Kobe Symposium 2014: CO2 refrigeration system for CVS in development by Sanden
04.12.2014 События

Kobe Symposium 2014: CO2 refrigeration system for CVS in development by Sanden

The International Symposium on New Refrigerants and Environmental Technology 2014 (Kobe Symposium), organised by the Japan Refrigeration and Air Conditioning Industry Association (JRAIA), took place on 20 and 21 October 2014 in Kobe. 489 attendees (includes 71 from abroad) attended to learn about the latest HVAC&R industry advancements from 38 presenters. R744.com was on the ground to report on new CO2 technology. In the first article we summarise Sanden's presentation.

Development of saving energy system using CO2 refrigerant for Convenience Store, Kosuke Miyagi

Miyagi presented on the technical development of a CO2 refrigeration system for convenience store (CVS), consisting of an indoor walk-in case and open showcases and outdoor 2HP and 6HP condensing units. System testing, which took place at Sanden’s Akagi plant from March 2013, used monthly power consumption of an R404A system installed in identical store (year 2012) as a benchmark. Test results were very promising. The total annual energy saving of the CO2 cascade refrigeration system was calculated to be 29%.

In conclusion, Miyagi stated that over the product’s 10-year lifespan, taking into consideration both direct and indirect emissions, the Sanden CO2 cascade system reduces CO2 emission by 205 tonnes of CO2-eq. 

Sanden addressed main challenges of using CO2 as a refrigerant

The Sanden presentation also addressed the following difficulties associated with CO2 refrigeration:

  • Low theoretical refrigeration cycle efficiency of CO2 compared to HFC
  • Thick-walled refrigerant pipe required for high pressure (market availability, on-site banding difficulty, high brazing technique)
  • CO2 is a small fluid molecule, so avoiding leakage requires high brazing technique
  • Management of exact amount of refrigerant charge required (high impact of charge on COP and cooling capacity)

In order to overcome the challenges of using CO2 as a refrigerant, Sanden developed a separated refrigeration system. The system is divided into three sub-systems. The 6HP unit system has two sub-systems for open showcases and the 2HP unit system has one sub-system for cooling coil units for the walk-in cases.

Configuration of 6HP unit

The 6HP unit system is a CO2-CO2 cascade refrigeration cycle composed of one high-stage cycle and two low-stage cycles. The low-stage cycle is connected with each showcase in store, and the high-stage cycle assists the refrigeration capacity of the low-stages by applied sub-cooling.

 The low-stage cycle of the 6HP unit consists of a compressor, an air-cooled gas cooler and an R744 plate type heat exchanger for sub-cooling. The showcase side is configured with an internal heat exchanger, an electronic expansion valve that controls refrigerant flow rate and throttle function, and an air-cooled evaporator and solenoid valve.

 In the high-stage cycle the gas is managed using a compressor, a gas cooler, electronic expansion valve and a plate type heat exchanger as an evaporator.

Advantages of split system

According to Sanden, the split system has the following benefits:

  • Reduction of one circuit’s refrigeration load, control of compressor speed according to the load of each showcase – increased efficiency
  • Smaller diameter of pipe is made possible by minimising the refrigerant flow rate of each circuit
  • On-site pipe bending possible, improved brazing workability and accuracy related to pipe diameter, improving air-tightness and reducing leakage
  • Reduction of refrigerant charge
  • In case of compressor failure or failure of another device it is possible to repair different sections part without stopping the system

Optimisation of the cascade refrigeration system

The simulation of results showed that by optimising the high and low pressure conditions of low-stage cycle and high-stage cycle, the cascade refrigerating cycle could have a of COP=1.52. This is approximately 1.5 times of the single-stage cycle (COP=1.02).

In order to maximise the COP of the cascade system, Sanden constructed a control logic and optimum pressure parameter for the high-stage cycle for each ambient temperature. In this way the COP has been improved by approximately 8%.

 The system deploys an all aluminium micro-channel gas cooler rather than a common fin-and-tube copper heat exchanger. The micro channel gas cooler helps reduce size (25%), weight (58%) and cost (60%) whilst increasing capacity by 15%.

r744.com

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