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Extended equivalent circuit one-diode model The simple equivalent circuit is sufficient for most applications. The differences between calculated and measured characteristics of real solar cells are only a few per cent. However, only extended equivalent circuits describe the electrical solar cell behaviour exactly, especially when a wide range of operating conditions is to be investigated. Charge carriers in a realistic solar cell experience a voltage drop on their way through the semiconductor...
Flatplate collectors
The most common collectors for solar domestic water heating systems in many countries today are flat-plate collectors. These mostly consist of three components An absorber is inside the flat-plate collector housing. This absorber converts sunlight to heat and transfers it to water in tubes, which passes through the system. The collector housing is highly insulated on the back and sides to keep heat losses to a minimum however, there are still some collector heat losses, which mainly depend on...
Wind turbines with horizontal rotor axis System components
Most wind turbines generating electricity today are horizontal axis machines. It is predominantly medium-sized enterprises that have pushed wind market developments. Wind power plants have reached a high technical level and Figure 5.12 Section through the Stall-controlled TW600 Wind Generator 600 kW Change-pole Asynchronous Generator, 43 m Rotor Diameter, 50-70 m Hub Height Figure 5.12 Section through the Stall-controlled TW600 Wind Generator 600 kW Change-pole Asynchronous Generator, 43 m...
Evacuated tube collectors
The high vacuum inside the closed glass tube of the evacuated tube collector is easier to maintain over a long period of time than that in an evacuated flat-plate collector. Glass tubes can resist the ambient air pressure due to their shape so that no supports are necessary between the back and front sides. A metal absorber sheet with a heat pipe in the middle is embedded inside a closed glass tube with a diameter of a few centimetres. A temperature sensitive working medium such as methanol is...
Synchronous generator with DClink converter
Modern power electronics can avoid the disadvantages of a synchronous generator that is directly coupled to the mains. The synchronous generator is connected via a DC link or frequency converter to the mains see Figure 5.38 . In this configuration, the system can operate the generator at a frequency independent of the mains frequency. Changing the generator frequency varies the generator speed. Hence it is possible to vary the speed over a wide range and to run at the optimal speed to obtain...
SpeedTorque Characteristics and Typical Generator Data
The connection between torque and speed for an asynchronous generator is also interesting because the rotor speed changes with the driving torque. The intrinsic torque of the machine is used for the following calculations. It is calculated from the air-gap power Pg and the synchronous speed nS of the stator field as The internal torque Mi is also given by the mechanical torque M negative sign in generator mode and the friction torque Mf positive sign With the equation of the current for the...
Lift devices
If wind, which circulates around a body, develops higher flow speeds along the upper surface than along the lower, an overpressure emerges at the upper surface and an underpressure at the lower. The result is a buoyancy force, according to Bernoulli The buoyancy force is calculated using the lift coefficient cL, the air density p, the apparent wind speed vA and the projected body area AP. Rotor blades of modern wind generators usually make use of the buoyancy force. The projected area of a...
Figures
1.1 Prices for Water Heating 4 1.2 Energy Conversion Chain and Losses for Water Heating with a 1.3 Energy Conversion Chain and Losses for Water Heating with an Electric Cooker 6 1.4 Evolution of Annual Crude Oil Production 7 1.5 World Primary Energy Demand by Region in 2001 8 1.6 Origin of the Anthropogenic Human-induced Greenhouse Effect 11 1.7 Annual per capita Carbon Dioxide Emissions from Fuel Combustion for Different Countries in 2001 14 1.8 Nuclear Power's Share of Electricity Generation...
Greenhouse Effect
Without the protection of Earth's atmosphere, the global mean ambient temperature would be as low as -18 C. Particular gases in the atmosphere such as carbon dioxide CO2 , water vapour and methane capture parts of the incoming solar radiation, acting like a greenhouse. These gases have natural as well as anthropogenic, or human-induced, sources. Figure 1.6 illustrates the anthropogenic greenhouse effect. Figure 1.6 Origin of the Anthropogenic Human-induced Greenhouse Effect Figure 1.6 Origin of...
