20131205 Introduction of Solar Energy

Introduction to Solar Thermal Energy

Prof. Keh-Chin Chang

Department of Aeronautics and Astronautics National Cheng Kung University

Outline Source of Solar Energy

Applications of Solar Energy

Introduction to Photovoltaic

Solar Thermal Energy Systems

Restrictions in Using Solar Energy

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

The Sun

A sphere of intensely hot gaseous matter Consist of H, He, O, C, Ne, Fe… Surface temperature: 5,800K Core temperature:13,600,000K

Source of Solar Energy

solstice solstice

equinox

equinox


Between the Sun and the Earth

Average distance:149.5 million km (1 astronomical unit, AU)

Elliptic Orbit

http://upload.wikimedia.org/wikipedia/commons/f/f0/Seasons1.svg


Between the Sun and the Earth


Position of the Sun (view from Earth)

Apparent placement of the Sun in the northern hemisphere

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.


Position of the Sun (view from Earth)

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

Solar Constant Entry point into atmosphere Intensity ~ 1350W/m2


Solar Radiation Budget (to Earth)


Solar Radiation Spectrum

Infrared

資料來源：http://en.wikipedia.org/wiki/Infrared

ISO分類系統

天文學分類方案

Thermal Radiation Spectra

Solar Energy Distribution Applications of Solar Energy

Annual global mean downward solar radiation distribution at the surface

太陽輻射光譜

台灣(Taiwan)的年平均日射量(yearly average insolation flux)為

11746kJ/m2day(21°~25°N) 日本(Japan)的年平均日射量為17608kJ/m2day(31°~43°N) 德國(Germany)的年平均日射量為21013kJ/m2day(34°~52°N) ●利用外國的日射量數據作為國內的依據，由於國內外雲量、溼度等氣候現象條件迥異，使得這些替代性的日射量數據與本地的實際情況發生誤差，更導致誤用此數據的研究結果產生偏差或誤解(林憲德，1986) ●台灣的日射量明顯低於同緯度地區(張鏡湖，1986) ●台灣只是氣溫高而非日射量高的地區(林憲德，1994) ●長江中下游及台灣地區，明顯是個日射量較低的地區(NASA，2005) ●台灣年平均日射量大概在3.0~4.5kWh/m2day之間，明顯低於同緯度地區的結果(NREL，2006)

Latitude

Altitude

Atmospheric transparency

Solar zenith angle


Factors affect the Solar intensity

Applications of Solar Energy Advantages of using solar energy Types of applications

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.

Application of Solar Energy

Advantages of using Solar Energy

Weiss, Werner, I. Bergmann, and G. Faninger. Solar Heat Worldwide–Markets and Contribution to the Energy Supply 2008. International Energy Agency, 2010.

Energy production prediction Application of Solar Energy

Advantages of using Solar Energy

Photovoltaic (PV) Solar cell

Solar thermal energy

Solar water heater Solar thermal power Solar cooling Solar thermal ventilation

Application of Solar Energy

Types of Applications

Introduction to Photovoltaic What is photovoltaic Solar cell

What is 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.

Photovoltaic

Solar Cell

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.

Photovoltaic

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

thermal energy

Solar Radiation Solar Thermal Energy Solar collector

Working fluid

qsun

qemit

qcond,panel

qconv,medium

qconv,air

Panel（metal）

absorbing film

Insulator

qcond,insulator

Heat Transfer Processes in a Solar Collector

Medium flow

Heat transfer modes

Three heat transfer modes in a solar collector: Radiation

𝑞𝑞𝑠𝑠𝑠𝑠𝑠𝑠: solar irradiation 𝑞𝑞𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒: emitted radiant energy from the panel

Convection 𝑞𝑞𝑐𝑐𝑐𝑐𝑠𝑠𝑐𝑐,𝑎𝑎𝑒𝑒𝑎𝑎 : heat loss due to wind 𝑞𝑞𝑐𝑐𝑐𝑐𝑠𝑠𝑐𝑐,𝑒𝑒𝑒𝑒𝑚𝑚𝑒𝑒𝑠𝑠𝑒𝑒: heat transfer to the flow medium

throughout tube wall Conduction

𝑞𝑞𝑐𝑐𝑐𝑐𝑠𝑠𝑚𝑚,𝑝𝑝𝑎𝑎𝑠𝑠𝑒𝑒𝑝𝑝 : heat transfer inside the metal panel 𝑞𝑞𝑐𝑐𝑐𝑐𝑠𝑠𝑚𝑚,𝑒𝑒𝑠𝑠𝑠𝑠𝑠𝑠𝑝𝑝𝑎𝑎𝑒𝑒𝑐𝑐𝑎𝑎 : heat loss to the insulator from the panel

(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)

𝑞𝑞𝑎𝑎𝑎𝑎𝑚𝑚 = 𝑞𝑞"𝑎𝑎𝑎𝑎𝑚𝑚 × 𝐴𝐴 = (𝜎𝜎𝑇𝑇4)𝐴𝐴 T: absolute temperature Stefan-Boltzmann constant

