Pyroelectric conversion—Effects of P(VDF–TrFE) preconditioning on power conversion

doi:10.1016/j.elstat.2006.07.014

Journal of Electrostatics

Volume 65, Issue 3, March 2007, Pages 182-188

https://doi.org/10.1016/j.elstat.2006.07.014 Get rights and content

Abstract

Large amounts of low-grade heat are emitted from various industries and wasted into the environment. This heat energy can be used as a free source for pyroelectric power generation. We first discuss the principle of pyroelectric conversion based on the differences in crystal structures of copolymer. Then we discuss the pyroelectric conversion of waste heat using copolymer films containing 60% vinylidene fluoride (VDF) and 40% trifluoroethylene (TrFE) and the impact of the preconditioning of the pyroelectric films on the net power output. We discovered that the pre-polarization (poling) of pyroelectric films has a significant beneficial effect by decreasing internal conduction during power conversion. This in turn increased net power output. We were able to reach 95–165 J/L of copolymer used in each cycle even including the internal leakage. We also showed it is critically important to operate the power conversion near the phase transition where electron discharge is the highest.

Introduction

Large amounts of low-grade heat are wasted at pulp and paper mills, steel works, petrochemical plants, glass manufacturers and electric power stations. However, due to the high cost of energy recovery and the lack of promising technologies, low-grade heat is normally discharged to the environment. When the temperature of a heat source is low, the amount of available heat for useful work is also low, i.e., inherently limited Carnot cycle efficiency. Further, equipment for improved heat utilization becomes bulky and the cost of installation becomes prohibitive. Nevertheless, the need and opportunity exist for converting waste heat to usable electricity. Existing technologies such as the organic Rankine cycle cannot function well below 80 °C [1]. Pyroelectric conversion technology could meet this challenge.

Olsen et al. [2] developed a pyroelectric conversion cycle and in this paper it will be referred to as the Olsen Cycle. It utilizes the charge–voltage characteristics (displacement versus applied electric field) of pyroelectric materials at different temperatures and voltages to achieve heat-to-electric power conversion. Pyroelectric films used in their study were a copolymer of 60% vinylidene fluoride (VDF) and 40% trifluoroethylene (TrFE), 60/40 P(VDF–TrFE).

Pyroelectric copolymers belong to a specific subgroup of dielectrics. Dielectrics fall in two groups: symmetric and nonsymmetric. Dielectrics without a center of symmetry exhibits piezoelectricity, i.e., charge generation upon stressing the material. Piezoelectric material can be nonpyroelectric or pyroelectric. As the name suggests, pyroelectric materials responds to change in temperature that causes internal strain which in turn results in electrical charging on the material surface.

Pyroelectricity has been observed in different crystals and ceramics [3]. In the early 1970s, it was discovered that poly (vinylidene fluoride) (PVDF) is pyroelectric [4]. PVDF is a semicrystalline high-molecular-weight polymer of a repeat unit (–CH₂–CF₂–) whose structure is essentially head to tail, i.e. –CF₂(–CH₂–CF₂–)_n–CH₂–, and has typically 50% amorphous content. Its simple formula hides a host of interesting properties, one of which is a number of crystal forms. It exhibits at least four crystalline phases (α, β, γ and δ) [5]. The stereochemical manner in which monomers are linked together defines polymer's chain configuration. In the melt or in the solution, polymer chains have randomly coiled shapes (conformations) but when cooled it forms many phases. The high piezoelectric and pyroelectric responses of PVDF are associated with the polar β phase [6].

PVDF melts at about 170–200 °C, depending on polymeric form and crystallization temperature, and when cooled from the melt without external stresses it crystallizes in the form of spherulites of the nonpolar α phase. The β phase is not usually produced from the melt since that requires high pressures or epitaxial techniques, but is obtained by mechanical deformation or electrical poling [6], [7].

The introduction of TrFE monomer into the VDF chain affects the structural stability of the crystals significantly. Copolymer P(VDF–TrFE) with VDF content between 50 and 80 mol% forms a ferroelectric β phase directly upon crystallization from the melt without any further stretching and shows higher piezoelectricity than PVDF [8]. Thus, polar β phase transforms to nonpolar α phase upon heating across the phase transition temperature. The temperature and electric field dependent property of pyroelectric materials can be used to convert heat to electricity as described by Olsen et al. [2].

Section snippets

Pyroelectric conversion cycle

The electrical energy production cycle (the Olsen cycle) may be described in terms of the charge–voltage behavior of a ferroelectric material. Fig. 1 shows a hysteresis loop of a typical pyroelectric copolymer at two different temperatures.

The direction of the path (either clockwise or counterclockwise) determines whether electrical energy is produced or dissipated. When voltage is applied to a ferroelectric material and is cycled isothermally, the material's natural response is to follow a

Film preparation

The pyroelectric film was prepared using 60/40 P(VDF–TrFE) pellets. The pellets were pressed into a 25 μm film in two stages at 200 °C for 5 s; initial pressing required 1275 kg/cm² to flatten the pellets but subsequent ones were done at 650 kg/cm² to obtain the targeted 25 μm thickness. A thin aluminum electrode was vacuum deposited on both sides of the film. Subsequently electrical leads were attached and high DC voltage (1000 V) was applied to the film at room temperature. This burned the defects

Resistivity and poling process

It is known that copolymer resistivity is highly dependent on temperature [6], [10]. When temperature increases, resistivity decreases and the internal conduction increases. This has a detrimental effect during pyroelectric conversion, because increased leakage current decreases the net output while pyroelectric films are discharging, i.e., generating power.

As the poling process proceeds at constant temperature and electric field, resistivity increases and thus the leakage current I_f decreases.

Conclusion

The pre-polarization (poling) of pyroelectric films has a very significant effect on the net power output by pyroelectric conversion. The poling process of a copolymer film was highly dependent on the strength of the electric field on the film, poling time and temperature. For a commercial 60%/40% P(VDF–TrFE) copolymer, satisfactory pre-polarization was achieved by applying an electric field of 20–37 MV/m for 2–4 h near 95 °C. The complete pre-polarization of the pyroelectric film significantly

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Pyroelectric conversion—Effects of P(VDF–TrFE) preconditioning on power conversion

Abstract

Introduction

Section snippets

Pyroelectric conversion cycle

Film preparation

Resistivity and poling process

Conclusion

Polymer

Pyroelectric conversion cycle of vinylidene fluoride–trifluoroethylene copolymer

J. Appl. Phys.

Ferro-, piezo- and pyroelectricity

Issue Series Title: IEEE Trans. Electr. Insul.