The secret to living off the grid
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Can one live entirely off the Grid?
Like it or not we have a world-wide shortage of power. In India and South Africa regular power outages have become the norm. Inverter and generator sales are going through the roof. However, getting off the grid is not always that simple.
Whilst generators burn fuel and make a noise they are or can be your best friend. If your household is running entirely off LPG for cooking and washing then all you need will be power to the lights and TV plus a couple of other low current draw items. A kettle is not one of these. A 1kW generator can supply a house lighting but not necessarily the TV and audio. 4kW generators have become very popular for this purpose and are available at a good price. Inverters are clean, can be coupled to a solar array to charge the batteries but in terms of cost, from the cheapest 5kW system to the most expensive, 2 to 5 times more expensive than a 4kW genset. These prices are related to sole inverter / batteries and the more expensive inverter / batteries and solar system. Some engineers peg current solar systems at about 2 dollars per watt (currently but dropping). A 5kW diesel generator costs about $1700.00 or 34c per Watt. Prices of gensets are more or less stable but the reality is, genset pricing does fluctuate according to demand and at present the more budget variety are shipped in from China. For a 5kW system powered by inverters, batteries and solar a buyer would be looking at approximately 10 000U$, not exactly chicken feed. However, a budget 5kW system could be had for about 4 000U$ without installation. The imported inverter may not last as long as that from a well established company but at present we would not know – inverter prices have dropped substantially as newer technology is incorporated. And they are very reliable.
Genset irritations – frequency
Users of gen-sets, myself including, have found some minor irritations when using a genset. UPS not locking, microwave ovens not working and over-heating of an APC line regulator. The problem in every case has been genset frequency – designed for 50 or 60Hz but running half load at a much lower frequency. This poses a problem when using appliances which utilise a mains transformer designed for 50 or 60Hz. The core, usually EI laminations of steel, is designed for this frequency and as the frequency drops so does the efficiency. Losses are incurred, released in the form of heat. One can get away with a 50Hz transformer running at 60Hz, but not always at 40Hz. And yes, gensets do sometimes drop to 40Hz. Generator inverters are more stable in this regard but cost a whack more. Large generators, gas or diesel, have very sophisticated mechanical and electrical governor controls. With a hefty price tag. You do not have this on portable and lower priced gensets.
Microwave ovens all have magnetrons, a self oscillating thermionic tube. The magnetron heater voltage, if designed for 6V, may not oscillate at lower heater voltages but usually at the very least, radiation energy drops substantially. On lower genset frequencies the platter or turntable motor also slows down. So make sure your generator output capacity has been rated properly. In our troubled times any mains frequency locked timers are just not reliable under standard generator power. (frequency fluctuations). A big irritation of course is that they are so much more reliable than the so called high current over the counter digital timers.
Inverter woes – output waveform
Inverters are not without their problems either. Most affordable inverters are rock steady at the given output frequency but the output is a modified sine wave. In simple terms, the output is a square wave derived from direct current and wave-shaped by inductors and capacitors or reactive components. This square wave can be run at many kHz or at 50/60Hz. Without embarking on a tedious technical explanation, inverters have improved in leaps and bounds over the last 5 years, especially with the use of IGBT (Insulated gate Bipolar Transistor) which allows huge switching currents at very fast switching speeds. When driving a switched mode power supply which just so happens to use similar circuitry they can be mated almost perfectly. However, driving a mains laminated core transformer from a UPS or modified sine inverter sometimes is always not that easy – a UPS is designed to power a computer as a back up supply.
SMPSU aren’t usually fussy about the frequency being too high or too low, neither the voltage sagging – they will control, switch and regulate. Mains transformers may not like the harmonic content output of the inverter (or UPS) and in some poor designs the low frequency transformers run hot (square wave content / harmonic energy dissipation) or switching can be heard through the loudspeakers of an amplifier. The general rule of thumb is not to connect your class A or AB amplifier to a UPS. If the amplifier is powered by a switching power supply this should then be OK.
