A study to determine the optimum tilt angle for installing photovoltaic solar panels in Zomba district, Malawi, has been conducted. The study determined the optimum monthly tilt angles of PV solar panels and the seasonal adjustments needed for the panels in order to collect maximum solar radiation throughout the year. In this study, global solar radiation (GSR) on four tilted surfaces was measured. The north-facing surfaces were titled at angles of 0°, 15°, 20°, and 25°. The GSR data was used to determine the daily and monthly optimum tilt angles for the PV panels. The optimum tilt angles were found to be 0° or 25° depending on the time of the year. From October to February, the optimum tilt angle has been determined to be 0° and, from March to September, the optimum tilt angle is observed to be 25°. There are only two seasonal adjustments that are needed for PV solar panels in Zomba district and these should be carried out at the end of February and at the end of September. For fixed solar panels with no seasonal adjustments, the optimum tilt angle for the PV solar panels that are northfacing has been determined to be 25°.
Non-renewable energy sources, such as fossil fuel, have been the major source of energy in many countries, including Malawi. But because of the problems associated with the use of these non renewable energy sources, there is a need for alternative energy sources that are sustainable and nonpolluting. Kalogirou [
There are several ways in which solar energy is being used, either directly or indirectly [
Solar energy is abundant in Malawi though locally measured data is not available for most locations. Internationally available remote sensed databases, however, provide sufficient data for most design purposes. Malawi receives maximum irradiation of 6.5 kWh/day/m² to 7.0 kWh/day/m² from September to October and a minimum irradiation of 4.3 kWh/m²/day to 4.6 kWh/m²/day from January to February or from June to July depending on the location. The solar energy resource in Malawi is thus well within the limits normally considered to represent technical and commercial exploitability [
The optimum tilt angle (inclination) of solar collectors such as PV solar panels is important for conversion of solar radiation into heat or electricity [
There are recommended tilt angles of solar panels that are based on the sun’s movement throughout the year that depends on the latitude of the location. In some cases, seasonal adjustments to the tilt angle may be necessary depending on the location of the area. Such information on the optimum tilt angles is normally based on data of the global solar radiation (GSR) for that particular area that is measured on tilted surfaces. Unfortunately for Malawi, such data is not available and all the installed solar panels have tilt angles based on the theoretical tilt angle. Thus there is always a need to determine the optimum tilt angles for various districts in Malawi. In this study, the optimum tilt angle
This section presents the description of the study area, the experimental setup used in the calibration of the pyranometers, and the experimental setup used in the measurement of the global solar radiation (GSR) on tilted surfaces.
The study was conducted at Chancellor College Meteorological Station in Zomba district, Malawi, located at latitude of 15.387 south and longitude of 35.338 east. Figure
Map of Malawi showing the location of Zomba district in Malawi.
Two types of solar pyranometers were used in this study. The first type is the Eppley pyranometer, which is a standard instrument for measuring GSR on a plane surface. The Eppley pyranometer used has a calibration factor of 8.53 × 10−6 V/Wm−2 and the GSR measured was obtained by dividing the voltage produced by this pyranometer with this calibration factor.
The other pyranometer used is the LI-COR pyranometer that uses a LI190SB Quantum Sensor. The LI-COR Quantum Sensor produces a current when GSR is incident upon it. Thus in order to convert the current to voltage, a high precision 1 Ω resistor is connected across its output terminals. Hence it is always necessary to relate the output voltage produced by the sensor to the DSR incident on the sensor. In this study, the three LI-COR pyranometers used were calibrated against the Eppley pyranometer in order to determine their calibration factors.
During the calibration process, three LI-COR pyranometers and an Eppley pyranometer were mounted on horizontal surfaces as shown in Figure
The experimental setup for calibration of LI-COR pyranometers.
Both the Eppley and the LI-COR pyranometers produce a voltage as an output when measuring the GSR. The calibration factor for the Eppley pyranometer was included in the data logger program so that it gave direct readings of the GSR. To determine the calibration factor of each LI-COR pyranometer, a graph of the GSR as measured by the Epply pyranometer was plotted against output voltage produced by the LI-COR pyranometer. A linear fit was fitted to the data in each plot in order to determine the calibration factor of the LI-COR pyranometer. The equation for the fitting line is given by
After the calibration process, all the pyranometers were then used to measure the GSR at various tilt angles as shown in Figure
Experimental setup for GSR measurement.
