde Dam  could be shifted to off-peak times using the iStorm system. The heat recovery from the iStorm system can produce hot water at a minimum of 50C, saving 0.28% of New Zealand's total electricity usage. This heat recovery is a by-product of the refrigeration cycle and is typically discharged to the atmosphere. This hot water undergoes further heating and can then be used to clean the milking equipment following milking.

Invensys process engineer Warwick Bines, who is in charge of the project, is fresh out of his food engineering degree course at Massey University, where he was supervised by New Zealand's "Dr Refrigeration", Don Cleland.

When looking for a fourth-year project, Warwick and three other food engineering students arranged with APV (now part of Invensys) to research efficient refrigeration for on-farm milk cooling.

Warwick has since been taken on by Invensys as a graduate, working on the iStorm ice slurry technology developed by EA Technology subsidiary company Thermetica. Another Invensys engineer, Steve Gray, has visited EA Technology in the UK to experiment with the technology.

The process relies on the latent heat of a suspended ice mixture to cool the milk indirectly using a heat exchanger.

Says Bines: "It's not a typical ice bank. It's flowing ice, retained in a liquid suspension. You don't take the energy out of the liquid  you take out the latent heat of the ice. So instead of increasing the temperature and the sensible [measurable with a thermometer] heat, you use latent heat." The liquid and ice in the tank are at 0C.

A small amount of heat transfer fluid is introduced into the ice slurry through a nozzle assembly in the side of the vessel  the "eductor", which has no moving parts. As the cold transfer fluid, chilled to 6 C, hits the eductor, it sucks in water and a small amount of air, forming flowable ice.

The size and shape of the nozzle, and the speed at which the transfer fluid is released into the vessel, are key to the effectiveness of the iStorm process.

The transfer fluid allows the ice slurry mixture to be maintained at 0C without freezing entirely. It also increases the refrigeration capacity of the liquid, so the vessel can be down-sized.

It is heavier than water and sinks to the conical bottom of the vessel, from where it is recovered, recooled to 6C and then re-introduced through the eductor.

The slurry mixture does not need stirring because the currents induced by the heat transfer fluid entering are sufficient.

An alternative to the ice slurry mixture is a "phase-change material", otherwise known as a eutectic material, which also uses latent heat as a source of cooling energy.

This is the material, contained in tennis ball-sized plastic nodules, that is presently being used on the trial farm to test the controller and the rest of the system while the technique of working with the ice is being ironed out in the UK.

However Bines says in his experience the nodules have not been as efficient thermodynamically as the ice slurry. "They have a water interface problem. The inside freezes, but as you warm up the outer surface, the ice within starts to melt, the sphere shrinks and energy has to cross the liquid interface [inside the nodules]. It then relies on conduction to give cooling, whereas in our system the thickness of the heat exchanger plate is the only barrier."

The eutectic balls will be replaced by ice slurry on the farm when the equipment and performance data arrive from the UK. 

The trial farm's 220 cows produce 23 litres of milk each, plus the milk for feeding the calves. The farmer milks from 5.307.00 am, and from 4.005.30 pm. The cooling load comes on from 5.30am9.30am, and from 4pm 8pm  just when everyone else in New Zealand wants electricity.

The ice storage tank enables a cooling bank to be set up during the night and other off-peak periods.

The PC controlling the system works out how much cooling is needed and when it will be needed, and switches on the refrigeration equipment accordingly.

A remote transmission unit (RTU) picks up the data and transmits it for downloading onto a website, viewable by Invensys, the farmer, the power company and other interested parties. Invensys can monitor and adjust the plant remotely.

The power company will be able to monitor and control the electricity supply to the unit.

If electricity demand is high in the district it, can ask to switch the unit off. But because the system measures the amount of stored cooling capacity and compares it with the amount required, the PC will be able to determine whether cutting the power would jeopardise the milk quality, and respond accordingly. 

The farmer would expect the power company to reflect this flexibility in its supply agreement and tariff.

The control equipment enables the dairy company to know in advance what it's getting from the farmer  how much milk, and at what temperature. Says Bines: "At the moment they guess in advance, based on historical trending."

Added value

Twice a day in the milking season on dairy farms throughout New Zealand the milking machines pump warm milk through a primary heat exchanger, using bore water or anything handy to pre-cool the milk, into vats where they are electrically refrigerated to await the tanker driver. The milk must be cooled to less than 7C in the vat within three hours of entering it.

The tankers call only every second day at many farms.

The tankers are not insulated or refrigerated, so the milk slowly warms as it sloshes around on its journey to the dairy factory.

It's essential to limit bacterial growth  the farmer is penalised if his milk does not meet the required standard. One bad lot can contaminate an entire tankload.

With the Invensys system, the farmer gets more reliable cold milk. It is snap-cooled from the cow and goes into the vat at 4C.

Invensys national operations manager Rex Mason says that New Zealand is "average to good" by international standards in terms of its bacterial count.

At present, New Zealand supplies 2% of the world's dairy products, and trades 30% of the world's tradeable dairy products such as casein and milk powder. It exports 95% of its dairy products.

But if low bacterial counts could be guaranteed, the milk could be processed at smaller specialised plants closer to their source, without the need for pasteurisation. This would enable more development of products specific to certain cow breeds or certain regions  like wine. These would raise the value of the milk from a commodity to an ingredient of a high-value product such as soft cheese.

How the iStorm System Works

to provide secondary cooling for milk, and heat water with the rejected heat

Investing in R & D

Invensys is a UK-based conglomerate of 1000 companies with a turnover of $30 billion a year.

Mason says the project plays on the "intelligent engineering" strengths of the New Zealand company, which incorporates Foxboro and APV, and also deals with Baan ERP software and the Wonderware factory software suite and PLC controls.

The company's multiple activities in New Zealand and its international connections make it possible to offer attractive career paths for young graduates like Warwick Bines.

Lateral thinking

Mason says a further stage the company is looking at is part-evaporating the milk on-farm. "Farmers get penalised for supplying too high a volume but they get paid according to milk fat and milk solids," he says. "If we can reduce the amount of water by 10% before it gets into the vat, you reduce the amount of refrigeration and volume needed. That means less refrigeration, transport and processing."

Says Bines: "We want to reduce the capital cost of the equipment. A high upfront cost is a big disincentive. We're modifying and simplifying it."

Mason says power companies are not really set up for iStorm at the moment, because the tariff system is coarse, differentiating only between peak and off-peak rather than the 5-10 tiers common overseas.

He says: "We are targeting the distribution companies that have to upgrade their lines."If electrical loads can be lopped with the Invensys package, the network companies can defer their upgrades.

In the next stage, Invensys is building 20 units and installing them in a district where they are of maximum benefit to power companies, dairy companies and farmers.

The iStorm technology was originally developed for cooling electrical distribution transformers, which work more effectively at cooler temperatures.

With a bit of imagination, it can be applied anywhere where an electrical demand can be deferred through storage of cooling energy  office blocks, hospitals, factories and schools.

Invensys' UK partner in the development EA Technology, is also involved with the LAMBS refrigeration project in August Energy-Wise News, issue 72.