# Solar (Thermal) Cooling

```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
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:




Convection



: emitted radiant energy from the panel
, : heat loss due to wind
, : heat transfer to the flow medium
throughout tube wall
Conduction


, : heat transfer inside the metal panel
, : 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.
= " ∙
heat flux
Fourier’s Law: " = −
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:
= " ×  = ℎ( − ∞ )
ℎ : convective heat transfer coefficient
ℎ = ℎ(,  )
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)
= " ×  = ( 4 )
T: absolute temperature
Stefan-Boltzmann constant
For real case:
" =  4
emissivity
,0 <  ≤ 1
Example: Glass (transparent material)
Reflection (G )
Emission (E= 4 )
Absorption (G )
Transmission (G )
G = G + G + G
or
G G G
1=
+
+
=++
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
, , , ,  =
intensity
, (,,,)
, (,)
blackbody
0 ≤  < 2

0≤≤
2
Spherical coordinate
Emissivity
b)
Monochromatic, hemispherical emissivity
,  =

2 2

0
0 ,

2 2

0
0 ,
=
c)

2 2

0
0 , ,
=

, (,)

1 2 2
(, , , )
0 ,
0
= , (T)
Total , hemispherical emissivity
=
∞
,  , ,
0
∞
, ,
0

1
=
4
∞
0
(, ), ,
Absorptivity
Definition:
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,  =  = 0)
⟹ 1 =  +  , 1 =  +
=
=

4
Looking for high  while small

A desired property for a good solar absorptance
> 0.9
1.0
visible light : 0.4-0.7μm
< 0.1
0
0.1
()
3
As Kirchhoff’s law for a diffuse (i.e., independent of direction)
surface
=
Source of Solar Energy





The Sun
Between the Sun and the Earth
Position of the Sun
Solar constant
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
Source of Solar Energy
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
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
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.
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 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
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
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
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)
Andasol solar power station (PTC)
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


DEC, Desiccant Evaporative Cooling
Closed cycle
Open cycle
Solar Thermal Cooling
Solar Thermal Energy
Conventional compression 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
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)
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. )
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
 8100
kcal
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).
Qc  H  A 
Qc  Heat provided from collector
H  Daily accum ulative nsolation
i
A  Effective collectorarea
η  Efficiency of solarwater 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?
A
A
Q
Qc
Q  Dem and Hea
t
Qc  Heat provided from collector per m2
A  Effective collectorarea
8100kcal
1720kcal
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).
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
```

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