For the measurement of GSR, the data logger was programmed to collect hourly average data for a period of one year. For each hour, the data logger scanned its channels every 3 seconds and an average was computed for that hour. The daily GSR data was then calculated from the hourly average of the GSR data from the period of 6 am to 6 pm because this is the period during which significant solar radiation was available.
This section presents results of the study. Presented first are the results on the calibration of the pyranometers and this is followed by the presentation of the results of the GSR measured on various tilted surfaces.
Figure
Solar radiation as obtained by the Eppley pyranometer plotted against the voltage produced by the LI-COR (serial number PHY32602).
A linear fit was fitted to the data in each plot in order to determine the calibration factor of the LI-COR pyranometer. The equation for the fitting line is given by (
A similar calibration was carried out for the other two LI-COR pyranometers and a summary of the results is shown in Table
Calibration factors for LI-COR pyranometers.
Pyranometer | Parameter | Value |
---|---|---|
LI-COR I (serial number PHY32602) | Mean |
|
Mean |
| |
LI-COR II (serial number PHY30406) | Mean |
|
Mean |
(7.295 ± 0.05) × 10−2 kWm−2/mV | |
LI-COR III (serial number PHY32951) | Mean |
|
Mean |
|
Table
The installation of solar panels at any location requires that they are tilted towards the sun. In this study, GSR data was measured at one horizontal surface and three tilted surfaces that were facing north for months of September 2011 to August 2012. For each day, the hourly average of the GSR (
The monthly average for the GSR was obtained from the daily averages by using
The monthly averages of the GSR were further used to determine the optimum tilt angle for the month.
For the month of September, shown in Figure
Global Solar radiation as measured on four surfaces at different angles 0°, 15°, 20°, and 25° for the months of (a) September, (b) October, (c) November, and (d) December.
Figure
Figure
Figures
Global Solar radiation as measured on four surfaces at different angles 0°, 15°, 20°, and 25° for the months of (a) January, (b) February, (c) March, and (d) April.
Figures
All plots in Figure
Global Solar radiation as measured on four surfaces at different angles 0°, 15°, 20°, and 25°for the months of (a) May, (b) June, (c) July, and (d) August.
The various optimum tilt angles were determined for all months in a year and are shown in Table
Optimum monthly tilt angle for solar panels in Zomba district.
Month | First best angle | Second best angle |
---|---|---|
September | 25° | 0° |
October | 0° | 25° |
November | 0° | 25° |
December | 0° | 25° |
January | 0° | 25° |
February | 0° | 25° |
March | 25° | 0° |
April | 25° | 20° |
May | 25° | 20° |
June | 25° | 20° |
July | 25° | 20° |
August | 25° | 20° |
Table
Optimum yearly-tilt angle for the solar panels in Zomba district.
Pyranometer | Tilt angle | Yearly average (kW/m2) |
---|---|---|
LICOR S1 | 0° |
|
LICOR S2 | 15° |
|
LICOR S3 | 20° |
|
EPPLEY | 25° |
|
The optimum tilt angle for the solar panels in Zomba district has been determined for each month. The optimum tilt angle has been determined to be 0° from October to February and to be 25° from March to September. Thus there are only two seasonal adjustments that are needed for tilting solar panels in Zomba districts. The adjustments should be carried out at the end of February (from a tilt angle of 0° to that of 25°) and at the end of September (from a tilt angle of 25° to that of 0°). In cases where the PV solar panels are fixed with no seasonal adjustments, the tilt angle for the solar panels facing true north should be 25°.
Optimum tilt angle
Average global solar radiation for an hour
Average global solar radiation for a day
Average global solar radiation for a month
Latitude
Photovoltaic
Global solar radiation
Direct solar radiation.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The first author is grateful to the Malawi Environmental Endowment Trust (MEET) in Malawi for providing financial support for his scholarship.