Determination of the Coefficient of Performance (COP) for Absorption Chillers: Principles, Methods,

The coefficient of performance (COP) is the most important energy-efficiency indicator for absorption chillers, quantifying how effectively a heat-driven refrigeration machine converts steam, hot water, fuel, or waste heat into useful cooling. Accurate COP determination requires precisely controlled test facilities, calibrated instrumentation, and strict adherence to recognized standards such as GB/T 18431, GB/T 18362, AHRI 560/590, and JIS B 8622. This article provides a comprehensive technical guide to absorption chiller COP testing, covering working principles, sample types, measured parameters, test procedures, calculation methods, and typical applications.

Technical Overview

An absorption chiller is a heat-driven refrigeration machine that replaces the mechanical compressor of a conventional vapor-compression system with a thermodynamic absorption cycle. Instead of consuming large amounts of electricity, it uses a thermal energy source—such as steam, pressurized or medium-temperature hot water, natural gas or oil combustion, engine exhaust, or industrial waste heat—to drive the refrigerant cycle. The most common working pairs are lithium bromide–water (LiBr–H₂O) for air-conditioning temperature ranges and ammonia–water (NH₃–H₂O) for low-temperature or industrial applications.

The coefficient of performance (COP) is defined as the ratio of the useful cooling capacity produced at the evaporator to the heat energy supplied to the generator (or to the burner in direct-fired units). In simplified form, the thermal COP is expressed as COP = Qₑ / Q_g, where Qₑ is the net cooling capacity in kilowatts and Q_g is the heat input in kilowatts. Because the absorption cycle is thermodynamically limited, typical values are far lower than those of electric chillers: single-effect machines achieve a COP of approximately 0.65 to 0.75, while double-effect machines reach approximately 1.10 to 1.45 under standard rating conditions.

COP determination is essential for product certification, energy-efficiency labeling, contractual acceptance, research and development benchmarking, and energy audits. Testing is typically performed by accredited third-party laboratories or by manufacturers' own EN ISO/IEC 17025-accredited facilities, in accordance with national and international standards.

Testing Samples

COP tests are conducted on complete, fully assembled absorption chiller units representing the actual product configuration, including solution pumps, purge systems, controls, and heat exchanger circuits. The principal sample categories are described below.

Steam-Fired Lithium Bromide Absorption Chillers

These units are driven by saturated steam, typically at gauge pressures of 0.1 MPa (single-effect) up to 0.8 MPa (double-effect). The test evaluates the machine's ability to convert steam enthalpy into chilled water cooling capacity. Sample preparation includes verification of steam generator condition, solution concentration, and control settings.

Hot-Water-Fired Absorption Chillers

These chillers use hot water—often from district heating networks, solar collectors, or industrial processes—at supply temperatures commonly between 85 °C and 150 °C. Testing confirms cooling output and COP at the specified hot-water inlet/outlet temperatures and flow rates.

Direct-Fired Absorption Chillers

Direct-fired units burn natural gas, liquefied petroleum gas, or light fuel oil in an integral burner to drive a double-effect cycle, frequently providing both cooling and heating. COP determination requires accurate measurement of fuel consumption and application of the fuel's low heating value (LHV), following standards such as GB/T 18362 and AHRI 590.

Exhaust-Gas and Waste-Heat Driven Chillers

These machines recover heat from gas engines, gas turbines, incinerators, or process exhaust streams. Testing involves controlled simulation of the exhaust source, with heat input calculated from exhaust mass flow and temperature difference across the generator.

Ammonia–Water Absorption Units

Used mainly for industrial refrigeration below 0 °C, these units require additional safety precautions due to ammonia toxicity. COP testing follows analogous energy-balance principles with adapted instrumentation and safety protocols.

