A benchmark comparative thermodynamic analysis of transcritical CO2 heat pump configurations for heating purposes

Although CO 2 as refrigerant is well known for having the lowest global warming potential (GWP), and commercial domestic heat pump water heater systems exist, its long expected wide spread use has not fully unfolded. Indeed, CO 2 poses some technological difficulties with respect to conventional refrigerants, but currently, these difficulties have been largely overcome. Numerous studies show that CO 2 heat pump water heaters can improve the coefficient of performance (COP) of conventional ones in the given conditions. In this study, the performances of transcritical CO 2 and R410A heat pump water heaters were compared for an integrated nearly zero-energy building (NZEB) application. The thermodynamic cycle of two commercial systems were modelled integrating experimental data, and these models were then used to analyse both heat pumps receiving and producing hot water at equal temperatures, operating at the same ambient temperature. Within the range of operation of the system, it is unclear which would achieve the better COP, as it depends critically on the conditions of operation, which in turn depend on the ambient conditions and especially on the actual use of the water. Technology changes on each side of the line of equal performance conditions of operation (EPOC), a useful design tool developed in the study. The transcritical CO 2 is more sensitive to operating conditions, and thus offers greater flexibility to the designer, as it allows improving performance by optimising the global system design.

Energy, exergy and exergoeconomic evaluation of the air source transcritical CO2 heat pump with internal heat exchanger for space heating

Heat pumps are one of the most energy efficient ways to supply heat and have traditionally used hydrofluorocarbons (HFCs) as refrigerants. HFCs, due to their high Global Warming Potential (GWP) are gradually being phased out globally and are being replaced by ultra-low GWP natural refrigerants like CO2. However, the optimal heat rejection pressure for CO2 heat pump for a given load return temperature is significantly larger than that for HFC based heat pumps. This results in a higher throttling exergy loss for CO2 heat pump system than that in an equivalent HFC based system and is one of the major reasons for reduced efficiency in CO2 heat pumps. Exergy analysis for the cycle suggests that the throttling losses associated with isenthalpic expansion across the high pressure expansion valve (HPV) over a large differential pressure are responsible for significant exergy destruction in CO2 heat pumps. To remedy this issue, this paper proposes a novel multi-phase rotary pressure exchanger (PXG) as a combined compression-expansion machine to recover pressure energy that would have otherwise been lost across the isenthalpic expansion and use it to compress the low pressure flash gas to the highest system pressure. This reduces the energy consumption of the system significantly. In addition to acting as a compressor, PXG also acts as an expander with relatively high isentropic expansion efficiency, thus increasing the amount of liquid produced after expansion compared to that produced by expansion through a traditional HPV. This further increases the COP of the cycle. In this paper, the gas dynamics taking place inside PXG and its effect on trans-critical CO2 heat pump cycle is investigated. The first law and the second law analysis establishes the thermodynamic limits on mass flow rate that can be compressed by PXG. A multi-phase species transport model is developed to gain insights into why PXG is capable of providing significant energy savings for trans-critical CO2 heat pumps. A multi-phase 3D CFD and species transport model of PXG provides insights into how the high pressure supercritical state of CO2 compresses the low pressure gaseous CO2 inside PXG as the two streams come in direct contact with each other. Experimental data on trans-critical PXG operation obtained previously is used to calibrate the PXG model and the PXG integrated cycle models. Key performance metrics like liquid content after expansion through PXG, amount of flash gas that can be compressed by PXG and the COP as a function of air source / ground source temperature and load return temperature are investigated. PXG can achieve the mass boost ratio of 0.27 and 0.4 at 50 C load return temperature for 37 bar and 48 bar receiver pressure respectively. COP lift and thus the energy savings provided by PXG are shown to increase with increased load return temperature as the density ratio between LP and HP inlet streams of PXG progressively increases. PXG is shown to provide as much as 40% COP improvement (COP Lift) for evaporator temperature of −10 C. COP lift is shown to increase from 30% to more than 50% with increase in the evaporating temperature from −30 C to 10 C. PXG is expected to provide a major stepping stone for developing ultra-low global warming, high efficiency and sustainable heat pump technology for applications in residential & industrial heating, waste heat recovery, geothermal heat pumps and energy storage applications.

