Power and energy management of grid/PEMFC/battery/supercapacitor hybrid power sources for UPS applications

TLDR: In this article, a hybrid power and energy source supplied by a proton exchange membrane fuel cell (PEMFC) as the main power source in an uninterruptible power supply (UPS) system is presented.

About: This article is published in International Journal of Electrical Power & Energy Systems.The article was published on 2015-05-01 and is currently open access. It has received 50 citations till now. The article focuses on the topics: Uninterruptible power supply & Hybrid power.

Figures

Fig. 3. Equivalent circuit of a PEMFC

Fig. 3. Equivalent circuit of a PEMFC

Fig. 10. Battery efficiency employed in the UPS system [17]

Fig. 4. Boundary conditions for macroscopic potentials in an SC

Fig. 14. Voltages and currents waveform of PEMFC and SCs when FC charges SCs if SC voltage is

Fig. 23. Power, current and efficiency characteristics of SC and UPS under the SC operating mode

Frequently asked questions

Q1. What have the authors contributed in "Power and energy management of grid/pemfc/battery/supercapacitor hybrid power sources for ups applications" ?

This paper presents a hybrid power and energy source supplied by a proton exchange membrane fuel cell ( PEMFC ) as the main power source in an uninterruptible power supply ( UPS ) system. 

Q2. What is the role of the fans in the power supply system?

Since using an air-breathing and self-humidified PEMFC in UPS system, and the PEMFC voltage heavily relies on the air stoichiometric flow rate, the role of the fans is very important to blow more air through the system. 

Q3. What is the main advantage of PEMFCs?

Since the PEMFCs can provide electrical power with high energy density, high efficiency and no pollution, they are considered as a promising technology for UPS products. 

Q4. What are the common modes of discharge of a battery?

For the nonlinear electrical response of a battery, the different common modes of discharge are constant current, constant load, and constant power. 

Q5. What is the open circuit voltage of a lead-acid battery?

Because the open circuit voltage of lead-acid battery is a function of charge and the current, a simple equivalent circuit for a battery can thus be written as0( ) ( , ) loadV V Q V The authorQ IR= − = (31)where 0( )V Q is the open circuit voltage when 0I = ; ( , )V The authorQ the cell overpotential(relative to the equilibrium), which is a positive voltage loss for a positive (galvanic) current 0I > , and negative for a negative (electrolytic) current 0I < . 

Q6. How does the power control and energy management control unit charge the SCs?

To prevent the PEMFC shutting down when charging, the power control and energy management control unit firstly turns on K3 and K4, which lets the battery charge the SCs, and then turns on K2 and K4, which lets the PEMFC charge the SCs. 

Q7. What is the efficiency of a UPS in the grid operating mode?

In grid operating mode, for a high frequency UPS system, its efficiency in the utility grid operating mode should be more than 90% under the rated load of 270W. 

Q8. How can The authorcalculate the output power of the stack?

Theoutput power of the stack can be calculated byIVP StackStack = (3)where PStack is the output power (W), VStack the output voltage (V), and The authorthe output current of the stack (A). 

Q9. Why are the PEMFCs more readily sized to fit different sites?

Because the PEMFCs are modular, UPS systems using them can be more readily sized to fit a wider variety of sites than thoseusing conventional generators [1] 

Q10. What are the theoretical analyses and experimental validations of the UPS system?

The theoretical analyses and experimental validations indicate that the developed power and energy management strategies in the UPS system with back PEMFC/SC/battery power sources are suitable for portable, backup and emergency power, and vehicles applications, and can guide the optimal design of the UPS system with hybrid PEMFC/battery/SC power sources. 

Q11. How can the power of the stack be calculated?

According to the real-time current and voltage measurements of PEMFC stack based on the loads of UPS, the power (ranging from 12 to 330 W) can be firstly calculated. 

Q12. What is the discharging voltage of a lead-acid battery?

Considering again a battery with open circuit voltage, which is 20 max( ) 1 QV t V Q = − , (34)and neglecting the internal resistance, then integrating equation (20), an implicit solution for( )Q t can be obtained as30 max max max1 3 Q QP V Q Q Q = − (35)Fig. 16 shows the discharging process diagrams of 3-cell lead-acid batteries when themain PEMFC fails, while the load of UPS is the rated power of 270W and the minimumdischarging voltage is 30V. 

Q13. What is the stoichiometric ratio of the air and hydrogen?

The air volumetric mass flow rate in SL/min is [14]22 4 608.22 OAir ref O SFINMQ ×⋅××= (6)where MAir is the air mass (kg) and 2OS is the required air stoichiometric ratio, whichis calculated theoretically as follows and selected within 20~30 by experience. 

Q14. What is the structure of a high frequency single-phase small-power UPS?

In this paper, in order to reduce the cost, improve the performance, and decrease the losses for the UPS system, a structure of grid/PEMFC/battery/SC hybrid power system is proposed in a high-frequency single-phase small-power UPS system as depicted in Fig. 

Q15. What is the air mass flow rate of the stack?

According to (7), (8), and (9), the required air stoichiometric ratio is222222 1)254.1(4N OO Opcell OOM rr MTc VFMS −+∆ −+ = (10)In the designed PEMFC generating system, the air mass flow rate guarantees the enough air supply for the stack, and the air supply and thermal controller keep the temperature of the stack in the range of the 46~55oC. 

Q16. What is the energy management strategy for a sustainable hybrid system?

Feroldi et al. [12] presented an energy management strategy for a sustainable hybrid system, which is based on wind-solar energy and bioethanol. 

Q17. What is the main structure of parallel structures for a PEMFC/battery/?

2. In Fig. 2, the outputs of PEMFC, batteries and/or SCs are linked in parallel, and the outputs of power and energy are controlled intelligently by power switches K0-K6 (thyristors) through the energy management and power control system.