The Wind Energy Generation Engineering Essay
The prospects of harnessing green power for the rural community and for integration into the national grid system using the low to moderate wind speed available in Nigeria are outlined. For a turbine rotor diameter of 10m, it is possible to optimally generate 8.5 kW and 11.6 kW for the two extreme wind speeds of 2.5 m/s and 4.48 m/s respectively. The economic viability and cost indices for the installation of wind turbine based on the kW and MW of nameplate capacity is also presented. The gradual transformation from vertically-operated power system (VOPS) to the horizontally-operated power system (HOPS) by which distribution network structure translates from passive to active are shown to be some of the benefits of integration of wind power into the grid system. The analyses of national budget allocations to power over the years and the consequent expected increase in power generation show that much money is wasted in the central power system without commensurate improvements in output. The paper highly recommends a shift from investing totally in CPS to implementation of RES, to stem the ‘wind energy phobia’ in Nigeria’s power system structure.
It is no longer a research topic as to the enormity of cost of adding new large central power plants, building new transmission lines and extending the traditional distribution systems. The manner in which to cob the high cost of expansion and yet meet the ever increasing electric energy demand of a growing economy still is a good research area (at least in a developing economy like Nigeria where the conventional generation of electricity still dominates).
The nation’s utility has largely been unable to meet its obligation in distribution and marketing of stable electricity to its numerous residential, commercial and industrial customers. This has been noted to be as a result myriad of problems which include dilapidated and obsolete equipment, lack of equipment maintenance and lack of spare parts, inadequate supply of fossil fuel and vandalism of equipment, illegal connections, high cost of foreign exchange and unbridled monumental corruption . Incidentally, electricity supply programme keep on expanding in the country without necessarily allowing the transmission grids to keep pace with the programme requirements . Besides, many of the associated equipment, machines and other facilities for generation, transmission and distribution had operated for several years beyond their normal life-span without adequate and regular maintenance, servicing and rehabilitation.
Table 1 shows the status of Nigeria’s electricity generation and consumption between 1990 and 1999. From the table it is observed that only between 31.3 to 40.7 per cent of the installed capacity were available for transmission within the period. Of the average available capacities for the years 1990 to 1999, a little above 50 per cent in each case was effectively utilized as a result of the high energy losses in transmission. In 2001, the federal government was prompted to promise an additional 1,485 MW through the expansion of the existing power stations. In 2002, few independent power plants sprang up with a total of about 290.8 MW and sell power to the then NEPA. The nation’s total installed capacity was by this beefed up to about 6210.20 MW. Of this number, the average availability for the year 2002 stood at 3211.44 MW, i.e. just 51.7% of the installed capacity .
Table1: Nigeria’s Electricity Generation and Consumption
2. The Way Forward
The power supply network in the country is centralized, and this has made transmission quite inefficient and expensive. About N10 million or even more is needed to build one transmission tower. With the length and breadth of the country covering over 3000 kilometers, this is by no means a joke. While central power systems remain critical to Nigeria’s energy supply, their flexibility to adjust to changing energy needs is limited. It therefore becomes an uphill task to supply electricity in the conventional power system structure. The increasingly difficulty in solving the transmission system capacity problem, for instance, due to the investment cost, the lack of available physical space for expansion and the rejection from the public , combined with the world-wide trend towards deregulation of the electricity markets created a boost towards the use of distributed energy resources in the power system . In today’s electricity market therefore, the responsibility for new power production units has, as a way forward, gone from central power systems to decentralized generations. Decentralized generation will reduce capital investment, lower the cost of electricity, reduce production of greenhouse gas, and decrease vulnerability of the electric system to extreme vandalism .
In the recent past, the federal and state governments, corporate bodies and individuals have made some attempts or shown some interests to implement decentralized independent power projects (IPPs) in Nigeria. These include but not limited to:
The signing of agreements in April and June 2000, between Nigeria and Italy’s ENI/Agip for construction of a 450 MW IPP in Kwale Delta State, and between Nigeria and US based Enron for an emergency 270 MW power supply project for Lagos state  respectively.
The contracting of Enron in August 1999 by Lagos State government to provide 90 MW power project. The initiation of a plan by Rivers State government to establish two new gas turbine power plants to be located in the districts of Omoku and Trans-Amadi.
The commissioning at Okpai in Delta State of a joint venture 480 MW IPP by NNPC/Agip. The signing of agreement between Eni, an Italian oil and energy company, and NNPC to add 450 MW to the national grid.
The intention by Prof. Bart. Nnaji, in 2005, to end the energy crisis in Aba by accepting to build a 105 MW power generating station in Aba at over 100 million dollars.
