Cryogenic storage usually means tanks for the keeping of cryogens (liquefied gases).

There are cryogenic storage tanks for both fixed and mobile applications. There is a very wide range of sizes. In each size category there is often a common design approach. The designs from one category to the next can be very different, but within each category there is a typical pattern of design and construction.

There are three main things to consider when determining the optimum cryogenic storage tank. The first thing is the cryogen itself: LNG, LO2, LN2, LH2, or LHe (and others). The thermodynamic and physical properties are vastly different between helium and oxygen, for example. The second thing is the quantity to be stored. The design approach and practicality of fabrication is obviously different for 10 liters versus 1,000,000 gallons. The third third thing is to really understand the motivation for cryogenic storage in the first place. What is the point? Is it to store up the molecules for later using those molecules for a fuel or other chemical process? Or it to store up the molecules as a kind of energy battery for later using the cold for an MRI machine or biological storage or semiconductor fabrication or wine bottling or tire recycling or any other of the numerous applications of the cold. Examination of the three things will drive out the thermal performance that is needed:

1. What cryogen is to be stored?

2. How much?

3. Why?

What is thermal performance? Cryogens are not storable in the traditional sense of the word. As long as the Sun is shining (and it is always shining somewhere) the heat is coming and the cryogen is boiling away (evaporating into the atmosphere). Just like household refrigerators came about in the early 20th century to keep the ice from melting away, we can also add a refrigeration system to keep the cryogen from boiling away. Such design is called Zero Boil-Off (ZBO) and requires an Integrated Refrigeration System (IRAS).

Just remember that cold costs money (hey, ice is not free in Florida and most inhabited places!) that the colder it goes, the more it costs and in exponential fashion. To balance it all out for the best solution may require some expert help. Help is available from the team of the Cryogenics Test Laboratory at NASA Kennedy Space Center where their theme is Energy Efficient Cryogenics. Other helpful sources for consultation include the Energy Evolution LLC network of thermal experts where their motto is We're Cold But We Care.

But let's get back to the normal non-refrigerated cryogenic tanks. There one major distinction to understand: vacuum jacketed (VJ) or non vacuum jacketed. The insulation system can include vacuum or no vacuum, that is a level of vacuum pressure or ambient pressure. There is a huge divide in performance between these two (think R-value 5,000 versus R-value 5). The vacuum jacket will drive the tank design and cost. Foregoing the vacuum will greatly lower the cost and weight. Then, the insulation materials come into play: multilayer insulation, powder, foam, aerogel, etc. etc. Which ones to choose? It all depends on everything. The bottom line is this: the performance must justify the cost.

Here are the main categories of commercial cryogenic tanks in the world today:

1. Liquid cylinders and small dewars.

2. Mobile tankers from 2,000 to 12,000 gallons.

3. Customer stations (fixed tanks) from 600 to 20,000 gallons capacity.

4. Large cylindrical tanks from 20,000 to 120,000 gallons capacity.

5. Large spherical tanks from 120,000 to 2,000,000 gallons.

6. Large to Giant flat bottom tanks from 2,000,000 to 50,000,000 gallons.

7. Giant prismatic tanks for LNG tanker ships from 5,000,000 to 10,000,000 gallons

Categories 1 through 3 are all vacuum-jacketed. Categories 4-5 can be either VJ or double-walled. The largest tanks have at least one flat surface and are therefore not VJ.

EXAMPLE: Cryogenic boiloff rate and heat leak rate for a cryogenic storage tank

The thermal performance can be described by the boiloff rate (or net evaporation rate) or heat leak rate: these two terms are synomonous. Below is an example for liquid oxygen storage in a 900,000 gallon tank insulated with 30 inches of non-evacuated (gas filled with gaseous nitrogen at 1 atmosphere pressure) perlite powder with a bulk density of 4 pounds per cubic foot. The boundary temperatures are approximately 300 K and 90 K.

Heat Leak Rate = 8,000 W (given)

Heat of Vaporization = 213 J/g

Density of LO2 = 1,141 kg/m3 (NBP)

Density of GO2 = 1.43 kg/m3 (STP)

Expansion Ratio (Liquid to Gas) = 877

Boiloff Rate = ???

Q = Mdot x Hfg

8000 J/s = Mdot x 213 J/g

Mdot = 37.6 g/s

Vdot = (37.6 g/s) / (1,430 g/m3) = 0.0263 m3/s = 26.3 liter/s = 1,577 liter/minute of GO2 at STP

Boiloff Rate = (37.6 g/s) / (1,141,000 g/m3) = 0.0000323 m3/s = 0.00198 m3/minute x 264 gallon/m3 = 0.523 gpm = 753 gpd of LO2 at NBP

So, a heat leak rate of 8 KW corresponds to a boiloff rate of 1,577 liter/min of gas or 753 gallons/day lost to the atmosphere.