field measurements of pv module performance using a handy tool

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of the performance of PV panels, using a very handy tool, for evaluation of the ... Keywords: PV modules testing, field tests, field evaluation, data acquisition system ... Since sunlight is an intermittent energy source, PV modules have to operate ... 25°C. The open circuit voltage Voc depends logarithmically on the irradiance ...
FIELD MEASUREMENTS OF PV MODULE PERFORMANCE USING A HANDY TOOL 1

A. Maheshwari1, C.S. Solanki1* and V. Agarwal2*

Department of Energy Systems Engineering, IIT-Bombay, Powai, Mumbai-400076 *1Corresponding author: Phone: +91-22-2576-7895, Fax: +91-22-2572-6875, E-mail: [email protected] 2 Department of Electrical Engineering, IIT-Bombay, Powai, Mumbai-400076 2 * Corresponding author: Phone: +91-22-2576-7422, E-mail: [email protected]

Abstract Si based PV technology is now matured and operating life of PV module in the range of 20 to 30 years is guaranteed. During this period, many times, it is required to find out the health of the PV module in the field in order to estimate the performance degradation after certain time period. This paper is concerned with field testing of the performance of PV panels, using a very handy tool, for evaluation of the effect of varying atmospheric conditions on device performance, i.e. to reveal the characteristics of PV modules in actual conditions. The issues that are of primary interest in these field tests are the module ratings, the modules' response to extreme weather conditions, and the possibility of performance degradation during long-term outdoor usage. The paper describes the preparation of the measurements including realization of the entire measurement system, which is in form of an automatic small size, easy to carry and usable handy tool. The most important part of the measurements is the IV-curve readings, and the construction of a functional IV-measurement system. Other part of the work includes the selection of a functioning sensor lineup and the programming of the data acquisition system (DAS). Environmental measurements are planned in a way that it facilitates the quality control of the measured IV-data as well as general analysis of the meteorological variables. Keywords: PV modules testing, field tests, field evaluation, data acquisition system

1.

Introduction

Renewable energy can play an important role in meeting the ultimate goal of replacing large parts of fossil fuels. One of the promising applications of renewable energy technology is the installation of PV systems to generate power without emitting pollutants while requiring no fuel. Despite relatively low power density of solar energy, this resource could potentially satisfy the global energy demand on its own. Increasing efforts are directed towards reducing the fabrication and installation costs and enhancing the performance of PV systems so that these systems can be deployed at a larger scale. PV modules can be designed for a variety of applications and operational requirements. A reliable PV module should perform its intended purpose adequately for 20 to 30 years, under the operating conditions encountered. But the PV module output performance is not independent as it varies over extended periods as well as depends on varying atmospheric conditions from the output rating quoted by the manufacturer, which is measured in the laboratory at Standard Test Conditions (STC, i.e. the solar spectrum AM1.5, 1000 W/m2 solar radiation intensity, 25°C module temperature and 2 mph wind speed) [Malik AQ et al., 2003]. So it is important to obtain knowledge how much energy these PV modules produce in different conditions, and how well they maintain their performance during longer periods of time, which is possible by testing of these modules. In literature, field experiments are performed by many people, giving examples of the changes in the PV performance after a certain time interval of outdoor light exposure [Malik AQ et al., 2003, Mitchell KW et al., 1986]. However, the test results are site-dependent as well as subject to the period of investigation. Thus, these data should be transformed to different situation with respect to meteorology (i.e. STC) in order to predict the time evolution. Therefore, a handy tool, which can perform these tasks, is proposed in this paper. The motivation of this paper is to allow one to find out the health of a PV module at any time in the field at a very cost-effective rate and to gain extensive field-experience from the modules operating outdoors in various operating climates, with the help of an automatic handy tool, which will be able to measure and display the instantaneous performance of the modules. The instantaneous performance of PV modules is characterized by module output parameters, module temperature, and meteorological data simultaneously [Malik AQ et al., 2003,

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Green et al., 1982]. This paper describes the information required about the variables which should be measured, the configuration of the measurement system, the method to derive relevant information from the raw data with statistical means, etc for testing the PV module health in the field at any given time.

2.

