Conventional MPPT methods

MPPT controlled energy harvesting systems have been widely implemented and their effectiveness is well proven (Simjee and Chou, 2006, Park and Chou 2006, Eakburanawat and Noonyaroonate, 2005). Ordinarily, these approaches can be roughly divided into the analog and digital parts. An analog MPPT circuit uses an analog circuitry and a classical feedback control to make the energy harvesting system approaching MPP. They are mainly characterized by their simplicity, low overhead and low cost, but are more problematic to control (Tanouti et al., 2010). A digital MPPT circuit use a MCU based with digital and adaptive algorithm to track MPP of the system. The benefits of this design are that it is normally more reliable and more efficient than an analog one. But the power consumption is the main challenge for using this approach in the micro energy harvesting systems. In this section, an overview of several existing MPPT approaches, which can be used for a micro energy harvesting system, is described below. tracking overhead is the main advantage of using this approach. But the drawbacks of this approach are obvious. Firstly, direct connection makes the system has very low

efficiency, because the solar panel stops charging the rechargeable batteries when the panel’s terminal voltage is lower than the rechargeable battery. Secondly, without MPP tracking system, the system can just work around the MPP of the system. This makes the MPPT efficiency of the system very low by comparing with other precise MPPT approaches.

(2) Fractional Open Circuit Voltage (FOC) approach/ Fractional Short Circuit current (FSC) approach current can be approximated. Based on these linear relationships, simple approaches to estimate or by momentarily disconnecting the solar cell from the load to sense open circuit voltage or short circuit current were proposed in (Masoum et al., 2002), (Simjee and Chou, 2006) and (Bekker and Beukes, 2004). An interface circuit, which is a charge pump or a boost converter, is used to adjust the operation point of the energy harvester. The MPPT is complete once the energy harvester’s output voltage or output current reaches the reference voltage or current. Because the simple open-loop control and does not require any intensive computation in these approaches, they are considered as suitable MPPT approaches for micro energy harvesting systems.

But the main drawback of these two approaches are that periodically disconnected the energy harvester from the system causes temporary energy loss, which restricts the efficiency of the system. In order to address this shortage, an improved design was presented in (Park and Chou, 2006), where an additional tiny solar cell is used in the energy harvesting system as a pilot cell. This is also called the sensor driven MPPT approach. The open-circuit voltage of the pilot cell is used in place of the open-circuit voltage of the main solar cell. This approach eliminates doing any open circuit voltage sensing on the main solar cell to improve system efficiency. While the shortage of this approach is straightforward, the pilot cell covers a much smaller area than the main solar panel that might not yield a representive MPP of the main solar panel, if dust or shadow on the panel does not cover the pilot cell in the same proportion. Thus, with

this approach, the pilot cell should be carefully chosen and placed to ensure that the voltage relationship is close to the main solar cell (Esram and Chapman, 2007).

(3) Hill-climbing/Perturb and Observe (P&O) approach

In order to solve the problems facing by previous approaches, an alternative tracking method by using a hill climbing or Perturb and Observe approach (P&O) has been proposed in (Esram and Chapman, 2007), (Eakburanawat and Noonyaroonate, 2005), and (Lu et al.,2010). The essential working principle of this method is adopting an iterative trial and error approach to track MPP (Esram and Chapman, 2007). It can be achieved by continuously sensing the output current and voltage at either the output of the transducers or the output power of the converter, and multiplying them to obtain the current power output of the system. A small perturbation is applied to the interface circuit by varying the duty cycle of the boost converter (Eakburanawat and Noonyaroonate, 2005) or the switching frequency of a charge pump (Shao et al., 2009), and then the effect on the output power of the energy harvester is noticed. The working principle is described as below. It assumed that the perturbation results in an increase in the terminal voltage of an energy harvester and the output power is recorded and compared to the previous power output before the perturbation. If the perturbation results indicates the power increasing, another perturbation in the same direction is applied, which results in a further increasing the terminal voltage of the energy harvester. Otherwise, a perturbation in the opposite direction is performed. The MPPT process is continuously repeated until the MPP is reached. There is a tradeoff, which is the relation between responding speed and MPPT efficiency, should be considered in the algorithm design. The large step size corresponds to a rapid response time to the environmental variances, but oscillates with a large swing near the MPP. A small step size can minimize the oscillation swing, but slow down the tracking speed (Lu et al., 2010). By contrasting to the advantages, the circuit complexity and the power consumption with respect to the FOC approach are considered as two main drawbacks of this approach. Moreover, because the algorithm has many steps to approach the MPP, this technique is unsuitable for those energy transducers under rapidly changing atmospheric condition. Hence, in the rapidly changing environment, an improvement P&O approach should be considered (Sera et al., 2006).

(4) Commercial DC-DC boost converter with built-in MPPT algorithm

Recently, several types of commercially available DC-DC boost converter with built-in MPPT algorithm have been developed, such as SPV1020 from ST Microelectronics (SPV1020, 2010), LTC3105 from LINEAR Technology (LTC3105, 2010), and SM72441 from TEXAS Instruments (SM72441, 2010). Based on their datasheets, they state that these devices can easily be used in low voltage, high impedance alternative power sources such as photovoltaic cells and TEGs, because of a small start-up capability and integrated MPP controller. The advantages of using this technology are simple design process and short time for debugging the circuit. But the MPPT efficiencies of the circuits are unknown.