A quick look at the heart of a solar street light

A Quick Summary on Solar Controllers for street lighting

The link holding the key to an efficient solar system is, without doubt, the control interface between the solar panel the battery, and anyhow the energy stored in the battery is used.

The technology used for this control is well established and there are some very ambitious claims regarding operating efficiencies made, which would seem to indicate that the actual selection of controller is of little importance as they are all roughly the same, but as usual, that is not the case, there are a number of features you need to understand before selecting a street lighting solar controller.

There are two types of solar controller on the market, PWM (Phase Width Modulation) and MPPT (Maximum Power Point Tracking)

To very quickly summarise the difference, PWM works well at low system wattages (<5 Watts), as it is a simple DC-DC converter that works very efficiently when the solar panel voltage matches the battery voltage. However, for the UK the solar panel voltage varies significantly throughout the day, so the MPPT technology is by far the preferred one. MPPT does as the name suggests maintain a steady charge voltage to the battery over a wide voltage range from the solar panel, thereby converting up to 20% more energy, especially at the beginning and end of each day when the solar voltage varies significantly.

PWM can only charge at the prevailing battery voltage which if it is low because of significant discharge it takes longer to recharge the battery. This technology is rarely offered today.

If we just consider the MPPT controller as the basic standard, then efficiencies of over 90% can be expected providing the rest of the connected components match the inputs and outputs of the specific controller. The critical algorithm for any charge controller is that it closely matches the battery charge voltage and current curve, known as the charge profile. If this profile is not accurate it is effectively a mismatch between the solar panel and the battery, which in most cases results in the battery not charging to its optimum capacity and possibly reducing the life span.

For a solar streetlight, we also need some other control devices to complete the system. An LED driver circuit, a sensor circuit to detect movement, a battery temperature monitor to match the charge characteristics of the specific battery to the ambient temperature, and possibly a wireless link to report on the battery condition.

Incorporating the LED driver circuit into the main controller should increase system efficiency purely through better system matching, and the same applies with any proximity control such as PIR sensors.

The latest PIR sensors can draw as little as 6μA, so are able to operate throughout the night taking little power from the battery.

The need to have a temperature sensor as part of the battery pack is critical to the charge profile for the battery and without it, the battery life can be greatly reduced through overcharging at higher temperatures.

Some complex solar systems have incorporated CCTV, air quality sensors, and even WiFi hotspots, but being realistic, these systems are only possible in countries with significant regular daily sunshine, as the drain on power would appear to eliminate their use in the UK

The monitoring of battery condition using wireless technology.

If a lighting authority has a central management system in place then it would be a logical move to include any solar products into that system. The current CMS available can both control and monitor individual street lights. If this were extended to solar units then the power drain of the CMS node becomes the critical factor. Battery monitoring is the primary aim followed by control, so there is plenty of scope to incorporate initially a battery monitor only requiring occasional communication.

I am not going into detail regarding CMS in this article as it’s a major subject in its own right, but to briefly summarise the three main options for solar control are:

1.      Short Range RF Mesh

2.      Long range RF

3. Cellular

Key note:

 In my last article about the batteries, it was evident that the technology is racing ahead and the potential for upgrading a battery must be considered as part of the whole system life expectancy.

The reality is, this will only be possible if the controller can be programmed to accept any new battery chemistry or technology.

A solar street lighting supplier must be able to offer this future-proof necessity if solar is to become an accepted option for street lighting.

Conclusion:

In summary, it is most likely the end-user will be specifying an MPPT controller matched to specific battery chemistry and solar panel performance. If in addition this controller has integral control and LED driver options then this will produce a more efficient overall system as well as an easy to maintain package.

Programmable light settings timings and dimming are also essential to fit in with the street scene today and the ability to future proof the battery source with onsite programming is essential.