An increasing demand for electric power in the twenty-first century and the need for more environmentally benign electric power systems are of critical concern to governments and stakeholders such as industries and end users. Electrical shortages, power quality, rotating outages, and increasing oil prices have motivated many utilities and consumers to look for alternative forms of highly reliable energy. Traditional utility ties have been deregulated, yielding room for new market structures and players. Regulatory commissions in different parts of the world have unbundled the vertical utility industry into separate business units that can be categorized into three broad categories: generation companies, which include utility and nonutility companies; transmission companies, which may be under state ownership; and distribution companies, which are privately owned business units. Distributed generation (DG) is a relatively new approach to describe the new wave of generation at the customer side, which is less than that of the typical control power station in a competitive electricity market. DG has been given a variety of definitions relative to its rating, power delivery area, environmental impact, penetration level, and point of connection. Although these criteria are necessary, they are not sufficient. We provide here a practical definition for DG as “an electric power source connected directly to the power network, preferably at the customer side of the meter, sufficiently smaller than the controlling generating plant.” This definition does not define the rating of the generation source, since the maximum rating depends on the voltage level of distribution on the sub-transmission network. DG involves the use of technologies such as microturbines, stirling engines, fuel cells, and renewable energy such as photovoltaic, wind, and biomass systems. DGs can meet the needs of a wide range of users, from residential to commercial and industrial sectors. Note that the DG definition does not specify the technology options. Hence, DG can include any of the renewable sources; combined heat and power (CHP) applications are modular. DG can help reduce investments in transmission and distribution capacity. From a planning point of view, DG can be placed close to load centers, thus minimizing loss in operating networks and reducing costs for operations and controls. DG is especially favored to help reduce losses in distribution networks and serve as stand-alone or back-up generation. Other benefits include: Policies are necessary to ensure that DG systems adhere to some quality of supply and to maintain system frequency. High voltage levels approved for DG connections relative to the utility company must also be known and properly controlled to achieve voltage security and respond to changing market conditions. This flexibility in construction of lines and centralized generation has made the DG market economically attractive. Major policy issues surrounding DG include: Renewable energy is energy derived from natural sources that replenish themselves over a short period of time. These resources include sun, wind, hydropower, organic plant and waste material (biomass), and earth heat (geothermal). Whereas renewable resources can generate both electricity and heat, the term “green power” is used in a narrow sense to mean electricity products generated from renewable sources that are environmentally and socially acceptable. Photovoltaic (PV) cells and modules are configurable from 1 to 5 MW. Figure 9.1 shows a typical modern PV system. In 1839, French physicist Edmond Becquerel was the first to discover that certain materials exposed to light produce current. Refinements at Bell Laboratory in 1954 led to the development of silicon-based PV cells, producing electricity conversion with more than 4 percent efficiency. Following the energy crisis in recent years, the use of solar power has become more widespread. Commonly known as solar panels, PV modules are commercially available, provide no emissions, are an alternative to other energy sources, are reliable, and require minimum maintenance to operate. However, they are expensive compared with other renewable-energy options and up to four to six times larger than complicated alternative technologies. Today, advances in material science have led to the engineering and fabrication of solar panels with about 30 percent efficiency. In the presence of sunlight, solar panels comprising discrete cells generate DC electricity, which after conversion to AC is connected to power the load or exported to the grid. PV cells can be part of a building, duplicating other building materials, and can have a wide range of application as DG ranging from residential and commercial users to remote power consumers (structures in school, homes, community facilities, and commercial buildings). The greatest potential for PV modules is as green power because they do not emit pollutants or CO2. However, they are a poor fit as a peaking power application source, since the unit outputs are not easy to control. PV systems produce better power during daylight periods of maximum available solar radiation and require battery storage for backup. They are not a good fit as premium power due to the unpredictable nature of power from solar cells. Manufacturers continue to reduce the cost of installation while increasing efficiency as new technology is developed for manufacturing materials and other operational and management costs. PV systems need to be developed and verified to optimize the output of the system at the design stage for maximum energy production and peak-sharing applications. Three models of a PV-equivalent circuit are shown in Figure 9.2. The equivalent circuit consists of a diode and source, which are in parallel. A simplified model has the following V-I equations:
