May 24 2019

Using Bacteria to Store Energy

Energy storage is now a critical technology for the future of our energy infrastructure. We want to move to renewable forms of energy, but many of them are intermittent sources, and so energy storage will be necessary. At low penetration, up to about 30%, we can essentially use the grid as if it were a battery – putting unused energy into the grid when producing excess, and then taking from the grid when demand exceeds production. Having an efficient electrical grid is essential for this strategy, but it has inherent limits. It requires that the majority of electricity in the grid comes from base load or on-demand sources.

There are many potential options for large scale energy storage, but no currently available options are ideal. A new study explores one such possible solution, using bacteria to produce energy storing molecules from sunlight. The authors of the study lay out the problem:

No present energy storage technology has the perfect combination of high power and energy density, low financial and environmental cost, lack of site restrictions, long cycle and calendar lifespan, easy materials availability, and fast response time.

We really need all of those features in one option for it to be an effective solution. If any one feature is bad enough it is a potential deal-killer. Right now pumped hydro has the greatest efficiency, but is very site restricted. Batteries are not site restricted and have good energy density, but have a limited life-span and use some expensive materials that are not harvested in an environmentally friendly way and cannot be easily recycled. Producing hydrogen for storage is not a bad option, but we need better storage technology for the hydrogen (that is what has delayed the hydrogen fuel cell car so far).

There are some new proposals out there as well, such as using excess energy to have cranes raise blocks of cement into a tower. The blocks can then be lowered to drive turbines and recover the energy.  You could also heat minerals that can hold onto the heat for a long time, or spin up fly wheels in a vacuum.

Again – none of these technologies are ideal. Likely we will by using a variety of methods in different locations and situations. The authors of the current review explore the potential of using engineered bacteria to store energy from sunlight in the form of carbon-based high energy molecules – essentially biofuel. Bacteria can split carbon from carbon dioxide using sunlight as their energy source. That carbon can then be used in other reactions to create hydrocarbons. Essentially the captured energy will be stored chemically.

This is the same principle as photosynthesis. Biological photosynthesis, however is only about 1% efficient. We will need to do better than that if this technology will play any significant role in energy storage. You can read the whole article if you want the technical details, but here is their conclusion:

Biology, and particularly rewired carbon fixation, could hold the answer to the large-scale storage of renewable energy. Several key challenges must be addressed: finding a mechanism for long-range electron transport that is efficient, supports high transfer rates, safe, and can be rapidly engineered; a mechanism of carbon fixation that can be expressed in a heterologous host, and is thermodynamically highly efficient, if not also fast; and finally, an energy storage system that is safe, convenient, and enables rapid dispatchibility. These innovations will require breakthroughs in systems biology of non-model exotic microorganisms, mining the genomes of exotic organisms, evolution tools for autotrophic metabolisms and in the development of synthetic enzymes and self-assembling and self-repairing biological nanostructures.

So – the technology is not yet available, but has potential. It sounds like we are at least 20 years away from having a deployable technology. It could be quicker depending on how much the research is funded and how many labs work on this problem. I have to wonder what the state of our battery technology will be in 20 years. That is the real question – which technology will get over the finish line first?

There are potential advantages to the bacteria approach, however. One is that it takes carbon out of the atmosphere, so it can be carbon negative. If all that carbon is then made into biofuel and burned, the carbon eventually goes back into the atmosphere, so it is ultimately carbon neutral. But – however much fuel exists in storage at any one time represents carbon that is not in the atmosphere. The authors also bring up the possibility of making bioplastics out of the resulting molecules. Plastics can be made from hydrocarbons. This is a more permanent form of carbon which can be used as a method for carbon capture and storage.

Imagine if all our plastics were made from carbon from the atmosphere, rather than fossil fuels. Further, at the end of the life cycle of those plastics they were buried deep under ground in permanent storage. That would be a way of sequestering carbon from the atmosphere.

Another potential advantage of using this method is that the energy stored in this way is in the form of fuel, which can be relatively energy dense and convenient to deal with. For some applications, such as jet engines, liquid fuel may be needed for many decades to come. For all applications where electrical engines are simply not powerful enough, biofuel from bacteria or another source may be the best option for the environment.

If I had to predict I would say that something like this, making biofuels and plastics or other solid material using bacteria or some other organisms, will likely have a niche but important role in our future energy infrastructure. I doubt it will have a starring role. I think I still put my nickle down on battery technology as having the main role for energy storage, with a host of other systems playing a supporting role.

The progress of battery technology has been fairly steady and predictable. No breakthroughs are required, just the continued progress that is being made. I follow science news about battery technology pretty closely, and there is no lack of significant advances. Sure, we don’t know which ones will pan out, many will not, and the ones we are reading about today will not come into play for 10-20 years. But the ones we were reading about 10-20 years ago are coming to fruition, resulting in that steady increase in battery storage. I think slow and steady is going to win the race.

But of course you cannot predict because there can always be a breakthrough with a disruptive technology that changes the game. I am just playing the odds.

I also think we should not put all our eggs in one basket. Let’s explore every possible option, because we don’t know which ones will pan out, and because there is room for many solutions in different applications. We don’t want to bet just on the wrong horse, so let’s bet on all of them.

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