Heat Rejection System Research Articles

Influence of the structure of a droplet sheet on the capacity of frameless space radiators and the efficiency of the power units

The development of a fine-dispersed droplet sheet of optimal structure for frameless space lowtemperature heat rejection systems is considered. The possibility of an optimal combination of the thermal capacity rejected by a droplet radiative cooler and the predicted efficiency of the space power unit is substantiated. The dependences of the basic thermal characteristics on the number of droplet planes used have been derived for low- and medium-temperature radiators with an ionic-liquid-based heat carrier.

Predicting the Effective Emissivity of an Array of Aligned Carbon Fibers Using the Reverse Monte Carlo Ray-Tracing Method

This study investigates the bulk radiative properties of absorbing and scattering fibers. This type of problem can be applied to a group of geometrically similar applications including thermal radiators, insulation, and tube-and-shell heat exchangers. The specific application studied here is an ordered array of carbon fibers acting as a radiating fin for a space-based heat rejection system. High total emissivity is beneficial for this application, so this study focuses on how geometric factors affect the effective emissivity of an array of fibers. Photon scattering among fibers in an array can result in an effective emissivity greater than the emissivity of the fiber surfaces themselves.

Thermodynamic analysis and optimization of a Stirling cycle for lunar surface nuclear power system

A model for the description of the thermal efficiency of a lunar surface nuclear reactor power system with eight free piston Stirling engines to generate nominal electrical power of 100kWe was developed. The heat loss of the hot heat pipes, finite rate heat transfer, regenerative heat loss, finite regeneration process time and conductive thermal bridging losses were considered. The results showed that the thermal efficiency increased and then decreased with the hot side temperature increase. The highest thermal efficiency was about 0.29 under the condition of the effectiveness of the regenerator being 0.9 and compression ratio being 2. Higher cold side temperature had bad effect on the thermal efficiency but could reduce the size of the heat rejection system. When the cold side temperature was designed as 500K, the lowest power system mass of 6.6 ton could be obtained. Enhanced heat transfer of the heat exchangers would increase the thermal efficiency but higher values of the nominal convection heat transfer coefficient of the heat exchangers would lead to a negligible thermal efficiency increase. The results obtained here may provide a new ideal to design lunar surface nuclear powered Stirling cycle.

Techno-Economic Analysis of Solar Cooling Systems for Residential Buildings in Italy

Abstract Solar cooling systems might represent a viable alternative for space cooling in residential buildings because the peak of cooling demand matches the availability of solar radiation. The use of adsorption chillers in this field gives another environmental benefit, since they employ natural refrigerants as water. However, the design of such systems is critical because it relates to several parameters and cannot be easily accomplished using traditional tools. In this work, a dynamic model for the simulation of a small solar cooling system employing adsorption chillers has been evaluated. The model, realized with the commercial software trnsys, has been implemented to quantify the effect of different operational and design parameters on the overall performances of solar cooling systems in three different Italian cities (Milan, Rome, and Messina). Particular focus was put on the comparison of different heat rejection systems, which was found to be a critical aspect in the design of such systems. In addition, an economic analysis has been performed for an optimized system, in order to evaluate the payback time of the systems compared to a traditional air conditioning system and provide indication on the possible outlooks by means of a sensitivity analysis.

In-Flight Performance of the OCO-2 Cryocooler

The Orbiting Carbon Observatory-2 (OCO-2) will have completed its first year in space on July 2, 2015. The OCO-2 instrument incorporates three bore-sighted, high-resolution grating spectrometers, designed to measure the near-infrared absorption of reflected sunlight by carbon dioxide and molecular oxygen. The cryocooler system design is coupled with the instrument's thermal control design to maximize the instrument's performance. A single-stage NGAS pulse tube cryocooler provides refrigeration to three focal plane arrays to ∼120 K via a high conductance flexible thermal strap. A variable conductance heat pipe (VCHP) based heat rejection system (HRS) transports waste heat from the instrument located inside the spacecraft to the space-viewing radiators. The HRS provides tight temperature control of the optics to 267 K and maintains the cryocooler at 300 K. Soon after entering the A-Train on August 3, 2014, the optics and focal planes were cooled to their operating temperatures. This paper provides a general overview of the cryogenic system design and reviews the in-flight cryogenic performance during the Observatory's first year.

