Solar Thermal Energy

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

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Outline

Department of Aeronautics and Astronautics National Cheng Kung University

Prof. Keh-Chin Chang

Solar Thermal Energy

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

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Source of Solar Energy

Consist of H, He, O, C, Ne, Fe… Surface temperature: 5,800K Core temperature:13,600,000K

A sphere of intensely hot gaseous matter

The Sun

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…

…

…

…

Source of Solar Energy

equinox

equinox

solstice

Elliptic Orbit

Source of Solar Energy

Between the Sun and the Earth

solstice

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

Source of Solar Energy

Between the Sun and the Earth

Zenith angle of the sun: Defined as the angle measured from vertical downward.

Azimuth angle of the sun: Often defined as the angle from due north in a clockwise direction. (sometimes from south)

Source of Solar Energy

Position of the Sun (view from Earth)

Apparent placement of the Sun in the northern hemisphere

Source of Solar Energy

Position of the Sun (view from Earth)

Source of Solar Energy

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

Solar Radiation Spectrum

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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

Latitude Altitude Atmospheric transparency Solar zenith angle

… … … …

Source of Solar Energy

Factors affect the Solar intensity

Source of Solar Energy

Solar Radiation Budget (to Earth)

Reserves of energy on Earth Solar energy distribution Advantages of using solar energy Types of applications

67

160755 Gm3 1.57 Mton

Gas Uranium

42

210

152 Gton

Oil

43

660.8 Gton

Available Period (year) Coal

Remaining Reserves

Applications of Solar Energy

Reserves of Energy on Earth

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Applications 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.

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. †

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.

Inexhaustible …

†

No pollution …

Application of Solar Energy

Advantages of using Solar Energy

Annual global mean downward solar radiation distribution at the surface

Applications of Solar Energy

Solar Energy Distribution

Energy production prediction

…

…

Solar cell

†

Solar water heater † Solar thermal power † Solar cooling † Solar thermal ventilation

Solar thermal energy

†

Photovoltaic (PV)

Types of Applications

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Application of Solar Energy

Application of Solar Energy

Advantages of using Solar Energy

Heat Transfer in a Solar Collector Heat Transfer Modes Conduction Convection Radiation

qcond,panel

absorbing film

qsun

qemit

Insulator

qcond,insulator

m

qconv,mediu

PanelȐmetalȑ

qconv,air

Mediumflow

Heat Transfer Processes in a Solar Collector

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Introduction to Heat Transfer

‫ݍ‬௖௢௡௩ǡ௔௜௥ : heat loss due to wind ‫ݍ‬௖௢௡௩ǡ௠௘ௗ௜௨௠ : heat transfer to the flow medium throughout tube wall

¾

¾

‫ݍ‬௖௢௡ௗǡ௣௔௡௘௟ : heat transfer inside the metal panel ‫ݍ‬௖௢௡ௗǡ௜௡௦௨௟௔௧௢௥ : heat loss to the insulator from the panel

Conduction

¾

¾

Convection

‫ݍ‬௦௨௡ : solar irradiation ‫ݍ‬௘௠௜௧ : emitted radiant energy from the panel

thermal conductivity

Fourier’s Law: ‫ݍ‬റ̶௖௢௡ௗ ൌ െ݇ߘܶ

heat flux gradient

area

‫ݍ‬௖௢௡ௗ ൌ ‫ݍ‬റ̶௖௢௡ௗ ή ‫ܣ‬റ

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.

Conduction

e

d

¾

¾

Three heat transfer modes in a solar collector: c Radiation

Heat transfer modes

௖௢௡௩



: convective heat transfer coefficient

௖௢௡௩

Newton’s cooling/heating law:

Convection



Knowledge of convective heat transfer needs to know both fluid mechanics and heat transfer

Definition: Heat transfer between a fluid in motion and a boundary surface

Convection

emissivity

‫̶ݍ‬௥௔ௗ ൌ ߝߪܶ ସ ǡ Ͳ

൏ߝ൑ͳ

Stefan-Boltzmann constant

or

reflectivity

Gഐ Gഀ Gഓ ͳൌ ൅ ൅ ൌ G G G

G = Gఘ ൅ Gఈ ൅ Gఛ

Reflection (Gఘ )

Absorption (Gఈ )

Irradiation (G)

absorptivity

transmitivity

Transmission (Gఛ )

Emission (E=ߝߪܶ ସ )

Example: Glass (transparent material)

For real case:

T: absolute temperature

Stefan-Boltzmann Law: for a blackbody (ideal case) ‫ݍ‬௥௔ௗ ൌ ‫̶ݍ‬௥௔ௗ ൈ ‫ ܣ‬ൌ ሺߪܶ ସ ሻ‫ܣ‬

Definition: Energy is emitted by matter via electromagnetic waves with the wavelengths ranging between the long-wave fringe ultraviolet (UV, 10-1m) and far infrared (IR, 103m).

