1. Fractional short circuit current MPPT
2. Fractional open circuit voltage MPPT

Figure 1: Characteristic curves of a typical PV module.

1. Fractional short circuit current (FSCC) MPPT 

If we examine the IV curve of a PV module (Fig. 1), it is evident that the current at maximum power point occur close to the short circuit current. This means that under a given environmental condition if the short circuit current is measured and compared with the photovoltaic current, then an approximated maximum power point can be reached. Mathematically, it can be written as

I_mpp= k₁ * I_sc

The constant k₁ can be calculated from the data sheet and typically has a value between 0.8-0.92. Figure 2 shows the block diagram of FSCC.

Similar to the fractional short circuit current MPPT , it is observed that the voltage at maximum power point occurs at a fraction of the open circuit voltage as shown in Fig.1. Therefore, if we measure the open circuit voltage and compare it with the photovoltaic voltage, then an approximated maximum power point can be reached. Mathematically, it can be stated as 

V_mpp= k₂ * V_oc

The value of k₂ varies from 0.7-0.85 and can be calculated from the datasheet of a particular PV module.The block diagram of FOCV is shown in figure 3.

Although, offline MPPT are approximated methods of tracking the MPP, yet they have several advantages that are listed below.

• They need only one sensor for their operation. So for FSCC only a current sensor and for FOCV a voltage sensor is required. Therefore, these methods are cheap.
• These methods do not suffer from the oscillations around the MPP.
• Their convergence speed is fast.
• They are easy to implement.
• They have a reasonable efficiency as reported in the [1].

Their use is limited in PV projects because of some serious issues that are given below.

• It is obvious that with the full day operation, the temperature and irradiance (sunlight) varies therefore, intermittent measurements of open circuit voltage / short circuit current is required. This intermittent/periodic measurement is a power loss. 
• These methods never operate at true MPP and hence are not suitable for efficient systems.

In reference [1,2] given below, an attempt is made to reduce the amount of intermittent measurements required for offline methods. The improvement works on the following pattern.

1. When the system starts, then the value of short circuit current / open circuit voltage i measured and stored in the memory.
2. The system has a predefined limit that allows the next measurement of the short circuit current /open circuit voltage if the difference between the stored value and the instantaneous value of photovoltaic current/ photovoltaic voltage exceeds. This means that the system no more works on the time based measurements. The results show that the measurement of offline parameters is performed only when there is a significant difference in the stored and instantaneous values.
3. Refer to the references for more detail understanding.

References:

[1] Hadeed Ahmed Sher et.al. "An intelligent control strategy of fractional short circuit current maximum power point tracking technique for photovoltaic applications" Journal of Renewable and Sustainable Energy, 7, 013114 (2015), 

[2] Sher, H.A.; et.al. "An intelligent off-line MPPT technique for PV applications" Systems, Process & Control (ICSPC), 2013 IEEE Conference on , vol., no., pp.316,320, 13-15 Dec. 2013