Transcritical CO2 Bottle Cooler Development

Transcription

1 Transcritical CO2 Bottle Cooler Development C. Rohrer Ingersoll Rand Climate Control, St. Charles Rock Rd. Bridgeton, MO, United States , ABSTRACT This paper contains an energy consumption comparison of four bottle cooler refrigeration systems using refrigerants R744 (CO 2 ) and the benchmark R-134a. A bottle cooler which uses a cassette refrigeration system was chosen for the comparison. The main objectives were to evaluate the bottle cooler performance with a variable speed two stage CO 2 rotary compressor, a single speed two stage CO 2 rotary compressor, and a single speed CO 2 reciprocating compressor all using a round tube, plate fin gas cooler, and also compare the CO 2 system efficiency with the benchmark R-134a system. 1. INTRODUCTION 1.1 Background The transcritical CO 2 bottle cooler project was initiated to design a bottle cooler that uses a non-ozone depleting and non-global warming refrigerant. Further the CO 2 bottle cooler was to be more efficient than the benchmark R134A system without significantly increasing the product cost. This paper includes a summary of testing performed on these environmentally friendly, cost sensitive systems. 1.2 Transcritical Cycle The transcritical cycle differs from the normal subcritical cycle, because the heat rejection process takes place above the critical point of the working fluid. Heat is rejected as single-phase gas cooling, rather than two-phase condensation as with common refrigerants. See Figure 1 below for pressure enthalpy diagrams of these cycles. Figure 1. P-H Diagram Comparison of Subcritical and Transcritical Cycles From the figure above, it can be seen the subcritical heat rejection process takes place under the saturation dome (condensing). In contrast, the transcritical process takes place above the dome (gas cooling). The transcritical cycle takes place under normal ambient conditions with CO 2 because its

2 critical temperature, 31.1 C, is much lower than the refrigerants commonly in use today. Normally, at ambient temperatures above approximately 23.9 C, CO 2 would operate in a transcritical cycle, but below this temperature it would operate in a subcritical cycle. The working pressures for CO 2 are also much higher than conventional refrigerants. The saturation pressure at a temperature of 31.1 C is 7.4 MPa. This high pressure requires changes in design: tube diameter, tube wall thickness, high-pressure safety, and compressor design. Heat transfer benefits can be found due to the high density of CO 2 at these pressures. 2. EQUIPMENT 2.1 Benchmark Equipment A bottle cooler with 900 can capacity was delivered to the R&D lab (Figure 2). Figure 2: Bottle cooler The benchmark bottle cooler consisted of the following components: Evaporator coil: 20 pass, single circuit, 7.94mm diameter copper tube, 5.72cm x 17.8cm aluminum fins, 17.8 fins/cm, 48.3cm finned length, and 53.3cm total length Capillary tube: 2.4m length, 1.12mm diameter Condenser: 44 pass, single circuit, 7.94mm diameter copper tube, 7.62cm x 27.94cm aluminum fins, 20.3 fins/cm, 35.6cm finned length R-134a reciprocating compressor (base compressor, low isentropic efficiency) Shelves: 53cm wire construction Refrigerant: R-134a Insulated Panels: Common isocyanate/polyol blend blown with R 141b Glass: double paned low E glass 2.2 CO 2 Equipment The following components were changed when tested with CO 2 (R744), although all other components remained the same: Expansion Device: Reciprocating unit: high side pressure regulator Rotary Units: capillary tube, 0.813mm diameter, 5m long

