Measurement of power loss during electric vehicle

Energy xxx (2017) 1e3

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Comments on “Measurement of power loss during electric vehicle charging and discharging” e Notable findings for V2G economics Yosef A. Shirazi a, *, David L. Sachs b a b

College of Earth, Ocean, and Environment, University of Delaware, USA Bright Power Inc., New York, NY, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 April 2017 Received in revised form 9 September 2017 Accepted 18 October 2017 Available online xxx

Apostolaki-Iosifidou et al. [1], published in the May edition of Energy, is an important contribution to the field of vehicle-to-grid (V2G). It is the first publication to document the roundtrip efficiency of V2G, a critical element for assessing its economic viability. Roundtrip efficiency was found to range between 53% and 62%. This contrasts with existing V2G economic analyses, which either assume roundtrip efficiency to be substantially higher, or ignore losses altogether. We highlight an additional source of energy loss not captured in the experimental design of Ref. [1]: unfavorable temperatures. Given the wide divergence between empirical findings and the assumptions employed in existing V2G economic analyses, we propose that existing analyses greatly underestimate the costs of implementing V2G and should be considered incomplete until re-specified with updated efficiency values. We show that even using the highest efficiencies found by Ref. [1], electricity losses offset more than half of the revenue from V2G frequency regulation in PJM, the most frequently studied ancillary service market for V2G operation. Researchers should employ findings in Ref. [1] for more accurately-specified V2G economic models. © 2017 Elsevier Ltd. All rights reserved.

Keywords: V2G Vehicle-to-grid Roundtrip efficiency Frequency regulation

1. Introduction The net utility that any technology offers to society can only be assessed through a full accounting of its costs and benefits. Yet two decades after the popularization of the vehicle-to-grid (V2G) concept [2], empirical estimates of electricity losses during V2G remained unreported. Apostolaki-Iosifidou et al. [1] deserves praise for identifying and beginning to fill this knowledge gap. The study quantifies electricity losses in a V2G storage system made up of a single vehicle and its supporting electrical infrastructure including the EVSE, breakers, and transformer. Losses for the V2G storage system were measured at two currents, 10 A and 40 A. Charging and discharging losses at 10 A were 17% and 36%, respectively. At 40 A, charging and discharging losses were 12% and 30%, respectively. These values are reported in the summary line of Table 7 in Ref. [1] and also feature in the paper's abstract. While data in Ref. [1] is presented only in terms of losses, V2G literature has coalesced around the language of efficiency (h). Roundtrip efficiency (hrt), the proportion of energy available after

* Corresponding author. E-mail addresses: [email protected] (Y.A. Shirazi), [email protected] (D.L. Sachs).

all charging losses (lc) and discharging losses (ld), is calculated as the product of charging efficiency and discharging efficiency. Therefore, the losses listed above can be expressed as hrt of 53% and 62% for the 10 A and 40 A scenarios, respectively. A formal expression of hrt is shown below:

hrt ¼ ð1 Ic Þ*ð1 Id Þ

(1)

The remainder of this communication is divided into four sections. First, we discuss select aspects of the methodology in Ref. [1] that influence loss measurements. Second, we discuss the importance of hrt for V2G operations and show that existing literature has greatly overestimated hrt. Next, we quantify the impact of hrt on V2G economics. We conclude with a summary of findings and offer suggestions for future research on V2G efficiency and V2G economics. 2. On experimental methodology Several aspects of the experimental design used in Ref. [1] may have influenced the results in underappreciated ways. The article suggests that the use of an over-sized transformer inflated measurements of system losses (see Section 4.1 in Ref. [1] for an example). While we agree that a sub-optimally sized transformer

https://doi.org/10.1016/j.energy.2017.10.081 0360-5442/© 2017 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Shirazi YA, Sachs DL, Comments on “Measurement of power loss during electric vehicle charging and discharging” e Notable findings for V2G economics, Energy (2017), https://doi.org/10.1016/j.energy.2017.10.081

