Page 308 - A Comprehensive Guide to Solar Energy Systems

312 A ComPreHenSIVe GuIde To SolAr enerGy SySTemS

From the I–V and P–V curves, some basic parameters such as fill factor (FF) and elec-
η
ηpv tricity conversion efficiency ( pv ) can be determined for the BIPVT under different ir-
radiances with different water flowrates by following some fundamental studies in the
literature [35]. The FF is essentially an index of the BIPVT panel quality. It is calculated in
eq. (14.3) by comparing the maximum output electricity (E mAX ) to the theoretically calcu-
lated vale (E T ) that would be generated at both the open circuit voltage and short circuit
current together. A larger FF is desirable, which corresponds to an I–V sweep that is more
square-like.
I MPP ⋅
FF = F MAX = V MPP (14.3)
FF=FmAXET=ImPP⋅VmPPISC⋅VoC E T I SC ⋅ V OC
η
ηpv The electricity conversion efficiency ( pv ) is defined as the ratio of the output electricity
Epv E pv with respect to the absorbed solar irradiance E IN by the BIPVT panel, which is repre-
E
I
n
Epv
Em AX sented in eq. (14.4). E pv can be taken as E MAX because the solar cell can be operated up to
its maximum energy output to get the maximum efficiency. In determining the efficiency,
the dimensions of the BIPVT frames were also considered in the calculations, as they are
necessary components that take up area though they are there for structural purposes.
η pv = E pv = E MAX (14.4)
ηpv=EpvEIn=EmAXIRA E IN IA
R
The determined parameters of the BIPVT panel under different irradiances with differ-
−2
ent water flow rates are summarized in Table 14.2. under a solar irradiance of 620 W m ,
compared with the case in which no water flows, the output electricity energy increases
from 18.45 to 22.65 W and the electric efficiency raises from 10.5% to 12.9% after a water
−1
Epv
ηpv flow of 120 ml min was introduced. Both E pv and η increased as the solar irradi-
pv
−1
Epv ance and water flow rates increased. With a water flow rate of 150 ml min , E pv reaches
η
ηpv 32.96 and 44.91 W and pv reaches 14.51% and 15.82% under solar irradiances of 800 and
−2
1000 W m respectively. The PV efficiency is enhanced by 21.1% and 24.0% for those two
solar irradiances respectively, comparing with the case in which no water flows. Based
on the thermal and electric performances provided in Tables 14.1 and 14.2, the total
ET=ηthermal+ηpv energy efficiencies E T = η thermal η+ pv of the BIPVT panel under different solar irradiances
ηT and water flowing rates can be estimated. The maximum T η evaluated in this study under
−2
−1
the solar irradiance of 620 W m with water flowing at a rate of 120 ml min is approxi-
−2
−1
mately 79.8%. With a solar irradiance of 800 W m and a water flowrate of 150 ml min
−2
the total efficiency drops to 77.3%, and with a solar irradiance of 1000 W m and a water
−1
flowrate of 150 ml min , and the value is approximately 75.2%.
14.4.4 Overall Efficiency and Comparisons With Other Relevant PVT
Collectors
For customers who want to utilize a larger amount of hot water through the BIPVT, they
may need to decrease the water flow rate and thus sacrifice the electricity gain, and vice