Energy harvestingfrom floor using organic piezoelectric modules

E. Bischur, N. Schwesinger Technische Universitaet Muenchen, Arcisstr. 21,

Munich, Germany bischur@tum.de; schwesinger@tum.de

Abstract-First, energy harvesting from floor is described as

a sustainable method to generate electrical energy. Different solutions, available on the market, will be considered and eva-luated. Second, the design, fabrication and test of organic piezoe-lectric harvesting modules will be discussed. These modules shall be used directly beneath the firstlayer of the floor to gain energy from people walking across this area.The harvesting modules consist ofa polymeric thin film of PVDF. PVDF shows piezoelec-tric properties if it was be stretched during or after the foil fabri-cation. Besides this stretching, it is required to polarize the PVDF, too. This polarization process is fortunately realized at tempera-tures above room temperature. The better the polarization process the better the conversion efficiencyof the material. Al-though the values of voltage conversion are not as high as with PZT-ceramics, PVDF offers some interesting properties making it very useful for such harvesting applications. Due to the use of organic polymers, the modules are characterized by a great flex-ibility and the possibility to create them in almost any geometric-al size. The fabrication process of the harvesting modules is very simple and suited for mass production. Harvesting modules are built upas roller-type capacitors.

The energy yield was determined depending on thedynamic loading force and the thickness of piezoelectric active material.An increase of the energy yield at higher loading forces and higher thicknesses of the modules is possible in general.

Keywords-PVDF; foil; piezoelectric; floor; walking; gener-

ate;energy yield;energy harvesting

I. INTRODUCTION Sustainability is no longer a catchword, it is a “must” in

terms of energy generation. Natural resources are limited and the mankind has to look for better energy efficiency especially due to the continuously growing energy demand. A lot of energy is lost in natural and technical processes without mea-surable effect. The use of these losses could help in the near future to bridge the gap between energy demand and the ener-gy which is available in reality.

In the following, a simple technique will be presented to gain electrical energy out of the progressive mobility and the resulting increase in traffic.

Generally there exists different ways to gain electrical energy from moving people or running traffic. An unusual approach is the use of pressure fluctuations in the ground, formed by crossing vehicles or people. In that case, the ground is exposed to permanently changing pressure amplitudes.

These pressure changes can be transformed into electrical energy.

For example, a company in the Netherlands, has developed an electromagnetic generator that uses the movement of a dance floor [1]. Unfortunately, the conversion principle used need, however, a relatively large deflection of the floor. Up to 10mm are common, to generate a noticeable electric power. Such large deflections of the floor may be acceptable for dancers but they cause a much fasterfatigueof the dancers. The mechanical design of the power converter has improved dur-ing the last years. Nevertheless, it is still a relatively compli-cated structure which requires high assembling costs. A Japa-nese company (soundpower) has installed piezoelectric harve-sters in the floor area of a subway ticket machine in order to supply it with electricity [2]. These harvesters consist just of piezoceramic. Complicated mechanical structures are not required. Similar products will be offered by an Israelian com-pany (innowattech). Again, piezoelectric ceramics are used, to generate electrical energy from rolling traffic across the harve-sters recessed into the road[3]. Nearby no deflection of the ground is necessary in these approaches, since the energy conversion is based on the piezoelectric effect.

However, the piezoelectric materials used have some tre-mendous disadvantages. First, high conversion efficiency requires a high-performance piezoelectric material. Only ce-ramics based on a composition of Lead-Zirconia-Titanium (PZT) fulfill this requirement. This high performance piezo-material is comparable very expensive due to special manufac-turing requirements. Second, PZT is a ceramic and possesses of a high degree of brittleness. It is a necessity to protect the material against mechanical stresses which could lead to a break. On the other hand pressure differences, applied to the material, should be used for power conversion i.e. stresses are wanted and omitted at the same time. Therefore, piezoceramic substrates in harvesting devices have to be protected in a way which allows the passing of compressive stresses onto the harvesting surfaceonly. Consequently, it is necessary to sur-round the active material with a protective sheath which leads to a further rise of the costs. Third, piezoceramic ages and loses its properties with increasing operating time. Harvesting devices of PZT need to be exchanged after a while. Although the life time of such aging harvesting devices is much longer as of batteries takes this behavior a main reason for the appli-cation of energy harvesters away. All these reasons make pie-zoceramic not really attractive for harvesting purposes. Alter-natively, MacroFiberComposits (MFC) are proposed for the energy harvesting. MFC’s consist of PZT-fibers sandwiched between Polyimide films. This basic design makes the devices unbreakable in limits [4]. Aging and basic costs of PZT still remain.

