The Key to Renewable Energy

Energy Storage Systems: Key to Using Renewable Energy

Bill Schweber for Mouser Electronics. Edited by Jon Gabay

Image Source: malp/Stock.Adobe.com

The push for renewable energy has never been so strong, especially in light of climate change and geopolitical tensions. The push to eliminate fossil fuels has gained relatively widespread acceptance.

One indicator is a global investment in renewable energies. When fossil fuel prices spike, investments in renewables are high. Once the fossil fuel costs go down, renewable energy becomes less cost-effective, and investment wanes. To meet climate and sustainability goals, better renewable energy technologies are required, and accelerated deployment of renewable energies needs to become a reality.

The key renewables are not always reliable. Wind and solar are intermittent, meaning larger arrays need to direct energy into energy storage arrays. Local and distributed energy storage systems (ESS) need to be in place. Presently, battery banks use lithium-based batteries, which present their own environmental concerns. Fortunately, lower-cost, cleaner, and higher-energy-density battery solutions are on the horizon. But for now, the formation of an infrastructure that can handle distributed generation, transmission, storage, and delivery to customers needs to be built out.

ESS Options Span a Range

As noted, the availability of the basic renewable energy source is only part of the broader energy puzzle. A complete system needs an energy source, storage, and transmission lines (Figure 1). When building a practical, complete system based on renewable energy, a significant concern is how to implement that interim energy storage.

Figure 1: A complete grid-connected ESS requires much more than just the energy storage subsystem; it also needs an energy source and transmission lines. (Source: Saft/Total Energies)

The need for energy storage applies to mobile and fixed-in-place installations, and the practical options are a function of the system size and setting. As the end application gets larger or is not required to be mobile, the preliminary list of possibilities expands (Figure 2).

 
Figure 2: While there are many apparent options for energy storage, their viability is a function of siting and capacity. (Source: Mouser Electronics)

Various energy storage ideas are under consideration (Figure 3), including pumping water to a higher elevation and letting it flow into a hydroelectric generator. The same approach can use an electric motor to lift a heavy weight and draw energy when needed by allowing it to descend. This technique can burst out high amounts of power for short durations. Another method is using a flywheel to store angular momentum. These are expensive to implement and maintain and have efficiency losses, in both delivering energy to store and drawing power to use.

 
Figure 3: Energy-storage schemes present a wide range of power and energy capacities. (Source: Elsevier/Science Direct)

Also examined are thermoelectric techniques like molten-salt technology, in which salts are heated to a degree that they melt. The thermal energy (heat) can then be drawn off and used to create electricity. This may be a viable way to store heat energy for hot water heating. But heating the salts to a high enough temperature and with enough heat storage to run a steam turbine for a sustained time is not a trivial task.

Compressed air storage is another possibility. Heavy tanks and compressors store air at very high pressures. The moving air is harnessed and turned into electricity, creating a clean generator. Implementing this on a large scale could be expensive, and again, mechanical systems have losses and maintenance.

Using intermittent renewables like wind and solar to split water into hydrogen and oxygen may provide a good alternative if it can be done more efficiently than standard hydrolysis. Experimental plating materials have shown promise in eliminating the need for expensive platinum used in current methods. And there have even been biological reactors that hydrolyze water. The benefit of separating hydrogen is that it can be used as a fuel for automobiles and can directly create electricity using fuel cells.

Electric-to-Electric Is the Attractive Option

Electrical storage is most desirable since it is potentially the most efficient. Switching power supplies are highly efficient; so with energy coming in the form of electricity, the best approaches are batteries and supercapacitors-based storage with switching high-power delivery techniques.

Fortunately, the electronics industry has developed new power technologies like onsemi’s wide bandgap diodes, MOSFETs, and drivers. These devices use the company's proprietary silicon carbide (SiC) technology, which can withstand much more punishment without failing.

The wide bandgap technology lets high-power diodes withstand repetitive reverse voltages (VRRM) of 1700V with only 80nA reverse current. The Schottky configuration has just a 1.5V forward voltage drop at 25A. Other family members can pass up to 100A with low loss and heat and up to 882A surge currents. These specifications are necessary for tying high-voltage and high-current panels together at a large local battery charger or a high-power, grid-tied inverter.

In addition, SiC technology results in fast switching speeds and is also used to make low-loss MOSFETs with up to 46A current carrying capacity. The rugged 900V reverse voltage rating helps stand up to spikes and switching noise, and this technology reduces electromagnetic interference (EMI) while providing the highest power density and smallest size.

