Introduction
Electric vehicles represent an alternative to internal combustion engine cars for reducing fossil fuel dependence, preserving the environment, and contributing to carbon emissions reduction [
1,
2,
3,
4]. Electric vehicles offer various configurations depending on the power source, with battery electric vehicles (BEV) representing the most popular option [
5,
6,
7].
An alternative solution for the electric vehicle (FEV) is the fuel-cell electric vehicle (FCEV), which operates under a hydrogen supply basis [
8,
9,
10]. Despite BEV being the most extended version of FEV, fuel-cell electric vehicles are gaining popularity as fuel-cell research is improving and car manufacturers are offering new models with competitive characteristics compared to BEVs [
11,
12,
13,
14].
Battery electric vehicles have been extensively studied and characterized and continue improving to achieve higher performance, longer lifespan, and extended driving range, one of the principal users’ worries [
15,
16,
17,
18]. Despite the low implementation of fuel-cell electric vehicles in the automobile market, the new advances in fuel-cell research and technology make them an alternative for the future electric vehicle since hydrogen is abundant, and the residue, water vapor, does not generate environmental impact [
14,
19].
Hybrid configuration in electric vehicles is not new since battery electric vehicles assisted by photovoltaic energy, called vehicle integrated photovoltaics (VIPV) [
20,
21,
22,
23], already exists in commercial versions [
24,
25]. Nevertheless, this hybrid configuration is not very popular due to the low energy density of PV panels compared to the battery and the complexity of the hybrid design. Despite these drawbacks, researchers continue investigating how to improve the PV-BEV’s performance, especially the driving range [
26].
Hybrid BEV-FCEV does not exist in commercial versions but is the subject of many research works, currently under the battery/fuel-cell/ultra-capacitor topology [
27,
28,
29]. The great advantage of this configuration resides in the benefitting effects of using the battery for high power demand and fuel cells for medium and lower power requirements when a high-rate battery and a fuel cell are combined to supply energy for vehicle traction [
30].
The hybridization of fuel cells and PV panels is barely studied. A prospective research deals with hybrid photovoltaic fuel cell/battery bank system [
31,
32,
33]. Nevertheless, a system including a built-in electrolyzer is not yet explored, maybe due to the technology complexity of installing an electrolyzer connected to the PV panel for in-situ hydrogen generation. Among the problems derived from the use of fuel cells in electric vehicles, we can mention the hydrogen storage system, which represents a security problem if operated in pressurized tanks [
34,
35,
36,
37,
38]. We can partially solve this problem by reducing the tank pressure; however, the lower the tank pressure, the shorter the FCEV autonomy.
This work proposes hybridizing a PV panel, a battery, and a fuel cell to power electric vehicles. Each power source is responsible for supplying energy under specific conditions, so they complement each other to optimize the hybrid system operation. The basis of this new idea is assembling the three systems in one, which makes the electric vehicle run under variable driving conditions for optimum performance, longer driving range, and higher durability.
Hybrid System Characteristics
-
a)
Photovoltaic panel
Electric vehicle motor operates at a voltage between 360 V and 480 V, although recent models elevate the working voltage to 800 V [
58,
59,
60,
61]. Power sources should match operating voltage to avoid energy losses in the voltage conversion process; indeed, the higher the voltage conversion rate, the higher the energy losses [
62,
63,
64].
Since photovoltaic panels currently operate in the range 12V to 48 V for a single panel, matching the electric motor operational voltage requires arranging in series PV panels until reaching the specific voltage; in our case, the number of panels in series is:
Vel and
VFV are the electric motor and photovoltaic panel working voltage.
In our case, considering an electric motor of 480 V, and a 48V PV panel:
PV panel standard size depends on output maximum power [
65,
66]. Considering that the average power density is constant, around 200 W/m
2, for a vehicle with an average rooftop size of 2.5 m
2, we can install a panel of 500 W [
67]. However, since the vehicle rooftop is not flat, the most adequate solar panel is a curved one (
Figure 3), which increases the effective area due to its curvature [
68].
Modern curved solar panel has a curving factor of 0.78, on average. Therefore, the effective area is:
This surface allows a PV panel of 640 W.
If we use the car hood for PV panel installation, the effective surface increases by a factor of 1.8, thus allowing a PV panel system of 1152 W.
The energy generated by a PV system depends on the peak sun hours (psh) value, which depends on the location and day of the year. Since location depends on latitude and day of the year on declination, we should know these two parameters to determine the energy generation by the PV system.
A current practice to simplify the calculation is using the typical day of the month, thus reducing the whole year to twelve values, one per month [
69].
