There are a number of ways in which solar lighting design solutions differ from conventional exterior lighting installations, so it is important to understand what additional information is needed and why it is required.
The artificial lighting component can be approached in the same way it would otherwise. Once lighting levels have been determined, suitable luminaires can been selected, together with corresponding beam distributions, wattages, and locations to satisfy the design.
However because the luminaires also have to generate their own power, additional factors need to be considered.
- The Project Location: City, ZIP/Post Code or installation address.
- Lighting operation schedule and control requirements.
- Number of autonomous days or operating hours from a full battery.
- Availability of grid power or if luminaires must be independent.
Once the location is confirmed, the most suitable solar cell technology can be selected. Each location has its own “Panel Generation Factor” which will influence the design and operation calculations of the solar lighting system. Knowing how much potential energy is available, enables the remaining variables – operation schedule, control requirements, autonomous hours and availability of grid power – to determine the best possible solution.
The operating schedule and controls are similar to a standard installation with a few additional options. The luminaires can be controlled via time clock, photoelectric cell or a combination of the two for powering on and off. Light levels can be reduced during low traffic times via automatic dimming or movement detectors, with the added possibility of controlling the output in relation to available battery power.
The basic operational principle of the battery-based solar PV system can be elaborated as follow. During the daytime, the PV panel generates electrical energy and the battery is used to store this portion of energy (line 1 in Figure 1). During the night, the energy is drawn from the battery to feed to the load (line 2 in Figure 1). Without the sunlight, the performance of the system depends solely on the capacity of the battery. The bigger the battery, the longer the system can operate without sunlight. This is known as autonomy, referring to the length of time in hours or days for which the battery-based solar PV system can operate without sunlight.
The autonomy can be calculated using the maximum capacity of the battery. For example, 3 days of autonomy means that the solar lighting system can operate for 3 days without the need to recharge the battery. However after 3 days the battery will be empty. If the system cannot be recharged due to overcast sky on the next day, the solar lighting unit will not operate since the energy stored in the battery is completely discharged during the 3-day autonomy.
The second possible scenario to restore the autonomy is where the power generated by the PV is greater than the power consumed by the LED load. In this case the battery will gradually be recharged. For example, if the 12 V, 16.5 Ah battery is empty, the charging electrical energy of 198 Wh is required to bring the battery once more to the full charging state. If the daily energy consumption by the LED load is about 100 Wh and the solar panel generates the electrical energy of 150 Wh. The difference of 50 Wh will be left over in the battery. Four days of this situation is enough to fill the battery to the full charging state.