# SolarEV City Concept for Paris

Paul Deroubaix, Takuro Kobashi, Léna Gurriaran, Fouzi Benkhelifa, Philippe Ciais, Katsumasa Tanaka

► To cite this version:

Paul Deroubaix, Takuro Kobashi, Léna Gurriaran, Fouzi Benkhelifa, Philippe Ciais, et al., SolarEV City Concept for Paris. Applied Energy, 2023, 350, pp.121762. 10.1016/j.apenergy.2023.121762. hal-04191442

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<figure><img src="image_1.png" /></figure>
# SolarEV City Concept for Paris<fnref n="1" />

Paul Deroubaix<fnref n="1" />,<fnref n="2" />, Takuro Kobashi<fnref n="3" />,*, Léna Gurriaran<fnref n="2" />, Fouzi Benkhelifa<fnref n="4" />, Philippe Ciais<fnref n="2" />, Katsumasa Tanaka<fnref n="2" />,<fnref n="5" />*

Emails: takuro.kobashi.e5@tohoku.ac.jp / katsumasa.tanaka@lsce.ipsl.fr

**Abstract.** Urban decarbonization is one of the pillars for strategies to achieve carbon neutrality around the world. However, the current speed of urban decarbonization is insufficient to keep pace with efforts to achieve this goal. Rooftop photovoltaics (PVs) integrated with electric vehicles (EVs) as battery is a promising technology scalable to supply CO₂-free, affordable, and dispatchable electricity in urban environments ("SolarEV City Concept"). Here, we evaluated Paris, France for the decarbonization potentials of rooftop "PV + EV" in comparison to the surrounding suburban area Ile-de-France and the reference city Kyoto, Japan. We assessed various scenarios by calculating energy sufficiency, self-consumption, self-sufficiency, cost savings, and CO₂ emission reduction of the PV + EV system or PV only system. We found that above a certain roof coverage 50-60% of the total roof area for Paris or 20-30% for Ile-de-France, PV electricity production regularly exceeds the demand. Above that roof coverage, feed-in-tariffs (FIT) or storage are needed to further realize the potential of the rooftop PV system. The combination of PVs with EVs is more effective in Ile-de-France than in Paris because large surplus electricity from rooftop PV in suburban area requires large storage.

With already low-carbon electricity of France by nuclear power, the maximum potential CO₂ abatement through the implementation of SolarEV City Concept in Paris (0.020 kgCO₂/kWh reduction from 0.063 kgCO₂/kWh) is limited in comparison to that in Kyoto (0.270 kgCO₂/kWh reduction from 0.352 kgCO₂/kWh). However, with declining costs of PVs and EVs in the coming decades, low-cost dispatchable electricity by the PV + EV system throughout Paris and Ile-de-France may facilitate the electrification of CO₂ emitting appliances, potentially allowing rapid CO₂ emission reductions from Paris.

<footnote n="1">¹ École Polytechnique, Palaiseau, France</footnote>

<footnote n="2">² Laboratoire des Sciences du Climat et de l'Environnement (LSCE), IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, Gif-sur-Yvette, France</footnote>

<footnote n="3">³ Graduate School of Environmental Studies, Tohoku University, Sendai, Japan.</footnote>

<footnote n="4">⁴ NEXQT – Liberté Living Lab</footnote>

<footnote n="5">⁵ Earth System Division, National Institute for Environmental Studies (NIES), Tsukuba, Japan</footnote>

<footnote>* Corresponding authors</footnote>

<footnote n="1">¹ A shorter version of the paper was presented at the 14th International Conference on Applied Energy (ICAE2022, Bochum, Germany), Aug 8-11, 2022. This paper is a substantially extended version of the short conference paper.</footnote>
**Keywords:** Urban decarbonization, electric vehicles, renewable energy, rooftop photovoltaics, Paris, Kyoto, SolarEV City

**Highlights**

* From roof coverage of 50-60% PV production regularly exceeds demand in Paris.

* PV + EV system is more effective in Ile-de-France than Paris for larger roof area.

* PV + EV system is less effective in Paris than in Kyoto for reducing CO₂ emissions.

**Nomenclature**

EV Electric Vehicles

ICE Internal Combustion Engine

DHI Diffuse horizontal irradiance

DNI Direct normal irradiance

FIT Feed-in-tariff

GHI Global horizontal irradiance

NPV Net Present Value

PV Photovoltaics

SOC State of Charge

V2B Vehicle to Building

V2H Vehicle to Home

**Introduction**

Many state governments including the US, China, India, and Japan have announced so-called “net zero” emission targets by mid-century to steer the world towards an emission pathway consistent with the long-term temperature goal of the Paris Agreement [1,2]. In July 2021, the EU established a legal framework to achieve net zero greenhouse gas (GHG) emissions by 2050 [3], with individual states setting varied target years (e.g. 2045 for Germany; 2050 for France). Net zero targets have also been put forward by regional and local municipalities including Paris and Kyoto. Decarbonization of urban areas is important because CO₂ emissions from cities around the world account for 71%-76% of the global CO₂ emissions [4], and because first-hand decarbonization measures are often performed through local governments [5]. Increasing zero carbon energy technologies (e.g., renewables) and improving energy efficiency, as well as electrifying the transport and heat system, are major pillars of decarbonization [6,7]. The SolarEV City Concept [8], a novel concept that makes use of synergies between rooftop photovoltaics (PVs) and electric vehicles (EVs) as battery (PV + EV), supports such pillars. When both technologies are combined, EVs can not only eliminate CO₂ emission from gasoline/diesel combustion through PVs, but also serve as electricity storage to address intermittency with rooftop PVs [9,10].

