Regardless of the direction of the wind, the yaw mechanism can help the turbine face the wind by changing the direction of the nacelle and the blades.
During the rotation of the nacelle, there is a possibility of twisting the cables inside of the tower. The cables will become more and more twisted if the turbine keeps turning in the same direction, which can happen if the wind keeps changing in the same direction. The wind turbine is therefore equipped with a cable twist counter which notifies the controller that it is time to straighten the cables -.
The gearbox, generator and the control electronics are all located inside of the nacelle. The nacelle is connected to the tower through the yaw mechanism. Inside the nacelle, two shafts connect the rotor of the turbine to the rotor of the electrical generator through the gearbox. The gearbox is the mechanical energy converter that connects the low-speed shaft of the turbine to the high-speed shaft of the electrical machine.
The control electronics inside the nacelle record the wind speed, direction data, rotor speed and generator load and then determine the control parameters of the wind operation system. If the wind changes direction, the controller will send a command to the yaw system to turn the whole nacelle and turbine to face the wind.
The electrical generator is the main part of the nacelle. It is the heaviest part and produces electrical energy which is transferred through the cables. There are different types of generators that are used for wind turbines and depending on the type of generator; wind turbines can operate with either fixed or variable speed. Fixed speed FS turbines use synchronous machines and operate at fixed speed.
These machines are not best solution for the wind turbines because the wind always changes its speed. Variable speed turbines use DC machines, brushless DC machines and induction machines. DC machines are not commonly used due to the maintenance problems with the brushes.
Induction and DC brushless machines are more suitable for wind applications -. Two-blade turbines have high stresses in cut-in speed; therefore the speed and power of the wind are insufficient for starting the rotation of the turbine and higher minimum wind speed values are required at the beginning. The radius of the blades is directly proportional to the amount of captured energy from the wind; hence and increased blade radius would result in a higher amount of captured energy.
The blades are aerodynamic and they are made of a composite material such as carbon or Plexiglas and are designed to be as light as possible. Blades use lift and drag forces caused by wind; therefore by capturing these forces, the whole turbine will rotate. The blades can rotate around their longitudinal axis to control the amount of captured wind energy. The pitch angle control is usually used for wind speeds above the nominal speed -. Based on axis position, wind turbines are classified as the horizontal axis and vertical axis turbines.
VAWTs have a few advantages over the horizontal axis wind turbines. VAWTs electrical machines and gearbox can be installed at the bottom of the tower on the ground, whereas in HAWTs these components have to be installed at the top of the tower which requires additional stabilizing structure for the system.
Another advantage of the VAWTs is that they do not need the yaw mechanism since the generator does not depend on the wind direction. There are a few disadvantages that limit the utilization of the VAWTs. Due to the design of blades, the sweep area of VAWT is much smaller.
Wind speed is low near the surface and usually turbulent; hence these wind turbines harvest less energy than horizontal axis ones. Additionally, VAWTs are not self-starting machines and must be started in motoring mode and then switched to generating mode -. Solar energy is one of the most important renewable energy sources that have been gaining increased attention in recent years. Solar energy is plentiful; it has the greatest availability compared to other energy sources.
The amount of energy supplied to the earth in one day by the sun is sufficient to power the total energy needs of the earth for one year . Solar energy is clean and free of emissions, since it does not produce pollutants or by-products harmful to nature. The conversion of solar energy into electrical energy has many application fields.
During the s, Europeans started to build solar-heated greenhouses and conservatories. In the late s, French scientists powered a steam engine using the heat from a solar collector. This solar-powered steam engine was used for a printing press in Paris in . A highly efficient solar-powered hot air engine was developed by John Ericsson, a Swedish-American inventor.
These solar-driven engines were used for ships . The first solar boiler was invented by Dr. Charles Greely, who is considered the father of modern solar energy . The first working solar cells were invented in by Charles Fritts . This accomplishment was achieved by following the fundamental work of Russel Ohl in the s . This breakthrough marked a fundamental change in the generation of power.
PV devices as solar cells are unique in that they directly convert the incident solar radiation into electricity with no noise, pollution or moving parts, making them robust, reliable and long lasting. In another words, PVs are arrays of cells containing a material that converts solar radiation into direct current DC electricity. A material is doped to increase the number of positive P type or negative N type charge carriers.
