If you’ve ever brewed beer, made old-fashioned sauerkraut, or experimented with one of the many other fermented comestibles that have gained popularity in the last decade or two, you know that microbes are experts in liberating energy from mundane substances.

Engineers should instinctively recognize that something important is happening when a jar of seemingly inert material gradually begins to bubble and fizz, and every once in a while the lesson is driven home in an unforgettable fashion. I once had a bottle of homebrew that somehow became (very) over-carbonated; the resulting fountain of golden ale was so powerful that it stained the ceiling in my dining room.

Voltage from Microbes

The idea of harvesting energy from microbial action is not new. Way back in 1911, a botanist by the name of Potter used ordinary brewing yeast (Saccharomyces cerevisiae) to produce electricity. Research into microbial fuel cells (MFCs) has continued between now and then, but MFC science is surrounded by feasibility obstacles, and practical applications remain scarce.

The goal of MFC technology is to use microorganisms to convert energy stored in soil, wastewater, etc. into another form which in turn can be converted into electrical energy. The details of this conversion vary from one system to another, and different systems employ different types of microbes, but the basic idea is to generate a potential difference by collecting electrons released via microbial metabolism. This potential difference then functions as the voltage from a battery and can power a circuit.

An example of soil-based microbial voltage generation. Image (modified) from MFCGuy2010 [CC BY-SA 3.0]

Microbial Energy for Electronic Devices

Energy harvesting is a well-known and increasingly viable technique for low-voltage electronic design. Planet Earth is teeming with energy—sunshine, wind, heat, sound, mechanical vibration, and ambient electromagnetic radiation. Why buy batteries when we could use energy that otherwise will simply dissipate in unproductive ways?

This diagram illustrates the concept of energy harvesting for electronic devices. Image used courtesy of Krishnan et al. 

Some types of energy harvesting are more practicable than others, and MFCs are on the lower end of the feasibility spectrum, especially for small devices such as sensors or displays. In most cases, a design team would choose sunshine or wind over microbes. However, some applications are not compatible with solar or wind power, and if there are also strong objections to using a conventional battery, an MFC might be the preferred solution.

This is exactly the type of application that was explored in research conducted by Gabriel Marcano and Pat Pannuto at the University of California San Diego. Their work is described in a publication entitled “Soil Power?: Can Microbial Fuel Cells Power Non-Trivial Sensors?” The objective was to assess the feasibility of using microbial energy to power an agricultural sensor system—especially one that, because of operational constraints, does not have access to sunshine or wind.

Microbial Fuel Cells vs. Biobatteries

Marcano and Pannuto differentiate MFCs from biobatteries:

Unless the medium or nutrients that an MFC uses to generate electricity are refreshed somehow, the apparatus should be referred to as a biobattery rather than a fuel cell. In the long term, we are interested in embedding MFCs in environments such as agricultural beds or wetlands, where extant processes can restore nutrients. The cell we study in this work, however, is self-contained. Thus, it is a biobattery.

The fundamental objective in both cases is the same—to convert energy liberated by microbial metabolism into voltage that can power a circuit. But if some sort of natural phenomenon doesn’t replenish the microbes’ food supply, the system functions more like a battery insofar as it eventually reaches a “discharged” state and no longer produces sufficient voltage for circuit operation.

MFCs Pair Well With Agricultural Sensors

An MFC is an appealing power source for agricultural sensors, especially if the sensor must be in contact with soil. First of all, soil already contains microorganisms and appropriate food (i.e., organic matter) for these microorganisms. Furthermore, a device that is on or near the soil surface will interfere with agricultural implements, and consequently, it must be buried. Sunshine, wind, and conventional batteries are not feasible sources of power for a buried sensor. An MFC introduces the possibility of a buried agricultural sensor that can function indefinitely by harvesting energy from the surrounding soil.

The system studied by the UCSD researchers included an off-the-shelf biobattery (called a MudWatt) fueled by potting soil. They combined this with an energy-harvesting power management unit from Analog Devices, a supercapacitor for energy storage, a microcontroller, and an e-ink display. The display functions as a convenient means of simulating the periodic bursts of load current required for the operation of a typical agricultural sensor.

This is a block diagram of the prototype system tested by researchers at UCSD. Image used courtesy of UCSD [CC BY-SA 4.0]

Marcano and Pannuto were able to create a functional system, and in this sense the research project was successful. Significant obstacles remain, however. The microbial fuel cell was sensitive to soil moisture content, and the researchers acknowledge that they achieved consistent power output by maintaining unrealistic levels of soil saturation.

Also, the controlled laboratory conditions did not provide insight into the effect of variables present in real agricultural environments. Power output might be significantly affected by temperature changes, tillage, or temporary disruptions in microbial activity caused by fertilizer or pesticide applications.

Finally, an MFC simply doesn’t generate much power, even in ideal conditions. Speaking of the system's capabilities, Pannuto concluded: “We’re not going to run a cell phone off soil anytime soon, but we can harvest enough energy to kick out a data packet a couple of times a day.”

Are MFCs Worth the Effort?

MFC technology has not made astounding progress over the last hundred years. UCSD’s research team has identified a scenario in which microbial energy harvesting may be a useful form of power generation, but on a large scale, MFC-based systems may never be both economically feasible and sufficiently robust.

Featured image (modified) used courtesy of Brian Boucheron [CC BY 2.0]

What are your thoughts? Should researchers continue investing time and money into MFC development, or would these resources be better expended elsewhere?