Energy Efficiency in Singapore’s Rapid Transit System
Energy Efficiency in Singapore’s Rapid
Transit System
Melvyn THONG and Adrian Cheong
Abstract
The Singapore Rapid Transit System (RTS) comprises the North South, East West and North
East lines, linking seamlessly to Light Rail Systems, namely, Bukit Panjang, Sengkang and
Punggol LRTs. After the Circle Line Extension opened in Jan 2012, the total rail length in
Singapore has reached 178km. The transport sector is one of the major energy consumers
in Singapore. With increasing global energy demand, greater effort is thus required to
make effective use of energy. In addition, the Singapore RTS network will double by year
2020. Hence, it is crucial that every effort is made in the design of the RTS to improve
energy efficiency in order to achieve a sustainable land transport system. This paper
describes the various strategies adopted in the design of the Singapore RTS to reduce
energy demand during operations.
Introduction
emphasis on effective use of resources.
Greater awareness of environmental issues
and rising fuel costs have made energy
efficiency the subject of much interest in
Singapore. In addition, ‘environmental
protection’ and ‘sustainable development’ are
buzzwords today in many global governmental
organisations. With ever growing demand
for natural resources in the face of economic
development, many environmental issues have
started to preoccupy countries around the
world. Efforts are being devoted to enhance
awareness of the importance of environmental
protection in both private and public sectors.
In Singapore, the Land Transport Authority
(LTA) has always been promoting the use of
public transport and supporting the green
movement, particularly in RTS. According
to the vision in the Land Transport Master
Plan (LTMP) in 2007, the RTS network will be
doubled by 2020 and this has placed greater
38
In Singapore, the Land Transport
Authority (LTA) has always been
promoting the use of public
transport as supporting the green
movement, particularly in RTS.
While most metro systems around the world are
essentially electric driven, there is a global trend
towards ‘greening’ the railways by adopting
measures, such as, renewable energy sources
for the train, avoiding ecologically sensitive
areas, ease of recycling train materials, the use
regenerative braking and the use of composite
materials for trains. Over the years, various
measures have been put in place in the RTS
design to make effective use of scarce energy
resources (Chew, T.C. and Chua, C.K. 1998),
(Windle, C J. 1986), (Thong, T.L., Ho, H.C. and
Sim, S.P. 2005). Technological innovations are
also analysed and meticulously mined for their
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Energy Efficiency in Singapore’s Rapid Transit System
energy conservation benefits. Such measures
have also resulted in reducing operating costs
in the long run.
The following sections depict the major
strategies adopted by LTA in the design of its RTS
network to improve energy efficiency and make
the land transport system sustainable. The areas
where these strategies are adopted include:
In the Singapore context, the DC traction
system (nominal voltage of 750VDC and
1500VDC) is adopted due to short interstation
distances. Transmission of traction power
along the track is achieved by means of a third
(conductor) rail laid close to the running rails or
via an overhead catenary line. The running rails
serve as the return rails (negative rails) for the
traction network to the various traction power
substations located along the RTS. 600VAC
systems with multiple conductor rails have also
been utilised for Singapore’s LRT systems. The
traction network for the RTS is supplied via the
internal AC HV network through the use of
diode rectifier systems. As the train accelerates,
or during coasting mode, power is delivered to
the train from the rectifiers via the third rail or
the overhead catenary line.
1. Electrical systems
Ideally, the most efficient type of train service
2. Lighting systems
pattern consists of rapid acceleration, coasting
period and period for braking at a high
deceleration rate. This type of service pattern
gives rise to the opportunity to save energy and
at the same time operate at the highest possible
speed. The regenerated energy from the trains
(when braking) comes in short bursts of high
intensity which greatly exceed the power
requirements of the train’s own auxiliaries and
quite often is not utilised by trains within the
vicinity. As such, the regenerated energy results
in excess energy which dissipates as heat.
Measures to improve energy efficiency require a
holistic approach as it spans multiple disciplines
and systems (for eg., mechanical and electrical
systems, rolling stock, civil design, etc.). To this
end, LTA has instituted many design efforts
in maximising energy efficiency and these
measures have been implemented across our
RTS infrastructure.
3. Air Conditioning systems
4. Platform screen doors
5. Rolling stock
6. Alignment – utilisation of hump profile
7. Escalator & Lift systems
8. Green Mark for RTS
LTA has instituted many design
efforts in maximising energy
efficiency and these measures have
been implemented across our RTS
infrastructure.
