Tom Murray
Abstract:
Renewable energy has been a buzz word for a larger number of years since the Oil peak of the mid 70’s, due to the inflating cost of fossil fuels such as Gas, Coal and Oil. This wiki starts by summarising energy sources, from fossil fuels which we have become so dependent on. Alternative forms of renewable energy such as Wind, Solar, Biomass, Thermal, Tidal and Hydro. Look at the pollution caused by fossil fuels directly and indirectly. Discuss the physics behind wind energy; derive the formulae to obtain the power from the wind. Get the theoretical values for efficiency using Betz law which was developed in early nineteen hundreds; determine the theoretical power of the wind? Show how we can obtain electricity from the wind, using mechanical energy to electrical energy conversion. Using an Aero4gen wind turbine and weather station to gather data to compare the theoretical to the actual results, from here we can determine the Capacity factor of a wind turbine. Breakdown a synchronous generator into its fundamental building blocks, use these blocks to calculate line voltage for a given wind speed. From the logged data compare the efficiency of the Aero4gen wind turbine to the Betz law efficient calculation. Look at the costs relating to wind farming in Europe for onshore and offshore. Determine how we can reduce the consumption of electrical energy using technology that’s available presently.
Introduction to Energy:
There are many different sources of energy available to us. We take the energy we use every day for granted to cook, light, transport and heat. We could be fooled into thinking that there's are many forms of energy, for instance, building are often heated using oil, gas, wind, solar or even electrical energy which is generated from coal, hydro, nuclear or even wind. There are only three fundamental sources of energy. The main focus is on Wind energy, using an Aero4gen Wind turbine and anemometer for wind speed.
Table 1 Energy Conversion
A summary of energy sources:
Fossil Fuel:
What is fossil fuel? This happens due to heat and pressure over a couple million years on dead plants and animals. The main source of energy for these animals and plants is the sun aka solar energy that becomes stored in the fossils of the dead animals or plants. Fossil fuel supplies approximately 90% of the world’s energy through oil, natural gas and coal. The energy we obtain from fossil fuels is chemical energy; it’s made up of electrical Potential Energy (PE) of the electrons and nuclei that make up atoms and molecules, and the Kinetic Energy (KE) of these electrons.
Figure 1 Types of fossil fuels [1]
Solar Energy:
Where does the sun get its energy? The sun is made up of hydrogen nuclei which are fused together in a multi-stage process called nuclear fusion, this creates large nuclei of helium and this energy is in the form of electromagnetic radiation which is release in packets called photons. Solar energy can be used directly to heat or create electricity use PV cells or solar collectors in the form of a parabolic dish.
Figure 2 [2]
Biomass Energy:
Refers to carbon-based materials produced by plants or animals over the past decade. Wood is the biggest contributor, which is burnt for heating and cooking. Other forms of Biomass: Crops of sugar cane which are turned into ethanol. Manure or garbage is converted to methane gas through digestion of bacteria. Woody biomass to produce either combustible gases through gasification process, or methanol, which is a liquid fuel. As oil prices increase Biomass energy is becoming more and more popular. This is a renewable form of energy because it can be grown over a shorter period of time when compared to fossil fuels.
Figure 3 Biomass Processing [3]
Hydro Energy:
The sun evaporates water; this forms clouds and eventually falls as rain or snow. With a large drop in elevation it’s possible to use falling water to produce energy. The energy in the water is an example of gravitational Potential Energy which is converted to Kinetic Energy. In hydroelectric power stations the Kinetic Energy is converted to electricity using turbines.
Figure 4 Hydroelectric Dam [4]
Geothermal Energy:
As we move down towards the earth’s core the temperature increases, geothermal energy can also occur on the earth surface in the form of hot water springs and geysers. The heat in the earth’s core is due to naturally radioactive and decay emitting high-energy particles that heat the surrounding materials.
Figure 5 Geothermal Energy [5]
Tidal Energy:
Where does Tidal energy come from? It’s basically comes from Kinetic and gravitational Potential energy from the earth-moon-sun system. The gravitational Potential Energy is associated with the separation of the earth-moon-sun and Kinetic Energy is associated with the rotation of the earth. In some locations around the world, the tide is high enough to allow water to be trapped at high tide and allow to flow through turbines to produce electricity.
Figure 6 Tidal Turbine [6]
Wind Energy:
Wind energy is an example Kinetic Energy that people have been harnessing for hundreds of year for sailing, milling and pumping. Wind energy is also indirect form of solar energy, when the sun heats the earth the heat is unevenly distributed thus causing hot and cold regions on the earth’s surface, this results in air current movements aka the wind. From table 2 below it can be seen that there are only three fundamental sources of energy available. The focus of this assignment will be on the Wind.
