Energy Storage Systems: Getting Solar Energy to the Batteries

How System Optimization Integrates Solar Energy with Battery Storage

Adam Kimmel for Mouser Electronics

Image Source: KanawatTH/Stock.Adobe.com

Solar energy technology is booming, with more capacity added each year. But how does the energy get from the source to the energy storage system (ESS) and then to the load? This process—power delivery—is simple in concept but complicated to implement, especially as the amount of energy and consistency of the energy source varies (often unpredictably) and the system power level changes.

Usable solar energy is precious. It is not enough to store energy in a battery and then send it to the load via an inverter; you must first collect that energy at maximum efficiency and route it to the energy-storage subsystem with a high-quality controller. This article will provide an overview of power delivery and look at popular ESS methods.

Energy Storage Systems

Three main components define an ESS (Figure 1):

• The path between an energy source and the energy-storage unit (often a battery energy storage system (BESS), though other forms are possible)
• The energy-storage unit and its management
• A DC/AC inverter between the storage unit and the load (i.e., end user or grids).

 
Figure 1: The battery accepts and accumulates energy from various sources and passes it as power to the loads via a DC/AC inverter. (Source: Mouser Electronics)

The ESS accumulates the source energy and distributes it as power to the load on demand. However, with solar power that energy is available only intermittently. This feature makes power resiliency, critical to applications such as residences and businesses, a significant challenge. Energy storage has emerged as an important intermediate to smooth the intermittency of renewable source energy.

Batteries are a leading solution to energy storage due to the rapid demand for advanced chemistries that electric vehicles (EVs) have created. The energy-storage battery and management system located between the source and load must mediate the amount of energy collected and available to satisfy the power demands.

Capacity-Driven Architecture

There are nearly infinite combinations of energy source and application loads. As a result, there is not one preferred architecture to deliver optimal performance. In addition, solar-based photovoltaic (PV) devices have widely varying segmentation based on the power's magnitude.

For solar, one widely used market segmentation has three divisions:

• Residential: For private spaces requiring up to 10kW of power
• Commercial: For offices and factories requiring up to 5MW of power
• Utility-scale: Installed in the field, providing more than 5MW of power

Solar Energy Scale-Up Considerations

Solar panels are constructed of multiple individual PV cells, each producing a single-digit voltage output. System designers connect these panels in series to maximize the efficiency of the architecture and deliver the required power. As a result, the size of a solar power system can provide the specific application power more accurately.

The following sections will focus on PV-based BESS for residential and even smaller commercial installations, as this is a common application that is familiar to consumers.

How Solar Energy Charges the Battery

A convenient benefit of battery-stored energy is the potential to use it independently or to connect it with the grid as backup or peak-demand support power (Figure 2).

 
Figure 2: Block flow diagram of critical functions required for a complete solar-powered energy storage system that powers local loads and the grid if needed. (Source: Infineon Technologies AG)

The electronic interface between the panels of PV cells and the batteries is a DC/DC converter with buck, boost, or buck/boost characteristics. The type of converter a designer selects depends on comparing the PV output's relative maximum voltage and the battery array's maximum voltages.

However, enabling optimal power delivery from solar panels requires a charge controller, like the Phoenix Contact AXC F 2152 PLCnext Controller. The charge controller transfers maximum power from the PV cell output first to the DC/DC converter and then to the storage battery at the maximum power point (MPP), where the source power matches the load. The AXC F 2152 controller is ideal for solar applications as it delivers maximum performance in rugged environments.

A solar cell produces current in proportion to the amount of sunlight falling on it while its open-circuit voltage remains relatively constant. Maximum power output occurs at the knee of each curve, where the cell transitions from a constant voltage to a constant current device, as shown by the power curves in Figure 3.

 
Figure 3: Maximum power output of the solar panel occurs when the cell transitions from a constant voltage to a constant current device. (Source: Analog Devices)

The MPP is a function of PV panel/solar source energy characteristics and ambient temperature. The highest-efficiency charger design aligns the solar panel's output voltage to the maximum power point when sunlight levels cannot support the charger's full-power requirements. This feature extracts more power from the conversion step, boosting energy efficiency.

