What does your off grid power system have in common with the power company’s cross-country high power lines? Both systems need to be designed to move electrical power from one place to another with minimal loss. In their case its across states, and in ours it may be across the house or property.
Getting a firm grip on this fixed loss will help you make good choices early on in designing your off grid power system.
Transporting Power involves loss
Unfortunately we don’t live in a lossless and frictionless world that makes the calculations in your physics class work. When power is moved between its source to distant locations where it is consumed, there will be some loss in distributing that power. To get the most from your remote power system, these losses must be minimized. However, line loss will be influenced by your budget sometimes because long, and large diameter wire can be expensive!
Loss calculations
Ohm’s law describes how electrical power can be calculated:
- Power(watts) = Volts x Current(amps)
- Volts = Current(amps) x Resistance (ohms)
In the ideal situation, there should be no voltage difference measured at either end of the same wire. In real life there is a small difference. An easy way to think of it is that the wire consumes a portion of the power it delivers. Using the two relationships above we have:
- Power (watts) = Current x Current x Resistance
The power lost in a wire increases really fast with increasing current. The key to keeping line loss low is to keep the current low, or have really big wire with very low resistance.
Since power is the product of Volts and Amps, the power company changes their voltage to very high values before sending them over the transmission lines. With the high voltage, less current flows over the lines for the same power delivered. Line loss is reduced, and more power is available to consumers.
DC Appliances or Inverter
In a similar way, our off grid power systems use this principle. Before inverters were practical, off grid homes would be wired for DC appliances at 12V. Much power is lost distributing this way because the current in the wires is higher.
When we use and inverter, we win in two ways. One, is the convenience factor because we can use our normal appliances. The second reason is that power loss in the home wiring is minimized when we use higher voltage and lower current.
When we want to move power from our solar array to the charging system, a 24V system will have less line loss and require a smaller diameter wire than its lower voltage cousin at 12V. In general the advantages of higher voltage systems is the fact that you can move them farther away from where you store the power. Things can be moved farther to that sunny patch on the property.
What do you think? What choices have you made about your battery bank voltages. Does the comparison to the power company lines make sense to you? Tell us about it in the comments below!
Related posts:
While doing an estimate for inverters on a ranch last year I noticed a utility style power line running half a mile from the generator building to the assembly hall and dormitory. Then I realised the generator contractor had done a smart thing. The normal 240/120 3 wire output had been fed to a transformer and kicked up to 8,200 volts to minimize losses over that half mile or so. A second transformer stepped the voltage back down to useable levels at the destination building. The high voltage line continued another mile down to some barns. Still cheaper than setting up a second generator at the barn and dormitory hall.
handy program I use a lot is found as a free download at
http://forums.iboats.com/showthread.php?t=257336
Program is called wire sizer 3.1 and saves a lot of headache in figuring out what size to use. It allows toy to quickly determine exactly how much percentage loss a give size at a given voltage Good for any voltage AC or DC within reason.
While 24 or 48 volt battery systems make sense from an efficiency perspective many people object because they still want to use 12V DC appliances. One solution is to use DC-DC converters to change the 24 or 48 volt battery voltage down to 12V at point of use. Several manufacturers now offer them at reasonable prices. I picked up a 20Amp unit, 24V to 12V for under $100 from Victron. Other readily available makes is Newmar, Mastervolt, and Analytic systems. Several more brands are available from some of-grid suppliers but I have no personal experience with these.
UPDATE
A nwere and improved wire size program is available here: http://www.midcoast.com/~aft/index2.html
This 3.2 version also has metric wires sizes listed and subtle refinements on th ehidden calculations. Plus you do not have to register and log into the website to download to program. I have used this improved version for a while now because /i do a lot of design wirk for Europeand and Asian based clients who use metric size wire.
My wife’s uncle upgraded his 12V system to a newer 48V system with an Outback 3.0 kW inverter. During sunless winter days his genverter runs less time and thus burns less fuel. System uses same number of L-16 batteries just connected differently and is thus more efficient.
Arlid, thanks for the links to the wire sizing programs.
The one thing about stringing all your L16s in series makes me nervous, one cell in one battery can mess things up. 2 sets of 24V would provide a little better insurance?
I understand your concern. However the redundancy of two 24V banks in parallel is offset by the problems with two strings in parallel. When there are two sets of batteries in parallel you always have slight differences between the two that cause one bank to get slightly less charger than the other. Either one bank gets undercharged or else the other gets over charged. Either can lead to trouble down the road after ever a couple of years.
In the case of having two 24V banks instead of one 48 Volt charge I think I would end up with a system for alternating them so one bank is only being charged as the other is discharging via the inverter. This would allow the bank being charged more fully as the other bank is providing the load current to the inverter. A simple 1-2- Both – OFF selector switch can handle the switching manually if you have a ganged pair of them.
This means the solar panel will come closer to fully charging the bank through to float stage as compared to simultaneously discharging and charging the same bank as is more normal. this should reduce or eliminate build up of lead sulfate.
The one big advantage of one single series string of battery cells is that each cell gets the same charge current (as per ohms law) meaning all the cells tend to get exactly the same amount of charge thus minimizing build up of lead sulfate.