Chiller Rating and Actual Operating Performance in Hot and Humid Region
With the increasing concerns on global environmental protection and carbon emission reduction, HVAC industry has enhanced the corresponding energy code. ASHRAE Standard 90.1 has made chiller minimum efficiency more stringent with effective from year 2015. The local codes in Asia are following up with this upgrading. Singapore green building scheme Green Mark focused on real chiller and chiller plant operating performance beside the minimum chiller efficiency requirement. While Malaysia strengthened the chiller rating requirement aggressively in the latest Malaysian standard: Energy efficiency and use of renewable energy for non-residential buildings-Code of practice (MS 1525: 2019) which requires all water chillers’ performance rating should not be less than the code specified full load Coefficient of Performance (COP) or Malaysia Part Load Value (MPLV) at Malaysia rating condition.
This paper will take a closer look at chiller performance rating by comparing full load rating and part load rating in tropical regions where some engineers claim chiller Integrated Part Load Valve (IPLV) or MPLV could not be representative of chiller operating performance in hot and humid climate region.
Chiller Ratings
COP and IPLV were designed by Air Conditioning, Heating, and Refrigeration Institute (AHRI) in standard AHRI 559/590 as an indicator to evaluate single chiller design performance in terms of the full load performance and unloading characteristic. IPLV is defined by weighted part load efficiencies which part load entering condenser water temperature (ECWT) varies linearly from 29.4°C at 100% load to 18.33°C at 50% and fixed at 18.33°C from 50% to 0%.
Since for tropical climatic regions, ECWT 18.33°C is not possible to be delivered from cooling tower, Malaysia made an adjustment on this part load metric and came up with MPLV in MS 1525:2019. MPLV denotes Malaysia Part Load Value which is calculated by weighted part load efficiency at Malaysia Standard rating conditions, which the ECWT temperature should vary linearly from 30.55°C at 100% load to 26.67°C at 50% and fixed at 26.67C for 50% to 0% load. IPLV is based on standard AHRI conditions and is typically referred to chiller part load performance rating during factory test and may not represent the real chiller off design operating conditions, especially in tropical climatic regions. While MPLV which is Non-Standard Part Load Value (NPLV) at Malaysia operation conditions could reflect the chiller off design conditions more in real world operation. By acknowledging this, MS 1525: 2019 states the chiller efficiency rating shall meet either minimum COP at 100% load at Malaysia standard rating conditions or minimum MPLV as shown in MS 1525:2019 Table 25, and not at both conditions.
MS 1525 table 25 refers to CSD chiller compliance path A clearly which has relatively high full load COP and average IPLV. On the contrast, variable speed drive (VSD) chiller would have relatively low full load COP due to VSD efficiency loss and high IPLV due to compressor speed optimization at part load conditions. It would be easier for VSD chiller to meet the MPLV requirement while easier for CSD chiller to meet the full load requirement.
ASHRAE standard 90.1 recognizes the different performance features between CSD and VSD chillers and specified two different compliance paths for CSD chiller and VSD chiller, while MS 1525 does not. Another major difference between MS 1525 and ASHRAE 90.1 regarding chiller minimum efficiency is the chiller rating condition. MS 1525 specifies water chillers standard rating conditions are leaving/entering chilled water temperature at 6.67°C/12.22°C and leaving/entering condenser water temperature at 36.11°C /30.55°C. The condenser water temperature is higher than AHRI 550/590 rating condition at 29.4°C /34.6°C which makes MS 1525 chiller efficiency requirement more stringent than ASHRAE 90.1 by about 5-6%.
In hot and humid climate region, there is an understanding that since IPLV is not applicable in real operation because condenser water temp cannot be reduced to 18.33°C, therefore deduced VSD centrifugal chiller is not able to save energy since there is no ambient relief. It is true in hot and humid climate region, there is no big ambient relief as per IPLV described. But there is still 5-6°C ambient relief available for significant operating hours and more importantly, chiller would be operating at part load conditions at most of times in most of applications because plants are designed with multiple chillers system and plant would be oversized due to safety factors from load calculation to equipment sizing. This is acknowledged by Malaysia code and therefore MPLV is defined to rate chiller part load performance. Although MS 1525 accepted MPLV for chiller efficiency compliance, the industry tends to specify full load COP compliance since the MPLV threshold is low and easy to be satisfied for VSD chiller, which implies industry has some bias on VSD chiller operating performance in hot and humid climate region.
Operating performance comparison between CSD and VSD chillers
To investigate the saving potential from CSD chiller and VSD chiller, a comprehensive chiller operating performance analysis for CSD and VSD big centrifugal chillers was conducted. MS 1525 specified higher condenser water temperature than AHRI 550/590 which makes MS 1525 chiller full load COP minimum requirement is about 5-6% more efficient than ASHRAE 90.1 path A compliance requirement. On the other hand, MS 1525 MPLV minimum efficiency requirement is quite loose compared with ASHRAE 90.1 VSD chiller path B compliance requirement because MS 1525 may not properly consider VSD chiller application in hot and humid climate region. So VSD chiller selected in this study was not based on MS 1525 MPLV minimum requirement, it was selected based on ASHRAE 90.1 path B VSD chiller minimum requirement instead whose full load efficiency is 6% better than ASHRAE 90.1 VSD path B minimum efficiency because MS 1525 full load COP minimum requirement is about 5-6% better than ASHRAE 90.1 CSD path A minimum efficiency at Malaysia rating conditions.Two building types, industrial and hotel complex in Kuala Lumpur, Malaysia, which have different cooling load profiles, are used in this comparison. The July cooling load profiles are illustrated in Graph 1 and Graph 2.