Table 2
Environmental effects of Fossil fuels.
We need to discuss the effects of using fossil fuels to produce energy, how the planet is becoming polluted. Air pollution from the fossil fuel can be categorized into six main types.
Particulate Matter (PM)
Sulphur Dioxide (SO2)
Nitrogen Oxides (NOx)
Hydrocarbons (HC)
Carbon monoxide (CO)
Carbon Dioxide (CO2)
Figure 7 Fossil fuel pollution[7]
These substances are released when fossil fuels are burnt or processed; some of the substances are due to secondary reactions. Both SO2 and NOx are examples that form acids, which fall as acid rain. Because air can move quickly a large area can become contaminated.
Particulate Matter:
Is made up of a wide range of particles – dust, smoke, droplets, microscopic bits of materials are so small that they can become air borne. Industrial activities such as mining and quarrying are major sources. Small diameter particles can be inhaled deep into the lungs causing respiratory problems. In addition, particles can carry damaging materials like sulphuric acid to surfaces, increasing the deterioration.
Sulphur Dioxide:
Power stations are the major contributing sources of SO2. When a fossil fuel is burnt almost all sulphur is released as SO2, this gas is colourless with a strong ordour. Sulphur Dioxide itself is very toxic to plants and can cause respiratory irritations to humans. The acid that SO2 forms when combined with water is sulphuric acid. These acids then fall in the form of acid rain or Particulate Matter.
Nitrogen Oxides:
Nitrogen oxides comprises of another important component of air pollution. Car engines, home boilers and coal fired electric plant heating the air above 500oC, causing nitrogen oxide to form. This nitrogen oxide can form nitric acid, and this acid settles to the earth as acid rain or Particulate matter.
Hydrocarbon:
The incomplete combustion of fossil fuel causes Hydrocarbons to be released in the form of photochemical smog.
Carbon Dioxide:
Transportation is the major cause of CO, with almost all of the contribution coming from road vehicles. This gas is colourless, ordourless, and highly toxic, it binds strongly to the hemoglobin in the blood, preventing the hemoglobin from carrying oxygen.
The Physics behind Wind Energy:
Mention earlier that the human race has been harnessing wind energy for hundreds of years for milling, sailing and pumping water. The first windmill in Ireland dates from 1281 (Kilscanlon, Co. Wexford). Now instead of milling or pumping we are now going to harness the mechanical energy and convert it to electrical energy to try and reduce a large amount of the fossil fuel usage. The sun is the greatest replacement of fossil fuel because this study is carried out in Ireland wind seems to be the more dominant renewable energy. There are many different types of wind turbines shown in figure 8. For the purpose of this assignment we can concentrate on the horizontal-axis (Modern HAWT) wind turbine. The power in a window turbine depends on a numbers of things, the blade radius R and the wind speed or velocity v.
Figure 8 Types of Wind turbines
If we take a hula hoop and move it at a constant speed a volume of air is now captured. The diameter of the hula loop is equal to that of the of the wind turbine blades, with this we can determine the volume is equal to the area of the blades by the length of air captured.
Figure 9 Volume of Air
Area of the loop or turbine blades
Density if air ρ=1.3kg/m3
To find out how much Kinetic Energy in the capture air:
The wind power for a given area A
Because of this v3 a doubling of v increases power by a factor of eight shown in table 3 for a wind speed of 4.55m/s and 9.09 m/s respectively, the turbine blades should have a large a length a possible; doubling the radius R quadruples the power for a 12.5m and 25m blade shown also in table 3.
Table 3 Wind Speed to Energy
Figure 10 Power Vs Wind Speed
Betz Law of efficiency:
A German physicist called Albert Betz in 1919 determined the maximum efficiency of a “hydraulic wind engine” or wind turbine. According to Betz law no wind turbine could capture more than 59% of kinetic energy from the wind, this law simple states that all energy coming from wind movement into the turbine were converted into useful energy then the turbine would stop. This is the Capacity Factor (CF) of the wind turbine we will look at this in detail n the assignment with actual data obtained from an Aero4gen wind generator.
Figure 11 Betz law[8]
Assumptions
The rotor does not possess a hub; this is an ideal rotor, with an infinite number of blades which have no drag. Any resulting drag would only lower this idealized value.
