Solar Thermal Heat Utilisation - Renewable Energy: Technology, Economics and Environment - page 125

4.1.4 Energy balance

General energy balance. Equation (4.3) describes the general energy balance of
a medium that absorbs radiation and converts it into heat.

& & & & &

&

= + + + +

Q Q Q Q Q

G (4.3)

G, abs

conv, abs rad, abs refl, abs cond , , abs useful

G . G,abs is the entire global radiation incident on the absorber surface; Q . useful is the
utilisable thermal flow. In addition there are four different loss flows:

− convection losses of the absorber to the ambient air Q . conv,abs ,

− long-wave radiation losses of the absorber Q . rad,abs ,

− reflection losses of the absorber Q . refl,abs ,

− thermal conductivity losses Q . cond,abs .

Energy balance of the collector. In solar thermal systems the absorber is nor-
mally part of a collector. Other components of the collector are frame, cover and
insulation. Given these conditions, the energy balance will be discussed further in
the following.
A collector removes the utilisable heat by a heat transfer medium flowing
through the collector (Fig. 4.2). The difference between the energy at the inlet and
the outlet of the heat transfer medium is the thermal flow removed by the transfer

medium Q . useful (Equation (4.4)), where c p is the specific heat capacity, m . is the
mass flow of the transfer medium and θ in and θ out are the inlet and the outlet tem-
perature of the transfer medium flowing into, or out of the collector.

. . . . . . .

Q Q G Q Q Q Q

rad,abs rad,cov g refl,cov conv,cov refl,abs conv,abs

Cover

τ cov

.

G

g,abs

Absorber Heat carrier

Heat carrier
m, c ,
p in

.

.

α abs ε abs ρ abs θ abs θ

m, c ,
p out

θ

System boundary absorber

.

Q

cond,abs

Frame and insulation

System boundary collector

.

Q

conv,frame

Fig. 4.2 Stationary energy balance at the collector or the absorber (for symbols see text)

( )

&

= & c m θ − θ

Q

(4.4) p out in

useful