Heat Rejection Rate Research Articles

Characterisation of practical air conditioning cycle by computer simulation

In this paper, characterisation of a practical cycle of the air conditioning unit is predicted by developing a computer simulation algorithm. Environment-friendly refrigerant R 404A was used. The refrigerant R404A properties of the mathematical model were analysed. Superheating, sub-cooling and non-isentropic compression processes were assumed. The simulated air conditioning cycle performance parameters, such as mass flow rate, refrigeration capacity, heat rejection rate, coefficient of performance and compressor discharge temperature, were carried out under various condensing and evaporating temperatures. Experimental results found in the literature for the performance parameters were introduced and compared to those obtained by the simulation technique. The experimental and the simulated results show good conformity.

Thermal management of a high temperature sodium sulphur battery stack

The sodium sulfur battery is an advanced secondary battery with high potential for grid-level storage due to their high energy density, low cost of the reactants, and high open-circuit voltage. However, as the operating temperature of the battery is high (about 300 °C), effective thermal management is required to prevent thermal runaway under high current density operation. To develop efficient thermal management strategies, a detailed, thermo-electrochemical, non-isothermal, distributed dynamic model of a sodium-sulfur battery stack is developed. Models of three thermal management strategies are developed and analyzed in this work: active cooling, passive cooling, and hybrid cooling. The active cooling strategy uses air as the cooling medium whereas, the passive cooling strategy uses a phase change material. In the hybrid cooling strategy, both active and passive cooling strategies are used. Due to the high operating temperature of the sodium sulfur batteries, the rejected heat can be utilized effectively and consequently a high variability in the heat rejection rate may not be acceptable. Performance of these strategies is analyzed under high current density operation by evaluating the variability in the cell temperature during charge/discharge cycles, temperature difference between the cells based on their locations, and variability in the heat rejection rate. The phase change-based hybrid thermal management strategy with an embedded controller maintained the temperature variation within ±/- 2.8 °C. Considering the tradeoff between the capital cost for PCM, fan power requirement, and variability in the heat rejection rate, the optimal quantity of the PCM and the maximum air velocity for the maximum current density of 260 mA/cm2 (0.55 C-rate) were found be 0.74 g/Wh and 3 m/s, respectively.

Performance Comparison of Propylene Glycol-Water and Ethylene Glycol-Water Mixtures as Engine Coolants in a Flat-Tube Automobile Radiator

This study aims at evaluating and comparing the thermal performance of five different engine coolants employed in an experimental engine cooling system with a flat-tube louvered-fin automobile radiator. For this purpose, a PLC-controlled test system was set up. The system could maintain the temperatures of the air and coolant at the radiator inlet, the speed of the air and flow rate of the coolant at the required values during the tests. The tested coolants were pure wa-ter, 30/70 ethylene glycol (EG)/water, 30/70 propylene glycol (PG)/water, 50/50 EG/water and 50/50 PG/water mixtures. In all tests, the coolant temperature at the radiator inlet was kept at 90°C, while the coolant flow rate was varied between 0.10–0.25 l/s with 0.05 l/s increments. Furthermore, the air temperature at the ra-diator inlet was kept at 25, 30 and 35°C, and the air speed passing over the radia-tor was varied between 1–4 m/s with 1 m/s increments. The thermal performance of the radiator was evaluated by locating the measured coolant flow rate and coolant inlet/outlet temperatures into the conservation of energy equation. It was found that water yielded the highest radiator heat rejection rates. Compared with water, 30/70 EG/water, 50/50 EG/water, 30/70 PG/water, 50/50 PG/water mix-tures yielded on average 3.50%, 7.89%, 8.28%, 11.46% lower radiator heat rejec-tion rates, respectively. Since PG has some advantages over EG such as lower cost and toxicity, PG mixtures can be employed as antifreeze instead of EG mix-tures in expense of a slight decrease in the thermal performance.

