Hydrogen A to Z Series: Q for Heat Transfer

August 4, 2022

Defining the Hydrogen Economy from A to Z: Q is for Heat Transfer

Continuing on in our defining the Hydrogen economy from A to Z series, we are focusing on the letter Q, the symbol for heat transfer.

Letter abbreviations and symbols are a large part of the scientific community: concepts, states, processes, and variables are much more easily represented as a letter when dealing with equations. One particular abbreviation we are quite familiar with in cryogenics is Q and its various forms: q, Q, and Q̇. In 1850, German physicist Rudolf Clausius described the symbol for heat transfer: “Let the amount of heat which must be imparted during the transition of the gas in a definite manner from any given state to another, in which its volume is v and its temperature t, be called Q.” From then, it has become commonplace to describe the amount of heat transferred (in Joules), as Q, the transfer rate (in Joules/second), as Q̇, and the heat rate per unit area as q (W/m2).

Q and q are especially important in low-temperature applications, as any heat leak into the system by conduction and radiation is described by a heat transfer rate or by an amount transferred over time. As we’ve seen, the most important thing in cryogenics is managing the heat. This is why insulation systems and vessel designs are essential in keeping the cryogens, like liquid hydrogen, in its liquid form. We also take it a step further by not only limiting the heat ingress into the system, but outright balancing it through active refrigeration. That way, with a properly designed controlled storage system, we can achieve zero losses due to boiloff and we can condition the tank temperature and pressure as we see fit. This is a powerful and quite often unappreciated step in modern low temperature refrigeration.

The Q̇ has also been incredibly important in thermal testing of insulation for decades. By measuring the heat ingress through calorimetry, particularly through boiloff calorimetry, we’re able to categorize the performance of a material or system by its conductivity. By knowing exactly how much energy (Joules) it takes for a gram of liquid to boil, we can tell how well or how poorly something conducts heat by measuring the boiloff. This is one of the few reliable ways to measure thermal conductivity of materials.

Although the letter and symbol might seem arbitrary, its use and significance are far reaching and important to every facet of our lives.  GenH2 is using advanced cryostat systems in thermal insulation testing and development and taking advantage of the management and control of heat in hydrogen liquefaction, storage and transfer to meet liquid hydrogen infrastructure needs.

Please continue to follow us next week as we explore the next letter of the alphabet and it’s contribution in Defining the Hydrogen Economy from A to Z.

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