Vapor Compression Cycle Research Articles

Influence of boundary conditions on the optimization of a geothermal heat pump studied using a thermodynamic model

This paper is the logical follow-up to a work [1] whose results were presented at the 28th French Thermal Congress which was to be held in Belfort in 2020. The model developed at that time is completed in this proposal to consider the specificity of the geothermal heat pump. This is a machine operating upon a mechanical vapor compression cycle, the limit of which is an inverse Carnot cycle. Its specificity consists of a cold loop at the source with the geothermal exchanger and the evaporator, then a hot loop at the sink with the condenser and a floor heat exchanger in the application considered here. We are particularly concerned with the optimal sizing of these heat exchangers through their effectiveness. The parametric sensitivity of this distribution to various boundary conditions is studied, especially by focusing on different conditions at the source: (1) imposed soil temperature, corresponding to a Dirichlet condition, (2) imposed heat flux (including adiabatic case), corresponding to a Neumann condition, (3) imposed mechanical power consumed by the heat pump, and (4) imposed coefficient of performance COP, to all cases being associated a finite thermal capacity in thermal contact with the geothermal exchanger operating in steady-state conditions.

Reinforcement Learning for Vapor Compression Cycle Control

Reinforcement Learning for Vapor Compression Cycle Control

Control-oriented Model of HVAC and Battery Cooling Systems in Electric Vehicles

Energy-efficient thermal management of heating, ventilation, and air-conditioning (HVAC) and battery cooling systems is essential for an enhanced driving range of electric vehicles. With recent advances in vehicle connectivity technologies and vehicle computation units, a predictive model has been emphasized in control design. However, the system complexity due to refrigerant circuits and coupling with the battery cooling system makes it challenging to develop a computationally inexpensive model for potential model predictive control applications. This paper presents a novel control-oriented model to capture the behavior of a combined HVAC and battery cooling system for electric vehicles. To reduce the complexity due to the nonlinear two-phase flow heat transfer, the refrigerant circuit is modeled based on the assumption of a quasi-steady ideal vapor-compression cycle. The proposed model is calibrated with a three-step optimization process and then validated against a high-fidelity physics-based model with open-loop simulations. The simulation results show the effectiveness of the proposed model with four states and five actuators: the root mean square errors (RMSE) of the battery temperature and the coolant temperature are 0.41°C and 0.35°C, respectively, and the normalized RMSEs of the heat transfer rate at the heat exchangers are within the range of 2.8% to 10.6%.

Vapor compression refrigeration testing on parabolic flights: Part 2 - heat exchanger performance

Propositions of vapor compression cycles for microgravity applications have a long history. In contrast, experimental efforts and implemented applications are rare in general and in recent years even less than in the 1980s and 1990s. The paper shows experimental results from a vapor compression cycle on parabolic flights. The focus lies on the effects of varying gravity forces on the condenser and evaporator. Flow visualizations clearly showed a change to an annular or slug-annular flow regime at the evaporator inlet shortly after the onset of gravity. The discharge pressure was found to consistently increase sharply by 1–3% across widely varying operating conditions. Oscillations were observed in the evaporator for some parabolas due to microgravity. Overall, the cycle response to varying gravity based on the cooling capacity can be categorized in four different groups, which appear to have a dependence on both the mass flow rate and the evaporator superheat.

Volumetric design for ORC-VCC compressor-expander units.

The combination of an ORC (Organic Rankine Cycle) with a VCC (Vapor Compression Cycle) is studied analytically. In this work, the two cycles use the same fluid and operate with a common condenser. Elementary transformations of the fluid between two consecutive points of the system are calculated using a one-dimensional thermodynamic approach considering real fluid properties, the pressure losses and irreversibilities present in the compressor, in the expander and in the pump. The performances and the recommended inlet and outlet diameters of the expander and the compressor are established for various operating conditions and 12 fluids. The system can produce a typical useful cooling capacity of 0.60 kW at 5.5 °C when using a 2 kW heating source at 65 °C and when the condenser temperature is 30 °C. The cooling capacity reaches 1.0 kW if the cold source temperature is 15.5 °C. Results show the required diameters ratios at inputs and outputs of the compressor and the expander. They indicate the situations for which the compressor is smaller than the expander and vice versa. They identify the particular cases in which the compressor and the expander have exactly the same inlet and outlet diameters. This particular case allows using two identical compressors, one working as a compressor, the second as an expander.

