Two-stage Heat Pump Research Articles

Thermodynamic analysis of a two-stage binary-fluid ejector heat pump water heater

Thermodynamic analysis of a two-stage binary-fluid ejector heat pump water heater

Part-Load Energy Performance Assessment of a Pumped Thermal Energy Storage System for an Energy Community

Research on pumped thermal energy storage (PTES) has gained considerable attention from the scientific community. Its better suitability for specific applications and the increasing need for the development of innovative energy storage technologies are among the main reasons for that interest. The name Carnot Battery (CB) has been used in the literature to refer to PTES systems. The present paper aims to develop an energy analysis of a CB comprising a high-temperature two-stage heat pump (2sHP), an intermediate thermal storage (latent heat), and an organic Rankine cycle (ORC). From a broad perspective, the CB is modeled considering two types of heat inputs for the HP: a cold reservoir in the ground (at a constant temperature of 12 °C throughout the entire year) and a heat storage at 80 °C (thermally-integrated PTES—TI-PTES). The first part defines simple models for the HP and ORC, where only the cycles’ efficiencies are considered. On this basis, the storage temperature and the kind of fluids are identified. Then, the expected power-to-power (round-trip) efficiency is calculated, considering a more realistic model, the constant size of the heat exchangers, and the off-design operation of expanders and compressors. The model is simulated using Engineering Equation Solver (EES) software (Academic Professional V10.998-3D) for several working fluids and different temperature levels for the intermediate CB heat storage. The results demonstrate that the scenario based on TI-PTES operation mode (toluene as the HP working fluid) achieved the highest round-trip efficiency of 80.2% at full load and 50.6% round-trip efficiency with the CB operating at part-load (25% of its full load). Furthermore, when the HP working fluid was changed (under the same scenario) to R1336mzz(Z), the round-trip full-load and part-load efficiencies dropped to 72.4% and 46.2%, respectively. The findings of this study provide the HP and ORC characteristic curves that could be linearized and used in a thermo-economic optimization model based on a Mixed-Integer Linear Programming (MILP) algorithm.

Research on Two-stage Humidification-dehumidification Heat Pump Desalination System

For the humidification-dehumidification desalination unit to perform more efficiently, a two-stage humidification-dehumidification heat pump desalination system was proposed. So as to improve the heat and mass transfer effect between seawater and moist air, the two-stage humidifier was employed to recover the waste heat of seawater discharged from the first stage humidifier. The models of a one-stage and two-stage humidification heat pump seawater desalination system were built using Aspen Plus simulation software. Multi-condition simulation studies were carried out for different inlet seawater temperatures, liquid-gas mass ratios, and refrigerant flow rates of the heat pump. According to the comparative examination of the simulation’s outcomes, it is concluded that: with the increase in inlet seawater temperature and liquid-gas flow ratio, the two-stage humidifier system’s recovery rate and gain output ratio are superior to those of the first-stage humidifier system, and the inlet seawater temperature and circulating airflow needed for the two-stage humidifier system to perform at its best are less demanding than those for the first-stage humidifier system., indicating that the two-stage humidifier dehumidification desalination technology of the heat pump has great performance improvement and a broad application prospect.

Performance assessment of a two-stage evaporation grade compression heat pump system for double temperature level drinking water production