Thermal–Hydraulic Analysis of ITER Component Cooling Water System Loop 2B

ITER cooling water system includes Tokamak cooling water system, component cooling water system (CCWS), chilled water system and the heat rejection system. The CCWS is further divided into CCWS-1, CCWS-2A, CCWS-2B, CCWS-2C, and CCWS-2D loops to provide independent chemical control and to prevent galvanic corrosion among the different clients’ materials (e.g. Cu, Al). The CCWS-2B is responsible to remove heat load generated by coil power supply components and the neutral beam injectors and diagnostics system during all the phases: commissioning, testing and conditioning and plasma operation. A CCWS-2B thermal–hydraulic analysis model was developed, by using the AFT Fathom code, to conduct the steady state thermal–hydraulic analysis of the system. In this thermal–hydraulic analysis model, the critical path with the largest pressure loss was used to size the pump head has been identified and the pressure loss on the control valve were used to establish the required flow balance at each piping connection points. This paper presents the results of this thermal hydraulic analysis which was composed by required pump head of the CCWS-2B loop and the main thermal–hydraulic parameters for each client (i.e. flow rate, velocities, pressure drops and outlet temperatures).

Thermal behaviour analysis on ITER component cooling water system loop 2B

ITER cooling water system is composed by several cooling loops, the primary heat transfer loops that form the Tokamak Cooling Water System (TCWS), the secondary heat transfer loops that form the Component Cooling Water System (CCWS) and the Chilled Water System (CHWS) and a tertiary heat transfer loop which is the Heat Rejection System (HRS). The CCWS is further divided into CCWS-1, CCWS-2A, CCWS-2B, CCWS-2C, CCWS-2D depending on the water chemistry needs of clients and wetted area material. The component cooling water system loop 2B (CCWS-2B) has the function to remove heat load from coil power supply component, Neutral Beam Injectors (NBIs) system component and diagnostic system which are located in different buildings. As the total number of the client connections for the loop is a few hundreds, simplified thermal hydraulic analysis model has been developed to perform thermal analysis on the component cooling water system loop 2B. The curve of the cooling water temperature at client inlet and outlet during one cycle of the most demanding plasma operation scenario was obtained and the cooling water flow rate can meet the thermal removal requirement of client was also confirmed from this analysis. In addition, operation behaviour of the main heat exchanger for CCWS-2B in this thermal analysis was depicted for main heat exchanger selection purposes. This study has been carried out with the AFT Fathom code.

Comparison of photovoltaic and solar thermal cooling systems for office buildings in different climates

Photovoltaic systems combined with electrical compression chillers offer a high potential for energy efficient cooling with a high economic feasibility. Due to much higher energy efficiency ratios of electric chillers compared to sorption cooling systems, the heat rejection system is much smaller and thus auxiliary energy and water consumption lower than in sorption cooling systems. The overall system efficiency, auxiliary energy consumption and achievable solar fraction depend on the photovoltaic module technology, the compression chiller energy efficiency ratio and obviously on the temporal correspondence of solar cooling production and cooling demand. A systematic simulation study was carried out to evaluate the overall performance of photovoltaic compression cooling systems in office buildings for different climatic conditions worldwide. For each location and cooling demand the solar energy production for different surface areas was calculated and solar fractions, EER and auxiliary energy demand was determined. It could be shown that the primary energy savings for solar electric cooling and heating are comparable to solar thermal systems. As solar thermal systems include hot storage, they mostly provide higher solar fractions and in some cases higher primary energy savings. If electricity export to the grid or for appliance use is included in the primary energy analysis, photovoltaic cooling systems always have higher primary energy savings.The total cooling costs for solar electric cooling are comparable to solar thermal cooling systems, if there is no feed in tariff for the excess PV electricity. If a grid export is possible and paid for, solar electric cooling systems are always more advantageous.

Performance Evaluation of Air-Based Heat Rejection Systems

On the basis of the Number of Transfer Units (NTU) method a functional relation between electric power for fans/pumps and effectiveness in dry coolers and wet cooling towers is developed. Based on this relation, a graphical presentation method of monitoring and simulation data of heat rejection units is introduced. The functional relation allows evaluating the thermodynamic performance of differently sized heat rejection units and comparing performance among them. The method is used to evaluate monitoring data of dry coolers of different solar cooling field projects. The novelty of this approach is that performance rating is not limited by a design point or standardized operating conditions of the heat exchanger, but is realizable under flexible conditions.