A quasi-two-stage trans-critical CO2 heat pump with in-cycle thermal storage for performance enhancement

Numerical performance of a water source transcritical CO2 heat pump with mechanical subcooling

Investigation of the performance of a transcritical CO2 heat pump system subject to heated water conditions: Perspective from the second law

Heat pump (HP) is one of the most energy efficient tools for address heating and possibly cooling needs in buildings. Growing environmental concerns over conventional HP refrigerants, chlorofluorocarbons (CFCs), and hydrofluorocarbons (HFCs) have forced legislators and researchers to look for alternatives. As such, carbon dioxide (R744/CO2) has come to light due to its low global warming potential (GWP) and zero ozone depleting characteristics. Even though CO2 is environmentally benign, the performance of CO2 HP has been of concern since its inception. To improve the performance of CO2 HP, research has been playing a pivotal role in developing functional designs of heat exchangers, expansion devices, and compressors to suit the CO2 transcritical cycle. Different CO2 HP cycles coupled with auxiliary components, hybrid systems, and refrigerant mixtures along with advanced control strategies have been applied and tested. This paper presents a complete overview of the most recent developments of transcritical CO2 HPs, their components, and applications.

Experimental and techno-economic analysis of transcritical CO2 heat pump water heater with fin-and-tube and microchannel heat exchanger

ABSTRACTTranscritical CO2 heat pump systems integrated with renewable energy sources and energy storage are being paid great attention to develop sustainable energy and energy savings in civil and industrial applications so as to achieve net‐zero carbon emissions by 2050. This paper presents a comprehensive review on the progress in research and technology development of transcritical CO2 heat pump technology with clean energy and energy storage systems. Transcritical CO2 heat pump systems are very competitive with conventional systems for space heating, hot water production, air‐conditioning, waste heat recovery and other engineering applications. This review focuses on the recent advances in the key research and technology development of transcritical CO2 heat pump integrated energy systems including sustainable ‘green’ heating, energy storage, electro‐thermal storage, waste heat recovery and disruptive electrification of renewable energy, combined heating and power, and new hybrid systems using transcritical CO2 heat pump systems. The electrification of heat is a key opportunity for industry and is crucial for tackling the climate emergency. Future research needs of transcritical CO2 heat pump integrated systems are identified according to this comprehensive review. Furthermore, Perspectives and deployment of transcritical CO2 heat pump systems integrated with renewable energy sources and energy storage technology are discussed.

CO2 heat pump integrated thermal storage for domestic hot water in hotels

Experimental investigation of the extreme seeking control on a transcritical CO2 heat pump water heater

Currently, at the current stage in the field of application of heat pumps in heat supply systems, it is promising to use low-potential heat from natural sources. The purpose of the work is to study the thermodynamic energy efficiency of heat transformation in the "water-water" heat pump (HР) cycle, the working agent of which is propane. A thermodynamic analysis of the energy efficiency of the use of modern heat pump technologies for the heat supply system when using natural, ecologically clean energy sources was performed. Factors that directly affect the energy efficiency of HР “W-W” have been identified, and the features of the operation of a water HР have been evaluated. Increasing the energy efficiency of a water HР depends not only on the perfection of the HР operation cycle and the choice of the HР working agent, but also on the process of heat transformation in the heat pump cycle. The results of the thermodynamic calculation of the energy efficiency indicators of the operation of the water heating plant using the natural energy source of water are presented. The energy efficiency of the water HР cycle, which implements the reverse thermodynamic Carnot cycle using a low-potential water heat source, is shown. The heat pump cycle "W-W" is accompanied by minimal losses when throttling the liquid working agent propane, and also solves the problem of useful heat use to increase the temperature of the coolant, which is heated for heat supply. A thermodynamic and exergy analysis of the energy efficiency indicators of the water HР with the environmentally safe agent propane (R290) was performed. The energy efficiency of the water HР cycle is estimated by the heat transformation coefficient HР (COP), which is calculated to be 3.72. The thermodynamic efficiency of the water HР in heat supply systems is considered using the exergy efficiency, it is 44 %. A comparative analysis of the thermodynamic energy efficiency of a water-based heat pump with other heat pumps operating on low-potential natural heat sources, such as ground and air, was conducted. For a ground-based heat pump, the coefficient of thermal transformation (COP) of the heat pump is 3.53, for an air-based heat pump 3.37. The thermodynamic efficiency of a ground-based heat pump is 40 %, for an air-based heat pump 36 %. Therefore, the thermodynamic energy efficiency of a water-based heat pump, based on the comparative thermodynamic analysis, is higher than the use of ground and air heat pumps in heat supply systems. Therefore, the use of water НР in heat supply systems is more appropriate in comparison with air and ground НР. Bibl. 42, Fig. 2, Tab. 1.

