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Power conversion prospects in solar-powered LED Lighting

Posted: 06 Aug 2012     Print Version  Bookmark and Share

Keywords:LEDs  photovoltaic solar cells  battery 

Fast-paced technology advances in light-emitting Diodes (LEDs) and photovoltaic solar cells are leading to notable improvements in their performance, which can then enhance the performance of the end-application. In applications that employ both technologies, such as solar-powered lighting, improvements in these two core technologies combine to offer substantial end-application performance improvement potential. In this case, higher-efficiency solar cells convert more of the Sun�s energy into electricity, which can reduce the amount of solar-cell area needed, and the high-efficiency LEDs run longer and brighter into the night. The challenge for solar-lighting solution manufactures�, however, is being able to leverage these advances quickly and cost effectively. One method to maximise the system�s performance is through its power-conversion strategy. A solid power-conversion strategy enables rapid development and deployment of solutions that utilise the latest technologies. In this article, we will review the components, develop a system, and provide a high-level method to analyse its behaviour.

Background
There is a wide array of solar-powered lighting examples. Whether you are in an area with an unreliable power grid and use a solar-powered lantern as a night-time reading lamp, or deploy full-scale, community-grade street lighting, the opportunities for a combined solar/LED lighting system are diverse, broad-based and global. The only difference is the scale of the end application (reading vs. general illumination have very different requirements).

The core components in all of these systems are the i) solar cells, ii) battery, and iii) the LED. As an aside, we can adopt a more generalized description of each of these components, including energy collectors (solar), energy storage (battery), and energy emitters (LEDs). While not exactly accurate, it serves to underscore the flexibility of the analysis. Figure 1a shows the most basic of system configurations.

Figure 1: Two system configurations.

In order for this implementation to work, however, the behaviours of each element must be compatible with each other. In this case, that means the output voltage/current behaviour of the solar cell must align with the battery-charging profile, and the battery-discharge profile must match the LED drive requirements. We quickly find that, for the configuration in figure1a, they do not.

Component overview
Reviewing the performance characteristics of each component, as found in the V-I characteristics in figure 2 (a through d), we find that, while they can be made to behave close to each other within a limited configuration set, it is virtually impossible to guarantee any reasonable level of performance. We quickly see the that maximum solar-cell voltage (per cell) is around 1V, while the NiMH battery operates in a range of .9V to 1.4V, and the LEDs require a constant current source, although their forward voltage is typically above 3V. Further, the NiMH battery has some specific charging requirements to extend its useful life.

While it is possible to develop a system that interfaces all of these components directly, it should be clear that there are significant limitations to that configuration, as well as ramifications for the overall system efficiency and its robustness.

Figure 2: Component V-I characteristics and drive requirements.

To address these limitations, let us review the alternative system diagram in figure 1b. Adding a power electronics interface between each of the three core elements allows a much higher degree of flexibility, and permits the overall system performance to be optimised. The microcontroller is not essential in this configuration. We can find a stand-alone battery-charger integrated circuit (IC) to address the needs of the NiMH charging profile, and similarly find LED driver ICs that convert the battery voltage into a constant current source.

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