Lecture Notes Of Solar Thermal Energy | Heat Transfer | Solar Energy

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Solar Thermal Energy Prof. Keh-Chin Chang Department of Aeronautics and Astronautics National Cheng Kung University 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 Introduction to Heat Transfer      Heat Transfer in a Solar Collector Heat Transfer Modes Conduction Convection Radiation Heat Transfer Processes in a Solar Collector qemit qconv,air qsun absorbing film qconv,mediu m qcond,insulator qcond,panel Insulator Panel（metal） Medium flow Heat transfer modes Three heat transfer modes in a solar collector:  Radiation    Convection    𝑞𝑠𝑢𝑛 : solar irradiation 𝑞𝑒𝑚𝑖𝑡 : emitted radiant energy from the panel 𝑞𝑐𝑜𝑛𝑣,𝑎𝑖𝑟 : heat loss due to wind 𝑞𝑐𝑜𝑛𝑣,𝑚𝑒𝑑𝑖𝑢𝑚 : heat transfer to the flow medium throughout tube wall Conduction   𝑞𝑐𝑜𝑛𝑑,𝑝𝑎𝑛𝑒𝑙 : heat transfer inside the metal panel 𝑞𝑐𝑜𝑛𝑑,𝑖𝑛𝑠𝑢𝑙𝑎𝑡𝑜𝑟 : heat loss to the insulator from the panel 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. 𝑞𝑐𝑜𝑛𝑑 = 𝑞" റ 𝑐𝑜𝑛𝑑 ∙ 𝐴റ heat flux gradient Fourier’s Law: 𝑞" റ 𝑐𝑜𝑛𝑑 = −𝑘𝛻𝑇 thermal conductivity area 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 Convection Newton’s cooling/heating law: 𝑞𝑐𝑜𝑛𝑣 = 𝑞"𝑐𝑜𝑛𝑣 × 𝐴 = ℎ𝐴(𝑇𝑠 − 𝑇∞ ) ℎ : convective heat transfer coefficient ℎ = ℎ(𝑅𝑒, 𝑓𝑙𝑜𝑤 𝑐𝑜𝑛𝑓𝑖𝑔𝑢𝑟𝑎𝑡𝑖𝑜𝑛) (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) 𝑞𝑟𝑎𝑑 = 𝑞"𝑟𝑎𝑑 × 𝐴 = (𝜎𝑇 4 )𝐴 T: absolute temperature Stefan-Boltzmann constant For real case: 𝑞"𝑟𝑎𝑑 = 𝜀𝜎𝑇 4 emissivity ,0 < 𝜀 ≤ 1 Example: Glass (transparent material) Reflection (G𝜌 ) Emission (E=𝜀𝜎𝑇 4 ) Irradiation (G) Absorption (G𝛼 ) Transmission (G𝜏 ) G = G𝜌 + G𝛼 + G𝜏 or G𝜌 G𝛼 G𝜏 1= + + =𝜌+𝛼+𝜏 G G G reflectivity transmitivity absorptivity Emissivity Defined as the ratio of the radiant energy rate emitting from a blackbody under identical condition a) Monochromatic (or spectral) , directional emissivity emitted 𝜀𝜆,𝜃 𝜆, 𝜃, 𝜙, 𝑇 = intensity 𝐼𝜆,𝑒 (𝜆,𝜃,𝜙,𝑇) 𝐼𝜆,𝑏 (𝜆,𝑇) blackbody 0 ≤ 𝜙 < 2𝜋 𝜋 0≤𝜃≤ 2 Spherical coordinate Emissivity b) Monochromatic, hemispherical emissivity 𝜀𝜆 𝜆, 𝑇 = 𝜋 2𝜋 2 ‫׬‬0 ‫׬‬0 𝐼𝜆,𝑒 𝑐𝑜𝑠 𝜃 𝑠𝑖𝑛 𝜃𝑑𝜃𝑑𝜙 𝜋 2𝜋 2 ‫׬‬0 ‫׬‬0 𝐼𝜆,𝑏 𝑐𝑜𝑠 𝜃 𝑠𝑖𝑛 𝜃𝑑𝜃𝑑𝜙 = 𝜋 = c) 𝜋 2𝜋 2 ‫׬‬0 ‫׬‬0 𝜀𝜆,𝜃 𝐼𝜆,𝑏 𝑐𝑜𝑠 𝜃 𝑠𝑖𝑛 𝜃𝑑𝜃𝑑𝜙 𝐸𝜆,𝑏 (𝜆,𝑇) 1 2𝜋 2 ‫׬ ׬‬0 𝜀𝜆,𝜃 (𝜆, 𝜃, 𝜙, 𝑇) 𝑐𝑜𝑠 𝜃 𝑠𝑖𝑛 𝜃 𝑑𝜃𝑑𝜙 𝜋 0 = 𝜋𝐼𝜆,𝑏 (T) Total , hemispherical emissivity ∞ 𝜀 𝑇 = ‫׬‬0 𝜀𝜆 𝜆, 𝑇 𝐸𝜆,𝑏 𝜆, 𝑇 𝑑𝜆 ∞ ‫׬‬0 𝐸𝜆,𝑏 𝜆, 𝑇 𝑑𝜆 ∞ 1 = න 𝜀 (𝜆, 𝑇)𝐸𝜆,𝑏 𝜆, 𝑇 𝑑𝜆 𝜎𝑇 4 0 𝜆 Absorptivity Definition: A function of the radiant energy incident on a body that is absorbed by the body a) Monochromatic, directional absorptivity, 𝛼𝜆,𝜃 (𝜆, 𝜃, 𝜙) b) Monochromatic, hemispherical absorptivity, 𝛼𝜆 (𝜆) c) Total, hemispherical absorptivity, 𝛼 For a solar panel (opaque material, 𝜏𝜆 = 𝜏 = 0) ⟹ 1 = 𝛼𝜆 + 𝜌𝜆 , 1 = 𝛼 + 𝜌 𝐼𝑠𝑢𝑛 𝑞𝑠𝑢𝑛 = 𝐴𝑝 𝛼𝑝 𝐼𝑠𝑢𝑛 𝑞𝑒𝑚𝑖𝑡 = 𝐴𝑝 𝜀𝑝 𝜎𝑇 4 Looking for high 𝜶𝒑 while small 𝜺𝒑 𝑞𝑒𝑚𝑖𝑡 𝑞𝑠𝑢𝑛 A desired property for a good solar absorptance 𝛼𝜆 > 0.9 1.0 visible light : 0.4-0.7μm 𝛼𝜆 < 0.1 0 0.1 𝜆(𝜇𝑚) 3 As Kirchhoff’s law for a diffuse (i.e., independent of direction) surface 𝜀𝜆 = 𝛼𝜆 Source of Solar Energy      The Sun Between the Sun and the Earth Position of the Sun Solar constant Solar radiation and intensity 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 Between the Sun and the Earth Source of Solar Energy Average distance:149.5 million km (1 astronomical unit, AU) equinox solstice solstice equinox Elliptic Orbit Between the Sun and the Earth Source of Solar Energy Position of the Sun (view from Earth) Source of Solar Energy Apparent placement of the Sun in the northern hemisphere 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. 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).  