Energy + Redox Flow Battery Storage is Less Expensive Than You Think

I recently sat down and priced out two battery systems. In doing so, I compared two very different technologies in an application of interest to me. You see, I would very much like to be independent of any grid system. I believe that there is no reason for anyone with the financing and desire (…oh and skills enough to bake a cake) to rely on a grid. Unfortunately, not everyone can afford this luxury (so financing is key); this is in part why I decided to write this piece. It is, after all, crucial to know what you are in for before you start to bake, right? This article is the first of several in a series of articles I plan to write in a sort of “how-to” tutorial. This particular piece is intended to introduce a couple of energy storage options, the benefits/drawbacks associated with each, and compare the lifetime cost of storage. Namely, I compare the capital cost of a lithium ion battery (LIB) with a redox flow battery (RFB) in a residential use case scenario. I will be honest, I have a bit of a bias towards RFBs, which are a personal favorite of mine and have been for many years (hard to forget your first love, if you will). However, I try to be battery agnostic when it comes to systems design, because I believe that each chemistry has its benefits and drawbacks. In fact, I proposed hybrid energy storage systems (hess) as a logical solution to meeting small and large-scale energy storage needs when coupled with intermittent power resources about 6 years ago; I was involved with a project group evaluating wind power generation support using a standalone microgrid supported by slow, medium, and fast response energy storage. That said, both LIBs and RFBs have a variety of different commercial chemistries available and each are ideally suited for somewhat different applications; for now, the important dividing factor to note is that one battery chemistry is aqueous (i.e., water-based) and the other is non-aqueous (i.e., organic—these include polymer electrolytes), which differ greatly in the energy density. Although there are major developments ongoing to make non-aqueous electrolyte RFBs commercially available.

First things first, to support a home off grid you need to know your approximate power draw. An important thing to consider is that a battery system should never be designed for peak power loads; a capacitor bank is much more ideally suited for meeting peak power needs. Capacitors have lower maintenance and per cycle cost. Besides, power spikes are few and far in between, so stringing up some capacitors on the front end might save you some money on the back end. For example, peak power draw situations might occur when you throw a cake in the oven right as your central heating/cooling system fires up. So, instead of building a massive battery to support these power surges, you can buffer the short power spikes with capacitors. In any case, a well-designed system will account for most of the possibilities (never all). More to the point, I highly recommend a little device called Kill-A-Watt (for the obsessive compulsive), which runs about $30 at Walmart. This is a device that will allow you to (quantifiably) asses your power usage and record real data on most of the major appliances in your home (and by natural deduction you may even get data for all major appliances). However, a simple (and free!) way to obtain sufficient information about your power usage is to just read your power bill. Be mindful that even utility companies make mistakes; I once paid my neighbors power bill for several months and by the time I finally resolved that fiasco I had accumulated about a year in credits on my account.

Once you have a good handle on usage, you can research options you have for power generation (these vary quite a bit from location to location). The three big ones are solar, wind, and hydro. With solar you are driving electrons through a work circuit by exciting them with light… cool... but inefficient and generally requires a large footprint; solar is simply impractical in certain places around the globe (where sunny blue skies may be scarce). Wind is another excellent source of power (but extremely hard to predict); essentially the wind turns a prop on the shaft of a generator producing power. Hydro is an excellent resource (if available); access to a flowing stream, consistent rain fall, or large body of water with elevation change in the surrounding area is a requisite. Hydro can be a seasonal resource, as streams tend to dry up and stop flowing. One major perk with hydro is that if it is coupled with any other reliable power source, it can serve as energy storage in lieu of a battery. Ideally, you should always establish more than one power source. One of my favorite combinations is solar with wind, because of the complementary nature of the two together. The thought behind this is that since it takes wind to move clouds, theoretically you would be generating power from at least one of those two resources, either all, or most of the time. All three—solar, wind, and hydro—are considered renewable power generation resources. The problem with renewable resources is that they are intermittent in nature. So, what happens when the wind brings the clouds in and then stops blowing? The prop slows to a halt and the cloud cover prevents any substantial solar generation. That is a major bummer, but leads into the underlying theme of this article. A battery can offset excess energy generated to supplement deficit power generation periods (or even provide power when there is a complete lack of generation). The question remains, how much excess do I plan to offset? Well that all depends on how much money I have, right? Wait what!? Yes, energy can be a costly metric.

