Vehicle-to-grid

Vehicle-to-grid (V2G) describes a system in which plug-in electric vehicles, such as electric cars (BEV) and plug-in hybrids (PHEV), communicate with the power grid to sell demand response services by either returning electricity to the grid or by throttling their charging rate.[1] [2]

Vehicle-to-grid can be used with gridable vehicles, that is, plug-in electric vehicles (BEV and PHEV), with grid capacity. Since at any given time 95 percent of cars are parked, the batteries in electric vehicles could be used to let electricity flow from the car to the electric distribution network and back. This represents an estimated value to the utilities of up to $4,000 per year per car.[3]

History

The company AC Propulsion Inc. coined the term V2G for vehicle-to-grid.[4]

Versions

V2G is a version of battery-to-grid power applied to vehicles.[5] There are three main different versions of the vehicle-to-grid concept, all of which involve an onboard battery:

A gasifier system on a pickup truck can be used to power the truck and produce electricity

It should also be noted that besides vehicles which have an onboard battery, vehicles without a large battery, but which connect to/recharge a battery placed at the house (for example being part of an off-the-grid electrical system or net metering system) could in effect form a vehicle-to-grid system. Even a renewable energy source (like wood gas) could be used.[7]

Types

V2G is classified based on the power flow direction: Unidirectional V2G and Bidirectional V2G.[8][9]

Peak load leveling

The concept allows V2G vehicles to provide power to help balance loads by "valley filling" (charging at night when demand is low) and "peak shaving" (sending power back to the grid when demand is high, see duck curve). Peak load leveling can enable utilities new ways to provide regulation services (keeping voltage and frequency stable) and provide spinning reserves (meet sudden demands for power). In future development, it has been proposed that such use of electric vehicles could buffer renewable power sources such as wind power, for example, by storing excess energy produced during windy periods and providing it back to the grid during high load periods, thus effectively stabilizing the intermittency of wind power. Some see this application of vehicle-to-grid technology as a renewable energy approach that can penetrate the baseline electric market.

It has been proposed that public utilities would not have to build as many natural gas or coal-fired power plants to meet peak demand or as an insurance policy against blackouts[10] Since demand can be measured locally by a simple frequency measurement, dynamic load leveling can be provided as needed.[11]

Carbitrage

Carbitrage is a portmanteau of 'car' and 'arbitrage'. When the electric utility would like to buy power from the V2G network, it holds an auction. The car owners are able to define the parameters under which they will sell energy from their battery pack. Many factors would be considered when setting minimum sale price including the cost of the secondary fuel in a PHEV and battery cycle wear. When this minimum price is satisfied, it is deemed as meeting carbitrage.[12]

Backup power solutions

Future battery developments[13] may change the economic equation, making it advantageous to use newer high capacity and longer-lived batteries in BEV/PHEVs. These newer batteries can be used in grid load balancing and as a large energy cache for renewable grid resources. Since BEVs can have up to 50 kWh worth of battery storage they represent somewhat more than the average home's daily energy demand. Even without a PHEV's gas generation capabilities such a vehicle could be used for emergency power for several days (for example, lighting, home appliances, etc. with combined load of 1 kW could be powered for 50 hours). This would be an example of Vehicle-to-home transmission (V2H). As such they may be seen as a complementary technology for intermittent renewable power resources such as wind or solar electric.

Utilities

These utilities currently have V2G technology trials:

Efficiency

Any conversion of energy has losses due to the laws of thermodynamics. Lower losses mean better efficiency. Most modern battery electric vehicles use lithium-ion with an 80–90% efficiency,[15] the lithium titanate version has a battery efficiency(less charger) from 90 to 98%, depending on circumstances like charge rate, charge state and temperature.[16][17]

Lead-acid batteries may be 76% efficient,[18] but are rarely used in modern road vehicles.

The charge/discharge cycle for the battery alone may be 80-90%.[19] The charge/discharge electronics also have losses.

Future Plans for Vehicle-to-Grid (V2G) in various Countries

A study conducted in 2012 by the Idaho National Laboratory [20] revealed the following estimations and future plans for V2G in various countries. It is important to note that this is difficult to quantify because the technology is still in its nascent stage, and is therefore difficult to reliably predict adoption of the technology around the world. The following list is not intended to be exhaustive, but rather to give an idea of the scope of development and progress in these areas around the world.