Electrical Systems
Both DC (direct current) and AC (alternating
current) traction systems are widely available.
Ideally, the most efficient type of
train service pattern consists of
rapid acceleration, coasting period
and period for braking at a high
deceleration rate.
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Energy Efficiency in Singapore’s Rapid Transit System
In Singapore, inverters are installed in the
RTS to recover the excess regenerative energy
from the braking of trains. This excess energy
is channelled back into the internal AC HV
network, which is then utilised by the station
auxiliary loads. The inverter consists of double
full wave thyristor bridge circuits which convert
the DC supply into AC supply.
Computer simulations are performed to
determine the optimum location of the
inverters (highest global energy recovered as
compared to the number of units of inverter
installed). Based on actual operation figures,
the inverters recover up to 5% of the total
energy used in the RTS, as shown in (Figure 1).
As an added advantage, the use of inverters in
Singapore’s RTS improves the receptivity of the
traction network and consequently reduces the
rate of wear on the train’s mechanical brakes.
This decreases the costs for train maintenance.
Lighting System
In the development of lighting concept
within a station, the importance of functional
lighting generally takes precedence over
other aesthetic requirements. The design of
the station lighting is in line with Singapore
Standard SS530 “Energy Efficiency Standard
for Building Services”. Strict lighting power
budgets are generally stipulated within the RTS
for different functional areas, which ensure
that energy efficient lighting is employed.
In the development of lighting
concept within a station, the
importance of functional lighting
generally takes precedence over
other aesthetic requirements.
In general, direct lighting through the use
of more energy efficient lighting source
is preferred over indirect lighting. In areas
where lighting is required due to architectural
considerations, energy efficient lighting, such
as, T5 fluorescent technology or the fast
emerging Light Emitting Diode (LEDs) lighting
source are considered. At areas such as station
entrances, the possibility of utilising natural
lighting is often explored to reduce the lighting
energy consumption.
Within the public areas, such as, the concourse
and platform, different lighting control levels are
implemented to cater for different operational
needs. This allows the appropriate level of
lighting to be selected at different periods in
order to minimise energy consumption.
Figure 1: Percentage of Overall Total Energy
Traction Load -26.5%
40
Regeneration Load -2.0%
Traction Load -50.5%
Regeneration Load -4.6%
Station Load -71.5%
Station Load -44.8%
North East Line
North South East West Lines
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Energy Efficiency in Singapore’s Rapid Transit System
for example, Service Transformer, 22Kv
switchroom, etc., to cut-in and cut-out the fans
so that the fans only run when necessary, thus
leading to further energy savings (Table 1).
...the possibility of utilising natural
lighting is often explored to reduce
the lighting energy consumption.
Table 1: Comparison of ECS Design Concepts
Air Conditioning System
Given the prevailing hot and humid weather
conditions throughout the year, air-conditioning
is introduced at underground stations to
provide Singapore’s commuters with a level of
comfort during their daily travel.
ECS Concept
Estimated Station Air
conditioning Load per
Station, kW
Estimated ECS yearly
Electricity Consumption for
15 Underground Stations
MWhr/year
Open System
2027
Closed System
1490
73,300
PSD System
490
36,500
Platform Screen Doors
As chillers are generally one of the major
energy-consuming equipments, efficiency
better than that stipulated in SS530 is specified
for the RTS projects.
Carbon Dioxide (CO2) Sensors are provided to
regulate outdoor air supply to the stations.
Installation of CO2 sensors automatically adjust
fresh air supply rates to the station public areas
while ensuring CO2 level is below 1000ppm. This
measure reduces energy consumption of the
air-conditioning system without compromising
air quality. The estimated reduction in energy
is up to 0.36% of a typical station’s power
consumption.
Variable Speed Drives are also provided for
Chilled Water Pumps and Cooling Towers. The
use of variable speed drives helps to reduce the
energy consumption of chilled water pumps
and cooling towers during part load operation,
with estimated reduction of up to 0.4% in
station power consumption.
Temperature sensors are provided in some
mechanically-ventilated
plant
rooms,
With the installation of platform screen doors
(PSDs) along the entire length of the station
platform, a separation is introduced between
the station and the tunnel. The heat from the
trains and warm, humid tunnel air are thus
prevented from entering the station, which
leads to the reduction of cooling load in the
station. In addition, PSDs act as barriers to
intrusions into the tunnels and isolate the
stations from the heat, dust and air blast
generated by the train movement.
