LITMIR.BIZ

1 Polymer Composites for Electrical Energy Storage

       Yao Zhou

       Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA, USA

1.1 Introduction

      Electrical energy storage in dielectric materials that involves the movement and separation of opposite charges between the two electrodes under the applied electric field is ubiquitous in electrical engineering. The electrostatic capacitors are widely employed in the power electronic devices in various applications, including grid‐connected wind farm and photovoltaic, hybrid electric vehicle, medical defibrillator, electric power transmission, etc. [1–4]. Polymer dielectrics have been widely adopted in electrostatic capacitors because of their high breakdown strength, light weight, ease of processing, low cost, and excellent flexibility [5–10]. However, the relatively low dielectric constant restricts the application of polymer dielectrics because of the low discharged energy density [11]. To overcome this issue, a polymer composite approach has been proposed to combine the advantages of polymers and inorganic nanofillers to attain both high dielectric constant and high breakdown strength, as well as low energy loss [12–14].

      Over the past decades, great achievements have been made in developing high performance polymer composites for electrical energy storage application. A variety of material structure designs and processing strategies have been proposed to increase the energy density of the polymer composites. This chapter provides a comprehensive understanding of the fundamentals and recent development in the field of polymer composites for electrical energy storage. The strategies of achieving high electrical energy storage performance such as controlling the nanofiller dimension and morphology, controlling the nanofiller distribution and orientation, modifying the nanofiller/polymer interface, introducing multiple nanofillers, and constructing multilayer‐structured composites are particularly discussed.

1.2 General Considerations

      In general, the stored electrical energy density (U_s) in the dielectric materials can be expressed as

(1.1)

where E is the applied electric field and D is the electric displacement [15]. To measure the stored energy density, discharged energy density (U_d), and charge/discharge efficiency (η) of the dielectrics, the electric displacement (D) is measured as a function of applied electric field (E) during the charging and discharging processes, which is also called as the D‐E loop. The schematic of typical D‐E loop of the dielectric material is shown in Figure 1.1. The stored energy density can be calculated by integrating the area bounded by the charging curve, the D‐axis, and the horizontal line y = D_max. Similarly, the discharged energy density can be calculated by integrating the area bounded by the discharging curve, theD‐axis, and the horizontal line y = D_max. The energy loss (U_l) during the charge/discharge circle can be represented by the area bounded by the charging and discharging curves. Apparently, the stored energy density equals to the discharged energy density plus the energy loss, that is, U_s = U_d + U_l. The charge/discharge efficiency is defined as η = U_d/U_s× 100%. For linear dielectrics, the electric displacement increases linearly with the applied electric field, i.e. D = ε₀ε_rE, where ε₀ is the vacuum dielectric constant (8.85 × 10⁻¹² F/m) and ε_r is the relative dielectric constant of the dielectric material [16–18]. As a result, the stored energy density of linear dielectrics can be simplified as

(1.2)

It is suggested from Eqs. (1.1) and (1.2)

Скачать книгу