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III. Calculation of the Total Available Energy: The amount of energy available for resale can depend on two factors. The first is the heat energy available given the maximum rate that at which gas can be generated on a continuous basis, which I shall refer to as the electric power potential of the farm. The second is the amount and timing of electricity use for the farm’s dairy operations – the load curve. Electric Power Potential: The gross amount of energy that may be generated on a farm is best approximated as linear in the number of cows. This is because the amount of gas generated per pound of manure and the heat value of that gas are not dependent in any well understood fashion on the amount of manure, and the amount of manure is linear in the number of cows. Despite the apparent simplicity, the range of values obtained in operation for the amount of energy available per cow varies tremendously. Robert Fick at Alliant energy reports that European generators obtain 0.15 kW of electrical power per cow on a continuous basis, while their American counterparts manage 0.2 kW (or 4.8 kWh/day). The difference probably owes to differences in animal size and feed. These figures correspond to generators using micro-turbines, with an assumed efficiency in electricity generation of 28%. This suggests that American digesters generate gas with a power potential of 0.71 kW on a continuous basis (or 17.14 kWh/day). These numbers are significantly higher than the conventional wisdom used to suggest. Parsons (1984) suggests a gas yield of 54 cubic feet per cow per day. Combined with an estimated heat value of 600 Btu/cubic foot (which may be a little high) this yields a power potential of 0.4 kW on a continuous basis (or 9.49 kWh/day). Assuming a standard heat engine and generator with an efficiency of 21%, we are left with 0.083 kW continuous (or 2 kWh/day). With a micro-turbine this would amount to 0.112 kW continuous (or 2.66 kWh/day). The discrepancy seems to arise from the estimated gas yield. Haubenschild Farm7 reports an average daily gas yield of 139 cubic feet per cow. This more than doubles their design estimate of 65 cubic feet/cow. They also report average electricity generation of 5.5kWh per cow per day, using a standard heat engine – which unsurprisingly also more than doubles their design estimate. Similarly, Craven farms used a design estimate of 65 cubic feet/cow, although they do not report the rates they have achieved.8 Based upon this information, it seems likely that Dr. Fick’s estimate is appropriate. While the reasons for the variation in gas yield are yet to be clearly explained and may be related in part to the addition small quantities of other digestible wastes such as newspaper, it seems likely that the gas yield attainable will, in the long run, converge on a figure that allows for continuous electrical output of approximately 0.2 kW per cow. 7 Nelson and Lamb (2000) 8Oregon Office of Energy 6PDF Image | The Economics and Feasibility of Electricity Generation using Manure Digesters on Small and Mid-size Dairy Farms
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