In DTI’s first presentation at Gustav Lorentzen, a test scenario for adding ice storage to a CO2 refrigeration system is described, with no disturbance to the existing installations in the sales areas, in order to move power consumption away from peak hours.
Load shifting through ice storage in retail CO2 systems
Looking to the future, grid operators, especially in Europe, will be faced with an increasing amount of challenges due to the introduction of renewable energies. One of the proposed solutions to these challenges is to utilise the demand side management through a Smart Grid setup, which relies on flexible power consumption.
Ice storage for a transcritical refrigeration system is suggested in order to meet demand for increased flexibility of the electrical grid and to benefit from the temperature difference between day and night operations.
- The ice storage will help to conserve the CO2 charge at moderate pressures in case of power failure
- The loss of efficiency seen in traditional ice storage systems for air conditioning is avoided due to the configuration of the ice bank between the high pressure side and the intermediate pressure receiver as well as the lower evaporation temperature level of supermarket systems
- The implementation of systems with only CO2, i.e. in Southern Europe, USA and Australia, will benefit from the increasing efficiency of transcritical CO2 systems in climates with higher summer temperatures
- The ice bank can increase the available heat for heat recovery at night
- The possibility of gain from secondary effects such as lower power consumption and reduced wear of compressors due to more stable running conditions
- Lower rates from the utility company will apply due to the reduced level of peak power usage (depending on the given country)
- Design of installations with smaller power supply (cables and transformer) is possible
Larger potential energy savings for warmer climates
Calculations based on the Danish Reference Year (DRY), representing a colder climate, compared to a temperature profile from San Francisco International Airport (SFO), representing a warmer climate, show a potential for larger energy savings for the warmer climate and significant savings from day and night price tariffs.
The amortisation period for the investment of the ice storage is more than 5 years; longer than generally accepted in the supermarket sector, despite a reduction of the power peaks on the warmest days to close to 50% and savings in the range of 5 to 15% in yearly compressor energy consumption. Factors such as heat recovery, down-sizing, and optimised operation modes are suggested as topics for future analysis in order to reduce the amortisation period for the investment in ice storage.
Vortex Flow Sensor in a CO2 refrigeration system
In DTI’s second presentation at Gustav Lorentzen, a commercially available Vortex Flow Sensor is described, which tested with good results in the liquid line of a CO2 supermarket refrigeration system. According to DTI, with fairly simple modifications, the sensor can be applied in refrigeration systems at a cost comparable to a conventional pressure transducer. The monitoring of energy efficiency, as well as operational characteristics of the components of a refrigeration system, can be greatly improved when flow measurements are available.
The Vortex Flow Sensor makes use of the physical phenomenon of vortex shedding. When a bluff body is placed in a pipe with its axis perpendicular to the direction of the fluid flow, a series of vortices will be periodically shed from alternate sides of the bluff body.
Tested with success in supermarket under normal operating conditions
The vortex sensor technology developed for use in aggressive aqueous fluids has been tested with success in a supermarket under normal operating conditions. The sensor operated continuously in the liquid line with CO2 under app. 38 bars pressure for 3 years and is still in operation.
The accuracy was controlled under field conditions and for accumulated flows over 24 hours it is better than -1% to +5%. The flow data was analysed and it seems probable that the sensor can be even further improved for widespread use in refrigeration systems and heat pumps.