United States

Current environmental issues in the US are playing a vital role in the demand for V2G technology. The decrease in costs for implementation of V2G will be directly related to the speed of adoption by consumers. As smart grid rollout continues and the population realizes the lower cost of electric vehicle ownership, demand will increase. Continued V2G testing and the further development of two-way communications standards will offer interoperability across systems. Fleets such as the US Postal Service will be crucial to V2G development. Private grid testing will continue as utilities, automakers, and colleges form partnerships. The University of Delaware has recently signed its first license for V2G testing with Autoport, Inc. They expect that by the second or third quarter of 2011, 100 electric vehicles on the road will be capable of V2G testing (Bryant 2010).

PJM interconnection has envisioned using US Postal Service trucks, school buses and garbage trucks that remain unused overnight for grid connection. This could generate millions of dollars because these companies aid in storing and stabilizing some of the national grid's energy. The US is projected to have one million electric vehicles on the road between 2015 and 2019. Studies indicate that 160 new power plants will need to be built by 2020 to compensate for electric vehicles if integration with the grid does not move forward (ZigBee 2010).

Japan

Japan currently is a leader in the electric vehicle industry. This may allow the country to pioneer new V2G technology for the mainstream. In order to meet the 2030 target of 10% of Japan's energy being generated by renewable resources, a cost of $71.1 billion will be required for the upgrades of existing grid infrastructure. The Japanese charging infrastructure market is projected to grow from $118.6 million to $1.2 billion between 2015 and 2020 (ZigBee 2010). Starting in 2012, Nissan plans to bring to market a kit compatible with the LEAF EV that will be able to provide power back into a Japanese home. Currently, there is a prototype being tested in Japan. Average Japanese homes use 10 to 12 KWh/day, and with the LEAF's 24 KWh battery capacity, this kit could potentially provide up to two days of power (Howard 2011). Production in additional markets will follow upon Nissan's ability to properly complete adaptations.

Denmark

Denmark currently is a world leader in wind power generation, with 20% of the country's energy coming from wind (there are enough installed turbines to meet up to 40% of the country's energy needs). Initially, Denmark's goal is to replace 10% of all vehicles with PEVs, with an ultimate goal of a complete replacement to follow. The Edison Project implements a new set of goals that will allow enough turbines to be built to accommodate 50% of total power while using V2G to prevent negative impacts to the grid. Because of the unpredictability of wind, the Edison Project plans to use PEVs while they are plugged into the grid to store additional wind energy that the grid cannot handle. Then, during peak energy use hours, or when the wind is calm, the power stored in these PEVs will be fed back into the grid. To aid in the acceptance of EVs, policies have been enforced that create a tax differential between zero emission cars and traditional automobiles. The Danish PEV market value is expected to grow from $50 to $380 million between 2015 and 2020. PEV developmental progress and advancements pertaining to the use of renewable energy resources will make Denmark a market leader with respect to V2G innovation (ZigBee 2010).

Following the Edison project, the Nikola project was started[21] which focused on demonstrating the V2G technology in a lab setting, located at the Risø Campus (DTU). DTU is a partner along with Nuvve and Nissan. The Nikola project is finishing in 2016, and lays the groundwork for Parker, which will use a fleet of EVs to demonstrate the technology in a real-life setting. this project is partnered by DTU,[22] Insero, Nuvve, Nissan and Frederiksberg Forsyning (Danish DSO in Copenhagen). Besides demonstrating the technology the project also aims to clear the path for V2G-integration with other OEMs as well as calculating the business case for several types of V2G, such as Adaptive charging, overload protection, peak shaving, emergency backup and frequency balancing. the project starts in August 2016 and runs for 2 years. Other notable projects in Denmark are the SEEV4-City Interreg project which will demonstrate V2G in a car sharing fleet in the north harbour of Copenhagen and the ECOGrid 2.0, which will not include EVs but build the aggregator software to fully integrate it into the Danish electricity markets.

United Kingdom

The V2G market in the UK will be stimulated by aggressive smart grid and PEV rollouts. Starting in January 2011, programs and strategies to assist in PEV have been implemented. The UK has begun devising strategies to increase the speed of adoption of EVs. This includes providing universal high-speed internet for use with smart grid meters, because most V2G-capable PEVs will not coordinate with the larger grid without it. The "Electric Delivery Plan for London" states that by 2015, there will be 500 on-road charging stations; 2,000 stations off-road in car parks; and 22,000 privately owned stations installed. Local grid substations will need to be upgraded for drivers who cannot park on their own property. By 2020 in the UK, there will be a smart meter in every residential home, and about 1.7 million PEVs on the road. The UK's electric vehicle market value is projected to grow from $0.1 to $1.3 billion between 2015 and 2020 (ZigBee 2010).