These reasons have prompted us to search for other solu-tions. Since we found that the piezoelectric effect is the easiest way to convert mechanical energy into electrical energy in view of very simple devices, we have focused our interests first to other materials. Unfortunately, we could not found

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really suited piezoelectric materials. Some of them are ceram-ics and brittle like PZT, others show much worse piezoelectric properties. At least we tended more and more to PVDF a pie-zoelectric polymer. PVDF has three main advantages. It is of orders less expensive as PZT, it doesn’t showaging and it is under harsh conditions unbreakable. Furthermore, PVDF is incombustible, chemical very resistant and can be processed by injection molding. Disadvantages lie in the pure piezoelec-tric coefficients, the relatively low Curie-Temperature (105°C) and the necessity to stretch and to polarize the material. Main reason for the use of PVDF was the planned application in floors. Heavy-duty mechanical loads as well as large areas don’t allow the use of other materials as of PVDF. Only PFDV offers under these conditions a long lasting functionality at a comparable low price.

II. HARVESTING PRINCIPLE The principle used in these investigationsbases on the di-

rect piezoelectric longitudinal effect which can be described with the coefficient d33. Dynamic compression of the piezoma-terial causes charges on the surfaces.They can be collected by electrodes on the surface and submitted to connected electric circuits.

Piezoelectric properties of PVDF were firstly discovered in 1969 by Kawai[5].If PVDF-foil is stretched uniaxialit gets partly semi crystalline and by forminga molecular chain struc-ture which is expressed as β-phase. This structure has a net dipole moment that causes the piezoelectric behavior of the whole solid. By means of an electrical polarization the mole-cule chains can be aligned uniformly and a noticeable piezoe-lectric behavior in the direction of stretching (d31) and perpen-dicular to the foil surface (d33) is measurable[6,7].

Since mechanical stress will be applied in the foreseen ap-plication perpendicularly to the PVDF-foilwe have considered mainly the longitudinal effect in our investigations. Thus,depend on the value of the piezoelectric coefficient d33 surface chargeshave been released. The efficiency of energy conversion is thereby determined by the thickness of the pie-zoelectric active material,the forceaffecting the piezoelectric material and the piezoelectric properties. Higher material thicknesses and higher forces generate a higher amount of surface charges generally. Thus, the ratio of the pressurized area and the whole area of the piezoelectric module is crucial. On the other hand, the effect of charge separation will be small if the piezoelectric properties of the material are worse. It is important therefore, to operate with materials possessing of best conversion properties, i.e. piezoelectric coefficients. They can be achieved in a successful polarization process.

III. DESIGN OF THE HARVESTING MODULES For all investigations asimple plate type capacitor configu-

ration was used.Self-supporting aluminum foils have been used as electrode material. These aluminum foils were ar-ranged on top and bottom of the PVDF-foil. Each 2 layers of PVDF-foil and aluminum-foil were stacked alternating above

each other. This basic designwas helpful to keep the manufac-turing easy, fast and cheap. The thickness of the PVDF-foil usedwas 55µm, determined by the manufacturer. Aluminum foil with a thickness of 10µm was used as basic material, too.Greater module thicknesses have been realized by winding the foil arrangement.Figure 1 shows a cross section of a har-vesting module.This assembling method allows for an easy change of the geometrical dimensions of the modules. Length and width of the modules were defined as desired by the foil with and the diameter of the coil produced, respective-ly.Thicknesses of the modules were determined by the number of windings.Finally,the PVDF in the modulewas polarized. Since polarization is the most important procedure to achieve sufficient piezoelectric properties we will consider this proce-dure in the next chapter.

