The present invention provides an electret sheet that retains high piezoelectricity under high-temperature condition. The electret sheet of the invention is characterized by including a charged porous sheet, in which the electret sheet has a volume resistivity of 1.0×10.sup.15 Ω.Math.cm or more at 25° C. and a dielectric breakdown voltage of 7 kV or more at 25° C. The electret sheet preferably having a volume resistivity of 1.0×10.sup.14 Ω.Math.cm or more at 37° C. and a dielectric breakdown voltage of 7 kV or more at 37° C. retains excellent piezoelectricity even after being left under a high-temperature atmosphere over a long period.

The Electret Energy Storage System (EESS) achieves an advancement in the energy storage field due to high energy/power level densities integrated into a long term energy storage solution. Combining the high density energy storage solution of the EESS with traditional battery storage technologies reduces the overall energy storage three dimensional footprint when compared to battery only footprints. This combined EESS/battery long term energy storage provides a solution in providing energy from renewable energy systems when the presence of the wind or sun are not available when compared to traditional battery only storage solutions.

The Electret Energy Storage System (EESS) achieves an advancement in the energy storage field due to high energy/power level densities integrated into a long term energy storage solution. Combining the high density energy storage solution of the EESS with traditional battery storage technologies reduces the overall energy storage three dimensional footprint when compared to battery only footprints. This combined EESS/battery long term energy storage provides a solution in providing energy from renewable energy systems when the presence of the wind or sun are not available when compared to traditional battery only storage solutions.

Provided is an energy conversion film excellent in charge retention performance and suppressed in deterioration of piezoelectricity even if it is exposed to a high temperature environment and an energy conversion element and the like using the film. An energy conversion element comprising: an energy conversion film at least comprises a charged resin film consisting of a resin film at least containing a thermoplastic resin and a metal soap; and an electrode provided on at least one of the two surfaces of the energy conversion film.

The present invention provides an electret sheet that has excellent durability against a pressing force that is repetitively applied, that is, excellent compression recovery properties, and can suppress stuffiness even when used by attaching it to the skin of a human body. The electret sheet of the present invention has an air permeability of 10 to 1,000 sec/100 mL. Thus, the electret sheet has excellent durability against a pressing force that is repetitively applied, that is, excellent compression recovery properties, and can suppress stuffiness even when used by attaching it to the skin of a human body.

Provision of a piezoelectric element which uses a lightweight and flexible electret material and an electrode layer which is also lightweight, has high conductivity and good flexibility to easily receive electrical signals from an electret, and has good durability. Additionally, provision of a musical instrument provided with such a piezoelectric element. A piezoelectric element comprising an electrode layer (B) on at least one surface of an electret material (A) having pores inside, wherein a porosity of the electret material (A) is 20 to 80%, the electrode layer (B) contains 20 to 70 mass % of a carbon fine particle, and a thickness of the electrode layer (B) is 2 to 100 m.

The present invention provides an electret sheet that retains high piezoelectricity under high-temperature condition. The electret sheet of the invention is characterized by including a charged porous sheet, in which the electret sheet has a volume resistivity of 1.010.sup.15 cm or more at 25 C. and a dielectric breakdown voltage of 7 kV or more at 25 C. The electret sheet preferably having a volume resistivity of 1.010.sup.14 cm or more at 37 C. and a dielectric breakdown voltage of 7 kV or more at 37 C. retains excellent piezoelectricity even after being left under a high-temperature atmosphere over a long period.

The present invention provides an electret sheet that exhibits excellent piezoelectricity even by light stress. The electret sheet of the invention is characterized by including a charged porous sheet, in which the electret sheet has a compressive elastic modulus of 80 to 300 MPa when compressively deformed at 25 C. and a 50% compression stress of 120 to 300 kPa at 25 C., and thus has the excellent piezoelectricity for light stress and exhibits the excellent piezoelectricity even by light stress (0.5 N or less) caused by a pulse wave or a breathing.

The present invention provides an electret sheet which can maintain a high generated voltage even if a large amount of pressure has been applied. Disclosed is an electret sheet of the present invention including an electrically charged propylene-based resin foam sheet which contains a crosslinked propylene-based resin that includes a propylene--olefin random copolymer having a weight average molecular weight of 370,000 to 420,000 at a proportion of 60% by weight or more, and has a gel fraction of 30% to 50% by weight, and in which the average thickness is 90 to 150 m, and the average number of foam cells in the thickness direction is 2 to 4 cells.

The present invention provides an electret sheet which can maintain a high generated voltage even if a large amount of pressure has been applied. Disclosed is an electret sheet of the present invention including an electrically charged propylene-based resin foam sheet which contains a crosslinked propylene-based resin that includes a propylene--olefin random copolymer having a weight average molecular weight of 370,000 to 420,000 at a proportion of 60% by weight or more, and has a gel fraction of 30% to 50% by weight, and in which the average thickness is 90 to 150 m, and the average number of foam cells in the thickness direction is 2 to 4 cells.