0004Lec11air by gstec

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									   Solar Air Heating Systems




       Focus of Today’s Lecture
• Solar Air Heating Systems
• SolarwallTM
• Rock bed storage
        Air as a Heat Transfer Medium
Advantages
• Safe - Frost protection and stagnation control are not
  necessary
• Less corrosive than water (longer life)
• Lower temperature level of the feed air compared to wet-
  based space heating


Disadvantages
• Low heat capacity and poor heat conductivity compared
  to liquids therefore larger duct diameters and exchanger
  surfaces are required.




                       Fluid Properties
     Properties of air and water at 25oC and 1 bar
                                             Air       Water
     Density (kg/m3)                        1.185      998.2
     Specific heat capacity, cp             0.28       1.16
     (mass) (Wh/kg·K)
     Specific heat capacity, cp             0.31       1158
     (volume) (Wh/m3·K)
     Thermal conductivity (W/m·K)           0.026      0.559


    Heat transfer from the absorber to the air is
    poorer than for collectors using liquid media
    due to the low thermal conductivity of air.
                    Designs of Solar Air Collectors




          (a) flow over absorber (least efficient)
          (b) flow under absorber (high efficiency)
          (c) flow on both sides of absorber (can
             be efficient if done properly)

From “Planning and Installing Solar Thermal Systems”, James & James/Earthscan, London, UK




                                         Standard Air Collector
      A typical solar air collector has a frame, thermal insulation on the
      back and sides, a transparent glass cover and an absorber. This
      absorber consists of a coated aluminium sheet that is designed
      as a U-profile.




      The ribs act as fins which increase the heat transfer. However,
      this may also increase the pressure drop.
From “Planning and Installing Solar Thermal Systems”, James & James/Earthscan, London, UK
                                              Collector Installation
      In-roof installation




                          In-roof collector installation between the rafters


        Facade integration




                                   Roof-integrated Solarwall column collector
From “Planning and Installing Solar Thermal Systems”, James & James/Earthscan, London, UK




                                       Solar Fresh Air System

                                                                                                      • Simplest sort of solar air
                                                                                                        system
                                                                                                      • Single pass system
                                                                                                      • Solar air system with
                                                                                                        fresh air operation (air
                                                                                                        heating with fresh air)
                                                                                                      • Exhaust air leaves the
                                                                                                        building by leakage or
                                                                                                        through exhaust air flaps
                                                                                                      • Minimal installation
                                                                                                      • This system is used
                                                                                                        mostly for retrofitting into
          From “Planning and Installing Solar Thermal Systems”, James & James/Earthscan, London, UK
                                                                                                        existing buildings
         Solar Supported House Ventilation

                                                                                                      • Single pass system with
                                                                                                        heat recovery unit
                                                                                                      • Heat recovery required
                                                                                                        due to high thermal
                                                                                                        insulation in new building
                                                                                                      • Heat recovery unit
                                                                                                        controls the feed and
                                                                                                        exhaust airflows into
                                                                                                        which the solar air system
                                                                                                        can be integrated
                                                                                                      • Heat recovery unit also
                                                                                                        provides additional
                                                                                                        heating for fresh air
          From “Planning and Installing Solar Thermal Systems”, James & James/Earthscan, London, UK




                              Solar Air Heating and Heat
                                    Recovery Units




From “Planning and Installing Solar Thermal Systems”, James & James/Earthscan, London, UK
             Solar Air Based Thermal Storage




                                                                                            From “Planning and Installing Solar Thermal Systems”, James & James/Earthscan, London, UK




                                                                                                                 To store the generated heat,
                                                                                                                 it must be transferred to a
                                                                                                                 suitable medium. For this
                                                                                                                 purpose gravel or stone
                                                                                                                 storage vessels can be used.


Source: “http://www.warmair.net/html/active_solar_heating.html”




             Solar Air Based Thermal Storage




    Radiant Heating
    Warm air stream is led through parts of the building such as walls
    or floors. The heat is transferred from the walls or floors to the
    adjacent rooms with a time delay.

From “Planning and Installing Solar Thermal Systems”, James & James/Earthscan, London, UK
                 Solar Air Heating and Domestic
                          Water Heating




                                                        Air-water heat transfer unit

From “Planning and Installing Solar Thermal Systems”, James & James/Earthscan, London, UK




                                                               Storage Walls
  A storage wall (e.g. Trombe wall) is a sun-facing wall built from
  material that can act as a thermal mass (such as stone, concrete,
  adobe or water tanks), combined with an air space, insulated
  glazing and vents to form a large solar thermal collector.

