Energy Recovery Methods

Vishwakarma Institute of Technology, Bibwewadi, Pune

Hybrid and Electric Vehicles

Prajwal Darade - 56, Dhanashree Daware - 57, Atharva Deore - 61, Roshan Dhadiwal - 71, Rushikesh Dhanedhar - 73

Guided by - Prof. Dr Dattatray. B. Hulwan

Energy Recovery Methods

Introduction

Energy recovery is any approach or procedure for minimizing energy input to an overall system by exchanging energy between the overall system's subsystems. Energy can take any form in each subsystem, though most energy recovery systems exchange thermal energy, which can be sensible or latent. In some circumstances, energy recovery necessitates the use of an enabling technology, such as diurnal or seasonal thermal energy storage (STES, which allows heat or cold to be stored between seasons). Waste heat from air conditioning equipment, for example, is collected in a buffer tank and used to assist with nighttime heating.

Energy Storage Systems

Electrochemical batteries are the most common energy storage technology utilized in electric vehicles today. Lead-acid, Ni-MH, Li-ion, Na-NiCl (ZEBRA), and Zn-O2 (from "Electric Fuel") are the most commonly used and also in the experimental stages at the moment. The last one relies on mechanical charging rather than electric charging. The specific energy, specific power, cycle life, and cost of all of these electrochemical batteries will be compared. Batteries, ultra-capacitors, and flywheels are all active energy sources in BEVs.

2.1 Batteries

The following are some of the battery requirements for electric vehicles: • Enough energy storage to achieve a desired driving range. •A high-enough input power capability to provide good acceleration, good regenerative braking for high-energy efficiency, and fast charging for vehicle convenience. • Long enough life to fulfil the industry standard for automotive component life. • Longevity in the face of environmental stresses (e.g., climatic stress, mechanical stress, etc.) so that EVs can operate in severe settings where conventional vehicles would not. • Tolerance to abuse in order to maintain the battery safe in extreme situations (e.g. overcharge, internal short-circuits, etc).

Fig 1. Li-Ion Batteries

2.2 Ultra Capacitor

An ultracapacitor, also known as a supercapacitor or electrochemical capacitor, is a device for storing electrical energy that is rapidly rising in popularity. A single ultracapacitor cell, like a battery, is made up of a positive and negative electrode separated by an electrolyte. Ultracapacitors, on the other hand, store energy electrostatically, like a standard capacitor, rather than chemically, like a battery - the electrolyte is divided by a dielectric separator, much like a capacitor.

This shape allows for a substantially better energy storage density than a traditional capacitor due to the narrow gap between electrodes. While an ultracapacitor stores less energy than a battery of comparable size, it can discharge it considerably faster because the discharge is not dependent on a chemical process. Ultracapacitors can be reused many times without degradation since no physical or chemical changes occur when charge is stored.

Figure 2. Schematic diagram of an ultracapacitor cell. Image Credits NREL.gov

2.3 Flywheel

Flywheel energy storage systems (FESS) use kinetic energy stored in a rotating mass to store energy with minimal frictional losses. The mass is propelled to speed by an integrated motor-generator that uses electric energy. The energy is released by bringing down the kinetic energy using the same motor-generator. The quantity of energy that can be stored is related to the moment of inertia times the square of the angular velocity of the item. The flywheel must spin at the fastest possible speed to maximise the energy-to-mass ratio. While dense materials may store more energy, they are also vulnerable to stronger centrifugal forces and may be more prone to failure at lower rotational speeds than low-density materials. As a result, material density is less important than tensile strength. Steel flywheels with speeds up to 10,000 rotations per minute are used in low-speed applications.

Fig 3. Structure & Components of FESS [2]

Types of Energy Recovery Systems

3.1 Regenerative Braking System (RBS)

3.2 Kinetic Energy Recovery System (KERS)

3.1 Regenerative Braking System (RBS)

Regenerative braking technology recycles the energy generated by the braking process by charging the battery, which can then be used again. In a regenerative braking system, energy lost while braking is transferred from the rotating axle to the generator, which is then transferred to the battery, saving energy.

Fig 4. Regenerative Braking System

3.1.1 Construction & Working

In a regenerative braking system, the motor that drives an electric car also acts as a brake. A dual-function electric motor is used in the system. It functions as a motor in one direction and as a generator in the other. When used as a motor, it converts electrical energy into mechanical energy and drives the wheels. However, when braking, it reverses direction and changes into an electric generator. When the brakes are applied on an electric or hybrid car, the electric motor reverses direction and enters generator mode.

