By:        Prof. Karina Garbesi              Asst. Prof. Environmental Studies Dept. of Geography and Environ. Studies San Jose State University San Jose CA, 95192-0116 USA              email: k_garbesi@lbl.gov

Karen Kello  Research Assistant  Dept. of Geography and Environ. Studies  San Jose State University  San Jose CA, 95192-0116  USA  email: rosen1@home.com

 

Robert Van Buskirk, Ph.D.*
Research Scientist
Department of Energy P.O. Box 5285
Asmara, ERITREA
email: robert@punchdown.org

*Currently Dr. Van Buskirk is Adjunct Professor of Environmental Studies, Department of Environmental Studies, San Jose State University, San Jose CA, 95192-0116.

1. Executive Summary
2. History & Current Status of Wind Power
2.1.     A Brief History of Wind Power
2.2     Wind Turbine Technology
3. Advantages and Disadvantages of Wind Power
3.1     Resource Availability
3.2     Power Management & Integration
3.2.1         Storage
3.2.2         Back-up Systems and Additional Power Generation
3.3     Environmental Issues
3.3.1         Land Requirement
3.3.2         Visual Impact
3.3.3         Noise
3.3.4         Electromagnetic Interference
3.3.5         Wildlife Disturbance
3.4     Economics
3.5     Production Installation & Maintenance
3.6     Political Issues
4. Eritrea Wind Power Possibilities
4.1     Preliminary Wind Resource Assessment
4.1.1         Objectives
4.1.2         Strategy
4.1.2.1             Identification and Collection of Wind Data
4.1.2.2             Preliminary Wind Power Classification for the Red Sea Coast
4.1.2.3             Baseline Meteorological Analysis of Eritrean Air Motions
4.1.2.4             Ground Search for Promising Sites
4.1.2.5             Follow-up Spot Measurements
4.1.2.6             Correlatoin Analysis of Wind Characteristics
4.1.2.7             Examination of Satellite Images
4.1.3         Results
4.1.3.1             Identification and Collection of Wind Data
4.1.3.2             Preliminary Wind Power Classification for the Red Sea Coast
4.1.3.3             Baseline Meteorological Analysis of Eritrean Air Motions
4.1.3.4             Ground Search for Promising Sites
4.1.3.5             Follow-up Spot Measurements
4.1.3.6             Correlation Analysis of Wind Characteristics
4.1.3.7             Examination of Satellite Images
4.1.4        Limitations
4.2     Energy Applications
4.2.1        The Eritrean Energy Sector
4.2.2        Wind Power Supply to the Electric Grid
4.2.3        Wind Systems for Isolate Development Projects
4.2.4        Wind Electricity for Cooking
4.3     Wind Power Economics in Eritrea
4.3.1        The Effect of Wind Speed on Cost
4.3.2        Variability in Turbine Cost
4.3.3        Land Costs
4.3.4        Financing Costs
4.3.5        Operation and Maintenance
4.3.6        Rough Cost Estimate for South Coast Project
5. Future Work
5.1    Wind Energy Policy Goals
5.2    Site Assessment
5.3    Pilot Project Formulation
5.4    Summary of Future Work
B. Appendix B: Preliminary Spot Measurement Results.
 
 

Eritrea as a relatively poor country which is just beginning its reconstruction process. Because of its depleted biomass resources and a low level of electrification, there is a national need for rapid expansion of both the electricity supply and of energy supply in general. In order to satisfy these needs and the large latent demand for electricity and energy, Eritrea needs to invest in a diversified portfolio of cost-effective energy developments. Our preliminary results suggest that wind energy can make an important contribution to a diversified Eritrean energy supply.

Wind energy has a number of advantages and drawbacks. Primary advantages of wind energy development include both its lower cost when compared to diesel-generated electricity and its lack of carbon emissions (which may allow Eritrea to obtain loan subsidies from industrialized countries). Further advantages include its rapid installation, modular configuration, and utilization of national rather than imported energy resources. Disadvantages include the intermittency of the supply, the unfamiliarity of the technology in Eritrea, and a potentially limited scale of resource development.

Another issue of concern is what type of wind technology should be implemented in the Eritrean context. The current tendency in Europe is the development of very large, technically complicated wind machines with a generating capacity of 500 kW or more. Such machines, because of their more complicated components and large size, will incur higher component replacement and installation costs. Another concern for the larger machines is whether or not current transportation and construction infrastructure in Eritrea can handle the installation in mountainous terrain in a safe and cost-effective manner. Consideration of these issues may lead to the use of smaller wind turbines for initial wind energy developments.

In order to convert the idea of wind energy development from concept to reality, Eritrea needs to engage in a variety of evaluation and planning activities. These include:

Currently, the Eritrean wind resource appears very promising, more than justifying continued study. Potential applications of wind energy development in Eritrea include not only utility-scale generation, but also electricity supply for small-scale isolated development projects, and fuel substitution for household cooking. The fact that the base supply of electricity in Eritrea will be produced by diesel generators during the next decade also increases the competitiveness of wind generation for electricity production. Another specific factor favorable to wind development in Eritrea includes the positive correlation between hourly wind speeds and hourly electricity demand. Electricity produced during periods of peak demand have added value due to the contribution they make to peak generating capacity and electric supply reliability during periods of greatest need.

To date, two regions of potential wind energy development have been identified: (1) the Southeastern coastal zone, and (2) Highland passes. The feasibility of the Southeastern coast for wind energy development seems clear at this stage based on data available from U.S. Navy studies of the meteorology of the Red Sea. Not enough data is available from potential sites in the Highlands to determine their feasibility with certainty, but spot measurements of afternoon dry season wind speeds ranging from 8 m/s to 15 m/s for some sites indicates a probable potential.

The results of this preliminary assessment provide justification for continuing more detailed work on the identification of sites for wind energy development projects. Detailed monitoring of potential sites should be able to begin in approximately six months if sufficient ground survey work can be performed to verify the potential of the identified high-wind locations. Furthermore, considering the wind energy potential indicated by this preliminary assessment, it is now important to develop the policy criteria with which wind energy projects are to be evaluated. A defined policy combined with background information to be collected on costs and technical specifications of wind energy equipment will lay the foundation for quick and efficient evaluation and implementation of wind energy pilot projects. If such pilot projects prove to be a success, then Eritrea may find that a larger-scale of wind energy development is in the nation's best interest. It is then possible that future large scale wind energy developments will make an important contribution to the reduced cost, quantitative expansion, and increased reliability of Eritrea's electricity production. Such improvements in Eritrea's electricity supply, if attainable, will benefit the national economy, the post-liberation reconstruction process and the Eritrean people.

