Design Study and Potential Implementation of Photovoltaic Noise Barriers for Sustainable Highways

Abstract

Photovoltaic noise barriers (PVNBs) are integrative structures that combine solar panels with noise barriers to harvest solar energy while abating noise from the highway. This study presents design studies and implementation issues of PVNB using highway assets to generate renewable energy. The energy estimation models were first developed at project level and then used for state-level analysis for top-mounted tilted, top-mounted bifacial, and shingles built-on designs. The case study of New Jersey, U.S., shows the energy output potential of 56,164 MWh per year when the shingles built-on configuration is adopted. Furthermore, a decision-making framework for site selection of PVNBs was proposed considering multiple economic, environmental, and social factors. The business models of PVNBs can be direct ownership or third-party ownership, which vary in infrastructure investment and economic gains. The implementation challenges related to risk of traffic accidents, efficiency loss because of soiling, noise absorption, and social impacts are also discussed.

Keywords

Photovoltaic noise barrier solar energy sustainable highway site selection business model implementation challenge

The U.S. wants to reduce greenhouse gas emissions by 50%–52% from 2005 to 2030 and reach 100% carbon-free electricity by 2035 ( 1 ). Renewable energy sources, specifically solar power, are a potential solution to the federal government’s environmental goals. Transportation agencies can benefit from renewable energy by utilizing various solar power technologies in highway infrastructure. Traditional solar farms require a large amount of land, especially when targeting high energy production. Transportation agencies can install solar plants on their owned but unused land, such as the right-of-way (ROW). Oregon Department of Transportation (ODOT) become the first agency in the U.S. to do so in 2008. After ODOT’s project, several other states, including California, Florida, Ohio, and Massachusetts, adopted the technology. However, it is desired to avoid creating a large land footprint by implementing solar integrated infrastructure on roofs, canopies, noise barriers, street lights, and traffic signs.

Photovoltaic noise barriers (PVNBs) use photovoltaic (PV) technology to produce renewable energy and simultaneously abate the noise generated from traffic. PV modules can be retrofitted onto existing barriers directly on their surface or mounted on top ( 2 ). The first PVNB was installed in 1989 in Switzerland, and, since then, in several European nations, including the Netherlands, Austria, France, and Germany ( 3 ). This technology allows multiple uses of the same road and consumes a limited amount of land. However, few studies have been conducted to evaluate the design configuration of PVNBs, especially how to install PV panels on the existing noise wall. The solar panels can be installed with different angles and positions, depending on the orientation of the noise wall, to improve solar energy production.

PVNBs can add value and function to existing noise barriers apart from the renewable energy benefits. For example, they may provide aesthetic improvements created by novel transparent solar panels, and they require minimal maintenance. Although there are no PVNB projects built in the U.S., experiences in Europe have shown that PVNBs can be designed to produce power without compromising the noise reduction function and require low maintenance throughout 20 to 25 years of system lifetime. There is little-to-no evidence to date that PVNBs significantly affect driver safety ( 4 ). However, PVNBs may be vulnerable to vandalism, can experience decreased efficiency incurred by shading effects and dust from the road, and can cause undue glare to drivers when the PV panels are 60° from the horizontal ( 4 ). From an economic point of view, PVNBs may suffer a low adoption rate because public agencies (such as DOTs) cannot directly benefit from tax-related financial incentives. However, public agencies can partner with the private sector to take advantage of those incentives. Therefore, more studies are needed to investigate the practical issues and implementation challenges of PVNBs.

Objective and Scope

This study presents a comprehensive study of PVNBs for evaluation of design configurations and discussion of implementation issues. Three retrofitting designs of PVNB were compared and evaluated through site-specific analysis, including top-mounted tilted, top-mounted bifacial, and shingles built-on configurations. Energy simulation was conducted to develop simplified models for state-level estimation using New Jersey, U.S., as an example. A decision-making framework of selecting the most appropriate site for PVNBs was proposed considering economic, environmental, and social criteria. Finally, the business models and implementation issues of PVNBs are discussed in detail.

Design Configurations of Retrofitted PVNB

There are four design configurations for PVNB when retrofitting existing noise barriers: top-mounted tilted, top-mounted bifacial, shingles built-on, and vertical built-on. The illustrations of PVNB configurations are shown in Figure 1. In the top-mounted tilted configuration, the solar panel is installed on top of the barrier with an angle designed to maximize solar absorption. Top-mounted bifacial PVNBs have their panels installed on the top of the barriers at an angle parallel to the face of the barrier, allowing the panels to harvest energy on both sides. In the shingles built-on configurations, rows of angled panels are installed along the face of the barrier. Lastly, the fourth configuration, vertical built-on, can be considered a simplified design of the shingles built-on, where the panels are installed at a 0° angle from the barrier.

Figure 1.

Retrofitting photovoltaic noise barriers with four design configurations.

Although the top-mounted titled and bifacial designs do not utilize the large surface area of the barrier face, these designs may have higher energy absorbed per panel. The top-mounted tilted design produces the largest energy output when facing south, and the top-mounted bifacial produces the largest energy output when facing east-west. The shingles built-on design takes advantage of the large surface area of the barrier, increasing the number of panels installed per unit length when compared with the top-mounted configurations. The tilted angle of the panels increases energy absorption; however, the panels create self-shading on the lower rows, thereby increasing energy loss. Lastly, the vertical built-on configuration has a large area of PV material, but the vertical position is not optimized to absorb the solar energy, which results in a small amount of energy harvested per panel. Table 1 presents the advantages and disadvantages of each PVNB configuration. The top-mounted tilted design is the most common PVNB in the project currently implemented in Europe because of lower installation cost and higher energy density ( 4 ). It is noted that the vertical built-on is not considered in further analysis since the shingles built-on configuration presents a similar design concept but generates more energy than vertical panels.

Table 1.

Photovoltaic Noise Barrier (PVNB) Configurations Advantages and Disadvantages

PVNB Configuration	Advantages	Disadvantages
Top-mounted tilted	Angled to absorb the most energy, higher energy density per panel	Small photovoltaic area, low energy output for the noise barrier facing east-west
Top-mounted bifacial	Absorbs energy from both faces, better for the noise barrier facing east-west	When facing north-south, the north face absorbs very little energy (in the Northern hemisphere)
Vertical built-on	Does not alter the noise barrier design	The vertical panels position absorbs lower energy than if tilted, not good for panel cooling
Shingles built-on	Angled panels can increase energy absorption, high energy output per length of barrier	Losses because of self-shading, safety related to solar panels projected toward the highway

Estimation Of Energy Performance of PVNBs

Analysis Methodology

The energy output of PVNB was analyzed using Sketchup, a 3D design software, using the Skelion plug-in. The Skelion plug-in allows the inclusion of PV solar panels in the design. The software allows for self-shading loss to be accounted for in the simulations, which is essential for the shingles built-on design. Skelion uses PVWatts for solar energy production estimation, a web-based application developed by the National Renewable Energy Laboratory ( 5 ). The energy production is estimated using the Perez 1990 algorithm to calculate the plane-of-array beam, sky diffusion, and ground-reflected diffusion irradiance components ( 6 , 7 ). PVWatts sub-models also include the thermal model by Fluentes, the hourly calculation of the sun position, and the angle of incidence ( 8 ). The data obtained from the simulations are plotted and fitted into a model for each configuration. The models are used to calculate the estimated energy output by inputting the barrier orientation. Each retrofitting configuration has a slightly different methodology for modeling the system energy output as a function of the barrier orientation. The energy estimation models were developed for three design configurations, as detailed below.

Top-Mounted Tilted PVNB

For the top-mounted tilted configuration, the optimal tilt angle should be calculated first. The optimal tilt angle depends on the orientation of the PVNBs, so a range of PVNB orientations were considered. The annual energy outputs for an array of angle values and a given orientation were found, the output of which was fit as a second-degree polynomial function of the angle. The global maximum of this function provided the optimal tilt angle for the given orientation. For simplification, this process was performed in orientations for every 20°, and the results were used for plus/minus 10° intervals. The panels were always assumed to be on the southern facing part of the studied barrier; therefore, the studied orientation (or azimuth) varies from 90° to 270°.

After the optimal angles were determined, the total energy output for each orientation of PVNB was calculated. This resulted in an equation where the annual energy output per meter is a function of the orientation. As expected, the orientation that produces the highest energy is 180° (south-facing), as shown in Figure 2.

Figure 2.

Energy outputs with solar panel azimuths for top-mounted tilted photovoltaic noise barrier.

Top-Mounted Bifacial PVNB

The top-mounted bifacial configuration does not require an optimum tilt calculation since the panels are installed parallel to the face of the barriers. The simulations resulted in a set of data pairing the orientation with the energy output. A second-degree polynomial equation (as shown in Figure 3) was fitted with R-squared values greater than 0.9. The total annual energy output was then calculated following the same process as the previous configuration.

Figure 3.

Energy outputs with solar panel azimuths for top-mounted bifacial photovoltaic noise barrier.

Shingles Built-on PVNB

To simulate the total energy output of a shingles built-on system, one must first determine the angle and size of the solar panels and the inter-row distance between them; however, shingles configuration design can be very complex. The panel size, angle, and inter-row distance can vary to minimize self-shading. For simplification, this study assumed the same solar panel size, inter-row spacing of 1 m, and angle tilt of 45°.

The simulation for different orientations was performed considering self-shading from the shingles. Figure 4 presents the fitted equation for 3-shingle and 4-shingle configurations. The data can be better fit using a third- or fourth-degree polynomial equation; however, the second-degree polynomial fitted equation has a satisfactory R-squared value higher than 0.95.

Figure 4.

Energy outputs with solar panel azimuths for shingles built-on photovoltaic noise barrier: (a) 3-shingle and (b) 4-shingle.

Verification of Energy Estimation Models

Two case studies were analyzed to verify the developed regression models for energy output estimation. The case studies are noise barriers from Springfield, NJ, and Parsippany, NJ, which were selected because the barriers have rather extreme orientations, mostly facing east-west and north-south, respectively. The Springfield case study was a noise barrier with 175.5° azimuth (facing south) along I-78. The barrier is 493 m long and approximately 5.2 m high (17 ft). The Parsippany barrier, along I-278, has a 116.1° azimuth (facing southeast) and is 1,068 m long and approximately 5.5 m high (18 ft).

Table 2 shows the results for the annual energy outputs versus their fitted values. For the top-mounted tilted, the percentage difference in outputs versus their fitted values are 0.1% and 6.0% for two case studies. These are acceptable levels of difference. Top-mounted bifacial also presented good results, the percentage difference between the outputs and fitted values from two case studies are 1.5% and 1.8%. For the shingles built-on configuration, the differences in results for the Springfield and Parsippany 3-shingle and 4-shingle cases are 5.4%, 0.6%, and 1.1%, respectively. This confirms the model provides a good estimate of the energy output.

Table 2.

Verification of Estimated and Simulated Energy Outputs

Photovoltaic noise barrier configuration	Case 1: Springfield, NJ		Case 2: Parsippany, NJ
Photovoltaic noise barrier configuration	Simulated (kWh/m/year)	Fitting model (kWh/m/year)	Simulated (kWh/m/year)	Fitting model (kWh/m/year)
Top-mounted tilted	198	199	180	170
Top-mounted bifacial	187	189	199	202
3-shingle built-on	496	470	429	427
4-shingle built-on	NA	NA	549	555

Note: NA = not available.

Another verification was performed to validate the assumption of using a 45° angle for the PVNB configuration with shingles built-on. The Springfield and Parsippany case studies were used again because of their orientations. Several simulations for each case study were performed, varying the angle of the shingles and aiming to find the combination of angles that harvest the most energy. Shingles with a small angle are close to a vertical position, absorbing less sunlight. However, this causes little shade on the following shingle row. Shingles with a large angle, close to a horizontal position, tend to absorb more sunlight, especially during summer months, but the flatter position creates shade on the following shingle, increasing the system losses.

Figure 5 shows the simulation models for the built-on shingles with the highest energy output, where Figure 5a is for 3-shingle in Springfield, NJ; Figure 5b is for 3-shingle in Parsippany, NJ; and Figure 5c is for 4-shingle in Parsippany, NJ. The simulated annual energy output is 275.6 MWh, 477.6 MWh, and 617.9 MWh, respectively. The model results with 45° angle are 0.3%, 4.0%, and 5.1%, respectively, smaller than the configuration with the highest energy output, proving that the 45° angles for the shingles are a good approximation and a great solution for simplifying state-level analysis.

Figure 5.

Built-on shingle designs with the highest energy outputs: (a) 3-shingle at Springfield, NJ, (b) 3-shingle in Parsippany, NJ, and (c) 4-shingle at Parsippany, NJ.

State-Level Energy Estimation Results

After verifying the estimation models and assumptions for energy outputs, state-level energy estimation was performed for retrofitting PVNBs on existing noise barriers in New Jersey. Currently, there are 72 noise barriers in New Jersey, totaling 10,268,832 square feet along 106.3 mi. The average height of the noise barriers is 17 ft., although a barrier of 13 ft is high enough to accommodate four shingles of solar panels, which represents 83% of the barriers in New Jersey. The vertical clearance of 5 ft from ground to panel was considered, which avoids damage on the solar-panel system in the case of a vehicle crash and prevents the parts of the solar-panel system falling on the crashed vehicles. These noise barriers are divided into 345 noise barrier segments based on their exact locations as provided by New Jersey DOT. The beginning and ending latitude and longitude of each barrier segment were used to find the average orientation and length of each segment. This information was used as the inputs for the developed regression models, and the energy estimation results are presented in Table 3.

Table 3.

State-Level Energy Estimation in NJ with Different Photovoltaic Noise Barrier (PVNB) Configurations

PVNB configuration	Energy output (MWh/year)
Top-mounted tilted	27,196
Top-mounted bifacial	21,246
Built-on 3-shingle	48,934
Built-on 4-shingle	56,164

The results are presented by each PVNB configuration; however, in the case of actual solar implementation, the most realistic scenario would be a mix of the designs. The configuration that produces the highest energy output is the shingles built-on, followed by top-mounted tilted and top-mounted bifacial. The estimated energy could provide electricity for 2,390 to 6,310 houses, assuming average energy consumption of 8.9 MWh per household per year ( 9 ). It can also save the electricity costs for 809 to 1,672 mi of streetlights, assuming that streetlights are installed every 20 m on both sides of roadway and have 50 W light bulbs working, on average, 11.5 h per day.

Potential Implementation of PVNB

Site Selection Framework

PVNB implementation can result in economic, environmental, and social benefits; however, selecting the locations for PVNBs along highways is crucial in guaranteeing the efficiency and safety of solar highway projects. For instance, the roadway should have the free space to implement the panels at a safe distance from the road and there should be no future road expansion plan that will encroach on that space. Additionally, PV panels need to have sunlight available at the proper inclination and orientation to guarantee maximum efficiency, while avoiding glare. The decision-making framework to select the appropriate site is proposed in Figure 6 to consider the various factors which should be considered, which this paper discusses in its decision-making framework for PVNB projects.

Figure 6.

Photovoltaic noise barrier (PVNB) decision-making framework.

The first step in the decision-making framework is to identify the criteria for the project and classify them into economic, environmental, and social categories. Several selection criteria at each category are summarized in Table 4. Then, a ranking or point-based method should be assigned based on each criterion. The weight for each criterion, which will then be multiplied by the points from the points-based method, can be defined by the project stakeholders. At the end of this process, the ranking of potential sites is presented. The sites with the highest rank should be analyzed for specific design configuration to maximize energy generation as well as economic and environmental benefits of the renewable energy. The metrics, or factors, can then be normalized and applied in multi-factor analysis. Finally, the most appropriate site is recommended for the implementation of PVNBs.

Table 4.

Potential Site Selection Criteria

Economic	Environmental	Social
• Solar irradiation • Barrier direction • Distance to power grid • Accessibility for maintenance	• Solar irradiance • Barrier direction • Region electricity source	• Highway traffic • Electricity rate • Electricity availability • Distance from barrier to highway shoulder

The first criterion of the economic category is solar irradiation. The main economic gain of a solar-panel system is from electricity, either from sales or cost savings. The barrier direction has a similar impact to that for solar irradiation. Except for the top-mounted bifacial configuration, all other configurations produce more energy when facing south. Therefore, south-facing barriers generate more electricity, increasing economic gains. For top-mounted bifacial, a barrier facing east-west is preferable. The distance from the barrier to the power grid will affect the initial investment. The greater the distance, the greater the cost for the electrical connections. The last criterion is the accessibility for maintenance. When the location is not easily accessible, an investment must be considered to either build access or create a closed lane for operation.

The solar irradiance and barrier direction also have an impact on environmental and economic categories. The higher the energy produced, the higher the environmental benefits. The environmental benefits can be measured by the amount of greenhouse gas (GHG) emissions avoided by using the solar-panel system subtracting those from manufacturing and installing the panels, compared with generating electricity from the regional electricity source. Therefore, when the system is installed in a location with high solar irradiation and on a noise barrier with an optimal orientation, the energy produced is higher. Additionally, a region where the electricity plant uses fuel types that emit high GHGs, such as natural gas and heavy oil, will also benefit more from the solar-panel system than the regions with clean electricity sources. On the economic side, the implementation of PVNBs can have a beneficial social impact when the region lacks electricity or has high electricity cost because of electricity transport costs.

Highway traffic volume has two impacts on the social category. First, the solar-panel system can cause an adverse reaction in the public; therefore, choosing a location with lower traffic volume may be beneficial. The second aspect is related to accidents; the solar-panel system presence can increase the severity of a collision because of components and electrical connections that may fail. The distance from the barrier to the highway shoulder also affects drivers’ safety. The farther the PVNB is from the highway, the better the drivers’ safety perception and the less likely the chances of a collision reaching the system.

The aforementioned criteria can be quantified using a point-based ranking method. An example is shown in Table 5, in which points are assigned for each identified criterion. After assigning points for each criterion, weight can be added to translate the priority of different agencies. The decision-making method using weighted criteria should be elaborated with different stakeholders, whereby each stakeholder assigns their weights for the criteria, and an average of the weights can be used for the final analysis. This method will identify potential barriers and will promote economic, environmental, and social benefits.

Table 5.

Example of Points-Based Method for Barrier Evaluation

Category	Criteria	Points
Category	Criteria	5	3	1
Economic and environmental	Solar irradiation (kWh/m²)	>4.5	4.5–4.25	<4.25
Economic and environmental	Barrier direction	S	SE and SW	N, NE, and NW
Economic	Distance to power grid	<0.5 km	0.5–1 km	>1 km
Economic	Accessibility	Yes—easily accessible	Yes—difficult access	No
Environmental	Electricity source	Natural gas and fossil fuel	Coal	Renewable
Social	Highway traffic (vehicles/day)	<5,000	5,000–20,000	>20,000
Social	Electricity rate (cents/kWh)	1st quartile	3rd and 4th quartile	4th quartile
Social	Electricity available	No	NA	Yes
Social	Distance from shoulder (m)	>2	0.5–2	0.5

Note: NA = not available.

Business Models

The implementation of PVNBs can be profitable for state DOTs. The economic benefits vary depending on the business models, such as net metering, renting the space to utility companies, and power purchase agreements (PPAs).

Renewable energy projects can utilize various financial benefits depending on the type of business arrangement. The project can be owned by the transportation agency or by a third-party through a public-private partnership (PPP). For instance, PPP projects can reap the benefits of federal investment tax credits, state tax credits, rebates, and renewable energy credits (RECs). Therefore, transportation agencies can avoid the cost of ownership (initial investment, operation, and maintenance). Additionally, the tax credits that cannot be used by public agencies will be received by the private partner. On the other hand, under a direct ownership model, the benefits can come from rebates, RECs, federal grants, and research funds.

There are different business agreements which make the project profitable for both the public and private agencies. When the agency owns the facility, the business agreement used is net metering. Net metering is a bill credit provided by the utility company for electricity generated by a grid-tied system in surplus of a customer’s on-site consumption ( 10 ). Under airspace lease, the PPP owns and operates the system and the transportation agency charges a fair market value lease rate for the use of the space. However, the agency may seek Federal Highway Administration approval to charge below market lease rates in the case where the facility is in the public interest for a social, environmental, and economic purpose. Any resulting revenue from leasing must be used for transportation purposes. A PPA is a long-term contract that commits the solar developer to finance, build, operate, and maintain the solar-panel system, and the transportation agency purchases the electricity produced for a pre-agreed rate ( 11 ).