In regenerative braking, complex electrical circuits are employed to select whether the motor rotates in the forward or reverse direction. Capacitors can be used by manufacturers to store electrical energy for later use. It is tremendously helpful to have a fully charged battery, particularly in electric automobiles. It enables them to travel longer distances. In the case of hybrid automobiles, it also aids in improving mileage and lowering pollutants.

Fig 5. Working of RBS

3.1.2 Regenerative Suspension System

A regenerative shock absorber turns parasitic intermittent linear motion and vibration into useful energy, such as electricity. Shock absorbers of the past merely dissipate this energy as heat. When utilized in an electric car or hybrid electric vehicle, the shock absorber's electricity can be redirected to the vehicle's motor, extending battery life. Electricity can be used to power accessories such as air conditioning in non-electric automobiles. Several new systems have recently been developed, albeit they are still in the early stages of development and have yet to be placed on production cars.

Fig 5. Regenerative Suspension System

3.1.2.1 Electromagnetic

In 2005, a patent for such a gadget was filed. To create energy, a linear motor/generator made up of a stack of permanent magnets and coils is used. Tufts University continued to develop this system, which was then licensed to Electric Truck, LLC. According to preliminary research, this method can recuperate 20 percent to 70 percent of the energy ordinarily lost in suspension.

Swinburne University of Technology developed a device that generates energy for storage in standard batteries by using DC electromagnetic machines as dampening elements. This method made use of a gadget that worked in a similar way to a'step-up' (boost) DC-DC converter. The design allowed the system's energy conversion efficiency to be optimised, as well as the damping coefficient of the damper to be controlled, allowing the system to function as a semi-active damper.

3.1.2.2 Hydraulic

Hydraulic pistons drive fluid through a turbine connected to a generator in a system developed at MIT. The system is controlled by active electronics that optimise damping, resulting in a smoother ride than a conventional suspension, according to the developers. They estimate that simply switching their trucks, a huge corporation like Walmart could save $13 million per year. A team from New York State University has created another system.

3.1.3 Advantages

This braking system will improve the fuel economy of the car.
It allows friction-based brakes to be used.
It helps to keep the battery charged longer.

3.1.4 Disadvantages

To manage the regeneration, additional equipment is required.
The expense of maintaining both the equipment and the machines is substantial.

3.1.5 Applications & Efficiency of a System

Because the technology works best on big, off-road vehicles that are travelling swiftly over rugged terrain, the companies are focusing on military applications.
Because commercial trucks carry high loads and little amounts of movement in shock absorbers can generate a useful quantity of electricity, this method can be used in the trucking business.
eROT System from Audi:
On normal German highways, power generation ranges from 100W to 200W.
On uneven roads, up to 600W of electricity can be generated.
Enhanced productivity:
Conventional automobiles account for 2% to 3% of total vehicle sales.
Military vehicles can get a discount of up to 6%.
Hybrid automobiles can save up to 8%.

Fig 6. Kinetic Energy Recovery System (KERS)

Kinetic energy recovery systems (KERS) are systems that are utilized in Formula 1 vehicles (for example, race cars) to collect kinetic energy and store it for later use. It works by turning the energy of motion (which would have been lost as heat if the automobile didn't have a recovery system) into electrical energy, which is stored in a battery, supercapacitor, or mechanical energy in a flywheel. The driver can then press a button on their steering wheel to discharge the battery to drive shaft action, boosting the vehicle's power.

3.1.6 Methods to Improve Amount of Regenerative Energy

Because of the motor capacity and the current limit of the battery, regenerative braking in EVs and HEVs is limited. As a result, mechanical as well as regenerative electric brakes must be used.
To boost regenerative energy, the motor and battery capacities must be increased; but, due to cost constraints and inverter capacity limitations, this is difficult to do.
As a result, without modifying the power train system, the regenerative energy is improved by enhancing the braking mechanism.

3.1.6.1 Averaging the Deceleration Method

When the car is decelerated, the described method averages the deceleration. (For example, in Japan, the JC08 mode is used to measure fuel consumption).
By averaging the deceleration, the proposed model has a smaller deceleration than the JC08 model.