The resulting P and N type semiconductors are then joined to form a PN junction that allows the generation of electricity when illuminated. Solar technologies are largely characterised as either passive solar or active solar depending on the way they capture, convert and distribute sunlight. Active solar techniques include the use of photovoltaic panels and solar thermal collectors while passive solar techniques include orienting a building to the sun, selecting materials with favourable thermal mass or light dispersing properties and spaces that naturally circulate air.
The more commonly used solar system applications are as follows :. PV system convert sunlight directly to electricity by means of PV cell made of semiconductor materials.
Transpired Solar Collectors or Solar Walls: In this system solar energy is used to preheat ventilation air for a building -. The basic operation of a solar cell is light shining on the solar cell produces both current and voltage to generate electric power. This process requires firstly a material in which the absorption of light raises an electron to a higher energy state and secondly the movement of this higher energy electron from the solar cell into an external circuit.
The electron then dissipates its energy in the external circuit and returns to the solar cell. Today solar photovoltaics are rapidly growing and increasingly important renewable alternative to conventional fossil fuel electricity generation. Small scale transportable applications such as calculators and watches were utilized and remote power applications began to benefit from photovoltaics. Moreover, due to the extensive improvement in inverter technologies, PV generation is now being preferred and deployed worldwide as DERs in a microgrid for expansion of local generation at distribution voltage level.
It has been studied that small PV installations are more cost effective than larger ones which indicates the effectiveness of feeding PV generation directly into customer circuits at low voltage distribution networks. Hence, they can be potential contributors to a microgrid --. In the UK, PV systems are also being used for a long time to provide high reliability power for industrial use in remote and inaccessible locations or where the small amount of power required is more economically met from a stand-alone PV system than from mains electricity ---.
The microgrid is a concept based small scale power generation system consisting of renewable energy sources together with fossil fuelled generators and local load. Microgrid is more modern way for utilizing the available potential of DG, not only in remote area electricity development but also in overcoming the short-fall of electricity commercially. Most commonly available renewable energy resources used for the development of microgrids are Wind, Solar, Biomass, Micro-hydro, Fuel Cell etc.
The micro sources must be equipped with power electronic interfaces PEIs and controls to provide required flexibility to ensure the operation.
The control flexibility of microgrid allows itself to present a main utility power system which meets local energy requirements for reliability and security.
So, the main task of microgrid is to obtain reliable and high quality electric power without any faults and abnormal operating conditions ----. Microgrids usually employ CHP combined heat and power equipment, due to more efficient energy strategy attempting to reduce energy demand through efficiency and smart controls while meeting these severity loads .
The concept of microgrid has made possible due to recent approaches in small scale reliable generators with power electronics and inverts the trend to large scale generation and bulk supply. Several types of micro generators can be considered for example photovoltaics and wind generators etc. So, if the environment is primarily residential the photovoltaic generators would be attractive for the main source of generation and for most remote locations wind generators are attractive.
The specific advantage of microgrid is that it facilitates with more inventive schemes for meeting the local requirements in elastic manner with small scale generators and consumers closely integrated. In some microgrid networks, the consumers can also be the suppliers, so a more inventive approach to load control may be possible in the mutual interests of cost and efficiency. In detailed, the microgrid system needs to inspire consumers to be involved in small scale cogeneration, photovoltaics and other renewable energy schemes.
Metering and charging arrangements would be agreed locally within the microgrid and would have to reflect the market for power within the microgrid .
The intention is that the microgrid is self-sufficient, but for security of supply and flexibility it would almost certainly be connected to the local electrical utility network, or even to adjacent microgrids.
These links may be bi-directional enabling the import or export of electricity, or, depending on commercial considerations, it might just be a unidirectional flow of power. From the point of view of the microgrid, the utility connection might be viewed just as another generator or load. This raises the question as to whether or not the microgrid should be linked to other networks over a synchronous alternating current AC connection.
The advantage of a synchronous link would be its simplicity, requiring only an electrical interconnection, circuit breakers and probably a transformer. Lasseter  has considered this possibility and shown that in principle it should be possible to run a microgrid with minimal central control of local generation which is able to operate connected to the utility, or, in the event of loss of the connection, move smoothly into stand-alone or island operation with no loss of power to the microgrid.
