One way or the other, the sun is the answer but … Wayne Gerdes – CleanMPG – April 13, 2017 The world may be upside down today but H2 and its production process' is one possible answer to a clean transportation future. Scientists at the U.S. Department of Energy's (DOE) National Renewable Energy Laboratory (NREL) recaptured the record for highest efficiency in solar hydrogen production via a photoelectrochemical (PEC) water-splitting process. The new solar-to-hydrogen (STH) efficiency record now stands at 16.2 percent, topping a reported 14 percent efficiency in 2015 by an international team made up of researchers from Helmholtz-Zentrum Berlin, TU Ilmenau, Fraunhofer ISE and the California Institute of Technology. A paper in Nature Energy titled Direct Solar-to-hydrogen Conversion via Inverted Metamorphic Multijunction Semiconductor Architectures outlines how NREL's new record was achieved. The authors are James Young, Myles Steiner, Ryan France, John Turner, and Todd Deutsch, all from NREL, and Henning Döscher of Philipps-Universität Marburg in Germany. Döscher has an affiliation with NREL. Direct solar-to-hydrogen conversion via inverted metamorphic multi-junction semiconductor architectures The record-setting PEC cell represents a significant change from the concept device Turner developed at NREL in the 1990s. Both the old and new PEC processes employ stacks of light-absorbing tandem semiconductors that are immersed in an acid/water solution (electrolyte) where the water-splitting reaction occurs to form hydrogen and oxygen gases. But unlike the original device made of gallium indium phosphide (GaInP2) grown on top of gallium arsenide (GaAs), the new PEC cell is grown upside-down, from top to bottom, resulting in a so-called inverted metamorphic multijunction (IMM) device. This IMM advancement allowed the NREL researchers to substitute indium gallium arsenide (InGaAs) for the conventional GaAs layers, improving the device efficiency considerably. A second key distinguishing feature of the new advancement was depositing a very thin aluminum indium phosphide (AlInP) "window layer" on top of the device, followed by a second thin layer of GaInP2. These extra layers served both to eliminate defects at the surface that otherwise reduce efficiency and to partially protect the critical underlying layers from the corrosive electrolyte solution that degrades the semiconductor material and limits the lifespan of the PEC cell. Turner's initial breakthrough created an interesting new way to efficiently split water using sunlight as the only energy input to make renewable hydrogen. Other methods that use sunlight entail additional loss-generating steps. For example: Electricity generated by commercial solar cells can be sent through power conversion systems to an electrolyzer to decompose water into hydrogen and oxygen at an approximate STH efficiency of 12 percent. Turner's direct method set a long-unmatched STH efficiency record of 12.4 percent, which has been surpassed by NREL's new PEC cell. Before the PEC technology can be commercially viable, the cost of hydrogen production needs to come down to meet DOE's target of less than $2/kg of H2. Continued improvements in cell efficiency and lifetime are needed to meet this target. Further enhanced efficiency would increase the hydrogen production rate per unit area, which decreases hydrogen cost by reducing balance-of-system expenditures. In conjunction with efficiency improvements, durability of the current cell configuration needs to be significantly extended beyond its several hours of operational life to dramatically bring down costs. NREL researchers are actively pursuing methods of increasing the lifespan of the PEC device in addition to further efficiency gains. While an alternative configuration where the device isn't submerged in acidic electrolyte and instead is wired to an external electrolyzer would solve the durability challenge, a techno-economic analysis commissioned by DOE has shown that submerged devices have the potential to produce hydrogen at a lower cost. The latest research was funded by the Energy Department's Fuel Cell Technologies Office in the Office of Energy Efficiency and Renewable Energy. The solar capture area necessary to produce 1 metric ton of H2 per day with this process is approximately equivalent to 5 football fields at an efficiency ratio of ηSTH = 15%. A 25% solar capacity factor, reasonable for a 2-D tracking system in the Southwest U.S., and 98% plant operating capacity factor are assumed. At hydrogen production rates of 1.702 x 10-6 kg/m2 for a ηSTH = 15% device, 20400 m2 of capture area, the area of about five regulation size National Football League fields (each 110 m x 49 m) are required for 1 metric ton of H2/day. 1 metric ton of H2 (1,000 kg) allows 200 H2 FCVs with 5 kg tanks to be filled per day. The wrench in the works - Solar to Electricity A caveat to this is that 5 football fields of straight 20 percent efficient solar panels would produce. A quick consumer case study reveals SunPower’s Consumer Home panels are 22.5 percent efficient w/ an overall avg. of 20.6 percent and an expected life of 40-year today. The brands latest lab cells have been verified by the NREL to output over 24 percent efficient today. On an industrial scale, consider U.C. Davis’ recent 16 MW SunPower system located on a 62 acre site produces approximately 33 million kWh/year or 90,400 kWh/day. Breaking this down to kWh/ft2 in order to compare against the Solar to H2’s lb efficiency calculations 5-football fields to refuel 200 cars per day looks like this. 2017 Toyota Mirai – 312 miles range from onboard 5-kg storage tanks 2017 Tesla Model S 100D – 335 miles range from 100 kWh of onboard battery storage PV to H2 According to the NREL report, 5-football fields of Solar to H2 Cell area is necessary to fill 200 (three hundred and twelve miles range) Mirai's. A football field of 360.8 ft x 160.7 ft size) * 5 or 290,000 sq. ft. is required for 1,000 kg H2 to fill 200 Toyota Mirai’s/day. The power necessary to compress, transport, and store is not included. Along with the H2 costs, the solar area needed for refueling just 200 cars is extraordinary. 3- to 5-minute fills are perfectly aligned with today's consumer expectations however. 2016 Toyota Mirai A future solution but very expensive fuel and car. PV to Electricity A 62 acre U.C. Davis/SunPower site that is now operational equates to 2.7 million sq. ft. and produces 90,400 kWh/day. This is enough energy to charge just over 900 Tesla Model S P100s/day with 100 kWh’s from flat to full each day. The problem being how to store that electrical power – more batteries for on-site storage at approximately $150/kWh – and the time it takes the consumer waiting for a complete full charge of 1 + hour. 2017 Tesla Model S P100D The P100D is certainly more attractive and much quicker than a Mirai/Clarity, but at almost twice the take home price depending on lease vs purchase, Fed and State incentives, this leaves most consumers out of the electric space. Early Conclusions Although still a ways off from the future target goal of $2.00/kg production or all in $3.00/kg incl. production, transportation, compression, and storage vs $15/kg at the H2 station today, this is one of the means by which we could eventually arrive at the H2 society. Sometime well into the future that is. The real battle for our personal future transportation dollars will be the 3 to 5 minute refueling for expensive $75 of H2 at today’s $15/kg rates for 300-miles range FCV range or 1-hour plus 100 kW Level3 fast charging for 100 kWh at $12 at $0.12/kWh (nationwide avg.) electricity cost for a similar 300 plus miles BEV range. I am not including Southern Calif. and the ridiculous $0.40/kWh rates after the basic 384 kWh threshold has been exceeded. 100 kW Level3 chargers are not in anyone's homes yet let alone few are on the road sans Tesla Super Chargers and all cost more than just a $0.12/kWh basic rate when including the charger costs.