Download Analysis of renewable energy sources utilization

SCYR 2010 - 10th Scientific Conference of Young Researchers – FEI TU of Košice
Analysis of renewable energy sources utilization
Ing. Martin Bačko, Ing. Anna Hodulíková
Dept. of Theoretical Electrical Engineering and Electrical Measurement Electronics, FEI TU of Košice, Slovak
Republic
[email protected], [email protected],
Abstract—This article deals with possibilities of using
renewable energy sources especially wind and solar energy for
gradual replacement of traditional energy gained from fossil
fuels which will be depleted relatively soon. Renewable energy
sources are available in much higher amount than the whole
world needs and their usage is imperative for keeping the
ecological equilibrium.
electron moves from valence to conductive zone. Electron can
freely move on the crystal lattice. When electron leaves the
valence zone, the free space is known as the hole, or so called
defective electron. Creation of these defective electrons is
responsible for inner semiconductors conductivity. Electrones
and holes are always in pairs, so in other words there is the
same amount of electrons and the holes.
Keywords—photovoltaics, renewable sources, solar energy,
wind energy
I. INTRODUCTION
In term renewable sources of energy we understand
ecologically clean direct or indirect form of solar energy,
which can be transformed by suitable technological solution
to electrical energy. Burning of fossil fuels probably caused
accumulation of harmful carbon dioxide emissions, which are
responsible for greenhouse effect and climatic changes. Direct
form of solar energy can be used for direct water heating, or
direct transformation to electrical energy using the
photovoltaics principle. Indirect form of solar energy is in the
form of water and wind energy which again can be with
suitable technical solution used for generating the electrical
energy.
II. PHOTOVOLTAICS
A. Basic principle
Photovoltaic systems use the principle of semiconductors.
Semiconductors are elements from IV group of periodic table
such as silicon (Si), germanium (Ge), tin (Sn) which have 4
valence electrons on sphere. Semiconductor systems consist
of 2 elements, for example III-V semiconductor galliumarsenic (GaAs) and II-VI semiconductor cadmium-tellurium
(CdTe).
Silicon is the most common material in photovoltaics. It is
the second most common element in Earth´s crust, but it is not
possible to find it in chemically clean form. It is the
fundamental semiconductor of IV group of periodic table,
therefore it have 4 valence electrons. Because it wants to keep
the most stable electron configuration, 2 electrons from
neighboring atoms in crystal lattice form a pair connection.
Such pairing (covalent bond) with 4 neighboring atoms
provides silicon with stable electron configuration similar to
rare gas argon (Ar).
From energetical point of view, the valence zone is fully
taken and the conductive zone is empty. By providing
additional energy, for example from light or heat source
Fig.1 Photovoltaics solar cell
Current flowing through PN crossing can be formulated as
algebraic sum of balanced heat flows of electric charge
carriers. In state of equilibrium is the sum of the heat flows
zero. Sums of electron and hole currents passing through the
PN crossing are also zero. Absolute values of electron and
hole currents from the N type semiconductor can be marked
I nN and I pN , from semiconductor type P can be marked as
as
I nP and I pP .
In equilibrium state:
N
n
N
P
P
- I + I p + In - I p =
0
(1)
N
N
(2)
P
P
(3)
- In + I p = 0
+ In - I p = 0
Illumination will cause the increase in concentration of
minority carriers. It will create the If current which flows
through the PN crossing. When illuminated, Fermi´s level
shatters to quasilevels for electrons and holes. Their
 , which was
difference φ resembles the voltage
Uf 
e
created as the result of illumination.
In stationary state, the current flowing through the PN
crossing equals to zero.
SCYR 2010 - 10th Scientific Conference of Young Researchers – FEI TU of Košice
I f  I nN  I pN  I nP  I pP  0
Majority carriers currents
(4)
I nN and I pP will change because
of illumination. Energetic levels are mutually shifted and
levels of potential barriers are changed:
I nN  I nP exp(
I pP  I pN exp(

kT

kT
)
(5)
)
(6)
describes exactly the solar cells behavior, especially in cases
where different operational conditions have to be considered.
Charge carriers in real solar cell show a voltage loss when
passing through the PN crossing. Serial connected resistor RS
allows representation of this voltage loss. Parallel resistor RP
represents inner loses in cell. RS value for real cells is about
few milliohms (fig. 3) and RP value is mostly higher than 10
Ω (fig. 4).
Using the equations (4), (5), (6) and after adjustments we get:
I f  I s [exp(

kT
)  1]  0
(7)
For photoelectromotoric force:
Uf 

e

I
kT
ln( f  1)
e
Is
(8)
If PN crossing is connected to circuit where current I is
flowing, using the (7), (8) equations we get:
I f  I  I s [exp(

