厄立特里亚风能资源、电力需求及其适宜的风力机组
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Eritrean wind resources and power needs with appropriate wind turbines
BERHANE Tedros， DAI Ren， GUO Xue yan
Abstract：
In this paper， wind resources in Eritrea are analyzed in three regions as the east lowland close to Red Sea，the central high land of mountains and the west lowland. Local annual records of wind speeds are used to compare wind energy potentials.Regional power needs are also predicted for its present status and near future.Eritrean government encouraging policies are introduced together with some possible risks of different cultural backgrounds.Study results show that there exist definite valuable wind resources and local power needs in the southeastern region of Eritrea.Estimated annual wind power production per turbine can reach 2 GWh for an equivalent of 2 500 h of full load per year，in which the ample land can accommodate a numerous turbines to offer more power than the entire country could absorb in the foreseeable future.Technologies of low speed wind turbine for wind speed of 8 m·s-1 is appropriate for this region.
Key words：
wind resources； power needs； wind turbine； Eritrea
摘要：分析了厄立特利亚东部红海低地、中部山区高原和西部低地三个地区的风能资源.采用当地的全年风速数据比较了各个地区的风能潜力.根据目前和未来若干年的用电状况预测了各地区的电力需求.也对厄立特里亚政府的风能鼓励政策和与文化背景有关风险提示作了介绍.研究表明，在厄立特里亚东南部地区存在可观的风能资源并且当地也具有电力需求.估计年发电量在2 GWh，相当于全年满负荷运行2 500 h，当地可以安装大量风力机组，全部发电量将超出厄立特里亚全国在可预见的将来的用电量.风速8 m·s-1 的低速机组是最适合该地区的风电机组.
关键词：风能资源； 电力需求； 风力机组； 厄立特里亚
中图分类号： TK 81文献标志码： A
1Introduction
Eritrea with a population of 4.5 million is located in the northeastern Africa.To the east it faces the Red Sea along its 1 200 km long coast line.To the west，it is boarded by the lowland desert area with a central range of mountains.Eritrea consists of two distinct lowland areas dissected by a central range of mountains.The eastern lowlands are relatively flat desert areas bordering the Red Sea where elevations range from 100 m below sea level to 500 m above sea level.The western lowlands consist of desert，hills，and savanna approximately 500～1 500 m in elevation.Since its special geography of the east low land close to sea and the west high land，seasonal wind energy resources are abundant in the east low land areas.It is anticipated to utilize them to satisfy needs at local and national power grid.Currently，all of Eritrea’s electricity is generated by diesel/fuel oil generation sets and distributed to the major towns and some villages through six independent regional grids.The electric power consumption in Eritrea increased from the 120 GWh in 1992 to the 260 GWh in 2009.[1] Eritrea has almost similar landscape as China，but in China 62.36 GW wind power generation units installed with an annual increase rate of 39.4% annual production of 76 billion kWh power.However，present wind power generation in Eritrea accounts to only 780 kW[2]，far less than 5%.If wind power proportion can be developed in 10% of national power supply，23 000 tons of oil per year will be saved and make a substantial contribution to reduction of CO2 emissions.Wind power is potentially the cheapest source for electricity in Eritrea and its applications are being introduced.Eritrea is fortunate to have some of the best wind energy resources in Africa.Wind measurements carried out over the past years have shown that there is a large wind resource potential，particularly in Southern Eritrea.In addition to this，locations with favorable wind conditions have also been identified in the middle and northern regions of Eritrea.[3] In this paper，it is going to present the natural features of wind resources in Eritrea，evaluate wind power generation potentials and predict power needs and appropriate wind turbine technology.Culture backgrounds and risks of investment are also mentioned for readers.
2Power Consumption
The electric power consumption in Eritrea was 260 GWh，in 2009.As shown in Fig.1，it is increased from 120 GWh in 1992 to 260 GWh in 2009.As it is shown in Fig.2，the value for electric power consumption （kWh per capita） in Eritrea was 51.00 as of 2009.From the year 1992 to 2009 a maximum value of 56.21 in 2003 and a minimum value of 38.63 in 1992 can be found.Normally in Eritrea a village can have 50 to 100 households.One household constitutes an average of 5 family members.The electrical appliances they mostly use are the electric lamp，television，ceiling fan and some families have a refrigerator.Assuming a maximum number of household in a single village，a total of 60.3 MW electric powers will be required annually.The consumption will definitely rise as we consider at a town level as the community is exposed to new electrical appliances.An approximated value of electricity consumption has been shown in Tab.1.
