Heat Rejection Research Articles

Numerical study of anisotropic scattering and solar radiation impact on liquid droplet radiator performance

The Liquid Droplet Radiator (LDR), a lightweight frameless radiant heat rejection system, is widely regarded as the optimal heat rejection solution for megawatt-scale space nuclear power systems. The LDR dissipates heat to space through its droplet layer, which plays a crucial role in determining its radiative rejection capacity. Radiation beams undergo a series of emission, scattering, and absorption processes within the layer before being emitted into space. Prior studies typically assume isotropic scattering in this process. However, this study develops a three-dimensional model for the LDR’s droplet layer based on Mie theory and Monte-Carlo methods to investigate the impact of anisotropic scattering and solar radiation. Our calculations reveal that the working fluid exhibits strong forward anisotropic scattering characteristics. Under the anisotropic assumption, the heat rejection capability of the LDR is 27–33 % higher compared to the isotropic assumption. Regarding the temperature distribution within the LDR, under the anisotropic assumption, the layer’s temperature is 6 K lower at the XY boundary surface and 3 K higher at the layer median, as opposed to the isotropic assumption. We introduce the concept of the probability of a light beam escape to elucidate the distinct beam transmission mechanisms and temperature distribution. When the distance between the LDR and the sun is in the vicinity of 1 AU, the impact of solar radiation on the LDR’s ability to operate is on the scale of 1E-5, which is almost negligible. This study lays down a theoretical groundwork for calculating the heat rejection capacity during the LDR design phase.

Analysis of thermal performance characteristics of rotating packed beds using response surface methodology

Cooling towers play a crucial role in various industries to dissipate the heat of circulating water. This process involves the direct contact of atmospheric air and hot water from the condenser within a porous packed bed. The cooling towers suffers from their large size which results in escalating construction and maintenance expenses. In the present work, a rotating packed bed has been investigated experimentally for its evaporative heat transfer performance and is proposed as an alternative to cooling towers. In the chemical and processing sectors, rotating packed beds are being explored as a potential alternative to conventional distillation towers due to their process intensification capabilities. The principal reason behind this adaptation is the enhanced mass transfer rates achievable by rotating packed beds in a compact size. The aim of this study is to lay the foundation for rotating packed beds to achieve similar process intensification in various heat transfer applications. For this purpose, the parameters that describe the performance of a cooling tower such as air pressure drop, cooling range, heat rejection rate from water, cooling effectiveness, and Merkel number were selected to assess the performance of the rotating packed bed. The effect of various factors such as mass flow rate of air, mass flow rate of water, water inlet temperature, and rotational speed on the performance parameters was observed. Response surface methodology was used for the design of experiments, development of the regression equations, and optimization of the performance parameters of rotating packed bed. The cooling effectiveness value of up to 0.59 and the Merkel number of up to 1.88 were achieved in the rotating packed bed.

Thermodynamic and economic analyses of the retrofit of existing electric power plants with fusion reactors

Electricity generation will need to reach net zero emissions globally in 2050. This will require an increase in share of renewable energy and the implementation of a controllable carbon-free base-load source. Nuclear fusion is a promising option to decarbonize base-load electricity production but its capital cost still doubles the one of technologically more mature alternatives such as large photovoltaic fields or off-shore wind installations. Within this framework, the retrofit of a dismissed power-plant could allow significant cost savings, thus facilitating the realization of a fusion electricity demonstrator. Among fusion reactors, stellarators are a valid alternative to tokamaks thanks to the higher blanket temperature and inherent continuous operation. In this scenario, we posit the challenge to use a nuclear fusion stellarator-based reactor to retrofit conventional power plants (PPs). Specifically, we select a nuclear fission plant in France and a supercritical coal fired site in Italy, by constructing 4 different retrofit scenarios as a function of the re-used components. We compare each option with a greenfield and optimized plant with the same reactor thermal power. through a thermodynamic, economic, and investment analysis.The results proves significant savings by retrofitting an existing plant, with a CapEx reduction up to 50% compared to the greenfield plants. Specifically, the most convenient retrofit strategy is to select a site that already implements cutting edge thermodynamic parameters while reusing the most existing systems (i.e. buildings, steam cycle, electricity generation, and heat rejection). This is the case of the 2 x 660 MWe supercritical coal-fired plant in Italy. Therein, the LCOEs are 39 $/MWh and 51 $/MWh, calculated with an interest rate of 2.7% and 6%, respectively, and compare with the conventional energy technologies. Moreover, such costs are competitive in the current European energy markets and yield significant net present values at the plant end of life.