The heat pump is widely used for space and water heating, but it has problems of low heating efficiency and inadequate heating at a large temperature span. According to the concept of energy cascade utilization and production, a two-stage evaporation grade compression heat pump (TE-GCHP) system is proposed in this paper for double temperature level boiled drinking water production. In this novel system, the application of grade compression matching multistage evaporation process realizes cascade utilization of the low grade thermal energy and cascade production of the hot drinking water, i.e. the refrigerant vapor from the intermediate-temperature water-cooling evaporator is sucked into the intermediate pressure suction port of the compressor to undergo the compression process of low compression ratio, while the refrigerant vapor from the low-temperature air-cooling evaporator is sucked into the low pressure suction port of the compressor to undergo the compression process of high compression ratio, so as to improve heating performance of the TE-GCHP system at large temperature spans. The energetic and exergetic analysis methods are used to evaluate heating performance of the proposed system and comparisons with the conventional waste heat-recovery single-stage compression heat pump (WH-SCHP) system are also discussed. The results indicate that there are the optimum water inlet temperature of medium-temperature water chamber and the optimum first-stage compression ratio to obtain the highest COP and exergy efficiency as well as the lowest power consumption, and energy-saving and economic performances of TE-GCHP system are markedly superior to those of WH-SCHP system and electric boiler. At evaporating temperature in the range of −10 °C to 30 °C, COP of TE-GCHP system is increased by1.06～1.29, and its exergy efficiency is improved by 2.17 %～10.62 %, while exergy loss of TE-GCHP system is reduced by 27.49 % on average, as compared to WH-SCHP system. In addition, as condensing temperature declines from 130 °C to 100 °C, power consumption of TE-GCHP system is decreased by 20.13–35.07 % compared with WH-SCHP system, whereas exergy efficiency of TE-GCHP system is on average 7.99 % higher than that of WH-SCHP system. In addition, the annual costs of the proposed system are 53.20 % and 69.68 % lower than those of the WH-SCHP system and electric boiler respectively.

Simulation and optimum control of a two-stage compression air source heat pump system: A comparison of two kinds of variable volume approaches

Two-stage vapor compression technology can effectively improve the performance of an air source heat pump, and a staged compression system with two separate compressors has a highly flexible modulation capacity. However, the effect of different volume modulation approaches on the heating COP requires further analysis. In this study, a Modelica-based dynamic simulation model of a two-stage air-source heat pump was developed and verified with experimental data. A numerical optimization method was used to obtain the optimum frequencies and low- or high-stage cylinder volumes of the compressors that maximize the COP. The results show that the three optimization approaches have different effects. At lower ambient temperatures, varying the high-stage compressor cylinder volume can yield the maximum COP among the three optimization methods, followed by varying the low-stage compressor cylinder volume and compressor frequencies. At higher ambient temperatures, varying low-stage cylinder volume can yield the maximum COP, followed by varying the compressor frequencies and high-stage cylinder volume. The further the working condition deviates from the design point, the greater the improvement in the COP. The maximum improvements in the COP achieved with optimization in this study were 8.23%, 6.39% and 5.39% by varying the high-stage cylinder volume, low-stage cylinder volume, and frequencies, respectively.

Experimental investigation of a novel two-stage heat recovery heat pump system employing the vapor injection compressor at cold ambience and high water temperature conditions

Heat pumps (HPs) are energy-efficient space heating devices that are key to global carbon reduction and carbon neutrality. However, current commercial HPs have performance issues in cold climates where space heating is needed most, including low COP and high energy consumption of defrosting. Aiming to tackle these issues, a novel two-stage heat recovery heat pump (THRHP) is therefore developed to enable heat recovery from exhaust air for improving COP and preventing dramatic energy use during winter defrosting. The performance of THRHP was optimized in the laboratory by investigating its expansion valve opening and exhaust air fans situation. Finally, the experiment results showed the prototype provided a heating capacity of 32.3 kW, generating 4 m3/h hot water of 55 °C with COP of 2.57 at outdoor temperature of 0 °C, achieving 20.1% higher COP than the commercial HPs, while the efficient and quick defrosting process only consumed 0.46 kW and 4 mins under outdoor temperature on −6 °C. The results gave more insights into the characteristics of THRHP and obtained the optimal control strategies of THRHP for better performance, thus promoting the wide deployment of HPs and achieving the ambitious carbon-neutrality targets.