 
II. HISTORY & CURRENT STATUS OF WIND POWER
 
 A. A Brief History of Wind Power

In the past 15 years utility-scale wind power has been transformed from an expensive, experimental, alternative energy technology, to a fully mature energy option. Installed wind capacity worldwide has increased from only 10 MW in 1980 to almost 5 GW by the end of 1995 (See Figure 1: World Wind Capacity). This exponential growth in wind capacity was spurred not surprisingly by dramatic reductions in the real cost of wind power (See Figure 2: Declining Costs of Wind power in California and Denmark). It is important to note that wind power achieved these great reductions in costs in two short decades with only a fraction of the government subsidies that have been poured into the development of nuclear and fossil fuel technologies over a much longer time period.

With an estimated cost for new installations of between 4 cents and 7 cents/kWh at appropriate sites, wind is now cost competitive with installed power from most energy sources (See Figure 3: Levelized Cost of Electricity) and the cost is expected to be substantially lower a decade from now [Swezey and Wan, 1995] . However, new combined-cycle (gas-turbine/steam turbine) generators are now dramatically lowering the cost of new power generated with natural gas or gasified coal. Cost estimates for new combined-cycle natural gas generation are as low as 2.5_/kWh, forcing wind to yet again reduce its target generation costs. It is important to note, however, that those costs apply to countries with an installed infrastructure for coal and natural gas (which Eritrea does not have). Furthermore, although fossil-fuel power generated with combined cycle technology is less damaging to the environment than conventional generation from fossil fuels, it is far more damaging than wind power. Therefore, it is also important to note that these costs reflect neither the environmental or public health damage from emissions of carbon dioxide and volatile organic compounds. In addition, wind power decommissioning costs, ultimately paid by society, are low for wind power when compared to decommissioning costs for other sources, especially nuclear power (which are not factored into the cost of nuclear energy).

Early development of modern utility-scale wind machines occurred primarily in California, spurred by attractive state and federal tax credits and concern for the environment. Up until 1994, California still maintained over half of the world's utility scale wind power. By 1995, 22 countries possessed more than 1 MW of installed capacity (See Table I: Installed World Wind Capacity by Country, 1980-1995 and Figure 4: Percentage of world wind capacity by country), with Europe holding just over 50% of the total [(IEA), 1996] . Of the European installed capacity, almost 90% was located in four countries: Germany, Denmark, the Netherlands, and the United Kingdom - with Germany having the lion's share.

Recently, a booming market has materialized in Asia, particularly in India, and to a lesser extent, in China. Although the 340 MW of new capacity installed in Asia in 1995 represented an enormous fractional increase of 160% in one year, the absolute increase was dwarfed by European additions of 781 MW [Anonymous, 1996] . The vast majority of Asia's current installed capacity is in the new projects in India. A number of analysts are concerned that the development in India may be too rapid to ensure that an adequate institutional structure is in place to protect the long term stable development of the industry.

While the European and Asian markets have been booming, the US market has declined precipitously in the last year. The decline has been attributed to various factors including a sharp fall in wind energy purchase prices in California, when many 10-year power contracts signed in the early 1980's terminated. Energy purchase prices have dropped from over $0.10 per kWh to below $0.03 per KWh. For some wind plants this resulted in a sudden 80% reduction in wind energy sales revenues. These low revenue levels that could not justify new investments, and even made it economically unjustifiable to replace old equipment. This has resulted in a large number of used and refurbished wind machines being available for relatively low prices.

The belief in the 1980's was that conventional energy prices would continue to rise. Instead they dropped in real terms, especially for new installed capacity of combined cycle gas plants, which has left wind vulnerable despite enormous cost reductions of the past 15 years. At the same time, ongoing discussion of deregulation of the electrical utilities industry in several states and at the federal level has created uncertainties for investors, and resulted in utilities abandoning many programs for renewables and energy efficiency.

The net effect of the great instability in the California and US markets has been punishing for US wind turbine manufacturers and wind power companies. Significant layoffs have occurred in the wind industry, and Kenentech Windpower, previously the world's largest developer of wind energy systems, filed for bankruptcy. Admittedly, Kenentech's problems were clearly exacerbated by overly ambitious investments in their 33-meter (blade diameter) variable-speed wind turbine, which had serious, unanticipated equipment failures in the field.

The prospects are now mixed for renewables in general, and for wind in particular in the US. Wind investments by the public sector were fueled by environmental concerns, however the current signal of the public's willingness to pay for environmental protection is unclear. It was industry's cry for low energy prices that spurred deregulation despite the fact that, in real terms, electricity prices have remained relatively stable for decades. This past year, there was enormous outcry and government concessions when gasoline prices rose abruptly, but modestly. Yet recently the US has committed to binding legal agreements for carbon emissions reductions, in the context of the United Nations Convention on Climate Change [Cushman, 1996] .

Although the rapid growth in the European market is expected to slow soon as public objections to wind machine appearance and noise grow [Garrad, 1996], the growth in the Asian Market is expected to remain strong. The enormous number of recent wind energy assessments for lands near the Mediterranean and Red Sea alone suggests rapidly growing interest among many smaller energy consumers in the region [Radhwan, ; Habali et al., 1987; (NREA), 1989; Pallabazzer and Gabow, 1991; Katsoulis and Metaxas, 1992; Katsoulis, 1993; Haralambopoulos, 1995; Incecik and Erdogmus, 1995; Pashardes and Christofides, 1995; Tolun et al., 1995; Turksoy, 1995] .

 
 B. Wind Turbine Technology

Wind power technology has improved dramatically over time. Modern wind turbines are 10 times more efficient at capturing the wind's power than the early Dutch windmills with the same rotor diameter [Gipe, 1995]. In more recent history, the capacity factor of California's wind farms has almost doubled between 1987 and 1990, from an average of 13% to 24% [Cavallo et al., 1993] .

Wind turbines may be categorized into three size ranges: microturbines (<1kW), small turbines (10s of kW), medium-sized turbines (100s of kW), large turbines (>1MW). Microturbines are a relatively new development designed to compete with very small scale remote solar PV at low cost. Small turbines are popular in both off grid and grid-connected applications, with an estimated 138,000 units (24 MW of capacity) operating in the United States, China, Great Britain, and the former Soviet Union alone. The medium-sized wind turbines make up the bulk of the world's approximately 5 GW of utility scale wind capacity. The large machines are mostly in the development and testing phase.

In addition to wind turbines (which by definition generate electricity), there are also mechanical wind systems used now 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 project focuses on wind electric potential, primarily at the small and medium scales. However, there may be useful future applications for microturbines displacing high-cost PV 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 by "free" labor. Thus, this option is not currently being pursued.

Most of the early development of modern utility-scale wind machines occurred in California in the late 1970s and early 1980s. Early experiments with multi-megawatt machines failed, leaving most of the successful machines in the field below the 200-kW range. In recent years, in the US, the trend has been toward somewhat larger machines, ~300 - 500 kW. In Europe, due in part to the lack of abundant and inexpensive land, there has been a long-term R&D commitment to larger MW-sized turbines. This interest waned briefly in the first half of the 1990's, but has been enthusiastically embraced once again [Garrad, 1996] . These larger machines, in US and Europe tend to be considerably more complicated, mechanically and electronically, than their smaller counterparts.