The flow into and out of the rotor is axial. This is a control volume analysis, and to construct a solution the control volume must contain all flow going in and out, failure to account for that flow would violate the conservation equations.
This is incompressible flow. The density remains constant, and there is no heat transfer from the rotor to the flow or vice versa.
The rotor is also mass less. No account is taken of angular momentum imparted to either the rotor or the air flow behind the rotor, i.e., no account is taken of any wake effect.
Applying conservation of mass to this control volume, the mass flow rate (the mass of fluid flowing per unit time) is given by:
where v1 is the velocity in the front of the rotor and v2 is the velocity downstream of the rotor, and v is the velocity at the fluid power device. Fluid density ρ and the area of the turbine are given by S. The force exerted on the wind by the rotor may be written as
Power and Work:
The work done by the force may be written incrementally as
and the power (rate of work done) of the wind is
Now substituting the force F computed above into the power equation will yield the power extracted from the wind:
However, power can be computed another way, by using the kinetic energy. Applying the conservation of energy equation to the control volume yields
Both of these expressions for power are completely valid, one was derived by examining the incremental work done and the other by the conservation of energy. Equating these two expressions yields
Looking back at the continuity equation, a substitution for the mass flow rate yields the following
Examining the two equated expressions yields an interesting result, mainly
Therefore, the wind velocity at the rotor may be taken as the average of the upstream and downstream velocities. This is often the most argued against portion of Betz' law, but as you can see from the above derivation, it is indeed correct.
Returning to the previous expression for power based on kinetic energy:
By differentiating (through careful application of the chain rule) E with respect to for a given fluid speed v2/ v1 and a given area S one
finds the maximum or minimum value for E. The result is that E reaches maximum value when.
Substituting this value results in:
The work rate obtainable from a cylinder of fluid with cross sectional area S and velocity v1 is:
The horizontal axis reflects the ratio v2/v1, the vertical axis is the "Capacity factor" Cf.
The “Capacity Factor" Cf (= P/Pwind) has a maximum value of: Cf.max = 16/27 = 0.593.
How do we convert Wind power to Electrical power?
To understand how a wind turbine converts Mechanical energy to Electrical energy we need to understand some basic principles of Electromagnetism induction. If we take a permanent magnet generator shown in figure 12 with a fix magnetic flux traveling from north to south. A conductor is then slowly moved through these lines of magnetic flux a current will be induced in that conductor, if the speed is increased so will the induced current increase proportionally. Reversing the direction of movement will result in the current also changing direction. Moving the magnet instead of the conductor will also result in the same result.
The resulting frequency can be calculated if the following parameters are known, the number of pole pairs (always in two's because we can't just have North pole or just a South pole) and the rotational speed of the generator.
Placing another two sets of conductors exactly 120 degrees apart from each other will result in a 3 Phase power; this supply is constant power when compared to a single phase pulsating supply. Most wind turbines use an inside out configuration to eliminate the use of electric brushes to extract the power from the windings, this means the conductor remain still while the permanent magnet rotates this can be seen in the Internal construction of the wind turbine, in figure 12 part 7 of the generator.
Figure 12 Simple AC Generator and Internal Construction of a wind turbine [11] [12]
The Aero4gen Wind Generator:
From the Aero4gen datasheet the only guide to understanding what performance the generator can produce is given in fig 13(b). That is to say it does not provide power curves for its products.
Figure 13(a) Aero4gen and Weather station
Figure 13(b) Amps output vs. Wind Speed (knots) extract from Aero4gen Datasheet
From the given data in Figure 13(b) it is possible to work out what load has been placed upon the generator in order to obtain the Amps vs. Wind Speed data line. The trend is to keep the voltage at 12V - as the load decreases in value and the wind speed increases more current and thus more power is generated. So at 60 knots the load will be approx 0.6Ω to maintain a 20A output at 12V.
Realistically the power generated is entirely dependent on the load on the generator at the time the loading on the Aero4gen wind turbine is normally controlled with a regulator.
The regulator only regulates the battery charge voltage and does not dynamically control the load on the generator. Essentially if the batteries are relatively low then more of a load will be placed upon the turbine which could essentially stall the turbine if the wind speed is low.
Synchronous Generator Breakdown:
The Aero4gen 12V system contains a synchronous generator, which is essentially an alternator that produces an AC current at a synchronous speed. The synchronous generator is constructed in a way deemed “inside out”, Fig 14 illustrates the basic concept: -`
Figure 14 (a) illustrates the rotor position with respect to the stators;
Figure 14(b) shows the basic elements of a four pole stator including the coil winding positions.