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Y.A. Shirazi, D.L. Sachs / Energy xxx (2017) 1e3

Nomenclature EVSE ld lc

hrt

PJM V2G

Electric vehicle supply equipment Percent losses during discharging Percent losses during charging Round-trip energy efficiency A regional transmission organization (RTO) in the Eastern USA Vehicle-to-Grid technology

electricity losses during V2G operation (Table 1). That peer review frequently failed to catch this substantial shortcoming is concerning. Of those analyses that do acknowledge efficiency loss, the median assumed hrt is 83.5% with a range of 73%e86%. In contrast, hrt measured by Ref. [1], the only empirical data available on the matter, did not exceed 62%. Stated differently, the highest hrt found empirically is far lower than any hrt assumed, and we are aware of no exceptions. See Table 1 for a comparison of empirical and assumed values in the peer-reviewed literature. 4. Quantifying economic costs

likely increased system losses, we also note that testing only in ideal temperatures likely suppressed system losses. The relative magnitudes of these two counteracting factors determine whether total losses for this system in a real-world implementation would be higher, lower or equal to values reported in Ref. [1]. As described in Section 3.2.1 in Ref. [1], all testing was performed near room temperature. However, battery systems experience elevated losses when operating much higher or lower than room temperature, as would often be the case for real-world V2G operations. Cold temperatures increase battery resistance [3e7], while hot temperatures necessitate power draws for thermal management systems to cool the battery and electrical components [4,5,8]. Both processes reduce system efficiency, but neither is reflected in the results from Ref. [1]. Temperature effects are especially critical because V2G is most beneficial to society and lucrative to participants during periods of high electricity demand, which tend to coincide with extreme ambient temperatures [9]. 3. Contextualizing the findings Losses during V2G are now empirically shown to exist. This may seem peculiar to state explicitly, but numerous peer-reviewed V2G economic analyses do not acknowledge efficiency losses. These omissions result in substantial over-estimation of the net benefits of V2G. Roundtrip efficiency loss has at least five deleterious effects on V2G economic viability. It increases electricity consumption, which must be paid by the V2G operator; it increases the energy throughput of the battery, thereby accelerating battery degradation; it decreases the maximum energy a vehicle can provide (kWh) before needing to charge; it decreases the maximum power a vehicle can provide (kW); and for frequency regulation specifically, it decreases the maximum achievable fidelity to the automatic generation signal, thereby incurring proportionally reduced revenue (see for example [10]). In a forthcoming research article, in which we survey the advances and shortcomings of V2G economic analyses, we identify twelve original peer-reviewed articles with transparent economic methodologies. One half of those analyses omit the cost of

Table 1 Comparison of empirical (first row) and assumed (all other rows) roundtrip efficiency (hrt) from select V2G economic analyses.

hrt

Study

53%e62% 73% 81% 85% 86% 100%

[1] [9,11]; [12] [13,14]; [15] [16,17,18,19,20,21]

