The RAM Standard

Previous renewable energy sources were predictable, but when it comes to wind energy, predictability is a challenge. Wind is variable in both time and velocity, making it difficult to accurately forecast its strength or timing. Additionally, wind power is dependent on wind velocity to the power of three, further complicating the ability to predict energy generation. Wind energy does not meet the three key criteria of reliability, availability, and maintainability in energy generation, failing to answer the fundamental questions of “how much, how long, and how often” electricity can be generated.

Utility coal-fired energy aims for 100% uptime at Nameplate Capacity over a 50-year lifespan, but in reality, performance is measured on a monthly basis. As the generating units age, they require more frequent repairs, leading to decreased Availability. For ESKOM power stations, the minimum was 67%, or “Two out of three ain’t bad!” – reminiscent of a Meatloaf song.

As of my last update in 2011 when I studied RE Systems, the availability had decreased to 60% and spare capacity had decreased from 15% to 8% to account for the aging fleet.

“The answer, my friend, is blowin’ in the wind” 

     – Bob Dylan

Wind Energy is a form of solar energy caused by a combination of three concurrent events:

• The sun unevenly heating the atmosphere, causing high and low air pressure regions
• Irregularities of the earth’s surface, causing barriers to wind glow
• The rotation of the earth, Carioles Forces cause wind to turn (almost like a draining bathtub)

For Capetonians, the constant presence of wind, whether it be a cold front from the NW or berg winds from the SE, greatly impacts their daily lives.

Power Coefficient of Wind Energy

In 1919, German physicist Albert Betz demonstrated that the maximum amount of kinetic wind energy that could be captured by an ideal wind-energy extraction machine was 16/27 (59.3%). Today’s modern turbines can capture up to 80% of this maximum, approximately 47%. For general calculations, it is typically assumed that 40% of the kinetic wind energy can be captured.

Currently there are two major types of Wind Turbines:

HAWT – Horizontal Axis Wind Turbines (HAWT) are windmill lift type turbines with aerodynamic rotors resembling propellers. The aerodynamic shape generates lift similar to a plane’s wing, increasing efficiency. However, the high wing tip speeds can be noisy and dangerous. HAWTs must be oriented towards the wind to operate effectively.

VAWT – There are two types of Vertical Axis Wind Turbines: the Savonius, known as the “Split-drum” drag type, and the Darrieus, which features a curved aerofoil for lift. Combining elements of both designs can result in a more efficient turbine. The Savonius turbine is less efficient but is directly associated with wind speed, quieter, and safer. It can accept wind from any direction. On the other hand, the Darrieus turbine has higher efficiency but is prone to vibrational issues.

The power generated by wind energy can be calculated using the formula: P = 0.5 x ρ x A x v^3, where ρ is the density of air, A is the sweep area, and v is the wind speed. This means that the power output of a wind turbine is proportional to the cube of the wind speed. For example, wind blowing at 4m/s will produce eight times more power than wind blowing at 2m/s. Air density also has a minor impact on power output, which is why wind turbines are typically located closer to sea level rather than at high altitudes.

Micro Power

Utility power is measured in MW (million watts), with the first three dry-cooled units at Majuba Power Station being 665 MW and the last three wet-cooled units being 716 MW nameplate rated. During the energy crisis, the average household consumption was 4,500 kWh, which has now decreased to 3,246 kWh per year. This means that hourly consumption has gone down from 513 watts to 370 watts per hour.

Utility power looks at MegaWatt [million Watt] so guess residential looking at Micro Power or a millionth? A Micro-Turbine for residential has the following Power curve.

Utilities typically operate on a MegaWatt scale, which is equivalent to one million Watts. In comparison, residential power usage is much smaller, often measured in Micro Power or one thousandth of a MegaWatt.

HAWT Wind Energy Output Curves for Micro Turbine

The turbine has a slow start-up speed of 9 km/h, with 60% of its power being generated at around 36 km/h. The maximum power is reached at approximately 47 km/h. The cut-out speed is typically set at 90 km/h, at which point the controller will activate the brakes. The turbine can survive wind speeds of up to 180 km/h.

