Heat Pump Mode Research Articles

Experimental Study on HVAC System for Electric Vehicles

An HVAC (heating ventilation and air conditioning) system for electric vehicles based on heat pump technology is designed and set up in this paper. Unlike the existing HVAC system for vehicles using fossil fuel, this experimental HVAC system can work in heat pump mode rather than directly heating the PTC thermistor by electricity when heating is needed. This experimental HVAC system takes use of parallel flow exchangers, enclosed DC vortex compressor and the low voltage DC speed controller, which is used for controlling the compressor`s working frequency. After the experimental system is set, the influence of compressor`s speed and the environmental temperature to the performance of the HVAC system are tested and researched in refrigeration mode and heat pump mode respectively. Parameters like the compressor`s power input, the system`s refrigeration/heating capacity, and the coefficient of performance (COP) varied with compressor`s speed are recorded. Curves of those parameters are drawn and the reasons of their changing trends are analyzed respectively. The testing results of this experimental HVAC system are compared with references` results. According to the testing results, this experimental HVAC system can achieve better refrigeration and heating effects, and the cooling performance of the system increases with compressor speed; Part of the outside heat-exchanger gets frosted when the system works in heat pump mode, but the COP remains greater than 1, which means this experimental HVAC system is more energy-saving than the existing HVAC systems which are directly heating PTC thermistor by electricity for heating; In addition, compared with the sliding-vane compressor, the vortex compressor used in this paper has a better comprehensive performance and hence has a better application prospect.

Refrigerant distribution in the vertical header of the microchannel heat exchanger – Measurement and visualization of R410A flow

This paper presents the R410A adiabatic upward flow in three vertical headers of microchannel heat exchangers. All microchannel tubes are inserted into the half depth. The objectives are to explore what affects R410A distribution and attempt to predict the distribution. R410A is circulated into the header through the (5 or 10) tubes in the bottom pass and exits through the (5 or 10) tubes in the top pass. It represents the flow in the outdoor heat exchanger (usually used as the condenser) when it is used as the evaporator in the heat pump mode of reversible systems. The quality was typically varied from 0.2 to 0.8 and the mass flow rate from 1.5 to 4.5 kg h−1 per tube. The best distribution was observed at high mass flux and low quality. The experiment and visualization reveals the flow pattern effects in terms of homogeneity and momentum. The churn flow had better distribution since the two-phase mixture was more homogenous and the distribution was better at high mass flux in the header because the higher momentum liquid was able to reach the top exit tube.

Experiment and visualization on R134a upward flow in the vertical header of microchannel heat exchanger and its effect on distribution

R134a upward flow in four different vertical headers and its effect on R134a distribution into parallel microchannel tubes were investigated experimentally. R134a was provided into the header by (5 or 10) tubes in the bottom pass and exits through the (5 or 10) tubes in the top pass representing the flow in the heat pump mode of reversible systems. The inlet quality was varied from 0.2 to 0.8, and the mass flux from 92.26 to 276.79kgm−2s−1 per tube. The best distribution was at high flux and low quality. Experiment and visualization reveals the flow pattern effect in terms of homogeneity and momentum: churn flow has better distribution than separated flow since the two-phase mixture is more homogeneous; distribution is better at high mass flux in the header (by increasing mass flow rate, tubes number and/or tubes protrusion) because the higher momentum liquid can reach the top exit tube.

Heat Pump Heating Systems Combined with Solar Energy and Lake Water Source for the Biogas Digester

A heating system combined with solar energy and lake water source heat pump was designed for 200m3 fermentation reactor．In it could be switched to solar energy power mode, solar power with lake water source heat pump mode and lake water source heat pump mode according to different conditions. Compared with coal fired boiler, initial investment is a little higher, but for annual operating cost , this heating system is only 50% of coal fired boiler’s. But in the next several years, the initial investment can be returned back. For energy saving and economic, it is a suitable heat source for the biogas engineering in suitable area.

