Clean Energy Technology

Flash steam plants, making up about two-thirds of geothermal installed capacity today, are used where water-dominated reservoirs have temperatures above 180°C. In these high-temperature reservoirs, the liquid water component boils, or “flashes,” as pressure drops. Separated steam is piped to a turbine to generate electricity and the remaining hot water may be flashed again twice (double flash plant) or three times (triple flash) at progressively lower pressures and temperatures, to obtain more steam.

Fuel cells are a further option to convert hydrogen into electricity and heat, producing only water and no direct emissions. Fuel cells can achieve high electric efficiencies of over 60% (above 80% overall efficiency when also including the heat output) and reveal a higher efficiency in part load than full load, which makes them particularly attractive for flexible operations such as load balancing. Molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs) operate with 600°C and 800-1 000°C, respectively, at higher temperatures, which allows them to run on different hydrocarbon fuels, without the need for an external reformer to produce hydrogen first. MCFCs are used in the MW scale for power generation (due to their low power density, resulting in a relatively large size). In general, these fuel cells offer a versatile and adaptable choice for producing huge amounts of power, with applications in a range of settings and industries: - Fuel cells may be used in microgrids to provide stable, dependable, and resilient electricity to communities, particularly in rural or off-grid areas. - Distributed power generation, which locates small power plants close to the location of consumption, such as colleges, military sites, and wastewater treatment facilities. - Fuel cells can also be used in combined heat and power (CHP) systems, which use the leftover heat from the generation of electricity to provide heating or cooling. CHP systems may be very efficient and cost-effective in buildings with large energy needs, such as hospitals or colleges.

Specifically designed gas turbines can run on pure hydrogen or a hydrogen-rich syngas/natural gas mixtures. These gas turbines using hydrogen-rich mixtures have accumulated millions of hours of operation at large scale, whereas in the case of using pure hydrogen, they have reached a few thousands of operating hours at precommercial scale. There are technical challenges associated with the high combustion temperature of hydrogen (NOx emissions, reliability/flame instabilities/flashback). For these reasons, the combustor of the gas turbine needs to be modified for gases with high hydrogen contents. Burning (undiluted) fuel gas mixtures with hydrogen concentrations of 5-60% is possible in certain gas turbines, depending on the degree to which they have been modified. Burning (diluted) fuel gas mixtures containing hydrogen up to concentrations of 100% is possible in certain gas turbines with the addition of significant amounts of diluents such as N2/steam to the fuel gas. Commercial operation of F-class gas turbines with mixtures of up to 20% of hydrogen in natural gas mixture without hardware modifications is expected by 2025. In the case of H-class gas turbines, commercial operation with mixtures containing over 50% of hydrogen is expected in new combined cycle by 2027. But many open questions and challenges remain, such as efficiency gains, NOx (reduced, for example, by steam injection, which can lead to reduced system performance) and response to variability in fuel flow composition. In many cases, the H2 mixing ratio will fluctuate, and the extent to which the combustion process can respond to large variations in %H2 requires further R&D.

At a natural gas-fired power plant with post-combustion capture using chemical absorption, the carbon dioxide is separated from the combustion flue gas by reaction of CO2 with a chemical solvent (e.g. amine-based) to form a weakly bonded intermediate compound, which may be regenerated with the application of heat to produce the original solvent (for further operation) and a concentrated CO2 stream.

Kalina cycle is one cycle option to use low-temperature geothermal resources, typically operating with temperatures varying from as low as 73°C to 180°C. The heat is recovered from the geothermal fluid using heat exchangers to vaporise a working fluid with a low boiling point (ammonia-water mixture) and drive a turbine.

