I began farming in 1998 after moving here from Washington State. Prior to moving here, I worked for Washington State University for 20 years at an agricultural research station in Puyallup (near Seattle). Our research centered mainly on nitrogen, phosphorus, and heavy metal chemistry in soil, as well as cover cropping. During that time I got my Masters in Soil Science from Washington State University. I currently farm 160 acres in North Central Iowa together with my brother. We use strip tillage/no tillage, intercropping, and cover cropping to grow mainly corn and soybean in rotation. Our farm has seven 3000-bushel grain storage bins all equipped with drying floors, which is an important asset for the proposed project.
Natural air grain drying can be a cheaper alternative to drying corn using LPG but is still energy intensive. Decreasing airflow rates by extending drying over a longer period in the fall and winter can save substantial amounts of energy in fan operation but can only be done if the drying air is heated enough to lower the relative humidity. In this project, ground-stored heat was collected by circulating antifreeze through buried polypropylene water lines to a heat exchanger in front of the drying fan allowing the drying air temperature and relative humidity to be optimized throughout an extended drying period. Because ground heat was so inexpensive to deliver to the fan and cutting airflow rate in half resulted in a much smaller fan horsepower requirement drying cost and energy use was substantially reduced.
Cutting drying airflow rates in half and doubling the drying period would move the same volume of air through a grain column but reduce the required fan to only 19% as large and would accomplish drying using about 40% of the energy for fan operation. During the winter soil temperatures remain warmer than air temperatures and closer to the average annual temperature. In early October air and soil temperatures are very close. But by late December soil temperatures 4 feet below the surface in northern Iowa are 15 to 20 degrees warmer than air temperatures. In this project, ground-stored heat will be collected by circulating antifreeze through buried polypropylene water lines to a heat exchanger in front of the drying fan allowing the drying air temperature to be optimized throughout an extended drying period for very little additional energy cost for pumping water. Flow from each loop will be controlled separately so that the size requirements of the heat field can be matched to needed airflow volumes and rates as determined by the ability to maintain temperature during the drying season. Cost of drying at static pressures will be determined by using two bins with fan plenums in close proximity. The bins will be filled with 2500 and 3000 (roughly 21% moisture content) bushels of corn and connected to a common plenum with a single heat exchanger that will heat air for both bins. A fan (1/2 h.p. for 2500 bushels and 3/4 h.p. for 3000 bushels) for each bin will deliver an air volume calculated to dry the corn by mid to late December. Incoming and outgoing water and air temperatures and progress of the drying front will be monitored throughout the drying period. Electricity consumption will be monitored through the Load Control Center provided by the utility.
System design continued to evolve following submission of the proposal, resulting in some modification of the original plan. Rather than design the entire experiment based on expected results, corn was dried in a single bin to prove the proposed concept was sound and corn could be dried in the manner proposed and in accord with predicted rates and costs. Experience from the initial year would then be used to suggest the most appropriate scenarios and design changes to implement in the second year.
In July, trenches were dug for placement of water lines to a depth of eight feet.
Three 3/4” high density polypropylene water lines were spaced one foot apart in the bottom of the trench. The three water lines made a circuit from the drying bin traveling out in one trench for 400’ and returning in another 400’ trench to make an 800’ loop. At the midway point the lines surfaced into a heated building where both entering and exiting water lines were individually valved before being manifolded together.
The three-speed circulating pump (Grundfos UPS15 58FC), flow meter, expansion tank, ports for measuring water temperature, and metering ports for adding antifreeze solution (23% propylene glycol) were located there. This was done in order to evaluate the rate of heat recovery and to assess in the future the number of water lines actually required. At the bin, entering and exiting water lines were again manifolded together using heat-fusion couplings before surfacing and passing through a 30” by 30” three-row air-to-water heat exchanger placed in front of a 3/4 horsepower Sukup bin fan.
Both the fan and heat exchanger were enclosed in a cabinet and connected to an adjacent open-front building using a thirty-inch diameter corrugated plastic culvert so that incoming air could be filtered to protect the heat exchanger from plugging with dust, leaves, or snow.
The cabinet was wood framed with cedar and skinned with twin-wall polycarbonate and designed to allow for air to bypass the heat exchanger within the cabinet to adjust air temperature without adding unnecessary static pressure which would decrease airflow. Temperature ports were placed where water entered and exited the heat exchanger to track ground heat supplied to the air stream. Incoming air temperature and plenum air temperature at the drying floor entrance were also tracked. A manometer was used to measure static pressure under the drying floor to gage airflow using the fan performance specifications. A watt-meter was used to measure electricity consumption rates for the circulating pump (87 watts) and drying fan (680 watts).
