The increasing adoption of electric vehicles (EVs) and electric bicycles, celebrated for their low emissions and high mobility, highlights a critical limitation: the finite energy storage capacity of their batteries, which directly constrains driving range. Addressing this challenge requires innovative charging infrastructure. A solar photovoltaic (PV) charging pavilion presents an elegant solution, leveraging renewable energy to provide convenient charging during vehicle parking hours, such as at workplaces or public squares. This approach not only optimizes space and time but also contributes significantly to energy conservation and emissions reduction. The integration of digital control systems further enhances its reliability and automation. This article details the comprehensive design, capacity calculation, and economic feasibility of a solar system for a charging pavilion capable of servicing 50 electric vehicles simultaneously.
The global context for such infrastructure is evolving rapidly. In Western nations, early adoption of electric cars spurred the development of small-scale charging stations along highways. Domestically, research and pilot projects have progressed from early studies on EV charging systems in 2001 to operational stations, such as a 10 kWp PV-powered station built in 2010. These developments underscore the growing necessity for dedicated EV charging infrastructure. Our design contributes to this field by proposing a scalable, self-sufficient solar system.
1. Charging-Discharging Schedule and Total Energy Calculation
The operational schedule of the pavilion is crucial for aligning solar generation, battery storage, and user demand. The system operates from 8:00 to 21:00 daily, with specific intervals allocated for charging from the PV array, discharging to the EVs, and providing power in the evening. The schedule is summarized in the table below.
| Period | PV Charging Activity | Battery Discharging Activity | EV Charging Window |
|---|---|---|---|
| Morning (8:00–12:00) | Active | Active | 8:00–12:00 |
| Midday (12:00–14:00) | Active (No EV load) | None | None |
| Afternoon (14:00–18:00) | Active | Active | 14:00–18:00 |
| Evening (18:00–21:00) | None | Active | 18:00–21:00 |
For this design, we assume each EV uses a 48V, 26Ah battery and arrives at the pavilion in a depleted state. All 50 charging spots are occupied, and vehicle turnover during charging periods is neglected for simplicity. The system’s DC power is inverted to 220V AC to accommodate various chargers.
The total daily energy demand (Q) for the entire solar system comprises three components:
1. Energy for EV Batteries (Q1): This is the energy directly supplied to the 50 vehicles during the morning and afternoon charging windows.
$$ Q1 = N \times V_{bat} \times C_{bat} \times t_{charge} $$
Where \( N = 50 \) (number of EVs), \( V_{bat} = 48V \), \( C_{bat} = 26Ah \), and \( t_{charge} = 6 \) hours (4 morning + 2 afternoon, assuming a 2-hour midday break for charging continues but not for EVs). Thus,
$$ Q1 = 50 \times 48V \times 26Ah \times 6h = 374,400 Wh = 374.4 kWh $$
Note: The original text’s calculation of 499.2 kWh appears to use an 8-hour charging time. We recalculate based on the described schedule (6 hours of active EV charging).
2. Evening Energy Supply (Q2): This is the energy supplied from the storage batteries during the evening period (18:00-21:00, 3 hours) to power the pavilion’s infrastructure or for potential ancillary loads.
$$ Q2 = P_{evening} \times t_{evening} $$
If we assume the evening load is simply maintaining the system and its sockets ready, and attribute a nominal value, a more systematic approach is to define it relative to daytime activity. For consistency with a total system load, we will derive it later from the overall solar system balance.
3. System Energy Losses (Q3): Estimated at 1% of the total generated energy.
A more holistic approach is to define the total daily load (L) that the PV array must satisfy. This includes the energy delivered to the EVs (Q1=374.4 kWh) and the energy needed to recharge the storage batteries that were depleted to supply the evening load. If we designate the evening load as \( L_{evening} \), then the storage batteries must store an amount equal to \( L_{evening} / \eta_{inv} \), where \( \eta_{inv} \) is the inverter efficiency. Furthermore, the PV array must produce enough to cover all losses. Therefore, the total daily energy requirement from the PV solar system Q can be expressed as:
$$ Q = \frac{Q1 + \frac{L_{evening}}{\eta_{disch}}}{\eta_{inv} \times \eta_{other}} $$
Where \( \eta_{disch} \) is battery discharge efficiency, and \( \eta_{other} \) covers other losses. Using representative efficiencies (\( \eta_{inv} = 0.92, \eta_{disch}=0.90, \eta_{other}=0.99 \)) and assuming an evening load \( L_{evening} = 50 kW \times 3h = 150 kWh \) (equivalent to a sizable ancillary load), we can calculate Q. For the purpose of continuing with subsequent capacity calculations, we will adopt the original article’s adjusted total of \( Q \approx 686.4 kWh \) as the target daily PV generation.
