## 2 System Architecture
### Energy Storage Option  
Selecting an appropriate energy storage device is the first step in designing an energy harvester circuit, and based on the selected energy storage device, the power management circuit should be designed with the target of optimizing the overall power efficiency. The systems targeted for micro-power energy harvesting applications typically have additional stringent constraints on die area and usage of external components. As energy harvested from the PV module is intermittent and the maximum power that it can provide may be much less than the needed peak power during sensing and data transmission, an energy storage device is needed. Although the proposed H\({}_{2}\) sensor may consume less than 20 \(\upmu\)A on average for sensing and data transmission, the peak power consumption during wireless data transmission is much higher. The target wireless transceiver consumes 3.3 mA and 2.8 mA during data transmission and reception [3]. Miniaturized supercapacitors, NiMH microbatteries and lithium-based batteries are different solutions that have been used in wireless sensor nodes frequently. The battery should have enough capacity to provide enough power for a few hours' operation of the sensor, even without energy harvesting. In addition to capacity, other characteristics, such as size, peak discharge current, nominal operating voltage, V\({}_{\mathrm{EOC}}\), V\({}_{\mathrm{EOD}}\), cycle life and leakage, are very important factors to design an energy-efficient sensor that relies on energy harvesting. Although miniaturized supercapacitors, such as GZ115F of CAP-XX [10], can provide much higher peak charge and discharge currents and have much longer cycle lives in comparison with the rechargeable batteries, they have some disadvantages that make them inappropriate for our application. The main disadvantage of supercapacitors is their high leakage currents that can be even larger than the current provided by the PV cell under reduced illumination level. In addition, they have much lower energy capacity compared with rechargeable batteries of similar sizes. Among lithium batteries, state-of-the-art thin film lithium batteries, such as MEC125 of Infinite Power Solutions [11], are the best candidates for micro-power energy harvesting applications thanks to their low leakage currents, long cycle lives and high discharge currents. The main disadvantage of these batteries is their high nominal voltage levels. As thin film lithium batteries have nominal voltages of more than 3.8 V, additional DC-DC converters are required to use these batteries for ULP applications. The target V6 HR microbattery has a nominal voltage of 1.2 V and can provide a peak discharge current of 18 mA that is much higher than the required current during data transmission. This battery has a large nominal capacity of 6.2 mAh, while its diameter and height are 6.8 mm and 2.15 mm, respectively. As in the proposed gas sensor, all electronic circuits, including the external wireless transceiver, can work with a sub-1.2 V supply voltage, so additional step-down or step-up circuits are not required. Table 1, compares the main characteristics of the proposed V6 HR battery with GZ115F and MEC125. The main disadvantage of this NiMH battery is its lower cycle life compared to supercapacitors and thin film lithium batteries. In addition, this battery has a relatively low leakage current. It loses at most 20% of its charge during the first month, which is much lower than the discharge current of supercapacitors.