Measurement and Modeling of 3D Solar Irradiance for Vehicle-Integrated Photovoltaic

Round 1

Reviewer 1 Report

the paper is presenting a vehicle-integrated photovoltaic system on a Toyota car. the shading objects and car-orientation were investigated by the mathematical modeling and experiments. 

the paper can be reviewed again after the essential revision comments below:

-description of fig. 4 is not clear to me: the curved module cannot receive the total the illumination area fora  flat module. how authors claim the curved module will receive unexpected more light coming from the outer region? the same reasoning is also repeated in fig. 5. in fact in fig. 8 we see this description is inconsistent with assumption of fig. 4, and 5. in fact, the more curvy, the more drop in performance. fig. 5 is opposite to fig. 11. we see the curved margin of the module abosrbing less.

-citation is essential to the degradation of the module by time: https://doi.org/10.1016/j.solener.2018.05.082

when authors take my comments in, i can reconsider in my decision.

Author Response

Author’s note for Reviewer 1.

For images and equations, please, find in the attached file. Unfortunately, this page cannot export images and other objects.

Point 1

-description of fig. 4 is not clear to me: the curved module cannot receive the total the illumination area fora  flat module. how authors claim the curved module will receive unexpected more light coming from the outer region? the same reasoning is also repeated in fig. 5. in fact in fig. 8 we see this description is inconsistent with assumption of fig. 4, and 5. in fact, the more curvy, the more drop in performance. fig. 5 is opposite to fig. 11. we see the curved margin of the module abosrbing less.

Author’s response

We appreciate the reviewer’s suggestion. The point from Reviewer 1 is a common question that frequently appeared in the technical discussion in standardization work for VIPV (IEC TC82 and the car-roof PV web-meeting). Shortly speaking, the over-estimation in standard indoor testing (used to labeling to the car-roof PV product) leads to an under-estimation of the outdoor performance. It is crucial to differentiate these two (outdoor performance vs. standard indoor testing). The previous manuscript was not clear on this point and the section 2.5.1 was completely revised.

2.5.1 Why is the curved PV modules are often overestimated in efficiency measurement?

Before discussing the curve-correction, let us clarify where the standard measurement method has a problem. The curved modules are often overestimated in efficiency measurement. It is because the input energy is often underestimated during the measurement by the indoor solar simulator.

To explain this, let us go back to the definition of efficiency measurement of the solar cells and modules (Equation (17)).

where,  is measured energy efficiency of the solar cell or module using the solar simulator.  is the input power to the solar cell or the module.  is the output power of the solar cell or the module. The measurement of  of the curved module is the same of the standard flat PV modules, and it is a simple electrical measurement. However, the trappy measurement is .

In the standard measurement of the solar cell and module,  is calculated by Equation (18).

where,  is an aperture area.  is irradiance in the aperture window. In the solar simulator measurement,  is adjusted as 1 kW/m² and uniform in the entire area of the aperture area . For the standard flat PV module, the aperture area  is the same as the module active area. However, this definition is not applied to the curved PV module, because, the aperture area is defined as the window of the flat plane, and not the curved surface.

Figure 4. Illustration of the underestimation of the input energy in the measurement of the curved PV module

The first cause of the overestimation is that the curved PV module often collects more light than that is do be defined by the aperture window (Figure 4). Since the illumination area of the solar simulator is always more extensive than the PV module, the curved module will receive unexpected more light coming from the outer region so that it generates unexpected more power. Alternatively, it is an excellent way to place an aperture mask on the curved PV module. However, it is often eliminated because the multiple-reflection between the aperture mask and optics in the solar simulator disturbs the uniform illumination in the zone of the aperture window and the solar simulator is often and repeatedly required time-confusion adjustment in the optics. Besides, there are no agreed standards in the position of the aperture mask for the curved PV module. Depending on the curved shape, the aperture mask interferences with the body of the curved module.

The second cause is the aperture area; namely, the module area in the active region of the curved module is not defined clearly (Figure 5) and may have multiple definitions. It is a cause of the fatal error in the estimation of the module efficiency in the dividing area of the module (Equation (17) and (18)).

Figure 5. Illustration of the multiple definitions of the module area

The third cause is the difference in the angular distribution in the indoor measurement (typical solar simulator) and outdoor operation. The curved surface generates more loss in the illumination by a higher incident angle (Figure 6). Unlike the case of the flat PV module, it cannot be applied to the indoor module efficiency to the outdoor operation. Correction by the curved shape is essential.