Long-term reliability of the thermal performance of a flat-plate heat pipe using a prognostics method

Fast development of electronic devices for satellites not only has led to integration of a large number of microchips in a confined area but also increased the heat flux dissipated by the electronic devices. So, it is challenge to achieve greater reliability as well as performance of the HRS (Heat-Rejection System) such as heat pipes, PFL (Pumped Fluid Loops), radiators, used for electronic devices installed to satellites. Moreover, the HRS needs to be evaluated with long-term reliability tests prior to being applied in the satellite. In this paper, accordingly, with a prognostics method that was recently performed with a particle filter, we focus on the long-term reliability of the thermal performance of a flat-plate heat pipe which is generally used as the HRS in a satellite. In addition, we predict the thermal performance trends for the heat pipe in use based on the measurement data. In particular, we take the thermal performance trends as estimated by the prognostics method with a particle filter using 30days of experimental data from a 42-day period, and we compare these trends against the actual experimental data to validate the prognostics method with the particle filter. Based on the results, we show that long-term reliability of the thermal performance can be obtained with limited experimental data considered all failure causes.

An Indirect Air Cooling System with Compound Refrigerating Cycle for CSP Plants

The geographical contradictions between plenty of water and high solar radiation constitute are hindering the development of concentrated solar power (CSP) plants, dry cooling systems are compelled to adopt to solve the contradictions for the CSP plants. This paper analysis the feasibility of a novel indirect air cooling with compound refrigerating cycle (CRC-IAC) system used in CSP plants. The solar field and power block in the analysis are modeled using NREL’s Solar Advisor Model (SAM), excluding heat rejection system. Thermodynamic simulations and optimizations for operation exhaust pressure with the changes of ambient dry-bulb temperature are presented. The ideal cycle performance for CRC-IAC system is evaluated and quantified. Moreover, the system was compared with circulating wet cooling (CWC) and air cooling condenser (ACC) in terms of electricity generation and levelized electricity cost (LEC). The results show that the LEC of CRC-IAC is increased by 6.44% and reduced by 5.38%.

ITER components cooling: Satisfying the distinct needs of systems and components

The ITER Tokamak requires multiple auxiliary systems to initiate, support, and monitor the fusion reaction. Heat produced by these systems, as well as the heat produced by the fusion reaction itself is collected by the ITER Cooling Water System (CWS) and rejected to the atmosphere. The CWS is composed of several systems designed for specific cooling roles. One of these systems is the Component Cooling Water System 2 (CCWS-2) whose function is to collect the heat from auxiliary client systems and components and transfer it to the Heat Rejection System. Clients are located throughout the site and have different requirements in terms of pressure, temperature, temperature variation, flow, metallurgy of wetted surfaces, and water quality. To satisfy these different requirements the CCWS-2 is divided into four separate loops, each of which has different operating parameters. For example, the CCWS-2A loop is designed to cool components with wetted surfaces of copper and primarily serves the radio-frequency heating systems, magnet power supplies, and neutral beam injector system components. This paper describes the evolution of the CCWS-2 system to match the needs of groups of compatible clients, and describes the development of the preliminary design of one of its loops, CCWS-2A, to meet individual client needs.

Probabilistic risk assessment of the energy saving shortfall in energy performance contracting projects–A case study

Lack of a proper assessment method on performance risks in Energy Performance Contracting (EPC) projects is one of the reasons hindering the further development of energy service companies (ESCOs) market. This paper proposes a simulation-based method to evaluate the probability of energy saving shortfall taking into account the variations in the influential parameters, including weather conditions, occupancy, operating hours, thermostat set-point, etc., during the contract period. The proposed method involves the use of a detailed building energy simulation programme, sensitivity analysis and Monte Carlo simulation techniques. Empirical data is also used to develop the probability distribution functions for the identified parameters to simulate the actual yearly variations in the post-retrofit conditions. A real case study of replacement of heat rejection system for a central chiller plant in Hong Kong is used to demonstrate the application of this probabilistic method. The result shows that the possible energy savings after a 1-year retrofit period ranges from 393,000kWh (2.86%) to 1098,000kWh (10.8%) with 90% statistical significance.