All the above mentioned IPPs and majority of others not mentioned are conceptualized with the aim of using the ‘abundant’ natural gas realized from the nation’s crude oil explorations as fuel. One of the prominent argument for choosing gas as the primary energy source is to reduce the incidence of gas flaring. To the extent of hoping to achieve the desired reduction in the gas flaring, to that extent is the consequent building-up of greenhouse gases (CO2, CH4, and N2O) resulting from the crude oil exploration and utilization. A gas turbine produces a high temperature, high pressure gas working fluid through combustion, to induce shaft rotation by impingement of the gas upon a series of specially designed blades. The shaft rotation drives an electric generator and a compressor for the air used by the gas turbine. Pollutant emissions, primarily nitrogen oxides, are a concern particularly as turbine inlet temperatures are increased to improve efficiency. These seaming constraints portend danger for the emerged and emerging IPPs in Nigeria, majority of which are gas-powered.
3. Prospects of Wind Energy in Nigeria
The most favoured sub-set of the distributed energy resources is the renewable energy system (RES). Renewable energy is commonly defined as ‘the energy from an energy resource that is replaced by a natural process at a rate that is equal to or faster than the rate at which that resource is being consumed’ . Wind energy, an example of RES, which derives from wind activity, which is generated by the sun’s uneven heating of the atmosphere, is the focus in this paper. It has been established that the wind power prospects in Nigeria is high with attendant cost effectiveness especially at the Northern boundaries , [9, 10]. Globally, Nigeria is located within small to moderate wind energy zone with average wind speed ranging between 2.5 m/s at the coastal areas to 4.48 m/s at the northern boundaries ; annual average increases from south to north. It is estimated that the maximum energy obtainable from a 25 m diameter wind turbine with an efficiency of 30% at 25 m height is about 97 MWH per year in Sokoto, and about 50 MWH per year in Kano, both in the northern areas, and about 24.5 MWH per year in Port Harcourt, a coastal town.
4. Available Wind and Wind Power Estimation
The power in the wind-energy system is a function of the cube of the wind speed. The anticipation, therefore, is that higher power will be generated in a region of high wind than in a region of small wind. An average wind speed of 13 miles per hour (5.81 m/s) and above are considered good wind resource, while an average wind speed below 10 mph (4.47 m/s) is considered a small wind resource . “Small wind’’, when tied to wind energy systems, starts with turbines with rotors (turbines blades and hub) that are about 2.44 meters in diameter . These turbines may peak at about 1 kilowatt (1kW), and generate about 75 kilowatt-hours (KWH) per month with a 4.47 m/s average wind speed.
On the other end of the ‘’small wind’’ scale, it’s reasonable to include turbines with rotors up to 17 meters in diameter. These turbines may peak at about 90 KW, and generate 3,000 to 5,000 KWH per month at a 4.47 m/s average wind speed. Turbines of this scale are appropriate for very large homes, farms, small businesses, schools, or institutions that use a lot of electricity, village power, and other major energy uses. In between 2.44 meters and 17 meters are various sizes of turbines that can accommodate a variety of energy appetites.
The aerodynamic power generated by a wind turbine is:
Paero = 0.5ρπR2V3Cp(λ) (1)
where R is the rotor radius in m, V is the average wind speed in m/s, ρ is air density (kg.m-3), and Cp is the power coefficient. The power coefficient Cp is inherently dependent on the tip-speed ratio λ, deified as:
where ωr is the rotor angular speed in rad.s-1. For a fixed or constant speed wind turbine considered in the paper, Cp may be estimated as:
The estimated maximum output power of the electrical generator, Pgen, is calculated using the following equation:
Pgen = η(ωrot)Paero (5)
where η(ωrot) is the efficiency of the electrical generator and gear-box as function of the rotor angular speed. Usually, the constant speed wind turbine utilizes squirrel cage induction generator whose rotor speed variations are very small, approximately 1 to 2 per cent. The slip, and hence the rotor speed of a squirrel cage induction generator varies with the amount of power generated.
In this analysis of wind power estimation, data is chosen to reflect the small wind resource as it applies to Nigeria. The two extreme wind speed values of 2.5 m/s and 4.48 m/s are considered. Turbine rotor diameter of 10 meters was used in the simulations. Fig. 1 is the power curve of the small wind system when the wind speed is 4.48 m/s. From the curve, an optimal tip-speed ratio, λopt, of 8.93 which corresponds to maximum power coefficient, Cpmax of 0.4824 is realized and this occurred at rotor speed, ωrot, of 8 rad/s. A similar simulation at 2.5 m/s wind speed produced a power curve that could not be differentiated from the former, with λopt and Cpmax of 9 and 0.4821, respectively. However, the only remarkable difference is in the rotor speed, ωrot which is 4.5 rad/s. Fig. 2 shows the aerodynamic power versus rotor speed for the two extreme wind speeds.
