Deveopment of a LabVIEW-based test faciity for standaone PV systems Aex See Kok Bin, Shen Weixiang, Ong Kok Seng, Saravanan Ramanathan and Low I-Wern Monash University Maaysia, Schoo of Engineering No.2, Jaan Koej, Bandar Sunway, 46150, PJ, Seangor Daru Ehsan, Maaysia. Te: +603-56360600, Fax: +603-56329314 Emai:{aex.see, shen.wei.xiang, ong.kok.seng, sram23, iow1}@monash.edu.my Abstract To quantify the potentia for performance improvement of a standaone photovotaic (PV) system, a test faciity has been instaed. This paper describes this deveopment of a prototype standaone PV system. Essentiay this entire system invoves the integration of a Persona computer (PC), Data Acquisition (DAQ), a battery array and a soar array simuator (SAS) to create a standaone PV system and to test and simuate the system. This new system boasts of high accuracy measurements couped with the commercia viabiity of ow cost. The basic idea of this faciity is that the SAS simuates soar power which is utiized to charge batteries. The information obtained by monitoring parameters, such as average battery s temperature, votage and current is fed to the PC via the DAQ for anaysis. This customized contro interface has been deveoped by utiizing LabVIEW software, which forms the programming backbone of inter-instrument communication via IEEE-GPIB bus. The software created for this system is highy generic and can be used for other instances where different hardware is used. This paper aso discussed further research pan, in utiizing this standaone PV system to perform oad anaysis and batteries charging or discharging with the inputs to the SAS with actua meteoroogica data obtained from the Maaysian meteoroogica department. 1. Introduction The rapid evoution of renewabe energies for sustainabe deveopment during the ast two decade has resuted in the instaation of significant amount of renewa power systems, e.g. photovotaic, wind, wave powers etc. a over the gobe. As natura resources, such as fossi fues are depeted prompty due to the huge demand of power, aternatives such as renewabe energies are desperatey needed to avoid economic breakdown and subsequenty poorer standards of ife around the gobe. With more soar array systems in the word, a free and unimited resource is tapped for practica usage, which coud be used in many appications saving imited and expensive ones such as natura gas and oi. In retrospect, the deveopment of a standaone PV system is critica in anaysis and mass production of soar array systems. A typica PV system [1-2] normay entais a battery array management and monitoring system as we, and this aso provides the framework for an accurate and commerciay viabe battery testing unit. Practica appications can be improved with the addition of a powerfu and reiabe battery monitoring system. In this aspect, the contro of the entire PV system via PC may be a cheap and viabe soution. In this work, the utiization of the PC competes a measurements, cacuations and anaysis. Firsty the user simuates a preset sequence of soar power to be generated by the SAS via the PC; the output of the SAS is subsequenty fed to the battery array. The DAQ monitors the battery data directy at reguar intervas, which is aso pre-set by the user. Using a computer directy instead of a microcontroers aows for a more controabe and fexibe operation of the system [3]. This paper is mainy structured into four major parts, with sections 2 and 3 discussing about the DAQ system, section 4 describes about the hardware setup, sections 5 and 6 describing about the operationa modes of the SAS. The discussion and future research pan are described in the ast section. 2. Data Acquisition The DAQ can measure a wide variety of readings, at the very east: votage, current and temperature. These three units are essentia for the purposes of a standaone PV system [4]. Temperature of the battery is taken as a secondary method of faut and overcharge detection in case of faiure or error in votage and current measurement. In this system, the DAQ is connected via IEEE GPIB bus interface to the USB port of the PC. This provides for ow noise and interference immunity as we as a fast means of data transfer. GPIB is used instead of the oder RS-232 due to the higher transfer rate (8Mbytes) and the fact that most industria appications in testing and anaysis are currenty in favor of the GPIB standard. In conjunction with the DAQ unit, it is the battery array. The DAQ is directy connected to the battery array via standard aboratory wiring for votage and current, and a J-type thermocoupe for temperature readings. The J-type thermocoupe is used due to its practica operating temperature range being that of 0 to 750 C with an error not exceeding 0.75%. Votage readings can be taken in as aternating current (AC) or direct current (DC) with an accuracy of 0.004%, whie current readings can aso be taken as AC or DC with an accuracy of 0.06%. The battery array consists of a pair of deep cyce NP38-12 ead acid batteries, which have a rated output of 12V at capacity of 38Ah. These specifications were found to be suitabe for the SAS used, which wi be discussed subsequenty.
