The design of a 65 m grid-independent, stand-alone solar pathway for park or area lighting was designed and operated on the campus of the Cape Peninsula University of Technology.
The development of solar roads to convert solar radiation on vast stretches of roadway in order to generate electricity on surfaces otherwise dedicated to transportationis showing promise. Great potential is seen for photovoltaic (PV) application with the maturing of solar road technology.
If all the highways, pathways and roads in a country were solar panelled, they would produce sufficient free energy to every home and industry in that country. This would not negate electricity bills as there will still be the cost of distributing the electricity, but that cost could be reduced by 90%.
Fig. 1: The first solar roadway opened in Amsterdam, the Netherlands: (a) close up; (b) showing the width of the path.
A solar road is a new concept for the generation of sustainable energy. Here, the road surface also acts as a solar panel. The generated electrical energy can be used for various applications such as road and public park lighting and traffic systems. Households may benefit from it as well. In time, electric cars might be able to make use of the energy. The energy will then actually be generated at the place where it is needed, a big step towards an energy-neutral mobility system.
The first solar roadway opened in Amsterdam, the Netherlands, in November 2014. It is part of a 100 m stretch of cycle path between two suburbs of the city. Electricity is generated by rugged, textured glass-covered photovoltaic cells exposed to solar irradiation [7 – 9].
SolaRoad, as it has been named by the Netherlands Organisation for Applied Scientific Research, is made up of rows of crystalline silicon solar cells. The cells are fully embedded into the surface of the concrete of the path, and then covered with a layer of translucent, tempered glass, as shown in Fig. 1. The road surface is treated with a special non-adhesive coating, and the road itself was built at a slight tilt to keep dust and dirt from accumulating and obscuring the solar cells [7].
Fig. 2: The world’s first solar-paneled road in the Netherlands.
The path cost about $4,3-million and was created as the first step in a project to be extended to 100 m. The local authority initiated a first phase of 70 m. The solar roadway is shown in Fig. 2. The conceptual, independent solar road lighting system design and its equivalent circuit for this study are shown in Fig. 3.
Case study
A 65 m metre stretch of walkway, located next to the Electrical, Electronic and Computer Engineering Department Building of the Bellville Campus of the Cape Peninsula University of Technology (CPUT) was chosen to set up a case study. The case study presented a simplified method for sizing and simulation of a stand-alone solar pathway to power public lighting on the campus. Fig. 4 shows the selected site.
Sizing and simulating the system
The steps of sizing of the stand-alone solar pathway SSP was executed by computer program called MATLAB. Climate data at the selected site was sourced from Solargis, and NASA Surface Meteorology and Solar Energy (SSE).
Fig. 3: A stand-alone solar lighting system.
This data, constituting global solar radiation, annual mean temperature, average sunlight hours and daylight hours/day, as well as the percentage of sunny and cloudy daylight hours, was taken over a 22-year period. Solargis data is regarded as the most reliable source of solar radiation data [11].
The NASA SSE is developing data sets from NASA Earth Science Enterprise (ESE) climate research to support renewable energy industries. The data sets contain solar parameters, principally derived from satellite observations and meteorology parameters from an atmospheric model constrained by satellite and sounding observations. These data sets cover a climatology history (July 1983 – June 2005) on a grid of 1˚ latitude by 1˚ longitude. The global coverage of the NASA Earth Science data set fills the gap where remote locations lack ground measurement data.
Fig. 4: Aerial photograph showing the selected site.
The main components in an SSP system are an array of PV modules, a battery, a charge controller and the DC lighting load. Fig. 5 shows the components and direction of the energy flow and sizing strategy for a stand-alone PV lighting system. The steps for sizing the Stand-alone solar pthway (SSP) are the following:
Evaluation of load needs (quantity and quality of lighting)
The requirements of the load must be known to design an SSP. This means that the quantity and quality of the power required for lighting must be known. To determine this, one must select lighting fixtures based on the application requirements, choosing the type and specifications of luminaire unit. The luminaire chosen was the GreenCobra Jr LED street light. This luminaire has become the industry standard for many cities, utilities and transport departments throughout the world. Its features include an aesthetic design, options for warm (3000 K), neutral (4000 K) and cool white (5000 K), and light trespass shielding.
