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

Problem

Grain drying is energy intensive. The rule of thumb for the use of LP for drying is that it takes about 0.02 gallons of LP (1840 BTUs) for each point of moisture dried from corn. Natural air drying can be done for less cost and energy use if grain columns are shallow enough, but can only be done during a limited period in the fall when air conditions are good enough to do this. During a drying period between mid-October and mid-November in the Upper Midwest the air conditions are nearly ideal to dry corn to a moisture content of 15% (the equilibrium moisture content or EMC for corn under those conditions). Beyond mid-November it is increasingly difficult to dry corn to 15% moisture content using ambient air drying without adding heat to lower the relative humidity. For this reason, fan size needs to be adequately large to meet recommended airflow rates to complete drying by about mid-November. Electricity or LP are often used to heat air, but the economic advantage of natural air drying diminishes to the extent that expensive heat sources are used in this way. Although natural air corn drying is less energy intensive and cheaper than LP drying, the time constraints that require large fans for drying still result in substantial electricity consumption to move the required air volume through the grain at the recommended airflow rate. An inexpensive heat source could permit the grain drying season to be extended at least into December, which would result in a small fraction of the energy needed during the normal season for corn drying while maintaining relative humidity of the drying air sufficiently low to dry corn to 15% moisture content.

Solution

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.