Cost-effective Energy Monitoring of Domestic Off-grid PV Systems

doi:10.4236/epe.2013.54B035

Paper Menu >>

Journal Menu >>

Energy and Power Engineering, 2013, 5, 182-188

doi:10.4236/epe.2013.54B035 Published Online July 2013 (http://www.scirp.org/journal/epe)

Cost-effective Energy Monitoring of Domestic

Off-grid PV Systems

Arthur James Swart, Ruaan Schoeman, Christo Pienaar

Institute of Applied Electronics, Vaal University of Technology, Private Bag, Vanderbijlpark, South Africa

Email: [email protected]

Received September, 2012

ABSTRACT

Domestic off-grid renewable energy systems have become common place in many areas of the world, as humanity

seeks to keep abreast with global technological changes and advancements. This paper aims to present a cost-effective

energy monitoring system which may be used to analyze and evaluate the operation of a domestic off-grid PV system.

Parameters which are monitored include the output voltage and current from a 55 W polycrystalline PV panel. The

output voltage and current from a power regulation circuit (which could be a DC-DC converter, solar charger or MPPT)

is also monitored with this singular system which incorporates a data logging interface circuit, a data logger and a per-

sonal computer.

Keywords: Off-grid PV System; Data Logging; Current Sensor; Differential Amplifier

1. Introduction

“Most people spend more time and energy going around

problems than in trying to solve them” [1]. These words,

by Henry Ford, well illustrate the inefficiency of trying

to solve existing problems by finding ways to circumvent

them. A current major environmental challenge which

exists is climate change [2], a challenge that cannot be

circumvented as a possible solution. The challenge needs

to be tackled head-on.

Fortunately, many nations have started to enforce

policies and regulations surrounding the reduction of the

global carbon footprint, which could, in the long run,

mitigate climate change. For example, in December

1997 the Kyoto Protocol was developed in which 160

nations signed an agreement in order to cut or reduce

carbon emissions through carbon taxes and the Clean

Development Mechanism (CDM) [3].

Many governments are also investing huge sums of

money into searching for alternative energy sources as a

means to replace current fossil fuel and nuclear energy

power plants. The worldwide venture investment in

“green technology” companies totalled US$ 2.57 billion

for the first quarter of 2011[4].

Subsequently some suggest that renewable energy

could supply as much as 77 percent of the world’s energy

demand by 2050, which would equate to a reduction of

one third of greenhouse gas emissions compared to busi-

ness-as usual projections [5]. It is predicted that solar

power will become the dominant energy source in the

future (see Figure 1). A branch of solar energy research

that has received worldwide attention is the photovoltaic

(PV) renaissance [6].

Solar power generation through PV arrays (strings), is

arguably the most eco-friendly, emission free and sus-

tainable source of energy known to man [7]. Many com-

mercial systems for domestic off-grid applications cur-

rently exist which could be purchased and installed within

a matter of hours. However, these systems rarely incor-

porate some or other type of monitoring, whereby the

user could analyze and evaluate not only the energy

consumption, but also the energy supplied [8].

Figure 1. Energy forecast according to the Scientific Advi-

sory Board [9].

A. J. SWART ET AL. 183

Numerous reasons for energy monitoring have been

documented [10-13], and include verification of energy

usage, determining where energy can be saved, avoiding

maximum demand loads, increasing reliability of energy

supplied, verifying mathematical calculations of energy

consumption, detecting faults and enabling effective in-

tervention to minimize possible energy losses.

The purpose of this paper is to present a cost-effective

energy monitoring system for domestic off-grid PV sys-

tems. It will firstly review some commercially available

data loggers and their required interfaces. The practical

setup used to validate the performance of the data logger

equipment is then provided. Results and discussions fol-

low with some succinct conclusions.

2. Data Loggers

A variety of commercial data loggers can be found via

the Internet today. Just one search on Google for “data

loggers” revealed over two million hits, while Google

Scholar presented 27 700 hits. Noteworthy too is the

number of research articles on IEEE Xplore which fea-

ture the words “data loggers”, being 354 in total [14].