The first study for the “PV + EV” in a city scale was conducted for Kyoto City, Japan (hereafter, “Kyoto”), which demonstrated that by using 70% of the rooftop area of Kyoto for PVs and converting all passenger vehicles to EVs, Kyoto’s CO₂ emission from electricity generation and internal combustion engine vehicles (ICEs) can be reduced by 60-74% with 22-37% energy cost reduction in 2030 [9]. Then, the analyses were extended to nine Japanese urban areas: Koriyama City, Sendai City, Okayama City, Kyoto City, Kawasaki City, Hiroshima City, Sapporo City, Niigata City, and Tokyo special districts [8]. It was found that the
effectiveness of “PV + EV” systems for urban decarbonization is highly variable, depending on the level of urbanization. For example, Kawasaki and Tokyo are highly urbanized areas with a small rooftop area and the number of vehicles per capita, resulting in relatively limited CO2 emission reduction (54-58%) by the PV + EV systems with energy cost saving of 23-26%. On the other hand, regional cities such as Niigata and Okayama have a larger rooftop area and the number of vehicles per capita, which can yield larger CO2 emission reduction (up to 95%) by the systems with higher energy cost saving of up to 34%.

The effectiveness of the “PV + EV” systems also depend on the country specific factors such as electricity tariff, climate, and urban structures. Chang et al. analyzed five South Korean cities, which indicated “PV + EV” systems can reduce CO2 emissions up to 86% with cost saving of 51% [11]. South Korean cities have generally higher buildings than those in Japanese cities, which reduces the benefits of “PV + EV” systems than those of Japanese regional cities [11]. Liu et al. analyzed Shenzhen, China, which is a highly urbanized new city, and found that CO2 emissions can be reduced by 42% and cost saving by 21% through the “PV + EV” system [12]. Dewi et al. evaluated the potentials of the “PV + EV” system for Jakarta, the capital of Indonesia, and found CO2 emission reductions by 76-77%, accompanied by energy cost savings by 33-34% [10].

The study by Arowolo and Perez analyzed Paris, Lyon, and Marseille, France for the effects of “PV + EV” systems [13]. For a half of rooftop available space installed with PVs and a half of passenger vehicles replaced with EVs, they found 20-42% of electricity demand reduction, 43-48% of CO2 emission reduction, and a payback period of 2-3 years by 2030. Although Arowolo and Perez analyzed Paris for the PV + EV system as in this study, they did not account for the electricity supply to the city from rooftop PV generation with hourly supply-demand balance (self-sufficiency), which is important for the analyses of variable renewable energy (VREs), particularly due to its the high-latitude location of Paris with strong seasonal variation of solar insolation.

None of the above studies and else have investigated PV + EV potentials in high latitude's cities with hourly supply-demand balance, where solar insolation and electricity demands are seasonally inversely correlated due to the increase in demand mainly driven by heating in winter. Therefore, in this study we assessed Paris for the potential of the SolarEV City Concept in comparison with the surrounding region of Ile-de-France and the reference city of Kyoto. Paris can be seen as an iconic city of climate change as it is the birthplace of the Paris Agreement, so is Kyoto for the Kyoto Protocol. Our analysis of Paris and Kyoto may thus be useful for raising awareness of the SolarEV City Concept.

Under the background above, this paper will strive to answer the following questions:

1. How can the SolarEV City Concept (rooftop PV system combined with EVs as battery) contribute to the decarbonization of the City of Paris (a high latitude city)?

2. What are the factors that can affect the potential of “PV + EV” for Paris in comparison to those of Ile-de-France and Kyoto?
### 3. What would be barriers to implement the SolarEV City Concept in Paris?

This paper is organized as follows. Section 2 describes the methods and materials of this study, including the City of Paris, techno-economic analysis, and scenarios. Section 3 illustrates the results of the analyses of Paris in comparison with those of Ile-de-France and Kyoto. Section 4 discusses the implications of the results and barriers to implement such technologies in a city scale. Section 5 concludes the paper.

## 2. Methods and materials

### 2.1. “Paris” and surrounding region, “Ile-de-France”

We studied two areas: Paris, i.e. the administrative jurisdiction of the City of Paris (or Paris “intramuros”), and Ile-de-France, which is defined as the region around Paris in this study unless noted otherwise (the administrative jurisdiction of Ile-de-France include the City of Paris). Paris is a highly urbanized area with population of over 2.1 million, located in high latitude (48.9° N, 2.4° W) (Fig. 1; Table 1). To fully elucidate the potential of rooftop PVs integrated with EVs for Paris, it is useful to consider the surrounding region of Ile-de-France as potential solar power suppliers (Fig. 1). Ile-de-France (including Paris) covers 18.9% of the population of France (i.e., mainland) [14], which is the largest metropolitan area within the EU. While Paris is an urban area with a high population density and energy intensity, Ile-de-France is mostly a suburban area with a lower population density and energy intensity (Fig. 1). Public transport is well developed in Paris. In Ile-de-France, on the other hand, residents more frequently use and own passenger vehicles. These differences are apparent in the number of registered vehicles per capita and average driving distances (Table 2). Data for Kyoto is also listed for comparison.

The French government will ban the sales of gasoline and diesel vehicles from 2040. EV share (BEVs and PHEVs) in France has already reached 21% for sales and 2.7% for stock in 2022 [15]. The Paris government will start an equivalent ban 10 years earlier, and the Ile-de-France government set a ban for diesel vehicle sale by 2030. Since 2018, a territorial climate-air-energy plan has been adopted in France, mandating inter-municipalities with more than 20,000 inhabitants to establish a global warming mitigation strategy based on renewable energy and energy consumption control. In this context, the City of Paris has announced that by 2050 it will achieve an energy mix of 100% renewables, of which at least 20% will be supplied locally [16].

