In recent years, the use of batteries with non-grid-tied, solar photovoltaic (PV) systems has grown immensely. Integrating battery banks with PV arrays enables owners to utilize solar energy at night when the sun is not shining. During the day, a portion of the output from the array (often excess output after meeting all load demands) goes towards charging a string of batteries. Stored energy in the batteries can then power electrical systems during non-generating hours. The result is a solar PV system that is more efficient and a commercial or residential building that is more environmentally friendly.
For owners and operators of battery-based PV arrays, an important decision that can have lasting effects on both the operation and return on investment of the system is the type of charge controller one uses. As its name indicates, a charge controller manages the charge of a battery bank. Its primary purpose is to prevent charge from flowing out of the batteries back into the solar array at night when voltage output is zero. It also plays a critical role in preventing overcharging, which can often cause damage to batteries from gassing and overheating.
Today, two types of technologies dominate the charge controller market for solar PV systems: Pulse-width modulation (PWM) and maximum power-point tracking (MPPT). Each technology possesses unique advantages and disadvantages that make it more or less suitable, dependent on variables such as array output, cost and space constraints, efficiency, ambient temperature, and others. The remainder of this blog will help owners and operators better understand how these variables impact controller selection so that they can maximize the efficiency and lifespan of their PV systems.
While PWM and MPPT charge controllers serve the same purpose of managing battery charge, they operate in two very different ways.
A PWM controller is more or less a switch that connects the solar array to the battery bank. This controller works by gradually reducing the amount of power applied to the batteries as they get closer to becoming fully charged. The controller allows the specific condition and charging needs of the battery bank to taper current from the solar array. In addition to reducing battery cycling and extending life, this keeps batteries in a fully charged state during generating hours—something many solar technicians refer to as “floating.”
MPPT controllers are more sophisticated direct current to direct current (DC/DC) converters that work by adjusting input voltage to a battery bank to allow the maximum harvest of power from a solar array. These controllers optimize the connection between the PV array and the battery bank by converting the high-voltage output from solar panels down to the low voltage required to charge batteries safely. In systems that use MPPT technology, generated power from the panels can meet voltage requirements for the external load, while simultaneously charging batteries.
Both PWM and MPPT controllers possess unique advantages and disadvantages that make them more or less suitable depending on the design characteristics of the solar array, cost, and environmental variables. Some important factors to consider when selecting one of the two technologies include:
While PWM controllers are far more advanced than the “on/off” regulated, state-of-charge switches that were used in primitive battery-based PV systems, their primary disadvantage when compared to MPPT controllers is efficiency. PWM charge controllers cannot use the maximum power produced by the PV array and their overall efficiency often ranges from 65-85 percent. MPPT controllers, on the other hand, ensure that the solar array is operating optimally always. In some cases, such as in cooler climates, they offer up to 30 percent more efficiency than PWM.
The overall size and output of the solar array have perhaps the most significant impact on which controller technology to use. Generally, a PWM charge controller is a cost-effective solution for smaller systems when solar cell temperature ranges from 45–75°C. MPPT controllers, on the other hand, are a better option for larger systems (150–200W), where higher efficiency can equate to substantial financial gains. Additionally, because MPPT enables owners to harvest the maximum amount of power from a PV array, they are often advantageous in situations when there are additional power needs, but land constraints don’t allow for physical expansion of the array.
On the basis of the initial capital investment, MPPT charge controllers are more expensive than PWMs. However, because MPPT controllers possess higher charging efficiencies, especially in cooler climates, that cost is often recoupable over time. MPPT controllers also provide the advantage of being able to wire many PV panels together in series. In these situations, the cross-sectional area of connecting cables can greatly decrease, which results in a corresponding reduction in overall system costs.
It’s important for owners and operators of battery-based PV arrays to understand that in some cases design characteristics of the system will necessitate the use of one controller or another. For example, in systems where a solar-input nominal voltage is higher than a battery-bank nominal voltage, an MPPT controller is always necessary. Such is also the case in systems that contain high-voltage “grid connect” panels where voltages exceed 35V.
Overall, however, no single technology is universally best, which makes it critical for owners to consult with manufacturers and PV system installers to ensure selection of the right controller for the unique variables associated with a specific system.
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