For real case: 𝑞𝑞𝑞𝑎𝑎𝑎𝑎𝑚𝑚 = 𝜀𝜀𝜎𝜎𝑇𝑇4 , 0 < 𝜀𝜀 ≤ 1

emissivity

Example: Glass (transparent material)

Reflection (G𝜌𝜌 ) Emission (E=𝜀𝜀𝜎𝜎𝑇𝑇4)

Irradiation (G)

Absorption (G𝛼𝛼)

Transmission (G𝜏𝜏)

G = G𝜌𝜌 + G𝛼𝛼 + G𝜏𝜏

or 1 = G𝜌𝜌

G + G𝛼𝛼G + G𝜏𝜏

G = 𝜌𝜌 + 𝛼𝛼 + 𝜏𝜏 reflectivity absorptivity

transmitivity

Emissivity Defined as the ratio of the radiant energy rate emitting from a blackbody under identical condition a) Monochromatic (or spectral) , directional emissivity

𝜀𝜀𝜆𝜆,𝜃𝜃 𝜆𝜆, 𝜃𝜃,𝜙𝜙,𝑇𝑇 = 𝐼𝐼𝜆𝜆,𝑒𝑒(𝜆𝜆,𝜃𝜃,𝜙𝜙,𝑇𝑇)𝐼𝐼𝜆𝜆,𝑏𝑏(𝜆𝜆,𝑇𝑇)

emitted

intensity blackbody

0 ≤ 𝜙𝜙 < 2𝜋𝜋

0 ≤ 𝜃𝜃 ≤𝜋𝜋2

Spherical coordinate

Emissivity

b) Monochromatic, hemispherical emissivity

𝜀𝜀𝜆𝜆 𝜆𝜆,𝑇𝑇 =∫ ∫ 𝐼𝐼𝜆𝜆,𝑒𝑒 𝑐𝑐𝑐𝑐𝑠𝑠 𝜃𝜃 𝑠𝑠𝑒𝑒𝑠𝑠 𝜃𝜃𝑚𝑚𝜃𝜃𝑚𝑚𝜙𝜙

𝜋𝜋20

2𝜋𝜋0

∫ ∫ 𝐼𝐼𝜆𝜆,𝑏𝑏 𝑐𝑐𝑐𝑐𝑠𝑠 𝜃𝜃 𝑠𝑠𝑒𝑒𝑠𝑠 𝜃𝜃𝑚𝑚𝜃𝜃𝑚𝑚𝜙𝜙𝜋𝜋20

2𝜋𝜋0

=∫ ∫ 𝜀𝜀𝜆𝜆,𝜃𝜃𝐼𝐼𝜆𝜆,𝑏𝑏 𝑐𝑐𝑐𝑐𝑠𝑠 𝜃𝜃 𝑠𝑠𝑒𝑒𝑠𝑠 𝜃𝜃𝑚𝑚𝜃𝜃𝑚𝑚𝜙𝜙

𝜋𝜋20

2𝜋𝜋0

𝐸𝐸𝜆𝜆,𝑏𝑏(𝜆𝜆,𝑇𝑇)

= 1𝜋𝜋 ∫ ∫ 𝜀𝜀𝜆𝜆,𝜃𝜃(𝜆𝜆,𝜃𝜃,𝜙𝜙,𝑇𝑇) 𝑐𝑐𝑐𝑐𝑐𝑐 𝜃𝜃 𝑐𝑐𝑠𝑠𝑠𝑠 𝜃𝜃 𝑑𝑑𝜃𝜃𝑑𝑑𝜙𝜙

𝜋𝜋20

2𝜋𝜋0

c) Total , hemispherical emissivity

𝜀𝜀 𝑇𝑇 =∫ 𝜀𝜀𝜆𝜆 𝜆𝜆,𝑇𝑇 𝐸𝐸𝜆𝜆,𝑏𝑏 𝜆𝜆,𝑇𝑇 𝑑𝑑𝜆𝜆∞0

∫ 𝐸𝐸𝜆𝜆,𝑏𝑏 𝜆𝜆,𝑇𝑇 𝑑𝑑𝜆𝜆∞0

=1𝜎𝜎𝑇𝑇4

� 𝜀𝜀𝜆𝜆(𝜆𝜆,𝑇𝑇)𝐸𝐸𝜆𝜆,𝑏𝑏 𝜆𝜆,𝑇𝑇 𝑑𝑑𝜆𝜆∞

0

= 𝜋𝜋𝐼𝐼𝜆𝜆,𝑏𝑏(T)

Absorptivity

Definition: A function of the radiant energy incident on a body that is absorbed by the body a) Monochromatic, directional absorptivity, 𝛼𝛼𝜆𝜆,𝜃𝜃(𝜆𝜆,𝜃𝜃,𝜙𝜙)

b) Monochromatic, hemispherical absorptivity, 𝛼𝛼𝜆𝜆(𝜆𝜆)

c) Total, hemispherical absorptivity, 𝛼𝛼

For a solar panel (opaque material, 𝜏𝜏𝜆𝜆 = 𝜏𝜏 = 0) ⟹ 1 = 𝛼𝛼𝜆𝜆 + 𝜌𝜌𝜆𝜆 , 1 = 𝛼𝛼 + 𝜌𝜌 𝑞𝑞𝑠𝑠𝑠𝑠𝑠𝑠 = 𝐴𝐴𝑝𝑝𝛼𝛼𝑝𝑝𝐼𝐼𝑠𝑠𝑠𝑠𝑠𝑠