Fuel cells are a further option to convert hydrogen into electricity and heat, producing only water and no direct emissions. Fuel cells can achieve high electric efficiencies of over 60% (above 80% overall efficiency when also including the heat output) and reveal a higher efficiency in part load than full load, which makes them particularly attractive for flexible operations such as load balancing. Molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs) operate with 600°C and 800-1 000°C, respectively, at higher temperatures, which allows them to run on different hydrocarbon fuels, without the need for an external reformer to produce hydrogen first. MCFCs are used in the MW scale for power generation (due to their low power density, resulting in a relatively large size). In general, these fuel cells offer a versatile and adaptable choice for producing huge amounts of power, with applications in a range of settings and industries: - Fuel cells may be used in microgrids to provide stable, dependable, and resilient electricity to communities, particularly in rural or off-grid areas. - Distributed power generation, which locates small power plants close to the location of consumption, such as colleges, military sites, and wastewater treatment facilities. - Fuel cells can also be used in combined heat and power (CHP) systems, which use the leftover heat from the generation of electricity to provide heating or cooling. CHP systems may be very efficient and cost-effective in buildings with large energy needs, such as hospitals or colleges.

Specifically designed gas turbines can run on pure hydrogen or a hydrogen-rich syngas/natural gas mixtures. These gas turbines using hydrogen-rich mixtures have accumulated millions of hours of operation at large scale, whereas in the case of using pure hydrogen, they have reached a few thousands of operating hours at precommercial scale. There are technical challenges associated with the high combustion temperature of hydrogen (NOx emissions, reliability/flame instabilities/flashback). For these reasons, the combustor of the gas turbine needs to be modified for gases with high hydrogen contents. Burning (undiluted) fuel gas mixtures with hydrogen concentrations of 5-60% is possible in certain gas turbines, depending on the degree to which they have been modified. Burning (diluted) fuel gas mixtures containing hydrogen up to concentrations of 100% is possible in certain gas turbines with the addition of significant amounts of diluents such as N2/steam to the fuel gas. Commercial operation of F-class gas turbines with mixtures of up to 20% of hydrogen in natural gas mixture without hardware modifications is expected by 2025. In the case of H-class gas turbines, commercial operation with mixtures containing over 50% of hydrogen is expected in new combined cycle by 2027. But many open questions and challenges remain, such as efficiency gains, NOx (reduced, for example, by steam injection, which can lead to reduced system performance) and response to variability in fuel flow composition. In many cases, the H2 mixing ratio will fluctuate, and the extent to which the combustion process can respond to large variations in %H2 requires further R&D.

While in conventional power plants flue gas or steam is used to drive one or multiple turbines, in supercritical CO2 (sCO2) cycles supercritical CO2 is used, i.e. CO2 at or above its critical temperature and pressure, where liquid and gaseous phases of CO2 are indistinguishable. sCO2 cycles offer many potential advantages, including higher plant efficiencies, lower air pollutant emissions, lower investment costs and high CO2 capture rates. In some cases, they could also allow for reduced water consumption. sCO2 cycles typically use nearly pure oxygen to combust the fuel gas in order to create a flue gas stream comprised primarily of CO2 and water vapour.

Organic Rankine cycle (ORC) is one cycle option to use low-temperature geothermal resources, typically operating with temperatures varying from as low as 73°C to 180°C. The heat is recovered from the geothermal fluid using heat exchangers to vaporise a working fluid with a low boiling point (butane or pentane) and drive a turbine.

In a solid oxide fuel (SOFC)-gas turbine (GT) system, the fuel (natural gas, syngas) is converted in an SOFC. The exhaust of the SOFC is still rich with fuel and unburned hydrogen, carbon oxides (CO/CO2) and water vapor and can be burned directly with oxygen or air in the GT in order to produce more power (electric output). In the case of oxy-combustion (use of O2 instead of air) one obtains a relatively pure CO2 stream for storage or use after condensation of the water in the exhaust gas of the GT. Alternatives for removing the CO2 are possible, e.g. removing the CO2 from the GT exhaust (only option in case of combustion of the remaining fuel with air), though possibly leading to lower CO2 capture rates.

Hydropower converts the energy from falling water into electricity. It is a mature and cost-competitive technology, today providing 16% of global electricity generation. Hydropower plants can be classified in three functional categories: run-of-river, reservoir (or storage), and pumped storage plants.

Nuclear fusion, the process that takes place in the core of the sun where hydrogen is converted into helium at temperatures over 10 million °C, offers the possibility of generating base-load electricity with virtually no CO2 emissions, with a virtually unlimited supply of fuel (deuterium and tritium, isotopes of hydrogen), small amounts of short-lived radioactive waste and no possibility of accidents with significant off-site impacts.