Basic Operating Characteristics of CO2 Refrigeration Cycles With Expander-Compressor Unit

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Purdue University

Purdue e-Pubs

International Refrigeration and Air Conditioning

Conference

School of Mechanical Engineering

2006

Basic Operating Characteristics of CO2

Refrigeration Cycles With Expander-Compressor

Unit

Hirokatsu Kohsokabe

Hitachi Ltd.

Sunao Funakoshi

Hitachi Ltd.

Kenji Tojo

Hitachi Ltd.

Susumu Nakayama

Hitachi Ltd.

Kyoji Kohno

Hitachi Ltd.

See next page for additional authors

Follow this and additional works at:

http://docs.lib.purdue.edu/iracc

This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact [email protected] for additional information.

Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories athttps://engineering.purdue.edu/ Herrick/Events/orderlit.html

Kohsokabe, Hirokatsu; Funakoshi, Sunao; Tojo, Kenji; Nakayama, Susumu; Kohno, Kyoji; and Kurashige, Kazutaka, "Basic Operating Characteristics of CO2 Refrigeration Cycles With Expander-Compressor Unit" (2006). International Refrigeration and Air Conditioning

Conference. Paper 789.

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Authors

Hirokatsu Kohsokabe, Sunao Funakoshi, Kenji Tojo, Susumu Nakayama, Kyoji Kohno, and Kazutaka Kurashige

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BASIC OPERATING CHARACTERISTICS OF CO

2

REFRIGERATION

CYCLES WITH EXPANDER-COMPRESSOR UNIT

Hirokatsu Kohsokabe1, Sunao Funakoshi2, Kenji Tojo3, Susumu Nakayama3, Kyoji Kohno3, Kazutaka Kurashige4

1

Hitachi, Ltd., Mechanical Engineering Research Laboratory, 832-2, Horiguchi, Hitachinaka-shi, Ibaraki 312-0034, Japan

(Fax: 81(29) 353-3869, E-mail: [email protected])

2

Hitachi, Ltd., Advanced Research Laboratory, 2520 Hatoyama, Saitama 350-0395, Japan

3

Hitachi Appliances, Inc., Air Conditioning Systems Division, 390 Muramatsu, Shimizu-ku Shizuoka-shi 424-0926, Japan

4

Tokyo Electric Power Company, R&D Center,

4-1 Egasaki-cho, Tsurumi-ku, Yokohama 230-8510, Japan

ABSTRACT

A transcritical CO2 refrigeration cycle with an expander was studied to improve the coefficient of performance

(COP) of the cycle of a simple expansion valve by recovering the throttling loss. We developed an expander-compressor unit. A scroll type expander and a rolling-piston type rotary sub-expander-compressor were adopted and

connected them with a shaft. With this unit, we made a CO2 refrigeration cycle for a small capacity air-cooled chiller

and investigated the basic operating characteristics of the cycle. In experiments, the expander-compressor unit was shown to be stable and to improve the COP of the CO2 chiller cycle under both cooling and heating conditions. The

test results indicated that the COP improvement of the cycle was more than 30%, while the total efficiency of the expander-compressor unit was 57%. Expander-compressor unit are a key technology for use in CO2 refrigeration

systems.

1. INTRODUCTION

Recently, natural refrigerants have been receiving attention due to global environmental problems. Carbon dioxide (CO2), a natural refrigerant, is a potential substitute for hydrofluorocarbons (HFCs) because of its incombustibility,

non-toxicity and low global warming potential (GWP). The coefficient of performance (COP) of CO2 refrigeration

cycles, however, is typically low compared to the HFC refrigeration cycles. The main reason for this low COP is large throttling loss that results from a transcritical refrigeration cycle. Many researchers have been trying to improve the COP of transcritical CO2 refrigeration cycles. The throttling loss can be reduced by replacing the

expansion valve with a work output expansion device (expander or ejector). Various types of expanders have been studied to recover the throttling loss of the CO2 refrigeration cycles. For example, Nickl et al. (2005) developed a

third generation CO2 reciprocating expander, which has three independent expansion stages. Hays and Brasz (2004)

analyzed a transcritical turbine-compressor for CO2 refrigeration and heat pump systems. Westphalen and

Dieckmann (2004) analyzed a scroll expander for CO2 refrigerant-cooling systems. Similar research is going on in

many other companies and universities with different types of expander designs. However, the amount of research for operating characteristics of the CO2 refrigeration cycle with expanders is relatively small.

