A Study on a Solar Simulator for Dye Sensitized Solar Cells

Dye-sensitized solar cells (DSSC) are emerging low-cost, simple alternatives to conventional solar cells. While there has been considerable study on improving the efficiency of DSSCs, there has not been sufficient research on a photovoltaic power conditioning system adaptable to DSSCs or on a solar simulator for DSSCs. When DSSCs are commercialized in the near future, the DSSC modules must be connected to an adaptable power conditioning system in order to manage the energy produced and provide a suitable interface to the load. In the process of developing a power conditioning system, a solar simulator with the characteristics of DSSCs is essential to show the performance of the maximum power point tracking. In this paper, a virtual DSSC is designed and simulated in PSIM. Irradiation factors, temperature and shadow effects are considered in dynamic link library block in PSIM which is linked to the external C routine. A 100 W converter is built to show the performance of a DSSC as the solar simulator controlled by a digital signal processor.


Introduction
Solar energy has emerged as a solution for problems such as depletion of fossil energy resources and environmental pollution.Solar energy is a clean and permanent energy source.In addition, solar power systems have many benefits such as very low maintenance, long life, and easy installation.That is why the solar power industry is rapidly developing [1].Most solar panels on the photovoltaic market are crystalline or multicrystalline silicon solar cells.However, in spite of their high efficiency, these are difficult to commercialize due to shortage of raw materials, high manufacturing costs, and complicated fabrication processes.DSSCs based on a photochemical mechanism are in the limelight as a promising alternative to silicon solar cells because of various advantages such as low manufacturing costs, simple fabrication process, flexibility, and transparency [2][3][4].However, to commercialize DSSCs, there are still many shortages to overcome [5].In particular, their low efficiency limits their practical application.Much DSSC research is currently focused on this problem.In addition, research on the installation of large-scale DSSC modules is important [6].
To develop a solar controller for DSSCs, researchers need to consider the output characteristics and conduct an experiment with DSSC modules.However, the output characteristics of DSSCs depend on the weather conditions.Therefore, an assistant power system that simulates the characteristics of DSSCs and that can be controlled arbitrarily by the researcher is required in the development process for reliable experimental data.
To solve these problems, in this paper, a simple and low-cost DSSC solar simulation of discontinuous conduction mode (DCM) boost converter is suggested, showing the performance versus such factors as irradiance, temperature, shading, and connection method.The algorithm is first simulated by the DLL block linked with the external C routine, referring to the extracted data from lab-made DSSCs.The simulation program shows the I-V and P-V curves versus conditions.The proposed solar simulator consists of the power supply, DC-DC boost converter, electronic load, and digital signal processor (DSP).That system generates the output power to 100 W level of DSSCs controlled by DSP with the SS algorithm.The SS is designed for trial experiments of large scale, and the reliability of the SS is verified in this paper.

Dye-Sensitized Solar Cell
2.1.An Equivalent Circuit of DSSCs.The simple equivalent circuit consists of the current source, diode, capacitances, and the resistances.The current source generates the I PH (photoelectric current) in proportion to the irradiation of sunlight.Figure 1 shows the equivalent circuit of a DSSC.The C Pt and R Pt are the impedance through carrier transport on the surface of the counter electrode.The C ELEC and R ELEC are the impedance with regard to carrier transport through the ions in the electrolyte, and those are worked as the diode with regard to electron transfer of TiO 2 /dye/electrolyte facing surface.R TCO is the resistance through the surface resistance.R SH is shunt resistance through the back electron transfer.The capacitance elements can be ignored because the values of the capacitances are fairly large.That is why the circuit will be simplified such as current source, diode, one of parallel resistance, and three of series resistances.The Equation (1) represents the I D which is the current flowing through a diode: Equation (2) represents the I SH which is current flowing through R SH .The R TCO , R ELEC , and R Pt are incorporated as R S : The output current I O can be explained like (3).The output current is same as all current excluding I D and I SH [7][8][9]: (3)

The Characteristics of DSSCs. The characteristics of
DSSCs are changed according to the factors such as irradiation, temperature, shadowing effect, and connecting style.In this paper, the characteristics of a 1 × 1 cm 2 size DSSC unit cell are explained according to some factors.Figure 2 shows 1 × 1 cm 2 size lab-made DSSC unit cell.

