Solar Thermal Energy

1. Solar Thermal Energy Prof. Keh-Chin Chang Department of Aeronautics and Astronautics National Cheng Kung University

2. Outline • Introduction to Heat Transfer • Source of Solar Energy • Applications of Solar Energy • Introduction to Photovoltaic • Solar Thermal Energy Systems • Restrictions in Using Solar Energy • Examples

3. Introduction to Heat Transfer Heat Transfer in a Solar Collector Heat Transfer Modes Conduction Convection Radiation

4. Heat Transfer Processes in a Solar Collector qconv,air qemit qsun qconv,medium absorbing film Medium flow qcond,insulator Insulator qcond,panel Panel（metal）

5. Heat transfer modes Three heat transfer modes in a solar collector: • Radiation • : solar irradiation • : emitted radiant energy from the panel • Convection • : heat loss due to wind • : heat transfer to the flow medium throughout tube wall • Conduction • : heat transfer inside the metal panel • : heat loss to the insulator from the panel

6. Conduction Definition: The transfer of energy from the more energetic to the less energetic particles (atoms or molecules ) of a substance due to interactions between the particles without bulk motion. heat flux area gradient Fourier’s Law: thermal conductivity

7. Convection Definition: Heat transfer between a fluid in motion and a boundary surface Knowledge of convective heat transfer needs to know both fluid mechanics and heat transfer

8. Convection Newton’s cooling/heating law: : convective heat transfer coefficient

9. (Thermal) Radiation Definition: Energy is emitted by matter via electromagnetic waves with the wavelengths ranging between the long-wave fringe ultraviolet (UV, ≈10-1μm) and far infrared (IR, ≈103μm). Stefan-Boltzmann Law: for a blackbody (ideal case) T: absolute temperature Stefan-Boltzmann constant For real case: emissivity

10. Example: Glass (transparent material) Emission (E=) Reflection ( ) Irradiation (G) Absorption () Transmission () G = transmitivity or absorptivity reflectivity

11. Emissivity Defined as the ratio of the radiant energy rate emitting from a blackbody under identical condition • Monochromatic (or spectral) , directional emissivity emitted intensity blackbody Spherical coordinate

12. Emissivity • Monochromatic, hemispherical emissivity • Total , hemispherical emissivity =(T)

13. Absorptivity Definition: A function of the radiant energy incident on a body that is absorbed by the body • Monochromatic, directional absorptivity, • Monochromatic, hemispherical absorptivity, • Total, hemispherical absorptivity,

14. For a solar panel (opaque material, ) , Looking for high while small

15. A desired property for a good solar absorptance As Kirchhoff’s law for a diffuse (i.e., independent of direction) surface 1.0 visible light : 0.4-0.7μm 1 3 0 0.1

16. Source of Solar Energy The Sun Between the Sun and the Earth Position of the Sun Solar constant Solar radiation and intensity

17. The Sun Source of Solar Energy • A sphere of intensely hot gaseous matter Consist of H, He, O, C, Ne, Fe… Surface temperature: 5,800K Core temperature:13,600,000K

18. Average distance:149.5 million km (1 astronomical unit, AU) equinox solstice solstice Elliptic Orbit equinox Between the Sun and the Earth Source of Solar Energy

19. Between the Sun and the Earth Source of Solar Energy

20. Position of the Sun (view from Earth) Source of Solar Energy Apparent placement of the Sun in the northern hemisphere

21. Position of the Sun (view from Earth) Source of Solar Energy Azimuth angle of the sun: Often defined as the angle from due north in a clockwise direction. (sometimes from south) Zenith angle of the sun: Defined as the angle measured from vertical downward.

22. Solar ConstantEntry point into atmosphereIntensity ~ 1350W/m2 Solar Constant Source of Solar Energy • Amount of incoming solar radiation per unit area incident on a plane perpendicular to the rays. • At a distance of one 1AU from the sun (roughly the mean distance from the Sun to the Earth). • Includes a range of wavelength (not just the visible light).

