DEVELOPMENT OF SOLAR PAVEMENT IN JAPAN AND ITSCONTRIBUTION TO CARBON NEUTRALITY
T. KAJI & T. YOSHINAKA
General Technical Department, Technical Division, NIPPO CORPORATION
kaji_tetsuri@nippo-c.jp
ABSTRACTIn road paving, solar pavement is being developed worldwide against the backdrop of theParis Agreement, which set a long-term goal of mitigating climate change by 2050 andbeyond, SDGs, carbon neutrality and other initiatives. In addition to providing traveling space,pavement, which covers a certain size of surface area in cities and regions, has the potential
to generate new values to create renewable energy. We are developing solar pavement withthe aim of developing road paving technology that contributes to carbon neutrality, and are
conducting demonstration testing, primarily, test paving, in places with vehicle traffic.
Pavement is warped and deformed by the load directly applied by people and vehicles.
Therefore, it is necessary to use paving materials that allow for warping and deformationand do not break. The solar pavement we are developing uses materials that allow for
warping and deformation to ensure that it withstands the load from people and vehicles andhas the skid resistance necessary for traffic safety.
To evaluate the durability of the solar pavement, we performed indoor testing of pavement
samples and test paving with vehicle traffic. In the indoor testing, we determined how thesolar pavement module would break under a load representing a vehicle and by directing
ultraviolet light at the module, we examined the relationship between the number of yearsof outdoor exposure to ultraviolet radiation and the amount of power generated. In the test
paving with vehicle traffic, we installed the solar pavement on roads with traffic of regularvehicles and in bends where large trucks slow down. In both cases, we showed that thesolar pavement module does not break and demonstrated that it can be easily installed,
including the wiring below the module, and generates power.
Through the testing, we have made progress in developing solar pavement for practicalapplication. This report describes our development activities, including an evaluation of thecontribution of road paving to carbon neutrality.

1. DEVELOPMENT OF SOLAR MODULES FOR PAVEMENT APPLICATIONS
A pavement is directly subjected to loads from pedestrians and vehicles and is constantly
subject to deformation and deflection. Therefore, solar cells installed on the pavement
surface must be made of materials that allow for deformation and do not fail. Therefore,loads from pedestrians and vehicles are not considered in the design of solar cells that are
widely used in rooftop solar and mega-solar systems. The authors started to develop a solar
cell (a solar module for pavement applications; hereinafter the “module”) that can be used
for pavement applications.
Figure 1 shows the construction of the module being developed by the authors. The module
is constructed with layers of materials that allow for deflection, is durable to withstandpedestrian and vehicle traffic, and provides the skid resistance required for a pavement.

[1]

XXVIIth World Road CongressSolar modules for pavement applications
Solar cell (a prototype for pavement)
Clear protective plastic plate
Surface leveling skid prevention

Unevenness-adjusting resin mortar
Asphalt pavement

Figure 1 - Solar pavement under development (example).

2. INDOOR TESTING
2.1 Evaluation of bearing capacity
To evaluate the bearing capacity of simulated roads, we conducted a loading test according
to the conditions specified in Japan’s method of immersed wheel tracking test [1]. Table 1
shows the test conditions and Photo 1 shows the test setup.
The test was conducted using a sample immersed in water at 60°C, which is the highest
road surface temperature in summer in Japan, and at the highest summer rainfall. The test
duration was set to 18 hours, which is three times the ordinary duration specified in the
Method of Immersed Wheel Tracking Test. To evaluate the feasibility of the module, a load
was applied to the sample while light was directed on it to generate power.
After the test, the module continued to generate power without short-circuiting of the solar
cells, showing that, in terms of durability, the layered structure of the module remains intact
without breaking away.
Table 1 - Test conditions(Method of Immersed Wheel Tracking Test)
Test)

Temperature

60℃

Sample condition

Water immersion

Load

49kN

Test duration

18 hours
Photo 1 - Test setup(Method of Immersed Wheel Tracking Test)[2]

XXVIIth World Road CongressIn the next step, we tested the wiring structure under the module. A test sample with a space
representing the wiring was placed under the module and subjected to cyclic compressive
loading equivalent to wheel loading from large vehicles. Table 2 shows the test conditions.
Photo 2 shows the loading setup.
To evaluate the bearing capacity under accelerated test conditions, the condition of the test
load was set to a value twice the contact pressure used in the Method of Immersed Wheel
Tracking Test of Japan [2].
The test results are shown in Figure 2. The figure shows that the module does not breakwhen subjected to more than 550,000 cycles of loading. In Japan, the impact of trafficloading on an asphalt pavement is said to be proportional to the fourth power of the wheelload [3]. Based on this, the number of loading cycles in the test is equivalent to 7,000,000
cycles of wheel loading from large vehicles. This indicates that the module canaccommodate a traffic volume equivalent to traffic category N6 (equivalent to national roads
and government-ordinance-designated city roads).

Table 2 - Test conditions(cyclic compressive loading test)

Test temperature

20℃

Control method

Load control

Loading jig

Diameter 40 mm

Setting load

1.26MPa

Loading speed

120mm/min

Before loading

Photo 2 - Test setup(cyclic compressive loading test)

After 300,000 loading
cyclesAfter 550,000 loading
cyclesFigure 2 - Results of cyclic compressive loading equivalent to wheel loadingfrom large vehicles.
[3]

XXVIIth World Road Congress2.2 Evaluation of weather resistance
The test for accelerated weathering of materials using an accelerated weathering tester was
conducted to evaluate how much the power generated by the module decreases when it is
exposed outside. Table 3 shows the test conditions. Figure 3 shows the results of a test to
determine the power generation rate versus the number of years of outside exposure.
The test shows that, with a 100% generated voltage before the test, the power generation
rate is 98.0% for 10 years of outside exposure and 94.2% for 20 years of outside exposure.
The decrease in the power generation rate is 1.14% per year for power generation devices
in widely used solar cell systems as shown in Reference [4] and 0.3% for the module. Thisshows that the decrease in the power generation rate due to outside exposure can be lower
for the module than for widely used solar cells.
Table
3 -3Test
conditions(accelerated
weathering
Table- Testconditions(accelerated
weatheringtest)
test)

Tester name

Metal-halide lamp type
(Weathering tester)Control mode

Auto dimming

Light intensity

150mW/㎡

Operation mode

Lighting on/off

Water spray mode

Before/after lighting off

Test
cycle

Lighting on

4.0 hours

Lighting off

2.0 hours

Water spray

60 seconds

Figure 3 - Results of the accelerated weathering test.

[4]

XXVIIth World Road Congress3. Evaluation by TEST PAVEMENT
3.1 Test pavement on the premises of our company
The authors laid a test solar pavement at a location with vehicle traffic on our premises in
Saitama Prefecture [5] and have been conducting follow-up monitoring (Photo 3, pavement
laid in November 2018). The durability and power generation characteristics of the module
were evaluated as a pavement by running vehicles with a total weight of up to 6 tons. Some
issues were identified with the adhesive strength between the components of the module.
These issues were resolved by changing the type of adhesive used and modifying the
fabrication process of the module.
In addition, we are evaluating the durability of the module on a road with large-vehicle traffic
on the premises. A test pavement on a road with traffic of large trailers and pavement
machines was laid on the premises of our motor pool in Saitama Prefecture (Photo 4). To
evaluate the durability of the module under various external forces, which cannot be fully
evaluated in laboratory testing, we selected sections where vehicles drive straight or turnleft or right. Particularly at a curve (Photo 4 (b)), a large vehicle with double rear axles runson the pavement while the tires slip laterally at low speed. Because of this, a vertical load
and a large shear load are applied. We are continuing to evaluate the solar panel installedon the pavement under severe external loading conditions. We identified issues with themodule, including the wiring structure under it, and made improvements to it. Thus, we are
improving the level of technical completeness of the module.

Photo 3 - Laid test pavement.
ModuleModule(a) Straight section(b) Curved section

Photo 4 - Sections with large-vehicle traffic.
[5]

XXVIIth World Road Congress3.2 Test pavement outside the company
We laid a test pavement on the premises of a private company’s factory in Aichi Prefecturein March 2021 and have been monitoring the pavement [6]. Photo 5 shows the pavement
laid on a parking lot.
A wiring structure with an inspection hole inside the pavement is tested to evaluate the easeof installation and in-service durability of the structure. The power generated by the module
is sent to the grid system connected to the adjacent factory. The power generation of themodule is monitored and the power conditioner is checked to ensure that it works stably. As
of January 2023, it has been 22 months since the module was put in service. No problem
was found with the power generation or the module. The module has been shown to havethe durability required for use on parking lots and can supply power stably.

Photo 5 - Pavement laid on the premises of a private company’s factory [6].
4. Power generation characteristics
4.1 Power generation performance in an environment with both sunny and shaded areas
Unlike rooftops and mega-solar systems, the road is an environment where shaded areas
tend to occur. Power generation devices are said to be susceptible to damage if solar cells
are partially shaded [7]. Damage-resistant power generation devices have been chosen andcircuits have been designed to prevent electrical damage. The test pavement location shownin Photo 6 is laid on the north side of the building that has both sunny and shaded areas.
The test is being conducted under conditions where a partially shaded area occurs. It has
been more than four years since the pavement was laid. Until today, the module hascontinued to generate power successfully with no damage to the power generation devices.

