MARCO LEGAL/NORMATIVO

Energy storage devices are a key part of EH powered wireless sensor systems and, particularly, of piezoelectric systems as ambient vibrations are often discontinuous and may not be present at all for a prolonged time. By storing energy it is possible to sustain the requirements of the electrical load for an intended application as the stored energy will be released all at once at a convenient time under higher peaks of power. The characteristics of the implemented energy storage devices have a significant impact on the overall EH system‟s performance.

A first group of characteristics is associated to the long-term lifetime of the EH system, which includes energy density (and its ageing effects) and limitations such as the maximum number of rechargeable cycles. In addition to these parameters, a second group of characteristics determine the efficiency of the energy storage device and, in turn, the short-term lifetime of the EH system; literally, how long the electrical load will remain active by only utilising the stored energy. Such parameters are, for example, the leakage current and the equivalent series resistance (ESR) of the energy storage device, which lead to power losses during charging and discharging.

Typical energy storage devices are capacitors/supercapacitors or rechargeable batteries. They provide different energy performance in a number of footprints and configurations. Batteries, in particular, advance on two fronts: 1) increased specific energy for longer time intervals (i.e., the battery capacity in Wh/kg); and 2) improved specific power for good power delivery on demand (i.e., the battery‟s powering capability in W/kg). Figure 2-17 illustrates the energy and power densities of lead acid, nickel-cadmium (Ni-Cd), nickel-metal-hydride (Ni- M-H), and Lithium-ion (Li-ion) batteries.

Figure 2-17 Specific energy and specific power of rechargeable batteries (Figure reprinted from [105])

Taking into account the small sizes aimed by integrated PEH systems, both high specify energy and specify power are an important factor. Up to date, Li-ion technology is then the most promising battery choice for energy- autonomous devices. A more detailed performance comparison of various rechargeable batteries is given in Table 2-2, where batteries of the Li-ion family are distinguished in liquid and polymer depending on their implementation.

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Table 2-2 Comparison of performance for various rechargeable batteries based on their average values [106]

More specifically, the Li-ion family can be divided into three major battery types named upon their cathode oxides, which are: cobalt, manganese, and phosphate. The characteristics of these Li-ion systems are summarised in Table 2-3.

Table 2-3 Characteristics of the most commonly used lithium-ion batteries [107]

There are many other lithium-ion based batteries. For instance, lithium-ion- polymer (Li-polymer) and lithium-metal (Li-metal), which may be the future of EH technology. Lithium titanate (Li4Ti5O12), for instance, is a promising alternative material for the negative electrode, which delivers better cycle stability than conventional mixed graphite anodes. A breakthrough of the recent years is represented by the Thinergy® _{MEC (Infinite Power Solutions, Inc. –}

Littleton, Colorado) thin film batteries. Such a battery technology adopts a solid state electrolyte called lithium-phosphorus-oxynitride (LiPON), where the extremely low electron conductivity results in ultra-low self-discharge (1%

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batteries, but they are made of thin materials in the nanometre or micrometre scale so that the overall thickness of the battery is of a few millimetres in total. Like other Li-on rechargeable batteries, however, there is also a low-limit voltage threshold that has to be taken into account for the design of a whole EH system-of-systems. For example, the aforementioned Thinergy® MEC battery has an operating voltage range between 2.1 V and 4.1 V. Compelling works involving the use of rechargeable batteries in the EH research community are those carried out by Ottman et al. [96] and Sodano et al. [109]. The use of capacitive energy storage, however, appears in the literature more often; especially with regard to the research community rather than commercially available architectures. The reason could be that off-the-shelf discrete components, such as capacitors, can more easily allow to show the EH capabilities of a new developed system. Energy is stored by capacitors through static charge (Q) rather than an electro-mechanical reaction: applying a voltage differential (V) on the positive and negative plates charges the capacitor as depicted in Figure 2-18.

