Solar photovoltaic cells (PV) provide a renewable method of both generating electric current and storing fuels as hydrogen gas. The existence of this technology offers the hope of replacing our increasingly scarce fossil fuels with a dependable, infinite alternative; however, the monetary cost of manufacturing these cells has blocked their implementation. Traditionally, the PV process required the use of “prohibitively expensive light-absorbing materials [e.g., (Al)GaAs and GaInP], and/or fuel-forming catalysts (e.g., Pt, RuO2, IrO2), and strongly acidic or basic reaction media, which are corrosive and expensive to manage over the large areas required for light harvesting” (Reece et. al 2011). The aim of Reece and his colleagues is to mimic the photosynthetic process with earth-abundant materials under neutral pH conditions. They were successful in achieving this goal for both wired and wireless solar photo-electrochemical systems (PEC), replicating leaf photosynthesis to fix hydrogen and oxygen gas. For the wired system they were able to obtain an efficiency of 4.7%, while the wireless system had an efficiency of 2.5%. The continual development of this technology offers greater potential for solar energy to replace traditional sources due not only to necessity, but due to market factors as well.—Donald Hamnett
The specific compounds Reece and his colleagues discovered that make this process work are: a cobalt oxygen evolving catalyst (Co-OEC), a nickel-molybdenum-zinc hydrogen evolving catalyst (NiMoZn), and a triple junction amorphous silicon (3jn-a-Si) interface between the catalysts which is coated with indium tin oxide (ITO). In the wired case, the NiMoZn catalyst was deposited on a nickel mesh substrate, which was wired to the 3jn-a-Si electrode. In the wireless case, the NiMoZn catalyst was deposited directly onto the silicone electrode’s adjacent stainless steel surface. Baseline values for the 3jn-a-Si cell were obtained by operating the cell in a three-electrode voltammetry setup. Under a no-light condition, the cell had a current of less than .05 mA/cm2, a low value. Upon illumination of 1 sun (100mW/cm2) of air mass (AM) of 1.5, the cell produced a current of .39 mA/cm2 at a potential of 0.55V when using a 1M potassium borate electrolyte (pH 9.2). To increase the efficiency in this example, they added the 0.25 mM Co2+(aq) catalyst. The Co-OEC increased the current to 4.17 mA/cm2. Bubbles at both electrodes indicated the formation of oxygen and hydrogen gas. The experiment went on to study the effect of differing thicknesses of Co-OEC films on the electrodes’ surfaces. It was found that at a 5 minute deposition time, the thin (85 nm) film created photoanodes with optimum performance. There is a give and take between Co-OEC presence and performance of the cell, because though it increases activity, the built up film also blocks incident radiation. When they tested the wireless cell, it was found to be stable for 10 hours. It was discovered that the stability of the wireless cell is dependent on the type of conductive oxide barrier layer used.
Though there is still work to be done to increase the efficiency and stability of these PEC cells, Steven Reece and his team has succeeded in replicating the photosynthetic functions of a leaf. Additionally, the fact that they have done so in a relatively neutral pH means that hydrogen and oxygen fuels could be generated without a membrane, as the two gases are very insoluble at a neutral pH. Lastly, the most notable aspect of this research is that they have replicated photosynthesis with low-cost, earth-abundant materials. The continued research in this area may very well spark an energy revolution, as the decreased overhead will incentivize investment in solar technology
Due to concerns about dwindling fossil fuel reserves and climate change, and to the falling costs of solar photovoltaic (PV) energy, solar power is being increasingly integrated into power grids. Unfortunately, several factors involving the composition of current grids limit solar power’s ability to be fully utilized. One such limitation is that the peak hours of solar availability do not coincide with the peak hours of demand. Another limitation is the current grid’s inflexibility, or inability to increase or reduce output from conventional energy sources to coincide with variable fluctuations in solar output. A solution to this problem has been proposed in the form of thermal energy storage (TES) deployed with concentrating solar power (CSP) (Denholm and Mehos 2011). This technology complements PV by storing otherwise wasted solar capacity as heat, which can be stored and used as output during periods where PV generation is minimal. Furthermore, PV with CSP stabilizes solar output to a firmer, more dependable source less subject to random fluctuations. This allows for solar to replace a portion of the grid currently occupied by conventional energy generation, and thus furthers the percentage of potential energy output due to renewables.—Donald Hamnett
Paul Denholm, Mark Mehos, and colleagues at the National Renewable Energy Laboratory used a REFlex model to predict the ability of CSP to increase grid flexibility and solar penetration in the Southwestern United States. This model compares the hourly load with renewable resources to calculate energy curtailment, based on the grid’s flexibility, or ability to change generator output to accommodate variable renewable energy sources. To determine the limits of PV, the researchers used weather data from 2005 and 2006 in the System Advisor Model (SAM), which converts the data into hourly PV output. These data were in turn used to model the interaction between solar and wind generated energy, using simulated data for 2005 and 2006 from the Western Wind and Solar Integration Study (WWSIS). Simulations of energy generation per hour were conducted over a year and the four-day period, April 7-10, was displayed as the paper’s example. The area under the curve was split into different sections representing contributions from the various energy sources. Simulations were run for the current PV system with an assumed 80% flexibility, and a PV with CSP system. In the latter example, CSP was added to the REFlex simulation using SAM produced hourly generation values.
The simulation under a PV-only scenario showed that a significant proportion of annual PV production, 5%, is curtailed due to a lower energy demand, since energy demand is low when PV output is high, and other generators cannot slow output down at fast enough ramp rates. In this situation, the PV curtailment and cost increase exponentially as the percentage of energy from PV increases. Clearly, this poses an obstacle to switching to an increasingly renewable grid because it puts a limit on the proportion of solar energy that is practical. In order for this PV to be utilized, a more flexible grid is required. When the researchers included CSP into the REFlex model, the data suggested that flexibility improved and curtailment reduced.
The CSP model was based on wet-cooled trough plant technology (Wagner and Gilman 2011). In this scenario, solar energy that would have otherwise been curtailed during low load hours was instead stored thermally during the day. At the same time, PV energy was incorporated into the grid; however, as PV lessened in the night hours and load increased, the stored CSP energy was run through the grid. Energy that would otherwise have been wasted during low load hours could now be stored and used during peak hours. This decreased the annual solar curtailment to less than 2%. Also, it increases the solar energy contribution to 25%, 15% PV and 10% CSP.
Denholm and Mehos also found that CSP/TES helped to lower the baseline amount of conventional energy needed, because CSP allows for solar energy to be utilized at all hours. This result has cascading effects, as the implementation of CSP lowers the minimum conventional generation requirements. CSP allows for additional solar power to be added at much lower marginal curtailment rates. Lowering the minimum load that relies on conventional energy opens up greater probabilities of increased solar and wind components. The researchers, through their complex grid analysis, determined that CSP technology has the ability to both increase solar efficiency and the possibility of the implementation of greater variable energy inputs.