Hybrid PV Systems (PV-Generator Hybrid) Study Notes)

Hybrid Energy System Overview and Classification

• Definition: A hybrid energy system, often called hybrid power, consists of two or more energy sources utilized together to ensure 100% system availability.

• General Classification Groups:

  • All-Renewable Hybrid: Combines multiple renewable sources (e.g., Solar and Wind).

  • Renewable-Nonrenewable Hybrid: Combines renewable sources with conventional generation (e.g., PV and Diesel Generator).

• PV Hybrid System Categorization:

  • Grid-Tied Hybrid: Systems connected to the utility grid with backup capabilities.

  • Stand-Alone Hybrid: Most popular for remote areas; operates independently of the utility grid.

• Hybridization Partners for PV: Photovoltaic systems can be hybridized with:

  • Wind turbines

  • Solar thermal systems

  • Biomass

  • Fuel cells

  • Conventional generators (Diesel, Petrol, Gas, Propane)

Advantages and Disadvantages of Hybrid Systems

• Key Advantages:

  • Uninterrupted Power Supply: Provides a greater balance in energy supply compared to single-source systems.

  • Efficiency and Resource Utilization: Increases overall system efficiency and utilizes untapped energy resources.

  • Economic Factors: Reduces long-term costs; fuel for renewable components is abundant, free, and inexhaustible.

  • Security and Environment: Offers diversity in supply, predictable costs (not influenced by fuel price fluctuations in all-renewable systems), and produces greener energy.

  • Geographical Suitability: Highly effective in areas near equatorial regions ($23^{\circ}$ N to $23^{\circ}$ S) or low altitudes where seasonal day lengths vary significantly.

• Key Disadvantages:

  • Storage Requirements: Needs storage systems (like hydrogen for PV-Fuel cell or battery banks) that can be costly.

  • System Complexity: Requires sophisticated control systems (though advanced inverters can sometimes manage this).

  • Integration Challenges: Integrating more than three sources is complicated; grid compatibility can be difficult to maintain.

  • Initial Costs: Infrastructure and storage system prices significantly influence the initial investment.

System Availability and Cost Dynamics

• Definition of Availability ($A$): The percentage of time a power system is capable of meeting load requirements. A design for 95% availability meets load needs 95% of the time.

• Design Determinants: Application requirements, solar variability at the site, and financial limitations.

• Availability Degradation Factors: Weather conditions, hardware failures, system maintenance, and excessive demand.

• Application Types:

  • Non-Critical Systems: Typically designed for 95% availability (e.g., standard households, PV cathodic protection units).

  • Critical Systems: Typically require 99% availability (e.g., telecommunication repeater stations).

• The Cost Corner: Costs increase rapidly when attempting to achieve the last few percentages of availability. For PV systems, availability below 80% generally implies no surplus capacity, as generation is less than consumption even on cloudless days. Between 0% and 80%, the relationship between cost and availability is approximately linear.

Hybrid System Indicators and Design Motivation

• PV with Generator Backup: Backup generators (Diesel/Petrol/Gas) allow the PV system to be designed for lower availability, leading to significant savings in battery capacity and panel counts.

• Key Indicators for Hybridization:

  • Large load size.

  • High seasonal insolation variability.

• The Hybrid Indicator Graph:

  • Vertical Axis: Load in Watt-hours per day ($Wh/day$).

  • Horizontal Axis: Array-to-load ratio ($W_p/Wh$).

  • Array-to-Load Ratio Definition: Peak array power rating (at $1\,kW/m^2$) divided by the daily load in $Wh$.

  • Interpretation: Locations with high array-to-load ratios (often due to cloudy climates requiring oversized arrays) and large loads are ideal candidates for hybrid systems.

• Economic Strategy: It is often more cost-effective to use a generator to supplement low-output months rather than sizing a pure PV system for absolute worst-case scenarios (e.g., January in the Northern Hemisphere).

Case Study: Bismarck, North Dakota Residential Hybrid System

• Location Coordinates: Latitude $46^{\circ}48'48"$ N, Longitude $100^{\circ}46'44"$ W.

• Climate Data:

  • Warmest Month: July (Daily mean $21.3^{\circ}C$; record high $46^{\circ}C$).

  • Coldest Month: January (24-hour average $-12.1^{\circ}C$; record low used for design $-40^{\circ}C$).

• Load Profiles:

  • Stand-alone residence, occupied all year.

  • All loads are energy-efficient (Fluorescent/LED lighting, high-efficiency fridge).

  • Heating is primarily passive solar with triple-pane windows; propane used for supplemental heating, water backup, cooking, and the generator.

• Design Consideration - Inverter/Battery Link: For efficiency, generators should operate at 90% capacity. Batteries are used to buffer this energy. To avoid inefficient fast-charging, systems are sized so the generator takes at least 5 to 10 hours to charge the bank.

Battery Bank Subsystem Design

• DC Voltage Selection: 48V battery bank is preferred.

• Corrected Load Calculation:

  • Step 1: Account for inverter and wiring losses (assumed 92% efficiency; 5% inverter loss + 3% wiring loss).

  • Step 2: Account for battery charge/discharge efficiency (assumed 90%).

  • Winter Example: $(150\,W \times 7\,h/day) / 48\,V = 21.875\,Ah/day$. Corrected for efficiency: $21.875 / 0.92 = 23.8\,Ah/day$. With battery losses: $23.8 / 0.9 = 26.4\,Ah/day$.

  • Total Corrected Loads: Winter = $207\,Ah/day$; Spring = $181\,Ah/day$; Summer = $143\,Ah/day$.

