The encasing of wind turbines in a duct to enhance performance is not new. A ducted wind turbine produces more power than an unducted wind turbine of the same parameters. A number of approaches in studying the effects of diffusers and other wind concentrating devices have been done and have resulted in a number of prototypes produced but without any commercialization. The aim of this paper is to investigate the failure of commercialization of ducted turbines. A technical and economic analysis of a ducted turbine is also presented. The work shows that traditional economic methods used to evaluate ducted wind turbines are erroneous; they do not account for external effects of power generation and individual and community benefits derived from this technology. Failure to penetrate the market is due to negative publicity as a result of the erroneous evaluation undertaken and lack of appropriate engineering techniques to protect ducted wind energy systems in extreme wind conditions.
The quest for sustainable and environmentally friendly methods of electric generation and the oil crisis experienced in late 1973 have fueled the shift to renewable energy sources. This has seen a tremendous growth in renewable energy technologies. Global power capacity from renewables increased by 75% between 2004 and 2008 [
South Africa’s energy intensive economy depends greatly on fossil fuels for energy generation and consumption with almost 90% coal based electricity generation and is ranked among the top 20 emitters of greenhouse gasses in the world and as the largest emitter in Africa [
It is from this background that the use of power augmentation devices becomes necessary to exploit low wind speeds and thus meet electrical needs of most rural people. The encasing of wind turbines in a duct or “shroud” in order to enhance their performance dates back to the 1950s [
A common decentralized power supply from small wind energy systems consists of wind electric generator, battery bank, inverter, wind turbine controller, control panels, interconnecting cables, and civil work. Two decentralized small wind energy systems are under construction. One of the systems is a common small wind energy system as explained above and is referred to as SWES in this paper. The other system is the ducted small wind energy system and is referred to as DSWES in this paper. Both systems have a 1 kW wind electric generator. The main difference between the two is that the DSWES has a shroud and its accompanying couplings which houses the wind electric generator. The shroud is being constructed from a 0.5 mm aluminium sheet. Figure
DSWES shroud under construction.
A schematic diagram of a common decentralized small wind energy system (DSWES/SWES) is shown in Figure
Schematic diagram of typical DSWES/SWES.
All the materials used to construct the DSWES/SWES were bought locally except the wind electric generators. The cost of these items was taken as part of the capital cost of the project as explained below. Table
Materials and corresponding costs for the DSWES and SWES.
Item | DSWES | SWES |
---|---|---|
Cost (US$) | Cost (US$) | |
Wind turbine generator | 575.00 | 575.00 |
Charge controller | 250.00 | 250.00 |
Battery bank | 200.00 | 200.00 |
Inverter | 115.00 | 115.00 |
Tower and accessories | 50.00 | 40.00 |
Aluminum sheets | 45.00 | — |
Aluminum bars | 9.00 | — |
Bearing | 5.00 | — |
Bracket | 3.00 | — |
Miscellaneous items (bolts, civil works, cables, labour, etc.) | 101.00 | 45.00 |
|
||
Total | 1 353.00 | 1 225.00 |
The technoeconomic analysis of wind turbines requires that the annual energy delivered by the wind electric generators be determined. The amount of electrical energy produced depends, among other factors, on the speed of the wind. Wind speed is site specific and depends on several geographical and climatic conditions. Wind resource assessment requires some considerable investment of time and money. In this study the mean annual wind speed obtained from our local weather station of 5 m/s and the corresponding full load hours (number of hours in a year that the wind turbine operates at rated power) of 1900 hours were used. The rated power output of the wind electric generator was used to estimate the annual energy delivered by the SWES according to (
Literature has alluded to the fact that ducted wind turbines’ slow commercialization is due to the high cost of energy attributed to this technology. In this section calculations of the cost of energy of a ducted wind turbine and a bare wind turbine are shown, compared, and contrasted. Unlike in conventional power plants, the cost of energy in both DSWES and SWES is determined by two major components, namely, capital costs and operation and maintenance costs. Capital cost comprises the cost of the wind electric generator, wind turbine controller, battery bank, inverter, and miscellaneous (control panels, interconnecting cables, civil works, etc.) and for the DSWES it adds the cost of the shroud (accessories and assembling). As shown in Table
Base values used for the comparison of the LUCE of the DSWES and the SWES.
