George M. Lloyd

Performance of high power metal hydride reactors

Metal hydride reactors were built with porous powder metal hydride (PMH) compacts. An improved reactor built with copper coated PMH compacts of LaNi5 with a 1.27 cm diameter produced a nominal specific cooling power of 1.5 kW/kg hydride. A similar reactor, built with copper coated PMH compacts of Ca0.4Mm0.6Ni5, showed 2.2 kW/kg hydride. Results with copper coated PMH compacts showed improved thermal conductivity. The compacts are structurally strong and prevent migration of fine metal hydride particles. Life-cycle tests were performed on the reactor with LaNi5 for over 3000 cycles and the cooling power of the reactor gradually decreased by approximately 55%.

Development of LaNi5/Cu/Sn metal hydride powder composites

Abstract Metal hydride powder composites (MHPC) were manufactured employing the copper-encapsulation technique. Thermal conductivity and permeability of the unactivated MHPC were measured at k eff ∼ 5 w/mK and K H2 ∼ 5 × 10 −15 m 2 . Preliminary measurements of permeability of an activated MHPC appear to show permeability is increased; hysteretic behavior was observed. Since there are several transport phenomena occurring when reaction is going on, it is not easy to separate them. Nevertheless, the measurements obtained from the activated MHPC clearly behaved in a different fashion than the unactivated MHPC. The MHPC were used in two prototype fast reactors. Falling pressure characterization of their performance confirms that the obtained values of k eff and K H2 are useful in applications such as heat pumps.

Compressor-driven metal-hydride heat pumps

Abstract Analysis and experiments are presented for a compressor-driven hydrogen metal-hydride heat-pump system. Such a system, utilizing fast hydride reactors, has the potential to achieve higher efficiency and competitive life-cycle costs with conventional refrigeration systems. Fast reactors utilizing copper-coated LaNi5 hydride compacts were designed with improved thermal conductivity. Reactors were built and tested and the results evaluated. The potential for hydride heat pumps is also described.

Thermal conductivity measurements of metal hydride compacts developed for high-power reactors

Thermal conductivity measurements on porous metal hybride compacts were performed to supplement the limited existing data. These materials are proposed for reactors that are designed for heat-pump applications requiring high specific powers. Previous computational studies have shown that the effective thermal conductivity k eff is a crucial optimization parameter. If it is too low, the reactors themselves limit the system performance. It is sufficiently high, external thermal resistances dominate and overdesign of the materials is unnecessary and unavoidably increases parasitic thermal losses. In this study nine samples were tested using the comparative method and careful attention was paid to ascertaining and propagating all errors into the final data reported.

Thermal analysis of the Ca0.4Mm0.6Ni5 metal–hydride reactor

Abstract Experimental results are presented for the coupled metal-hydride reactors of Ca 0.4 Mm 0.6 Ni 5 (Mm=misch metal) with hydrogen pumped by the compressor. In order to augment the heat transfer in the reactor, metal hydride powders were copper-coated and compressed into a porous metal hydride (PMH) compacts. The reactors, packaged with these PMH compacts, produced continuous cooling output of approximately 0.8 kW/kg of Ca 0.4 Mm 0.6 Ni 5 when the cycle time was set at 4 minutes. The experimental results indicate that optimized reactors of viable specific powers can be fabricated. In addition, comparisons between experimental and theoretical work are in a good agreement.

Formulation and Numerical Solution of Non-Local Thermal Equilibrium Equations for Multiple Gas/Solid Porous Metal Hydride Reactors

The assumption of local thermal equilibrium (LTE) is very common in the study of reacting flows in porous media. The assumption simplifies the structure of the solutions and places fewer constraints on computational methods for the domain and boundary conditions. However, in certain systems, such as gas/solid metal hydride reactors, the boundary conditions may impose high energy transfer rates which produce slowly evolving phase change fronts coupled with rapid kinetics. Overall performance of the systems is proportional to the release or absorption of hydrogen, and this is sensitively related to temperature. Thus, capturing local departures from LTE is required. This paper directly evaluates the influence of these effects by solving an NLTE (non-local thermal equilibrium) formulation for coupled reactors as a function of the interphase heat transfer coefficient, h sf . The reactor dynamics and overall energy balances are compared to solutions previously obtained from LTE calculations. The results appear to be the first NLTE results for coupled reactors. They confirm the existence of NLTE effects and suggest the magnitude of h sf for which they can be minimized.

Systematic numerical analysis of the damage index method used for bridge diagnostics

The damage index method is an intuitively attractive method for detecting localized stiffness perturbations through their influence on mode shapes. In spite of its attractiveness, and a large literature on the formulation of the damage index method for different types of structures and its application to specific test problems, the practical consequences which can result from its limitations, as well as its overall effectiveness, have not been demonstrated or discussed. This is an important problem since, at minimum, the qualitative characteristics and performance of a proposed diagnostic algorithm should be understood. In this paper, the authors review the damage index formulation and examine the traditional assumption step by step. Then this method has been used for numerous cases at different locations and degrees of stiffness perturbation for a large pre-stressed segmental concrete bridge. The finite element models have been used as test structures. The object is to evaluate the feasibility of the damage index method.

Development of a remote coil magnetoelastic stress sensor for steel cables

Despite the increasing popularity of cable-stayed bridges there is no convenient and accurate means available to measure the forces in the cable stays. The measurement of the forces is important for monitoring excessive wind or traffic loadings, to gage the redistribution forces which may occur after seismic events, and for detecting corrosion via loss of the cross-section. Although magnetoelastic stress sensors have been extensively tested on many types of prestressing cables, and have demonstrated accuracies of < 1%, to-date they have been based upon a solenoid geometry, which is not practical for cable force measurements in existing bridges having hundreds of cables. In order to address this problem a magnetoelastic sensor for the direct measurement of stress in steel cables is currently under development. The sensor differs from previous magnetoelastic sensors in that the cable is magnetized by a removable C- shaped circuit, rather than by a solenoid. We report preliminary results on measurement of the initial permeability curve indicating adequate sensitivity to stress with this geometry, but further work is necessary to understand the influence of the more complicated field geometry on data reduction and calibration procedures.

Transitional reactor dynamics affecting optimization of a heat-driven metal hydride refrigerator

This paper summarizes our research on modeling of metal hydride systems, examines the fundamental limitations imposed by geometrical and thermodynamic considerations, and presents a theoretical model for a complete system throughout an entire cycle. Apart from the assumption of local thermodynamic equilibrium no major simplifying assumptions are made and the non-ideal behavior of the hydrides is included. The nature of the heat and mass transfer processes as two coupled reactors are optimized is elucidated. The consequences for future metal hydride heat pump design are investigated.

Design and experimental validation of a wireless PVDF displacement sensor for structure monitoring

Polyvinylidene fluoride (PVDF) is a piezoelectric polymer material. One of its most attractive applications is being used as a sensor for structure monitoring. A suitable circuit interface plays an important role in sensor design. PVDF sensor can be used in a large variety of situations according to different design of circuit. The approach to a special circuit interface, which enables PVDF sensor to be utilized as a wireless “dynamic strain gage”, is presented in this paper. The wireless PVDF sensor was then tested and all the results have been compared with strain gage output for strain and displacement measurements.