Improvement of Air Compressor Cooling with Intercooler Fine Pruning

An intercooler serves as a heat exchanger between the several stages of the working compressor, assisting in the transmission of thermal energy between fluids of varying temperatures. This article is about the experimental analysis of the effectiveness of an intercooler. Multiple variables oversee the performance evaluation under different circumstances. Standard operational values are used to calculate performance evaluation metrics such as total heat transfer coefficient and others. Heat rejection of intercoolers has been enhanced from 61% to 65% by the variation of different fin lengths. Furthermore, the recently added intercooler's isothermal efficiency, which reached an astounding 56.5% significantly, outperformed the earlier unit. This serves to highlight how well the intercooler design was modified. Furthermore, the effectiveness of the Intercooler was assessed considering the circumstances during operation. The intercooler fin is primarily concerned with the performance of the air compressor. This work analyses the many characteristics of fin length, fin number, and fin diameter. When compared to the existing intercooler, this modified intercooler has a high performance

Waste heat recovery of a combined Brayton and inverse Brayton cycle for gas turbine based multi-generation hydrogen and freshwater purposes: 4E comparison with a simple coupled Brayton and inverse Brayton cycle

Using gas turbine cycles in power generation layouts can lead to a significant amount of waste energy. The combined Brayton and inverse Brayton cycle (IBC), which are used in such systems, has a considerable amount of waste energy in the heat rejection stage and exhausted gas, which has not been considered in previous studies. In the present research, a simple coupled Brayton and IBC (Configuration 1) is compared with a multi-generation system (Configuration 2) in which a hot water unit, a thermoelectric generator (TEG), and an absorption chiller are added to Configuration 1 for the waste energy utilization of combined Brayton and IBC. Furthermore, the power produced in IBC and TEG is directed to a proton exchange membrane electrolyzer and a reverse osmosis desalination unit for hydrogen and potable water outputs. Results show that although the total investment cost rate of Configuration 2 is higher than that of Configuration 1, the fuel cost rate, environmental cost rate, and exergy destruction cost rate of Configuration 2 are lower. Furthermore, at the best performance point, Configuration 2 has exergy efficiency and unit cost of products equal to 40.77% and 63.19 $/GJ. They are higher than Configuration 1 by 5% and 2%, respectively. Hence, with Configuration 2, a higher exergy efficiency with a lower fuel consumption and environmental cost rate is accessible.

Experimental Investigations on the Impacts of Aluminum Nanoparticles on a Low Heat Rejection Engine Running on a Blend of Soya Biodiesel and Diesel Fuel

Experimental Investigations on the Impacts of Aluminum Nanoparticles on a Low Heat Rejection Engine Running on a Blend of Soya Biodiesel and Diesel Fuel

Innovative and efficient integrations of desalination plants coupled absorption, adsorption, and humidification-dehumidification desalination units employing external heat recovery techniques