While the idea of providing full height PSDs is
not new, issues regarding reliability of the door
operating mechanisms and accuracy in train
stopping positions in the stations are some of
the major obstacles to its widespread adoption.
With technological advancements, these issues
no longer hinder the implementation of the
PSDs and these are now a standard feature
within Singapore’s underground RTS stations.
PSDs act as barriers to intrusions into
the tunnels and isolate the stations
from the heat, dust and air blast
generated by the train movement.
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Energy Efficiency in Singapore’s Rapid Transit System
With the adoption of PSDs, the station’s
cooling load and ECS electricity consumption
is reduced by more than 50% as compared to
those without PSDs. Table 1 gives a summary of
the estimated station cooling load and annual
ECS electricity consumption for the different
ECS concepts evaluated.
Apart from substantial savings in the electricity
costs, savings in capital cost associated with
providing smaller plants and station footprint
are also realised with the adoption of PSDs.
Rolling Stock
There are two major factors which contribute
to effective energy usage in Singapore’s trains.
They are:
a. Lightweight cars, which can be achieved
through lighter carbody shells, bogies, electrical
propulsion and auxiliary equipment.
b. Intensive use of regenerative braking and
enhanced efficiency of traction equipment.
This can be achieved through improved system
design, optimisation of traction motor design
and providing need based energy storage
arrangement.
In the following sections, it is shown how
weight management of the passenger vehicle,
coupled with improved traction and auxiliary
systems and energy efficient driving, together
with energy regeneration, contribute to
effective energy usage in rolling stock.
• Weight Management
As energy consumption is in direct proportion
42
to train weight, every effort has been made
to keep the train weight as low as possible.
Studies have been conducted to explore
advancement in rolling stock technology,
especially those on weight reduction of
passenger vehicles for the future rail projects.
As energy consumption is in direct
proportion to train weight, every
effort has been made to keep the
train weight as low as possible.
Therefore, it is effective to focus on weight
reduction of these sub-systems in order to
improve energy efficiency.
i Carbody and Miscellaneous Equipment
The carbody is a major weight contributor
of the passenger vehicle, as seen in Figure 2.
It constitutes about 21% of the Motor-car
weight. Therefore, the choice of material
plays a vital role in determining the energy
consumption pattern of a vehicle. With the
adoption of aluminum alloy in the MRT
trains, significant improvement is noted in
the weldability, mechanical strength and
corrosion resistance of the train body. This
has also helped manufacturers to reduce the
carbody weight significantly.
For the Singapore Circle Line (CCL) vehicles,
weight reduction of the carbody were also
reaped on other different aspects, namely,
by reducing the amount of glass; utilising
skeletal cable trays, aluminium diffusers
and aluminium body side interior plates;
and providing lighter seats which required
less underside reinforcements. Through
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Energy Efficiency in Singapore’s Rapid Transit System
With the adoption of aluminum
alloy in the MRT trains, significant
improvement is noted in the
weldability, mechanical strength
and corrosion resistance of the train
body.
this carbody optimisation, the weight of
the trailer car and motor car have been
trimmed by 3.6% and 4.6% respectively.
Besides this, huge weight savings were also
reaped from the auxiliary inverters, battery
boxes and gangways as a result of improved
configurations (Figure 2).
Figure 2: Breakdown of train weight
CCL Train Weight Breakdown (M Car)
10%
Bogie Mounted
Wheel Set
19%
Car Body Structure
Car End
Pneumatic
Air Conditioning
6%
11%
6%
2%
3%
iii Auxiliary Power Converter
The conventional APS galvanic isolation
between the DC line voltage and the 3-phase
AC 400V output is realised by a 50Hz output
transformer, which is big and heavy. These
transformers also have high losses at low
frequencies.
Bogie Frame
8%
14%
traction drives with IGBT based power
electronics for our newer fleets of trains,
there has been a significant weight saving
with enhanced performance and power,
as compared to conventional DC drives
GTO choppers used in the older stock. The
IGBT technology is expected to remain as
the undisputed state-of-the-art choice for
inverters in the near future.
Doors and Windows
Electrical and
Electronics Equipment
21%
Interior and
Exterior Fittings
ii Train Propulsion
Enhanced efficiency of traction equipment
also plays an important role in the energy
efficacy of passenger vehicles. Over the last
30 years, traction technology has moved
rapidly through diode rectifiers, thyristor
choppers, GTO choppers and inverters,
and most recently, the IGBT inverters and
permanent-magnet motors.
