Energy efficiency of trams

Trams are an expensive outlay for a city. They require permanent tracks and overhead wires. At least that’s how the figures can often appear initially, but there is more to it than basic installation cost. What of the energy required to operate, maintain and recycle our transport network. Once installed trams are one of the lowest energy and most sustainable mass transit systems a city can operate. Much lighter than trains, without the need for often cost prohibitive and energy-intensive underground tunnels that a metro system requires. Trams run on hard wheels and rails that can be fully recycled and have much lower rolling resistance than soft rubber tyres. They are plugged directly into the mains, negating the need for energy and resource intensive batteries that need their own separate and often more expensive charging infrastructure.

Below is a breakdown comparing trams and buses. Because the length of trams can vary, a single carriage, which is also comparable with the weight of a double-decker bus, has been used in these calculations. In reality, a three-car tram can carry as many as 140 passengers, standing and seating, compared with the new electric London buses, which propose to have a capacity of 90.

The main characteristics affecting energy consumption are rolling resistance, drive efficiency, drag coefficient and frontal area and weight.

Figure 1 – Breakdown of key parameters of buses and trams

Rolling resistance

The rolling resistance of a hard, almost inflexible tram wheel, on a similarly inflexible track has a Coefficient of rolling resistance of approximately 0.001, approximately ten times lower than a bus tyre (0.01) and as much as twenty times lower than a correctly inflated car tyre on asphalt (0.02).

A double decker bus travelling at 30mph along a smooth tarmac road would need 24.9 kW to keep it moving. The same bus running on tram wheels on metal tracks would need only 3.5 kW. More than 7 times the power.

Drive efficiency

An electric motor converts the electricity supplied to it into motion with around 90% efficiency. A diesel combustion engine loses 60% of the energy in the fuel (energy density 10 kWh per litre) as heat before it is converted into motion. That means the bus above, needing 24.9 kW of power to keep moving at 30mph, actually needs 62.2 kW of fuel.

An electric bus has an electric motor, just like a tram, so benefits from the improved efficiencies. However, it also needs to carry it’s own energy supply in the form of a battery. In theory a battery has minimal losses, however in practice charging with different currents can incur significant losses, up to 50% in some cases. On average for, the life of a battery the charge and discharge cycles are estimated to be approximately 80% efficient. That means using a battery adds in losses of 20% to our equation. 80% x 90% gives a drive efficiency of 72% for an electric bus.

A tram is effectively plugged into the mains and avoids these storage losses.

Weight

Trams weigh significantly more than buses. Per passenger, they need more energy to get moving from a standstill. If a single carriage tram (12 tons) plus 48, 76kg passengers (16 tons gross weight) undertook a journey of 5km with a stop at each kilometre, it would use approximately 2.15 kWh accelerating, compared to an electric bus (13 tons) plus 90 passengers (gross weight 19 tons), 2.96 kWh and a diesel (gross weight 18 tons) 6.05 kWh. However, per passenger, the electric bus comes out best at 33 Wh, compared to 47 Wh for the tram and 76 Wh for the diesel engine bus.

Summary of energy to travel 5km in kWh

Tram tracks also spread this weight. Installing a narrow, hard metal rail significantly reduces road break up compared with allowing buses to traverse the entire width of a road.

Drag coefficient and frontal area

Drag coefficient (Cd) refers to the “slippyness” of a shape through air. A flat fronted double decker bus is not particularly drag friendly and has a Cd of approx. 0.8. A modern tram is much smoother , alebit with the additional drag of the pantograph, but with a teardrop rear shape, is much lower at approx. 0.3 (estimated from studies on trains).

The frontal area (A) of a bus required to push through the air is approximately 12m2, with a tram almost half this at 6m2.

Together the two are multiplied together to give the drag area. 0.8 x 12 = 9.6 for a double decker bus, compared with 0.3 x 6 = 1.8 for the tram.

This, combined with the rolling resistance, means a trams energy expenditure once up to speed is a fraction of that of a double decker bus. For our 5km journey, the tram only uses 11 Wh cruising, whilst the electric and diesel buses use 49 Wh and 117 Wh respectively.

In the period 2016 to 2017, buses in Bath (all Diesel, and mostly double-decker) covered 7.09 million miles, 11.4 million km. Factoring up our 5km calculation (some stops will be closer, some further away) to give a very rough estimate. These bus journeys are using 2281987 x 15.44 kWh = 35.2 million kWh or 3.5 million litres of diesel fuel. If these routes were replaced with trams the figure would be 6.1 million kWh, nearly 6 times less.

Sustainability

Not only do trams wheels have a fraction of the rolling resistance of “rubber” tyres, they produce far less particulates. The particulates they do produce are particulates of Iron, known as FePM. One study on such releases from trains, show the majority are generated from the braking phase and exacerbated in confined areas. Particulate emissions from trams are a fraction of tyre clad automobiles on tarmac.

