The balance-of-system cost for any grid-tied PV system can be quite high. Given that inverters for 10 kW systems cost several thousand dollars – a significant up-front cost to a homeowner or anyone undertaking a solar panel installation – traditional system design, with a single, large DC-AC inverter, can result in a rethinking of the size and requirements of the system.
An alternative approach is to distribute the inverter electronics by adding a microinverter to each panel. (See the previous TechZone articles “Simplified Topology is Key to Solar PV Microinverter Design Reliability” and “Micro inverters Improve Efficiency”). Among the advantages of this approach is that the system can grow in a completely modular fashion since each panel offers an AC output. This lets a small solar installation get up and running with a lower initial investment while still allowing for future expansion.
A second advantage often cited by microinverter proponents is the ability to optimize the overall output of the system when input solar power across the installation may vary widely from panel to panel. Maximum power point tracking (MPPT) electronics in the microinverter improve the efficiency of energy harvested by maximizing the extracted power for the conditions present at each individual solar panel.
Since not all panels in even relatively small systems will receive the same amount of light energy at all times during operation, the ability to individually optimize power output is an essential ingredient to creating efficient energy harvesting capability. Consider the situation as the sun comes up. Part of the rooftop will be receiving direct sunlight while other parts remain in shadow.
Besides the improvement to performance throughout a 24 hour cycle, adding intelligence to each panel will pay dividends over the life of the system. With a return of investment of 10 years or longer, any solar power installation is expected to operate for at least 20 years. Although different photovoltaic materials and solar panel types exhibit a wide range of degradation characteristics, changes to output over time are a given. Furthermore, each cell in a module will behave differently, so the panel-to-panel differences can be quite pronounced. Pushing the charge control electronics out to the panel level improves the overall system performance, as the characteristics of each panel change over the life of the system.
Panel output can be optimized through the use of MPPT electronics coupled to a DC-DC converter dedicated to each individual panel in the system. One good example is the Texas Instruments (previously National Semiconductor) SolarMagic SM3320 chipset. The SolarMagic design includes the SM72442MT-ND MPPT IC along with the DC-DC converter (Fig. 1). A complete evaluation kit with part number SM3320-BATT-EV/NOPB-ND is also available. This design employs a SolarMagic DC-DC converter that replaces conventional circuits with a buck-and-boost topology along with a panel mode switch. TI's design allows four operational modes to improve the energy harvesting performance.
Figure 1: Dedicated MPPT and DC-DC converter electronics for each photovoltaic module.
There are three DC conversion modes to expand the conventional buck-boost operation. Depending on the solar panel output determined by the MPPT IC, buck-only, boost-only or buck-boost interleaved modes are chosen. For times when the incidence of solar power is higher, the SolarMagic MPPT will switch to panel mode operation. If the converter output is within 2 percent of the input, the DC-DC conversion electronics are bypassed by a panel mode MOSFET switch. Although the typical efficiency is 98.5 percent in power tracking mode, bypassing the converter in panel mode allows even more harvested energy out of the system with 99.5 percent panel mode efficiency.
The SM72442MT-ND MPPT IC uses the perturb and observe method of determining the maximum power point while the MPPT algorithm keeps the power extracted from the power constant regardless of the battery voltage. Perturb and observe offers a good compromise between maximum power point tracking capability and the ripple voltage once the maximum power has been determined. The system defaults to voltages and currents appropriate for lead-acid batteries, but it can be reprogrammed for other battery chemistries.
For panels in a string receiving low levels of solar input, the DC-DC converter output will eventually drop out. To ensure continued operation of the entire string of panels, the design should include diodes across each panel. Dark panels can be bypassed while the rest of the string continues to output electrical power (Fig. 2).
Figure 2: Bypass diodes prevent dark modules from affecting series string output.
Power point tracking to control DC-DC conversion at each site clearly offers a way to optimize the entire PV installation given varying conditions at the specific site of each panel. However, including electronic control into the junction box for each panel allows more than just site-specific feedback and control circuits. It can allow remote access and monitoring for each panel in a system.
This next step is taken in the SolarMagic SM3320-RF-EV/NOPB-ND evaluation kit (Fig. 3). This system includes the MPPT and DC-DC converter electronics and adds a wireless link. The SM3320-RF-EV/NOPB-ND incorporates the Nordic nRF24LE1 chip which integrates a 2.4 GHz radio and an 8051 microcontroller on a single piece of silicon. Even with the addition of RF electronics, the kit profile is still suitable for incorporation into a solar panel junction box.
Figure 3: SolarMagic SM3320-RF-EV/NOPB-ND MPPT Evaluation Kit with RF link.
The main anticipated use of the RF link is for remote shutdown of individual solar panels in a system. The ability to disconnect individual panels remotely is useful in many situations, from the initial installation to maintenance or emergencies. Theft detection and deterrence is another feature. Although the principal application for the link is this remote shutdown capability, the RF link can be used for monitoring each panel’s performance (Fig. 4). From the operator’s perspective, this will improve the ability to anticipate and schedule maintenance of the system.
Figure 4: Solar energy harvesting system with smart panels and remote monitoring and control.
Performance of the RF link in the SolarMagic SM3320-RF-EV/NOPB-ND is not dependent upon the junction box material or positioning. No additional antenna is required either. Although the evaluation kit is intended to be mounted and completely enclosed inside the junction box enclosure, the antenna is external. The SM3320-RF-EV/NOPB-ND design uses the DC power lines as the antenna to maintain a robust RF link to the remote transceiver.
Adding intelligent electronics at the module or panel level in a solar power installation increases the potential energy harvested in day-to-day use, as well as over the lifetime of the system. Remote monitoring and control using panel-level electronics adds a level of connectedness to photovoltaic systems that we have become accustomed to in just about every other aspect of 21st century life.