where Ip is the photon current, Id is the diode current, Is is the diode ideal factor, K is the Boltzmann constant, T is the absolute temperature, e is the charge of an electron, and Isc is the solar insulation. The open-circuit voltage, Voc, of the PV is given as
At 25°C, Equation 9.2 becomes
The modified equivalent circuit (Figure 9.3) accounts for a real solar cell with external contacts, and voltage loss (drop) is accounted for through leakage currents and resistances. Using the V–I and Kirchoff's law
Also consider a simpler way (parallel connection): for parallel resistance
and for series connection
Example 1 Combine PV to series and parallel resistances Rs and Rp, respectively (Figure 9.4). The generalized PV equivalent circuit for both series and parallel resistances will be:
Note that
To obtain from cell to module, we use
where n is the number of cell modules. A PV system can be connected to a grid (utility system), can stand alone, or can be integrated. Different types of loads affect the PV system, as shown from the performance characteristics of the V–I curves. PV systems connected to different loads exhibit different V–I characteristics, as seen in Figure 9.5. For a simple resistive load
To achieve Rmax, we can use a maximum power tracker, which keeps a PV system operating at its highest efficiency point at all times. Example 2 Consider a solar panel rated 20 W at 1.0 V. Determine: Solution: Thus, each path of panels connected in a series will provide 2 A. Therefore,
Hence, five parallel paths each having twelve panels in a series are required to form an array to meet the power requirement. The total power that can be obtained from the solar array is
The major challenge to solar energy is the cost of equipment and devices. The full cost of installation of a solar energy system is still three to six times as high as the installed cost of conventional nonrenewable generation. The cost of a PV cell module is expressed as the cost per peak watt output at standard insolation. Standard insolation (energy from sunlight on a flat surface perpendicular to the sun's rays) is defined as 1 kW per square meter. The key to developing solar-PV electricity generation is to reduce the cost of PV cells by increasing the production volume of cells so that the benefits of mass production can be realized. Solar concentrator systems include power towers, linear, and dish/engine towers. These renewable-energy systems are based on energy conversion of solar to thermal energy to run conventional steam or gas turbines for power generation. Windmills have been used for many years to harness wind energy for mechanical work such as pumping water for farms and ranches. Today, wind power is one of the fastest-growing sources of energy. After the energy crisis of the 1970s, wind energy was considered the most economically viable choice in the portfolio of available renewable-energy options. Wind turbines can produce electricity at an affordable cost without additional investments in infrastructure such as transmission lines. Wind turbines basically include the rotor, generator, turbine blades, and driver or coupling device as shown on Figure 9.6. The operation is simple. Wind blows through the blades, with pressure exerted on the cross-sectional area of the blade. Aerodynamic force causes the blades to turn the rotor. The gearbox and the generator shown in Figure 8.8 are all in a single unit behind the machine blades. The output of the generator is passed through a unit for appropriate conversion from DC to AC. The windmill comes in different configurations, normally horizontal or vertical. The wind speed and the height of a pole-mounted windmill above the ground contribute to the power output of the wind-turbine system. The location of the system is equally important in sizing the output of the windmill. Wind turbines produce no emissions. They have a variety of sizes and applications and are classified into utility scale and individual scale. For large-scale utility projects, they can range from 1.5 to 5 MW loads. A small system can be as simple as a single pole and a blade. Wind turbines come in two basic types: The mathematical model for wind power stems from aerodynamic power, given in
where P is the air density, R is the radius of the circular area swept by turbine blade, in meters, v is the actual wind speed, and Cp is the turbine-power coefficient, which represents the power conversion efficiency of a wind turbine. Tip speed ratio of the machine's blades to wind speed is