Corn harvest was later in the fall than usual. The eighteen-foot diamater bin was filled to a depth of thirteen feet (approximately 2600 bushels) with corn (20.2% moisture content). The fan was started November 5th, and the heat exchanger employed beginning November 16th. The air temperature increase due to the fan motor operation was calculated to be about 1.25 degrees F. The remainder was from ground heat. Water and air temperatures entering and exiting the heat exchanger were measured almost daily and over a broad range of temperatures. The relationships (analyzed by regression analysis) between the measured drop in water temperature as it passed through the heat exchanger and the corresponding air temperature increase was used to calculate an average temperature increase of drying air throughout the drying period based on ambient air temperatures measured by the National Weather Service for our location. Ground temperatures (and water temperature entering the heat exchanger) declined over the course of the drying period. In addition, a fraction of the air was allowed to bypass the heat exchanger during some of the drying period to limit the temperature increase and keep relative humidity near the target. So the above relationships were determined in two-week intervals. The slopes of the regression curves changed slightly with time. The equations appropriate for each period were used to calculate drying air temperature increases from climatology data. As the end of the projected drying period approached, periodic sampling of grain at different depths from the top of the grain column were obtained to track the drying front.
Drying was continuous for the most part. 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 drying days. 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 little impact on trial results.
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 the first year of this experiment.
Delivering ground heat to the dryer air stream was even less expensive than it was projected to be. The fan operated without the heat exchanger for the first ten days of the drying period, elevating the air temperature just 1.3 degrees F. Then the heat exchanger was enclosed within the air stream and the pump turned on. The three-speed circulating pump was kept on the highest setting (87 watts), which circulated the antifreeze solution at a rate of 6.3 gallons per minute. During the next 65 days heat supplied by the fan together with ground heat increased air temperatures an average of 8 degrees F. The elevation of drying air temperature averaged 7 degrees F over the entire drying period, including the first ten days before ground heat was included. 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.
As of this report (1/29/18) the drying front is moving through the top of the grain column, and the drying period will be terminated. Below the drying front the measured moisture content was 14.2%. The average moisture content for the entire grain mass will not be known until it is hauled to the elevator but is currently estimated to be 14.5%. 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. Assuming an average moisture content for the 2600 bushels dried, the energy cost is $0.006/point of moisture. For comparison, LP drying at $1.25/gal LP would cost $0.0275/point of moisture from each bushel. 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. An attempt to assess capital costs, which is likely to be the largest fraction of total cost when drying using ground heat, will be made for the final report.
Work plan for next year
Whether the heat removed from the ground during drying, estimated at 32 MBTU, is entirely replaced during the summer by natural reheating of the ground or whether a new slightly lower equilibrium temperature is established is unknown. But it is likely to be beneficial if the ground loop can be used for cooling during the summer months or warm air passed through the heat exchanger during the heat of summer solely to help recharge the ground with heat. It would be beneficial to discover the ease to which ground temperatures could be increased during the summer and maintained into the winter months. The degree to which solar energy during the summer can be stored in the ground in such manner will be explored next year.
Although this project was successful, the results of the first year drying trial suggest some alteration of the goal. The initial goal was to extend the drying season into early winter to decrease drying costs. It may be that winter should be considered the optimal drying season when drying in this manner, using low airflow rates and much less energy and at temperatures that are safe for grain storage. If the problem of ice formation blocking airflow in extreme cold temperatures can be adequately addressed there is still room for further substantial reductions in cost and energy use.
For the coming year, along with the efforts to increase the ground temperature at the beginning of the drying period, the University of Minnesota Fan Selection for Grain Bins will be used to select the most efficient 1/3 hp fan. The duration of drying will be extended from approximately November 1st to February 15th, which should be a long enough period to allow for power interruptions. The expected equivalent of 85 fan operation days needed during this 107-day period is estimated to consume about 746 kWh of electricity to dry 2600 bushels of corn, further decreasing the cost of drying from $0.006/ point used in this years trial to $0.0031/point of moisture removed from each bushel.
Educational & Outreach Activities
Much of the outreach effort will be conducted with the cooperation of the County Extension Directors (please see Letter of Reference for more of their thoughts). A few ideas that they suggested are as follows:
— A tour of grain facilities could be conducted with a Program Specialist from ISU to discuss grain drying.
— Demonstrations about grain drying could be conducted in the late summer to educate producers.
— Coordinate with utilities, ethanol companies and grain drying companies to conduct a program on proper drying.
— Data from the project could be posted on the internet connected with Extension web site, provide press releases and interviews with media.
— Provide information at Extension programs as part of crop production events.
— Host a display at a farm show to display how the system works.