Solar Designs Focus On Low Power

By Pallab Chatterjee
This year’s Semicon West showed growth in both attendees and exhibitors, but the big growth—at least from a percentage standpoint—was in the associated Intersolar show, which featured advances in PV and solar thermal systems.

This year’s Intersolar show covered areas that were not just commercial grid-tied PV systems. Rather, it also had off-grid systems with new battery technologies, grid interactive systems with both batteries and power grid connection, and full multi-site monitoring solution software.

These systems have spawned a number of new technologies and applications for solar that all share a common theme—low power consumption. Low power in solar is relative. Most of the common panels are 200W, plus or minus 10%, based on the exact footprint of the panel and connection method. These are basically 30V peak voltage panels, or 0.5 volts per cell, with 55 to 65 cells per panel connected in series. Delivering the voltage necessary for the current generation of portable devices and charging their batteries is not a problem, as most systems are 3.3v and below. The challenge comes from the ability to generate the current needed for these devices.

The large 60-cell solar panels generate about 8.5 amps. Typical wireless systems (such as 802.11n radio) operate with peak current is in the 1.2-amp range for a single-antenna setup. This current requirement is also a key to battery recharge systems, which use high currents to reverse the chemical reactions in rechargeable batteries and store new charge.

The key to using solar power to support new sensors and control devices is to improve the maintenance cycle and operating life of the devices. In industrial control, public access areas, lighting control, and parking control, access to service the sensor and control electronics is difficult, limited and expensive. For these applications the preferred solution is a solar powered system with some sort of short-term power storage technology to allow the system to work 24 hours a day with only 6 to 12 hours of sun exposure. The leading power storage technology for isolated sensors and individual devices is a supercap, which is a combination battery-capacitor system. These are generally “off-grid” systems that do not connect to other power systems.

Some of the larger applications in the “smart device” or “Internet of Things” (IOT) arena have PV systems that are either grid-tied or grid-interactive. These have driven new circuits to do power management and conversion from the PV cells themselves. The three new technologies shown at the show, which are now scaled for smaller power application rather than building scale PV, are solar thermal conversion, micro-inverter and grid-interactive inverters.

The solar thermal systems (also called PV/T systems) are typically found on buildings, which need the power for supplementing the main energy draw or providing a separate power grid for the sensor and monitor products. The advantage of the Thermal PV systems is the rise in net efficiency in power for a given space. Standard PV produces 10W per square foot, while standard thermal produces 20 to 30W per square foot. The resulting combination is 30 to 40W per square foot. This is much more than the standard 14% efficiency of bulk Si PV.

Micro-inverters are designed to channel power per panel rather than for a bank of panels. They eliminate the conversion loss from sub-par panels (due to clouds and shadows) from the banks of series connected panels, which get connected to a standard inverter. These micro-inverters now offer 95%-plus conversion efficiency. They also are extremely effective at allowing single panels to be connected to IOT devices without complex wiring to a center, or simplified lower voltage interconnect to a centralized storage or distribution system. Most of these micro-inverters use single-chip SOCs or mixed technology (HV and standard CMOS) system-in-package designs. The micro-inverters are a key enabler of distributed and isolated power systems. These systems are almost always off-grid or grid-interactive with local storage.

On the grid-interactive side, new high-efficiency control systems and inverters were shown. These allow for easy design expansion and planning of grid-tied systems, while allowing for flexibility in both the expansion of panel sizes tied to the inverter and the size/depth/type of battery systems used. This is important in conjunction with advances in battery technology, which are allowing much higher power densities per cell and changing the charge characteristics needed from the inverter.

This solar interaction and the low-power operation of the IOT devices are a combination that must be deployed simultaneously for the large-scale applications to flourish.