where ω = rotor rotational speed in radians/second is the rotor rotational speed in radians/second and Cp is maximum at λoptimal. The wind-turbine system uses induction generators that are independent of torque variation while speed varies between 1 to 2 percent. A taller tower is expected to provide higher-speed winds to the turbine. Surface winds can also easily be affected by the irregularities or roughness of the earth's surface or forests/buildings. This is given as
where v is the wind speed at height H, v0 is the reference speed at reference height H0, and α is the friction coefficient. In the United States,
whereas in Europe
Example 3 If the average wind speed on an open plain is known to be 6 m/s at 10 m height, determine the wind speed at 50 m height if the roughness coefficient α is 0.15. Solution: We know that
Thus
Wind turbines produce no emissions. As a green-power application, the efficiency of wind turbines is superb, since they do not emit CO2 or pollutants. However, wind power has an obvious disadvantage. Because its output cannot be controlled, it is mostly suited for peaking applications, producing power only when there is sufficient wind. Wind power cannot serve as premium power because of its unpredictability. Moreover, it is unsuitable for CHP applications. Despite these drawbacks, windmills remain a subject of continuing research, with the principal focus on: Wind turbines are a relatively inexpensive way to produce electricity compared with PV, which is the most competitive green power to date. Example 4 Determine the amount of power that is present in a 10 m/s wind striking a windmill whose blades have a radius of 5m. Solution: The area swept by the turbine blade is
Power present in wind is
If the turbine-power coefficient Cp is 0.2, then the amount that will be converted to usable electric power is
Bioenergy is the energy derived from biomass organic matter such as corn, wheat, soybeans, wood, and residues that can produce chemicals and materials that we normally get from petroleum. Bio-power is product of biomass gasification, which converts biomass to a gas that can be used to power a turbine and generate electricity. The biomass-gasification process is shown in Figure 9.7. The energy-conversion process for biomass also utilizes pyrolysis oil, produced when biomass is converted directly into fluid fuel. The most common fuels are ethanol alcohol or biodiesel derived from corn ethanol. Biomass power plants are commercially available in the United States for up to 11 GW of installed capacity. Biomass power ranges from 0.5 to 3.0 GW using landfill gas and forest products, respectively. Biomass has traditionally been used for domestic cooking and heating, and such use is still widely practiced in developing countries. Biomass power is viable only when a sufficient quantity of bioproducts is available and a conversion process is done. Truly continuous applications are likely for biomass systems, and it appears to be a good fit for CHP applications. Since the output of these units cannot be controlled, they are not suitable for peaking applications. Biomass power is not a premium power due to the limited availability of bioproducts. It is also not an ideal green power due to the emission of CO2 and other pollutants. Several research efforts are under way to improve the quality of biomass power and reduce its environmental impacts. The form of energy input is very inexpensive. However, the efficiency of biomass power is low (typically less than 20 percent), and is a relatively expensive way to produce electricity compared with PV. Several advances in technology are being used to reduce CO2 emissions and improve the green-power nature of biomass fuels. The success of biomass energy depends on the continuity of fuel supplies. Hydropower or waterpower is power derived from the energy of falling water or fast-running water, which may be harnessed for useful purposes. Since ancient times, hydropower from many kinds of water mills has been used as a renewable-energy source for irrigation and the operation of various mechanical devices, such as gristmills, sawmills, textile mills, trip-hammers, dock cranes, domestic lifts, and ore mills. A trompe, which produces compressed air from falling water, is sometimes used to power other machinery at a distance. There are three types of hydropower facilities. Hydropower plants range in size from small systems for a home or village to large projects producing electricity for utilities. The size of a hydroelectric power plant can also be categorized in terms of its power-generating capacity and the overall head of the system. Facilities range in size from large power plants that supply many consumers with electricity to small and micro plants that individuals operate for their own energy needs or to sell power to utilities. Although definitions vary, the US Department of Energy has defined the various categories of hydropower plants as follows: Once we have the flow rate and the head figures, we can roughly estimate the potential power available in kW with the following formula:
