Distribution of COVID-19 vaccines may require ultralow temperatures, something not easily achieved
You may have noticed in the media recently that one of the challenges with widescale COVID-19 vaccination is the distribution of the vaccine itself. Manufacture and distribution of billions of vaccine doses would be complicated enough in normal circumstances but the distribution for the COVID-19 vaccines may be more problematic because most of the initial vaccine candidates require storage at approximately -80C instead of the more typical 4C. There are people who are far more knowledgeable than I am who can explain why the vaccine needs to be stored at such a low temperature, but I thought this would be good opportunity to take a look into how these low temperatures can be achieved.
Note: This article is using mixed units. Temperature is shown in Celsius because this is how we interface with users, particularly in the applications that need very cold refrigeration. However, pressure is shown in “psi absolute” (psia) since that is by far the most common unit used amongst technicians and refrigeration in Canada.
It’s all about compression
The vast majority of refrigeration systems operate at temperatures above -35C and your typical food storage freezer doesn’t usually operate much colder than -20C. We start running into more challenges the colder we go and one of the first challenges is compression.
All compressors have operating envelopes and, from a compression standpoint, this is usually discussed in terms of compression ratio. The compression ratio is the ratio between the absolute discharge pressure and the absolute come from mechanical limitations, efficiency requirements, and temperature constraints. The larger a compression ratio is, the higher the temperature gets in compression. Typically, reciprocating compressors are limited to compression ratios in the 10-15 range and screw compressors can be used with compression ratios of up to approximately 20-25 if they are economized. These compression ratio limitations mean that we are limited in how much we can “lift” a refrigerants pressure at once. Said another way, if we wanted to refrigerated something to -80C using single stage compression and a typical condensing temperature of about 50C, we would need a fluid that had less than a 25x saturated pressure difference at these conditions. Unfortunately, we don’t have a refrigerant with these properties available; this makes sense when you consider that compression ratio of any substance goes very high and approaches infinity as the suction pressure approaches a perfect vacuum, or zero psi absolute. Figure 1 shows how the compression ratio would change for R404A at 50C condensing and different evaporating temperatures. Note that the compression ratio is well over 100 for -80C evaporating.
One method we can use to achieve higher compression ratios is to do the compression in stages. If we compress the refrigerant from a lower temperature to an intermediate pressure, desuperheat the vapour, and then compress it again from the intermediate pressure to the condensing pressure, we can essentially double the compression ratio of a cycle using reciprocating compressors (or any compressors for that matter). Even for temperatures where we technically “can” use single stage compression, it is often more energy efficient to use two stages instead.
If we take a good look at R404a we can see that the limit for single stage reciprocating compressors is in the -35C to -40C range if we need to condense at 50C. If we split the compression into two stages, we are able to achieve lower temperatures while maintaining appropriate compression ratios in each stage. Figure 2 shows the approximate limits for R404a with heat rejection at 50C for each of the different technologies. We can evaporate at -70C and use the pressure associated a -25C suction temperature as an interstage pressure and then compress from that interstage at 50C, which would result in compression ratios of 10 or less for both compressors.
Interestingly, R404a doesn’t quite get us where we need to be if we want to hit -80C and operation at -70C is uncommon. It would make sense then to try to find a refrigerant that did not have to be in a deep vacuum in order to evaporate at -80C. Figure 3 shows the pressure curves of several common refrigerants. As you can see, refrigerants that perform well in the typical temperature range we use, do not appear to be great candidates for really low temperatures. However, there are refrigerants that do not need to operate in a deep vacuum at low temperatures. Currently, the most common one of these refrigerants is R508B; as you can see, it evaporates at 22 psia at -80C. What is not immediately obvious looking at Figure 3 is that it has a critical temperature of about 12C and a critical pressure of 555 psia. Additionally, the compression ratio is already at 25 at the critical point. Ultimately, this means R508B is best suited for use in a cascade arrangement where it is using another refrigeration system to provide a lower temperature heat sink for it to use as a condenser.
Ultra-low temperatures
Cascade refrigeration systems can be very useful and efficient since they allow us to design two refrigeration systems that individually operate in the most suitable conditions for the refrigerant. For example, we can use R404A (or R448A/R449A) to create a -20C heat sink that can be used as a condenser for an R508B cycle. This creates optimum conditions for energy use and operation for both the systems.
There are two significant downsides of using a refrigerant like R508B: It has an extremely high GWP of over 13,000 (recall we are phasing down R404A and it has a GWP of about 3900) and it is very expensive. When I used to fix ultra-low systems, I called this stuff liquid gold. In hindsight, since the temperature was almost always above 12C, I should have called it supercritical gold—but that doesn’t have the same ring to it.
There are other options available to reach ultra-low temperatures. CO2 can be used directly to refrigerate spaces to about -78C. There can be several safety issues associated with high CO concentrations in occupied spaces that need addressed. For temperatures of between -50C and -100C we can use an air cycle system. There has been advancement of this technology lately, but it is expensive. Air cycles can be relatively efficient at low temperatures and they have no has no direct environmental consequences.
For even colder temperatures, it is possible to use a special type of refrigeration system called an auto cascade to achieve temperatures in the -150C range. These systems are similar to the cascade systems, expect there are several “levels” and the refrigerant is all mixed together. The evaporation of one part of the fluid at a particular pressure, condenses another. The specific details about the refrigerant blends in these systems are often proprietary and the systems are usually quite small. And finally, for even colder temperatures, liquid nitrogen can be used as consumable evaporating at -210C.
As you can see, there are several ways for us to achieve ultra-low temperatures, but there are significant barriers in technology and many of these systems do not scale well to large installations or large refrigeration loads. If it turns out that the COVID-19 vaccines do require transportation and storage at -80C, that adds a significant level of complexity to their distribution.