For the operation of the DAQ, the user needs to specify the means, whereby the instrument communicates with the PC via the GPIB. Instrument communication, in this case, aso supports LPT (the RS-232) and the ASRL formats. In theory, any standard, which is accepted by the VISA function in LabVIEW, is perfecty usabe. Once the user specifies parameters specific to the measurement wanted, the data acquisition can commence. instrument (VI), the user has to enter the mandatory VISA resource name and channe to be scanned. However for this measurement, the user has the option to choose the type of thermocoupe to be used, the defaut being J-type. A typica LabVIEW Virtua instruments (VI) is shown in figure 3. 3. Deveopment of LabVIEW Based Contro for the DAQ 3.1 Data Acquisition: The three important measurements, which are critica to the anaysis and operation of a standaone PV system, are votage, temperature and current of the battery storage units. In the SAS unit, the user has to specify certain parameters as preset vaues so that the system knows what to do. Certainy, there are preset defaut vaues, which are present for this system but to cater for versatiity, the user has the option to change those parameters. For a three measurements, the user shoud provide the type of communication modue used between the DAQ and the computer, the defaut method being GPIB. The DAQ channe utiized for measurement has to be specified so the system knows which channe to monitor, where each channe corresponds to the physica wires connecting the DAQ to the BAU. There are up to 22 channes for this system, where ony two of them are abe to measure current due to hardware restrictions, namey channe 21 and channe 22. The hardware DAQ mode used is the Agient 34970A [5]. This wi be highighted in section 4. After receiving data from the DAQ, LabVIEW does a fast check to see if the decoded data can be considered vaid measurements (no error) and whether they are within imits of the preset vaue hard coded into the program. In the case, a vaue precedes or exceeds the imits, LabVIEW wi send a coded instruction to the DAQ to sound the aarm (For exampe, this happens in the case of battery overcharge or neary empty). Figure 1: Votage Acquisition VI Figure 2: Temperature Acquisition VI Graphica User Interface (GUI): The software for system operation of the DAQ was created with ease of use in mind. The user is prompted to enter the necessary detais. Figure 1 shows the front pane for votage acquisition, the user has to enter the communication method (under VISA resource name), channe to be scanned (under scan ist), AC or DC measurement (togge switch under AC/DC) and a rough range of votages to measure. Figure 2 shows the front pane for temperature acquisition. As in the votage acquisition virtua Figure 3: Bock Diagram for Votage Acquisition
A sampe fowchart for votage measurement is shown in figure 4, depicting a genera idea of what happens during data acquisition: Figure 5: Experimenta Set Up 5. SAS Figure 4: Fowchart for temperature acquisition Data Storage: Data acquired from measurements are automaticay stored into a preset text fie, which can be modified by the user. The data coected can be utiized for postprocessing purposes, athough rea-time data processing and anaysis is possibe. The user has the option of overwriting the data aready in the fie, or appending the fresh data to the data previousy coected. The SAS is actuay a DC power suppy that simuates the output characteristics of a soar array pane. It is used to simuate different current-votage (I-V) curves of different arrays under various conditions. Some of the conditions are irradiance, temperature and oads. The SAS has three different operating modes as foows [6]: i. Fixed Mode ii. Simuator Mode iii. Tabe mode Foowings are brief descriptions of the three different modes of operations. i. Fixed Mode This mode is the defaut mode, which is set when the unit is first powered on. The I-V characteristics for this mode is rectanguar in shape as shown in figure 6, which actuay foows a DC power suppy. This mode aso aows users to program the unit from the front pane and brings the convenience when there is a need to do certain testing, which does not need the I-V curves. 4. Standaone PV set-up Figure 5 shows the set-up that has been used in the investigation of the standaone PV system. As shown, the foowing instruments are required: i. Persona Computer ii. Soar Array Simuator (Agient E4350B) (Max power: 480 Watts) iii. Data Acquisition Unit (Agient 34970A) iv. Resistor pack as oad Figure 6: Fixed Mode Characteristics