where Q is the discharge, m3/sec, H is the net head, η is the system efficiency, and γ is the specific weight of water, kN/m3. Hydroelectric sites are broadly categorized as low- or high-head sites. Low head typically refers to a change in elevation of less than 10 feet (3 m). A vertical drop of less than 2 feet (0.61 m) will probably make a hydroelectric system unfeasible. A high flow rate can compensate for low head, but a larger and costlier turbine will be necessary. Environmental and climatic factors as well as human activities in the watershed determine the amount and characteristics of stream flow on a day-to-day and seasonal basis. A storage reservoir can control flow but unless a dam already exists, building one can greatly increase cost and legal complications. The following are some of the problems related to hydropower development: Example 5 During the design stage of a small hydropower station, the following data were collected for the site: If the combined efficiency of the water conveyor system from dam to turbine, turbine system, and generator is estimated at 0.77, estimate the capacity of the system. Let g = 9.82 m/s2. Solution:
The fuel cell was first developed by Sir William Grove in 1839 and put to practical use in the 1960s by NASA to generate fuel for electricity needed by the Apollo and Gemini spacecraft. Fuel cells are quiet, clean, and highly efficient on-site generators of electricity that use the electrochemical process to convert fuel into electricity. In addition to generating electricity, fuel cells can also serve as a thermal-energy source for water and space heating or for cooling absorption. Fuel cells can run using hydrogen, natural gas, methanol, or gasoline. The efficiency for conversion of fuel to electricity can be as high as 65 percent, as it does not depend on Carnot limits. This efficiency is what makes fuel cells environmentally friendly. Fuel cells come in a variety of different forms, all of which are under development. Examples include phosphoric acid fuel cells (PAFC), proton-exchange membrane (PEM) cells, solid-polymer molten carbonate fuel cells (MCFC), solid oxide fuel cells (SOFC), alkaline (a direct methanol) fuel cells, regenerative fuel cells, and botanical ceramic fuel cells (BCFC). Fuel cells produce virtually no emissions of air pollutants or greenhouse gases. However, their costs are significantly higher than those of conventional technologies. Although fuel cells use different types of fuels, they operate using the same basic principle. A fuel cell consists of two electrodes: an anode and a cathode, separated by an electrolyte, as seen in Figure 9.9. Through the hydrogen catalyst, atoms split into a proton H + and an electron, and the proton passes through the electrolyte to the positive cathode. A DC current results. Using a converter, we can easily generate an AC current. The combined hydrogen and oxygen at the cathode produce water and heat. Details on the differences between fuel cells are based on materials and manufacturing costs, operating temperature, efficiency, and power-to-volume (weight) ratio. Additional models of fuel cells and their distinguishing features are available in the literature. The topology of a fuel cell is defined as a stack that consists of the part of the fuel cell that holds the electrodes and the electrolytic material. Hydrogen is extracted from gasoline, propane, and natural gas refineries to operate commercial fuel cells. Emissions from fuel cells are very low, so they have minimal environmental impacts. Their high efficiency leads to lower fuel costs and minimal maintenance due to a lack of moving parts. They have virtually no pollutant emissions, and CO2 is rather low. Fuel cells are a good fit for green power and premium power. In addition, they provide a moderately high thermal-quantity output and hence are ideal for CHP applications. However, they perform poorly as peaking power due to extremely high capital cost. Developmental research work to refine the effectiveness of fuel cells is receiving greater attention. Fuel cell efficiency ranges from 40 to 80 percent. Two of the most commonly used fuel cell types are PAFCs, which operate at relatively high temperature and use an external water-cooling system to cool the stack and PEM cells, which operate at a lower temperature than most of the other fuel cells and contain no chemicals such as liquid acids or molten bases that would cause concerns about the materials of construction.
The energy of chemical input and output, i.e., of H2, O2, and H2O, is defined as a chemical energy given as enthalpy, Helmholtz function, or Gibbs free energy. Fuel-cell energy can also be expressed in terms of calorific value. However, Gibbs free energy (Gf) is the preferred measure of fuel-cell energy, defined as the energy to do external work, neglecting any work done by changes in pressure or volume. Gibbs energy represents the external work involved in moving electrons around an external circuit. Gf energy is used to represent the zero-energy point, and a change in Gf is given by
Consider 2H2 + O2 →2H2O, which is equivalent to the following (after chemical interaction) as
where the new product is one mole of H2O. Reactants are 1 mole of H2 and 1/2 mole of O2, which gives a balance of energy
i.e.,
However, the Gf of the function is not constant. It changes with temperature and the state of the liquid or gas. Continuing the calculation in terms of electrical work, we get 2N electrons around the equivalent fuel-cell circuit. N is the Avogadro number, i.e., the charge on one electron.
where F is the Faraday constant or charge on 1 mole of electron.