What is perhaps less clear is how the synchronous connection would be re-synchronised once the utility was ready to re-establish the connection. The alternative approach would be an asynchronous connection using a direct current DC coupled electronic power converter. This might be bi-directional, enabling import and export of power or simply a device to import power when local resources were inadequate. An advantage of this approach is that it isolates the microgrid from the utility as regards reactive power, load balance, etc.
Only power is exchanged with the utility, the microgrid is entirely responsible for maintaining the power quality frequency, voltage and supplying reactive power and harmonics within its area. With an asynchronous link the microgrid might be unusual that all its power will be supplied through electronic inverters. Some generators, such as photovoltaic cells are intrinsically sources of DC and hence need inversion to connect them to an AC network.
Others, for example, micro-turbines or Stirling engines may generate AC but are not well suited to operate a synchronous generator because the frequency is unsuitable or variable. Voltage source inverters with suitable control schemes will be required to permit stable operation of the network with many small generators attached. Fortunately, advances in power electronics and digital controllers mean that sophisticated control strategies are possible and the cost need not be excessive.
Which of these approaches is more appropriate may well depend on the size of the microgrid. It may also depend on the regulatory environment governing the interchange of power between the microgrid and the utility .
Microgrids require wide range control to ensure security of whole power system, optimal operation, emission reduction and transferring form one operating mode to the other without violating system constraints and regulatory requirements. However, the technical problems of a microgrid must be managed for the concept to become a reality.
The control of a microgrid is thoroughly tied with the energy and power balance in the microgrid, and the question of energy storage. There are three main parameters frequency, voltage and power quality that must be considered and controlled to acceptable standards whilst the power and energy balance is maintained -.
A power system usually contains no significant energy storage; the generated and dissipated power must, therefore, be constantly kept in balance. This power balance must be maintained on a cycle-by-cycle basis if the system is to maintain its frequency. Too much generation and the system accelerate, too little and it slows; neither situation is acceptable. The permissible frequency deviation is defined by Statute and in the UK it is the responsibility for the NGC to ensure that this deviation is not exceeded.
Since the whole of the UK is run as one synchronous system, any new generator means the disconnection of another or a rise in load, if the system frequency is to remain constant. Power balance in a microgrid is therefore essential for frequency control. In a microgrid, frequency stability becomes critical; therefore, control is a major concern. There are a number of techniques used to restore the power balance and hence correct the frequency: All of these are available within a microgrid, but because the system is small the problem is much more difficult to manage to the same standard as is normal in a utility system.
Short term storage of energy is needed to cope with the fluctuations in power demand or accommodate the sudden loss of some generation. A microgrid with many small generators will not be an intrinsically stiff system, unlike a national interconnected utility. The small generators will neither store significant energy in their mechanical inertia, nor will they necessarily respond quickly to sudden changes of load.
Short term storage, probably distributed with the generators, will permit the inverters to follow the rapidly changing demand while giving time for the generators to respond, or extra generation to be brought on line or for generators to be closed down. This same storage could be used to help accommodate the diurnal variation of demand. There are two related issues, firstly quite small power imbalances will produce large frequency excursions and secondly they will happen much more quickly.
The first issue may also be an advantage for a microgrid since small energy stores will have significant effects.
The second issue means that stored energy recovery must be fast and precise. Since the most probable store, in the near future is likely to be a battery with an inverter, this does not pose an insurmountable problem; such a system is quite fast enough to ensure adequate frequency control . Electric power system in the UK operates at frequency of 50Hz and definitely there are advantages adopting this frequency, whether there is to be a synchronous connection or not. The frequency limits are set it and operates by law, relatively tight and standards are not the same like other power systems.
So, there are not any reasons or relaxation possible for not adopting these standards. In the UK electric power system the generally used frequency control method is by the control of rotational speed of synchronous machines supplying power. In large interconnected system with several synchronous generators, no single machine can control the frequency, so there would be flow of synchronising power into any machine which is slowed in order to keep it in synchronism.
For altering the frequency and speed, large power imbalance required of the order of MW per Hz for the UK. It would be less technical issue if there are few machines then the less stiff, system and frequency control. In such a system the machines must be able to respond quickly to load variations in respect to preserve the power balance at all times. It means fast detection of frequency change and accurate control of load generation or both.
