Parasitic Heat Load Research Articles

Economic impact of thermal energy storage on natural gas power with carbon capture in future electricity markets

As policies evolve to reflect climate change goals, the use of fossil fuel power plants is expected to change. Many fossil fuel power plants may opt to incorporate carbon capture and storage (CCS) technologies in response to evolving emissions policies. Additionally, fossil plants may need to be operated flexibly to accommodate the growing concentration of renewable energy systems. Unfortunately, most CCS technologies are very expensive, and they impose a parasitic load on the power plant, thereby decreasing net power output and the ability to operate flexibly. This investigation evaluated the economic potential of using hot and cold thermal energy storage (TES) to supplement the peak power output and flexibility of a natural gas combined cycle (NGCC) power plant with CCS capabilities. The two TES subsystems (hot and cold) were charged during periods of relatively low electricity prices and discharged during NGCC operation: resistively heated hot TES to offset the CCS parasitic heat load and vapor compression cooled cold TES to chill the inlet air to the power plant. Thermodynamic models were created for the base NGCC + CCS power plant, the hot TES equipment, and the cold TES equipment, to determine key performance and cost parameters such as net power output, fuel consumption, emissions captured, capital costs, and operational costs. These parameters were then used to simulate the operation of the power plant with and without the TES technologies in accordance with fourteen electricity pricing structures predicted for different future electricity market scenarios. The difference in net present value (NPV) between the base NGCC + CCS power plant and power plant with the TES technologies was used as the primary economic metric in this analysis. The NPV benefit from increased revenue due to TES utilization was found to outweigh the NPV penalty from the additional capital costs. This economic result was attributed to the low cost of the TES equipment and the ability to charge the storages using cheap electricity from high levels of renewable output. The results have shown that hot TES increased NPV in 12 of 14 market scenarios while the cold TES increased NPV in 14 of 14 market scenarios. A combination of both hot and cold TES yielded the largest increases in NPV.

Parasitic heat load in a miniature pulse tube cooler

The parasitic heat load represents the heat flow disturbing the cold end of cryocoolers due to conduction, convection and radiation. On-board space missions, cryocoolers such as pulse tubes are typically employed in a cold redundancy scheme. Therefore, the heat flow through a pulse tube cooler in OFF state directly affects the cold performance of the operating cooler, bringing in heat and limiting the total available cooling power at the cold tip. The reference pulse tube is a miniature pulse tube cooler, developed for 2 W cooling at 80 K with a low input power. This study aims first to measure the parasitic heat load through three methods, which results are compared and confronted with numerical evaluation based on well-known correlations. Then a novel way in design is proposed, integrating an element of lower conduction replacing a part of the regenerator in order to reduce the heat load. The impact of this modification on the performance is measured and compared with its influence on the heat load.

Performance of nitrogen pulsating heat pipes as passive thermal switches in a redundant cryocooler application

A novel redundant cryocooler system, featuring two cryocooler cold heads linked via separate nitrogen pulsating heat pipe (PHP) thermal switches to a common cold plate, is developed and performance tested. The setup allows the cold plate to be maintained at the design temperature when one of the two cryocoolers is inactive by leveraging the low thermal conductivity of the static vapor in the associated dry PHP as insulation from the warm inactive cryocooler. The PHP thermal switches operate passively and require no mechanical actuation, inherently offering a highly reliable switching mechanism. PHP heat load, effective thermal conductivity, effective conductance, evaporator temperature, and condenser temperature measurements are presented for a range of fill ratios and applied cold plate heat loads. The tested PHPs can transfer at least 16 W before dryout and have effective thermal conductivities in the ON and OFF switch states ranging from 10,000 W/m-K to 55,000 W/m-K and 8 W/m-K to 50 W/m-K, respectively. Thermal switch performance is characterized by the switching ratio – the ratio of its effective conductance in the ON state to that in the OFF state – and is demonstrated to approach about 2500 for the tested PHPs. The parasitic heat load through the PHP switch in the OFF state (about 0.6 W) is found to be about 4 percent of its maximum tested ON state heat transfer capacity (about 16 W).

