The Sun's Two Paths to Power: Understanding Solar Hydrogen Production and Concentrated Solar Power

While most individuals think about solar electricity as via photovoltaic panels using sunlight for energy; there are many more ways where you can derive value through the sun than just photovoltaic panels. For example, you can utilize the sun to produce clean hydrogen fuel and generate utility-scale power through heat. There are a couple exciting new technologies that are pushing the envelope on solar energy: hydrogen from solar energy sources and concentrated solar power (CSP).

Hydrogen is known as the "fuel of tomorrow" for several reasons. Hydrogen has approximately 142 MJ/kg energy content, and if you use hydrogen in a fuel cell, the only emissions produced are by water. However, producing large amounts of the hydrogen fuel cleanly, is still a major challenge for producing hydrogen. One way to resolve this issue is to use sunlight to split water as a means of hydrogen production, this process has zero greenhouse gas emissions.

There are three main types of hydrogen production from solar energy which are currently at different maturity levels:

The first technology (most mature) uses photovoltaic (PV) panels coupled with electrolyzers. Electrolyzers are electric devices that take electricity and convert water to hydrogen and oxygen by using heat and heat transfer. PV systems are most developed and readily available; PV systems are very modular and reliable; when PV and electrolyzer are connected without any power conversion devices, the STH conversion efficiency of the entire system has approached the theoretical limit.

Research shows that concentrated PV systems significantly outperform conventional ones. Using InGaP/GaAs/Ge cells under 750 suns concentration, scientists achieved STH efficiencies of 18-21% with production rates of 0.8-1.0 liters of hydrogen per minute per square meter of module area. Conventional silicon modules under one sun, by comparison, achieved only about 9.4% STH efficiency with production rates around 0.3 L/min·m². This represents a performance advantage of 1.5 to 3 times for concentrated systems.

Water electrolysis has an effective range of use between 70-80%, making this option more appealing when considering future renewables electricity prices. The only major challenge right now is the high price of electrolyzers and the unpredictability of solar radiation, resulting in the need for careful integration into the system.

Photoelectrochemical (PEC) systems utilize a more integrated approach than previous methods for electrolysis of water by first generating electrical energy and then utilizing that energy for generating hydrogen from water. PECs utilize semiconductor materials submerged in water, which are capable of absorbing light from the sun and converting it directly to store energy chemically in the form of hydrogen through the electrolysis of water. This occurs when light hits the semiconductor creating pairs of electrons/holes. The electrons in the semiconductor mechanism reduce protons to form hydrogen; the holes created will oxidize the water molecules producing oxygen.

PECs were first researched approximately 50 years ago by Shinichiro Fujishima and Honda when they found that a titanium dioxide (TiO2) electrode could split H2O into H2 and O2 when coupled with a platinum cathode/alloy and illuminated with UV light. (This is what is referred to as "Honda-Fujishima effect")

Currently, PEC systems have an attractive, compact design with the ability to achieve direct solar-to-hydrogen conversion through a simple and elegant mechanism. Despite these positive design features, PEC technology is still in its relative infancy and must overcome some significant challenges before commercialization can occur, such as low efficiency in their solar-to-hydrogen conversion, degradation of materials used to create PEC cells, and scalability of performance. Thus, ongoing research is being conducted into advanced materials and nanostructured photoelectrodes designed to address these issues.

One of the more creative ways of doing this is to utilize nanoscale sized semiconductor materials (also called quantum dots) dispersed in an aqueous medium as photocatalysts. Upon illumination with sunlight they produce electrons (and holes) that can migrate to the interface of the particle and initiate the respective oxidation and reduction half reactions referred to as hydrogen evolution and oxygen evolution respectively.

The single particle photocatalyst system, or one-step excitation system, requires that the bandgap of the semiconductor straddles both the hydrogen evolution potential and the oxygen evolution potential. There is also a two-part photocatalyst system or "Z-scheme" photocatalyst configuration where two different photocatalysts are tied together by a chemical mediator (i.e. redox couple) so that water splitting occurs in two distinct steps or half reactions. This significantly lowers the energy needed for each reaction, while allowing a greater variety of visible light to be utilized.

Recent breakthroughs demonstrate the potential of this approach. A Chinese research team led by Liu Gang at the Institute of Metal Research enhanced titanium dioxide-the key photocatalytic material-by adding scandium through "structural reshaping" and "element substitution." The scandium ions fit smoothly into the material's lattice, removing "trap zones" that normally snag electrons, and reshape the crystal surface to form "electronic highways" that guide charge carriers efficiently.

The enhanced material uses over 30% of ultraviolet light and achieves a hydrogen production rate under simulated sunlight 15 times higher than earlier versions. According to the research team, a one-square-meter photocatalytic panel could produce around 10 liters of hydrogen per day under sunlight.

