Improving discharge characteristics of indirect integral collector storage systems with multi-element storage

Desired performance of unpressurized integral collector storage systems hinges on the appropriate selection of storage volume and the immersed heat exchanger. This paper presents analytical results expressing the relation between storage volume, number of heat exchanger transfer units and temperature limited performance. For a system composed of a single storage element, the limiting behavior of a perfectly stratified storage element is shown to be superior to a fully-mixed storage element, consistent with more general analysis of thermal storage. Since, however, only the fully-mixed limit is readily obtainable in a physical system, the present paper also examines a division of the storage volume into separate compartments. This multi-element storage system shows significantly improved discharge characteristics as a result of improved elemental area utilization and temperature variation between elements, comparable in many cases to a single perfectly-stratified storage element. In addition, the multi-element system shows increased robustness with respect to variations in heat exchanger effectiveness and initial storage temperature.

A study of capacitance-voltage characteristics of lead zirconate titanate ferroelectric thin films and their usage to investigate polarization and coercive field.

Capacitance-voltage (C-V) characteristics of lead zirconate titanate (PZT) thin films with a thickness of 130 nm were measured between 300 and 533 K. The transition between ferroelectric and paraelectric phases was revealed to be of second order in our case, with a Curie temperature at around 450 K. A linear relationship was found between the measured capacitance and the inverse square root of the applied voltage. It was shown that such a relationship could be fitted well by a universal expression of C/A = k(V+V(0))(-1/2) and that this expression could be derived by expanding the Landau-Devonshire free energy at an effective equilibrium position of the Ti/Zr ion in a PZT unit cell. By using the derived equations in this work, the free energy parameters for an individual material can be obtained solely from the corresponding C-V data, and the temperature dependences of both remnant polarization and coercive voltage are shown to be in quantitative agreement with the experimental data.