| August 2018 |
| Coventry University |
| Faculty of Engineering, Environment and Computing |
| M08AAE |
| Alternative Energy and Smart Grid |
| Instructions to candidates |
| Time allowed: 2 Hours 0 minutes |
| Answer: Any 3 Questions Total number of questions : 4 All questions carry equal marks |
For this examination you will be supplied with: Answer Books |
| Important: You must hand this question paper in at the end of the examination. |
Q1.
a) Critically discuss why some wind turbines have large numbers of blades while others have only two or three. Give examples of the applications of both types of wind turbine. (6 marks)
Answer : The main objective of wind turbine design is to maximize the aerodynamic efficiency, or power extracted from the wind. Wind turbines with large number of blades have highly solid swept area and are known as high-solidity wind turbines. Wind turbine with small number of narrow blades is referred to as low-solidity wind turbine. The major factors involved in deciding the number of blades are power coefficient, tip speed ratio and yawing rate to reduce the gyroscopic fatigue. Modern wind turbine are neither built with many rotor blades nor with very wide blades even though turbine with high solidity have the advantage of enabling the rotor to start rotating easily because more rotor area interacts with the wind initially.
In theory the more blades a wind turbine rotor has the more efficient it is. However large numbers of blades can interfere with each other so high solidity wind turbines tend to be less efficient overall than low solidity turbines. Of low solidity machines three bladed rotors tend to be the most energy efficient, two bladed rotors are slightly less efficient still. Almost 70% modern wind turbine use 3 blades. ( 4 marks)
2 or 3 bladed wind turbines are used for electricity generation because they operate at high tip speed ratios and therefore do not require as high a gear ratio to match the speed of the rotor to that of the generator. Multi-bladed turbines are generally used for water pumping because of their low tip speed ratios and resulting high torque characteristics. ( 2 marks)
b) If the wind turbine has blades that are 35 m in length and average wind speed is 10 mph, how much power is produced by the wind turbine? Consider the mechanical to electrical efficiency; the power coefficient and the air density are 80%, 40% & 1.2 kg/m3 respectively. (4 marks)
Answer :
Velocity, V = 10 mph = 10*1.609*1000/(60*60) = 4.47 m/s
Turbine blades area, A = pi()*r^2 = 3.14*35^2 = 3848.45 m2
Air density, ρ = 1.2 kg/m3
Generated power from Wind turbine, P = 0.8*0.4*1.2*3848.45*4.47^3 = 131.98 kW
c) Critically discuss the advantages of using bio-mass energy and explain why the bio-mass system is abundant in the developing countries? (5 marks)
( 3 marks for advantages & 2 marks for 2nd part)
Biomass energy is generated from organic material, plant or animal waste, which is burned to provide energy, e.g. heat & electricity. Since they come from living sources, these products potentially never run out which makes biomass a renewable energy source. There are several advantage for using bio-mass energy:
- Better for the environment than fossil fuels
The burning of biomass does release carbon dioxide but captures carbon dioxide for its own growth. Carbon dioxide released by fossil fuel is released into the atmosphere and are harmful to the environment. Many energy sources struggle to control their carbon dioxide emissions as these can cause harm to the ozone layer and increase the effects of greenhouse gases.
- Less Dependency on Fossil Fuels
Using biomass as an alternate source of fuel reduces our dependency on fossil fuels which is better for the planet and more cost effective.
- Very Easily Available
Biomass is cheap and readily available source of energy. If the trees are replaced, biomass can be a long-term, sustainable energy source.
- Reduce Landfills
By burning biomass for energy, we can take waste that is harmful to the environment and turn it into something useful. Around six million tonnes of wood is wasted by being sent to landfill in the UK each year. This wood could be used in biomass boilers to heat homes and factories etc.
- Renewable Heat Incentive
By installing a biomass boiler, you can get paid under the Renewable Heat Incentive, which is a Government-backed financial incentive to promote the use of renewable heat for both the domestic and commercial markets.