Wind turbines may have either a horizontal or a vertical axis. Most modern machines are horizontal axis and have two or three blades, although there are still a number of vertical axis machines-mostly of the two-bladed Darrieus design-in operation. Being symmetric about the vertical axis while spinning, the turbine of vertical axis machines is insensitive to changes in horizontal wind speed direction. In contrast, horizontal axis wind turbines (HAWT), must have the plane of the rotor perpendicular to the wind to capture its power. Modern HAWT systems are therefore designed so that the horizontal axis can rotate about the vertical axis of the tower on which it is mounted. This rotation is referred to as "yawing".

Wind machines may yaw passively or actively. In passive-yaw machines the rotor aligns itself downwind of the tower. In active yaw systems, the turbine orientation is computer controlled to be always into the wind (experiments with passive aerodynamic control of upwind systems have not been highly successful) . These systems are therefore alternatively referred to as down-wind or up-wind systems, respectively. The justification for the additional cost and complexity of the up-wind systems is the avoidance of power reducing turbulence down-wind of the tower.

Another new option in wind machines, allowed by the current sophistication and reduced cost of electronics, is the variable-speed generator, which generates electricity at variable rotor rotation speeds. The argument for variable speed machines is that they can operate over a wider range of wind speeds, thereby increasing the capacity factor of the system and reducing the costs. In addition, running the turbine at variable speed reduces the mechanical stresses on the rotor and drive train, which is hoped will increase the life span of the system. These machines were introduced into the field in the early 1990s and offered great hope for further reducing wind power costs (primarily by increasing capacity factors). Now that there are field data available on variable speed machines, it is not clear that those gains have materialized [Milborrow, 1996] . Critics claim that, although the variable speed machines do indeed generate power over a larger range of wind velocities, their capacity factor is reduced by low availability (because of repair and maintenance) and inability of the active-yaw system to track the wind rapidly enough given their enormous inertia. It is yet unclear whether these problems can be eliminated so that the full potential of this technology can be captured.

Another factor that varies among turbines is whether the pitch of the blades is fixed or variable. Altering the pitch of airfoil blades alters the relative lift and drag forces, thereby changing the amount of power the turbine extracts from the wind. Lift is increased to allow start up at lower wind speeds, or decreased to dump excess power to protect the turbine in high winds and to provide aerodynamic rotor breaking. Pitch control mechanisms experience very large forces at the blade-hub connection and thus must be carefully and robustly designed. Fixed-pitch blades, also known as stall-controlled blades, naturally lose lift in high winds when the laminar flow over the blade surface becomes separated and turbulent. Stall-controlled systems are attractive for their simplicity, but do have a number of limitations: they experience larger stress loads at high wind speeds than variable speed machines and must have additional means of aerodynamic or mechanical breaking. [Cavallo et al., 1993]

To us, the important question is: Which technologies are most appropriate in the Eritrean context? Choosing among the enormous diversity of machines could be a bewildering task. Fortunately, there are factors that can be useful guides. The very large machines (500 kW or more) are likely to be unsuitable for a number of reasons. First, their large size makes them unsuitable for transportation on small roads. A 500 kW turbine has a rotor diameter of about 35 meters, compared to a rotor diameter of only about 18 meters for a 100 kW turbine. Similarly, installation of the larger machines requires the availability of very large cranes. Finally, the use of the new variable speed machines seems imprudent at this point given their limited field experience and their relatively unimpressive performance to date. In general, it appears prudent to opt for the smaller range of medium sized machines (~80 - 200 kW) with simpler technologies and long field experience. Variable pitch machines, although more complicated than fixed pitch machines, might be the exception since they have received considerable field experience (22 of the 35 models of wind turbines in the Altamont Pass in 1992 are of variable pitch design).

When considering the purchase of individual machines, there are some compiled data available that should prove useful. Lynette (1989) performed a detailed evaluation of the operation and maintenance of 4,500 turbines installed in California between 1981 and 1987. Current information is available in quarterly reports entitled the "Wind Stats Newsletter" on the performance of 7000 individual turbines installed in 6 countries in Europe (Wind Stats Newsletter, Vrinners Hoved, 8420 Knebel, Denmark, Fax: +45 86 36 56 26, Tel: +45 86 36 59 00). The data for Denmark and Germany are available in digital form. In addition, the California Energy Commission compiles data on turbine performance from field experience in the California.

But storage issues arise only when more than 30% of the energy supplied is from intermittent renewables. Below this level the supplied energy provides a variable, negative load, and as such reduces the fuel consumption requirements on the other generators in the system. The energy provided by the wind resource is essentially free and inexhaustable except for the capital, maintenance, and repair costs for the turbine and grid integration electronics.
 

If the installed wind capacity is larger than this, then the intermittence problem may be reduced by having many wind turbines spread out among different sites in different wind regimes. This will smooth the overall output. In addition, if energy demand is not well correlated with the timing of wind supply, financial incentives can be used to shift demand to periods of larger wind power supply. Alternatively, back-up power systems and/or storage facilities can be installed.

 
Storage

Since wind power is an intermittent energy source, some method of storing energy is desirable for high wind penetration systems to ensure adequate supply during peak consumption. Such storage at present is probably too costly to be feasible for large-scale projects. Batteries are the simplest method of storing energy, but can be large and costly. Other possible methods include using excess energy to pump water from a low reservoir to a high reservoir during times of low demand, and then allowing the water to flow back down through a turbine to generate electricity when it is needed. Storing excess energy as compressed air in subterranean chambers has also been shown to be feasible, and may be less costly than storage using the elevation of water [Cavallo, 1994)]. Other possibilities for future utility-scale wind generation include fly-wheel storage (for short term variations) or chemical energy storage in the form of hydrogen fuels. The latter may be a particularly promising application for coastal wind plants in the long-term future.
 

Back-up Systems and Additional Power Generation

Another solution to intermittency is the installation of additional power sources, either as backup systems or to work in conjunction with the wind system. This increased diversity increases system reliability considerably. Consistency is best improved through the use of hydropower generators and gas turbines when available. Because they can be quickly turned on and off in response to wind power fluctuations. Diesel generators can also be run when wind velocity is low and stored electricity is not available, but their response time is not as quick as hydropower and gas turbines.
 

Compared to most other sources of energy, wind energy is environmentally benign. No air pollutants, radioactive materials or any hazardous substances result from the production or use of wind turbines. That is not to say, however, that wind energy does not have any negative qualities. Following is a list of negative environmental impacts commonly associated with wind farms.

Land requirement

Because of the nature of wind, many machines must be installed to catch large amounts of energy. In addition, these machines must be spread out over a large area, to ensure that the turbines do not interfere with each other. Turbines in large wind farms are typically separated by 5 to 10 rotor diameters, each turbine occupying an area of about 50 square diameters of land. Although these requirements seem demanding at first glance, they actually compare favorably with land requirements for coal or nuclear generation when the full life-cycle requirements of mining, processing, transportation, and waste disposal are considered.