Fukuta et al. (2001) studied the theoretical performance of the CO2 cycle with a vane compressor-expander

combination. Taking account of the loss due to miss-matching of the operating condition, the theoretical COP of the cycle with the expander was found to be approximately 1.5 times larger than the cycle without the expander. Hiwata et al. (2003) investigated the theoretical performance of the cycle with the expander for both cooling and heating operations. A new refrigeration cycle (change over cycle) was proposed as a cycle with an expander for CO2

air-conditioning systems.

This paper shows the experimental performance of a transcritical CO2 refrigeration cycle with an

expander-compressor unit and presents its operating characteristics for both cooling and heating conditions.

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2. CO

2

REFRIGERATION CYCLE WITH EXPANDER-COMPRESSOR UNIT

A CO2 refrigeration cycle with an expander-compressor unit is shown in Figure 1. This cycle is for an air-cooled

heat pump chiller. The specifications of the cycle components are shown in Table 1. The refrigeration cycle consists of a main compressor (MC), an oil separator, a four-way valve, an air-side heat exchanger, an expander-compressor unit, a water/refrigerant heat exchanger, check valves, and etc. The expander-compressor unit includes an expander (EX) and a sub-compressor (SC).

In Figure 1, a solid arrow shows the flow of the refrigerant during cooling operation and a broken arrow shows the flow during heating operation. The four-way valve changes these two operating modes. In both cooling and heating operation, check valves fix the flow direction of the refrigerant to the expander so that the expander always recovers the energy. This recovered energy directly drives the sub-compressor. Here, the sub-compressor acts as a boosted compressor and reduces the power of the main compressor.

Figure 2 shows an ideal P-h diagram of the CO2 cycle with the expander-compressor unit. An expansion process and

a compression process are assumed to be isentropic. The line from point A to B indicates the compression process of the sub-compressor, and the line from point B to C indicates the compression process of the main-compressor. The line from point D to E indicates the expansion process of the expander. The total efficiency of the expander-compressor unit ηt is defined by

ηt =Δhsc／Δhex (1) where Δ hsc is the isentropic enthalpy difference of the sub-compressor, and Δ hex is the isentropic enthalpy difference of the expander. In Figure 2, the evaporating temperature Te is 0.8℃, the suction super-heat degree is 0℃,

the heat rejection pressure Ph is 9.0MPa and the expander inlet temperature Ti is 36℃. The theoretical COP

improvement of the cycle with the expander-compressor unit is shown in Figure 3. It can be seen that for the given conditions, the cycle with the expander-compressor unit has a 30% improvement in COP over the basic transcritical CO2 cycle, while the total efficiency of the expander-compressor unit ηt is 50%.

3. EXPANDER-COMPRESSOR UNIT

Figure 4 shows a sectional view of the expander-compressor unit (ECU). We used a scroll type expander and a rolling-piston type rotary sub-compressor. Both elements were connected with a crankshaft, so both rotate at the same speed. The ECU design condition is a cooling rated condition. The ECU is lubricated by polyalkylene glycol (PAG) oil, which is stored in the bottom of the casing. The end of the crankshaft is under the oil. The oil pick up supplies the oil to the axial bore of the crankshaft and then feeds it to bearings and other sliding surfaces. The oil separator supplies oil to the casing oil sump (see Figure 1).

4. EXPERIMENT

The test conditions are shown in Table 2. The experiments were carried out at different refrigerant charges, ambient temperatures, and water inlet temperatures for both cooling and heating conditions. The performance of the CO2

cycle with the ECU was compared with the performance of the CO2 cycle without the ECU (with expansion valve).

In one testing cycle, the temperature and pressure at each component, the refrigerant mass flow rate, the compressor power input, and the rotating speed of the ECU were measured. The system capacity calculated to the amount of heat exchange from the water side. The cooing or heating capacity Q is given as

Q = ρw･Cw･Vw･ΔT (2)

where ρw is the water density, Cw is the heat capacity of water, Vw is the volume flow rate of water, andΔT is the

average temperature difference between the inlet and outlet water of the water/refrigerant heat exchanger. The COP is the ratio of the cooling or heating capacity to the compressor power input, which is defined by

COP = Q／Wi (3)

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In experiments, the ECU was stable under both cooling and heating conditions.