In wind turbines, induction generators are often used and solar photovoltaics arrays are connected with inverters. For controlling the frequency, inverters can be utilized and inverter frequency can be controlled independently, but inverters require various viewpoints due to not rotational devices synchronous generators -5]. The system voltage within a large multi generator system is controlled by initially the voltage of the machines but also by the reactive flow.
In general, the reactive balance becomes more critical in a smaller system. For example, all reactive demand must be supplied from one generator in a single machine system. This is not strictly true, but adds significantly to cost and control problems if reactive demand has to be compensated by extra static plant.
A conventional distribution system is usually a feeder network, and there is little interconnection. Voltage drop along feeders becomes an issue, as it will vary with load and distance along the feeder. This dictates that any simple microgrid will have to be either small to be satisfactory or be specially designed as an interconnected network. With proper design, production of the correct voltage should not be an insurmountable problem . Control of power quality will be the biggest issue for a microgrid.
Voltage dips, flickers, interruptions, harmonics, dc levels, etc. As has been discussed by Venkataramanan and Illindala , the distributed generation within the microgrid could enable better control of power quality. With electrical storage together with the distributed generation power quality could be maintained in much the same way as is achieved by Uninterruptible Power Supply UPS systems.
The electronic inverters can not only supply power at the fundamental frequency, but also generate reactive power to supply the needs of reactive loads, cope with unbalanced loads and generate the harmonic currents needed to supply non-linear loads . There is no economic general purpose method for the storage of electricity per se in the quantities required for public utility use. There are off course methods involving capacitors and super conducting magnets; both of which are technically complex and with present knowledge, rather expensive but nevertheless used in specific situations.
Because the direct storage of electricity is not very practical, the storage of energy by other methods for later use in electricity generation is employed. These are many and varied, depending upon the situation and the purpose for which the electricity is to be used. It is likely that a microgrid will rely on chemical energy storage in the form of electric batteries.
In the simplest of systems this will mean lead acid cells which are well developed, available, predictable and robust. For more sophisticated applications, redox batteries are becoming available and development will continue. In critical situations where cost is not an issue, the application of super conducting energy storage has been used. Again, continued development is expected to both reduce costs and to increase reliability.
The calculation of battery size energy and inverter rating power will depend on the size of the loads and generators within the microgrid as well as its topography. As an alternative to storing energy, the shedding of load is more likely to be used in a microgrid, rather than a large scale public utility because it is easier to identify those loads which are least critical. Where co-generation is used, some of this energy storage may well be in the form of heat.
This storage could be in the form of domestic hot water or stored for use in space heating. Innovative control strategies can be developed to make use of this storage and if necessary the plant may be run to meet the electrical load when there is no demand for thermal energy . The software details are specified below. National Renewable Energy Laboratory NREL to assist in the design of micro-power systems and to facilitate the comparison of power generation technologies across a wide range of applications.
HOMER allows the modeler to compare many different design options based on their technical and economic merits. It also assists in understanding and quantifying the effects of uncertainty or changes in the inputs. HOMER simplifies the task of evaluating designs of both off-grid and grid-connected power systems for variety of applications.
When a user designs power system, the user must make many decisions about the configuration of the system. HOMER performs the analyses to explore a wide range of design questions that are below:. Conceptual relationship between simulation, optimization and sensitivity analysis. The above figure 4. The optimization oval encloses the simulation oval to represent the fact that a single optimization consists of multiple simulations.
Similarly, the sensitivity analysis oval encompasses the optimization oval because a single sensitivity analysis consists of multiple optimizations. HOMER simulates the operation of a system by making energy balance calculations for each of the 8, hours in a year. For each hour, HOMER compares the electric and thermal load in the hour to the energy that the system can supply in that hour. For systems that include batteries or fuel-powered generators, HOMER also decides for each hour how to operate the generators and whether to charge or discharge the batteries.
HOMER performs energy balance calculations for each system configuration that we want to consider. It then determines whether a configuration is feasible i. A user can then view hourly energy flows for each component as well as annual cost and performance summaries. After simulating all of the possible system configurations, HOMER displays a list of feasible systems, sorted by lifecycle cost.