Performance Analysis of Two Systems Combining Heat Pump and Water Vapor Compression for Waste Heat Recovery

In the area of the heating industry, a heat pump is an efficient alternative technology to achieve energy saving and carbon emission reduction. The conventional heat pump has gradually been applied to replace the traditional direct electrical heating method while the required temperature is below 100 °C. A heat pump with temperatures between 100–140 °C is in the stage of rapid development. However, a heat pump with temperatures above 150 °C has received relatively little attention. In this paper, two systems combining a heat pump and water vapor compression (CHPVC and HPTVC) have been studied for waste heat recovery from 45 °C to a water vapor supply with a temperature above 150 °C. A thermodynamic model has been proposed to analyze the performance of the two systems, and a twin-screw compressor model has been developed to calculate the isentropic efficiency of the compressor applied in the heat pump. Four different parameters have been used to analyze the energy efficiency. The simulation results show that while the inlet water temperature is 45 °C and the required vapor temperature is 150 °C, the optimal COPs of CHPVC and HPTVC are 2.432 and 2.436, respectively. Moreover, CHPVC is more suitable for the large saturation temperature lift, and HPTVC is more suitable for a relatively small temperature difference between the inlet water and the required vapor. Compared with the direct electrical heating method or the conventional two-stage heat pump, these two systems are remarkably efficient and show good energy-saving potential.

Seeking a new throttle to substitute the thermostatic expansion valve in vapor-injection process for an air-source heat pump: An experimental study

Air source heat pump (ASHP) as a sustainable energy technology is playing an essential role in achieving carbon neutrality for almost countries. The operational stability of ASHP is related to throttles. The selection of thermostatic expansion valve (TEV) types is sometimes limited by a wide range of outdoor temperature conditions. In order to find a substitutable throttle of TEV, this paper compares a pulse width modulation driven (PWM-driven) solenoid valve coupled with a capillary with TEV to control vapor-injection based on a two-stage compressed air-source heat pump (TCASHP) prototype. The experiment results show that the TCASHP unit can operate in a wider temperature range if the TEV capacity is selected properly to control vapor-injection. While the on-off periods and times of PWM-driven set out of place make the TCASHP situate at a hunting problem state. The solenoid valve should be installed behind the capillary to control vapor-injection by experiments and entropic analysis. Furthermore, the shorter on-off periods and longer opening time of PWM-driven solenoid valve reduce the risk of hunting problems. Besides, the operation stability is the best for TCASHP unit with PWM-driven solenoid valve coupled with capillary to control vapor-injection, which has the same performance as the TEV.

Ammonia-based compression heat pumps for simultaneous heating and cooling applications in milk pasteurization processes: Performance evaluation

Ammonia-based heat pumps were investigated to deliver heating (65–95 °C) and cooling (2 °C) simultaneously for the pasteurization of raw milk. Two compression heat pump configurations using a pure refrigerant (ammonia) and zeotropic mixture (ammonia/water) as working fluids were integrated with raw milk pasteurization process: two-stage ammonia compression heat pump with closed intercooler, and two-stage ammonia/water compression heat pump with a solution circuit. The integrated heat pump systems performance simulations were carried out at the base-case design conditions and parametric study on the influence of key operational variables on the system's first- and second-law efficiencies. These heat pumps are attractive and more efficient solutions than current heating and cooling systems used in the dairy processing industries (fossil-fuel-fired boiler and ammonia compression chiller) by up to 59% of a primary energy saving. Besides, the two-stage ammonia/water heat pump operates at a lower system high pressure than the ammonia heat pump, which allows reducing the cost of the system components, especially for the compressor. The development of such types of multi-stage compression heat pumps (with large-temperature-lift) using zeotropic mixture with wide temperature glide, as a working fluid, significantly contributes towards the decarbonization of the dairy processing industry.

Operating cost comparison of state-of-the-art heat pumps in residential buildings across the United States

