materials for passive solar heating
TRANSCRIPT
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ASSIGNMENT PRESENTATION
PASSIVE SOLAR HEATING
PRESENTED BY:-SWAPNIL NIGAM
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PASSIVE SOLAR HEATING???
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.
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PRINCIPLE DIAGRAM OF “PSH”
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KEY COMPONENTS OF “PSH”
1. Aperture
(Collector)
2. Absorber
3. Thermal mass
4. Distribution
5. Control.
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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
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The orientation of the APERTURE.
Thermal mass location.
Insulation and air sealing.
Material required for thermal mass.
Local climate conditions i.e. seasonal variation of sun shine.
REQUIREMENTS OF “PSH” DESIGN
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MATERIAL REQUIREMENTS OF “THERMAL MASS”
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.
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TABULATION OF REQUIREMENTS ACC. TO MATERIAL SELECTION PROCESS
ATTRIBUTES REQUIREMENTS
Function Heat storing medium
Constraints• Heat diffusion time ≈ 12 hours• Wall thickness ≤ 0.5 m• Adequate working temperature Tmax.> 100°C
Objective Maximize thermal energy stored per unit material cost
Free variables • Wall thickness, w• Choice of material
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THE TRANSLATION INTO MATERIAL INDICES
1. The heat content “Q” per unit area of the wall,
Q = w ρ Cp ΔTwhere,
ρ Cp = Specific heat per unit volume
ΔT = Temperature interval
2. 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
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3. On eliminating the free variable, w,
Q = (2 α t)1/2 ρ Cp ΔT
4. Using,
α = λ / ρ Cp
5. Finally, we obtain,
Q = [ (2 t)1/2 ] [ΔT] [ λ / (α)1/2 ]
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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
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THE SELECTION OF MATERIAL
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Area of the
Graph,
between Thermal
conductivity (λ)-
Thermal
diffusivity
(α), representing
the materials
satisfying the
requirements.
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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.
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Materials
M1= λ/√α (W.s1/2/ m2.K)
Approximate cost ($/m3) Comments
Concrete 2.20 * 103 200 Best choice
Brick 3.50 * 103 1400
Better than concrete, due to more specific heat.
Glass 1.00 * 103 1400Not as good as concrete
Stone 1.60 * 103 10,000Useful in some cases
Titanium 4.60 * 103 2,00,000
Unexpected but valid.
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