Upgrades of the tore Supra water cooling system for the WEST project

In ITER, it is foreseen to use an actively-cooled tungsten (W) divertor likely from the beginning of operation. This Plasma Facing Component (PFC) will be subjected to high energy deposition during the plasma operations that severely limit component lifetime. Tungsten has been less extensively studied in tokamaks than carbon. Unlike most present day short pulse fusion devices, Tore Supra is able to reach the ITER pulse length and provide relevant discharges conditions for tungsten PFCs technology validation.A new upgrade of the machine aiming at testing a W divertor under the steady state heat fluxes is being studied in the framework of the WEST (Tungsten (W) Environment in Steady-state Tokamak) project.As the PFC requirements will change, the PFC Primary Heat transfer System (PHTS) must be upgraded. And, even if the injected power in the plasma will not be significantly increased for the WEST project, an upgrade of the Heat Rejection System (HRS) is also needed. This paper presents the studies carried out for these upgrades and the technical solutions to be implemented.

Application of Magnetocaloric Heat Pumps in Mobile Air-Conditioning

Air-conditioning (AC) is an important sub-system in electric vehicles (EVs). AC is responsible for the highest energy consumption among all the auxiliary systems. As the energy is delivered by the batteries, the power consumption for air-conditioning can imply a significant reduction of the vehicle autonomy. Given the actual state of the art and the temperature and power requirements, electrically driven compressors are the most feasible solution. However, vapour-compression systems are reaching their maximum efficiency. Using innovative technologies can improve the performance of standard systems and hereby increase the vehicle autonomy. This paper presents the first steps in the design of a magnetocaloric air-conditioner for an electric minibus. The system will include two reversible magnetocaloric heat pumps, one in the front part of a minibus and one on the rear. The heat rejection system of the power electronics will be coupled to the air-conditioning system. In order to assist the design of the system, a dynamic model has been developed for the cabin, the hydraulic loops and heat exchangers, and the magnetocaloric units. An integrated design of the complete system is necessary, as it will work under dynamic conditions which depend on the thermal load in the cabin. In this paper, the operation conditions of the magnetocaloric units are presented and the design of the magnetocaloric air-conditioner is discussed. This work has been developed under the frame of the European Project ICE which aims to develop an innovative mobile air-conditioning system for EVs based on a magnetocaloric heat pump.

Computation of Tore Supra cooling system limits

Tore Supra is an actively cooled superconducting tokamak allowing long pulse operation. In the last decade, two major upgrades were achieved. In 2002, all the Plasma Facing Components (PFCs) were replaced to withstand an injected power of 25MW in steady state in the framework of the CIEL (Composants Internes et Limiteur) project. In 2012, a significant increase of the capabilities of the additional heating systems will be achieved, mainly on the LHH (Lower Hybride Heating) system, which will be able to inject up to 7MW in the plasma during 1000s (CIMES Chauffage et Injection de Matière En Stationnaire project). This project also included the development and implementation of a new continuous pellet injection system. One of the objectives of the experimental program in 2012 and the following years is to perform long pulses with an injected power of up to 10MW during 1000s. The thermal power to be exhausted to the environment by the water cooling system, comes from the primary loops cooling the PFCs (up to 10MW) and mainly from those that cool the RF heating system generators (10–20MW). The secondary loop, the Heat Rejection System (HRS), has two cooling towers with an exhaust capacity of 11MW in continuous operation.With the completion of CIMES, the water cooling system capability to withstand the overall injected energy in the new configuration of the machine and RF generators is questioned. This requires a detailed assessment of the operational limits of the cooling system.These limits depend on the wet-bulb temperature which governs the lowest achievable temperature level of the HRS. They will be different depending on whether the pulse is performed in summer or in winter.For this purpose, a transient thermo-hydraulic computer model of the cooling system was developed with matlab (CATSYS model). The simulations of already achieved long pulses with high injected energy were compared to the experimental measurements. The tool was then used to perform simulation and to compute the limits of the Tore Supra cooling system and to identify the equipments which have to be upgraded.The paper presents a description of the Tore Supra cooling system and main components as well as the CATSYS model. It will give the results of the experimental validation performed and the simulation results giving the limits of the analysed system.A new upgrade of the machine aiming at testing ITER W divertor components under the steady state heat fluxes expected in ITER is being investigated within the WEST project (tungsten (W) Environment in Steady-state Tokamak). It will lead to the replacement of the major part of the existing PFC's and of the dedicated PHTS. The CATSYS model will be used for the design of the new cooling system. Some preliminary results of the changes required for WEST will also be presented.