Figure 6. Illustration of the multiple definitions of the module area: (a) Principle ray direction in testing, although the illumination by the typical solar simulators is not collimated, and the field of view is typically ±10° to ±45°; (b) Principle ray direction in the outdoor operation. The outdoor illumination is the mixture of the collimated light (direct sunlight) and diffused sunlight (illumination from the sky and reflection by surroundings). The ratio of the collimated and diffused light varies by climate; (c) Example of the distribution level in the outdoor operation calculated by the ray-tracing simulation. The light-green arrow lines correspond to the direct sunlight. The blue arrow lines correspond to the scattered sunlight. The color gladiation on the curved surface indicates the non-uniformity of the irradiance on the curved surface. The darker color indicates the lower irradiance.

Point 2

-citation is essential to the degradation of the module by time: https://doi.org/10.1016/j.solener.2018.05.082

Author’s response

We appreciate your suggestion. Since it is not a paper for the solar cell, but testing and outdoor operation modeling, we did not include the contents of the “degradation.” For readers interested in the application of various solar cells for VIPV, we added texts related to degradation in the Introduction section and added citations to various types of solar cells and modules.

The above calculations assume that the solar cells are stabilized and no or negligible degradation. However, some solar cells degrade by time, and it is to be considered, in case such type of solar cells are used [6-9]. The photovoltaic is also useful for auxiliary powers and range-extension of other low-carbon energies [10-15].

Stutzmann, M. Role of mechanical stress in the light‐induced degradation of hydrogenated amorphous silicon. Applied Physics Letters, 1985, 47(1), 21-23. Moeini, I.; Ahmadpour, M.; Mosavi, A.; Alharbi, N.; Gorji, N. E. Modeling the time-dependent characteristics of perovskite solar cells. Solar Energy, 2018, 170, 969-973. Lindroos, J.; Savin, H. Review of light-induced degradation in crystalline silicon solar cells. Solar Energy Materials and Solar Cells, 2016, 147, 115-126. Meyer, E. L.; Van Dyk, E. E. Assessing the reliability and degradation of photovoltaic module performance parameters. IEEE Transactions on reliability, 2004, 53(1), 83-92. Letendre, S.; Perez, R.; Herig, C. Vehicle integrated PV: a clean and secure fuel for hybrid electric vehicles. In PROCEEDINGS OF THE SOLAR CONFERENCE 2003 June, (pp. 201-206). AMERICAN SOLAR ENERGY SOCIETY; AMERICAN INSTITUTE OF ARCHITECTS. De Pinto, S.; Lu, Q.; Camocardi, P.; Chatzikomis, C.; Sorniotti, A.; Ragonese, D.; ... & Lekakou, C. (2016, October). Electric vehicle driving range extension using photovoltaic panels. In 2016 IEEE Vehicle Power and Propulsion Conference (VPPC) (pp. 1-6). IEEE. Kim, J.; Wang, Y.; Pedram, M.; Chang, N. Fast photovoltaic array reconfiguration for partial solar powered vehicles. In Proceedings of the 2014 international symposium on Low power electronics and design (2014, August) (pp. 357-362). ACM. Alhammad, Y. A.; Al-Azzawi, W. F. Exploitation the waste energy in hybrid cars to improve the efficiency of solar cell panel as an auxiliary power supply. In 2015 10th International Symposium on Mechatronics and its Applications (ISMA) (2015, December). (pp. 1-6). IEEE. Fujinaka, M. (1989, August). The practically usable electric vehicle charged by photovoltaic cells. In Proceedings of the 24th Intersociety Energy Conversion Engineering Conference (pp. 2473-2478). IEEE. Ezzat, M. F.; Dincer, I. Development, analysis and assessment of a fuel cell and solar photovoltaic system powered vehicle. Energy conversion and management, 2016, 129, 284-292.

We also polished English and corrected several editorial mistakes.

Author Response File: Author Response.docx

Reviewer 2 Report

Very good, detailed and high level article. Can be published as is.

Author Response

I appreciate your review.

Reviewer 3 Report

The subject is extensively studied and models are well thought out. There are few minor improvements that are required. 

For such an extensive piece of work the conclusion is too brief. Please expand the conclusion to accommodate findings of each section. It may not be a bad idea to be a little more specific and somewhat expand the point. 

