Week 9 Challenge

Week 9 Challenge

Energy Management system

Aim:

To design the energy management strategy model with battery, ultracapacitor and fuel cell for positive power and to distribute the power demand among these components.

Explanation:

Energy Management Strategy:

Energy management strategies are the algorithms that decide the power split between engine and motor in order to improve the fuel economy and optimize the performance of HEV’s. Energy management strategy is mainly required for the hybrid electric vehicle which uses more than one energy storing elements. It is mainly used to distribute the load demand among the energy storage elements to operate the vehicle in the high efficiency mode.

Every energy storage element has the optimum operating range where they can deliver the power at high efficiency. If battery, fuel cell and ultracapacitor are used in a vehicle as energy storage elements. Then it is high priority to use the energy management system.

Battery cannot be able to supply the high power for long time and sudden peak load. For high power demand, the battery pack size and cost also increase. If the battery is used to supply the high power for long time, it leads to degradation of battery. It is not favorable condition. Hence battery can be operated for low power demands more efficiently.

Ultracapacitors cannot be used efficiently to deliver the low power demands and medium power demands for long time. Ultracapacitors complement a primary energy source which cannot repeatedly provide quick bursts of power, such as an internal combustion engine, fuel cell or battery. But it can be efficiently used to meet the sudden high peak load and to use the high regenerative current for charge.

Fuel cells only be operated in the optimum operating zone for high efficiency. If it is operated below the operating range (low power region) it is in activation loss region where the efficiency is very less. If the fuel cells are operated above the optimum operating area in mass concentration loss region (high power region) the efficiency is very less. Only if the fuel cells are operating in ohmic loss region (medium power region) the efficiency is high. Hence fuel cells are used to deliver the medium power loads.

Flow chart:

The above flow chart explains the condition for load sharing by energy management systems. It is used to design the Simulink model for energy management system.

Software used:

• MATLAB/ Simulink

Blocks Used:

• Constant
• Signal builder
• Switch
• Inerval test
• Gain
• Greater than
• Equal to
• AND
• OR
• Add
• Goto
• From
• In
• Out
• Scope

Model:

Energy management System subsystem:

Procedure:

• The input to the energy management system is power demand which is provided by the powertrain.
• Here for simplification and understanding, we use the signal builder for the power demand.

Signal Builder:

• The SOC of ultracapacitor and battery can be given manually to analyze the result and for better understanding of the working algorithm.
• If the power demand is positive, that is used for propulsion.
• The load demand sharing algorithm has to be used to deliver the load.

Positive power Subsystem:

• The operating range for the fuel cell is 23kW to 55kW. It is derived from the polarization curve.
• The active area of fuel cell is 420cm^2.
• The operating range starts from current density of 0.24A/cm^2 and ends at 0.76A/cm^2.
• The average voltage of each cell is 0.6V.
• If 380 cells are used in series, then then total voltage is 380*0.6 = 228V.
• Lower limit of current = 420*0.24 = 100.8A
• Upper limit of current = 420*0.76 = 319A
• Lower power limit = 228*100.8 = 22.9kW
• Upper power limit = 228*319 = 72.7kW
• At 72.7kW, the efficiency is less.
• So the upper power limit is taken as 55kW for high efficiency.
• If the power demand is low i.e., in our case if it is less than 23kW and if SOC is greater than minimum value of battery then this power has to be delivered by battery.

Battery backup subsystem:

• If the SOC is less than minimum value, then this power has to be delivered by fuel cell.

Fuel cell backup Subsystem:

• If the power demand is between 23kW and 55kW then this power can be delivered by fuel cell.
• If the power demand is above 55kW and if SOC of ultracapacitor is greater than minimum value then this power has to be delivered by ultracapacitor.
• If the SOC of Ultracapacitor is less than minimum value, then this power has to be met by battery.
• And if both SOC of battery and ultracapacitor is less than the minimum value then that power has to delivered by fuel cell.
• For the negative power i.e., regenerative braking, if the SOC of ultracapacitor is greater than battery then the regenerative power is used to charge the battery.

Negative power subsystem:

• If the SOC of ultracapacitor and battery is equal then 0.5C charging is done for both ultracapacitor and battery.
• If the SOC of Battery is greater than ultracapacitor then the negative power is used to charge the ultracapacitor as the same way as battery.
• The model is simulated and visualized in the scope.

Output:

Scope:

If SOC_UC = SOC_batt:

Summary:

From the above plot, if both SOC_UC and SOC_batt are equal, then the negative power is shared equally between ultracapacitor and battery for charging. And the positive power is shared among the components according to the energy sharing algorithm. The low power (0kW-23kW) is met by battery, medium power (23kW-55kW) is met by fuel cell and peak power (>55kW) is met by ultracapacitor.

If SOC_UC < SOC_batt:

Summary:

From the above plot, if SOC_UC < SOC_batt then negative charge is completely used to charge ultracapacitor.

If SOC_UC > SOC_batt:

Summary:

From the above plot it is observed that, if the SOC_UC > SOC_batt, then the negative power is completely used by battery for charging.

If SOC_UC < 0.35:

Summary:

From the above plot, it is observed that, if the SOC of ultracapacitor is less than minimum value then the peak load cannot be met by ultracapacitor, so the battery is used to deliver the peak load and the negative power is used to charge the ultracapacitor.

If SOC_UC & SOC_batt < 0.35:

Summary:

From the above plot it is observed that, if the SOC of both ultracapacitor and battery is less than the minimum value of 0.35 then the total power demand is delivered by fuel cell.