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.


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.


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:

London Routemaster Bus info:

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)

Car Sick

Last week BBC Scotland showed a half hour programme introduced by David Miller called Car Sick. It looked at how, despite the rhetoric on climate change and sustainable living, people in Scotland are relying on the car more than ever.

Yet Scotland, like the rest of the UK, is increasing its spending on road transport. The largest road building programme since the 1970’s is underway across the UK, whilst public transport faces huge cuts. There is however no evidence that this money will improve our lives. In fact the evidence suggests it will do the opposite whilst costing us a lot more.

Air pollution is only one of five main categories through which we pay for road transport

Car Sick highlighted Air Pollution from exhausts and the cost to the health services from inactivity (£600 million in Scotland) as the main impacts of our car addiction. Using data from TRACCSEMEP/EEA and a report by Ricardo-AEA, Wattsthecost estimates exhaust air pollution from road transport costs the UK £1.5 billion per year and Non-exhaust emissions around £1 billion (figure 1). The RAC and Defra put these costs at between £4.5 and 10.6 billion. Yet Air Pollution is only one of five main categories through which we pay for road transport, not including climate change.

Breakdown of pollution costs in the UK
Figure 1 – Breakdown of air pollution costs in the UK

Figure 2 below shows estimates for external costs of congestionaccidentsinfrastructure and noise from different vehicles. These figures are huge underestimates when compared to the UK governments own statistics, which put the annual accident cost at £16.3 billion – 11% higher than the previous year and noise at up to £10 billion. Whilst these figures are varied and estimated, there is no doubt there are a huge range of hidden costs that we fork out for every day, simply because there is no convenient alternative.

TRACCS estimates car owners in the UK are paying an average of £2300 a year to run a car. The Wattsthecost external cost estimations shown above amounts to each person paying £4000 a year of our taxes essentially subsidising the problems they cause. The average income tax contributions are a little over this at £5000. People Baulk at the cost of annual travel cards in London, which in 2016 range from £972 to just over £4000. Yet London is one of the few cities in the UK with reliable public transport alternatives. In most, an amount equivalent to the upper fare range is coming out of the governments, businesses and our own pockets in funding lower property prices, increased hospital admissions and time lost in gridlock, because driving is seen as the only option.

Car Sick interviewed Klaus Bondam, mayor of Copenhagen from 2006 to 2010 (who in 2012 featured in the short film Brussels Express, discussing Europe’ then most congested city: Brussels (knocked off the top in 2014 by London)). He highlights that Copenhagen cannot afford not to invest in public transport and cycling, and points out one of main factors in Copenhagen’s rejection of the car is the absence of a Danish car industry and the powerful lobbying forces that entails.

Having spent 5 years living in Vienna, I can also attest to the benefits of living without a car. Despite paying around double the income tax I would on the same income in the UK, an annual transport pass costs a measly €366 a year (€365 for the bus, underground, overground train and tram network and €1 for the comprehensive city bike scheme). This enabled me to easily travel to and from work, home from a night out, to the shops and to training in the evening. The city authorities no doubt have a huge subsidy bill, but businesses who need the roads can do their jobs efficiently, the road repair bill is lower with fewer cars fatiguing the surfaces meaning less tarmac going to landfill and overall less casualties to deal with.

In Scotland and the rest of the UK we are essentially all paying a fortune to trip over parked cars, listen to incessant traffic noise, worry if our car has been broken into, if the meter has run out or when the next massive repair bill will be due and waste hours of our week sitting in traffic and driving around finding parking spaces, but are absolutely adamant that this is the best and only option.

External transport
Figure 2 – Cost estimates for Congestion, Accidents, Infrastructure and Noise taken from INRIX, TRACCS, EMEP and Ricardo-AEA.

Non-Exhaust Emissions

The health effects of particulate matter have become increasingly recognised. Headlines last summer over diesel manufacturers emission fraud have led us to believe that exhaust emissions are the main problem. Yet non-exhaust emissions actually contribute just as much if not more to the problem than the much talked about vehicle emissions.  The figure below shows the estimated annual emissions of PM10 and PM2.5 from exhaust (E) and non-exhaust (NE) from a 1km radius in central Bristol.

Non-exhaust emissions are composed of dust from road abrasion, tyre, brake and engine wear. Using data from the European Environment Agencythe handbook of external costs of transport and transport data from 2010wattsthecost estimate non exhaust PM2.5 and PM10 cost the UK nearly £1bn in health effects.

A recent report questions the safety of artificial football pitches made from old car tyres. It highlights the potentially lethal hidden compounds in “rubber” tyres. Figure 2 shows the elements contained in their dust. Arsenic, Cadmium, Lead and even gold are included. This combined with brake, engine and road surface wear creates a cocktail of particles, which not only do we all inhale but also contaminates 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, the tax payer has to pay for them to be sent to landfill where they slowly turn into Methane, a Greenhouse gas 84 times more warming than CO2.

So, what can we do about this? The simple solution is to use lighter, smaller vehicles. A bicycle also has tyres and brakes made of the same materials as that on cars, but the forces involved are a fraction of that in a car and negligible compared with a HGV or Bus, so far less material is worn and re-suspended from the road.