Field studies in California indicate that wind projects occupy about 7.1 ha (0.071 square kilometers) per MW [Gipe, 1991]. Using these figures, a wind farm which produces power at the rate of an average nuclear power plant (about 1 GW) may require an area of over 70 square kilometers. However, since only about 10% of this area is actually disturbed by the turbine base and access roads, the remainder can be used for farming or grazing. In California, the addition of wind turbines actually increased revenues and land values for the farmers who owned the land, allowing ranching which was otherwise not economical to continue.
 

Visual impact

Because wind turbines are typically between 40 and 70 meters in height, a significant impact of a large number of turbines in an open area is unavoidable. However, the effect of the turbines' presence is subjective and opinions vary widely. Surveys have shown that uniformity of size and spacing improves aesthetic acceptability considerably [Clarke, 1991]. In addition, tolerance of other large structures such as pylons, utility towers and telephone poles suggests that negative opinions may be more a matter of habituation than the appearance of the objects involved.
 

Noise

Noise created by gearboxes and blades can be a problem in densely populated areas, and can limit settlement within audible range. This issue has received considerable focus in densely populated Europe where wind development has been very rapid in recent years. Recent studies show that the use of large wind turbines over medium sized turbines limits noise output per watt, but these large machines have many other drawbacks discussed in the section on turbine technology. Although people do not generally choose to settle in areas with unusually high wind speeds anyway, care should be taken to ensure a distance of several hundred meters between the turbines and the nearest habitation. Placing turbines some distance from residences is useful for both noise and safety reasons.
 

Electromagnetic interference

To avoid problems of interference with vital radio transmission services such as aircraft navigation systems, the relative locations of such facilities and wind turbines should be studied carefully. Where potential for problems exists, careful choice of turbine design and materials can reduce radio interference, or, a booster receiver / transmitter can be installed.
 

Wildlife Disturbance

At this point, birds seem to be the only wildlife adversely affected by wind turbines, and even this disturbance is rarely a problem. Studies show that, in most cases, the effect is negligible under conditions of good visibility. At Altamont Pass in California, however, bird strikes occur at a rate of about one per month, which is often enough to raise concerns over the issue. This problem can be limited by avoiding known bird migration routes, and perhaps more importantly, locations of known rare and endangered species. Considerable research is underway on rotor and tower modifications that might be used to limit the disturbance.

It should be noted that studies in the United States and Europe indicate that bird deaths attributable to collisions with wind turbine rotors are significantly lower than deaths from collisions with motor vehicles and windows. Also, environmental benefits provided by wind power over nuclear and fossil fuels also accrue to birds.
 

Optimal conditions for the economical development of wind power include high wind speeds, low interest rates, and low labor costs. Due to the high capital costs associated with wind development, no country has successfully built up a wind generation industry without subsidies. At over $1,000 per installed kW, high capital costs can be a real problem when such capital or loans are not available, or where interest rates are high. However, because wind power is a clean energy source, loan subsidies may be obtained from industrialized countries to help in financing development.

The relatively low cost of decommissioning of a wind power plant is one of its often unaccounted for economic advantages. Decommissioning costs for a medium-sized wind turbine (including removal of the turbine, tower, foundation, and wiring, and re-vegetation of soil) is estimated by the US Bureau of Land management at about $50 per kW [Gipe, 1995]. In addition, the wind decommissioning costs cited here may be significantly overestimated by not taking into account the fact that old turbines can be upgraded by modern turbines at the end of their useful lives, reusing the tower and platforms, as well as recycling many of the other parts.

Finally, unlike many other capital-intensive energy technologies, such as solar or geothermal, wind plants require a fair amount of labor. According to the Worldwatch Institute, results from California indicate that wind is the most labor intensive of the energy technologies it examined. This is a disadvantage in that it implies a fairly intensive maintenance and operation routine is required. It may be a benefit in the case of Eritrea in that labor costs are low, and as such maintenance and operation may cost less in Eritrea and provide additional employment.
 

Regular maintenance, consisting mainly of simple procedures such as lubricating turbine parts, can be carried out by the on-site operator, but visits which require parts or all of the turbine to be lowered to the ground may require assistance. Thorough maintenance checks should be made at least once per year, more frequent visits are recommended, especially before and after severe weather conditions. Maintaining a log of the condition of fasteners and any corrosion is recommended, especially in coastal areas where salt and moisture can be especially corrosive.

For more remote populations, roads from larger cities to remote villages may need to be built or improved. Larger wind machines must be delivered by truck, and future maintenance may require roadways also. Of course, small wind machines may be delivered to remote locations by helicopter or airplane, in which case roads would be unnecessary. Telephones or some other method of communication would also be helpful for maintenance.
 

Approaches to wind mapping depend on the nature of regional wind flows, the complexity of the terrain, and the availability of a reasonably dense network of meteorological monitoring stations. In regions with very simple terrain, predictable regional wind flows, and reasonably dense monitoring networks, wind maps may be generated using simple extrapolations of meteorological station data. This was the approach taken in the wind assessment of Somalia (Pallabazzer and Gabow, 1991).

For somewhat more complex terrain, station data may be extrapolated using simple meteorological models that account for the effect of topography on flow. Such models are commercially available (WAsP from RISOE, Denmark), but are only applicable for gently sloping terrain in regions with well-defined regional winds (geostrophic winds) (Petersen et al., 1994). They use linearized versions of the equations of fluid dynamics which cannot simulate conditions in Eritrea, where thermally driven winds disrupt regional wind patterns and extreme elevation changes in the highlands channel flows and concentrate wind energy. Here, locations only a few kilometers apart may show little or no correlation in wind speed and direction. Standard methods are not available for such sites. Approaches in these regions range from applying meteorological intuition, given information on topography and regional wind flows, to using state-of-the-art mesoscale models employing the full set of non-linear equations of fluid dynamics. In general, these models have not been validated under the extreme tropical conditions present in Eritrea.

For these reasons, it is sensible to pursue a wind resource assessment in stages in Eritrea, where the ensuing stage is pursued if the previous stage provides promising preliminary results. Figure 5 is a schematic presenting the different phases and the interactions between them. They are briefly described here.

There are several distinctive factors that might justify Eritrea taking a relatively advanced position with regards to wind energy development. First and foremost, Eritrea's economic development is constrained by the lack of long-term investment capital. Due to current political concerns with regards to global carbon emissions, wind energy development will provide access to capital funds at subsidized rates (perhaps even as grants) that otherwise would not be available for electricity development. And since energy supply is also one of the major constraints on national economic development, such additional investments will have an economic multiplier effect in addition to increasing the pace of national reconstruction.

