Taking Control of Transcritical CO2
New Technologies Aid New Equipment
The following article was prepared by Parker Hannifin Corp. Micro Thermo Technologies and describes one of the approaches for use of CO2 in transcritical systems for refrigeration.
“We needed a robust and reliable control system to ensure that this pilot store test was successful.” — Director of equipment purchasing, maintenance and energy for a New England food retailer.
The challenge was to provide complete store control for a grocery store using transcritical CO2 as a refrigerant. The install was to be a technology demonstration for the entire corporation of the ability to use environmentally safe refrigerants while still meeting or exceeding energy management expectations.
TC CO2 systems are desirable because they use no HFC refrigerants. They also have good heat rejection (which is captured and reused for space and water heating) and overall lower cost of ownership due to lower energy and refrigerant costs.
This particular customer decided to pilot a TC CO2 store in order to begin the process of proving that this technology worked. It was important to this customer because of internal corporate initiatives to reduce the environmental impact of operations, as well as external commitments within the industry.
So the business challenge for this organization was to pilot the first-ever transcritical CO2 refrigeration system for food retail in the U.S., do so in an ‘energy neutral’ way, and meet the internal and external metrics for reducing greenhouse gases (GHGs) and HFC usage.
Once the decision was made to move forward with a pilot store utilizing TC CO2 as a refrigerant, a number of challenges had to be overcome. Among these were the need for specialized compressors, a way to control them, and the very high pressure that is utilized in TC CO2 systems.
Since both of the subsystems were new and relatively unfamiliar to this food retailer, the support needed to specify, install, commission, and support these systems was critical. Micro Thermo Technologies has experience providing advanced controls to manage not only transcritical CO2, but also subcritical CO2 and glycol — both of which were also used in this particular installation.
In addition, the company had worked closely with the chosen rack manufacturer for a number of years and had dozens of transcritical rack controls in the field with that manufacturer.
Micro Thermo provided rack controls, case controllers, condenser controls, temperature and pressure monitoring and the MT Alliance™software management suite to tie all the subsystems together and monitor and collect data from the entire system. The system control strategiesincluded set points, operating conditions, and alarms that can be managed at one location via Alliance.
The customer was immediately able to lower its overall carbon footprint by moving to CO2 refrigerant because of its significantly lower global warming potential (GWP). This fact alone immediately satisfied several corporate initiatives that were noted as drivers for this project.
In addition, the Alliance system controls all aspects of the store’s HVAC, refrigeration, and lighting — so this allows a highly integrated, enterprise-wide system that is designed to deliver significant energy savings due to enhanced alarm management, optimized valve control (for electric valves), and other energy saying schemes such as floating head pressure. In addition, there is energy savings from the heat reclaim aspect of TC CO2 which is being tracked and measured through the heating season.