A ban on older Vespa Mopeds in the Italian City of Genoa that don’t meet modern emissions regulations has recently made the headlines. However the Italians, whilst known for their love of the petrol engine, are far more sensible with their private transport choices, opting for smaller vehicles.

Why is it we don’t all pedal round 5 seater cars? Because the energy to shift 1000kg to work and back is phenomenally greater than to move 20kg. So why are we using fossil fuels to do the same?

Next week wattsthecosts takes a closer look at vehicle efficiency and weight.


Vehicle Weight

This week Parkrun was in the headlines as a local council decided it should contribute to the upkeep of the paths it was held on.  The council cites £60,000 it has to spend in park path maintenance as the reasons behind the change in stance.
If councils want to save money on infrastructure, cracking down on people running around parks isn’t the first place they should start.
In the south west it is estimated it would cost £1.1 billion to get roads back into an acceptable condition. The UK as a whole spends over £4 billion on road infrastructure damage, more than three quarters of which is from the damage caused by passenger cars and HGVs (figure 1, taken from

Roads are often perceived as the cheaper more flexible option, aside from their “carbon emissions”, but when it comes to the environment there is more than just exhaust emissions to consider. As discussed in wattsthecost’s previous blog, the majority of particulate matter emissions from road transport are from non-exhaust emissions, of which dust from road abrasion is a significant contributor.

When a road wears down, cracks up and can no longer be patched, it goes to landfill. Some reuse options are available, but like car tyres the product deteriorates after each cycle.

A few weeks ago it was reported that autonomous trucks will be trialled on the M6 as part of a drive to “cut traffic congestion”. This seemingly highly sophisticated technology is similar to a 19th century British Invention: the train.

But even in the 1800’s train/tram technology was far superior to these autonomous trucks. In 1885 Frank Sprague invented the regenerative electric motor, made famous with its introduction to Formula 1 more than 100 years later in 2009. Coupled with solid steel wheels on hard tracks, Sprague’s trams used comparatively tiny amounts of energy.

In addition to energy savings, Kintetic energy recovery (KER) using electric motors means we don’t have to use friction to brake at all, eliminating the dust and metal waste produced by disc brakes. Trains and trams run on solid steel wheels, producing a fraction of the dust of a soft car/bus tyre and these wheels and tracks can be melted down and completely re-used once their life is over.

Despite the efficiency of vehicles improving by around 20% since 2005 a car still only converts around 17 – 21% of the energy in the oil into motion. Compared with an electric motor which can convert upwards of 90%.

The association of American rail roads estimates a train can, on average, move 1 ton of freight 479 miles on 1 gallon (4.54 litres) of fuel. Using trains cargos far heavier and of similar sizes to trucks regularly reaching over 2km, in some cases 6.5km in length can be transported with only one driver.  No autonomous technology is needed, although in recent decades modern electronics have made rail travel one of the safest methods we have for moving people and goods around. In the UK trucks, buses and coaches are responsible for 1 in 5 road deaths despite making up only 1 in 50 vehicles on the road.

Up until the second world war Bristol had a tram network that would rival any modern continental city. However the main supply cables were destroyed by a Luftwaffe bomb and like most UK cities it embraced the motorcar.

Despite the huge potential energy and air quality improvements coupled with the associated cost savings to the public, Bristol city council considers trams too expensive. Instead Metrobus, a dedicated route for a low efficiency combustion powered bus is perceived as the cheap option, simply because less upfront physical infrastructure has to be installed.

Britain is one of the few countries in Europe to have double decker buses in city centres. This is because we use them as our primary public transport mode, when other major cities use rail, and it is costing us a fortune.


Noise is something we deal with and accept as part of everyday life. But do we really appreciate the impact it has on our life?
Here are some of the negative impacts noise can have (EEA, 2014b):

  • Sleep disturbance
  • Cardiovascular and physiological effects
  • Mental health effects
  • Annoyance
  • Cognitive impairment, particularly in children
  • Impacts on wildlife
  • Economic impacts

Road traffic is the most dominant source of environmental noise, affecting an estimated 125 million people (>55 decibels (dB) Lden, contributing to at least 10,000 premature deaths, over 900,000 cases of hypertension and 43,000 hospital admissions in Europe every year (EEA 2014b).

In 1996 the EC presented its “Green paper on future noise policy” (EC 1996). This estimated that due to external costs such as reduced house prices and possibilities of land use, increased medical costs and loss of productivity in the workplace, noise was having a negative impact ranging from €13 million to €30 billion on the EU economy.  The World Health Organisation has identified it as the second largest environmental health risk in Western Europe (WHO 2011).
In the UK the government estimates the cost of urban road noise is £7 to 10 billion. Similar to the cost of road accidents (£9 billion) (Defra 2014).
In Sweden the social cost of road noise was put at SEK16 billion, rail SEK 908 million and aircraft noise SEK 62 million (EEAb 2014).

Below are the external costs of different modes of transport during the day and night in Urban, Suburban and rural areas, calculated using a combination of EEA external cost estimates combined with TRACCS road usage data.