Fig.1
Eritrean annual power consumption
Fig.2
Annual power consumption per capita
Tab.1
Indicator of the running time and power consumption of some commonly used electrical equipment in Eritrea
3Evaluation of Wind Resources
Wind is significantly affected by topography and weather，with seasonal，daily，and hourly variations.Much of the wind resource is located along coastlines （including offshore） and in mountain regions，but significant resources are also found on plains[4].
The national wind resource assessment was one of the initial goals of the Department of Energy Program.Early research in wind characteristics included the development and application of techniques for estimating the magnitude and distribution of wind resource over a selected area.Areas are often described by their “wind class” ranking，rather than their range of Wind Power Densities （WPD） or mean wind speeds.Tab.2 shows the ranges of WPD and associated classes，at 10 m height above ground.It also gives the ranges of mean wind speed for each class.
Tab.2
Classes of wind power density at 10 m height[5]
3.1Eastern and South Eastern Stations
As shown in Fig.3，Eritrea occupies the southernmost 1 200 km of the western Red Sea shoreline，including hundreds of islands near the major ports of Mitsiwa’e （Massawa） in central Eritrea and Aseb （Assab） in the south.All of the stations mentioned in this group are either on the coast of the Red Sea or somehow related to the Red Sea.They are well scattered so as to study the potential of wind along the entire Red Sea coast.The results from the stations are arranged by starting station in the northern tip of Red Sea （Hasmet） and then the station which follow along the Red Sea as you go south[6].
Fig.3
Landscape of Eritrea： three areas of Eritrea are
studied with respect to plain，mountain and offshore regions
As in Fig.4 （to be continued），since Class 2 areas are marginal for utility scale applications but may be suitable for small wind projects.Assab，Idi and Gahro stations are favorable for wind power.But other stations will remain for wind mill and water pumping.At Assab，monthly wind speed is shown in Fig.5 on which it is clear that from May to September，Assab has an average wind speed lower than in the other time period of year.
Fig.4
Mean annual wind speed of eastern and south eastern stations
Fig.5
Monthly wind speed of Assab
3.2Central Highland Stations
Central highland is a high plateau that bisects Eritrea into two with elevation usually over 2 000 m.To the right of this highland lays a wide arid coastal plain that stretches the entire length of Eritrea （bordering the Red Sea）.On the other side there are western lowlands which have a semiarid climate and consist chiefly of desert hills and savanna.In this group，there are 12 stations scattered all over the highland.The results from these stations are shown in Fig.6 [6].
As it is seen on Fig.6，since Class 2 areas are marginal for utility scale applications but may be suitable for small wind projects，most of the central highland stations are favorable for wind power.
3.3Western Lowland Stations
There are four stations in this part of Eritrea.Agordat，Aligdir and Kerkebet have similar weather and they are at similar altitude.Keren is at an altitude almost double of the other 3 places.Despite of Keren’s high altitude，it is Kerkebet which usually scores higher annual wind speed [6].Based on Fig.7，since Class 2 areas are marginal for utility scale applications but may be suitable for small wind projects，all of the western lowland stations are not favorable for wind power rather they still have powerful resources for pumping water.
3.4Wind Power Potential
Wind power density D is a true indication of the wind energy potential of a site than wind speed alone.Its value combines the effect of the wind speed distribution and its dependence on air density and wind speed.Wind power density is defined as the wind power available per unit area swept by the turbine blades and is given by
A1 Adobha；N Nakfa；G Gizgiza；D1 Debresina；E Embatkala；A2 Asmara；D2 Dekemhare；H Halay；K Kahaito；M Mailaham；A3 Areza
Fig.6
Mean average wind speed of Central Highland Stations
Fig.7
Mean average wind speed of western lowland stations
D=0.5ρVm3（1）
where ρ is air density and Vm is mean wind speed value.
Based on the above formula the four regions of Eritrea，the western lowlands，the central highlands，the northwestern and the southwestern lowlands have a wind power density at 10 m hub height as shown in Tab.3.
Tab.3
Population and wind power potentials of
four regions of Eritrea
Considering the population of each region as it is in Tab.3，the west lowlands，central high land，northeast lowlands and southeast lowlands requires a minimum annual power of 0.101 GWh，0.129 GWh，45.77 MWh and 20.3 MWh respectively.
4Eritrea’s Technology Selection and Government Policy
Wind power technology has improved dramatically over time.The capacity factors of California’s wind farms have almost doubled between 1987 and 1990，from an average of 13% to 24% [7].
Wind turbines may be categorized into four size ranges as follows：
micro turbines （Class Ⅰ）：less than 1 kW；
small turbines （Class Ⅱ）：10～100 kW；
medium sized turbines （Class Ⅲ）：100～1 000 kW；
large turbines（Class Ⅲ）：greater than 1 MW.