Fig. 5: The direction of the energy flow and sizing strategy for a
stand-alone PV lighting system.
The number of hours between sunset and sunrise had to be established. The determination of monthly averaged daylight for each month was based on the monthly average day from the NASA Earth Science data base, which presents the monthly average, averaged for a particular month over the 22-year span of the data base. Fig. 6 presents the monthly averaged night hours between sunset and sunrise. A maximum of six hours of operation was selected per night for the SSP system.
An equivalent number of no-sun or “black” days for which a battery or backup storage would be needed to supply power was then considered. This parameter is very important for sizing the battery or any other energy storage system.
Fig. 6: Monthly averaged night hours from sunset to
sunrise at the selected site.
Calculating energy needs
An estimate of the required load is achieved by listing the power demand of all loads together with the number of hours of use per day and the operating voltage of each of these loads. The number of hours of use per day between sunset and sunrise for the lighting load was determined (see Fig. 6).
The number of monthly averaged night hours for each month is determined based on the “monthly average day”. A 65 m stretch of walkway at the selected site needs twelve LED street light units, distributed evenly along the length of the pathway. A total of 288 W is needed for the lighting load. The total daily power and energy requirements for the luminaires were determined as shown in Table 1.
| Type of load | Max. power and energy needs |
| Total number of solar lights | 12 |
| One LED frame load | 24 W |
| Lighting load (DC load) | 0,288 kW |
| Load including charge controller losses (10%) | (0,0288 kW) 0,317 kW |
| Lighting hours of operation (average, per day) | Max. 6 at night (till midnight) |
| Total lighting load energy reqs/day | 0,317 x 6 = 1,92 kWh |
| Auxiliary loads (Wires, …) |
0,0144 kW |
| Total auxiliary energy reqs/day | 0,0144 x 6 = 0,864 kWh |
| Total power needs | 0,3314 kW |
| Total system energy reqs/day | 2 kWh |
PV system sizing
The following design methodology was applied for the PV system to cover 100% of energy needs, with energy recovery.
Since the pathway cannot be adjusted to the position of the sun, the panels will generate approximately 30% less energy than those placed at suitable angles on roofs [7, 8,10]. However, the road is tilted slightly to aid water run-off and to achieve a better angle to the sun. The energy needs from the PV arrays and their sizing are shown in Table 2.
| Type of load | Max. power and energy needs |
| Daily total system energy requirements/day | 2 kWh |
| System losses (controller, … ) 5 % | 0,1 kWh |
| Total energy requirements from PV array | 2,1 kWh |
| Hours of PV array operation | 6 h |
| Daily total PV power needs | 0,35 kW |
Sizing the battery storage system
The batteries are the main components in the SSP system after the PV arrays. It is not possible to meet the needs of night lighting if the battery capacity is too small. Conversely, if the battery capacity is too large, a large solar array is needed to ensure that the batteries are charged fully during the day. The oversized array and battery capacity will result in increased and unnecessary costs. If the solar array is not large enough, the battery cannot be charged fully and will always be in a state of power deficit, which affects the battery life adversely.
The battery type selected for the system is the Aspen 24S-83, manufactured by Aquion Energy. Operation and performance of the battery are depicted in Table 3. This battery is a clean, 24 V saltwater battery outperforming and outlasting traditional lead-acid batteries [11].
| Parameter | Specifications value |
| Nominal capacity (10-hour charge/ 20-hour discharge) |
83 Ah |
| Nominal voltage | 24 V |
| Life cycles | 3,000 cycles (to 70% retained capacity) |
| Ambient operating temperature | -5°C to 40°C |
| Voltage range | 20,0 to 29,7 V |
| Peak power | 750 W |
| Continuous current | 30 A |
| Usable depth of discharge | 100% |
Aquion’s proprietary Aqueous Hybrid Ion (AHI) technology uses no heavy metals or toxic chemicals and is non-flammable and non-explosive. These batteries are also robust in partial state-of-charge cycling and over a wide range of ambient temperatures. It is important to choose a quality battery rated at a minimum of 12 000 Ah storage capacity.