Second, the main constraint on wind energy development in the economically more advanced countries is the competition from natural gas generators [Grubb, 1993]. Because of the very high capital cost of transportation and distribution facilities for natural gas, such generators are not a feasible option for Eritrea in the next decade. Meanwhile, compared to electricity generation from diesel and fuel oil which costs $0.065 per kWh, standard cost estimates indicate that for good wind sites, electricity generated from wind is cheaper than electricity generated from diesel

Third, Eritrea has taken steps to perform a wind resources evaluation which is a major constraint on wind energy development [Grubb, 1993]. The lack of wind resources evaluation expertise and technical capacity in wind electricity generation is probably one of the main factors limiting wind energy utilization in developing countries. Current technical capacity building activities are removing this constraint for Eritrea.

Fourth, Eritrea has some unquestionably favorable wind resources along the Southern coast and may have other good sites in highland passes in the center of the country. For highland sites, some of the physical properties of the wind resource increase its potential value. These include a highly periodic wind pattern with low variability [Yosief, 1996], and a positive correlation between wind power availability and electricity demand. A positive correlation between wind power and demand is a rare quality of wind energy resources [Grubb, 1993].

Due to these unusually favorable factors for Eritrean wind energy development, it may be to Eritrea's advantage to plan a relatively high level of wind power penetration for the energy sector. What remains is the technical question of the economic and physical feasibility of wind energy development in the Eritrean context.

In the evaluation of wind energy feasibility in Eritrea, several subsidiary technical questions arise:

Virtually all electricity is generated with diesel and fuel oil with perhaps 100 kW of solar electric capacity in remote areas. There exist both central generators in the 45 Megawatt Interconnected System (ICS) with the rest being generated in small to medium-sized towns which are part of the 15 MW Self-Contained System (SCS). The cost of electricity production in Eritrea is approximately Birr 0.45 or US$ 0.065. As much as 80% of the population is without electricity, and one of the national priorities is to expand electricity supply by at least 6% per year. From 1992 to 1995 actual electricity consumption has expanded at a rate of 10% per year. These expansions are occurring primarily through improvements and expansion of the ICS because the economies of scale will provide for greater efficiency in a central electrical grid. The Eritrean government has approved the construction of a 84 MW heavy fuel oil fired power plant which is expected to be completed at Hirghigo, near Mitsiwa'e in 1998. If Eritrea decides to invest in a wind electricity generating capacity, it could be used to provide a 10%-20% expansion of the electric supply after 1998.

In more isolated locations, there exist selected PV applications for systems of 3 kW peak demand or less, while most larger applications use small-scale diesel generating units. The solar electric systems in these projects have a very high capital cost of over $7 per watt of peak capacity. Meanwhile, the diesel systems suffer from high operational costs and fuel supply problems. For those sites with good wind resources and for applications that can utilize intermittent electricity supply, wind generators may provide a more economical source of electricity with capital costs of $1-$4 per watt of peak generating capacity for systems in excess of 10 kW capacity.

In the biomass sector, because of shortages and environmental degradation produced by the collection and use of biomass fuels, current national policy is to replace biomass use in the household energy sector with Liquefied Petroleum Gas (LPG) which is viewed as the most economical and desirable replacement fuel.

Therefore; corresponding to these three active areas of energy development, there are three priority areas of wind energy development in Eritrea:

For expansion of electricity supply for the ICS, wind farms would be developed near the ICS that would feed electricity into the central grid. Being an intermittent and variable supply of electricity, only a portion of ICS demand could feasibly supplied with wind energy. But if wind energy supply has a lower net cost of production than diesel generation, then it can provide generation cost reductions proportional to its fractional electricity supply contribution.

There are also specific applications in isolated locales where wind electricity supply will be the most economical option in high wind locations when compared with solar PV and small scale diesel generators. Feasibility evaluations and cost analyses of these smaller-scale applications together with the development of a technical support infrastructure can aid in the implementation of less costly and more efficient smaller-scale rural development projects in such isolated areas.

And finally, it may also be feasible to use wind generated electricity to provide energy for household cooking applications. This application surprisingly may be competitive with LPG supply under certain conditions due to peculiarities of the Eritrean cooking sector.

These three possible wind power applications will be discussed in more detail below.
 

Preliminary identification of some potential wind energy sites in the central highlands region has been made, based primarily on a few spot measurements and biological indicators. The most promising site so far is the mountain pass at Dekemhare at which spot measurements during the dry season indicate wind speeds of 8 m/s to 15 m/s (at 2.5 m. height) during the day. If the daily wind pattern for this location is similar to that of other highland areas, then this implies a mean wind speed in the range of 5 m/s to 9.5 m/s during the dry months. Spot measurements during the rainy season indicate a distinct decrease in wind speeds during June to September (similar to the decrease in winds at Aseb during this time), with mean wind speeds of only 3 m/s to 5 m/s during the rainy season. Therefore, preliminary indications from these spot measurements are that overall mean speed at 10 m. height for the better highland spots is likely to be in the range 5 m/s to 10 m/s. Feasible wind generation sites have mean wind speeds in excess of 6 m/s at 10 meters height. Hence it is likely--but not certain--that feasible wind generation sites exist in the highland passes near the main electricity load in Eritrea. Other sites that show promise based on cursory evaluation and biological indicators include Ma'ereba, Nefasit, and hills near Adi Teklezan.

For highland locations, daily fluctuations in wind energy supply seem to be fairly well correlated with load. Typically in the highlands the winds abruptly begin at 8 am in the morning, reach a moderate value before 10 am, and then gradually increase until 3 p.m., after which they decrease to their low night-time values sometime after 7 p.m. Electricity loads begin earlier than wind power supply, but during most of the day, they appear to be fairly well matched since peak loads are in the midday to evening times as is peak wind. The biggest drawback of wind power supplies may be their low production during the rainy season. There is relatively little seasonal variation in Eritrean electricity demand, and the seasonal variations in wind power will produce a significant mismatch between wind electricity supply and demand.

Whether or not wind generation should be used to supplement power supply in the ICS depends on the location and confirmation of nearby feasible wind generation sites. It is likely, but not certain, that such sites will be found in highland passes in Eritrea. These sites are meteorologically similar to the wind generation sites in California which are created by the airflows generated from sea/land temperature differences of the order of 10° C. Such wind-generating temperature gradients also exist in Eritrea during the dry season. For ICS power development prospecting and verification work for feasible sites in the Central highlands needs to continue.

Meanwhile for components of the SCS in other areas of Eritrea the prospects are mixed. For sites along the Southern Red Sea coast wind electricity seems feasible, and the remaining question is which of the several possible generation sites are best. More studies need to be performed examining the detailed wind distributions both North and South of Aseb to locate the most beneficial sites. In the Southwestern Lowlands, no indications of feasible wind sites have been found, while for the Northern mountains windy passes like those in the Central highlands may very well exist. The Northeastern coastal plains seem clearly unsuitable for wind power development while the far Northwestern lowlands remain unexplored. Therefore components of the SCS which could benefit from wind energy developments include Aseb, and potentially Nacfa. Elements of the SCS in the Central highlands will soon be integrated into the ICS, and hence do not warrant individual wind power development investments. Supplement of the Western components of the SCS with wind power appears less likely since wind speeds appear to be consistently moderate in that region.
 