Experimental Study on the Multiple Energy Assisted Heat Pump

A multiple energy assisted heat pump (MEAHP) system for radiant floor heating was proposed, which integrated a domestic air-assisted heat pump with a solar water heater. The system mainly employed a vacuum tube solar collector with a surface area of 7.44 m2, an electrical rotary-type hermetic compressor, two capillary tubes and three heat exchangers. All operation modes have functioned successfully in Lanzhou city, China. The experimental data of air-source heat pump (Mode 2) and double-source heat pump (Mode 3) have been discussed. The results show that the coefficient of performance (COP) of the MEAHP system and the COP of the space heating system (COPs) are around 2.2 and 1.9 with the room air at 19 °C, respectively, and the temperature difference of the inlet and outlet of the radiant floor heating pipes is 2.8 °C. Some methods to improve the system performance are put forward.

Comparative study of transcritical vapor compression configurations using CO2 as refrigeration mode base on simulation

Despite of the excellent efficiency of the CO2 transcritical cycle in heat pump mode, there is a loss of efficiency in refrigeration mode compared with the subcritical vapor compression cycle with HFC refrigerants. This reason has lead several researches to propose ways to improve the transcritical cycle performance using CO2 as refrigerant in refrigeration mode, mainly associated to cycle configurations changes. In this paper, a comparative study and energetic simulation of most common configurations for transcritical single stage cycle using CO2 as refrigerant has been carried out. In order to make the comparison, a cycle components modelization has been proposed and the simulation results are used to find the optimum configuration for a single stage vapor compression transcritical system.

Experimental study of a photovoltaic solar-assisted heat-pump/heat-pipe system

A practical design for a heat pump with heat-pipe photovoltaic/thermal (PV/T) collectors is presented. The hybrid system is called the photovoltaic solar-assisted heat-pump/heat-pipe (PV-SAHP/HP) system. To focus on both actual demand and energy savings, the PV-SAHP/HP system was designed to be capable of operating in three different modes, namely, the heat-pipe, solar-assisted heat pump, and air-source heat-pump modes. Based on solar radiation, the system operates in an optimal mode. A series of experiments were conducted in Hong Kong to study the performance of the system when operating in the heat-pipe and the solar-assisted heat-pump modes. Moreover, energy and exergy analyses were used to investigate the total PV/T performance of the system.

A novel process for engines or heat pumps based on thermal-hydraulic conversion

A novel concept for heat engines or heat pumps is presented. The cycle followed by the working fluid is very similar to the Carnot cycle (reverse or engine). However, the isothermal steps of the cycle are achieved by the liquid/vapour phase change of the working fluid, like in the Rankine cycle. The work exchange involved during each step is realised by the alternating movement of an inert liquid, which acts as a liquid piston. This work-transfer liquid flows either through a hydraulic motor (for the engine mode) or through a hydraulic pump (for the heat pump mode). This intermediary hydraulic fluid avoids the main irreversibility inherent to conventional engines or heat pumps. The estimated performances of the engine (efficiency) and heat pump (COP) are very high and close to the corresponding Carnot efficiencies. An example of trigeneration (Combined Cooling, Heating and Power) using thermal solar energy at a low temperature (70 °C) and R-1234yf as a working fluid is given. This scenario requires associations between basic thermal-hydraulic elements (e.g., the engine or heat pump), such as a thermal cascade or mechanical coupling.