A flurry of news reports a few years ago questioned the safety of artificial football pitches made from old car tyres. It highlighted the potentially lethal hidden compounds in the “rubber” tyres, which contain traces of Arsenic, Cadmium, Lead and even gold. The various polymers tyres are made of have been found washing into water courses. Brake, engine and road surface wear also add to a cocktail of particles, which are not only inhaled, but also contaminate the land.

The vast Autumn crop of leaves that falls around the country has a “contaminated waste” classification from the environment agency. Instead of being sent to the nearest anaerobic digester to generate heat and electricity, or composting, they end up in landfill where they slowly turn into Methane, a Greenhouse gas 84 times more warming than CO2.ead of being sent to the nearest anaerobic digester to generate heat and electricity, or composting, they end up in landfill where they slowly turn into Methane, a Greenhouse gas 84 times more warming than CO2.

A car tyre starts it’s life with around 8mm of tread, weighing in the region of 7 and 15kg. It ends its life with a few mm of that tread worn, weighing almost the same. Car tyres are one of the most difficult items to “recycle”. Detaching steel bead from plastic and textile is not easy. These days it is common to shred them and potentially give them a second life. However, this is not recycling. Once rubber crumb as it is known, is laid on a sports pitch, that will be it’s final use, with landfill the only viable option after that. Novelty goods can also be made out of car tyres, but they themselves are difficult to recycle and so the cycle ends.

Similarly tarmac roads eventually crack and require replacement. Once dug up most road surface ends up in landfill and virgin material used to replace. A few bespoke applications are available, but these are far from the closed loop they are made out to be. The recent trend of adding plastics, such as car tyres to asphalt will only make the task of dealing with the final product even more complex.

Trains and trams with solid steel wheels running on steel tracks will also reach the end of their lives with not much less material worn off as tyres have. However the wheel and track can be melted down and completely recycled. As with most metals, this can be carried out repeatedly, without deterioration of material quality.

Regenerative Braking

In 1885 Frank Sprague invented the regenerative electric motor, made famous with its introduction to Formula 1 more than 100 years later in 2009. With Sprague’s system, a tram would use the electric motor to brake, generating electricity which was sent back down the overhead line to the supply.

In the days of Sprague’s trams, the electrical supply was connected directly to the electricity source. With modern grids, it is not possible for trams to “regenerate” back to the grid. The only case where it will work is if another tram is in the vicinity. However, other solutions are available, such as a small battery on the tram to assist with acceleration from a station, or even a flywheel at the tram stop, meaning the tram has no weight penalty.

An extreme example of regenerative braking in action is the Iron Ore train in Sweden, which transports coal from Kiruna to Narvik. The iron ore is mined at 530m above sea level and transported to the port of Narvik. During the journey down, the electric motor is used to brake instead of friction brakes and the energy stored in a system of batteries on board the train. This power is then used to return the train and empty wagons on the 3 hour journey back up to Kiruna.

Batteries and Hydrogen

Although battery and hydrogen powered buses and even trams themselves can seem attractive, there is a significant energy penalty, not only in manufacturing the battery, but also charging and discharging. In fact all vehicles have to be charged somewhere. The charging infrastructure necessary to supply all the energy a bus needs for one day is not insignificant. Try “filling” an 18 kWh bus battery using a domestic standard mains supply and it would take days. Upgrades to the grid can be extremely expensive and not always possible.

Similarly the best hydrogen manufacturing processes are still only approximately 65% efficient. Hydrogen is three times less dense than petrol/diesel, meaning three times the journeys with tankers are needed to move it around. In the future the current gas network may be upgraded to pipe this hydrogen around, but until then the only energy source available on tap is electricity.

If we are planning to stick around our cities for at least the next few decades, wiring in steel wheeled shod vehicles makes sound energy and ultimately economic sense.

References and working out

Bombardier Tram info: https://en.wikipedia.org/wiki/Bombardier_CR4000

London Routemaster Bus info: https://en.wikipedia.org/wiki/New_Routemaster

Drag energy (Joules) = 0.5 x density of air (kg/m3) x drag coefficient (CdA) x velocity (m/s)3

Kinetic Energy (Joules) = Energy lost in Braking (Joules) = 0.5 x mass of car (kg) x velocity (m/s)2

Rolling resistance (Joules) = Coefficient of rolling resistance (Crr) x (gravitational constant (9.81) x mass (kg) x Velocity (m/s)

1000 Joules (J) = 0.27777 Watt Hours (Wh)

Energy (Wh) = Power (W) x time (hours)

Average weight of person = 76.9 kg (source: bbc)

Density of air = 1.3 kg/m3

Gravitational constant = 9.81 m/s2

Energy in 1 litre of petrol = 9.7 kWh (source: sewtha)

Coefficient of rolling resistance of wheels = 0.01 truck, 0.001 tram (source: engineering toolbox)

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