Testing Items

A complete COP determination comprises the following measured and calculated quantities:

• Net cooling capacity at the evaporator (kW), derived from chilled-water flow rate and inlet/outlet temperature difference
• Heat input to the generator (kW), derived from steam flow and enthalpy drop, hot-water flow and temperature difference, fuel flow and LHV, or exhaust-gas energy
• Thermal coefficient of performance (COP = Qₑ / Q_g)
• Auxiliary electrical power consumption of solution pumps, refrigerant pumps, purge units, burners, and controls (kW)
• Comprehensive (integrated) COP, in which auxiliary electrical energy is added to the denominator
• Chilled-water and cooling-water inlet/outlet temperatures, flow rates, and pressure drops
• Steam pressure, steam flow rate, and condensate temperature (for steam-fired units)
• Fuel composition, low heating value, and consumption rate (for direct-fired units)
• Solution concentrations and cycle internal parameters, where instrumented, including absorber and generator solution temperatures
• Part-load performance at 25 %, 50 %, 75 %, and 100 % load, and the calculated Integrated Part-Load Value (IPLV)
• Stability verification, including absence of solution crystallization and abnormal purge operation during the test

Testing Methods

Reference Standards

COP testing is generally performed in accordance with one or more of the following standards, depending on the product type and target market:

• GB/T 18431 — Steam and hot water type lithium bromide absorption chiller units (Chinese national standard)
• GB/T 18362 — Direct-fired lithium bromide absorption chiller (heating and cooling) units
• AHRI 560 — Absorption water chilling and water heating packages (North American rating standard)
• AHRI 590 — Direct-fired absorption water chilling and water heating packages
• JIS B 8622 — Absorption refrigerating machines (Japanese standard)
• ISO/IEC 17025 — General requirements for the competence of testing and calibration laboratories (applies to the laboratory itself)

Test Apparatus and Instrumentation

Testing is conducted on a calibrated laboratory bench equipped with controllable chilled-water and cooling-water loops, a heat-source supply system (steam boiler, hot-water loop, or fuel supply with calibrated metering), and a data-acquisition system. Minimum instrumentation accuracy requirements typically include:

• Temperature measurement with Class A platinum resistance thermometers (PT100) or equivalent, resolution 0.01 °C and accuracy within ±0.1 °C, with differential pairs matched to reduce uncertainty
• Liquid flow measurement by electromagnetic or coriolis flow meters with accuracy within ±1 % of reading
• Steam flow measurement by condensate collection (gravimetric or volumetric) or calibrated vortex/condensate flow meters
• Fuel flow measurement by calibrated gas meters or gravimetric liquid-fuel measurement, combined with gas chromatographic or reference LHV data
• Pressure measurement with transducers accurate to ±0.5 % or better
• Electrical power measurement with power analyzers meeting class 0.5 accuracy or better

Test Conditions and Tolerances

Nominal rating conditions specified by the applicable standard must be established and maintained. For chilled-water systems, common full-load conditions are chilled-water inlet/outlet temperatures of 12 °C/7 °C and a cooling-water inlet temperature of approximately 30 °C with the standard-design flow rates. Deviations during the steady-state measurement period must remain within the tolerances defined by the standard—for example, ±0.3 °C on water temperatures and within a few percent on heat-source input. Where ambient conditions are specified (for air-cooled components or direct-fired burners), they must also be controlled or corrected.

Measurement Procedure

• Pre-test inspection: verify unit integrity, solution charge and concentration, purge operation, control settings, and instrument calibration status
• Start-up and stabilization: bring the unit to the specified load point and maintain operation until all monitored parameters remain within tolerance, typically requiring a stabilization period before data acquisition
• Steady-state data acquisition: record all temperatures, flows, pressures, fuel/steam consumption, and electrical power at frequent intervals over the measurement period prescribed by the standard (commonly one hour or more of stable operation)
• Averaging and energy balancing: compute mean values, cross-check the heat balance across the machine (heat input plus electrical input versus heat rejected to cooling water plus cooling output), and confirm the balance closes within the allowable deviation
• Part-load sequence: repeat the procedure at each required part-load condition with the corresponding reduced heat input and modified cooling-water conditions per the standard
• Report preparation: document test conditions, instrumentation, raw data, calculated capacity, heat input, COP values, uncertainty assessment, and conformity with the specified limits

