Winds of change start at around nine miles per hour

July 24, 2008


Image courtesy of www.metaefficient.com

The first known use of wind power dates to inscriptions on an Egyptian vessel from 3500 BC that shows a ship under sail, thus beginning globalization trends that nearly five thousand years later the modern world is still refining.  The conventional windmill did not appear until the ninth century AD in what is now known as Iran, where it was invented by the Persian geographer Estakhri for milling grain and was soon thereafter adapted for pumping water.  While windmills and sailing ships declined in the dawn of the industrial age in favor of steam, internal combustion, and the fossil fuel economy, wind power has enjoyed a popularity resurgence in recent years as the environmental costs of power generation rise and global climate change is promoting closer examination of emissions-free power alternatives.  Energy present in the atmosphere as a result of the heating and cooling of the earth's surface takes the form of wind and may be captured by wind turbines to produce alternating current.  This power can be directly fed into a substation and thence a regional power grid or can be inverted to direct current to charge batteries for stand-alone residential and small commercial power sources.

Wind power has been a dramatic weapon in Costa Rica's war against carbon emissions; as of 2007, the nation rated 33rd in the world in the penetration of wind power into the domestic power grid, where it currently accounts for one half of one percent of the nation's total electricity output.  In its drive to become the world's first carbon-neutral nation by the year 2021, Costa Rica races with similarly-minded Monaco, Norway, New Zealand, and Iceland.  Large-scale wind-farming in Costa Rica is projected to be an important part of fossil-fuel independence.  For the moment, large-scale wind farms are restricted to the Guanacaste region, mostly surrounding Lake Arenal, where ample wind resources provide dramatic returns on investment.  The success of wind farms in Guanacaste has led to plans for similar wind farms in the Escazu and Santa Ana regions of the Central Valley.

While wind farms are clearly an important part of the greening of Costa Rica, residential-scale wind power is an equally exciting prospect both for residential-scale energy independence as well as for reducing the nation's power demands as a whole.  For individuals and small businesses able to tap wind power, it is one way to retire a pesky monthly power bill through a non-trivial capital investment up front.  Still, because of wind's intermittency it rarely offers a stand-alone solution and is best when tied into a regional or national power grid, though stand-alone applications can also use solar power and fossil fuel power generation to fill in gaps when power demands exceed what the wind can provide.  The rest of this article outlines how to determine if wind power may be right for you.

First, you have to have the resource.  Even in the windiest places, there will be times when the wind falters and the turbines still.  And most places aren't windy enough to sustain a robust household without either excessive capital costs or without depending strongly on alternate power supply for when the wind stills.  Since wind is intermittent, stand-alone wind power applications all depend at a minimum on batteries to store excess energy and a power inversion system for distributing the power in the form used by conventional electrical appliances.  Even with a robust battery bank, there is still a limiting threshold of average wind-speed necessary to make a given facility self-sustaining.  Though there is some question about this and technology is bringing this limiting threshold lower, rules of thumb put the minimum sustained wind speed threshold at between 9 and 13 mph for project viability. 

The first information required to assess wind power potential, therefore, is wind speed.  Anemometers are relatively inexpensive and can be purchased for this purpose.  Still, single measurements are of little value.  Overall viability depends on how sustained winds are.  Therefore, it is particularly helpful to refer to the nearest weather stations to see if measurements are comparable to allow for time series analysis of wind speed records.  Also, wind speed varies significantly as a function of height above ground surface.  So, it is vital for a comprehensive analysis to measure wind speeds as high above the ground as capital limitations may allow for a tower to be built.  It is rarely practical to gather even a single year's data of wind speeds at different heights in a target area, and commonly residential scale wind power decisions are based on a few wind speed measurements and on regional wind-speed characteristics as compiled by local or regional weather stations.

The mathematical function that defines wind power potential is given as Equation (1)


where P = Power in watts, α is an efficiency coefficient, ρ  is the density of the air in kilograms per cubic meter, r = radius of the area swept by wind turbine in meters, and v = velocity of the air in meters per second.  Betz's law states that the maximum theoretical value for α is 0.59, though turbine manufacturers report the value which below the Betz Law limits varies as a function of turbine configuration. 