Thermal desalination technologies have been developed recently to solve some pressing problems, such as high energy expenditure, increasing fuel costs, and environmental problems related to conventional plants. However, these technologies still need to be enhanced in terms of water production and gain output ratio (GOR). This paper presents innovative and efficient combined thermal desalination plants consisting of absorption (ABDS), adsorption (ADDS), and humidification-dehumidification (HDH) desalination units to enhance energy utilization, water production, and overall system performance. Four innovative combined desalination plants employing different external heat recovery scenarios are presented and compared. The result from each scenario is evaluated and compared with those of the standalone ABDS and with other studies found in the literature at the same heat source temperatures. The heat source is mainly utilized to drive the ABDS plant. The ADDS and HDH are driven by the heat rejection from the ABDS components with different external heat recovery scenarios. Results illustrated that the maximum freshwater production generated from the proposed scenarios ranges between 1.57 and 2.94 m3.day−1 with a GOR of between 1.38 and 1.75 at 120 °C. Raising heat source temperatures from 55 to 120 °C decreased the improvement in the water productivity from 618.5 to 224 % and decreased the enhancement in the GOR from 222.8 to 99 %. In scenarios where the ADDS and HDH are driven by the heat rejection from the absorption and condensation process of the ABDS, the maximum freshwater production results from the ABDS unit. On the other hand, in scenarios where the ADDS and HDH are driven by the outlet hot water from the ABDS-generator, the maximum freshwater production is obtained from the HDH unit. The finding provided that the proposed innovative scenarios have an effective improvement in overall system performance indicators.

Novel Lightweight Loop Heat Pipe Radiator for High-Power Space Systems

High-power space systems reject a large amounts of waste heat via medium- or high-temperature radiators. For this application, the heat pipe radiator is desired for its simplicity, efficiency, and reliability. It is commonly composed of cylindrical heat pipes and planar fins. Although typically made of advanced materials with low density and high thermal conductivity, the pipe–fin combination suffers from a series of drawbacks, such as large thermal resistance in component joints, thermal expansion rate incompatibility, and high acquisition cost. This study innovates the heat pipe radiator configuration by developing an all-metal loop heat pipe (LHP) radiator. The evaporator and compensation chamber are formed in a cylindrical container with traditional sintered wick layers and a plug. The LHP applies dozens of parallel capillaries arranged in two planar arrays as condensers to spread heat to radiating panels, realizing lightweightness and efficient heat transfer. A theoretical study was performed to analyze the nonuniform liquid–vapor distribution in multicondenser lines. Experimental studies were carried out to investigate startup performance, quasi-steady-state operation, and temperature hysteresis. The proposed design is proven to be capable of fulfilling the heat rejection requirement of space systems and exhibits technical benefits over existing ones.

Thermo‐hydraulic analysis of wavy microchannel heat sinks with porous fins based on field synergy principle

AbstractIn order to enlarge the area and intensity of convective heat transfer among the coolant and heated surface, the vertical fins of microchannel heat sinks (MCHSs) with microencapsulated phase change material slurry (MPCMS) as coolant are arranged into wavy porous channels to realize more heat being dissipated to the outside. The phase transition of microencapsulated particles in laminar flow state is described, and the Brinkman–Forchheimer–Darcy model based on volume average approach and the energy equation for local heat equilibrium are adopted to portray flow and heat transfer in porous medium. The impacts of geometrical parameters on flow and heat transfer behaviour of wavy porous MCHS are numerically analyzed, and performance evaluation factor (PEF) is defined to estimate the thermo‐hydraulic capability of heat exchanger. The numeric outcomes match well with the experiments. Results indicate that MPCMS has a significant heat transfer improvement in the newly designed channel configuration compared with the coolant fluid flowing in the straight microchannel. Based on field synergy principle, the comprehensive capability enhancement mechanism of MPCMS in new MCHS is explored, and its superior thermal performance can be attributed to the improvement of the synergistic degree among flow and temperature fields, and its reasonable structural design can effectively improve the heat rejection capacity in the limited space.

