OK, enough negativity

There's no doubt that I've spent a lot of keystrokes pointing out things that won't work or won't live up to their hype, from solar chargers to harvesting of kinetic energy. And, while I have posted favorably or at least neutrally on a few things, I seem to have had more fun debunking than bunking.

So I'd like to spend a bit of time discussing what appears to me to be a promising technology and one that, if it fulfills its promise and comes to fruition, would be a potential game changer.

It's no secret that I'm of the opinion that, whether for the reason of the finite amount of fossil fuels available to be exploited ("peak oil") at prices that can sustain an economy or for the reason that the extraction and burning of fossil fuels for energy results in the release of massive amounts of CO₂ and other greenhouse gases, we need to reduce and ultimately eliminate our reliance on fossil fuels for other than production of products needing their chemistry (almost everything really, from pharmaceuticals to plastics to fertilizers).

Thus, I want to see the increasing penetration of renewable energy for all purposes (including transportation). For a couple of the leading candidates, wind and solar, intermittency is a big issue (though others will argue that it's not such a big deal). This is the case not only because intermittent sources will, on occasion, generate more energy than can be used and not enough at others, but also because the grid relies on very fine adjustments of both energy and frequency.

So, what is required to solve this issue? Without a doubt, the ability to store solar or wind generated electricity at times when supply exceeds demand would be outstanding. It would also enable renewable energy to be supplied at a steady rate.

There are a variety of technologies for grid scale storage of electrical energy:

• Pumped hydro
• Compressed air
• Batteries (various varieties, a promising type for this use is the "flow battery")
• Molten salt, hot oil, etc. in solar thermal plants
• Power to gas (e.g., electrolyze water to produce hydrogen for fuel)
• Others (see here)

I want to look at compressed air energy storage and a startup called "LightSail Energy." Their concept is to utilize a variant of compressed air energy storage ("CAES"). The idea is that, typically, compressing air heats the air and, when the air is stored, in cooling the air

loses much of its total energy. LightSail adds a fine mist of water during compression. The water (with its very high specific heat) absorbs the heat of compression and the hot water is stored for process heat or whatever. Then, when the air is utilized, heat (either from the original heating or elsewhere), is added so that the escaping air, which cools upon expansion, has increased energy to be utilized in the piston compressor/expander. So the idea is to use the renewable source to compress air, remove the resultant heat with a water mist, store the hot water for use in a heat exchanger, add heat when the air is discharged through a turbine piston expander. A schematic drawing from a LightSail patent application is to the right.

It should be noted that there are other firms out there using other media to capture and reuse the heat of compression of air in energy storage systems, one such is Energy Storage Power Corporation who uses oil for this purpose. A schematic diagram of their process is at left.

LightSail's Chief Scientist and one of its founders is Danielle Fong. Fong is a prodigy who dropped out of junior high school at age 12 to attend college, from which she graduated at 17. She was accepted into a Ph.D. program at Princeton but dropped out (or took leave) to move into something with more immediate applicability in the energy field than plasma physics.

There are a LOT of videos out there with Fong carrying the flag for LightSail (see here, here, here, here, and here for examples). She's photogenic, is "aww shucks" humble, wears interesting clothes, and what media outlet doesn't love an attractive female child prodigy Ph.D. candidate dropout who's founded a startup that can be stated in the blurbs to have the potential to "change the world" (and, as can be seen in the intro photo at the top left, I'm not immune)? One thing I respect about Fong is that, in articles about her firm, she'll address naysayers directly and without obfuscation in the comments.

LightSail claims, I would assume potentially, a round trip thermal efficiency of 90% and shows a working 200 bar, 250 kW prototype (200 bar is about 20 mPa (megapascals) (and, by the way, can one nest parentheses in textual material?)) at a one-way efficiency of 71%. 

LightSail's Technology page (interestingly, the landing page for the site) describes smaller "Power Units" of 250 kW power and "Storage Units" of 750 kWh capacity, clearly highlighting the distinction between rate of energy delivery (250 kW in the "Power Units") and storage capacity (750 kWh "Storage Units"). When I simply assume an ideal gas and evaluate the work that can be done by isothermally expanding [edit: a cubic meter of compressed air] from 300 bar (about 30 mPa) that LightSail is aiming for and ignoring the work against the atmosphere, about 48 kWh is available and so 30kWh certainly seems realistic if a close approximation of isothermal expansion can be achieved. Long term storage, though, will require lots of thought and engineering with respect to the thermal energy carried off by the mist.

According to Fong, LightSail's storage technology is capable of storing and delivering 30 kWh (kilowatt hours) per cubic meter of storage space. Thus, we can infer that the 750 kWh Storage Unit will occupy 25 cubic meters. For reference, a standard shipping container (to the extent that such a thing exists) has a volume of 38.5 cubic meters. Ok, how much is 750 kWh? At my house, my family of four uses electrical energy at the rate of, on average, about 2 kW (embarrassingly enough) or about 1,460 kWh/month. So these 750 kWh would last me a bit over half of a month at my house. My Company's headquarters laboratory facility uses, on average, just short of 750 kWh/day. That facility has about 15 people working there. I don't have (and LightSail probably does not have, though I can't imagine that they haven't made some estimate) any figures on what such a Storage Unit will cost.

The California Public Utilities Commission (CPUC) has mandated that the State bring 1.325 gigawatts of storage on line by 2020. So, at 30 kWh/m^3 storage capacity, how much volume would be needed to meet this mandate? Infuriatingly, there's no way to know! California uses electricity at an average rate of about 34 gigawatts, so 1.325 gigawatts represents the ability to deliver a bit under 4% of the State's electrical energy but there's no way to know for how long. The state has said 1.325 gigawatts but did not (so far as I can find) mandate capacity so there's no telling how long the systems to be implemented could deliver these 1.325 gigawatts. An hour? A day? I've posted on this frustrating situation before.

Such details aside, LightSail has discussed storing the compressed air in underground caverns for utility scale storage and that's a VERY big step up from a 750 kWh Storage Unit. But with funding from Vinod Khosla's VC firm, Bill Gates, Peter Thiel, and Total Energy, there's a fair amount of confidence in LightSail.

A deeper excursion into the thermodynamics of this system will follow and I believe that that will shed light on possible integration of such a system into our generation/transmission/distribution system. My initial feeling is that it's likely to be most useful for such things as frequency regulation rather than solving the intermittency problem thwarting confidence in the wide scale implementation of solar and wind generated energy.

I'll acknowledge that the only connection between this video and LightSail is the word "Sail." But I just wanted to use any excuse to embed this song. It's hard to believe that it's the Beach Boys.