Energy Storage Cost Spitballs

Energy
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Insert quarter for a squirt of “Inspired by Hydrogen Technology discussions!” brand fragrance!

The purpose of this thread is to spitball at energy storage costs of various technologies that are currently available. Please cite your sources when possible. While mostly a bundle of creative lies, I’m happy to use numbers available in public datasheets and standard online purchasing costs if there’s nothing better out there.

I’ll start. Flooded lead acid batteries, a personal favorite of mine.

Trojan SIND 02 2450, $726, 4.9kWh (100hr rate).
Assuming 100% round trip efficiency, which isn’t right but is actually close at lower charge rates where you’re not gassing hard:
@ 30% DoD, 6250 cycles, $0.079/kWh
@ 50% DoD, 3600 cycles, $0.082/kWh
@ 80% DoD, 2000 cycles, $0.093/kWh

In comparison, a Tesla Powerwall (which is somewhat hard to find specs for…):
13.5kWh storage, “10 year warranty” (so I’m calling it 3650 cycles @ 100% DoD), works out at $7000/unit (a bit hard to compare apples to apples, as this includes an inverter) is $0.14/kWh. This isn’t a straight apples to apples comparison as the lead acid system is just the battery, no inverter, but inverters aren’t that expensive, especially for longer term energy storage systems.

Please ballpark other things!

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Let’s see if I’m doing this right…

I think on a small scale cryogenically stored gasses are a non-starter. Liquified natural gas requires so much infrastructure to create, contain, and transport safely and efficiently that it pretty much only works on levels of ‘oil tanker’ in size. Too bad, it would be nice if it would just condense like propane…

So how about compressed natural gas? There’s a bit of infrastructure being built out for use in transportation. I’m curious how it compares to gasoline either for vehicle propulsion or home scale electricity production (like those <10kw dual fuel generators but with CNG instead of propane).

Gasoline Gallon Equivalent

GGE is the measure of how much non-gasoline fuel it takes to equal one gallon of gasoline in energy storage. As far as I can tell, this is purely a measure of BTU’s and does not consider comparative costs of extraction, refinement or storage.

According to the US DOE [State & Alternative Fuel Provider Fleets: Fuel Conversion Factors to Gasoline Gallon Equivalents], CNG measured in gallons at 3600PSI is 0.287. For reference diesel is 1.155*, and LP(liquid propane) is 0.758.

GGE for natural gas is also a bit of a moving target since the mass of CNG you can stuff in a tank at a given pressure is widely variable on temperature- of the ambient air, of the gas as supplied, and even by the rapid temperature and volume changes that depend on how fast the tank is filled.

Storage weight

There are 4 often cited storage tank types, so named types 1-4. I’m going to use the type 1 numbers, as that’s your standard deep-drawn steel industrial tanks. They are heavy, and the other 3 tank-types are some variation of lighter weight aluminum core with kevlar/carbon composite wrapping.

Just for kicks I’m going to imagine converting my F150 to CNG, which currently holds about 22 gallons of gasoline. I get a maximum range of roughly 400 miles on a full tank. Let us also assume engine power outputs are 1:1 between CNG and gasoline.

Cenergy solutions [CNG Tanks | CNG Cylinders | CNG Storage by Cenergy Solutions] type-1 tank with a storage capacity of 133 liters @ 3600psi has a gge of 11.8 and a weight of 295 pounds.

So about the same fuel quantity in GGE would require two of those

  • 22gal gasoline * 6lb/gal = 132lbs (tank weight not included)
  • 295lb type-1 11.8gal CNG tank * 2 = 590lbs

So by weight we’re about 4.4 times heavier for CNG. I can see now why it’s only been trucks and large construction equipment converted at quantity to CNG thus far.

BTU conversion and kWh?

So as far as I can tell, I should be able to use the GGE number as a direct conversion between energy storage of gasoline and that of CNG.

If so, then 114,000 BTU/gal of gasoline * 0.287 = 32,718 BTU/gal @ 3600psi

  • 114,000 BTU = 33.41kWh
  • 32,718 BTU = 9.589kWh

By price

At time of writing the national average of CNG is $2.11, gasoline is at $3.39, and diesel is #3.89 [Average Prices].

So that is saving 62% of cost per gallon, somewhat tempered by increased fuel weight and possibly engine efficiency (which I haven’t researched yet)

Conclusion

By weight and by volume CNG is a bit of a bummer for such applications where it’s a concern, but for large vehicles that can handle the weight and perhaps smaller scale fixed-location storage the cost is quite a bit less even if there are increased losses in weight, space, storage/transfer complexity, and whatever comparative efficiencies there are between internal combustion engines for either transport or electrical power generation.

* Which is less than I thought it would be. I guess that says a lot for the ability of diesel engines to extract more of that energy efficiently, between increased compression ratio, longer engine life, and so on. Certainly diesel provides better performance to industry than a mere ~12% increase in power output by GGE.

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@Canem What about the embodied energy of the tank technologies? Surely, those high pressure tanks cost a lot of energy to make and require more complicated maintenance due to the higher tolerances/purities/etc…

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Great question. I couldn’t find much on the actual manufactured cost of such tanks. A few retail value estimates, but that varies wildly depending on say, an OEM truck manufacturers requirements for size, type, and quantity of tanks delivered.