Figure 2-18 Energy storage on a capacitor

Capacitors are better suited to deliver very high power peaks and several voltage ratings. They can be grouped into three main family types:

 electrostatic capacitors, which use a dry separator and have a very low capacitance, from a few pico-Farad to low microfarad;

 electrolytic capacitors, which use a moist separator and are typically rated in microfarads;

 supercapacitors, whose capacitance is rated in farads, are ideal for energy storage that undergoes frequent charge and discharge cycles at high current and short duration.

Supercapacitors are also known as ultracapacitors, because of their high capacitance density properties, or as electrochemical capacitors (EC). As shown in Figure 2-19, supercapacitors store charge in an electric double layer set up by ions at the interface between a high-surface area carbon electrode and a liquid electrolyte [110-111].

Figure 2-19 Internal structure of a supercapacitor [112]

Because of their internal structure, however, the operating voltage of supercapacitors is limited by the specific potential at which the electrolyte undergoes chemical reactions (e.g., 1 to 3 V per cell). For high-voltage applications, supercapacitors‟ cells like batteries can be series-connected but voltage balancing techniques may be required to prevent any cell from going into over-voltage. Specific energy of supercapacitors is relatively low and ranges from 1 to 30 Wh/kg. Although high compared to a regular capacitor, 30 Wh/kg is one-fifth that of a consumer Li-ion battery. In addition, whereas electrochemical batteries deliver a steady voltage in the usable power band, the voltage of supercapacitors decreases on a linear scale from full to zero voltage as shown in Figure 2-20.

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Figure 2-20 Voltage vs charge for battery and capacitor technology; same behaviour can be consider for discharging [113]

Nevertheless, the highly reversible electrostatic charge storage in supercapacitors does not produce the changes in volume that usually accompany the redox reactions of the active masses in batteries, thus causing a limited cycle life of batteries in the range of several hundred to a few thousand cycles versus the millions full charge-discharge cycles of supercapacitors. It is also worthwhile to mention that the energy density of a battery decreases with the decrease of the battery size. Novel lithium battery manufacturing process have been developed to mitigate such an issue, for example: using secondary thin-film deposition [114], downsizing laser welded metal casings and glass feed [115], 3d assembly [116], improving conductivity through nano-sized silicon columns [117], adopting Li intercalation [118], integrating substrate of short length and high surface area [119]. Despite the battery capacity can be increased upon using the above methods, the timing cost of the manufacturing process increases. More recently portable fuel cell system have also been investigated, although scaling down the components used by current fuel cell technology is not simple such as for the water management systems.

Hybrid Energy Storage solutions also represent a valid solution for energy- autonomous WSN applications. The combination of super-capacitors and thin film solid state batteries has been proposed to practically achieve an enhanced performance of EH applications [120]. This is mainly due to the long-term lifetime characteristics of both super-capacitors and thin film batteries, which have demonstrated more than 20 years of operation and over one million

rechargeable cycles [121] and more than 10 years and over 10 thousands rechargeable cycles [122], respectively. However, in terms of efficiency, both technologies present some issues. Super-capacitors with 1-10 F capacitance have a typical leakage current of 0.01-0.1 mA. For example, for the Maxwell PC10 2F supercapacitor, the average leakage current is measured at 40 µA [123] in the first 12 hours of discharge. For applications with charge/discharge of 0.1-10 A current, e.g. camera ash light, such a level of leakage current is negligible. However, for EH powered wireless sensor systems, the charge/discharge current is several orders of magnitude lower at 0.1 mA level. For example, at 3.3 V node‟s operation, 132 µW of power would be dissipated by the Maxwell PC10 2F supercapacitor. Similarly, thin film batteries typically have a leakage current less than 1 µA but the ESR is 50-75 Ω. When active mode currents of wireless sensor nodes of around 15-17 mA are drained from the battery, then 11-22 mW of conduction loss can be attributed to the internal resistance.