ENS4445/ENS5445 SUSTAINABILITY AND RENEWABLE ENERGY LECTURES 7-9

List the three main types of PV systems.

• Grid-connected systems

• Stand-alone off-grid systems

• Directly coupled systems (e.g., water pumping, desalination, filtering)

What are the benefits of stand-alone PV systems?

They can significantly change people’s lives by providing electricity in remote or off-grid areas.

What conditions must users of stand-alone PV systems meet?

• Willingness to check and maintain batteries

• Adjust energy demands

• Fuel and fix a noisy generator

• Take responsibility for safe operation

What is the first step in designing a stand-alone PV system?

Estimating the load (iterative process).

What key design choices affect stand-alone PV systems?

• Efficient appliances vs more PVs/batteries

• Essential loads vs luxuries

• AC vs DC or AC+DC

• Backup generator or no generator

How is energy demand estimated?

Energy = power rating × hours in use (but must also consider standby consumption and motor surges).

Why do standby electronic devices matter in load estimation?

~2/3 of residential electricity is consumed by devices in standby mode.

Why do motor surges affect PV system design?

Starting currents affect inverter, wire, and fuse sizing.

What role does the inverter play in stand-alone PV systems?

Converts DC battery power to AC for loads.

How does inverter efficiency vary?

It depends on the magnitude of the load being supplied at that moment.

What is a drawback of standby devices with inverters?

They keep the inverter running continuously (5–20 W) without delivering real energy services.

Why are batteries essential in stand-alone PV systems?

They store energy during good solar conditions for use during low or no solar input.

What are exotic alternatives to batteries?

Flywheels, compressed air, hydrogen production.

Why are lead-acid batteries the “workhorse” of PV systems?

They are mature, affordable, and reliable compared to alternatives.

What is an SLI battery?

A starting, lighting, and ignition (car) battery.

Why are SLI batteries unsuitable for PV systems?

• Designed for short bursts (400–600 A)

• Fail quickly after deep discharge cycles

• Thin plates optimized for surface area, not deep cycling

What are deep-cycle lead-acid batteries?

Batteries designed for steady current over long periods, with thicker plates and ability to withstand repeated deep discharges.

Where are deep-cycle batteries commonly used?

RVs, boats, golf carts, and PV systems.

How is battery storage capacity usually expressed?

In amp-hours (Ah).

What is the nominal voltage of a lead-acid cell?

2 V per cell (e.g., 12 V = 6 cells).

Example: A 200 Ah, 12 V battery at C/10 delivers what?

20 A for 10 hours until fully discharged to 10.5 V.

Why does battery capacity depend on discharge rate?

Higher discharge rates reduce total deliverable Ah.

What discharge rate is typically used for deep-cycle PV batteries?

C/20 (sometimes C/100).

What are the pros of series vs parallel battery connection?

• Series: higher voltage, smaller wires/fuses, easier connections.

• Parallel: easier expansion (add one battery at a time).

What does Coulomb efficiency mean in batteries?

The ratio of charge extracted to charge put in.

What is the role of a charge controller?

• Slow charging as battery nears full

• Prevent overcharging by disconnecting PV array

• Prevent over-discharging by disconnecting loads

How is PV array sizing for stand-alone systems done?

Based on peak sun hours, applied to current rather than power.

Why must standalone PV systems be sized carefully?

To ensure sufficient storage and avoid premature battery failure.

What percentage of solar energy is converted into wind?

~1–2%.

How does wind energy compare to biomass in terms of energy availability?

Wind energy is 50–100 times greater.

What natural processes create wind?

Uneven solar heating, Earth’s rotation, and terrain.

Who built the first organ powered by wind and when?

Heron of Alexandria, ~1 AD.

When were windmills first used in Persia?

Around 800 AD.

When was the golden age of European windmills?

1200–1850.

Who pioneered modern wind turbines in the 1890s?

Poul la Cour in Denmark.

When did Australia see many small wind generators?

1936–1970.

What revived wind power research in the 1970s?

The 1973 oil crisis.

What are the two main wind turbine designs?

Horizontal-axis (HAWT) and vertical-axis (VAWT).

Advantages of VAWT?

No yaw system, nacelle on ground, lighter, cheaper.

Disadvantages of VAWT?

Poor torque, low efficiency, difficult to control.

Pros of downwind HAWT?

No yaw mechanism needed.

Cons of downwind HAWT?

Tower shading fatigue.

Pros of upwind HAWT?

Smoother power, more output.

Cons of upwind HAWT?

Needs yaw mechanism.

How do blade numbers affect turbine design?

More blades = lower speed, less noise, more torque.

Why are 3-blade turbines most common?

Stable, quiet, efficient balance between cost and performance.

Formula for power in the wind?

P=1/2ρAv³

What is air density used in calculations?

1.225 kg/m³.

How does blade length affect power?

Power ∝ blade length squared (via swept area).

What is the Betz limit?

The theoretical maximum efficiency of a wind turbine = 59.3%.

Why is variable speed control useful?

Higher efficiency, less stress, grid support (reactive power, voltage).

What does pitch control do?

Adjusts blade angle to maximize lift/drag ratio and efficiency.

What are the key characteristics of wind energy?

Intermittent, variable, site-specific, three-dimensional, least latitude-dependent.

How does wind power density depend on wind speed?

It is a cubic function of wind speed.

What is the typical cut-in speed for a turbine?

3–5 m/s.

At what wind speed is rated power usually reached?

Around 12 m/s.

What is the cut-out speed for most turbines?

25 m/s.

What control methods are used in turbines?

Active/passive pitch control, stall control, yaw control.

What are the main environmental concerns of wind farms?

Acoustic noise and visual aesthetics.

Why are bird kills not a major issue for wind farms?

Studies show minimal impact.

How can land be used around onshore wind farms?

Farming and grazing continue.

How do wind power costs compare to conventional sources?

Almost equal to fossil fuels and more competitive than solar for bulk power.

Why is wind already economical in remote areas?

High wind speeds make it cheaper than diesel generation.

Typical size range of new offshore wind farms?

50–100 MW.

Advantages of offshore wind?

Higher average wind speeds, less turbulence, low shear, lower tower heights, reduced visual impact.

Disadvantages of offshore wind?

Higher capital cost, difficult maintenance, need for subsea cabling.