2. Capacity Design of the Solar System
2.1 PV Array Capacity
The sizing of the PV array is based on the average daily peak sun hours (PSH) at the installation site. The required nominal PV power (P) is calculated as:
$$ P = \frac{Q \times f_{safe}}{PSH} $$
Where \( f_{safe} \) is a safety factor (often 1.2-1.5) accounting for dust, temperature, and other losses. Using \( Q = 686.4 kWh/day \), \( f_{safe} = 1.5 \), and \( PSH = 6 \) hours:
$$ P = \frac{686.4 \times 1.5}{6} = 171.6 kW $$
We select 250W PV modules. The number of modules required is:
$$ N_{modules} = \frac{P}{P_{module}} = \frac{171,600W}{250W} \approx 687 $$
The configuration involves connecting modules in series to reach the desired system voltage and in parallel to achieve the required current. Assuming a system voltage of 48V for battery charging and modules with a Vmpp of ~30V, about 2 modules in series are needed per string. The number of parallel strings would then be \( 687 / 2 \approx 344 \). The original design’s number of 884 modules (29 series x 31 parallel) suggests a different safety factor or PSH assumption. We will proceed with the design target of ~687 modules for our revised solar system.
2.2 Battery Bank Capacity
The battery bank must store sufficient energy to power the loads during periods of low or no solar insolation. The capacity (C) in Ampere-hours (Ah) at the system voltage is calculated as:
$$ C = \frac{L \times D}{DOD \times \eta_{bat} \times V_{sys}} $$
Where:
- \( L \): Total daily load energy (kWh). This includes the evening load and any daytime load not directly met by PV. For autonomy calculation, we use the critical load. If we consider the evening load \( L_{evening}=150kWh \) as the critical load, and assume the EV load is only served when sun is available, then \( L \approx L_{evening} \).
- \( D \): Number of autonomy days (e.g., 3 days of cloudiness).
- \( DOD \): Maximum permissible Depth of Discharge (e.g., 0.8 for lead-acid).
- \( \eta_{bat} \): Battery round-trip efficiency (e.g., 0.85).
- \( V_{sys} \): System voltage (e.g., 48V).
$$ C = \frac{150 kWh \times 3}{0.8 \times 0.85 \times 48V} \approx \frac{450}{32.64} \approx 13,780 Ah $$
This is a very large capacity (13.78 kAh). The original article’s calculation of 780 kAh uses a different ‘L’. If we take the total daily generation Q (686.4 kWh) as the energy that must be stored for 3 days, the formula \( C = \frac{686.4 \times 3}{0.8 \times 0.9 \times 48} \approx 1560 kAh \) is obtained. This approach sizes the battery to store nearly all PV output, which is characteristic of a large off-grid solar system. For a realistic design, the battery is sized for the critical nighttime/cloudy day load, not the total PV production.
3. Pavilion and Control System Design
The physical design of the pavilion integrates structure and function. The upper section features a canopy constructed from PV modules, installed at the optimal tilt and azimuth angle to maximize solar harvest. Beneath this canopy are the charging stations equipped with outlets for the EVs. The lower section, potentially a basement or secured enclosure, houses the core components of the solar system: the battery bank, the system controller, inverters, and distribution panels. This design protects sensitive equipment from the elements and theft.
The control system is the brain of the operation. A sophisticated charge controller manages energy flow. Its primary functions include:
- Maximum Power Point Tracking (MPPT): Extracting the maximum available power from the PV array.
- Battery Charging Management: Regulating the charging of the battery bank through multi-stage protocols (bulk, absorption, float) to ensure longevity.
- Load Control & Prioritization: Directing power either directly from the PV array to the loads (when sun is available) or from the batteries (at night or on cloudy days). It can prioritize charging the EVs during peak sun hours.
- System Protection: Providing safeguards against overcharge, deep discharge, overcurrent, and short circuits.
- Grid Interaction (if grid-tied): Managing bidirectional flow if the system is connected to the utility grid.
A conceptual control diagram involves switches (Q1, Q2, Q3, Q4—representing contactors or solid-state relays) managed by a central controller (P1-P4). These switches connect the PV array (S1) to the battery bank (B1, B2), and the battery bank to the load (L1). The controller’s logic ensures that PV power charges the batteries and services the load simultaneously when possible, disconnects the PV when batteries are full, and can isolate battery banks for maintenance while keeping the solar system partially operational.
4. Economic Feasibility Analysis
An economic assessment is vital for evaluating the practicality of the proposed solar system. The analysis focuses on initial investment, recurring costs, revenue, and key financial metrics like Return on Investment (ROI) and Payback Period.
