Automatic data acquisition system for a photovoltaic solar plant

Solar & Wind Technology Vol. 3, No. 4, pp. 25%265, 1986 0741~83X/86 $3.00+.00 Printed in Great Britain. Pergamon Journals Ltd.

AUTOMATIC DATA ACQUISITION SYSTEM FOR A PHOTOVOLTAIC SOLAR PLANT

A. SANTOS, C. LOPEZ BARRIO a n d A. GARCiA ~GUERRA

Dptos Sistemas Digitales y Electr6nica, ETSI Telecomunicaci6n, Univ. Polit6cnica de Madrid, Madrid, Spain

(Received 18 November 1985; accepted 15 January 1986)

Abstract--An autonomous monitoring system for photovoltaic solar plants is described. The system is able to collect data about the plant's physical and electrical characteristics and also about the environmental conditions. It may present the results on a display, if requested, but its main function is measuring periodically a set of parameters, including several points in the panel I-V characteristics, in an unattended mode. The data are stored on a magnetic tape for later processing on a computer. The system hardware and software are described, as well as their main functions.

1. INTRODUCTION

One of the purposes of the research being carried out in several countries on photovoltaic solar energy is to obtain information on the actual plants so as to optim- ize future systems in terms of performance, reliability and cost. To fulfil these objectives an automatic monitoring system is mandatory. It should measure in situ the plant characteristics, gathering and storing information that will lead us to evaluate the plant efficiency during long periods. Environmental conditions, including, obviously, the solar insolation, should also be collected so as to know their influence on the plant efficiency.

This paper describes a system with this aim. It measures several physical and electrical plant characteristics, as well as environmental conditions, storing their results on a magnetic tape. Later calculations on a computer will allow the plotting of parameters, such as the energy produced, averaged over the hour, day, month or year, the peak power under different conditions, variation of module temperature related with wind speed, etc. The system is also responsible for the battery maintenance, switching a load to keep its charge within the adequate limits.

The system is now working in the R. Areces Solar Plant [1] developed by the Instituto de Energia Solar in the ETSI Telecomunicaci6n (Madrid) and it can be easily adapted to other photovoltaic plants.

2. SYSTEM OBJECTIVES

The parameters to be measured are of three kinds :

(1) Environmental conditions (direct and global

solar radiation, wind speed and direction, and ambient temperature) ;

(2) Physical parameters of the panel (temperature in several cells and in cooling fins, and active time of the motors) ;

(3) Electrical characteristics (battery voltage, load current and battery temperature).

All these measurements have to be acquired periodically with a period to be determined by the user. Additionally, some other data, such as the panel I - V characteristics, are also taken when the solar radiation reaches certain values.

All these results are stored on standard magnetic tape cassettes so that they can be recovered and evalu- ated by a minicomputer.

The user can also demand some data in console, so as to know the instantaneous value of any of the above parameters and to print its result. The console also allows the user to modify the measurement period and some other conditions.

One additional system function is the calculation, at sunset, of the total energy produced during the day, considering the load current measured until then. According to this result, it switches on a load (a bulb panel) for as much time as is needed to spend the stored energy and discharge the battery.

3. H A R D W A R E S T R U C T U R E

The system has two main parts : a control unit and a data-acquisition unit. The first part commands the data acquisition, processes the results to present them in an adequate format and stores them on a tape or

259

260 A. SAN[OS e l tt[.

Environment sensors

11 Conditioning I

tt .. " I

111 Conditioning

SoLar panel Motors

I

11" I " AnaLog inputs

Data acquisition unit

I0

Digital T/D

VIAs

01-

Load

1

EPROM AIM - 6 5

ConsoLe

Fig. 1. System block diagram.

in the display/printer. The other main unit in which the hardware has been divided, is in charge of the measurements, giving a certain conditioning to the signals coming from the transducers. The whole system is shown in Fig. 1.

3.1. Digital hardware The control unit, a microprocessor, in order to gain

flexibility and compactness, is in charge of the following tasks: controlling the data acquisition and conversion, communication with the user and storing the measured data in a tape. It also commands the switching of the panel load and some other conditions in the measurements. This control unit is an AIM-65 microcomputer, based on the Rockwell's 6502 CPU [2]. It has several of the components needed for this application : an interface with the user (a keyboard, a 20-digit display and a thermal printer), an interface with an audio tape, plus 4 kbyte RAM memory, sockets for ROM/EPROM memories and some digital I/O lines. In addition, this is a development system, so it allowed us to codify the software in assembly language, debug it and test the additional hardware needed.

