Past paper Qs

Question 1

A student wants to test a 1:100 scaled wind turbine model in a wind tunnel. The power
coefficient Cp for a given wind speed and a given tip speed ratio is extracted from the wind
tunnel test. Comment on the potential difference between the Cp from the experiment and
the actual Cp for the wind turbine and give your reason.

Answer 1

The potential difference between the Cp obtained from a wind tunnel experiment with a scaled wind turbine model and the actual Cp for a full-scale wind turbine can be attributed to factors such as scale effects, limitations in replicating atmospheric turbulence, flow blockage effects, discrepancies in the model’s representation, and uncertainties in measurements. Validation through computational simulations and field measurements is essential to obtain more accurate Cp values for practical application.

Question 2

At an urban location, while there is a good wind speed level, the direction of the wind is
rather unpredictable and changes quickly. Recommend a type of wind turbine for this
location for power extraction and give your reasons for your choice.

Answer 2

A vertical axis wind turbine (VAWT) would be recommended for an urban location with unpredictable and quickly changing wind directions. VAWTs are capable of capturing wind from any direction, have better start-up performance, and their compact design, lower noise levels, and visual aesthetics make them well-suited for urban environments.

Question 3

Comment on the large difference in the design of the traditional windmills and the modern
large wind turbines and explain why.

Answer 3

The large difference in design between traditional windmills and modern large wind turbines can be attributed to their distinct purposes and technological advancements. Traditional windmills were primarily used for mechanical power generation, whereas modern wind turbines are designed for utility-scale electricity generation, incorporating advanced materials, aerodynamics, and control systems to optimize efficiency and reliability.

Question 4

For a weather station at a remote location, a small wind turbine is required for electricity
supply. For the installation of the wind turbine, you may choose to erect the wind turbine
either besides the building or on the roof top at the same height. Give your
recommendation for the installation and estimate the difference in the medium wind speed
and power generation.

Answer 4

For a small wind turbine installation at a remote weather station, I recommend placing the turbine on the rooftop of the building rather than beside it. This rooftop installation would likely provide higher wind speeds and potentially greater power generation due to increased elevation and reduced obstructions compared to a ground-level installation.

Question 5

For a weather station at a remote location, a small wind turbine is required for electricity
supply. For the installation of the wind turbine, you may choose to erect the wind turbine
either besides the building or on the roof top at the same height. Give your
recommendation for the installation and estimate the difference in the medium wind speed
and power generation.

Answer 5

For a small wind turbine installation at a remote weather station, I recommend placing the turbine on the rooftop of the building rather than beside it. This rooftop installation would likely provide higher wind speeds and potentially greater power generation due to increased elevation and reduced obstructions compared to a ground-level installation.

Question 6

For a HAWT rotor blade, which section of the blade, i.e. near root, mid-span or near tip, is
generating the most torque and therefore contributes the most to power generation? Give
the reasons for your answer. Why is the blade tapered with reduced chord near the tip?

Answer 6

In a Horizontal Axis Wind Turbine (HAWT) rotor blade, the section near the tip generates the most torque and contributes the most to power generation due to the higher wind speed experienced at the tip. The blade is tapered with a reduced chord near the tip to reduce drag, distribute structural load, and optimize aerodynamic efficiency.

Question 7

Wells turbines are often used for wave energy systems for oscillating flows. Use a sketch
to show why a Wells Turbine blade section can work for flow from opposite directions (bidirectional).

Answer 7

The symmetric blade produces rotation no
matter which direction the air approaches
from.

Question 8

What are the reference wind speed and turbulence intensity for a wind turbine designed
according to IEC Class IIA?

Answer 8

A wind turbine designed according to IEC Class IIA has a reference wind speed of 42.5 m/s and a turbulence intensity of 10%.

Question 9

For large wind turbines of 8 MW power rating to be installed in deep waters in an offshore
wind farm, what type of substructures would you recommend?

Answer 9

• Semisubmersible
  platform;
• Tension‐leg
  platform;
• Spar buoy

Question 10

Two wind turbine designs have the same power rating of 3 MW and the same rated rotor
speed of 14 rpm. If one wind turbine adopts fixed rotor speed control and another uses
variable rotor speed control, which wind turbine should produce more annual energy?
Briefly explain your answer.

