Swine Digester for RNG 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. 

There are 15 visible sheets. 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  (where you are now)
• 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 fine-tune the analysis
• Nutrient inputs- see the extra inputs in this sheet to fine-tune the analysis
• Winter Biomass Crop Budget - extra inputs for the cost of growing a cereal rye winter biomass crop as an extra feedstock
• Prairie Grass_Key_Inputs - extra inputs for the cost of growing prairie grass as an extra feedstock
• Prairie Grass_Budget - crop input quantities and prices related to growing and harvesting prairie grass or perennial grass
• 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
• Nutrient calculations - 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
• Complex Inputs - an area to enter a more detailed list of capital cost items than is allowed in the Financial Inputs sheet.

This spreadsheet does not calculate the biogas output of a digester from manure.  The EPA AgSTAR AD spreadsheet screening tool is recommended for that calculation if the information is not available elsewhere.  That tool and its user manual are available at the AgSTAR website. 

Some parts of this spreadsheet are repurposed from the Cost of Renewable Energy Spreadsheet Tool (CREST) version 1.4 by Sustainable Energy Advantage, LLC under contract to the National Renewable Energy Laboratory.  The model output has been changed to investment criteria such as net present value and internal rate of return at a user-entered electricity sale price and other inputs, rather than the cost of energy as in that model.  Some of the prairie grass enterprise budget format is from the To Grow or Not Grow switchgrass budget spreadsheet by Jacobs, Mitchell, and Hart.

This research is part of a regional collaborative project supported by the USDA-NIFA, Award No. 2020-68012-31824, Consortium for Cultivating Human and Naturally reGenerative Enterprises.  Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.