MATERIALS FOR PASSIVE SOLAR HEATING

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MATERIALS FOR PASSIVE SOLAR HEATING

  1. 1.  It is one of several design approaches collectively called “Passive Solar Design”. Typically, passive solar heating (PSH) involves:  The “collection of solar energy” through properly-oriented, south-facing windows.  The “storage of this energy in thermal mass," comprised of building materials with high heat capacity such as concrete slabs, brick walls, or tile floors  The “natural distribution of the stored solar energy back to the living space”, when required, through the mechanisms of natural convection and radiation  “Window specifications” to allow higher solar heat gain coefficient in south glazing.
  2. 2. 1. Aperture (Collector)2. Absorber3. Thermal mass4. Distribution5. Control.
  3. 3.  The APERTURE (collector) is a large glass (window) area through which sunlight enters the building. The hard, darkened surface of the storage element is known as the ABSORBER. This surface sits in the direct path of sunlight. Sunlight then hits the surface and is absorbed as heat. The THERMAL MASS is made up of materials that store the heat produced by sunlight. Distribution is the method by which solar heat circulates from the collection and storage points to different areas of the building. Elements to help control under- and overheating of a passive solar heating system include roof overhangs, which can be used to shade the aperture area
  4. 4.  The orientation of the APERTURE. Thermal mass location. Insulation and air sealing. Local climate conditions i.e. seasonal variation of sun shine.
  5. 5.  The material should act as a “HEAT STORING MEDIUM”. The heat should flow from one end of the wall to other end of this THERMAL MASS, only after 12 hours. Materials should be having nominal thickness. The material should be cheap, and the thermal energy stored per unit material cost, should be maximum.
  6. 6. 1. The heat content “Q” per unit area of the wall, Q = w ρ Cp ΔT where, ρ Cp = Specific heat per unit volume ΔT = Temperature interval2. The time constant (t) is estimated by the approximation used for the heat-diffusion distance in time t, w = (2 α t)1/2 where, α = diffusivity
  7. 7. 3. On eliminating the free variable, w, Q = (2 α t)1/2 ρ Cp ΔT4. Using, α = λ / ρ Cp5. Finally, we obtain, Q = [ (2 t)1/2 ] [ΔT] [ λ / (α)1/2 ]
  8. 8. Hence, the heat capacity of the wall is maximized by choosing material with a high value of, M = [ λ / ( α )1/2 ]6. But, we have assumed a material thickness restriction of w ≤ 0.5 m & t = 12 hrs. = 4 * 104 seconds. So, along with the above material property another attribute to be looked upon is, α ≤ 3 * 10 - 6 m2/s
  9. 9. Area of the Graph,between Thermalconductivity (λ)-Thermal diffusivity(α), representingthe materialssatisfying therequirements.
  10. 10.  The materials satisfying the graphs are, 1. Epoxies 2. Brick 3. Soda glass 4. Concrete 5. Stone 6. Ti alloy The materials as can be seen are only SOLIDS and not the POROUS MATERIALS & FOAMS (generally used in walls). Finally, the materials are selected on the basis of their cost per unit volume.
  11. 11. M1= λ/√α ApproximateMaterials Comments (W.s1/2/ m2.K) cost ($/m3) Concrete 2.20 * 103 200 Best choice Better than concrete, Brick 3.50 * 103 1400 due to more specific heat. Glass 1.00 * 103 1400 Not as good as concrete Stone 1.60 * 103 10,000 Useful in some cases Titanium 4.60 * 103 2,00,000 Unexpected but valid.
  12. 12. THANK YOU

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