Experimental investigation of ternary blends on performance, and emission behaviors of a modified low-heat rejection CI engine

This study investigated the performance, combustion, and emissions of a modified low heat rejection (LHR) diesel engine fueled with a blend of 90 % coconut waste cooking oil (CWCO) biodiesel and 10 % diethyl ether (DEE). The engine combustion chamber components were coated with 300 μm lanthanum-doped partially stabilized zirconia for thermal insulation. Engine testing was performed at varied loads from 0 to 100 % using an eddy current dynamometer. Exhaust emissions, including hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), and smoke were measured. Compared to conventional diesel, the CWCO-DEE blend showed a 3 % higher brake thermal efficiency of 33.4 % and 2.42 % lower brake-specific fuel consumption at full load. HC, CO, and smoke emissions decreased by 18 % (39 ppm), 11 %, and 19 % at higher loads with the blend. However, NOx emissions increased slightly by 21.2 %. The DEE compensated for CWCO's lower cetane number and viscosity, while the LHR coating enhanced combustion by providing thermal insulation, raising exhaust gas temperatures by 13 %. The improved efficiency and reduced emissions demonstrate the potential of optimized biodiesel-additive blends in conjunction with LHR engine modifications to sustainably utilize inexpensive waste cooking oil feedstocks as renewable diesel replacements. However, further optimization of blend compositions, additives, and coatings is needed to balance performance benefits against possible NOx increases. This study highlights a promising combined approach leveraging engine design and fuel advancements.

Thermodynamic analysis of rotary pressure exchanger and ejectors for CO2 refrigeration system

Natural refrigerant CO2 has become a viable choice for refrigeration units for land-based and offshore applications due to its environment-friendly nature and compactness. The CO2 transcritical cycle allows operating in colder climates and in elevated ambient temperature conditions and with significant heat recovery. However, the energy efficiency of the system suffers at higher heat rejection conditions mainly due to expansion losses. This work theoretically investigates and proposes the implementation of a new expansion work recovery device, a pressure exchanger (CO2-PX), for the transcritical CO2 cycle. The numerical models are developed in the Engineering Equation solver (EES) to compare the performance of CO2-PX configuration with standard booster-, parallel-, and ejector- configurations for various conditions. The analysis is carried out for the evaporation temperature of 0 ℃ and the gas cooler outlet temperature of 33 ℃ to 37 ℃. The results indicate that the coefficient of performance (COP) is improved by 17.7–23.5%, 16.3–20.3%, and 2.4–5.5% to the standard booster, parallel and ejector configurations, respectively, at the investigated conditions.

Performance Evaluation of Water Cooled Condenser of Air Conditioner Working on VCRS

VCRS commonly known as (Vapour Compression Refrigeration System) is widely adopted method for air conditioning of buildings, in pharmaceutical industry for storage of medicine, in hospitals, automobiles etc. There are several methods for increasing the efficiency% (COP) of an VCRS cycle. Some of the modification can be done in Compressor, Condenser, Evaporator, and in Refrigerant . In this experiment, we adopted method of Subcooling to reduce Compressor work. This study investigates the implementation of subcooling in the condenser of a Vapour Compression Refrigeration System (VCRS) to enhance its efficiency and reduce power consumption. The traditional condenser design is modified by submerging it in a water tank, allowing for subcooling of the refrigerant. The subcooling process involves lowering the temperature of the refrigerant below its saturation temperature, which improves the system’s Coefficient of Performance (COP) by increasing the density of the refrigerant entering the expansion valve. This modification aims to optimize the heat rejection process and enhance the overall performance of the refrigeration system .Experimental result demonstrate a significant reduction in power consumption and an improvement in COP, highlighting the effectiveness of the condenser subcooling method in enhancing the efficiency of VCRS. Keywords : Compressor, Condenser, Expansion Valve, Evaporator.

Assessment of performance and sustainability of waste heat dryer coupled with air conditioner unit during drying of banana slices

The present work presents the effective application of waste heat rejected from window air conditioning (AC) in a waste heat dryer (WHD) for drying plantain banana slices. The comparison between drying with the waste heat dryer coupled with an AC unit (ACWHD) and a natural convective indirect solar dryer (SD) reveals significant differences in drying kinetics, supply air conditions, and thermodynamic performance. The drying kinetics of banana slices obtained with ACWHD and SD were validated by revisiting the concepts of dryer performance index (DPI) and generalized drying curve. The temperature of the hot air supplied to the drying chamber in ACWHD was 48 °C while the relative humidity of the supply air was below 30 %. The thermodynamic performance assessment revealed that the exergetic efficiency was about 45 % and 16 % for ACWHD and SD respectively. The results of exergy based sustainable index and environment impact factor indicated the potential application of the waste heat of residual air from AC units for drying applications. Thus, the results of the present study imply that this novel idea has appreciable potential for commercialization with large-scale central air conditioning units in commercial complexes and industries where the magnitude of heat rejection will be multi-fold.