<table><caption>Table 1. General statistics of Paris, Ile-de-France (including Paris), and Kyoto. Data for Kyoto are from [17].</caption><thead><tr><th></th><th>Paris</th><th>Ile-de-France</th><th>Kyoto</th></tr></thead><tbody><tr><td>Population (million)</td><td>2.18</td><td>12.2</td><td>1.47</td></tr><tr><td>Area (km²)</td><td>105</td><td>12,011</td><td>827</td></tr><tr><td>Density (1000 ppd/km²)</td><td>20.6</td><td>1.0</td><td>1.8</td></tr></tbody></table>
<table><caption>Table 2. The number of cars, the state of car use, and the average travel distance in Paris, Ile-de-France (including Paris), and Kyoto. Data for Kyoto are from [17].</caption><tr><td>Roof area (km²)</td><td>31</td><td>402</td><td>52</td></tr><tr><td>Roof area per capita (m²)</td><td>14</td><td>33</td><td>13</td></tr><tr><td>Annual Electricity consumption per capita (kWh)</td><td>6031</td><td>6277</td><td>5678</td></tr></table>

<table><thead><tr><th></th><th>Paris</th><th>Ile-de-France</th><th>Kyoto</th></tr></thead><tbody><tr><td>Number of diesel cars (thousand)</td><td>250</td><td>2,802</td><td>-</td></tr><tr><td>Number of gasoline cars (thousand)</td><td>334</td><td>2,525</td><td>-</td></tr><tr><td>Total number of cars (thousand)</td><td>585</td><td>5,327</td><td>485</td></tr><tr><td>Number of cars per capita</td><td>0.27</td><td>0.44</td><td>0.33</td></tr><tr><td>Proportion of cars used during weekdays</td><td>0.35</td><td>0.63</td><td>-</td></tr><tr><td>Average distance of driving cars (km per car per day)</td><td>21.8</td><td>26.3</td><td>8.0</td></tr></tbody></table>

<figure><img src="image_2.png" /><figcaption>Figure 1. Map of Ile-de-France including the City of Paris ("Paris Intramuros").</figcaption></figure>

## 2.2. Techno-economic analysis
The viability of variable renewable energy (VRE) projects depends on various factors such as climate, electricity tariff, costs of renewable energy technologies, electricity demand, supply-demand matching, degradation of technologies, feed-in-tariff (FIT), discount rate, urban structures, etc. [18–21]. A techno-economic analysis is a useful tool to evaluate all these factors and can provide information on whether to invest in projects under consideration [20,22]. We employed a methodology developed in the previous studies [17,23] that has been applied to investigate the future potential of the SolarEV City concept elsewhere. The methodological description here is kept concise at the level required for presenting our results.

Our analysis was performed with a publicly available energy-economic software, SAM (System Advisor Model; version 2020.11.29 Revision 2), developed by the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) [24]. SAM is designed to evaluate the viability of various types of renewable energy projects [25]. The model runs with a given set of parameters or with a range of values for a subset of parameters iteratively. SAM can analyze the system at the scale of the city (or the region) and considers the surplus and deficit only at this level. The model does not account for the surplus and deficit at the household level, which may well occur without any surplus or deficit at the regional level. Thus, the basic assumption is that rooftop PV generated electricity was consumed in a city in the most-efficient way within the framework we considered. Grid constrains are not considered, which requires more detailed studies. The PV + EV system can have positive and negative influences on the grids depending on how it is managed [26]. The SAM files used for our analysis is available at Mendeley Data [27].

We calculated the cost savings in terms of net present value of cashflow for a project period of 25 years [17], with a nominal discount rate of 2.5%. The potential changes in the costs of centralized electricity production, in particular the cost of maintaining the current means of production, are not considered. The costs of replacing existing gasoline and diesel vehicles with EVs and the resulting saving of fuel costs are treated as the background in our scenarios and are thus not considered in our cost calculations unless otherwise noted. The net present value is expressed in 2019 Euros or 2030 Euros, depending on the first year of the project. In addition to cost savings, we calculated energy sufficiency, self-sufficiency, self-consumption, and CO₂ emission reduction as decarbonization indicators [17]. Energy sufficiency is defined as the total amount of electricity produced by PV in one year divided by the annual electricity demand. Self-sufficiency is the actual electricity coming from PV consumed in a year divided by the annual demand. Self-consumption is the proportion of PV electricity that is consumed within the region considered [17] (or the ratio of self-sufficiency to energy sufficiency). More precisely, these indicators [28] can be expressed as:

$$ Cost \ saving \ (%) = \left\{ 1 - \frac{\frac{NPV}{N}}{AnnualEnergyCost_{Base}} \right\} \times 100 $$

where NPV is net present value of cash flow, N is project period, and AnnualEnergyCostBase is annual energy
expense of a base scenario (i.e., annual electricity from power company and gasoline cost).

$$ \text{Energy sufficiency} = (E_{\text{PV-load}} + E_{\text{battery-load}} + E_{\text{PV-grid}}) / (E_{\text{load}}) $$

$$ \text{Self-sufficiency} = (E_{\text{PV-load}} + E_{\text{battery-load}}) / (E_{\text{load}}) $$

$$ \text{Self-consumption} = (E_{\text{PV-load}} + E_{\text{battery-load}}) / (E_{\text{PV-load}} + E_{\text{grid}}) $$

where

$E_{\text{load}}$ = Total electricity load (kWh·yr⁻¹) in the city.

$E_{\text{PV-load}}$ = Electricity (kWh·yr⁻¹) supplied from PV directly to load in the city.