Solar Constant Entry point into atmosphere Intensity ~ 1350W/m2 Solar Radiation Spectrum Source of Solar Energy Solar Radiation Budget (to Earth) Source of Solar Energy Factors affect the Solar intensity Source of Solar Energy  Latitude  Altitude  Atmospheric transparency  Solar zenith angle Applications of Solar Energy     Reserves of energy on Earth Solar energy distribution Advantages of using solar energy Types of applications Reserves of Energy on Earth Applications of Solar Energy Remaining Reserves Available Period (year) Coal 660.8 Gton 43 Oil 152 Gton 210 Gas 160755 Gm3 67 Uranium 1.57 Mton 42 Solar Energy Distribution Applications of Solar Energy Annual global mean downward solar radiation distribution at the surface Advantages of using Solar Energy Application of Solar Energy  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. Weiss, Werner, I. Bergmann, and G. Faninger. Solar Heat Worldwide–Markets and Contribution to the Energy Supply 2008. International Energy Agency, 2010. Advantages of using Solar Energy Application of Solar Energy  Energy production prediction Types of Applications Application of Solar Energy  Photovoltaic (PV)   Solar cell Solar thermal energy Solar water heater  Solar thermal power  Solar cooling  Solar thermal ventilation  Introduction to Photovoltaic   What is photovoltaic Solar cell 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. 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. Solar Thermal Energy Systems      How to use solar thermal energy Types of solar collectors Solar water heater Solar thermal power Solar thermal cooling How to Use Solar Thermal Energy Solar Thermal Energy Working fluid Solar Radiation Solar Thermal Energy Solar collector thermal energy working fluid Types of Solar Collectors Solar Thermal Energy   Collectors and working temperature Low temperature Medium temperature High temperature 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. Flat-plate collector Solar Thermal Energy  Glazed collector  Unglazed collector Flat-plate collector Solar Thermal Energy 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. 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. Evacuated tube collector Solar Thermal Energy  Heat pipe  Sydney tube Collector efficiency Solar Thermal Energy http://polarsolar.com/blog/?p=171 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. 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 downwardfacing linear, fixed receiver.  Simple design of flexibly bent mirrors and fixed receivers requires lower investment costs and facilitates direct steam generation. 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. 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. 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) Solar Water Heater Solar Thermal Energy Direct (open loop) Indirect (close loop) User User Passive (Thermosyphon) User User Active Heat exchanger Solar Water Heater Solar Thermal Energy  Installation direction For northern hemisphere → Facing south  For southern hemisphere → Facing north   Installation tilt angle  The angle of the collector is roughly equal to the local latitude Solar Water Heater Solar Thermal Energy L=local latitude Direction shifted from south (angle) Tilt angle of the collector Annual heat collection(%) Annual heat collection(%) Increasing collection area Annual heat collection vs. direction/tilt angle (in north hemisphere) Increasing collection area  Solar Water Heater Solar Thermal Energy  Residential hot water system Hot water production  House warming  “Solar Thermal Action Plan for Europe”, ESTIF, 2007  Large-scale system Dormitory hot water  Swimming pool  Industrial process heating  Solar Water Heater Solar Thermal Energy  Industrial process heating  In EU, 2/3 of the industrial energy demand consists of heat rather than electrical energy.  About 50% of the industrial heat demand is located at temperatures up to 250°C. Solar Water Heater Solar Thermal Energy  Market potential of industrial process heating Solar Thermal Power Solar Thermal Energy  Conversion of sunlight into electricity Direct means : photovoltaics (PV),  Indirect means : concentrated solar power (CSP).  