Harnessing of geothermal energy for a greenhouse in Ecuador employing a heat pump: design, construction, and feasibility assessment

Globally, the greenhouses' farming area comprises 500 000 ha, and they efficiently produce more than half of the vegetables consumed around the world. Nevertheless, high-yield crops tend to be incredibly energy-intensive. This study proposes designing and building a coupled geothermal heat pump for a 470 m2 greenhouse in the Andean zone conditions addressing a requirement of 15 °C at night and 30 °C during the day. Firstly, the study determined the energy potential of the solar and geothermal sources employing actual measurements and contrasting the results with theoretical models. Then, it developed an energy balance in the greenhouse to size the geothermal heat pump using the vapor compression cycle. Finally, the comprehensive system was built and evaluated through the Leveled Cost of Heat (LCOH). The operation requires a potential of 29.56 and 65.76 kW for heating and cooling; this is technically feasible when running the system with a heating flow driven by an optimized temperature ramp of 1.64 °C h−1. Also, the capacity factor (CF) shows that a lifespan between 12 to 14 years is required to reach acceptable LCOH when CF is as low as 0.45. Financially, it is necessary to foster customs exemptions to make it competitive versus more traditional sources such as electricity and LPG since the main components of the heat pump and the geothermal exchanger are not produced locally and represent nearly 70 % of the upfront costs.

Proper orthogonal decomposition for reduced order dynamic modeling of vapor compression systems

A computationally efficient but accurate dynamic modeling approach for vapor compression systems is important for many applications. Nonlinear model order reduction techniques which generate reduced order models based on high fidelity vapor compression cycle (VCC) models are attractive for the purposes. In this paper, a number of technical challenges of applying model order reduction methods to VCCs are described and corresponding solution approaches are presented. It starts with a reformulation of a standard finite volume heat exchanger model for matching the baseline model reduction structure. Reduced order models for evaporator and condenser are constructed from numerical snapshots of the high fidelity models using Proper Orthogonal Decomposition (POD). Methodologies for system stability and numerical efficiency of POD reduced order models are described. The reduced order heat exchanger models are then coupled with quasi-static models of other components to form a reduced order cycle model. Transient simulations were conducted over a wide range of operating conditions and results were compared with the full order model as well as measurements. The validation results indicate that the reduced order model can execute much faster than a high-fidelity finite volume model with negligible prediction errors.

Experimental validation of an organic rankine-vapor compression cooling cycle using low GWP refrigerant R1234ze(E)

There is a significant global opportunity to capture and utilize low grade waste heat to reduce fossil fuel consumption, greenhouse gas emissions, and improve energy efficiency across a wide range of industries. In this work, an advanced type of thermally activated cooling system, an organic Rankine-vapor compression cycle (ORVC) with novel heat integration strategies, was designed and tested at a relevant scale for industrial waste heat recovery (300 kWth cooling capacity). The ORVC linked an organic Rankine power cycle and a vapor compression cooling cycle using a turbine and compressor that shared a single shaft. The ORVC test facility absorbed waste heat from a liquid stream at 91℃ to simulate engine coolant in diesel generator sets, rejected heat to a glycol stream at 30 ℃, and generated chilled water at 7 ℃. A cooling capacity of 264 kW ± 3.5 kW was experimentally validated with a COP of 0.56 ± 0.01 during steady-state operation at the design temperatures. The thermal efficiency, accounting for pump work, of the Rankine cycle was 7.7% ± 0.22% and the COP of the vapor compression cycle was 5.25 ± 0.09. The centrifugal turbo-compressor operated at 31.5 kRPM ± 0.3 kRPM, with a turbine and compressor isentropic efficiencies of 76.7% ± 0.90% and 84.8% ± 0.54%, respectively, with near-perfect power transmission between these components. The pressure drop in the piping and heat exchangers were significantly larger than expected which had a detrimental impact on the performance of the ORVC. In addition, the condenser on the cooling cycle could not deliver the subcooling as specified from the design point modeling. The results from the sensitivity analysis showed that the higher condenser glycol outlet temperature had the largest impact on performance, which is consistent with other analytical models in the literature. When the ORVC simulations were updated with experimental values for isentropic efficiencies of the turbomachinery, the thermal COP was 0.66 which represents an estimate of the predicted performance if test facility limitations are overcome.

