Solar Thermal Heat Utilisation - Renewable Energy: Technology, Economics and Environment - page 128

( − )

U θθ S

η τα = −

coll abs e abs

(4.14)

cov abs

&

G

g

( − )

U θ θ

coll abs e

η τα = −

(4.15)

cov abs

&

G g,rel

2

( − ) ( −

)

C θθ S C θθ S

1 abs e abs 2 abs e abs

η τα = − −

(4.16) cov abs

& &

G G g g

2

( − ) ( − )

C θθ C θθ 1 abs e 2 abs e

η τα = − −

(4.17)

cov abs

& &

G G

g,rel g,rel

In many cases the solar fractional saving F s is significant. It is defined in dif-
ferent ways by the relevant literature. In this context, it is the ratio between the
utilisable emitted energy through conversion of solar radiation by the solar instal-
lation ex-storage to the actual demand for heating, domestic warm water or proc-
ess heat that is to be covered partly or entirely by solar energy (Equation (4.18)).
All heat losses of the heat storage are allocated to the solar system when using this

definition. It is thus defined as the conventional energy carrier saving Q .

aux , in

relation to the corresponding demand for heat Q . demand . If the substitution of the
conventional energy carrier is the starting point - as it is common - then for this
exclusively conventional system no storage, or only a very small storage, is neces-
sary. Thus the storage would only be used by the solar system and the definition
in Equation (4.18) is correct.

&

(4.18) Q

F = 1 −

aux

&

s

Q

demand

4.2 Technical description

Besides the collector a solar thermal system also consists of other system compo-
nents. Essential are a liquid or gaseous heat transfer medium and pipes to trans-
port the heat transfer medium. Normally, a heat store with none, one or several