London’s metro network requires a significant amount of all the electricity needed in the UK capital; owner Transport for London is currently investing in new rolling stock and improvements to stations and infrastructure to increase capacity and efficiency. An S Stock train approaches Rayners Lane with a Metropolitan line service to Uxbridge. Image courtesy of Matt Buck / CC BY-SA 2.0.
As cities shift their primary fuel sources for urban transportation from fossil fuels to electrification, emissions are being reduced – not only direct combustion-related emissions (carbon monoxide (CO), carbon dioxide (CO2), and nitrogen oxides (NOx), but also other harmful particulate matter. While this is obviously a positive step towards improved air quality and addressing climate change, the next challenge is to make sure that we are using this electricity for traction power efficiently.
With increased usage, greater pressures are being placed upon our power supplies. Without careful management, the coming decades could see instabilities in the energy grid caused by an excess demand in peak hours. As electricity is the lifeblood of tramways, light rail and metro systems, these are real challenges that face cities around the world as they drive the shift towards electrically-powered travel modes.
The scale of demand
A 2008 Transport for London (TfL) report identified that the London Underground’s electricity consumption equated to 2.8%
of the capital’s total and 0.4% of all the energy used in the UK.1 Over 80% of this is related to traction power, forming a large
part of TfL’s annual GBP120m (EUR135m) energy bill.
In the Austrian capital, Wiener Linien’s network (metro, tram and bus) consumed 630GWh of electricity in 2017, costing the authority EUR60m.2 So managing electricity flow more efficiently offers possibilities for not only avoiding losses, but also reducing overall energy consumption. These can be significant, not only in terms of the impact on operational costs – which are linked to potentially volatile market fluctuations from year to year – but also in removing the need to place further loads on existing fossil fuel-driven power stations. Currently oil, gas and coal are still the world’s primary source of electricity, providing around 80% of our power.3
This demand will be exacerbated by the inconsistency of supply from renewable energy sources (although positive steps are being made in this area) and an increased need from industry, as well as the uptake in electric vehicles. More specifically, an increase in onboard battery sources for EVs, buses and light rail vehicles has the potential to cause huge peak demand on the energy grid. There are three key ways to address this issue: significant investment in new fixed grid connections, ‘smart charging’ to reduce peak loads and balance the load, and a requirement for greater energy efficiency and less ‘waste’.
Yet with the rapid growth in urbanisation allied to a growing demand for travel, how can operators carry more passengers while at the same time using less electricity – and in many cases without substantial rebuilding of decades-old infrastructure and vehicles?
In this article, we take a look at some of the technologies and strategies that are being implemented not only within existing light rail networks, but also with new-build systems.
Going 100% green
Ever since the first experiments with electrification of street railways around 140 years ago, the mode has improved its efficiency greatly. New materials and designs have resulted in lighter weight and more efficient components. This includes everything from improved insulation, to composite overhead support masts and improved construction of the contact wires they support. These innovations have simplified construction, and brought greater reliability so less energy is wasted and more
is ‘recycled’.
As more and more systems make use of renewable energy – 60% in the case of Vienna and 100% in the case of the trams in Greater Manchester and Melbourne – the arguments for light rail become even stronger. One of the great advantages often cited in favour of urban rail systems is that they are emissions-free at point of use, but increasingly many can now claim to be 100% green even back to grid level.
It is a happy coincidence that solar photovoltaic arrays typically output dc energy at 600-800V, while tramways and urban railways normally use 600-750V, so the cost of the power electronics needed to connect such arrays to dc traction networks are cheaper than creating new grid connections.
Co-operation for common goals
Cities and operators are also working closely with industry suppliers to drive efficiencies. One example is the pan-European E-LOBSTER programme that received European Union funding of EUR4m in June 2018. Bringing together a consortium of academic and commercial partners, this two-year programme seeks to address the common challenges of urban electricity distribution networks and light rail systems.
Currently operating largely independently but still sharing the need for reliable power supplies, E-LOBSTER seeks to find commonalities in the requirements of local power grids and rail operators to find integrated solutions to reduce losses, improve the stability of power grids and accommodate the needs of local electrical storage, while remaining mindful of the relatively new entrants to the urban mobility spectrum such as electric vehicles.
The work of the consortium focuses on the creation of new tools for real-time monitoring of losses and energy consumption of power distribution and rail electrification networks; the introduction of advanced power electronics to manage these interactions; development of a new real-time Railway-to-Grid/Grid-to-Railway management system using shared assets; and identification of suitable storage technologies to allow greater harmonisation of renewable energy sources, transferring knowledge from the automotive sector. Finally, a demonstration project using all this accumulated knowledge is planned for Metro de Madrid in 2020.
The project partners claim that better integration will result in a reduction of the losses at both the distribution level (claimed to be 5%) and at the rail network level (8%), while also improving the stability of the wider local power grid. They hope that such developments will lead to efficiencies when building and upgrading existing networks, leading to reduced operating expenditure and new opportunities for the enlargement of renewable energy capacity at tramway and metro stations.
The first results are expected in 2021, with Giannicola Loriga of project co-ordinator RINA stating that “the project will have a relevant impact on the growth of participating organisations, who will be able to deliver competitive products and services on the market in four to five years after completion of the project.”
