Fire and Air: Combined Biochar and Compressed Air for Resilient Energy Storage
If you’re tuned into the world of renewable energy, you’ll know that rapidly building out global energy storage capabilities is a hot topic.
With the rapid and long-overdue deployment of wind, solar, and other renewable energy production sources, the focus is now shifting to how to effectively store their often intermittent production for when its needed on the grid. There is a lot of exciting, necessary work going into researching and deploying advanced concepts, and massive grid-scale storage systems. However, there’s an equal need for smaller scale systems which can be deployed rapidly, using technology which already exists. Some of the communities which would benefit most from energy storage systems are those least likely to be reliably served by large national grids. These places tend to be rural, remote, or otherwise unable to rely on consistent grid energy (as in much of the developing world). Solutions which can store energy in a carbon-neutral way that use off-the-shelf technologies would be excellent for these communities. It would allow them to build local resilience and offsetting the carbon-intensive alternatives (such as diesel/gasoline generators) that many of these communities currently rely on.
Compressed Air Energy Storage
Compressed air is a well-understood technology with over a century of use in power applications. Famous examples of its use include the city-wide compressed air network of Paris, and its use in mining operations to power tools and vehicles. With modern materials and manufacturing techniques, it’s possible to use off-the shelf pieces to compress and store air at several thousand PSI. Where Compressed Air Energy Storage (CAES) falls short is in its efficiency. When air is compressed, some of that energy turns into heat as the gas molecules are crowded together. Conversely, when compressed air is allowed to expand, the temperature of the air will drop. This effect is proportional to both how big the pressure difference is, and how fast you do it. The upshot is that it takes more energy to compress hot air, and cold air has less energy in it. This is a problem for energy storage, as you generally want to be able to store a lot of air in a small space, and you want to be able to store/release it relatively quickly.
Existing CAES facilities often address this efficiency problem by reheating the air as it’s being released by burning natural gas. This provides the energy boost needed to get the most out of the stored air, but using fossil fuels to accomplish the process is less than ideal. Additionally, most systems like this don’t (to my knowledge) have much use for the heat generated during the compression process and is treated as a waste product. While there are more advanced CAES systems being developed which capture the heat of compression and reuse it to heat the air later, these systems aren’t the kind of thing which can be widely replicated by small independent operators looking to build a system now.
Biochar As An Alternative
A carbon-neutral process which could be a fitting replacement for the natural gas reheating strategy is biochar pyrolysis. This process takes bio matter (most often agricultural and forestry waste), and converts it into a charcoal product. This charcoal has a lot of compelling properties. The two primary properties of interest here are related to is its ability to be added to soil. There, its carbon structure can stay intact for hundreds of years (possibly up to a millennia), in effect sequestering the atmospheric carbon dioxide harvested by plant matter over its lifetime. Additionally, this char acts like a “microbial reef” in the soil, creating a porous structure in which fungi, microbes and nutrients can occupy. This has the effect of improving soil health and, in turn enhances the ability of new plant life to grow. As a result biochar is becoming a sought-after soil amendment for agriculture, fostering the early growth of a distributed producer network.
The process of creating biochar is a combustion process that burns the gases let off from the heated biomatter without allowing that matter to actually burn. It generates a significant amount of heat, though only about half of what a traditional biomass energy system would produce. This is due to the fact that the other half of the energy (and emissions) is embodied in the remnant biochar. While this makes the process less appealing from a traditional energy generation standpoint, it pairs nicely with CAES.
Combined CAES and Biochar
With a combined pyrolysis and CAES system, the features of the two systems compliment each other. When compressing air using available renewable energy, the waste heat generated can be utilized to dry biomatter being prepared for pyrolysis. The less moisture this matter contains, the more efficiently the pyrolysis process can operate. Utilizing waste heat helps accelerate the drying process and improves overall process efficiency.
When discharging with the pyrolysis system active, the expanding air can be heated twice. The first pass of heating can occur by passing the air through a heat exchanger for the “raw” pyrolysis gasses being emitted from the cooking biomatter. These gasses contain water vapor, tar, and a cocktail of useful wood alcohols that have uses if condensed and removed from the gas stream prior to combustion. The expanding air can do a lion’s share of this, with possibly a secondary gas-to-water heat exchanger to ensure condensation. The second opportunity to reheat the expanding air is with the exhaust gases from the combustion proper, after they have been passed by the biochar pyrolysis retort. These two separate heating opportunities allow for the possibility of multi-stage expansion, where the compressed air does work multiple times on its journey down to atmospheric pressure. The exact machinery used for extracting this work is flexible, in part because the working gas can have little to no corrosive flue gasses. It is not outside the realm of possibility for internal combustion engines to be adapted as compressors and expanders, though far more efficient purpose-built machinery exists.
The system could also be theoretically operated in a “desyncronized” or “cold” mode, where the compressed air release can take place some time after the pyrolsis burn. With this mode, the pyrolysis heat is transferred into a large thermal mass such as a water tank. The water can do the same job as the expanding air with regards to condensing the volatile pryolysis gas products, and in extracting energy from the flue gasses. When compressed air is needed, the air is passed through a set of heat exchangers which interact with the thermal mass. This would likely be an ideal mode for slower air discharges where a longer run at lower power is favored over a short energy discharge. A working model of a water-based thermal system is the biochar system at Living Web Farms in North Carolina. That operation utilizes the captured thermal energy for radiant heating of both a greenhouse and a biomatter drying space.
An additional element of symbiotic efficiency comes from the need for temperature regulation of the biochar process. The optimum temperature range for a high yield of quality biochar is within a specific range, and the use of blowers to help oxidize the combustion process to that end is common in larger pyrolysis systems. The exhaust gasses from the CAES expander could be used to replace blowers, and excess heat from the combustion process could also be shunted dynamically to the expanding air if temperatures get too high. The compressed air flow provides a useful way to both increase and decrease the reaction temperature.
While elements of the system described above can get quite complicated, simple deployments of this kind of combined system could be quite readily deployed. It is also the kind of system which is friendly to small-scale “backyard” tinkering, as it doesn’t require much in the way of advanced materials or techniques. The system also works best in places with access to large amounts of waste biomatter and a suitable footprint for storage. These requirements often stack up well with rural agricultural communities which often are those most vulnerable to energy insecurity. Because of the nature of compressed air the system is also scalable, allowing an operation to start small and add additional tanks, compressors, generators and burners as needed.
In summary, there appears to be a good symbiosis between the biochar pyrolysis process and compressed air energy storage. Together, they form a system which can store dispatchable renewable energy, and release it with a carbon-neutral-to-negative footprint. It’s built out of well-understood components, and produces a valuable agricultural product in the process. The system is well suited for rural and agricultural locales, and could be rapidly iterated upon and scaled to meet local needs.
Open Source Notice
As with my other posts, I’m offering this idea up as open source under a Creative Commons Attribution - Share Alike license. Please feel free to take this idea, pick it apart, adapt it and implement the parts which make sense.