3 Gas cooler: 44 pass, single circuit, 6.35mm diameter copper tube, 8.73cm x 27.94cm aluminum fins, 22.9 fins/cm, 27.9cm finned length The rotary unit used 10 passes for the intercooler and 34 for gas cooling Refrigerant: R-744 (CO 2 ) Food Grade CO2 Compressors 600 watt input (120V, 60Hz), variable speed, two stage, rotary compressor Prototype Inverter 400 watt input (120V, 60Hz), single speed, two stage, rotary compressor 1 kilowatt refrigeration capacity (220V, 50Hz), single speed reciprocating compressor 3. TEST SET UP/PROCEDURE The bottle cooler was installed in a controlled ambient test room. The bottle cooler was fully instrumented with pressure and temperature sensors. Type T thermocouples were used to measure all temperatures. A watt transducer was used to measure electrical inputs. Refrigerant mass flow rate was not measured because this would impart a significant addition of charge to the system. Since we did not measure mass flow, energy use was measured with a watt transducer while maintaining equal product temperatures during each of the test runs. The bottle cooler was loaded with 900, 355ml soda cans, per customer specifications. The data collected was recorded by the data acquisition system every 10 seconds. A background program calculated saturated condensing and evaporating temperatures. Each of the CO 2 systems used the same components as the benchmark with the exception of the compressor, gas cooler, and expansion device. The systems with rotary compressors used a capillary tube designed for CO 2. The reciprocating compressor system used a high side pressure regulator, which could be adjusted to optimize the high side pressure for lower energy consumption. Each of the CO 2 systems used the same gas cooler with cross counter flow circuiting. The gas cooler piping diameters were smaller to deal with the pressure, which increased the heat transfer coefficient without penalizing the efficiency due to pressure drop. 4. RESULTS 4.1 Summary of Results Table 1 below gives a summary of energy consumption over 24 hours at 32.2 C/65% RH. Table 1: Energy comparison Ambient R-134A Benchmark CO 2 Rotary CO 2 Recip. (kwh) CO 2 Variable Speed Rotary (kwh) (kwh) (kwh) 32.2 C/65% RH % Energy Reduction -- 30% 15% 17%

4 4.2 Benchmark Results As the benchmark, the bottle cooler was tested with R-134a as received from the factory. The thermostat temperature and differential setting were adjusted to obtain product temperatures no lower than 0 C and no greater than 7.2 C, while achieving a product average no greater than 3.3 C. The charge was then adjusted to low superheat at the end of the compressor run cycle. At 32.2 C/65% RH, the bottle cooler produced an average product temperature of 2.8 C, and coldest and warmest product temperatures of 0.8 C and 6.9 C, respectively. Energy consumption at this ambient was 10.4 kwh/day. 4.3 CO 2 Variable Speed Rotary Results The bottle cooler refrigeration system was then converted to the 600 W variable speed two-stage rotary compressor, inverter, CO 2 capillary tube, and gas cooler, as seen in Figure 3, below. The evaporator remained the same as the benchmark. CO2 2 Stage Compressor Figure 3: 2 stage rotary compressor with intercooler/gas cooler From the figure, it can be seen the gas cooler was circuited in such a way to use 10 of the 44 passes for intercooling between compression stages. The refrigerant suction gas is compressed in the first rotary compression stage and then discharged to the shell of the compressor, which returns to the second stage. The second stage discharged to a condensate pan/pre-cooler, and then to the gas cooler. This design allowed the compressor supplier to design the shell for intermediate pressure and not discharge pressure, while gaining efficiency by intercooling between stages. The cooled gas from the gas cooler is then expanded through a capillary tube. The capillary tube is brazed to the suction line to create a suction line heat exchanger before the evaporator. The suction line is coiled vertically to serve as an accumulator, which is located behind the compressor, which can be seen in Figure 3.

