Heat pumps could reduce biogas carbon footprint

New research suggests that an alternative source of heat could reduce the carbon footprint of a process that turns food waste into power. A University of Glasgow-led team of scientists has demonstrated that using air-source heat pumps to support anaerobic digestion could cut the carbon emitted during biogas production by more than a third, writes the university in a press release.

Anaerobic digestion uses microorganisms in oxygen-free conditions to break down biodegradable materials like food waste and sewage sludge to release biogas. This gas is a mixture of methane and carbon dioxide which can be burned to turn turbines, so as to generate low-carbon electricity. Machines called bioreactors maintain the optimal temperature during anaerobic digestion to maximize the amount of biogas generated.

The researchers compared the footprint of bioreactors heated by conventional heating systems with one of those heated by air-source heat pumps. In a new paper, they outline how they created a computer model of the thermodynamics of heat pumps. In fact, they coupled the model with machine learning-based anaerobic digestion modeling and trained the new system from existing research databases.

Testing the model

To compare the two kinds of bioreactors, scientists compared the carbon footprint over the course of their respective lifespans. Particularly, it is a standardized approach called life cycle assessment. They found that the heat pump system would emit less carbon than the baseline natural gas system.

The modeled carbon reduction was up to 28.1 percent in an anaerobic digestion process maintained at a temperature of 55°C. At a lower temperature of 37.5°C, the carbon footprint of the process went down by 36.1 percent.

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Heat pumps to support low-carbon anaerobic digestion

Dr. Siming You, from the University of Glasgow’s James Watt School of Engineering, is the paper’s corresponding author. “This model is the first technical and environmental assessment of how useful air-source heat pumps could be in decarbonizing the process of producing biogas. The results suggest that there’s a significant role for heat pumps in supporting low-carbon anaerobic digestion.”

“That kind of decentralized waste recycling could go a long way to helping people produce their own local source of electricity. The research is also part of a larger effort to decarbonize water and wastewater treatment in rural communities,” he concluded.