Study of the optimal control problem formulation for modulating air-to-water heat pumps connected to a residential floor heating system

For implementation in controllers with limited computational power, simplified optimal control formulations are desirable. For the case of a modulating heat pump system, the optimization problem is nonlinear due to the supply water temperature and load dependency of the heat pump coefficient of performance (COP). By neglecting these nonlinearities, e.g., by assuming a constant COP, the optimization problem becomes convex and fast convergence to the global minimum is ensured. This study quantifies the impact of these simplifications for the case of a modulating air-to-water heat pump connected to a residential floor heating system. A first comparison suggests a significant deterioration of the control performance. While the nonlinear formulations result in continuous operation at part load, the convex approximations give rise to large power fluctuations, increasing the energy cost with 7% to 16%. However, by penalizing power peaks in the cost function, the achieved control performances are almost identical. This reveals that the cost function is flat near the optimum. Compared to a conventional heating curve control strategy, the energy cost savings amount to 5% for optimization-based control strategies (in the case of a day/night tariff structure). Although less than expected, this improvement is still significant given the wide spread presence of the application.

Extremal work of an endoreversible system with two finite thermal capacity reservoirs

Extremal work of two finite thermal capacity reservoirs is determined. It is obtained by using optimal control theory. It is shown that the temperature of driving fluid changes exponentially with respect to flow velocity and process duration. The extremal work for heat engine mode is different from that for heat pump mode. The obtained results are compared with those obtained with infinite low temperature heat reservoir. The model and analysis in this paper provide a means for improved evaluation of the mechanical energy limits in practical systems.

Optimum work in real systems with a class of finite thermal capacity reservoirs

The optimum work in real systems with two finite thermal capacity reservoirs is determined. It is obtained by using optimal control theory. It is shown that the temperature of external working fluid changes exponentially with respect to flow velocity and process duration. The analysis proves that the optimum work is different for heat engine mode and heat pump mode. The recently obtained results are compared with those obtained previously. The models and results in this paper provide an approach to improve calculations of energy limits in real systems.

Numerical simulation of an advanced energy storage system using H 2O–LiBr as working fluid, Part 1: System design and modeling

The advanced energy storage technology proposed and patented by authors can be applied for cooling, heating, dehumidifying, combined cooling and heating, and so on. It is also called the variable mass energy transformation and storage (VMETS) technology in which the masses in one or two storage tanks change continuously during the energy charging and discharging processes. This paper presents an advanced energy storage system using aqueous lithium bromide (H 2O–LiBr) as working fluid. As one of VMETS systems, this system is a closed system using two storage tanks. It is used to shift electrical load and store energy for cooling, heating or combined cooling and heating. It is environmental friendly because the water is used as refrigerant in the system. Its working principle and process of energy transformation and storage are totally different from those of the traditional thermal energy storage (TES) systems. The electric energy in off-peak time is mostly transformed into the chemical potential of the working fluid and stored in the system firstly. And then the potential is transformed into cold or heat energy by absorption refrigeration or heat pump mode when the consumers need the cold or heat energy. The key to the system is to regulate the chemical potential by controlling the absorbent (LiBr) mass fraction or concentration in the working fluid with respect to time. As a result, by using a solution storage tank and a water storage tank, the energy transformation and storage can be carried out at the desirable time to shift electric load efficiently. Since the concentration of the working solution in the VMETS cycle varies continuously, the working process of the VMETS system is dynamic. As the first part of our study, the working principle and flow of the VMETS system were introduced first, and then the system dynamic models were developed. To investigate the system characteristics and performances under full-storage and partial-storage strategies, the numerical simulation will be performed in the subsequent paper. The simulation results will be very helpful for guiding the actual system and device design.

Performance evaluation of an integrated automotive air conditioning and heat pump system

This study deals with the performance characteristics of an R134a automotive air conditioning system capable of operating as an air-to-air heat pump using ambient air as a heat source. For this aim, an experimental analysis has been performed on a plant made up of original components from an automobile air conditioning system and some extra equipment employed to operate the system in the reverse direction. The system has been tested in the air conditioning and heat pump modes by varying the compressor speed and air temperatures at the inlets of the indoor and outdoor coils. Evaluation of the data gathered in steady state test runs has shown the effects of the operating conditions on the capacity, coefficient of performance, compressor discharge temperature and the rate of exergy destroyed by each component of the system for both operation modes. It has been observed that the heat pump operation provides adequate heating only in mild weather conditions, and the heating capacity drops sharply with decreasing outdoor temperature. However, compared with the air conditioning operation, the heat pump operation usually yields a higher coefficient of performance and a lower rate of exergy destruction per unit capacity. It is also possible to improve the heating mode performance of the system by redesigning the indoor coil, using another refrigerant with a higher heat rejection rate in the condenser and employing a better heat source such as the engine coolant or exhaust gases.