5 The inverter for the variable speed compressor used a thermister to control the speed of the compressor. This thermister was initially located on the evaporator refrigerant outlet where the thermostat bulb was located. This location was unacceptable because it caused excessive product temperature fluctuation, probably due to CO 2 having quick pull down (high heat transfer coefficients). It was decided to locate the thermister at the evaporator coil air outlet so the compressor could change speed and cycle more closely to the required air temperature. This solved the fluctuation issue. The thermister varied the speed of the compressor by +/- 10 hz based on the evaporator air outlet temperature. When the desired evaporator air outlet temperature was reached, the thermostat turned the compressor off until the next cycle. After the necessary adjustments were made to the thermister settings, the charge was adjusted to obtain a flooded condition (0 C superheat) at the end of each cycle. At 32.2 C/65% RH, the bottle cooler with the variable speed compressor produced an average product temperature of 3.2 C, and coldest and warmest product temperatures of 1.2 C and 6.8 C, respectively. Energy consumption at this ambient was 7.3 kwh/day, which is 30% less than the benchmark system. The increase in efficiency can be explained because of the high efficiency variable speed motor, twostage compression with intercooling, and CO 2 s higher heat transfer coefficients. The gas cooler also had 1.2% more air side area than the condenser, but this should only account for a very small amount of efficiency gain. In space constrained (heat exchanger constrained) systems, CO 2 has a heat transfer advantage, because it can reject heat more efficiently due to the higher heat transfer coefficients and cross counter flow arrangement. Further, CO 2 compressors typically have higher isentropic efficiency, because the compression ratio with CO 2 is lower than current HFC compressors. 4.4 CO 2 Rotary Results The system was then converted to the CO 2 single speed, rotary compressor, but all other components remained the same. The single speed used a larger less efficient motor, however the rotary compression components remained the same as the variable speed unit. At 32.2 C/65% RH, the bottle cooler with the single speed compressor produced an average product temperature of 3.0 C, and coldest and warmest product temperatures of 34.2 F and 7.1 C. Energy consumption at this ambient was 8.8 kwh/day, which is 15% less than the benchmark system. 4.5 CO 2 Reciprocating Results After single speed rotary testing was completed, the system was then converted to the CO 2 single speed, reciprocating compressor. The expansion device was also changed from the original CO 2 capillary tube to a high side pressure-regulating valve. This valve has the ability to regulate the high side pressure to optimize the system efficiency. For small increases in compression work, there is a notable gain in heat of rejection and cooling capacity at the same refrigerant exit temperature, but is limited by the properties of transcritical CO 2. For the test with this compressor, the gas cooler and intercooler were piped in series, because this compressor is single stage. Thus, the gas cooler used all 44 passes for gas cooling, where the two stage rotary compressors used 10 passes for intercooling and 34 passes for gas cooling. The reciprocating compressor system charge and thermostat were set to achieve desired conditions. At 32.2 C/65% RH, the bottle cooler with the reciprocating compressor produced an average product temperature of 2.5 C, and coldest and warmest product temperatures of 0.6 C and 6.7 C. Energy consumption was 8.6 kwh/day, which was 17% less than the benchmark system. The single speed

6 reciprocating compressor system used 2% less than the single speed rotary system. This is probably due to the ability to optimize the high side pressure with the pressure regulator. Table 1 below contains a summary of each of the units tested. Table 1: Summary of Test Results CO 2 Bottle Cooler Test Unit Tested: Room Conditions: R-134a Benchmark CO 2 CO 2 CO 2 Recip. Rotary Rotary Recip. 2-Stage Variable Speed 2-Stage 1-Stage Capillary Tube Capillary Tube Capillary Tube High side valve 850Watt Capacity 600W input 400W input 1KWatt Capacity Dry Bulb: C Relative Humidity: % 63.6% 67.5% 63.2% 63.6% Refrigeration: Evaporator Temperature (Avg.): C Evaporator Temperature (Low.): C Air Temperatures: Air Into Evaporator (Low): C Air Off Evaporator (Low): C Air Into Condenser (Average): C ASHRAE Simulators: Average Temp: C Coldest Test Simulator: C Warmest Test Simulator: C Pressure (psi) High Side Pressure: Mpa Middle Stage Pressure: Mpa KWH: Energy: Kwh CONCLUSIONS Two stage variable speed CO 2 compression yields the most efficient operation of the units tested, although the most expensive. Single stage operation still yielded substantial energy savings while only being marginally more expensive. When component quantities increase for transcritical CO 2 systems, there is a sufficient chance to reach cost parity. Pipe diameters are smaller, refrigerant cost is less, and compressors should reach parity over time, which creates an attractive green solution for the bottle cooler industry.