How does voltage develop across a prickly pear stem? Results from the preliminary study
Before we can figure out how much electricity we can harvest from prickly pear cacti, we need to first understand how voltage develops across the stem. We can explain this theoretically by examining how the prickly pear photosynthesizes.
Prickly pear cacti, also known as Opuntia spp., undergo what is known as Crassulacean Acid Metabolism, or CAM. Because cacti are found in deserts where water is scarce, they have adapted to the droughted environment by opening their stomata, or pores used in gas exchange, only at night rather than during the day like most plants do. They do this because water vapor escapes when stomata are open, so opening the pores at night when it is cooler minimizes water loss. When the stomata are open, the cactus absorbs carbon dioxide (CO2) to use in photosynthesis. But photosynthesis requires sunlight, meaning the cactus has to hold on to the CO2 until morning. Because CO2 absorption happens through diffusion, the cactus needs to fix the CO2 as malate (also known as malic acid) to absorb as much CO2 as possible.
The malic acid is broken down during the day to release CO2 for use in photosynthesis. The prickly pear stem is opaque, meaning sunlight does not pass from one side through to the other. This means that each side of the cactus intercepts a different amount of light, or photosynthetically active radiation, and breaks the malic acid down at different rates. As the side facing into the sun breaks down the acid faster, the pH begins to rise. The side of the cactus that is not directly facing the sun breaks down the acid slower and has a more acidic, or lower, pH for longer. This creates a concentration gradient, which causes protons, or positively charged hydrogen ions, to move from the shaded side with more acid to the sunny side with less acid. This flow of protons is what we can measure as a voltage potential.
Based on these mechanisms, I hypothesized that when the differences between the two sides of the cactus is greatest (e.g., the amount of light intercepted, acid concentration, and pH), voltage development will be highest. I tested this hypothesis by building a device to measure voltage development across the stem and placed quantum light sensors on either side of the cactus stem. I collected data from 10 prickly pear stems planted in pots for 4 days. With the help of the Living Solar Panel undergraduate students, we also took samples of each stem every 4 hours (e.g., from 6am to 10pm) and separated the sides (sunny and shaded sides). We are currently determining the pH and acid concentrations of each sample (backers can check back later for a Lab Note describing this process!).
The figure above shows some of our initial results. The yellow sections of the graph indicate the times between sunrise and sunset and the orange dotted line is solar noon (the time the sun is highest in the sky). The red line shows the difference in intercepted light (photosynthetically active radiation, PAR) between the two sides of the stem, denoted as △PAR and measured as micromoles per meter squared per second. The blue line is the average millivolts (mV) measured across each stem and averaged for the hour (two devices stopped working, so this data comes from 8 devices that recorded data each minute). The light blue bars show the maximum and minimum values for each hour.
Although this is only a few days worth of data, we can begin to see a pattern emerge: as the difference in △PAR increases, so does the voltage development across the stem. Voltage development decreases and is close to zero at night. The voltage development may not seem like much, but it's important to remember that this data comes from a single set of wires placed into the cactus and we can expect the recorded voltage development to increase with the use of electrodes and wires placed in series.
You may notice that voltage development decreases in subsequent days - we believe this is because the cactus is healing over the wires and altering the data. We are currently working with the UC Santa Cruz engineering department to find a way to trick the cactus stem into not healing over the wires.
I am also currently working to improve the data collection devices to include an in situ pH sensor, meaning I will be able to constantly monitor the difference in pH between the two sides of the cactus stem and see how this relates to voltage development. Once the devices are improved, I will repeat the experiment described above in a controlled setting using known light intensities to create a more robust dataset. Backers will be kept updated on device improvements and the results of the controlled light experiment!
Have any lingering questions? Feel free to ask them below!
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