Lecture 11 Solar Energy : Part 8

Summary

The lecture classifies photovoltaic (PV) systems, explains the components of a standalone system, and demonstrates a bottom-up method for sizing one for a small house.

Types of PV Systems

PV systems are classified by grid availability, intended use, and whether backup power is needed.

• Standalone (off-grid) systems operate independently of the utility grid. They often use batteries to supply power at night or during cloudy periods, though some loads can be connected directly to PV without storage.
• Grid-connected systems feed into or draw from the utility grid, commonly to offset electricity bills through net metering. Standard grid-tied systems generally shut down during a grid outage. A grid-connected system with a separate battery can provide backup.
• Hybrid systems combine solar PV with battery storage and/or another energy source, such as wind, hydro, a diesel generator, or a fuel cell. They may also be connected to the grid.

The lecture also mentions several specialized configurations:

• Building-integrated PV (BIPV): PV materials incorporated into building elements such as walls or glass.
• Concentrated PV: Mirrors focus sunlight onto PV panels.
• Floating PV: Panels installed on bodies of water. Potential benefits include saving land, reducing evaporation, and helping cool panels.
• Agrivoltaics: PV panels installed on agricultural land, with ongoing work to determine suitable crops, panel spacing, and mounting arrangements.

Standalone PV System Components

A standalone system typically includes:

1. PV modules or array: Convert sunlight into DC electricity.
2. Charge controller: Regulates voltage or current to protect batteries from overcharging and excessive discharge.
3. Battery bank: Stores surplus daytime energy for use when sunlight is unavailable.
4. Inverter: Converts DC electricity to AC for compatible loads.
5. AC and DC loads: Appliances and equipment that consume the electricity.

The lecturer recommends keeping the PV array, battery bank, and inverter at similar system voltages where practical. This can reduce conversion equipment, cost, and losses.

Battery Concepts Used in Sizing

• Efficiency: Energy output divided by energy input.
• Depth of discharge (DoD): The portion of a battery’s capacity that can be used while respecting its operating limits. For example, 50% DoD means using half the nominal capacity. The lecture notes that lithium-ion batteries commonly allow a higher DoD, around 80–90%.
• Autonomy: The number of consecutive days a system must serve its load without energy from the PV array. In the worked example, an autonomy of two days is treated as storage for three days in total: the current day plus two additional days.

Standalone System Design Method

The design proceeds from the load upward, accounting for component efficiencies:

1. Determine the load and daily energy use.

   • For each appliance, calculate: Power rating (W) × hours of use per day = energy use (Wh/day).

   • Sum the appliances’ daily energy use.

2. Size the inverter.

   • Assume the appliances may run simultaneously.
   • Add their power ratings to estimate the required inverter output rating.
   • Account for inverter efficiency when calculating how much energy must be supplied to it.

3. Size the battery bank.

   • Use the system voltage to convert daily energy into ampere-hours.
   • Increase the required capacity to account for DoD and autonomy.
   • Account for battery efficiency.
   • Choose series and parallel connections to meet the required voltage and capacity.

4. Size the charge controller.

   • Account for its efficiency when determining the energy that must be supplied from the PV array.

5. Size the PV array.

   • Estimate peak sun hours by dividing daily solar radiation by the standard irradiance of 1,000 W/m².
   • Divide the required daily PV energy by peak sun hours to estimate array peak power.
   • Select a practical number of modules and connect them in series or parallel to meet the system voltage and power needs.

Worked Example: Small Rural House

The example uses a daily solar radiation value of 6,000 Wh/m², a 24 V system, and these loads:

• One 100 W TV used for 2 hours: 200 Wh/day
• Two 50 W fans used for 8 hours: 800 Wh/day
• Four 10 W lights used for 5 hours: 200 Wh/day

The total load is 1,200 Wh/day.

Inverter

With all appliances assumed to run together, the load is 100 + 100 + 40 = 240 W. At 90% inverter efficiency, the inverter must receive about 1,333 Wh/day to deliver 1,200 Wh/day.

Battery Bank

At 24 V, the inverter’s daily energy requirement is about 55 Ah/day. Applying 50% DoD gives about 111 Ah; designing for three days of storage gives about 333 Ah.

With 12 V, 100 Ah batteries, the example selects four batteries, connected as two batteries in series per string and two strings in parallel, giving 24 V and 400 Ah. At 80% battery efficiency, the battery input requirement is about 1,667 Wh/day.

Charge Controller and PV Array

At 95% controller efficiency, the PV array must provide about 1,754 Wh/day. The radiation value corresponds to approximately 6 peak sun hours, so the calculated array size is about 292 Wp.

With 60 W, 12 V modules, the design uses six modules (360 Wp), arranged as two modules in series per string and three strings in parallel to obtain 24 V.

The example illustrates the overall sequence: calculate the load, then size the inverter, battery bank, controller, and PV array while accounting for efficiencies and practical series/parallel arrangements.

Speakers and Sources Featured

• Speaker: One lecturer, identified in the video metadata as Prof. Sonal K. Thengane; no other speakers are apparent in the subtitles.
• Other material presented: Lecture diagrams, a comparison table of PV system types, and a worked numerical design example.