Flow Battery

From Cell to Stack to System

Flow batteries separate the storage of electrical energy from the charge/discharge process; power and energy can be scaled independently of each other to meet the needs of many different applications. Very few technologies are realistically applicable to utility-scale electrical energy storage (EES); flow batteries are in an elite category. But they are also scalable to home and building applications for peak shaving or peak shifting, or for emergency backup power.

In a flow battery, energy storage is accomplished in arbitrarily large tanks full of electrolytes, while separate stacks of cells convert electricity into and out of different electrochemical (redox) states of those electrolytes. The tank on the left contains the catholyte, and the tank on the right contains the anolyte.


By Nick B, benboy00 - https://commons.wikimedia.org/wiki/File%3ARedox_Flow_Zelle_Deutsch_Farbverlauf.png, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=35143999

Of course, the single cell in the above diagram is actually a stack of about 14-19 cells in a real battery, a few of which are shown expanded below with both the membrane (typically DuPont NafionTM 117) and the porous graphite fiber electrode pads visible.

The charge collector plates also contain a serpertine "flow field" in the picture below, which is used to assure uniform distribution of electrolyte within the cell for minimum overpotential and reduced pumping losses.

There are many such plates in a stack, and many stacks are then plumbed in parallel in a battery. Time scales within a cell are on the order of fractions of a second, but any simulation of a flow battery must cover many days of charge/discharge scenarios.

This means that the complexities of modeling a realistic flow battery involve more than just combining fluid flow, thermal energy, electrical networks, and electrochemical treatment of redox reactions. They must also involve handling multiple time and distance scales simultaneously: you need to be able to zoom in on the fast time-scale multiphysics within a cell, while at the same time zooming out to the full system as it moves through its daily operational cycle.

Vanadium Redox Flow Batteries (VRFB) represent the current state-of-the-art, with 20+ years of reliable and safe operation demonstrated. Many research projects are underway to find alternate electrochemistries or membranes, or to reduce the cost and increase the performance of VRFBs.

Or just enjoy the diurnal variations of temperature gradients within a single cell of a 17-cell stack in a 6-stack battery over the course of a typical day:

 

Click here to fetch the VRFB case study on our User Forum

 

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.