In addition to an engineering analysis, Ref. [1] also includes an economic analysis of a hypothetical V2G system. The analysis models the provision of 1 MW of frequency regulation in PJM for 24 hours and assumes a frequency regulation price of $30/MW-hr. Electricity losses (payable to the power company) are found to offset 17% of revenues earned from V2G. However, $30/MW-hr pricing for frequency regulation is not representative of prevailing prices in PJM. Frequency regulation pricing has fallen in recent years alongside increased deployment of fast-responding natural gas generation and utility-scale storage systems [22,23]. Rather than a frequency regulation price of $30/ MW-hr as used in Ref. [1], the trailing 6, 12 and 24 month average prices for frequency regulation in PJM are $11.85, $12.50 and $14.53/MW-hr, respectively [24]. We have recalculated the value of electricity losses during V2G frequency regulation in PJM using a more representative $13/MWhr frequency regulation price and the highest hrt reported in Ref. [1], 62%. We assume a $0.12/kWh electricity rate for commercial sector prices in the Middle Atlantic region [25] and hold other inputs the same as Ref. [1]. Results from our model indicate that providing 1 MW of frequency regulation in PJM for 24 hours yields $312 in revenue and consumes 1382 kWh of electricity due to efficiency losses. These losses result in an imposed cost of $166, or 53% of V2G revenues. In other words, an amount greater than half of all revenue earned from V2G is offset (payable to the power company) by the incremental increase in electricity consumption. While the absolute monetary value of electricity losses will vary with the quantity of service contracted into the frequency regulation market, the percentage of revenue offset by efficiency losses will remain constant under these assumptions.1 Fig. 1 displays the relationship between the value of electricity losses and frequency regulation price at three different values of hrt. These hrt values represent the lowest hrt from Ref. [1] (53%), the highest hrt from Ref. [1] (62%), and the median hrt from those studies that acknowledge efficiency losses (83.5%). Note that at frequency regulation prices under $7/MW-hr, the value of electricity losses exceeds total V2G revenue for hrt values of 62% and lower. Because hrt is critical for accurate V2G economics, and the gap between previously assumed and empirically determined hrt values is so wide, we recommend that existing economic analyses are incomplete without re-specification. References [18e21] all model the economic viability of V2G in

1 Even in an idealized scenario where transformers are 100% efficient and ambient temperatures are always ideal, hrt would be 68% as measured by Ref. [1] instead of 62% at the 40 A scenario. This is calculated by dividing the original nrt at 40 A by the measured charging efficiency and discharging efficiency of the transformer at 40 A from Table 7 in Ref. [1]. Assuming hrt of 68% in the economic model described above yields $312 in revenue and $138 in electricity losses, indicating 44% of V2G revenue is offset by electricity losses.


Losses as % of Revenue

Y.A. Shirazi, D.L. Sachs / Energy xxx (2017) 1e3

100% 80%

rt = 53% rt = 62% rt = 83.5%

60% 40% 20% 0% 5

15

25

35

Frequency Regula on Price ($/MW-hr) Fig. 1. The value of electricity losses as a percentage of revenue compared to frequency regulation price at three values of roundtrip efficiency (hrt). The dashed vertical line represents $13/MW-hr.

the exact market described here, yet all omit the cost of electricity losses during V2G. We believe these studies are the most likely to see the greatest reduction in estimated net benefits of V2G after respecification. In an upcoming paper, we aim to show how existing V2G economic analyses share numerous other substantial shortcomings in their models. 5. Conclusion and outlook Apostolaki-Iosifidou et al. [1] should be widely recognized as the first to report empirical efficiency of a storage system during V2G operation. Electricity losses likely represent the single largest cost element for V2G operations in the frequency regulation market and should be emphasized in future research. We recommend that future research on V2G efficiency assess newer vehicle models and follow actual frequency regulation signals. Additional research refinements should include conducting trials in ambient and variable temperature conditions with appropriately sized transformers. We encourage publication of empirical hrt values from purpose-built stationary storage systems as well. Stationary systems are the closest substitutes to vehiclebased storage and may yield meaningfully higher values of hrt. Various initiatives across the world are considering or actively incorporating V2G programs into electrical grids for frequency regulation services (for example [26e28]). However, if these programs are looking to existing peer-reviewed literature for accurate economic estimates, results of actual V2G programs are likely to fall short of expectations. Improved economic analyses, relying on empirical data like those provided by Ref. [1], can yield more reliable estimates of the net costs and benefits of this potentially promising technology. Acknowledgments The authors are indebted to three anonymous reviewers for their contributions to this discussion piece. The authors would also like to thank Lauren Knapp and Dr. Rebecca Anthouard for improving clarity. Any errors that remain are the fault of the authors alone. References [1] Apostolaki-Iosifidou E, Codani P, Kempton W. Measurement of power loss during electric vehicle charging and discharging. Energy 2017;127:730e42.

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