Using a UPS on an inverter to prevent drop-out
A problem that does arise through inverter use though is when a user connects a UPS to the inverter to prevent drop-out when applying the mains load transfer switch. Some inverters, usually the line interactive variety, switch to battery power. Manufacturers claim that the inverter output is dirty. Some say the frequency is off. Whatever. A UPS is micro-controlled and looks at many variables, voltage and frequency being two of the criteria. Contamination of either (voltage distortion and frequency lockout) will cause the UPS to switch to battery. In my experience some UPS do not see a pure sine wave input, glitches or any unusual line activity which will cause the UPS to transfer to batteries. Where we do pick up problems is that the power supplies, and these are predominantly switched mode units of television receivers do not always like the inverter or UPS output waveform and in one particular case, a plasma screen power supply blew twice before the owner realised it was in fact the UPS causing the failure. (An APC UPS cured the problem – the problem was the plasma movistor, or anti-surge component in the power supply). Sounds ambiguous but remember this is not for all cases – many TV receivers run happily on a UPS, in fact most do. If you are fortunate enough to have a friend with electronic test equipment it’s a great experiment to check your UPS or inverter output wave shape. Don’t do this if you have no experience in using an oscilloscope though – one may need to use isolated or differential probes to make the tests. Carry the tests out to pure resistive and reactive loads to get a good idea as to how bad it can get.
SCR switched inverters
Years back most inverters used SCRs for switching. Although they could muster up enough courage to drive huge inverter switching transformers many engineers shied away from this technology – bear in mind that the currents were huge, cables were 200A welder gauge and they were cumbersome to work with, using point to point connections. Modern fast switching devices are all PCB mount with the only heavy cabling taking place between batteries and IGBTs. One 600W pure sine wave inverter I was tasked to repair had 30 2N3055s (15 per leg) in the output switching stage. The unit weighed over 200kg. (with batteries). Modern 1000VA UPS and inverters can weigh as little as 5kg depending on battery size. It must be stated that modern technology has made these devices very reliable – indeed the only thing eventually failing over a 5 year period would be the battery. Most electronic failure is due to user ignorance. A typical problem waiting to happen is connecting anti-surge devices after the UPS.
Although engineers will often argue the merits of line interactive UPS over double conversion units, the trend is that double conversion can eliminate many of the weaknesses associated with the use of gensets but not necessarily that of inverters. Inverter power is essentially the same as that of online UPS (double conversion) AND line interactive – both UPS types work equally well if only used when there is a power outage. Since we are living off the grid who really cares – charge the batteries through solar, wind or generator, switch the UPS or inverter on when you need it. Online UPS are favoured for genset use. They also often have a higher continuous current rating. If you do have an online UPS of considerable power, rather use the UPS as the sole source of power, possibly with extended run time batteries and not connect them to an inverter. This gets switched on when you need the power otherwise both online and line interactive draw power from the battery, no matter how small. Use batteries to power LED lighting. Use the online UPS to power the TV receiver. This may all sound confusing but what is the end result you are looking at? Light and heat. Heat means huge current drain. Generating heat in most cases is heavy on LPG as well. Always have a backup supply handy – you may not have light and you may not have wind. To my mind the solution is always around the use of LPG, a small reliable DC generator and of course, solar panels.
Going off the grid means you will have to get a generator, no matter how small, for real emergencies. This would be to power critical lighting and charge batteries. Solar arrays are great if you have the finances but there will be a time when nature takes it’s own direction and there may be inadequate power to charge the batteries for critical circuits. Interestingly enough, if one had to look at real emergencies and people whom have come back from life threatening ordeals unscathed, the best fraternity to gain insight from would be your deep sea fishermen or yachtsmen. They live off the grid – DC power is king. There are many forums associated with living off utility power, a wealth of real-world experience and of course, serious solution providers. A yacht out in the ocean is a house on an island.
Interesting topics and head scratchers:
- NIFE cells
- Rotary converters
- Briggs and Stratton / automobile alternators
- Early submarines and DC power
And again, my Kipor 6700T refuses to start this morning.
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DC power is king. I wonder? We still battle to find cheap storage devices but not to generate huge amounts of AC or DC power. So the rational conclusion should be that DC storage is king because we cannot store AC. DC is also definitely preferred in transmission lines because of the lower losses. The outcome may just be that one day our homes may be all supplied with DC which goes through an inverter to generate AC power to feed inductive loads. As there are no phase angles (synching) to worry about a tie-in system from your home PV should be easier to design and implement.