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the negotiation of a contract with a power distributor is high, but inestimable. Maintenance costs are similarly elusive. In fact, the only input into the process that it is possible to cost is energy. Engineers at two independent organizations have confirmed that digesters consume about one third the amount of electrical energy4 they are capable of producing, regardless of size. The constancy of this relationship is because the vast majority of the input is used to achieve digester temperature, which is roughly linear the amount of manure to be regulated.5 Benefit-Cost Analysis: Given the dearth of sensible estimates, it is impossible to conduct a full cost benefit analysis. Instead, given differing assumptions regarding the electricity sale and purchase contract I calculate the net revenues that a farm could achieve on its electric generation, inclusive of saved electricity costs and net of energy inputs. From here on, I will refer to this figure as the electric margin. This is all that may be calculated with any sense of realism, given the numbers available to me. Given the electric margins, I analyze the likely interactions between the terms of the contract to buy and sell electricity on farms of different sizes. II. A Description of the System:6 A digester is often described as an extension of the digestive system of the herd itself. Manure is flushed or scraped from the stalls along with a certain amount of bedding. This waste is then heated and added to a series of tanks (usually two), where other waste, such as newspaper, may be added. In the tanks, solids may be separated out and water removed. The temperature of the manure is typically maintained at 95-105°F, although two stage digesters exist that start the manure off at about 130°F and then move it down to 95°F. Bacteria in the tanks digest the manure in an anaerobically (in the absence of oxygen), releasing gas. The gas is composed of roughly 55 percent methane, with the remainder composed mainly of carbon dioxide, as well as some hydrogen sulfide and traces of other gases. The collected gas is transferred to some sort of engine and generator. Some farms use a standard engine with a generator, while others use a micro- turbine. A micro-turbine is essentially a jet engine connected to a generator. The micro- turbine enjoys a slightly higher electrical efficiency than the heat engine: about 28% vs. 23%. It also is efficient in the capture of waste heat, capturing about 37% of the heat potential of the gas as usable heat energy. Despite these advantages several farms use heat engines. This is because micro-turbines cost more to install and because parts and technicians to work on micro-turbines are not easily found. The following flow-chart depicts the system along with its most important state and control variables. The loading rate of manure, its temperature, dilution and the proportion of it that is frozen dictate the amount of energy needed to heat the digester to 4 The somewhat surprising fact that a number of farms use electricity rather than gas to maintain digester temperature, is due to the favorable electrical price structure that helps make digesters financially viable in the first place. 5 Parsons (1984), p.25. See section IV of this paper for details. 6 The reader interested in a more detailed description of the digestion process is referred to Parsons (1984). 4PDF Image | The Economics and Feasibility of Electricity Generation using Manure Digesters on Small and Mid-size Dairy Farms
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