However, only six (6) articles from the IEEE Xplore

consider data loggers in the PV environment, with one of

these six articles referring to a PICO based logger [8].

A data logger, or energy meter, should be cost-effec-

tive, user-friendly, easily and quickly accessible and re-

liable. Three commercially available data loggers in

South Africa were reviewed as possible candidates for

the cost-effective energy monitoring system. These three

data loggers are given in Table 1 with selected advan-

tages and disadvantages.

The PICOLOG 1012 is manufactured by PICO Tech-

nology in the United Kingdom [15] and is available from

two local approved distributors in South Africa [16,17].

They are currently the most cost-effective loggers avail-

able (12 single-ended input channels with a maximum

input voltage of 2.5 V), but need to be permanently con-

nected to a personal computer (PC) to enable data storage

and analysis. A general purpose notebook (price of

around Є400 in South Africa) is the more logical choice,

as it is portable, compact and provides battery backup in

case of power disruptions.

The DAQPro 5300 is manufactured in the United

States (USA) by Fourtec Fourier Technologies [18], and

is available from a number of local suppliers in South

Africa. They are modestly priced and do not require a

permanent connection to a PC for logging purposes.

They are limited to eight analogue input channels with a

maximum input voltage of 10 V.

The CAMPBELL Scientific CR800 is the Rolls Royce

of data loggers and is manufactured in the USA [19]. It

has six single-ended analogue inputs with a maximum

input voltage of 5 V. It can store data over an extended

period of time and be remotely activated for data

downloads.

Due to the advantageous listed in Table 1, the PI-

COLOG 1012 was selected as the preferred choice for

the cost-effective monitoring system.

3. PICOLOG 1012 Setup

The PICOLOG 1012 is relatively easy to install with the

supplied software. The user interface comprises the PI-

COLOG PLW Recorder which needs to be setup for data

to be recorded directly to the hard drive of a PC. The

main window of this software program is shown in Fig-

ure 2, while the parameter window is shown in Figure 3.

Real time graph analysis of the results may be viewed

instantaneously, while comments or notes may be added

to each individual log file. The spread sheet button may

be used for data transfer to other software packages such

as MS EXCEL or WORD.

Table 1. Data logger summary.

Data loggerAdvantages Disadvantages

PICOLOG

1012

 Relatively cheep (Є200)

 Very user friendly inter-

face

 Repetitive unlimited sam-

ples with unique file

names

 Real time mathematical

calculations

 12 input analogue / digital

channels

 2 output digital channels

 Remote download

 Time stamp per sample

missing

 Must be permanently

connected to a PC

DAQPro

5300

 User friendly interface

 Onboard memory for

medium data storage

 Modestly prices (Є800)

 8 input analogue

channels

 Limited number of

samples

 Mathematical

calculations after data

collection

 One alarm output

reduces input channels

by 1

 No remote download

CAMPBELL

Scientific

CR800

 User friendly interface

 Onboard memory for

extensive data storage

 Remote download

 4 output digital channels

 Resistance measurements

 Pulse counter

 Very expensive

(Є1000)

 6 single-ended

analogue input

channels

 Mathematical

calculations after data

collection

A. J. SWART ET AL.

184

Add note or comm ent

Real time gr aph analysis

Spread sheet for data transfer

Unique channel / unit labels

Samples per time / count

Personal directory / log file name

Figure 2. PICO 1012 software main window.

Scaling data to true values

Alarm parameters, activation and outputs

Unit values and display

Figure 3. PICO 1012 software parameter window.

The log file can be saved to a personal directory with a

specified file name. This file name should have a digit

value at the end which will automatically be increased by

1 for each repetitive log saved to the PC. Sampling can

be either per time (maximum time for measuring and

recording) or per count (set a maximum number of

counts). The sampling time interval must also be stipu-

lated, and must never cover more than a few days in or-

der to avoid data file corruption.