Answer 10

The wind turbine with variable rotor speed control is expected to produce more annual energy compared to the one with fixed rotor speed control. Variable rotor speed control allows the turbine to adapt and optimize its performance based on the varying wind conditions, resulting in higher energy capture and overall annual energy production.

Question 11

Define what is meant by the term AM0 when used in photovoltaics and where is it most
applicable.

Answer 11

In photovoltaics, AM0 refers to Air Mass 0, representing the solar spectrum at sea level under direct overhead sunlight. It is primarily applicable to space-based solar cells and serves as a reference for ideal solar energy measurements outside Earth’s atmosphere.

Question 12

A PV solar panel comprises 60 identical solar cells connected in series. Six of these cells
are damaged such that both their short circuit currents and open circuit voltages are now
half what they were before. The other 54 cells in the panel are unaffected. Explain
qualitatively (i.e. without undertaking a proper calculation) what happens to the panel’s
output power due to this damage.

Answer 12

Qualitatively, the damage to the six cells in the PV solar panel will significantly impact the overall output power of the panel. Since the short-circuit currents and open-circuit voltages of the damaged cells are now halved, their individual power outputs will also be reduced by a factor of four. As the cells are connected in series, the overall output power of the panel will be limited by the performance of the weakest cell. Therefore, with six cells experiencing reduced output, the panel’s total output power will be significantly diminished compared to its original capacity.

Question 13

Give two examples of thin-film deposition techniques that can be used to produce solar
cells. Give two advantages that PV cells produced this way have over conventional silicon
PV cells.

Answer 13

• Molecular beam epitaxy (MBE)
• Metal-organic vapour phase epitaxy (MOVPE)

Question 14

What is the Betz limit for wind turbine power? Give two assumptions in deriving this limit.

Answer 14

The Betz limit states that the maximum power a wind turbine can extract from the wind is 59.3% of the kinetic energy in the wind. This limit is based on assumptions that the turbine does not create turbulence or affect the wind flow and that the wind is an ideal fluid with uniform velocity.

Question 15

For rotor blades of large horizontal axis wind turbines (HAWT), the blade sectional pitch
angle β reduces from root to tip. Explain why

Answer 15

The reduction in blade sectional pitch angle β from root to tip in large horizontal axis wind turbines (HAWT) is done to achieve a more uniform lift distribution along the blade span and improve overall aerodynamic efficiency.

Question 16

Describe how stall regulation works at higher wind speed for a HAWT with fixed rotational
speed in 1 sentence

Answer 16

At higher wind speeds, stall regulation in a horizontal axis wind turbine (HAWT) with fixed rotational speed is achieved by increasing the pitch angle of the turbine blades, which alters the aerodynamic forces and causes the blades to operate near or beyond the point of stall to limit power output.

Question 17

For a new wind turbine of 6 MW power rating to be installed in deep water of 45 meters in
an offshore wind farm, what type of substructures do you recommend?

Answer 17

• Monopile;
• Four‐legged jacket;
• Twisted jacket;

Question 18

Give one advantage and one disadvantage of increasing the number of junctions in a multi-junction solar cell.

Answer 18

Increasing the number of junctions in a multi-junction solar cell can improve overall efficiency by capturing a wider range of the solar spectrum, but it can also increase manufacturing complexity and costs.

Question 19

AM0, AM1 and AM1.5 are all examples of the solar spectrum used to characterise PV
panels. Which of these is used to characterise domestic PV panels and why?

Answer 19

Among the three examples of the solar spectrum, AM1.5 is commonly used to characterize domestic PV panels. This is because AM1.5 represents the solar spectrum at sea level and is considered a standard condition for testing and comparing the performance of PV panels in typical terrestrial environments, including residential applications.

Question 20

Which of the following semiconductors could be stacked to form an effective multi-junction
solar cell and why? Using the band-gap information that is given, in which order would they
be stacked, starting with the top layer facing the sun: InAs (Eg = 0.35 eV), Ge (Eg = 0.7
eV), Si (Eg = 1.1 eV), InP (Eg = 1.34 eV), GaAs (Eg = 1.42 eV), CdTe (Eg = 1.5 eV), GaInP
(Eg = 1.85 eV)

Answer 20

InAs (Eg = 0.35 eV)
Ge (Eg = 0.7 eV)
GaInP (Eg = 1.85 eV)
GaAs (Eg = 1.42 eV)
InP (Eg = 1.34 eV)
Si (Eg = 1.1 eV)
This stacking order allows the top layers to absorb lower-energy photons, while the bottom layers absorb higher-energy photons, maximizing the overall efficiency of the multi-junction solar cell.