Microturbines are a relatively new development designed to compete with very small scale photovoltaic technology.Small turbines are popular in both off grid and grid connected applications，with a combined 138 000 units （24 MW of capacity） operating in the United States，China，Great Britain，and the former Soviet Union.Medium sized wind turbines comprise the majority of the world’s approximately 5 GW of utility scale wind capacity，while the large machines are mostly in the development and testing phase.In addition to wind turbines，which generate electricity by definition，there is also mechanical wind systems used primarily for pumping water.In 1993，there were more than one million wind pumps estimated to be operating worldwide，with an equivalent capacity of over 250 MW.This Eritrean wind study focuses on wind electric potential，primarily at the small and medium scales.However，there may be useful applications for microturbines displacing high cost photovoltaic power in village settings.In the Eritrean context，low head water pumps are unlikely to be popular because installation would require monetary charges for an activity that is normally carried out manually.Thus，this option is not currently being pursued.
The major drawback of wind energy is its lack of reliability.It is varying with nature conditions.The four most promising storage methods are compressed air，pumped hydro，flow battery and flywheel storage.Addition of the storage system to the wind farm will definitely increase the initial coast of the investment but it is a must to introduce such storage to have enough energy production in time of high demand.The pumped hydro storage will best suit the Eritrean wind farm considering the proximity of the site to the sea.
The International Electrotechnical Commission （IEC） has developed a classification system for wind turbine systems，which specifies the design conditions for particular wind turbines，as shown in Tab.4.Class I turbines are designed to operate in the harshest climates，with strong annual average wind speeds and turbulent wind.Class II turbines are designed for most typical sites and Class III turbines are designed for low wind resource sites.Typically Class II and III turbines have a larger turbine rotor （longer blades） to capture more of the wind energy at lower wind speeds.They may look more appealing from an energy capture point of view，even at high wind speed sites； but this should not encourage people to install higher class turbines for lower class sites.The class of wind turbine should be selected based on the conditions at a particular site.
Tab.4
IEC wind turbine classification
Combining the information from the wind resource assessment with the information from the load energy budget will provide the needed information for selecting a turbine model and size.This information will need to be shared with several turbine companies to insure that the best turbine for the job can be bought for the best price.Incentives can also play a role in determining turbine size.Based on the International Electrotechnical Commission （IEC） Eritrean wind resources are best for class I and class II wind turbines.
The main energy sector development policies and strategies pursued by the Eritrean government have been aiming at：
（1） Promoting economically and environ mentally sound energy sector development through the application of appropriate technology of energy production，conservation and usage optimization.
（2） Implementing a policy of appropriate energy pricing structures that avoid all forms of subsidy，
（3） Diversifying sources of energy in order to minimize the strategic dependence on the dwindling biomass energy resources and imported oil by way of promoting private capital participation in hydrocarbon exploration and developing renewable energy resources potential.
（4） Modernizing and expanding the country’s power generation and distribution system and creating enabling situation for private participation in energy development and market.
（5） Developing capacity through training and establishment of the necessary institutional and legal framework in order to competently manage the sector.
Investors are also reminded to consider the following risks：
（1） Transporting turbine components to the site may not be easy as the roads size is limited.
（2） Proximity to existing infrastructure including transmission lines and roads with adequate capacity to serve the wind plant.
（3） Scarcity of large cranes during installation.
（4） Community acceptance and compatibility with adjacent land use.Even though，Eritrean’s awareness towards clean energy growing from time to time，education is the most important step to gain community support for a wind project.
5Concluding Remarks
Wind power development throughout Eritrea would probably be very economical，either yielding low electricity costs for government lead development，or potential high returns for private investors.The measurements as reflected in the wind class classification show that Assab and Gahro，close to the border with Djibouti to the south，have among the best potentials in Africa.Windy passes that are suited for electricity generation also exist in the central highland stations.The critical average wind speed for pumping of about 2.5 m·s-1 is available all over Eritrea.Wind power is to be expected to substitute 50% of Eritrean existing thermal power production.
Acknowledgements
We are very grateful to those who made useful comments on an earlier version of this manuscript.Thanks are also extended to Chinese Scholarship Council （CSC） for providing financial support.Furthermore，we appreciate Eritrean Ministry of Energy and Mines for archiving the up to date Eritrean wind data and documents.We also thank for anonymous colleagues for their timely help.
References：
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Received date： 2012-11-12
Biography： Berhane Tedros，male，master degree gratuated student，