𝑞𝑞𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 = 𝐴𝐴𝑝𝑝𝜀𝜀𝑝𝑝𝜎𝜎𝑇𝑇 4 𝑞𝑞𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 𝑞𝑞𝑠𝑠𝑠𝑠𝑠𝑠

𝐼𝐼𝑠𝑠𝑠𝑠𝑠𝑠

Looking for high 𝜶𝜶𝒑𝒑 while small 𝜺𝜺𝒑𝒑

A desired property for a good solar absorptance B As Kirchhoff’s law for a diffuse (i.e., independent of direction) surface

𝜀𝜀𝜆𝜆 = 𝛼𝛼𝜆𝜆

3

0

0.1 𝜆𝜆(𝜇𝜇𝑚𝑚)

1.0 𝛼𝛼𝜆𝜆 > 0.9

𝛼𝛼𝜆𝜆 < 0.1

visible light : 0.4-0.7μm

Types of Solar Collectors

Collectors and working temperature Solar Thermal Energy

Low temperature

Medium temperature

High temperature

]

Flat-plate collector

Use both beam and diffuse solar radiation, do not require tracking of the sun, and are low-maintenance, inexpensive and mechanically simple. A

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.



Glazed collector

Unglazed collector Solar Thermal Energy

Flat-plate collector Solar Thermal Energy

Evacuated tube collector

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

Heat pipe Sydney tube Solar Thermal Energy

Parabolic trough collector

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

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.


Parabolic dish reflector

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

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

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

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

Annual heat collection vs. direction/tilt angle (in north hemisphere)


Direction shifted from south (angle)

Annual heat collection(%)

Incr

easi

ng c

olle

ctio

n ar

ea

Tilt angle of the collector

Annual heat collection(%) In

crea

sing

col

lect

ion

area

L=local latitude

“Solar Thermal Action Plan for Europe”, ESTIF, 2007

Solar Water Heater

Residential hot water system Hot water production House warming

Large-scale system Dormitory hot water Swimming pool Industrial process heating


Solar Water Heater

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

Market potential of industrial process heating Solar Thermal Energy

Solar Thermal Power

Conversion of sunlight into electricity Direct means : photovoltaics (PV), Indirect means : concentrated solar power (CSP).

High temperature applications (by means of sun-tracking, concentrated solar collectors)


Solar thermal power

Solar Thermal Power

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.


http://climateprogress.org/wp-content/uploads/2009/04/albiasa.gif

Solar Thermal Power

Types of solar thermal power plant

Technology roadmap concentrating solar power, IEA, 2010.


Solar Thermal Power Solar Thermal Energy

PS10 and PS20 solar power tower (HFC) (Seville, Spain). 2007 and 2009


Kimberlina solar thermal energy plant (LFR) (Bakersfield, CA), 2008.

http://upload.wikimedia.org/wikipedia/en/7/7f/Fresnel_reflectors_ausra.jpg


Calasparra solar power plant (LFR) (Murcia, Spain) 2009.


Solar Thermal Power

Andasol solar power station (PTC) (Granada, Spain), 2009

Puertollano solar power station (PTC) (Ciudad real, Spain), 2009

Solar (Thermal) Cooling

Active cooling Use PV panel to generate electricity for driving a

conventional air conditioner Use solar thermal collectors to provide thermal energy for

driving a thermally driven chiller

Passive cooling Solar thermal ventilation


Solar thermal cooling

Solar Thermal Cooling

Solar cooling benefits from a better time match between supply and demand of cooling load

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.

2



Active cooling Use solar thermal collectors to provide thermal energy for

driving thermally driven chillers.


Heat source

Cooling distribution

Cooling tower

Chiller

http://www.yazaki-airconditioning.com/en/applications/solar_thermal.html


Basic type of solar thermal chiller Absorption cooling－LiBr+H2O

Adsorption cooling－silica gel+H2O

DEC, Desiccant Evaporative Cooling


Open cycle

Closed cycle


COPthermal=QC/Qg

COPelect=QC/We

Conventional compression cooling Adsorption/absorption cooling

high pressure vapor

expansion valve

We

QC

QL

condenser

compressor

evaporator low pressure vapor

Qa

We

QL

desorption

absorption

(switch)

Qg

expansion valve

condenser

evaporator QC

high pressure vapor

low pressure vapor


COPelect=QC/We

"Solar Assisted Cooling – State of the Art –,“ESTIF, 2006.

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.



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


Geographical aspects Financial aspects

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


Geographical Aspects

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


Financial Aspects

20131205 Introduction of Solar Energy

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