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  • Writer's pictureMike Casey

Electric Frost Fans - how good are they and do they save you money?

Updated: Dec 4, 2022

In 2020, we imported two 30 kilowatt (kW) AGI electric frost fans, which are the first and currently the only electric frost fighting fans of their type in New Zealand. We received an EECA demonstration grant and have just provided EECA with a report on their performance over the 2021 Frost Fighting season.

After writing the final report for the project, we had plenty of juicy data and results to share. We thought the best way to share this would be via a public blog post people can find when searching for electric frost fighting fans!

We installed two 30kWh, 57amp frost fighting fans on our 6 hectare Central Otago Orchard. These fans were installed instead of traditional diesel burning fans, which can burn anywhere between 30 and 40 litres of diesel an hour, depending on the model.

But first, let's take a step back and examine what these frost fans actually do.

Why are frosts dangerous?

There are different critical temperatures at different stages of fruit bud development. We are unconcerned about frosts until our fruit buds begin to show signs of life after being dormant all winter.

At each stage, from swelling through blossom through to fruit set, we have different critical temperatures.

Critical frost temperatures for 10% fruit bud loss during frosts. -8.8 at Swollen bud, -5.56 at side green, -3.89 at green tip, -3.33 at tight cluster, -2.78 at open cluster and first white, -2.22 at first bloom and beyond
Globally considered critical temperatures and rate of bud loss across different stages of cherry fruit bud development (red is celcius conversion)
Note: that there is vast amounts of anecdotal evidence that our fruit buds in New Zealand are actually far more susceptible to cold temperatures, because we have less hardy winters than the likes of North America which means our buds don't harden off as much. It's generally accepted that we need our critical temperatures are a degree warmer.

As we move further into spring, we have less severe frosts, but we also become more vulnerable to frosts when they occur. This is why frost protection of our 9300 cherry trees (25km in total) is so important.

What is a frost and how do they form?

Frosts are usually caused by a warm layer of air called the inversion layer. This warm layer of air traps a cold layer below, and it's this cold layer of air that becomes dangerous for fruit buds. What frost fans (sometimes called wind machines) do is pull the warmer inversion layer down from above and mix it into the cold frosty layer at ground level. This brings the average ground level temperature up and keeps our fruit buds nice and toasty.

The graph below shows one such frost where our electric fans turned on at close to midnight and stayed on to just after dawn.

The green line is the temperature of the air at 8.5 metres, and the light blue line is the temperature at 1.5 metres. During this particular frost, there was a strong inversion layer with which to work. The air above was much warmer than the air below.

The red arrow shows the temperature trajectory at 1.5 metres, the purple line shows a much improved temperature trajectory due to operating the electric frost fighting fans, and finally the yellow arrow shows the effect of the sun at daybreak.

The frequent peaks and troughs are caused by the rotation of our electric wind machines. They spin a full 360 degrees over a 9 minute period, and therefore distribute air in all directions, while the temperature sensors are in a static location.

Frost Season: August to November 2021

Frost season began for us on the 18th of August 2021, and ended with a small frost on the 4th of November 2021.

In total, the Western fan ran for 122 hours, 13 mins and 52 seconds across 89 starts, and the Eastern fan ran for 115 hours, 6 minutes and 44s and 69 starts.

Note: there are increased starts on both fans, and particularly on the Western fan because of demonstration and testing. Total runtime is largely reflective of the total frost time fought during the season, with 119 total frost hours measured during this period.

The conclusion drawn is that the AGI fans met expectations and were able to lift frosts to the same level as other incumbent frost fighting fans operating on neighbouring properties. As results were comparable, the real benefits of going electric are decarbonisation and operational cost savings.

Electric fan power consumption and operational costs

Across both the orchard and our corresponding household, 20MWh of consumption was recorded using our Power Radar monitoring system, partially funded by EECA.

Frost Fighting power consumption vs other power consumption August to November.

Katie Fendall from Simply Energy, our account manager, ran the numbers for us over this period. She recorded 15.5MWh taken from the grid over the 2021 frost season, meaning roughly 25% of our power consumption was self generated through our solar panel array.

In total, 10.5 MWh of power was consumed during frost fighting, which represents roughly 50% of total power consumption on site over the period. Frost fighting happens at night when solar generation is impossible. Therefore, it is estimated that 67% of grid take was for frost fighting or 7MWh.

A very rudimentary calculation using a kWh price of $0.3 would put the total price for energy at $2100. Compare this with a diesel price of $1.61 per litre (average price during frost season), and the same diesel fans would have cost $13,411 and $15,372 to operate for 119 hours.

Note: If we were using today's diesel prices (as of 23 Sept 2022) the total cost to run fossil fuel powered fans would range from $18,278 and $24,371. This is a huge saving, pretty much the entire additional costs of the fans within 1 -2 seasons.

Actual recorded operational cost at Forest Lodge

You don't need batteries or solar to run frost fighting fans - you can run from the grid. However, this last section is about how we have driven the price down even further with our award winning renewable grid connected system.

Because Forest Lodge has a large battery array, we can actually go one step further. Instead of having a fixed price, we buy a kWh from our retailer, but instead buy at the wholesale market price. This is because we can use the power stored in our batteries when the spot price goes high, and therefore drive our average price per kWh down.

Here are the actual results compared to diesel prices for our 119 frost fighting hours this season for a single fan (remember we have two).

Electric fan 2021 operational costs 2021 (actual)

Break down of total cost of electric 30kWh wind machine in 2021 by month. Total cost was $621
2021 Frost fighting costs per month for electric 30kWh wind machine

Diesel wind machine operational costs 2021 (theoretical)

Theoretically the total cost to run a diesel frost fighting fan is over $7000 for the 2021 season. Table shows break down of frost events and price of diesel on that day.
Total cost for a diesel fan, assuming 40L of diesel per hour, is well over $7000 with 2021 price, and could be as higher than $12000 with 2022 diesel prices.

What about the carbon savings?

It wouldn't be a Forest Lodge blog without talking about carbon emissions!

119 hours using two fossil fuel equivalent fans are estimated to consume a total of 9520L of diesel.

In terms of what this means for carbon emissions, using the formula that a litre of diesel is 2.68kgs of CO2 and a kWh from the grid is 90 grams of CO2, diesel fans would have resulted in ~25T of carbon, whereas we only produced 666kgs of emissions or ~0.7T.

Note: Our electric frost fans resulted in a 97% reduction in carbon emissions!

What is the payback period on electric wind machines?

Electric frost fan table showing the length of the incremental payback period is 2 years
Electric Frost fan payback period

Above is a table from a recent presentation we did at the Sustainable Energy of New Zealand conference. It shows that there is a difference in the actual cost of installing two electric fans compared to the estimated cost of installing two diesel wind machines. With an additional $32k in capital cost, and an annual energy savings of nearly $20k ($2.70 price per diesel), you can see that these fans will pay off their additional capital expense in under 2 years, and their whole capital expense in 9 years. I

In other words, electric wind machines are a "no-brainer" for new developments, but with a 10 year expected lifetime, it's hard to justify ripping down existing infrastructure from a CapEx vs OpEx perspective. It's definitely worth it for the climate, though!

Note: The price of diesel is a huge factor in the payback period calculations. If you expect the price of diesel to continue to rise, this payback period will get even better.
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