While particulate photocatalysis remains in the laboratory, its potential for large-scale deployment is compelling. Powder-form photocatalysts are simpler to handle and more amenable to spreading over large areas using potentially inexpensive processes compared to PV-electrolysis or PEC systems.

Concentrated Solar Power (CSP) takes a fundamentally different approach to harnessing the sun. Instead of converting light directly into electricity, CSP uses mirrors to concentrate sunlight, generate high-temperature heat, and then drive conventional turbines to produce electricity.

The fundamental concept is very straightforward. Heliostats, or arrangements of mirrors, follow the Sun's daily course and reflect the Sun's rays to a collector located at the top of a tower. This concentration of sunlight is used to heat a working fluid to very high temperatures, and once the heat is produced, the heated working fluid is used in the generation of steam that will rotate a turbine driving the generator.

The ability to incorporate thermal energy storage into a CSP system is what makes CSP of such value. The heat produced by the process of concentrating the Sun's rays can be captured and stored for hours, meaning that electricity generation from the CSP system can occur long after sunset. The dispatchable aspect of CSP-that is, when you need electricity you can produce it-is what distinguishes CSP from PV solar systems, which cease to produce electricity when it starts to cloud over or at night.

The technology found at the top of the pyramid currently (Gemasolar in Spain, Crescent Dunes in Nevada and Noor III) features liquid molten salt being used not just for transferring heat, but also storing energy. All three systems have successfully demonstrated the ability to operate continuously for a full 24 hours while maintaining more than 15 hours of energy storage with liquid molten salts alone.

The U.S. Department of Energy's Concentrated Solar Power Generation 3 (CSP Gen3) program will advance this technology beyond the existing commercial level CSP systems. One of the design approaches being explored under the CSP Gen3 program is the "Liquid Pathway" system, which uses relatively low-cost liquid chlorides as energy storage, and a liquid sodium receiver at approximately 740oC to transfer heat to the supercritical carbon dioxide (sCO2) power cycle. The entire sCO2 power cycle will operate at a higher efficiency than traditional steam Rankine type cycles, as well.

This represents a significant advance from current plants, which typically operate at around 565°C using nitrate salts. Higher operating temperatures enable greater efficiency and lower the levelized cost of energy-the Gen3 target is below $60 per megawatt-hour.

A two-tank molten salt system allows operators to circulate salt through solar receivers to charge (heating the "hot" tank), and then through heat exchangers to generate steam when discharge is required. The thermal efficiency of storage itself is high-storing heat in insulated tanks exceeds 90% efficiency for daily cycles.

The round-trip efficiency for electricity storage, however, faces a fundamental limitation. Converting heat back to electricity through steam turbines typically achieves only 35-42% thermal efficiency. Even advanced supercritical CO2 turbines struggle to exceed 50% . For comparison, lithium-ion batteries routinely exceed 85% round-trip efficiency.

This efficiency penalty means CSP is best suited for applications where thermal storage's value-long duration, low cost per kilowatt-hour of storage, and the ability to provide synchronous generation-outweighs the conversion losses. For grid-scale storage lasting 6-12 hours, the economics can still work.

The development of renewable sources of energy to generate electricity, CSP's contribution to the decarbonization of industrial processes, and the creation of thermal storage have all allowed CSPs to provide services beyond just electricity. Many industrial processes require continuous, on-demand supplies of steam or direct heat within a temperature range of 300 to 550 degrees Celsius, which includes processes such as paper manufacturing, oil refining, and chemical processing.

By using very large-scale molten salt thermal energy storage systems, CSPs can achieve this goal by providing process steam and/or superheat air for industrial applications as required in real-time. The large capacities of these molten-salt thermal energy storage systems also offer a verycost-effective alternative to electrochemicals batteries, having a cost of less than $35 per kilowatt-hour (kWh) of usable thermal energy storage.

There are complementary methods for harnessing the sun's energy, including solar hydrogen production and concentrated solar power(CSP). The sun's energy is converted into chemical fuel (hydrogen) via photovoltaic (PV) electrolysis and photocatalytic systems which can be stored indefinitely. Hydrogen can be used for transportation, industry, and electricity generation. Alternatively, CSP uses sunlight to generate heat. CSP then converts that thermal energy into electricity for dispatchable (orderly) delivery.

Rapid advances in both technologies are occurring. Increased solar-to-hydrogen conversion efficiencies result from improved materials and system integration; CSP continues to push for higher operating temperatures and lower costs. When combined, PV electrolysis and CSP allow for a solar-powered world in which not only does the sun provide energy where required but also produces an easily-stored form of fuel to provide energy in the off-peak periods throughout the day.

Earth receives a massive supply of energy from the sun. This is roughly the equivalent of 173 trillion watts (1 trillion = 1,000,000,000,000) hitting the earth each second. The challenges and opportunities for engineers include finding ways to utilize multiple modes to capture this vast supply of energy from the sun.