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| Fig. 1: A section of Solaroad in the Netherlands. (Source: Wikimedia Commons) |
A few years ago there were reports of potentially revamping the entire US interstate system with roads containing some solar-powered batteries which could then inductively charge electric vehicles that pass over them. Clearly a huge infrastructure project, but is it even possibly feasible? How long would it take, and would it be outdated the moment it was completed?
The solar road envisioned is a combination of two technologies. First, the photovoltaic (PV) road wherein the road is fitted with solar panels and batteries to absorb and store sunlight. In the Netherlands, in 2014, a project called Solaroad was opened, where one such road was tested, as a bike path. [1] One section of this road is shown in Fig. 1. Second, inductive roads, wherein the roads can inductively charge electric vehicles above ground. Some work has been done in Korea at the Korea Advanced Institute of Science and Technology (KAIST) in having a bus that is charged in just this way. [2]
However, the perennial problems that plagues both of these technologies are efficiency and obsolescence. The first is a killer because of the myriad loss mechanisms, which compound on one another. The PV cells themselves do not perfectly absorb all incident radiation, then there are losses in storing the energy and finally major losses in the inductive transmission. Secondly, PVs are still a rapidly developing technology, for which the technologies and prices are still significantly improving. This means that major solar projects which may take years to complete may be far behind contemporary technology by the time it is completed.
The goal of this study will be to study the technology and see if it is possible to overcome these problems. We will estimate the cost of an Interstate Highway System (IHS) scale solar road project using the price of current technologies. Then, the inductive power output capability of such a project will be estimated, followed by a discussion of its forecasted obsolescence.
First, let us consider the cost of such a road. The Solaroad project cost about $3 Million per for the entirety of the track, corresponding to about $1200 per square meter. [1] According to data by the Solar Energies Industries Association (SEIA), PV prices have fallen about 50% in the last 8 years since the Solaroad's construction. [4] Let us assume, optimistically, that this is reflected in the price of constructing a larger Solaroad at this point, and that the installation of these panels is the dominating cost of construction (i.e. that adding induction coils, pavement etc. is significantly less expensive than the panels themselves). Then, we estimate that the IHS was 78.5 × 106 m long, and approximately 24 m wide. Then, the total cost of the solar road project would be
We can compare this to the original estimate price for building the IHS, $1.17 × 1011 (in 2020, adjusted for inflation, which ballooned up to over 4.5 times that cost by project's end, and took 35 years rather than the estimated 12. [5] Heeding this warning, we make the final estimate of 4.5 × $1.13 × 1012 = $5.1 × 1012 ($5.1 Trillion) for the cost of such a solar road. This is over 4 times larger than the entirety of the Bipartisan Infrastructure Bill (IIJA) passed in 2021. [6] Clearly this would be an enormous investment.
Would this, however, satisfy our electric car energy needs (disregarding the storage and electric vehicle issues)? Optimistically, current PVs can produce a maximum of about 0.2 kW/m2 when the weather is clear. [7] Taking, on average, that the weather is clear about 30% of the total time of year, we can estimate that the total output of nation-wide Class 2 80 ft (24 m) wide highways per year would be approximately
However, not all of this energy will be directly useable. Further considering a 90% efficiency when inductively charging EVs, optimistically. [7,8] This brings our usable energy output to be about 8.9 × 1011 kWh y-1.
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| Fig. 2: Cost of a NREL Photovoltaic system benchmark ($/Watt). [4] (Source: D. Kuldinow) |
According to the U.S. Department of Energy, America's total transportation energy consumption is about 8.8 × 1012 kWh y-1. [9] Therefore, even at enormous cost, the Solaroad would be unable to meet the entirety of the nation's transportation energy needs. However, only about one-quarter of all travel is done on the IHS, bringing the amount of energy consumed on the IHS to be 2.2×1012 kWh/year, still about a factor of two out of range from our solar road capabilities [10]. To this number, there are quite a few mitigating factors. Firstly, the current efficiency of solar panels is about half of the maximum theoretically allowable and furthermore, it is possible to supplement these roads with arrays of solar panels to the sides of the roads to contribute to the power input. Both of these, in theory, could boost the output to the requisite power consumption on the IHS. Even in this case, it would still mean that the majority (three-quarters) of transportation energy would need to be transferred using other means (charging stations, gasoline hybrid etc.). However, there are a number of mitigating factors. The most obvious drawback is the obscuration of roads due to the residual rubber from tires or other rubble. This is, indeed, an issue and would require regular cleaning of the roadways to prevent them from becoming unusable, and would probably be quite expensive to perform on the national level. One option to avoid this would be to place all of the solar panels beside the road, and have them connected up to storage and inductive charging systems placed beneath the road. This would actually most likely be less expensive than the Solaroad, because it would not require specialized road surfaces, but rather just the addition of batteries and inductive chargers. Nevertheless, it seems that within a few years, as the technology for photovoltaics continues to improve, a small scale test of the technology might be warranted, like those at KAIST, to experimentally determine feasibility.
Now we need to consider how reasonable it would be to build such a project. Let's consider the time-dependent cost of PV technology, displayed in Fig. 2 according to the SEIA, along with an asymptotic exponential fit, a generalization of Moore's Law. Observing this fit, it would seem that we are very near the minimum price of about 2.57 $/W. However, we note that in the span of the last ten years, the price of PVs has more than halved. Thus, without more data over the next five years or so, it is impossible to know if we should expect a further significant drop in PV prices. Because even a 25-50% decrease would warrant pause, it is untenable to begin such a large project until the price of these devices has stabilized.
We have shown that, with current technologies, an IHS-scale solar road project with inductive chargers would not be sufficient to satisfy the needs of highway travelers in the United States. Until there is a factor of two increase in solar cell efficiency, or the solar panels occupy greater than twice the area of the road surface itself, the project will be impossible regardless of price. However, if such a breakthrough can be made, such a project could enable country-wide long distance EV travel (not only personal but also shipping) without the need for lengthy recharge stops. This would become especially useful as driverless trucks become commonplace and will thus require no stop at all. However, due to the current state and rapid improvement of these technologies, such a project cannot be started now and expected to have a long lifetime and should not be considered until the price of the construction materials has stabilized and a small scale long- term validation test has been completed.
© Derek Kuldinow. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] P. Oltermann, "World's First Solar Cycle Lane Opening in the Netherlands," The Guardian, 5 Nov 14.
[2] S. Lee et al., "On-Line Electric Vehicle Using Inductive Power Transfer System." 2010 IEEE Energy Conversion Congress and Exposition, IEEE 5618092, 12 Sep 10.
[4] D. Feldman et al., "US Solar Photovoltaic System and Energy Storage Cost Benchmark: Q1 2020," U.S. National Renewable Energy Laboratory, NREL/TP-6A20-773224, January 2021.
[5] "Federal-Aid Highway Act of 1956," Pub. L. 84-627, 70 Stat. 374 (1956).
[6] "Infrasructure Investment and Jobs Act," Pub. L. 117-59, 135 Stat. 429 (2021).
[7] D. Ginley, M. A. Green, and R. Collins. "Solar Energy Conversion Toward 1 Terawatt," MRS Bull. 33, 355 (2008).
[8] H. H. Wu et al., "A 90 Percent Efficient 5 kW Inductive Charger for EVs," IEEE 6342812, 12 Sep 12.
[9] "Monthly Energy Review October 2022," DOE/EIA-0035(2022/10), October 2022, Tables 2.1, 2.5, 3.7c, 4.3, and 6.2.
[10] "Traffic Volume Trends: December 2021," U.S. Federal Highway Administration, December 2021.
