Overview of High Temperature Solar Power Production

1. Overview of High Temperature Solar Power Production

2. 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

3. 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: • That collects solar energy and converts it to heat, and • That converts heat energy to electricity.

4. Electric Power Generation • Concentrating solar power systems can be sized from (10 kilowatts up to hundreds of 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.

5. 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.

6. 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.

7. 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.

8. Trough systems • Parabolic concentrators have been successfully operating commercially since 1984, including the largest solar power plant of any kind, followings are key countries having Solar Energy Generating Systems. • 1st Germany 3.8 GW • 2nd Japan 1.9 GW • 3rd US 814 MW • 4th Spain 632 MW

9. 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.

10. 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. • 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.

11. 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%.

12. 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.

13. 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.

14. 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.

15. 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%.

16. Uses high heat capacity fluids as heat transfer storage mediums 12 to 17 hours of storage will allow plants to have up to 60% to 70% capacity factors. Thermal Storage

17. Thermal Output of Hybrid Plant with Thermal Storage

18. Solar Hydrogen Production Example: Water splitting: 2H2O → 2H2 + O2

19. 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. Heat required for this process is provided by solar energy.

20. 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.

21. 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.

22. 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:

23. 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

24. PARABOLIC DISH & ENGINE LENS CONCENTRATORS PARABOLIC TROUGH PARABOLIC DISH SOLAR FURNACE SOLAR FURNACE CENTRAL RECEIVER

25. Comparison of Technologies (2006)

26. Solar Photovoltaic Systems

27. What is a solar cell? • Solid state device that converts incident solar energy directly into electrical energy • Efficiencies from several percent up to 20-30%. • No moving parts • No noise • Lifetimes of 30-40 years or more

28. PV Cells • Photovoltaic energy is the conversion of sunlight into electricity. A photovoltaic cell, commonly called a solar cell or PV, is the technology used to convert solar energy directly into electrical power. A photovoltaic cell is a non mechanical device usually made from silicon alloys. • Sunlight is composed of photons, or particles of solar energy.  These photons contain various amounts of energy corresponding to the different wavelengths of the solar spectrum. • When photons strike a photovoltaic cell, they may be reflected, pass right through, or be absorbed.

29. Only the absorbed photons provide energy to generate electricity.  When enough sunlight (energy) is absorbed by the material (a semiconductor), electrons are dislodged from the material's atoms.  Special treatment of the material surface during manufacturing makes the front surface of the cell more receptive to free electrons, so the electrons naturally migrate to the surface. • When the electrons leave their position, holes are formed.  When many electrons, each carrying a negative charge, travel toward the front surface of the cell, the resulting imbalance of charge between the cell's front and back surfaces creates a voltage potential like the negative and positive terminals of a battery.  When the two surfaces are connected through an external load, electricity flows.

31. How Solar Cells Work • Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon. • Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce electricity. • An array of solar cells converts solar energy into a usable amount of direct current (DC) electricity.

33. The photovoltaic cell is the basic building block of a photovoltaic system.  Individual cells can vary in size from about 1 centimeter (1/2 inch) to about 10 centimeter (4 inches) across.  However, one cell only produces 1 or 2 watts, which isn't enough power for most applications.  • To increase power output, cells are electrically connected into a packaged weather-tight module.  Modules can be further connected to form an array.  The term array refers to the entire generating plant, whether it is made up of one or several thousand modules.  The number of modules connected together in an array depends on the amount of power output needed. 

36. Photovoltaic Systems consist of several electronic components which work together to collect solar radiation and convert it into electricity. • Collectors • Energy Storage System • Inverter • Distribution Center

37. Available Cell Technologies • Single-crystal or Mono-crystalline Silicon • Polycrystalline or Multi-crystalline Silicon • Thin film • Ex. Amorphous silicon or Cadmium Telluride

38. Monocrystalline Silicon Modules • Most efficient commercially available module (11% - 14%) • Most expensive to produce • Circular (square-round) cell creates wasted space on module

39. Polycrystalline Silicon Modules • Less expensive to make than single crystalline modules • Cells slightly less efficient than a single crystalline (10% - 12%) • Square shape cells fit into module efficiently using the entire space

40. Amorphous Thin Film • Most inexpensive technology to produce • Metal grid replaced with transparent oxides • Efficiency = 6 – 8 % • Can be deposited on flexible substrates • Less susceptible to shading problems • Better performance in low light conditions that with crystalline modules

41. Solar Cell Market Estimate

42. Selecting the Correct Module • Practical Criteria • Size • Voltage • Availability • Warranty • Mounting Characteristics • Cost (per watt)

43. Effects of Temperature • As the PV cell temperature increases above 25º C, the module Vdecreases by approximately 0.5% per degree C

44. Effects of Shading/Low Insolation • As insolation decreases amperage decreases while voltage remains roughly constant

45. Grid-Tied System • Advantages • Low: Easy to install (less components) • Grid can supply power • Disadvantages • No power when grid goes down

46. Grid-Tied System(With Batteries) • Complexity • High: Due to the addition of batteries • Grid Interaction • Grid still supplements power • When grid goes down batteries supply power to loads.

47. Off-grid stand alone systems operate independent of the electrical grid. You can purchase individual components to build your own system or purchase pre-packaged systems (pre-packaged systems coming soon). Off-Grid Stand Alone Systems

48. System with Inverter

50. Battery Basics • Battery • A device that stores electrical energy (chemical energy to electrical energy and vice-versa) • Capacity • Amount of electrical energy the battery will contain • State of Charge (SOC) • Available battery capacity • Depth of Discharge (DOD) • Energy taken out of the battery • Efficiency • Energy out/Energy in (typically 80-85%)