Scenario analysis of a bioethanol fueled hybrid power generation system
© Wu et al.; licensee Chemistry Central Ltd. 2014
Received: 27 November 2013
Accepted: 14 February 2014
Published: 4 March 2014
Regarding most of FC/PV/Battery based hybrid power generation systems, the photovoltaic (PV) power usually dominates the main power supply and the water electrolyzer is used to produce hydrogen. The energy efficiencies of these hybrid power generation (HPG) systems are usually low due to the low conversion efficiency of PV cell and the extra power consumption to hydrogen production. To reduce the electricity demand by the PV system and improve the energy efficiency of hydrogen production unit, a scenario-based design of the HPG system is necessary.
This paper proposes an EFC/PV/Battery based hybrid power generation system to meet 24-hour power demand. An ethanol-fueled fuel cell (EFC) power generator not only dominates the main power supply, but also a combination of the stand-alone EtOH-to-H2 processor and PEMFC can ensure higher energy efficiency. A PV system is treated as an auxiliary power generator which can reduce the (bio)ethanol consumption. A backup battery not only stores excess power from PV or EFC, but also it can precisely satisfy the power demand gap. Finally, scenario analysis of the hybrid power generation (HPG) system in regard to the hybrid power dispatch and energy efficiency is addressed.
An optimized fuel processing unit using ethanol fuel can produce high-purity hydrogen. The simulation shows that the stand-alone EtOH-to-H2 processor not only guarantee the high energy efficiency, but also it can continuously produce hydrogen if the fuel is enough. According to scenarios for the daily operation of the HPG system, the EFC power dominates the power supply during the night, the PV system dominates the power supply during the day and the backup battery aims to instantly compensate the power gap and store the excess power from PV or EFC. According to the hybrid power dispatch, the distribution of the HPG system efficiency is specified.
KeywordsHybrid power system Scenario analysis Ethanol-fueled fuel cell system Energy efficiency
Recently the interest in renewable and sustainable power generation techniques such as photovoltaic, wind, and fuel cell powered systems has grown. The off-grid hybrid power system is increasingly popular for remote area power generations, even though the photovoltaic (PV) or wind system is highly dependent on weather conditions and locations. Muselli et al.  developed a stand-alone hybrid power system which was composed of the PV panels, an electrical generator using fossil fuels, and battery storage, and Elhadidy  studied the feasibility of using wind/solar/diesel energy conversion systems to meet the power demand of the specific community in Saudi Arabia. Notably, a gasoline or diesel engine is usually treated as the backup power generator.
The PV system generates electricity during daylight hours, but the fuel cell system produces electricity as long as hydrogen is supplied. To address the clean power generation system, Hwang et al.  proposed a solar/fuel cell hybrid power system to meet the daily load demand of a typical family in Taiwan, Uzunoglu et al.  investigated dynamic models and simulation of this kind of hybrid power generation systems with appropriate control strategies, and Zervas et al.  provided an optimal decision framework for managing the hybrid energy systems. Similarly, these hybrid systems all involve the water electrolyzer for producing hydrogen. The electrolyzer is simpler and does not require complex units, but there is no solution to reduce its large electricity consumption. For most of hybrid systems, the PV panel is larger so as to carry out water electrolysis by using the excess power. Therefore, the overall energy efficiency is quite low .
The hydrogen fuel cells have been considered the most promising for automotive application, but the guidelines for the hydrogen storage, safer operation and transportation are quite rigorous . The fossil fuel processing unit could produce the high purity of hydrogen [8, 9], but the external energy supply and large carbon emissions are not avoided. If biomass can replace fossil fuels as the feedstock, the net-zero greenhouse gas (GHG) emissions of hydrogen production systems can be achieved. Liquefied ethanol is a popular feedstock because it is easily made by fermentation, no compressed storage is needed, and lower operating temperatures are required for reforming processes [10, 11].
In this paper, an ethanol fueled HPG system is proposed. The modeling, optimization and design of the EtOH-to-H2 processor is achieved by using Aspen Plus. The scenario analysis of the HPG system in regard to the daily load demand of a typical family in Taiwan  is investigated in Matlab. Since the EFC power unit cannot rapidly meet the load demand due to complicated chemical reactions in the ethanol reforming process, the lithium-ion battery is considered to meet the power gap when the excess power from PEMFC and PV units have been completely stored in the battery in advance. According to prescribed hybrid power dispatch, it is verified that the energy efficiency of the proposed HPG system is superior to the other renewable hybrid energy system.
Hybrid power generation system
where T ESR is the temperature of the ESR reactor, P i (i = C2H5OH, CO, H2O, …) is corresponding partial pressure, and R is the universal gas constant. Moreover, the one-dimensional, pseudo-homogeneous model is built by the Aspen Plus. The assumptions for the steady-state simulation of the ESR reactor include gas phase reactions, plug-flow reactor, and no pressure drop. The thermodynamic properties of some species are evaluated by using the Peng-Robinson equation of state. Other parameters include the fluid density = 0.02 kmol m-3, catalyst weight = 5 kg, and sizes of the reactor with the diameter and length of the reactor D = 0.5 m and L = 1.5 m.