Solar thermal power  High temperature applications (by means of sun-tracking, concentrated solar collectors) Solar Thermal Power Solar Thermal Energy  Electrical power is generated when the concentrated light is converted to heat and, then, drives a heat engine (usually a steam turbine) which is connected to an electrical power generator. Solar Thermal Power Solar Thermal Energy  Types of solar thermal power plant Technology roadmap concentrating solar power, IEA, 2010. Solar Thermal Power Solar Thermal Energy  Combination of storage and hybridisation in a solar thermal plant Solar Thermal Power Solar Thermal Energy PS10 and PS20 solar power tower (HFC) (Seville, Spain). 2007 and 2009 Solar Thermal Power Solar Thermal Energy Kimberlina solar thermal energy plant (LFR) (Bakersfield, CA), 2008. Solar Thermal Power Solar Thermal Energy Calasparra solar power plant (LFR) (Murcia, Spain) 2009. Solar Thermal Power Solar Thermal Energy Puertollano solar power station (PTC) (Ciudad real, Spain), 2009 Andasol solar power station (PTC) (Granada, Spain), 2009 Solar (Thermal) Cooling Solar Thermal Energy   Active cooling  Use PV panel to generate electricity for driving a conventional air conditioner  Use solar thermal collectors to provide thermal energy for Solar thermal cooling driving a thermally driven chiller Passive cooling  Solar thermal ventilation Solar Thermal Cooling Solar Thermal Energy International Journal of Refrigeration 3I(2008) 3-15 Solar Thermal Cooling Solar Thermal Energy  Solar cooling benefits from a better time match between supply and demand of cooling load 2 1 "Renewable Energy Essentials: Solar Heating and Cooling," International Energy Agency, 2009. 2 B.W. Koldehoff and D. Görisried, "Solar Thermal & Solar Cooling in Germany," Management. Solar Thermal Cooling Solar Thermal Energy  Active cooling  Use solar thermal collectors to provide thermal energy for driving thermally driven chillers. Heat source Cooling tower Cooling distribution Chiller Solar Thermal Cooling Solar Thermal Energy  Basic type of solar thermal chiller  Absorption cooling－LiBr+H2O  Adsorption cooling－silica gel+H2O  DEC, Desiccant Evaporative Cooling Closed cycle Open cycle Solar Thermal Cooling Solar Thermal Energy Conventional compression cooling Adsorption/absorption cooling QL high pressure vapor Qg condenser We QL high pressure vapor condenser desorption expansion valve compressor (switch) We expansion valve absorption low pressure vapor Qa evaporator COPelect=QC/We QC low pressure vapor evaporator COPthermal=QC/Qg COPelect=QC/We QC Solar Thermal Cooling Solar Thermal Energy COPthermal of different type of chiller Henning, H. “Solar assisted air conditioning of buildings – an overview.” Applied Thermal Engineering 27, no. 10 (July 2007): 1734-1749. Solar Thermal Cooling Solar Thermal Energy "Solar Assisted Cooling – State of the Art –,“ESTIF, 2006. Solar Thermal Cooling Solar Thermal Energy A. Napolitano, "Review on existing solar assisted heating and cooling installations," 28.04.2010 – Workshop Århus, Denmark ABSORPTION, 2010. Solar Thermal Cooling Solar Thermal Energy D. Mugnier, "Refrigeration Workshop Market analysis Market actors Systems costs Politics : incentives & lobbying Conclusion Introduction," 28.04.2010 – Workshop Århus, Denmark ABSORPTION, 2010. Solar Thermal Cooling Solar Thermal Energy D. Mugnier, "Refrigeration Workshop Market analysis Market actors Systems costs Politics : incentives & lobbying Conclusion Introduction," 28.04.2010 – Workshop Århus, Denmark ABSORPTION, 2010. Solar Thermal Cooling Solar Thermal Energy  Passive Cooling (solar ventilation, solar chimney)  A way of improving the natural ventilation of buildings by using convection of air heated by passive solar energy.  