Analysis of the effects of thermal anchors on the reduction of the parasitic load to the EU-DEMO TF coils

The design of the superconducting magnet system of the EU-DEMO tokamak reached the end of the pre-conceptual design phase, and a careful investigation of the parasitic heat load from the room-temperature environment is needed. A possible route for the parasitic heat to enter the toroidal field (TF) coils is by conduction through their Gravity Supports (GSs), which provide mechanical support to the TFs, the Vacuum Vessel (VV) and attached coils, acting as a thermal bridge to the TF casing. A thermal anchoring of the GS using Supercritical helium (SHe) at 4.5 K, re-routing a fraction of the coolant from the TF casing cooling loop, downstream of the casing, is addressed in this study. A 3D thermal model is developed for a plausible design of the EU-DEMO GS, coupled to fluid lines that account for the active cooling by SHe. The effect of the location of the Thermal Anchor (TA) at different heights of the GS plates is investigated parametrically and the beneficial reduction of the parasitic heat load to the TF coil is demonstrated. The reduction of the static heat load to the TF winding pack, when the TA is enabled, is computed and discussed.

Systematic approach to determine the transient cooling power and heat leak of a commercial pulse tube cryocooler

In a typical pulse tube cryocooler application, modeling the cool-down dynamics of a load attached to the cold tip require knowledge of the transient cooling power. Although this data may be calculated for in-house developed cryocoolers, the only data provided in commercial off-the-shelf cryocoolers is the steady state cooling power. In this paper, we show a systematic approach to derive transient cooling power data by performing heating and cooling experiments. The data obtained is verified with cooling data of an attached mass, which showed a good agreement. We also present a method to determine the parasitic heat load in a steady and transient operation of a commercial pulse tube cryocooler.

Development of a high-effectiveness slotted plate cryogenic heat exchanger

High-effectiveness heat exchangers are a ubiquitous component of cryogenic systems, but their performance typically falls short of model-based expectations. The following paper summarizes modeling efforts of a heat exchanger designed to achieve an effectiveness in excess of 99% within a prescribed envelope of volume, weight, pressure drop, and operating conditions. Simulation efforts focus on employing minimal assumptions and accounting for many loss mechanisms to avoid overestimation of predicted effectiveness. Axial conduction and parallel passage flow imbalance are major contributing factors to heat exchanger inefficiency. Consequently, a staggered stacked slotted plate geometry, also known as a matrix heat exchanger, was chosen as the most promising type to achieve the desired effectiveness. High thermal conductivity, high surface area finned copper plates separated by low thermal conductivity, low cross-sectional area stainless steel spacers achieve high stream-to-stream heat transfer while limiting axial conduction and allowing for fluid redistribution between plates. The number of design parameters was reduced based on manufacturing limitations and conceptual reasoning. ANSYS Fluent computational fluid dynamics was used to form Nusselt number and Darcy friction factor correlations for the geometry and flow conditions. A finite difference MATLAB model accounted for axial conduction, parasitic heat loads, and material property variation with temperature and pressure. The Fluent-derived correlations were used as inputs to the MATLAB model. The MATLAB model was validated by comparing its results to an analytic, constant property effectiveness-NTU solution for stacked plate heat exchangers, as well as experimental data collected from a similar heat exchanger. Trade-offs between weight, pressure drop, and effectiveness were studied.