South Korea

South Korea has set a goal that by 2030, 100% of electric customers will be using smart grid technology. Beginning in March 2010, the government will invest $23.3 billion in development and rollout of smart grid technology. Grid revenues are projected to increase from $4.8 to $53.2 million between 2015 and 2020 (ZigBee 2010).

Current projects

University of Delaware

Dr. Willett Kempton, Dr. Suresh Advani, and Dr. Ajay Prasad are the researchers at the US University of Delaware who are currently conducting research on the V2G technology, with Dr. Kempton being the lead on the project. Dr. Kempton has published a number of articles on the technology and the concept, many of which can be found on the V2G project page.[23] The group is involved in researching the technology itself as well as its performance when used on the grid. In addition to the technical research, the team has worked with Dr. Meryl Gardner, a Marketing professor in the Alfred Lerner College of Business and Economic at the University of Delaware, to develop marketing strategies for both consumer and corporate fleet adoption [24] A 2006 Toyota Scion xB car was modified for testing in 2007.[25]

Lawrence Berkeley National Laboratory

At Lawrence Berkeley National Laboratory, Dr. Samveg Saxena currently serves as the project lead for Vehicle-to-Grid Simulator (V2G-Sim).[26] V2G-Sim is a simulation platform tool used to model spatial and temporal driving and charging behavior of individual plug-in electric vehicles on the electric grid. Its models are used to investigate the challenges and opportunities of V2G services, such as modulation of charging time and charging rate for peak demand response and utility frequency regulation. V2G-Sim has also been used to research the potential of plug-in electric vehicles for renewable energy integration. Preliminary findings using V2G-Sim have shown controlled V2G service can provide peak-shaving and valley-filling services to balance daily electric load and mitigate the duck curve. On the contrary, uncontrolled vehicle charging was shown to exacerbate the duck curve.

In another research initiative at Lawrence Berkeley Lab using V2G-Sim, V2G services were shown to have minor battery degradation impacts on electric vehicles as compared to cycling losses and calendar aging.[27] In this study, three electric vehicles with different daily driving itineraries were modelled over a ten-year time horizon, with and without V2G services. Assuming daily V2G service from 7PM to 9PM at a charging rate of 1.440 kW, the capacity losses of the electric vehicles due to V2G over ten years were 2.68%, 2.66%, and 2.62%.

Nissan and Enel

In May 2016, Nissan and Enel power company announced a collaborative V2G trial project in the United Kingdom, the first of its kind in the country.[28] The trial comprises 100 V2G charging units to be used by Nissan Leaf and e-NV200 electric van users. The project claims electric vehicle owners will be able to sell stored energy back to the grid at a profit.

One notable V2G project in the United States is at the University of Delaware, where a V2G team headed by Dr. Willett Kempton has been conducting on-going research.[23] An early operational implementation in Europe was conducted via the German government-funded MeRegioMobil project at the "KIT Smart Energy Home" of Karlsruhe Institute of Technology in cooperation with Opel as vehicle partner and utility EnBW providing grid expertise.[6] Their goals are to educate the public about the environmental and economic benefits of V2G and enhance the product market.[23] Other investigators are the Pacific Gas and Electric Company, Xcel Energy, the National Renewable Energy Laboratory, and, in the United Kingdom, the University of Warwick.[29]

Completed projects

Edison

Denmark's Edison project, an abbreviation for 'Electric vehicles in a Distributed and Integrated market using Sustainable energy and Open Networks' was a partially state funded research project on the island of Bornholm in Eastern Denmark. The consortium of IBM, Siemens the hardware and software developer EURISCO, Denmark's largest energy company DONG Energy, the regional energy company Østkraft, the Technical University of Denmark and the Danish Energy Association, explored how to balance the unpredictable electricity loads generated by Denmark's many wind farms, currently generating ~20% of the country's total electricity production, by using electric vehicles (EV) and their accumulators. The aim of the project is to develop infrastructure that enables EVs to intelligently communicate with the grid to determine when charging, and ultimately discharging, can take place.[30] At least one rebuild V2G capable Toyota Scion will be used in the project.[31] The project is key in Denmark's ambitions to expand its wind-power generation to 50% by 2020.[32] According to a source of British newspaper The Guardian 'It's never been tried at this scale' previously.[33] The project concluded in 2013.[34]