The Shadowing Effect.
The shadowing effects affect the characteristics of a DSSC.When the DSSC is under the shadow, power loss is generated as heat because of reverse current flows [10][11][12].But a DSSC is less sensitive to partial shade compared with the Si solar cells.A 1 × 1 cm 2 DSSC was prepared for experiments on the shadowing effect.The metal-coated foil was also prepared to create a shadow.The shadowing effects were given to 0, 25, 50, and 75% in order.The photovoltaic performances of the cell were evaluated by a source meter unit (Model 2400, Keithley Instruments Inc.) under the standard test conditions (STC), which is the temperature of 25 • C and irradiation of 1000 W/m 2 , using a solar simulator (Sun 2000, Abet Technologies Inc., USA).Table 1 shows the characteristics of the unit cell according to the shadowing effects such as opencircuit voltage (V oc ), short-circuit current (I sc ), fill factor (FF), efficiency, and maximum power.The current is greatly reduced by increasing the shadowing rate.But the voltage is a little reduced.The power is reduced according to the shading rate.The V oc and I sc of each shading condition are considered in SS algorithm.

The Temperature and Irradiation
Effect.The DSSCs in the field operate under the variable STC due to the irradiation and temperature changes throughout the day.The change of temperature and irradiation affects the efficiency and performance of a DSSC.shows the I-V characteristics of a DSSC according to the change of the irradiation.At this time, the temperature is constant as 25 • C. The voltage is almost constant, but the current is linearly changed according to the irradiation.The maximum power point is also changed by the value of irradiation.

The Series-Parallel Connection.
We assumed that there is no loss in series-parallel connection.The value of the voltage increases in proportion to the number of DSSCs in the series connection.At this time, the current is constant.And the value of the current increased in the parallel connection.At this time, the voltage is constant.Therefore, the I-V and P-V characteristics of DSSCs according to the connection method can be simply obtained by multiplying N s (Number of series connections) and N P (Number of parallel connections) to STC data.4 shows PSIM circuit with a DLL block and the window for conditions input.Since characteristic equations of DSSC are of nonlinear form, the implementation using equivalent equations take lots of data memory size and calculation time in microcontroller unit (MCU).So, the proposed method uses only extracted data from lab-made DSSC according to the factors and interpolation method to obtain the new reference curve in any conditions [14].To simplify the process of getting I-V curve in selected new conditions, relationship variables are calculated from Figure 5:

Simulation Program
where V IrrL is the voltage of the lower range according to the selected irradiation, V IrrH is the voltage of the higher range according to the selected irradiation, V IrrN is the new reference voltage according to the selected irradiation, Irr is the selected value of irradiation, V TL is the voltage of the lower range according to the selected temperature, V TH is the voltage of the higher range according to the selected temperature, V TN is the new reference voltage according to the selected temperature, T is the selected value of temperature, V SHL is the voltage of the lower range according to the selected shadowing rate, V SHH is the voltage of the higher range according to the selected shadowing rate, V SHN is the new reference voltage according to the selected shadowing rate, SH is the selected value of shadowing rate, β Irr is the relationship variable of irradiation, β T is the relationship variable of temperature, β SH is the relationship variable of shadowing rate, and V STC is the voltage at STC.For example, if the irradiation is selected to 700 W/m 2 , the higher range and lower range are each 800 W/m 2 and 600 W/m 2 .
Finally, the new reference voltage (V REF ) is calculated by Similarly, the new reference current (I REF ) is obtained through the same procedure like (4) ∼ (8): where α Irr is the relationship variable of irradiation, α T is the relationship variable of temperature, α SH is the relationship variable of shadowing rate, and I STC is the current at STC.
After the calculation is finished according to the conditions, the collection of the results will be made.After the collection is finished, the results of simulation will be displayed at the Simview of the PSIM.

The Result of the Simulation Program. The output of
DSSCs is changed according to some factors.The SP model represents the output of DSSCs in the PSIM.The results of simulation are displayed at the Simview of the PSIM according to some factors.Figure 6 shows the I-V and P-V curves of the simulation for DSSCs.The user can obtain any output curves according to any value of the factors from SP.  device, DC-DC boost converter, electronic load, voltage conversion circuit, and DSP.The power supply device supplies the power to the entire system.The DC-DC boost converter in DCM is used to obtain the power of the DSSCs.The basic converter usually uses the hard switching which causes the switching loss and diode recovery loss when the switch is turned on and off [15].The DCM is not perfect soft switching.However, it is simple to control, only need output voltage to feedback, and is sometimes the best topology given the capacity and switching loss.That is why the DC-DC boost converter in DCM is applied in this paper [16].