With increasing concerns about the long-term global warming effect of HFC refrigerants, low-GWP refrigerants and non-vapor compression heat pumps are gaining attention to be the replacements for current vapor compression heat pumps that rely on HFCs. In addition, advanced heat pump solutions, such as heat pump water heaters, cold climate heat pumps, and low-GWP heat pumps are emphasized by regulators to make the heat pump market cleaner, more efficient, and more affordable. However, current research efforts lack comparisons of the new technologies under realistic building loads, climate conditions, and utility rates. In addition, combinations of emerging technologies have also not been considered. This study developed a methodology to systematically evaluate conventional and emerging space conditioning and water heating technologies both from performance and operating costs points of view. This study analyzed eight HVAC configurations for space heating, cooling, and water heating in terms of annual operating cost when coupled with an actual building affected by weather conditions. The eight HVAC configurations covered advanced solutions such as heat pump water heaters, ground source heat pumps, cold climate heat pumps, and membrane heat pumps, and they were compared with traditional vapor compression heat pumps and gas furnaces. The operating cost assessment was conducted based on building energy simulation using the real utility structure. The results revealed the trade-offs between HVAC combinations for the eleven climate zone covered in this study. The two-stage cold climate heat pump can save 10%–20% heating costs than single-stage heat pumps in cold regions. A membrane evaporative cooler can provide cooling cost savings in eight out of the eleven climate zones analyzed. A gas furnace should only be used in cold places where the electricity price per kWh to gas price ratio is higher than 3. HPWH should be recommended to places where the electricity price to gas price ratio is below 3.

Thermodynamic and economic analysis of two-stage CO2 Heat Pump with reverse Brayton cycle

At present, the output water temperature of most CO2 heat pumps is below 95°C. Herein, the reverse-Brayton cycle is innovatively applied to the CO2 heat pump to increase its output water temperature. A numerical model of a two-stage reverse-Brayton cycle with refrigerant injection and internal heat exchanger is proposed. The effects of discharge pressure, suction temperature, injection mass ratio, and pressure of refrigerant injection on COP of this system at different heat source temperatures are analyzed. Simulation results show that the highest COP can reach 3.05 with a discharge temperature of 129.1°C. The highest discharge temperature is 157.1°C and the temperature lift is 87.1°C with a COP of 2.85 correspondingly. High temperature hot water or steam is supplied by the proposed cycle with a respectable COP, which will broaden the application scope of CO2 heat pumps in industries. Moreover, relationship between COP and the payback period is revealed under typical conditions to evaluate the benefit of the proposed system.

Performance and economic evaluation of a solar-air hybrid source energy heating system installed in cold region of China

This paper proposes a solar-air source energy storage heating system (SASES-HS), which can solve the problems of high energy consumption and difficult defrosting when the ambient temperature is low. By coupling solar energy,air energy and phase change energy, the system heats the end of the user through a two-stage heat pump. In order to analyze the feasibility of the system, the mathematical model of the system is established, and an experimental platform is built. A typical experimental condition was chosen to analyze the performance of the system in detail. The results show that the system can continuously provide high-temperature hot water of nearly 60°C for buildings in a severe cold environment of −20°C for 24 h, and the heat exchange temperature difference is as high as 80°C. In addition, the system operates with a higher coefficient of performance than a single-stage heat pump system. The COP can reach the level of 4.1 during the day and as low as 2.5 at night. The operating cost of the system only accounts for 65.8% and 88.4% of the coal-fired boiler and the air source phase change heat pump, respectively. And the CO2 emissions of this system only account for 57.5% and 88.4% of the coal-fired boiler and the air source phase change heat pump, respectively. In view of the analysis of operational performance and economic and environmental benefits, it is found that the system is worth popularizing and using in the northern cold regions.

The Influence of Extratropical Weather Regimes on Wintertime Temperature Variations in the Arctic during 1979–2019

In this study, the Arctic sea ice cover in the sector 30° W–60° E in February, and the monthly mean temperature (averaged over the polar cap north of 70° N and 700–1000 hPa, Tcap) in winter during 1979–2019 were analyzed using established change-point detection methods. Step changes were found in 2004, with lower sea ice cover and higher air temperature during 2005–2019 than 1979–2004 (with Tcap anomalies of 1.05 K and −0.63 K, respectively). Two combinations of weather regimes were associated with the anomalously warm months (1.61 K): (1) Scandinavian trough and Ural blocking, and (2) Atlantic ridge and Ural blocking. The first causes a “polar express” for the poleward transport of heat and moisture from mid-latitude East Europe. The second causes a “two-stage heat pump” that transports heat and moisture from the subarctic Atlantic. Their opposite combinations were associated with the anomalously cold months (−0.73 K), which occurred more frequently during 1979–2004. These trends in weather regimes could account for 25% of the step-change in Arctic winter temperature, with the remainder likely caused by changes in sea ice cover, ocean heat transport, and concentrations of aerosol and greenhouse gases.

