Energy Estimates
Example energy estimate calculations
All calculations are approximates only. Anyone seriously interested in building an anaerobic digestion system should undertake lab analysis of his/her intended feedstock. Only a lab analysis will provide you with an exact enough estimated to determine the economic feasibility of your project.
Key to the calculations: A kW is a unit of energy, while a kWh is the amount of energy produced in an hour. To determine total kWh in a year, you must multiply kW by 8,760 (number of hours in a year).
Manure only
Basic facts: A milking cow plus replacement produces 40 tonnes manure/year. Therefore, a herd of 200 milking cows plus replacements produce approximately 8,000 tonnes manure/year.
Energy conversion: 1 tonne of manure = 0.0055kW electricity 0.0055kW electricity = 48kWh electricity/year.
Electricity production: Therefore 8,000 tonnes manure = 384,000kWh electricity/year. However, realistically, anaerobic digestion systems only operrate 90% of the time (due to maintenance, etc). Therefore, a herd of 200 milking cows plus replacements will produce: 384,000kWh x 0.9 = 345,600kWh electricity/year.
Heat: When the biogas is combusted roughly 35% is converted to electricity, while 50% is converted to heat. As such, a herd of 200 milking cows plus replacements will produce: 345,600 kWh x 1.4 = 483,840kWh heat/year. Throughout the year, 30 – 50% of this heat will be required to maintain the anaerobic digestion system’s temperature. The rest is available, if possible, for use on-farm and at neighbouring facilities.
Revenue:
- If the electricity is sold for $0.10/kWh, the revenue from electricity sales will be: 345,600kWh x $0.10 = $34,560/year.
- If the electricity is sold for $0.14/kWh, the revenue from electricity sales will be: 345,600kWh x $0.14 = $48,384/year.
Manure and off-farm feedstock
Basic facts: An on-farm anaerobic digestion system can blend up to 10 – 25% off-farm feedstock and work effectively. The following calculations are for the dairy farm described above (200 milking cows plus replacements) with the addition of 25% (2,650 tonnes/year) dough, fats, oil and grease (high-energy feedstock).
Energy conversion: 1 tonne of high-energy feedstock = 0.055kW 0.055kW = 482kWh electricity / year
Electricity production: Therefore 2,650 tonnes of high-energy feedstock = 1,277,300kWh electricity/year. Once again, realistically, anaerobic digestion systems only run 90% of the time (due to maintenance, etc). Therefore, 2,650 tonnes of high-energy feedstock will produce: 1,277,300kWh x 0.9 = 1,149,570kWh electricity/year.
Heat: 1,149,570kWh x 1.4 = 1,609,398kWh heat/year.
Total electricity and heat production: When combined with the electricity and heat generated by the dairy manure from 200 milking cows plus replacements, total electricity generated from this anaerobic digestion system is:
- 345,600kWh (manure) + 1,149,570kWh (high-energy feedstock) = 1,495,170kWh electricity/year.
- 483,840kWh (manure) + 1,609,398kWh (high-energy feedstock) = 2,093,238kWh heat/year.
Total revenue:
- If the electricity is sold for $0.10/kWh, the revenue from electricity sales will be: 1,495,170kWh x $0.10 = $149,517/year.
- If the electricity is sold for $0.14/kWh, the revenue from electricity sales will be: 1,495,170kWh x $0.14 = $209,324/year.