Considering the memory requirements, the system uses 8 kbytes of EPROM to store the application

program. The 8-kbyte ROM memories that contain the AIM-65 monitor are also kept so that this monitor is available, if desired, and some of their routines like the drivers for the display, printer, keyboard and cassette can be used.

The 4 kbytes of RAM memory are sufficient for the variables used by the program and to store the data measured during 30 min. Data must be stored on the cassette every 30 min; this time being considered sufficient not to waste tape, as it would if data were stored too often (there are headers and synchronizing characters) so as not to lose too much data in case of power failure.

The system also needs, besides the analog part that will be covered in the following section, several digital input/outputs: it needs digital outputs to trigger the panel 1-V sweeping, to switch the panel load and to disconnect the plant from the battery in certain measures. Three inputs were needed : one to have an echo from the device that produces the I - V sweeping and two to measure the panel motors' active time.

The motors' active time is measured by producing pulses that reach one of two counters when the corresponding motor is active. Two more timers are used to actualize the information of the current date and time. All these needs, digital input and outputs plus

Data acquisition system for

timers, are covered with three VIAs R6522 (versatile interface adapters).

3.2. Analog hardware Regarding the analog part of the system, there are

several transducers to measure the different parameters needed, whose signals are conditioned and carried to the A/D converter, so they can be treated and stored by the processor. The signal coming from every sensor is normalized, as a voltage varying from 0 to 1 V, so that the converter can use its full range for all of them without overflows. A data acquisition unit, such as the Burr Brown's SDM 857, is used. It is a hybrid-technology integrated circuit that contains all components necessary to multiplex and convert analog signals, including an instrumentation amplifier and a sample and hold circuit, besides some control circuitry. It produces the digital signal that is treated by the microprocessor.

The transducers and sensors used to measure the parameters need to produce different kinds of signals : low level voltage variations, resistivity changes, etc. All these signals are transformed into voltages varying from 0 to 1 V. The sensors and their conditioning circuits are described below.

For the global and direct solar radiation, a pyran- ometer and a normal incidence pyrheliometer from Eppley Labs, Inc., are used. As they produce a voltage in the 0-8 mV range, their signals are treated similarly to the outputs of the thermocouples that will be seen next: they are carried through a differential channel to an instrumentation amplifier.

For the wind speed and direction, the sensors are a cup anemometer and a lightweight vane from Weather Measurements. The first one produces a d.c. voltage, that is carried directly to the converter. The second one is a linear potentiometer from which voltage changes are also obtained.

For the ambient temperature, the sensor is a linear thermistor from Meteorology Research; so a source current is used to condition the signal. It was made with discrete components and it also includes a linear amplifier (with two operational amplifiers).

To measure the temperature in different parts of the panel and in the battery, several copper-constantan thermocouples are used. They produce a low level voltage that varies from -1 .2 mV to 5 mV in the expected range of temperatures (from - 10 to 90°C). To measure these small signals, they are carried to an instrumentation amplifier with differential inputs, so that the common-mode noise will be rejected. The signal wires are twisted and a low-pass filter (with cut frequency of 0.8 Hz) is included for each thermocouple. The amplifier is a high accuracy one (AD 522) ;

a photovoltaic solar plant 261

a gain of 100 has been chosen and a d.c. level shift of 1.3 mV at the input is forced, so that its output is always positive as the A/D converter requires. Pre- viously, the signal from every thermocouple is passed through two eight-channel multiplexors, working as an eight-channel differential input system, as can be seen in Fig. 2.

To complete the thermocouple measurement system, all thermocouple wire pairs terminate in a connector strip whose temperature is monitored to allow cold junction compensation [3] for all of them. This reference temperature is obtained with the AD 590, a temperature transducer that produces an output current proportional to its absolute temperature. Its conditioning circuit is just a resistor, to produce voltage variations, plus an amplifier made with two operational amplifiers.

The electrical parameters are investigated as described below. The panel is connected as shown in Fig. 3. The panel voltage is measured between points 1 and 2 with a voltage divider. The load current is obtained by measuring the voltage in an SH200 shunt. This voltage is amplified as close as possible to the shunt and carried with a coaxial wire. The battery voltage is also measured, as it may differ from the panel voltage when they are disconnected, as hap- pens during the null insolation periods.