5. TEST RESULTS

5.1 Effects of refrigerant charge

Figure 5 shows the effects of the refrigerant charge on system performance. Figure 5(a) is a cooling rated condition and Figure 5(b) is a heating rated condition. The relative COP, the system capacity, and the compressor power input were charted. In Figure 5(a), overall, when the refrigerant charge increases, the cooling capacity increases, but its rate of increase is gradually saturates. On the other hand, the compressor power input slightly increases, so the COP has a maximum point at a specific refrigerant charge. The optimum refrigerant charge is 6.4 kg with the ECU and 6.7 kg without the ECU. In Figure 5(b), the results of the heating rated condition are similar to but smaller than the refrigerant charge of the cooling rated condition. The optimum refrigerant charge is 3.35 kg with the ECU and 3.6 kg without the ECU. In Figure 5, the CO2 refrigeration cycle with the ECU improved the COP by more than 30%.

5.2 Effects of inlet water temperature

Figure 6-7 shows the effects of inlet water temperature. The refrigerant charge remained at the optimum value. Figure 6 shows the relative COP at the cooling condition. Figure 7 shows the relative COP at the heating condition. The COP with the ECU is above the COP without the ECU. These results indicate that though the ECU was designed for the cooling rated condition, it improves the COP of the cycle under the different operating conditions. Figure 8 shows a P-h diagram of the CO2 cycle with the ECU at cooling rated condition. In this figure, the line from

point A to B indicates the compression process of the ECU, and the line from point D to E indicates the expansion process of the ECU. A two-dot chain line means a constant entropy S. Based on this figure, the results from Equation (1), the total efficiency ηt is 57%.

6. CONCLUSIONS

We have developed an expander-compressor unit and investigated the basic operating characteristics of the CO2

refrigeration cycle. As a result, the following conclusions were obtained:

• The expander-compressor unit improves the COP of the CO2 chiller cycle under both cooling and heating

conditions.

• The COP improvement of the cycle with the expander-compressor unit was more than 30%, while the total efficiency of the expander-compressor unit was 57%.

• Expander-compressor unit are a key technology for use in CO2 refrigeration systems.

REFERENCES

Nickl, J., et al., 2005, Integration of a three-stage expander into a CO2 refrigeration systems, Int. J. Refrig., vol. 28,

no. 8: p. 1219-1224.

Hays, L., Brasz, J.J., 2004, A transcritical CO2 turbine-compressor, International Compressor Engineering

Conference at Purdue, C137.

Westphalen, D., Dieckmann, J., 2004, Scroll expander for carbon dioxide air conditioning cycles, International Refrigeration and Air Conditioning Conference at Purdue, R023.

Fukuta, M., et al., 2003, Performance prediction of vane type expander for CO2-cycle, Proceedings of the 21st IIR

International congress of the refrigeration Washington, ICR251.

Kosuda, O., et al., 2005, Performance of scroll expander for CO2 refrigeration cycle, Proceedings of the 2005

JSARE Annual Conference, C315 (in Japanese).

Okamoto, M., et al., 2005, Development of two-phase flow expander for CO2 air-conditioners, Proceedings of the

2005 JSARE Annual Conference, C316 (in Japanese).

Fukuta, M., et al., 2001, Cycle performance of CO2 cycle with vane compressor-expander combination, IMechE

Conf. Trans. of Int. Conf. on Compressors and their Systems, p. 315-324.

Hiwata, A., et al., 2003, A study of cycle performance improvement with expander-compressor in air-conditioning systems, IMechE Conf. Trans. of Int. Conf. on Compressors and their Systems, p.339-347.

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SC EX 空気側熱交換器水側熱交換器主圧縮機膨張／圧縮機 M MC 　　負荷　　モータオイルセパレータ調整弁 P P 逆止弁四方弁冷却運転加熱運転ポンプファン Cooling Heating Main Compressor Expander-Compressor Unit Pump Oil Separator Motor Load Water/Refrigerant Heat Exchanger Check valve

Air-side Heat Exchanger

Fan

4-way valve

Figure 1: CO2 air-cooled chiller cycle with expander-compressor unit

Table 1: Specifications of CO2 cycle components

Type Type Sub-compressor Expander Rolling-Piston Rotary Type Scroll Type Expander-Compressor Unit AC, 200V, 3.75kW Motor Cross-Fin Tube Air-side Heat Exchanger

Plate and Tube Water/Refrigerant Heat Exchanger Scroll Type Main Compressor Type Type Sub-compressor Expander Rolling-Piston Rotary Type Scroll Type Expander-Compressor Unit AC, 200V, 3.75kW Motor Cross-Fin Tube Air-side Heat Exchanger