Heat-pipe enhanced solar-assisted heat pump water heater

A heat-pipe enhanced solar-assisted heat pump water heater (HPSAHP) is studied. HPSAHP is a heat pump with dual heat sources that combines the performance of conventional heat pump and solar heat pipe collector. HPSAHP operates in heat-pump mode when solar radiation is low and in heat-pipe mode without electricity consumption when solar radiation is high. HPSAHP can thus achieve high energy efficiency. A prototype was designed and built in the present study. An outdoor test for a HPSAHP in the present study has shown that COP of the hybrid-mode operation can reach 3.32, an increase of 28.7% as compared to the heat-pump mode COP (2.58).

Exergy analysis of a vapour-compression refrigeration/heat pump cycle with chimney

The greatest speciality of this work is using chimneys with different heights above the condenser instead of a condenser's fan on the vapour-compression refrigeration/heat pump system chosen as an experimental set. Exergy analyses of natural convection with and without chimneys and forced convection on the same experimental set have been made with acceptances of steady state-steady flow system on each part of the set. Temperatures and pressures on each point of the system are measured and then specific enthalpies and entropies of them are determined. The coefficients of performance in refrigeration and heat pump modes and the exergies of each point, irreversibilities and second law efficiencies for each part of the system are calculated and given in tables comparatively. In this study, it is seen that the values for natural convection with chimney are between the forced convections and natural convections.

Work Limits in Imperfect Sequential Systems with Heat and Fluid Flow

This paper analyses physical limits of multistage production or consumption of mechanical energy (work) in sequential heat-mechanical operations characterized by finite rates. The benchmark system, where these limits are evaluated, is a cascade of imperfect stages through which a resource fluid flows with a finite rate. Each stage consists of a fluid at flow, an imperfect work generator or consumer and the environment. The problem investigated is that of limiting yield or consumption of work by the fluid that interacts sequentially with the environment in a finite time. A discrete, finite-rate model subsumes irreducible losses of work potential caused by thermal resistances. Dynamic limits on work are obtained which bound one-stage pr multistage energy convertors with production or consumption of power. These limits are expressed in terms of classical exergy and a residual minimum of entropy generation. A discrete generalization of classical exergy is found for systems with finite number of imperfect stages and finite holdup times. For this generalized exergy a hysteretic property is valid, meaning a difference between the maximum work delivered from engine mode and the minimum work added to the corresponding heatpump mode of the system.

