[This is a preprint of my Cool Fuel column in Cold Facts Magazine from the Cryogenic Society of America.]
I’m throwing in the towel on academia and starting my own bank. Let’s call it the 1st Hydrogen Bank. You invest your money and I’ll bank it as pure hydrogen energy for later use. It’s not only the coolest bank around but will be the greenest, fastest (10x), largest (10x), and have the lowest exchange rate among energy banks. Standard terms & fees apply:
- Open an account for as little as $500,000
- Reserve limit up to $50,000
- Compounding losses near 3% per day
- Transfer fee only 10%
- Zero-balance re-activation fee of $10,000
- Apply now to start in 2023!
Jokes aside, those are some hefty terms and fees. Such is the predicament of organizations looking to bank or store energy in the form of liquid hydrogen. No matter how hard you try, you can’t stop heat transfer in cryogenics. Heat boils stored cryogenic fluids, causing tank pressure to increase and Pressure Relief Valves (PRVs) to vent, resulting in daily losses. These losses are coming under increased scrutiny, not just for the economic losses as above, but because a new study in preprint indicates hydrogen leakage may contribute to short term global warming.[i] While these side effects may be smaller than other fuels, and the long-term benefits of hydrogen still far outweigh the short-term setbacks, the negative results could become significant because of the required scale of hydrogen. Regardless of economic or environmental reasons, we need to get serious about reducing liquid hydrogen storage losses.
Zero-Boil-Off (ZBO), a.k.a. no-vent storage is achieved through combinations of 1) effective insulation, 2) passive cooling, and 3) active cooling. Industry has settled on a baseline configuration of 4 tonne cylindrical liquid hydrogen storage tanks with vacuum jacketed insulation achieving daily boil-off rates near 3%.[ii] For reference, 4300 kg of hydrogen has a raw energy equivalent of 143 MegaWatt-hours. With this insulation, the heat load on the liquid is near 500 W, depending on fill percentage. It’s difficult to substantially improve upon vacuum jacketed super-insulation. Let’s consider what we can do with passive and active cooling.
Passive cooling utilizes cool hydrogen boil-off vapors to intercept heat on the way to the liquid. If we are very good at utilizing cold hydrogen vapors to intercept incoming heat, we will warm hydrogen all the way to room temperature before exiting the tank. This hydrogen carries a total cooling capacity of up to 4,500 kJ/kg for a total of 7,800 W of cooling power at a similar venting rate caused by the heat inflow of just 500 W. Yes, you read that right. Passive cooling has incredible potential to reduce the heat load resulting in hydrogen venting. We’ve developed small-scale 3D printed tanks designed with two full vapor shrouds, shown in Figure 1, to demonstrate this concept. However, these small-scale tanks are nowhere near the scale needed for bulk storage. It is expensive to manufacture large scale tanks with 4-5 fully nested shells for passive cooling as each vapor shroud should be insulated from the next. As 3D printing technologies continue to advance though you can imagine a future where multi-walled monolithic tanks are printed with vapor shrouds, at scale, in rapid succession.
Wouldn’t it be nice if nature collected the energy coming into the liquid hydrogen tanks and concentrated it somewhere small for efficient removal? Surprisingly, it does! Heated hydrogen rises quickly to the top of the vapor ullage space within tanks. Wes Johnson’s recent SHIIVER Report from NASA-Glenn shows that the temperature of hydrogen within the ullage space can easily be 50 to well over 100 K higher in temperature than the liquid below.[iii] A Boil-Off-Gas (BOG) compressor can be used to extract this concentrated high temperature hydrogen. If we assume an extraction temperature near 77 K such a compressor would lift 1100 kJ/kg of thermal energy with hydrogen from the tank. This means that just 0.45 g/s of hydrogen out of the tank is enough to balance the 500 W incoming heat load. If para-orthohydrogen conversion is completed before the hydrogen leaves the tank the required mass flow rate is 0.35 g/s. We can scale up compressors to meet this demand, provided we have either a high-pressure storage area to accumulate the hydrogen, or a fuel cell to utilize the hydrogen, for example.
All passive cooling approaches require continuous boiling and extraction of hydrogen. What if we wanted to simply bank the hydrogen over summer without losses for use during winter? Active cooling is required to intercept the 500 W of heat load with no-venting and ZBO. Cooling to 20 K is incredibly limited by a poor Carnot Coefficient of Performance (COP) near 0.07. This implies that 14 W of electrical energy must be input for every 1 W of heat lifted at 20 K for an ideal refrigerator. Sadly, even our very best refrigerators are only operating near 30-40% of ideal. A power input of 23.8 kW is required to run such a refrigerator. While this may seem considerable, a 20×10 m solar array shielding the storage tank from the sun could output up to 30 kW. If you’re not a fan of solar you can use the extra hydrogen from the BOG compressor and a 60% efficient fuel cell to produce 25.5 kW, also enough to run a cooler.
What can be seen from this simple analysis is that we have several tools at our disposal to dramatically reduce and potentially eliminate venting from liquid hydrogen storage facilities. The challenge quickly becomes which path, or combination of paths, yields the quickest return on investment. I’d very much enjoy the opportunity to develop such a research facility here on WSU’s campus for freight and logistics refueling. With your help, may’be we really will start a hydrogen bank.