  During the day, sunlight would
  shine through the glazing and warm
  the surface of the thermal mass. At
  night, if the glazing insulates well
  enough, and outdoor temperatures
  are not too low, the average
  temperature of the thermal mass
  will be significantly higher than
  room temperature, and heat will
  flow into the house interior.

                                                                                            From “Solar Engineering of Thermal Processes”, Duffie & Beckman
    Examples of a Solar Air Heating System




    -     Installed in the summer of 1991
    -     420 m2 SOLARWALL® heating and ventilation system
    -     Air is drawn by two vane-axial fans.
    -     The ventilation air is delivered via 400 ft of perforated ducting.
    -     Airflow rates are approximately 40,000 cfm.
    -     Savings of 317,000 kWh each year.
Source: “www.solarwall.com”




    Examples of a Solar Air Heating System




                                              Source: http://www.rockymtsolar.com/




                                              Source: http://oee.nrcan.gc.ca/
Source: “www.solarwall.com”
        Three Types of SolarwallTM

 3. High Performance SolarwallTM
A perforated, dark-coated metal
absorber sheet is used as the
outside jacket for a façade. Air is
drawn in through the perforations
and heats as it flows up behind
the absorber.




 unglazed aluminum absorber




            The SolarwallTM System,
            Transpired Air Collector




                   From “Planning and Installing Solar Thermal Systems”, James & James/Earthscan, London, UK




SolarwallTM is an air based thermal system that
pre-heats the outside air before it enters the building
to provide fresh air changes and natural humidification.
                    High Performance SolarwallTM




Source: Conserval SolarwallTM Air Heating System Design Manual




                                                          Bypass Damper
                                                                    • the damper in front of
                                                                      the fan intake allows
                                                                      the solar collector to be
                                                                      bypassed

                                                                    • the damper is closed
                                                                      during the heating
                                                                      season and opened in
                                                                      the summer months
                                                                      when solar heating is
                                                                      not required

                                                                    • the operation of this
                                                                      damper is automatic
                                                                      and controlled by an
                                                                      ambient air temperature
                                                                      sensor via a
                                                                      temperature controller
Source: Conserval SolarwallTM Air Heating System Design Manual
                             Three Types of SolarwallTM
         2. Canopy Solar Wall




    The 30,000 ft2 (2,787 m2) hangar doors were converted to a solar
    heater which heats the outside air. An existing south wall overhang
    was used as the canopy section that collects rising heated air for
    five 23,000 cfm fans, each with 270 ft (82 m) of perforated flexible
    ducting.

Source: “http://www.solarwall.com”




                              Three Types of SolarwallTM
         2. Canopy Solar Wall




    Air is heated along the wall and rises to the canopy and is drawn
    in by the fan. There are no perforations in the absorber.
                                   The SolarwallTM System,
                                   Transpired Air Collector




              Nighttime Winter Operation – Tempered outside air is mixed
              with plant air and distributed in a building. Auxiliary unit heaters
              may be added to supplement the Solarwall.
Source: Conserval SolarwallTM Air Heating System Design Manual




                            Three Types of SolarwallTM
        1. Glazed SolarwallTM




        Energy
        House                                                            glazing




      Glazing reduces heat loss from the surface of the solar collector
      allowing higher temperatures to be achieved (usually used for
      residential applications where make-up air requirements are
      lower (low flow rates))
Air Preheating Systems




                                   The SolarwallTM System,
                                   Transpired Air Collector




              Daytime Winter Operation - Solar heated outside air is brought
              into a building and distributed.

Source: Conserval SolarwallTM Air Heating System Design Manual
                                   The SolarwallTM System,
                                   Transpired Air Collector




               Day/Nighttime Summer Operation – Solarwall is bypassed and
               fresh air is brought directly into a building.

Source: Conserval SolarwallTM Air Heating System Design Manual




                                  Make-up Air Fan Systems




                                   Fabric ducting                Metal ducting
From “http://www.solarwall.com”
• Significant advances
  in development of
  open loop air based
  BIPV/T systems:
  prototype
  prefabricated BIPV/T
  roof module design;
  numerical models and
  convective heat
  transfer correlation;
  full scale demo of
  transpired collector
  with PV in a fully
  integrated system.

                          29

								
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