Solar Simulator
The inductor of DCM boost converter was designed: where L is the value of inductor in DCM, R O is the output load, V i is the input voltage, V O is the output voltage with maximum value of 40 V, and f is the switching frequency of the converter with the value of 30 kHz.
A converter was implemented on lab-made PCB.The electronics load is used to change the output load condition.In this paper, that DSP (TMS320F28335 EVM) is used to control the current and voltage of the converter.The voltage is measured to the ADC of the DSP through the voltage interface board.A DSP compares the voltage with the reference curve of DSSCs according to the conditions such as the irradiance, temperature, connection method, and shadowing effects and generates the PWM signal to control the reference current in boost converter through the simple PI control.Finally, the converter makes the output of DSSCs and then the measurement is implemented.The output of  DSSCs can be changed according to change of the value of the electronics load as explained previously.

Experimental Results
. Figure 8 shows the I-V curve (red line) of DSSCs module under STC and the I-V curve (blue line) of DSSCs module under non-STC.The module consists of 54 series-318 parallel connection without shadowing effects.Six points from A to F are marked on Figure 8, which is the experiment target.As shown in Figure 9, there are three different wave forms.The first wave form shows the wave form of the gate (V gs ).The second and third wave forms show the wave forms of the output voltage and current.Figure 9(a) shows the wave forms of 21 V and 2.54 A, which is in Point A. Figure 9(b) shows the wave forms of 25 V and 2.44 A, which is in Point B. Figure 9(c) shows the wave forms of 30 V and 1.87 A, which is in Point C. The outputs of DSSCs are confirmed according to the DSP control with the proposed algorithm.
Figure 10 shows the wave forms of the results according to selected irradiation, temperature, and shadowing effect.This module is also DSSCs of the 54 series-318 parallel connected module.At this time, the irradiation is 800 W/m 2 , the temperature is 25 • C, and the shadowing rate is 25%. Figure 10(a) shows the wave forms of 21 V and 1.47 A, which is in Point D. Figure 10(b) shows the wave forms of 25 V and 1.43 A, which is in Point E. Figure 10(c) shows the wave forms of 30 V and 1.17 A, which is in Point F. The outputs of DSSCs under non-STC are also confirmed with the proposed SS system.

Conclusion
In this paper, the simple and low-cost SS is suggested for trial test of the DSSCs for developing adaptable power conditioning system.For the first time, the simulation program is designed to simulate the DSSCs.That consists of the DLL block which is linked with the external C routine in the PSIM.The proposed method is based on the actual data from lab-made DSSC unit cell and simple calculation using interpolation method.The SS consists of the power supply, DC-DC boost converter in DCM, electronics load, and DSP.The performance of SS for DSSC modules was verified through the 100 W level of DCM boost converter according to the factors of irradiation, temperature, and shadow effects.

Figure 1 :
Figure 1: An equivalent circuit of a DSSC.

Figure 3 (
a) shows the I-V characteristics of a DSSC according to the change of the temperature.At this time, the irradiation is constant at 1000 W/m 2 .The current is almost constant, and the voltage is changed by the change of the temperature.The voltage decreases by approximately 2.84 mV when the temperature increases by 1 • C[13].The maximum power point is also changed by the alternation of the value of voltage.Figure3(b)

Figure 3 :
Figure 3: (a) The I-V characteristic curves of DSSCs according to the temperature.(b) The I-V characteristic curves of DSSCs according to the irradiation.

Figure 4 :Figure 5 :
Figure 4: (a) The SP model with DLL block.(b) The window for conditions input.

4. 1 .
The Design of the Solar Simulator.

Figure 7 :Figure 8 :
Figure 7: The block diagram of the SS.

Figure 9 :
Figure 9: (a) The wave forms of 21 V and 2.54 A at Point A. (b) The wave forms of 25 V and 2.44 A at Point B. (c) The wave forms of 30 V and 1.87 A at Point C.

Figure 10 :
Figure 10: (a) The wave forms of 21 V and 1.47 A at Point D. (b) The wave forms of 25 V and 1.43 A at Point E. (c) The wave forms of 30 V and 1.17 A at Point F.

Table 1 :
The characteristics of the unit cell according to the shadowing effect.