Compressor COP (Coefficient of Performance) energy efficiency testing measures the ratio of cooling or heating output to electrical power input, providing the definitive benchmark for compressor efficiency. Conducted in accordance with international standards such as ISO 917, EN 12900, and GB/T 5773, COP testing supports energy labeling, regulatory compliance, and product development for refrigeration and air-conditioning compressors. This article explains the testing principles, sample requirements, measurement methods, and typical applications of COP energy efficiency evaluation in accredited third-party laboratories.
Technical Overview
The Coefficient of Performance (COP) is the fundamental energy efficiency metric for refrigeration and heat pump compressors. It is defined as the ratio of the useful thermal output produced by the compressor system to the electrical power consumed to produce it. For refrigeration duty, COP equals the refrigerating capacity divided by the total power input; for heat pump duty, it equals the heating capacity divided by the power input. A higher COP indicates a more efficient compressor, meaning more cooling or heating is delivered per unit of electricity consumed.
Compressor COP energy efficiency testing determines this ratio under tightly controlled, standardised operating conditions. Because compressor performance varies strongly with evaporating temperature, condensing temperature, superheat, subcooling, and supply voltage, reproducible comparisons between products are only possible when tests follow recognized rating procedures. Key standards governing this field include ISO 917 (testing of refrigerant compressors), EN 12900 (rating conditions and presentation of performance data), GB/T 5773 (test methods for positive displacement refrigerant compressors), and related national and regional energy efficiency regulations. Related metrics include EER (Energy Efficiency Ratio), which is numerically equivalent to COP when expressed in W/W, and seasonal indices such as SEER, IPLV, and APF used at the system level.
Accredited third-party laboratories perform COP testing to provide manufacturers, regulators, and purchasers with independent, traceable evidence of efficiency claims. Results are used for energy efficiency labeling, market access, engineering design selection, and benchmarking of competing compressor technologies such as reciprocating, rotary, scroll, and screw types.
Testing Samples
Hermetic Refrigeration Compressors for Household Appliances
These include small hermetic reciprocating and linear compressors used in household refrigerators, freezers, and wine coolers. Typical samples are rated in the low-back-pressure (LBP) range, with rating conditions around -23.3 °C evaporating temperature and 55 °C condensing temperature. Samples are usually submitted as complete units with factory-specified refrigerant and lubricant charge.
Rotary and Scroll Compressors for Air Conditioning
Fixed-speed and variable-speed (inverter-driven) rotary and scroll compressors for room air conditioners, split systems, and light commercial units fall into the high-back-pressure (HBP) category. Common rating points include 7.2 °C evaporating and 54.4 °C condensing temperatures for standard cooling duty. Inverter compressors are tested at multiple frequencies to map efficiency across the operating envelope.
Semi-Hermetic and Screw Compressors for Commercial Refrigeration
Larger semi-hermetic reciprocating, scroll, and screw compressors used in supermarket refrigeration racks, cold storage, chillers, and heat pump plants are tested at medium- and high-capacity rating conditions. These samples often require external oil cooling, flexible mounting, and careful alignment with the test bench drive and piping interfaces.
Prototype and Research Samples
Development-stage compressors, alternative refrigerant configurations, and modified production units are tested to compare design iterations, validate simulation models, and quantify efficiency gains from new valve systems, motor designs, or lubricants before production release.
Testing Items
A comprehensive COP energy efficiency evaluation typically includes the following measured and derived parameters:
- Refrigerating capacity (or heating capacity for heat pump duty), expressed in watts
- Total electrical input power, including motor losses, measured with a precision power analyzer
- Coefficient of Performance (COP) and Energy Efficiency Ratio (EER), where EER in Btu/(W·h) equals 3.412 multiplied by COP in W/W
- Refrigerant mass flow rate, determined by primary or secondary measurement methods
- Suction and discharge pressures and corresponding saturated temperatures
- Discharge gas temperature and suction gas temperature, with controlled superheat
- Liquid subcooling at the test condenser outlet
- Volumetric efficiency (actual flow relative to theoretical displacement flow)
- Isentropic (adiabatic) efficiency of compression
- Operating current, voltage, power factor, and motor winding temperature
- Full or partial performance mapping across multiple rating conditions, where required for variable-speed or multi-rating certification
Testing Methods
Secondary Refrigerant Calorimeter Method
The secondary refrigerant calorimeter is the primary reference method described in ISO 917 and GB/T 5773. The compressor under test refrigerates a calorimeter vessel containing a charge of secondary refrigerant with an embedded electric heater. At thermal equilibrium, the heater input power plus calibrated heat leakage equals the refrigerating capacity delivered by the compressor. Because heat balance is measured electrically with high accuracy, this method achieves low measurement uncertainty and serves as the arbitration method when other methods disagree.
