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Overview of High Temperature Solar Power Production. Prepared by. Prof. Dr. A. R. El-Ghalban. Department of Mechanical Engineering. University of Engineering and Technology. Taxila, Pakistan. Technology Overview .
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Overview of High Temperature Solar Power Production Prepared by Prof. Dr. A. R. El-Ghalban Department of Mechanical Engineering University of Engineering and Technology Taxila, Pakistan
Technology Overview • High temperature can be achieved by concentrating solar radiation using various mirror configurations. High temperature heat can be used in the following applications; • Electric power generation. • Hydrogen production
Electric power generation • Using concentrating systems solar power plants produce electric power by converting the sun's energy into high-temperature heat. • The heat is then channeled through a conventional generator. • The plants consist of two parts: One that collects solar energy and converts it to heat, and Another that converts heat energy to electricity.
Electric power generation • Concentrating solar power systems can be sized from (10 kilowatts up to 350 megawatts). • Some systems use thermal storage during cloudy periods or at night. • Others can be combined with natural gas and the resulting hybrid power plants provide high-value, dispatchable power. • Concentrating solar power plants generate their peak output during sunny periods when peak electricity demand occurs as air conditioning loads are at their peak.
Technology Overview • These attributes, along with world record solar-to-electric conversion efficiencies, make concentrating solar power an attractive renewable energy option in the sunbelt regions worldwide. • There are three kinds of concentrating solar power systems. This classification according to the way how they collect solar energy. • Trough systems, • Dish/engine systems, and • Power tower systems.
Trough systems • In such systems the sun's energy is concentrated by parabolic curved, trough-shaped reflectors onto a receiver pipe running along the inside of the curved surface. • This energy heats oil flowing through the pipe, and the heat energy is then used to generate electricity in a conventional steam generator. • A collector field comprises many troughs in parallel rows aligned on a north-south axis.
Trough systems • This configuration enables the single-axis troughs to track the sun from east to west during the day to ensure that the sun is continuously focused on the receiver pipes. • Trough designs can incorporate thermal storage—setting aside the heat transfer fluid in its hot phase—allowing for electricity generation several hours into the evening. • Currently, all parabolic trough plants are "hybrids," meaning they use fossil fuel to supplement the solar output during periods of low solar radiation.
Trough systems • Parabolic concentrators have been successfully operating commercially since 1984, including the largest solar power plant of any kind, the 350 MW plant Solar Energy Generating Systems.
Dish/Engine Systems • A dish/engine system uses a mirrored dish (similar to a very large satellite dish). • The dish-shaped surface collects and concentrates the sun's heat onto a receiver, which absorbs the heat and transfers it to fluid within the engine. • The heat causes the fluid to expand against a piston or turbine to produce mechanical power. • The mechanical power is then used to run a generator or alternator to produce electricity by an electric generator or alternator.
Dish/Engine Systems • Dish/engine systems use dual-axis collectors to track the sun. • The ideal concentrator shape is parabolic, created either by a single reflective surface or multiple reflectors, or facets. • There are many options for receiver and engine type, including Stirling engine and Brayton receivers. • Dish/engine systems are not commercially available, although ongoing demonstrations indicate good potential.
Dish/Engine Systems • Individual dish/engine systems currently can generate about 25 kilowatts of electricity. • More capacity is possible by connecting dishes together. • These systems can be combined with natural gas and the resulting hybrid provides continuous power generation. • The dish-Stirling system works at higher efficiencies than any other current solar technologies, with a net solar-to-electric conversion efficiency reaching 30%.
Dish/Engine Systems • One of the system’s advantages is that it is “somewhat modular,” and the size of the facility can be ramped up over a period of time. • That is compared to a traditional power plant or other large-scale solar technologies that have to be completely built before they are operational.
Solar Power Towers • By collecting solar energy during daylight hours and storing it in hot molten salt, solar power towers give utilities an alternative method for meeting peak loads. • The receiver collects the sun's heat in a heat-transfer fluid (liquid salt), which is used to generate steam for a conventional steam turbine located at the foot of the tower for production of electricity. • The liquid salt at 290°C is pumped from a cold storage tank through the receiver, where it is heated to 565°C and then on to a hot tank for storage.
Solar Power Towers • When power is needed from the plant, hot salt is pumped to a steam generating system that produces superheated steam to power a turbine and generator. • From the steam generator, the salt is returned to the cold tank, where it is stored and eventually reheated in the receiver. • They are unique among solar technologies because they can store energy efficiently and cost effectively. • They can operate whenever the customer needs power, even after dark or during cloudy weather.
Solar Power Towers • Power towers operate by focusing a field of thousands of mirrors onto a receiver located at the top of a centrally located tower. • With thermal storage, power towers can operate at an annual capacity factor of 65%, which means they can potentially operate for 65% of the year without a backup fuel source. Without energy storage, solar technologies like this are limited to annual capacity factors near 25%.
Solar Hydrogen Production • Steam Methane Reforming • Steam methane reforming is a possible process to produce hydrogen. Methane is reformed at elevated temperature and pressure to produce a syngas (mixture of H2 and CO). CH4+H2O Heat (206 kJ/ mole) 2H2 + CO • The reforming reaction is carried out in a reformer containing tubes filled with nickel catalyst at temperatures between 500ºC and 950ºC and a pressure around 30 atmospheres.
Solar Hydrogen Production • Coal Gasification • Like steam methane reforming, coal gasification proceeds by a treatment of coal feedstock with high temperature steam (1330ºC) to produce syngas (mixture of H2 and CO). Coal (carbon source) + H2O H2 + CO + impurities • The heat required for this gasification step comes from controlled addition of oxygen, which allows partial oxidation of a small amount of the coal feedstock.
Coal Gasification • Because of this, the reaction is carried out in either an air-blown or oxygen-blown gasifier. The oxygen blown gasifier must be supplied with O2 from an independent air purification system.
Solar Hydrogen Production • Sulfur-Iodine Cycle • In the sulfur-iodine cycle, a heat source, possibly a solar dish, provides the heat necessary to drive three coupled thermo-chemical reactions. • The coupled reaction system takes water as an input and through a series of reactions involving sulfur and iodine produces H2 and O2 as output. Process flow sheets have been developed for heat sources at 850 °C and 950°C. • The S-I cycle is described by the reactions:
Solar Hydrogen Production 2H2O + SO2 + I2 Heat (-216 kJ/ mole) H2SO4+ 2 HI (< 120oC) H2SO4Heat ( 371 kJ/ mole) H2O +SO2 + ½ O2 (> 800oC) 2HI Heat ( 12 kJ/ mole) H2 + I2 (> 300oC) Net Effect H2O H2 + ½ O2
Solar Power Generation The Idea of Solar Driven ORC
Dish/Engine Systems • The cost for such prototype unit (25 kW) is about $150,000. Once in production the cost could be reduced to less than $50,000 each, which would make the cost of electricity competitive with conventional fuel technologies.
