Questions & explanations
1. Explain how liquid organic hydrogen carriers (LOHCs) store and release hydrogen.
Liquid organic hydrogen carriers (LOHCs) are organic compounds that can be hydrogenated (add hydrogen) and dehydrogenated (release hydrogen) reversibly. For example, dibenzyltoluene (DBT) is a common LOHC. In the hydrogenation step, hydrogen gas is reacted with the LOHC at moderate temperature (150-200°C) and pressure (20-50 bar) using a catalyst, forming a hydrogen-rich liquid. To release hydrogen, the liquid is dehydrogenated at higher temperature (250-300°C) and low pressure, producing hydrogen gas and the original LOHC. The LOHC is recycled. The hydrogen storage density is about 6-7 wt% for DBT. LOHCs are non-toxic, non-flammable, and can be stored in existing fuel infrastructure. The main challenge is the energy required for dehydrogenation and catalyst stability.
2. How does the efficiency of thermochemical water splitting compare to other hydrogen production methods?
Thermochemical water splitting can theoretically achieve higher efficiencies than electrolysis because it uses heat directly, avoiding electricity generation losses. Practical efficiencies for thermochemical cycles are around 20-50%, depending on the cycle and heat source. In comparison, electrolysis has efficiencies of 60-80% (based on electricity), but when considering the full chain from primary energy, thermochemical may be better. However, thermochemical requires very high temperatures and complex reactors, which increase cost. Steam methane reforming, the current main method, has about 65-75% efficiency but produces CO2. Thermochemical cycles are carbon-free if the heat is from renewable sources. They are still in the research stage and not yet commercial.
3. What factors affect hydrogen yield in biological production and how can they be optimized?
Key factors include pH, temperature, substrate concentration, and light intensity (for photofermentation). For dark fermentation, optimal pH is around 5.5-6.5 and temperature around 35-40°C. High substrate concentration can inhibit bacteria, so it must be balanced. For photofermentation, light intensity and wavelength matter; too much light can cause photoinhibition. Nitrogen limitation is needed to activate nitrogenase. Also, the presence of oxygen must be avoided. Optimization involves controlling these parameters and using genetically modified strains to increase hydrogen production. Bioreactor design, such as using immobilized cells, can improve yields. Combining dark and photofermentation in a two-stage process can achieve higher overall hydrogen yields.
4. Explain the pressure-composition-temperature (PCT) diagram for a metal hydride.
A PCT diagram shows the relationship between hydrogen pressure, composition (hydrogen-to-metal ratio), and temperature. At a given temperature, as hydrogen is added, the pressure increases until a plateau region where the metal transforms into the hydride phase. During this plateau, the pressure remains constant while the composition changes. Once the metal is fully hydrided, pressure rises again. The plateau pressure depends on temperature; higher temperature gives higher plateau pressure. The diagram helps determine the operating conditions for storage and release. For example, to store hydrogen, you apply pressure above the plateau; to release, you lower pressure or heat the material. The width of the plateau indicates the storage capacity.
5. What are the main challenges in using metal hydrides for hydrogen storage in vehicles?
The main challenges are weight and heat management. Metal hydrides have low gravimetric density (typically 1-7 wt% hydrogen), meaning the storage system is heavy. For vehicles, a target is 5.5 wt% system capacity, but current materials fall short. Also, releasing hydrogen requires heat, which must be supplied quickly for fuel cell operation. The heat of absorption/desorption can be up to 30-80 kJ/mol H2. Managing this heat adds complexity and weight. Additionally, some hydrides degrade over many cycles due to pulverization or contamination. Cost is another issue, especially for alloys like LaNi5. Research focuses on lightweight hydrides like Mg-based or complex hydrides (e.g., alanates) to improve capacity and kinetics.
6. What is thermochemical water splitting and how does it differ from electrolysis?
Thermochemical water splitting uses high-temperature heat to drive chemical reactions that split water into hydrogen and oxygen. Unlike electrolysis, which uses electricity, thermochemical cycles use heat, often from concentrated solar power or nuclear reactors. The process involves multiple chemical steps that recycle all reagents except water. For example, the sulfur-iodine cycle uses heat to decompose sulfuric acid and hydroiodic acid. The overall reaction is water splitting, but each step occurs at different temperatures. Thermochemical cycles can achieve higher efficiencies than electrolysis if the heat source is very hot. However, they require materials that withstand high temperatures and corrosive chemicals.