Performance of a Flat-Tube Louvered-Fin Automotive Condenser with R1234yf

Numerical simulation of a mini-channel, flat-tube, louvered fin, automotive condenser is performed to study the heat rejection rate, pressure drop and performance of the heat exchanger. The simulation study is carried out for the refrigerant R1234yf. The properties of R1234y are obtained from REFPROP software. The moist air properties are calculated from those of dry air and water vapor using suitable correlations. To select the input data, the cycle performance is carried out for a standard vapor compression refrigeration system working with R1234yf between the temperature limits of [Formula: see text]C on the low-pressure side and [Formula: see text]C on the high-pressure side. The condensation process is taken into account in three sections, namely, the superheated, two-phase and the subcooled regions. A custom code is prepared in MATLAB to solve the simultaneous equations of heat transfer from refrigerant to inside tube wall, inside tube wall to outside tube wall and outside tube wall to moist air. The simulation results show the sensible heat transfer during desuper heating to be very small compared to the condensing region. Results are reported for the pressure variation along the refrigerant flow passage in the desuper heating, two-phase and subcooling regions. The heat-transfer coefficient is found to be the highest in the two-phase region for higher dryness fractions. The effect of inlet air velocity is less compared to that of the inlet air temperature on the heat rejection rate.

Performance Enhancement techniques in vapour Compression refrigeration system by various evaporator loads and thermoelectric sub cooling

Part load performance analysis on simple VCR system using selected refrigerant mixture at different loading conditions is completed to identify the economic mode of loading in the evaporator. In this research work, evaporator has been loaded at different loading (thermal loading – cooling of liquid substance in the evaporator) conditions like 25, 50, 75 and 100% of its capacity based on its volume and water is considered as a substance to be cooled in the evaporator for analyzing the performance of the refrigeration system at different loading conditions. The cooling temperature C and the condensation process has range of water is considered from 26 to 2 been carried out by Atmospheric Air Circulation (AAC) mode. The various performance indexes of refrigeration systems namely work input to the compressor, condenser heat rejection rate, refrigerating effect and COP are obtained based on unit mass flow rate of the refrigerant in this analysis. Experimental results. Experimental analysis has been carried out with 3oC of subcooling in the VCR system with thermoelectric subcooling and various performance indexes are investigated with reference to water temperature in the evaporator at different loading conditions. Significant improvements in various performance indexes have been obtained for VCR system with thermoelectric subcooling. The experimental results clearly state that 75% loading condition gives the highest COP at lowest temperature of evaporator substance. Among various loading conditions, COP of 75% loading condition is about 6.7, 3.9 and 25.13% higher than that of 25, 50 and 100% loading conditions respectively. suggest that 75% loading condition is the significant one among other loading conditions in the evaporator.

Subcooling effect on air-conditioning system by simulation analysis

Air-conditioning systems are mainly used in industries and residences to maintain the low temperature and required humidity level. More energy is consumed in air-conditioning systems. Coefficient of Performance (COP) is an important factor for the measure of air-conditioning system performance. Sub cooling is the concept used to reduce the condenser temperature. Sub cooling concept can be obtained by many methods like Humidification, Evaporative cooling etc. In order to optimize the performance of air conditioning systems, the condenser temperature is decreased by evaporative cooling. This approach can be applied to existing air-conditioning system to increase its performance and reduce the energy consumption. Simulation work has been carried out to study the performance of air-conditioning system with sub cooling concept. The condenser heat rejection rate is increased about 5.37%. By reducing condenser temperature about 2 °C, COP is increased about 1.86%. When the condenser temperature is reduced by 4 °C, COP is increased about 2.65%. Similarly same process is carried out for 6 °C, 8 °C and 10 °C; the COP improved about 3.98%, 5.5%, and 6.86% respectively. This simulation work provides an improvement in COP value and the results are compared with and without subcooling concept.

Operator Action–Induced Two-Phase Flow Condition Resulting in Performance Degradation of Interfacing Passive System

This study investigates the degradation of the heat transfer performance of a closed-circuit intermediate natural circulation heat transport loop used as a passive safety system in a nuclear power plant (NPP). The degradation arises from the strong thermal-hydraulic (TH) coupling of the loop operating characteristics, saturation temperature and pressure, and natural circulation flow rate, which determine the heat rejection rate to the TH boundary conditions imposed on the hot side of the loop by the transitory state of the primary reactor coolant system (RCS) of the NPP. Several operator actions related to a feed-and-bleed emergency operating procedure (F&B) are postulated, and system TH code simulations are performed to demonstrate how the F&B can induce two-phase flow conditions in the RCS. Natural circulation two-phase flow regimes in the RCS hot leg can significantly reduce the heat transfer to the circulating working fluid of the interfacing heat transport loop over long periods, sometimes lasting over 24 h, of passive system mission time. A transient performance indicator for the passive system mission is introduced for use in the passive reliability assessment and quantitative comparison of transient simulations. The need to consider human factors in the design and operation of NPP passive safety systems is stressed.