Most developing countries have abundant renewable energy resources, including solar energy, wind power, geothermal energy, and biomass, as well as the ability to manufacture the relatively labor-intensive systems that harness these. By developing such energy sources developing countries can reduce their dependence on oil and natural gas, creating energy portfolios that are less vulnerable to price rises. In many circumstances, these investments can be less expensive than fossil fuel energy systems
Q2.
a) A commercial multi-crystalline silicon solar cell with a so called H-pattern as shown in below figure has the following specifications:
Open circuit Voltage, Voc = 0.6 V
Short circuit current, Isc = 5.0 A
Voltage at maximum power point, Vmpp = 0.5 V
Current at maximum power point, Impp = 4.6 A
Solar cell size: 150 cm2
Consider 9% metallization coverage and the intensity of solar radiation is 1 kW/m2. Calculate the total area efficiency, active area efficiency & the fill factor (FF) of the cell. (5 marks)
Figure for Q2 (a): H pattern multi crystalline silicon solar cell
Total area efficiency, ( 2 marks)
Active area efficiency,
(2 marks)
Fill factor,
FF = Vmpp*Impp/(Voc*Isc)= 0.5*4.6/(0.6*5) = 0.76 (2 marks)
b) Critically discuss how one can improve open circuit voltage of a solar cell. (3 Marks)
The open-circuit voltage, VOC, is the maximum voltage available from a solar cell, and this occurs at zero current. The open-circuit voltage corresponds to the amount of forward bias on the solar cell due to the bias of the solar cell junction with the light-generated current. The open circuit voltage can be calculated using the following formula;
It can be seen from the above equation that Voc depends on the saturation current of the solar cell and the light-generated current. While Isc typically has a small variation, the key effect is the saturation current, since this may vary by orders of magnitude. The saturation current, I0 depends on recombination in the solar cell. Open-circuit voltage is then a measure of the amount of recombination in the device. Silicon solar cells on high quality single crystalline material have open-circuit voltages of up to 730 mV under one sun and AM1.5 conditions, while commercial devices on multicrystalline silicon typically have open-circuit voltages around 600 mV. Also the short-circuit current (ISC) decreases with increasing bandgap, the open-circuit voltage increases as the band gap increases.
c) Critically compare 1st, 2nd and 3rd Generation solar cells considering performance, cost and manufacturability? (6 marks)
Solar cell technologies are traditionally divided into three generations. First generation solar cells are mainly based on silicon wafers and typically demonstrate a performance about 15-20 %. These types of solar cells dominate the market and are mainly those seen on rooftops. The benefits of this solar cell technology lie in their good performance, as well as their high stability. However, they are rigid and require a lot of energy in production. (2 mark)
The second generation solar cells are based on amorphous silicon, CIGS and CdTe, where the typical performance is 10 – 15%. Since the second generation solar cells avoid use of silicon wafers and have a lower material consumption it has been possible to reduce production costs of these types of solar cells compared to the first generation. The second generation solar cells can also be produced so they are flexible to some degree. However, as the production of second generation solar cells still include vacuum processes and high temperature treatments, there is still a large energy consumption associated with the production of these solar cells. Further, the second generation solar cells are based on scarce elements and this is a limiting factor in the price. ( 2 mark)
Third generation solar cells uses organic materials such as small molecules or polymers. Thus, polymer solar cells are a sub category of organic solar cells. The third generation also covers expensive high performance experimental multi-junction solar cells which hold the world record in solar cell performance. This type has only to some extent a commercial application because of the very high production price. A new class of thin film solar cells currently under investigation are perovskite solar cells. Polymer solar cells or plastic solar cells, on the other hand, offer several advantages such as a simple, quick and inexpensive large-scale production and use of materials that are readily available and potentially inexpensive. Polymer solar cells can be fabricated with well-known industrial roll-to-roll (R2R) technologies that can be compared to the printing of newspapers. Although the performance and stability of third generation solar cells is still limited compared to first and second generation solar cells, they have great potential and are already commercialized. (2 marks)
Q3
- Synergise a “Smart Grid”, i.e. fundamental operational features and abilities (4 marks)
A smart grid is a modernized electrical grid, which uses information/communications technology to determine the behaviours of suppliers and consumers, in an automated fashion to improve the efficiency, reliability, economics, and sustainability of the production and distribution of electricity. The importance of the smart grid is to ensure that electricity distributed reliably, securely, efficiently as the global energy demand increases. (1.5 marks)
Some key constituents of the Smart grid are: (2 marks)
