In my last blog it was established that the grain should not be heated by more than 5 C and that a 50,000 btu furnace could do this in 12 hours. What if we used the heat from a small gas driven generator set? The electric power generated could be used to power the aeration fan, and this set up would be ideal for remote bins where power from the grid is not available. OK let’s consider this. But how big should the gen set be? What will it cost for the gen set, and the fuel to dry Jim S’s 3500 bushels of wheat that is now 17%?
Jim is using a 5 HP fan. 1 HP is .7457 kW, so the gen set must be able of producing at least 5 x .7475 = 3.7 kW. At Princess Auto, I found a 7500 Watt, Westinghouse, gas Generator that had a 6.6 gallon (25 liter) fuel tank that would last 11 hours at half load. It cost about $1000. We would be burning 25/11 or 2.27 liters per hour.
The specific energy of gasoline is 46.4 MJ/kg and 1 liter of gas weighs 1.64 lbs/2.2 = 0.743 kg. One liter of gas will produce 46.4 x .743 = 34.47 MJ or 34,470 kJ. But we burn 2.2 liters per hour, 2.2 x 34,470 = 75,845 KJ expended as heat and electricity in one hour. We said we need 3.7 kW to power the fan, but a Watt is in terms of per second and we want it as per hour, so multiply by 3600. 3.7 x 3600 = 13,320 kJ per hour. The energy produced as heat will be the total energy produced by the burning gas, minus the electrical energy: 75,845 kJ – 13,320 kJ = 62,525 kJ per hour.
In the previous blog, we saw that our 50,000 btu furnace put out 52,750 kJ/hr and we saw that this would be the energy necessary to raise the temperature of the wheat by 5 C. That is at 100% efficiency. However we will be losing some of the heat; not all the heat energy will be going into raising the temp of the grain. Our gen set produces a little more heat than the furnace, and doing 11 hour runs would be enough. A 5 C rise in temp, is like pulling the temperature of the grain down by 5C. Using our rule of thumb that 15 C decrease reduces the MC by 1%, then 5C would reduce it by 0.3 percentage points. To go from 17 to 14.4 would take 2.6/.3 = 8.6 eleven hour runs. Let’s say 9 runs. Each run uses 25 liters, and let’s say a liter costs us $1. The cost for the fuel would be 9 x 25 = $225. One must bear in mind that this is for both the electricity to drive the fan but also for the supplemental heat.
One way to make this better would be to convert the gas gen set to natural gas. With an inexpensive conversion kit, this can easily be done. The cost of gas is about 2.5 times that of natural gas, so we could easily get our price of fuel down to $100.
Another idea, would be to use a car, truck or tractor that runs on gas, and simply park it right up against the fan so that the radiator is right next to the input of the fan. Shroud the whole thing with a tarp, so that the fan sucks in all the heat from the idling vehicle. If it is burning about 2 liters per hour, it will be giving off about the same amount of heat as our gen set, and will raise the temperature of the wheat by 5 C. We would avoid the cost of the gen set, but we would be paying over $200 in gas to get the grain dried.
In all of this estimating, it was assumed that the daily temp was constant. But we all know that the temp changes by 5, 10 or even 15 deg over a day. Can we use this variation in temp to save some money. Also on any given day we will have better and worse drying conditions. The humidity can be high or it can be low. Maybe we could do some careful cherry-picking to only do our runs when we will maximize our drying and reduce our overall cost? Should we be cycling heating and cooling? Just more to consider for future blogs.