Up to 10 channels may be used for recording (units are

stipulated in the channel setting screen), while an unlim-

ited number of channels may be used for basic mathe-

matical calculations using the recorded data. A bonus of

this software is its alarm capability (see red light next to

DC voltage) which can be used to alert the user to possi-

ble faults or undesirable conditions. It may also provide a

digital output signal via the PICOLOG 1012 to an exter-

nal alarm or switching circuit.

The PICOLOG 1012 has the disadvantage of not in-

cluding a time stamp per sample. However, a time stamp

is included for the final sample within a given log file.

The process of determining the individual time stamps

for previous samples within the log file proves tedious

and laborious.

4. Data Logging Interface

The average energy consumption for domestic homes can

vary between 2 kW/h and 16 kW/h per day [20-24] de-

pending on appliance usage, household size, occupancy

profile and seasonal variation. The maximum load pro-

file would require a 16 kW DC-AC inverter, such as the

VICTRON QUATRO 24/8000/200 available from Cur-

rent Automation in South Africa [25]. This inverter has a

maximum input DC voltage of 66 V, which is inline with

other research reporting on output PV arrays or string

voltages for domestic usage [20]. However, it is possible

that these PV voltages could be as high as 400 V, but this

would rarely be found in general domestic applications

where the roof structure will not allow for large numbers

of PV panels. Considering the 66 V parameter then re-

veals that the data loggers mentioned above will not be

able to accommodate these high voltages, as most of

them have a maximum input voltage of 2.5 V. A data

logging interface (DLI) circuit is subsequently required

to condition the voltage to make it less than the maxi-

mum input voltage required by the data loggers. The DLI

must also be able to provide DC current monitoring using

hall-effect current sensors. The advantages of the DLI

used in this research include the following:

 Internal battery backup for current sensors;

 Generic use for solar chargers, MPPT’s and DC-DC

converters;

 Dual channel monitoring on one-board;

 Current sensor interchange-ability according to current

demands;

 Easy calibration using a multimeter; and

 Protection for data loggers that have a maximum input

voltage of 2.5 V.

The circuit diagram for two channels is shown below

in Figure 4. Channel B is used to monitor the voltage

and current from the PV panel to the MPPT, while chan-

nel A monitors the same parameters from the MPPT to

the battery. Points 1-9 and A-H are the connection points

A. J. SWART ET AL.

185

for the hall-effect current sensors, which could be either

the LTS 6-NP (LEM product) or the ACS756 (AL-

LEGRO product) (see Table 2 for selection). The inline

fuse ensures safety in the case of a short circuit and is

highly recommended for PV system installations on ei-

ther side of the power regulation circuit (being the MPPT).

The additional on-board DC-DC converter provides an

additional power source for circuits requiring isolation

from the main power regulator (certain data loggers re-

quire an isolated power source). The battery serves a

two-fold purpose, being a power source to the load dur-

ing the night and a constant power source to the current

sensors.

It is important to note that the negative rails on the

circuit board may not be connected to a common ground

point. This is due to the fact that most MPPT’s have a

common positive rail. A differential amplifier is there-

fore required to provide a common ground point required

for data logging (achieved using the operational amplifi-

ers TL072).

Voltage calibration was performed using a 55 W poly-

crystalline PV panel, a 12 V 12 Ah battery and a digital

multimeter. The battery was initially connected to the

output of channel A on the DLI (X8). The battery termi-

nal on the MPPT was connected to the input of channel

A (X5), with no load being attached. The differential

amplifier produces an output voltage which is reduced by

R3/R10 to give a voltage below 2.5 V. The output of the

DLI circuit was then measured at point X2-3 (0.970 V)

which becomes the input to channel 1 in the PICOLOG

1012. This corresponds to a true battery voltage of 12.81

V (measured with the multimeter). The output of the DLI

circuit for channel B was also measured (point X3-3

giving 0.5 V representing a true value of 0 V; these val-

ues were assumed to be the same for channel A).

Figure 4. Circuit diagram of the DLI.

Table 2. Hall-effect current sensors.

