4. Power Generation and Energy Budget
Each CO₂ extraction unit must be entirely self-powered, meaning it cannot rely on an external electrical grid or tethered power cable. We achieve this by integrating renewable micro-generation technologies. The primary power source in our design is solar photovoltaic, augmented in some cases by wave energy or microbial fuel cells, depending on deployment context. In this section, we describe the power system and analyze the energy budget to show that the unit can generate and store enough energy to run the CO₂ extraction process continuously.
Primary Power Source – Solar PV
Solar panels are a natural choice for any surface or near-surface ocean device. Our design uses a small solar panel (~0.1–0.2 m² area) mounted on the top of the device (for a buoy) or on the structure (for a pier-mounted unit, one could attach a panel to the pier rail). Assuming a 20 W peak panel, in a sunny location (say 6 equivalent sun hours per day) it can produce about 120 Wh per day. In many locations, especially equatorial or mid-latitudes, daily solar energy could be higher (200+ Wh). We assume on average 100–200 Wh per day of captured energy per unit from solar. The panel charges a battery to allow night-time operation. The battery (e.g. a ~20 Wh LiFePO₄ cell) can store enough to run the device through the night or a couple of cloudy days, albeit perhaps at reduced duty cycle in extended bad weather. Solar is reliable and cheap, but of course is limited at high latitudes or in polar seasons; in those cases, we might lean more on the alternative sources below.
Auxiliary Power – Wave Energy
The constant motion of waves and swell can be harvested by a small point absorber or oscillating mechanism attached to the device. For example, a floating buoy device can have a magnet-coil linear generator that moves with wave motion. Research on small wave energy converters for buoys has shown outputs of a few watts are achievable in modest sea statessciencedirect.com. One design demonstrated ~5.6 W output on average in lab wave conditionssciencedirect.com. If our device is in a wave-rich environment, incorporating a simple pendulum or float that drives a generator could provide on the order of 1–5 W continuously (24–120 Wh per day). While this adds mechanical complexity, it could significantly boost energy availability in rough seas or during winter months when solar is low. In our base design, wave energy is optional. The device's architecture can accommodate adding a wave module (perhaps as an attachment that plugs into the power system). For now, we assume solar is the main source, but note that wave power can complement solar (being available day and night and more seasonally consistent in some regions).
Auxiliary Power – Microbial Fuel Cell (MFC)
In some deployments (particularly if the device is anchored near the seabed or in nutrient-rich waters), a benthic microbial fuel cell could continuously generate power from organic matter in the seawater/sediment. MFCs use bacteria to create an electron flow from oxidation of organic compounds. Typically, MFCs have only produced milliwatts historicallyjmu.edu. However, new research efforts (including a DARPA-funded project) are making strides toward 10 W scale microbial fuel cellsjmu.edujmu.edu. If an MFC anode is placed in anoxic sediment and a cathode in the oxygenated water, a continuous trickle of power can be drawn. In our context, an MFC could provide a small base load power (~0.1–1 W) to keep the sensors and control running indefinitely, and maybe contribute to the electrolysis needs. Because MFC technology for this power level is still experimental, we treat it as a future enhancement. Nonetheless, the design could allow plugging into an MFC module (for example, if a project wants to anchor the device with an MFC unit on the sea floor). Notably, MFC power could be very useful for deep-sea deployments where solar is unavailable. For shallow, surface units, solar is much simpler and cheaper, so MFC is not needed.
Energy Storage
A rechargeable battery is included to buffer the energy supply. We favor LiFePO₄ chemistry due to its long cycle life (over 2000 cycles) and good safety in varying temperatures. A ~20 Wh battery (for instance, a 4-cell 18650 pack or a single 12 V 2 Ah pack) can store the day's solar energy to run at night. If the device harvests ~150 Wh on a sunny day, it might use ~50–100 Wh during the day and store the rest to cover ~50 Wh at night. In prolonged low generation periods (stormy weather, overcast), the device can default to a low-power standby (pausing CO₂ extraction) until energy is available. This ensures it doesn't kill its battery. The system's power management will always maintain a reserve for running the microcontroller and communication so it can restart extraction when power returns.
Power Consumption Breakdown
Electrochemical Process
The major energy consumer is the electrochemical process – specifically, driving current through the water to release CO₂. Based on the chemistry, removing 1 mole of CO₂ (44 g) requires on the order of 120 kJ of electrical energypubs.rsc.org (this includes both phases of the cycle).
Daily CO₂ Removal Estimates
Low-End Power Scenario
High-End Power Scenario
Optimistic Scenario (with efficiency improvements)
Design Target Range:
0.1 - 0.2 tons CO₂/year
Other Power Consumers
Microcontroller & Sensors
In sleep mode, the microcontroller and LoRa radio draw microamps. Active sensor readings and transmissions might draw a few tens of milliamps but only for short bursts. For example, a LoRa transmission of a few bytes might take 50 mA for 50 ms. If it transmits once a minute, that's negligible (<1 mAh per day).
Daily consumption: <1 Wh
Water Pump
The water pump, if used, might draw 2–5 W but it would run intermittently (maybe a few minutes each hour to refresh water).
Daily consumption: 0.5-1 Wh
Electrolysis
The main power consumer, running at approximately 5W when active for several hours per day, depending on available energy.
Daily consumption: 50-150 Wh
Energy Budget Balance
Thus, the energy budget can be balanced: On a typical day, solar provides ~100 Wh (worst-case) to 300 Wh (best-case). The device might use ~50–150 Wh for CO₂ extraction (depending on conditions) and a couple Wh for everything else, and store any excess. If energy is limited, the device can throttle the electrolysis (e.g., run at 50% duty cycle) to match what's available. The microcontroller will implement such an energy-aware schedule, possibly performing most of the heavy CO₂ extraction during midday when solar is abundant, and slowing down at night.
Environmental Considerations
Solar panels will generate less in cloudy high-latitude winters; wave power might generate more during storms but maintenance is trickier then. For consistency, in polar or very cloudy environments (or if mounted in shade under a pier), an alternative might be needed. One alternative is hooking into a microbial fuel cell if the device is anchored (which could provide baseline power continuously). Another is connecting to a small wind turbine (some off-the-shelf micro-wind generators can be $50 and provide ~5–10 W in wind, which could be viable on buoys). These options can be region-specific. For instance, an Arctic deployment could rely on an MFC and wind since solar is absent in winter.
System Longevity
In summary, each unit is designed to harvest on the order of 0.1–0.5 kWh per day, and this supports removing on the order of 0.1–0.2 kg CO₂ per day. The self-power strategy using solar (and possibly wave/MFC) ensures autonomy. Importantly, all chosen power technologies have no fuel cost and low maintenance (solar panels might just need occasional cleaning of bird droppings or salt, wave energy devices might need mechanical checks). The battery is sized for daily cycling and chosen for longevity (LiFePO₄ can easily handle >5 years of daily cycles with little capacity loss). Overall, the energy budget closes: we can power the CO₂ extraction process at a rate that matches our design goals, making each unit truly self-sustaining in the field.