$E_{\text{battery-load}}$ = Electricity (kWh·yr⁻¹) supplied from battery to load in the city.

$E_{\text{PV-grid}}$ = Electricity (kWh·yr⁻¹) exported from PV to grid out of the city.

The outputs from SAM were used to calculate these indices. We calculated CO₂ emission reduction from vehicles and the consumption of electricity to assess the environmental benefits. [17]

## 2.3. Scenarios

We focused on two illustrative scenarios: “PV only” and “PV + EV”. The PV only assumes a penetration of building rooftop PV. The PV + EV scenario assumes a penetration of residential rooftop PV combined with electric vehicles with bi-directional charging (e.g., V2H). In the PV + EV scenario, all vehicles are assumed to be electric and connected to vehicle-to-home (V2H) or vehicle-to-building (V2B) systems (bi-directional EV chargers), in which electric vehicles can be used to charge electricity from the rooftop PV and to discharge to the building. We studied these two scenarios for two areas: Paris and Ile-de-France, comparing them also with Kyoto [23]. We considered two periods: 2019 (for PV only) and 2030 (for PV only and PV + EV). The choice of area affected technical characteristics in the analysis, such as the roof area, the number of cars or the electricity demand. In our analyses, the choice of period only affected the prices, with projections used for the 2030 scenarios, to illustrate the effects of cost decline. For each scenario, we tested a different set of hypotheses on the roof coverage, different technologies (PV only or PV + EV), cost of technologies (2019 or 2030), and feed-in-tariffs (with and without). As for the techno-economic analysis, we compared the scenarios with a “base” one with the uses of gasoline/diesel engine vehicles and grid electricity [23].

## 2.4. Meteorological data

We used hourly data for the following parameters: diffuse horizontal irradiance (DHI; W/m²), direct normal irradiance (DNI; W/m²), global horizontal irradiance (GHI; W/m²), temperature (°C) and wind speed (m/s). We obtained this dataset for Paris and Ile-de-France (49°N, 002.5°E) for 2019 with a tool SIREN, which calculates these parameters from climate reanalysis data MERRA-2 [29]. However, a direct application of this data can lead to an overestimate of the PV electricity production [9]. Thus, DHI, DNI, and GHI were reduced with a coefficient of 0.8, which was inferred from the analyses for Japanese cities [17]. The calculated capacity factor
of 11.1% by SAM was a good agreement with the observed value of 11.0% in 2019 for Paris. The year 2019 was the one with the highest capacity factor in the period 2014-2020 (minimum 9.6% and mean 10.3%). In SAM, we chose the tilt angle of the panel to be 40° and its azimuth to be 180° for Paris and Ile-de-France.

## 2.5. Electricity demand and cost

The hourly electricity demand for Ile-de-France is available for the year 2019 from "Réseau de Transport d'Electricité" [30](Fig. 2). For 2030, we rescaled the 2019 demand by a factor 1.08 to match the electricity demand of EV for the "PV + EV" scenarios. The increased electricity demand was calculated under an assumed current EV efficiency of 17.2 kWh/100km. This efficiency corresponds to the mean use (mean between winter and summer conditions) of a Renault Zoé in city driving conditions. The driving patterns were assumed to be the same in 2019 and 2030. On the other hand, data for hourly electricity demand were not available for Paris. However, data for annual demand are given by Enedis, the power grid operator for the whole city [31]. For Paris, we thus used the hourly electricity demand of Ile-de-France multiplied by a factor of 0.18 based on the annual demand data of two regions. For 2030, we calculated that the demand increase due to EV charge is 2%, from deriving distance and EV efficiency resulting in the same scaling factor of 0.18 for Paris.

Concerning the prices of electricity, all our analysis for Paris and Ile-de-France used a weighted average price of 0.16 €/kW from the prices for household (0.178 €/kW) and industry (0.105 €/kW) [32] with annual electricity consumptions of 22.7 TWh and 7.3 TWh in 2019 [33], respectively. When we considered FIT, we used 0.04 €/kWh for both regions, which was an average day-ahead market price of electricity in 2019 for France [34]. We used the same price of electricity in 2019 and 2030. As the price of electricity has been rising in France for the past 30 years [32], particularly strongly in the recent past in the midst of geopolitical turmoil, it is likely to continue rising in the foreseeable future, which adds additional values on decentralized energy systems [18].
<figure><img src="image_3.png" /><figcaption>Figure 2. Temperature, average daily electricity demand, and daily PV generation for Paris in 2019 and Kyoto in 2018. The data for Kyoto are from [23].</figcaption></figure>
## 2.6. PVs

The roof areas of Paris and Ile-de-France are 31 and 402 $km^2$, respectively [35]. To convert the roof area into the maximum capacity of PV, we used an estimate of 7 $m^2$ (i.e., 5 $m^2$ for PV panels and 2 $m^2$ for space) for 1 kWh of PV for both 2019 and 2030, which corresponds to the use of about 70% of the total rooftop area of the city [17]. In 2019, the cost of a PV-system depends on the surface area of panels for each roof. We used the 5-10 $m^2$ price (Table 3), which is higher than the price of 10-100 kW (thus 50-500 $m^2$) based on the dataset given by the Apur [36]. Using the higher price, our results on energy cost saving by rooftop PV system should be considered conservative. Maintenance prices were given as 22.5 €/kW/yr [36] and assumed to be the same in 2030. BloombergNEF (BNEF) estimates the global price for fixed-axis utility scale PV systems in 2030 to be 69% of 2020 prices (all costs included) [37]. We applied the same reduction to our residential prices, which leads to the prices for 2030 given in Table 3. A currency conversion rate of 1.12 US$/€ was used (2019 mean).