Please comment in the feasibility of the solar powered car based on the presented study. Authors have put in a lot of effort to analyse a fairly extensive range of conditions. It makes the manuscript appear incomplete with no comments on the feasibility of the idea in practice. Of course this can be accompanied by a statement clarifying that this feasibility is only based on the current results. 

Page 18, figure 17 not figure x - section 4.2 2nd paragraph.

GHI is used as an abbreviation across the manuscript but is not defined in the text. Please use the full form when the abbreviation is used for the first time. 

While latitude and longitude are briefly touch upon towards the end of the manuscript, these are limited to very small region as shown in figure 17. It may be good to include, at least a simulation or a calculation of the effect of latitude and longitude. Also, it may be important to point at the possible relationship of distance from equator which in some sense summarize the combination of latitude and longitude.

Consider summarizing the limitations of this model in the discussion section. 

In general, the discussion and conclusion sections need a little more work to do justice to rest of the extensive work. Take into account the unfortunate fact that there are very few people that actually read the entire publication especially when it is this extensive. For such readers (and there are a lot of them) it would be important to make these two sections more detailed. 

Author Response

Authors note to reviewer 3

Point 1

For such an extensive piece of work the conclusion is too brief. Please expand the conclusion to accommodate findings of each section. It may not be a bad idea to be a little more specific and somewhat expand the point.

Author’s response

I appreciate your constructive suggestion that lead our work more constructive. Now, the conclusion section is updated.

Conclusions

Increasing the concern of the greenhouse gas emissions in the transportation sectors, the fuel, and engine of the car is required to consider the total emission of CO₂ by a well-to-wheel basis. The typical approach is an electric vehicle (EV) charged by the electricity generated by renewable energy like solar power. However, this approach relies on the infrastructure of the distribution of the clean electricity (PV power station installation, grid construction and connection, distribution of the clean electricity, and installation and operation of the EV charging station). It is much more convenient than the car collects solar energy and runs by it (or at least extending the range of mileage supported by its PV modules to reduce the frequency of charging).

The solar-powered vehicles are now seriously discussed, and we had several feasibility reports [2-3]. In principle, it is feasible by the improvement of both PV and car technologies to run a majority of electric vehicles on solar energy. However, as it is often the case of the early feasibility study, it is not supported by the real solar resources of the car running in the real environment. The motivation for our study is to know the real and active solar energy for the car through both modeling and measurement.

For modeling, we developed a simple shading model using a uniformly distributed (density and height) shading objects and random driving orientation using a simple combination of the collimated direct sunlight, uniform diffused sunlight, and reflection by road and shading objects. For the validation of the model, we used an array of pyranometers mounted on the car in 5 axes and monitored the solar resource around the car for one year. The model was validated in x, y, and z-direction local to the car as well as the angular distribution of the main beam of the solar irradiance on the car-roof.

Additionally, we developed by the correction method of the curved surface of the car-roof and car-body. Commonly, the car-roof and car-body is curved, and the car-manufacturers wanted that the PV panels are also curved fit to the car-body. The measurement (indoor testing) and performance (outdoor operation) in the standard PV panel was established by the fact that the PV panel always has a flat surface. We had to construct both the testing method and operation model from the beginning considering the three-dimensional density of the photon energy absorbance. Our solution is to introduce a curve-correction factor with keeping compatibility with the current testing equipment and standards. The curve-correction factor can be calculated by the ray-tracing calculation, once both the curve profile (possibly by CAD file) and angular distribution of the solar irradiance (also given by those mentioned above by the new solar resource model on the car-body).

Another crucial factor that affects energy yield from the VIPV is the mismatching loss either by the partial shading and non-uniform distribution by the curved surface. Currently, we developed a Monte Carlo simulation assuming that random numbers give the non-uniformity and ratio of the partial shading.

Here is a summary of the conclusion in technical issues;

A simple shading model to VIPV was developed and validated by one-year monitoring on the solar irradiation on the car-roof and car-body in 5 axes. The curve-correction model of the curved surface of VIPV was developed. Mismatching model using Monte Carlo simulation was developed to analysis on the partial shading of VIPV.

Point 2

Please comment in the feasibility of the solar powered car based on the presented study. Authors have put in a lot of effort to analyse a fairly extensive range of conditions. It makes the manuscript appear incomplete with no comments on the feasibility of the idea in practice. Of course this can be accompanied by a statement clarifying that this feasibility is only based on the current results.

Author’s response

I appreciate your constructive suggestion. We agree and we though that the most appropriate place to add text is the discussion section. The following subsection was added.

