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Biogas in Winter: Heating a DIY Biogas Digester

There is no wrong way to build a first biogas digester, so long as it results in the unforgettable experience of seeing the miraculous blue flame of biogas for the first time. Soon afterwards, however, most peoples’ thoughts turn to a continuously operating system for regular daily energy at their home or small farm. If your dream homestead or community happens to lie in a cold region, it used to be believed biogas was not a viable option for you unless it was buried deep underground. Our understanding of the biogas process has improved a lot in recent years together with the introduction of new technology such as evacuated tube solar water heaters, so with a little bit of planning and DIY know-how, it is possible to thumb your nose at Old Man Winter and enjoy biogas in subzero temperatures.

We all know there is no free lunch, heating energy has to come from somewhere. There are a number of different options for heating a biodigester with or without fossil fuels, but before we consider them, it is important to keep in mind two important considerations for wintertime biogas. In the winter and early spring there will naturally be less organic waste available and accessible, requiring lower operating temperatures. Put simply, yes, biodigesters do benefit from heating to operate in wintertime, however, they do not need to be anywhere near peak performance temperatures of 100 degrees Fahrenheit. They need only minimal operating heat between 55 degrees and 75 degrees. Secondly, what heat that is produced needs to be focused in the “active zone” in a biogas digester, which is located on the bottom of the digestion chamber, half-way between the center and the inlet.

activezone

Some of the most common methods for heating home biogas digesters are on the chart below. We want to utilize waste heat if it is available and avoid dedicated electric heaters if possible. The following chart is divided by what heating options are appropriate where utility rates are low and where utility rates are high, which includes off-grid applications. The perfect cold climate off-grid combined heat and power system in my opinion would be a home gasifier or boiler burning annually-renewable wood chips or pellets together with a biogas digester. Waste heat from the gasifier heat exchanger or gen-set could be used to heat the biodigester, and in return biogas could provide year-round cooking energy, as wood gas has too much CO to safely burn indoors.

Methods for Heating Biogas Digesters

Where utility prices are low.

Where utility prices are high + off-grid

Home hot water system

Dedicated Electric

(Hot tub heater)

Dedicated Solar

Combined heat & power

Connected to the home hot water system.

Circulates heated water directly back into digester on

Flat panel for warm regions

Waste heat from gen-set, boiler or gasifier.

Evacuated tube for cold regions

If you intend to use your home hot water system to heat your biodigester, you must have your heat exchanger outside the digestion chamber to avoid the possibility contaminating your fresh water supply through a leak or ruptured pipe. For dedicated heater applications, a 50/50 water to planet and pet-friendly glycol is recommended to avoid freezing.

Biogas and solar thermal form a winning team to help reduce fossil fuel use, whether it is with a dedicated solar heater or solar heat supplements your home hot water system. For our two cubic meter home biogas units, I recommend 20-inch x 48-inch evacuated tube heater for Zones 4 & 5, or 10 tubes per cubic meter of digester volume. Provided the heat exchanger is located directly beneath the active zone. In summer months, solar heaters can be covered or disconnected.

In addition to heating, there are some additional steps you can take to maximize your heating energy.

Additional Steps to Improve Wintertime Performance

Insulation

Thermal blanket

Hoop house

Greenhouse

I hope everyone has a happy holidays and has the opportunity to enjoy a fossil fuel-free Christmas dinner prepared with 100 percent natural, clean burning biogas from ordinary household waste.


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