With the development of asynchronous
With the Medium Frequency Transformer
proposed for the Downtown Line trains,
which can operate at a higher frequency of
approximately 20 kHz, it allows for a more
efficient and much lighter alternative due
to its smaller size and lighter weight, hence
improving the energy efficiency of the train
by means of reduced body weight.
• Energy Efficient Driving
With the system designed to work at a
schedule speed of 40 km/h, the signalling
equipment and the rolling stock have to be
capable of achieving end to end speeds with
all out running of approximately 44 km/h,
depending on which sections and which
lines are considered. This gives a margin for
operational purposes and also has allowed
the introduction of coasting, which in itself
saves a very significant amount of energy.
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Energy Efficiency in Singapore’s Rapid Transit System
In order to minimise propulsion energy
requirements,
the
highest
possible
acceleration and deceleration rates are
required, and this leads to a regime in which
a high acceleration rate extending over
quite a short period is followed by a longer
coasting period and then a braking period
at a high deceleration rate. This minimises
power consumption for a given average
speed.
The amount of coasting is defined as the
difference in time between coasting and
all out run as a proportion of the coast run
time. Introduction of coasting saves a large
amount of energy. But as the proportion of
coasting is increased, there are diminishing
returns (Figure 3).
Figure 3: Relationship between Schedule Speed, Coasting
Allowance and Energy Saving
Trade-off Between Energy Savings and Schedule Speed Lost
Schedule Speed Lost (%)
6.0
Schedule Speed Lost
45.0
Energy Savings Gained
40.0
5.0
35.0
30.0
4.0
25.0
3.0
20.0
2.0
15.0
10.0
1.0
0.0
Energy Savings (%)
50.0
7.0
5.0
0
1
2
3
4
5
6
7
8
9
0.0
Coasting Allowance (%)
In the case of the MRT, the motors are
rated at 145 kW (1 hour rating) and two
thirds of the axles are motored. The whole
question of schedule speed, proportion of
motored axles, peak power available and
control regime is very complex, and their
optimisation requires careful design and
experimentation.
44
•
Energy Regeneration
Regenerative braking has also produced
significant
improvements
in
energy
efficiency on our rail systems. From the
C651 generation of trains onwards, our
passenger vehicles are equipped with threephase induction motors that allow for the
recovery of energy while braking. These
recovered braking energy, which otherwise
would have been lost in brake resistors, can
be utilised by an accelerating train nearby.
However, the amount of useful energy that
can be recovered is strongly influenced
by the supply system and traffic density.
Hence, in order to make the supply system
more receptive for energy recovery, our DC
traction network has equipped sufficient
substations with thyristor inverters.
Alignment – Hump Profile
The design of the RTS alignment also takes
into consideration the need to reduce
energy consumption for the train operation.
Whenever feasible, the use of hump profile is
considered in the alignment to reduce energy
requirements for braking and accelerating by
slowing the trains when entering the station
on the up-grade and facilitating acceleration
when departing on the down-grade. This is
achieved by raising the station above the interstation alignment, as shown in Figure 4.
Figure 4: Hump Profile
Station A
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Station B
Energy Efficiency in Singapore’s Rapid Transit System
Based on the simulation studies and assuming
inter-station distance of 1.5km with a hump
profile gradient of 3%, the difference in
the energy consumption travelling between
stations with and without the hump profile by
a 6-car train having a loading of 6 passengers/
m2 is 5.5kWH. The simulation studies have
also shown that there is some reduction in the
inter-station run times.
...the use of hump profile is
considered in the alignment to reduce
energy requirements for braking and
accelerating by slowing the trains
when entering the station on the
up-grade and facilitating acceleration
when departing on the down-grade.
Control of Escalators
In the current design of escalators for RTS,
an energy saving device, such as, an inverter
system, is being provided to conserve energy
when the escalator is operated at no load by the
reduction of speed (0.75m/s) to standby speed
(0.2m/s). During an extended period of no load,
the energy saving device will also be able to
further reduce the speed from standby speed
to stop. This speed reduction is achieved by use
of sensor detection at the escalator landings.
Provision is also made to allow the escalator to
be operated without this energy saving device,
for example, by using star/delta starter.
Adjusting the speed of the escalator to slow
down when not in use can save energy by up
to 30%.
Green Mark for RTS
In recent years, LTA has been working with
train manufacturers and system suppliers
to improve the energy efficiency of its MRT
system. To leverage on the best practices and
to ‘go even greener’, the Green Mark for Rail
Transit System (RTS) framework was drawn up
as a holistic approach to ensure that the design
of RTS takes into account environmental and
sustainability considerations.
