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CMU researchers find controlled charging of PHEVs can cut cost of integration into electricity system by 54-73%; higher benefits with wind power

In a new study published in the journal Applied Energy, Carnegie Mellon University (CMU) researchers found that controlled charging of plug-in hybrid electric vehicles (PHEVs) reduces the costs of integrating the vehicles into an electricity system by 54–73% depending on the scenario.

More specifically, controlled charging can cut the cost of integrating PHEVs approximately in half. The magnitude of these savings is ~5% to 15% higher in a system with 20% wind penetration compared to a system with no wind power, and the savings are 50–60% higher in a system that requires capacity expansion.

As one of the fastest growing electricity sources in the United States, wind can be expected to meet a large proportion of the renewable portfolio standards. To compensate for the increased amounts of these inherently–variable sources of electricity, the power grid requires additional flexibility to manage fluctuations in generation. For systems incorporating high levels of wind power, ramping natural gas combustion turbine plants in response to changes in output from variable resources has typically provided this flexibility. Recent research has shown that ramping gas turbines to manage the variability of wind power can increase NOx emissions and reduce the greenhouse gas benefits associated with wind power production.

Plug-in electric vehicles (PEVs), including plug-in hybrid electric vehicles (PHEVs) and battery electric vehicles (BEVs), create additional electricity demand, resulting in additional air emissions from power plants. But they have also been proposed as a means for increasing grid flexibility in order to integrate renewables, with much emphasis on the possibility of using the vehicles for grid storage via a bidirectional electrical connection between the vehicle and the electricity grid, referred to as vehicle-to-grid (V2G). … However, it has been shown that the market for V2G in the energy market and ancillary services market is small, arbitrage potential is limited, and participation can significantly reduce battery life by increasing the total energy processed by the battery.

… As an alternative, electricity demand can be partially managed by modulating the charging rate of PEVs—for example, following variations in wind supply. Such an approach does not increase the energy processed by the battery, and it is possible that such an approach could actually extend battery life by lowering average charge rates and thus heat generation. Controlled charging can also take advantage of the high levels of wind generation that commonly occur at night in the US. At these times other load is likely to be low, and coal plants would likely need to be cycled, adding costs and emissions that could be saved with smart charging of PEVs. Alternatively, ramping of thermal plants could be reduced by building excess wind capacity, curtailing wind energy when it is not needed, and taking it when most cost effective for the system.

—Weis et al.

Engineering and Public Policy Department (EPP) graduate student Allison Weis and professors Paulina Jaramillo and Jeremy Michalek evaluated the potential cost savings from controlled charging in scenarios with vs. without additional wind power in order to understand whether PEVs can provide cost savings in systems with increased levels of wind power, or whether controlled charging only limits the impact of the vehicles themselves on the system.

System overview – energy is provided by conventional power plants and wind plants and must meet the demand from plug-in vehicles and non-vehicle load in each time step. Weis et al. Click to enlarge.

In the study, they focused on PHEVs, which do not require changes in current driving patterns. They examined the benefit of controlled charging of PHEVs relative to convenience charging (vehicle charges at maximum rate upon arrival); delayed charging (vehicle begins charging at maximum rate just in time for its next use); and no charging (no PHEVs) under alternative scenarios of high vs. low wind penetration in the power generation fleet, high vs. low PHEV penetration in the vehicle fleet, and high vs. low initial power generating capacity.

They constructed a mixed integer linear programming model for capacity expansion, plant dispatch, and plug-in hybrid electric vehicle (PHEV) charging based on the NYISO system. They used hourly data for wind and load and assumed perfect information (no forecast error) to focus on capacity expansion and unit commitment decisions.

They then compared results using a 15-min resolution to test the importance of sub-hourly trends. They did not evaluate the entire range of power plant fleets, but instead focused on comparing the difference between a system with sufficient capacity and one requiring investment in new capacity.

Broadly, they found that controlled charging of PHEVs reduces peak load and can reduce wind curtailment used to mitigate extreme generation fluctuation.

  • Given a 10% penetration of PHEVs (totaling 900,000 PHEVs in this model), controlled charging—which allows for shifting generation to cheaper plants and to off-peak hours—reduces power generation costs by $65–$110 million dollars a year compared to the uncontrolled charging scenario, representing 1.5– 2.3% of total system costs and 54–73% of the cost of integrating PHEVs.

