1. 2-4. Solar Panels

2. Solar Panel Power Rating
Rated in Watts Peak (Wp). This is the number of Watts power that the panel should produce if it is exposed to 1000 W/m2 of sunlight coming straight into the clean panel while there is a cell temperature of 25ºC and an air mass of 1.5. Power is delivered to the load at the maximum power point of the panel (the optimum loading)

3. Rating Measurement Standards
1000 Watts per square meter of solar energy
Energy is falling at right angles onto the panel
Cell temperature of 25 degrees Celsius
Air Mass of 1.5 (sun angle about 46° from vertical)
Clean panel surface
Load taken from panel at maximum power point

4. Actual Panel Output
Solar energy almost never is great enough to provide 1000 W/m2 of solar radiation. Typically W/m2 is the highest seen on clear days at noon.
In the tropics, solar cells are 50ºC to over 60ºC. Higher cell temperatures result in lowered output of 10% to 15% over rated values
Panels rarely face directly toward the sun, surface reflections increase and output decreases as a result
There is often a mismatch between the load and the panel resulting in a few percent reduction from the rated value.

5. Panel Types
Single Crystal construction. Each cell is a single crystal of silicon. Oldest design, highest efficiency. Round cells initially but may be cut square. Panel made up of many (usually 36) cells connected in series. Very reliable.
Polycrystralline construction. Each cell includes several large crystals of silicon. Cells can be any shape. Almost as high efficiency as single cells. Panel made up of many (usually 36) cells connected in series. Excellent reliability.
Thin film construction. Silicon or other PV material is put in a very thin layer onto metal or plastic. Mass production is relatively easy and theoretically can be cheaper than crystal based panels. Efficiency low to medium. Reliability varies from poor to good. Sometimes called “amorphous” panels.

6. Panel appearances Single crystal cell Polycrystalline Cells
Thin Film panel

7. Panel construction
Top layer is glass or in some cheap panels, clear plastic.
The middle layer is the active PV material. In the case of crystalline cells, 36 individual cells are connected in series to make a panel for charging a 12V battery. Each cell produces about 0.5 to 0.6 volts. The area of the cell determines the Amperes it can produce.
Backing. Typically a special plastic called Tedlar though sometimes glass. Thin film panels may have a backing that is ceramic or metal as well as possibly glass or plastic.
Cells are embedded in a clear plastic material between the top layer and the bottom layer. This is called the encapsulant.

8. Panel construction
Typical solar PV panel construction cross section

9. Terminology

10. Recommended specifications
Panels
Must have an output adequate to bring the battery to full charge several times a week
Screw type wire terminals with lock washers
36 cells or more
Meet international standards for construction and are certified by testing at an international test center

11. Note that Impp X Vmpp = Wp
Key Characteristics
Number of cells (must be at least 36 for SHS) or it will not be very effective in charging a 12V battery
Voc = the open circuit voltage which is the voltage across the terminals with no load attached
Isc = Short circuit current which is the Amperes measured directly across the terminals with no load attached
Impp= Current delivered at the maximum power conditions under standard conditions
Vmpp= Voltage delivered at the maximum power conditions under standard conditions
Note that Impp X Vmpp = Wp

12. Voltage-Ampere Relationship (IV Curve)

13. Solar intensity affects mainly current

14. Connecting panels for increased voltage and amperage
Connecting panels in series (+ terminal of one panel connected to – terminal of the next) results in adding the voltage of the series connected panels
Connecting panels in parallel (+ terminal of one panel connected to + terminal of the other and – terminal of one panel connected to – terminal of the other) results in adding the amperes produced by each panel

15. Sun’s movements over the year

16. Proper Orientation
Facing toward the Equator (South in the North Pacific)
Tilted about the same number of degrees as the latitude of the site
Never tilt less than 5 degrees because fast water runoff is necessary for cleaning. 10 degrees is better.

17. Shading
Output from panels in the shade is a small fraction of the output from a panel in the sun
Even shading a few cells on the panel will greatly reduce the output from the panel
No shade should be on the panel from 0900 to 1500

18. Be careful of shading due to changing sun position

19. Heat effects
Every three degrees C (5.4F) of temperature rise in a PV cell lowers the output of a PV panel by 1%.
Keep panels as cool as possible to prevent power loss due to overheating
NEVER mount solar panels flush on any surface, if at all possible provide at 6 inches (150mm) or more of ventilation space underneath panels, especially on metal roofs and never less than 2.5 inches (60mm).

20. Diodes

21. Use of Diodes
Bypass Diode - Used to take shaded portions of a panel or array out of the circuit by bypassing cells that are producing electricity at a much lower rate than others. Many panels come with by-pass diodes already installed.
Blocking Diode – used to prevent back flow of electricity from the battery to the panel or to isolate the panel connections from the battery so a short at the panel does not short the battery. Often not needed

22. Use of Blocking and Bypass Diodes

23. Increasing Array Voltage
So two panels intended to charge a 12V battery can be connected in series to charge a 24V battery. Series connection will work well only if the panels have the same ampere rating.

24. Increasing Array Current
To increase the amperes available, connect the panels in parallel. As long as the two panels have the same voltage it will work ok

25. Panel Mounting Rules
Face south and tilt to latitude but no less than 10 degrees
Must have ventilating air passing underneath the panel
No shade any time of the year between 0900 and 1500
Mounting must use marine grade stainless steel fasteners that isolate aluminum panel frames from the roof. No aluminum can be allowed to touch a metal roof.
Mounting arrangement must be strong enough to survive storms yet simple enough to allow access to panel connections without major dismantling of the array.

26. Adequate Panel Size
Panel size needs to be fitted to the requirement of the load that will be powered.
Modest oversizing of the panel can dramatically increase system reliability, increase battery life and lower maintenance costs