Number of PV panels, arrangement of PV array
Select the PV module. The module chosen for this project is an STPS040-12, manufactured by Suntech Power Company. Table 4 shows the technical specifications of a mono-crystalline silicon PV module. The STPS040-12 is a 40 W, 12 V solar panel which will provide enough power to the deep cycle battery. The modules are composed of 36 monocrystalline silicon solar cells of similar performance, interconnected in series. The solar road consists of 40 Wp Soltech PV modules.
| Parameter | Specifications value |
| STC: 1000 W/m2, 25 °C, AM 1.5 | |
| Electrical and Physical Characteristics | |
| Peak Power (Pmax) | 40 Wp |
| Optimum operating voltage – Vmp | 17,2 V |
| Optimum operating current – Imp | 2,32 A |
| Open circuit voltage – Voc | 21,6 V |
| Short circuit current – Isc | 2,62 A |
| Operating temperature | -40 °C to +85 °C |
| Maximum system voltage | 1000 V DC |
| Length & width | 1,619 m x 0,814 m |
| Thickness, | 0,039 m |
| Weight | 12 kg |
Next, calculate the number of modules connected in series. Modules in series are also referred to as a “string”. The number of modules to be connected in series is:
(1)
Calculate the number of strings required in parallel:
(2)
Calculate the number of PV panels needed to cover the power needs:
(3)
It follows that the number of PV panels and the arrangement of the PV array are:
Calculate the average daily energy output from the PV array:
(4)
Pmx is calculated by this equation:
(5)
The monthly global radiation in array panels and average monthly temperature at the selected site must be known to calculate the average daily energy output from a PV array. Fig. 7 depicts the monthly global radiation on array panels at the selected site versus Cape Town City (Cape Town Data from NASA). This global radiation was calculated in the selected site and the average monthly data is taken over the 22-year period.
Fig. 8 shows the daily average minimum (blue graph) and maximum temperatures (red graph) at the selected site, with percentile bands (inner band from 25th to 75th percentile, outer band from 10th to 90th percentile). The temperature typically varies from 7 to 27°C over the course of a year and is rarely below 4 or above 31°C.
The warm season lasts from 9 December to 27 March with an average daily high temperature above 25°C. The hottest day of the year is 15 February, with an average high of 27°C and low of 17°C. The cold season lasts from 26 May to 14 September, with an average daily temperature high below 19°C. The coldest average day of the year is 5 July, with an average low of 7°C and a high of 18°C.
January is, on average, most sunny while June has the least sunshine.
Fig. 7: Comparison of the average monthly irradiation on array panels at the selected site, with the City of Cape Town, obtained from a global NASA database.
Monthly average daylight is based on the “monthly average day” (SSE methodology). July and December were therefore chosen to represent winter and summer respectively at the selected site.
December
June
The average monthly energy output from the PV array (Ea) and energy load (EL) for each month were calculated and then simulated in Matlab for each month of the year.
Conclusion
The study indicated that the model was able to supply adequately the power needs of the public lighting on a 65 m standalone pathway network, and represents a good alternative for public lighting and energy savings.
Fig. 8: The daily average temperature at the selected site: Minimum (blue) and maximum (red) temperature with percentile bands.
Acknowledgement
This article is based on a paper presented at the 2017 Industrial and Commercial Use of Energy Conference, and is reproduced here with permission.
References
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Contact Abaid Abdulrauf, Cape Peninsula University of Technology, Tel 074 826- 2070, abdul.abaid78@gmail.com