Usually, wind turbines are controlled optimally to maintain the output power even when wind speed is below the nominal wind speed and the power is below the nominal power in a process known as power optimization. Power limitation is adopted if the wind speed or the electrical output power is above the nominal. From Eq. (5), the optimal generator power for the two extreme wind speeds of 2.5 m/s and 4.48 m/s are 8.5 kW and 11.6 kW, respectively. The data used for the simulations are as presented and used in .
Fig. 1: Power curve.
Fig. 2: Aerodynamic power against rotor speed.
5. Economic Viability and Installation Cost
Energy from the wind is a viable candidate for the generation of green power for the rural community and for integration into the national grid system. The energy resource is free from depletion or extinction so long as a good wind site is selected. A propeller type Windmill, installed in Jos, Plateau State, in 1970 for small scale electricity generation, and a 5kW 3-blade horizontal rotor Wind Turbine, installed in Sayya Gidan-Gada, Sokoto State, in 1998, for electricity generation were viable for a number of years but for lack of adequate maintenances and proper skills.
Wind turbines can be found to have many shapes and sizes. To talk about the cost of wind turbine starts with the knowledge of its components.
I. System Components
Wind energy systems, small wind system inclusive, comprise a number of components which together function to provide the needed green power. The wind generator (or turbine) is only one component in a wind energy system, and very often is not even the most expensive component. A complete wind energy system includes:
Turbine – generates electricity using the wind’s energy. It is a collection of blades and hubs they are attached to, both of which are together known as rotor, high speed and low speed shafts, gear-box, and alternator. The rotating blades convert the wind’s kinetic energy into rotational momentum in a shaft. The rotating shaft turns an alternator, which in turn makes electricity.
Tower – supports the turbine, getting it up out of the turbulent zone created by trees and buildings, and exposes the turbine to much more ‘’fuel’’.
Wiring and conduit – carries the electricity down the tower and to power-conditioning equipment.
Controller – controls switching of converters and charging of battery.
Power Electronics Converter – converts alternating current (AC) to direct current (DC), or vice versa. Fig. 3 shows concrete and steel base for the tower and the controller.
Metering – allows user to understand and manage system operation.
Total costs for installing a commercial-scale wind turbine will vary significantly depending on the cost of financing, when the turbine purchase agreement was executed, construction contracts, the type of machine, the location of the project, and other factors . Experts agree that a larger wind turbine will cost less per KW as compared to the small wind turbines [15, 16]. Studies have shown that the economies of scale start to affect the cost per KW from around 500 KW turbines to larger turbines. It means that the larger the wind turbine, the less the cost per KW. Most of the commercial-scale turbines installed today are 2 MW in size and cost roughly $3.5 Million installed. Smaller farm or residential scale turbines cost less overall, but are more expensive per kilowatt of energy producing capacity. Wind turbines under 100 kilowatts cost roughly $3,000 to $5,000 per kilowatt of capacity. That means a 10 kilowatt machine (the size needed to power an average home) might cost $35,000-$50,000. It’s a large initial investment but maintenance costs are generally quite low and, and of course, it pays for itself in the money you are no longer spending on electricity. Most will balance themselves out in less than 15 years, depending on which system you buy (larger ones cost more but pay back faster), and whether or not you are offered government incentives such as rebates .
Fig. 3: Concrete and steel base for the tower and the controller .
6. Wind Power Integration Benefits
The conventional power system (CPS) structure is a vertically-operated power system (VOPS) in which electrical energy generated by a relatively small number of large power plants is transported to the desired end-users. The distribution networks are passive with radial structure through which the electrical power flows from the higher to the lower voltage levels. The global benefit of wind power integration, perhaps, is the possible transition from VOPS to horizontally-operated power system (HOPS) in which in addition to the large CPS connected to the medium voltage (MV) and low voltage (LV) networks, medium- to small-scale wind turbine units (or wind parks) are also connected at the MV and LV networks (distribution systems). In this case, the distribution networks transit from passive to active networks. The transmission system acts as an energy bus that interconnects the different active distribution systems and the remaining large CPS. The integration of wind power for the supply of electricity broadens the energy base and reduces environmental pollution .
The integration of wind power especially in the rural areas will have far-reaching benefits on the socio-economic activities of the rural dwellers:
It will generate short and long term employments, stimulate local economic activities, and thus fix people in the rural areas and consequently mitigate migration to urban centers.
It will usher in sustainable growth in the agricultural sector.
It will stimulate emphasis on small scale and medium scale industries.
It will provide increased access to basic infrastructure.