Utilizing waste heat from data centers with adsorptive heat transformation – Heat exchanger design and choice of adsorbent

The electricity and water consumption of data centers is growing on a global scale. A shift towards liquid cooled racks in combination with thermally driven cooling can help to reduce the electricity and water demand associated with the necessary heat rejection. To quantify the potential of adsorptive heat transformation devices in reducing the electricity and water demand, the prediction of thermal efficiency, heat flow rates and energy efficiency ratio is required. To this end, a numerical model is newly developed using basic adsorption heat exchanger theory. This model can predict the necessary performance indicators with respect to temperatures and volume flow rates, heat exchanger design and adsorbent. The full performance map of a market-available adsorption chiller (71 points) and own measurements are used for calibration and rigorous validation of the model. An average deviation (experiment vs. calculation) of 8.3 % in terms of thermal efficiency and 7.2 % in terms of heat flow rates is achieved, indicating a very good agreement for a wide range of temperatures. At a moderate liquid cooled rack outlet temperature of 50 °C, a heat rejection temperature of 26 °C and a cold aisle inlet temperature of 18 °C the cooling power of the silica gel reference chiller of 5.3 kW can be increased by 59 % to 8.5 kW at a partial energy efficiency ratio (pumps and control, no fans) of > 20 by assuming MIL-100(Fe) as adsorbent on a flat-tube lamella heat exchanger. The model can be used in subsequent annual system simulations to quantify the savings in electrical power and water consumption, which strongly depend on the ambient conditions.

Impact of in-process cooling and tool rotation speed on the mechanical and microstructural characteristics of friction stir processed AA2024

Despite significant grain size refinement, friction stir processing (FSP) of heat-treatable aluminum alloys is found to degrade the alloys of strength and hardness qualities. The drop in strength brought on by coarsening or the dissolution of the strengthening precipitate(s) could not be made up for by the improvement in strength brought about by the refinement of grain size following FSP. In order to get around this, the current work looks into a practical method of heat rejection from the FSP zone in addition to grain size refinement during FSP of AA2024 to regulate the precipitate evolution. Under natural cooling, air cooling, and CO2 cooling conditions, friction stir processing (FSP) was used to change the microstructure of Solutionized AA2024 at three distinct rotation speeds of 750, 1180, and 1500 rpm (fixed traverse speed of 50 mm/min). The microstructural evolution (through optical microscopy and SEM) and mechanical properties of FSP samples under different conditions of rotation speed and cooling and their differences were mainly discussed. All the FSP samples have a higher Vickers hardness than the base metal (minimum variation of 16.79% and maximum variation of 41.05%). The maximum hardness, yield strength, tensile strength, and elongation have been obtained under air cooling conditions and a rotation speed of 1180 rpm, which is a result of the simultaneous and effective effect of grain boundary strengthening and precipitation hardening.

Characterization testing of the Air Liquide large pulse tube cryocooler (LPTC)

An Air Liquide large pulse tube cryocooler (LPTC) was parametrically performance tested for cold tip temperatures between 30 K and 200 K and heat rejection temperatures between 0°C and 40°C. The testing utilized a single off-the-shelf power supply to drive the compressor motors in parallel. The effect of compressor temperature and pulse tube inclination angle are also presented. In addition, the exported forces were measured at ambient conditions and the results are shown and discussed.