For current solar PV applications, the capital cost of the power supply is in excess of $7 per watt of peak supply. Furthermore, the average utilization or capacity factor for such solar systems is approximately 20% of peak supply. In contrast for larger scale wind systems the capital cost of the system is approximately $1 / watt peak supply, while for smaller-scale systems the capital system cost is $3-$10 per watt. The capacity factor large-scale wind systems is about the same: 20% of peak supply for sites with mean wind speed of 6.6 m/s (& mean wind power density of 333 W/m²). Therefore, for good wind sites, wind electric supply may be approximately half the expense of solar systems for small-scale systems.

The constraining factor for small-scale wind applications is the availability of `good' wind sites. A poorer wind site will have a smaller capacity factor than a good wind site. This means that the amount of power actually generated will be a smaller fraction of the rated peak generation capacity. The power generated is approximately proportional to the third power of the mean wind velocity. We can therefore approximately calculate the mean wind speed for sites which will have electricity generation costs which are equivalent to solar PV.

        Vav = 6.6 m/s * ([$3/watt]/[$7/watt])^(1/3) = 5 m/s

Sites with such mean velocities may be found all along the Eritrean coast, at many sites near passes in the highlands and in regions of the Western lowlands. Such sites may exist in regions with predominant wind speeds of 3 m/s which have local topographic features than can accentuate and accelerate prevailing winds. According to Bergey Windpower Co., a major small-scale windturbine manufacturer, feasilbe wind speeds can be as low as 4 m/s.

In order to utilize wind power for small-scale applications, a technical infrastructure for designing, installing, and maintaining such systems would have to be developed. But such wind systems are quite similar to solar PV systems which both provide intermittent power supply. Small scale systems can be purchased which produce 12 V DC power that can be integrated into the solar PV systems using the same batteries, inverters, etc. Small to medium scale wind systems would be especially useful for applications which have an inherent storage capacity such as water pumping or ice production. For these applications the initial capital investment can be reduced by avoiding energy storage costs.

One drawback of such systems is that they require a detailed knowledge of the wind energy resource. Common use of small-scale wind energy would require fairly detailed wind energy mapping in order to guide technicians and designers who would do the design, installation and maintenance of these systems. It may be advisable to install one or two such small-scale wind systems as pilot projects to test the feasibility and benefits of including a wind generation as one of the small-scale energy supply options.

On the other hand, the use of wind-generated electricity changes the comparative advantages of LPG and electric cooking by eliminating the generation inefficiencies for the electric cooking option. With a distribution efficiency of 85% and use efficiency of 60%, the over-all efficiency of wind electricity use in cooking is 50% (or up to 60% if improved electric cookers with insulation and 70% efficiency are used). LPG stoves in contrast probably have cooking efficiencies of the order of 25%-40%. [Note: Cooking efficiencies in boiling water tests for LPG stoves with metal pans is about 60%. Meanwhile LPG enjera cooking applications use a 1.5 cm thick clay cooking plate. Comparison tests between clay and metal cooking pots over a flame conclude that clay cooking pots require almost twice as much fuel as metal cooking pots. Therefore, we conclude that the probable efficiency of LPG enjera cookers is 60% * 0.5 = 30% or less. Soon cooking tests will be conducted on LPG enjera cookers to test this hypothesis.]

Another advantage of using electricity for cooking energy is that in terms of safety and convenience it is the energy source of preference in Eritrea for cooking the traditional bread (enjera).

The question, then becomes, which is most economical? Wind electricity for cooking or LPG for cooking? A rough calculation will show that the two energy supplies are comparable and both are much more cost-effective than wood.

Recent studies of enjera thermodynamics indicate [Negusse, 1996] that for enjera the energy required for cooking is 1.5 MJ/kg for a 60% efficient electric cooker. This means that for a 30% efficient LPG cooker the required energy should be approximately 3.0 MJ/kg and for a 10% efficient wood stove the energy required is 6.0 MJ/kg.

For wind electricity with 85% transmission efficiency and $0.05 /kWh generation cost, the cooking costs is as follows:

            $0.05 /kWh * (1 kWh/3.6MJ) * (1.5 MJ/kg)/0.85 = $0.025/kg

In contrast for LPG, the fuel cost is $0.0087/MJ [Government of Eritrea, 1995] which implies the following cooking cost:

            $0.0087/MJ * 3.0 MJ/kg = $0.026/kg

Meanwhile for wood at 80 Birr/quintal (or $11/100kg and 16 MJ/kg) we have a cost of:

            $0.0069/MJ * 9.0 MJ/kg = $0.062/kg

We summarize these and similar calculations of enjera cooking costs in the following table:
 

TYPE OF SUPPLY COOKING ENERGY COST ($/kg)
 

Wind Electric (production cost)                 $0.025
LPG (retail cost)                                      $0.026
Diesel Electric (production cost)                $0.032
Wind Electric (retail cost)                         $0.033 *
Diesel Electric (retail cost)                       $0.043
Wood (retail cost)                                   $0.062

These calculations imply that with respect to retail cost, LPG is the best alternative fuel with wind electric second, diesel electric third and wood being the most expensive. It also indicates that three different strategies for mitigating the wood fuel crisis: LPG substitution, wind electricity substitution, and improved traditional stoves (an increase from 10% to 20% efficiency), are approximately equivalent in terms of final cost.

But it should also be noted that under improved conditions for wind electricity and LPG can provide approximately equal cooking cost reductions of 30% to 50%. Improved conditions for LPG would consist of high-efficiency cookers. Improved conditions for wind might be either technological improvements, an improved wind site (for example with mean wind speed of 8 m/s), or subsidized interest rates on investment and long project life (e.g. 3% interest and a 20 year project life). Under such conditions, the price per kWh may decrease to $0.04 or below. This combined with added insulation in the electric cookers (or improved cooking strategies) that raise cooker efficiencies to 70% [Negusse, 1996] would reduce wind electric cooking costs (of production) to:

$0.04 /kWh * (1 kWh/3.6MJ) *(1.28 MJ/kg)/0.85 = $0.017/kg

For the wind electricity option, the added convenience and safety of the electric cookers might prompt consumers to spend greater amounts of money on electric cooking supplied by wind. This is an option that perhaps should be considered if LPG continues to have distribution safety and supply difficulties, if capital at low interest rates becomes readily available for such projects, or if there are difficulties in obtaining rapid LPG adoption rates.

While such theoretical arguments can provide guidance on evaluating biomass fuel replacement strategies based on LPG and wind electric supply, practical economic and operational comparisons need to be performed through pilot projects. When good wind sites are identified, the government or donors may want to consider a pilot project whereby wind electric generators are installed to provide supplementary electricity for electric mogogo use. In such a project, reduced electricity rates might be charged during periods of peak supply to encourage people to perform their cooking activities at such times. This will help modify the load curve to better match the supply curve on average. The reduced rates will presumably create sufficient demand to produce both a demand and income increase. Then the net income from the increased demand can be compared to the capital investment for the wind generators to see if the project is economically viable. Furthermore, the demand increase can be analyzed to determine how much conversion from other fuels to wind electricity has occurred.