It will pave the way for reduced mortality and morbidity rates through access to quality health services.
7. Budgetary Allocations and Implementations
Over the years significant percentage of the annual national budget has always been allocated to power in a bid to beef up electric power delivery to the people. Table 2 shows the appropriation bills, the amounts and percentages allocated to power between 2006 and 2010. From the table, it is evident that above the sum of N500 billion has been spent on power in the last five years. The implementation of the budget allocated to power will in this context be viewed in terms of the corresponding increase in the generation capacity within the period under investigation. In the penultimate year (i.e. 2005), the peak generation stood at 3,500 MW, while within the period under investigation the peak generation seldom attained 4000 MW despite the huge investments in the sector. Fig. 4 is the variation of PHCN average generated capacity between 2006 and 2011. Fig. 5 shows the variation of peak generated capacity between January and May in 2010.
Table 2. Budgetary Allocations to Power between 2006 and 2010.
Allocation to Power
Several reasons have been adduced for the inability of PHCN to have matched generation capacity with the amount of investments in the power sector in the last five years:
(i) The existence of in-built minimum of 25 percent distribution losses and 10 percent generation losses in PHCN facilities . That means if say 6,000 megawatts is generated and you subtract 35 percent as losses, you have less than 4,000 megawatts; barring other conditions that are external to the PHCN such as vandalisation, supply of gas and other factors.
(ii) In order to generate 6,000 MW, about 1,424 million standard cubic feet of gas per day will be required . Yet available statistics show that only about 400 to 700 mscf/d is supplied to PHCN.
(iii) Owing to the crumbling of PHCN transmission and distribution equipment having been in use for between 18 and 45 years, if 6,000MW were to be available today, the PHCN infrastructure could only handle 3,500MW. The fragile grid network has been disclosed as the reason for the frequent overloading, system collapses and transmission losses of up to 40 per cent .
Fig. 4: PHCN average generated capacity between 2006 and 2011.
Fig. 5: Variation of peak generated capacity between January and May in 2010.
In 2005, the Federal Government of Nigeria through the Energy Commission of Nigeria (ECN) and with support of UNDP developed the National Renewable Energy Master Plan for Nigeria. In this arrangement, the target is for solar to contribute 5.0 MW, 75MW, and 500MW in 2010, 2015 and 2025 respectively; wind power generation to contribute 1.0MW, 19 MW and 38MW for short term, medium term and long term as in the case of solar . The Master Plan is far from being realistic as higher capacity is much realizable as against the projection.
Table 3 shows the installed wind capacity for China, USA, and our own Egypt between 2006 and 2010. In perspective, South Africa, with a population a third the size of Nigeria’s has a total installed wind capacity of over 40,000 MW launched in the second quarter of 2010. Companies such as Vestas, Goldwind and Wind Prospect have all opened offices in the South Africa. Ironically, while our sister African countries seek to invest in and develop green power, the Nigerian government seek to invest in the build-up of greenhouse gases. In the 2009 fiscal year, the federal government allocated over N2 billion for the purchase, maintenance and fueling of power generators for Aso Rock, the National Assembly and ministries, departments and agencies (MDAs) ; an evidence that investing in the existing PHCN infrastructure with a view to tracking the electric power problem in Nigeria is a mirage.
Table 3: Installed wind power capacity (MW)
8. Conclusion and recommendations
The prospects of harnessing green power for the rural community and for integration into the national grid system using the low – to – moderate wind speed available in Nigeria have been presented. Decentralized IPPs hold the key to a way forward. The prospects of wind energy in relation to the two extreme wind speeds of 2.5 m/s and 4.48 m/s available in Nigeria is investigated through simulation study where it is shown that 8.5 kW and 11.6 kW respectively are realizable. The economic viability, installation cost and wind turbine components are also presented. The global as well as rural benefits of wind power integration are outlined. Budgetary allocations to power, inabilities to meet the ever increasing electric power demands of Nigeria despite the huge investments and the reasons for the failure are succinctly presented. Finally, strong wills of few African countries to develop wind power (juxtaposed with few developed countries) as against Nigeria’s investment in the build-up of greenhouse gases is presented.
Investments in IPPs should be pursued vigorously. Greater percentage of yearly allocations to power should be channeled towards IPPs with renewable resources as the primary source of energy. Investing in IPPs with gas as fuel should be seen as palliative measure as such plants will become extinct when the oil well dries up. The global efficacy of renewable energy is realized when operated in hybrid; in the context of our peculiar environment, therefore, wind-solar hybrid power plant is highly favored and recommended. It is believed that wind power integration holds the key to sustainable generation capacity for Nigeria since the benevolence of the nature is limitless.
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