<table><caption>Table 3. Prices of PVs assumed in this study.</caption><thead><tr><th></th><th>Paris</th><th>Ile-de-France</th></tr></thead><tbody><tr><td>PV, 2019, installation and inverter included (€/W)</td><td>1.9</td><td>1.9</td></tr><tr><td>PV, 2030, installation and inverter included (€/W)</td><td>1.31</td><td>1.31</td></tr></tbody></table>

## 2.7. EVs and vehicle statistics

We assumed that EVs are equipped with a 40kWh battery [17], and set the power charge to 6 kW. Discharge can be expected with the same power for a V2H system [17]. It was projected that Europe would reach price parity between EV (including home chargers) and ICE during the period 2025-2030 [38]. The additional costs of having EV plus V2H system is estimated to be around 25 €/kWh in 2030 [17]. The cost of battery replacement of 91 € per battery (when the capacity is reduced to 80% of the initial capacity) is included in the techno-economic analysis [17]. We limit the use of EV batteries only in the range from 50 to 95% (state of charge: SOC) to prevent battery degradations and to allow EV owners to use their EVs for short trips anytime.

Number of cars and annual driving distance are given in Table 2. We assumed that the number of cars, the proportion of cars used during weekdays, and the average distance of moving cars in Paris and Ile-de-France would be the same between 2019 and 2030. It should be noted that while Paris and Ile-de-France replace ICEs with EVs, the number of vehicles may be reduced. The city of Paris is promoting the use of bicycles instead of cars, which is also an important decarbonization measure [39]. A decrease in the use of vehicles, therefore an increase of the parked time of vehicles, might raise the potential of “PV + EV”. However, a significant change in the number of vehicles would reduce the total battery capacity and thus could affect the results of the “PV + EV” scenario.
## 3. Results

### 3.1. Paris and Ile-de-France compared

For Paris, “PV only” scenario can bring benefits already in 2019, and the benefits can further increase in 2030 (Fig. 3a). There are economically optimum PV capacities of 2.7 GW and 3.6 GW with FITs for 2019 and 2030, respectively (Fig. 3a; Table 4). Excessive roof area PV coverages lead to negative NPVs owing to large capital costs and excess PV generation. Figure 3b shows that in the “PV only” case, surplus PV electricity start increasing from around 40%. From this point, FIT starts to play an important role to increase NPVs and optimal PV capacity. In addition, above this threshold, self-sufficiency diverts from energy sufficiency, and self-consumption starts to decline (Fig. 3b). Concerning “PV+EV”, NPVs become larger than zero above 10% rooftop coverage in 2030, and NPVs continue rising above 70% (Fig 3a). Thus, the rooftop PV economics reaches the maximum with the maximum usage of the rooftop area (71%) in Paris by coupling with EVs. Self-consumption is 100% even at the maximum rooftop usage (71%) for “PV + EV” system with self-sufficiency and energy sufficiency being equal (Fig. 3c).

For Ile-de-France, there are optimums for PV capacity in all the scenarios (Fig. 3d). For “PV only”, FITs make a difference with a roof coverage of above around 20% (Fig. 3d), which is much smaller than those for Paris owing to larger rooftop area per capita (Fig. 3a). When the rooftop coverage of PV becomes larger than around 20%, PV electricity production becomes greater than the demand in many hours of the year, resulting in a lower self-consumption and a larger difference between self-sufficiency and energy sufficiency (Fig 3e). In addition, NPVs reach peaks in 54% and 60% rooftop coverage, and abruptly drops, which is the result of the added EV battery replacements and incurred costs owing to the increased usage of EV batteries for large surplus electricity (Fig 3a). It is noted that energy sufficiency in Ile-de-France reaches 78%, which is much larger than 34% of Paris. Concerning “PV + EV”, NPVs become larger than zero above 8-10% coverage in 2030. In contrast to Paris, self-consumption exhibits a slight decrease from 62% of the roof coverage with increasing surplus electricity. Accordingly, self-sufficiency is lower than energy sufficiency above 62% of the rooftop coverage (Fig. 3f).

The results above indicates that Paris, where energy demand per area is high and the rooftop area is relatively limited, is better suited to have “PV only” below 50% of the rooftop coverage. For Ile-de-France with a relatively large rooftop area, on the other hand, PV-generated electricity can be quickly saturated with a small roof coverage (Fig. 3e). In this instance, PV + EV scenario has more advantages than PV only scenario, as long as the systems will be implemented at a substantial scale (i.e., above the threshold rooftop area). If PV only scenario proceeds further in Ile-de-France, it could lead to an overproduction of electricity that could result in negative NPVs, requiring a curtailment, as is already happening in some areas with a large amount of renewables. It is noted that NPVs in Figure 3 do not include cost saving from elimination of gasoline/dieses expenses. Thus, rooftop PV + EV systems have larger cost benefits than the results shown in Figure 3.
<figure><img src="image_4.png" /><figcaption>Figure 3. “PV only” and “PV + EV” potentials for Paris (left panels) and Île-de-France (right panels). Note that NPVs do not include cost savings from gasoline/diesel expenses.</figcaption></figure>
<table><caption>Table 4. Decarbonization indicators for Paris and Ile-de-France in 2019 and 2030. Cost-savings include those from gasoline/diesel expenses as in [23]. Optimal PV capacity is defined by maximum NPVs. The percentages indicated for the optimal PV capacity are the corresponding rooftop area in the city.</caption><thead><tr><th rowspan="2"></th><th colspan="2">2019</th><th colspan="2">2030</th><th colspan="2">2030</th></tr><tr><th colspan="2">"PV only"</th><th colspan="2">"PV only"</th><th colspan="2">"PV+EV"</th></tr><tr><th><strong><u>Paris</u></strong></th><th>with FIT</th><th>without FIT</th><th>with FIT</th><th>without FIT</th><th>with FIT</th><th>without FIT</th></tr></thead><tbody><tr><th>Optimal PV capacity (GW)</th><td>2.7 (44%)</td><td>2.5 (44%)</td><td>3.6 (58%)</td><td>3.2 (52%)</td><td>4.4 (71%)</td><td>4.4 (71%)</td></tr><tr><th>Self-consumption (%)</th><td>97</td><td>98</td><td>87</td><td>92</td><td>100</td><td>100</td></tr></tbody></table>
<table><tr><td>Self-sufficiency (%)</td><td>20</td><td>19</td><td>24</td><td>23</td><td>31</td><td>31</td></tr><tr><td>Energy sufficiency (%)</td><td>21</td><td>19</td><td>28</td><td>25</td><td>31</td><td>31</td></tr><tr><td>Cost saving (%)</td><td>2</td><td>2</td><td>5</td><td>5</td><td>12</td><td>12</td></tr><tr><td>CO<sub>2</sub> emission reduction (%)</td><td>18</td><td>17</td><td>21</td><td>20</td><td>51</td><td>51</td></tr></table>

