## 2 State-of-the-Art Technologies  
In the literature, a hybrid system incorporating photovoltaics (PV), fuel cells (FC), and ultra-capacitors (UC) is reported for stand-alone applications. In this system, the FC provides additional power when PV cannot meet load demand. If the demand further exceeds the FC capability, the UC bank starts and joins the supply. Reference [11] reports an experimental study based on a hybrid system including PV panels, wind turbines, biomass gasification plants, and a battery bank for energy storage. The feasibility of the hybrid system was verified, and the control strategy was set up according to battery charging status. If the state of charge (SOC) goes down to 50% of its nominal value, the biomass gasification plant starts to work to provide power. In reference [12], a hybrid syngas/solar PV/battery power system was developed for off-grid applications. During the running tests, the solar PV/battery system with a standby syngas genset ran reliably to meet the changing power demand. Similar to [11], the batteries aid in the power supply if the demand exceeds the intermittent solar power. The operational strategy of the genset was based on the batteries' SOC. The genset automatically started to operate and supply power to both of the load and the batteries when their SOC dropped to 40% and stopped charging the batteries when their SOC reached 80%. Reference [13] developed a hybrid PV-engine system and an optimization strategy for SOC control combined with minimum costs. Another study presented in [14] showed a hybrid system combining PV, wind, and bio-gas genset to power a sewage plant. In general, these systems are effective in providing required power to meet the demand but suffer from the lack of systemic optimization in coordinating the different power generators to achieve the best system performance (e.g., utilization of renewables, energy efficiency, and operating cost). Moreover, the genset as a backup supply is bulky and expensive. In this regard, fuel cells based on proton exchange membrane (PEM) provide much promise. These are more efficient and environment friendly. They can conveniently convert the chemical energy stored in hydrogen and oxygen into electricity [15; 16; 17; 18; 19] and be interfaced with an AC or DC distribution system using different power electronic converters [20]. In reference [21], the authors studied energy efficiency when the FC supplied electricity to the load and/or the AC grid in residential environments. Their operational strategy was based on the phase angle and voltage variation of the output so as to control a FC-based power system [22]. Steady state behavior and the dynamic voltage variation of the FC were predicted based on empirical and test results. In [23], an energy management optimization strategy for a hybrid wind-batteries-FC system was designed to realize cost-effective operation of the hybrid system. It was found that the FC system can meet the excessive power demand for a reasonable time if the wind power cannot satisfy the load demand whilst the ultra-capacitor can only supply power for a shorter duration. In this work, the system can tolerate the rapid changes in wind speeds and suppress the effects of these fluctuations. The optimization and integration of hybrid renewable energy hydrogen fuel cell energy systems are critically reviewed in reference [24]. The effect of the fluctuation range of the power output on the hydrogen production devices, in terms of equipment lifespan and hydrogen purity, in a wind-hydrogen system cannot be omitted [25]. The data analysis in reference [26] indicated that the hydrogen content was stabilized between 1.04% to 1.10% in the load range of 20% to 100% in the water electrolysis system, which meets the safety standards. The current status and development trend of wind power generation-based hydrogen production technology have been recently reviewed in reference [27].