Plate and Tube Water/Refrigerant Heat

Exchanger

Scroll Type

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Figure 2: P-h diagram of CO cycle with ex2 pander-compressor unit 2 4 6 8 10 12 250 300 350 400 450 500 thalpy /kg] P re ssu re P [ MPa ] S : Entropy 0 10 20 30 40 50 60 70 0 20 40 60 80 100 膨張/圧縮機の機関効率ηｔ（％）ＣＯＰ向上率（％） COP Improvement [%] R744 Te = 0.8 ℃ Ph = 9.0 MPa Ti = 36 ℃

Figure 3: COP improvement of CO2 cycle with expander-compressor unit

Total Efficiency of Expander-Compressor Unit ηt [%]

Figure 2: P-h diagram of CO2 cycle with expander-compressor unit

En h [kJ A B C D E ⊿h_sc ⊿h_ex S const. S const. 36℃ T_e= 0.8℃

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Rotary Sub-compressor Cylinder Roller Scroll Expander Fixed Scroll Orbiting Scroll Balance Weight Crankshaft Lubricating Oil Casing Oil Pick Up

Figure 4: Cross-sectional view of expander-compressor unit

Table 2: Test conditions

9.7~15.5 35 6.4 9.7~15.9 35 6.7 12 35 4.2~7.0 Cooling Expansion Valve 35~45 7 3.6 40 7 3.2~4.1 Heating 12 35 4.5~6.4 Cooling Expander-Compressor Unit 35~45 7 3.35 40 7 2.8~3.45 Heating Water Inlet Temperature: T_wi[℃] Ambient Temperature: T_a[℃] Charge of CO2 [kg] Operating Mode Expansion Device 9.7~15.5 35 6.4 9.7~15.9 35 6.7 12 35 4.2~7.0 Cooling Expansion Valve 35~45 7 3.6 40 7 3.2~4.1 Heating 12 35 4.5~6.4 Cooling Expander-Compressor Unit 35~45 7 3.35 40 7 2.8~3.45 Heating Water Inlet Temperature: T_wi[℃] Ambient Temperature: T_a[℃] Charge of CO2 [kg] Operating Mode Expansion Device

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0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 2.5 3.0 3.5 4.0 4.5 CO2 charge[kg] Re la ti ve C O P [-] ●with ECU ▲without ECU ●with ECU without ECU ▲ 0.0 0.2 0.4 0.6 0.8 1.0 1.2 2.5 3.0 3.5 4.0 4.5 CO2 charge[kg] R e la ti ve H e at in g C ap ac it y[-] ●with ECU ▲without ECU 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Re la ti ve C O P [-] ●with ECU ▲without ECU 0.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 CO２　charge[kg] 0.0 0.2 0.4 0.6 0.8 1.0 1.2 2.5 3.0 3.5 4.0 4.5 CO2 charge[kg] R e la ti ve Co m p. I n pu t[ -] ●with ECU ▲without ECU 0.0 0.2 0.4 0.6 0.8 1.0 1.2 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 CO2 charge[kg] R e la ti ve C o o lin g C ap ac ity [-] ●with ECU ▲without ECU 0.0 0.2 0.4 0.6 0.8 1.0 1.2 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 CO2 charge[kg] R el at iv e C o m p. I np ut [-] ●with ECU ▲without ECU T_Ta = 7 ℃ wi = 40 ℃ Vw= 38 l/min Ta = 35 ℃ Twi = 12 ℃ Vw = 30 l/min

(a) Cooling Rated Condition (b) Heating Rated Condition Figure 5: Effect of refrigerant charge on performance

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0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 30 35 40 45 50 Water Inlet Temperature Twi [℃]

R e la ti ve CO P [ -] 　　●ｗｉｔｈ　ECU 　▲ｗｉｔｈｏｕｔ ECU 2 4 6 8 10 250 300 350 400 450 500

Enthalpy

h

[kJ/kg]

Pre ss u re

P

[ M P a]

S

const.

S

const. Δ

h

ex A Δ

h

sc B C D E

Figure 8: P-h diagram of cooling rated condition Figure 6: Effect of water inlet temperature on

COP (cooling mode)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 8 10 12 14 16 18

Water Inlet Temperature Twi [℃]

Re la ti ve C O P [-] Ta = 35 ℃ Vw = 30 l/min 　●with ECU ▲without ECU Ta = 7 ℃ Vw= 38 l/min

Figure 7: Effect of water inlet temperature on COP (heating mode)