Energy recovery and stability
From John Smith Raworth’s first experiments with regenerative braking control – where the electric motor also acts as a generator under braking – in the early years of the 20th Century, the amount of the energy recovered from braking has improved to 15-20% through the use of modern electronics and energy storage. While the use of regenerative braking on all forms of rail transport is now commonplace, it is increasingly – via developments in motorsport – making its way to the buses and road cars that many of us use today.
There are many innovative new solutions to increase this further and make better use of the power that is supplied back into the wires (or rails). One of these is promoted by Dutch start-up Hedgehog Applications. Its idea is to harvest the energy generated through regenerated braking of trains at Nederlandse Spoorwegen (NS) railway stations for use to power urban electric buses and bolster local power grids.
Maarten Klein Geltink described the situation at the recent EU LIght Rail conference in Brussels: “We are already seeing increased electrification in the bus sector… but the buses themselves won’t be the bottleneck, reliable energy supply to charge them will.”
This year the company hopes to implement its first ‘Energy House’ at the NS station at Apledoorn. The structure will include a battery and supercapacitor to not only convert the energy from braking trains, but also draw energy from local renewable sources to create a bi-directional smart grid connection for use by four electric buses and EV charging points at the station.
This stored energy is then fed to smart- chargers to make it available for electric bus charging points at the station. Mr Geltink added: “The entire system is free floating, which means that it operates at the same voltage as the overhead line, reducing the inefficient conversion of energy.”
The company argues that the proximity of railway stations to bus and tram termini is a key opportunity for the technology as there is often limited space in urban environments for new fixed grid connections.
The idea of such ‘reversible substations’ is already being used elsewhere. One product that has seen a wide degree of success is Alstom’s HESOP (Harmonic and Energy Saving Optimiser) technology. Introduced first on Paris’ T1 tramline in 2012 with positive results, HESOP is used to both supply traction voltage and recover 99% of the braking energy from vehicles, the majority of which is usually lost as heat. A 1500V dc version was installed on line 3 of the Milan metro in a pilot in 2017, co-funded by the European Commission’s Life+ programme that supports environmental initiatives.
During the braking phase, the recovered energy is used by other vehicles as a priority, with any surplus transferred to other sources in the immediate vicinity such as station lighting, escalators or air conditioning and then back into the grid after that. This has the added benefit of decreasing heat build-up in tunnels and confined spaces, reducing the need for investment in additional ventilation or air-conditioning equipment. With its increased efficiency, HESOP allows for the wider spacing of substations, reducing their number and the associated capital expenditure.
The results from Milan speak for themselves. Installed in the Rogoredo substation on line 3, the system recovered around 2MWh/day (a 15% reduction and equivalent to 171 tonnes of CO2).
A similar system implemented by Wiener Linien (WL) to recover energy from braking resistors on its U2 metro line saw inverters placed in the substation at Hardeggasse station in September 2014. Incorporating a double converter in parallel with the substation’s rectifier to the existing transformer, and advanced control mechanisms, the system – installed at a cost of EUR600 000 – constantly monitors energy transmission. It converts recovered power into a ‘clean’ ac source (i.e. reducing the harmonic distortion that can affect sensitive power control equipment and the flow of energy) and re-injects it to the three-phase grid. Over a three-year period, 1.6m kWh per year was recovered, a monthly average of 145 000kWh, and fed back into the medium-voltage network.
According to WL analysis, the benefits of the installation include a better consistency of current quality, easy integration with existing infrastructure and high reliability; the noted downsides were an increase of heat and noise within the substation.
An innovation from UK-based Ultra Electronics PMES has seen great success in the area of supply stability, a key issue for any urban rail operator. Its Mobile Traction Power Module (MPTM) is a portable substation that has been used by London Underground to supply energy in the event of transformer rectifier failure, to keep services running during routine maintenance, or even just to provide an additional boost to the system when required.
As a self-contained ‘plug and play’ unit, the 20-tonne MPTM can be delivered on the back of a lorry and features its own hydraulic legs to make it a freestanding structure. With input voltages configurable from 11kV and 22kV AC, it can provide 2MW-rated power at 750V or 630V dc and can be deployed in as little as two hours.
Flywheels and depots
But what do you do for lesser-used routes or at termini where ‘regen’ energy is often lost if no other vehicle is in the near vicinity to use it? One way of resolving this issue is through the use of a flywheel to convert the regenerated energy into kinetic energy for later use. The process is reversed and energy is released back into the wires when the voltage drops.
German tramway operator Freiburger Verkehrs AG installed the first such flywheel on its tramway at Wendeschleife Landwasser. It went into operation in 2013. A second was introduced on the stretch between Komturplaz and Gundelfinger Straße in December 2018. Weighing 1.9t, the one-metre flywheel rotates at up to 3450rpm. According to the operator’s calculations, approximately 250 000kw/hour can be saved through use of the flywheel per annum – roughly equivalent to the energy requirement of 65 households (or 1500t of CO2).
Making depots and control centres more efficient users of energy (and other natural resources) is also a route taken by many operators. Relatively simple and cost-effective interventions such as improved insulation and the addition of LED lighting reduce energy demand, while the usage of heat pumps and waste heat recovery can lead to significant long-term savings. Turning vehicles off where possible also realises significant savings.
Although depot and control centre sites’ electricity usage is less than 5% of a system’s overall energy bill on average, every little helps.
Article appeared originally in TAUT 979 (July 2019).