With rising energy costs and concerns over fossil fuel usage, more homeowners are looking into alternative energy sources like manure-fueled generators to meet some or all of their home’s power needs. Manure fuel, also called biomass gas, is a renewable energy that can be produced on farms and rural locations with livestock.
In this comprehensive guide, I’ll explain in detail how a manure-fueled generator system works, the benefits and considerations, and provide step-by-step instructions on how to size, setup and operate one properly to power a home.
Overview of Manure-Fueled Generator Systems
A manure-fueled generator uses animal manure as a fuel source to produce electricity through a process called anaerobic digestion. Here’s a basic overview of how these systems work:
Manure Collection: The raw manure from cows, pigs, chickens or other livestock is collected and stored until needed. This provides the fuel source.
Anaerobic Digester: The manure is fed into an airtight tank called a digester where bacteria break down the organic matter in the absence of oxygen and produce a mixture of gases, mainly methane and carbon dioxide.
Gas Processing: The biogas from the digester goes through cleaning and scrubbing to remove impurities and condense the methane gas content.
Generator: The purified methane gas is then used to power an engine generator to produce electricity. This powers the home’s electrical system.
Waste Heat Capture: Waste heat from the generator can optionally be recovered and used for heating water or spaces.
The entire process from manure collection to electrical power generation takes place onsite at the property with the livestock manure source.
Benefits of Using Manure-Fueled Generator
Switching to a manure-powered generator has several excellent benefits:
Renewable Energy Source: Manure is a renewable fuel that can be produced continuously from livestock operations. This provides energy independence and security.
Reduce Electricity Bills: After the initial system cost, the free renewable fuel can drastically cut monthly electric utility bills.
Lower Environmental Impact: Manure power avoids fossil fuels. It also properly processes the manure for odor control and nutrient management rather than raw spreading.
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However, there are some downsides to consider as well before switching to manure power which will be covered later.
Sizing a Manure-Fueled Generator System
The first step in planning your manure generator is properly sizing the system components to match your property’s livestock manure production and electrical demands. Here are the key factors in sizing:
Estimate Manure Quantity Available
The number and type of livestock you have determines how much raw manure is available as fuel:
- A dairy cow produces 80-100 lbs of manure per day.
- A pig produces 15-20 lbs per day.
- Chickens produce 0.11 lbs per day per bird.
Table of typical manure output:
To find your total daily manure production, multiply the number of each animal by the daily manure values.
For example: 30 dairy cows x 100 lbs/day = 3,000 lbs of manure produced per day.
Estimate Methane Production
The methane gas that can be produced from the manure depends on the animal type:
- Cattle manure – On average, counts on 10-20 cubic feet of methane per pound of manure.
- Swine manure – Expect approximately 8-12 cubic feet per pound.
- Poultry manure – Generates around 4-10 cubic feet per pound.
Take your total manure weight and multiply it by the methane factor for your livestock type.
In the example of 3,000 lbs/day from dairy cows, assuming 15 cubic ft/lb would produce 45,000 cubic ft of methane per day.
Determine Gas Requirement
Next, estimate the amount of gas required to meet your home’s electricity needs:
- Natural gas generators need 850-950 BTU per kWh generated.
- Methane has an energy density of approximately 1,000 BTU per cubic foot.
Check your average daily kWh electricity usage, divide by the BTU/kWh needed, and multiply by 1,000 BTU/cu-ft to get the required daily gas production.
For example, if 30 kWh/day is used, 30 kWh / 900 BTU/kWh x 1,000 = 33,333 cubic feet of methane required per day.
Compare your livestock methane output to the home gas requirement to make sure you have adequate manure resource available.
Digester System Design Considerations
There are several key factors that go into designing the digester component:
- The digester tank must provide at least 15-30 days retention time for full manure breakdown.
- Size in cubic feet = manure volume per day x retention time.
- For example: 2,000 cu-ft manure/day x 20 days = 40,000 cu-ft minimum digester size.
- Mesophilic digesters operate at 95-105°F.
- Thermophilic digesters operate at 130-140°F.
- Thermophilic is more efficient but requires more energy input.
Manure Total Solids
- Optimal is 11-13% total solids.
- May need to add water to dilute manure if solids are too high.
- Optimal C:N ratio is 15-30:1 for digestion.
- May need to add a carbon source like straw or saw dust.
- Minimum 15 days retention time.
- Potentially up to 60 days retention for maximum gas production.
- Mixing prevents solids settling and scum layer
- Helps distribute nutrients and bacteria.
Electrical Generator System Components
The purified biogas from the digester is used to fuel an engine generator set to produce electricity. Here are the key system parts:
- Sized based on power output needed for the home.
- Natural gas, propane, or dual-fuel engines modified to run on methane.
- Look for high-efficiency models (over 30%).
- Filters and scrubbers to remove moisture, CO2, H2S.
- May require chilling or pressurization.
Power Control System
- Converts and regulates biogas energy into usable electricity.
- Matches generator output to electrical loads.
- Battery bank to store excess power for peak usage times.
- Deep cycle lead-acid batteries are commonly used.
- Gas alarms, explosion-proof components, ventilation.
- Emergency shut-offs and backup power source.
- System to provide data on gas production, engine function, etc.
- Helps optimize and troubleshoot issues.
Step-by-Step Installation Guide
Once the manure-powered generator system is designed, here are the key steps to install it:
1. Prepare Site
- Select adry, level area with good drainage.
- Consider noise and ventilation for generator location.
- Layout all plumbing lines and electrical connections.
2. Build Digester
- Excavate pit or setup tank based on the digester sizing.
- Line with sealed impermeable liner to prevent leaks.
- Add inlets, outlets, and piping for manure feed and gas collection.
3. Install Generator
- Mount genset on concrete base with vibration isolation.
- Connect fuel line to conditioned biogas supply.
- Connect to electrical load control system.
4. Connect Components
- Connect digester to generator with gas transfer line.
- Install biogas conditioning systems on line.
- Link generator power to battery bank and electrical loads.
5. Startup and Test
- Inoculate digester with active bacteria culture.
- Begin feeding manure on startup mode.
- Monitor and test all systems until operating normally.
Operation and Maintenance
To keep a manure-fueled generator running properly long-term, follow these operating guidelines:
- Maintain consistent manure input and loading rate.
- Monitor biogas production and composition.
- Remove digested solids; check C:N ratio.
- Filter gases and condensate buildup.
- Keep engine serviced; change oil, plugs, filters.
- Inspect lines, fittings, gaskets for leaks/damage.
- Keep batteries conditioned; test backup generator.
- Log all system data for performance tracking.
Some maintenance can be done yourself, but also have a qualified technician provide yearly inspections.
Manure-powered home generators require proper planning and installation but can provide an excellent source of renewable energy. This article has provided a complete overview of how these systems work, what’s involved in designing and installing one, and how to operate it successfully as an alternative energy system. Let me know if you have any other questions!