<table><thead><tr><th rowspan="2">Ile-de-France</th><th colspan="2">2019</th><th colspan="2">2030</th><th colspan="2">2030</th></tr><tr><th colspan="2">"PV only"</th><th colspan="2">"PV only"</th><th colspan="2">"PV+EV"</th></tr><tr><th></th><th>with FIT</th><th>without FIT</th><th>with FIT</th><th>without FIT</th><th>with FIT</th><th>without FIT</th></tr></thead><tbody><tr><td>Optimal PV capacity (GW)</td><td>15 (19%)</td><td>14 (17%)</td><td>20 (25%)</td><td>18 (22%)</td><td>43 (54%)</td><td>47 (60%)</td></tr><tr><td>Self-consumption (%)</td><td>97</td><td>98</td><td>88</td><td>92</td><td>99</td><td>100</td></tr><tr><td>Self-sufficiency (%)</td><td>20</td><td>19</td><td>24</td><td>23</td><td>53</td><td>50</td></tr><tr><td>Energy sufficiency (%)</td><td>21</td><td>19</td><td>27</td><td>25</td><td>53</td><td>50</td></tr><tr><td>Cost saving (%)</td><td>1</td><td>1</td><td>4</td><td>4</td><td>21</td><td>22</td></tr><tr><td>CO<sub>2</sub> emission reduction (%)</td><td>13</td><td>12</td><td>14</td><td>14</td><td>79</td><td>77</td></tr></tbody></table>
## 3.2. Paris and Kyoto compared

The potential of the SolarEV City Concept differs not only by the geographical extent as exemplified by the comparison between Paris and Ile-de-France but also climate, latitude, electricity tariff, etc. Therefore, it is useful to compare Paris with Kyoto, Japan, where the SolarEV City Concept was first investigated [9]. There is a large difference in latitude between Paris and Kyoto. As Earth is round and the rotating axis is tilted, higher latitudes experience annually a lesser amount of solar irradiance and larger seasonal change of solar irradiance, which affects energy yields of PV. The slope and azimuth of PV panels also affect energy yields differently in different latitudes (Fig. 4). Flat PV panels in Paris produces about 24% less energy in a year than that of Kyoto (Fig. 4). Maximum energy yields can be obtained in a 30°-50° slope in Paris and in a 30° slope in Kyoto with south faced azimuth (180°) (Fig. 4), where Paris has still 21% less energy yield than that of Kyoto. However, energy yields on southern faced façades (a slope of 90%) in Paris are only 9% less than that of Kyoto (Fig. 4), indicating that façade PVs are relatively good options in higher latitudes.

<figure><img src="image_5.png" /><figcaption>Figure 4. Energy yields with different slopes of rooftop PVs with various azimuths. Red lines represent Paris, and blue lines represent Kyoto.</figcaption></figure>

On the seasonal variation of PV energy yields, Paris shows a notable decline in electricity generation in winter compared to Kyoto (Fig. 5). The variation can be alleviated by having some slopes, but the winter decline in Paris is rather large, considering large electricity demands for heating. It is noted that wind power generates more electricity in winter in Europe [40] and may compensate the decline in solar power.
<figure><img src="image_6.png" /><figcaption>Figure 5. Monthly PV generation with various southern faced PV slopes in Paris and Kyoto.</figcaption></figure>

Figure 6 illustrates average hourly electricity demand variations in a day for different months for Paris and Kyoto [23]. In Paris, hourly demand variation shows some consistent characteristic through the year with a first peak around noon then another peak around 19:00. During summer, demands are smallest and in winter demand reaches peaks (Fig. 2 and 6). On the other hand, daily demand profiles in summer and winter are different in Kyoto (Fig. 6). The largest electricity demand comes in summer with one large peak in the afternoon. In winter, two peaks develop, a morning peak around 9 pm and an evening peak around 7 pm. It is interesting to note that a drop in the lunch break only develops in Kyoto not in Paris (Fig. 6). Figure 6 also shows how electricity is supplied from the “PV + EV” system for both cities. In Paris, owing to smaller rooftop area per capita and less PV generation, the “PV + EV” system can only supply 31% of its demand (annual-mean). On the other hand, the “PV + EV” system in Kyoto can supply 76% of electricity demand, which includes the demand from EVs (Table 4 and 5).

<figure><img src="image_7.png" /><figcaption>Figure 6. Hourly demand in a day for different months and annual-mean supply by “PV + EV” in a day. Electricity supplies from grid, PV, and battery are shown in orange, red, and grey, respectively for the “PV + EV” scenario with</figcaption></figure>
FIT in 2030 for both cities.