Thus, to extract maximum power from the PV panel over the use, engineers should monitor MPP and the panel load to control the converter and optimize power output dynamically (Figure 4). This action is called maximum power-point tracking, or MPPT.

 
Figure 4: Basic MPP management (here, for a lead-acid battery) requires adjusting the DC-DC converter as the load the panel sees depends on the panel output. (Source: ResearchGate)

MPPT requires a strategy or algorithm in the charge controller to determine the MPP and then track it. Engineers employ two techniques to track maximum power: constant panel voltage and perturb and observe.

Constant Panel Voltage
The most straightforward tracking technique sets panel voltage to a constant level determined by the cell's open-circuit voltage (VOC), provided by the cell's datasheet. Design engineers estimate a solar panel's voltage at maximum power (VMP) at a fixed voltage just below VOC. To simplify the approach, the design team considers the temperature coefficient at VMP equal to VOC and linear across the expected temperature range. These approximations enable a simplified temperature-compensated resistor to set panel voltage at VMP.

Perturb and Observe (P&O)
The constant panel voltage carries a drawback: It cannot continue to provide maximum efficiency as conditions change, such as exposure to varying cloud densities and normal wear and tear of the PV components.

A more advanced approach that adapts to the conditions for MPPT tracking is called "perturb and observe" (P&O). P&O MPPT assesses the slope of change in power versus change in voltage (ΔP/ΔV), which is positive to the left of MPP, negative to the right of MPP, and zero at the local maximum, denoting the optimal voltage. The dynamic MPPT algorithm maps any changes in MPP by deliberately "perturbing" the panel load slightly around its nominal value and then observing the changes (for better or worse) in the output.

The controller’s embedded MPPT algorithm offers maximum efficiency in terms of power harvested from the cells and transferred to the output, regardless of varying environmental conditions such as irradiation, dirt, and temperature. Once the controller has finished its start-up mode, it begins an MPPT mode to search for the maximum power point. Figure 5 shows how the pulse-with-modulation (PWM) signal duty cycle changes to locate the zero-slope point on the curve.
 

Figure 5: The MPPT approach assesses the slope of change in power versus change in voltage characteristics for the PV panel around its nominal operating points. (Source: SN Applied Sciences)

Getting the Power Out

Directing energy into the battery is only half the challenge in a BESS. The system aims to get the accumulated power from the batteries to the load, usually a 120/240VAC line that can power line-operated devices and systems.

The output function requires a DC/AC inverter, which takes the battery's DC output and transforms it to line-compatible AC. Like the electronics between the source and the batteries, this inverter is not a "one-size-fits-all" unit. Engineers must consider the topology and design of the inverters and the many design challenges and tradeoffs. Although no formal set of definitions exists, industry experts often classify inverters into three power and attribute categories: low, medium, and high.

Micro-Inverter (Low Power)
Rated between 50W and 400W, a low-power micro-inverter integrates a separate inverter and MPP tracker in each solar panel and is more efficient than string inverters. There is minimal DC cabling, but it requires extensive AC cabling. So it is only a good, economical fit for small systems.

String (Medium Power)
A string is a medium-power configuration for between 1kW and 20kW. In this approach, the solar panels connect with multiple inverters in series (or strings), most often one per string. The process enables high efficiency because every string can operate independently at its maximum power point.

Central Inverter (High Power)
A central inverter is a high-power configuration operating at 20kW and above. This category arranges strings in parallel, with only a single inverter for a set of solar panels. Because the strings are at different voltages, engineers add special diodes to drive the panels to maximum power. However, the diodes carry inherent loss, which reduces efficiency. Therefore a central inverter may not allow all solar panels to achieve their maximum power point.

Takeaways

Renewable energy brings new opportunities to power control, in areas such as intermittent supply and power delivery architecture. Connecting a solar panel to a battery with a crude controller and using the battery for power may work occasionally, but it also carries performance shortcomings, safety concerns, and efficiency issues.

Instead, selecting an appropriate controller and DC/DC topology for the energy source–storage battery–battery management path provides a better method. Engineers should optimize the selected DC/AC inverter to ensure performance efficiency, consistency, longevity, and resilience.