Sensor number LTS 6-NP ACS756

IMAX ±19 A ±50 A

Cost Є10 Є6

Sensitivity 104 mV/A 40 mV/A

Advantage/Disadvantage Finer resolutions based on wire taps

Greater accuracy for smaller currents

Rectangular mounting bars make PCB mounting difficult

Less accurate for smaller currents

A. J. SWART ET AL.

186

The PV panel was subsequently connected to the input

of channel B, with the output being left open. This results

in the open circuit voltage of the PV panel being meas-

ured (being 20.51 V in this case). The output of the DLI

for this open circuit voltage was then measured at point

X3-3 (1.737 V).

Current calibration was then done by measuring the

output voltage of channel B (not yet connected) of the

DLI circuit (X3-2). This output voltage (1.245 V) repre-

sents a 0 A flow of current, and is assumed to be the

same for channel A. The maximum current which can be

detected by the current sensors is 19 A, which is then

represented by the maximum input voltage to the data

logger, being 2.5 V.

Next, the output of channel B was connected to the PV

panel point on the MPPT which results in current flow,

as the MPPT starts charging the battery. This step can

only occur around midday when a clear sky condition (no

clouds) exists. An additional measurement is taken for

channel A and B of the DLI, as the output of the differ-

ential amplifier is non-linear. These additional measure-

ments were taken after 30 minutes of charging, and are

shown in Table 3. These parameters were then entered

under the “Table Lookup” option in the Scaling window

of the PICOLOG 1012. Maximum input voltage to this

DLI is limited to 250 V, in order to prevent an over volt-

age condition on the data logger.

The reason for not using a power supply for calibration

is due to the fact that most home owners would not have

a power supply. However they could still make use of the

voltages available from the PV panel and battery by us-

ing a commercially available digital multimeter.

5. Practical Setup

Figure 5 shows a block diagram of the practical setup

which was used to verify the operation of the DLI and

PICOLOG 1012.

The practical setup comprises a 55 W polycrystalline

PV panel, a 12 V 20 A MPPT, a 12 V 12 Ah lead-acid

battery and a 12 V 3 W LED load. A basic solar charger

or DC-DC converter may also function as the power

regulation circuit [26]. The PV panel was set to a tilt an-

gle of 36° at a fixed orientation angle of 0°(true north).

This tilt angle was justified by another research project

currently underway at VUT, and was recently presented

at an international conference in China [27]. The PI-

COLOG 2012 recorded the input and output voltages and

currents for each channel. Calibration was done using a

digital voltmeter, and was discussed in the previous sec-

tion.

6. Results and Discussion

The battery and PV voltage over a 24 h time period is

shown in Figure 6, while Figure 7 shows the current

distribution graph of the battery and PV panel for the

same time period. Figure 8 presents the input and output

power of the MPPT used in the practical setup. These

graphs are copied straight from the PICO 1216 software,

and have not been edited in any way.

Table 3. Scaling parameters.

PICOLOG 1012 Channel A Channel B

DLI out 0.50.970 1.188 0.5 1.5721.737

Scale

True value0 12.81 14.07 0 17.1220.51

DLI out 1.245 2.5 1.245 2.5

Scale True value0 19 0 19

Battery

Notebook12 Ah

12 V

Load

LED 3 W

Orientation =0°12 V

Tilt =36°

MPPT

20 A

12 V

PICOLOG 1012

PV panel

p-Si 55 W

DLI DLI

Figure 5. Practical setup.

B a tte ry vo ltagePV v oltage

06:00 12:00 18:00

Figure 6. Battery and PV voltage for a 24 h day.

0.5

1.5

Battery currentPV current

06:00 12:00 18:00

Figure 7. Battery and PV current for a 24 h day.

A. J. SWART ET AL. 187

Pow er inputPow er output

06:00 12:00 18:00

Figure 8. Input power (from the PV panel to the MPPT)

and output power (from the MPPT to the battery).

The PV panel’s voltage rises at 08:00 and falls at

18:00, giving rise to 12 hours of possible sunlight (see

Figure 6). However, this does not mean that optimum

output power is available for 12 h, as is indicated by the

gradual rise of the current graph shown in Figure 7.