Jumping right into it, larger LIBs can be (and traditionally are) built by stringing together a bunch of smaller cells (e.g. 18650 cells that range in price from $1.50 to over $5 per cell on Amazon). I know, how primitive, right? When taking this approach, there are two “best practice” safety measures to consider, (1) isolate a certain number of these cells in fire proof pouches (to reduce risk loss or damage to hardware), and (2) establish a voltage cut-off for each cell (yes, I said each cell, not string). When you have hundreds of these cells strung together it can become a “power electronics” challenge to manage all of them on an individual basis, but safety should always come first and there are tools available to do this (e.g. the Ultra Power Balance Charger available online). It is important that the voltage of each cell is at least monitored, because a single run-away cell could set off the rest of the string. On a more positive note, lithium ion batteries—no matter what the cathode chemistry is (no pun intended)—are considered a power and energy dense chemistry. So, if this is the system of choice you may bypass the need for a capacitor bank as part of the overall storage system. This, of course, depends on the number of cells you string up, but generally speaking if you build out a 30 kWh LIB system (which is what I recommend for any decently sized single-family home) you should have ample instantaneous power capability to meet the peak power demands of that home. Note I use the energy metric (kWh) to determine roughly the power capability (kW) of a LIB system; this is because a good rule-of-thumb is that the rated power and energy are about 1:1 under normal operating conditions. There are more power dense LIBs that are commercially available, capable of delivering much higher power for a shorter duration. However, these particular cathode and electrolyte composition cells are better suited for UAV’s and other mobile and remote-control type applications, where rapidly re-charging might be especially of interest. This is simply not the case with a stationary support system, where the charge/discharge happens over longer time periods, spanning hours (not minutes) and size of the device is not necessarily a huge concern.

A typical RFB comprises (1) reaction cores, (2) at least two electrolyte tanks, and (3) pumps and power electronics. The reaction core (or reactor—I know sounds uber advanced) is essentially the battery minus the energy part of it. If the reactor core is flooded with an electrolyte it would perform like a traditional battery pack, i.e., could undergo charging/discharging like a normal battery and would have a fixed energy capacity (that would depend on how much electrolyte could fit into the void volume of the reactor). The beauty of this type of battery is that the power and energy can be designed separately (i.e., more or less decoupled). So, if I want more energy (for say a longer deficit support duration) then I increase the volume of electrolyte in my tanks. This is where the pumps come in; whatever volume I decide on, I would need to circulate from one tank, through the reaction core, and back into the tank. This is the case for both the catholyte and anolyte (i.e., both tanks are circulated through the reactor). A major drawback, of course, is that you are relying on the pumps to launch the battery in order to harvest the energy stored as chemical potential in your tanks. That said, there are few clever ways to cold start a RFB, and the subject of another article I plan to publish (so, stay tuned—at any rate feel free to comment if there is a desire to discuss). In any case, a cold start would be a rare event, especially with a dual power source generation system.

As far as safety goes, RFBs have extremely low risk of a fire hazard, but do have some risk pertaining to electrolyte spillage. Three of the most popular commercial chemistries are zinc bromide, all-vanadium, and all-iron, which are very much acidic (pH < 2). Nothing that serious in my opinion; in all honesty, the zinc bromide would probably serve as a good laxative and save you a few bucks at the pharmacy next time you prepare for your next colonoscopy. Besides, a simple solution to mitigate the spillage risk is to use secondary containment, e.g. placing your small buckets into a larger second bucket should do the trick. Fortunately, since we’ve been using buckets for a long time now, this is very straightforward. However, this aspect of the RFB should not be taken lightly. I speak from experience, when I say nothing puts a damper on your day like getting sprayed with some concentrated acid; we had an intern once that engaged the pumps prior to checking all the port connections—enough said. That said, once you design and build the RFB system, it will last you a very long time, and the energy is expandable (like having an SD card slot on your mobile device—how convenient, right?). The reactor(s) and pumps are the limiting factors in service life, with evidence for extremely long cycle life (approaching 6000 charge/discharge cycles with no evidence of permanent degradation on the reaction cores—so you’d better not skimp on the pumps).

Besides the aforementioned hardware cost, another key determining factor in battery price is the raw materials cost of the key energy carrier(s), which often set the price. For example, mining and refining lithium salt from minerals is a huge bottle neck in the industry today (even though lithium is a relatively abundant element). As far as abundance and annual production rates go, here is the breakdown (according to Wikipedia): (1) iron (by far the most abundant) with an annual production of just over 1.1B tonnes, (2) vanadium at 76,000 tonnes, (3) zinc at 11.9M tonnes, and (4) lithium at 35,000 tonnes. To add another dimension on cost of lithium, 3000 18650’s will cost about $4500 off-the-shelf (but if you are picking these up in bulk you may be able to get them cheaper). It is unlikely that these will drop in price too much, despite valiant efforts by manufacturers around the globe. The cost is in manufacturing; large vacuum and drying chambers are a requisite in manufacturing lithium ion cells. There is still room for reducing cost with larger configuration cells, which are a promising approach to larger LIB systems. With RFBs the bulk of the cost is not in the chemicals (or energy), but the novelty reaction stack components. These components are projected to drop in price dramatically over the next several years.

