Around 2010, the first refrigeration systems operating on transcritical (TC) CO2 (R744) – commonly called booster systems – began cropping up in North American supermarkets. Since then, usage of TC CO2 systems has grown steadily and they’ve become viable options for places like supermarkets, food warehouses, and ice rinks, as more organizations move away from traditional HFC refrigerants. Natural refrigerant market accelerator Shecco estimates that TC CO2 systems grew from 140 in 2008 (all in Europe) to more than 35,000 systems worldwide today.
This blog will cover some specifics of transcritical carbon dioxide systems, then touch on some primary benefits and the reasons for its ascendant popularity before finally switching gears to manufacturing, where we’ll talk about some particulars related to our design and material choices for CO2 system components.
Specifics of transcritical CO2 systems
Transcritical CO2 systems are distinguishable by the fact that their working fluids go through subcritical and supercritical states. The fluid (CO2) is first boiled and expanded into a superheated vapor by an evaporator. Then, a compressor is used to increase the heat and pressure of the superheated vapor. Once that pressure exceeds the critical point of CO2 (1055 PSIG/74 BAR), the CO2 is transformed into what’s described as an undefined gas. It has properties of both a liquid and a gas, but is technically neither and is indistinguishable as either state.
There are a few important differences to note between subcritical and TC systems. First, there’s no condensing function in TC systems. Instead, TC systems include a gas cooler to dissipate heat. Another difference involves the use of a high-pressure expansion valve to controls the introduction into the evaporator.
There are a few notable benefits associated with opting for transcritical CO2. First, when compared to other refrigerants, carbon dioxide is much cheaper on a per-pound basis. For example, R-410A, a popular HFC refrigerant, averages between $6 and $8 per pound. On the other hand, we regularly buy CO2 for less than a dollar a pound.
Changing HFC or HFO systems over to CO2 isn’t practical, and it will take time for the return on investment to be realized, but the cost savings of CO2 compared to traditional refrigerants are real and significant. These cost savings come in the form of cheaper fluids as well as higher attainable efficiency for the system as a whole.
Unlike synthetic refrigerants like R-134 and R-404A, which have Global Warming Potentials of 1400 and 3260, respectively, CO2’s is negligible with a Global Warming Potential of 1. Because of this, CO2 is often referred to as “future proof,” meaning that as regulatory agencies continue to evaluate and outlaw synthetic refrigerants due to environmental reasons, CO2 will remain a viable option.
Because CO2’s index of compression greatly exceeds that of synthetic refrigerants, its discharge temperature (roughly 100 -120°C) is higher than that of traditional HFC refrigerants. Additionally, CO2’s higher enthalpy means more of the rejected heat can be reclaimed, therefore making TC CO2 systems more attractive for heat reclaim purposes. As a general rule of thumb, the higher the system’s ambient temperature, the more heat will be available for reclaim, however CO2 systems still provide usable quantities of recoverable heat even in winter conditions. The reclaimed heat can then be routed to other areas of the operation, such as building HVAC or domestic hot water, improving the system’s efficiency.
Design and material considerations
Given the high pressure of transcritical CO2 systems, more rugged materials are required. At Super Radiator Coils, we design and manufacture three products for TC applications: gas coolers, hot gas reheat coils, and evaporators.
To do so, we rely on tube materials like stainless steel and copper alloy strengthened with nickel and tin along with copper alloy (K65) headers and connections. While stainless steel is typically seen more often, copper alloy is an excellent option for TC CO2 applications for a couple reasons. First, it can be brazed, allowing for easy integration into systems with copper connections. It’s also far less costly than stainless steel. The tube material we use is UL listed up to 1740 PSIG (120 bar).