Difference between revisions of "Solar assisted ups"

Latest revision as of 16:33, 12 April 2013

taken from....

http://strike.coloradolinux.com/~sjg/solar-assisted-UPS.html

A Solar-Assisted Uninterruptable Power Supply What is a solar-assisted UPS? Like a regular UPS it has a battery, inverter, charge controller and transfer switch. Unlike a regular UPS there is also a solar panel to charge the battery. The control circuitry is different and the battery larger, because the battery is only charged partially by the AC power supply, leaving enough capacity for daily solar charging. Why might you want to build one? Because you'd like a low-budget solar system that doesn't involve any red-tape, letting you offset retail electricity with all the solar power you can generate, and still use AC at night or on cloudy days. Because you'd like an alternative to a generator when the power goes out for extended periods (e.g. to keep a freezer running). Because you want to learn about solar power and would enjoy live plots on your website of solar power generated and used. Because you'd like to invest in something more dependable than the stock market. Because solar-assisted UPSs are unfortunately not yet commercially available. Why might you not want to build one? Because the sulfuric acid in the battery is corrosive, the hydrogen gas it produces is explosive, and the lead is a neurotoxin. Because wiring together a 12V system (like the ones found in cars) requires skill, mistakes could damage equipment or cause injury. Because you don't have a computer or need a UPS. What parts are needed? How much might it cost?

   conventional UPS - hopefully surplus with a dead battery.
   17V solar panel(s) - big enough to provide between half and 100% of your intended load ($330 for 80W)
   12V deep cycle lead-acid battery, twice as large as the maximum daily output of the solar panels ($110 for 100AH)
   control system - including a computer, digital voltmeter and computer-controlled AC outlet ($112 without PC)
   panel rack, fuses and cabling - about $100 

How to Build a solar-assisted UPS

   Order automation/monitoring hardware and get it working on your PC. I bought my Phidgets voltmeter from Trossen Robotics and used an x10 appliance module for the outlet. Since I wanted full details on the productivity of my system, I also got 2 Phidgets ammeters.
   Purchase a fuse block and fuses from an automotive parts supplier or from Del City . Get 16 gauge wire by cutting up an outdoor extension cord. For the solar panel wiring, you can use an extra heavy outdoor extension cord with 12 gauge wire.
   Obtain a conventional UPS (ask your computer people at work or school) open up its battery compartment, remove the old battery, and wire the cabling there to the fuse block. I connected the UPSs crimp connectors to my own crimp connecters with two small tabs of sheet metal.
   Purchase a new and much larger battery. Make sure the UPS can charge and use the battery without any solar connections yet. When shopping for your battery be careful to purchase a fresh one according to the manufacturing codes explained here If you are not using a solar charge controller, the battery must be flooded, not AGM or sealed.
   Wire your digital voltmeter to the fuse block, and plug the UPS into a computer controlled outlet. Write a control script to regulate the battery voltage, only powering the UPS when the battery is less than half charged (less than 11.9V).
   Order a PV panel and optionally a solar charge controller. Wire these into the fuse block, and watch the voltage climb as the sun charges your battery. Install ammeters if desired to monitor how much current is produced and consumed by your system. If you have no solar charge controller, then be sure to use enough power to keep the battery below 14V most days. The battery does need to be equalized monthly, at higher voltages, but very high voltages could burn out the inverter in the UPS. You can manually move your load to utility power, turn off and disconnect the UPS and let the battery equalize on a sunny day when you are home and can stop it before it gets above 15.5V.
   Build a rack for the panel using ordinary construction materials. I cut apart a 10 foot antenna mast and bolted it to the long sides of the panel, letting the ends stick out several inches. I put the ends into small cinderblocks that hold them tightly, and placed 2 of the small blocks on top of larger ones to give the panel the proper tilt and face it towards the equator. I built a reflector from foil-faced plastic insulation, wired to hardware cloth which is in turn wired to additional cinder blocks. This rack is low cost, adjustable, stable in high winds, and doesn't require a permanent structure. 

The above task list is designed to prevent disasters in the form of equipment purchased that doesn't ever become productive. The PV panel is the most expensive item, and its purchase is put off until the rest of the system is functional. Also, the computer-related things are bought first, so you know right away if they will work for you. The battery needs to be purchased after your charging system is in place, because batteries self-discharge and can be damaged if kept at a low state of charge for months. The suggestion that you half-discharge your battery is intended for true deep cycle batteries, which will not be damaged by such a deep discharge. With computer control, you could even use a bank of starter batteries, if you cycled them very shallowly.

About the Economics

I spent about $750 building my 80W solar-assisted UPS, not counting the conventional UPS and the controlling computer. Normally a UPS is intended to power a computer, so I am presuming that you have one of these already. If you are buying a computer only to control the UPS, the economics and productivity of the system are drastically reduced, and I'd recommend against it. A conventional UPS costs only about $100, so you could purchase one of those without blowing the budget. My solar-assisted UPS powers a laptop computer, which is also serving as a webserver, mailserver, television, security system and wireless access point. Today's machines are so powerful that the machine in question is very lightly loaded and draws only 18W. If the primary goal of your project is to save electricity, and you are running a desktop server I'd urge you to replace it with a laptop, because the electricity you save by doing so is much more than you'll be able to produce with the modest solar-assisted UPS described here, and the cost is less. My server cost only $450 including shipping, and the solar panel on my UPS is providing about 70% of the power it uses, or $15 worth of electricity per year. That means my solar-assisted UPS is producing approximately a 2% yield, tax free and increasing in value as electricity rates increase. I spent a little extra money on additional monitoring of my system. A basic solar-assisted UPS could have a slightly higher yield, but a lot depends on how much sunshine you have in your climate, and how much you spend on solar panels.

When commercially produced solar-assisted UPSs become available their economics should be more favorable, because they should contain their own control circuitry, and you could use them without a computer. Since the yield on investment for a home built solar-assisted UPS isn't that different than the yield on a savings bond (1.3% for series E November 2008) it could be argued that small scale solar power is close to grid parity. In fairness, I should admit that a solar-assisted UPS requires a lot more spare parts and effort on the part of the owner than a savings bond. True grid parity isn't quite here yet. The economics of a solar-assisted UPS may be more favorable than the economics of a grid inter-tie solar system, (apart from tax credits) because the engineering standard and documentation requirements are far more extensive for a power plant than for an appliance. A solar-assisted UPS doesn't require a building permit, because no permanent alterations are made to the building, and the connection to building wiring is through an ordinary plug. This makes it practical to install one if you're renting or living with family.