<table><caption>Table 5. Decarbonization indicators for Kyoto. Cost-savings include those from gasoline/diesel expenses as</caption><thead><tr><th rowspan="2"></th><th colspan="2">2019</th><th colspan="2">2030</th><th colspan="2">2030</th></tr><tr><th colspan="2">"PV only"</th><th colspan="2">"PV only"</th><th colspan="2">"PV+EV"</th></tr><tr><th><u>Kyoto</u></th><th>with FIT</th><th>without FIT</th><th>with FIT</th><th>without FIT</th><th>with FIT</th><th>without FIT</th></tr></thead><tbody><tr><td>Optimal PV capacity (GW)</td><td>2.3 (22%)</td><td>1.8 (17%)</td><td>7.4 (72%)</td><td>2.8 (27%)</td><td>7.4 (72%)</td><td>5.9 (57%)</td></tr><tr><td>Self-consumption (%)</td><td>88</td><td>96</td><td>40</td><td>80</td><td>84</td><td>94</td></tr></tbody></table>

in [23]. The percentages indicated for the optimal PV capacity are the corresponding rooftop area in the city.
<table><tr><td>Self-sufficiency (%)</td><td>29</td><td>25</td><td>43</td><td>32</td><td>76</td><td>68</td></tr><tr><td>Energy sufficiency (%)</td><td>33</td><td>26</td><td>106</td><td>40</td><td>90</td><td>73</td></tr><tr><td>Cost saving (%)</td><td>4</td><td>4</td><td>18</td><td>11</td><td>31</td><td>29</td></tr><tr><td>CO2 emission reduction (%)</td><td>27</td><td>23</td><td>40</td><td>30</td><td>78</td><td>70</td></tr></table>

Figure 7 shows cross-spectral coherence [41] between electricity demand vs. temperature and PV generation. Cross-sectional coherence for temperature vs. demand shows positive coherence (or correlation) in summer and negative coherence (correlation) in winter for periods longer than around 64 hours (or a few days) for both Paris and Kyoto (Fig. 7). Paris shows wider area of significant correlation both in winter and summer for the periods of 64-512 hours and 1024-2048 hours than those of Kyoto (Fig. 7). For the plot of demand vs. PV generation, cross-spectral coherence become less significant except daily periods. Positive coherence becomes significant in Kyoto in summer for the periods of 512 hours, indicating that space cooling demands are coinciding with higher PV generation (Fig. 7).
<figure><img src="image_8.png" /><figcaption>Figure 7. Cross-spectral coherence [41] for demand vs. temperature and PV generation. Arrows pointing right indicate in-phase relationships, and vice versa. The areas enclosed with lines are significant in a 95% confidence. X-axis represents time, and y-axis represents periods.</figcaption></figure>

Figure 8 shows Kyoto's decarbonization potentials for “PV only” and “PV + EV” in 2018 and 2030 as for Paris and Ile-de-France in Fig. 3. Kyoto's profiles are more similar to those of Ile-de-France than that of Paris in terms of higher self-sufficiency and surplus electricity (Fig. 3 and 8). In Kyoto, the “PV + EV” system can provide 76 % (self-sufficiency) of electricity demand including the demands from EVs. In comparison with “PV only” system, coupling PV systems with EV battery can help increase self-consumption from 40 % to 84 % (Fig. 8 and Table 4).
<figure><img src="image_9.png" /><figcaption>Figure 8. “PV only” and “PV + EV” potentials for Kyoto. “w.” and “w/o” indicate with and without, respectively.</figcaption></figure>

The differences between Paris and Kyoto on the effects of “PV + EV” originate from energy structure of the cities and climate. To understand the influence of climate including the latitudinal effect on the “PV + EV” system for Paris, we apply the energy demand, the number of vehicles per capita (also annual driving distance), and rooftop area per capita of Kyoto to Paris as a sensitivity analysis, such that the energy structure becomes comparable with that of Kyoto in the analysis [28]. [28]With these settings, self-consumption, self-sufficiency, and energy sufficiency of the “PV + EV” system for Paris is 87%, 62%, and 71% for the “PV + EV” system with the full usage (71%) of rooftop area in the city. The self-sufficiency in Kyoto is 76% (Table 5), which is higher by 14 points than that of Paris. Our sensitivity analysis indicates that the effect of the “PV + EV” system in Paris is lower by around 20% than that of Kyoto owing to the difference in climate.
### 3.3. CO₂ emission reductions from power sector

France is already quite low carbon in terms of electricity generation owing to predominant use of nuclear power plants. In 2019, the grid CO₂ emission factor for France was 0.063 kgCO₂/kWh [42], which was much lower than 0.352 kgCO₂/kWh in 2019 for Kyoto [43]. Thus, “PV + EV” system can potentially reduce CO₂ emission from electricity consumption by 0.020 kgCO₂/kWh (≈ 0.063*0.31, where 0.31 is self-sufficiency for PV + EV) with the prices of 2030 for the Paris power system and 0.270 kgCO₂/kWh (≈ 0.352*0.76, where 0.76 is self-sufficiency for PV + EV) for the Kyoto power system. Therefore, the “PV + EV” system in Kyoto, where the current electricity generation has high CO₂ emission from coal- and gas-fired power plants, is 13.5 times more effective to lower the mean CO₂ content (/kWh) of power generation than that in Paris when the full potential (our assumption of 70 % rooftop area) of “PV + EV” system is realized in both regions.

