AReCoS

Embedded controller · CO2

Pressure optimization for transcritical CO2 racks.

On transcritical CO2, efficiency is won or lost on pressures set by the machine's actuators: the gas cooler pressure and the receiver pressure. The AReCoS controller is designed to choose them together, from a physics model of the machine, on the equipment.

In the AReCoS cycle model, on a 35 °C (95 °F) day, running ten bar below the best CO2 pressure costs about 26% of the cooling per unit of power. That is the cost of the wrong pressure, not a saving against a well-tuned controller.

  • U.S. Patent 12,768,557
  • A hardware test on a vapor-compression system (US 2024/0295337 A1, FIGS. 4 to 7)
  • Under test in a research agreement (CRADA) with UT-Battelle, LLC, which manages Oak Ridge National Laboratory for the U.S. Department of Energy
  • Patent and CRADA record
Request a technical evaluationDownload the CO2 brief PDF

Every circuit hides a pressure someone has to choose

In an ordinary system the weather, load and fans set the condenser pressure, and the optimum is wide and flat. Run CO2 above its critical point, or compress in two stages, and a pressure appears that someone has to choose.

Carbon dioxide

Supermarket CO2 racks

One controller designed to run the whole CO2 plant, in under two minutes: a design explainer, not footage of an installation.

Transcritical CO2 racks: the gas cooler pressure, the three valves, heat reclaim, a leak, and the grid.

Carbon dioxide

The high side stops being automatic

Carbon dioxide above about 31 °C (88 °F) does not condense at all. Heat leaves through a gas cooler, the pressure there comes loose from the temperature, and it becomes a decision the machine has to make again every time the weather moves.

A pressure-enthalpy chart of a transcritical carbon dioxide refrigeration circuit. A shaded dome marks where the refrigerant is part liquid and part gas, ending at a marked point labeled: above 31 degrees Celsius, 88 Fahrenheit, carbon dioxide no longer condenses. The circuit is drawn as a loop around and above that dome. The compressor raises pressure to about 94 bar, well above the dome. The gas cooler runs across the top at constant pressure. That pressure is above the top of the dome, so the refrigerant never condenses on this leg. A high-pressure valve then drops the refrigerant to a receiver at 45 bar, where about two fifths of it flashes to gas. A gas bypass valve returns that flash gas to the compressor inlet. A separate evaporator valve feeds the remaining liquid to the evaporator at about 37 bar, which runs across the bottom. Three labels with arrows name the high-pressure valve, the gas bypass valve and the evaporator valve.
Where the pressure goes in a transcritical CO2 circuit, and which valve sets what, for a machine evaporating at 2 °C (36 °F) with 35 °C (95 °F) outdoor air.

Drawn with the AReCoS refrigerant cycle tools, with the gas leaving the cooler 2 °C (3.6 °F) above the outdoor air, a flash tank at 45 bar and no suction-line heat exchanger. The next figure is the same machine.

Carbon dioxide

And the decision is lopsided

In the model of this machine, the best pressure sits near 94 bar on a 35 °C (95 °F) day and about 125 bar on a 45 °C (113 °F) one. Ten bar above it costs under four percent of the cooling per unit of power; ten bar below, about twenty-six. Running low feels safer for the hardware, but it is the expensive side.

A line chart with gas cooler pressure in bar absolute along the bottom and cooling delivered per unit of power up the side. Two curves. The 35 degree Celsius, 95 Fahrenheit, outdoor air curve climbs steeply from 74 bar, peaks near 94 bar, and then falls away slowly. The 45 degree Celsius, 113 Fahrenheit, outdoor air curve starts from zero at about 83 bar, below which the receiver holds no liquid, is lower throughout, and peaks much later and far more broadly, at 125 bar. A filled dot marks each peak. Two open rings on the warm-day curve sit ten bar either side of its peak and show what missing it costs. Ten bar below loses about twenty-six percent of the cooling per unit of power. Ten bar above loses under four percent. A shaded band covers the ten bar below the peak. On the warm day, the fall below the peak is far steeper than the fall above it.
Cooling delivered per unit of power against gas cooler pressure, on a warm day and a hot one, for the machine above.

On a CO2 machine that is also making hot water there is no single best value: the same pressure trades cooling against heat delivered, and the right answer depends on what the building wants at that moment.

Carbon dioxide

Three valves, three different jobs

A transcritical circuit with a flash tank has three interacting valve functions. The high-pressure valve after the gas cooler sets the gas cooler pressure. The flash-gas bypass valve returns the receiver's flash gas to the compressors. The expansion valve at each evaporator feeds that evaporator.

Each moves the other two. In many installations each holds its own target, correcting after the fact what the others just did; this controller is designed to set all three together from one model of the machine.

Carbon dioxide

A hot afternoon, or a leak?

On a CO2 booster the receiver level moves with weather, load, defrost and heat reclaim, and a fixed level alarm cannot tell that from a leak. The AReCoS controller is designed to predict the level from a model of the gas cooler, valves and cases, and to read what the prediction misses as charge lost, well before the receiver runs low.

The gas is cheap. A receiver that runs dry is not: flash gas reaches the case valves, and the cases go warm.

Carbon dioxide

When the grid asks

On CO2, cutting load for the grid is designed to come from compressor capacity and a floated suction, never from pushing the high side below its best pressure, where it loses cooling far faster than it saves power.

One controller: efficiency on CO2, cooler compressors on ammonia

On carbon dioxide the decision buys efficiency, and it moves with the weather. On ammonia it buys cooler compressors; what that is worth in reliability is for plant tests to show. Same controller, same kind of decision, different payoff.

The same controller on two-stage ammonia

Ready for evaluation on your machine

The controller is available for evaluation today, fitted to your CO2 rack, and licensed per unit.

The controller is designed to make this decision on transcritical CO2, as it does on HFC and HFO systems.

In the evaluation, AReCoS builds your machine's model from its components, fits it to your plant and checks it against your own test data.

The cycle work behind this page is the AReCoS team's own.

Let's talk.

Tell us what you run: a refrigeration system or a machine you build.

Email info@arecos.net Download the CO2 brief PDF
625 Kenmoor Ave SE, Suite 350
Grand Rapids, Michigan 49546