Seasonal energy performance assessment of a hybrid HVAC system driven by solar and biomass energy for space heating and cooling in residential buildings

The H2020 project Hybrid-BioVGE aims to develop a sustainable HVAC system concept for buildings’ climatisation entirely based on renewable energy sources. The Hybrid-BioVGE system provides space heating/cooling and DHW production of small- and medium-scale buildings and deploys solar and biomass heat as energy inputs. During the heating period, thermal energy is provided directly by solar thermal collectors and a biomass boiler. The cooling energy demand is met by a thermally-driven ejector cycle based on a Variable Geometry Ejector (VGE), which adjusts its geometry according to two degrees of freedom. A demonstrator of the Hybrid-BioVGE system was designed and installed in a residential building in Porto (Portugal). The demonstrator’s seasonal energy performance was assessed numerically by a simulation model implemented in TRNSYS. Simulation results show that up to 98% of the system energy input is provided by renewables during the heating season, with a solar fraction higher than 60%. Conversely, the system energy performance is lower than expected during the cooling season due primarily to the limited performance of the heat rejection loop.

Characterization testing of the Thales LPT9310-HP cryocooler

Two Thales LPT9310-HP commercial off-the-shelf (COTS) cryocoolers were parametrically performance tested for cold tip temperatures between 50 K and 250 K and heat rejection temperatures between 0°C and 40°C. Both coolers had identical transfer line configurations. The test results revealed small unit-to-unit thermodynamic performance variation. They are compared to test results of a COTS LPT9310 cooler with the same transfer line configuration. The effect of compressor temperature and pulse tube inclination angle are also discussed. In addition, the off-state heat leak as a function of inclination angle is presented. Finally, the feasibility of using a single LPT9310-HP COTS cooler in a two-stage configuration is discussed.

Development and Validation of a Novel Zero-Dimensional Heat Rejection Model for High-Efficiency Engines

In recent years, the trend towards the performance maximization of modern internal combustion engines has led to the creation of accurate simulation models to optimize the engine design and operating conditions. Temperature management is crucial to achieve the performance goals of an internal combustion engine without affecting the component’s reliability. Formula 1 mandates that only a limited number of experimental tests can be performed, which leads to the necessity of simulators capable of substituting empirical tests. Furthermore, the requirement of adapting the vehicle setup before each race weekend to maximize the performance on each circuit layout necessitates short computational time. To address this, the development of a zero-dimensional model of the thermal flows within an engine is presented in this paper. This model allows to precisely compute the dynamic variations of all the heat flows inside the combustion engine, excluding only the radiative ones and the engine components’ temperatures. The new simulation approach has been developed and validated on a Formula 1 engine and shown to be precise and fast. The results demonstrate the value of the proposed model with an average engine fluid temperature error of less than 1 °C for a computational cost comparable with on-board applications.

Anthropogenic heat from buildings in Los Angeles County: A simulation framework and assessment

Anthropogenic heat (AH), i.e., waste heat from buildings to the ambient environment, increases urban air temperature and contributes to the urban heat island effect, which leads to more air-conditioning energy use and higher associated waste heat during summer, forming a positive feedback loop. This study used a bottom-up simulation approach to develop a dataset of the annual hourly AH profiles for 1.7 million buildings in Los Angeles (LA) County for the year 2018 aggregated at three spatial resolutions: 450 m, 12 km, and the census tract. Building AH exhibits strong seasonal and diurnal patterns, as well as large spatial variations across the urban areas. Building AH peaks in May and reaches a maximum of 878 W/m2 within one of several AH hotspots in the region. Among the three major AH components (surface convection, heat rejection from HVAC systems, and zonal air exchange), the surface convection component is the largest, accounting for 78% of the total building AH across LA County. Higher AH is attributed to large building density, a high percentage of industrial buildings, and older building stock. While AH peaks during the day, the resulting ambient temperature increases are much larger during the night. During the July 2018 heatwave in LA County, building AH (excluding the surface component) leads to a daily maximum ambient temperature increase of up to 0.6 °C and a daily minimum ambient temperature increase of up to 2.9 °C. It is recommended that reducing summer building AH should be considered by policy makers in developing mitigation measures for cities to transition to clean energy while improving heat resilience.