## 4. Discussion

Our model calculations showed that developing rooftop PV could bring economic benefits for both Paris and Ile-de-France, although it is smaller than those of Kyoto in a lower latitude. It was estimated to be the case already in 2019 and the benefits could increase in 2030 owing to declining costs. Up to a certain roof coverage, 50-60% of the total roof area for Paris and 20-30% for Ile-de-France, the production does not exceed the demand, resulting in a nearly 100% self-consumption at the macroscopic level. Above such threshold, FIT was shown to play a role. EVs used as batteries are another option, which can be more profitable thanks to the combination of EVs with PV through V2H or V2B systems developed at the city or region level. V2H or V2B systems can add value to PV systems, allowing a high self-consumption and self-sufficiency, when the roof coverage exceeds the threshold coverage for the region. The systems also allow bypassing the classical battery storage, which is too expensive to be profitable. Finally, because electricity production is already low carbon in France due to the reliance on nuclear power (70.5% of generated electricity in 2019) [44], solar panels alone do not allow substantial CO₂ emission abatements in the power sector. But the “PV + EV” scenario facilitates further CO₂ emission abatements, directly or indirectly from the use of EV relative to that of ICE.

Technical difficulties may hinder the implementation of the PV systems on complicated roof surface and V2H and V2B systems in densely populated districts [45]. Grid constraints could also be a limiting factor [46]. These factors will require further technical studies as well as demonstration projects for a widespread use of rooftop PV integrated with EV. Furthermore, there are also practical/social constraints for the implementation of “PV only” and “PV + EV” systems [47], which require further social/policy studies. For example, wind turbines are not well accepted in some French regions, and solar panels may not be socially accepted in a historic city such as Paris [48]. Most buildings in Paris are under co-ownership that would require an agreement on the PV installation and use, as well as associated cost and benefit sharing. Legal difficulties could also arise since buildings situated less than 500 m away from a historical monument or landmark must ask for a specific permit for a PV installation. The present roof coverage of PVs is less than 0.3% in Paris. While the use of EV is becoming more widespread in Paris these days, not all buildings have sufficient parking space for EVs. Such
infrastructure constrain may hinder the implementation of V2H systems in Paris. However, such barriers may be lower in Ile-de-France. Even if the systems bring net benefits at some point of the long-term project period, the requirement of a large initial investment could also be an economic barrier. To address those issues, strong political leaderships, accompanied by legal and financial support and stakeholder engagement, may be needed.

Although Paris faces various barriers for the penetration of rooftop or building integrated PV (BIPV) systems, technical developments of PV, battery, and relevant technologies are rapidly advancing, including costs [49], design [50], efficiency [51], etc., opening windows of opportunities to realize a SolarEV City in Paris. Projected cost decline of PV and EV [49,52] will produce low-cost dispatchable electricity within the city (i.e., levelized cost of electricity (LCOE) of less than €0.04/kWh [13]), which could make necessary electrification faster in Paris, Ile-de-France, and beyond [53].

In comparison to power sector (6% of the total CO2 emission in France), other sectors such as industry (23%), building (21%), transport (40%), and energy (7%: own use) sectors emits a large amount of CO2 totaling about 300 Mt-CO2 in 2021 in France [54]. Energy efficiency improvement and electrification is the first options for the transitions towards clean energy [4]. Paris and other cities in France will require a large amount of additional clean electricity toward carbon neutrality [6]. Rooftop PVs as the least invasive renewable energy (LiRE) [9] provide effective energy toward carbon neutrality, integrated with EV batteries. As the birthplace of “Paris Agreement”, Paris has an important mission to play a leading role for urban decarbonization toward net-zero.

## 5. Conclusions

Our analyses indicated that synergies between PVs and EVs cannot be expected much if it is performed in Paris alone as its effect is only activated above the rooftop PV coverage of 50%-60%. In addition, rooftop PVs can face certain implementation barriers in Paris, and may only lead to a modest reduction in CO2 emissions from the power sector in France owing to the predominant nuclear power. However, our analysis also suggested that if the SolarEV City Concept can be implemented in all the Ile-de-France administrative region or jointly for Paris and Ile-de-France, large surplus PV electricity can be generated in the suburban area of “Ile-de-France”, which can be supplied to the center of the region, “Paris”. The storage brought by EVs in the suburban area is highly effective, nearly doubling economically optimal PV capacity compared to that of “PV only”. In addition, Ile-de-France is less susceptible to implementation challenges (i.e., historical buildings) for Paris. Finally, we found that the high-latitude locations of Paris and Ile-de-France are not an advantage for the “PV + EV” system in comparison to Kyoto, but it may be complemented with higher wind power potentials in winter in the region. For these reasons, the SolarEV City Concept for Paris jointly with Ile-de-France can be one of the important pillars for urban energy transition toward sustainability.

## Acknowledgement
K.T. benefited from State assistance managed by the National Research Agency in France under the Programme d'Investissements d'Avenir under the reference ANR-19-MPGA-0008. The project is supported by JSPS KAKEN: 23K11520 to T.K.

**CRediT authorship contribution statement**

Paul Deroubaix: Methodology, Formal analysis, Investigation, Writing - Original Draft, Visualization. Takuro Kobashi: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - Original Draft, Writing - Review & Editing, Visualization, Project administration. Léna Gurriaran: Investigation, Writing - Original Draft. Fouzi Benkhelifa: Investigation, Writing - Review & Editing. Philippe Ciais: Investigation, Writing - Review & Editing, Funding acquisition, Project administration. Katsumasa Tanaka: Conceptualization, Investigation, Writing - Original Draft, Writing - Review & Editing, Supervision, Project administration.

**Declaration of Competing Interest**

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

**Data availability**

Supplementary data is available at https://data.mendeley.com/datasets/t93bh6bthj/1.

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