ii. Simuator Mode The SAS s interna agorithms are used to simuate the I- V curve. The curve can be generated by keying in the foowing four input parameters: a. Open circuit votage (Voc) b. Short circuit current (Isc) c. Current at the maximum power point on the curve (Imp) d. Votage at the maximum power point on the curve (Pmp) Figure 7 shows the I-V characteristics curve generated by the simuator mode [6]: 6. Deveopment of LabVIEW Based Contro for the SAS LabVIEW is used to contro and monitor the performance of stand-aone PV system. In the LabVIEW software, there are huge varieties of ibraries. Some of the ibrary functions that are usefu are data acquisitions, data anaysis, waveform generation, arrays, ooping function for continuous operation, data storage, and reading data fies from externa source. Each LabVIEW created is caed a Virtua Instrument (VI). These VIs comprises of two segments, one segment is for user interface caed Front Pane, the other is the back end segment caed Bock Diagram which houses a the program code for execution in the form of bocks and connection paths for data fow [4]. The LabVIEW software is executed on PC and the contro signas are sent over to the SAS via IEEE-GPIB interface bus. Data coection and processing: iii. Tabe Mode Figure 7: Simuator Mode Characteristics This mode is the fastest mode compare to the earier two modes mentioned. The operation on this mode wi provide an accurate I-V simuation of the SAS simuator. Another added advantage of using this mode is that it provides 60 tabes with a tota of 33,500 I-V points of storage and a maximum of 4,000 I-V points per tabe. The tabes are easiy retrievabe from its stored ocation. About 30 ookup tabes amounting to a tota of 3,500 points are stored in a non-voatie memory. These data wi be retained when the power is switched off. This mode aows using the current and votage offsets to the seected tabe to simuate a change in the operating conditions of the soar array [6]. Figure 8 depicts the tabe mode output characteristics: Data transmission between the PC and the SAS is carried out via a Genera Purpose Interface Bus (GPIB) which aows parae communication. This device uses the Universa Synchronous Bus (USB) port of the PC. When the GPIB is pugged into the PC s USB port, an eectrica handshake wi take pace internay and the Ready signa, which is indicated by a LED is seen. After the program code was sent to the SAS, the GPIB unit wi show an Active signa indicating that the program has been successfuy downoaded to the SAS and waiting for execution. The SAS unit s address, which is a defaut vaue set by the manufacturer of the unit itsef, needs to be keyed in the bock program prior to execution of the code. When the program is executed, the data generated by the SAS wi be on rea time basis, this vaid data chain wi be channeed back to the PC from the SAS. The LabVIEW software decodes this received data, and if it is recognized as measured vaues, then it wi be stored in a buffer and aso dispayed as a graph on the Front Pane. The simuator and tabe modes are empoyed to simuate the standaone PV system. These two modes are successfuy created and executed. A fow chart is shown in figure 9, it depicts how the program wi be executed by stages and the respective data fow path: User interface and resuts: Figure 8: Tabe Mode Characteristics Figure 10 depicts how the bock program code ooks ike for the SAS contro system. Figure 11 and 12 indicate two screen shots of the graphica user interface (GUI) caed Front Pane together with the resuts obtained from the simuation [4]. Figure 11 shows the screen shot of the simuator mode GUI and its respective resut in the graph form. Figure 12 shows the tabe mode GUI and its respective resut. The important points to note for the tabe mode is that its output graph dispays two different
potting, the first pot is the curve generated from the user-defined tabe of current and votage pair array, the second pot, which overaps the first curve, is the curve generated by varying the oad. These output graphs vaidated that the impementation of the simuator and tabe mode operation were successfu. Figure 11: Simuator Mode Operation Figure 9: Program Fow Chart for SAS Contro Figure 12: Tabe Mode Operation 7. Discussions Figure 10: Bock Diagram for SAS contro The integration of the SAS and the DAQ system together using LabVIEW to create the standaone PV system has been successfu. The data from the DAQ, which measures the battery average temperatures, wi be feedback into the SAS. The SAS wi process this data and carry out the respective task such as either to continue charging the battery or to stop charging battery. Having the different I-V curves for the 12 hour operation or onger wi enabe the user to study how a particuar PV array behaves or is affected, based on the oad requirement or even the sun radiation at certain area.
