Rethinking corn drying: drying throughout the winter using ground-stored heat
A summary of on-farm research funded by USDA-SARE
Grain drying is energy intensive. Most of the corn grown for grain is dried using large quantities of LP- or natural gas-heated air at or soon after harvest. The rule of thumb using LP for drying is that it takes about 0.02 gallons of LP for each point of moisture dried from corn. Alternatively, some corn is dried in storage bins equipped with drying floors during a short period in the fall using large volumes of air that is heated only several degrees by the heat from fan operation (often called "natural air drying")1. Natural air drying can be done for less cost and energy use if grain depth is not excessive. In the Upper Midwest, ambient air conditions between mid-October and mid-November are nearly ideal for drying corn to a moisture content of 15 percent.
Although average temperatures continue to be low enough from November through March2 for safe storage and drying of corn that is less than 20 to 22 percent moisture content, during much of the period ambient air temperatures and humidities prevent corn from drying below 18 to 19 percent moisture without supplemental heat. Because the economic advantage of natural air drying diminishes to the extent that expensive heat sources are needed to heat air, it has not been considered to be economically practical beyond late November. Although natural air corn drying is less energy intensive and usually less expensive than LP drying, the time constraints that require large fans for drying result in substantial electricity usage to move the required air volume through the grain at the recommended airflow rates.
A different approach to drying corn
The cold winter temperatures in the Upper Midwest provide opportunities to prolong the drying period, at lower airflow rates and at safe storage temperatures, which would save substantial costs and energy use for fan operation. Cutting the airflow rate in half or to one third of the full airflow rate can be done for about 20 percent or eight percent, respectively, of the fan power required for the full rate3. The same volume of air can be moved through the grain operating a smaller fan at half the airflow rate for twice as long or a third of the airflow rate for three times as long for 40 or 24 percent, respectively, of the energy required for full flow. Yet, meeting targeted goals of 13 to 15 percent moisture content would require drying air temperature increases of six to 12 degrees F to sufficiently lower the relative humidity of ambient air. Using electricity, LP, or natural gas at current prices for heating air would negate much or all of the savings resulting from the lower fan operation’s energy usage. Despite the fact that colder air will hold less moisture than warmer air, drying corn to 15 percent moisture content during the winter would take very little additional air volume compared with drying during the fall. A low cost energy source for heating drying air is key to expanding the drying season from a limited time during the fall to utilizing the entire winter and into the spring.
A solution lies in ground heat (ground-stored solar energy). Ground heat is inexpensive to access and is the reason that ground source heat pump systems are so efficient and have become popular. In the fall, soil temperatures drop slower and reach minimum values later than air temperatures. The difference between cold winter air temperatures and warmer soil temperatures can be used to heat air for low- temperature grain drying, even without using a heat pump, which is the most expensive part of a ground- source heat pump heating system.
On-farm research results
A project funded by USDA-SARE (Project FNC17- 1080, Decreasing Energy Use and Cost of Grain Drying by Extending Drying Period Using Ground-Stored Heat) has provided ample demonstration that low-temperature drying using ground heat to warm the drying air can be done well into the winter using substantially less energy. The drying system included a ground loop (three 800- foot 3/4-inch water lines buried to a depth of 8 feet), an air-to-water heat exchanger, and a 3/4 hp fan that replaced the three hp fan that had been used on the 3000-bushel bin for natural air drying. Operation of a small circulating pump for the ground loop delivered enough heat to the heat exchanger in the drying air stream for an average eight to 10 degree F air temperature increase (Table 1). The energy cost for the heat was two to three percent of what the cost would have been using LP at $1.00/gallon. The objective of the first year of the project was to extend the drying period into the winter. The conclusions after the first year of drying were that drying could be continued 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. The second year of the project is currently underway. A quarter hp fan (309 watt) replaced 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 watt) to low (66 watt). The drying progress in the second year is on schedule to meet the objectives for further reduction in energy use and cost as predicted after the first year (projected to be $0.004/point of moisture removed compared to $0.02/point using LP at $1.00/gallon).
Potential limitations experienced
Drying can continue through the winter at temperatures below freezing, but precautions should be taken when air temperatures are extremely cold to keep frost formation on exhaust ventilation openings in the bin from restricting or stopping airflow. It may be necessary to stop the drying fan at temperatures near zero. There are a number of other reasons that it might be advantageous to interrupt the drying process by operating the fan only within certain parameters:
- A short interruption each day can assist the local utility to reduce demand during peak demand times, often in exchange for a lower electricity rate for the user.
- When ambient air temperatures are warm, ground temperatures and ambient air temperatures may be too similar, so that the humidity of the drying air cannot be lowered enough to dry grain to a targeted moisture content. Since the “shelf life” of corn is inversely proportional to temperature and starting corn moisture content, operating the fan when ambient temperatures are too high would unnecessarily shorten the allowable period for drying the corn. Although operating the fan at night if needed to keep grain cooler might still make sense.
- Another way to increase the drying air temperature besides artificially heating it is to operate the fan only during a warmer or less humid fraction of each day. Although this might require a larger fan to compensate for the shorter duration of fan operation, total cost may remain low because of the decreased capital cost for heating.
Under any of these circumstances it may be necessary or advantageous to exercise control (e.g. with thermostat or humidistat) over when to shut the drying fan off.
Cost and energy savings
Unlike with conventional dryers, energy costs have become so minimal for the system developed in this project that capital cost is the larger component of the total cost of grain drying. The research focus will shift to optimizing capital and total costs.
Energy use comparisons among drying systems are difficult because net energy values for delivered LP or natural gas are difficult to find and electricity can be produced with either fossil or alternative energy. Direct energy use comparisons between electric and LP heat sources have been avoided in this article. Emphasis has been on energy costs rather than energy usage, with the assumption that the correlation of cost and energy use is reasonably close.
It should be emphasized that both cost and energy consumption are very important, and the intent of the research has been to decrease both.
The ultimate goal of the research is to develop a low-cost and energy efficient drying system that is easy to use and provides grain with safe conditions throughout the drying process. All producers are concerned with controlling costs and should want to assess whether this system can fit into their operations. Most people are concerned with energy conservation for reasons beyond simply lowering costs, either for ecological reasons such as climate change or geopolitical reasons, such as excessive dependency on foreign fossil fuel sources. The shift in the proposed drying system from most of the cost being for purchased energy inputs (mainly LP or natural gas) to most of the cost being for capital cost (using local contractors) results in greater support for local economies. Whether for one or all of the above reasons, drying corn in the manner proposed should lead to a more sustainable and profitable agriculture and less impact on the environment.
- Low Temperature & Solar Grain Drying Handbook (Midwest Plan Service #22)
- Iowa Environmental MESONET, Iowa State University
- University of Minnesota Fan Selection for Grain Bins
Eric Jellum, Jellum Farm, research project coordinator, firstname.lastname@example.org