Solar thermal energy systems

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rzn95

Engineering

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Discuss all the possible energy losses from a flat plate solar thermal water heater? (page 40 onwards)

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Module-2-3 Solar Thermal Energy Pawan Tyagi Last lecture summary • What is in the solar radiation? Wavelengths and energy distribution? • What is absorbed by the earth? • What is the angle at which maximum radiation is received at various points on earth? • Solar insolation =>Direct versus diffused solar radiation • Concentrating solar power Plank’s Law • Plank’s law – wavelengths emitted by a blackbody depend on temperature 3.74 108 E   14400   5   exp    1  T    (7.1) • λ = wavelength (μm) • Eλ = emissive power per unit area of black body • (W/m2-μm) T = absolute temperature (K) Extraterrestrial Solar Spectrum Masters, Figure 7.2 Air Mass Ratio Masters Figure 7.3 h2 1 air mass ratio m  = h1 sin  As sunlight passes through the atmosphere, less energy arrives at the earth’s surface • Air mass ratio of 1 (“AM1”) means sun is directly overhead • AM0 means no atmosphere • AM1.5 is assumed average at the earth’s surface (7.4) Solar Intensity: Atmospheric Effects Sun photosphere Intensity Extraterestrial sunlight (AM0) Sunlight at sea level at 40° N Lattitude at noon (AM1.5) “AM” means “air mass” Solar Noon and Collector Tilt • Solar noon – sun is directly over the local line of longitude • Rule of thumb for the Northern Hemisphere - a south facing collector tilted at an angle equal to the local latitude • During solar noon, the sun’s rays are perpendicular to the collector face Masters, Figure 7.8 The Earth’s Orbit Masters Figure 7.5 For solar energy applications, we’ll consider the characteristics of the earth’s orbit to be unchanging Solar Declination • Solar declination δ – the angle formed between the plane of the equator and the line from the center of the sun to the center of the earth • δ varies between +/- 23.45˚ • Assuming a sinusoidal relationship, a 365 day year, and n=81 is the spring equinox, the approximation of δ for any day n can be found from  360    23.45sin  n  81   (7.6)  365  Altitude Angle βN at Solar Noon • Altitude angle at solar noon βN – angle between the sun and the local horizon  N  90  L   (7.7) • Zenith – perpendicular axis at a site Masters, Figure 7.9 Quantifying available sun energy • Direct beam radiation IBC – passes in a straight line through the atmosphere to the receiver • Diffuse radiation IDC – scattered by molecules in the atmosphere • Reflected radiation IRC – bounced off a surface near the reflector Figure 7.18 Extraterrestrial Solar Insolation I0 • Starting point for clear sky radiation calculations • I0 passes perpendicularly through an imaginary surface outside of Figure 7.19 the earth’s atmosphere In one year, less than half of I0 reaches • Ignoring sunspots, I0 can be earth’s surface as a written as direct beam   360n   2 I 0  SC  1  0.034cos  (W/m ) (7.20)   365    On a sunny, clear • SC = solar constant = 1.377 day, beam radiation kW/m2 may exceed 70% of • n = day number I 0 Attenuation of Incoming Radiation • Can treat attenuation as an exponential decay function I B  Ae km • IB = beam portion of the radiation that reaches the earth’s surface (7.21) • A = apparent extraterrestrial • flux • k = optical depth m = air mass ratio from (7.4)   360n   I 0  SC  1  0.034cos    365    (W/m2 ) (7.20) The realistic representation of I0 is better represented by another mathematical form A Solar Insolation on a Collecting Surface • Direct-beam radiation is just a function of the angle between the sun and the collecting surface (i.e., the incident angle q: I BC  I B cos q Solar thermal energy In this lecture on solar thermal systems • Fundamental working knowledge of the scientific and engineering principles of solar collectors and solar systems. • Conventional equipment such as heat exchangers, pumps, and piping layout are important but not the focus • Because of time limitations, certain equations/correlations had to be omitted What solar thermal energy can do? What solar thermal energy can do? • Produce electricity. • Domestic water heating-typical temperature with flat plate 100 C. • Heating air in winter-simple set up Electricity: Solar Central Receiver – Solar Tower • 10 MW • Two-tank, molten-salt thermal storage system • Barstow, CA • Demolished in November 2009 • Solar Tres is now being built in Spain – will be 15 MW Source: http://www.trec-uk.org.uk/csp.htm Electricity:Parabolic Troughs Source: http://www.flagsol.com/SEGS_tech.htm • Mojave Desert, California • Aerial view of the five 30MW parabolic trough plants • Solar Electric Generation System (SEGS) Source: http://www.flagsol.com/SEGS_tech.htm • Largest solar energy facility in http://en.wikipedia.org/wiki/Solar_ Energy_Generating_Systems the world – 354 MW Water-air heating for household use Typical lay out of solar thermal system In the dark room Solar collectors A solar thermal collector is a heat exchanger that converts radiant solar energy into heat. In essence this consists of a receiver that absorbs the solar radiation and then transfers the thermal energy to a working fluid. • Solar collectors categories: • Based on working fluids (a) water, (b) oil, and (c) air Common fluid: water with glycol and air • Based on tracking type (a) tracking (b) non tracking Design for liquid and air system What are the advantage and disadvantages of using liquid and air based system? Liquid vs. air solar thermal system What are the components of a solar thermal panel for water heating? The anatomy Can there be other optimal configurations to yield better energy extraction for the solar energy? Liquid collectors Air collectors Losses How many types of losses occurs with a solar thermal panel? Energy losses Losses • ~4 % reflected or absorbed by the glass • ~75% heat radiated back by the absorber layer. 98% sun radiation is with
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Explanation & Answer

Attached.

Possible Energy Losses from a Flat Plate
Solar Thermal Water Heater
Student’s Name
Institution
Date

Possible Heat Losses
Heat may be lost through:
❖Reflection by the Glass
❖Absorption By Glass
❖Heat Loss through Conduction
❖Heat Loss through Convection

Reflection by the Glass
❖Sun rays are reflected in the atmosphere
❖The a...


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