Decreasing Energy Use and Cost of Grain Drying by Extending Drying Period Using Ground-Stored Heat

Drying was continuous for the most part during the first year. However, there were two periods of interruption for cold weather. When extremely cold temperatures were encountered by the relatively warm and moist air exhausting from the grain at the top of the bin, frost accumulated to the extent that ventilation pathways were becoming completely iced shut and an ice layer on the surface of the grain began to form.

Twice during the drying period the fan was shut off because of temperatures falling below zero. The two periods were each six days in length. Opening the larger doors on top of the bin would allow ventilation when the vents frost over but would exacerbate the problem of cold air creating an ice skin on the surface of the grain that blocks air flow. This is a significant problem that needs to be addressed, although in this case it was fairly easily solved by turning the fan off during these periods.

The drying period was between 11/05/17 until 01/29/18, 85 days. With the interruptions for cold weather there were actually 74 days of fan operation, with an average ambient temperature of 25.1 F. Although the interruptible power rate was used to calculate cost, which would have required shutting power off for up to four hours per day, fan and pump operation was continuous because the small cost of drying in this trial did not warrant the monthly charge for separate metering and control of power supply. Interruption of power would have extended the drying period accordingly, from 74 days (24 hr/day) to 89 days (20 hr/day) but would have had little impact on trial results.

The conclusions after the first year of drying were that drying could be continued not just into the winter but throughout the winter into the spring and that better fan selection for the static pressure at the desired airflow rate could further reduce energy costs for drying. A quarter hp fan (Cincinnati Fan Company, 309 watts as measured by a watt meter) was purchased to replace the larger fan (680 watt) used during the first year, and a second heat exchanger was added to increase heat extraction from the ground loop, which made it possible to decrease the water flow rate by turning the circulating pump from high (87 watts) to low (60 watts). (It should be emphasized that the smaller fan was not necessarily more efficient than the larger fan, just a better match for the application. At 0.5" static pressure the smaller fan can move about 80% of the air volume the larger fan moves, but at 1.5" the larger fan's airflow rate would be over four times that of the smaller fan.) The new fan and heat exchangers were also relocated so that the duct elbow was further from the fan, in response to duct layout recommendations that placed elbows no closer than 30" from the fan in order to optimize airflow through the fan.

On November 6 the bin was filled to a depth of about 15' with corn (about 3000 bu) that averaged 18.7% moisture content. The manometer measured a combined positive and negative static pressure of about 0.5" water, corresponding to an airflow rate of about 1100 cfm. The fan was estimated to contribute about 0.75 F to the air stream temperature. Operating the circulating pump on low resulted in a water flow rate of 3.8 gpm. Entering and exiting water temperatures were recorded for both heat exchangers in order to calculate ground heat flux to the air stream. Fan operation was controlled manually to exclude periods that were too warm (upper 30s F) or too cold (below 0 F). Drying was terminated March 12, and the corn was hauled to the elevator for an accurate measure of bushels and moisture content. The ambient air temperatures during the time the fan operated was 21.4 F.

Delivering ground heat to the dryer air stream was even less expensive than it was projected to be. During the first year of the trial heat supplied by the fan together with ground heat increased drying air temperatures an average of 7 degrees F. Ground-heat provided about 32 million BTUs over the drying period using a quantity of electricity for the pump (136 kWh) that alone would have supplied only 0.463 million BTUs (1.4% of the heat provided). The ground-heat supplied to the air stream was estimated to cost 2% of what it would have cost using LP (assuming an electricity rate of $0.064 cents/kWh and 383 gallons of LP at a cost of $1.25/gal). Electricity used by the fan (680 watts) was by far the greatest fraction of the energy used.

Another surprise was how far into the winter drying is possible in this manner. Since the difference between air temperature and soil temperature becomes bigger as winter progresses, winter is even more optimal for drying in this manner than fall. By the end of January ground temperatures as measured at the heat exchanger inlet had declined about 12 degrees F since the start of the drying period, but the drying airstream temperature could still be increased 10 degrees F on a day of average temperature.

After the first year the corn was hauled to the local elevator in early March. Below the drying front the measured moisture content was 14.4%. Because the drying front had not completely moved through the top of the grain mass the overall average moisture content of the corn was 15.0%. There was a total of 2659 bushels. The energy cost of drying was determined by multiplying the kilowatt-hours used (1356 kWh) times the local electricity rate ($0.064/kWh for the interruptible rate). The cost for electricity used by the fan and pump was $85. The energy cost for drying was $0.006/point of moisture removed. For comparison, LP drying at $1.25/gal LP would cost $0.0275/point of moisture from each bushel.

The drying period in the second year of the experiment was 127 days, with an average temperature of 17.8 F. The fan operated only 90 days during this period, mainly because of extremely cold weather. Average air temperature during the time the fan operated was 21.4 F. An unanticipated problem developed as a result of a power interruption resulting in freezing in the heat exchangers before power was restored. The extended cold period without a good means of artificially heating the exposed components of the ground loop resulted in more down time of the system than would have been necessary based simply on extremely cold temperatures. (Better enclosure to allow artificial heating or perhaps a solar operated pump to ensure continuous pump operation even during power outages would be a good idea.) The drying period was terminated earlier than desirable (March 12) because of a warm extended forecast. The average temperature increase from ground heat was 9.2 F. As determined after hauling the corn to the elevator, there were 2930 bu of corn averaging 15.3% moisture content. The drying front was evident in the upper third of the bin. The dried corn in the lower part of the bin averaged 14.4% moisture content.  The average BTU/hr value from ground heat was 12,144, requiring only 60 watts costing $0.0038 using $0.064/kWh electricity. This is equivalent to $0.181 worth of LP using LP at $1.25/gal but costing only 2.1% as much.

The ability to use climate normals for air temperature and relative humidity, fan performance specifications, and psychrometric equations or graphs to predict costs, energy use, and rates of drying under different drying scenarios has been corroborated in this experiment. Air volumes and time requirements for drying were initially estimated using psychrometric and climate data for the appropriate time periods. The close correspondence between estimated and measured time needed for drying provides a basis for cost comparisons under different methods and exploring different scenarios.

For the second year of the trial, the cost of drying was anticipated to be reduced from $0.006/point used in this years trial to $0.0031/point of moisture removed from each bushel. The beginning corn moisture content in the second year was 18.7% rather than 20.4% as in the first year. Although a smaller air volume is required to dry fewer points of moisture from corn, at this moisture content a higher air volume/point of moisture is required because the drying air removes less water per volume of air. In addition, the deeper grain column in the second year made drying somewhat more energy intensive by comparison with the first year since fan horsepower requirements increase exponentially as depth and static pressure increase. So although the anticipated cost reduction after the first year to $0.0031/point was not achieved, the cost was reduced to $0.0051/point and would have been close to the anticipated cost given the same corn moisture content, depth, and climatic conditions as in the first year.