PV Module Parameters and Their Dependencies

Since sunlight is an intermittent energy source, PV modules have to operate under conditions that vary a lot. This places certain restrictions on their use, because they cannot produce energy at a constant rate and the power delivered at a certain instant is still very much a function of the weather conditions at hand. Two critical parameters are the solar irradiance and the temperature of the module. The other parameters that also affect the performance are wind speed, module degradation and module aging [Green et al., 1982, Gxasheka et al., 2005]. The short circuit current Isc is proportional to the irradiance on the PV module. This Isc rises with increasing temperature, as the rise is less than 0.1% per ºC, though the standard temperature for reporting Isc is usually 25°C. The open circuit voltage Voc depends logarithmically on the irradiance and decreases at a faster rate with rising temperature (-2.3mV per ºC) than the Isc increases. Therefore, the PV modules’ maximum power decreases with rising temperature, as the decrease is 0.4% per ºC and the module efficiency also decreases. Use of the modules over extended periods also affects the module performance as the output degrades with ageing of the modules [Dixon et al., 1978]. Voc and Isc depend on parameters like temperature and irradiance, and the dependency is shown in the following equations [Green et al., 1982]:

VOC =

kT ⎛ I SC ⎞ ln ⎜ ⎟ q ⎝ IO ⎠

I SC = bG

(1) (2)

Where Io is the saturation current, q is the electronic charge, k is the Boltzmann constant, T is the absolute temperature, G is incident light intensity, and b is a constant, depends on the properties of the semiconductor junction, the geometry of the detector and the size of the collector area. With the known dependency of Voc and Isc on the parameters like temperature, irradiance, wind speed, it would be possible to estimate the values of PV module parameters at any environmental condition. This feature can be built-in in the proposed handy tool and would enable to estimate the status of a given module in different times of day or different seasons, even though measurement is performed just once.

3.

Description of the Proposed Measurement Tool

The measurement system for PV modules described in this paper consists of sensors for measuring both environmental and module parameters, a data-logger for data acquisition, and a display unit to display the measured values instantly or there can be an option for storage in which the read values can be stored permanently. The whole measurement process is automated so that the measurements can be conducted for prolonged intervals. The sensors are calibrated to ensure that they give correct values. Simply assuming that sensors give the right value can be very disadvantageous, because faulty values are hard to spot afterwards in the data analysis stage. The features that are taken care of in this tool are that it should be compact, should meet the technical requirements of the measurements, and satisfies the budget constraints.

Figure 1: Basic block diagram of the measurement system [Dalimin, 1987]

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Figure 1 shows the basic block diagram of the proposed measurement system [Dalimin, 1987]. Data from the PV module and the meteorological data are collected through various sensors and electronic circuits. All these data is in analog form and converted into digital form by an Analog to Digital Converter (ADC). A multiplexer selects one data at once to convert into digital format and display. For displaying purpose, a 16 character x 2 line display LCD unit is used. All the electronic units and circuits are controlled and programmed by an 89C51 Microcontroller unit.

Figure 2: Basic schematic diagram of the measurement system Figure 2 shows the basic schematic diagram of the proposed measurement system. It also gives an idea about the structure of the measurement system. In a block, shown by dotted lines in Figure 2, the electronic circuitry part and the displaying unit is situated, and this is connected to the panel and the sensors via wires and cables.

3.1

The Variables to be Studied in Field Experiments

PV-related variables can be grouped into module variables and environmental variables. Module variables include the parameters related to the IV-curve (ISC, VOC, VMPP and IMPP), the module temperatures and the power output of the module [Green et al., 1982, Gxasheka et al., 2005]. On the other hand, the most common environmental parameters include ambient temperature, global irradiance, as well as wind speed and wind direction. The following sections contain brief descriptions of why these parameters are important for the operation of PV modules, and how they are measured. 3.1.1

Module Temperature

PV module parameters are sensitive functions of module temperature. The real significance of measuring the temperature of the modules is to evaluate how it changes with irradiance, wind speed and wind direction, or ambient temperature. Module temperature can change relatively quickly if the weather conditions change, so it has to be monitored on a continuous basis. Since the actual temperature inside a PV module cannot be measured, the temperature sensors are usually attached directly to the back surfaces of the modules. The measured temperature is not quite equal to the temperature inside the module, but it provides a good starting point for an estimate. One of the most popular sensors used for this purpose is the thermistor sensor, which provides a rugged and inexpensive choice for sensor. The fundamental property of a thermistor (dependence of resistance on temperature) is used to convert temperature into voltage. Amplified and calibrated output of the thermistor circuit in is send to microcontroller after multiplex and digitization for displaying purposes. 3.1.2

Solar Irradiance

A standard solar module circuit is used to measure the global irradiance. Photodiodes have the unique property that the output current, i, is directly proportional to the incident light intensity G expressed in W/m2. Mathematically this is shown in equation (2). Knowing the factor b, suitable electronic circuit is used to convert