Calculations with return on capital
In addition to the calculations described above without return on capital, investors usually expect payments of interest for their investment of the order of usual paid rates for comparable investments. Starting with the payment A0 in the initial year, the capital cn after a time of n years, with the interest rate ir, can be estimated using compound interest formulae. With For an initial capital of A0 6500 following the example of the photovoltaic system in the section headed 'Costs for a...
Irradiance gain for surface tilt or tracking
If a solar energy system tracks the sun so that the angle of incidence is virtually zero, the energy yield increases significantly. The higher direct irradiance on a surface perpendicular to the solar radiation causes the increased energy yield. During days with high direct irradiation, tracking can achieve energy gains over horizontal orientation in the order of 50 per cent in summer and up to 300 per cent in winter, depending on the latitude of the location see Figure 2.15 . However, tracking...
Integral collector storage systems
For integral collector storage systems ICS the hot water storage tank is integrated with the collector itself. A technically robust system is not easy to Table 3.4 Heat Transition Coefficient k and Total Energy Transition Coefficient g-value of Various Conventional Materials and Transparent Insulation Materials TIMs Conventional glass covers k in g-value W m2 K TIM between low-iron glass with air gap Conventional glass covers k in g-value W m2 K TIM between low-iron glass with air gap 2 layers...
Photovoltaic inverters
Photovoltaic inverters differ from inverters for other applications. This is because they must operate the photovoltaic generator at the optimal operating point to generate the maximum power. Grid-connected inverters are therefore often combined with MPP trackers, i.e. DC-DC converters, to set a voltage at the generator that differs from the grid voltage. A battery charge controller is integrated into most stand-alone inverters. Photovoltaic inverters operate at the rated power PN for only very...
Info Azh
Figure 4.7 Space Charge Region Formation at a p-n Junction by Diffusion of remain. They create a positive space charge region. Where holes have diffused into the n-region, negatively ionized atoms remain and create a negative space charge region. An electric field between the n-region and p-region is thus created. It counteracts the charge carriers and hence the diffusion cannot continue indefinitely. Finally, a diffusion voltage is created. The charge neutrality within the boundaries dn and dp...
Electrical description of a synchronous machine
All three phases of the stator windings produce a main field that is connected to the rotor. The useful inductance Lh and the useful reactance Xh 2nfjLh reflect this. Besides the useful field there are leakage fields that are not connected to the rotor. The leakage reactance Xa describes these leakages. The rotor field of a rotating rotor induces a voltage leak in the stator. This voltage is also called the synchronous internal voltage Vp. The rms value of the induced voltage is proportional to...
Squarewave inverter
A very simple inverter circuit is the two-pulse bridge connection B2 connection shown in Figure 4.53. It consists of four thyristors. A transformer connects the circuit to the grid. Thyristors 1 and 3 work together, as do thyristors 2 and 4. If these two groups switch periodically, they generate a square-wave alternating current at the transformer. Thyristors 1 and 2 can be replaced by non-controllable diodes for simplification in this case, only half of the bridge must be controlled. This...
Solar thermal domestic water heating systems
The heating of domestic water involves much higher temperatures than for swimming pool water. The simple absorbers used for swimming pool heating are in most locations not suited to domestic hot water systems because the absorber losses due to convection, rain and snow as well as heat radiation are unacceptably high. Domestic water heating systems typically use collectors that have much lower losses at higher water temperatures. These are either flat-plate, evacuated flat-plate or evacuated...
Introduction 1
Solar thermal systems are an important use of solar energy. The use of solar thermal applications has a long history Archimedes is reported to have boiled water using a concave mirror as early as 214 BC thermal systems today still represent the most cost-effective use of solar energy. Solar thermal signifies the thermal use of solar energy in general. Therefore, a number of different technical applications exist. Besides building space heating, water heating or industrial processes, solar...
Solar Irradiance on the Surface of the Earth
Values measured on the surface of Earth are usually lower than the solar constant. Various influences of the atmosphere reduce the irradiance. They are reduction due to reflection by the atmosphere reduction due to absorption in the atmosphere mainly O3, H2O, O2 and CO2 reduction due to Rayleigh scattering reduction due to Mie scattering. The absorption of light by different gases in the atmosphere, such as water vapour, ozone and carbon dioxide, is highly selective and influences only some...