Humidification dehumidification saline water desalination system utilizing high frequency ultrasonic humidifier and solar heated air stream

An experimental investigation on the solar desalination system was conducted according to humidification dehumidification methodology hybrid with hot air stream flow and vapor compression cycle as a condensation unit. A high-frequency ultrasonic atomizer (HFUA) was used as a humidifier which rapidly reaches 100% Relative Humidity. The tests were done during Aug. and Sept. 2020 under the climate of Suez city, Egypt 29.9668°N, 32.5498°E. The effect of high-frequency ultrasound atomizer situation, water height, and hot air stream flow rate on distillate yield were studied. The results illustrated that increasing atomizer number and decreasing water height increases the daily distillate production. The maximum daily freshwater productivity occurred at atomizer number N = 6, rising 38.6% and 115% compared with N = 4 and N = 2, respectively. Furthermore, water height at H = 1 cm is the most efficient with 16% and 28.6% increment compared with H = 2 cm and H = 3 cm, respectively. The optimum hot air stream flow rate ṁa = 0.01156 kg/s with an increment of 55.95%, 31%, 6.48%, 11.3%, and 38.5% compared with 0.0086, 0.01011, 0.013006, 0.0144, and 0.0173 kg/s, flow rates, respectively. The daily production reaches 7.72 kg\\day, the system efficiency 33.84 % and the estimated cost per unit liter is 0.03437 $/L.

A mixture of carbon dioxide and dimethyl ether as a refrigerant for ground air conditioning system

BACKGROUND: The selection of refrigerants for modern air conditioning systems (ACS) in ground facilities is a multidisciplinary task. Particularly, meeting the required energy efficiency of the refrigeration cycle as well as ensuring ecological safety of production, operation, and utilization of the refrigeration system. Herein, the working pressure levels of the refrigeration cycle considerably affect the availability, cost, and safety of the refrigeration equipment. The fire safety of the working substance is also important.
 AIM: To investigate the feasibility of a mixture of dimethyl ether and carbon dioxide as refrigerant for energy efficient and safe application of ACS in ground facilities.
 METHODS: Comparative analysis of a simple one-stage vapor–compression cycle using traditional working substances (R22 and R410A) and the proposed working substance, which is in the form of a mixture of dimethyl ether and carbon dioxide, using packages, such as Mathcad, HYSYS, CoolPack, and REFPROP, was performed.
 Results: An ecofriendly mixture of dimethyl ether and carbon dioxide with low global warming potential and zero ozone depletion potential was proposed as refrigerant. Increasing the percentage of dimethyl ether in the blend reduces the temperature glide in the gas cooler, a property of CO2, and pressures at which the blend operates. The mixture has limited operational properties due to the flammability of dimethyl ether, but its environmental performance makes the material of some practical interest.
 CONCLUSION: Fire safety of the proposed working substance was calculated. The concentration of dimethyl ether in the mixture at which it becomes flammable and unsafe for ACS was determined to be 8.3%.
 With an increase in the dimethyl ether content in the mixture with CO2 from 4% to 8%, the refrigeration coefficient of the cycle increases from 2.53 to 2.88, but it is 1.57 times less than that of R410A.
 The difference in operating pressures between the used non-ecological refrigerants and proposed mixture was determined. Results indicate that the mixture of dimethyl ether and carbon dioxide is currently inapplicable to mass production compressors, which use R410A as refrigerant. The condensation pressure of the most effective and nonflammable mixture of dimethyl ether and CO2 (with dimethyl ether concentration of 8%) is 101 bar against 30 bar for R410A.
 Therefore, we intend to evaluate test mixtures of dimethyl ether with other substances in the future.

