Rooftop Wind Solutions

Rooftop Wind Solutions

 Introduction

Micro-wind turbines are used in micro-wind generation and are much smaller in scale than those used in conventional wind generation making them more suitable for residential energy production. Micro-wind generation is a method of micro generation that uses the flow of wind energy to produce electricity for a house or farm. Broadly speaking, there are two types of wind turbines that can be installed: vertical axis wind turbines and horizontal axis wind turbines.

The installation of a micro-wind turbine usually consists of the turbine and an inverter. Wind causes the blades of the wind turbine to rotate, generating mechanical energy. The mechanical energy from the rotation is converted to direct current (DC) in the turbine and using the inverter, is converted to alternating current (AC). The inverter output is connected to a breaker panel where the electricity can be shared among the electrical equipment in the home. Excess electricity can be exported from the home to the electrical grid using a bidirectional meter and credits will be provided accordingly by the retailer based on the electric current tariff for electricity.

The generation of electricity is mostly based on the rotational wind speeds of the wind turbines. Certain geographic locations are more suitable for producing electricity compared to others. Depending on the amount of wind that can be obtained from a region, the generation can vary. Another factor that can affect the performance of wind turbines is the obstructions in the area of the installed turbine. Obstructions from trees or other buildings will hinder the turbines from producing at its optimum capacity.

Rated Power

Wind turbines are advertised with a rated power. Small turbines, like those you’d see on a roof, are generally rated at 400W to 1kW.  The rated power of a turbine is a best-case scenario. It’s a measure of how much power the turbine will generate at the highest wind speed that the turbine can tolerate. To get a more accurate estimate, look at the turbine’s power curve. Here’s a typical power curve for a 1 kW turbine:

The curve shows that the turbine starts generating power at around 3 m/s (6.7 mph) - the cut-in speed. Slower winds don’t have enough power to make the rotor spin. As wind speed increases, there’s a rapid increase in power, but the power output only hits 1 kW (the rated output) when the wind speed is around 11 m/s (nearly 25 mph). To put that into perspective, if your land had average wind speeds of 25 mph, all of your trees would be permanently bent. It’s more likely that you’ll see winds in the 3 to 5 m/s range, which means that a 1 kW turbine is usually producing less than one-tenth of its rated value. The shut-down speed is the speed at which the turbine will apply a braking mechanism to prevent damage. A typical shut-down speed is just a few m/s higher than the rated speed, so the “sweet spot” - the range where the turbine produces its rated power - is pretty narrow.

Sustained Winds versus Turbulent Winds

To make wind power cost-effective, turbines need access to strong and sustained winds. A residential rooftop offers neither. Wind speed increases with altitude, and the top of a house is pretty close to ground level. Even worse, all the obstacles - trees, other buildings, even the house itself - cause turbulence in the wind. So instead of a fast steady breeze flowing in a mostly constant direction, you get short choppy gusts of wind coming from random directions. Turbulence not only decreases the turbine’s output, it also causes mechanical stress that shortens the turbine’s life. The rule of thumb is that the turbine should be at least 9 m (30 ft) higher than any obstacle within 150 m (500 ft): rooftop turbines should be mounted near the center of the roof rather than along the perimeter, because turbulence is greater around the outside of the roof than in the center. Fair enough, but that study only looked at turbulence, not overall production. Following its recommendations means that your turbine will last longer. That’s a good thing, because it could take a very long time for the turbine to generate enough electricity to offset the cost of the turbine itself. A better suggestion is to avoid putting wind turbines on the roof.

Payback Period

  A 400 watt horizontal axis wind turbine (HAWT)   with a 10 m (33 ft) tower, since higher altitudes are where you get the sustained winds, .with average wind speeds of 10 m is 3.6 m/s, or about 8 mph. With 3.6 m/s winds, the 400W turbine generates 50W. Assuming it runs 24/7/365, the turbine will generate 438 kwH per year. At a utility cost of  $0.12/kWh,  the turbine saves the owner $52/year in electricity cost. A typical 400W turbine costs about $400 - and that’s just the turbine, not the tower. The least expensive tower kit that I could find sells for just under $400, not including the concrete base. So at a minimum, it costs upwards of $800 for one of these turbines and its tower. That’s more than a 15 year payback period, which wouldn’t be terrible for a long-term investment, except that the turbine comes with a one-year warranty.  

What If the Grid Isn’t an Option?

It’s easy to say that these turbines aren’t worth the money when compared to grid power, but what about remote locations where grid power isn’t available? Let’s look at the 1 kW turbine and tower for $8800. We determined that it produces 675 kWh per year. Is there a better renewable source of energy? Solar?