There is one important economic disadvantage of fuel replacement in the home cooking sector. Currently biomass production is a national economic activity where the energy is produced in-country. Converting to either LPG or wind electric will take money that the population is currently paying to national wood and biomass producers and spend it on LPG imports or loan repayments on wind generator investments. Such a change from spending on national production to the purchase of imports or the repayment of loans will adversely affect the balance of payments and make Eritrea more dependent on the international economy. LPG purchases will be a constant drain until Eritrea develops its own sources of oil. Meanwhile if wind investments can be funded through grants or revolving loans, then there might be the possibility of reinvesting funds which are used for capital cost recovery for the wind investments. Capital fund reinvestment could produce a reduced foreign exchange drain in the wind electric case.
 

Installation Costs for 50MW Wind Farm
(1995 installation year)

Component Unit cost (1992$/kW)*

Wind turbine 888
Substation 68
Transmission 6
Service Center 5
Land 40
Indirect/Permitting 26
Total 1032

Source: Brower, 1993; Gipe, 1995

*Costs assume a 10.5% financing charge.
 

Installation Expenses for Danish and California Wind Plants
(% of Total Costs)

Zond ELSAM
Turbines             73         68
Foundation         4             9
Installation         4
Roads               3              2
Electrical         10             8
Grid Connection 4             6
Land                                3
Control Building                1
Miscellaneous     2           3
 

The turbine cost is the largest portion of project cost, and fortunately is probably the least uncertain of the component costs. It can be fairly well predicted for a given project. All of these other factors could vary change considerably in the Eritrean context and are discussed individually below.
 

Another factor that could change the cost per installed watt is the size and type of the wind machine chosen. Prices quoted above were based on approximate averages of US and European experience. The trends in US and European wind machines has been and continues to be toward larger and more complicated machines (>500 W) (variable-speed, upwind machines). These were anticipated to result in substantial cost reductions relative to their simpler single-speed, downwind counterparts, due to economies of scale and because of increases in capacity factors anticipated to accrue from being able to operate over a wider range of wind speeds. Now that there is field-data to analyze, however, it is not clear that those gains have materialized Milborrow, 1996 #20 . Critics claim that, although the variable speed machines can indeed generate over a larger range of wind velocities, their capacity factor is reduced by low availability (repair and maintenance) and inability of the active-yaw system to track the wind rapidly enough given their enormous inertia.

These larger more complicated machines may well not be appropriate in the Eritrean context. Transportation and installation requirements are more demanding and costly, requiring larger cranes and larger and better highways, and greater technical expertise. Furthermore, for smaller plant capacities, smaller (100 - 200 kW) machines are more appropriate since they can offer redundancy by incorporating a larger number of machines thereby reducing output variability if one machine is out of service. These simpler systems may also require less maintenance.

An important question is whether these smaller machines will have higher installed costs because of the lost opportunity of economies of scale? The following data from Gipe (1995), organized only by size of machine, suggest there will be little cost compromise from using smaller machines.
 

Typical Wind System Installed Cost

Rotor diameter (m) kW Approx. installed cost $/W

Microturbines
1     0.25     $2,500     $10.00
2     0.75     $5,000     $6.67

Small Turbines

3     1.5     $10,000     $6.67
7     10     $30,000     $3.00

Medium-Sized Turbines

18     100     $125,000     $1.25
25     200     $250,000     $1.25
35     400     $500,000     $1.25
40     500     $600,000     $1.20

 Although economies of scale are clearly apparent in the microturbines and small turbines, at about 100 kW these economies of scale fall off, so that the smaller medium-sized turbines produce almost as cost-effectively as their large cousins. Furthermore, Gipe (1995) cites many individual examples of German wind turbines for which the price per kW is actually less for the smaller turbines (80 - 100 kW) than for their larger 450 - 500 kW counterparts. Production and field data are not available for the new generation of 1MW and larger machines being designed in Europe. Therefore, no relevant cost comparisons can currently be made.
 

We summarize the effect of interest rate and project lifetime on loan repayment costs in the following table. These calculations assume an inflation rate of 0%, and provide results in terms of loan repayment costs per kWh of wind electricity produced. The calculation assumes that the capital costs of project equipment, installation and land acquisition are $1200 per kWh, and that the capacity factor for the wind turbines is 0.28.
 

Interest Rate Project Life (years) Repayment Cost ($/kWh)
2%     10     0.054
2%     20     0.030
3%     10     0.058
3%     20     0.032
6%     10     0.066
6%     20     0.042
10%   10     0.080
10%   20     0.058

 5. Operation and Maintenance

More research needs to be done to determine what fraction of US and European-based installation and operation and maintenance costs are from labor. Those costs, which are usually substantial in the US and Europe, should be lower in Eritrea, and might result in substantial cost savings. In addition any components, such as wind towers, that might be manufactured locally could further reduce costs. In the long-term, if manufacture and production of other wind turbine parts are localized, further cost reductions should accrue. Studying the cost breakdowns for the new wind developments in China and India will be instructive.
 

6. Rough Cost Estimate for South Coast Project

Project #1: New Wind Turbines
Capital Costs $1200/kW
Maint. & Oper. $0.010/kWh
Interest Rate 3%
Project Lifetime 20 years
Cost of Electricity $0.052/kWh

Alternatively, we can consider the project costs using reconditioned wind turbines which cost approximately 1/3 the price of new turbines [Zond, verbal price quote for 80 KW Vestas turbines]. We can presume that the project life would be shorter (10 years) and maintenance costs may be higher ($0.015/kWh). The the following table describes project costs:

Project #2: Reconditioned Wind Turbines
Capital Costs $ 400/kW
Maint. & Oper. $0.015/kWh
Interest Rate 3%
Project Lifetime 10 years
Cost of Electricity $0.034/kWh

Apparently under this scenario the cost of electricity is much lower though using reconditioned turbines may entail additional risks and jeapordize the attainment of loans.

A south coast project that is constructed at Id should according to the data have a significantly higher capacity factor. It is estimated that with a 30% higher wind power density, the cost of electricity would be proportionally lower. This would correspond roughly to a cost of electricity of $0.039/kWh and $0.028 respectively for projects #1 and #2, though these estimates do not account for the added cost of transmission facilities.

In the near term, the wind prospecting and site identification will continue, wind energy policy goals will be defined, and the pilot project will be initiated. In the longer term, wind resource development will proceed according to the process illustrated in the Schematic for Wind Resources Development in Eritrea. The near-term steps are described below.

