Electric cars: the future?
By Shehza Shafeek, Faye Zhao, Helienke Yoong and Leah Pettit
introduction |
Electric cars have been gaining a lot of momentum worldwide as the more efficient and environmentally-friendlier alternative to diesel and petrol fuelled cars. They work by replacing the gasoline engine with an electric motor and are powered by fuel cells. The electric motor gets its power from a controller which gets its power from an array of rechargeable batteries. Battery electric vehicles, or BEVs, use electricity stored in a battery pack to power an electric motor and turn the wheels. When depleted, the batteries are recharged using grid electricity, either from a wall socket or a dedicated charging unit.
With electrical vehicle sales in countries like China and Sweden increasing by over 100% in the past year alone, it is an industry that is expanding massively and raises the question: are electric cars truly more efficient than petrol-fuelled cars? We wanted to explore the environmental impacts and efficiency of electric vehicles compared to petrol-fuelled cars and, to do so, we conducted two investigations. The first was a calorimetry experiment; we combusted hexane to heat up water and calculated the enthalpy of combustion of hexane as a result. The second was an experiment using an electric motor to determine the energy used to raise a mass of 300g. This allowed us to compare the energy outputs of each fuel and, consequently, the efficiency of each fuel. |
calorimetry experiment
METHOD
- Set up experiment as shown in diagram
- Measure out 50mL of water using a measuring cylinder and pour into the copper can. Note the starting temperature of the water and place a lid onto the copper can to minimise heat loss.
- Measure the mass of the hexane spirit burner with the lid
- Light the spirit burner and let it heat the water for 5 minutes before extinguishing the flame.
- Measure the final temperature of the water
- Measure the final mass of the spirit burner with the lid
results
Analysis
Test 1
Thermal energy given out by the hexane = ΔQ = mcΔT
ΔQ = 0.00288 x 4.18 x 32
ΔQ = 0.385 kJ
Moles of hexane burnt = m/Mr
Moles = 2.88/86.18 = 0.03342 mol
Thermal energy given out per mol = 0.385/0.03342
Thermal energy given out per mol = 11.53 kJ mol-1
Test 2
Thermal energy given out by the hexane = ΔQ = mcΔT
ΔQ = 0.335 kJ
Moles of hexane burnt = m/Mr
Moles = 0.031 mol
Thermal energy given out per mol = 10.8 kJ mol-1
Test 3
Thermal energy given out by the hexane = ΔQ = mcΔT
ΔQ = 0.255 kJ
Moles of hexane burnt = m/Mr
Moles = 0.0277 mol
Thermal energy given out per mol = 9.20 kJ mol-1
Thermal energy given out by the hexane = ΔQ = mcΔT
ΔQ = 0.00288 x 4.18 x 32
ΔQ = 0.385 kJ
Moles of hexane burnt = m/Mr
Moles = 2.88/86.18 = 0.03342 mol
Thermal energy given out per mol = 0.385/0.03342
Thermal energy given out per mol = 11.53 kJ mol-1
Test 2
Thermal energy given out by the hexane = ΔQ = mcΔT
ΔQ = 0.335 kJ
Moles of hexane burnt = m/Mr
Moles = 0.031 mol
Thermal energy given out per mol = 10.8 kJ mol-1
Test 3
Thermal energy given out by the hexane = ΔQ = mcΔT
ΔQ = 0.255 kJ
Moles of hexane burnt = m/Mr
Moles = 0.0277 mol
Thermal energy given out per mol = 9.20 kJ mol-1
The efficiency of burning hexane in releasing heat energy in heating up water, as we measured, is equal to the average measured and calculated value for the enthalpy change of combustion of hexane, divided by the actual value of the enthalpy of combustion of hexane, which is -4163 kJ mol-1.
This is equal to ((11.53 + 10.8 + 9.2) x 1/3) / 4163 = 0.252%.
We note that the efficiency of burning hexane in a spirit burner in heating water was very low. This was because, as shown, a copper can had to be used in order to maximise heat transfer from the flame to the copper can, but, because copper is a conductor of electricity, this meant that there were high energy losses to the surroundings through the conduction of heat through the copper can. In addition, heat from the flame was lost through convection to the air, which would not have been the case in a petrol engine. Also, as shown, hexane did not undergo complete combustion, as we could tell from the formation of soot on the copper plan and the yellow flame.
This is equal to ((11.53 + 10.8 + 9.2) x 1/3) / 4163 = 0.252%.
We note that the efficiency of burning hexane in a spirit burner in heating water was very low. This was because, as shown, a copper can had to be used in order to maximise heat transfer from the flame to the copper can, but, because copper is a conductor of electricity, this meant that there were high energy losses to the surroundings through the conduction of heat through the copper can. In addition, heat from the flame was lost through convection to the air, which would not have been the case in a petrol engine. Also, as shown, hexane did not undergo complete combustion, as we could tell from the formation of soot on the copper plan and the yellow flame.