Wind speed ranges
The design tip speed ratio depends closely on the number of rotor blades. The maximum power coefficient of a three-bladed rotor is reached at a tip speed ratio of between 7 and 8, whereas for two-bladed rotors the figure is 10 and for a one-bladed rotor about 15. However, these values can vary depending on the system design. The optimum tip speed ratio Aopt also defines the design wind speed along with the rotor radius r and the rotational speed of the rotor n in min-1. For example, the optimum...
Responsibility of capital
It is nearly impossible for an investor to realize all the possible consequences of an investment. However, the higher the rates of return the higher the risks this is a well-known rule. The risk for investments in the energy sector does not only include the invested capital but also much greater assets as well. Certainly, not all investments that receive a return on capital will eventually cause losses in capital, but all investments do have responsibilities, especially in the energy sector....
Photovoltaics
The word 'photovoltaic' consists of the two words, photo and Volta. Photo stands for light Greek phos, photos light and Volta Count Volta, 1745-1827, Italian physicist is the unit of the electrical voltage. In other words, photovoltaic means the direct conversion of sunlight to electricity. The common abbreviation for photovoltaic is PV. The history of photovoltaics goes back to the year 1839, when Becquerel discovered the photo effect, but in that century the technology was not available to...
Synchronous machines Design of synchronous machines
A synchronous machine consists of a stator and a rotor. The stator is the stationary part of the machine with the three-phase windings to produce the rotating field as described in the previous section. The casing is made of Figure 5.23 Cross-section through a Synchronous Machine Left Cylindrical Rotor, Right Salient-pole Rotor Figure 5.23 Cross-section through a Synchronous Machine Left Cylindrical Rotor, Right Salient-pole Rotor stacked sheets containing the three-phase windings of the...
Collector performance and collector efficiency
After explaining the assembly of collectors in the previous sections, this section will describe the calculation of the collector performance and efficiency. This is necessary for the further estimation of the output of the whole collector system. The collector converts solar irradiance E, which is transmitted through the front glass cover with transmittance t onto the collector surface AC, directly to heat. The power output of the solar collector Qout is reduced by losses due to reflection...
Limiting power output and storm protection
The power that can be taken from the wind varies with the wind speed. After reaching the nominal power, the power should remain constant for wind speeds greater than the nominal wind speed because the turbine and generator cannot handle more power. Therefore, a wind power plant must limit the power with one of the two following methods Figure 5.14 Stall Effect at Higher Wind Speeds Figure 5.14 Stall Effect at Higher Wind Speeds Stall control takes advantage of the stall effect that occurs at...
Drag devices
If an object is set up perpendicularly to the wind, the wind exerts a force FD on the object. The wind speed v, the effective object area A and the drag coefficient cD, which depends on the object shape, define the drag force Figure 5.5 shows drag coefficients for various shapes. With PD FD v, the power to counteract the force becomes If an object moves with speed u by the influence of the wind in the same direction as the wind, the drag force is l'd-0.4 dosed l gt back open Figure 5.5 Drag...
Calculation of the Solar Angle of Incidence
The solar angle of incidence 0hor on a horizontal surface is a direct function of the sun height yS. This angle is also called the zenith angle dZ The calculation of the angle of incidence 0tilt on a tilted surface is more complicated. The surface azimuth angle at describes the deviation from the south. If the surface faces to the west, at is positive. The inclination angle yt describes the surface tilt or slope of the surface. If the surface is horizontal, yt is zero. Figure 2.13 visualizes...
Reserves of Fossil Energy Sources
The current energy supply depends mainly on fossil energy carriers as described in the previous section. Fossil fuels such as natural gas, petroleum, hard and brown coal needed many thousands of years to form. Organic substances i.e. animal or vegetable residues were the base materials. Hence, fossil fuels are stored biomass of the ancient past. A huge amount of these fossil fuels has already been consumed in the 20th century. However, due to the increasing exploitation of the fossil...