Transient simultaneous heat and mass transfer model to estimate drying time in a wetted fur of a cow

To reduce heat stress that contributes to complications in milk production and breeding, dairy cows are typically cooled by intermittent water sprays coupled with fan-induced air flow. Industry currently utilises fixed duration on and off-cycles; however, water efficiency may be improved by matching the sprinkler off-time to the cow drying time that corresponds to current outdoor environmental conditions. Electricity use can also be reduced by varying fan speeds to achieve the required heat rejection rate for a given set of conditions. To achieve savings in water and electricity, a transient, one-dimensional simultaneous heat and mass transfer model of evaporation within the wetted fur layer of a dairy cows was developed to estimate drying time and heat rejection rate based on ambient conditions. Water vapour concentration, liquid water content, and temperature within the fur layer were used to estimate a global drying time for fur. Model results was compared against a steady-state model in literature and the predicted net heat load is within 15% with a mean average error of 6.0%. Parametric analyses were performed to estimate drying time as a function of outdoor air temperature, air speed, humidity, and mean radiant temperature. Simulation results were used to develop a correlation for use in a control algorithm to predict the fan speed and sprinkler operation frequency needed to meet specified cooling load thresholds given outdoor conditions. Estimated savings demonstrated that the model-based controller could reduce annual electricity and water consumption by 20% and 40%, respectively. • Transient one-dimensional model for prediction of drying time from wetted cow fur. • Drying variation with outdoor temperature, humidity, air speed and roof temperature. • Predictive controller correlation based on regression of model simulation runs. • Controller shown to lower annual electricity by 20% and water consumption by 40%.

Experimental Investigation on Performance of Vapor Compression Refrigeration System using Nanorefrigerant (R134a+Al2O3) with Evaporative Condenser

In traditional refrigeration system the focus of researchers is always to improve its performance by reducing work of compression & enhancing the heat transfer rate in different heat exchangers and this can be achieved by adopting suitable technique. In this research work experiment is carried out on vapor compression refrigeration test rig using nanorefrigerant of R134a as base refrigerant and nanoparticles of aluminium oxide Al2O3 in different concentration so that variation in heat transfer rate can be observed. The different concentration of Al2O3 (gm/litre) in R134a were prepared, studied, and tested experimentally in VCR test rig. The test rig consists of evaporative condenser as conventional air-cooled condenser does not perform well in hot and dry climatic conditions. This evaporative air cooling offers desired heat rejection rate in condenser at high pressure and enhances system performance. The investigations show that nano refrigerant (R134a+ Al2O3 0.5gm/L Concentration & 50-60nm size) with evaporative condenser recorded the highest coefficient of performance among all other combination of nanorefrigerant. This indicates that nanorefrigerant can be safely experimented without any blockage or difficulty. The conventional VCR system operated with evaporative condenser without nanorefrigerant also shows improvement in performance compared to simple VCR with air cooled condenser. This system is suitable for hot and dry climate conditions.

Modelling a simple-vapour compression refrigeration cycle for Fish-Storage boxes

In this paper, a computerized modelling of a simple single-stage vapour compression refrigeration cycle has been carried out. The aim is to find a cycle performance that is suitable for fish preservation applications. The simulation was completed using the CoolPack software on two different refrigerants and analysed at various condensation temperatures. As a fixed variable, a specific compressor power has been chosen for all cases in meeting the evaporation temperature designed. The results of this study inform the comparison of cycle performance including heat rejection rate, cooling capacity, COP, and pressure-enthalpy diagram. The main conclusion is that both types of refrigerant that are simulated can produce high COP which is useful for handling the cooling load that is later available. The report on this study will be used further to design a fish-storage box.