Refrigerant Flow Meter Methods
Alternative methods determine capacity by directly measuring refrigerant mass flow using calibrated flow meters installed on the liquid line or suction line. Refrigerating capacity is then calculated from the measured flow rate and the enthalpy difference between evaporator inlet and outlet states, obtained from pressure and temperature measurements and validated refrigerant property data. Coriolis-type mass flow meters are commonly used for their accuracy and independence from fluid density assumptions.
Test Bench Configuration and Control
A COP test bench comprises a controlled environment chamber, an evaporator circuit, a water- or air-cooled test condenser with adjustable capacity, expansion devices, and instrumentation for pressure, temperature, and electrical power. Suction pressure is regulated by adjusting evaporator conditions, and discharge pressure by adjusting condenser heat removal. Superheat is typically held at a specified value, often between 8 K and 11 K or as defined by the applicable rating standard, while ambient temperature is maintained at the required setpoint, commonly 32 °C or 35 °C depending on the rating condition.
Steady-State Criteria and Data Acquisition
Testing is conducted only after steady-state conditions are achieved, with all monitored parameters held within the tolerances permitted by the standard over a sustained observation period. Data are then averaged over a defined measurement interval. Instrumentation is calibrated against traceable references, and measurement uncertainty for refrigerating capacity is typically required to remain within approximately ±3 %, with agreement between two independent capacity determination methods within ±4 % as required by ISO 917.
Rating Conditions and Result Presentation
Test results are reported at the rating conditions defined by the applicable standard, for example low-temperature refrigeration ratings near -23.3 °C evaporating and 55 °C condensing, and air-conditioning ratings near 7.2 °C evaporating and 54.4 °C condensing, with standardized superheat and subcooling. The test report states capacity, power input, COP, operating conditions, refrigerant type, supply voltage and frequency, instrumentation uncertainty, and the measurement method used.
Applications
Compressor COP test data serve a wide range of engineering, regulatory, and commercial purposes:
- Energy efficiency labeling and minimum efficiency compliance for regulatory market access in major economies, supported by GB-series energy efficiency standards, EN and EU ecodesign requirements, UL and IEC safety-adjacent performance programs, and JIS-aligned regional schemes
- Manufacturer product development, including comparison of prototype iterations, validation of thermodynamic simulation models, and optimization of valve geometry, motor efficiency, and lubrication
- System integrator component selection, allowing chiller, heat pump, and refrigeration rack designers to match compressor efficiency maps to real duty profiles
- Benchmarking of competing technologies, such as fixed-speed versus inverter-driven compressors, or transitions to low-GWP refrigerants with equivalent efficiency verification
- Procurement qualification and tender documentation requiring independent verification of efficiency claims by accredited third-party laboratories
- Corporate energy management and carbon reduction reporting, where verified compressor efficiency data feed into lifecycle energy and emissions calculations for refrigeration and air-conditioning fleets
FAQ
What is the difference between COP and EER?
COP and EER describe the same ratio of thermal output to electrical input. COP is a dimensionless ratio expressed in W/W, while EER is historically expressed in Btu/(W·h) in North American practice; numerically, EER equals 3.412 times COP. Both are determined under the same steady-state compressor test conditions.
Which standards govern compressor COP testing?
The principal standards are ISO 917 for refrigerant compressor testing, EN 12900 for rating conditions and presentation of performance data, and GB/T 5773 for positive displacement compressor test methods, supplemented by IEC, UL, and JIS documents covering related safety and system-level performance requirements.
How long does a COP test take?
A single rating point typically requires several hours of conditioning to reach steady state followed by a defined measurement period. Full performance mapping of a variable-speed compressor across multiple frequency and temperature conditions may take several days of laboratory time.
What sample preparation is required before testing?
Samples should be production-representative units with specified refrigerant, lubricant, and motor protection, supplied with nameplate data, recommended operating envelope, and piping connection details. Prototype units require clear documentation of any non-standard features so that results remain representative of the intended production configuration.
How is measurement accuracy ensured?
Accredited third-party laboratories use calibrated pressure transducers, platinum resistance thermometers, precision power analyzers, and traceable mass flow measurement. Capacity is preferably determined by two independent methods, and the test is accepted only when the methods agree within the tolerance permitted by ISO 917, typically about ±4 %.
Can COP testing be performed on heat pump compressors?
Yes. For heat pump duty, the heating capacity at the condenser is measured against electrical input to yield the heating COP, using the same calorimetric or flow-metering principles with condenser-side heat balance instrumentation.