Optimal Scheduling for a District Cooling System with Chilled Water Storage: Comparative Assessment of Vapour Compression and Absorption Refrigeration Cycles

District cooling (DC) systems are able to exploit locally available energy in order to supply cooling to local end-users through an integrated DC network. The present work aims at obtaining the critical decisions in the performance of a DC network at the operating level. End-users can be of different type (e.g., residential or commercial) with different demand profiles and an optimal strategy needs to be employed in order to satisfy the demand and minimise energy usage. In published literature the vast majority of the studies employ simplified mathematical models in order to describe the thermodynamic cycle used and only a handful of studies take into account the actual dynamics of the system transition from one state to the other as well as the time delay attributed to the cooling medium transfer through the network. The critical decisions involve the operating conditions and optimal allocation for the cooing of existing units in a suitable time interval, which form the basis for the satisfaction of requirements in cooling under demand variability. Although reduced order models of the process of ABR cycle are used, the time delay of the cooling medium transfer through the pipes is considered through the employment of validated first order plus time delay models. The proposed optimisation framework enables the identification of demand driven, highly performing solutions through the direct consideration of all interacting factors over a pre-defined time horizon. Optimal results include the definition of the optimal allocation of produced cooling effect in such way that cooling demand is always satisfied.

Potential of MWCNT/R134a nanorefrigerant on performance and energy consumption of vapor compression cycle: a domestic application

Potential of MWCNT/R134a nanorefrigerant on performance and energy consumption of vapor compression cycle: a domestic application

Performance and Economic Investigation of Solar Regenerated Liquid Desiccant Room Air Conditioning Systems using Eco-Friendly Desiccants

Traditional air conditioning systems run on the vapor compression cycle, which utilizes electricity generated from fossil fuels, the reserves of which are fast depleting. Moreover, the refrigerants used in such systems have exacerbated ozone layer depletion and climate change. Liquid desiccant air conditioning (LDAC) systems appear as a favorable option in light of these drawbacks. This paper has developed the numerical model of an LDAC system using a dehumidifier that is internally cooled and has a finned coil. The study of this particular model has been limited in the past. The model has been validated against a reference study available in the literature. Moreover, the commonly used desiccant materials tend to be corrosive and detrimental to the air quality. Therefore, the feasibility of an ionic liquid (65% [Emim]OAc) and an organic salt solution (73% HCOOK) as desiccant materials has also been investigated in this paper. With the proposed scheme for room air conditioning, the air temperature and humidity levels within the range of comfort (21∘C, 53% R.H.) could be supplied to the conditioned space. Performance analysis revealed that the eco-friendly desiccants could achieve dehumidifier and regeneration effectiveness similar to that of the traditionally used corrosive salt solution of calcium chloride (CaCl2). Furthermore, the estimated dehumidifier efficiency of about 76% is found to be higher than that available in the previous studies. Economic analysis provides a comparison of total cost between the conventional and the novel desiccant air conditioning systems and also reveals HCOOK to be a more economical choice as desiccant material.

An experimental study of a newly designed freezing desalination unit equipped with reversed vapor compression cycle

A newly designed freezing desalination system was constructed. The system operates on the principle of reversed vapor compression. It is equipped with two identical heat exchangers, one of them works as an evaporator and the other as a condenser. A three-way valve was used to reverse the operation of heat exchangers, causing the ice in the evaporator to melt. The performance of the newly designed system was evaluated in both the forward and the reversed cycles. Electric current, power consumption, pressure drop, water productivity, and specific consumption were all investigated. The experiments were conducted with ice ratios of 0.27, 0.39, 0.45, 0.54, and 0.65 with cycle operating periods of 110, 135, 180, 240, and 300 min. The results showed that the electrical current, power consumption, and pressure drop were decreased with the reversed cycle. The maximum energy-saving percentage was 13% for cycle operation time 300 min. The amounts of freshwater reached 19 L for cycle operation times of 300 min from 30 L saline water. It is recommended to run our newly designed system at an ice ratio of 0.45 since these results save 1.5% in the reversed cycle, which is the most used cycle in the new design.

