Solar Energy, amongst all non-conventional or renewable energy sources, still remains relatively easier to harness, store and use. While the usage of solar energy dates back to quite a few decades, powering space crafts and satellites, it has now reached a stage where even a backpacker uses it to charge his mobile phone while on a trek and gigantic solar farms are pumping gigawatts into the electrical grid.
It is worth noting that:
- The fuel is free: it is a renewable, clean and ecologically harmless energy source
- It produces no noise, harmful emissions or polluting gases.
- The systems are modular and can be quickly installed anywhere
- The installation cost is negligible in new building realization
- In the building, it is possible to cover the external surfaces (roof, walls)
- The energy is produced close to the load: no energy lost on the distribution line
- Only minimal maintenance is required to keep the system running
Fundamental improvements have happened in the way of solar cells becoming more efficient than what these were at inception, and with it, semiconductor technologies that have evolved by leaps and bounds. The presence of semiconductors in the entire value chain starts with the solar panel itself. From the source of energy to the end consumer, in whatever form, and all the conversion in between, it is semiconductor technology which has brought about a sea change. In the next few paragraphs, the role of semiconductors is highlighted in the solar/alternate energy value chain.
The use of semiconductors, starts from the junction box itself (while, solar cells themselves being semiconductors, in the first place). The junction box is a hermetically sealed box which provides the electrical termination for the solar panel. A solar panel is a series-parallel combination of many solar cells arranged in strings. The strings are then interconnected and have bypass diodes connected across each and every string. This is because the panel must allow for the current to flow through it when it is partially shaded or even fully shaded. If the bypass diodes would not have been present, the shaded sting or panel, would behave like a very high resistance and render the whole panel or the combination of panels ineffective. Just any diode won’t work: The diodes need to have very low forward drop at the rated maximum current to prevent static losses and reduction in efficiency. This loss appears as heat. It must also be noted, that the bypass diode when not functional, i.e. when there is full sunlight, it is subject to a reverse voltage stress.
It is necessary for the diode not to have a very high leakage current under this condition: since this reduces efficiency and in sunny geographical areas, this is a constant loss factor. Standard diodes have low leakage but very high drops. Schottky diodes have very low drops but high leakage currents. A trade-off has now been developed, and devices with both low forward voltages drop and low leakage currents are commercially available, saving huge powers collectively. In addition to these diodes, active bypass devices are also available, where the drop is even lower.
Going beyond the junction box, there is a need to convert the power available at the output to a more meaningful voltage and current levels. It is also a known fact that the solar panel follows a unique power delivery curve. Put in simpler words, the panel delivers maximum power to the load, under certain conditions. So, it is important to maximize the output from the panel in a systematic and scientific manner. The process is called the MPPT or Maximum Power Point Tracking. This is done in two ways – either by mechanically tracking the azimuth or by elevation of the solar panels (large systems, complex, very expensive) or use electronic tracking.
The latter is the most popular for domestic and commercial scales as it is now easy to implement, deploy and service. This system is commonly designed around a microcontroller, power devices like MOSFETS, diodes, and gate drivers. Essentially the system is a buck or boost or a buck-boost converter, depending on the end application, and controlled through the algorithm in the microcontroller. In essence, the microcontroller takes the reading of the panel input power, the system output power and tracks them in such a way that the ratio of the power output to the power from the panel is maximized under all conditions of ambient sunlight.
So this means:
- The microcontroller needs to calculate and calculate fast
- Take decisions based on observations
- Do the general housekeeping
- The power conversion block has to convert voltage and current levels to usable levels
- The system has to work hardest when it is most sunny and hot outside.
Thus, a system which is smart, robust and efficient, is what is needed. Smartness comes from the fast processing capabilities of the new microcontrollers with many peripherals inside, reducing the physical footprint and real estate. Efficient power conversion is possible using the latest generation of IGBTs, MOSFETs, and Diodes. Moving away from classical silicon only, to exotic (better a simpler replacement, can be new also) technologies like SiC and GaN makes systems much more efficient and robust than what it was, say ten years ago.
Another figure of merit is the power density: big is not better anymore. The industry demands more and more power output per cubic inch of electronics than what was available a year back and growing. This is possible with newer technologies of fabrication which allow for higher and higher frequencies of operation. This, in turn, means, the passive components like capacitors and inductors become smaller and smaller. The system becomes smaller, cheaper and more efficient.