We can easily find the least cost system at the top of the list or we can scan the list for other feasible systems. Sometimes we may find it useful to see how the results vary with changes in inputs, either because they are uncertain or because they represent a range of applications.
We can perform a sensitivity analysis on almost any input by assigning more than one value to each input of interest. HOMER repeats the optimization process for each value of the input so that the user can examine the effect of changes in the value on the results. We can specify as many sensitivity variables as we want and analyze the results -.
In a microgrid power system, a component is any part of a whole power system that generates, delivers, converts or stores energy. The microgrid comprises in four major components that are wind turbine or solar photovoltaics, generator, converter and storage batteries. There are two intermittent renewable sources for electricity generation that are wind turbines and solar photovoltaics.
Wind turbines convert wind energy into ac or dc electricity and PV modules convert solar radiation into dc electricity. Generator is a dispatch-able energy source, meaning that the system can control it as needed and it consumes fuel to produce AC or DC electricity.
Converter is used to convert electrical energy into another form and it converts electricity from ac alternating current to dc direct current or from dc to ac. Finally, storage batteries are used for storing the DC electricity. This is the minimum wind speed needed to start the wind turbine which depends on turbine design and to generate output power.
The cut-out wind speed represents the speed point where the turbine should stop rotating due to the potential damage that can be done if the wind speed increases more than that . This is the wind speed at which the wind generator reaches its rated output.
Note that not all wind generators are created equal even if they have comparable rated outputs. This measurement is taken at an uninformed wind speed that the manufacturer designs for. It tends to be at or just below the governing wind speed of the wind generator. Any wind generator may peak at a higher output than the rated output. The faster you spin a wind generator the more it will produce until it overproduces to the point that it burns out.
Manufacturers rate their generators at a safe level well below the point of self-destruction. This figure may be the same as rated output, or it may be higher. Wind generators reach their peak output while governing, which occurs over a range of wind speeds above their rated wind speed . The mean wind speed for a usual day of a month can be calculated by averaging all the recorded wind speeds for the month.
The mean wind speeds are then upgraded to the hub height. Wind speeds increase with height . The calculated mean wind speeds are speeds recorded near the ground surface.
Since the hubs of wind turbine are usually more than ten meters high, the mean wind speeds at a particular height will be greater than V i. Therefore, to obtain mean wind speeds, V i has to be projected to the hub height. The projected V i is calculated using the power-law equation shown -. The power-law exponent, x depends upon the roughness of the surface. A random variable v can be expressed with a Weibull distribution by utilizing the probability density function pdf as given by Stevens and Smulders  and shown below:.
Where c is a scale parameter with the same units as the random variable and k is a shape parameter. The electric power output of a wind turbine is primarily a function of wind speed  and as shown below:. The average wind power output from a wind turbine is the power produced at each wind speed multiplied by the fraction of the time that wind speed is experienced and integrated over all possible wind speeds.
The average power output of a turbine is a very important parameter for any wind power system since it determines the total energy production and hence the total income.
It is a much better indicator of economics than the rated power, which can easily be chosen at too large a value. The formula of average wind power output can be obtained by substituting 3 and 4 into 5 , which gives equation 6 below :.
The software HOMER models a wind turbine as a device that converts the kinetic energy of the wind into AC or DC electricity according to a particular power curve, which is a graph of power output versus wind speed at hub height. An example of power curve is shown in figure 4. HOMER assumes that the power curve applies at a standard air density of 1. First, it determines the average wind speed for the hour at the anemometer height by referring to the wind resource data.
Fourth, it multiplies that power output value by the air density ratio, which is the ratio of the actual air density to the standard air density. Standard Atmosphere  and assumes that the air density ratio is constant throughout the year.
The engineering software package HOMER is used for modelling the hybrid power system, in the software the size of PV array is always specified in terms of rated capacity.
The rated capacity accounts for both the area and the efficiency of PV module, so neither of those parameters appears clearly in the software.
The derating factor is a scaling factor meant to account for effects of dust on the panel, wire losses, elevated temperature or anything else that would cause the output of the PV array to deviate from that expected under ideal conditions. The HOMER software does not account for the fact that the power output of a PV array decreases with increasing panel temperature but we can reduce the derating factor to crudely correct for this effect when modelling systems for hot climates.
