Exam Flashcards
(17 cards)
1a. Calculate wind turbine efficiency
Use graph for P(actual) @ given speed
Calculate P(theoretical) using the equation
Efficiency = actual/theo
1b. Calculate torque with given speed, given tsr and generator efficiency.
V = TSR x U(wind speed)
omega = V/r
P(mech) = P(actual)/generator efficiency
Torque = P(mech)/omega
1c. calculate the required pitch with given angle of attack and TSR
Theta = tan-1(1/TSR)
Pitch angle = theta - angle of attack
1d. Breifly discuss the key differences in the operation and performance between variable speed HAWT using permanent magnet generator compared to fixed speed HAWT with induction generators.
Variable Speed
can adjust rotor speed according to wind speed, optimising the tip speed ratio for maximum aerodynamic efficiency across a range of conditions. Leading to higher energy yield due to better adaptation to wind speed fluctuations. Provides better control over voltage, frequency and harmonics, improving power quality.
Fixed Speed
Operates within a small range of wind speeds, making it optimal at a specific range, leading to sub optimal performance outside that range. Lower energy capture, especially under varying wind conditions. More susceptible to grid disturbance and contributes less to grid support during faults.
1e. Explain the principles of pitch control in HAWT, stating 3 conditions where this would be used.
Pitch control in HAWT involves rotating the blades around their longitudinal axis to change the angle of attack of the wind on the blades. This riectly effects the lift and drag, thus the rotational speed and power output of the tubines.
- Optimise power output at varying wind speeds
- Limit mechanical loads
- Protect itself from damage at high wind speeds
2a. Provide a brief overview of 3 important considerations within an initial assessment of a wind turbine site.
- Determine if the site has sufficient and consistent wind to make the project viable.
- Ensure compliance with environmental laws and minimise ecological impact. Asses impact on wildlife and evaluate noice/visual imapct.
- Evaluate feasability of grid connection/proximity to transmission lines, as well as road access for part delivery
2b. Briefly describe the operation of two types of anemometer, stating how the wind speed is measured and highlight any advantages/disadvantages.
Cupped
Wind rotates a set of hemisoherical cups mounted on arms, the rotational speed is proportional to wind speed. Often paired with a wind vane for direction tracking.
ADV: Simple, robust and commonly used.
DADV: Subject to wear and tear, less effective at low speeds.
Ultra Sonic: Uses ultrasonic pulses between transducers to measure the speed of sound in air. Wind effects the time taken for the sound to travel between the probes. From this, both wind speed and direction can be determined precisely.
ADV: High accuracy and fast response, no moving parts so not subject to wear and tear
DADV: Much more expensive than other methods
2c. Calculate u2 from anemometer data
u2 = u1 x (h2/h1)^a
2d. Discuss how a large set of wind speed data for a site can be converted into an estimated yield from a wind turbine with a known power curve. State one reason why calculations using average (mean) wind speed are prone to error.
To estimate energy yield from a wind turbine, wind data is matched agaisnt the turbines power curve, which shows output at different wind speeds. each data points corresponding power is calculated and summed over time to determine total energy production, with losses then considered like downtime and electrical losses.
Using only the average wind speed is innacurate because power depends on the cube of wind speed, meaning output is highly sensitive to variations in wind, not just the mean.
2e. Estimate the annual generation of a class2 1mw turbine……..
Assume: 50m diameter, cp = 0.35
P = cp x 0.5 x p x pi(r)^2 x V^3
Per day: multiply by 24h
Per year: multiply per day by 365
Provide an overview of the operation of a pumped storage hydroelectric system. Include sketch
PSH systems operate by storing energy as potential energy in water held at height in an upper reservoir. During periods of low electricity demand water is pumped from a lower reservoir to an upper reservoir using excess grid energy. When demand increases, this water is then released back down through a penstock, converting its potential energy into kinetic and pressure energy, which drives turbines connected to generators to produce electricity.
3b. For a run of river hydro electric system with a head of 260m experiencing a flow rate of 36Ls-1. Calculate the power output considering an overall system efficiency of 80%.
ms-1 = Ls-1 x 10-3
P = efficiency x p x g x Q x h
3c. For the case in (b), if the pipe internal diameter is 250mm. Calculate the speed of the water and the static pressure jsut before it enters the turbine unit.
V = Q/A
P = phg
3d. Provide a breif explination of the sensing and control aspects of a run of river hydro system that regualtes flow and pwoer to match the availability of the resource. Support with a block diagram or sketch.
In a run of river system, power control is closely linked to the available water resource, as there is minimal storage capacity. To prevent over extraction, pressure sensors are placed near the bottom of the water body to monitor water level. The sensor data feeds to a PID controller which adjusts the inlet valve to regulate the flow into the turbine. the controller aims to keep the rate of change of water level near zero, maintain a stable resource and allowing the system to operate at a relatively constant power output. This feedback system ensures that power generation aligns with real-time resource availability, protecting the system and maximising efficiency.
4a. Describe one type of energy converter system. Include a sketch of the system including key components and the principle of how it extracts power.
Oscillating water column
A type of wave energy converter that captures the kinetic and potential energy of ocean waves and converts it into electrical by using the movement of air driven by wave action to spin a turbine.
As waves rise and fall inside a partially submerged chamber, they push and pull air through a turbine located at the top.
This bi-directional air flow drives a Wells turbine, which rotates in the same direction regardless of air flow direction, powering a generator.
4b. Dicuss two of the significant challenges that are apparent in the wave energy sector.
Harsh marine environment
Extreme and unpredictable nature of marine environment. Wave energy converters must operate in coditions that include strong waves and winds, and corrosive sea water. These factors impose tough demands on material and structural durability, making it difficult and expensive to design systems that can survive long-term exposure.
Complexity and cost of energy conversion and transmission
Wave energy systems involve multiple stages of energy conversion. It has to capture mechanical energy from waves, convert it into usable forms, then into electrical energy and finally trasnmitting the elctricla energy to to shore and integrating it into the grid. Each of these steps required specialised components, increasing both compelcity and cost.
4c. A proposed tidal energy site has typical ………….
i. Calculate suitable electrical power rating for the generator assuiming system conversion eff. of 35%.
ii. With a cut in speed of 0.7 calculate the approcimate annual energy produced by the system.
i. P = eff(gen) x eff(conversion) x 0.5pAV^3
ii. Plot Power (max from part i.) vs Time graph (max 6 hours)
Graph of sinusoidal wave peaking int he middle and symmetrical.
E = 0.5 x (a+b) (h)
h = Power from part i
a = (6 - cut in) - (cut in)
b = (cut in)
There are 4 cycles in 1 day so times the value by 4.