These days you can find plenty of equipment to generate power, even found this really cool cross flow turbine generator on eBay (very useful in small elevation change hydro generation scenarios). What seems to be elusive, however, is pricing for a good battery system. That said, you can find the Tesla Powerwall offered through a variety of third party vendors; you can pick up a 1st generation for roughly $3,000 (at $600/kW and about $470/kWh) and the 2nd generation for $5500 (at $1,100/kW and about $410/kWh—and these may not be all the costs). Unfortunately, the Powerwall only comes in two flavors, and neither of which actually have a sufficient energy capacity for off grid support. You can find alternative LIB systems as low as $300/kWh; please note I don’t care about the price per kilowatt because I am more interested in how long I will have those killowatts (i.e., energy) with LIBs. Still, even at $300/kWh, the energy price point doesn’t come close to the low cost of energy with a RFB system. If you know your chemistry and can mix your own electrolyte you are looking at about $10-20/kWh. The big expense with RFBs is in reactor cores (and where the cost on a per kilowatt basis becomes important); breaking down the system into individual components (subject of another paper) I estimate the price at about $2,500/kW (using off-the-shelf components), which is more than double what you might spend for a LIB setup on a per kilowatt basis. That may be shocking at first glance, but when you consider the desired specifications for an ideal battery support system, the two are very competitive. Ideally you want a 3-4 kW system capable of about 10-20 hours at that nominal power (or ca. 30-60 kWh), and about 15-30 minutes of peak power capability. So, the LIB system would suffice, while the RFB would need a capacitor bank to support peak power events of 10-15 kW. To build out an appropriate capacitor bank I estimate you will need about 36 Maxwell 3000F 2.7V capacitors (these run about $40-50/unit on EBay). In which case, the LIB system would end up costing about $18,000, while the RFB + capacitors would cost about $13,000 (for roughly a 20-hour battery support system). A fantastic perk with the RFB system is if you want a 40-hour system the price difference is miniscule, e.g. doubling the volume of electrolyte brings the sticker price to just under $14,200.

A simple cost per month and break even can be determined based on cycle life and capacity degradation. Some of the LIB systems I am preferential to are reported to have less than 20% capacity loss over 4500 cycles. Assuming you will cycle the battery once every day (this is a big assumption), the system will last anywhere from 10-13 (suffering only about 20% performance degradation with virtually zero maintenance—which is a huge plus). I have personally tested RFBs (viz. the all-vanadium and zinc bromide chemistry) out to 6000 (and even 10,000 cycles). This means you are looking at about 16-30 years of operation, and quite possibly much longer (with little to no performance loss when maintained properly). For the purposes of comparing the cost lets be optimistic and give the LIB a 15-year life span and the RFB a 20-year life span (Note: This may be a gross underestimation with RFBs assuming the owner operator can perform the necessary maintenance and repairs themselves, which is not really an option with LIBs). With those life expediencies, the LIB will cost you about $100/month and RFB will cost about $54/month; neither of which will lower your monthly power bill, unless you live someplace like Hawaii. So, if your generation runs about a buck a watt, your monthly “off-the-grid” power bill is estimated to be about $128/month with a LIB support battery and $75/month with an RFB systems. Unfortunately, the LIB system doesn’t ever really break even on the mainland (according to national monthly power bill averages). In Hawaii, however--where the average monthly power bill is $203--you will break even in the summer (although you won’t hardly notice that is summer) of year 9 with a LIB system and April of year 7 if you go with an RFB. You actually break even at about year 14 with an RFB system on the mainland (where the average monthly power bill is $107).

In conclusion, I want to reiterate that you can find both solar and wind power generation systems for about a buck a watt off-the-shelf, and if you are an extreme “do-it-your-selfer” you may be closer to about $0.70/watt. Hydro is a bit more expensive (and much more reliable—mind you), if you have access to a sufficient water source of course. So, at the end of the day you are looking at spending roughly what you might invest on a new economy car, i.e., no reason why this can’t be financed. The one disclaimer is that the battery system I recommend would keep your lights on for about 1-2 days. So, should generation cease entirely for a few (or more) days, you are on your own. Say you only put up solar and there is cloud cover for any number of consecutive days, then you are dealing with some serious under generation. This is even possible with dual power generating systems (e.g. wind plus solar) as well and can only be managed with commodity storage, i.e., gas or liquid fuel storage (and a means to burn this fuel of course). In a later piece, I plan to discuss how you can implement onsite liquid and/or gas fuel generation by utilizing excess power generated by your power station. So, stay tuned! For now, suffice it to say, it is possible to get off the grid for roughly the price of a new Hyundai sedan. It costs the average American $2.50 per gallon of fuel in transportation, and if (for whatever reason) you want to store that fuel anywhere besides your vehicle fuel tank it will cost you anywhere from $5 to $15. This shocked me as well. I believe that if the general public doesn’t take an ownership initiative the same will happen with our electricity. So, if we build it they will come… until next time!

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