  • In scenarios requiring capacity expansion, controlled charging offers the opportunity to change which types and how many new power plants are built, in addition to influencing plant operation.

  • In a fixed capacity scenario, controlled charging allows the additional vehicle load to be accommodated without any new capacity, as the system is already operating with more capacity than required by the 15% reserve margin.

  • In all cases, delayed charging is able to capture some, but not all, of the cost reductions offered by controlled charging.

  • Regardless of the capacity scenario, when there is a 20% wind penetration, controlled charging offers 6–13% greater cost reduction compared to the same system without wind. However, the researchers noted, system operators should not rely on controlled vehicle charging to cut wind integration costs.

  • In most of the scenarios, at 10% PHEV penetration or higher, controlled charging provides enough system benefits to save $100/vehicle/year for many vehicles.

It is already cheaper to charge an electric vehicle than fill up a gasoline vehicle. But allowing grid operators to control electric vehicle charging speed could reduce these costs further. We see additional savings up to $70 per vehicle each year or even higher for systems that expect new power plant construction and systems with a lot of wind power.

—Dr. Jeremy Michalek, director of the Vehicle Electrification Group

The researchers warn that more study is needed to understand all the implications of controlling electric vehicle charging.

We need to keep in mind that fossil fuel plants are typically cheap to operate, so reducing cost can sometimes result in increased emissions. In our next study, we plan to quantify the health and environmental impacts of these changing emissions.

—Allison Weis


  • Allison Weis, Paulina Jaramillo, Jeremy Michalek (2014) “Estimating the potential of controlled plug-in hybrid electric vehicle charging to reduce operational and capacity expansion costs for electric power systems with high wind penetration,” Applied Energy 115, 190–204 doi: 10.1016/j.apenergy.2013.10.017


A.C. R.

If there are benefits with wind, there will be far bigger benefits with nuclear powerplants, that produce excess power far more reliably and consistently than wind.

Sadly, it appears the researchers can't mention that. Nuclear energy is still taboo, even in most energy research circles.


Like coal, nuclear is very much a base load option. The problem with wind has always been coping with highly variable local conditions without destabilising the grid. An integrated system using vehicle batteries for storage is one way to solve the latter problem, a problem that gets much worse as wind capacity increases as a percentage of the total grid. Flow batteries located at the wind farm is the other option that is worth pursuing.


Better Management of energy production and specialy demand is no longer a real problem.

More and more future loads can be managed (i.e. can be varied, delayed or temporary turn off) to match energy produced by various sources such as wind, solar etc., without affecting comfort level.

All the technologies required exist and are not very costly.


The public information on the paper didn't mention anything about the grants or other financing for the research.  If it was NREL, the omission of nuclear would be expected.

With nuclear, excess generation for vehicle charging would be available almost every night and weekend (save during conditions of extreme cold where electric heat is prevalant) and the peaks will be known and constant.  This is inherently easier to handle than wind with a capacity factor of 0.35, or solar at 0.25 and downward.  Maybe it's possible to give all wind installations a set of flow batteries the size of refinery tank farms, but one such will be on the order of the size of a 1 GW nuclear plant and its auxiliary buildings.  The nuke's impact on the landscape is vastly smaller.  That's truly the "green" option, but anti-nukes would rather burn lignite.


If they can find additional ways to incorporate the grid with EVs, the grid may just survive. I say that because I fully expect wider adoption of EVs to decrease battery cost so much as to make them economical for wind and solar storage on an individual home basis. On the other hand, it may be desireable to at least mantain local grids such that any individual who suffers a set back with their system can temporarily buy from the group on the grid until they get their stuff fixed.

This eventuality of low battery costs is precisely why our government, press, financial institutions and many engineers and scientists oppose electric vehicles still. It's a huge change, but mostly a huge change in who makes the money and who has the power.


Not really a question of nuclear vs wind, more one of "all of the above" and making sure the grid can handle the variations in supply and demand. As far as physical foot print is concerned, small modular reactors, which are basically buried, look ideal, but face uphill work against the NIMBY crowd and bureaucrats. Big tank farms for flow batteries are not faced with that.