(Thermal) Radiation

c)

b)

Ͳ൑ߠ൑

ߨ ʹ

Ͳ ൑ ߶ ൏ ʹߨ

blackbody



ଶగ



௖௢௦ ఏ ௦௜௡ ఏௗఏௗథ



ߝ ܶ ൌ

‫׬‬଴ ‫ܧ‬ఒǡ௕ ߣǡ ܶ ݀ߣ



‫׬‬଴ ߝఒ ߣǡ ܶ ‫ܧ‬ఒǡ௕ ߣǡ ܶ ݀ߣ



మഏ

‫׬‬బ



ாഊǡ್ ሺఒǡ்ሻ

ஶ ͳ න ߝ ሺߣǡ ܶሻ‫ܧ‬ఒǡ௕ ߣǡ ܶ ݀ߣ ൌ ߪܶ ସ ଴ ఒ

=ߨ‫ܫ‬ఒǡ௕ (T)

‫׬‬బమ ఌഊǡഇ ூഊǡ್ ௖௢௦ ఏ ௦௜௡ ఏௗఏௗథ

ൌ ‫׬‬଴ ‫׬‬଴మ ߝఒǡఏ ሺߣǡ ߠǡ ߶ǡ ܶሻ ܿ‫߶݀ߠ݀ ߠ ݊݅ݏ ߠ ݏ݋‬ గ



ഏ మഏ మ ‫׬‬బ ‫׬‬బ ூഊǡ್

‫׬‬బమ ூഊǡ೐ ௖௢௦ ఏ ௦௜௡ ఏௗఏௗథ

Total , hemispherical emissivity

ߝఒ ߣǡ ܶ ൌ

‫׬‬బ

మഏ

Monochromatic, hemispherical emissivity

Emissivity

ூഊǡ್ ሺఒǡ்ሻ

ூഊǡ೐ ሺఒǡఏǡథǡ்ሻ

Spherical coordinate

intensity

ߝఒǡఏ ߣǡ ߠǡ ߶ǡ ܶ ൌ

emitted

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

Emissivity

௣ ௣

௘௠௜௧



Looking for high ࢻ࢖ while small ࢿ࢖

௣ ௣ ௦௨௡

௦௨௡

‫ݍ‬௘௠௜௧



‫ݍ‬௦௨௡

Total, hemispherical absorptivity,

c)

For a solar panel (opaque material, ఒ ఒ ,

Monochromatic, hemispherical absorptivity,

b)

‫ܫ‬௦௨௡

)



Monochromatic, directional absorptivity, ߙఒǡఏ ሺߣǡ ߠǡ ߶ሻ

a)

Definition: A function of the radiant energy incident on a body that is absorbed by the body

Absorptivity

…ሻ

„ሻ

ƒሻ

, ఒ௕ , ఒ௕ ఒ௕ curve moves to higher decreased

ఒ௕



௠మ ήఓ௠

sub-range as T is

ଶగ஼భ ఒఱ ୣ୶୮ ஼మ Τఒ் ିଵ

Plank’s spectral distribution of emissive power

Plank’s Spectral Distribution

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Plank’s Spectral Distribution

Source of Solar Energy

0.1

ߣሺߤ݉ሻ

visible light : 0.4-0.7m

ߙఒ ൐ ͲǤͻ

3

ߙఒ ൏ ͲǤ1





As Kirchhoff’s law for a diffuse (i.e., independent of direction) surface

0

1.0

A desired property for a good solar absorptance

Solar Radiation Spectrum

What is photovoltaic Solar cell

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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.

