Solar Thermal Energy

SOLAR THERMAL ENERGY

Prof. Keh-Chin Chang

Department of Aeronautics and AstronauticsNational 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 CollectorHeat Transfer ModesConductionConvectionRadiation

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 modesThree heat transfer modes in a solar collector:① Radiation

: solar irradiation : emitted radiant energy from the panel

② Convection : heat loss due to wind : heat transfer to the flow medium

throughout tube wall③ Conduction

: heat transfer inside the metal panel : heat loss to the insulator from the panel

ConductionDefinition: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

ConvectionDefinition: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

ConvectionNewton’s cooling/heating law:

: convective heat transfer coefficient

(Thermal) RadiationDefinition: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 temperatureStefan-Boltzmann constant

For real case:

emissivity

Example: Glass (transparent material)

Reflection ( )Emission (E=)

Irradiation (G)

Absorption ()

Transmission ()

G = or

reflectivity absorptivity

transmitivity

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

emissivity emitted

intensity blackbody

𝜃

𝜙0≤ 𝜙<2𝜋0 ≤ 𝜃≤ 𝜋

2Spherical coordinate

Emissivityb) Monochromatic, hemispherical

emissivity

c) Total , hemispherical emissivity

=(T)

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

a) Monochromatic, directional absorptivity,

b) Monochromatic, hemispherical absorptivity,

c) Total, hemispherical absorptivity,

For a solar panel (opaque material, ) ,

𝑞𝑒𝑚𝑖𝑡 𝑞𝑠𝑢𝑛

𝐼 𝑠𝑢𝑛

Looking for high while small

A desired property for a good solar absorptance

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

3

0

0.1

𝜆 (𝜇𝑚)

1.0 𝛼𝜆>0.9

1

visible light : 0.4-0.7μm

Source of Solar EnergyThe SunBetween the Sun and the EarthPosition of the SunSolar constantSolar 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

solsticesolstice

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 ConstantEntry point into atmosphereIntensity ~ 1350W/m2


Solar Radiation Spectrum


Solar Radiation Budget (to Earth)

Latitude

Altitude

Atmospheric transparency

Solar zenith angle


Factors affect the Solar intensity

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

Reserves of Energy on EarthApplications 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 DistributionApplications of Solar Energy

Annual global mean downward solar radiation distribution at the surface

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 predictionApplication 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 EnergySolar Thermal Energy

Solar Radiation

working fluid

thermal energy

Solar RadiationSolar Radiation

Solar Radiation Solar Thermal EnergySolar collector

Working fluid

Types of Solar Collectors

Collectors and working temperatureSolar 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.

Solar Thermal Energy


Glazed collector Unglazed collectorSolar Thermal Energy

Flat-plate collectorSolar 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 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 tubeSolar Thermal Energy

Collector efficiency

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


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 HeaterSolar Thermal Energy

Direct (open loop) Indirect (close loop)

Passive

Active

User

(Thermosyphon)

User User

Heat exchanger

User

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(%

)

Incr

easi

ng c

olle

ctio

n ar

ea

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 heatingSolar 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.



Combination of storage and hybridisation in a solar thermal plant

Solar Thermal Power

Solar Thermal PowerSolar 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

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



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

evaporatorlow pressure vapor

Qa

We

QL

desorption

absorption

(switch)

Qg

expansion

valve

condenser

evaporator QC

high pressure vapor

low pressure vapor


COPelect=QC/We


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

Solar Thermal CoolingSolar Thermal Energy


A. Napolitano, "Review on existing solar assisted heating and cooling installations," 28.04.2010 – Workshop Århus, Denmark ABSORPTION, 2010.



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

Facade integration (roof)

http://science.howstuffworks.com/earth/green-technology/sustainable/home/solar-water-heater.htm/printable

http://www2.macleans.ca/2009/11/11/ten-ways-to-green-your-home/ab380292/

Conventional installation way in Taiwan

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

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

Reduction of -40%


Restrictions in Using Solar 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

Restrictions in Using Solar Energy

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

Restrictions in Using Solar Energy

Financial Aspects

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

TCMQ p

p

Q Heat Demand M Hot Water QuantityC specific heat capacity of water

ΔT temperature difference between hot and cold water

daykcal

CCCkg

kcalpersondayperson

kg

CCCkg

kcalpersondayperson

lQ

8100

23501560

23501560

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

AHQc

224 1 0.5

136004.1862 2 7200 7200

1720

ckWhQ m

m daykJ s kcalkWh kJs

day day day daykcalday

erwater heat of solar Efficiencyηareacollector Effective A

nsolationmulative iDaily accuHollectorded from cHeat proviQc

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

cQQA

areacollector Effective Aer mollector pded from cHeat proviQ

tDemand HeaQ

c

2

2

2

764.41720

8100m

daymkcal

daykcal

A

piecesm

m 6955.58.0

764.42

2

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

2 22

121000

2110000

4.186 12 21000 3600

2 21 110000 10000

420 2 420 2

(1) 420 2 4200

4.186(2) 420 2 420 2 9.767

ly calm mday cm day

kcal kcalmm day day

kW hrW s kWhm mm day m day day

Solar Thermal Energy

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