Backup Electric Power Design Considerations- Part 2, by Duliskov

Solar Energy Generation

For any significant solar power generation, plan to cover your entire roof with panels. Consider installing a few panels on the roof of your trailer, if you have one; this will give you mobile power and better concealment. If you make the panels tilt or slide out from under each other, you can significantly increase the total surface exposed to the sun while stationary. Alternatively, install the panels on a ground support for easy access and scalability. Make sure that the selected location does not have structures or trees casting a shadow over, and take photos in different times of the day and all seasons. Power generation capability of slightly shaded solar panels drops dramatically, even if the shadow covers only 5% of their surface. If you don’t have the luxury of time, you can use an application to simulate where the shadows of each object will be at any given moment in time, summer or winter. (For example, you can use “Light Trac” iPhone app.)

The distance between the panels and the battery bank is essential, especially in case of low voltage, 12-24 Volt transfer to battery charger. The cable will dissipate part of the current as heat, wasting it, and the cables will be extremely expensive. A better option is to use a higher voltage, for example 96 Volts, or installing an inverter at the panels and sending an AC current down the line to the battery bank. Another option is to install individual self-synchronizing inverters for each panel. They will synchronize frequency with each other and the utility power, so you can simply plug them into your house grid via a special switch and feed your house simultaneously from the solar panels and utility power at the same time. The battery bank will be charged from any outlet in your house. This will make solar contribute to your energy bill without re-arranging your incoming utility lines and will be transparent to your utility company. Alternatively, it is possible to isolate few circuits on a separate panel to be only fed by solar and the battery bank.

Wind Energy Generation

Wind power is noisy. If you install wind turbines on a mast connected to the frame of your house, you will hear the grinding noise throughout your house, especially with larger turbines. Smaller turbines are not going to satisfy your power needs of a larger bank, but they can supplement a solar array or generator. Specialists in wind energy say “go big or forget about it”, because wind energy is extremely inefficient in the case of small scale generators. I never installed a wind turbine, but I read many books and distilled for myself the following: don’t do wind under 5 KW; for the cost of installation and maintenance, you could buy a larger solar system and have less trouble over time.

Hydro Energy Generation

If you are lucky enough to have a creek or river flowing through your property, you can tap into hydro power. Obviously, in summer the creek may dry up and in the winter it may freeze up, but otherwise it is a constantly available source of power. Be mindful of the ecosystem of the creek when deciding how much water to divert to your project. Also, make sure local regulations allow for what you are planning to do. The water/energy of the river flowing through your land may not be legally yours. Check with local authorities before you invest in hydro generation.

Generally there are two most important considerations in selecting the micro-hydro turbine. Firstly, how high is the water column. (This depends on the steepness of the terrain or the height of the dam.) Twice the height is twice the energy the water will produce. Secondarily is the volume of water you can divert to the turbine. Again, power output is proportional to volume. This will drive to either go with an impulse turbine (optimal for the high velocity low volume) or with a reaction turbine (low velocity of water but high volume). Unfortunately, I don’t have direct experience with hydro myself. Interestingly, the 5KW rule seems to apply here as well, based on literature: forget about it if you can’t go big.

Gas/Propane Generators

Gas/propane generators sometimes have a 240V split phase output. It is best to load both sides equally; otherwise you will be wasting half of the power of your generator. Be careful buying cords. The majority incorrectly only tap into one side of 240V outlet. Buy something like this to optimally use generator power. And when wiring into your house using a transfer switch, ensure that the circuits you wired to one side or another are about equal in terms of power consumption under normal conditions.

Propane has indefinite shelf life versus gasoline. Keep in mind that in winter the propane tank will have less pressure inside. If the propane is used at a high rate, the expansion cools the tank further. You may end up having access to only about half of the propane in the tank, unless you warm your tank while using it with, for instance, this blanket. The 120 Volt heater with magnetic head does not make good enough contact with the convex surface of the tank to transfer enough heat to keep it warm. I tried two 400W heaters attached to a 20lb tank and they barely raised the temperature by a few degrees, which was not sufficient. They work well on flat paramagnetic surfaces though.