What is Photovoltaic

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Introduction to Photovoltaic

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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

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How to use solar thermal energy Types of solar collectors Solar water heater Solar thermal power Solar thermal cooling

Solar Thermal Energy Systems

Solar Cell

Solar collector

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@ Collectors and working temperature

High temperature

Medium temperature

Low temperature

Solar Thermal Energy

working fluid

thermal energy

Solar Thermal Energy

Types of Solar Collectors

Solar Radiation

Working fluid

Solar Thermal Energy

How to Use Solar Thermal Energy

…

Solar Thermal Energy

Weiss, Werner, and Matthias Rommel. Process Heat Collectors. Vol. 33, 2008.

Main losses of a basic flat-plate collector during angular operation

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

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Flat-plate collector

Glazed collector …

Flat-plate collector

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Flat-plate collector

Solar Thermal Energy

Unglazed collector

Solar Thermal Energy

…

Solar Thermal Energy

Heat pipe

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Sydney tube

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

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Evacuated tube 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.

…

…

Solar Thermal Energy

http://polarsolar.com/blog/?p=171

Parabolic trough collector

Collector efficiency

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.

…

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Solar Thermal Energy

Simple design of flexibly bent mirrors and fixed receivers requires lower investment costs and facilitates direct steam generation. …

Parabolic dish 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 downwardfacing linear, fixed receiver.

Solar Thermal Energy

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Linear Fresnel reflector

…

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(Mainly using flat plate collector or evacuate tube collector)

Most popular and well developed application of solar thermal energy so far Low temperature applications

Solar Thermal Energy

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

A heliostat is a device that includes a plane mirror which turns so as to keep reflecting sunlight toward a predetermined target.

Solar Thermal Energy

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Heliostat field collector

(Thermosyphon) User

…

…

†

The angle of the collector is roughly equal to the local latitude

Installation tilt angle

†

Solar Thermal Energy

Heat exchanger

User

User

Solar Thermal Energy

Indirect (close loop)

For northern hemisphere  Facing south † For southern hemisphere  Facing north

Installation direction

Solar Water Heater

Active

Passive

User

Direct (open loop)

Solar Water Heater

Increasing collection area

Direction shifted from south (angle)

Increasing collection area

…

…

Solar Thermal Energy

†

Dormitory hot water † Swimming pool † Industrial process heating

Large-scale system

†

Hot water production † House warming

Residential hot water system

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

Solar Thermal Energy

Tilt angle of the collector

L=local latitude

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

Solar Water Heater

…

Solar Water Heater

Annual heat collection(%)

About 50% of the industrial heat demand is located at temperatures up to 250ƒC.

†

…

Market potential of industrial process heating

Solar Thermal Energy

In EU, 2/3 of the industrial energy demand consists of heat rather than electrical energy.

Solar Thermal Energy

†

Industrial process heating

Solar Water Heater

…

Solar Water Heater

…

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 Energy

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

High temperature applications

†

Direct means : photovoltaics (PV), † Indirect means : concentrated solar power (CSP).

Conversion of sunlight into electricity

Solar Thermal Power

…

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Solar Thermal Power

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Solar Thermal Energy

Combination of storage and hybridisation in a solar thermal plant

Solar Thermal Energy

Technology roadmap concentrating solar power, IEA, 2010.

Types of solar thermal power plant

Solar Thermal Power

…

Solar Thermal Power

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

Solar Thermal Power

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

Solar Thermal Power

Solar Thermal Energy

Solar Thermal Energy

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

Solar Thermal Energy

Solar Thermal Energy

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

Solar Thermal Power

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

Solar Thermal Power

†

Solar thermal ventilation

International Journal of Refrigeration 3I(2008) 3-15

Solar Thermal Energy

Use solar thermal collectors to provide thermal energy for Solar thermal cooling driving a thermally driven chiller

†

Passive cooling

Use PV panel to generate electricity for driving a conventional air conditioner

Solar Thermal Energy

†

Active cooling

Solar Thermal Cooling

…

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Solar (Thermal) Cooling

†

Solar Thermal Energy

Chiller

Cooling distribution

Cooling tower

Use solar thermal collectors to provide thermal energy for driving thermally driven chillers.

Active cooling

Heat source

…

Solar Thermal Cooling

2

Solar Thermal Energy

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.