Looking for Energy Losses of a Rotary Permanent Magnet Magnetic Refrigerator to Optimize Its Performances

In this paper, an extensive study on the energy losses of a magnetic refrigerator prototype developed at University of Salerno, named ‘8MAG’, is carried out with the aim to improve the performance of such a system. The design details of ‘8MAG’ evidences both mechanical and thermal losses, which are mainly attributed to the eddy currents generation into the support of the regenerators (magnetocaloric wheel) and the parasitic heat load of the rotary valve. The latter component is fundamental since it imparts the direction of the heat transfer fluid distribution through the regenerators and it serves as a drive shaft for the magnetic assembly. The energy losses concerning eddy currents and parasitic heat load are evaluated by two uncoupled models, which are validated by experimental data obtained with different operating conditions. Then, the achievable coefficient of performance (COP) improvements of ‘8MAG’ are estimated, showing that reducing eddy currents generation (by changing the material of the magnetocaloric wheel) and the parasitic heat load (enhancing the insulation of the rotary valve) can lead to increase the COP from 2.5 to 2.8 (+12.0%) and 3.0 (+20%), respectively, and to 3.3 (+32%), combining both improvements, with an hot source temperature of 22 °C and 2 K of temperature span.

Thermodynamic performance comparison of a two-stage cryocooler and cascade cryocoolers with thermal anchoring for precooling purpose

This paper describes the prospective issue of actively incorporating a cryocooler for thermal anchoring purpose to block the heat ingress from room temperature and reduce the everlasting parasitic heat load. Two systems, one with a single two-stage cryocooler and the other with two independent cascade cryocoolers, are examined in this paper to elucidate the involved thermodynamic inefficiencies in terms of energy and entropy flows. The overall system efficiency is greatly affected by the percent Carnot efficiency of the actual cryocooler covering the large temperature span. The single two-stage cryocooler configuration is more efficient than the cascade cryocoolers in general because it can transfer the heat load from the primary cryogenic load temperature to room temperature directly by thermomechanical coupling. The cascade cryocoolers, however, have to handle the heat load at the intermediate temperature section when the lower temperature unit dumps heat to the cold end of the upper temperature stage, which often creates significant entropy burden to the upper one. On the other hand, the true cascade cryocoolers can also attain higher efficiency than the single two-stage cryocooler if the heat pumping process from the primary cryogenic load temperature to room temperature is too inefficient with excessively large temperature ratio. This efficiency reversal is possible by virtue of dividing the large temperature span into two small ones. In this case, the appropriate intermediate temperature must be selected and a highly effective external heat exchanger needs to be carefully incorporated in between. Parametric studies have been performed to discuss the impact of the intermediate temperature on thermal anchoring and different parasitic conduction or radiation heat leak.

Numerical study on the operating characteristics of cryogenic loop heat pipes based on a one-dimensional heat leak model

The cryogenic loop heat pipe is an effective and reliable heat transfer system in the space technology. A global model of cryogenic loop heat pipe is developed by full consideration of the subcooled effect and the mass flow rate variation of the working fluid along the evaporator core. The heat leak is calculated on the basis of one-dimensional temperature distributions in the evaporator core. The calculation of the proposed model improves the prediction accuracy relative to traditional zero-dimensional models. The effects of filling pressure, parasitic heat load, auxiliary heat load, and use of bayonet on the CLHP performance are investigated. A normal “U-shape” of the evaporating temperature curve appears at a low filling pressure (1.45 MPa). At moderate and high filling pressures (1.465 MPa and 1.5 MPa, respectively), two new evaporating temperature curves with “flat bottom U-shape” and “L-shape”, which shows the unique operating characteristics of cryogenic loop heat pipes, are found. The two new profiles are attributed to the coupling of mass and energy between gas reservoir and primary compensation chamber. The determinant factor for the minimum working heat load is the void fraction of the primary compensation chamber. By contrast, the maximum working heat load is determined by the void fraction at a low filling pressure (below 1.4 MPa) and the maximum capillary pressure at a high filling pressure (above 1.4 MPa). The use of a bayonet can decrease the maximum vapor quality in the evaporator core and effectively avoid burnout.

