Optimizing Solar Power Systems

One of the major drawbacks for renewable energy sources, such as wind and solar, is that the wind doesn’t always blow and the sun doesn’t always shine. Therefore, these systems require energy storage to handle the times where the renewable energy source cannot meet the power demands.

The traditional approach to this problem is to connect the renewable energy source to a battery bank to store energy for the still and the cloudy days. When the weather does not cooperate and the power demands are high, energy is bled off the battery stack instead of relying entirely on the renewable energy source.

On the opposite end of the spectrum, there are times where the sun is shining brightly and the wind is blowing fiercely. This surplus energy must be controlled. It cannot be fed into the battery network and the power system directly without a regulating mechanism. All of these systems require a charge controller to limit the energy that flows into the system.

Hard-Wired Battery Configurations

Existing battery stacks and charge controllers are inherently inefficient. The battery stack in a renewable energy system, like all battery stacks, is wired in a fixed manner. The batteries are often connected in a combination of parallel stacks of series connections. To be of service, they must be over designed to meet the high current demands of inrush currents, but then have the capacity to run for hours at a fixed voltage.

In general, solar power systems are wired such that the battery stack is in parallel with the load. In this way, the solar panels can charge the battery and power the load. While it keeps the batteries topped off during the sunny days, it tends to shorten the life of the batteries. More advanced systems will allow the batteries to drain and only charge them again when they drop below a certain level. These more complicated systems require a direct current to direct current (DC to DC) converter to step up or down the voltage as needed, as well as a regulator of some sort to prevent overcharging.

The solar panels and the batteries output direct current (DC) voltage. Most residential and commercial installations require 120 V or 240 V, 60 Hz alternating current (AC) voltage. To convert the DC to AC, an inverter must be installed in line with the system. Standard inverters can be lossy and inexpensive ones tend to produce a square wave instead of a true arc pure sine wave, which eventually destroys power supplies on sensitive devices.

Configurable Connections via Software Switching

Besides controlling the battery output, software switching can be implemented directly to the solar cells themselves. In traditional solar power installations, the solar cells and the storage batteries are wired in a fixed configuration, limiting system flexibility. Because a solar cell simply produces a DC voltage based on the sunlight, these solar cells can be thought of as DC voltage sources, just like batteries. The battery software switching system can be installed between individual solar panels so that they can be switched from series to parallel to adapt to the solar energy and the demands of the load.

Increased Efficiency of Charging and Discharging Cycles

Simple solar power systems have the solar panel array output connected in parallel with the battery stack and the load. A few switches control the output between the solar panel array and the stack or the load, depending on the situation. During peak sunlight, a charge controller bleeds off excess energy to protect the batteries. During peak demand, or during times of low sunlight, the solar panels must be entirely disconnected from the load or else the batteries are applying the voltage to the solar panels.

Instead of these inherent inefficiencies, the software controlled configurable connections can reconfigure the solar panels and batteries as needed. If the solar panel array has a low voltage output due to cloud cover, it can still be used to charge batteries, instead of being disconnected from the batteries entirely.

Built-In Power Conversion

The configurable connections made possible through software switching mean that the source can be reconfigured to form an arbitrary waveform generator. The name “arbitrary” is a bit misleading, as for this application, the waveform will be very carefully controlled. By switching connections at regular, timed intervals, it is possible to produce a clean sinusoidal wave, such as one at 60 Hz, which is required by most residential and commercial applications in the United States, or 50 Hz, which is used in many other countries. This means of switching connections is a much more efficient way of performing a DC to AC conversion, and produces a clean waveform with little distortion, as contrasted with cheaper inverters which often produce a square wave, or a wave with other distortions.

In a world that just used an AC frequency of 50 Hz or 60 Hz, it might be simple enough to build inverters to convert DC to AC. However, with the ability to produce virtually any waveform desired through software switching, there are many more possibilities.

Application Example: Refrigeration and Air Handling

Consider refrigerated air handlers in a commercial building, or freezers in a grocery store. Often, the compressors are powered by an AC motor that adjusts the speed of the motor based on the feedback loop provided by the temperature sensors mounted inside the building or the freezer. To convert this system to run on a solar panel array, DC power is generated by the solar panel. It then runs through a Variable Frequency Drive (VFD) controller to alter the frequency output (which also converts the signal from DC to AC) to control the speed of the motor that drives the compressor. Each one of these steps is lossy, converting much of the energy to heat.

Not only are multiple power conversion lossy, but the VFD must also account for the change in output power, not just the change in output frequency. Asynchronous AC motors try to maintain the same voltage to frequency ratio, as changing this changes the magnetic properties in the motor. Therefore, as the frequency drops, the output voltage drops. If configurable connections are used, the system adapts to the lower frequency, changing the output voltage to meet the demands. Furthermore, switching connections can be used to reduce the effects of “stepped” voltages that come from an integer number of batteries. Instead of reducing the output voltage by a set of defined steps as a set number of batteries are turned on or off, the switching system can produce a smooth transition, allowing custom voltage outputs that would be nearly impossible to produce using a fixed number of batteries.

With configurable connections, a bunch of these “middle men’’ can be eliminated. Instead, the solar cells or the batteries (depending on the output of the sun) can be reconfigured to produce a waveform of whatever frequency is required by the AC motor to drive the compressor. The feedback loop from the temperature sensors can be fed directly into the software and it will adjust the waveform as needed.

For DC drives, there are advantages as well. DC motors are often controlled by a Variable Speed Drive (VSD) which alters the duty cycle of the motor to adjust the speed. Instead of requiring a VSD, there is the potential to use software control to reconfigure the solar cell and battery network in a more efficient manner, perhaps sending energy to the battery when the motor requires low duty cycles. Depending on the load and the solar conditions, it could be optimized to draw power from the battery, solar cells or some combination during higher duty cycles.

Final Thoughts

Renewable energy sources offer the promise of a cleaner, greener future. However, some of the inherent inefficiencies must be addressed before their widespread adoption. One of the leading complaints about solar power systems is the variability of the sunlight. Even with an existing energy storage system, such as a battery network, there are losses due to the power conversion as well as reduced life expectancy on most batteries. All of this adds to the cost of the system. Furthermore, the over design required to meet the peak loads requires additional batteries that also add to the expense and complexity.

With configurable connections via software switching, the system does not need to be over designed to the same degree. The software smartly alters the connections on the solar cells as needed by the demands of the battery status and the load, making the entire system much more robust and maintenance-free.

The real advantage of the configurable connections via software switching is the ability of the system to adapt to many systems, eliminating unnecessary power conversions and lossy steps. With software control, feedback loops can be integrated with the power source instead of being run through a separate controller. VFDs and VSDs may not be required with the proper feedback and optimization routines.

Software configurable connections in the solar power system stand to improve the efficiency of the systems and smooth out the ebbs and flows of the sunny days, cloudy days, peak demands and battery-draining nights. To realize the full potential of renewable energy sources, these problems must be resolved, and software configurable connections are a key component in reducing inefficiencies.

Written By: Stephen Horowitz & Seth Price