The digester_econ.xls spreadsheet on my web page has been fairly popular because of all of the interest in greenhouse gases and the destruction of methane that a digester provides. One limitation of this spreadsheet has been that it didn't provide much information on how much biogas a digester might produce. Last week I updated this spreadsheet and added a sheet that calculates biogas output based on the Chen-Hashimoto formulas that are also described in the EPA AgSTAR's Farmware software user instruction paper. As far as I can tell, my spreadsheet version gives a result close to what the Farmware software itself gives.
Of course, why use the digester-econ.xls spreadsheet at all when Farmware is available? That is a fair question. I developed the spreadsheet originally because it was called for under a contract with the Minnesota Department of Agriculture, several years ago before the current Farmware version was available.
Now that Farmware is available, my view is that the "spreadsheet vs. Farmware" choice is a matter of personal preference, sort of like the old "Ford vs. Chevy" question. The formulas are all available for viewing in the spreadsheet, so if you are trying to understand the calculations, that might be useful. But, Farmware is applicable to all of the major livestock species and digester types, while the spreadsheet is applicable mainly to dairy and to plug-flow or mixed digesters.