IV. POLARIZATION OF THE PVDF Polarized PVDF can be achieved by applyingsufficient

electric field strength. The process of polarization is supported by the temperature. The higher the temperature the higher is the remanent polarization. Temperatures are allowed as far as the values are below the Curie-Temperature. The physical nature of the modules allows for a simple application of a polarization voltage onto the electrode layers. In case of an AC-voltage, it is possible to generate hysteresis loops as shown in figure 2. The polarization was estimated by means of a simple calculation. It is obvious, that the height of the rema-nent polarization value Pr (@E=0MV/m) increases with an

increase of the temperature. Temperatures far below 100°C don’t let expect high values of polarization. Similar hysteresis

Figure1: Basic principle of the assembly method

Electric Field E [MV/m]

Figure 2: Hysteresis loops at different temperatures @ an electric field strength of 140MV/m

loops were found for different values of the electric field with a variation of the Temperature. Highest Values of Pr have been found @ E=140MV/m. Unfortunately, it wasn’t impossible to work with these optimal values. Due to electric breakdown of the PVDF the polarization was carried out at room tempera-ture and electric field strength of 60MV/m.

V. EXPERIMENTS In all experiments we used modules with a basic area of

1.6cm2. The number of windings varied between 1 and 41. Therefore, different areas have been observed during the mea-surements. A load resistor was connected to the electrodes. The whole surface of the module was compressed by weight pulses. The voltage across the resistor was measured during this pulse loading and unloading, respectively.

VI. RESULTS

A.Influence of polarization Theoretically, the electrical energy should increase with in-creasing remanent polarization Pr of the piezoelectric materi-al.The reason is - more piezoelectric domains are aligned perpendicular to the plane surface with a higher polarization level. Thus, more free charges per volume could be created during mechanical stress pulses.Figure 3 shows the remanent polarization Pr of PVDF as a function of the electric polariza-tion field strength. The regarding electrical energy output Wel

is additionally shown.Although both curves are not identical one can observe a very good agreement between polarization value and electric power output. It is obvious that for practical applications the PVDF has to be optimal polarized. Otherwise the relatively low power output would be further reduced and would make the modules more inefficient.

B. Influence of number of windings (layers) The energy generated Welby a module can be estimated us-

ing following equation

Wel = 12

CV2 = 12εAtg33

2 σ332 (1)

whereby the Voltage V is given by

V=g33σ33 t (2)

with g33 - piezoelectric Voltage-coefficient, t - thickness, 33- mechanical tension, A –compressed area, ε - permittivity The design of the generator modules lead to an increase of the surface with increasing windings.Thus, the capacity of the modulesrises with increasing number of layers. If a device like that will be mechanically stressed by compres-sion/decompression, a much higher number electric charges will be generated as in a single layer device under the same stress. Figure 4 shows this behavior clearly. Figure 4 shows also a difference between estimated values and measured values. Additional investigations have confirmed the mea-

surement results. The reason for this great difference is not really clear yet. Possibly piezoelectric transversal effects could increase the amount of energy. To utilize the piezoelectric transverse effect even better under the same load, a modification of the design was considered. If the electrical energy generated increases with each additional winding, than the energy yield should increase significantly with a high number of windings. This assumption requires that each additional PVDF winding generates a constant amount of electrical energy, regardless of the total number of windings (equivalent to the energy converted per volume PVDF). However, since PVDF is deformable and not as rigid as ceramic, this assumptioncan be fulfilled only within narrow limits. Itis obvious to investigate the real behavior to find out, if such characteristic could be observed, or whether there is an optimal configuration to certain load condition. For this reason, the energy generated was based on the PVDF-volume for the different modules. Due to this normalization it is easy to compare different module sizes. Furthermore, only the energy density can be taken into consideration.

