Let's talk wind power for your homestead; to avoid surprises!
When homesteaders look to power, they usually begin with solar; due to it's 'canned' simplicity. I will start there when we get moved myself; but the sun does NOT always shine; so alternatives should be considered.
The most common alternate is wind power, and it is often misunderstood. The first item to address is the rated power output. Wind generators are rated, for wattage output, at higher wind speeds than you will normally see. When considering a wind generator, you must ask for the power generation curve where they have tested power generated Vs wind speed. Then look up the average wind speed in the area your Homestead covers. Combining this average wind speed with the power generation curve; will tell you what you can expect from this specific wind generator, when installed on your homestead.
THIS is the most common disappointment from people who install wind generators. They think they will get 1000 watts, and they get 400 watts. BUT if they do this basic step, they would have been prepared for the local power generation wattage.
There are two basic types of wind generators, a vertical, and a horizontal machine. Each has advantages, but the homesteader should be aware of both; in order to make a optimal generator choice. Here are a couple of generators as examples; I am NOT recommending them, only using them for definitions of type! I have no business affiliation with any of these links....
Horizontal wind generator example:
This type must turn into the wind before it can generate power, but once it faces the wind, it can spin several times faster than the wind pushing it. This is called the tip speed ratio, and the better this number, the more efficient it will be, especially in low wind velocities. For the homesteaders use, the horizontal axis machine is usually more efficient, but slightly harder to use, so it is a trade off. This is the most common type of wind generator, and one of the older types; as it was used for centuries to pump water and grind grains. It is popular, because it works. In this case, the easiest use is harvesting electricity for storage and use on the homestead.
It is ironic that at my homestead, I will be using electricity to pump water; emulating the windmills on a homestead a century ago.
I will discuss voltage in another post to shorten this one some, so readers don't starve to death before they finish reading, LOL!
Vertical axis machines are usually easier to install, and this design is the most common type. This is a savonius wind turbine often seen being used to measure wind speed, because the turbine blades turn at the exact same speed as the wind pushes them (tip speed ratio of one to one). These wind generators start at lower wind speeds, and survive better on the top end when exposed to high wind speeds. The tradeoffs are that they are not as efficient, and they suffer from the Magnus effect. The Magnus effect in short is the same force that causes a curve ball to curve, and on a wind generator pushes that generator at 90 Degrees to the incident wind, at Twice the force the wind provides!
For a detailed review of the Magnus Effect:
All this means is that the mount must be braced so it doesn't fall over in high winds.
A Homesteader needs to buy the highest capacity wind generator they can afford, understanding that they will provide about a third of the rated wattage in real use conditions. Just be aware that unless you are in a storm shelter, your wind generator will not deliver rated output. The big advantage is that when the solar primary system is reduced by clouds, the wind generator will step and provide more output, due to the normal increase in wind associated with cloudy Skies.
You will need a dump load for a wind generator. When the wind is up, and the batteries are full, you can't just shut off the charge with a wind generator. IF you totally unload the wind generator, it will over-speed the assembly, and it WILL come apart under the increased centrifugal force!
Over speeded assemblies will explode! Both kinds of wind generators, are subject to the laws of Physics. Full charge usually occurs when the wind is fastest; which is a worst case scenario! The 'fix' is to either add a resistive load, to dump this extra power into, or build a mechanical brake assembly. The dump load is a lot simpler, and less expensive, but Either way will work.
Dump loads are overpriced, and some are badly designed; that said, I will try to explain what is needed to make your own dump load on the homestead.
There are three parts in a dump load: the control, the load, and the switching.
The control is simple, only because the existing controllers have a bypass or over voltage output that can be used to trigger the dump load. So I will semi ignore this, because it is more complex and already available.
The Load is easiest to build using ceramic power resistors stacked in parallel. I chose 100 watt power resistors, and I am loading them to about 75% of their capacity. Please NOTE, these resistors get HOT, that is why they are made from ceramic, so don't touch them to see if they are working!
Let's talk power. Power is equal to the voltage times the current (or P=EI to burn 24 volts at 71 amps EI), and resistance is equal to Voltage divided by current (or R=E/I).