Both kevlar/carbon wound and deep-draw steel tanks capable of containing several kpsi of gas are not new to industry, they’ve been around a while; but while not being ‘bleeding edge’ technology, they are certainly still more expensive than ‘stamped and welded sheet metal tube’ for containing a liquid fuel like gasoline or diesel.

I’d be curious to know what takes more energy to produce- the steel tanks or the composite tanks. Steel takes a lot of energy to form, heat, deep-draw, and weld. On the other hand it takes a lot of energy to autoclave a carbon-fiber pressure vessel together, and that’s on top of the cost of the carbon fiber materials itself (by just about every metric carbon fiber is more expensive- by weight, by volume, by strength per component, by difficulty of manufacturing, etc).

That’s kinda why I mostly just ignored issues of storage/transfer complexity. It takes more, and more-expensive components to pipe compressed gassed around properly than liquid fuels. Maintenance certainly is a factor, composite tanks and even the steel pressure tanks have a schedule to follow (usually defined by DOT) where they must be removed from service and sent off for hydro-testing. The simplified version of which is ‘pump it full of liquid and see if it survives at rated pressure’.

Certainly not a thing that’s ever needed doing for your car’s gas tank. Now what I’m also having a time getting any good numbers on is how long do such tanks last. The steel tanks likely will outlast us. So long as they don’t corrode by being left in the mud they’ll pretty much never fail a hydro-test, generally it’s just valving that needs replaced.

Composite tanks I’m fairly certain will not outlast us. I realize the stresses influencing a pressurized fuel tank and those on an aircraft fuselage are different, but carbon fiber being prone to sub-surface delamination and UHMW composites slowly decomposing when exposed to heat are just some of many ongoing struggles in the composite industry. To my knowledge neither such problem has yet been solved.

Maintenance costs on the engines themselves might go down though. Liquid ICE engines are fairly tolerant of contaminants in fuel, but not to an unlimited degree. They can degrade in the presence of un-burned byproducts floating around in there, and never ever let water get into you’re diesel injectors. Gasoline can varnish, especially causing problems in carburated engines. Diesel BTW, is hygroscopic; also it can grow bacteria that causes even more of a problem.

So I’m just thinking once you spend the energy to get the properly-purified natural gas or propane, and stuff it into a tank, it generally stays that way.

I’d imagine such infrastructure could sort-of be built out like propane transportation, but with the inability of making natural gas a liquid just by pressure like propane, and with only about 1/3rd the energy density of said propane it would take vastly more material and energy to move around an equivalent amount of CNG than LP.

I couldn’t say if those costs would exceed the 62% savings over gasoline, but I bet it would shrink it a lot.

Perhaps using natural gas compressors at point-of-use by feeding them with existing low-pressure underground NG piping? Don’t transport it compressed, just compress it on site. Just a guess…

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There’s also the post-use considerations. I know steel is trivial to recycle and smelt. Carbon fiber, as I understand it, can be ground up for bulk fill material, but that’s about it - there’s no recycling path available for it.

People who fuel CNG vehicles at hiome typically do that, and the energy costs aren’t that great because you don’t need to go to insane pressures.

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Since it’s mixed in with epoxy for a binder (or some other plastic resin) it’s particularly awful to try to recycle. Even the bulk fill material leaches toxins for potentially decades.

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I imagine “burn it rich and recover the carbon that way” is about as much as you’ll get in terms of recycling, and that sound gross…

Sounds like most plastic recycling, actually!

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Going another route, micro-scaled pumped hydro is certainly possible, if you keep your power needs low and have just the right geology so you can get good head pressure and easy source of water.

Analysis of a micro-site in Belgium, https://iopscience.iop.org/article/10.1088/1742-6596/813/1/012033/pdf 1500m3 upper reservoir, lower reservoir only 625m3 capacity, stated storage of 17 kWh total. And using a pump as a turbine to pull the power out. According to Table 1 on page 6, I think they could generate between 3.4-7.5 kW depending on needs, I think is what it’s saying.

Pipe is 60m, so seems like there isn’t a huge height difference, just something reasonably moderate. Would still be a lot of excavation if you don’t have the proper geology. Not sure about costs, but I’m sure it’s something you’d need to look at possibly decades payback time.

Here’s something from a forum talking about it https://forum.solar-electric.com/discussion/6104/micro-pumped-hydro-storage, basically you need a crap ton of water. A simple pair of 5-10K gallon tanks, even with pretty good head of pressure (which decreases as you get towards the bottom of the top tank!) doesn’t last all that long, even just pulling 1-2kW out.

Now if you access to cheap digging, you probably can get a upper and lower ponds just fine, but the pipeline and pump/turbine setup in there probably isn’t going to be the cheapest. Maybe if it’s just a simple 1-2" pipe you can use some SCH80 or something, if it’ll stand the pressure you’ll be at. Possibly run it above ground, possibly need to bury it. Still, you just need masses of water. So if you’re in just the right geology, you could make it work well enough. Cover both the upper and lower if you have an area with high evaporation or limited precipitation ideally.