In reality the output of a PV array does depend strongly and nonlinearly on the voltage to which it is exposed. The maximum power point the voltage at which the power output is maximized depends on the solar radiation and the temperature. If the PV array is connected directly to a dc load or a battery bank then it will often be exposed to a voltage different from the maximum power point and performance will suffer. A maximum power point tracker MPPT is a solid state device placed between the PV array and the rest of the dc components of the system that decouples the array voltage from that of the rest of the system and ensures that the array voltage is always equal to the maximum power point.
By ignoring the effect of voltage to which the PV array is exposed, HOMER effectively assumes that a maximum power point tracker is present in the system. To explain the cost of PV array the user specifies its initial capital cost in U. The replacement cost is the cost of replacing the PV array at the end of its useful lifetime which the user specifies in years.
By default the replacement cost is equal to the capital cost but the two can differ for several reasons --. A generator consumes fuel to produce AC or DC electricity. The generator can be AC or DC and can consume a different fuel.
The principal physical properties of the generator are its maximum and minimum electrical power output, its expected lifetime in operating hours, the type of fuel it consumes and its fuel curve which relates the quantity of fuel consumed to the electrical power produced. A diesel generator is used for the microgrid system.
Where F 0 is the fuel curve intercept coefficient, F 1 is the fuel curve slope, Y gen the rated capacity of the generator kW and P gen the electrical output of the generator kW. The units of F depend on the measurement units of the fuel.
In the same way the units of F 0 and F 1 depend on the measurement units of the fuel. For a generator that provides heat as well as electricity, the design engineer also specifies the heat recovery ratio.
HOMER assumes that the generator converts all the fuel energy into either electricity or waste heat. The heat recovery ratio is the fraction of that waste heat that can be captured to serve the thermal load. The design engineer can schedule the operation of the generator to force it ON or OFF at certain times.
During times that the generator is forced ON, HOMER decides at what power output level it operates which may be anywhere between its minimum and maximum power output. The fixed cost of energy is the cost per hour of simply running the generator without producing any electricity.
The marginal cost of energy is the additional cost per kilowatt-hour of producing electricity from that generator. The effective price of fuel includes the cost penalties if any associated with the emissions of pollutants from the generator.
HOMER calculates the marginal cost of energy of the generator using the following equation: Where F 1 is the fuel curve slope in quantity of fuel per hour per kilowatt-hour and C fuel,eff is the effective price of fuel including the cost of any penalties on emissions in dollars per quantity of fuel . Although renewable resources are attractive, they are not always dependable in the absence of energy storage devices.
As a result, renewable resources are often used together with energy storage devices. However, in many cases, such systems are the least understood and the most vulnerable component of the system . Among different types of energy storage devices, lead-acid batteries are still the most commonly used devices to store and deliver electricity in the range from 5V to 24V DC -. The battery bank is a collection of one or more individual batteries. The software package HOMER models a single battery as a device capable of storing a certain amount of dc electricity at a fixed round-trip energy efficiency with limits as; how quickly it can be charged or discharged, how deeply it can be discharged without causing damage and how much energy can cycle through it before it needs replacement.
HOMER assumes that the properties of the batteries remain constant throughout its lifetime and are not affected by external factors such as temperature.
In HOMER, the key physical properties of the battery are its nominal voltage, capacity curve, lifetime curve, minimum state of charge and round-trip efficiency. The capacity curve shows the discharge capacity of the battery in ampere-hours versus the discharge current in amperes.
Manufacturers determine each point on this curve by measuring the ampere-hours that can be discharged at a constant current out of a fully charged battery. Capacity typically decreases with increasing discharge current. The lifetime curve shows the number of discharge-charge cycles the battery can withstand versus the cycle depth.
The number of cycles to failure typically decreases with increasing cycle depth. The minimum state of charge is the state of charge below which the battery must not be discharged to avoid permanent damage. In the system simulation, HOMER does not allow the battery to be discharged any deeper than this limit. The round-trip efficiency indicates the percentage of the energy going into the battery that can be drawn back out.
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Hybrid power systems depending on renewable energy sources with a diesel generator used as a stand-by or back-up supply, especially when the load demand is high and renewable energy sources are less. So, designing of a hybrid power system rely on site specification, load demand and available energy .
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