Evaporation and wetting behavior of silver-graphene hybrid nanofluid droplet on its porous residue surface for various mixing ratios

Droplet evaporation offers high heat rejection rates and is widely used in the form of spray cooling or dropwise cooling of various heat dissipating devices. However, due to the limiting heat flux removal capacity of conventional fluids, such as water, these cannot be used in thermal management of high heat flux devices. In this research, the evaporation of silver (Ag)-graphene (GNP) hybrid nanofluid droplet and its residue effects on the evaporation of following Ag-GNP hybrid nanofluid droplet, due to its synergistic thermal properties, is experimentally investigated for various mixing ratios, from MR-1 (0.1(Ag):0.9(GNP)) to MR-5 (0.9(Ag):0.1(GNP)), and different residue sizes. A theoretical model is also proposed for hybrid nanofluid droplet evaporation and semi-empirical relations are developed to estimate the hybrid nanofluid droplet spreading over its residue surface. The results show a substantial increase in the droplet evaporation rate with increasing residue size and decreasing mixing ratio. MR-1 hybrid nanofluid droplet gives the highest evaporation rate (up to 370%) on its highly wetted residue surface, while the evaporation rate significantly drops moving from MR-2 to MR-5 hybrid nanofluid droplets on their partially wetted residue surfaces. Moreover, the evaporation rate substantially increases (up to 240%) with increasing residue size for MR-1 hybrid nanofluid droplet resting on its residue surface, however, the effect of residue size on droplet evaporation rate considerably diminishes moving from MR-2 to MR-5 hybrid nanofluid droplets resting on their respective residues.

Theoretical Study of the Absorption Refrigeration Cycle Using Water-Lithium Bromide as Working Pair for Cold Storage Application

This study aims to investigate the performance of a cooling absorption of the water-lithium bromide cycle for a cold storage application which has a capacity of five tonnes of fish. The cooling absorption system utilizes the waste heat of a 6 MW steam power plant as the heat source for the generator. The thermodynamics analysis is carried out to determine both the heat rejection rate of the power plant and the performance of the refrigeration absorption cycle. The performance of the cooling system is investigated by varying the temperature of each component of the refrigeration absorption cycle (i.e generator, absorber, condenser, and evaporator). Also, the influence of temperature on the heat input (to the generator), heat output (from the absorber), and the pump power, is examined. The results show that the Coefficient of Performance (COP) of the refrigeration absorption system increases when the temperature of the absorber, condenser, and evaporator rises. On the contrary, increasing the temperature of the generator leads to the reduction of the COP of the system. It is also found that the pump power increases when the temperature of the absorber rises, while on the contrary decreases when the generator temperature increases. The results also reveal that the amount of heat input into the generator is directly proportional to the temperature of the generator. While the heat output of the absorber decreases when the temperature of the absorber increases.

Using a capillary mat as a shallow heat exchanger for a ground source heat pump system

The ground heat exchanger is an important part of a ground source heat pump system. The site selection and arrangement affect the thermal performance of the entire system. A horizontal (shallow) ground heat exchanger has lower initial cost than a borehole heat exchanger. However, it has a larger physical footprint and its thermal performance is strongly affected by surface heat flux and rainfall infiltration. The capillary mat heat exchanger comprises small parallel capillary tubes and provides larger effective heat transfer area per unit footprint installation area. Additionally, it is more easily prepared and installed. This paper reports experimental result and numerical validation of ground source heat pump cooling application using shallow horizontal capillary mat ground heat exchangers. From the experiment, it was found that the installed capillary mat heat exchangers provide sufficient heat rejection rate for the heat-pump's thermal load. A numerical model for capillary mat heat exchangers is developed and the numerical result are validated using experimental data, within 1.54% root-mean-square error. A new and simple numerical relationship is also proposed to express the rate of heat transfer from capillary pipes to the surrounding soil. The extended numerical simulation found that currently installed ground heat exchangers can provide higher thermal load. The estimation of the pressure drop suggests that for a given pressure drop and flow rate, a single capillary mat having a surface area of 13.18 m2 is equivalent to a 40-m high-density polyethylene pipe with a standard diameter of 3.2 cm and a surface area of 8.05 m2.

Influence of geology and hydrogeology on heat rejection from residential basements in urban areas

Urbanization and limited land availability have resulted in the increased utilization of underground structures including residential basements in largely populated cities such as London, with an average addition of 200 basements per year in some boroughs. Residential basements kept at a comfortable temperature level throughout the year significantly contribute to heat fluxes in the subsurface as well as an increase in ground temperature. Understanding the ground thermal status is crucial in managing the significant geothermal energy potential in urban areas as well as the sustainable development of the urban underground, and in maintaining the energy efficiency of underground structures. In this proof-of-concept study, a 3D finite element approach accounting for coupled heat transfer and groundwater flow in the ground was used to investigate the influence of ground conditions on the heat rejection rate from basements. A detailed analysis was made of ground, above ground and underground built environment characteristics. This study demonstrates that the amount of heat from basements rejected to the ground constitutes a significant percentage of the total heat loss from buildings, particularly in the presence of groundwater flow. The extent of thermal disturbance in the ground varies depending on the ground characteristics. The volume of thermally disturbance ground inversely correlates with the groundwater flow rate in ground mainly consisting of highly permeable material. However, a direct correlation exists when the thickness of permeable soil layer decreases. A larger horizontal to vertical ratio of ground thermal disturbance is observed when the thickness of permeable soil layer increases.

