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Studies on sky-therm cooling

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Abstract

Passive cooling by the sky-therm technique presents a very attractive prospect for environmental conditioning in India because of its inherent simplicity and low cost. Sky-therm control has been proposed for storage rooms for the sericulture industry (which is a major cottage industry in Karnataka State). The temperature within these rooms is to be kept at 25 ± 3°C throughout the year. A thermal model for buildings equipped with roof ponds and movable insulating covers as proposed in the sky-therm process has been developed. The thermal behaviour of a building has been simulated to study the influence of such parameters as the heat capacity, depth of water in the pond, roof thickness and its thermal conductivity, air infiltration rates etc. Experiments have been carried out to check which of the several commonly used empirical relations for the sky temperature is valid. A simple technique for measuring air infiltration has been developed. It has been found that even for the most severe cooling requirement in Bangalore (for April), sky-therm cooling alone is adequate to maintain a temperature below 28°C. In places where the requirements are more demanding, it may be necessary to augment sky-therm with evaporative cooling.

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Abbreviations

A:

area (m2)

a, b:

view factors for the window to sky and ground respectively

C:

heat capacity (kcal/K)

c:

specific heat (kcal/kg K)

D:

diffusion coefficient (m2/hr)

d:

depth of water in pond (m)

F:

view factor for the pond to sky

h:

heat transfer coefficients (kcal/hr m2 K)

hm :

mass transfer coefficients (m/hr)

Is :

solar influx (kcal/hr m2)

l:

characteristic dimension (m)

LE :

latent heat of vaporisation (kcal/kg)

mE :

mass flux due to evaporation (kg/hr m2)

Pw,p Pw,a :

partial pressure of water at the temperature of pond (T p ) and air (T a ) respectively, ″H g

Q:

heat flux (kcal/hr)

Ram :

mass transfer Rayleigh number [β (~xw,p - ~xw,a) l3/vD]

S:

shading factor

Sh:

Sherwood number (h′ m l/D)

T:

temperature (K)

U:

conductance (kcal/hr m2 K)

V:

volume of the building (m3)

v:

velocity of air (m/s)

xw :

mass concentration of water vapour (kg/m3)

xw :

mass fraction of water vapour

α:

absorbitivity

β:

expansion coefficient

ε:

emissivity

εe :

equivalent emissivity = (1/ε1 + l/ε2 - 1)-1

ε1, ε2 :

emissivity of surfaces 1 and 2 which are parallel

λ:

air infiltration factor

ρ:

density (kg/m3)

σ:

Stefan-Boltzmann constant (4.89 × 10-8 kcal/hr m2 K4)

v:

kinematic viscosity (m2/s)

p∼:

reflectivity of window glazing

a:

ambient air

AI:

air infiltration

b:

room

c:

insulating cover

d:

ground

f:

floor

g:

glass

i:

inside

m:

moisture

o:

outside

p:

pond

r:

roof

s:

sky

w:

walls

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Author information

Author notes

  1. G. S. Dutt

    Present address: Centre for Environmental Studies, Princeton University, New Jersey, USA

Authors and Affiliations

  1. Department of Mechanical Engineering, Indian Institute of Science, 560 012, Bangalore

    C. R. Prasad, G. S. Dutt, S. R. C. Sathyanarayan & V. Kuppu Rao

Authors
  1. C. R. Prasad
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  2. G. S. Dutt
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  3. S. R. C. Sathyanarayan
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  4. V. Kuppu Rao
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Prasad, C.R., Dutt, G.S., Sathyanarayan, S.R.C. et al. Studies on sky-therm cooling. Proc. Indian Acad. Sci. 2, 339–356 (1979). https://doi.org/10.1007/BF02848931

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  • DOI: https://doi.org/10.1007/BF02848931

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