The panel ~ V characteristics is measured with power circuitry, represented in Fig. 3 as a transistor, that is a variable load that carries the panel from open circuit to almost short circuit; our system starts this sweeping and measures the adequate current-voltage pairs during it.

All these analog signals are carried to the SDM- 857, using its 16-channel multiplexor, the internal instrumentation amplifier, with a gain of 10, and the sample/hold circuit. The last step is the high-accuracy converter producing a 12-bit output.

To minimize errors, there is a method to com- pensate for any offset voltage in the circuit : when any measurement is taken, a short-circuited channel is also measured; the data measured in this last channel is the actual offset voltage, so it can be subtracted from the requested channel data. To allow this offset compensation, there is a differential channel with its inputs short-circuited and a single-ended channel with its input to ground.

The instrumentation and conditioning circuits used allow the achievement of the accuracies recommended in [4] for every parameter. However, the user should check them periodically, and calibrate the several adjustment potentiometers provided.

From the microprocessor, the data acquisition unit acts as several memory positions (memory map

262 A. SA:~ ros ~'1 a/.

C C

u

O

MI i

MUX

I.A

MU×

2 03

m- CS 4 Latch

"°teh I AB3..ABO

AB7..AB4

Fig. 2. Analog channels interface.

I .A.

A/O converter

addressing). Every analog channel is selected addressing (for reading or writing) a determined address. This selection also activates the sample/hold circuit and, some microseconds later, the A/D converter. To minimize the' decoding circuits, four bits in the addresses bus are used to select a channel in the SDM multiplexor, and another four bits to select a differential channel in the external multiplexors. Reading another memory position, eight bits of the converted data are obtained and, in another position, the

remaining four bits plus the converter status bit, that signals the end of conversion.

4. S O F T W A R E S T R U C T U R E

The application software is organized as a monitor program which allows the user to execute from the console several commands, the most important being the "automatic program", that takes care of the system measurements in the unattended mode.

®

SH200 I I

0.3 m&2

Control i Control " I

/ ^ / ^

l ® 2b 'ive ttery

Fig. 3. Photovoltaic plant, battery and load.

Load I

®

D ®

Data acquisition system for a photovoltaic solar plant

There are also some internal routines, called by these commands, that take care of the interface with the data acquisition unit and of the data conversion to obtain results in the particular units for each parameter.

4.1. Monitor program and commands The interface with the user is the monitor program.

It simply explores the keyboard, and when it finds one of the keys D, P, M, T, L, S, / or A pressed, it passes the control to the corresponding command routine.

This monitor takes control when the ESCAPE key is pressed and when a RESET is produced. In this case it waits for 30 s for the user to press any key. If this does not happen, it is assumed that a power failure occurred in the unattended mode, and the "automatic program" is executed, initializing several parameters by default.

The commands the user may execute are the following :

Command D: to display and/or change the date and time.

Command P : to print the date and time. Command M : to take a measurement of any par-

ameter, indicated by a code the user must type. The result is presented on the display or printer with a text that identifies the parameter and the corresponding units in which the result is presented. This command also allows the measurement and printing of the value of several pairs of current-voltage of the panel I - V characteristics.

Command T : to periodically measure and print up to five different parameters. It is similar to command M, except that the requested parameters are measured several times.

Command L: to measure a parameter once every second and display its result.

Command /: to take up to seven parameters' measurements once a second, and every 10 min print their mean values in the period.

Command S : to exit to the AIM-65 monitor. Command A : this is the "automatic program" that

will be seen in the next section.

4.2. Automatic program In the unattended mode, the system takes period-

ically a set of measurements and stores their results on the tape.

There are two modes of functioning, a diurnal one and a nocturnal. During the day, with the period indicated by the user, it measures the following parameters: direct and global solar radiation; mean, maximum and minimum wind speed; mean wind direction ; ambient temperature ; temperature at eight

263

points in the panel, battery voltage, panel load current and active time for each motor. Additionally, five times a day, when the global solar radiation reaches five critical values, the panel I -V characteristics is also measured. At any moment, the user may print the last values measured and the energy produced until then. This routine flowchart is shown in Fig. 4, including some more details about its operation.

When the global solar radiation decreases below 5 W m 2, sunset is assumed and the nocturnal routine takes control. This one first calculates the total energy produced during the day, according to all the data obtained about load current, and determines the time the panel load has to be switched on to discharge the battery. During the night, several paramerers are also measured once an hour and at the load switching on and off times. These parameters are the battery voltage and temperature, the mean and maximum wind speed and the mean and minimum ambient temperature. In the case of eclipse, the system will be operated normally.