Finite time generalization of thermal exergy

An extended exergy allowing some inevitable dissipative properties (e.g. those occurring in boundary layers) can be formulated for finite rate transitions, which lead a system not in equilibrium with the environment (or “dead state”) to equilibrium with it in a finite time. This exergy is a function of the end states and the process duration or a process intensity index. This exergy simplifies to classical thermal exergy in the limiting case of infinite duration, when the property of reversibility is recovered due to the vanishing rates. The functional of the extended exergy can be derived either as a finite time extension of the classical thermodynamic work extracted from a system and its environment or by evaluating a functional of the dissipated classical exergy. With finite time exergy, one can effectively optimize various continuous and multistage processes encountered in the theory of energy conversion and energy exchange in systems with a finite exchange area or with a finite contact time. However, the most important application of the dissipative exergy is the enhanced bounds predicted for the work delivery (consumption) in finite rate processes. These bounds are stronger than classical thermostatic bounds. In our formulation, nonlinear thermodynamic models are linked with ideas and methods of variational calculus. The variational formalism, with certain energy like and momentum like quantities strongly analogous to those known from analytical mechanics and optimal control theory, is an effective tool in the optimization of work. In this theoretical framework, one can easily discuss the role of finite process intensity and finite duration. The optimality of a specific irreversible process for the finite time transition of a fluid from one thermodynamic state to another is pointed out as well as a connection between the process duration, optimal dissipation and the optimal process intensity measured in terms of a Hamiltonian function. Our analysis proves that finite time exergy is different for processes approaching equilibrium with the environment, in which work is released (engine mode) and for processes departing equilibrium in which work is supplied (heat pump mode). However, the coincidence between the dissipative exergies and classical exergy is shown for the quasistatic case. The hysteretic properties of the finite time exergy cause a decrease in the maximum work received from a system in engine mode and an increase of work added to a system in heat pump mode, features which are particularly important in high rate regimes or for short duration thermodynamic processes. These results prove that the limits implied by the classical exergy theory should be replaced by the more realistic, stronger limits which are obtained for finite time processes.

The maximum coefficient of performance of internally irreversible refrigerators and heat pumps

A class of irreversible refrigeration cycles is investigated to determine the maximum coefficient of performance in the heat pump mode and the refrigerator mode. For the purpose of generality and simplicity of the results, finite-time heat transfer in the condenser and evaporator is expressed in terms of arithmetic mean temperature differences. The generic source of internal irreversibility is measured by a single irreversibility factor which transforms the Clausius inequality into an equality to simplify the cycle model. These optimum cycle performances are obtained as closed form analytical expressions in which the irreversibility factor has been shown to be simply related to the ratio of the actual and endoreversible cycle coefficients of performance.

96/00870 World supply/demand survey of A/Cs in '94

This chapter analyzes limits on multistage production or consumption of mechanical energy (work) in sequential heat-mechanical operations characterized by finite rates. The benchmark system, for which these limits are evaluated, is a cascade of imperfect stages through which a resource fluid flows at a finite rate. Each stage consists of a fluid at flow, an imperfect work generator or consumer, and the environment. Cascade models serve to analyze the dynamical behavior of engines and heat-pumps when the resource reservoir is finite and the power yield cannot be sustained at a steady state. Based on equations of nonequilibrium work, enhanced bounds are found for real processes. An essential decrease of the maximum work received from an engine system and an increase of the minimal work added to a heat-pump system occurs in the high-rate regimes and for short durations. These works are quantified in terms of finite-rate exergies, which exclude some evolutions allowed by the classical exergy. The practical value of multistage solutions refers to energy limits enhanced due to the inclusion of imperfect thermal machines with a small number of stages and internal dissipation. For short durations the heat-pump mode exergy, which defines the lower bound on the work consumption, can be significantly higher than the minimal work of classical thermodynamics. It explains the restrictive applicability of classical thermodynamic bounds when they are applied to real processes, and it shows that these bounds should be replaced by stronger bounds obtained from nonequilibrium thermodynamics.

Operation data of a directly fired absorption heating unit with periodically operating absorber

AbstractThis contribution presents the layout, construction, and experimental investigation of operation behaviour of a directly fired absorption heating unit with periodically operating absorber. The working system is CH3OH‐LiBr/LiOH. The unit is designed to generate a maximum heat load of 10 kW. The field of application is mainly in domestic heatIng. A special feature of the absorption heating unit is the provision of the major part of required process heat in the heat pump mode of operation with high efficiency. At extremely low ambient air temperatures, heat is supplied in the direct mode. A numerical simulation routine is also described. The simulation program describes both steady‐state and dynamic processes in the unit. With models for each component, the performance of individual components can be studied and optimized. The results presented here consist of the first experimentally obtained efficiencies in direct heating mode, results of numerical simulation of the component generator, and a comparison of measured and predicted characteristic system temperatures.