The key values of selected 1,000RT CSD and VSD centrifugal chillers are shown in table 1. The blue efficiency highlighted areas show that 1,000RT CSD chiller meets MS 1525 minimum COP requirement, and the yellow highlighted areas show that 1,000RT VSD chiller meets MS 1525 minimum MPLV requirement and 5% higher than ASHRAE 90.1 VSD chiller full load COP requirement at Malaysia rating conditions.
The chiller part load efficiencies at different loadings and different constant ECWT were compared in Graph 3 . CSD chiller shows better efficiency than VSD chillers when ECWT at 30.5°C from 85% to 100% while VSD chiller efficiency becomes much better than CSD chiller when loading and ECWT are reduced. It shows unless chillers are operating at 85%-100% at ECWT 30.5°C at most of times, VSD chiller real operating performance will be better than CSD chillers.
The hour-by-hour chiller operating efficiency was calculated based on hourly cooling load and the web bulb temperature in Kuala Lumpur, Malaysia. Since CSD chiller and VSD chiller have different efficiency sweet spots, the chiller sequencings were optimized differently to minimize both CSD and VSD chillers operating energy usage. The monthly chiller average operating efficiencies were compared and plotted in Graph 4 and Graph 5.
The results demonstrated that VSD Chiller operating efficiency is 7% better than CSD chiller plant for industrial building, and 15% better than CSD chiller plant for hotel complex. Industrial building has more operating hours when chillers were relatively high loaded while hotel building has more operating hours when chillers are partially loaded, which make VSD chiller gained more efficiency benefit from part load as compared with industrial building.
Even for hot and humid climate region like Kuala Lumpur, VSD chiller still could gain efficiency benefit from reduced ECWT until about 27°C, which indicates chiller part load efficiency is also a critical performance indicator. VSD chiller normally would make chiller full load efficiency drop around 3-5% due to VSD loss, but part load efficiency would be improved significantly from compressor speed optimization. Taking VSD chiller out of the consideration during project tendering due to the full load efficiency gap from CSD chiller would not be a wise decision. CSD centrifugal chiller and VSD centrifugal chiller are compared in this case analysis because industrial has an opinion that VSD centrifugal chiller will not be working efficiently in hot and humid climate region and argued that the centrifugal compressor lift could not be reduced so as the compressor speed. It is true centrifugal chiller and positive displacement chiller would react differently to lift and load.
Centrifugal chiller uses dynamic rotating compressors, which makes its variable drive speed variation dependent on the amount of lift also and not just the amount of load in order to achieve stable pressure energy. While positive displacement chillers use two mechanical parts to trap refrigerant vapor, which makes its variable drive speed variation dependent on the amount of load. Some engineers argued that in hot and humid climate region, the condenser water temperature cannot be reduced to 24°C and 18°C as per IPLV described, so there is not significant lift reduction at all loads and centrifugal chiller cannot speed down. Based on this argument, they conclude that full load COP of centrifugal chiller will be the best value to represent chiller performance in hot humid climate regions.This argument is quite misleading. From this case analysis, it is shown that centrifugal chiller efficiency would be improved significantly when condenser water temperature is lower than 30.55°C, from 29 °C to 27°C even lower during part load. Multiple compressors also contribute a lot to part load efficiency in this case analysis. The trend would be similar for different VSD centrifugal chiller designs.
The intention of this case analysis is to show that the potential energy saving from VSD chiller should be well evaluated by comparing with CSD chiller at actual operating conditions. Evaluating chiller performance just based on single value full load COP without the consideration of MPLV is not propriate and incomplete because chiller would be operating at part load at most of times.
Summary
IPLV/MPLV and COP are all important merits to determine how a single chiller is performed at full load and how well a single chiller is unloaded at specified operating conditions. It should be recognized that better COP or better IPLV/MPLV do not simply deduce better energy and economic saving since plants normally are not single chiller plants and chillers do not operate at full load or at same part load conditions as per IPLV/MPLV assumed.
In real chiller plant, chillers typically will be operating at multiple chillers system at part load at off design conditions at most of times. The real chiller and plant operating performance should be evaluated and optimized case by case based on plant design and application. One chiller which is considered as efficient in one project could be considered as inefficient in another project.
Chiller real operating performance could not be evaluated by single full load COP value or by single MPLV value. There is no single value which can predict and represent chiller energy consumption in real project. Both VSD chiller and CSD chiller can perform efficiently even in hot and humid climate region with the right application and optimized strategy. Industry should recognize the VSD minimum efficiency compliance path (MPLV) same as full load COP path and conduct comprehensive evaluation on chiller and total plant operating performance based on project load profiles and operating scenarios to make the right and responsible decision.
References:
- Air Conditioning, Heating, and Refrigeration Institute: AHRI 550/590, Standard for Water Chilling Package Using the Vapor Compression Cycle Appendix D
- ASHRAE Standard 90.1, Section 6.4.1.1
- Malaysian Standard: Energy efficiency and use of renewable energy for non-residential building- Code of practice, MS1525:2019, Section 8.11.1