Battery Charger

When selecting the solar battery charger, make sure you pick an MPPT (maximum power point tracking) type. They will allow for fluctuation in the input voltage, while flexibly adjusting the voltage conversion ratio, so they utilize the full power of the solar panel, not wasting any energy. In other words, their peak conversion efficiency is close to 100%, no matter how much sun is hitting your panels. There is a good technical explanation of PWM vs MPPT technology online. The same is true for wind turbines. However, many wind turbines come with a built-in battery charger, so pay attention to what is built in. Not all are equal.

Sensitive Equipment and Battery Care

Lower quality generators may not operate at exactly 60 Hz (50Hz in other countries) and can cause electronics to misbehave or UPSes to switch to internal batteries due to low quality of current received. You may want to consider protecting sensitive equipment with an isolation transformer.

Figure-2

If your batteries have multiple types of posts, it is usually less expensive and more secure to use the ones with holes in them to connect wires. Cover the car type posts with short pieces of garden hose, just a bit longer that the post itself; this will prevent shorting if you accidentally drop a wrench onto the battery. You can also use shrink tubing on cables to reduce the risk of shorts.

Some battery manufacturers provide a max torque rating for screws attaching cables. Be careful and measure your torque wrench, because you can ruin your battery by twisting the soft lead post with excessive force.

Apply shrink wrap or terminal caps boots to cover as much of the exposed conductor as possible to prevent shorts.

Tips

It is a good idea to install a 12 volt outlet right next to your batteries, for instances, where you connect them to the inverter, so that you can power a 12 volt tool or light directly without power conversion back and forth. A 12V outlet can be used without powering an inverter.

If you have power tools, it is useful to have a car charger for the tool batteries to recharge your tools without running the inverter, which can use as much power as it deposits into the batteries.

Energy Storage

The only practical way to store electric energy is in chemical form in batteries. There is loss of energy while it is being deposited into batteries and converted into chemical energy and then also while the battery sits in storage (self-discharge). The following is applicable to 12 Volt lead acid batteries.

The output from a battery will be DC current. Batteries are built from units of 2 Volts each. For example, six of these units make a 12 Volt battery. Physically larger batteries are more susceptible to damage from rough handling (drops, vibration) because they use larger plates. Some batteries are of a sealed type of manufacturing and require no maintenance, but be aware however that the pressure regulators on sealed batteries will vent above 5 psi typically, resulting in permanent loss of electrolytes that is impossible to replenish due to their sealed nature. These batteries needs to be charged very carefully with temperature corrected charging regime, especially at high ambient temperature. Other batteries are of open type and will evaporate water while being charged, requiring a periodic refill with distilled water (manual or automatic). The open type also emit hydrogen gas, which may accumulate in the battery compartment and explode with a spark; therefore, adequate ventilation is needed. Because hydrogen is lighter than air, ventilate at the top not bottom of the compartment. They can also spill acid more easily. There is no consent amongst specialists which type is best for deep cycle energy backup systems. While sealed batteries require less attention if “ruined” by overcharging at high temps, they can’t be corrected as easily as open types.

Due to low voltages, when supplying power to pumps, microwave ovens, welding equipment, and other large loads, the current flowing through the cables from batteries to the inverter is very high, easily in the hundreds of amperes. This requires cables with very little resistance– beefy expensive ones. (There is good information on cables online.) Generally speaking, anything below 1.0AWG will be inadequate to power a 2kW or larger inverter. I suggest using welding cables or similar, because they are pure copper, not alloys, with less resistance, and are built of hundreds of thin wires, that makes them very flexible, relatively speaking of course, and easy to work with. Also their outer shell is multi-layer and much more durable. Thick cables require capable crimping tool. If by any chance your connectors will be exposed to salt water, only use a tinned copper connector, as copper “rusts” in salt water quickly and tin protects the connectors and wire. Alternatively, you can cover the surface of your connections with dielectric silicone grease, but that will make re-arranging your batteries a little messier. You can buy premade connecting cables, but I suggest developing the skill and acquiring the equipment to do this yourself.

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