…

Solar Thermal Cooling

Absorption coolingƐLiBr+H2O Adsorption coolingƐsilica gel+H2O DEC, Desiccant Evaporative Cooling

† † †

Basic type of solar thermal chiller

:H

evaporator

4&

expansion valve

condenser

COPelect=QC/We

low pressure vapor

compressor

high pressure vapor

4/

Conventional compression cooling

4D

Solar Thermal Energy

Open cycle

Closed cycle

Solar Thermal Energy

4/

evaporator

4&

expansion valve

condenser

COPthermal=QC/Qg COPelect=QC/We

low pressure vapor

absorption

(switch)

:H

high pressure vapor

desorption

4J

Adsorption/absorption cooling

Solar Thermal Cooling

…

Solar Thermal Cooling

Solar Thermal Energy

Solar Thermal Energy

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

Solar Thermal Cooling

Henning, H. “Solar assisted air conditioning of buildings – an overview.” Applied Thermal Engineering 27, no. 10 (July 2007): 1734-1749.

COPthermal of different type of chiller

Solar Thermal Cooling

Solar Thermal Energy

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

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

…

Solar Thermal Energy

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.

†

†

Passive Cooling (solar ventilation, solar chimney)

Solar Thermal Cooling

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

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 Energy

HDH is based on evaporation of brackish water and consecutive condensation of the generated humid air, mostly at ambient pressure.

†

Solar humidification-dehumidification (HDH)

Solar Thermal Applications

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Solar desalination/distillation

Conventional installation way in Taiwan

Facade integration (roof)

Damage due to typhoon invasion

Conventional installation way in Taiwan

Roof integrated flat-plate collectors on house in Denmark (Source: VELUX)

Damage due to typhoon invasion

Summary, Executive, Werner Weiss, and Peter Biermayr. Potential of Solar Thermal in Europe - Executive Summary, 2009.

Reduction of -40%

Solar Thermal Energy

Contribution of solar thermal to EU heat demand by sector

Facade integration (balcony)

Geographical aspects Financial aspects

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Restrictions in Using Solar Energy

Solar radiation has a low energy density relative to other common energy sources

†

Solar Energy supply is restricted by time and geographical location † Easily influenced by weather condition

Unstable energy supply

†

Low energy density

Geographical Aspects

…

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Restrictions in Using Solar Energy

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Restrictions in Using Solar Energy

The capital cost in utilization of solar energy is generally higher than that of traditional ones, especially for PV.

†

†

Examples

Most economically competitive technology by now The need of SWH is inversely proportional to local insolation

Solar water heater

†

Higher cost compared with traditional energy

Financial Aspects

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 ʚ, while the annual average temperature of cold water is 23 ʚ. 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?

T

temperature difference between hot and cold water

Q Heat Demand M Hot Water Quantity C p specific heat capacity of water

M u C p u 'T

§ · l kcal Q ¨¨ 60 u 5 person ¸¸ u1 u 50qC  23qC © person u day ¹ kg u qC § · kg kcal ¨¨ 60 u 5 person ¸¸ u1 u 50qC  23qC person u day kg u q C © ¹ kcal 8100 day

Q

Answer 1

(Note: water specific heat Cp is assumed to be 1 kcal/kg-к, water density is 1 kg / l. )

…

Example 1

(Note: 1cal = 4.186J = 4.186 W × s).

Qc

Qc

Heat provided from collector

4

kWh u 1m2 u 0.5 2 m ˜ day kJ ˜ 3600 s kWh s 2 2 day day kcal 1720 day

7200

H Daily accumulative insolation A Effective collector area  Efficiency of solar water heater

Qc

H u A uK

kJ day

1 kcal 4.186 7200 day

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?

Answer 2

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Example 2

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?

1720 kcal

m 2 ˜ day

4.764m 2

Effective collector area

A

day

Heat provided from collector per m 2 Qc

8100 kcal

Demand Heat

Q

4.764m 2 | 5.955 Ÿ 6 pieces 0.8m 2

A

A

Q Qc

Answer 3

…

…

Example 3

420

(Note: ly = Langley = cal/cm2).

420

kcal u 2 m2 m ˜ day

1 1000 2 1 10000

cal u 2 m2 2 cm ˜ day

4.186W ˜ s (2) 420 1 2 u 2 m2 10000 m ˜ day

(1) 420

ly u 2 m2 day

420

1 kW ˜ 3600 hr u 2 m2 2 m ˜ day

kcal day 4.186 1000 1 10000

4200

9.767

kWh day

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)

Answer 4

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Example 4

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