Should batteries be considered as power sources or are they just transitional energy-storage vessels? This is not a trick question: the answer depends on how and where they are being used as well as semantics and an understanding of their role.

If you are using a battery alone to power your device, gadget, or whatever, you likely regard it as a source. But in the bigger picture, unless you recharge or replace that battery, it is just a depleting reservoir for stored energy. In other words, as the cliché goes, where you stand on the matter depends on where you sit. (Note that I implicitly include supercapacitors in this discussion; even though they use a very different energy-storage mechanism, they still represent a rechargeable, battery-like device.)

I was thinking about this because of all the attention I see being given to batteries as never-ending power sources, even on a larger scale. For example, there was a recent research project which discussed a new form of battery. In this case, the project “Micro-supercapacitive stone module for natural energy storage” discussed how researchers at Kookmin University and the Kumoh National Institute of Technology (both in Korea) fabricated stone materials into supercapacitors to function as energy storage systems (ESS) units which were rugged enough to be used for kitchen counters and so could be built into the house (Figure 1).

Figure 1 By fabricating a high-performance asymmetric microsupercapacitor (MSC) on a natural stone surface, researchers envision a scalable energy storage wall applicable as a three-dimensional energy station inside or outside a whole-building interface. Source: American Chemical Society

Okay, so far so good. But some of the public-relations messaging associated with this research went beyond that role, implying they have come up with a novel way to power future smart home systems using these stones that can turn countertops and walls into batteries. There were headlines such as “Future smart homes could be powered with electronics built on stones” and some reports called these supercapacitor arrays “power supplies” which they are and they are not, depending on your perspective.

Even if the energy-storage capacity (watt-hours) of these supercapacitor countertops was sufficient to power the house for an unspecified amount of time, what’s the source of energy needed to charge them: solar, wind, other? There’s a “something for nothing” aspect to these stories which implies that batteries are infinite energy sources. Yet as we know, when it comes to energy and power sources, there are very few free lunches.

I have the same concerns with discussion of hydrogen as alternative fuel for fuel cells. Yes, the amount of available hydrogen is enormous, but getting it to be a fuel doesn’t come easy. Whether you use basic electrolysis of water or a more advanced process, that extracted hydrogen is still a stored-energy resource. Yet many discussions about the benefits of hydrogen have the implication that it is a primary energy source rather than a storage medium.

It gets even more confusing with the attention given to using electric vehicles and their batteries’ “power sources” in what is called vehicle-to-grid (with the snappy acronym V2G). In many ways this seems like a great idea: use all EV batteries to support a home or even the grid when the grid is down; even better, when EVs are collecting “extra” power they won’t be needing immediately, they can send that excess back to the grid (Figure 2).

Figure 2 In a vehicle-to-grid topology, the batteries in the vehicle can be use as the basis for an energy storage system (ESS) which can send power to the grid as well as charge from the grid. Source: Elsevier/Science Direct

Except for this: if I step back and try to get an overview, this discussion has some aspects of a perpetual-motion machine with energy going back-and-forth as cars send energy to the grid and the grid charging the cars in a circular relationship. Each party in the arrangement assumes the “other guy” will provide power but that may not be a viable long-term plan. In addition, V2G also requires that each vehicle be equipped with a bidirectional power converter which adds to design complexity, thermal issues, space requirements and, of course, cost.

There’s no doubt it makes sense in some cases to have all those vehicles willingly dump their precious battery energy onto the grid under the right conditions, but I’m not seeing it happen often, or on a wide scale. It does seem somewhat ironic or wishful that people would spend big dollars for a vehicle with larger-capacity battery and to achieve greater driving range, only to then divert some of that energy to the anonymous grid.

This is not to say that there is no role for using the batteries of a vehicle as a source of AC power, unrelated to supporting the grid. The popular Ford F-150 with an integral Pro Power Onboard system has a built-in generator and full power panel that uses the truck’s engine as a generator when battery power is getting low and so keeps the household-style 120-V outlets in the bed powered and useful, Figure 3.

Figure 3 The Ford F-150 is offered with a complete battery-based ESS capable of proving 120/240 VAC with automatic charging of the battery pack from the main engine as needed. Source: Ford Motor Company

The engine kicks on and off as needed to keep the system powered; with the hybrid model F-150, it provides up to 7.2 kW of available power for running tools, lights, and even appliances. Still, using it as a sophisticated generator for local jobs or power during a blackout is very different than using it in V2G mode to support the grid.

You can see some interesting V2G perspectives and predictions in many online sources including a trio of recent articles in IEEE Spectrum (see References). Note that V2G is also getting considerable attention in non-technical media and sites but much of it is simply wrong on the technical facts and confused or muddled on key issues.

What’s your view on the viability of V2G as a broad-based approach to using vehicle batteries to store excess power generated from various sources such as renewables, then sending this power to the grid if needed? Technically feasible? Worthwhile? Going to happen on large scale? An “academic” exercise but not much more?

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.

I don’t see the V2G capability being used for anything beyond offering a frequency stabilization resource, or as a means to power your home during a blackout. I don’t think actually powering the grid is going to happen without significant return benefits, benefits that are largely absent in the solar energy export arena.

The concept of a “primary power source” seems elusive to me. All energy “sources” are really storage systems. The only real distinction here is how fast the storage device can be refilled. Some are very fast, based on either a liquid fuel (hydrogen, or petrol), or the ability to charge very quickly (super-capacitors). The remainder tend to be significantly slower (battery charging, pumped storage, gravity storage).

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