Electric motor experiment
METHOD:
- Set up the equipment as shown in Figure 1 and Figure 2 - set up the motor and pulley system, clamp the stands to the table to make sure they cannot shift, attach the ammeter in series in the circuit and the voltmeter in parallel. Take care to ensure that the pulley system and motor are correctly aligned.
- Check that the string is rolled around the pulley system in the right direction to allow the motor to work properly.
- Attach a 300g mass to the string and place the masses on the ground. Roll up the string around the line shaft until it is pulled taut.
- Measure 130 cm from the ground up using two one-meter rulers. Read the value off the ruler at eye-level to avoid parallax error and mark this height onto the stand using a board pen.
- Simultaneously switch the power pack on at 10 volts and start the stopclock
- Measure the current and potential difference using an ammeter and voltmeter respectively. Record these results.
- Once the bottom of the masses has reached the 130 cm mark on the stand, switch off the power pack and stop the timer.
- Record the time it took for the 300g masses to be pulled up 130 cm.
- Carry the experiment out a total of four times.
results
analysis
Because the energy supplied in an electrical circuit, E, is given by E = VIt, the energy inputs were calculated.
As efficiency is given by useful output/total input, it is necessary to calculate the useful output energy. This is given by the gravitational potential energy gained by the mass. Because the equation for gravitational potential energy, GPE, is GPE = mgh, the useful energy output is given by 0.3 x 9.81 x 1.3 = 3.8259J.
Therefore, the efficiency that we measured from the electric motor is equal to 3.8259/36.97 = 10.3%.
The efficiency we obtained for the electric motor, however, was much lower than that of electric motors in electric cars, which is between 85% and 90% (Hanley, n.d.). This is likely due to our experimental design, where there was significant room for energy loss. For example, there was work done in overcoming the friction in the system, and the elastic band used would sometimes slip.
As efficiency is given by useful output/total input, it is necessary to calculate the useful output energy. This is given by the gravitational potential energy gained by the mass. Because the equation for gravitational potential energy, GPE, is GPE = mgh, the useful energy output is given by 0.3 x 9.81 x 1.3 = 3.8259J.
Therefore, the efficiency that we measured from the electric motor is equal to 3.8259/36.97 = 10.3%.
The efficiency we obtained for the electric motor, however, was much lower than that of electric motors in electric cars, which is between 85% and 90% (Hanley, n.d.). This is likely due to our experimental design, where there was significant room for energy loss. For example, there was work done in overcoming the friction in the system, and the elastic band used would sometimes slip.
CONCLUSION
Despite our results giving efficiencies which are much lower than those given for petrol engines and electric motors used in cars respectively, our results did confirm the information online that electric cars are more efficient – our results showed that the electric motor was over 40 times more efficient than burning hexane in giving out useful work.
However, the amount of pollution released by an electric vehicle depends on how the electricity is made. Battery electric cars charged off the dirtiest coal-dominated grid still produce less pollution than their gasoline-powered counterparts. In addition, electric cars can take up to a whole night to recharge and the batteries can still go flat after a few hundred kilometres. The batteries themselves are also difficult to safely dispose of.
Nevertheless, we remain hopeful as, in the long-term future, there is evidence to suggest that electric cars will overtake petrol-fuelled cars in popularity and will be a huge factor in reducing greenhouse gas emissions and helping the environment. Work is actively being done currently to make the batteries recyclable. BEVs powered by renewable energy sources like wind or solar are virtually emission-free and electric vehicles can save 50-60% of greenhouse gas using the well-to-wheel method of calculation. When taking well-to-wheel emissions into account, all-electric vehicles emit an average of around 4,450 pounds of CO2 equivalent each year – by comparison, conventional gasoline cars will emit over twice as much annually. There are also no tailpipes so no tailpipe emissions – they don’t directly release greenhouse gases - and they have lower life cycle greenhouse gas emissions.The reduction of emissions that contribute to climate change improves public health and reduces ecological damage, and these are two goals that we must always be working towards.
However, the amount of pollution released by an electric vehicle depends on how the electricity is made. Battery electric cars charged off the dirtiest coal-dominated grid still produce less pollution than their gasoline-powered counterparts. In addition, electric cars can take up to a whole night to recharge and the batteries can still go flat after a few hundred kilometres. The batteries themselves are also difficult to safely dispose of.
Nevertheless, we remain hopeful as, in the long-term future, there is evidence to suggest that electric cars will overtake petrol-fuelled cars in popularity and will be a huge factor in reducing greenhouse gas emissions and helping the environment. Work is actively being done currently to make the batteries recyclable. BEVs powered by renewable energy sources like wind or solar are virtually emission-free and electric vehicles can save 50-60% of greenhouse gas using the well-to-wheel method of calculation. When taking well-to-wheel emissions into account, all-electric vehicles emit an average of around 4,450 pounds of CO2 equivalent each year – by comparison, conventional gasoline cars will emit over twice as much annually. There are also no tailpipes so no tailpipe emissions – they don’t directly release greenhouse gases - and they have lower life cycle greenhouse gas emissions.The reduction of emissions that contribute to climate change improves public health and reduces ecological damage, and these are two goals that we must always be working towards.