Additional research may be useful for informing policy makers and for developing an optimal technical and institutional infrastructure to support wind energy development in Eritrea. In order to assure the sustainability of wind energy development, a study of the organizational structure required to support utility-scale wind capacity, including an analysis of training and staffing requirements needs to be determined. Also an analysis of the experience of current wind energy development in China and India, will be usefuo to wind project planners and staff. Furthermore, an analysis of the operations and maintenance history of medium-sized utility-scale wind machines will help facilitate low-risk investments in Eritrean wind power.
 

After several more months of prospecting work and data collection, an evaluation report will be written with an update on the specific sites that have been identified. This report will also include a compilation of the field data that has been collected. Copies of the report will be distributed to the relevant branch ministries .

The sites that are most promising for accommodating wind power applications, as deemed by the Eritrean energy planners, should have a ground-based evaluation of local wind distributions performed. These evaluations should consist of two teams of data collectors: one team remains at a reference location while the other team samples wind speeds at representative locations in the same locale in order to find the best location for the site evaluation instruments .

In late 1996 or early 1997, the detailed site assessment program with automated data logging equipment will begin. At this stage, at least five promising sites should be identified for further monitoring both in the highland passes and on the Southern coast. Arrangements for installation, security, maintenance, and monitoring of the data collection equipment should be arranged with local and regional authorities. Procedures for station monitoring, and maintenance need to be developed. Monitoring activities will adhere to international sampling standards to ensure maximum reliability for the utility or prospective investors. Similarly, a data processing and distribution system needs to be established and institutionalized. Site monitoring should continue for at least one year, with perhaps a longer monitoring period for better sites. After one year, equipment installed at some of the sites can be relocated to other locations as part of a continuing site evaluation program .

During the site selection and data collection phases, other factors important for site evaluation can be considered. These include ease of access, distance to transmission and distribution facilities, logistical impediments to maintenance and technical support, land use and land availability, security, and local community reactions and concerns regarding wind energy development .
 

 If these four activities are actively pursued over the next two years, Eritrea will soon be in a good position to take advantage of its indigenous wind energy resources in a rational, effective, and efficient manner.
 

APPENDIX B: PRELIMINARY SPOT MEASUREMENT RESULTS

During the past six months a moderate intensity effort to make ground measurements of wind speeds at a wide range of locations is being pursued. At the initial stages the only available measuring device has been a pivoted flap anemometer. And later two used cup anemometers were acquired. The cup anemometers produce an AC output voltage that is proportional to the wind speed. These anemometers were calibrated relative to a newly acquired anemometer that was installed with a meteorological data logging station at the Energy Center in Asmara. No adjustments were made for altitude. Calibrations were performed at 2300 m.

Approximately 15 locations in the highlands have been examined with spot measurements with most sites being visited only once. Most sites have only moderate wind speeds. But several sites showed promising signs of either high winds speeds, biological indicators of high winds or both. These sites include: the pass at Dekemhare, the valley near Ma'ereba, hills near Adi Teklezan, and the pass at Nefasit. Dekemhare, Ma'ereba, and Nefasit have trees with flagging and have been observed to have winds of approximately 10 m/s in the afternoon during the dry season. The hills near Adi Teklezan have trees with flagging. Below we discuss the results of measurements near Dekemhare in more detail.

In February a series of measurements were made with the flap anemometer at locations between 30 and 50 km south of Asmara. These measurements indicated afternoon wind speeds of 10 m/s or above in two areas: near Dekemhare and near Ma'ereba. Both locations had abandoned windmills for water pumping that were installed during Italian colonial times in the 1930's. The measurements were not adjusted for altitude.

On 17 March 1996, a series of follow-up measurements were made in the Dekemhare area which indicated wind speeds of 8 m/s to 15 m/s from 11 a.m. to 4 p.m. in the afternoon.

Site #1 Site #2

Time Wind Speed Time Wind Speed
(hr:min) (m/s, 5 min mean) (hr:min) (m/s)
11:05     9.8     12:03     10.1
11:18     8.8     12:33     11.9
13:08     8.7     16:15     11.7
15:38   10.3     16:25     14.7
16:57     10.3

These measurements were taken at 2.5 m height with a flap anemometer. Both sites are on top of a hill or ridge. Winds were from the North. Adjustments were made for measurement bias from the anemometer, but no adjustments were made for altitude (such adjustments would increase the wind speed estimates).

Of course such measurements should be interpreted with caution, but the high wind speeds were supported by nearby biological indicators which included eucalyptus trees with complete flagging (no medium-sized branches on the upwind side of the tree). Furthermore, statistical analysis of temporal wind speed variations (Yosief, 1996) indicate that wind speeds in Eritrea have a highly periodic daily cycle. For Asmara, the nearest station with detailed data, the root mean square difference between a periodic wind signal and two-hour average wind data is 40% of the mean wind speed and less than 25% of the average daily peak wind speed. Therefore our estimate of the daily peak wind speed for site #2 is 12 m/s, with an error of this estimate of approximately 25% (4 m/s). For Asmara, the meteorological station closest to Dekemhare, mean peak daily wind speeds are approximately 1.7 times the average wind speed. If we assume that the same holds true for site #2 near Dekemhare, then our measurements indicate a mean wind speed of approximately 5 m/s to 9.5 m/s during the dry season. Note that along the Southern coast during the dry season, mean wind speeds are in the range of 7 to 9 m/s.

Another series of spot measurements were taken for site #2 near Dekemhare on 4 June, 1996. This was a rainy day in the highlands, and the meteorological conditions were those of the rainy season with winds from the South. The data were taken with a calibrated cup anemometer at 3.5 m height. This series of measurements yielded the following:
 

Site #2

Date: 04 June 1996 Date: 05 June 1996

Time Wind Speed Time Wind Speed
(m/s) (m/s)
15:00-16:00 5.2 07:00-08:00 2.7
16:00-17:00 6.2 08:00-09:00 5.1
17:00-18:00 6.1
18:00-19:00 5.6
19:00-20:00 5.6
22:00-23:00 3.8

It should be noted that the wind was observed to be quite calm before 7:24 am on 05 June, and at this time the wind speed abruptly picked up. If one takes this data, plots it over a 24 hour cycle, and uses linear interpolation (fixing the speed at about 0.5 m/s at 7:20 am) one obtains an average wind speed of approximately 4 m/s. This indicates that the mean wind speed during this time is between approximately 3 m/s and 5 m/s during this period, or slightly higher than Asmara. Comparison of this data with observations at 12 m height in Asmara at the same time indicated that the observed wind speeds were 15-30% higher than speeds at 12 m height in Asmara.

 The decrease in wind speed during the highland rainy season is mirrored by the decrease in wind speeds along the Southern coast where mean winds are approximately 3-4 m/s at this time.

Overall, these data indicate a promising potential at the Dekemhare location with an estimated annual mean wind speed of approximately 6.5 m/s at approximately 3 meters height. We expect winds farther above the ground to be significantly stronger indicating a potentially excellent wind site. But the errors in these measurements are large and many more measurements need to be taken before  conclusions about site potential can be made.

  [end of report]