There are several reasons for an inside out design some of these are listed below:
A large proportion of the heat is generated in the armature windings, and since these are on the outer part of the generator these can be cooled more effectively than an inner winding design.
Most Synchronous generators are built in large sizes, hence a large generator would require a thicker Cross Sectional Area (CSA) of copper in its windings to generate more power, and minimise copper losses (I2R).
The output of a synchronous generator is AC and since the armature conductors in the stator can be connected to the transmission line the need for slip rings is removed.
The illustration below figure 15 is an extract from the Aero4gen manual; it depicts the number of slots the generator has and also shows that it is a six pole design, and has two coils per phase.
Fig 15 illustration of the stator windings connected in star format [13]
From the diagram above some information can be obtained easily by direct observation of the drawing: -
Since a pole spans 180o electrical, the slot span (γ) is 180/6 = 30o electrical. In other words the electrical angle from the centre of one slot to the centre of an adjacent slot is 30o. The coil pitch is determined by the number of slots or teeth that the coil spans, in this case it is 5 – thus the coil pitch is equal to 30o × 5 = 150o electrical. The illustration below figure 16 is called a “developed diagram” it shows the placement of one of the phases, it is clearer to see the number of coils per pole per phase (=2), the number of slots in total (=36), the number of slots required per pole (A1-A6) and the number of coils per phase (=12).
Figure 16 developed diagram of the generator
Figure 18 illustrates the internal schematic diagram Aero4gen wind turbine. The rms line voltage EL (or voltage between two lines before the 3 Phase bridge rectifier) is:
Thus the induced emf in each fractional pitch coil is 96.5% of the induced emf in a full pitch coil.
The frequency of the three phase AC voltage (assuming the generator is turning at 350 rpm with no load):
Since is unknown, however, for each revolution of the generator it generates 0.06V; so at 350rpm the generator should be producing 21V. Plotting the calculated flux values under load when the generator is operating at 350rpm from 20Ω-0.5Ω appears to be a straight line graph as shown in figure 17.
Figure 17 Line Voltage vs. Calculated Flux Density
Based on the equation {y=0.004+1E-16 } the flux density value calculated at 21V is 84mWb.
Figure 18 Internal Schematic of Aero4Gen Wind turbine.
Calculation of the Aero4gen Generator Efficiency:
To get the overall efficiency of the wind generator the power from the wind (mechanical power) through the generator blades must be calculated. This is quite a comprehensive task involving obtaining the dynamics of the blades and the kinetic energy from the wind over a range of wind speeds.
Wind is made up of moving air molecules. Any moving object with mass carries kinetic energy in an amount which is given by the equation:
Kinetic Energy = 1/2 x Mass x Velocity2
Where the mass is measured in kg, the velocity in m/s, and the energy is given in joules.
So the mass of air hitting the wind generator (which sweeps a known area) each second is given by the following equation:
Mass/sec (kg/s) = Velocity (m/s) x Area (m2) x Density (kg/m3)
And therefore, the power (i.e. energy per second) in the wind hitting a wind turbine with a certain swept area is given by simply inserting the mass per second calculation into the standard kinetic energy equation given above resulting in the following equation:
The generator blades swept diameter = 800mm
The Air Density = 1.3 kg m3 (+5oC)
The Wind Velocity = 9 m/s
Figure 19 Wind Speed data vs. power (averaged over 1 min bins)
By using the 5th order polynomial equation given from the trend line from the 4.5Ω load (Figure 19) we can obtain the power value for 9m/s: - (x=9 m/s)
European Wind Farmming:
Global Resources
Figure 21 European wind resources [10]
If we place a number of wind turbines in an area of land or sea to harvest the energy from the wind we can only place another wind turbine within a certain distance so no wind-shadows are cast. Experts say that wind generator can't be placed within 5 time their blade length.
Important calculations for a wind speed of 6 m/s
In brief the main elements determining the cost of wind energy is the investment are the turbine cost, foundations, electrical installation, connection to the electrical grid, consultancy fees, land cost, financing, security and operation maintenance. These costs vary depending on different countries in Europe. The estimated value of a turnkey wind turbine is said to cost 1000 Euro/kW for onshore and 1200 to 2000 Euro/kW offshore wind farms. Onshore wind farms dominate the turbine cost because the ease of installation and operation maintenance when compared to offshore turbines. The major cost of onshore is the turbine. The price of wind turbines are said to reduce further over time, this is due to 3 to 5% for each new development in turbine design. Typical progress ratios for the wind turbines are 80-95% meaning that the wind turbine costs decrease by 5 -20% when total installed capacity doubles. There is enough wind energy resources to power all of Europe, whose current consumption is 2,900 TWh.