Irradiance on a Horizontal Plane
As described above, solar radiation is scattered and reflected when passing through the atmosphere. Rays of extraterrestrial sunlight are virtually parallel. Terrestrial sunlight, on the other hand, consists of direct and diffuse components see Figure 2.6 . Direct solar radiation casts shadows, because it is directional, coming directly from the sun diffuse irradiation, on the other hand, has no defined direction. The total irradiance on a horizontal surface on Earth is also called global...
Wind Turbine Design
Figure 5.11 Rotors with Vertical Axes Figure 5.11 Rotors with Vertical Axes opposite sides. Near the axis, the blades overlap so that the redirected wind can flow from one blade to the other. The rotor blades also utilize the lift principle so that the efficiency of the Savonius rotor is little better than that of simple drag devices. However, the efficiency is much worse than that of good lift devices, reaching maximum power coefficients of the order of 0.25 Hau, 2000 . Savonius rotors have...
Diffuse irradiance on tilted surfaces
There are two approaches for estimating the diffuse sky irradiance Edf tilt on a tilted surface the isotropic approach and the anisotropic approach. The isotropic approach assumes that the diffuse radiance is constant across the whole sky. An important conclusion from this approximation is that the isotropic diffuse irradiance on a tilted surface is always lower than the diffuse irradiance on a horizontal surface because a receiver does not see the diffuse irradiance from behind the tilted...
Wind speed distributions
Wind speed distributions are commonly used to indicate the annual available wind energy. These distributions are estimated using measurements, wind maps or computer analysis. Tables or statistical functions can give the distribution. Figure 5.1 shows the relative distribution h v of wind speed v in Karlsruhe, which is located in south Germany near the Black Forest. This distribution indicates how often a certain wind speed occurs. It is immediately obvious that the wind energy potential in...
Domestic hot water storage tanks
Only annual usage simulations can estimate the exact dimensions of a hot water storage tank. The size depends on the hot water demand, solar fraction see Equation 3.59, p112 , collector performance, collector orientation, pipes and last but not least on the annual solar irradiation. For central European climates, a rough estimate can be made. Here, the storage volume should be 1.5-2 times the daily demand. Besides the storage volume for the daily demand, a standby volume of 50 per cent and a...
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Figure 3.7 Energy Conversion in the Solar Collector and Possible Losses The use of special materials for the front cover can increase the collector efficiency. These materials should let incoming radiation through, but reflect the outgoing infrared emissions coming from the absorber plate back inside. Infrared reflecting glass such as In2O3 or ZnO2 with high transmittance for visible light but high reflectance for infrared meets these requirements. Table 3.5 shows the parameters of these...
Estimation of the degree of shading of diffuse irradiance
For the estimation of the degree of shading of diffuse irradiance, the object polygon is considered as being projected on a hemisphere. The objects represented by the object polygon block the diffuse irradiation that passes through the object polygon on the hemisphere. As a simple example, Figure 2.23 shows a polygon with four points, with two of them on the horizontal ground plane. The two remaining points define the polygon clearly. y a m a n describes the connecting line of the points pj and...
Equivalent circuits and circle diagrams for the stator current
The electrical structure of an asynchronous machine is similar to a transformer that consists of two coupled windings. The windings of an ideal transformer are coupled free of leakages. A real transformer has leakage fields and ohmic losses that can be considered by additional resistances and reactances in the equivalent circuit see Figure 5.27 . Hence, the equations for the voltages of the transformer become With the number of turns per phase w1 and w2 and I V2tr-1 R2 2 Xhas well as Xh1 become...
Optimum distance of solar energy system support structures
Photovoltaic and solar thermal systems are often installed on the ground or on flat roofs. Support racks are used to mount the solar energy systems. Usually these are tilted to get higher annual irradiances, as explained in the section on irradiance on tilted surfaces see p60 . Furthermore, horizontally installed solar energy systems have relatively high losses due to soiling, for example, through deposition of air pollutants, bird excrement or other dirt deposits on the surface of the...