Figure 4 shows the evolution of the energy generated by the rooftop and hood car photovoltaic installation. We consider the average declination value for the entire month.
We observe that photovoltaic energy generation moves in the range 2.6 kWh to 6.6 kWh. Considering an average latitude of 30º, and averaging the energy generation for the entire year, we obtain 5.2 kWh.
-
b)
Lithium battery
Lithium batteries are responsible for vehicle traction at the most demanding power-driving conditions, acceleration and uphill road; therefore, we can determine the battery energy capacity from the required maximum dynamic force and average vehicle speed. Mathematically:
The maximum dynamic force corresponds to the route segment where the vehicle is accelerating and/or climbing an uphill road:
m,
a,v and
<v> are the mass, acceleration, current and average vehicle speed,
κ and
μ are the drag and rolling coefficient, and
α is the road slope.
Expressing the drag coefficient in terms of aerodynamic coefficient:
Cx is the vehicle aerodynamic coefficient,
Af is the front area, and
ρair is the air density.
A light-duty vehicle, including all the elements configured in
Figure 2, weighs approximately 1900 kg [
70]. This value differs depending on the manufacturer’s model, although the weigh reduces by 200 kg if we select the appropriate fuel cell system [
71].
Considering a moderate driving pattern, characterized by an acceleration of 1.75 m/s
2 [
72], an aerodynamic coefficient of 0.29 [
73], a rolling coefficient of 0.015 [
73], and an average road slope of 2% (α=1.1º) [
74], for average vehicle speed of 40 km/h in urban route [
75], we have:
The required maximum power for a peak vehicle speed of 70 km/h, a current traffic limit in many urban areas [
76], is:
This power corresponds to 105.3 CV (103.9 HP), which matches many light-duty vehicles’ engine power of similar characteristics [
77].
Electric vehicle battery supplies the required energy for the electric engine. Nevertheless, to obtain the battery power and energy capacity, we should apply energy losses derived from transmission system and battery discharge; therefore:
In current conditions, transmission and battery discharge efficiency is 0.855 [
78,
79,
80,
81,
82] and 0.95 [
83,
84]; replacing in Equation 10 and using data from Equation 9:
If we apply the WLTP protocol [
85] to determine the time interval the vehicle submits to the maximum power requirement, we obtain a time fraction of 8.494%. Retrieving data from the literature, the average daily trip distance is 35 km [
86]; therefore, for an average speed of 40 km/h, the daily travelling time is:
If we apply the battery fraction use to the daily traveling time, the daily battery time use is:
The required energy from the battery during this working time is:
Equation 14 provides the theoretical minimum battery energy capacity to cover energy demand for acceleration and uphill road.
Batteries, however, suffer from capacity modification depending on discharge rate according to the expression [
87]:
Cr and
Cn represent the real and nominal battery capacity. Coefficients
a and
b are empirical constants depending on battery type; for lithium-ion a=0.9541 and b=0.0148 [
87].
Battery energy capacity depends on battery capacity and voltage as:
Because the battery voltage remains constant:
Replacing values from our prototype, we have:
Combining Equations 15, 16, and 18 and considering the battery voltage, the battery capacity results:
We must determine the battery discharge rate, X
C, to evaluate if the battery stands for the maximum required power; applying data from our prototype:
A discharge rate of 12.3 is fully compatible with lithium battery performance.
-
c)
Fuel cell
Fuel cell power for electric vehicles depends on the type of cell used for electricity generation; in our case, we use an electrolyzer whose power range is 5 W to 250 kW [
88], matching the required maximum power for our prototype. An alternative is a methanol reformer unit.
According to chemical reactions, the methanol reformer is more efficient since it produces three hydrogen moles per methanol unit, while the electrolyzer only produces two hydrogen moles per mole of water. Nevertheless, considering a fuel tank, methanol, or water of identical volume, the electrolyzer is more efficient since it produces 55.6 moles of hydrogen when using the electrolyzer and only 49.5 moles in the case of a methanol-reformer.
The gap in hydrogen generation comes from the difference in density between methanol and water, 792 kg/m
3 and 1000 kg/m
3 [
89,
90]. Considering methanol and water molecular mass [
89,
90]:
For a conventional light-duty vehicle 40 liters tank:
We consider hydrogen as an ideal gas, occupying 22.7 liters per mole.
Fuel cell operates when battery is deactivated, and vehicle is not working on energy recovery mode. Previous work studied the required energy distribution associated to inertial, drag, rolling and weight forces [
91]; the results analysis shows that the vehicle only requires 20% of the maximum power to stand for drag and rolling forces. Therefore, the fuel cell should have a supplying power of:
The energy supplied by the fuel cell is:
Which corresponds to the following daily hydrogen flow:
eH2 is the hydrogen specific energy.