8. Concusions In this paper, the rea time contro strategies were successfuy empoyed to integrate the data acquisition unit, the soar array simuator, battery and oad into the standaone PV system. The LabVIEW is used as the software to create a GUI for the data acquisition unit and soar array simuator. The preiminary testing resuts show the simuator mode operation based on the parameters of an existing soar array pane and the tabe mode operation based on a random data have been impemented. These two modes can be used to generate the IV curves which simuate the characteristics of the soar array. Furthermore, the system can monitor and measure the system parameters, such as votage, current and temperature. However, further deveopment of the system is sti needed to enhance the system functions. 9. Further Deveopment Further studies on how the standaone PV system responds to the changes ike oad requirement, battery state of charge, battery and PV array ce temperature and sun irradiance. Therefore as part of the further deveopment phase, the pan is to use the data coected from the meteoroogica department Maaysia with regards to sun radiation data and the foowing I-V characteristic equations [1], to generate the different I-V curves for ong term operation. To generate these I-V curves, two important parameters are needed to be cacuated, which is the short circuit current (I sc ) and the open circuit votage (Voc). qv kt I = I I 0 e 1 ---------------- Equation (1) where by the I is the component of the ce current due to photons q = 1.6x10-19 couomb k = 1.38x10-23 Joues/K T = ce temperature in K The above equation needs to be used to find the short circuit current (I sc ) of any PV ce, by setting the V = 0 in the equation and this wi ead to I sc = I. Based on the principa that the ce current is proportiona to the ce irradiance, therefore under a test condition, G 0 = 1 kw/m 2 at AM 1.5, then the ce current at other irradiance eve is given by equation [1]: kt I + I 0 kt I Voc = n n ------- Equation (3) q I q I 0 10. Acknowedgement This project is supported by Monash University Maaysia under project no. 2004-ES-7-04. The authors woud ike to thank the University for the financia support of this work. 11. References [1] Roger A. Messenger and Jerry Ventre, Photovotaic Systems Engineering, 2 nd edition, CRC Press LLC, Boca Raton, Foida, USA, 2004. [2] Ahmad Zahedi, Soar Photovotaic Energy Systems: Design and Use, The New Word Pubishing, Mebourne, Austraia, 1998. [3] T. Eswarakhanthan, J. Bottin, A. EL-Sassi. R and S. Raveet, Micocomputer-Controed Simuator of Photovotaic Generator Using a Programabe Votage Generator,Soar Ces, 17 (1986) 383-390. [4] Stephan Buer, Eckhard Karden, Andreas Lohner, Rik W.De Doncker Lab View-Based Universa Battery Monitoring Management System, IEEE, 12 th Int. Teecommunications Energy Conference, pp 630-635, 1998. [5] Agient 34970A Data Acquisition/Switch Unit User s Guide, Agient Technoogies. [6] Agient E4350B, Soar Array Simuator Operating Guide, Agient Technoogies. [7] Agient, E4350B Data Sheet, May 2004, Avaiabe at: http://www.home.agient.com/upoad/cmc_upoad/a/s AS_datasheet_May04.pdf (Accessed 9:05PM, 25 May 2005.) [8] B. Wichert, M. Dymond, W. Lawrance, Deveopment of a test faciity for photovotaic-diese hybrid energy systems, Renewabe Energy 22 (2001) 311-319. 0 I ( G) = ( G / G0 ) I ( G0 ) ------------------ Equation (2) For the open circuit votage (Voc) computation equation 3 need to be used and aso with setting the ce current to zero [1]: