Actively Cooled Current Leads for Superconducting Electronics Using Mixed-Gas Joule-Thomson Refrigeration

Electrical leads used for the supply of current to superconducting magnets and electronics must span the temperature range from room temperature to cryogenic temperatures. Because the conventional materials used for such purposes (e.g., copper and aluminum) have both a finite electrical resistance and a significant thermal conductivity, operation of the leads results in both thermal generation and conductance. The resulting thermal loads must be removed from the cryogenic environment. This paper describes a method for integrating cryogenic refrigeration technology with current leads in an efficient and practical manner. The key to this concept is the use of a mixed-gas cooling cycle that absorbs the distributed refrigeration load continuously over the temperature range that it is generated, as opposed to allowing it to pass down to the cold end of the lead where the same energy flow constitutes a much higher entropy load on the cryocooler. Additional benefits of this technology include a more isothermal electronic package, as well as improvements in reliability, and reduction in size and mass. Mixed-gas working fluids can be used within Joule-Thomson devices to achieve a greater refrigeration effect for the same pressure span than is possible with a pure substance. This paper describes a computational tool that allows the composition of gas mixtures to be optimized for the case where the refrigeration load is not completely concentrated at the cold end, as is typically the case, but rather the refrigeration load is distributed over the entire temperature range. A genetic optimization algorithm was found to be the most robust and reliable technique for identifying optimum gas mixture composition. The thermodynamic advantage associated with accepting the refrigeration load at the temperature of its origin, rather than at the cold end, is quantified.

The Modeling of Air-Cooled Absorption Chiller Integration in CHP System

Absorption chillers are well suited for the use of exhaust heat from prime movers, and they improve the heat utilization of Cooling, Heating, and Power (CHP) systems. An air-cooled absorption chiller eliminates the cooling tower and brings considerable advantages as compared to water-cooled chillers. However, the expensive capital cost and crystallization of LiBr (Lithium Bromide) solution in certain operation conditions restrict the commercialization of air-cooled LiBr absorption machines. This paper discusses the feasibility of air-cooled absorption in CHP systems, where the control strategies based on the application can avoid the occurrence of crystallization. By using the fundamental thermodynamic principle, steady-state thermodynamic modeling and simulation have been done in Engineer Equation Solver (EES) to predict the operation of air-cooled absorption chiller integration in CHP systems with special consideration of the crystallization limits. The data of field operation acquired from a CHP system at UMD are used for validation.

Aircraft Power System Options and Thermal Management for High, Pulsed Power Application

A general thermodynamic analytical evaluation tool was developed to investigate the impact of technological improvements on mission effectiveness and weapon power generation in an aircraft based pulsed power system. The power system investigated consists of six major components, the prime power source, the power generator, the power conditioner, the pulsed power source, the pulsed power processor and the thermal management with a total estimated payload restriction of 4600 kgs. based on a USAF cargo aircraft. The analysis was based on a 2.5 MW pulsed power source output and a notional mission profile with an engagement period of 60 minutes during which several duty cycle scenarios were considered. Six power system architectures were evaluated with a baseline power system model that incorporated current off-the-shelf technologies for each component. A helicopter engine was used as the primary power source because of its high power density but the engine performance is very sensitive to increasing altitude where the output power diminishes rapidly. As a result of this and the necessity to accommodate load-following during engagement, the investigations were extended to a hybrid power system architecture with turboalternator-battery and turboalternator-flywheel combinations. Preliminary analysis based on prorated values of specific power and power density for all the components revealed that the overall mass of the power system could be brought down from 13,330 kgs. for the baseline architecture to 4075 kgs. for the conceptual load-following turboalternator-battery hybrid power system. Coolant requirements for an open thermal management system ranged from 2007 kgs. of Ammonia or 1127 kgs. of water for a heat load of 2.9 Mwt corresponding to a 30% duty cycle pulsed power source operation.

Experimental Study on Performance Enhancement of Heat Pump With Screw Compressor

The air-source heat pump has been widely used in industrial refrigeration and central air-conditioning applications because of its unique superiority. An important consideration in the design of heat pump is improving its COP (coefficient of performance). In this paper, the results of experimental investigation on the effects of alternative refrigerants (R22, R134a, R404A and R407C) and economizer on the performance of heat pump are presented. The COP of the heat pump used R134a is up to 4.5% higher than R22, but its capacity got a 37.08% decrease. The refrigerant R407C applied in heat pump can improve the capacity up to 7.86% than R22, but its COP shows a decrease up to 5.92%. The refrigerant R404A used in heat pump will result in poor capacity and COP compared to R22. The economizer system used in heat pump will improve the COP, but as the superfeed pressure of the economizer increases, the system COP increases first, and then drops. So there exists an optimal superfeed pressure of the economizer for the best COP. Also, the effect of the economizer on the screw compressor performance is analyzed by recording the P-v indicator diagram.

Investigation of Fractional Characteristic of Molecular Motion by Molecular Dynamics Simulation

Molecular dynamics simulation (MDS) is adopted to investigate the characteristic of fractional motion of molecules in liquid phase, vapor phase and liquid-vapor interface in the paper. Based on the theory of mean free path and Shannon sampling theorem, the way to determine a universal criterion of time step of simulation is presented. It is shown that there exists difference in the regular pattern of molecular motion in the state of liquid and vapor phase. The fractional features are different for different matter states. Under the condition of same temperature, the characteristic fractional number of molecular motion in liquid state is greater than one in vapor state. It is shown that the fractional dimension numbers in the X, Y and Z direction of the liquid-vapor interface are different. This proves that the liquid-vapor interface has anisotropic character.

