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					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
   Radiation
Heat Transfer Processes in a Solar Collector

                          qconv,air
                  qemit
           qsun

 absorbing film                         qconv,mediu      Medium flow
                                        m


                                       qcond,insulator
                                      Insulator
   qcond,panel            Panel(metal)
Heat transfer modes

Conduction

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

(Thermal) Radiation

Example: Glass (transparent material)

                              Irradiation (G)




                                transmitivity


        reflectivity   absorptivity
Emissivity




                                 emitted




             intensity
                                    blackbody




                  Spherical coordinate
Emissivity

Absorptivity







    1.0




            visible light : 0.4-0.7μm



      0

          0.1                           3
Source of Solar Energy
   The Sun
   Between the Sun and the Earth
   Position of the Sun
   Solar constant
   Solar radiation and intensity
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




                                                          Elliptic Orbit
                                     equinox
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
Solar Radiation Spectrum
                           Source of Solar Energy
Solar Radiation Budget (to Earth)
                               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
   Advantages of using solar energy
   Types of applications
Reserves of Energy on Earth
                                          Applications of Solar Energy




          Remaining    Available Period
          Reserves          (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
Advantages of using Solar Energy
                                                                                      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.
Advantages of using Solar Energy
                                   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 Radiation                       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 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
                                         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
                 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
                                        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
                   receiver placed atop a fixed
                   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

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




                                                                                                                                                                    Annual heat collection(%)
                                                                        Annual heat collection(%)


                                                                                                    Increasing collection area
Increasing collection area




                                                                                                                                 L=local latitude


                                 Direction shifted from south (angle)



                                                                                                                                 Tilt angle of the collector
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)
                                    (Ciudad real, Spain), 2009




Andasol solar power station (PTC)
(Granada, Spain), 2009
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
        driving a thermally driven chiller          Solar thermal cooling




   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
                                             Closed cycle
       Adsorption cooling-silica gel+H2O
       DEC, Desiccant Evaporative Cooling   Open cycle
 Solar Thermal Cooling
                                                                                Solar Thermal Energy


 Conventional compression cooling                   Adsorption/absorption cooling

                                 QL                                                    QL
                                                    Qg
      high pressure vapor                                 high pressure vapor
                            condenser                                            condenser

We                                                    desorption
     compressor                    expansion                            We                expansion
                                   valve                    (switch)                      valve

                                                      absorption

                            evaporator         Qa
                                                                                 evaporator
                                                                                              QC
       low pressure vapor
                                         QC               low pressure vapor



             COPelect=QC/We                                  COPthermal=QC/Qg
                                                             COPelect=QC/We
         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
Facade integration (roof)
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)
Facade integration (balcony)
        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. )
Answer 1
 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


                               
                                 kg  C  50C  23C 
             l                       kcal
  Q   60
       person  day  5 person  1
                               
                               
                                 kg  C  50C  23C 
              kg                     kcal
      60
       person  day  5 person  1
                               
           kcal
     8100
           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).
Answer 2
 Qc  H  A 
      Qc  Heat provided from collector
      H  Daily accumulative i
                             nsolation
      A  Effectivecollector area
      η  Efficiencyof solar water heater

         kWh
  Qc  4          1m2  0.5
        m2  day
                  kJ                              1
                      3600 s                         kcal
        kWh                          kJ
    2        2 s             7200      7200 4.186
        day          day             day           day
            kcal
     1720
            day
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
                   Q  Demand Heat
     Q
  A               Qc  Heat provided from c
                                           ollector per m2
     Qc            A  Effectivecollector area


        8100 kcal
                     day
  A                            4.764m 2
       1720 kcal
                   m 2  day


  4.764 m 2
        2
             5.955  6 pieces
   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).
      Answer 4

       ly               cal
420        2 m2  420 2        2 m2
      day             cm  day

                              1
                               kcal                 kcal
             (1)  420      1000
                                       2 m2  4200
                           1
                         10000m  day
                                 2
                                                    day


                       4.186W  s              1000 kW  3600 hr
                                               4.186       1
                                                                               kWh
             (2)  420 1 2           2 m  420 1 2
                                         2
                                                                  2 m  9.767
                                                                      2

                      10000 m  day              10000 m  day                 day

				
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