Tim McVeagh
With the new milk cooling regulations coming in to effect in June 2018, there has been a lot of interest in how to meet these requirements.
While many farmers will have to invest a significant amount to upgrade cooling systems, others will comply by making their existing system more efficient. This starts with primary cooling. But improving the system with a silo insulation wrap may get some farmers across the line, be comparatively cheap, reduce refrigeration running costs, and help to maintain milk quality.
About 80% of New Zealand dairy farm silos are uninsulated and silo insulation wraps have been around for at least 10 years, so are they a worthy investment?
The obvious benefit of silo insulation is less energy required to chill and store milk. Energy Efficiency and Conservation Authority (EECA) data suggests farmers can save up to 25% of the energy used for refrigeration and speed milk chilling by up to 20% in summer months. They estimate the payback time to be between three and eight years.
On a national scale, according to EECA, the uninsulated silos on NZ farms waste up to $7 million of electricity (about 3000 households); and emit about 4000 more tonnes of carbon dioxide a year, (similar to 1800 cars).
Silo insulation, unlike some other measures taken to meet the new regulations, does not increase peak power demand, which is critical in some areas.
Where insulating the silo is enough to meet the new milk cooling regulations, this can save spending a considerable amount on upgrading cooling systems.
In an uninsulated silo, a milk deposit tide mark which the CIP system cannot remove, can build up over time. This potential source of grades does not occur with an insulated silo.
There is less heat loss of cleaning solutions during the CIP wash, meaning more-effective cleaning.
When electricity charges increase, the savings associated with silo insulation increase.
Indoor silos are unlikely to benefit significantly from a silo wrap and the return on investment would be marginal.
Development, design, composition
Early wraps were glued to the silo, so when a wrap was to be removed and fitted to another silo, it could take a day for one person to remove all the glue (a requirement by most dairy companies). Now dairy companies prohibit gluing to the silo, so modern wraps strap to the silo and are easily removed to be fitted to another silo of the same diameter.
Some early silo wraps were marketed by fly-by-nighters with poorly fitting wraps that were not properly waterproofed and not vermin proof.
Silos that have a refrigerated section on their lower sides also have insulation in this area, so wraps do not cover this section.
Most of the heat exchange to the milk is through the wetted surface. The transfer of heat from the silo wall to air is not as high as that from the silo wall to milk. While the air in the head space does become warmer, it rises and goes out the vent at the top of the silo. So wrapping the top of the silo cannot be justified.
The wraps from Dairycool and Tru-Test consist of three layers. The outer PVC material provides protection from damage and the elements. The middle waterproof closed cell foam provides the bulk of the insulation. The inner aluminium foil acts as a vapour barrier to stop moisture getting into the foam.
There are two common silo diameters, with wraps available for both.
Factors affecting feasibility
The feasibility of a silo insulation wrap is dependent on the average milk volume, silo refrigeration, milk temperature, milk collection time, variable electricity price and the outside air conditions. Outside air conditions include the effect of shading versus direct sunlight, ambient temperature, and wind speed.
Jim Miller of Millbridge Consulting provides independent advice to dairy farmers on energy requirements including milk cooling.
“Farms on skip-a-day collection get a much bigger return on investment of a jacket”, Miller says. “And wind has just as big an effect on heat transfer as average temperature. Somewhere like Taranaki that doesn’t have particularly high temperatures but does have a prevailing wind, is a case where the heat transfer can be more than people expect.”
Because the outside air conditions and collection time are beyond the control of farm staff and can vary significantly, the financial benefit of a silo wrap can also vary widely and be difficult to predict. Situations where a silo wrap is likely to be more beneficial include where there is marginal cooling of milk prior to entry into the silo, hot windy days, and night or skip-a-day collection.
EECA Tool:
EECA have on their web site an online tool which allows farmers to assess energy savings by insulating the silo, and an understanding of how much faster milk can be chilled in an insulated silo. By entering the silo size, electricity cost, and farming region, the calculator will display money saved per year, electricity saved per year, silo chilling time saved as a per cent, estimated insulation cost, and payback time. See
https://www.eecabusiness.govt.nz/tools/milk-vat-insulation-savings-calculator/
Note that the silos of 21,500 litres and greater have refrigerated wall sections, and so the trends evident for the smaller silos changes. There is less electricity and chilling time saved due to insulation, as a significant proportion of the milk is stored below the level protected by the side wall refrigeration pad which is already insulated. Payback is not affected as much, as the silo wraps are smaller and therefore cheaper.
“I believe the gains in cooling speed in the EECA calculator are for a sunny afternoon. But a lot of farms need peak refrigeration for the morning milking when the gains are not so great, with lower temperatures, generally less wind, and less direct sunlight. I normally work on about 10% saving in chilling time over all,” Miller says.
“Farmers who are only just meeting the current milk cooling standards will need an extra 35 to 40% more cooling capacity to meet the new regs,” he says. “You can get a gain of about 10% with a jacket, but won’t be enough to get you over the line. Farmers who are pretty close to meeting the new regs may find the 10% gain with fitting a jacket will be enough. For them, it’s a really good investment and it will also generally pay for itself in energy savings in four to six years’.
Silo wraps are supplied in NZ by Dairycool and Tru-Test. They are all made in NZ, and are fitted by the respective companies or their agents.
Dairycool have been making wraps in the agricultural hotbed of Ashburton for 12 years, with the earliest ones still in service and showing no sign of wear. Improvements in materials and design mean the latest products and are expected to last at least 20 years.
Dairycool make wraps in white or grey. While white should be a better colour with respect to reflecting solar radiation, some customers prefer that the wrap is less evident on the silo.
Dairycool supply nationwide, with a turnaround time of five days.
“Farmers who have wrapped their silos report that where refrigeration has previously been running for 25 to 30% of the time on a hot day maintaining the chilled temperature, with insulation it will run for a very short time if at all,” Murray Hollings of Dairycool says. “The general consensus is that the wrap will pay for itself in two to three seasons in terms of power saved.”
Dairycool insulation wraps range in price from $2888 for a 11,500 litre silo to $3950 for a 30,000 litre large silo. Prices include fitting, and are GST exclusive. Smaller wraps to 2000 litres are made to order.
Tru-Test Polar Wraps are a product line that were acquired by Tru-Test with Dairy Technology Services.
Tru-Test provide a no-obligation milk cooling assessment service. Data loggers record milk temperatures for four milkings, allowing the most cost effective milk cooling solution to be recommended. This may include fitting a polar wrap or one of the other Tru-Test milk cooling systems.
For polar wraps, Tru-Test provides a nationwide service, with a one year warranty. Prices range from $2343 for a 3400 litre vat to $4279 for a 30,000 litre silo. These prices include freight and fitting and are GST exclusive.
Tru-Test farm trials monitored chiller running times for a 16,000 litre silo with, and without a Tru-Test polar wrap. Average day ambient temperature for the trial period was 180C, and tanker collection was during the day. Over a 24 hour period, with the polar wrap fitted, the chiller ran for 109 minutes less. This equates to a savings of about $4 a day under these conditions. During hotter periods and with night or skip-a-day collection, the savings would be considerably greater.
It appears insulating milk silos is a worthwhile investment in some situations, with benefits beyond reducing electricity costs. Each case must be judged on its merits. The EECA calculator is an appropriate place to start.