Advanced Dual Level Vehicle Heat Rejection System for Passenger Cars

Advanced Dual Level Vehicle Heat Rejection System for Passenger Cars

Heat rejection and primary energy efficiency of solar driven absorption cooling systems

Efficient heat rejection is crucial for the overall primary energy balance of sorption systems, as it dominates the auxiliary energy consumption. Low ratios of cooling power to auxiliary electricity of 3.0 or less are still common in sorption system, so that the primary energy efficiency is not always higher than for conventional compression chillers.Whereas dry heat rejection systems require electricity for fan operation, hybrid or wet cooling systems in addition need pumping energy for the cooling water and the water itself. The energy efficiency can be improved for heat rejection to the ground, where only pumping energy is needed for the geothermal heat exchange.Dynamic simulation models were used for a single effect absorption chiller powered by solar thermal collectors via a hot storage tank. The chiller models were coupled to a three dimensional numerical ground heat exchanger model or to cooling tower models. The models were validated with operating data of a 15kW solar cooling system installed in an office building.Primary energy efficiency ratios were determined for different heat rejection systems and improved control strategies were developed. The installed system primary energy ratios varied between 1.1 and 2.2 for auxiliary heating and between 1.2 and 2.5 for auxiliary cooling depending on the heat rejection and control strategy chosen. The low electrical energy consumption of the geothermal heat rejection saves 30% of auxiliary electricity and results in an electrical coefficient of performance of 13. The maximum primary energy ratios for solar fractions up to 88% are 2.7 for auxiliary heating and 3.2 for auxiliary cooling, i.e. nearly three times higher than for the reference electrical compression system of 1.2.

The ITER Heat Rejection Challenge

ITER is an international fusion facility being built in France to demonstrate the scientific and technological feasibility of fusion power. Fusion power at ITER is generated using a Tokamak machine in which burning plasma at temperatures of 150,000,000°C is confined within a vacuum vessel by magnetic fields. The enormous amount of heat generated by the Tokamak and its auxiliary systems is removed by the cooling water systems, consisting of the Tokamak Cooling Water System (TCWS), the Component Cooling Water System (CCWS), the Chilled Water System (CHWS), and the Heat Rejection System (HRS). These systems are designed to remove an initial peak heat load of about 1100MW.ITER is an experimental facility that will operate in a cyclical fashion. High levels of fusion power will be generated during repeated plasma pulses with specified durations. Heat produced by the fusion reaction will not be used to generate electricity, but will be rejected to the environment.The cyclical nature of the ITER machine presents distinct challenges to the design of the HRS which must reject normal facility heat loads plus large, intermittent heat loads from Tokamak pulse operations, while maintaining stable and predictable cooling tower basin water temperatures to meet the needs of cooling water system clients. This paper explores these challenges to the HRS design and describes the selected solutions.

Performance Analysis of Automated Control System for Condenser Water Treatment Unit

The air conditioning system in the Umm Al-Qura University (Albdiya Campus) was conceived to be a district cooling by a remote chilled water plant. Recently, there are two chilled water plants in the university installed strategically to provide chilled water to all the academic and administrative buildings of the university through distribution network with total capacity approximately of 12,000 tons of refrigeration. The plants were built based on cooling towers with open water cycle as heat rejection system. Water treatment chemicals has been used to protect the cooling systems from corrosion, scaling and microbiological fouling accompanied with dissolved and suspended water impurities. Different methods are being used to determine and control the treatment chemical concentrations and system performance indicators. Traditional chemical controller has drawback of indirect measurements and set points. The purpose of this paper is to present a solution to overcome the problems of traditional and conventional chemical treatment and control sys-tems. Central cooling plant number (1) assigned to perform experimental setup using new chemical treatment technology. Advanced automatic chemical treatment controller installed on condensers (1, 2 and 3), and certain key performance indicators were selected and monitored such as chemical and water consumption, power, energy saving, and maintaining system integrity and efficiency. Satisfactory results were obtained in terms of performance and cost of operation.