7. What is metal hydride storage for hydrogen and how does it work?
Metal hydride storage involves absorbing hydrogen into a metal or alloy to form a hydride. The hydrogen is stored in the solid material, which can release it when heated. The process is reversible: at moderate pressures and temperatures, hydrogen bonds with the metal. For example, LaNi5 absorbs hydrogen to form LaNi5H6. The amount of hydrogen stored is given by the pressure-composition-temperature (PCT) diagram, which shows the equilibrium pressure at different hydrogen concentrations. The storage density can be higher than compressed gas on a volume basis. The main advantage is safety, as the hydrogen is chemically bound and not under high pressure. However, the weight of the metal reduces the gravimetric density.
8. Give an example of a complex metal hydride and explain its hydrogen storage mechanism.
Sodium alanate (NaAlH4) is a complex hydride that stores hydrogen through a two-step reaction. First, NaAlH4 decomposes to Na3AlH6, Al, and H2 at around 180-230°C. Second, Na3AlH6 further decomposes to NaH, Al, and H2 at higher temperatures (above 250°C). The theoretical capacity is about 5.6 wt% for the first step and 3.0 wt% for the second, total 7.4 wt% if both steps are used. However, the second step requires high temperature and is less reversible. Doping with titanium catalysts improves kinetics and reversibility. The mechanism involves breaking Al-H bonds and forming Al metal. Complex hydrides offer higher capacities than simple metal hydrides but still face challenges with temperature and reversibility.
9. How does the choice of well casing material affect corrosion in geothermal systems?
Geothermal fluids often contain corrosive gases like hydrogen sulfide and carbon dioxide, as well as dissolved salts. If the well casing is made of ordinary steel, these chemicals can quickly eat through it, causing leaks and well failure. Using corrosion-resistant alloys like stainless steel or nickel-based metals can greatly extend the life of the casing. However, these materials are more expensive. Operators must balance cost with the expected corrosion rate based on the fluid chemistry. Regular inspection and cathodic protection, which uses an electric current to stop corrosion, are also used to protect the casing. Proper material selection is one of the most important decisions in geothermal well design.
10. How does the energy efficiency of chemical hydrogen storage (ammonia or LOHC) compare to compressed hydrogen gas?
Chemical hydrogen storage has lower round-trip energy efficiency than compressed gas because of the energy needed for conversion. For ammonia, the cracking step requires about 15-20% of the hydrogen's energy content. For LOHCs, dehydrogenation consumes 25-30% of the energy. In contrast, compressing hydrogen to 700 bar uses about 10-15% of its energy. However, chemical carriers offer higher volumetric density and easier handling, which can reduce transport costs. The overall efficiency depends on the entire chain: production, storage, transport, and release. For long-distance shipping, ammonia or LOHCs may be more economical despite lower efficiency. Research aims to improve catalysts and reduce energy losses.
11. Give an example of a bioreactor design for biological hydrogen production and explain its advantages.
A common bioreactor for dark fermentation is a continuous stirred-tank reactor (CSTR). It has a constant flow of substrate and removal of products, keeping conditions stable. The stirring ensures good mixing and contact between bacteria and substrate. Advantages are high productivity and easy control. For photofermentation, a flat-panel photobioreactor is often used. It has a large surface area for light capture and thin layers to ensure light penetration. This design maximizes light utilization and hydrogen production. The flat-panel reactor can be made of transparent materials and placed outdoors. Both designs require careful temperature and pH control. The choice depends on the microorganism and scale.
12. How does certification of a wave energy converter differ from certification of a tidal turbine?
Both wave energy converters and tidal turbines follow similar IEC standards for safety and performance, but the specific tests differ. Wave energy converters must be tested for varying wave heights and periods, while tidal turbines are tested for water flow speeds and turbulence. The environmental impact assessment also differs: wave devices may affect wave patterns and marine life differently than tidal devices. Certification for tidal turbines might emphasize rotor blade safety and marine mammal collision risks. However, both require verification of power output and survivability in extreme conditions. The core certification process is the same, but the technical details are tailored to each technology.