The recommendations found in [4] regarding the sampling and recording intervals are not fully met at the moment, as this standard was not published when the system development was started. For this reason the wind direction is not stored with the accuracy recommended in that reference (_+5%). However, only minor software changes will allow the system to meet the proposed intervals and accuracies for every parameter.

4.3. Data acquisition routines There are two basic routines that obtain the binary

data corresponding to a data measurement in a single- ended or a differential channel. They previously measure the short-circuited channel and compen- sate for the existing offset voltage. They take care of addressing the multiplexors, waiting for the end of conversion and reading the result.

When the binary data is obtained, it must be converted as decimal data in the corresponding units (volts for voltages, m s 1 for wind speed, ~C for temperatures, etc.). For most parameters, there is a linear relation between the voltage measured and the corresponding physical parameter. So just a product, and in some cases, an addition has to be performed to obtain the data. These arithmetical operations are calculated working with integers in double precision (16 bits). The linearity factors have been scaled con- veniently so that the results may be contained in 16 bits. The last process to present the data is to convert these results into four ASCII digits. The scale factor is also taken into account to include the decimal period in its appropriate position.

264

l Reset l

Initialize J parameters I

A. SANTOS et al.

J InitiaLize parameters I "!

I DispLay time

Wind measurements

Yes

I ./'-Vchart measurements

I Dump to cassette

I

m

I r N, ht /

Fig. 4. Automatic program flowchart.

I J Periodical measurements

Dump to cassette

I

A particular case is the thermocouples, as there is no linear relation between voltages and temperatures. However, the relation has been decomposed into seven zones in which linear factors are applied with min imum errors. The reference temperature is also measured; another linear relation gives the corresponding cold-junction compensation voltage, which is subtracted from the measured one.

To obtain the panel I - V characteristics, five oper-

ations are carried out : (1) the battery is isolated, the offset voltage and current are measured and the sweeping is started ; (2) the maximum current is determined, by taking continuous current measurements to locate the beginning of its decrease; (3) the current interval between pairs of measurements is calculated, to obtain the requested number of points evenly spaced; (4) pairs of current voltage values are measured and stored and (5) data measured are con-

30

27

24

J 9

6

Data acquisition system for a photovoltaic solar plant

Ponek I - V chorocteristics

++ ++ +++++

I I I 0 2.5 5 7.5

#+++

I0 12.5 15 17.5 20 22.5 25

Voltoge ( V )

265

Fig. 5. Typical I -V panel characteristics.

periodically takes a set of measurements and stores their values on a tape. It also monitors the energy produced during a day and during the night it switches on a load for the time required to discharge the battery. Another function is the measurement of the panel I - V characteristics, taking pairs of voltage current values.

The system has a great flexibility and allows us to have a complete set of data on the functioning of the solar plant during long periods of time, without any human intervention, allowing the evaluation of its performance and the external factors that influence it. An estimate has also been made of the cost of the system and it was found to be about 4% of the total plant cost.

verted to ASCII values, and printed or stored in the cassette. The maximum power is also calculated among all the current-vol tage pairs measured. Figure 5 shows the data obtained in a typical measurement.

5. CONCLUSIONS

An automatic and manual measurement system for a solar plant has been developed. It measures and presents in a display or printer the value of several physical and electrical parameters under command by the user. It also has an unattended mode, in which it

REFERENCES

1. G. Sala, G. L. Arafijo, A. Lugue, J. M. Ruiz, A. Coello, E. Lorenzo, F. Chenlo, J. Sanz and A. Alonso, The Ram6n Areces concentration photovoltaic array. Sih, er Jubilee Conyress of the ]SES. Atlanta (1979).

2. AIM-65 User's Guide. Rockwell International. Anaheim, CA (U.S.A.) (1978).

3. Manual on the Use of Thermocouples in Temperature Measurement. American Society for Testing and Materials, Philadelphia, PA (U.S.A.) STP 470A (1974).

4. F. C. Treble, J. M. Tubcke and K. Trader. Rec- ommendations for data monitoring and processing. Photovoltaic power generation. Proceedings of the Final Design Meetin9 on EC Photovoltaic Pilot Projects. D. Reidel, Brussels (1981).

Documents

Automatic data acquisition system for a photovoltaic solar plant