Roger Pham

Actually, wind speed can be predicted for hours and even days in advance, within a few mph accuracy. Same is true for degree of cloud coverage, as well as electricity demand curve can be predicted ahead of time. An utility operator can simply plug these prediction data into a computer to determine how much power output out of CCGT or even coal power plant in order to avoid rapid ramping up or down of fossil energy power plants.

However, 100% replacement of fossil fuel energy with renewable energy and nuclear energy will require seasonal-scale energy storage, possibly by using a combination of waste biomass and electrolytic H2. PHEV's usage is a good way to cut way down on petroleum consumption, but this falls short of permitting eventual 100% replacement of fossil-fuel energy.

A.C. R.

Predicting the wind is not the real problem. If you predict with 100% accuracy that the wind of tomorrow will only result in a 5% capacity factor for your wind fleet, then you have a PROBLEM. Even if you get 20/20 vision and a crystal ball, you have to actually go and do something about the lack of wind. There will be mild weather spells for weeks on end sometimes. Burn fossil? Or go nuclear and don't need fossil nor wind in the first place. Those are the options.

Like E-P says, the nuclear option is reliably available at night just when the plugins need charging. Wind is not. It may be available on one night, but completely absent on another. Last night here was VERY quiet, not even a breeze. Needless to say solar output was ZERO. Because, uhm, it was nighttime. That means fossil electricity for the plugins charging last night. Plugin hybrids are decent for schedulable demand on the timescale of hours, but not days, not to mention weeks. With solar you have near zero output most of the world in an entire season, better known as "winter".

An elementary school child can understand these things. For some reason entire regiments of renewable academics and other impractical people cannot understand it.


"An elementary school child can understand these things. For some reason entire regiments of renewable academics and other impractical people cannot understand it."
Excellent observation! But what is the point of criticizing yourself?


Last night here was VERY quiet

Yeah, it was very quiet THERE. The thing about the wind it that it is always blowing SOMEWHERE so the key to getting reliable wind power is to cast a wide net and think of it as a regional/national power source instead of a local one.
"Interconnecting wind farms through the transmission grid is a simple and effective way of reducing deliverable wind power swings caused by wind intermittency. As more farms are interconnected in an array, wind speed correlation among sites decreases and so does the probability that all sites experience the same wind regime at the same time. The array consequently behaves more and more similarly to a single farm with steady wind speed and thus steady deliverable wind power."
"The wind is always blowing somewhere so if widely distributed farms are linked together, we can
always count on some wind production. How much? A study in the US mid-west showed that one
third or 33% of annual power production from distributed wind farms could be counted on to supply
base load with the same reliability as a coal power plant. Because generation sites would also be
closer to demand, grid distribution losses would be cut from 7% to less than 2%. In Spain, where
wind farms are distributed over the entire country, hourly variations in the supply from wind are
effectively eliminated - smoothing out the electricity supply from wind. On a particularly windy day
in March 2008, Spain reached a point where 40% of its power was coming from wind."

Also, we need to stop thinking of wind OR solar OR nuclear OR etc.

This idea has been tested;

The thing about the wind it that it is always blowing SOMEWHERE so the key to getting reliable wind power is to cast a wide net and think of it as a regional/national power source instead of a local one.

This only works if you have enough machines to collect the power where it IS and enough transmission lines to feed it to where it ISN'T, and a net wide enough that your minimum needs are always satisfied.  There is massive resistance to siting of even essential short-run lines, and even if nobody objected you'd still have the problem of the cost.  Good luck with that.

Someday you have to admit that the dream of the all-Green grid relies on technical elements that fall somewhere between "never demonstrated" and "hopeless".  We can't wait for the dream, we need to fix things NOW.  We are already going on 30 years too late.

"Interconnecting wind farms through the transmission grid is a simple and effective way of reducing deliverable wind power swings caused by wind intermittency.

I recall a weather map where the entire central USA was somewhere between calm and barely above the cut-in speed of current wind turbines.  Interconnections broad enough to fix THAT intermittency problem may be effective, but are far from simple (or inexpensive).

