where Ei and Eo are respectively the energy associated with mass
entering and leaving the system, Qj is heat transfer to system from
source at Tj, and Wnet is net work developed by the system.
The exergy balance for steady flow process of an open system is
given by
XExi þXExQ ¼ XExo þ ExW þ IR (3)
where Exi and Exo are respectively the exergy associated with mass
inflow and outflow, ExQ is exergy associated with heat transfer, ExW
is exergy associated with work transfer and IR is irreversibility of
process. The irreversibility may be due to heat transfer through
finite temperature difference, mixing of fluids and mechanical
friction. Exergy analysis is an effective means, to pinpoint losses
due to irreversibility in a real situation.
The energy or first law efficiency hI of a system or system
component is defined as the ratio of energy output to the energy
input of system or system component i.e.
hI ¼ Desired output energy
Input energy supplied (4)
The exergy or second law efficiency is defined as
hII ¼ Desired output
Maximum possible output ¼ exergy output
exergy input (5)
The percentage energy loss lc of system or system component is
calculated as the ratio of energy loss in the system or system
component to the energy entering in the whole system. Similarly
the percentage exergy loss or efficiency defect dc of system or
system component’s is defined as the ratio of exergy loss IRc in the
system or system component to the exergy Exinput entering in the
whole system. The dc is given by
dc ¼ IRc=Exinput 100 (6)
3.1. Analysis of conceptual DSG STPP
The analysis for the individual components of DSG STPP (Fig. 1)
has been carried out by ignoring the kinetic and potential energy
change and assuming steady state operation. The energy Ej and
exergy Exj at state point j are represented respectively by mjhj and
mjjj. Table 2 shows the relations for the energy loss, hI, exergy loss
and hII for DSG STPP and its components by choosing each
component in Fig. 1 as a control volume. The total energy input to
DSG STPP is the energy QI received by the collector system or falling
on the aperture plane of collector. The QI by considering only the
beam component of solar radiation is given by
QI ¼ IbrbBLNcNr (7) where Ib is beam radiation falling on horizontal surface, the tilt
factor rb for beam radiation is rb ¼ cos q/cos qz, the minimum
angle of incidence q for N–S horizontal axis tracking
is cos q ¼ ½ðsin fsin d þ cos fcos dcos uÞ
2 þ cos2dsin2u
1=2, zenith
angle qz is cos qz ¼ sin d sin f þ cos d cos f cosu and the declination
angle in degrees is given by d ¼ 23:45sin½360=365ð284 þ nÞ.
The total exergy input to DSG STPP or the exergy ExI received by
the collector system is calculated by
ExI ¼ QIð1 Ta=TSÞ (8)
where TSy5600 K is apparent black body temperature of sun.
The energy absorbed Qa by receiver/absorber of solar collector
field is given by
Qa ¼ hoIbrbBLNcNr ¼ hoQI (9)
The heat energy Qa is transferred to water as useful heat gain
rate Qu by water flowing through receiver tube and remaining
amount Qa Qu is lost from the receiver to ambient as heat loss Ql.
The Qu is given by
Qu ¼ ðm11h11 m10h10Þ ¼ m10ðh11 h10Þ (10)
The exergy Exa of heat absorbed by receiver at mean receiver
temperature Tr is given by
Exa ¼ Qað1 Ta=TrÞ (11)
The exergy gain Exu by water flowing through receiver is
given by
Exu ¼ m10½j11 j10 ¼ m10½ðh11 h10Þ Taðs11 s10Þ (12)
The mean receiver temperature Tr is calculated by
Ql ¼ UlpDoðTr TaÞLNcNr (13)
The heat loss coefficient Ul is correlated in terms of Tr by
calculating the Ul for various values of Tr ranging from 523 to 723 K.
The Ul for a Tr is given by
Ul ¼ q0
loss=ðpDoðTr TaÞÞ (14)
where q0
loss is heat loss rate per unit length of receiver tube. The Ul
for a Tr is calculated iteratively [17] by solving the following
equations:
q0
loss ¼ q0
cos ¼ pDcohwðTco TaÞ þ ecpDcos
T4
co T4
S
(15)
q0
loss ¼ q0
cico ¼ 2pkcðTci TcoÞ=ðlnðDco=DciÞÞ (16)
q
0
loss ¼ q
0
rci ¼ pDos
T4
r T4
ci.
1
er
þ
Do
Dci1
ec
1
(17)
Table 2
Relations fo