VAWT Output Curves

The attractiveness of VAWT type turbines remains. VAWTs have several advantages that make them ideal for an urban environment:

• VAWT are omnidirectional and can use wind coming from any direction.
• The gearbox and other equipment can be located closer to the ground.
• VAWTs start to produce power at lower wind speeds, which is ideal for the urban environment.
• With Magnetic Levitation (MagLev) generator friction is reduced, increasing efficiency.

However, on a commercial scale, this type of turbine is not typically implemented due to its lower efficiency. They are best suited for low wind speeds (typically less than 2-6 m/s), but considering that wind power is proportional to the cube of the wind speed, this also means that the power output is significantly less than that of a horizontal axis wind turbine.

Theoretical power coefficients for different types of wind turbines are provided in the graph below: HAWT=0.48, Darrieus=0.4, and Savonius=0.15. The Darrieus turbine utilizes curved airfoils around a central mast to generate lift, similar to the HAWT. However, it has been known to experience issues with vibrational failure. The Savonius turbine, on the other hand, is a drag type turbine typically made by splitting a drum, resulting in lower efficiencies but the ability to generate power at low wind speeds.

A new hybrid system has been developed that combines the curved airfoils of the Darrieus turbine with the drum design of the Savonius turbine. This hybrid system aims to improve low-speed torque, reduce vibrations, and increase wind speed utilization. Real-time data for locations like Natal and Transvaal will hopefully be available soon to evaluate the performance of this new hybrid system.

Cost of Conventional Coal Fired Energy

Looking at the latest Sixpack – Medupi, its Name plate capacity is 4764MW [6×794 MW units].

The initial expected cost of R80 billion (2007 Rands), was revised to R154 billion (2013 Rands). By 2019, the cost of Medupi was independently estimated at R234 billion (2019 Rands).

Wikipedia

That is a lot of zero’s!!! R234 000 000 000/4764 000 000W = R 234 000/4764W = R49. 12/Watt.
Therefore the current cost of constructing Utility Coal power in South Africa is R50K per kW. It would have been more cost-effective to install PV with LiFePo4 battery backup at the same price, with increased availability.

Determining Wind Energy Load Factor:

This issue is significant because when I began this “course” in 2021, there was a lack of information available for South Africa. There was limited real data to work with. The concept is to obtain historical data specific to your region (such as a wind map for South Africa), which is available in other countries. By collecting wind speed data over a period of time and utilizing the graph depicting wind speed versus power, you can calculate your Theoretical Load Factor.

Load Factor = Power Produced / Nameplate power.

The only historical data available pertains to the Eskom Klipheuwel test farm. The load factor at this farm is 28%, indicating that it produces 28% of its nameplate power in a year. It is worth mentioning that one wind turbine at the Klipheuwel Test farm broke down within six months, and maintenance issues affected the others due to the lack of local representation, possibly contributing to the low figures. In contrast, our 400Watt pilot installation achieved a load factor of over 60%.

SAWEA Wind Map

UPDATE: SAWEA has recently made the Wind Resource Atlas public, which is commendable. Local tests on small wind turbines in our area have shown a 50-60% load factor, with Gansbaai being highlighted in red on the map. The release of the Wind Energy map will make it simpler to install Wind Energy devices.

The Wind Resource map eliminates the need for guesswork, allowing for the installation of an engineered solution.

Enter the Hybrid

The smallest and least known of Spain’s Canary Islands, El Hierro, is making a splash by becoming the first island in the world fully energy self-sufficient through combined water and wind power.

28 April 2014

The power station on the smallest of the Canary Islands succeeded in covering 100% of the local demand for almost 25 days in a row between July 13 and August 7, beating its own 18-day record achieved in 2018.

These systems, known as Wind-and-Water Hybrid systems, are primarily suitable for municipal use, where Hydro serves as a backup battery for Wind Energy. In residential settings, a Hybrid Wind-and-Solar generation system is utilized, allowing for one controller to manage both the turbine and auxiliary input for PV panels. The term “Hybrid” is used due to the differing electrical outputs of Wind Energy (AC) and PV (DC), creating a combination of AC and DC energy sources.

