Tropical Design Module 8: Solar Heat Factor

SOLAR HEAT FACTOR

The percentage of incident solar radiation which is transmitted through a wall, assuming equal air in temperatures on both sides.
The solar heat factor is proportional to the inside rise in temperature and is related to the radiant heat from the ceiling or walls.
Radiation from the walls can be controlled by controlling the reflectivity and the “U” value of the construction.
To avoid excessive radiation, the solar heat factor should not exceed 3% for roofs and 4% or walls in the warm humid tropics.

q / I = 5 U*X %

Where q = heat transfer due to radiation = Q / A (heat flow per unit area)

q = U*A*(tsa-to) / A

q = U*( tsa –to ) from sol-air temp formula tsa = to + (X*I*ro)

q = U*( X*I*ro)

q / I = U*( X*ro ) if ro = 0.05 ----- 5%

Then q / I = 5 U*X %

Example:

From previous example of brickwall, “U” value is 2.6 W / m2 oC.

Absorptivity for external concrete rendering = 0.6

q / I = 5 U*X % = 5 *2.6*0.6% = 7.8% > 4% max for walls

To reduce the solar heat factor to acceptable level;

reduce the absorptivity of rendering by using white wash or lime wash with an absorptivity of 0.30
q / I = 5 U*X % = 5 *2.6*0.3% = 3.9% < 4% max for walls OK

or reduce the transmittance “U” to 1.6 by adding 1cm of expanded polystyrene with resistivity of 30 (resistance is 0.01*30=0.30)
“U” = 1 / (0.385+0.30) = 1.46
q / I = 5 U*X % = 5 *1.46*0.6% = 4.38% > 4% max for walls
1.5cm of polystyrene (resistance 0.015*30=0.45), “U”=1/(0.385+0.45)
q / I = 5 U*X % = 5 *1.20*0.6% = 3.60% < 4% max for walls OK

TIME LAG

The time taken per fluctuation in temperature to pass through a material of unit thickness (hrs/m3oC).
Time lag depends on two properties:
- thermal capacity (amount of heat required per unit volume per degree rise in temperature)
- Rate of flow of heat

Time lag = thermal capacity / rate of flow of heat

The time constant of a composite temperature is the sum of the time constants of the individual layers.

STANDARDS FOR WARM HUMID CLIMATES:

Walls and Roofs should have:

A reflective surface (low absorptivity)
Light weight material (low heat capacity)
Insulation (low “U” value)

Solar Heat Factor: Roofs 3%
Walls 4%

Time Lag: 3 hours max.

To achieve this,

- Roof should have a cavity between the roof and ceiling. A reflective surface together with a cavity will achieve standard.
- Heavy external walls should be restricted to day rooms, and should be as well shaded as possible.