Now let me walk through a basic design:
I will add a wind generator that will generate a maximum of 1700 watts on a 24 volt system. I must design for this Maximum condition, even though at my local wind velocity this generator will make 500 watts on the average. SO, at 1700 watts and 24 volts we need to burn 71 amps (1700w/24v=71 amps). Therefore we need a resistor of 0.34 ohms (24v / 71amps) that will handle the 1700 watts, which is usually hand wound and expensive. BUT we can cheat, because resistors have a couple of advantages if placed in parallel. First, resistors in parallel share the power dissipation, so ten in parallel will have ten times the power capacity, so we can us smaller resistors together. Secondly, resistors in parallel divide the resistance values by the number of resistors, so in the ten in parallel, the value would be 1/10th of the individual values.
I need .34 ohms so if I start with 8 ohm resistors; and I put 24 in parallel I will see .33 ohms, which satisfies the resistance need. When I look at power across an eight ohm resistor, at 24 volts we see a current of 3 amps. With 3 amps times 24 volts we see 72 watts. Therefore, in order to build this dump load, I will need 24, 100 Watt, 8 ohm resistors; in parallel. These are an off the shelf part, and are easily ordered from ebay. Solder these on a copper pipe on each end (we will talk about the switch part later), remembering that at full power there will be 71 Amps of current in this assembly! Spread these resistors out, because they will dump a lot of power in the form of heat, so air circulation is important!
Now, let's talk switching. It is difficult and expensive to buy a 71 amp Dc switch, so we need to cheat again. There are relays that will handle 10 amps each, so you can include eight of these in parallel, to switch the full load. What I would recommend is using a solid state switch. NOW before you panic, let me walk you through the switch. They have a single device that will handle 12 amps at 60 volts that acts like a relay. It is called a T-MOS Power FET (T chanel Metal Oxide Semiconductor Field Effect Transistor) that turns on when voltage is applied to the gate (think of it as a 'coil'). The FET you should use is a MTP3055E:
Here is the specifications for the MTP3055E:
Type Designator: MTP3055E
Type of Transistor: MOSFET
Type of Control Channel: N -Channel
Maximum Power Dissipation (Pd): 70 W
Maximum Drain-Source Voltage |Vds|: 60 V
Maximum Gate-Source Voltage |Vgs|: 20 V
Maximum Drain Current |Id|: 14 A
Maximum Junction Temperature (Tj): 175 °C
Drain-Source Capacitance (Cd): 450 pF
Maximum Drain-Source On-State Resistance (Rds): 0.15 Ohm
DO NOT run screaming into the other room, these specifications only have a few important pieces. The most important is the current capacity (Id) of 12 Amps; that means that you can tie 3 resistors together, and let it switch 9 amps. That way, with 8 devices, the full resistor bank can be switched. I intend to use one MOS FET for two resistors, because they are inexpensive and will last forever at 50% load.
The next one to look at is the Rds on (this is the resistance across the 'contacts' Drain to Source) because the internal resistance dissipates power too, and causes device heating. With the planned two resistors loading, I will have 6 amps of current, and 0.15 ohms for 0.9 volts drop; so 0.9 volts at 6 amps dissipates 5.4 watts. This would run without a heat sink, but heat sinking will make it last a lot longer. Mounting it to a piece of copper or aluminum makes it easier to connect things electrically. too.
The last thing to look at is the Maximum voltage that can be applied to the gate (coil) Called VGS Max which is 20 VDC. For me, with a 24 volt system, this could be a problem. This problem is easily solved by a simple input circuit, by placing a 1,000 ohm resistor from the Gate to source, and another one from the control signal to the Gate, the maximum voltage I can apply with a 24 volt control signal, is 12 volts. This will protect the gate from overdrive, when the gate is fired. The source on all the MOS FETs ties to the ground, while the Drain ties to the Resistor to be switched.
These devices will cost about $0.50 each, and other MOS FETs can be used as long as the Vds is at least twice your system voltage, and the Rds on is low enough, and the drain current is twice the needed. Make sure it is an "N" channel and not a "P" channel MOS FET!
Now you can choose a wind generator, and make a dump load; Happy Homesteading!