Validation of CO2 heat pump using Dymola/ThermalSystems with open dataset

1.Introduction

A CO2 heat pump is a type of heat pump that uses CO2 as a refrigerant, which has almost no damage to the ozone layer, and its Global Warming Potential (GWP) is also low 1. These heat pumps can reduce carbon emissions while helping maintain temperature control in buildings. CO2 heat pumps are available for various sectors including agriculture, commercial, data centers, residential, and industrial and manufacturing.

Finding an open dataset about operation of CO2 heat pump can be challenging. Although research papers often contain valuable data and insights, not all authors make their datasets publicly available. In some cases, researchers may not have collected or recorded the necessary data to create a comprehensive dataset. Additionally, even when a dataset is available, it may not be well-documented or easy to use. However, Wu [1],[2] has conducted a comprehensive series of research studies on the performance of CO2 heat pump, which have yielded highly detailed information pertaining to the modeling of the system. In the above research works, a prototype residential liquid-to-air ground-source heat pump (GSHP) using CO2 as the refrigerant was tested in National Institute of Standards and Technology (NIST), which has a nominal cooling capacity of 7 kW. The tests were performed in an environmental chamber and followed the ISO 13256-1 standard for rating GSHP. The CO2 heat pump operated either in a subcritical or a transcritical cycle, depending on the entering liquid temperature (ELT). Only operation data of cooling mode was considered for modeling and validation. In the cooling mode, the finned-tube heat exchanger operated as an evaporator. Plate heat exchanger was used as the gas cooler/condenser.

ThermalSystem library, which is developed by TLK GmbH is used for modeling of CO2 heat pump, the schematic of model is shown as below:

2.Validation

2.1 Input

The main components in this CO2 heat pump are the semi-hermetic reciprocating compressor, a finned-tube evaporator, two plate heat exchangers (one as gas cooler/condenser, one as suction line heat exchanger), and electrical expansion valve. Detailed geometry information for the main components can be found in the reference [1]. For validating the CO2 heat pump model, two test conditions was selected, which included subcritical and transcritical operation. The conditions were described as below:

Detailed comparison results between test and simulation were shown in the below table:

The results show that for the key thermodynamic states in both cycles, the relative error between test and simulation are below 5%. The results indicate that the compressor power for both subcritical and transcritical cycles exhibits a relative error of -1.63% and -3.17%, respectively. Likewise, the cooling capacity demonstrates a relative error of -2.94% and -5.04% for subcritical and transcritical cycles, respectively. Additionally, the COP shows a relative error of -1.29% and -1.69% for subcritical and transcritical cycles, respectively.

The PH diagrams for the above subcritical cycle and transcritical cycle are plotted for comparison between test and simulation in the below figure. It can be seen that the numerical model in the present report predicts successfully the real thermodynamic behavior of the CO2 heat pump under different operation condition.

3.Reference

[1] Wu, W., Skye, H. M., & Dyreby, J. J. (2021). Modeling and experiments for a CO2 ground-source heat pump with subcritical and transcritical operation. Energy Conversion and Management, 243, 114420. https://doi.org/10.1016/j.enconman.2021.114420

[2] NIST Technical Note 2068, Laboratory Tests of a Prototype Carbon Dioxide Ground-Source Air Conditioner