Biogas vs. Solar Energy
While in Freiburg we visited two farms, one which produced milk and was equipped with a large solar array, and one which produced biogas in order to create electricity and heat. While both are green sources of energy, I would like to provide an argument for solar over biogas in (almost) every situation.
But first, how is biogas produced, and how does it make electricity? Biogas is made by composting organic material and depriving it of oxygen. If the material is composted without oxygen, only anaerobic bacteria survive and decompose the material. These anaerobic bacteria produce methane as their by-product instead of carbon dioxide. This methane is then collected and is the resulting biogas. At the biogas farm we visited, the methane was then stored at atmospheric pressure and fed into a combustion engine. The combustion engine turned an electric motor in order to create AC electricity like a classical generator. The extra heat from the combustion engine was harvested and piped to neighbors and a swimming pool for heating.
So what are the advantages and disadvantages of a biogas plant? First, I will cover the advantages. One big advantage is that the plant can generate electricity through the night and day. Because the bacteria produce methane independent of the sun, biogas can meet the demands of electricity usage at any time of the day. Another advantage of biogas is the energy density of the product. If the methane is compressed it can far exceed the energy density of any battery currently available. However, at the biogas farm we visited, the biogas was being directly converted to electricity so this advantage was somewhat irrelevant. The last advantage is that the process creates a rich fertilizer as its byproduct. After the bacteria has finished processing the organic material, the leftover product can be used by farmers to fertilize their soil. Now, what are the disadvantages of biogas? Their first big disadvantage is the complexity of the process. Many steps have to be gone through in order to produce electricity, and with each step there is more manual labor involved and more possibilities for mechanical failure. Compared to solar, which is a solid state process, biogas production has a lot of moving parts. Farmers have to harvest and shred organic material to feed the composters, the composters have to be mechanically agitated, and the biogas has to be burned in a combustion engine. All of these steps require machines that wear out and must be maintained and lubricated. Another disadvantage is the land required to grow the crops to sustain the biogas plant is much larger than the land required to put a similar output solar array. This makes it far more inefficient from a surface area perspective. Furthermore, building a biogas plant is much more expensive than a solar array because of the complexity of the system. Another reason is the hazard involved with storing and burning large amounts of methane, which is more hazardous than battery electricity storage.
So what are the advantages and disadvantages with solar? Solar has many advantages, including its low price, ease of installation, solid state technology, low maintenance, and carbon independent operation. The main disadvantage with solar is the problem of energy storage, however that problem has been mostly solved with the introduction of lithium iron phosphate (LFP) batteries. These batteries have less energy density than a traditional lithium-ion battery, but have greater cycle life, lower price, similar current output capacity, and greater stability. These batteries are already widely implemented in off grid solar systems and standard range EV’s, and their adoption will continue to increase as more grid scale batteries are installed. Because of this battery technology, solar has biogas beat on basically every front. Solar is cheaper, safer, more reliable, easier to maintain, and longer lasting. The only advantage biogas has over a system with solar and batteries is the potential energy density of methane. Compressed methane could be produced in a carbon neutral process and then used to power things that require high energy density, such as planes or rockets.
Overall I thought the biogas farm was fascinating to visit, but I couldn’t help being skeptical about why the technology was not more widely adopted. Before battery and solar technology was as advanced 20 years ago, I think biogas was a viable option, but now in 2023 it should not even be considered for power generation.