• Battery Selection: Trojan T105 or 12V sealed lead-acid (295 Ah @ C/72 rate).

• Sizing Calculation:

  • Days of Autonomy ($D_{non}$): 3 days selected for non-critical residential use. Equation: $D_{non} = 0.1071(PSH)^2 - 1.869(PSH) + 9.4286$.

  • Configuration: For a Winter target of 970.31 Ah, strings of 4 batteries in series (for 48V) and 3 strings in parallel. Total = 12 batteries.

  • Performance: Provides ~2.74 days of supply in winter and nearly 5 days in summer.

PV Array Sizing and Optimization

• Sizing Approach:

  1. Size for 100% PV (no generator).

  2. Iteratively reduce module count and calculate the annual energy percentage met by PV vs. Generator.

  3. Perform life-cycle cost analysis on these options.

• Example PVWatts Calculation:

  • Location: Bismarck, ND.

  • Target: Meet December daily need of $11.2\,kWh$ ($54\,V \times 207\,Ah$).

  • Conversion Factor: 0.85 (accounting for 15% losses in wiring, controller, and array).

  • Result: 3.6 kW array size at a $70^{\circ}$ tilt (latitude + $23^{\circ}$).

• Monthly Performance (3.6 kW Array):

  • March: Corrected load $9.8\,kWh/day$. PVWatts production ~$15.4\,kWh/day$. Result: Daily excess of $5.6\,kWh$.

Module Selection and String Configuration

• Design Constraints: Source circuit $V_{OC}$ must be < 150\,V at $-40^{\circ}C$.

• Module Temperature Coefficient Calculation ($\Delta V_{OC}/\Delta T$):

  • At $-40^{\circ}C$, temperature difference from STC is $65^{\circ}$.

  • Example 150W Module: $V_{OC} = 40.26 + (65 \times 0.155) = 50.34\,V$. Three in series is > 150\,V, necessitating 2-module strings (higher wiring cost).

  • Final Selected 200W Module: $V_{OC} = 32.9\,V$; Temp Coeff = $-123\,mV/K$. At $-40^{\circ}C$, $V_{OC} = 32.9 + (65 \times 0.123) = 40.9\,V$.

  • String Layout: 3 modules in series = $3 \times 40.9 = 122.7\,V$ (< 150\,V). Total 6 strings to reach 3.6 kW level (18 modules total).

Generator Selection and Performance Modeling

• Generator Sizing Factor ($C/10$ Rate):

  • Winter battery capacity = $708\,Ah$.

  • Charging current = $70.8\,A$.

  • Charging voltage = ~120% of nominal = $48 \times 1.2 = 57.6\,V$.

  • Required power at charger output = $70.8\,A \times 57.6\,V = 4078\,W$.

  • Input power to charger (90% eff): $4078 / 0.9 = 4531\,W$.

  • Generator Recommendation: A 5000W generator running at ~91% rated output.

• Operating Logistics:

  • Generator used primarily for battery charging from 20% to 70% state of charge (takes ~5 hours).

  • Small generators yield ~5 kWh per gallon of fuel.

  • Running time calculation: Approx $0.294\,h$ to deliver $1\,kWh$ to the batteries.

  • Fuel consumption: $0.906\,gal/h$ at $4531\,W$ output.

Generator Maintenance and Cost Analysis

• Maintenance Intervals:

  • Oil Change: Every 25 hours.

  • Tune-Up: Every 300 hours.

  • Engine Rebuild: Every 3000 hours.

• Example (6-module PV array): Requires 554 hours of annual generator operation.

  • Yields ~22 oil changes per year.

  • Yields ~2 tune-ups per year.

  • Rebuild needed every ~5.4 years ($3000 / 554$).

• Fuel Cost Example: 12-module array in November requires 87 battery kWh from the generator. Running time = $25.6\,h$. Fuel needed = $23.2\,gal$. Cost at $2.30/gal$ = $53.56$.

Control and Balance of System (BOS)

• MPPT Charge Controller: Maximizes PV output and manages battery charging.

• Off-grid Hybrid Inverter: Functions as the brain of the system.

  • Connects Battery (DC IN) and Generator (AC2/GEN IN).

  • Monitors battery voltage; triggers generator start/stop signals.

  • Supplies household AC loads (Design requires min 5110W; 6000W rating with 48V input is common).

• Inverter Efficiency Caution: Peak efficiency is ~95% (at 1000-2000W output). Efficiency drops below 90% for loads under 300W. If a 100W load runs at 70% efficiency, it draws 143W from batteries, increasing total daily load.

• BOS Components: Battery container, array mount, surge protection, grounding, and wiring consistent with National Electrical Code (NEC) requirements (distribution panel min 100 A).

Questions & Discussion

• Question: What is the array-to-load ratio calculation for a specific site in the USA?

  • Response: If January is the design month with a daily load of $5.73\,kWh/day$ and the array is 24 modules totaling $2.63\,kW_p$, the ratio is $2.63 / 5.73 = 0.46$.

• Question: Why is 48V preferable for the battery bank voltage?

  • Response: This minimizes current (reducing wire size and copper losses) and is a standard for large residential inverters and charge controllers.

• Question: How does the generator's operating efficiency change with load?

  • Response: Generators are most efficient at 80% to 90% of their rated output. Running at a small fraction of capacity significantly decreases efficiency.

• Question: What is a hybrid indicator?

  • Response: It is a graph of Daily Load vs. Array-to-Load Ratio that helps designers determine if adding a generator backup is economically preferable to oversizing the PV array.