Solar farms along the ROW have been successfully implemented by several state DOTs, such as Oregon, Colorado, Massachusetts, Ohio, Florida, and California. Table 6 presents the year, the number of panels, power capacity, energy destination, and business model of different solar highway projects in the U.S. The first highway solar project built along interstate I-5 in Oregon in 2008 has 104 kW capacity. The project generates one-third of the required electricity to light the interchange and avoids 43mt/year of CO₂ eq. from non-renewable electricity sources ( 12 ). Because of the success of the project, 4 years later ODOT implemented its second project with 17 times more power following the same business model of PPP ownership. In the same year, two more solar projects were implemented, one in Colorado (also under PPP ownership) and one in Florida state highway following a direct ownership business model. Colorado also had a project built in 2012, but with much smaller capacity located at the toll highway. Florida has a legal barrier that prevents PPAs; any party selling power in Florida must adhere to the same rules as large utility companies ( 13 ). Ohio also has a direct ownership project with a power capacity of 117.5 kW.

Table 6.

Existing Solar Farms on Right-of-Way and Business Models

State	Year	No. of panels	Power capacity (kW)	Energy destination	Business model
Oregon	2008	594	104	Light interchange	Third-party ownership
Oregon	2012	6,994	1,750	Serves Portland General Electric	Third-party ownership
Colorado	2011	22	62	Under-deck bridge lights, roadside and overhead signs, tolled ramps	Third-party ownership
Ohio	2011	1,164	117.5	NA	Direct ownership
Colorado	2012	N/A	707	Road surveillance cameras, streetlights, variable message signs, toll collection equipment, toll plazas, maintenance facilities	Third-party ownership
Florida	2012	468	112	NA	Direct ownership

Note: NA = not available.

The economic return of PVNBs can be a concern for project implementation. It is important to avoid costly, over-engineered systems, and ineffective designs resulting from the lack of PV system quality standards and guidelines ( 14 ). Previous analyses have shown that the electricity benefit of PVNBs alone cannot fully pay for the barriers themselves; however, by reducing the net present value when compared with the noise barrier without a solar-panel system, and when considering ecological benefits, the project can be even profitable ( 4 ).

Implementation Challenges

The implementation of PVNBs faces some challenges, including the consequences of traffic accidents, glaring, efficiency loss because of soiling, noise absorption, social impacts, and business partnerships.

The consequences of traffic accidents increase when PVNBs are present. The electric components are susceptible to fire and falling panels can increase the accident severity. Locating the safest place to accommodate the electric components is essential, and the panels’ attachment infrastructure should be designed to resist high impacts. The panels must be constrained to the structure; suitable retention devices should be added inside the glass and designed to withstand any stone impact from passing vehicles ( 14 ).

Glare is a potential cause of accidents and should be analyzed for high elevation sites, where the panels reflect direct light from the incident angle. Although there are a few cases where glare is an issue, solar panels are designed to absorb and not reflect sunlight. Solar panels have a similar solar reflection rate to that of roadside vegetation. Based on the ODOT report on the Solar Highway Program, solar panels typically have an albedo of 30%, compared with roadside surface materials such as dry sand at 45%, grass-like vegetation at 25%, and broadleaf deciduous trees at 10% ( 10 ). Based on the recent feasibility study of using PV panels on noise barriers and snow fences, conducted by Minnesota DOT, PVNBs could cause potential light damage to drivers without sunglasses for only 373 min in a 1-year period ( 15 ). Another potential problem for traffic safety is snowdrift from panels. Areas susceptible to large snowdrifts should either be avoided, or the panels should be installed on the leeward side of the roadway.

Energy loss because of soiling may decrease the efficiency of PVNBs. The soiling loss depends on geographic region, local environmental, level of development (rural, suburban, and urban), tilt angle, and rainfall. The loss rate is specific for each location and varies throughout the year. Kimber et al. studied the effect of soiling in California, where rainfall was limited during an annual, months-long dry season ( 16 ). The study concluded that, for each day without rainfall, the energy losses incurred by soiling increased on average by 0.2%, which represents annual loss rate between 1.5% and 6.2% ( 16 ).