A decade back, people were happy if efficiencies reached 85%. Today, >95% is not uncommon, with 97-98% being reported as well. This is thanks to newer topologies, in conjunction with the enormous computing power of modern microcontrollers and DSPs, which makes it easy for the power electronics designer to come up with previously unexplored power conversion topologies.
Grid-tied and standalone power plants are also in high demand. These are inverters which convert the limited voltage available from the solar panel to 230V AC so that usage of domestic appliances is possible. This implementation is a little more complicated than the MPPT we explained earlier. While the challenges of more power from a smaller size remain, the other challenge is multiple levels of conversion. To illustrate, let us take the case of a standalone solar generator.
There is the panel, then there is a battery charger, there is a suitably sized battery bank, and finally, there is the inverter. The high-frequency inverter again is composed of a DC-DC converter, which raises the battery voltage to around 400V DC or so, and then it is further chopped to provide a 230V 50Hz sine wave, by the DC-AC converter. So we can observe, that there are multiple stages of power conversion, working in cascade, and efficiencies can only multiply: so even with 95% efficiencies in each block, the overall end to end efficiency will be still lower. This is the reason why people strive to have as high efficiency as possible in the individual stages.
This is where low loss IGBTs, MOSFETS or high-performance diodes come into the picture. Every percent gained, is a step forward. To circumvent the problem of cascaded power conversion, there is a concept of grid tied inverter where the consumer consumes from the grid and whatever is generated from rooftop solar power plant is fed back to the grid. So if a user consumes X units but gives back Y units, he is charged for X-Y units (or paid back for Y-X units if generation was more than the consumption).
Here in addition to power conversion, a system which can synchronize itself with the utility grid is needed for proper power feedback, and also to isolate or island itself for safety reasons, should grid become unavailable. This calls for an even more powerful processor, in addition to good and robust power conversion topologies and devices. To accurately meter the power consumed and delivered, new metering techniques called net metering has also been developed and commercialized.
Inverters called micro inverters, typically in the range of 200-500W have now been developed which sit right at the back of the panel, often serving the purpose of the junction box as well. So here it is more plug and play, and suitable for low priced real estates. Consider a pole mounted panel of about 2.5 square meters, with a 250W micro-inverter, the output of which is directly plugged into the mains, to feed back to the grid. Plus, there are systems called hybrid Smart Solar Streetlights, which can take power from solar, the grid or a battery bank.
The panel charges the battery during the day which is suitably sized to meet autonomy of 2-3 days (self-reliant to provide lighting for 2-3 nights even with no charging) and even if the battery is depleted, it automatically falls back to mains grid connection for uninterrupted power supply. The streetlights are connected to the cloud for remote control, maintenance, and monitoring.
In certain cases, the use of a DC grid has also been thought of and deployed. This means at least one intermediate conversion stage may be done without. In smart nano-grids, this may be quite suitable.
In the entire value chain, we must also appreciate the fact that there is work also needed at the load side. The solar panel can supply a limited power to charge a storage battery: so, how to make it last longer? Make the loads efficient as well. For example, conventional lamps are now being replaced with LED lamps with great improvement in luminous efficacy, power quality, and consumption figures.
There are now instances where a captive installation could be designed in a manner that most of the loads operate on DC, and this avoids a few conversion stages. This further adds to efficiency. The ceiling fans are now being slowly replaced with BLDC motors from standard induction motors, resulting in power saving of up to 60% in some case, 40 – 50% being more typical. Fridges, ACs, and other machines are also having their motors converted to BLDC motors for much better efficiency figures. Our LED TVs are much lesser power hungry than earlier generations. It is ONLY advanced semiconductor processes, devices, and solutions that have made this huge energy saving and efficiency improvement possible.
With intelligence and sensing on the chip also being easily available, systems are made smart these days as part of an IoT scenario. This can lead to further energy savings at the load side. Consider a bulb, which senses human presence and turns itself ON or OFF, as an example!
We can conclude that higher efficiencies, more features, reduced sizes and form factor and inherent smartness have been the game changers in the solar power conversion industry and generally, in the electronics industry over the last few years. This trend has been made possible by breakthrough innovations in discrete semiconductors and on-chip processing capabilities. This trend is expected to continue over the next several years with even higher performance semiconductors making the unprecedented, possible.