A solar-assisted hybrid air-cooled adiabatic absorption and vapor compression air conditioning system

Absorption-compression hybrid systems which consist of absorption and vapor compression cycle have advantages on both energy efficiency and cost-effectiveness. In the open literature, a great amount of research on such hybrid systems focuses on the water-cooled system which requests a larger area for the whole cooling unit due to the size of the cooling tower. In this paper, an air-cooled solar-assisted hybrid system consists of an adiabatic absorption (ABS) and a vapor compression refrigeration system (VCR) with subcooling mode and cascade mode is proposed for miniaturization. Steady-state experiments of the proposed system in the cascade mode were carried out and subsequently a numerical model is developed to evaluate performance difference and possible application for the two kinds of coupled modes. Simulation results show an increment of 63.9%∼166.7% in the electric COP of the system (COPele,sys) in the cascade mode, which is similar to from the experimental findings. Although the simulated COPele,sys increases by only 15.9% ∼ 29.8% in the subcooling mode, electricity saving rate brought by unit thermal energy input (defined as ‘r’) and the exergetic efficiency (COPex,sys) are significantly greater than those of the cascaded mode. So is the primary energy efficiency (PEE). It is concluded that the subcooling mode driven by the compound parabolic collector (CPC) is applicable to residential buildings due to its distinct advantages —— lower levelized cost of cooling (LCOC) of 0.06 US $/kWh, shorter payback time (PBT) of 8.54 years and smaller collector area of 11.41 m2, in comparison with other configurations. The study of this paper can function as guidance for design of the air-cooled absorption-compression integrated air conditioning system.

Desiccant cooling assisted vapor compression system: A double stage desiccant cooling cycle via evaporative condenser

To promote wider distribution of desiccant cooling technologies, a desiccant cooling assisted vapor compression system (DCVCS) is proposed. As a continuation of desiccant cooling (DC) systems development, DCVCS was built upon combination of a DC cycle and a vapor-compression (VC) cycle in a serial manner. In DCVCS, air-cooling is firstly produced in the DC cycle and the chilled air is used to the evaporative condenser of the VC cycle. Then through the pipework, the condensed refrigerant is delivered to the conditioning space and air-conditioning of the space is achieved via the evaporator in the space. In the DCVCS composed of the two cycles, a proper match between the two cycles is particularly important for the optimal performance of the combined system. The performance of the DCVCS is optimized by cycle simulation and comparatively analyzed with respect to a reference desiccant cooling system (RDCS). It shows that, for the optimally matched system configuration of DCVCS, the energy consumption reduces by 20–50% and the cooling capacity increases 3–4 times, compared with RDCS.

Comparative analysis of battery electric vehicle thermal management systems under long-range drive cycles

Due to increasing regulation on emissions and shifting consumer preferences, the wide adoption of battery electric vehicles (BEV) hinges on research and development of technologies that can extend system range. This can be accomplished either by increasing the battery size or via more efficient operation of the electrical and thermal systems. This study endeavours to accomplish the latter through comparative investigation of BEV integrated thermal management system (ITMS) performance across a range of ambient conditions (-20 °C to 40 °C), cabin setpoints (18 °C to 24 °C), and six different ITMS architectures. A dynamic ITMS modelling framework for a long-range electric vehicle is established with comprehensive sub models for the operation of the drive train, power electronics, battery, vapor compression cycle components, and cabin conditioning in a comprehensive transient thermal system modelling environment. A baseline thermal management system is studied using this modelling framework, as well as four common thermal management systems found in literature. This study is novel for its combination of comprehensive BEV characterization, broad parametric analysis, and the long range BEV that is studied. Additionally, a novel low-temperature waste heat recovery (LT WHR) system is proposed and has shown achieve up to a 15% range increase at low temperatures compared to the baseline system, through the reduction of the necessary cabin ventilation loading. While this system shows performance improvements, the regular WHR system offers the greatest benefit, a 13.5% increase in cold climate range, for long-range BEV drive cycles in terms of system range and transient response without the need for additional thermal system equipment.