This equation gives the fundamental emf = reversible open-circuit voltage of the fuel cell. Ocean energy is another type of renewable energy. The ocean can produce thermal energy from the sun as well as heat and mechanical energy from the tides and waves. Ocean thermal energy has a variety of applications involving electricity generation. It uses a simple conversion from warm surface water or boiled seawater to turn a turbine that activates a generator. The conversion of ocean power to electricity involves heavy use of a mechanical turbine. A dam is usually used to convert sea-power energy to electricity. Active research on ocean energy is under development in the Pacific West of the United States and in Europe. The energy available from the ocean's surface-wave motion is almost unlimited, but it has proved difficult to capture in any appreciable quantity. Many ingenious systems have been proposed but, except for small installations, very few are generating electricity commercially and most have been thwarted by practical problems. Some of these proposals are outlined below. Most are still in an experimental phase and many are not scalable into high-capacity systems. Formidable technical challenges are involved in designing practical systems for capturing wave energy. A number of solutions have been proposed to solve these technical challenges. The wave power per unit length of the wave front PL is given by
where ρ is the density of the water (103 kg/m3), a is the wave amplitude (half of the wave height), g is the gravitational constant (10 m/sec2), λ is the wavelength of the oscillation, and T is the period of the wave. Example 6 Estimate the power per meter in a wave with amplitude 1.82 m, length 82.15 m, and period 3.5 seconds. Take density of seawater as 1,029 kg/m3 by
Another minor source of renewable energy is the geothermal heat pump. This form of power is based on accessing underground steam or hot water from wells several miles into the earth. Pumping hot water to drive conventional steam turbines, which drive the electrical generator to produce electrical power, does the conversion. The water is then recycled back to earth to recharge the reservoir for a continuous energy cycle. There are several types of geothermal power plants, namely dry steam, flash steam, and binary cycle. Dry-steam plants draw water from the reservoirs of steam, while both flash-steam and binary-cycle plants draw their energy from the recycled hot water reservoir. Geothermal power is currently under development in the United States, and some reasonable levels of power have been produced in California, Utah, Nevada, and Hawaii. Various applications of geothermal power exist, such as heat pumps, agricultural applications, fishing farms, and food processing. Geothermal projects force significant up-front capital investment for exploration, drilling wells, and capital equipment. Exploration risks and environmental impacts are also considered in geothermal power plant projects. Microturbines are a new generation of gas turbines that are small in size, typically producing between 25 and 500 kW of power. The technology is derived from the auxiliary power systems used in aircraft, diesel engines, turbochargers, and automotive designs. It consists of a compressor, combustor, turbine, and generator, as shown in Figure 9.12. Incoming air is compressed to about 3 atm of pressure and sent to a heat exchanger called a recuperate, where hot exhaust gases raise the temperature. The heated steam is mixed with fuel and burned with enough energy to drive the turbine, which subsequently powers the electrical generator. The turbine has only one moving part, which drives the generator at 96,000 rpm on the air bearings and hence does not require lubrication and cuts down on operational/maintenance costs. The generator creates AC current but can be rectified for DC output as needed. It can easily be operated for 60 Hz and reverted to a 50 Hz supply. Microturbines can also easily be started in parallel for increased power output to 30 to 60 kW. In general, microturbine emissions are comparable with those of large turbines. Manufacturers base NOx levels on field tests and projections. Emission control to achieve an acceptable standard is focused on combustion design and flame control. Microturbines are moderately applicable for peaking power but can be used as a stand-alone in limited areas. Their inverter-based generators offer a high premium of power quality. If efficiency degrades as temperature increases, it leads to CO2 emissions, which further degrade its efficiency. It is a moderate fit for CHP applications. Further development of microturbines to lower costs due to electronics, power conditioning, and grid connection are