Figure 1: Remanent polarization depending on the electric field strength and regarding electric energy output @ the same mechanical stress level (5kg)

Figure 4: Energy generation depending on the number of wind-ings

The energy density is shown in Figure 5. Theoretically, the value should be constant for the same load as shown in the red line. Practically, a rise of the energy density was observed with an increase of the numbers of layers. The behavior is not linearly and seems not to approach a saturation value. Surprisingly is the height of the deviation. Sin

Figure 5: Energy density depending on the number of PVDF windings, mechanical load: 5kg

ce this behavior was found also for different load values one has to take into account an additional effect. Unfortunately, it is difficult to estimate this effect clearly. It is believed presently that additional charges can be produced by a combination of the transversal effect occurring at the edges and an electret effect occurring in the gaps between the electrode layer and the PVDF-layer. Further investigations are on the way to clarify this unexpected behavior.

VII. CONCLUSION Generator modules have been developed, which are able to generate electrical energy from mechanical loading, which occur in the shape of compressive forces in the ground. Compressive forces, for example, canbe caused by the weight of people or vehicles moving across the ground. The conversion principle of the generator modules bases on the piezoelectric longitudinal effect of PVDF. The design of the generator modules isvery simple. Therefore, it will be possible to reduce perspective production and assembling costs tremendously in comparison with harvester modules of ceramic.Furthermore, costly processes, such as thermal evaporation for the deposition of electrodes are not required for functioning systems. Aluminum foil serves as electrode material and possesses additionally of an advantageous reliability. Experiments have shown that the modules were able to convert dynamic compressible mechanical loads into electrical energy. It was found that the polarization process of the PVDF film had a decisive influence: The higher the remanent polarization, the better the energy conversion. Due to production lags of the foil it was not possible to polarize PVDF with optimal parameters. Dielectric breakdowns of the foil allowed only very poor polarization parameters. It was found that electricity generated rises with an increasing number of windings. This showed surprisingly values which

exceeded by far the theoretical estimated values. Although, one could assume an additional transversal effect, it is impossible to explain these high values yet. Piezoelectric PVDF-foils appears to be very promising in comparison with similar solutions of PZT ceramic. PVDF as a polymermaterialis flexible, very robust and resists easily mechanical destruction forces. In contradiction, PZT as an inorganic ceramic is extremely brittle and requires auxiliary protection in a harsh environment. This is counterproductive in view of the application described above. PVDF harvesting modules offer a wide range of application in terms of sustainability. They could be placed, even in crowded areas on the ground and used as additional long-lasting and less/(low)-maintenance power supply for e.g. emergency lights, alarm systems, self-powered level-crossings, self-powered traffic sensors, etc. The average energy density of the modules is about 3µWs/cm3@ a load of 5kg. Since the energy converted fol-lows the mechanical loading more than proportional an energy density of about 0,1mWs/cm3 can be found @ 70kg of me-chanical load. This value is close to photovoltaic generators and very promising in view of perspective large scale applica-tions. Besides this, the harvester modules can be used for different functions simultaneously. In a previous publication we presented a parquet floor with included PVDF harvesting modules [9].This arrangement allowed for the counting of persons crossing the floor, for estimation of their walking direction as well as for the generation of electricity.

References [1]http://www.sustainabledanceclub.com [2] http://www.soundpower.co.jp [3] http://www.innowattech.co.il/index.aspx [4] http://www.smart-material.com [6]Kawai, H., “The Piezoelectricity of Poly (vinylidene Fluoride)”, Jap.J. Appl. Phys.8, 975-976 (1969) [7]Fukada, E.; Furukawa, T: “Piezoelectricity and ferroelectricity in polyvinylidene fluoride”, Ultrasonics 19 (1), 31–39 (1981). [8]Schewe, H., 1982, “Piezoelectricity of Uniaxially Oriented Polyvinylidene Fluoride”, 1982 Ultrasonics Symposium, 519-524 (1982). [9] Bischur,E., Schwesinger,N., 2011, “Piezoelectric energy harvester under parquet floor”, Proceedings of SPIE, SPIE, S. 79770M

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