Theoretical and experimental analysis of solar multi-pass air solar heaters

Abstract

Expressions

for the performance

of Solar

air

heaters operating in four modes

were

derived

and

solved using a computer

and

the

results

from a

series of experiments used to check the validity of

the expressions.

The expressions derived

were

simpler than the generally accepted exponential relationships but it was found that they described the performance of the col lectors reasonably we 11. It was also found that the four modes differ in

their performances

over

the parameter ranges

considered. The parameters varied

were

mass flow

rate of the air, co I 1 ector length, flow channel

height and insolation. The four modes analysed were

single

pass,

double (top first> pass, double

(middle first) pass and triple pass.

In the single pass mode,

air

flowed between

the

absorber plate and the back plate. In the double

(top first) pass mode, air flowed between the two

covers in the first

pass

and then between

the

absorber plate and the back plate in the second

pass. In the double (middle f' i na t ) pass mode,

flowed between the lower

cover

and the absorber

.'

plate in the

first

pass and then between the

absorber plate and the back plate in the second

pass. In the triple pass mode, air

flowed

between

the two covers in the first pass, between the lower

cover and the absorber plate in the second pass and

then between the absorber plate and the back plate

in the third pass.

It was found that at no added cost, the triple pass mode gave the best performance fa I lowed by the double (top first) pass mode and the double (middle first) pass mode. The single pass mode was found to give the most inferior performance as compared to other modes. This order of performance was found to be independent of the varied parameters. The gOO? performance of the triple pass as compared to the

other modes and of the double (top first) pass mod~

as compared to the double (middle first) pass mode

were attributed to a lower

top cover

temperature

which. meant a lower heat loss to the surroundings.