Success and CO2 Savings from Appliance Energy Efficiency Harmonisation

Page 15

4. CHILLERS FOR COMMERCIAL BUILDINGS

Overview of the product Chillers are a mainstream product in developed economies. Chillers are large cooling systems that use the vapour compression cycle to cool water which is then circulated in a loop via a pipe network around a building to provide cooling for indoor spaces via secondary air handling systems which are located as required. The secondary air handling systems are not part of the chiller design and are usually specified by building designers. The heat removed from the circulating chilled water loop is discharged to the outside via a cooling tower or an air cooled condenser. The temperature of the water loop supplied by chillers is typically between 4C and 9C. Chillers are commonly used in larger commercial buildings.

Chillers are a global commodity and there is significant international trade. The main differences in products are to suit local requirements regarding capacity and operating conditions, but generally there are only small differences in product design at a regional level.

Chillers range in size from 10kW to more than 2MW. Chillers may have one or more compressors, evaporators and/or condensers.

Comparison of energy performance test procedures

Key parameters to consider for efficiency testing and measurement Chillers are a heat pump that that removes heat from a circulating water loop and discharges this heat to the outside using the vapour compression cycle. The main components are a compressor, refrigerant, an evaporator, a condenser, a water loop and a pump. Some systems use cooling towers in association with the condenser.

The key parameters used for efficiency testing and measurement are the input energy (electricity) and output (total heating or cooling capacity). These two parameters can be used to define overall efficiency of the product during operation. The ratio of these two parameters is defined as the system Coefficient of Performance (COP).

As with any refrigeration based system, the efficiency of a refrigeration system is affected by indoor/outdoor temperatures and the temperature difference.

In order to obtain accurate comparative efficiency value of the chiller, the conditions for measurement need to be carefully controlled, including outside conditions and water loop temperatures. Accurate determination of the output of a chiller requires measurement of the water flow and temperature difference in the water loop during operation.

Overview of the international test method

At this stage there is no international test method for chillers. ISO have approved a new work item ISO/NP 19298 called Water chilling packages using the vapor compression cycle in July 2007 under ISO TC86 SC6, although work on this standard has not yet advanced to the next stage.

However, this proposed standard is based on and is likely to be technically equivalent to the US industry standard ARI 550/590 Water chilling packages using the vapor compression cycle which is prepared and published by the US Air-conditioning and Refrigeration Institute (ARI).

In terms of efficiency the standard defines the Coefficient of Performance (COP) as the ratio of cooling output (W) to electrical input (W) this variable is dimensionless (W/W).

Success and CO2 Savings from Appliance Energy Efficiency Harmonisation

Page 16

The standard is likely to define the following parameters for output, energy consumption and energy efficiency (COP):

Standard rating conditions: defines the water inlet temperature for water cooled condensers and air inlet temperature for air cooled condensers as well as the water temperature leaving the evaporator (Condition A).

Part load rating conditions: defines the water inlet temperature for water cooled condensers and air inlet temperature for air cooled condensers at load output conditions of 75%, 50% and 25% of the rated output of the system (Conditions B, C and D respectively).

The draft standard specifies test setup for energy and capacity determination.

Adequacy of the international test method

Although the international standard is only an early draft and has not yet been issued for public comment, it will draw on the main international approaches to testing and rating for chillers currently in force in the USA and Europe and is likely to be adequate in addressing the performance of the product across a range of conditions. The values for Conditions A, B, C and D are separately reported, which could allow a local weighting to be performed if required, although the COP at rated output (Condition A) and the IPLV under ARI550/590 is widely used internationally as a comparative performance value.

Regional differences in products and testing approaches The two main regions that specify tests method for chillers are North America and Europe. North American test methods are based on ARI550/590. Europe runs a certification program called Eurovent which specifies test conditions for testing, rating and certification of chillers. The two methods are generally quite similar in their requirements but there are some minor differences in the rating conditions as noted below. Some of these differences are likely to have arisen from rounding of imperial and metric units and it is hoped that these can be resolved during the development of the relevant ISO standard. In any case, these current differences will not result in any significant differences in energy efficiency measurements in most cases. The key differences between ARI and Eurovent are set out below:

Table 4: North American SEER test conditions

Condition ARI550/590 Eurovent

Liquid cooled condenser inlet temperature

29.4C (85F) 30C

Liquid cooled condenser operation condition

0.054 L/s/kW 5K temp drop

Air cooled condenser outdoor air temperature

35C (95F) 35C

Water temperature leaving the evaporator

6.7C (44F) 7C

Liquid cooled condenser operation condition

0.043 L/s/kW 5K temp drop

Success and CO2 Savings from Appliance Energy Efficiency Harmonisation

Page 17

The ARI standard defines an Integrated Part Load Value (IPLV) which is defined as a weighted average of the 4 test points COP values at Conditions A to D as follows:

IPLV = 0.01A + 0.42B + 0.45C + 0.12D

Where Condition A, B, C and D are COP at load outputs of 100%, 75%, 50% and 25% respectively. Under ARI, part load conditions are as follows:

Condition A: 29.4C (85F) inlet temperature for water cooled condensers and 35C (95F) for air cooled condensers, 100% rated output.