Numerical and Experimental Studies of Parasitic Heat Losses in Coldfinger of a Pulse Tube Cryocooler in Off-State Condition

ABSTRACTA pulse tube cryocooler (PTC) for future metrological satellites has been developed at one of the lead centers of the Indian Space Research Organisation in Bangalore, India for cooling on-board Infrared (IR) detectors to 80 K.A study has been conducted on the coldfinger of PTC to understand the off-state heat loads on the cooler by varying the value of gravity numerically in ANSYS FLUENT and experimentally by orienting the setup with respect to gravity. The off-state parasitic losses represent a major heat load in on-board applications that include redundant, viz. nonoperating coolers. To find out the amount of off-state parasitic heat losses in a nonoperating coldfinger of the PTC experimentally, transient warm-up technique was used. Various heat loads were applied experimentally on the cryo-tip at temperatures ranging from 80 to 100 K for determining the parasitic losses. The effect of orientation of PTC on the off-state parasitic heat load with respect to gravity is studied and presented in this paper. Enhancement due to free convection heat flow normalized by gas molecular conduction in pulse tube is analyzed using computational fluid dynamics to verify and compare with experimental results. The best orientation angle where the parasitic is low is when the cold end of the coldfinger of pulse tube cryocooler faces down (0°) and high when the cold end of the coldfinger is oriented to 135°.

Modeling of Two-Phase Heat Exchangers With Zeotropic Fluid Mixtures

Heat exchangers are important components in many engineering applications. This paper proposes a numerical two-phase heat exchanger model with simultaneous heat transfer and pressure drop calculations. The presented model provides a modeling framework compatible with numerous different correlations for both single- and two-phase flow of pure fluids and fluid mixtures. Furthermore, it considers nonconstant fluid properties as well as longitudinal heat conduction and parasitic heat loads, which is particularly relevant in mixed refrigerant cycles for cooling of low-temperature applications. The governing equations are derived and the solution strategy is presented, followed by the model validation against analytical solutions in the corresponding limits. Finally, an exemplary heat exchanger is analyzed using both homogeneous and separated flow models, and the results are compared with experimental data from literature.

Dynamic Resistance Measurements in a GdBCO-Coated Conductor

Dynamic resistance is a phenomenon which occurs when a superconducting wire carries dc transport current whilst experiencing an alternating magnetic field. This situation occurs in a range of HTS machinery applications, where dynamic resistance can lead to large parasitic heat loads and potential quench events. Here, we present dynamic resistance measurements of a 5-mm-wide Fujikura coated conductor wire at 77 K. We report experimental values obtain through varying the field angle (the angle between magnetic field and normal vector of the conductor wide-face), the dc current levels, and the magnetic field amplitude, and frequency. We show that the dynamic resistance in perpendicular magnetic field can be predicted by using a simple analytical equation. We also show that across the range of field angles measured here the dynamic resistance is dominated by the perpendicular component of the applied magnetic field.

Through-Wall Excitation of a Magnet Coil by an External-Rotor HTS Flux Pump

High-temperature superconducting (HTS) magnet systems conventionally require normal-conducting current leads, which connect between the HTS circuit and an external power supply located at room temperature. These current leads form a thermal bridge across the cryostat wall, and they represent the dominant heat load for many magnet applications. The use of a superconducting flux pump device is an alternative approach to exciting a magnet coil, which can eradicate this parasitic heat load, as such devices do not require direct physical connection to the HTS circuit. However, earlier proposed flux pump designs have required power-dissipating active components to be located within the cryogenic envelope, thus imposing their own parasitic heat load. Here, we report the successful demonstration of a mechanically rotating HTS flux pump, which operates entirely outside of the cryogenic envelope. This prototype device projects flux across a cryostat wall, leading to the injection of a direct current into a thermally isolated closed HTS circuit. This is achieved through the implementation of a flux-concentrating magnetic circuit employing ferromagnetic yoke pieces, which enables flux penetration of the HTS circuit at large flux gaps. We have demonstrated the injection of direct currents of > 30 A into a closed HTS circuit while operating this device across a cryostat wall.