The profiles of H2/C2H5OH vs. at different TESR,in is depicted in Figure 3(a). It shows that the maximum hydrogen yield can be achieved by adjusting the water flow at constant ethanol flow. The profiles of H2/C2H5OH vs. (S/C)|EtOH at different TESR,in is depicted in Figure 3(b). Similarly, it shows that the maximum hydrogen yield is obtained by adjusting the ethanol flow at constant water flow. The corresponding operating ranges are bounded by and 600K ≤ TESR,in ≤ 800K. Notably, the manipulation of ethanol flow can ensure the larger hydrogen yield than the water flow at the same TESR,in, and the increase of TESR,in cannot induce the high hydrogen yield. According the heat recovery design, the waste gas temperature at the outlet of heating jacket decreases when the preheated temperature of ESR increases. The low reactor temperature would reduce the conversion of ethanol reforming reactions.
According to the sensitivity analysis by adjusting one variable (S/C)|EtOH, the optimization tool of Aspen Plus is employed to determine the optimal operating conditions, = 2.14 at TESR,in =600 K. The steady-state simulation of the system at optimal conditions is shown in Figure 4, where the flowrate of hydrogen product can achieve 55.62 kgmol/hr, i.e. H2/C2H5OH =4.38 and the ethanol conversion can achieve 95%.
PEM fuel cell system
0.065 mol s− 1atm− 1
0.065 mol s− 1atm− 1
178 × 10− 4 cm
8.314 Jmol− 1K− 1
0.035 × 232 F
should be satisfied by increasing the inlet flow rate of oxygen () or air flow .
Remark 2: The stack requires a circulating water system to keep the relative humidity of membrane as well as regulate the stack temperature. It is assumed that the stack system is isothermal, the relative humidity is kept at a desired level, and the hydrogen flow in the inlet of fuel cell (). In fact, the fuel processing delays, including reaction time and transportation delay exist such that the power gap between supply and demand is inevitable.
1000 W m− 2
1.3854 × 10− 23 J K− 1
2.96 Am2W− 1
− 8.6 × 10− 4 m2W− 1
0.0037 K− 1
1272.3 AK− 3
Hybrid power dispatch: Figure 8(a) shows that the PV and PEMFC systems are dispatched to precisely meet the daily household load demand . In our design, the daily PV power distribution shown in Figure 8(b) is fixed according to prescribed patterns of solar irradiance. A scenario shows that both PV and PEMFC are integrated to instantly cope with the peak power. Especially, the EFC power should dominate the main electricity supply during the day and night.
Power gap compensation: If the EtOH-to-H2 processor cannot produce pure hydrogen immediately due to slow reactions, then the EFC power decay at this moment. It is caused by the fuel processing delay such that the power demand gap (desired Pfc-actual Pfc) shown in Figure 8(b) appears. Assumed that the rapid response of Li-ion battery can instantly compensate the power demand gap and the battery charging from PV and EFC units is always larger than the battery discharging, Figure 9(a) and 9(b) show that the HPG system can precisely meet the daily load demand. In this perfect scenario, the hybrid power dispatching shown in Figure 9(a) is confirmed where the battery contribution (green area) aims to compensate the power demand gap during each time period. Figure 9(b) shows that the battery is recharged by the excess power from PV and PEMFC during day and night.
Regarding hybrid power dispatching problem, the PV unit (blue bars) provides 34.5% total power, the battery (green bars) is 12.4% and the EFC (red bars) is about 53.1%. Notably, the battery is charged from the EFC is about 56.1% and 43.9% from the PV. It implies that the PV power is used to save 39.9% fuel consumption.
Photovoltaic efficiencyAccording to the formulation of solar cell efficiency limits , the PV efficiency is described as(32)
According to above efficiency of each unit, the (overall) system efficiency of HPG system is expressed as
An optimized fuel processing unit using ethanol fuel can produce high-purity hydrogen. The simulation shows that the stand-alone EtOH-to-H2 processor not only guarantee higher energy efficiency, but also it can continuously produce hydrogen if the fuel is enough. According to scenarios for the daily operation of the HPG system, the EFC power dominates the power supply during the night, the PV system dominates the power supply during the day and the backup battery aims to instantly compensate the power gap and store the excess power from PV or EFC. According to the hybrid power dispatch, the distribution of the HPG system efficiency is specified.
a f , ideality factor
A fc , effective fuel cell area, cm2
A s , surface area of the solar panel, m2
C dl , Double layer capacitance, F
E0, Reference solar radiation, W m− 2
E g , Gap energy voltage for silicon, eV
E s , solar radiation, Wm-2
i, current density, Acm-2
I s , terminal current of PV cell, A
k an , k ca , flow constant of anode and cathode, mol s− 1atm− 1
l m , Membrane thickness, cm
c p , specific heat capacity, J K− 1kg− 1
P i , partial pressure of i component, atm
P BPR , back pressure, atm
P, cell power, W
T a , ambient temperature, K
T0, reference junction temperature, K
T NCOT , normal cell operating temperature, K
R s , series resistance, Ω
R sh , parallel resistance, Ω
u, superficial velocity, m s− 1
V s , terminal voltage of PV cell, V
The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract No. NSC 100-2211-E-006-264.
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