Southwest Research Institute

In 2014, Southwest Research Institute (SwRI) developed the first vehicle-to-grid aggregation system qualified by the Electric Reliability Council of Texas (ERCOT). The system allows for owners of electric delivery truck fleets to make money by assisting in managing the grid frequency. When the electric grid frequency drops below 60 Hertz, the system suspends vehicle charging which removes the load on the grid thus allowing the frequency to rise to a normal level. The system is the first of its kind because it operates autonomously.[35]

The system was originally developed as part of the Smart Power Infrastructure Demonstration for Energy Reliability and Security (SPIDERS) Phase II program, led by Burns and McDonnell Engineering Company, Inc. The goals of the SPIDERS program are to increase energy security in the event of power loss from a physical or cyber disruption, provide emergency power, and manage the grid more efficiently.[36] In November 2012, SwRI was awarded a $7 million contract from the U.S. Army Corps of Engineers to demonstrate the integration of vehicle-to-grid technologies as a source for emergency power at Fort Carson, Colorado.[37] In 2013, SwRI researchers tested five DC fast-charge stations at the army post. The system passed integration and acceptance testing in August 2013.[38]

Skepticism

There is some skepticism among experts about the feasibility of V2G. As the New York Times states:

In 2007 an Environmental Defense representative stated: "It’s hard to take seriously the promises made for plug-in hybrids with 30 miles (48 km) all-electric range or any serious V2G application any time soon. It’s still in the science project stage."[29]

The Vehicle-to-grid potential of Honda’s full hybrid vehicles is unexplored, but Honda is doubtful of using them to power homes. "We would not like to see stresses on the battery pack caused by putting it through cycles it wasn’t designed for," said a Honda spokesman. "Instead, they should buy a Honda generator that was made for that purpose."[29] However, in December 2013, Honda announced a partnership with the University of Delaware where they delivered an Accord Hybrid with on-board bidirectional charger to enter into the PJM Interconnection's frequency regulation market.[39]

The more a battery is used the sooner it needs replacing. Replacement cost is approximately 1/3 the cost of the electric car.[40] Over their lifespan, batteries degrade progressively with reduced capacity, cycle life, and safety due to chemical changes to the electrodes. Capacity loss/fade is expressed as a percentage of initial capacity after a number of cycles (e.g., 30% loss after 1,000 cycles). Cycling loss is due to usage and depends on both the maximum state of charge and the depth of discharge.[41] Tesla Motors' JB Straubel discounts V2G because battery wear outweighs economic benefit. He also prefers recycling over re-use for grid once batteries have reached the end of their useful car life.[42] A 2017 study found decreasing capacity,[43][44] and a 2012 hybrid-EV study found minor benefit.[45]

Efficiency comparison

Charging a fairly efficient battery system from the grid is at best 70 to 80% efficient. Returning that energy from the battery to the grid, which includes "inverting" the DC power back to AC with efficiencies of about 90% yields 63–72% energy return to the system. This needs to be factored against potential cost savings as well as the additional wear and tear on the batteries (current batteries last a few thousand cycles at maximum) and especially increased emissions if the original source of power is fossil based. This cycle of energy efficiency may be compared with the 70–80% efficiency of large-scale location-restricted pumped-storage hydroelectricity.[46] However, pumped storage is limited by geography so it could be practical to take a small amount of energy from a large number of batteries if there are enough PHEV/BEV vehicles on the grid. 1 kW from 1000 vehicles is 1 megawatt of power.

Vehicles

This list is incomplete; you can help by expanding it.

The REV 300 ACX vehicle includes a V2G system.

Boulder Electric Vehicle 500 series and 1000 series trucks (in production: 2012-2014).

The ACPropulsion T-Zero, E-box and MINI-E all have V2G systems.

The Nissan Leaf has a Vehicle To Home system in Japan with an external inverter.

The Mitsubishi Outlander PHEV has a Vehicle To Home system in Japan that is also planned for roll out in Europe.