I admit, I'm fascinated by ideas like floating wind farms anchored in the "Roaring Forties" producing juice to be converted to liquid fuels and sent to the rest of the world by tanker (possibly sail-powered).  Having run some numbers on the energy cost of recovery of CO2 from seawater, I am forced to admit that these can only be dreams for a long, long time.

Also, we need to stop thinking of wind OR solar OR nuclear OR etc.

We need to stop thinking about "some of EVERYTHING".  We need to toss out the elements that cannot pull their weight in energy production, dispatchability and carbon elimination.  This will leave us with mostly nuclear and hydro.

This idea has been tested

Testing is fine.  Relying on elements (like biogas) that cannot scale is insane.  If they cannot pull their weight in the major leagues, they have to be taken out of the batting order.  Facts are facts.

A.C. R.

ai fin, get real. Here's REAL data from ALL wind farms in an entire windy country, the Republic of Ireland (one of the windiest countries in the world).

That's a LOT of variation, even on an aggregated nation wide level.

Lika Mackay said, "we must not kid ourselves". Look at the numbers.


@ EP,
One thing of many we Americans are "renowned for" world wide is our attitude towards waste.
First of all we waste junk food on ourselves to make us obese and physically retarded (mental retardation is not too far off).
We waste our resources that will never return during our existence e. g. (Dust Bowl). I learned sixty years ago that contour plowing was essential to consistent farming. Where is that being consequently practiced today? We drive big fat vehicles synonymous to our own appearance and waste precious fuels and energy - and even worse - pollute our environment beyond repair. We have devastated our own forests that were the lungs of our country and made a cesspool of the Great Lakes region. But that not enough, we invade developing countries and continue to ravage and rape their resources to satisfy our thirst for continual wastage.
Will we ever learn that this planet and everything in and on it and subsequently our own country are finite.

"The Ugly American" is a devastating interpretation of how the American diplomatic corps acted and presented itself abroad. This behaviour of the past has only been topped recently by the activities of the NSA. Anyone considering us as their friends are punished sufficiently so they need no enemies. Decades ago when I ventured abroad, I was proud to be an American, nowadays my sense of shame overweighs.

If we, as a nation, have no desire to change and propogate the resolute intention to retain or even enhance our detractable behaviour, then I must concede that you are right with your conclusive deductions.

On the other hand, if we decide that we can, and change our behaviour and stop all this waste, then our children and grandchildren will have a fair chance of survival. In this sense, all those proposals from ai_vin are possible and by far - a better alternative.


You know it's almost laughable that, to show there's still "a LOT of variation, even on an aggregated nation wide level," ACR picks Ireland as an example. Ireland is so small it's national level IS our local level: The island of Ireland covers 84,431 square kilometers (32,599 square miles). At its widest the island is about 280 km (174 mi). At its longest (north-south) it is 486 km (302 mi).

As a comparison, the island of Ireland is smaller than the State of Indiana which is 35,910 square miles in area.


Well said yoatman and ai_vin.

USA and Canada is already producing twice as much e-energy than we really need if we stopped wasting 50+% of it.

A good start would be reducing (50+%) of the wasted energy used for our inefficient buildings and houses.

Changing buildings/houses construction codes is the first step that our politicians should but will not take. A progressive approach can reduce energy required for all new constructions by 50+% over a ten year period.

Most existing buildings/houses can be modified to reduce energy consumption by up to 50%.

The same can be done with existing and new industries and all new vehicles.

So, the easier lower cost solution is to consume 50% less over 10 to 20 years and no new NPP or CPP would be required. The existing HGPP, Nuke and Hydro power plants would be enough for the reduced base loads. Most CPP could be progressively phased out. Added Solar + Wind with storage + overnight off peak base load could easily handle the need of future electrified vehicles.


@ Harvey
Is this what you mean?


There's another reason I like the idea of a distributed power supply - the centralized power/plus grid system we currently use isn't as secure as we like to think it is.


Then obviously it's a bad idea to try to transfer power thousands of miles to balance regional differences in wind and solar generation.


There will be little inconvenience, if you can't recharge you BEV because the wind was too low, or it was too cloudy. So simply don't go to work tomorrow and maybe the next day, until the wind blows enough or the sun shines, to provide the energy to recharge your BEV.

That is an easy fix for intermittent wind and solar power. Sure Harvey. It's easy. No Problemo...

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