The Use of Train-Generated Mass Airflow to Produce Electricity
When wind power is mentioned, most of us think of the ‘classic’ tower plus blades design of above-ground onshore or offshore wind turbine generators. But, as this article about using the airflow generated by underground trains illustrates, there is potential to adapt wind turbine generator designs to harness ‘wind’ energy from other sources.
By Norman H. Holley, COO, WWT Tunnel, USA
WWT Tunnel, LLC (WWT) of Pasadena, California, recently completed the third in a series of studies on behalf of the Los Angeles County Metropolitan Transportation Authority (LACMTA). The purpose of these studies was to confirm the feasibility of using the mass airflow generated by a subway train as it moves through an underground tunnel as a source for generation of renewable energy.
The first two studies established the general speed, duration and airflow characteristics of the mass airflow generated by a LACMTA train as it moves through the Red Line Tunnel. The purpose of the recent ‘proof of concept’ project was to establish that the mass airflow generated by a ‘train initiated event’ (a ‘TIE’) could be collected and utilised to power an underground adaptation of a wind turbine and, thereby, generate usable electrical power. The goal was to convert the unused ‘energy’ from the mass airflow generated by the TIE into reliable ‘green’ energy which the Metro could use (in either AC or DC format) in place of power purchased from third party providers. Fabrication of the equipment to be used was governed by three overriding principles: (1) safety, (2) production efficiency and (3) use of recycled materials as much as possible.
The ‘MACE’ Unit Design
For this project, WWT fabricated a 10-foot (just over 3 metre) section of multi-blade mass airflow collection equipment (the MACE). The MACE is a modular unit which is attached to the tunnel wall and consists of five components:
- a co-axial shaft;
- a series of three-bladed rotors which are attached to the common shaft and, when hit by the airflow from a train coming from either direction, cause the shaft to rotate;
- an electrical generator which converts the rotational speed of the shaft into electricity;
- a steel frame, and a steel mesh cage which encloses the entire MACE Unit for safety and strength purposes;
- a computer controller which monitors and controls all aspects of the equipment and its operation. The controller also includes a series of safety elements which will immediately shut down the MACE Unit in the event of an anomaly or emergency.
All design and patent rights for the MACE are held by MACE Energy, Inc. and licensed to WWT Tunnel, LLC, as are the trademark rights to the ‘MACE’ name.
Use of Recycled Materials to Construct the MACE
The blades of the three-bladed rotors are fabricated from polycarbonate – both for its strength and durability, and the fact that it is a ‘green’ material made from recycled water and soda bottles and recycled plastic bags. The hubs connecting the blades to the co-axial shaft are made from recycled aluminium.
Above-Ground Testing and Installation
Prior to installation in the tunnel, the MACE was subjected to rigorous computer modelling and above-ground testing over a period of several months to verify both the performance and safety of the unit (see Figure 1). These tests culminated in an extended session lasting almost 4 hours with maximum airflow speeds in excess of 70 mph (110 kph) and cycling and braking sessions much more strenuous than what the MACE would be subjected to in the tunnel. The MACE passed all pre-set criteria, generating electricity at a rate consistent with the power curve of its generator. Once above-ground testing was completed, the unit was installed in LACMTA’s Red Line Tunnel between the North Hollywood and Universal Studios stations.
Data Collected During the In-Tunnel Test
During the 10 days the MACE was actively collecting data, over 700,000 data points were collected, each of which was subsequently analysed and incorporated into the official Project Report (see www.wwttunnel.com). This data established that the MACE Unit performed even better than expected, as it was able to extend what had been anticipated to be a 90-second TIE event to an event with an average length of 147 seconds – capturing the airflow both in front of and behind (vacuum effect) the passing train, and it was able to generate power at lower train speeds than anticipated.
Interrelation of ‘Wind’ Speed and Power Produced
The average highest speed of the trains passing the MACE was 56.7 mph (91.25 kph). Since the amount of power produced goes up geometrically as the ‘wind’ speed goes up arithmetically, trains traveling at a higher speed would generate a higher ‘wind’ speed as well as a longer TIE, and therefore greater power production. The effect of the speed of the passing train on the MACE is summarised in Figure 2, where the horizontal axis lists the average ‘wind’ speed generated by passing trains in miles per hour and the vertical axis shows the average power generated per TIE in watts.
General Findings and Conclusions from the Project
The data collected during this project established:
- a baseline profile of the mass airflow in the tunnel, and its usefulness in meeting LACMTA’s goal of increasing reliance on renewable energy;
- that the mass airflow generated by subway trains (which is currently unutilised energy) can be safely collected and used to generate clean, self-generating electrical power;
- that the mass airflow in the tunnels is a consistent and reliable source of energy – due to the regularity of the speed and schedule of the trains, the power generated is far more reliable than above-ground wind and solar power, which are subject to weather conditions and other variables; and
- that the ‘plant capacity’ of the MACE (over 18%) is comparable, if not better, than the ‘plant capacity’ of many established above-ground wind farms while the capital cost of installing MACE equipment is substantially lower on a price per kilowatt-hour basis than the cost of developing and building above-ground power sources.
Potential Uses for MACE-Generated Power
The MACE produces the most power during peak hours when a transportation authority needs the most power (i.e. more trains = more mass airflow = more power generated). Therefore, MACE-generated power can be used to offset ‘peak’ demand (‘peak shaving’), making it even more valuable in terms of power purchase avoidance. Likewise, MACE-generated power can be stored and used for ‘demand response’ or other emergency or non-emergency uses including:
- in DC format: to offset spikes in usage from, for example, a train accelerating out of a station, to assist in dynamic braking as a train approaches a station, and to provide power for electric vehicle charging facilities at stations;
- in AC format: to provide power for lights, escalators and other station uses.
As shown in Figure 3, based on the results of this pilot project, a single 10-foot MACE Unit could generate over 28,000kWh per year.
Carbon Emission Avoidance
The carbon footprint of a MACE Unit is zero. Based on the data given in Figure 3 and typical carbon footprints for US natural gas and coal consumption, in one year a single 10-foot MACE Unit could avoid approximately 17 tonnes of CO2 emissions from burning natural gas or 30 tonnes of CO2 emissions from burning coal.
Further Study Required
The MACE Unit is not a ‘perpetual motion machine’. There may be some minor impact on a train from pushing the air ahead of the train through the MACE rather than simply pushing that air down the tunnel. However, the MACE is primarily open space, consisting of less than 30% ‘fill’ – an amount significantly less than the solid signs, electrical boxes and other items already impinging on the airflow moving through the tunnel. Further study is needed to quantify the amount of measurable air resistance, if any, the MACE may create for a train. But, more importantly, it is clear from the results of this project that the extra energy required to overcome any ‘wind’ resistance created by the MACE for a train will be a small fraction of the electrical power which the MACE will be consistently producing on a dependable basis.
About the Company
Founded in 2007, WWT Tunnel, LLC focuses on the development and commercialisation of previously underutilised renewable energy sources. With over 25 years of experience in wind energy and related technologies, Mr Holley and WWT Tunnel, LLC recognise the need for finding creative solutions to present and future energy needs.
All design and patent rights for the MACE are held by MACE Energy, Inc. and licensed to WWT Tunnel, LLC, as are the trademark rights to the ‘MACE’ name.
