|
|
|||
|
|
|||
| Articles | Utilties in Baja |
|---|---|
| September
5th, 2003 Utilities in Baja by Roberto
Being a person who has been there and done that, I thought I would share some of the lessons I have learned, so that others may avoid my mistakes. The first one is the temptation to set up SOMETHING, quickly. This is a mistake, because it will undoubtedly lead to the same expense being incurred multiple times, or in a system that falls short of what we would like. So, here goes. First, some background information. The heart of an off-grid system is the inverter. This device converts DC to AC, and optionally charges batteries from an external source like the grid or a genset as well as provide management and switching of the external source. Basic inverters provide AC from a DC battery bank and output a modified sine wave. This form of AC will drive MOST appliances but not all. In the simple case, the problems are a simple whine from the appliances' power supply, in others the complete failure of the appliance, with the possibility of overheating of the powe supply and even fire. A high-end inverter provides a true sine wave output, which is at least as good or better than what is provided by your U.S. utility. It's often better because there is less variation in the output. In addition, a high-end inverter provides for the connection of an external power source, like a genset. With this type of setup, you connect your casa, trailer, whatever to one place and one place only - the output of the inverter. The inverter takes care of how to provide the power, whether by inverting the DC or directly from the external source. When the external source is being used, the inverter will typically also charge your batteries. For example, I use a Xantrex ProSine 2500 sine wave inverter. The 30 AMP connector on my RV is connected to same. I also have a Honda EU3000i generator connected to the inverter. In the summer, when it's VERY HOT, I run the generator at nighttime. At that time the inverter switches off, the power from the generator is directed to the RV and some of the power (configurable and set at 10 AMP HOURS) is used to recharge the battery bank. No special wiring outside of the power shed. I also run 110 for power outlets in the shed and 110V lighting on the Ramada. Prices for inverters range from <$100 to >$2,000, depending on features, waveform and rated output. The inverter, of course, is connected to a battery bank. When purchasing a set of batteries, it is important to consider the type of batteries to use and their capacity. Firstly, always use deep-cycle batteries. These batteries are designed to survive multiple cycles where the bank is discharged to a low level (25% charge or less). A starting battery is designed to provide max cranking amps for a short time to turn starters, and would quickly lose it's ability to hold a charge under such use. There are some very complex ways to measure required capacity, but I use a fairly simple formula. Decide which appliances you are likely to use during the course of 24 hours. Check their rating in terms of watts used. Then figure how many hours/day you will use each one. Then, for each appliance divide the required wattage by 10 and then multiply by the number of hours used. This will give you the DC Amp Hours required to run that appliance. This works because amperage = wattage / voltage, but the conversion efficiency (or inefficiency) of an inverter increased the required amperage, otherwise you would divide by 12. At that point, add up the requirements of all the appliances and you will have the required amp hours your battery bank must provide in the course of 24 hours to feed your inverter. That will tell you how many batteries to buy. I buy 6V flooded golf-cart batteries, each of which provides 220 AH and are very cost effective. An alternative would be a sealed battery, either gell-cell or AGM (Absorbed Glass Mat), each of which use a semi-solid electrolyte. They are more expensive, but offer some real advantages, especially in placement (they do not evaporate flammable gas) and improved resistance to failure due to overcharging in hot weather. A pair of flooded, 6V (Golf Cart) batteries) start around $122, a pair of AGM batteries will cost approximately 2.5 times that. The third element are solar panels. Ideally, you will need sufficient power produced by the panels to replace the ampere hours used during the course of 24 hours. Certainly, this is true if you are going to stay down for more than a week at a time. Otherwise, you can get away with a very large battery bank that cannot get depleted during the entire time you are there once you add back the output of the panels. This is a good way to save money, since solar panels are quite expensive, much more so than batteries. To decide how many panels you need, perform a calculation similar to the one you used to compute the battery requirements. The gotcha there is panels are rated at 17-20 volts. So, a 120 watt panel will provide 120 watts at 17-20 volts at a temperature of approximately 70 degrees. The hotter it gets, the more that output will decrease. In addition, battery charging occurs at approximately 14 volts, so some amperage is lost during the conversion from 17 to 14 volts. The rule of thumb I use is compute the output using the maufacturers' ratings, and buy 20-30% more panels. The price for Kyocera 120 watt panels is around $480. The final piece of the puzzle is the solar charge controller. This device is connected between the solar array and the battery bank. It monitors the state of charge of the batteries and directs the required charging voltage from the solar array to the battery bank. Modern controllers are three-stage chargers, where the charging voltage is lowered as the statge of charge rises. This is because the ability of a battery to accept a charge dimishes inversely to the state of charge. Modern controllers also have the ability to monitor the temperature of the battery, and lowers the charging voltage at higher battery temperatures. This prevents flooded batteries from losing all the electrolyte and all batteries from being "cooked". Batteries are less able to accept a charge as temperatures rise. A trace 60 AMP charge controlle costs about $165. These are the components of a solar system. When I recover from typing I will tell you what I learned about installing the parts and wiring them togther. Wiring It All Together Ok, so we now have a 1. Inverter The first step is to wire the Inverter to the battery bank. The correct way to do this is to take 1. The maximum current flow the inverter is rated for (this is
typically at LEAST twice the continuous rating)
using the table above, apply the following formula to decide the wire gauge you should use to do the installation: CM = I * L * 10.75 / E CM = Wire size in Circular Mils * to compute the current, take the peak rating and divide by 10. This will provide the peak amperage between the inverter and the batteries. * Note that you must multiply the distance between the inverter and the battery bank by two so you use the ROUND TRIP distance between the two. Round up the result of the calculation to the next value in the table. For example: 1000 Watt inverter continuous (2000 peak) CM = (2000 / 10) * 20 / 0.36 You will need 00 or 2/0 wire for your installation. The most cost-efficient way to do this correctly is to purchase the required cable from a welding supply shop. Along with it buy crimp terminals, a crimp tool and shrink-wrap tubing. Crimp-and-shrink, and connect the inverter to the battery bank. For additional safety, given the high amperage flow across the circuit, install a high-amperage DC fuse between the inverter and the bank. I like to use Marine rated fuse blocks, because the are designed for the purpose and highly resistant to marine corrosion (again, we're talking about Baja here). Here is an example: http://www.westmarine.com/webapp/wcs/stores/servlet/ProductD isplay?storeId=10001&langId=-1&catalogId=10001&c lassNum=119&subdeptNum=118&storeNum=9&productId= 17898 The same computation and procedure applies to connecting the solar panel array. You will be connecting the panels in parallel (positive-to-positive and negative-to-negative) so you must add the panel wattages together. For the purpose of the installation, divide the cumulative panel wattage by an average of the nominal voltage rating of the panel (typically around 17V). You connect the output of the Solar Array to the Charge Controller, and the output of the controller to the battery bank, using the same cable size. Again, good crimp-and-shrink procedure is very important in order to make sure you have good connections and max corrosion protection. The next step is to connect your RV, casa, whatever to the inverter output. This is much simpler, divide the rated inverter output by 110. This will give you the rated output in amperes. Go to the local Home Depot and buy Romex cable rated appropriately. Connect that to a standard outdoor outlet and you're done. Connecting the (optional) generator is very similar to the previous
step. Divide the max rated generator output by 110, providing
the amperage the generator is rated for. Then buy the Romex, connect
and you are good to go. Discuss This Article in The "Baja Tech" Forum. |