Replacing data for our prototype:
Considering the energy required by the fuel cell and an electrolyzer efficiency of 0.8 [
92], the necessary power to make the electrolyzer work,
Pely, is:
On the other hand, retrieving efficiency data from commercial electrolyzer units [
93,
94,
95], we obtain:
We observe the good agreement between the two calculation methods, within 93.5% accuracy, which proves the quality of the calculated value. Comparing data from Equations 29 and 30 with the expected power demand from the fuel cell (Equation 25), a deviation of 8.6% occurs, which is attributed to inaccuracy in calculating the fuel cell operating time fraction from WLTP protocol.
Energy Balance
For a daily cycle, the proposed prototype requires a total power of 107.8 kW to operate the battery and fuel cell, with a combined energy capacity of 20.54 kWh. The solar photovoltaic system generates 5.2 kWh daily.
Table 1 resumes the power and energy hybrid system characteristics.
We realize that the photovoltaic system cannot stand for battery or fuel cell power or energy demand; besides, even with a charged battery, the combined energy of the PV system and battery cannot match the fuel cell energy demand. Therefore, since we cannot enlarge the PV system size or reduce the fuel cell energy requirements, we should increase the battery capacity to compensate for the system energy unbalance.
Since the energy imbalance is 3.51 kWh, the new battery capacity should be:
The extended energy capacity is 9.8 kWh. Super-index o accounts for the extended capacity.
Nevertheless, to prevent sudden lack of energy due to cloudy days, the battery extended capacity should cover the PV energy supply, thus having a capacity of:
The new extended energy capacity is 15.0 kWh. Super-index FV,o indicated battery extended capacity for cloudy days.
The total energy balance, however, should include the battery and the fuel cell since the PV system is an additional power source not currently included in the vehicle powering system; computing these three elements results in 51.4 kWh. Comparing this value with the average conventional battery energy capacity over 120 different EV models for the same driving conditions and driving range [
96], corresponding to a value of 51.6 kWh, we have a 0.4% deviation, which is negligible.
The high agreement between reference and calculated battery energy capacity shows that the design of the hybrid system is coherent and compatible with driving conditions.
Driving Range Evaluation
Hybrid power system intends to improve energy efficiency, enhance battery and fuel cell management, and optimize the use of PV panels. This goal also aims to extend the vehicle driving range as a result of energy use optimization and efficiency increase.
Driving range evaluation depends on the power source use, battery, or fuel cell. Since we design the prototype with the fuel cell as a regular power source for low energy demand and the battery as an auxiliary source for high power requirements, we determine the driving range from the combined use. To do so, based on data from
Table 1 and considering a 45-liter tank to supply water to the electrolyzer, which is the standard size for light-duty vehicles [
97,
98], we obtain the following driving range:
The driving range associated with battery use depends on the vehicle energy consumption rate under battery driving mode and the battery energy capacity. In modern electric light-duty vehicles, the average energy consumption rate is 164.96 Wh/km [
99,
100,
101,
102]; therefore, for a battery of 15 kWh like the one proposed for our prototype, we have:
ζr accounts for the energy consumption rate, in Wh/km.
The driving range in Equation 33 corresponds to the standard average energy rate. Nevertheless, we used for our hybrid system moderate driving conditions, which are not the standard ones for the average energy consumption used in the calculation of conventional battery energy capacity, since in moderate driving pattern, the energy consumption is 10% lower, on average, than the standard value [
103,
104]; therefore, the control unit protocol uses a value of 140.2 Wh/km [
105], which results in the following driving range:
Applying a more conservative driving pattern extends the driving range by 16.1 km, 17.6% longer driving distance.
Repeating the process with the fuel cell, considering the fuel (water) tank size, the hydrogen flow required to make the fuel cell work, the cycle operating time, and the daily travelled distance:
Now, replacing values from our prototype characteristics:
This value is within the standard range for commercial fuel cell electric vehicles of similar characteristics, according to manufacturer data [
106,
107]. Compared to the average value reported by the literature [
108,
109], 520 km, we realize that our prototype shows a slightly longer driving range, 8.7 km, meaning a 1.7% longer distance.
Applying the reduced energy consumption rate, which corresponds to the operating conditions for the prototype performance simulation:
The driving range is extended by 93 km, 17.5% longer travelled distance, the same percentage as for the battery system.
