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EETE MAY 2014

POWER design Simplified solar-based battery charging By Steve Knoth and Albert Wu Solar power is green and abundantly “free,” but often times it can be less than reliable. Varying temperature effects that shift the solar panel’s optimal power delivery point, in addition to device aging, partial shading, the sun going down, animal waste, etc. can all impede a panel’s performance. Due to these reliability and variability concerns, nearly all solar-powered devices feature rechargeable batteries for backup power purposes. Once just lead-acid based, these batteries have now expanded to include Lithium-based chemistries too. The goal of the solar-based recharging system is to extract as much of the solar power as possible to charge the batteries quickly, as well as maintaining their state of charge. Furthermore, drain on the battery when the panel is lightly, or not illuminated, is important and should be minimized whenever possible. Clearly, solar powered applications are on the rise. Solar panels of various sizes now power a variety of innovative applications from crosswalk marker lights to trash compactors to marine buoy lights. Some batteries used in solar powered applications are a type of deep cycle battery capable of surviving prolonged, repeated charge cycles, in addition to deep discharges. These type of batteries are commonly found in “off grid” (i.e., disconnected from the electric utility company) renewable energy systems such as solar or wind power generation. System up time is paramount for off-grid installations due to proximity access difficulties. Solar panel basics For a given amount of light energy and operating conditions, a solar panel has a certain output voltage at which peak output power is produced. Figure 1 shows the characteristics of a 72 cell panel at a panel temperature of 60ºC. The blue line shows the I-V curve of the panel with the x-axis being the panel voltage. The dashed red line shows the resulting output power of the panel as the panel voltage is swept from 0V to the open circuit voltage of the panel using a simple load box to accomplish the sweep. For this particular case of conditions, the maximum power point is at 32V and the panel can deliver 140W. Once the panel temperature is allowed to vary, which it certainly will in a real world setting, the maximum power point can vary between 28V on a hot day to 44V on a cold winter’s day. Many simpler solar charging systems set the panel voltage operating point to a fixed level. In the case of this particular panel, these simpler systems would set the operating point of the panel to be 32V in order to extract the most power at a given temperature, 60°C in this case. However, when the panel temperature changes, significant power is wasted because the panel is no longer operated at its true maximum power point. Upwards of 20% to 30% of the available power can be wasted in these cases. To make matters worse, most panels are required, by safety standards set in place, to have bypass diodes built into the solar cell array. The reason for this has to do with what occurs when only portions of the panel are shaded from sunlight, while other areas get full sun. When this occurs, Fig. 1: With no partial shading, a simpler power curve exist for a given solar panel the solar cells that are shaded become reverse biased but still have high currents flowing through them because the other illuminated cells are providing the current. High temperatures in the shaded cells can occur and this can pose a fire hazard. To help lower the risk of fire, manufacturers place bypass diodes throughout the panel. Figure 2 shows how bypass diodes can be placed in the 72 cell panel. With bypass diodes in the panel, complex power versus voltage characteristics can occur when partial shading is present. Figure 3 shows such a scenario where two local power maxima are present, one at 21V and the other at 37V. If the above 32V simple power point method were used, 79.4W of power would be available compared to 90.1W available at the true maximum power point of 21V. This represents a significant loss of 13.5% in this case. Clearly, a system that can operate and track the true maximum power point would be a superior approach. Design challenges Typical solar panel efficiencies range from about 5% to 15% Combined with the fact that larger (i.e., more powerful) panels cost more, solar powered designs must maximize efficiency to minimize total system cost. To effectively harvest energy from the sun in a solar based product, the design must manage a widely varying input while also finding a way to operate the solar panel at or near its maximum power point. Furthermore, the design must safely charge the battery chemistry of choice used in the product. Steve Knoth and Albert Wu are Senior Product Marketing Engineer and Design Manager respectively, for the Power Products Group at Linear Technology Corporation – www.linear.com Fig. 2: 3 bypass diodes placed in a 72 cell solar panel for safety considerations 26 Electronic Engineering Times Europe May 2014 www.electronics-eetimes.com


EETE MAY 2014
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