The framework covers various aspects of a RTS
line from E&M systems and station design,
to operational considerations, as well as,
assessing areas where green technology or
advanced methods can lead to better use of
resources from a whole-life-cycle approach.
It also provides the necessary benchmark to
incentivise continual improvement beyond the
current state (Figure 5).
Figure 5 Green Mark for Rapid Transit System framework
Green Mark for Rapid Transit System framework
• Aim
• Objective
Green Strategy 1
Green Strategy 2
Green Strategy 3
Effective Use
of Energy
Environmental
Protection and
Sustainable
Development
Water Conservation
The aim of the framework is to enable and
to facilitate the design of RTS network to
minimise the environmental impact and
enhance sustainability.
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Energy Efficiency in Singapore’s Rapid Transit System
The framework was developed with the
following objectives:
contribution of the particular strategy versus
the overall sustainable development effort.
a) To promote sustainable and environmental
friendly design that meets the performance
and operational requirements of RTS
networks.
With this framework, LTA would be able to
assess the energy efficiency of the RTS network
more effectively.
b) To provide guidance in the formulation
of LTA’s engineering standards in the
conceptualisation, design and construction
of new RTS networks.
c) To identify areas for continual improvement.
Adjusting the speed of the escalator
to slow down when not in use can
save energy by up to 30%.
In addition, it is supported by three green
strategies:
a)effective use of energy;
b)environmental protection and sustainable
development; and
c)water conservation.
The framework incorporates a scoring system
for different criteria defined per strategy. The
strategies are in turn weighted based on the
Conclusion
With an extensive and expanding rail transit
network, energy efficiency will always be an
important component of environmental impact
and operation costs.
In addition, LTA recognises that its transport
system must not be provided at the expense of
the environment and therefore, continues in its
pursuit of new technology and innovative ideas
to make our rail transit system more energy
efficient.
As the growth of RTS will have an impact on
the environment, LTA has also encouraged
practitioners in the planning, design,
construction, and operation of the RTS, to take
due recognition of any potential environmental
impact and for appropriate countermeasures
to be taken.
References
Chew, T.C and Chua, C.K. 1998. Development Of
Singapore’s Rapid Transit System And The Environment.
Japan Railway & Transport Review, No. 18 1998, 26-30.
Windle, C.J. 1986. Electricity saving strategies employed
on the. Singapore MRT. International conference on Mass
46
rapid transit, Singapore, 1986, pp. 397-403.
Thong, T.L, Ho, H.C, and Sim, S.P. 2005. Energy
Conservation Measures For Rapid Transit System In
Singapore. Paper presented at the 7th Inetrnational Power
Engineering Conference (IPEC), 2005. Singapore.
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Energy Efficiency in Singapore’s Rapid Transit System
Melvyn Thong is the Director of Mechanical & Electrical Services (MES) for
the upcoming Thomson Line (TSL) and existing lines in the Land Transport
Authority, Singapore. He has been with the LTA since 1999 with over 20
years experience, and is currently responsible for the design development
and implementation of TSL and Eastern Region Line (ERL). In addition,
he is also responsible for projects related to the upgrading of Electrical
& Mechanical Systems for existing lines that comprise both driver and
driverless heavy metro systems. Prior to this appointment, he was the
Deputy Director in MES Division, instrumental in the development of in-house design capabilities
within the organisation for traction power systems and tunnel ventilation systems for RTS, as well
as the development of the BCA-LTA Green Mark for RTS launched in 2010.
He is a recognised Chartered Electrical Engineer (UK), Professional Engineer (Singapore) and
Licensed Electrical Engineer approved for 22kV switching operations in Singapore.
Adrian Cheong is the Deputy Director of Mechanical & Electrical Services
at the Land Transport Authority (LTA) and a Senior Consultant in MSI
Global Pte Ltd, Singapore. He has been with LTA since 1996. Mr Cheong
is responsible for the delivery of the Mechanical and Electrical Services
inclusive of Power Supply Systems and Tunnel Ventilation Systems for the
railway network in Singapore. He is also involved in areas of consultancy
work for overseas projects under MSI Global. Holding a Master’s Degree in
Electrical Engineering from the National University of Singapore, Mr Cheong
is involved in several Research and Development projects in his division to enhance sustainability
in rapid transit projects. He is also one of the key drivers behind the recently launched BCA-LTA
Green Mark for RTS in 2010 which is used as a benchmark for sustainability practices within LTA.
JOURNEYS | May 2012
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