4.5 Feasibility of the VIPV based on our measurement and modeling

Compared with the past report by Toyota Motors [2] and NEDO [3] that did not consider the shading effect by surrounding objects and loss by the curved surface, our research gave a more realistic solar resource on the car-roof and car-side. These two reports gave an overestimation of the energy yield expectation. The rough number suggested by this research is around 3/4 using highly curved PV modules on the car-roof and shading effects (taken by the capital of the local government in Japan).

As it is measured and modeled in this study, the energy yield of VIPV is undoubtedly less than that of the PV power plant that is constructed maximizing the energy yield by a given amount of the solar panels. However, considering that the related soft-cost and other balance of system cost (construction, structure, connecting to the grid, distribution, and the cost of the constructing and operation of the EV charging station) that are to be eventually shared by car-owners or car-manufacturers, VIPV will be attractive option both in cost and user-friendliness.

Pint 3

Page 18, figure 17 not figure x - section 4.2 2nd paragraph.

Author’s response

I appreciate your suggestion. It was corrected.

Figure 17 indicates the map of the practical solar resource on the car-roof normalized to the GHI, affected by climate conditions.

Point 4

GHI is used as an abbreviation across the manuscript but is not defined in the text. Please use the full form when the abbreviation is used for the first time.

Author’s response

I appreciate your suggestion. The word GHI appears both the caption and the main text. The definition of GHI was added in the first appearance in the caption and the main text.

One is the urban zone in Bangkok (N13.7°and 4.87 kWh/m2/day Global Horizontal Irradiance, GHI),

The typical monitored result in the route in Figure 13 is shown in Figure 14 (clear sky day) with comparison to the Global Horizontal Irradiance (GHI) of fixed pyranometers (mounted on a roof of one of the buildings of the University of Miyazaki).

Point 5

While latitude and longitude are briefly touch upon towards the end of the manuscript, these are limited to very small region as shown in figure 17. It may be good to include, at least a simulation or a calculation of the effect of latitude and longitude. Also, it may be important to point at the possible relationship of distance from equator which in some sense summarize the combination of latitude and longitude.

Author’s response

I appreciate your constructive suggestion, and we agree it is one of the common questions of readers. The fact is that it does not have a good correlation. We added a correlation trend to latitude and corresponding text.

Generally speaking, both the shading impact and curve impact increases with the decrease of the sun-height, namely, increasing the latitude. However, both curve-correction factor and effective solar resource to the car-roof normalized to GHI (including the loss by the curved surface), do not show a strong correlation to latitude (thus sun-height), unlike other typical solar resource parameters, it is affected by local meteorological conditions. Also, note that both the curve-correction factor and the effective solar resource relative to GHI are strongly affected by the distribution (both special and height) of shading objects.

Figure 17. Map of the effective solar irradiance for the car-roof: (a) Curve-correction factor in typical car-roof; (b) Effective solar resource to the car-roof normalized to GHI, including the loss by the curved surface; (c) Correlations between latitude (related to the sun height) and the curve correction factor; Correlations between latitude (related to sun height) and the effective solar resource to the car-roof normalized to GHI, including the loss by the curved surface

Point 6

Consider summarizing the limitations of this model in the discussion section.

Author’s response

I appreciate your constructive suggestion. We agree it is useful. We added one subsection for this discussion.

4.4. Limitation of the model

First, the model discussed in this article relied on the random number and assumed that every parameter affecting the solar resource on the car-roof and car-side is distributed by a simple rule, for example, ranged uniform distribution. It may be meaningful to the averaged or integrated energy yield, but cannot be applied to the power prediction in specific driving point and direction, specific climate as well as specific surrounding conditions. The given distributions may not be equal to the real situation that varies every position and every time. In this sense, it is to be applied to annual or other long-term integration like annual energy yield.

Distribution of the shading objects (both spacial distribution and height distribution) is essential to the model. Our model is based on a linear trend (Figure 2) but was too simplified to the shading in the high-rise section in Figure 13, where the trend of Figure 2 may become trapezoid or other complicated shapes. Namely, the energy yield in the urban area may give optimistically-biased value.

For the moment, the impact by the partial shading has not been considered yet. We only estimated the impact of using a Monte Carlo simulation. Depending on the string configuration or the power-conditioner connected to the car-mounted module, the output power may drop by the mismatching loss that was not anticipated to the model.

We also polished English and corrected several editorial mistakes.

Round 2

Reviewer 1 Report

authors took my comments in, i can now accept the paper for publication.