Energy costs for corn drying and cooling

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.

• Operating temperature.

• Controls.

• 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 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.

View recommendations for natural-air corn drying in the Upper Midwest

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.

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.

How to use dryeration and in-storage cooling to dry corn

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

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