Fig.2 Solar cell equivalent scheme
) ]
kT
(9)
Uf 
I I
kT
ln( f
 1)
e
Is
(10)
If PN crossing is connected to resistor
R
Uf
I
, equation
(9) will be:
If 
In
Uf
R
 I s [exp(
the
case
I  Is[exp(

kT

kT
of
Fig.3 V-A Characteristics of Rs resistor
)  1]
small
(11)
external
resistors
when:
)  1] we get I f  I .
In the case of big external resistors when I   0 , we get
I f  I s [exp(

kT
)  1]  0 .
If we connect the source of voltage to PN crossing we get:
If 
U f U
R
 I s [exp(

kT
)  1]
(12)
For the solar cell power we use this formula: P=U.I For the
maximum output:
eU
d (UI )
e
 Ik  Is  Is
exp m  0
dU
kT
kT
(13)
For idle connection:
U0 
I
kT
ln( k  1)
e
Is
(14)
B. Solar cell equivalent scheme, V-A characteristics
One diode enhanced model
Simple equivalent circuit is sufficient for most
applications. The difference between measured and calculated
values is only few percents. Only enhanced model (fig.2)
Fig.4 V-A Characteristics of Rp resistor
C. Photovoltaics on KTEEM – module and program
For measurements on department the photovoltaics cell
QX6926 (fig.6) and SFH203 infra diode are used. We can
calculate the power P [W] because of 4Ω resistor which is
connected to cell and simulates the load.
Program (fig.5) was written for the measurement, which
can collect the voltage or current values simultaneously from
4 devices. The results are daily written to *.csv file (fig.7) and
are sent to specified ftp server at midnight, where they can be
further evaluated (fig.8).
Figures 9 and 10 shows the classic bigger photovoltaic
module which is commonly used for household or industrial
applications and its technological parameters.
SCYR 2010 - 10th Scientific Conference of Young Researchers – FEI TU of Košice
Fig.5 Measuring program main screen
Fig.9 Photovoltaic panel
Fig.6 Solar cell QX6926
Fig.10 Photovoltaic panel – technical parameters
Fig.7 Example of output file in *.CSV format
Fig.8 Example of daily graph - Voltage
Fig.11 Example of solar cell system
SCYR 2010 - 10th Scientific Conference of Young Researchers – FEI TU of Košice
III. WIND ENERGY
Wind energy is the indirect form of solar energy. Solar
irradiation causes temperature differences on Earth and these
are the origin of winds. Wind can achieve much higher energy
concentration than solar radiation (10 kW/m2 during violent
storm and more than 25 kW/m2 during hurricane, compared to
maximum value of solar irradiation 1 kW/m2). Slow wind
speed about 5 m/s however has energy concentration about
only 0,075 kW/m2.
B. Wind/solar energy system
A. Wind from energetic point of view
Kinetic energy W in wind with speed v is equal to:
W 
1
m.v 2
2
(15)
Fig.12 Example of combined solar/wind system
Power P of the wind with constant speed v, is:
P W 
1
m.v 2
2
Density ρ and content V of air determine its weight:
m   .V
(17)
Weight of air with density ρ, which flows through the area
S with speed v on trajectory š, can be calculated using this
equation:
m   .V   .S .š
(18)
Power P of the wind will be:
P
1
 .S .v 3
2
(19)
Wind density ρ is changing due to air pressure p and
temperature υ It is directly proportional to pressure nex to
temperature.
Ratio between wind power taken by turbine PT and total
wind power P0 is called power coefficient CP:
CP 
PT 1  v 2
 .1 
P0 2 
v1
  v 22
.1  2
  v1



(20)
Betz calculated ideal wind ratio which is:
v2 1

v1 3
(21)
After using the equation (20) we get so called Betz power
coefficient:
C P _ Betz 
16
 0,593
27
(22)
If wind turbine slows the wind from initial speed v1 to one
third v1 (v2 = (1/3).v1), then it is theoretically possible to
achieve the maximum power which in the case of the wind
turbine is 60%.
Real wind generators cannot achieve this theoretical
optimum, however good systems have CP coefficient between
0,4 a 0,5.
Ratio between used wind power PT and ideal power Pid
defines efficiency η of the wind generator.

PT
CP

Pid C P _ Betz
IV. CONCLUSION
(16)
(23)
Solar and wind energy can be considered as a real alternative
because systems which utilize it already exist (fig.11,12) and
achieve good results. Hopefully it will be a matter of few
decades until they achieve a wide range utilization in every
sphere of industry and household applications, as they are the
only way how to get clean and harmless energy.
Acknowledgment
The paper has been prepared by the support of Slovak grant
projects VEGA 1/0660/08, KEGA 3/6386/08, KEGA
3/6388/08
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