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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
24 January 2014
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
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