Point A, in Figure 6, illustrates the PV panel’s output

voltage rise from approximately 11 V (diffused radiation

received) to 22 V (its maximum power point with direct

radiation received) in accord with the movement of the

sun. Point B shows the effect of cloud movement result-

ing in the PV panel’s output voltage varying considerable.

Point C (in Figure 7) highlights a concern in that the PV

panel’s output current is not precisely 0 A during the

night. This suggests that current calibration still requires

attention. Point D verifies the fact that the PV current

will always be higher than the battery current during the

day, as the MPPT also consumes energy.

Point E, in Figure 8, reveals the change-over in power

consumption. During the night the battery provides

power to the MPPT for driving the load, while the PV

panel takes over during the day to fulfil this role. The

negative power simply indicates that current is flowing in

the reverse direction (from battery to the MPPT). Point F

denotes the maximum power drawn for the day, being

approximately 32 W, at approximately 10:15 the morning.

7. Conclusions

A cost-effective energy monitoring system for domestic

off-grid PV system installations was presented. The PI-

COLOG 1012 was identified as an ideal real time data

logger and a suitable DLI circuit was designed and con-

structed. The results prove that the input and output pa-

rameters of the power regulation circuit can be success-

fully recorded and used to analyse the PV system’s per-

formance.

REFERENCES

[1] At Task, “Six Common Mistakes that Plague IT Projects

and Three Keys to Avoiding Them,” 2011.

[2] S. V. Larsen, L. KÃ¸rnÃ¸v, and A. Wejs, “Mind the Gap

in SEA: An Institutional Perspective on Why Assessment

of Synergies amongst Climate Change Mitigation, Adap-

tation and Other Policy Areas are Missing,” Environ-

mental Impact Assessment Review, Vol. 33, No. 1, 2011,

pp. 32-40. doi:0.1016/j.eiar.2011.09.003

[3] S. M. Shaahid and M. A. Elhadidy, “Economic Analysis

of Hybrid Photovoltaic-diesel-battery Power Systems for

Residential Loads in Hot Regions-A Step to Clean Fu-

ture,” Renewable and Sustainable Energy Reviews, Vol.

12, No. 2, 2008, pp. 488-503.

doi:0.1016/j.rser.2006.07.013

[4] IPCC, Special Report on Renewable Energy Sources and

Climate Change Mitigation, [Online], Available at:

http://www.ipcc.ch/. Accessed: 20 November 2011

[5] R. Pichs, “Statement by a Colleague of IPCC Working

Group Co-chair Prof Ottomar Edenhofer,” 2011.

[6] T. P. Chang, “Optimal Tilt Angles for Photovoltaic Mod-

ules Using the PSO Method with Nonlinear Time-varying

Evolution,” Energy, Vol. 35, No. 5, pp. 1954-1963.

doi:0.1016/j.energy.2010.01.010

[7] E. Dursun and O. Kilic, “Comparative Evaluation of Dif-

ferent Power Management Strategies of a Stand-alone

PV/Wind/PEMFC Hybrid Power System,” International

Journal of Electrical Power and Energy Systems, Vol. 34,

No. 1, 2012, pp. 81-89. doi:10.1016/j.ijepes.2011.08.025

[8] L. K. Biswas, M. H. Rahman and S. Haque, “A Low Cost

Caracteristics Analyzer of PV Module,” Developments in

Renewable Energy Technology (ICDRET), 2012 2nd In-

ternational Conference, 2012, pp. 1-5.

[9] Energy Revolution-YUDU, [Online], Available at:

http://content.yudu.com/Library/A1tlhk/Rexelrenewables

Sharp/resources/12.htm. Accessed: 2012-04-12,

[10] N. Lemmer, “Using Energy Meters as a Tool to Improve

Energy-efficiency,” 25 Degrees in Africa, 2012, pp.

28-29.