Air at sea level has a density of 1.225 kilograms per cubic meter.  Therefore a 15 mph breeze (6.67 m/s) and a rotor diameter of 4 meters (radius of 2 meters) would have a frictionless potential power generation of 727 watts.  Applying Betz's law means that the practical limit for such a configuration is actually somewhat less than 428 watts.  If this wind speed were sustained across 24 hours per day on average, that would amount to a total of 10 kw-hours of daily power production, which is about the amount of power used in a small household with modest power needs.  In this hypothetical example, there are some things that can be adjusted and some that cannot be.  Wind constancy is largely beyond control, but wind speed can be increased by placing the turbine higher above ground.  Also, it is possible to increase the swept area by increasing the turbine's rotor length.  In the preceding example, doubling the length of the rotor arms, for instance, changes the power generation four fold.  Even more dramatic, each doubling of wind velocity boosts power by eight times.  This reveals that even small adjustments to optimize wind speed and swept area can result in dramatic performance enhancements.

The optimal approach to design is to converge toward the result from both ends of the analysis.  In the above example, we have shown the power that can be produced from a given wind-speed.  Now, let us examine power requirements.  I recommend filling out a spreadsheet of household appliances and the period of time each is to be used.  I have published an example here and here to assist in the design of alternative energy applications like hydroelectric, solar, and wind power.  I have summarized a modestly anointed household in the summary pasted below.  This household presumes that water heating, cooking, and clothes drying are done with gas and hybrid appliances since they are large electrical users.

Item Qty Watts Hours Smlty Demand SAME TIME
DC Refrigerator 1 120 10 1 1200 120
dish washer 1 1350 3 1 4050  
Microwave 1 1000 0.5 1 500  
Blender 1 300 0.25 1 75  
Coffee Maker 1 900 0.5 1 450 900
Toaster 1 1000 0.25 1 250  
Slow Cooker 1 750 1 1 750  
Rice Maker 1 750 0.5 1 375  
Computer 1 250 4 1 1000 250
Monitor 1 150 4 1 600 150
Television 1 100 4 1 400 100
Internet modem 1 60 4 1 240 60
satellite tv or cable box 1 40 4 1 160 40
stereo system 1 100 2 1 200  
Light bulbs wattage 2 8 75 4 0.5 1200 300
Ceiling Fan 8 65 15 0.5 3900 260
Washing machine 1 500 1 1 500 500
Hybrid dryer 1 400 1 1 400  
          16250 2680

The total daily power demand under this usage pattern is 16.3 kw-hours (total shown in red in the above table).  In our example above, a 4-meter turbine in a 15 mph breeze sustained for 14 hours can generate only 10 kw-hours, or about two thirds of the total daily demand.  If we boost the turbine rotor length from 4 to 6 meters in diameter, the wind potential of 23.2 kw-hours easily satisfies the home's anticipated power demands.  Clearly, optimal wind-power system design requires as much information as possible about the variation in wind speeds at all hours of the day in order to reasonably project the amount of energy that can be produced.  Since both wind speed and swept area are variables that can be favorably boosted through added capital investment, the convergent analysis is the best approach to settling on the system that best fits:  1)  power demands;  2)  local wind patterns;  and 3)  capital budget. 

In our example, presuming we are confident about the average wind speeds for the area, a six meter diameter rotor size is reasonable.  The remainder of the self-standing system design is not so simple, comprising an inverter, charge controller, battery bank, and secondary power source.  In the far right hand column of the table above I have left a series of appliances that may be on simultaneously.  The total amount of simultaneous power demand is the design criterion for inverter sizing.  In this case, an instantaneous power requirement of 2680 watts (shown in blue in the table above) would logically presume the deployment of a 3000-watt inverter.  Additional design decisions revolve around the input voltage, which can be either 12-, 24- or 48-volt.  In most cases, 24-volt systems offer the best balance between functionality and economy.  Twelve-volt systems are less expensive but offer less amperage and less overall versatility.  Forty-eight volt systems, on the other hand are top-end, requiring twice as many batteries and greater expense but having a number of advantages over 24-volt systems.