Controlling Wind Turbines

Utility wind

The power generated by kinetic wind energy is directly proportional to the speed cubed, while the speed of the rotor tip is determined by the square of the rotor radius. AC generators must maintain a constant speed in order to achieve the desired frequency, such as 3600 rpm for a 60 Hz grid or 3000 rpm for a 50 Hz grid with a 2-pole motor. The number of poles in the motor can affect the required speed, with a reduction in speed for motors with more poles. Most generators have 4 to 6 poles, so a gearbox is used to increase the speed of the generator rotor. AC/AC converters are then used to convert the rectified AC to DC and then invert it back to the correct AC frequency and voltage.

The original towers, known as FSIG (Fixed Speed Induction Generators), were controlled by the grid frequency and rotated at a constant speed of 15rpm for 50hz and 20rpm for 60hz. Utility Horizontal Axis Wind Turbines (HAWT) are designed to operate at a single RPM to generate power at the grid frequency, which is why all turbines in a wind farm will be turning at the same speed. Utility HAWTs are designed to operate at 50 km/h (13-14 m/s) to achieve their nameplate power output.

The main issue with wind turbines was the prevention of overspeed, leading to the popularity of FSIG turbines that use the grid for braking. Essentially, these turbines were connected directly to the grid and spun at the grid’s frequency. Utility Turbines also feature variable pitch blades, similar to Kaplan Hydro Turbines or helicopter blades, allowing them to increase power output by adjusting the angle of attack to maximize the utilization of wind energy at same rotational velocity.

Micro Wind Turbines:

Micro wind turbines do not have the luxury of add-ons like variable gearboxes or rotors. Instead, they use direct drive permanent magnet AC generators designed to rotate at 300 rpm at rated power speed and supply 3-phase AC power. Solar now utilizes MPPT controllers that are better suited to the variable output characteristics of wind turbines. Previously, PWM was used, but it required the generator to be turning fast enough to generate voltage above battery voltage before it could start charging.

When using MPPT technology, it is important to remember that power equals voltage multiplied by amperage. This means that you can charge a 12V Lead Acid battery even when generating power with a voltage lower than 12V. The digital control (MOSFET) in MPPT controllers ensures that the outgoing DC charging amperage is slightly lower than the incoming AC amperage – but at 12V. These controllers are often referred to as MPPT-Boost controllers, which expand the range of power generation and make residential wind energy a feasible option. Additionally, the MPPT controller acts as a brake for the turbine when wind speeds and energy levels exceed safe limits, applying current to electromagnetic brakes. This safety feature is typically set up for wind speeds of 25m/s or 90 km/h, determined by the power output of the turbine.

In certain cases, micro wind turbines may have more than the ideal three blades in order to boost start-up torque in low wind conditions, particularly in smaller residential installations. These turbines prioritize affordability and ease of maintenance over maximizing efficiency.

Calculating Wind Energy Costs

Here is a quick calculator to work out the power you will get out of Wind Energy, as noted above there many variables.

Wind Turbine Calculator

400Watt 3-bladed model Case Study:

Wind Turbine size: 400-Watt HAWT S3, with controller

Cost: R4 162

Shipping: R2 515

Vat @ 15%: R1 002

Installation Costs: R2 000

Total Cost: R 9 679

Nameplate power generation: 3504 Units

Load Factor: 50%

Usable Power generated annually: 1 752 Units

Generation income (@ R3.30/unit): R5 782

Payoff Period: 1.67 years

fREe Renewable energy cost.

So time for a quick summation of the Renewable Energy generation technologies covered, and some very interesting values:

NOTE: All the figures above are for electrical power generation, with the exception of SWH which heats water directly, without storage.

Next will be looking at electrical storage – critical so that your RE sources suddenly have an availability of 100%. Always on when you need it.

NEXT: Tidal Energy and Battery Storage

PREVIOUS: Solar Energy

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