Improving Efficiency of Cooling Coil Chiller Units

Air Cooled condensers were first introduced in US power industry in early 1970’s, but only during last 10-15 years number of installations greatly increased largely due to growing attention being paid to environmental safety. Also, growing demand for water for both domestic and industrial use has brought an increased interest in use of Air Cooled condensers. This is a review paper which studies the performance of Air-cooled condenser under various operating conditions it is found that there is degradation in performance of air cooled condenser under high ambient temperatures and windy conditions. The heat rejection rate of ACC also depends on surface condition of fins and thus its performance is reduced due to external fouling of finned tubes due to weather conditions and by internal fouling from condensate (Ammonia corrosion). A Hybrid (dry/wet) dephlegmator achieves major enhancement in performance when ambient temperatures are high. Also shading of condensers is done for air-conditioning units to mitigate the adverse effect of high ambient temperatures due to solar radiation. Now a day’s wind walls are used to reduce the effect of high wind velocity .second option is to increase the fan speed Fin cleaning plays an important role in heat rejection. External cleaning improves air side heat transfer coefficient. In order to improve the performance of an ACC Flat tubes inclined at some angle to horizontal can also be used in place of conventional circular horizontal tubes so that an improvement in heat transfer rate occurs.

Cooling mode experimental analysis of a direct expansion geothermal heat pump with a control depending on the discharge pressure

This work presents an experimental study of a direct expansion geothermal heat pump in cooling mode. The three geothermal loops are installed in parallel. To improve the system performance, a specific control depending on the discharge pressure was implemented to select the geothermal loops to serve as condensers. The fundamental question is whether this particular heat pump can operate continuously? So the goal was to determine the operating limits in order to develop the best strategies for the most efficient use of this recent geothermal heat pump.The analysis of experimental results enabled us to reach a maximum of 12 h continuous for this heat pump with the implemented control. During this optimal period of operation, the coefficient of performance reaches an average of 2.60 with maximum values of up to 3.40. The average cooling capacity is 9.60 kW for an average ground heat rejection rate of about 290.8 W m−1.Beyond this period, the performance of the DX GHP decreases and some phenomena like the oscillation loop, soil saturation, were identified and presented.The experimental results allowed also generally reveal that the controls implemented promotes geothermal loops cooling and helps increase the performance of the DX GHP.This article represents a logical continuation of our previous published study on heating mode.

Investigation of Steady-State Heat Extraction Rates for Different Borehole Heat Exchanger Configurations from the Aspect of Implementation of New TurboCollector™ Pipe System Design

When considering implementation of shallow geothermal energy as a renewable source for heating and cooling of buildings, special care should be taken in the hydraulic design of the borehole heat exchanger system. Laminar flow can occur in pipes due to the usage of glycol mixtures at low temperature or inadequate flow rates. This can lead to lower heat extraction and rejection rates of the exchanger because of higher thermal resistance. Furthermore, by increasing the flow rate to achieve turbulent flow and satisfactory heat transfer rate can lead to an increase in the pressure drop of the system and oversizing of the circulation pump which leads to impairment of the seasonal coefficient of performance at the heat pump. The most frequently used borehole heat exchanger system in Europe is a double-loop pipe system with a smooth inner wall. Lately, development is focused on the implementation of a different configuration as well as with ribbed inner walls which ensures turbulent flow in the system, even at lower flow rates. At a location in Zagreb, standard and extended thermal response tests were conducted on three different heat exchanger configurations in the same geological environment. With a standard TRT test, thermogeological properties of the ground and thermal resistance of the borehole were determined for each smooth or turbulator pipe configuration. On the other hand, extended Steady-State Thermal Response Step Test (TRST) incorporates a series of power steps to determine borehole extraction rates at the defined steady-state heat transfer conditions of 0/−3 °C. When comparing most common exchanger, 2U-loop D32 smooth pipe, with novel 1U-loop D45 ribbed pipe, an increase in heat extraction of 6.5% can be observed. Also, when the same comparison is made with novel 2U-loop D32 ribbed pipe, an increase of 18.7% is achieved. Overall results show that heat exchangers with ribbed inner pipe wall have advantages over classic double-loop smooth pipe designs, in terms of greater steady-state heat extraction rate and more favorable hydraulic conditions.