Having settled on a 24-volt system in our hypothetical case, the remaining design variable is the amount of reserve energy storage to be allocated in batteries.  While rigorous designs might call for three days of battery backup power, that amounts to 34.9 kilowatt-hours of backup, which for a 24-volt system amounts to a capacity of 2020 amp-hours once 80% battery draw-down and 90% inverter efficiency safety factors are included in the calculations.  For 520 amp-hour L-16 6-Volt batteries, this would presume a parallel array of four such batteries to achieve the amp-hour battery backup and a series of four batteries along each leg to achieve the voltage.  This adds up to 20 batteries, which at $500 average cost in Costa Rica is a $10,000 investment, considerably more than most homeowners are willing to consider.

Most people, rather than spend so much on batteries, are more inclined to purchase four batteries to achieve the voltage and use a fossil fuel generator or solar panels for supplemental power generation when winds fall, rather than capitalizing substantial battery backup capacity.  Even with three days of battery backup, it is likely that across the life of a project installation that the winds will periodically fall beneath design plans and periodically require supplemental power.

Wind power solutions make ideal grid tie-ins.  For a grid-tie wind turbine application, homeowners do not require inverters, batteries, charge controllers, nor must they be concerned about periods of low wind speed.  In grid-tie systems, homes provide power to the grid when producing more than the house is consuming.  And when winds are still, the house pulls power from the grid to meet its needs.  The whole balance of power is automated, so that no disruptions or attention is required, and the power meter measures the balance of energy flow.  For homes that produce more power on average than they consume, the extra power is fed into the national power grid and available for other users, and the homeowner receives a monthly check in compensation.  And for those users that use more power than they are able to generate, they nevertheless achieve savings on their power bill and are able to capitalize installation costs much more easily than if they were forced to include inverter, battery bank, and backup power generation in the original capital outlay.

Costa Rica does not have a grid-tie alternative for small-scale producers of hydro, solar, or wind power.  Until the national power company normalizes grid-tie connections for all manner of micro-power generators, it will continue to be a capital challenge for first time renewable-energy developers to rationalize complete stand-alone systems.  At present ICE has a policy whereby a credit can be extended to such consumers in proportion to the amount of grid power that is offset by home power generation.  Still, this is a needlessly bureaucratic and tedious alternative, when the state of the art in most societies is an electric meter that spins in both directions and allows home generators to achieve savings on par with power costs and to even sell power back to the utility when power generation potential is particularly high in comparison to energy demands.

While Costa Rica's leadership in wind farming has been inspirational and a significant investment in the greening of Costa Rica, this nation must liberalize its policies on micro-energy production.  In seven years, Costa Rica has reduced its dependence on oil-generated electricity from 30% of the national total in 2000 to a mere 7% of the total in 2008, this in spite of yearly increases in national power demands throughout that time period.    Grid tie systems mandated by governments in many countries and stimulated with large tax rebates for the purchase and installation of alternative energy sources, mostly photo-voltaic panels.  The most practical way to continue to reduce Costa Rica's dependence on foreign oil is to borrow a scene from the playbook of such planetary energy leaders like Germany and welcome small-scale producers into the national power equation by normalizing grid-tie options that directly reward small scale energy producers with either reduced electrical bills or even net revenues for those small scale power generators that produce more than they consume.  Without assurance that excess electrical power will be purchased by ICE, it is more difficult to economically justify investments in residential-scale power generation projects.  By making it easy to sell power back to the government, individuals are presented with firm returns that increase with the degree of capital investment.

The liberalization of grid-tie ICE policy would immediately spawn investment in solar, hydroelectric, and wind power systems.  With careful government coordination, it is likely that micro-alternative energy generation may completely supplant the remaining eight percent of the national power supply that comes from oil.














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