Ancillary Services in Vehicle-to-Grid (V2G)
Hao Liang
Department of Electrical and Computer Engineering
University of Waterloo
Waterloo, Ontario, Canada, N2L 3G1
h8liang@bbcr.uwaterloo.ca
BBCR Smart Grid Subgroup Meeting
2011.1.12
Outline
• Introduction
• Categories of V2G Applications
• Bidirectional V2G for Frequency Regulation (Phase 2)
• Unidirectional V2G for Frequency Regulation (Phase 1)
• Non-Ancillary Service: Load Shaving (Phase 3)
• Summary
1
Introduction
• Conventional View of Battery Vehicles (BVs)
– The conventional view expects battery vehicles to be plugged in to charge their
batteries. Hybrid and fuel cell vehicles generate electricity from the fuel in their
tanks. Plug-in hybrids can either run from fuel or can charge from the grid. In the
conventional view, electricity never flows from vehicle to the grid
[1] The Grid-Integrated Vehicle with Vehicle to Grid Technology. University of Delaware. http://www.udel.edu/V2G/.
2
Introduction
• V2G Concept
– The V2G concept is that battery, hybrid, and fuel cell vehicles all can send power to
the electric grid. For battery and plug-in hybrid vehicles, the power connection is
already there. For fuel cell and fuel-only hybrids, an electrical connection must be
added
3
Introduction
• University of Delaware
• Utility Trials
PG&E, USA, converting a number of company-owned Toyota
Prius to be V2G PHEVs at Google's campus
Xcel Energy, USA, converting six Ford Escape Hybrids to PHEVs
with V2G
4
Introduction
• Other Types of Grid Storage
[2] Energy Storage and Frequency Regulation. http://www.pjm.com.
5
Categories of V2G Applications
• Load Shaving
– Aims at using the energy stored in electric vehicles to compensate for the peak load
of the grid
– From the vehicle owners’ point of view, since electricity price is determined by demand,
the transport cost can be relatively reduced by drawing “cheap” energy from the grid,
and vice versa
• Ancillary Services (Frequency Regulation)
– An ancillary mechanism to fine-tune the frequency of the grid in a small time scale,
e.g., minutes
– May not necessarily involve energy delivery but simply the use of the capacity of
vehicle batteries
6
Categories of V2G Applications
7
Categories of V2G Applications
• Ancillary Services (Frequency Regulation, Cont’d)
– Bidirectional V2G: “Load + generator” regulation
– Unidirectional V2G: “Load only” regulation:
May reduce profits to less than 1/4 of what they could gain using bidirectional V2G
All legacy EVs can participate without any retrofit to the EVs themselves or substantial
additional infrastructure in the charging stations
A logical first step in V2G implementation
8
Bidirectional V2G for Frequency Regulation (Phase 2)
[3] S. Han and K. Sezaki, “Development of an optimal vehicle-to-grid aggregator for frequency regulation,” IEEE Trans. Smart Grid,
vol. 1, no. 1, pp. 65–72, Jun. 2010.
9
Bidirectional V2G for Frequency Regulation
• Locational Marginal Pricing (LMP) and Regulation Market
Clearing Price (RMCP)
10
Bidirectional V2G for Frequency Regulation
• Analysis on Charging Rate Control
– A vehicle is considered as providing regulation service when being idle and the vehicle
is paid from the grid operator. On the other hand, when a vehicle is charging, it has to
pay for purchasing the power from the grid. Thus, a revenue function could be defined
as follows:
11
Bidirectional V2G for Frequency Regulation
• Analysis on Charging Rate Control (Cont’d)
– Rewrite the revenue function
– Assuming that the prices are given as hourly data, the second integral term in a discrete
form yields
where N is the number of increment in expected plug-in duration T
12
Bidirectional V2G for Frequency Regulation
• Analysis on Charging Rate Control (Cont’d)
– A “Theorem”: charging control should be on or off at maximum charging rate to
maximize the revenue
– Thus, we no more need to be concerned about the charging rate. Instead, we focus on
how to determine the charging sequence
13
Bidirectional V2G for Frequency Regulation
• Analysis on Charging Sequence Control
– The continuous variable r(t) can be replaced by a step function C(t), which is only
1 or 0 for the entire duration
– Then the revenue function (3) is revised as
14
Bidirectional V2G for Frequency Regulation
• Analysis on Charging Sequence Control (Cont’d)
– Optimal solution: An optimal control sequence could be obtained simply by turning
on (making it 1) from the minimal point of
until the charging duration constraint (8) is satisfied
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Bidirectional V2G for Frequency Regulation
• Consideration of SOC
– For example, from the moment SOC reaches 100%, regulation “down,” which
corresponds to charging of the battery, cannot be performed at all
– The regulation price PR(t) in (7) should be
modified to reflect the energy constraint
as follows
16
Bidirectional V2G for Frequency Regulation
• Consideration of SOC (Cont’d)
– Solution: Dynamic Programming
– In order to perform the dynamic programming, the revenue function was employed as
a performance measure with a slight modification to reflect the desired final SOC
control as a cost
where x(t2) is an actual SOC at the end of the control, xT is a desired final SOC,
and  is a proportional factor which reflects the relative importance of the desire to
drive the system to the final SOC xT
17
Bidirectional V2G for Frequency Regulation
• Numerical Results
– Regarding the vehicle parameters, we assumed 20 KWh battery with maximum
charging and discharging rate of 0.1 C, which in turn is 2 KW for each direction
– The vehicle was considered with 10% initial SOC, and the desired final SOC was
set to 90%
– The total plug-in duration was set to 12 h. The aggregator should control the
charging to be on for 8 h to transfer the SOC to 90%
18
Bidirectional V2G for Frequency Regulation
• Numerical Results (Cont’d)
19
Unidirectional V2G for Frequency Regulation (Phase 1)
[4] E. Sortomme and M. A. El-Sharkawi, “Optimal charging strategies for unidirectional vehicle-to-grid,” IEEE Trans. Smart Grid,
vol. 2, no. 1, pp. 131–138, Mar. 2011.
20
Unidirectional V2G for Frequency Regulation
• Graphical Description
21
Unidirectional V2G for Frequency Regulation
• Regulation Algorithm
• Research Problem
– Aggregator profits come from two sources: The first is a fixed markup over market price
on energy which is passed on to the consumers. The second is a fixed percentage of the
revenue obtained from providing regulation services
– Optimizing: POP, MxAP, and MnAP, for all vehicles connected to the aggregator
22
Non-Ancillary Service: Load Shaving (Phase 3)
[5] S. B. Peterson, J. F. Whitacre, and J. Apt, “The economics of using plug-in hybrid electric vehicle battery packs for grid storage,”
J. Power Sources, vol. 195, no. 8, pp. 2377–2384, Apr. 2010.
[6] H. Liang, B. J. Choi, W. Zhuang, and X. Shen, "Towards optimal energy store-carry-and-deliver for PHEVs via V2G system," in
Proc. IEEE INFOCOM'12, to appear.
23
Non-Ancillary Service: Load Shaving
• Graphical Description
24
Non-Ancillary Service: Load Shaving
• Research Problem
– Daily energy cost minimization of vehicle owners
– Consider: Bi-directional energy flow, vehicle mobility pattern, realistic battery model,
and time-of-use (TOU) electricity price
– Solution: Dynamic programming, specifically, stochastic inventory theory
25
Summary
•
Unidirectional V2G for Frequency Regulation (Phase 1)
•
Bidirectional V2G for Frequency Regulation (Phase 2)
•
Non-Ancillary Service: Load Shaving (Phase 3)
26