Warm-up calorimetry of Dewar-Detector Assemblies

Boil-off isothermal calorimetry of Dewar-Detector Assemblies (DDA) is a routine part of their Acceptance Testing Procedure. In this approach, the cryogenic liquid coolant (typically LN2) is allowed to naturally boil-off from the Dewar well to the atmosphere through a mass flow meter; the parasitic heat load is then evaluated as the product of the latent heat of vaporization and the "last drop" boil-off rate.An inherent major limitation of this technique is that it may be performed only at the fixed boiling temperature of the chosen liquid coolant. A further drawback is related to the explosive nature of "last drop" boiling, manifesting itself as an uneven flow rate. This especially holds true for advanced High Operational Temperature Dewar-Detector Assemblies, typically featuring short cold fingers and working at 150 K and above.In this work, we adapt the well-known technique of dual-slope calorimetry and show how accurate heat load evaluation may be performed by comparing the slopes of the warm-up thermal transients under different trial added heat loads. Because of the simplicity, accuracy and ability to perform calorimetry literally at any temperature of interest, this technique shows good potential for replacing traditional boil-off calorimetry.

Performance of a fast response miniature Adiabatic Demagnetisation Refrigerator using a single crystal tungsten magnetoresistive heat switch

The performance of a fast thermal response miniature Adiabatic Demagnetisation Refrigerator (ADR) is presented. The miniature ADR is comprised of a fast thermal response Chromium Potassium Alum (CPA) salt pill, two superconducting magnets and unconventionally, a single crystal tungsten magnetoresistive (MR) heat switch. The development of this ADR is a result of the ongoing development of a continuously operating millikelvin cryocooler (mKCC), which will use only magnetoresistive heat switches. The design and performance of the MR heat switch developed for the mKCC and used in the miniature ADR is presented in this paper; the heat switch has a measured Residual Resistivity Ratio of 32,000±3000 and an estimated switching ratio (on thermal conductivity divided by the off thermal conductivity) of 15,200 at 3.6K and 38,800 at 0.2K when using a 3T magnetic field. The performance of the miniature ADR operating from a 3.6K bath is presented, demonstrating that a complete cycle (magnetisation, cooling to the bath and demagnetisation) can be accomplished in 82s. A magnet current step test, conducted when the ADR is cold and fully demagnetised, has shown the thermal response of the ADR to be sub-second. The measured hold times of the ADR with just parasitic heat load are given, ranging from 3min at 0.2K with 13.14μW of parasitics, to 924min at 3K with 4.55μW of parasitics. The cooling power has been measured for operating temperatures in the range 0.25–3K by applying an additional heat load to the ADR via a heater, in order to reduce the hold time to 3min (i.e. approximately double the recycle time); the maximum cooling power of the miniature ADR (in addition to parasitic load) when operating at 250mK is 20μW, which increases to 45μW at 300mK and continues to increase linearly to nearly 1.1mW at 3K. To conclude, the predicted performance of a tandem continuous ADR utilising two of the miniature ADRs is presented.

Analytical Study on Multi-stream Heat Exchanger Include Longitudinal Heat Conduction and Parasitic Heat Loads

High performance heat exchangers are critical component in many cryogenic systems and its performance is typically very sensitive to longitudinal heat conduction, parasitic heat loads and property variations. This paper gives an analytical study on 1-D model for multi-stream parallel-plate fin heat exchanger by using the method of decoupling transformations. The results obtained in the present paper are valuable for the reference on optimization for heat exchanger design.