Features of modeling a highly efficient multistage vapor compression heat pump unit

The increase in the cost of fuel and energy resource and the deterioration of the environment from the combustion of traditional fossil fuel, have led to a great interest in energy-saving technology by using secondary energy resources in the thermal energy of industrial, housing and communal services using heat pump units in Russia and abroad. This paper analyzes the well-known two-stage heat pump units, and reveals their advantages in comparison with single-stage. The modeling of a highly efficient multistage vapor compression heat pump unit is proposed. Moreover, a method for calculating a multistage heat pump unit with a high coefficient of performance is presented. In addition, an example of calculating the thermodynamic cycle of a four-stage heat pump unit is presented. The influence of the number of stages on the increase in coefficient of performance in relation to a single-stage heat pump unit, the effect of the temperature difference between the temperature of the high-potential heat source and the temperature of the low-potential heat source on the coefficient of performance were analyzed. In addition, the influence of the initial value of the temperature of the high-potential heat source before heating during the course in the heat pump unit on the value of coefficient of performance for a different number of stages is analyzed under the condition of a constant difference between the heating temperature of the high-potential heat source at the outlet of the heat pump unit and the temperature of the low-potential heat source.

Operation strategy and performance investigation of a high-efficiency multifunctional two-stage vapor compression heat pump air conditioning system for electric vehicles in severe cold regions

Cooling and heating are indispensable for electric vehicles (EVs) in severe cold regions due to the hot summer caused by global warming. A multifunctional two-stage vapor compression (MTSC) heat pump air conditioning system (HPAC) with its operation strategies is proposed, which can operate in not only single-stage vapor compression (SSC) cooling and heating mode, but also two-stage vapor compression (TSC) heating mode under ambient temperatures as low as −35 °C. Then a simulation model is developed, and comparative analyses among the MTSC HPACs with different refrigerants, and between MTSC HPACs and SSC HPACs are conducted. The influences of intermediate pressure and pressure drop on performance are discussed. Besides, exergy analysis and second law performance are presented. It is found that the MTSC HPAC attains a 6% to 30% higher coefficient of performance (COP) than the SSC HPAC when the evaporating temperature changes from −10 °C to −20 °C, and 156% to 85% higher energy efficiency than the positive temperature coefficient (PTC) heaters when the evaporating temperature varies from −25 °C to −45 °C. The COP of R290 is slightly higher than that of R410A, R32 and R454C. Furthermore, the main exergy destruction appears in compressors, accounting for 40% to 45%.

Quantification and comparison of COP improvement approaches for large-scale ammonia heat pump systems

• Compares five approaches to improve the COP of large-scale ammonia heat pumps. • Evaluates both one-stage and two-stage HP cycles connected in series. • A pinch analysis optimizes the heat exchanger network between HP and heat sink. The number of large-scale heat pumps (HP) implemented in Danish district heating networks has increased in recent years. The installations are often custom-made solutions designed carefully to obtain a high Coefficient of Performance (COP). This study aims at quantifying and compare five approaches for improving the COP of a heat pump, with a one-stage ammonia cycle used as the reference case with sink temperatures of 50 °C/80 °C (in/out) and source temperatures of 10 °C/3 °C. The approaches include increased complexity of cycle layout, optimization of intermediate pressure in two-stage cycles, a connection of HP cycles in series, optimization of load share between heating capacity for HPs in series, and optimal design of heat exchanger networks (HEN). The latter is based on an implementation of pinch analysis to HP HENs. The results revealed an improvement in COP by +15.8% obtained with increased complexity of cycle layout (two-stage cycle with a low-stage desuperheater) compared to a COP of 2.98 for the reference case. Connecting up to three one-stage HPs and two-stage cycles in series improved COP by 4.42% and 24.5%, respectively. The benefit of optimal load share and intermediate pressure was less than 0.5 % points for all configurations compared to reference values based on theoretical approaches. Exploiting the potential for optimal HEN increased the potential to 10.4% for one-stage cycles in series and nearly 29% for two-stage cycles increasing the COP to a maximum of 3.84.

