Energy costs for corn drying and cooling
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Grain needs to be dry to be stored through warm weather, but it takes energy to remove moisture from grain.
Fortunately, there are things growers can do to estimate the costs of drying and cooling corn and manage energy use.
Energy use per bushel per percentage point of moisture removed for gas-fired drying varies widely among dryers. Plus, the percent of moisture to be removed varies widely from field to field and year to year.
However, energy use per bushel per percentage point of moisture removed is fairly consistent for a dryer that’s similarly managed from one year to the next.
The best source for energy use information about drying is actual records on the quantity of grain dried, amount of moisture removed and energy used to remove that moisture.
We encourage managers of gas-fired dryers to collect the information necessary for calculating annual drying costs. In some cases, you may want to install extra gas and/or electric meters to get the needed data.
If you don’t have information from actual records, use the following figures to roughly estimate energy use for gas-fired drying.
These figures do not factor in the energy savings that would result from recirculating part of the dryer’s exhaust air. Note that liquefied petroleum gas (LPG) is mostly propane.
Estimated energy use for gas-fired drying:
Burner: 0.02 gallons LPG per bushel per percentage point of moisture removed
For dryers that use natural gas, the equivalent number would be about 0.0184 cubic feet of natural gas per bushel per percentage point of moisture removed.
Fans: 0.01 kWh electricity per bushel per percentage point of moisture removed
Factors that affect gas-fired dryer energy use
The type and design of the dryer is the most important factor, as energy use is affected by factors such as:
Airflow per bushel.
Direction of airflow relative to grain direction.
Grain depth or column thickness.
Grain stirring or mixing.
Rapid cooling in the dryer vs. slow cooling in a separate bin.
Using partial exhaust air recirculation.
Frequently, there are trade-offs between energy efficiency, capacity (bushels dried per hour or day) and grain quality. For example, things that improve energy efficiency can sometimes reduce capacity.
Unfortunately, we don't have good data for specific dryer types or brands and no organization currently conducts independent tests on dryer energy use. The best we can do is to make sure the manufacturer's data makes sense and that we're comparing the same things when looking at data for different brands.
As you'd expect, dryers use less energy in warm, dry weather, and more energy in cold weather. Gas use is roughly proportional to the number of degrees it heats the air.
For example, heating air from 0 to 240 degrees Fahrenheit takes about 20 percent more gas than heating air from 40 to 240 degrees.
The following factors offset the energy savings for heating drying air during warm conditions:
Warm air is less dense than cold air, so fans move less air. This tends to reduce drying capacity when outdoor air is warm.
Warm outdoor air usually holds more moisture than colder air, which also tends to reduce drying capacity.
Less moisture is lost during cooling when outdoor air is warm, because moisture loss is a function of the difference between the temperature of the hot grain and outdoor air.
It takes slightly more gas per point of moisture removed at low moisture contents than it does at higher ones. And of course it takes more gas per bushel for more total points of moisture removed.
This means that anything growers can do to get corn out of the gas-fired dryer sooner – for example, using dryeration or combination drying – will improve the efficiency of gas used and reduce total gas usage.
Don't dry grain more than necessary. Overdrying for safe storage, marketing or the grain’s final use increases energy use, reduces weight of grain available for sale, increases breakage susceptibility of grain kernels and reduces overall profits.
Careful management and/or high-quality moisture sensors and dryer controls can reduce problems with overdrying.
Researchers and farmers are finding differences among hybrids in energy requirements for artificial drying. Unfortunately, we don't yet know enough to recommend one hybrid over another on this basis.
However, corn yield trials do indicate some high-yielding hybrids have consistently lower harvest moisture. Using these hybrids would reduce drying costs.
The more corn that drying air must pass through before exhausting from the dryer, the more saturated the air will be and the lower gas use per bushel will be.
However, increasing grain depth or column thickness also increases moisture variation when the dryer is unloaded. Increasing grain depth also tends to reduce dryer capacity.
Losing dryer capacity with increasing grain depth is especially critical for in-bin continuous flow dryers, which usually give the best combination of capacity and efficiency with grain depths of four to six feet.
In theory, drying is more efficient at higher drying temperatures because it takes less energy to evaporate water at higher temperatures. In reality, the drying temperature’s effect on energy efficiency probably depends more on factors like airflow per bushel and grain depth or column thickness.
However, we do know that reducing drying air temperature reduces dryer capacity and improves grain quality – specifically, fewer cracked and broken kernels, better test weight and less starch damage.
Increasing airflow per bushel increases drying rate and tends to decrease moisture variation in the dried grain. However, increasing airflow also increases gas use per bushel.
Energy use for natural-air drying – specifically, the electricity for operating natural-air drying fans – is very weather- and airflow-dependent, but can be predicted for given weather conditions and airflow.
The best information source about energy use for natural-air drying is several years of farm records for moisture removed, bushels dried and electricity used. The more years of records, the better, because energy use per bushel can vary greatly from year to year.
University of Minnesota drying studies, based on many years of weather data, indicate the following electric energy use values are reasonable averages for drying corn from 21 percent moisture to 16 percent. This assumes the drying bin is equipped with a fan that can supply 1.0 cubic foot of air per minute per bushel (cfm/bu) of grain in the bin.
Note that you can hold 16-percent-moisture corn through the winter, but you’d need 15 percent moisture or less for storage in warmer weather.
