Experimental investigation of factors influencing seasonal performance of water-cooled chiller and heat pump

Inampudi, Sugun Tej

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https://hdl.handle.net/2142/124479 Copy

Description

Title

Experimental investigation of factors influencing seasonal performance of water-cooled chiller and heat pump

Author(s)

Inampudi, Sugun Tej

Issue Date

2024-01-12

Director of Research (if dissertation) or Advisor (if thesis)

Elbel, Stefan

Doctoral Committee Chair(s)

Wang, Sophie

Committee Member(s)

• Miljkovic, Nenad
• Zhang, Yuanhui

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Compressor Capacity Modulation
• Charge Optimization
• Bphx Sizing
• Refrigerant Drop-in
• Seasonal Performance
• Advanced Exergy Analysis
• Dedicated Subcooler
• Water Cooled Chiller

Language

eng

Abstract

Rising cooling needs for the residential and commercial air conditioning sectors and the demand for higher seasonal efficiencies require HVAC&R to handle cooling loads below the design condition. Although several studies exist focusing on a particular compressor capacity modulation strategy, there is no extensive study that explores all these available modulation strategies in the same experimental facility. This study experimentally compared a single speed, a two-stage compressor, tandem combinations of two single-speed compressors, a single speed and two-stage compressor, and a variable-speed compressor using the same R410A water ethylene glycol (WEG) system for cooling application. These modulation strategies were tested according to AHRI Standard 551/591 (2020), and their seasonal performance is given by Integrated Part Load Value (IPLV.SI). A two-stage compressor with lower LMTD at the heat exchangers and a single speed compressor with higher isentropic efficiency have the same performance at one of the part load conditions. Additionally, a single speed compressor with an improved compressor motor had a 3% higher IPLV.SI than a two-stage compressor with the baseline motor. Different methods of interpolation that are available when the capacity is in between two compressor stages of operation were discussed. The tandem combinations with different number of stages had the same seasonal performance, showing that not all the stages available might be useful. The variable speed compressor with the best IPLV.SI was selected to further study the performance under heating application using EU 14825 (2013) standard with constant and variable condenser outlet conditions. For heating application, variable speed compressor with a wider range of modulation was recommended when selecting the compressor for variable condenser outlet temperature conditions while it might not be the best option due to compressor operating envelope limitation when operating with constant condenser temperature outlet. Due to the new restrictions and ongoing phase-down of high GWP refrigerants, there is an increased focus on the amount of refrigerant used. In the current study, the optimum charge was compared for two different scroll compressors: a variable speed compressor and a single speed compressor in the same R410A which is installed with a condenser, receiver and dedicated subcooler. The results showed that COP maximizing charge corresponds to a when some part of the two-phase heat transfer happens in the subcooler and not when the receiver is partially filled as intended. The charge optimization was conducted not only at full load conditions but also at part load conditions to determine if there is any effect of continuous capacity modulation like variable speed compressor on the optimum charge. Optimum subcooling and the improvement potential for compressor which does not have continuous capacity modulation and cannot match the required load without cycling was found to be higher at part load conditions than full load conditions. The effect of operating at peak COP charge on seasonal performance was also analyzed under cooling operation using a variable speed compressor. Although cooling capacity obtained when the system is operating with COP maximizing charge was 6% lower than when operating with typical operating charge, IPLV.SI was 11% higher. Additionally, charge optimization was done for the first time in the same experimental facility with different configurations of the condenser with/without receiver and with/without subcooler to understand the contribution of these components. The abnormal peak in COP corresponding to when the receiver was empty of liquid refrigerant was not observed for any of the three configurations without a dedicated subcooler. This study also provided for the first time insights into how a receiver and dedicated subcooler can be used to not only obtain optimum subcooling but also account for any potential refrigerant leaks by holding additional charge in the receiver. Results indicated that an ideal charge optimization curve can be achieved by sizing the condenser and subcooler to ensure only the optimum subcooling happens in it. The impact of dedicated subcooler and condenser sizing on both charge optimization as well as the system, seasonal performance was also studied. This analysis helped in understanding how these components should be sized and when the diminishing returns occur from oversizing. This work also showed that when the system is operating with enough refrigerant to fill the receiver with saturated liquid refrigerant, using a dedicated subcooler decouples the cooling capacity increment from power consumption increase typically observed with increasing subcooling. Results showed that for the investigated system increasing the dedicated subcooler area beyond 8% of the total available heat rejection area will not yield any additional seasonal performance improvement. Seasonal performance studies usually focus on compressor capacity modulation methods; however, the current study explored the idea of improving the seasonal performance of two compressor modulation techniques: a two-stage and a variable speed compressor with different capacity modulation ranges by using larger condensers in an R410A water ethylene glycol chiller system. The analysis showed that oversizing the heat exchangers has larger benefits with the compressor modulation strategy with lower range. Additionally, the seasonal performance of a two-stage compressor with oversized condenser was higher than a basic single speed compressor with a better motor indicating that oversizing BPHX might yield higher performance than improving the compressor motor efficiency. Due to the high GWP of R410A, there is a focus on replacing it with low GWP refrigerants such as R32 to reduce CO2 emissions. This study also compared the performance when using R32 as a drop-in refrigerant in the two different R410A compressor capacity modulation strategies with comparable cooling capacity. When the refrigerant was changed from R410A to R32, IPLV.SI for variable speed compressor remains relatively constant at 5.5 while it increased from 3.9 to 4.4 for the two-stage compressor. It was hypothesized that low refrigerant flow rate provided by variable speed compressor when matching the load at the lower part load conditions led to refrigerant maldistribution in BPHX and reduced IPLV.SI. Compressors are compared using efficiency and BPHX are evaluated based on their LMTD and pressure drops. It is difficult to compare the contribution of these two to the system COP using traditional first law analysis. Advanced exergy analysis (AEA) can help with this estimation by using exergy destruction as a metric. In the current study, AEA framework was used to analyze the different factors that can impact the system performance and seasonal performance. Three factors included are compressor motor efficiency, condenser size and compressor capacity modulation strategy. AEA showed that optimizing the compressor followed by condenser has the highest potential of improving system performance. Exogenous total exergy destruction dropped by 18% with upgraded compressor while it dropped by 28% with oversized condenser showing that that the condenser has higher impact on the exergy destruction. AEA comparison of a two-stage compressor and a variable speed compressor showed that endogenous exergy destruction of the two-stage compressor is 39% higher indicating that variable speed compressor is more efficient at matching the cooling load. Based on the relative magnitude of avoidable endogenous exergy destruction, upgrading the compressor and condenser would yield higher system performance when using two-stage and variable speed compressors respectively.

Graduation Semester

2024-05

Type of Resource

Text

Handle URL

https://hdl.handle.net/2142/124479

Copyright and License Information

Copyright 2023 Sugun Tej Inampudi

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