Photo 6 - With both sunny and shaded areas.
[6]

XXVIIth World Road Congress4.2 Relationship between received solar radiation and power generation
To properly determine the power generation characteristics in a road environment whereshaded areas tend to occur, we developed a method of measuring the received solar
radiation on the module [8]. Figure 4 illustrates how to measure the received solar radiation.
In Figure 4, solar radiation in the sunny area and solar radiation in the shaded area were
measured separately, and the received solar radiation was calculated from the solar
radiation and the receiving area. Figure 5 shows the relationship between the received solar
radiation on the module and the generated power. The figure shows that the generated
power increases as the solar radiation increases and a trend in power generation at specificlocations can be identified.

Actinometer (sunny)

Actinometer (shaded)

Shadedarea

Generated power (Wh)

Sunny
area

North side of the building

ModuleTime-lapse
cameraReceived solar radiation (kW)Figure 4 - Example of received solar
radiation measurement.

Figure 5 - Relationship between received
solar radiation and generated power.

4.3 Effect of road surface levelling
In order to ensure safe traffic, the surface of the module is leveled using clear aggregate toprovide the required skid resistance (Photo 7). Compared to widely used solar cells with aflat surface, the power generation performance of the module may degrade after leveling.
Using a solar simulator, the relationship between the angle of the light source and the power
output of the solar cells was investigated (Figure 6). The results show that, even when the
sun’s altitude (the angle of incidence on the module) is low, it is possible by choosing anappropriate material for surface leveling to make it easy to capture the sun’s light and send
it to the solar cells and thereby to ensure that the solar cells generate power.
Table 4 shows the power output with and without surface leveling, which we estimated fromthe relationship in Figure 6 between the incident angle (the sun’s altitude) and the solar cell
output power, based on the distribution of the sun’s altitude on the first day of the month,which was calculated from the solar altitude data provided by the National Astronomical
Observatory of Japan. The table shows that, in January to April and in October to December,when the sun’s altitude is low, surface leveling increases daily power generation. The annual
average power generation also increases. This shows that degradation of power generation
performance can be prevented by choosing an appropriate method of surface leveling.

[7]

XXVIIth World Road CongressPhoto 7 - Leveling the surface of the module.

Figure 6 - Relationship between the angle of the light source and the power output.

[8]

XXVIIth World Road CongressTable 4 - Annual change in the sun’s altitude and trend in the estimated power
generation associated with surface leveling
Date of sampling

2018
Total
1-Jan 1-Feb 1-Mar 1-Apr 1-May 1-Jun 1-Jul 1-Aug 1-Sep 1-Oct ##### 1-Dec

Altitude 70～

2： 15 2： 15 1： 15

Altitude 60 ～ 69°

3： 15 1： 45 2： 00 2： 30 1： 45

Sun’s
Altitude 50 ～ 59°
3： 45 1： 45 2： 00 1： 30 1： 45 2： 30 1： 15
altitude*and hours Altitude 40 ～ 49°
3： 30 2： 00 2： 00 1： 30 2： 00 1： 45 2： 00 3： 15
of daylight
Altitude 30 ～ 39° 1： 45 4： 00 2： 30 1： 30 1： 30 1： 30 1： 30 1： 15 1： 30 2： 00 4： 30 2： 15(hours:minutes)

4：08

Altitude 20 ～ 29° 3： 30 2： 30 2： 00 2： 00 1： 30 1： 45 1： 30 1： 45 1： 45 1： 45 2： 30 3： 30
8：01
Altitude 10 ～ 19° 2： 30 2： 00 1： 30 1： 30 2： 00 1： 45 2： 00 1： 45 1： 45 2： 00 1： 45 2： 00
Altitude0 ～ 9°

2:00

2： 00 2： 00 1： 45 1： 45 2： 00 1： 45 1： 45 1： 45 1： 30 2： 00 2： 00

Comparison ofW/ surfacing
13.14 14.86 17.26 19.85 22.18 23.83 24.00 22.62 20.89 18.20 15.42 13.38(surface irregularities)
solar cell*
power output
W/o surfacing
12.38 14.19 16.85 19.78 22.32 23.98 24.19 22.78 20.89 17.94 14.75 12.66(Wh)(individual cells)