Direct gain warms air inside the chimney causing it to rise out the top and drawing air in from the bottom. Solar desalination/distillation  Solar humidification-dehumidification (HDH)    HDH is based on evaporation of brackish water and consecutive condensation of the generated humid air, mostly at ambient pressure. The simplest configuration: the solar still. In sophisticated systems, waste heat is minimized by collecting the heat from the condensing water vapor and pre-heating the incoming water source. Solar Thermal Applications Solar Thermal Energy Facade integration (roof) Conventional installation way in Taiwan Conventional installation way in Taiwan Damage due to typhoon invasion Damage due to typhoon invasion Roof integrated flat-plate collectors on house in Denmark (Source: VELUX) Facade integration (balcony) Contribution of solar thermal to EU heat demand by sector Solar Thermal Energy Reduction of -40% Summary, Executive, Werner Weiss, and Peter Biermayr. Potential of Solar Thermal in Europe - Executive Summary, 2009. Restrictions in Using Solar Energy   Geographical aspects Financial aspects Geographical Aspects Restrictions in Using Solar Energy  Low energy density   Solar radiation has a low energy density relative to other common energy sources Unstable energy supply Solar Energy supply is restricted by time and geographical location  Easily influenced by weather condition  Financial Aspects Restrictions in Using Solar Energy  Higher cost compared with traditional energy   The capital cost in utilization of solar energy is generally higher than that of traditional ones, especially for PV. Solar water heater   Most economically competitive technology by now The need of SWH is inversely proportional to local insolation Examples Example 1  A family with 5 members plans to install a solar water heater which is mainly used for bath. The hot-water temperature required for bath is 50 ℃, while the annual average temperature of cold water is 23 ℃. Assuming that each person needs 60 liters of hot water for taking bath a day. How much heat should be provided by the solar water heater to satisfy the family’s demand for bath? (Note: water specific heat Cp is assumed to be 1 kcal/kg-℃, water density is 1 kg / l. ) Answer 1 Q  M  C p  T Q  Heat Demand M  Hot Water Quantity C p  specific heat capacity of water ΔT  temperature difference between hot and cold water   l kcal Q   60  5 person  1  50C  23C   person  day  kg  C   kg kcal   60  5 person  1  50C  23C   person  day  kg  C kcal  8100 day Example 2  A solar water heater is equipped with an ​effective collect area of 1m2, and the daily cumulative insolation onto the collector is 4 kWh/m2-day in February. If the average efficiency of the solar water heater is 0.5, how many kilo-calories (kcal) of heat can be collected by this solar water heater during a day? (Note: 1cal = 4.186J = 4.186 W × s). Answer 2 Qc  H  A  Qc  Heat provided from collector H  Daily accu mulative insolation A  Effective collector area η  Efficiency of solar water heater kWh 2  1 m  0.5 2 m  day kJ 1  3600 s kcal kWh kJ 2 2 s  7200  7200 4.186 day day day day kcal  1720 day Qc  4 Example 3   The minimum heat demand is 8100 kcal/day, and there is a certain solar panel which can offer a heat supply of 1720 kcal/m2 in a day. With the absence of auxiliary heating device, calculate the required installation area of the solar panel. If the effective arer of this solar panel is 0.8 m2 /piece, how many pieces of solar panel should be installed to collect this heat demand? Answer 3 Q A Qc A Q  Demand Heat Qc  Heat provided from collector per m 2 A  Effective collector area 8100 kcal 1720 kcal day  4.764m 2 m 2  day 4.764m 2  5.955  6 pieces 2 0.8m Example 4  From meteorological data, the average daily accumulative insolation in Tainan is 420 ly/day (i.e., langley / day). For a solar collector that faces south with a area of 2 m2 and tilt angle of 0 degree, what is the daily accumulative insolation onto the collector surface? (in kWh and kcal, respectively) (Note: ly = Langley = cal/cm2). Answer 4 420 ly cal  2 m2  420 2  2 m2 day cm  day (1)  420 1 1000 2 1 10000 kcal kcal  2 m2  4200 m  day day 4.186 1 4.186W  s kWh 2 1000 kW  3600 hr (2)  420 1 2  2 m  420 1 2  2 m2  9.767 day 10000 m  day 10000 m  day