[11] S. C. Brofferio, A. Antonini, G. Galimberti and D. Galeri,

“A Method for Estimating and Monitoring the Power

Generated by a Photovoltaic Module Based on Super-

vised Adaptive Neural networks,” Smart Measurements

for Future Grids (SMFG), 2011 IEEE International Con-

ference, 2011, pp. 148-153.

[12] D. Sera, R. Teodorescu and P. Rodriguez, “Photovoltaic

Module Diagnostics by Series Resistance Monitoring and

Temperature and Rated Power Estimation,” Industrial

Electronics, IECON 2008, 34th Annual Conference of

IEEE, 2008, pp. 2195-2199.

[13] N. Pearsall and B. Atanasiu, “The European PV System

Monitoring Guidelines - Modernisation under the PER-

FORMANCE Project,” Photovoltaic Specialists Confer-

ence (PVSC), 2009 34th IEEE, 2009, pp. 000256-000261.

[14] IEEE Xplore, Data Loggers, [Online], Available at:

http://ieeexplore.ieee.org. Accessed: 1 July, 2012

[15] PICO Technology, Homepage, [Online], Available at:

http://www.picotech.com/index.html. Accessed: 11 July,

2012

A. J. SWART ET AL.

188

[16] AP Electronics, Homepage, [Online], Available at:

http://www.apelectronics.co.za/. Accessed: 11 July, 2012

[17] RS Components, Homepage, [Online], Available at:

http://za.rs-online.com/web/. Accessed: 11 July, 2012

[18] Fourtec Fourier Technologies, Homepage, [Online],

Available at: http://www.fouriersystems.com/. Accessed:

17 July, 2012

[19] Campbell Scientific, Homepage, [Online], Available at:

http://www.campbellsci.com/. Accessed: 20 July 2012

[20] M. Abdel-Salam, A. Ahmed, M. Amery, M. Swify, A.

El-kousy and K. Sayed, “Design and Implementation of

Stand-alone Residential PV System,” Applied Electrical

Engineering and Computing Technologies (AEECT),

2011 IEEE Jordan Conference, 2011, pp. 1-6.

[21] R. A. R. Kilpatrick, P. F. G. Banfill and D. P. Jenkins,

“Methodology for Characterising Domestic Electrical

Demand by Usage Categories,” Applied Energy, Vol. 88,

No. 3, 2011, pp. 612-621.

doi:10.1016/j.apenergy.2010.08.002

[22] U. Deichmann, C. Meisner, S. Murray and D. Wheeler,

“The Economics of Renewable Energy Expansion in Ru-

ral Sub-Saharan Africa,” Energy Policy, Vol. 39, No. 1,

2011, pp. 215-227. doi:10.1016/j.enpol.2010.09.034

[23] S. K. Firth, R. A. Buswell, and K. J. Lomas, “A Simple

Model of Domestic pv Systems and Their Integration

with Building Loads,” Eleventh International IBPSA

Conference, Glasgow, Scotland, 2009.

[24] Z. Li, F. Boyle, and A. Reynolds, “Domestic Application

of Solar PV Systems in Ireland: The Reality of Their Eco-

nomic Viability,” Energy, Vol. 36, No. 10, 2011, pp.

5865-5876. doi:10.1016/j.energy.2011.08.036

[25] Current Automation, Homepage, [Online], Available at:

http://www.rectifier.co.za/index.html. Accessed: 20 July,

2012

[26] O. Asowata, J. Swart and C. Pienaar, “Optimum Tilt An-

gles for Photovoltaic Panels during Winter Months in the

Vaal Triangle, South Africa,” Smart Grid and Renewable

Energy, Vol. 3, No. 2, 2012, pp. 119-125.

doi:10.4236/sgre.2012.32017

[27] O. Asowata, A. J. Swart and H. C. Pienaar, “Optimum

Tilt and Orientation Angles for Photovoltaic Panels in the

Vaal Triangle,” Asia-Pacific Power and Energy Engi-

neering Conference, Grand Mercure Baolong Hotel,

Shanghai, China, 27-29 March, 2012.