Calculation of COP

The key calculations are performed as follows:

• Cooling capacity: Qₑ = m·cₚ·(T_chilled,in − T_chilled,out), where m is the chilled-water mass flow rate and cₚ the specific heat of water
• Steam heat input: Q_g = m_steam·(h_steam − h_condensate), using enthalpy values at measured pressure and condensate temperature
• Hot-water heat input: Q_g = m_hw·cₚ·(T_hw,in − T_hw,out)
• Direct-fired heat input: Q_g = V_fuel·LHV, corrected to reference fuel conditions
• Thermal COP: COP = Qₑ / Q_g
• Comprehensive COP: COP_comp = Qₑ / (Q_g + W_electric), where W_electric is the total auxiliary electrical input
• Integrated Part-Load Value: IPLV = 0.01·A + 0.42·B + 0.45·C + 0.12·D, with A, B, C, and D representing COP (or efficiency) values at 100 %, 75 %, 50 %, and 25 % load respectively, as defined by the applicable standard

Applications

COP determination results serve a wide range of engineering, commercial, and regulatory purposes:

• Product certification and energy-efficiency labeling for market access and compliance with minimum energy-performance requirements
• Factory acceptance testing and site acceptance testing prior to commissioning of new installations
• Verification of contractual performance guarantees between chiller manufacturers, contractors, and end users
• Research and development support for cycle optimization, heat-exchanger design, solution chemistry, and control strategy improvement
• Energy audits and energy-performance contracting, where verified COP values quantify savings from waste-heat recovery and cogeneration projects
• Contribution to green building certification documentation and corporate carbon/energy reporting
• Diagnostic benchmarking of aging units to support retrofit, rebuild, or replacement decisions
• Independent evidence for dispute resolution and warranty claims, provided by accredited third-party laboratories

FAQ

What is a typical COP value for an absorption chiller?

Single-effect lithium bromide machines normally achieve a thermal COP of about 0.65 to 0.75, while double-effect machines achieve about 1.10 to 1.45. Direct-fired double-effect units typically rate between 1.25 and 1.45 based on fuel low heating value. Values outside these ranges under standard conditions may indicate measurement issues or machine faults such as fouling, air leakage, or solution contamination.

How is COP different from EER or electrical COP?

Thermal COP considers only the heat input to the generator, whereas comprehensive COP also includes auxiliary electrical energy consumed by solution pumps, purge systems, and controls. Standards define precisely which inputs are included, so test reports should always state the calculation basis.

Which standard applies to a direct-fired absorption chiller?

Direct-fired units are typically rated under GB/T 18362 in China, AHRI 590 in North America, and related provisions of JIS B 8622 in Japan, with heat input calculated from fuel consumption multiplied by the low heating value.

How long does a COP test take?

A single full-load COP determination generally requires several hours, including start-up, stabilization to steady state, and a prescribed measurement period of one hour or longer. A complete part-load series at four load points, including transitions between conditions, typically takes one to two days.

What accuracy is required for the test instrumentation?

Standards commonly require water-temperature measurement within ±0.1 °C using matched differential sensor pairs, flow measurement within ±1 %, and correspondingly tight requirements for steam, fuel, and electrical measurements. All instruments must be traceably calibrated, and the laboratory should operate under an ISO/IEC 17025 quality system.

Can COP testing be performed on site instead of in a laboratory?

Yes. Field performance tests using portable calibrated instrumentation can verify installed performance against design values, although tighter control of ambient and load conditions in a laboratory generally yields lower measurement uncertainty. Field tests are widely used for acceptance and energy-audit purposes and should follow the tolerance and methodological provisions of the applicable rating standard wherever possible.