See also

References

  1. ↑ Cleveland, Cutler J.; Morris, Christopher (2006). Dictionary of Energy. Amsterdam: Elsevier. p. 473. ISBN 0-08-044578-0.
  2. ↑ "Pacific Gas and Electric Company Energizes Silicon Valley With Vehicle-to-Grid Technology". Pacific Gas & Electric. 2007-04-07. Retrieved 2009-10-02.
  3. ↑ "Car Prototype Generates Electricity, And Cash". Science Daily. 2007-12-09. Retrieved 2007-12-05.
  4. ↑ Emadi, Ali (2005). Handbook of Automotive Power Electronics and Motor Drives. p. 34.
  5. ↑ Willett Kempton (2007-12-08). "Download Limit Exceeded". citeseerx.ist.psu.edu. doi:10.1016/j.jpowsour.2004.12.025. Retrieved 2017-05-20.
  6. 1 2 Brinkman, Norm; Eberle, Ulrich; Formanski, Volker; Grebe, Uwe-Dieter; Matthe, Roland (2012). "Vehicle Electrification - Quo Vadis?". Research Gate. Retrieved 2014-12-20.
  7. ↑ Dave Nichols using a wood gas generator to power his car and produce electricity when he parks it at his house
  8. ↑ Yong, Jia Ying, et al. "A review on the state-of-the-art technologies of electric vehicle, its impacts and prospects." Renewable and Sustainable Energy Reviews 49 (2015): 365-385.
  9. ↑ Sortomme, Eric, and Mohamed El-Sharkawi. "Optimal charging strategies for unidirectional vehicle-to-grid." Smart Grid, IEEE Transactions on 2.1 (2011): 131-138.
  10. ↑ Woody, Todd (2007-06-12). "PG&E's Battery Power Plans Could Jump Start Electric Car Market". Green Wombat. Retrieved 2007-08-19.
  11. ↑ US 4317049, SCHWEPPE, FRED C., "Frequency adaptive, power-energy re-scheduler", published 1982-02-23
  12. ↑ "RMI Smart Garage Charrette Report" (PDF). Rocky Mountain Institute.
  13. ↑ "Toshiba's New Rechargeable Lithium-Ion Battery Recharges in Only One Minute" (Press release). Japan: Toshiba Corporation. 2005-03-29. Retrieved 2007-12-05.
  14. ↑ Fang, X.; Misra, S.; Xue, G.; Yang, D. (2011). "Smart Grid - The New and Improved Power Grid: A Survey". IEEE Communications Surveys and Tutorials. doi:10.1109/SURV.2011.101911.00087.
  15. ↑ Valøen, Lars Ole; Shoesmith, Mark I. (2007). The effect of PHEV and HEV duty cycles on battery and battery pack performance. 2007 Plug-in Highway Electric Vehicle Conference.
  16. ↑ Tatiana Minav (2014-03-26). "Energy Regeneration and Efficiency in an Electro-Hydraulic Forklift with Lithium-Titanate Batteries, Chapter 5 Analysis. (PDF Download Available)". ResearchGate. Retrieved 2017-05-20. battery efficiency during performed testes in average is 98 %
  17. ↑ Charging Lithium-ion Quote: "Charge efficiency is 97 to 99 percent"
  18. ↑ "Section 1. Energy, Work and the Storage Battery". evbatterymonitoring.com. Retrieved 2017-05-20. Generally, the energy efficiency of lead acid batteries is about 76%, meaning that 76% of the energy that was put into the battery during charging is all that is available for release during discharge.
  19. ↑ "Car Battery Efficiencies". large.stanford.edu. Retrieved 2017-05-20.
  20. ↑ Briones, Adrene; Francfort, James; Heitmann, Paul; Schey, Michael; Schey, Steven; Smart, John (2012-09-01). "Vehicle-to-Grid (V2G) Power Flow" (PDF). Idaho National Laboratory. Retrieved 2015-04-29.
  21. ↑ "Home". Nikola. Retrieved 2016-07-12.
  22. ↑ Andersen, Peter Bach; Marinelli, Mattia; Olesen, Ole Jan; Andersen, Claus Amtrup; Poilasne, Gregory; Christensen, Bjoern; Alm, Ole (2014). "The Nikola project intelligent electric vehicle integration" (PDF). Technical University of Denmark. Retrieved 2016-07-12.
  23. 1 2 3 "V2G : Vehicle to Grid Power". June 2001. Retrieved 2008-02-05.