A new heat supply strategy for CO2 capture process based on the heat recovery from turbine exhaust steam in a coal-fired power plant

A new heat supply strategy for the CO2 capture process based on the heat recovery from turbine exhaust steam was proposed and evaluated. A water-steam cycle, a CO2 capture unit, a CO2 compression unit, and an enhanced two-stage heat pump were simulated for a 600 MW coal-fired power plant using Aspen Plus. Effects of the working medium, the exhaust pressure of the turbine, and reboiler temperature on the heat pump efficiency were studied. The integrated configurations were simulated and the effects of reboiler duty, exhaust steam pressure, and reboiler temperature on the thermodynamic performance of the plant station were discussed. Results indicated that the utilization of internal heat exchangers improved effectively the coefficient of performance (COP) of the two-stage heat pump by 13.3–22.0%; the heat pump recovered effectively the latent heat of exhaust steam, and a lower CO2 desorption temperature would significantly contribute to a higher COP of the heat pump system and greater thermodynamic performance of the proposed integrated configuration with heat pump. A net electric efficiency penalty of 7.07% and a fuel utilization efficiency of 51.58% were obtained when the reboiler temperature was 90 °C.

Modeling and performance analysis of a combined thermal Brownian heat pump cycle

<p indent=0mm>Based on finite-time thermodynamics theory, this paper proposes a two-stage thermal Brownian heat pump cycle model combined with an intermediate reservoir. The model is formed by two Brownian heat pump cycles operating in series. Two heat pump cycles operate between three heat reservoirs with constant temperature. In this regard, finite-rate heat transfer processes exist among heat pump cycles and reservoirs. In consideration of the heat transfer losses among reservoirs and heat pump cycles, the expressions of heating load and coefficient of performance (COP) are derived. The influence of design parameters on system performance is analyzed by numerical calculations. Results show that heating load decreases monotonically with the increase in the barrier height of the potential and increases monotonically with the increase in the external load and the asymmetry of the potential. The COP reaches the maximum values with the barrier height of the potential, the asymmetry of the potential, and the external load. When the heat reservoir temperatures are constant, the closer the intermediate heat reservoir temperature is to the topping cycle heat reservoir temperature, the higher the heating load and the COP. Compared with one-stage thermal Brownian heat pump cycle, which works between the heat source and sink with the same temperatures, the combined Brownian heat pump cycle has a larger heating load but a reduced COP.

RETRACTION NOTICE: A novel configuration and performance for a two-stage thermoelectric heat pump system driven by a two-stage thermoelectric generator

RETRACTION NOTICE: A novel configuration and performance for a two-stage thermoelectric heat pump system driven by a two-stage thermoelectric generator

Method of linear approximation of COP for heat pumps and chillers based on thermodynamic modelling and off-design operation

Often, simple estimates of the coefficient of performance (COP) for heat pumps (HPs) and chillers are used. Depending on the purpose, this may not be sufficient. There are more accurate methods for determining COP, but they may not be used due to complexity, nonlinearity, or limited application of advanced COP estimation methods. Here, we present a new COP approximation method suitable for HPs and chillers. It is based on linear relationships and was developed from a thermodynamic two-stage HP model for design and off-design operation using ammonia as a refrigerant. This approximation method was then applied to a case study investigating the potential of district heating supplied by HPs in Tallinn, Estonia. Groundwater, sewage water, seawater, river water, lake water, and a district cooling return line were explored as potential heat sources. The results show a deviation in COP of less than 1.5% compared to the thermodynamic model. Annual calculations show the applicability of the COP approximation method for calculating hourly COPs at different heat source and heat sink temperatures, as well as changing heat loads, seasonal COP, heat demand ratio, and hourly Lorenz efficiency.