Click here for Defras transport noise modeling tool.


Infrastructure external costs reflect the damage to the road which heavy vehicles inflict. These effects can differ by country type, road type and vehicle class.

Chart showing the contribution per vehicle type to total marginal infrastructure costs in Austria using traffic data from 2010 and cost data from 2014.

Road Accidents

How do Road Accidents cost us money?

  • Lost Output – Loss of productivity capacity of an individual as a result of an injury or fatality.
  • Medical and Ambulance – cost of medical facilities and personnel.
  • Human – pain and distress felt by the accident victims and relatives. Costs based on peoples Willingness to Pay (WTP) for reductions in risk of occurrence.
  • Police – time spent attending and reporting accidents by police officers.
  • Insurance and administrative – based on the average time spent processing insurance claims, plus overheads and expenses.
  • Damage to property – this includes damage to vehicles and other third party property.

Source: A valuation of road accidents and casualties in Great Britain: Methodology note by the Department for Transport

Global burden of road traffic accidents

  • Nearly 1.3 million people die in road crashes each year, on average 3,287 deaths a day.
  • An additional 20-50 million are injured or disabled.
  • More than half of all road traffic deaths occur among young adults ages 15-44.
  • Road traffic crashes rank as the 9th leading cause of death and account for 2.2% of all deaths globally.
  • Road crashes are the leading cause of death among young people ages 15-29, and the second leading cause of death worldwide among young people ages 5-14.
  • Each year nearly 400,000 people under 25 die on the world’s roads, on average over 1,000 a day.
  • Road crashes cost USD $518 billion globally, costing individual countries from 1-2% of their annual GDP.
  • Road crashes cost low and middle-income countries USD $65 billion annually, exceeding the total amount received in developmental assistance.
  • Unless action is taken, road traffic injuries are predicted to become the fifth leading cause of death by 2030

Source: Association for Safe International Road Travel (2015)

Table below: Marginal accident cost estimates, €ct/vkm (based on 2010 prices)
Taken from Table 12 of Update of the Handbook on External Costs of Transport, Ricardo-AEA, 2014

Accident Costs

Wind Lifecycle Costs

Estimated lifecycle emissions of wind energy are put at 34.1 gCO2e/kWh (Nugent and Sovacool, 2014).

Resource inputs and technology

Lifecycle CO2e emissions are significantly higher for geared onshore turbines than for gearless and offshore counterparts. Turbines with gearboxes require significantly more stainless steel and reinforced concrete than lighter synchronous models. Despite increased materials to reach the seabed and larger nature of offshore turbines, their emissions intensity is found to be lower. (Nugent and Sovacool, 2014).


An average wind speed of 7.5 m/s compared to 8.5 m/s is estimated to have a 4 gCO2e/kWh lifecycle emissions difference. Clearly not the most important factor when compared with sizing and longevity (Nugent and Sovacool, 2014).


The lifecycle emissions trend with respect to turbine life is approximately 40 gCO2e/kWH for a 20 year lifespan, 28.53 gCO2e/kWh for 25 years and 25.33 for 30 years (Nugent and Sovacool, 2014). It is important that any such figure accounts for the increase frequency of maintenance and grid curtailment of the turbine as its lifetime.
A recent study of UK wind farms found that the output decreased by 1.6% +/- 0.2% for every year of operation, meaning they could operate for up to 25 years and possibly beyond for modern machines (Staffell and Green, 2014).


Some studies have estimated transportation to be as high as 28% of the total lifecycle emissions of wind energy (Nugent and Sovacool, 2014). Very often getting the large components to site can be difficult – in some cases requiring helicopters to lift kit. However in the offshore industry manufacturing locations are increasingly being sited next to the coast enabling the large components to be taken straight to site.


As with PV and other renewable technologies that rely on hardware, the biggest factor in manufacturing emissions is the energy mix used. When it is a renewable energy mix, the emissions are very small. Using a Brazilian electricity mix (eight times lower than the global average at 64gCO2/kWh) gives manufacturing emissions of 7.1 gCO2e/kWh and using 566gCO2e/kWh gives 9-11 gCO2e/kWh (Nugent and Sovacool, 2014).

Sizing and Capacity

The laws of physics dictate that larger rotor diameters and taller hub heights (generally meaning higher wind speeds) will give exponentially higher power outputs than their smaller counterparts.

P = 1/2 ρ E A v3
P = power (W)
ρ = density of air (kg/m3)
E = efficiency of the turbine (e.g. 20% is 0.2)

A = area wind passing through perpendicular to the wind (m2)
v = wind velocity (m/s)

Studies have found that 20 x 5kW turbines have lifecycle emissions of 42.7 gCO2e/kWh, 5 x 20kW 25.1gCO2e/kWh and a 100kW 17.8gCO2e/kWh.

The current record for size is the 8MW Vesta V164 turbine installed off the Danish coast, generating 192,000 kWh in a 24 period (Vestas, 2014).

Click here for a map of some of the largest wind installations in the world.

And information on how wind energy is forecast

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