18.80
18.56

5. Issues with the module as a road pavement
It is important in developing a power generation module to ensure that it has the bearing
capacity and skid resistance required for a pavement, does not break when impacted by tire
chains in winter, and is protected against electric shock because it is laid on the groundsurface. Table 5 shows examples of the key properties required for a solar module forpavement applications, which were found through development of the module.
The materials, electrical wires, and cables used for the module have never been used forroad pavement. Importantly, they need to go through cyclic laboratory testing and pavement
testing. We showed, for example, that the type of damage is different for laboratory testing
and for field testing, depending on the loading and pavement-laying conditions. Many of thefundamental issues with the solar pavement have been solved. The remaining issues are:improving the ease of laying the pavement, the durability of the wiring structure under themodule, and the power generation efficiency; reducing the cost of the pavement, which isdependent of the scale of production; and increasing the reliability of the pavement.
Table 5 - Examples of key properties required for a solar module for pavement
applications
Requiredproperties

Skid resistance

Requirements

Skid resistant whenraining

Examples of test
methods
Examples ofcriteria

Impact resistance Bearing capacity Water resistanceNo cracking, no
breaking from
impact load

Remaining functional Remaining
to generate powerfunctional towith no breaking awaygenerate power
and no breaking downwhen raining
when loaded

Shade tolerance

No warping, no
breaking away, no
discoloration

No damage topower generation
devices whenpartially shaded

Immersed wheel
Outdoor power
Immersed wheel
Accelerated
tracking test
generation
tracking test
weathering test
Cyclic loading test
demonstration test
No module
Road: BNP > 60
No solar cell shortNo module cracking cracking, no solar
No module warping No solar cell failure
Sidewalk: BMP > 40
circuitingcell short-circuiting
Skid resistance test
(BPN)

〇

×

Labeling test

〇

×

〇

×

Widely used solar～～～
cell (tempered
W/o crackW/ crack
W/ crack W/o crack
W/o crackW/ crack
glass)
prevention
prevention Possibility of cracking prevention
prevention prevention prevention〇
Our technology

Weather
resistanceCompliant withstandards

×〇×

No warping

Susceptible to failure

〇

〇

〇

〇

〇

No breaking

Durable to withstandlarge-vehicle traffic

No breaking

No warping

No failure[9]

XXVIIth World Road Congress6. Possibility of development into the future
One movement towards transforming energy systems is a smart community initiative [9].
Developing energy management systems, such as a community energy management
system (CEMS), is being considered, but utilizing road space is not included. From theperspective of decarbonization, a solar pavement can be potentially used as a road energy
management system (provisionally named REMS). We expect that the solar pavement canmake a significant contribution to carbon neutrality (Figure 7).

7. Closing remarks
After giving form to a solar pavement for practical applications, through a test pavement,
we accepted visits from various sectors, including energy infrastructure companies,
automotive manufacturers, and several research organizations. While being still in the
research and development phase, the solar pavement has attracted attention, exceeding
our expectations. We will try to meet expectations for its future and feel strongly
responsible for completing the development of the module as soon as possible.

8. References[1] Japan Road Association: Pavement investigation and examination method handbook, B004, March 2019.
[2] Tamotsu Yoshinaka, Toshio Hiratsuka, Shigenobu Sato, Tetsunori Kaji: “A study to put into practical use asolar pavement to create sustainable energy,” 34th Japan Road Conference, 2021.
[3] Journal of the Japan Society of Civil Engineers, Pavement Engineering, February 1995.
[4] Seiyu Sai, Ritsuko Sato, Tetsuyuki Ishii, Yasuo Chiba, Atsushi Masuda: “Indoor measurement ofdegradation rate of solar power modules exposed outside,” National Institute of Advanced Industrial Scienceand Technology, AIST Research Findings on Solar Power Generation, 2018.
[5] Tamotsu Yoshinaka, Tetsunori Kaji, Toshio Hiratsuka, Shigenobu Sato: “A study on development of a solar
pavement on which vehicles can travel,” 33th Japan Road Conference, 2019.

[10]

XXVIIth World Road Congress[6] Maari Hirata, Issei Suzuki, Katsuya Ohta, Tamotsu Yoshinaka: “Demonstration testing of a solar pavement
to achieve carbon neutrality,” 34th Japan Road Conference, 2021.
[7] Japan Photovoltaic Energy Association: Design and construction of solar power systems, September 2015.
[8] Tetsunori Kaji, Maari Hirata, Tamotsu Yoshinaka: “A study on the power generation characteristics of asolar pavement during daylight hours under actual road conditions,” Japan Society of Civil Engineers National
Convention, Annual Academic Lecture, 2019.
[9] Websites of the Ministry of Economy, Trade and Industry and the Agency for Natural Resources and Energy.

[11]

XXVIIth World Road Congress