  24. ↑ Boyle, Elizabeth (2007-11-28). "V2G Generates Electricity--And Cash". UDaily.
  25. ↑ Kempton, Willett; Udo, Victor; Huber, Ken; Komara, Kevin; Letendre, Steve; Baker, Scott; Brunner, Doug; Pearre, Nat (November 2008). "A Test of Vehicle-to-Grid (V2G) for Energy Storage and Frequency Regulation in the PJM System" (PDF). University of Delaware. Retrieved 2016-03-08.
  26. ↑ "V2G-Sim". Lawrence Berkeley Lab. Retrieved 2016-11-20.
  27. ↑ Wang, Dai; Saxena, Samveg; Coignard, Jonathan; Iosifidou, Elpiniki; Guan, Xiaohong (2016-07-21). "Quantifying Electric Vehicle Battery Degradation from Driving vs. V2G Services". 2016 IEEE Power and Energy Society General Meeting (PESGM).
  28. ↑ "Nissan and Enel Launch Groundbreaking Vehicle-to-grid Project in the UK". Nissan Newsroom UK. Retrieved 2016-11-19.
  29. 1 2 3 4 Motavalli, Jim (2007-09-02). "Power to the People: Run Your House on a Prius". New York Times. Retrieved 2014-12-20.
  30. ↑ "Intelligent power grid". Zurich: IBM Research.
  31. ↑ "WP3 - DISTRIBUTED INTEGRATION TECHNOLOGY DEVELOPMENT". Edison. Retrieved 2011-08-30.
  32. ↑ "Danish Climate and Energy Policy". Danish Energy Agency. 2013. Retrieved 2016-03-08.
  33. ↑ Graham-Rowe, Duncan (2009-06-19). "Denmark to power electric cars by wind in vehicle-to-grid experiment". London: The Guardian. Retrieved 2011-08-30.
  34. ↑ Rasmussen, Jan (2013-07-11). "The Edison project is successfully closed!!!". Edison. Retrieved 2016-03-08.
  35. ↑ "SwRI develops first ERCOT-qualified vehicle-to-grid aggregation system". Southwest Research Institute. Retrieved 2015-02-26.
  36. ↑ "SPIDERS: The Smart Power Infrastructure Demonstration for Energy Reliability and Security" (PDF). Sandia National Laboratories.
  37. ↑ "SwRI will participate in a U.S. Army program to demonstrate alternative sources for an emergency electrical power grid". Southwest Research Institute. Retrieved 2015-02-26.
  38. ↑ "SwRI deploys novel vehicle-to-grid aggregation system". Southwest Research Institute. Retrieved 2015-02-26.
  39. ↑ "Honda Joins Vehicle-to-Grid Technology Demonstration Project in Partnership with University of Delaware and NRG Energy" (Press release). US: Honda. 2013-12-05. Retrieved 2013-12-06.
  40. ↑ "Frequently Asked Questions". Electric Vehicles. Canadian Automobile Association. Retrieved 2016-03-08.
  41. ↑ "Lithium Ion UF103450P" (PDF). Panasonic. 2012. Retrieved 2016-03-08.
  42. ↑ Shahan, Zachary (2016-08-22). "Why Vehicle-To-Grid & Used EV Battery Storage Isn't Logical". Clean Technica. Retrieved 2016-08-22.
  43. ↑ "Green Car Congress: Hawaii study finds vehicle-to-grid discharge detrimental to EV batteries". www.greencarcongress.com. 2017-05-15. Retrieved 2017-05-18.
  44. ↑ Matthieu Dubarry, Arnaud Devie, Katherine McKenzie (2017) “Durability and reliability of electric vehicle batteries under electric utility grid operations: Bidirectional charging impact analysis,” Journal of Power Sources, Volume 358, Pages 39-49, doi: 10.1016/j.jpowsour.2017.05.015
  45. ↑ Peterson, Scott B. "Plug-in hybrid electric vehicles: battery degradation, grid support, emissions, and battery size tradeoffs" (2012). Dissertations. Paper 167. page 8
  46. ↑ Levine, John. "Pumped Hydroelectric Energy Storage and Spatial Diversity of Wind Resources as Methods of Improving Utilization of Renewable Energy Sources" (PDF). US: Colorado University. Retrieved 2014-08-28.

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