Flow Battery, Flow!

Brent Cullimore

The field of positive psychology is devoted to making so-called “normal people” happier. A key observation is that people tend to be happiest when they are in a state called Flow. (As much as I would like to live the State of Flow, Colorado comes close!)

In this state of relaxed concentration, people are performing an activity (knitting, fly fishing, solving crossword puzzles etc.) to which they can devote themselves exclusively ... at least for a little while, and which they do well.

So is a flow battery a happy battery because it is doing just one thing, and doing it well?

Or was I just experiencing flow while building math models of flow batteries? After all, just adding the word “flow” to anything gets my attention, living as I do on the Wet Side of Mechanical Engineering Boulevard.

So why should a battery flow? Other than the American Declaration of Independence asserting that pursuing happiness is everyone’s unalienable right, of course.

Perhaps the biggest advantage of a flow battery is that there is no design constraint between the maximum rate of energy that you can put in or take out of one, and the amount of energy that you can store inside. You can design a flow battery that accepts a mere 1MW charge or discharge rate, yet holds 5GWh of energy. Or you can design one that can charge at an astounding 5GW rate but which only holds 1MJ of energy. Why you do either of those extremes is beyond me, but the point is: you could.

If that isn’t impressive, then please learn to be disturbed when some press release says “a 50 MW battery” but doesn’t say how big it is. Or if it says “a 50 MWh battery” but doesn’t say how fast you can discharge it. C’mon people, we’re engineers. We have full right to get upset when a journalist can’t tell amps from amp-hours!

Do a search on “flow battery” maybe sprinkling in keywords like vanadium (VRFB) or iron-chromium (ICFB) or zinc-bromine (ZNBR). You’ll be amazed at what is already available and has been life-tested for decades, and what is being developed for the next generation. You might start with a little overview, such as this:

As I have mentioned in a prior blog, we need grid-scale batteries, and most technologies struggle to reach that scale and yet stay cost-effective.

There is an incredible amount of research and investment happening in the energy storage world, and flow batteries are but one recipient. A majority of the attention (and R&D money) is still flowing into dry cells, especially that stalwart: the lithium-ion battery. The press is full of stories of advances in that technology.

In fact, it is darn hard to wax effusive about flow batteries in the same week that Tesla and Panasonic showcase their Nevada-based lithium-ion battery Gigafactory.

That’s OK, CRTech software can model lithium-ion batteries too, at least in regard to thermal management and investigations of runaway. In fact, here’s a model derived from a Panasonic battery that happens to be a lot like the ones that Tesla uses:

Modeling a dry cell may not bring the same joy, but if I were a betting man (meaning: "if I were not an engineer") I would bet on some variation of a lithium-ion battery winning the Charge Wars, especially for vehicles.

Still, I’d hedge with a side bet on flow batteries, since it really is too soon to tell which one will win the race for utility-scale batteries. Both types have already been deployed for that application. I personally think that extra research money should flow toward flow batteries for grid-scale electrical energy storage because of scalability, longevity, and safety. Yes, you read that right: I’m OK betting someone else’s money on flow batteries!

May you find happiness in whatever you enjoy and excel. And may both you and your batteries flow.

dispersed vs. coalesced front

Tuesday, June 26, 2018, 1-2pm PT, 4-5pm ET

This webinar describes flat-front modeling, including where it is useful and how it works. A flat-front assumption is a specialized two-phase flow method that is particularly useful in the priming (filling or re-filling with liquid) of gas-filled or evacuated lines. It also finds use in simulating the gas purging of liquid-filled lines, and in modeling vertical large-diameter piping.

Prerequisites: It is helpful to have a background in two-phase flow, and to have some previous experience with FloCAD Pipes.

Register here for this webinar

FloCAD model of a loop heat pipe

Since a significant portion of LHPs consists of simple tubing, they are more flexible and easier to integrate into thermal structures than their traditional linear cousins: constant conductance and variable conductance heat pipes (CCHPs, VCHPs). LHPs are also less constrained by orientation and able to transport more power. LHPs have been used successfully in many applications, and have become a proven tool for spacecraft thermal control systems.

However, LHPs are not simple, neither in the details of their evaporator and compensation chamber (CC) structures nor in their surprising range of behaviors. Furthermore, there are uncertainties in their performance that must be treated with safety factors and bracketing methods for design verification.

Fortunately, some of the authors of CRTech fluid analysis tools also happened to have been involved in the early days of LHP technology development, so it is no accident that Thermal Desktop ("TD") and FloCAD have the unique capabilities necessary to model LHPs. Some features are useful at a system level analysis (including preliminary design), and others are necessary to achieve a detailed level of simulation (transients, off-design, condenser gradients).

CRTech is offering a four-part webinar series on LHPs and approaches to modeling them. Each webinar is designed to be attended in the order they were presented. While the first webinar presumes little knowledge of LHPs or their analysis, for the last three webinars you are presumed to have a basic knowledge TD/FloCAD two-phase modeling.

Part 1 provides an overview of LHP operation and unique characteristics
Part 2 introduces system-level modeling of LHPs using TD/FloCAD.
Part 3 covers an important aspect of getting the right answers: back-conduction and core state variability.
Part 4 covers detailed modeling of LHPs in TD/FloCAD such that transient operations such as start-up, gravity assist, and thermostatic control can be simulated.