To comply with the development of the “dual-carbon” goal, a new refrigerant for heat pump products is necessary. Firstly, mathematical models of enhanced vapor injection air energy heat pump and transcritical CO2 heat pump systems were established, and they were calculated using thermodynamic methods. The results show that when CO2 is used as the refrigerant, the discharge temperature is 30% and 22.7%, which is higher than that of R410A and R134a, and 4.9% lower than that of R32. At the compressor outlet, the volume flow rate of CO2 is 59.0%, 60.8%, and 64.2% less than R410A, R32, and R134a, respectively. In terms of power consumption, the difference between CO2 and R410 A is within 6%. The power consumption of the CO2 system is 0% ∼ 6.4% higher than that of the R32 system and 29.2% ∼ 46.5% lower than that of the R134a system. The COP of the CO2 system is within 6% of that of the R410A and R32 systems, which is 41% ∼ 87% higher than that of the R134a system. When the ratio of CO2 / R32 is 0 ∼ 0.2 and 0.9, the COP of the mixed refrigerant is better than that of the single refrigerant CO2.

The objective of this paper is to describe an energy-based approach to visualize, identify, and monitor faults that may occur in a water-to-water transcritical CO 2 heat pump system. A representation using energy attributes allows the abstraction of all physical phenomena present during operation into a compact and easily interpretable form. The use of a linear graph representation, with heat pump components represented as nodes and energy interactions as links, is investigated. Node signature matrices are used to present the energy information in a compact mathematical form. The resulting node signature matrix is referred to as an attributed graph and is populated in such a way as to retain the structural information, i.e., where the attribute points to in the physical system. To generate the energy and exergy information for the compilation of the attributed graphs, a descriptive thermal–fluid model of the heat pump system is developed. The thermal–fluid model is based on the specifications of and validated to the actual behavioral characteristics of a physical transcritical CO 2 heat pump test facility. As a first step to graph-matching, cost matrices are generated to represent a characteristic residual between a normal system node signature matrix and a faulty system node signature matrix. The variation in the eigenvalues and eigenvectors of the characteristic cost matrices from normal conditions to a fault condition was used for fault characterization. Three faults, namely refrigerant leakage, compressor failure and gas cooler fouling, were considered. The paper only aims to introduce an approach, with the scope limited to illustration at one operating point and considers only three relatively large faults. The results of the proposed method show promise and warrant further work to evaluate its sensitivity and robustness for small faults.

Heat pumps are a more efficient method to heat water compared to conventional electrical heating systems. However, the harmful environmental effects of traditional CFC and HCFC refrigerants require more sustainable alternatives to be adopted. In marine applications, leaked HCFCs can cause substantial harm to sensitive coastal ecology and are powerful greenhouse gases. Standard CO2 (R744 when used as a refrigerant) is a promising replacement for HCFCs due to its low global warming and ozone depletion potential. This paper investigates adapting the traditional ground loop of a geothermal heat pump to a marine loop for use with seawater or coastal applications. New analytical results on the use of thermal energy from marine water for domestic water heating with a transcritical CO2 heat pump are presented in this paper. Substantial research has recently been performed on transcritical CO2 heat pump cycles, including cycle enhancements and optimization techniques. However, the research has been primarily focused on traditional heat pump configurations, with ambient air usually being the heat source. Geothermal temperatures vary on average between 8°C to 12°C throughout the year. A large body of water in this temperature range can provide a consistent source of thermal energy and thus effectively replace the ground source used in typical geothermal heat pumps.