In the same location, the average solar resource is 4.5 peak sun hours (PSH) per day, with a worst case of 2.6 PSH in the winter. To generate 675 kWh/year, a photovoltaic (PV) array would need to produce 1.85 kWh/day. In the winter, a 1 kW solar array would cover that, even if the overall system were only 75% efficient. (85% is a more realistic number.) A small PV system costs about $4/watt installed, so the total investment, including installation, is about $4000. Even better, the PV system has no moving parts so it requires no annual maintenance. PV seems the better option. 

Wind: Go Big or Don’t Bother

Wind power is a great source of renewable energy - at the utility scale. Large turbines are more efficient than small ones, and higher towers reach those energy-rich high speed winds. If you’re considering a small turbine, think again. Unless you’re in the Arctic region, you’re better off spending the money on solar panels.

Capacity Factor

The average capacity factor for rooftop wind turbines was 5%. This poor performance is currently a major factor limiting the feasibility of small urban wind turbines and is primarily due to inappropriate siting of the wind turbines. An urban environment contains turbulence caused by buildings and obstructions, and careful siting is required to avoid these less efficient winds. With proper siting, higher capacity factors can be achieved, making urban wind turbines more feasible. Capacity factors as high as 14% have been achieved in urban settings. Greater capacity factors are also possible considering that rural wind turbines see percentages in the 20-35% range and higher. These reasonable capacity factors provide a more assuring outlook for rooftop wind

Factors that inhabit Urban wind roof tops

The low capacity factor achieved in an urban environment is the main hindrance in rooftop wind turbine feasibility; however, it is not the only factor affecting it. The price of electricity and value of RECs also affect how much income is generated from energy produced with a wind turbine. These three factors have a significant impact on the economic feasibility of rooftop wind turbines. Using future projections for these factors, and   small increases in each of them could significantly reduce the payback period of rooftop wind turbines, making them more economically feasible.

Siting Factors

Certain areas and building types are more promising for rooftop wind turbines. Buildings that meet the following criteria would be easier and more effective to install rooftop wind turbines:  Above 150 feet tall, and taller than buildings upwind; Roof area of at least 5,000 square feet; Supported by columns to which the turbine can be mounted

 1 kW wall mounted wind turbine

Recently developed 1 kW turbines can be mounted on a wall and does not need a structure for mounting. 1000 W power while only 1.50 m in diameter and 15 kg in weight, An integrated dump load and braking system along with the following ;

·  2x patented aluminum rotor

·  Nacelle incl. wind direction following system

·  Grid inverter incl. on-board computer and display

·  Dump load incl. ceramic screws and mount

·  Automatic storm control system incl. sensor, actuator and relay

·  Protective wax coating for corrosive sites

·  Manual and detailed installation guide including pictures for each step

   The wind turbine can be plugged into the next wall socket. The new inverter will synchronize with your local grid. You start saving on your next electricity bill as soon as the wind starts blowing. The   inverter automatically adjusts to both 230 V and 110 V AC power. The energy generated directly feeds your appliances, only the missing power is bought from the grid. The power generation ability of the turbine is presented as follows:

The complete turbine ready for installation is priced at Euro 1400 (US $ 1620). This seems a more attractive solution which given better than average wind conditions could be feasible.

Why Micro Wind Turbines don’t work.

Size

A simple equation gives the power of the wind. Power = 0.5 x collection area x the wind speed cubed. What it tells us is that the power of a turbine is related to two factors: the size of the turbine and the strength of the wind.  The area of the circle is equal to the constant pi (3.14) times the radius of the circle squared. What that means is that as you increase the length of a turbine blade, the collection area increases disproportionately.  A micro turbine with Its blades 1.75m long, would give a collection area of just under 10sq m. Tiny. Small turbines have disproportionately smaller collection areas and therefore generate dramatically less power.

Wind speed

The key here is that cube function on the wind speed. The power of the wind is related to the cube of the wind speed. So, at low wind speeds you get virtually nothing. When it really blows it you get a lot of power. Double the wind speed and you get eight times the power. Quadruple it and you get 64 times as much. Eight times the speed and we're talking more than 500 times the power. Halve the wind speed to six meters per second (a moderate breeze) and - thanks to that cube law - you now get just 120 Watts - that's two standard incandescent light bulbs with average wind speeds likely to be between 4m and 5m per second the turbine would generate 25 Watts. That is barely enough for two energy saving light bulbs.  

Conclusion

Micro wind turbines are unable to generate significant amounts of electricity, at this point in time, to be financially feasible.  Wind solutions with small and micro turbines that are to be mounted on walls, will cost less than the mast mounted turbines but wall mounting will have disadvantages of lower wind speed. Work on micro turbines is still in progress and in the future we may have designs that are financially feasible. Small wind turbines in urban areas today are only profitable under very favorable conditions. The coupling of the small wind turbine to a storage system is crucial for cost-effectiveness.