23. Solar Radiation Spectrum Source of Solar Energy

24. Solar Radiation Budget (to Earth) Source of Solar Energy

25. Factors affect the Solar intensity Source of Solar Energy • Latitude • Altitude • Atmospheric transparency • Solar zenith angle

26. Applications of Solar Energy • Reserves of energy on Earth • Solar energy distribution • Advantages of using solar energy • Types of applications

27. Reserves of Energy on Earth Applications of Solar Energy

28. Solar Energy Distribution Applications of Solar Energy Annual global mean downward solar radiation distribution at the surface

29. No pollution Inexhaustible Contribution to energy supply and CO2 reduction The annual collector yield of the world was 109,713 GWh (394,968 TJ). This corresponds to an oil equivalent of 12.4 million tons and an annual avoidance of 39.4 million tons of CO2. The annual collector yield of Taiwan was 918 GWh (3306 TJ). This corresponds to an oil equivalent of 101,780 tons and an annual avoidance of 322,393 tons of CO2. Advantages of using Solar Energy Application of Solar Energy Weiss, Werner, I. Bergmann, and G. Faninger. Solar Heat Worldwide–Markets and Contribution to the Energy Supply 2008. International Energy Agency, 2010.

30. Advantages of using Solar Energy Application of Solar Energy • Energy production prediction

31. Photovoltaic (PV) Solar cell Solar thermal energy Solar water heater Solar thermal power Solar cooling Solar thermal ventilation Types of Applications Application of Solar Energy

32. Introduction to Photovoltaic • What is photovoltaic • Solar cell

33. What is Photovoltaic Photovoltaic • A method of generating electrical power by converting solar radiation into direct current electricity through some materials (such as semiconductors) that exhibit the photovoltaic effect.

34. Solar Cell Photovoltaic • Sun light of certain wavelengths is able to ionize the atoms in the silicon • The internal field produced by the junction separates some of the positive charges ("holes") from the negative charges (electrons). • If a circuit is made, power can be produced from the cells under illumination, since the free electrons have to pass through the junction to recombine with the positive holes.

35. Solar Thermal Energy Systems • How to use solar thermal energy • Types of solar collectors • Solar water heater • Solar thermal power • Solar thermal cooling

36. How to Use Solar Thermal Energy Solar Thermal Energy Working fluid Solar Radiation Solar Thermal Energy Solar collector Solar Radiation Solar Radiation thermal energy Solar Radiation working fluid

37. Collectors and working temperature Types of Solar Collectors Solar Thermal Energy Low temperature Medium temperature High temperature

38. Flat-plate collector Solar Thermal Energy • Use both beam and diffuse solar radiation, do not require tracking of the sun, and are low-maintenance, inexpensive and mechanically simple.

39. Unglazed collector Flat-plate collector Solar Thermal Energy Glazed collector

40. Flat-plate collector Solar Thermal Energy

41. Flat-plate collector Solar Thermal Energy • Main losses of a basic flat-plate collector during angular operation Weiss, Werner, and Matthias Rommel. Process Heat Collectors. Vol. 33, 2008.

42. Evacuated tube collector Solar Thermal Energy • A collector consists of a row of parallel glass tubes. • A vacuum inside every single tube extremely reduces conduction losses and eliminates convection losses.

43. Heat pipe Sydney tube Evacuated tube collector Solar Thermal Energy

44. Collector efficiency Solar Thermal Energy http://polarsolar.com/blog/?p=171

45. Parabolic trough collector Solar Thermal Energy • Consist of parallel rows of mirrors (reflectors) curved in one dimension to focus the sun’s rays. • All parabolic trough plants currently in commercial operation rely on synthetic oil as the fluid that transfers heat from collector pipes to heat exchangers.

46. Linear Fresnel reflector Solar Thermal Energy • Approximate the parabolic trough systems but by using long rows of flat or slightly curved mirrors to reflect the sun’s rays onto a downward-facing linear, fixed receiver. • Simple design of flexibly bent mirrors and fixed receivers requires lower investment costs and facilitates direct steam generation.

47. Parabolic dish reflector Solar Thermal Energy • Concentrate the sun’s rays at a focal point propped above the centre of the dish. The entire apparatus tracks the sun, with the dish and receiver moving in tandem. • Most dishes have an independent engine/generator (such as a Stirling machine or a micro-turbine) at the focal point.

48. Heliostat field collector Solar Thermal Energy • A heliostat is a device that includes a plane mirror which turns so as to keep reflecting sunlight toward a predetermined target. • Heliostat field use hundreds or thousands of small reflectors to concentrate the sun’s rays on a central receiver placed atop a fixed tower.

49. Solar Water Heater Solar Thermal Energy • Most popular and well developed application of solar thermal energy so far • Low temperature applications (Mainly using flat plate collector or evacuate tube collector)

50. User User User User Heat exchanger Solar Water Heater Solar Thermal Energy (Thermosyphon)