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the short-circuit current to voltage, amplify and calibrate the signal and convert the solar radiation measurements into binaries digits and the result is displayed by the LCD unit [Benghanem et al., 1997]. 3.1.3 IV-curve Parameters IV-curve parameters contain information of the electrical characteristics of the modules. The most important parameters are ISC, VOC, VMPP and IMPP. These parameters are used directly in characterizing the performance of the modules, and degradation of module performance is also shown through them. The actual measurement of IV-curves requires the module to be connected to an electronic circuit in which the load changes at a certain rate with the help of microcontroller control. The acquisition of voltages is obtained directly on each module while the currents are measured with shunts with a little energy consumption. The module power output is obtained just by measuring the voltage and current of the module and the required multiplication task is done in the microcontroller. By gathering most of the data points close to the load value that corresponds to the maximum power point on the IV-curve, the MPP can be determined more precisely. The maximum power is determined using the hill climbing algorithm, which is shown in a flow chart in Figure 3. The instantaneous power is estimated and a step-by-step search maximizes the solar panel power transfer. The lastly measured powers readings are averaged, the power variation is compared with the last variation in order to decide if the duty-cycle must be increased or reduced. When the power variation is close to zero, the duty-cycle of the converter circuit remains in its optimum value since the maximum power has been attained [Simaes et al., 2000]. Figure 3: Hill climbing algorithm structure [Simaes et al., 2000] 3.1.4 Other Variables of Interest Other variables that one may choose to observe are wind speed and direction, UV-radiation, the spectrum of sunlight, and the air moisture level. Wind speed plays an important role in determining module temperature, since convective heat losses at the surface of the module are much larger at high wind speeds. Wind direction has to be related to module placement to determine how strong this effect is at any given time. A rotating wind speed sensor is called an anemometer that outputs electrical pulses at a rate that is proportional to its rotation frequency, and wind direction is measured with a windvane, whose output voltage depends on the direction it points to.

3.2

The Data Acquisition System (DAS)

The DAS consists of an Analog to Digital Converter (ADC), a multiplexer and a microcontroller unit interfaced with the various measuring circuits and the displaying unit. Figure 4 shows the basic flow chart and Figure 45 shows the state diagram of the DAS [Eftichios et al., 2003, Mukaro et al., 1999]. Figure 4 and 5 shows that when power is first applied or a reset is signaled, the first state entered is the Initialize state. This state ensures that all internal variables have a defined initial value and that the input/output lines are properly configured. The system then goes into the Wait state. In this mode, the oscillator remains active to keep track of time but the system does nothing except to wait for the interrupts. Instruction execution is stopped, internal power consumption is decreased, however, and internal RAM contents are preserved. The program then starts the timer and reads for any output device to be connected to the data acquisition system. If it is not connected the timer awakens the system from the Wait mode. A set of readings is then taken and stored after which the DAS goes back into the Wait state to wait for another data acquisition and storage cycle. If display device is connected, the system makes a transition into the Display mode in which the data is send to that device. When the system start time has been entered, data and reference voltage corresponding to this start time is immediately sampled and stored. After taking this initial set of readings, the DAS goes back into the Wait state to wait for another data acquisition and storage cycle. After this state, the microcontroller goes into the Measurement state where the system increments lapse time and lights an LED to indicate that samples are being

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taken. In this Measure state, reference voltage is sampled and A/D converter readings from the sensor are taken and averaged. If the data from sensor is less than a predefined value, the system assumes it is night time and does not record this data. This is done to save memory. Only lapsed time is recorded, and the data acquisition system returns to the Wait mode. The system repeatedly sleeps, awakens and keeps track of time until the data are valid. If the sampled value is above the predefined value, the system goes into the Store state where the data are written in the external EEPROM chip. Upon completion of data storage, the system switches off the LED to indicate that the acquisition and storage processes are complete. It then returns to the main program where it will go back into the Wait mode again to wait for the next data acquisition [Mukaro et al., 1999].

Figure 4: Basic flow chart of the DAS [Eftichios et al., 2003]

4.

Figure 5: State diagram of the DAS [Mukaro et al., 1999]

Current Status and Future Work

Sensor circuit realizations for temperature, solar radiation, and I-V curve with their interface with the microcontroller and displaying circuit are underway. The devices and the circuits have been decided. Some of the measuring circuits has been discussed and implemented but yet to be calibrated and tested. The monitoring of different variables will be performed in future for some period to pursue the goal of monitoring the behavior of the module parameters as functions of the environmental variables.

5.

Conclusion

The motivation of the paper is to allow one to know the health of the PV module in field and monitor their performance over extended period of time. To achieve the objective, the proposal for a DAS in form of an automatic handy tool for PV modules field testing is described. This automated field-testing tool will allow evaluating, analyzing, and testing PV modules under actual operating conditions in the field itself and will show the interdependence between the electrical parameters of a PV module and the environmental parameters and the need to take modules in laboratories for testing purposes will be eliminated.

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