Experiments and exergy analysis for a carbon dioxide ground-source heat pump in cooling mode

A prototype CO2 ground-source heat pump (GSHP), i.e. a water-to-air heat pump designed to couple with a ground heat exchanger, was tested in cooling mode per ISO standard 13256-1 to provide experimental data for assessing and improving CO2-based GSHPs. The GSHP consisted of a vapor-compression cycle with a liquid-line/suction-line heat exchanger (LLSL-HX). The system operated in a subcritical cycle for antifreeze entering liquid temperature (ELT) ≤ 25 °C and a transcritical cycle for ELT > 25 °C. The ‘Standard’ condition metrics were: coefficient of performance (COP) 4.14, total capacity 6690 W, sensible capacity 5400 W, and sensible-heat ratio (SHR) 0.81. The ‘Part-load’ performance was: COP 4.92, total capacity 7240 W, sensible capacity 5640 W, and SHR 0.78. The GSHP was also tested at additional ELTs ranging (10 to 39) °C, where COP ranged (7.3 to 2.4). Compressor efficiency correlations were shown for these and 118 additional tests. The LLSL-HX was estimated to reduce COP by (0 to 2)% for ELTs ranging (10 to 25) °C, and increase COP by (0 to 5)% for ELTs ranging (30 to 39) °C. At the ‘Standard’ condition the major exergy defects were compressor 0.32, ground heat exchanger 0.19, condenser 0.11, evaporator 0.11, EEV 0.07, pump 0.07, and fan 0.04. The exergy defects were sensitive to ELT, though the compressor was always the largest. Estimated performance of the cycle without the LLSL-HX showed for ELT (10 to 39) °C the HX reduced the EEV throttling defect by (0 to 0.08) and compressor defect by (0 to 0.015), but increased the condenser/gas-cooler defect by (0 to 0.06).

Experimental Investigation on Performance ByPass Cycle Multi-Split Cooling System-VRF

Variable refrigerant flow air-conditioning (VRF) systems are important and widely used building energy systems around the world. In this study, the performance of a multi-split VRF system using the bypass cycle is evaluated using the experiment as a possible sub-cooling method to prevent flash gas generation in liquid pipelines. The experimental for the multi-split VRF system is developed by considering the applications of the bypass cycle, and is validated with experimental data. The results of the tests that have been carried out are obtained in the vapor compression cycle with a bypass cycle, an increase in cooling capacity of 2.96% and an energy saving ratio (EER) of 1.72%. The input power of the bypass cycle is reduced by up to 2.45% when the performance of the multi-split VRF systems with bypass cycle cooling capacity conditions.

Performance Assessment and Working Fluid Selection for Novel Integrated Vapor Compression Cycle and Organic Rankine Cycle for Ultra Low Grade Waste Heat Recovery

This paper presents the performance assessment and working fluid selection for a novel integrated vapor compression cycle-organic Rankine cycle system (i-VCC-ORC), which recovers ultra-low-temperature waste heat rejected (50 °C) by the condenser of a vapor compression cycle (VCC). The analyses are carried out for a vapor compression cycle of a refrigeration capacity (heat input) of 35kW along with the component sizing of the organic Rankine cycle (ORC). The effects of the operational parameters on integrated system performance were investigated. The integrated system performance is estimated in terms of net COP, cycle thermal efficiency and exergy efficiency by completely utilizing and recovering the heat rejected by the condenser of the VCC system. R600a-R141b with COPnet (3.54) and ORC thermal efficiency (3.05%) is found to be the most suitable VCC-ORC working fluid pair. The integration of the vapor compression refrigeration cycle with the organic Rankine cycle increases the COP of the system by 12.5% as compared to the standalone COP of the vapor compression system. Moreover, the sensitivity analysis results show that there exists an optimum operating condition that maximizes the thermal performance of the integrated system.