concerns. Applications of fuel diversity (low BTU) and digester gas are under development. Microturbine efficiency is up to 30 to 40 percent for non-recuperated units and 20 to 30 percent for recuperated units. The available natural gas pressure level mostly impacts microturbine efficiency. Further work that hybridizes microturbines with fuel cells will facilitate the generation of additional electricity of up to 60 percent efficiency. There are many models of microturbines produced by a number of manufacturers, e.g., Elliot Group, Ingersoll Rand, Honeywell, and Capstone Turbine Corporation. Microturbines are rated in terms of rated power, fuel input, heat rate, efficiency, emissions (NOx, CO2), turbine rotation, weight, noise, and size. Stirling engines (Figure9.13) [12] are classified as external combustion engines in which energy is supplied to the working fluid inside the engine from a source outside the engine. They are scaled systems with an inert working fluid, either helium or hydrogen. They are generally found in sizes of 1 to 25 kW. The efficiency of stirling engines is typically less than 30 percent. Potential applications include use as small-scale portable power for battery chargers and as co-generators for electricity and thermal/cooling. Stirling engines have low emissions when natural gas is used. Table 9.1 compares the various types of renewable energy we have reviewed in this section. TABLE 9.1 Summary of Renewable Energy Resources *PEM: Polymer electrolyte membrane With the analysis of the energy needs complete, the next step is an evaluation of the power options available. This raises questions such as: The answers to these questions would be individual to the user's circumstance and location. On-site generation brings with it the need for an up-front investment but also a long-term reduction in the consumption of conventional energy and increased reliability of the power supply. Limitations to options available to the consumer stem from factors such as the electricity market structure, which varies by state as well as the availability and quality of resources such as solar, wind, or biomass fuel. Storage technologies are summarized on Table 9.2 as shown below. TABLE 9.2 List of Storage Technologies Penetration refers to the proportion of a power source on a system, expressed as a percentage. There are several ways that this can be calculated, with the different methods yielding different penetrations. It can be calculated as: The level of penetration of intermittent variable sources is significant for the following reasons: Renewable electricity supply in the 20–50+ percent penetration range has already been implemented in several European systems, albeit in the context of an integrated European grid system. In 2010, four German states, totaling 10 million people, relied on wind power for 43 to 52 percent of their annual electricity needs. Denmark is not far behind, supplying 22 percent of its power from wind in 2010 (26 percent in an average wind year). The Extremadura region of Spain gets up to 25 percent of its electricity from solar, while the whole country meets 16 percent of its demand from wind. From 2005 to 2010, Portugal vaulted from 17 percent to 45 percent renewable electricity. Discussion of acceptable or unacceptable penetration figures should be treated and used with caution, as the relevance or significance will be highly dependent on local factors, grid structure and management, and existing generation capacity. There is no generally accepted maximum penetration of wind energy that would be feasible in any given grid. Rather, economic efficiency and cost considerations are more likely to dominate as critical factors; technical solutions may allow higher penetration levels to be considered in the future, particularly if cost considerations are secondary. High-penetration scenarios may be feasible in certain circumstances: Due to high fuel and electricity prices and low supply reliability in developing countries, some renewable-energy technologies are more feasible in some places than others. Another constraint facing renewable energy is the difficulty of adequate funding for renewable-energy projects, some of which can be very cost intensive (e.g., solar). Most developing countries cannot afford to finance significant renewable-energy projects. In most developing countries, government policies are not supportive of renewable-energy development. There is therefore a need for policy review to favor renewable-energy development across the world. Consider a solar module rated 5.3 V, 38 W. If it is used to provide power to an application that needs 30 V and 25 A current, determine Solution: Solar module rating 5.3 V, 38 W Peak rating = no. of modules × rated power = 