For example, the onsemi NTHL060N090SC1 EliteSiC MOSFET has a drain-source resistance of only 84 milliohms at 46A and can dissipate up to 221W. As a switching element, it can withstand up to 900 reverse volts. A companion gate driver like the NCP51705MNTXG makes it easy to interface with the EliteSiC MOSFETs and take advantage of the technology’s fast rise and fall times. This driver can put out up to 6A gate drive with fast 50ns turn on and off delays. The small QFN24 can dissipate 2.9W and draws just 12mA.

What makes this technology so immediately exciting is the availability of power modules like the NXH100B120H3Q0STG IGBT modules, which can handle up to 80kW and 100A (Figure 4). These modules have built-in bypass diodes and boost diodes and snubbing IGBT protection diodes, all included in the low-inductance layout (Figure 5).

Figure 4: The onsemi NXH100B120H3Q0 dual-boost power modules simplify design, implementation, and maintenance of high-energy, renewable energy, high-voltage, and high-current systems. (Source: Mouser Electronics)

Figure 5: Ideal for solar inverters, uninterruptable power supplies, and energy storage and metering systems, onsemi SiC power modules are available now and ready to use. (Source: onsemi)

Batteries Take the Lead

Rechargeable batteries offer many favorable factors. They require very little site preparation and have no moving parts—once installed and operating, they are virtually maintenance-free. Even though classic lead-acid cells are 90% completely recycled, lithium-based chemistries are often the preferred choice. Lithium chemistries offer higher energy density by weight. Like any battery technology, lithium-based batteries can be scaled to needs, in series for higher voltages or in parallel for higher currents. If designed right, individual cells can be replaced without interrupting the rest of the array.

Batteries require very little site prep and have no moving parts. Once installed and operating, they are virtually maintenance-free. One key reason lithium-based battery units are being adopted for utility energy use is the increasing popularity of electric vehicles (EVs). As demand grows for lithium-based batteries—for EVs as well as for consumer devices—increases in high-volume mining, manufacturing, assembly, and use of high-energy-density power packs lessen barriers to the production process.

Once again, the electronics industry has met these challenges with small and cheap battery management chips that charge and meter energy safely. These devices can also protect the rest of the battery array in case of single-cell failures.

Another option is to build a fixed-in-place system using low-cost, used batteries that have been recovered from older or wrecked vehicles, which can offer considerable capacity. Generally, batteries are considered "done" and no longer fit for their initial applications when storage capacity drops to 80% of the original value. That still leaves significant capacity for a fixed-site installation to reuse and recycle such second-life cells (Figure 6).

Figure 6:  The growth in electric vehicles is also yielding capacity via used “second life” batteries from these vehicles. (Source: Circular Energy Storage Research and Consulting)

Combining vehicles and utility grids, the potential exists to tie an electric vehicle to the grid during power outages. With the proper electronics and distributed infrastructure, vehicles can now take the place of substations for short periods.

On-Grid or Not?

As mentioned, a battery-based ESS can stand alone for local use only or can be connected to the grid. Topologies are in use now to allow energy inserts and drops in a distributed environment.

A sophisticated ESS algorithm can balance the allocation of power flow for a combination of the highest availability and lowest operating cost. That is, when the source is available and power costs are the lowest, the system uses the grid to charge the batteries. In turn, when grid costs are high or when renewable sources are unavailable or insufficient, the system uses the batteries.

More advanced ESS designs allow for further benefits and overall cost reduction by putting power back into the grid when the batteries are charged and the grid is no longer needed, as the renewable source can supply the power required. They use a bi-directional ESS power unit, which transparently directs the flow of power from where it is available to where it is needed or can be stored.

If energy is available from distributed sources, facilities and homes can be supplied energy as if an uninterruptable power supply (UPS) is present. The end user never sees an outage (unless physical damage to the grid occurs). Minor brownouts can occur, but local UPS systems can be used to keep critical devices like medical machines alive until the facility regains power from the grid.

The standardization of the constituent components’ technologies has now advanced in performance and dropped in cost to the level where these online systems are now available for residential use, such as the Generac PWRcell system (Figure 7).
 

Figure 7: New residential systems, such as the Generac PWRcell, seamlessly integrate grid power, solar-cell power, and battery-based storage; it can also incorporate an optional generator (not shown). (Source: Generac Power Systems, Inc.)

Conclusion

Energy storage systems are an essential building block of any power-sourcing and delivery arrangement that relies partly or entirely on an intermittent or unpredictable source. Users have many options for providing this storage, each with tradeoffs in critical electrical, mechanical, and physical performance and installation parameters. Battery-based energy storage is desirable due to its availability, modularity, scalability, energy density, manageability, and noise-free operation.