4.1 Initial Investment Cost
The major capital expenditures for the off-grid solar charging pavilion are itemized below. Costs are estimates based on typical market prices.
| Component | Quantity | Unit Cost (USD) | Total Cost (USD) |
|---|---|---|---|
| 250W PV Module | 687 | 80 | 54,960 |
| Mounting Structure & Canopy | 1 Set | 30,000 | 30,000 |
| Battery Bank (48V, ~14kAh) | 1 Set | 120,000 | 120,000 |
| MPPT Charge Controller(s) | 4 | 2,500 | 10,000 |
| Inverter(s) (48V DC to 220V AC) | 4 | 3,000 | 12,000 |
| Wiring, Connectors, Protection Devices | 1 Lot | 15,000 | 15,000 |
| Total PV System Cost | ~241,960 | ||
| Pavilion Construction & Civil Works | 1 Set | 40,000 | 40,000 |
| Grand Total Initial Investment (I) | ~281,960 |
4.2 Recurring Costs and Revenue
Recurring Costs:
- Battery Replacement: With a lifespan of 4-5 years, the battery bank (costing $120,000) may need replacement 5-6 times over a 25-year project life. Assuming 5 replacements at 60% of initial cost: \( 5 \times (120,000 \times 0.6) = $360,000 \). Annualized cost: \( 360,000 / 25 = $14,400 \).
- Operation & Maintenance (O&M): Estimated at 1% of initial PV system cost per year: \( 0.01 \times 241,960 \approx $2,420 \).
- Total Annual Cost (C_annual): \( 14,400 + 2,420 = $16,820 \).
Annual Revenue (R_annual):
Assuming 50 charging spots, each used once per day for a fee. With an average fee of $2 per charge session:
$$ R_{annual} = 50 \times 2 \times 365 = $36,500 $$
Annual Net Profit (P_annual):
$$ P_{annual} = R_{annual} – C_{annual} = 36,500 – 16,820 = $19,680 $$
4.3 Financial Metrics for Off-Grid System
Simple Return on Investment (ROI):
$$ ROI = \frac{P_{annual}}{I} = \frac{19,680}{281,960} \approx 0.07 \text{ or } 7\% $$
Simple Payback Period (PP):
$$ PP = \frac{I}{P_{annual}} = \frac{281,960}{19,680} \approx 14.3 \text{ years} $$
Given a typical project lifespan of 25 years for PV modules, a payback period of 14.3 years is lengthy, primarily due to the high capital and replacement cost of the battery storage within the solar system.
4.4 Economic Impact of Grid-Connection
A grid-tied configuration dramatically improves economics. The battery bank is eliminated or drastically reduced (kept only for short-term backup), removing the largest cost component. The PV array now acts as a net-metered system, offsetting grid consumption for charging.
Revised Initial Investment for Grid-Tied System:
- PV Array & Mounting: $54,960 + $30,000 = $84,960
- Grid-Tied Inverters & Safety Equipment: $20,000
- Electrical Integration & Metering: $10,000
- Construction: $40,000
- Total Initial Investment (I_grid): ~$154,960
Recurring Costs & Revenue:
- O&M: ~1% of PV system cost = ~$850/year.
- No battery replacement costs.
- Revenue remains $36,500/year from charging fees.
- Additional Savings/Revenue: Excess solar energy fed back to the grid may generate credits or feed-in tariff (FIT) income. Assuming a modest FIT or offset value.
Net Profit with Grid-Tie (ignoring FIT initially):
$$ P_{annual\_grid} = 36,500 – 850 = $35,650 $$
Financial Metrics for Grid-Tied System:
$$ ROI_{grid} = \frac{35,650}{154,960} \approx 0.23 \text{ or } 23\% $$
$$ PP_{grid} = \frac{154,960}{35,650} \approx 4.35 \text{ years} $$
Considering a government subsidy for distributed solar generation (e.g., $0.042/kWh for all PV production), the payback period shortens further. If the solar system produces 686.4 kWh/day, annual subsidy income is:
$$ Subsidy = 686.4 \times 365 \times 0.042 \approx $10,520 $$
Total annual income becomes \( 36,500 + 10,520 = $47,020 \). Net profit becomes \( 47,020 – 850 = $46,170 \). The payback period then reduces to:
$$ PP_{grid+subsidy} = \frac{154,960}{46,170} \approx 3.36 \text{ years} $$
This demonstrates the transformative economic benefit of grid interconnection and supportive policies for such a solar system.
5. Conclusion
This article presents a detailed design and analysis of a solar-powered charging pavilion for electric vehicles. The proposed solar system is engineered to simultaneously charge 50 electric vehicles, with a calculated daily energy generation of approximately 686.4 kWh. The design specifics include a PV array of about 687 modules (250W each) and a substantial battery bank for off-grid operation, though the capacity is highly dependent on the chosen load profile and autonomy requirements. The integrated design featuring a PV canopy, underground storage, and an intelligent control system offers a functional and aesthetically pleasing solution for parking lots, campuses, and public squares.
The economic analysis reveals a critical insight: the off-grid configuration, while fully autonomous, suffers from a long payback period (over 14 years) primarily due to battery costs. However, transforming the design into a grid-connected solar system radically improves its financial viability, reducing the payback period to under 4.5 years, and to roughly 3.4 years with common solar energy subsidies. Therefore, a grid-tied or hybrid solar charging pavilion is not only a technically sound solution for supporting electric mobility but also an economically attractive investment that promotes sustainable energy use and infrastructure development.