On-Farm Digester Assessment Tool

This spreadsheet tool has been designed to assist stakeholders in evaluating the economics of a farm-based anaerobic digester utilizing manure as the main feedstock but with additions of food waste and warm season grasses, and generating electricity from the biogas.  These instructions assume that you have a basic understanding of the use of MS Excel, as well as a basic understanding of how a dairy farm with an anaerobic digester functions.

This spreadsheet does not calculate the biogas output of a digester. The EPA Agstar AD spreadsheet screening tool is recommended for that calculation if the information is not available elsewhere. 

There are 14 visible sheets in the spreadsheet. The tabs are color-coded. Yellow tabs are for data entry. Green tabs are for data entry for the switchgrass calculations. Grey tabs contain calculations. The sheets are:

• Welcome - summarizes the purpose of the model and the individual sheets
• Background - authorship, version number, and related information
• Key inputs & outputs - starting point for a digester generating electricity from manure alone or with food wastes
• Financial inputs - see the extra inputs in this sheet to finetune the analysis
• Nutrient inputs - see the extra inputs in this sheet to finetune the analysis
• Switchgrass_Key_Inputs - extra inputs for the cost of growing switchgrass as an extra feedstock
• Switchgrass_Budget - crop input quantities and prices related to growing and harvesting switchgrass
• Sensitivity analysis - graphs and a few tables showing the sensitivity to key inputs
• Saved scenarios - manually copy and paste the results into the additional columns for side-by-side comparisons
• N_Availability - Table 1.2-12 from the 2023-24 Penn State Agronomy Guide with N availability default values
• Condensed cash flow - inflows and outflows with graphs
• Detailed cash flow -  inflows and outflows with supporting operational and financial information
• Nutrient chart - flow chart of the nutrient flows, and table showing the nutrient valuation calculations
• Nutrients - looks at the impact of digestion and solids separation on the fertilizer nutrient value of the digestate, and contains the calculations for the numbers shown in the Nutrient_chart sheet

Review the Welcome sheet, and then start at the Key inputs & outputs sheet. Review the defaults in the yellow cells for the 12 inputs in this sheet. The inputs include the planned volumes of manure and food waste that will feed the digester along with the expected rates of biogas generation from each. Three key financial inputs here are the electricity price, the tipping fee for the food waste, and the installed cost per kilowatt of electrical generation capacity. 

The economic analysis values the nitrogen and phosphorus nutrients in the manure, food waste and grass based on what it would have cost to purchase those nutrients as commercial fertilizer for crop production. Cropland available for nutrient application is needed for that calculation, along with how much of the cropland is double-cropped.

Use the two grey menus to select whether the system will include a solids separator and whether warm-season grasses such as switchgrass will be added to the digester.  If adding grass, indicate how many acres of grasses will be grown and added.

The estimated economic performance of the digester is summarized below the inputs in this sheet. Overall performance is indicated by the internal return to equity (IRR) before and after taxes and by the net present value. The net present value is also annualized for users who are accustomed to economic measures that are expressed on an annual basis.  A table and graph show a breakdown of the cash inflow and outflow components.

If that quick look has encouraged you to go further, you can customize the analysis for your own situation using the next four sheets. The Financial Inputs sheet displays the parameters for the conversion of biogas into electricity. Installed cost can be entered in more detail here along with expenses for operations and maintenance. The rest of this sheet relates to financing, taxes, and government incentives.

The Nutrient Inputs sheet contains the inputs relating to the total volumes and solids volumes and the nutrients nitrogen, phosphorus, and potash that move through the digestion process and solids separation. These inputs are used in the calculations in the Nutrients sheet and summarized in the Nutrient chart sheet. The availability of nitrogen to crops is important for valuing this nutrient.  See the copy of table 1.2-12 from the 2023 Penn State Agronomy Guide that is contained in the N_availability sheet for availability estimates appropriate for your situation.

The Switchgrass_Key_Inputs sheet contains inputs related to the cost of adding a warm-season grass such as switchgrass as an extra feedstock. These inputs are used in the main calculations only if the "Include warm season grasses …" toggle in the Key inputs & outputs sheet is set to "Yes."  The top part of this sheet shows inputs that relate specifically to the grass. Further down is a list of inputs from the previous sheets that can be modified here in case they change as a result of adding the grass.

The Switchgrass_Budget sheet contains a crop enterprise budget for the warm-season grasses.

The Sensitivity Analysis sheet contains several tables and a graph showing the impact of a 25 percent increase in each input on the net present value of the cash flows.

The main categories of cash flows are shown in the Condensed Cash Flow sheet.  See the Detailed Cash Flow sheet to drill down further into the cash flow calculations, including details of tax depreciation and debt payments. 

The Saved Scenarios sheet allows the user to copy the key performance measures from the current analysis over to a separate area where multiple runs can be compared side by side.