a low-cost microcontroller-based maximum power

7/27/2019 A Low-Cost Microcontroller-Based Maximum Power

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Thus, Ln is calculated as

( )1om cmn

s Lm

V D L

f I

(3)

where f s=1/ T s is switching frequency, D cm is duty cycle atmaximum converter output power, I Lm is peak-to-peak ripple of the inductor current, V om is maximum of the dccomponent of the output voltage and I om is dc component of the output current at maximum output power. The inputcapacitor value is calculated to be

( )10.02om cm cm

in pvm pvm s

I D DC

I R f

(4)

where I pvm is the converter input current at maximum input power, while R pvm is the PV array internal resistance at theMPP and is defined as

inm pvm

pvm

V R

I = (5)

where V inm is the PV array output voltage at the maximum power point. B. PV Module

The PV equivalent circuit is shown in Fig. 2 [7-8].This module is commonly known as a single diodemodel. The mathematical model of solarPV system isdiscussed here as under. Ideally the voltage currentequation of the solar cell is given by

( )

( )

/exp 1

/

s s p ph p rs

s s

sh

q V n IR I n I n I

kAT

V n IR

R

+ = +

(6)

where

( ), pv pv n I T n

G I I K

G= + (7)

, , s p

pv n sc n p

R R I I

R

+= (8)

,

,exp 1

sc n I T o

oc n V T

t

I K I

V K

aV

+ =

+

(9)

st N kT V q

= (10)

where I pv is photovoltaic currents (0.9006A (@ STC)), I pv,n isnominal light-generated current (0.9006A), R s is equivalentseries resistance (23.74 ), R p is equivalent parallel resistance(503.209 ), I sc,n is nominal short circuit current at STC(0.86A), V oc,n is nominal open circuit voltage at STC (93.4V),

K I is current coefficient (-0.2762A/K), K V is voltagecoefficient (3.39710

4V/K), T =T -T n, T is actual temperature(K), T n is nominal temperature at STC (0K), G is actualirradiation (1,000W/m 2), Gn is nominal irradiation at STC(1,000W/m 2), I o is saturation current of the photovoltaic cells(4.303210

28A), a is diode ideality constant (usually

1a1.5), V t is thermal voltage of the array (1.4906V), q iselectron charge (1.6021764610

19 C), k is boltzman constant(1.380650310

23 J/K), N s is number of cells connected inseries (58), N pp is number of PV module connected in parallel(15).The power output of a solar cell is given by

pv pv pv P V I = (11)

Also the input energy to PV system is solar radiation and totalsolar radiation on an inclined surface is estimated as

( )T b b d d d b r I I R I R I I R= + + + (12)

where I b and I d are direct normal and diffuse solar radiations, Rb, Rd and Rr are the tilt factors for the beam, diffuse andreflected part of the solar radiations. Hourly power outputfrom PV system with an area A PV (m

2) on an average day of jth month, when total solar radiation of I T (kWh/m

2) isincident on PV surface, is given by

sj Tj pv P I A = (13)

where is PV-system efficiency is given by

m pc f P = (14)

and the modular efficiency m is given by

( )1m r c r T T = (15)

where r is the module reference efficiency, pc is the power conditioning efficiency, P f is the packing factor, is the arrayefficiency temperature coefficient, T r is the reference

temperature for the cell efficiency and T c is the monthlyaverage cell temperature.

(a) PV equivalent circuit.

+

1exp0 aV

N IR

V

N I N I t

pp

s

pp pp pv

(b) Simulation model

Figure 2. PV equivalent circuit and simulation model of the proposed system.

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Figure 3. Characteristic of a proposed PV module with temperature and irradiation: (a) constant temperature and varying irradiation and (b) constantirradiation and varying temperature.

Power Meter

Power Module

Oscilloscope Computer and Simulation tool

Figure 4. The implementation of a laboratory prototype.

C. Simulation Results and Laboratory prototypeA detailed simulation was developed using

Matlab/Smulink to model singlediode solar cell and alsoused to create V pv- I pv and P pv-V pv characteristics. Figs. 3illustrate the V pv- I pv and P pv-V pv characteristics under fivedifferent levels of irradiance. The temperature effects areshown in this study as well.

Fig. 4 shows a photograph of a laboratory prototype andthe measurement system. The first operation of the proposedsystem was tested with the implementation of a laboratory

prototype, which is shown in Fig. 5. The PV module utilizedin the practical test was the Bangkok Solar BS50 withmaximum power equal to 50W. The implementation of the

control algorithm was achieved with the microcontroller PIC18F4431. The data acquisition was obtained through serialtransmission from the digital power meter (WT-1600) to acomputer. The main control information, such as current,voltage, power, and duty cycle, is stored during MPPToperation.

The simulation and experimental results of the MPPTsystem using the PV module BS50 are shown in Figs. 5,considering a load resistance that is equal to 10.3 and

battery 125Ah (48V).

Figure 5. The experimental and simulation results of the proposed MPPTlaboratory prototype.

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a low-cost microcontroller-based maximum power

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