The primary function of a noise barrier is to abate the sound from traffic, yet the inclusion of PV material by itself does not aid this purpose. The analysis performed here assumed an existing noise wall with no requirement for more noise absorption. When noise absorption is required, the most appropriate configuration can be shingles built-on ( 17 ). It is beneficial to use glassy material in the PV system to reflect noise, combined with sound-absorption materials in the non-PV areas ( 4 ). Vallati et al. performed an acoustic and energy study with five different PVNB configurations, including top-mounted with 60° tilting angle, vertical built-on, and three other configurations (T-shaped, and two curve-shaped) ( 18 ). The results showed that the T-shaped and the top-mounted tilted presented the best acoustic results ( 18 ).

Other environmental issues include the possible impact of a solar-panel system on the natural environment, such as the effect of glare on birds. Social issues may also be considered for project implementation. Community support is essential for project acceptance and to avoid vandalism. Publicizing the electricity destination and disclosing the benefits of the solar-panel system for the region is crucial for public approval.

At present, the U.S. does not have any PVNB projects that have been constructed. Consequently, the available information regarding solar highways is primarily from solar arrays placed in the ROW of roadway. Massachusetts DOT (MassDOT) has plans to install the first PVNBs in the U.S. The project will be retrofitting an existing noise barrier. The motivation is to learn about this technology and then to mitigate the costs of future noise barriers with incentives from PV programs. After the implementation, the noise will be tested to guarantee low or no acoustic impact of the PV material. The location chosen for the project was based on the orientation, noise barrier age, convenience for maintenance, and high elevation. The high elevation avoids the need to trim trees, snow blockage resulting from plowing, vandalism, and impact from car accidents. The project will be implemented through PPP using PPA. Therefore, the developer will build and maintain the system, while selling the electricity to MassDOT at a lower rate than the market ( 19 ).

The transportation agency can avoid the costs associated with retrofitting and operating the noise barrier through PPP. The partnership can follow PPA, in which a third-party private company builds, operates, and maintains the solar-panel system, while the agency purchases the produced electricity for a predetermined rate ( 11 ). As a result of this agreement, the transportation agency avoids ownership costs (initial investment and operation), and has the project benefits from tax credits that a public entity cannot utilize.

Conclusions

This study presented design studies, practical issues, and implementation challenges for using highway assets to generate renewable energy and reduce carbon emissions. Energy simulation was conducted for both project and state levels. In the project level, the design configuration with the shingles built-on has the highest energy output; while the energy outputs of top-mounted tilted and top-mounted bifacial configurations vary depending on barrier orientation. The simplified regression models provide a quick way to estimate total energy output for each design configuration considering different orientations of noise barriers. For the state-level estimation of PVNBs in New Jersey, the shingles built-on design can produce energy output of 56,164 MWh per year.

PVNB implementation can result in economic, environmental, and social benefits. The proposed decision-making framework for site selection includes factors that impact these three categories to increase monetary gains, decrease environmental impacts, and/or increase public welfare. The framework can be used to recommend the appropriate list of noise barriers for PVNBs, which can then be further analyzed with life-cycle assessment and life-cycle cost analysis for sustainability assessment ( 20 ).

Agencies can implement PVNB projects through direct ownership or third-party ownership as business models. The latter provides tax-related benefits and shifts the infrastructure costs to the private partner. However, the economic gains may be lower than those from direct ownership. Implementation issues should always be considered during project planning phase to mitigate negative impacts and increase system performance and efficiency.

Footnotes

Author Contributions

The authors confirm contribution to the paper as follows: study conception and design: H. Wang; data collection: L. Soares; analysis and interpretation of results: L. Soares, H. Wang; draft manuscript preparation: L. Soares, H. Wang. All authors reviewed the results and approved the final version of the manuscript.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors would like to acknowledge the financial support provided by New Jersey Department of Transportation through the research project: “Energy Harvesting from Roadways in New Jersey,” Contract number or Grant Number: NJDOT TO 361.

ORCID iDs

Laura Soares

Hao Wang

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10.

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