1. Safe and secure integration of renewable energy sources to the grid
2. Communications devices providing more detailed real time information about the load on the grid such as Smart meters.
3. Energy storage to mitigate the effects of intermittent sources of renewable energy.
4. More efficient methods of transmission such as HVDC.
b) Why are energy storage techniques of importance when considering renewable energy supplies? Describe three different methods for storing energy and briefly state how they might be used.
(2 marks + 4 marks)
The future Smart grid will consist of increased quantities of grid connected renewables. This will consist of wind power and solar power. Both forms of energy are intermittent and variable in nature. Therefore storage technologies will be needed to regulate the supply from these sources. Energy storage can also be used to mitigate voltage fluctuations and improve power quality issues such as harmonics. (2 marks)
Give 3 examples of the different storage technologies and briefly explain its principle of operation;
- Batteries; (2 marks)
Simply based on two electrodes and an electrolyte.
Large utility applications use Lead acid and Sodium Sulphur (NaS).
Lead acid have been used for many years in utility applications providing excitation for synchronous generators and as back up auxiliary power supplies. These are cheap but require significant maintenance. Their lifetime is particularly short if discharge too deeply.
NaS batteries operate at very high temperatures (300-400 deg C) and have large capacity per unit volume and weight.
Li-ion is progressively being used also but primarily for Electric vehicles.
NiCd is a robust technology with high energy density. It was traditionally used in power tools and mobile phones etc however it has been displaced by other technologies recently. It has been chosen for a flagship storage project in the USA.
- Fuel Cells; (1 marks)
Using hydrogen and oxygen are another possible storage technology. One of the challenges is the energy required to electrolyse water to generate the hydrogen.
- Flywheels; (1 marks)
Store energy in the form of kinetic energy in rotating mass. Energy can be released when energy is required. Mainly used for UPS and power quality requirements. Normally used at building level rarely at grid level.
c) Briefly discuss the different types of solar inverter architectures/types for grid connection and explain the benefit of each approach? (5 marks)
Solar architectures:
0.5 marks for any of the following (maximum of 2 marks):
- Central inverter
- String inverter
- Multi string inverter
- Power Optimiser or Micro inverter
0.5 mark for any point/diagram per device (maximum of 2 marks):
Central inverter:
- Is used for solar power plants,
- Up to 1.5MW with string voltages of 1kV
- 3 phase interconnection
- Can avoid use of boost or step up transformer
- Connect to MV transformer
String inverter:
- From 1-10kW, one string of modules per inverter
- Smaller and lighter than central inverters
- Often wall mounted progressively being used in utility scale PV systems
Multi-string:
Features the optimal MPP tracking for a single string of PV modules
•DC-DC converter is implemented for each string for MPP tracking
•Beneficial when strings of different angles/orientations and different ratings are combined.
Power Optimiser:
- DC-DC converter are connected to PV modules
- Potential to improve energy harvest with MPPT for each module (25%)
- Visibility at module level with appropriate monitoring systems
- Requires connection to an inverter for grid connection
- Safety benefits (arc protection, fires)
Q4.
- Discuss the main factors used to evaluate the location of a hydro power system.