Improved Performance of an Indigenous Stirling Type Pulse Tube Cooler and Pressure Wave Generator

Sustained efforts have been made in our laboratory to improve the performance of an indigenously developed pressure wave gen- erator by reducing the mechanical losses and the required input power. An acoustically matching pulse tube cooler, with a design target of 0.5W at 80K, was designed using Sage and experience gained from previous studies. The pulse tube cooler was fabri- cated and tested. The effect of regenerator stacking pattern on the cooler performance was studied by filling the regenerator with mesh of the same size #400 and with multi meshes #250, 325, 400. In present experiments, regenerator with #400 mesh at 30bar filling pressure performed better with more energy efficiency. A no load temperature of 74K was achieved with input power of 59W corresponding to a cooling power of 0.22W at 80K. Parasitic heat load to the cooler was measured be 0.68W. This heat load is primarily by heat conduction through the regenerator and pulse tube wall. By reducing the wall thickness from 0.30mm to 0.15mm, the parasitic loads can be reduced by 50%.

The effect of elevated water content on swirl-stabilized ethanol/air flames

Ethanol is currently being considered as a potential alternative to traditional fuels. This study seeks to validate the use of hydrous ethanol in lieu of fossil fuels or anhydrous ethanol in order to reduce the production cost associated with ethanol. Experiments are conducted in a swirl-stabilized combustor, representative of a gas turbine and hydrous ethanol ranging from 0% to 40% water by volume are tested. A stable flame was achieved for fuels up to 35% water and the Lean Blow Out limit was determined for these fuels. Fuels ranging from 0% to 20% water were tested in greater detail which included thermal mapping of the flame, exhaust temperature measurements, exhaust NOx, CO2, and O2 measurement, as well as CH* and OH* imaging of the flame. Equivalence ratio within the combustor was varied to include 0.6, 0.8, 1.0 and 1.1, representing extremely lean, lean, stoichiometric, and rich test conditions, respectively. Results revealed that the exhaust heat rate, combustion efficiency, and combustor thermal efficiency were not affected negatively by elevated water content up to 20%. However, the flame temperature did generally decrease as a result of water addition, particularly in the lower flame region. CH*/OH* emissions in the lower-flame region were also appreciably reduced due to the parasitic heat load of water vaporization and local quenching in the lower parts of the flame. The practical consequence of burning hydrous fuel was reduced exhaust temperature. Reduced peak temperatures lead to reductions of exhaust NOx at all test conditions. This study indicates that hydrous ethanol with up to 20% water can potentially be used in lieu of the more expensive anhydrous ethanol for combustion applications.

Experimental study of a nitrogen-charged cryogenic loop heat pipe

Cryogenic loop heat pipes (CLHPs) are effective and efficient cryogenic heat transport devices suitable for many space applications. In this work, a miniature cryogenic loop heat pipe (CLHP) with nitrogen as the working fluid was designed and experimentally investigated. An auxiliary loop was employed to assist the supercritical startup of the primary evaporator. The operational characteristics of the CLHP and the matching characteristics of heat loads applied to the primary and secondary evaporators were investigated experimentally. The results show that the CLHP can achieve reliably the supercritical startup when the heat load applied to the secondary evaporator is no less than 3W; when the heat load applied to the primary evaporator is no less than 2.5W, the primary evaporator can operate independently, otherwise a proper selection of the heat load applied to the secondary evaporator should be considered to overcome the parasitic heat load from the ambient. The CLHP is working at the variable conductance mode and can achieve smooth operational transition subject to a large step change of the heat load applied to the primary evaporator.

Modeling and Analysis of Supercritical Startup of a Cryogenic Loop Heat Pipe

Supercritical startup of cryogenic loop heat pipes (CLHPs) has been investigated both analytically and experimentally. Mathematical model of the supercritical startup has been established using the nodal network method, and parametric study is conducted where the effects of working fluid charged pressure, parasitic heat load from the ambient, etc., on the supercritical startup characteristics are incorporated and evaluated. The result improves understanding of the effects of these parameters on supercritical startup and identification of those conditions under which supercritical startup can and will occur. In addition, the modeling effort has led to an enhanced understanding of supercritical startup performance.