30 × 25 = 750 W Consider a wind turbine with blade radius 7.2m. At the location of installation, wind speed is 15 m/s for 3 hours and 25 m/s for another 3 hours. Determine the amount of energy that can be intercepted by the wind turbine Solution: Amount of energy intercepted by the wind turbine = 1,030.73 + 4,772 = 5,802.7 kWh. A wind turbine installed in a desert where the average wind speed is 5.5 m/s is to generate power for a small settlement. If the length of each turbine blade is 2.6 m, efficiency of the turbine-gear system is 0.62 while generator efficiency is 0.72. Determine whether the machine will be able to meet community needs if daily energy demand is 45,000 kWh. Solution: where Cp is the Betz limit taken to be 0.59 If the community requires a daily energy of 45,000 kWh, then the daily energy produced by wind turbines is less than 45,000 kWh, hence the wind turbine cannot meet the energy needs of the community. Chapter Summary Renewable-energy systems have attracted a lot of attention in the past decade owing to environmental friendliness and sources that are freely available in nature. In this chapter, we discussed some of the key renewable-energy resources and associated energy-conversion systems. Different evaluation criteria of renewable energy in terms of efficiency, cost, interoperability, etc., were given and serve as a guide for selecting which renewable energy to use in a typical microgrid design. This is important because of the roles of renewable-energy resources in future electric power systems. Because of their easy deployment on a stand-alone basis, renewable-energy resources are readily deployed in a microgrid. Consider the rooftop of a home measuring 15 × 20 m, which is covered with PV cells to provide the electrical energy required by that home. If the peak sun per day is four hours and the efficiency of a solar cell is 22 percent, find the average daily electric energy converted by the rooftop. If a unit of energy costs $0.1, what is the cost of energy harvested by the home in one year? A circular cell has a diameter of 6cm. If its rating at 25° is 1,200mA and 0.45V in full sunshine, estimate the efficiency. Compare the performance of an onshore and offshore wind turbine in terms of return on investment, assuming that the two turbines are the same in all respects. Consider all possible factors and make appropriate assumptions. Consider a wind turbine that is rated at 100 kW in a 10 m/s wind speed in air at standard conditions. If power output is directly proportional to air density, what is the power output of the wind turbine in a 10 m/s wind speed at an elevation 2,000m above sea level at a temperature of 20°C? Discuss the following subjects with respect to renewable energy:9.1 INTRODUCTION
9.2 DISTRIBUTED GENERATION CONCEPTS
9.3 DG BENEFITS
9.4 WORKING DEFINITIONS AND CLASSIFICATIONS OF RENEWABLE ENERGY
9.4.1 Solar
Application
Cost Implications
Modeling
Modified Equivalent Circuit
PV Systems
V–I Characteristics
Future Challenge of Solar Energy
9.4.2 Solar Concentrator Systems
9.4.3 Wind-Energy Systems
Types of Wind Turbines
Modeling of Wind Power
Impact of Tower Height on Wind Power
Emission-Control Technologies for Wind Power
9.4.4 Biomass Bioenergy
Advantages and Disadvantages of Biomass Power
9.4.5 Hydropower
Hydroelectric Plant Size
Problems Related to Hydropower
9.4.6 Fuel Cells
Operation of Fuel Cells
Power and Energy Relationships
9.4.7 Ocean Energy
Supply Characteristics
Technical Challenges and Solutions
Available Power
9.4.8 Geothermal Heat Pumps
9.4.9 Microturbines and Stirling Engines
Description
9.4.10 Stirling Engine
9.4.11 Summary of Renewable-Energy Classifications
Technology
Size Ranges (kW)
Efficiency (%)
Generation Electricity ($/MWh)
Environmental Issues/Emission Control
Reliability Drawbacks on Fuels Source
Reciprocating diesel
30–5,000
26–43
85–90
7.1–14.2
Controls required for NOX and COX
Yes
Turbines/ microturbines
5–10
20–30
60–75
11.9–18.9
Low impact
Yes
Fuel-cell PEM*
1–250
27–40
40–75
21.9–31.3
Nearly zero emissions
Yes
Photovoltaic (PV)
5–5,000
9–14
8–35
18.0–36.3
Zero direct emissions
No
Wind
5–1,000
20–40
20–26
0.2–28.5
Zero direct emissions
No
Biomass
20,000–50,000
5–10
5–20
0.05–0.09
Indirect emission
No
Geothermal
5,000–10,000
5–15
5–25
0.03–0.05
Low emission, only excess steam
No
Ocean
100–10,000
5–45
5–60
5–7
Zero direct emissions
No
Small hydro
100–1,000
50–55
60–90
0.03–0.25
Zero direct emissions
No
Technologies
Properties
Solid-state batteries
Thermal storage
Flywheel
Pumper hydro storage
Compressed-air energy storage
9.5 RENEWABLE-ENERGY PENETRATION
9.6 MAXIMUM PENETRATION LIMITS OF RENEWABLE-ENERGY RESOURCES
9.7 CONSTRAINTS TO IMPLEMENTATION OF RENEWABLE ENERGY
Illustrative Problems and Examples
EXERCISES
BIBLIOGRAPHY