Question 21

Explain why having glass windows in a building gives rise to the greenhouse effect?

Answer 21

Glass windows in a building give rise to the greenhouse effect by allowing sunlight to enter and trapping the resulting heat inside, creating a warming effect similar to a greenhouse.

Question 22

Besides the PV panels, name three other components that a domestic PV system may
need? State what each of these components does.

Answer 22

In addition to PV panels, a domestic PV system may require an inverter to convert DC electricity to AC electricity, a charge controller to regulate battery charging (if present), and a mounting and racking system to securely install the panels.

Question 23

Describe the two main intrinsic loss mechanisms in a solar cell. Which one will dominate
in a GaN solar cell (Eg ~ 3.4 eV) and why?

Answer 23

The two main intrinsic loss mechanisms in a solar cell are recombination and thermalization. In a GaN solar cell with a relatively large bandgap energy of 3.4 eV, thermalization is expected to dominate as the high bandgap allows efficient absorption of high-energy photons, but excess energy from lower-energy photons is more likely to be dissipated as heat rather than utilized for generating electrical current.

Question 24

An exact 1:100 scale model of a large HAWT is tested at the same wind speed
and tip speed ratio in a wind tunnel. The power coefficient measured is much
lower than the actual wind turbine. Which non-dimensional parameter is
significantly different for the two cases? What is the flow physics reason for this
lower performance for the scale model?

Answer 24

The non-dimensional parameter that significantly differs between the scaled model and the actual wind turbine is the Reynolds number. The lower Reynolds number in the scaled model results from scaling down the size while maintaining the same wind speed and tip speed ratio. This lower Reynolds number leads to increased viscous effects, higher drag, flow separation, reduced lift, and increased turbulence, resulting in lower performance and a lower power coefficient compared to the actual wind turbine.

Question 25

For a horizontal axis wind turbine, why the blade is twisted with decreasing local
pitch angle from root to tip of the blade? Why the blade chord is reduced towards
the tip?

Answer 25

The blade of a horizontal axis wind turbine is twisted with decreasing pitch angle from root to tip to maintain an optimal angle of attack and prevent stall conditions as the wind speed and angle of attack change along the blade’s span. The reduction in blade chord towards the tip is done to balance the aerodynamic forces and ensure a more even distribution of loads, improving the turbine’s overall performance and structural integrity.

Question 26

Describe how stall regulation works at higher wind speed for a HAWT with fixed
rotational speed, using velocity triangle diagrams at the blade section.

Answer 26

At higher wind speeds, stall regulation in a Horizontal Axis Wind Turbine (HAWT) with a fixed rotational speed is achieved using velocity triangle diagrams at the blade section. As the wind speed increases, the relative wind velocity at the blade section becomes more perpendicular to the chord line, resulting in increased angle of attack. At a certain critical angle of attack, stall occurs, causing a separation of airflow and reduced lift. This stall regulation prevents the blade from operating at excessively high angles of attack, maintaining stable and efficient operation of the turbine.

Question 27

To determine the availability of each wind turbine in a wind farm during operation,
what are the essential data you would recommend to collect? Briefly explain your
answer.

Answer 27

Failure rate and repair rate therefore you would need the mean time between failures and mean time to repair.

Question 28

Making reference to the currents and voltages generated, explain briefly why
multi-junction solar cells are generally more efficient that single junction solar
cells.

Answer 28

Multijunction solar cells are capable of absorbing different wavelengths of incoming sunlight by using different layers, making them more efficient at converting sunlight into electricity that aa single junction cell.

Question 29

The performance of a photovoltaic module usually changes with ambient
temperature for the same solar irradiance. Name two properties of the
photovoltaic junction that change with temperature, briefly explain why and
hence say how the overall performance changes.

Answer 29

The open-circuit voltage (Voc) of a photovoltaic module decreases with increasing temperature due to the temperature dependence of the semiconductor material’s bandgap energy. In contrast, the short-circuit current (Isc) typically increases with higher temperatures due to the enhanced mobility of charge carriers. These temperature-dependent changes result in a trade-off: while the decrease in Voc reduces the available voltage output, the increase in Isc provides a higher current output, influencing the overall performance of the photovoltaic module.