Average fan energy use – shown as kilowatt hours per bushel of grain dried (kWh/bu) – for natural-air drying corn in southern Minnesota, from 21 to 16 percent moisture at 1.0 cfm/bu:
Oct. 1 harvest: 0.75 kWh/bu
Oct. 15 harvest: 1.0 kWh/bu
Nov. 1 harvest: 1.25 kWh/bu
Factors that affect natural-air drying energy use
Energy use per bushel can be twice as large in a cool, wet fall as in a warm, dry one.
Grain won’t get as dry during cool, wet weather, but the chances of spoilage are low in cool weather. Adding supplemental heat – to heat drying air beyond outdoor temperatures – would speed drying in cool years, but supplemental heat greatly increases total energy use and cost.
Contrary to popular belief, spoilage in natural-air dryers is actually less likely during cool weather because molds grow more slowly at low temperatures.
If using lower-than-recommended airflows, the risk of spoilage increases. If using higher-than recommended-airflows, energy use per bushel increases.
You can deliver the same airflow per bushel with smaller fans in large-diameter, shallow bins compared to narrow-diameter, deep bins. Although large-diameter, shallow bins are initially more expensive than narrow, deep bins, energy savings make up the difference over the life of the bin.
Layer filling, which involves filling a bin a few feet at a time over a period of several weeks, can also save energy in natural-air drying.
The first layers put in the bin rapidly dry because airflow per bushel is quite high in a partly filled bin. Because these layers dry rapidly, layer filling also allows you to start filling the bin with wetter corn.
Hybrids that mature earlier and dry down faster in the field can be harvested and dried earlier in the fall, when energy use per bushel is lower.
Combination drying uses a gas-fired dryer to dry corn from high harvest moisture down to about 21 percent moisture, then uses a natural-air dryer to dry the corn to a safe storage moisture.
Use the energy values given earlier to arrive at a total energy cost for combination drying.
Cooling and aeration
The energy needed to cool hot grain from a gas-fired dryer or cooling grain for winter storage depends primarily on the fan motor’s size and the number of hours the fan operates. Note that hot corn transferred directly into storage should be cooled to outdoor temperatures within about 24 hours after it leaves the dryer.
The fan size, fan operation time and fan energy use required for changing grain temperature are much smaller than the fan size, fan operation time and fan energy use required for changing grain moisture.
Time required to move a cooling front through a bin of grain – called a cooling cycle – depends on airflow per bushel, measured in cubic feet of air per minute (cfm) per bushel.
Use the following calculation to get a rough estimate of the hours required to cool grain:
Cooling time in hours = 15 / Airflow in cfm per bushel
To reduce the stored grains’ harvest temperature to the temperature recommended for winter storage (20 to 30 degrees Fahrenheit in the upper Midwest), it’s best to cool the grain in stages, reducing the temperature by 10 to 15 degrees at a time. Completely cooling a bin of grain can require two to four cooling cycles.
Motors on grain drying and aeration fans usually draw about 1 kW of electrical power per rated horsepower (hp). The total kilowatt hour (kWh) of electric energy use is kW of electrical power drawn multiplied by the fan motor and the number of hours of fan operation.
Thus, the fan motors’ electric energy use is approximately:
Fan energy use in kWh = Motor power in hp x 1 kW per hp x hours of operation
Example energy cost calculations
Estimate energy cost per bushel for drying corn from 21 to 16 percent moisture when liquefied petroleum gas (LPG) costs $2.00 per gallon and electricity costs $0.10 per kilowatt hour (kWh). It’s assumed that the final point or two of moisture would be removed during the cooling process.
Points removed = 21% - 16% = 5 points moisture removed
Energy cost = (5 points x 0.02 gallon LPG per bushel per point x $2.00/gallon)+ (5 points x 0.01 kWh per bushel per point x $0.10 per kWh)
= (0.1 gallons per bushel x $2.00 per gallon) + (0.05 kWh per bushel x $0.10 per kWh)
= $0.20 per bushel (LPG) + $0.005 per bushel (electricity) = $0.205 per bushel
Estimate the average fan energy cost for drying 21-percent-moisture corn harvested Oct. 15 and dried at 1.0 cubic foot of air per minute per bushel (cfm/bu) in a natural-air dryer in Minnesota when electricity costs $0.10/kWh. Note that you may need to operate the fan in the spring to complete drying.
Electrical energy cost = 1.0 kWh per bushel x $0.10 per kWh = $0.10 per bushel
Using equations from the “Cooling and Aeration” section, estimate the cost per bushel to aerate a 10,000-bushel bin of dry corn using a 1/3 hp fan that delivers an airflow of about 0.15 cubic feet of air per minute (cfm) per bushel. Assume it takes four cooling cycles to completely cool the grain and that electricity costs $0.10/kWh.
Time for cooling cycle = 15 / 0.15 cfm per bushel = 100 hours
Total fan operation time = 4 cycles x 100 hours per cycle = 400 hours
Fan energy use = 0.33 hp x 1.0 kW per hp x 400 hours = 133 kWh
Fan energy cost = 133 kWh x $0.10 per kWh = $13.30
Fan energy cost per bushel = $13.30 / 10,000 bushels = $0.00133 per bushel
Other drying costs
Although energy costs for grain drying are very important, don’t forget to consider other grain drying costs when comparing drying systems.
The total for the other costs of owning and operating a grain dryer can sometimes be as much as energy costs. Other costs include things like:
Equipment costs for dryers.
Controls, such as depreciation, taxes, insurance, maintenance, and repairs.
Labor to operate drying systems.
Additional factors that might be harder to quantify include:
Quality of dried grain (quality is usually better for slower, lower-temperature systems).
Lost marketing opportunities for grain that’s not dried immediately after harvest.
The value of storage space you get with in-storage drying systems.
Reviewed in 2018