(5 marks)
The dam also called as water reservoir is the most important part of the hydroelectric power plants. All the water that is used for generation of electricity in the hydroelectric power plants is stored in the dam. Since huge quantities of water are stored in the dam, it is very important that the bed and walls of the dam should be able to sustain all the hydraulic pressures of water. Water has mass and large quantities of water have huge weight which is exerted on the bed and the walls of the dam. If the walls of the dam are not strong enough to sustain the forces of water, the walls will break and water will spread to the surrounding areas producing devastating floods that have potential to cause large scale destruction of human, animal and plant life.
Apart from the construction of the dam, selecting proper site for the dam is very crucial. Selecting the proper site will help carrying out construction of the strong dam and it will also help reduce risks due to natural disasters like earth quake. Here are some of the important factors to be considered while selecting the site for the dam for hydroelectric power plants:
1) Good topographical location along the path of river: The best location along the path of the river is river canyon or at the location where there is narrowing of the river. If the aim is to store maximum amount of water, then the volume of basin above dam should be calculated so that sufficient quantity of water can be stored in it. The perfect site is one where there is wide and flat valley.
2)Right geological structure: The rock structure on which the dam will be constructed should be strong enough to sustain the weight of dam and water stored in the dam. The rock structure should be able to sustain all the visible and invisible forces. The rock structure should be stable and there should be least occurrence of the earthquakes in the region. The rock structure should not allow the seepage of water and it should be waterproof.
3) Sufficient water flow
The flow of water where dam is constructed should be sufficient enough to fill the dam. There is lots of loss of water from dam due to evaporation, the flow of river water should be able accommodate this loss of water without affecting the production of electricity from the hydroelectric power plant. People living around the areas where storage basin is going to be constructed and the areas that will be submerged should be convinced to move from there and they should be given proper compensation and suitable resettlement areas. If this factor is ignored the chances of the success of the hydroelectric power plant will reduced.
- A Hydroelectric power station has a reservoir of area 2.4 km2 and the capacity 5 x 106 m3. The effective head of water is 100 meters. The penstock, turbine and generation efficiencies are respectively 95%, 90% and 85%. Calculate the total electrical energy that can be generated from the power station ( 4 marks)
Weight of water available, W = 5 x 106 * 1000*9.81
Overall efficiency, η = 0.95*0.9*0.85 = 0.726
Electrical energy that can be generated = W *H*η= 5 x 106 * 1000*9.81*100*0.76/(1000*3600) = 989175 kWh
- Critically evaluate the similarities and differences between pure battery vehicles and hydrogen fuel cell vehicle. (6 marks)
Battery Electric vehicles (BEVs) use electricity to power a motor located near the vehicle’s wheels. Hydrogen fuel cell vehicles (FCVs) are similar to battery electric vehicles (BEVs) in that they use a high-voltage electric motor to propel the vehicle. Unlike BEVs, however, FCVs are equipped with a hydrogen fuel tank and a fuel cell system that generates electric power to drive the electric motor. Fuel cell vehicles produce their primary electricity using a fuel cell powered by hydrogen, rather than a battery. During the vehicle design process, the vehicle manufacturer controls the power of the vehicle by changing the fuel cell size and controls the amount of energy stored on board by changing the fuel tank size. This is different than a battery electric vehicle where the amount of power and energy available are both closely tied to the battery size.
The most common type of fuel cell for vehicle applications is the polymer electrolyte membrane (PEM) fuel cell. In a PEM fuel cell, an electrolyte membrane is sandwiched between a positive electrode (cathode) and a negative electrode (anode).. FCVs provide the environmental benefits of a BEV but they have a longer driving range and significantly shorter refueling time. In an FCV, an automotive fuel cell propulsion system runs the vehicle by converting hydrogen and oxygen into electrical current through an electro-chemical reaction in the fuel cell stack. It emits just water vapor and heat, without other tailpipe pollutants. Therefore, FCVs are considered to be zero-emission vehicles. FCVs can also be hybridized with a high-voltage battery, to improve vehicle performance and better optimize the cost and robustness of the fuel cell propulsion system.
End