Condition B: 23.9C (75F)inlet temperature for water cooled condensers and 26.7C (80F) for air cooled condensers, 75% rated output.

Condition C: 18.3C (65F) inlet temperature for water cooled condensers and 18.3C (65F) for air cooled condensers, 50% rated output.

Condition D: 18.3C (65F) inlet temperature for water cooled condensers and 12.8C (55F) for air cooled condensers, 25% rated output.

Comparability of regional testing approaches

The two main systems that are in force, based on ARI and Eurovent, are quite comparable and will give similar results for efficiency and capacity. Results determined under one of these rating systems should be comparable with the other under most circumstances.

Subjective assessment of the level of international harmonisation for testing Chillers are large specialised pieces of equipment that are generally used in commercial buildings. Rating for energy efficiency is specified in North America, Europe and Australia. The two main rating systems in force in North America and Europe are quite similar and fairly comparable, so subjectively there is a reasonable degree of international harmonisation for the testing and rating of chillers.

Prospects and key directions for international harmonisation of testing The two main testing approaches for chillers are fundamentally similar but with a number of small differences. The impact of these differences in most case will be small. Some have arisen from rounding between imperial and metric temperature measurements. While these differences have a small impact, it is desirable that they be eliminated if possible in the longer term.

The best prospects for a harmonised international approach to the testing and rating of chillers is through the development of the new standard ISO/NP 19298 Water chilling packages using the vapor compression cycle. This work should be supported and key stakeholder input from North America and Europe encouraged.

Comparison of energy efficiency metrics

Common Efficiency Metrics and Regional Approaches

The most widely used efficiency metric for chillers is Coefficient of Performance (COP). This measure is essentially output power over input power and provides a direct measure of operating efficiency. This is measured under defined operating conditions. The major test methods define conditions for determination of COP at rated capacity (Condition A) under part load conditions of 75%, 50% and 25% of rated output (Condition B, C and D respectively). When stated with the cooling output and testing conditions, the COP at these points provides all of the data required for efficiency comparisons.

Success and CO2 Savings from Appliance Energy Efficiency Harmonisation

Page 18

COP at rated output is the primary comparative efficiency value for chillers. The Integrated Part Load Value (IPLV), which weights data for all 4 test conditions, is also widely used as a comparative efficiency value.

The IPLV is a pre-defined weighting of the efficiency of 4 load conditions to give and integrated average annual efficiency. The weightings used in the standard IPLV formula may not suit all applications and regions. However, it is widely used as an international benchmark for performance. The separate reporting of COP at Conditions A to D would allow the standardised test outputs to be weighted in a way that was more relevant to local conditions (if required) without the need for any retesting.

COP at rated output and the IPLV are generally used together to define minimum energy performance standards (MEPS) where applicable.

Performance Issues

The measurement of chiller energy efficiency is a relatively straight forward determination of output energy over input energy. There only significant performance issue during operation is the fouling of the evaporator and where applicable the condenser (where this is water cooled). The main standards have allowances to estimate the performance degradation due to fowling during operation. These range from standard fixed allowances to simulation approaches based on parameters such as capacity, surface area of the heat exchanger and operating range of the liquids.

Comparability of efficiency metrics At this stage, there is good comparability of efficiency data under the current North American and European rating systems for chillers. To maintain flexibility at a regional level it is recommended that values for each of the Conditions A to D be separately reported together with the IPLV values.

Recommended directions Two major rating systems for chillers are in existence. While slightly different conditions are specified in the North American and European systems, the practical differences in the measured efficiency results will be small in most cases. Fortunately, this means that there is already a good level of global harmonisation in the testing and reporting of chiller energy efficiency.

Part of the issue is that there is no international test procedure at this stage. However, work on an ISO standard has been approved and it is hoped that this will draw on existing approaches in North America and Europe to form the basis of a uniform global approach to testing and rating of chillers. This should be supported as the basis for providing a basis for the global convergence of testing approaches for chillers.