We can compare the driving range for the proposed topology with the one for an electric vehicle powered by a lithium battery or a fuel cell.
Table 2 resumes the results of this comparison.
The average, maximum, and minimum values are obtained from the manufacturer technical sheet for different models of light-duty electric vehicles of similar characteristics to our prototype [
110,
111].
We can evaluate the driving range gain, simulating an electric vehicle operating with two independent power sources, a battery, and a fuel cell. The working time of every power source is the same as for our prototype. Since the battery operates for 12% of the total time and the fuel cell for the remaining 88%, the simulated electric vehicle driving range is obtained by applying the fraction time to the driving range shown in
Table 2, thus obtaining (
Table 3):
We realize that the combined use of battery and fuel cell extends the driving range compared with the battery or the fuel cell option, which proves the enhancement of the electric vehicle performance operating under the hybrid power system basis.
Economic Analysis
Implementing a dual battery and fuel cell hybrid system as a power source for electric vehicle traction, plus installing photovoltaic panels on the vehicle rooftop and hood, may represent a significant investment related to the conventional topology of a single battery or fuel cell unit.
Flexible PV panels are more expensive than flat ones, but the continuous growth of the photovoltaic market reduces the manufacturing cost and the gap between flat and flexible PV panel prices [
112,
113,
114,
115].
Fuel cell cost is higher than lithium batteries [
116,
117]; however, new advances in fuel cell research forecast a promising future cost lowering [
118,
119]. A recent study predicts a drop in fuel cell cost by 2025 and a continuous dropping until 2030 [
120]. Nevertheless, according to actual prices, a fuel cell costs around 400 USD per kW [
121,
122], although the price depends on the production rate [
123].
Lithium-ion batteries, which are the most widely used for electric vehicles, have shown a dramatic declining cost in the past decades, from an initial price of nearly 8000 USD per kWh in 1990 to 100 USD per kWh in present days [
124], with expecting lowering to less than 40 USD per kWh in few years [
125].
Considering the current prices for lithium batteries and fuel cells, the total cost of a single battery or fuel cell installation compared with a hybrid system like the one we propose in this work for equal power and energy capacity is shown in
Table 4.
We realize that the hybrid system is cheaper than the fuel cell but more expensive than the lithium battery. Nevertheless, since we reduce the frequency of battery recharge due to extended driving range for a standard lifespan of 15-20 years [
126] and considering the daily traveled distance, 35 km, from the simulation, the number of battery recharges for every case, battery, and hybrid system is:
Considering an average electricity price of 0.2095 €/kWh for the European market [
127] and 0.1272 USD/kWh (0.1351 €/kWh) for the American market [
128], the additional cost for battery recharging is:
Now, adding this additional cost to the initial investment, we have (
Table 5):
Analyzing the results from
Table 5, we realize the hybrid system is cheaper for the European market by 1158.91 €, representing a relative cost saving of 14.8%. For the American market, the cost is similar, with a moderate cost saving of 214.58 € (227.90 USD), representing a reduction of 2.6%.
Conclusions
Hybrid battery and fuel cell power sources for electric vehicles represent a feasible and reliable alternative to current topologies. The PV panel implementation on the vehicle rooftop and hood helps to increase the energy supply and reduce the risk of sudden energy breakage and vehicle stops.
Hybrid system optimum performance is achieved when running the electric vehicle on battery for high power demand like medium or high acceleration processes and uphill road segments and on fuel cells for low power requirements such as low acceleration steps, constant vehicle speed, and flat and downhill route.
Power system management is optimized through a control unit that commutes the power source from the battery to the fuel cell and vice versa, depending on driving and road conditions. The control unit decision is based on continuous information provided by specific installed sensors (solar radiation, vehicle speed, and road tilt).
Electric vehicle performance is enhanced due to the optimized power source control, resulting in an extended driving range. The driving range gain related to an electric vehicle equipped with a dual independent battery and fuel power source block depends on the energy consumption rate of this latter vehicle configuration.
Driving range extension may reach up to 276.3 km, with a minimum and average gain of 127.2 km and 237.8 km. This driving range extension represents a significant improvement in the electric vehicle’s performance, increases user confidence in the electric car, and reduces the number of recharges of both the battery and the fuel cell.
The proposed hybrid system topology for electric vehicles is cheaper than a single fuel cell power source of equivalent energy capacity by 17.2%; the hybrid system is more expensive than the battery power source system if we consider only the initial investment. Nevertheless, if we analyze the total cost, including the battery recharge price for the whole battery lifespan, the hybrid system saves money, 2.6% for the American market and 14.8% for the European one.