Input parameter | SWES | DSWES |
---|---|---|
Useful life of wind electric generator (years) | 20 | 20 |
Useful life of wind turbine inverter and controller (years) | 10 | 10 |
Useful life of civil works (years) | 20 | 20 |
Useful life of battery bank (years) | 7 | 7 |
Annual O&M of DSWES/SWES as a fraction of its capital cost | 0.02 | 0.04 |
Discount rate as a fraction | 0.10 | 0.10 |
Source: Nouni et al. [
The comparison of the cost of energy of the DSWES and the SWES was done using the levelized unit cost of energy (LUCE). The following expression as suggested by Kandpal and Garg [
The recovery factor (
The annual energy delivered by the wind electric generator is estimated by [
However, for this study, the rated power (1 kW) of the wind electric generators and the full load hours (1900 hrs) as explained above were used to determine the annual energy delivered by the bare wind turbine and the annual energy delivered by the DSWES is 1.4 times that of the bare wind turbine as explained in Section
One important factor which is thought to have contributed to slow commercialization of ducted wind turbines is the immense wind loading on the duct during storm conditions [
In high winds, conventional wind turbines “feather” away the blades to protect them from damage. This principle does not work in ducted wind turbines since the blades are housed in the duct. FloDesign after its disastrous experience has put flaps on the duct which close down in a storm [
The duct is of high solidity and has poor drag characteristics. This leads to poor response to wind direction changes. The ducted system can turn to face the wind direction but cannot accurately adjust itself if wind direction changes frequently in a short period [
The price of wind energy depends very much on the institutional setting in which wind energy is delivered. This is a key element to include in any debate about the cost of wind energy; however, this element falls away in this study because the institutional setting has been assumed to be the same for both systems. Table
Estimated annual energy output and corresponding LUCE of the DSWES and the SWES.
System type | Annual energy output (kWh) | LUCE (US cents/kWh) |
---|---|---|
SWES | 1 900 | 30 |
DSWES | 2 660 | 26 |
The study shows that the LUCE of ducted turbines compares favourably with that of bare wind turbines. At a wind speed of 5 m/s, the cost of energy from the DSWES is 13% cheaper compared to the SWES. This results from the output energy of the DSWES which is 1.4 times that of the SWES.
The cost of wind energy varies with wind speed. Figure
LUCE for the DSWES and SWES for varying mean wind speeds.
As shown in Table
Annual carbon dioxide avoided from the DSWES and SWES for varying annual energy production.
Producing electricity from wind energy reduces the consumption of fossil fuels and therefore leads to emission savings. As indicated above each kWh of electricity produced from wind avoids 696 g of carbon dioxide. Conventional power plants also emit nitrous oxides, sulphur dioxide, and methane. These wastes are freely dumped but generate costs for others in the form of lung diseases and damage from acid rain and global warming [
Current engineering cost models do not take these savings into account. They also do not consider disease risk reduction from wind energy. If all these factors could be considered the cost of energy from a ducted wind turbine will be even cheaper than that from the bare wind turbine because it avoids more carbon dioxide and saves more water, and its susceptibility to diseases reduction is higher.
The lack of state-of-the-art engineering techniques to protect ducted wind turbines in extreme wind conditions and the inability of ducted wind turbines to accurately follow wind direction changes are thought to have in a way contributed to the slow commercialization of ducted wind turbines.
The study has illustrated that the average LUCE of the ducted wind turbine is US 0.26/kWh compared to US 0.30/kWh of the bare wind turbine. The calculation did not take into account savings in CO2, wastes associated with conventional power production and their related external effects, and saved amount of water. Taking these factors into account would make ducted wind turbines even more favourable. It is regrettable that current engineering economic models do not take these elements into account. Failure of ducted wind turbines to penetrate the market is in part due to negative publicity as a result of the erroneous evaluation undertaken. Lack of appropriate engineering techniques to protect ducted wind turbines in extreme wind conditions and the inability of ducted wind energy systems to accurately follow the wind direction are also thought to have contributed to the slow commercialization of ducted wind turbines.
The authors declare that there is no conflict of interests regarding the publication of this paper.