Coupled behavior of shape memory alloy-based morphing spacecraft radiators: experimental assessment and analysis

Thermal control is an important aspect of spacecraft design, particularly in the case of crewed vehicles, which must maintain a precise internal temperature at all times in spite of significant variations in the external thermal environment and internal heat loads. Future missions beyond low Earth orbit will require radiator systems with high turndown ratios, defined as the ratio between the maximum and minimum heat rejection rates achievable by the radiator system. Current radiators are only able to achieve turndown ratios of 3:1, far less than the 12:1 turndown ratio requirement expected for future missions. An innovative morphing radiator concept uses the temperature-induced phase transformation of shape memory alloy (SMA) materials to achieve turndown ratios that are predicted to exceed 12:1 via substantial geometric reconfiguration. Developing mathematical and computational models of these morphing radiators is challenging due to the strong two-way thermomechanical coupling not present in traditional fixed-geometry radiators and not widely considered in the literature. Although existing simulation tools are capable of analyzing the behavior of some thermomechanically coupled structures, general problems involving radiation and deformation cannot be modeled using publicly available codes due to the complexity of modeling spatially evolving boundary fields. This paper provides important insight into the operational response of SMA-based morphing radiators by employing computational tools developed to overcome previous shortcomings. Several example problems are used to demonstrate the novel radiator concept. Additionally, a prototype morphing radiator was designed, fabricated, and tested in a thermal environment compatible with mission operations. An associated finite element model of the prototype was developed and executed. Model predictions of radiator performance generally agree with the experimental data, giving confidence that the tools developed are able to accurately represent the thermomechanical coupling present in morphing radiators and that such tools will be useful in future designs.

Development and Testing of a Shape Memory Alloy-Driven Composite Morphing Radiator

Future crewed deep space missions will require thermal control systems that can accommodate larger fluctuations in temperature and heat rejection loads than current designs. To maintain the crew cabin at habitable temperatures throughout the entire mission profile, radiators will be required to exhibit turndown ratios (defined as the ratio between the maximum and minimum heat rejection rates) as high as 12:1. Potential solutions to increase radiator turndown ratios include designs that vary the heat rejection rate by changing shape, hence changing the rate of radiation to space. Shape memory alloys exhibit thermally driven phase transformations and thus can be used for both the control and actuation of such a morphing radiator with a single active structural component that transduces thermal energy into motion. This work focuses on designing a high-performance composite radiator panel and investigating the behavior of various SMA actuators in this application. Three designs were fabricated and subsequently tested in a relevant thermal vacuum environment; all three exhibited repeatable morphing behavior, and it is shown through validated computational analysis that the morphing radiator concept can achieve a turndown ratio of 27:1 with a number of simple configuration changes.

STEADY-STATE HEAT REJECTION RATES FOR A COAXIAL BOREHOLE HEAT EXCHANGER DURING PASSIVE AND ACTIVE COOLING DETERMINED WITH THE NOVEL STEP THERMAL RESPONSE TEST METHOD

At three locations in Zagreb, classical and extended thermal response test (TRT) was conducted on installed coaxial heat exchangers. With classic TR test, thermogeological properties of the ground and thermal resistance of the borehole were determined at each location. It is seen that thermal conductivity of the ground varies, due to difference in geological profile of the sites. In addition, experimental research of steady-state thermal response step test (SSTRST) was carried out to determine heat rejection rates for passive and active cooling in steady state regime. Results showed that heat rejection rate is only between 8-11 W/m, which indicates that coaxial system is not suitable for passive cooling demands. Furthermore, the heat pump in passive cooling mode uses additional plate heat exchanger where there is additional temperature drop of working fluid by approximately 1,5 °C. Therefore, steady-state rejection rate for passive cooling is even lower for a real case project. Coaxial heat exchanger should be always designed for an active cooling regime with an operation of a heat pump compressor in a classical vapour compression refrigeration cycle.