Table 4 Cost estimates for onshore and offshore installation
Table 5 Payback years [13]
Saving Electrical Energy:
Variable Speed Drives
Throughout the world more than 70% of electrical energy is used for electric motors, over 60% of these motors are used for fluid distribution using pumps or air circulation using fans. At present less than 10% of these motor are equipped with Variable Speed Drives (VSD). Traditional installation the motors are connected directly to the mains via a Direct on Line (DOL) starter. Motor starting generate high current peaks up to 6 to 9 times the nominal current. The motor can only run at full speed. On air circulation flow variations can only be performed using mechanical valves with no reduction in the power consumption , a 20% reduction in air flow only produces a 3% power saving. By using a VSD you reduce starting current, flow variation is obtained by vary the speed of the motor, without the need for a mechanical valve devices no longer required considerable saving are achieved both for the installation and maintenance. Now a reduction in 20% air flow results in a 50% decrease in power consumption compared to the 3% for traditional DOL starter. In a large electrical installation the energy saving in a year is 30%.
Light to save the Environment
Type 1 Naturally ventilated cellular
Type 2 Naturally ventilated open-plan
Type 3 Air-conditioned; standard
Type 4 Air conditioned, prestige
Table 6 Lighting energy per square meter[15]
Annual consumption of energy for lighting offices is approx:
3.5 – 4.5 GWh spewing out 1.5 million tonnes of CO2!
Lighting controls have the potential to reduce lighting energy consumption significantly and to moderate peak demand in commercial buildings. Lighting controls reduce lighting energy consumption by exploiting one or more strategies. The most common and, arguably, most successful lighting control strategy is occupant sensing, which employs an occupant sensor to switch lights on and off according to detected occupancy. Despite their relatively widespread use, there are surprisingly few well-documented studies in the US that demonstrate that occupant sensors actually reduce lighting energy use sustainably. Daylighting is another lighting control strategy that has been investigated in a few monitored sites. With the advent of inexpensive manual dimmers and handheld remote controls, occupant controlled manual dimming is becoming an affordable option, and has been shown to have some energy savings potential and high occupant satisfaction rating in one installation. The most humble of lighting control strategies, bi-level switching, has not been seriously evaluated even in those states where it is required by energy code.
Given the many ways that lighting controls can reduce lighting energy waste and potentially improve occupant satisfaction, the shortage of well-monitored installations showing the sustained benefits of different lighting control strategies is surprising, and is probably a contributor to the relatively slow adoption of lighting controls in nonresidential buildings.
Conclusion:
As engineers it’s our responsibility to design systems that are friendly to the environment during manufacture and when used. The key solution is the sun; we can’t depend on just one renewable energy system we need a combination of number of different systems together depending on geographical location. Wind, Biomass and Solar are the key pieces of the jigsaw puzzle for Ireland. So what can we do in the future? This is extremely difficult thing to answer, because consumption of individual energy sources and the consumption of energy depend on a large number of factors:
Population
Economic activity
The price of energy
Availability of energy
Technological advances
Development of alternative sources of energy.
Government policies
World political stability
Public attitude.
References:
Notes EE535 Renewable Energy Systems
[1] http://www.qmmc-wisdom.com/fossil-fuel-and-nuclear-energy-vs-renewable-energy.html
[2] http://mapawatt.com/tag/solar-thermal-diagram/
[3] http://www.alternate-energy-sources.com/biomass-energy.html
[4] http://ga.water.usgs.gov/edu/hyhowworks.html
[5] http://www.biofuelswatch.com/how-does-geothermal-energy-work/
[6] http://www.ecoseed.org/water-power-blog/article/10-water-power/1686-ireland%E2%80%99s-openhydro-to-build-tidal-energy-pilot-project-in-washington-state
[7] http://m.eb.com/assembly/87019
[8] http://en.wikipedia.org/wiki/File:Betz_tube.jpg
[9] http://serc.carleton.edu/earthlabs/climate/5.html
[10] http://www.no-fuel.org/index.php?id=243
[11] http://www.electronics-tutorials.ws/accircuits/acp14.gif
[12] http://www.enerpower.ie/wind_turbines.html
[13] Aero4gen user manual attached
[14] http://www.enerpower.ie/wind_turbines.html
[15] The Association for the Conservation of Energy (ACE)