Fig 7. Averaging the Deceleration

By averaging the deceleration, the proposed model's deceleration is half that of the previous model's maximum value.

Fig 8. Deceleration vs Time

As a result, not only is the deceleration reduced by two-thirds, but the maximum demand power is also reduced by two-thirds.
As a result, even if the battery's input current limit limits regenerative braking, the suggested model's regeneration energy is higher than the JC08 model.

Fig 9. Power Demand vs Time

3.1.6.2 Experimental Results (for i-MiEV)

○Limitation of the regenerative brake:

■The regenerative torque is limited to 24 Nm.

■This is because “i-MiEV” does not control the regeneration brake associated with the mechanical brake, which results in difficulty of the precise braking control.

■Therefore to keep the safety also to avoid much de-acceleration the regenerative torque is limited.

Fig 10. Regenerative Torque vs Motor Speed

3.2 Types of KERS

3.2.1 Electronic KERS

3.2.2 Mechanical KERS

3.2.3 KERS in Formula 1

Fig 11. KERS

3.2.1 Electronic KERS

An electric motor / generator unit (MGU) fitted to the engine's crankshaft captures braking rotational force in electronic KERS. This MGU transforms kinetic energy to electrical energy, which it then stores in batteries. The boost button then activates the batteries' electrical energy to power the MGU. How to store electrical energy is the most difficult component of building electronic KERS. A lithium battery, which is effectively a huge mobile phone battery, is used in most racing systems. Because batteries get heated while they're charged, many of the KERS cars include more cooling ducts because they'll be charged many times during the race. Super-capacitors can be used to store electrical energy instead of batteries; they run cooler and are perhaps more efficient.

3.2.2 Electro-Mechanical KERS

Energy is not stored in batteries or super-capacitors in electro-mechanical KERS; instead, it is stored kinetically by spinning a flywheel. This device functions as an electromechanical battery. In a racecar, space is limited, so the unit is tiny and light. To generate sufficient energy density, the flywheel spins at rates ranging from 50,000 to 160,000 rpm. The spinning flywheel is contained in a vacuum atmosphere, which reduces aerodynamic losses and heat accumulation. This system's flywheel is a magnetically loaded composite (MLC). Because it is woven with high-strength fibres, the flywheel retains one piece at these high speeds. Metal particles incorporated in the fibres allow the flywheel to be magnetised permanently as a permanent magnet.

The flywheel will work in the same way as an MGU. When current flows from the stator, the flywheel can generate a current in the stator, releasing electricity, or it can spin like a motor. This flywheel is used in conjunction with an MGU attached to the gearbox, which transfers electrical energy from the road to the flywheel and back to the gearbox at the push of a button to accelerate the vehicle. Permanent magnet flywheels are not employed in all electro-mechanical KERS flywheels. Instead, two MGUs are used in these systems, one near the flywheel and the other near the gearbox. To store energy, some systems combine flywheels and batteries.

3.2.3 Mechanical KERS

The mechanical KERS system features a flywheel as the energy storage device but it does away with MGUs by replacing them with a gearbox to manage and transmit the energy to and from the driveline. The kinetic energy of the vehicle end up as kinetic energy of a rotating flywheel through the employment of shafts and gears. This method of storage, unlike electronic KERS, eliminates the need to convert energy from one form to another. When compared to mechanical storage, each energy conversion in electronic KERS incurs its own set of losses, resulting in a low overall efficiency. A continuously variable transmission (CVT) is used to deal with the constant change in speed ratio between the flywheel and the road wheels, which is controlled by an electro-hydraulic control system. When the gadget is not in use, a clutch permits it to be disengaged.

Fig 12. Schematic Diagram of KERS

4. Conclusions

1.Mechanical KERS is more Efficient.

2.Electronic KERS are used in F1 cars Mostly including Red Bull, Toro Rosso, Ferrari, Renault and Toyota.

3.Batteries replaced by Super/Ultra Capacitors in EKERS.

4.Regenerative Suspension Systems can be effective in saving energy loss at suspensions and can increase battery life of EVs in the long run.

5. References

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Efficiency Improvement of Regenerative Energy for an EV (Takuya Yabe 1 , Kan Akatsu 1 , Nobunori Okui2 , Tetsuya Niikuni2 and Terunao Kawai2 1 Shibaura Institute of Technology 2 National Traffic Safety and Environment Laboratory.)
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Energy Recovery Methods