Entropy Generation in Natural Convection of Water Near Its Density Maximum in a Square Cavity

This paper is focused on the entropy generation due to heat transfer and viscous flow in natural convection of water near its density maximum in a square cavity. The present hydrodynamic and temperature fields are obtained by solving numerically the mass, momentum and energy balance equations, using the finite difference method. Local entropy generation distributions are obtained based on the resulting velocity and temperature fields by solving the entropy generation equation. The effect of the Grashof numbers on the total entropy generation is studied. Local entropy generation distribution was found to be dependent on the Grashof number and the dimensionless initial temperature. The results also show that thermal entropy generation is relatively dominant over viscous entropy generation.

Performance of Fuel Cell and Engine Based Cogeneration Systems in Heating and Cooling Applications

A generalized thermodynamic model is developed to describe cooling, heating and power generating systems. This model is based on reversible power generation and refrigeration devices with practical, irreversible heat exchanger processes provides valuable information on a system’s performance and allows easy comparisons among different systems at different loading conditions. Using both the first and second laws as well as the carbon dioxide production rate allows one to make a first order system assessment on its energy usage and environment impact. The use of the exergy destruction rate and insuring that its behavior be consistent with that of the first law performance is a important to insure that the thermodynamic system boundaries are correctly and completely defined. The importance of the total thermal load to required power ratio (HLRP) as a scaling parameter is demonstrated. While the reported results confirmed that generalized trends are not possible identify, a number of trends for limited conditions have been identified. The results have shown that a combined vapor compression/absorption refrigeration has higher first law utilization factors and lower carbon dioxide production rate for system with higher refrigeration to total thermal load ratios for all HLRP values. Fuel cell based subsystems outperform engine based subsystems for systems with large refrigeration loads.

Predictive Model and Application of a Hybrid GSHP System for Space Conditioning, Water Heating and Deicing of a Seniors Independent Living Center

A parametric estimation algorithm is described for system design criteria selection in Ground Source Heat Pump (GSHP) Heating, Ventilation and Air Conditioning (HVAC) applications requiring dissipation of annual cycle excess thermal energy to the air, i.e., a hybrid mode. The model applies a combination of order-of-magnitude scaling (OMS) and classical non-dimensional flow and heat transfer methods. The objective is to develop a simplified parametric range display for selection of design values. Application requirements and constraints map into the solution space for specific design value selections. The annual thermal budget cycle is configured to time-phase the earth-stored excess energy from the cooling season for dissipation in the heating season using water to ambient forced-air (fan-coil) cooling. Estimators for an application are developed for the quantity, spacing and depth of well bores and the loop flow rate range based on Reynolds number and Nusselt number correlations for water and earth thermal properties. A case study application in two parts is described. The GSHP system uses a common working fluid (water) in a closed loop serving all 70 zones to furnish heating, cooling, domestic water heating, and exterior walkway deicing for an 80,000 sq. ft. area, 54 apartment, senior, center in Dallas Texas, USA. In 1999, the initial Phase I facility of 55,000 sq. ft. area was occupied using the full capacity flow system design without the dissipation coolers. In 2004 the coolers were included with the expansion to full occupancy. Design parameter values, operating experience, energy use, and the rationale for the demonstrated compatibility of the single solution for essentially two applications are described.

Energy Savings and Pollution Reduction by Coupling a Three-Kiln Brick Production System for the Construction Industry

Atmospheric pollution is one of the most important environmental problems, becoming a phenomenon that could reach levels of serious consequences with irreversible environmental impacts. In Mexico, like in several other countries, brick makers carry out brick production by burning mixtures of different heavy fuels. Because of the wide variety of fuels used it is necessary to determine what types of residual gases are generated, in order to propose remedial treatments in production or to introduce substitution technologies. These preventive actions need to be put in place in order to comply with the Ecological Balance and Environment Protection General Law. Brick kilns emit pollutant gases and particles that remain in the air causing a serious health hazard to the near-factory residents. Amongst these pollutants are carbon monoxide (CO), sulfur oxides (SOx) and hydrocarbons. This paper presents the results of the analysis of a novel 3-kilns coupled system. This experimental analysis includes the determination of the combustion products for gases and particulate matter generated from the burning of the heavy fuels using Gas Chromatography and Scanning Electron Microscopy. Field data also allowed the determination of energy savings for this system, mainly due to the reduction of the consumption of fuel. Fortunately these results show a combined 30% energy savings and a reduction of pollutants and particle emissions.

Thermally Driven Mechanical Actuators by Using Absorption/Desorption of Metal Hydrides: A Comprehensive Simulation

Metal hydrides have been investigated for use in a number of applications, such as heat regenerators, thermal compressors, and hydrogen storage. McKibben actuators are pneumatic actuators that have unique characteristics, such as biomimeticity (having force/deflection characteristics similar to natural muscles), compactness, high force-to-mass ratios, and moreover are lubricationless, noiseless, soft actuating, and environmentally benign. Actuators with these characteristics are ideal for many industrial, space, defense, robotic, and biomedical applications. The combination of metal hydride technology with McKibben actuators builds on the advantages of both technologies, while mitigating the deficiencies of each. In this paper, we report results from a comprehensive simulation strategy for a LaNi4.3Al0.7 based McKibben actuator. The simulations are able to predict and characterize the performance bounds of the actuator in terms of actuator time/thermal input, power/efficiency and force/displacement diagrams. The advantages and disadvantages of the design are discussed from these perspectives.