Off Grid system sizing advice

[OzarkBrandon]

I am looking at a piece of land that currently has no electricity.  Utility will run service once a primary residence is under construction, but that won't happen for several years.  I would like to immediately build a barn with a small wing finished to accommodate overnight stays.  My thought was that I could buy some batteries, an inverter, charger to provide basic power for the occasional weekend stay.  I'd buy a PV panel or two to keep the batteries charged while away, but would plan on using a generator for the heavy recharging.  An online energy calculation tool suggested that my max power draw would be .45 KW and I'd use 3 KWh

I'd welcome any suggestions for the type, size, components selection for this scenario.  Don't need anything too specific as the costs will help me decide if this is the right property to buy...

Thanks in advance for any input!

[OzarkBrandon]

Specifically, I would like to know if I should go 12 or 24V, and how many AH the battery bank should be...

[UK4X4]

12/24/48 all depends on your true system size and requirements, you have to start from the load and work backwards


if your going to be using 12V appliances and lights then stick to 12v for your batteries, if your going to be using an invertor and 110V,AC, higher voltages make for smaller wiring sizes to provide the same power and less losses in your cabling


here's something I wrote at work for a solar power project design

As with most things in solar power, equipment load is not always as it seems

What is written on appliances is often not what the equipment actually draws

but is the rating of the power supply suitably big enough to power the item with some spare.

So when you take your 8 amp computer power supply off and measure its actual draw
its usually surprisingly less.

So the preferred method is to measure actual power consumption in order to make the system
as efficient and compact as you can.

Beware with laptops and other computing gear as depending on their operational status, power consumption changes, try and use the equipment in the same state as you would expect it to be in general use.

So in our test install we intend to have two NHQG gauges, a Uniconn system running
both an Ifield and standard communications card.

Being that you can't separate the individual items and the Uniconn just has a generic wattage as per its datasheet

o AC: 100–250 V rms, 1A, 50/60 Hz (300 V rms max. over-range)
o DC: 24 V +/– 2%, 2 A

When this was measured with both gauges connected in the workshop and the fan on

It was seen under test to only consume 0.7 amps at 24 volts

Others reported seeing a higher amperage on other installed systems and a 1.5amp capable supply was deemed sufficient for the Uniconn, gauges and cards.








Power system design               





As with many engineering solutions there is no one factual way of doing it
a solar power system has many nuances from the capturing a natural entity and estimates in how efficient your panel will be on day 1 or 2 years down the road, losses in cabling and the consumption of the charge controller.

There are general guidelines and work methods to design your system so that it functions
and checks that can be done when the system is installed or at least tested in conditions similar
to where it will be finally installed.

Here is the worked example using data from the previous sections

According to the product data sheet the unit would consume 2 amps for every hour the unit was going to operate, but our test was 0.7 amps, so lets take a mid route to insure we have
sufficient power, but not end up with a huge array

1.5 amps X 24 hrs = 36 AH/day

Sunlight was determined for the area of installation at 5.5 hrs per day

So in order for the system to function for 24hrs a day

I will need the solar panels to provide 36 amps in 5.5 hrs

Panels are sold with many different output voltages and currents depending on the physical
quantity and efficiency of the individual cells and are usually listed by Power
ie 1 off 180 watt panel

So we need to change our AH/day to power in order to choose a suitable panel

36 amps required/ 5.5 hours  = 6.5 amps per hour at 24 volts

P=VI

Power required would be 24V X 6.5 amps= 156 watts required



Being that panels never come in exactly the
power you require, you may need to use one
greater to or just smaller than you require.


With that we have already taken liberties with higher than the required amps V's measured
and lowering the max sun...from 5.9 to 5.5 our system is over specification for now.

We can be pretty sure that if we use 150 watts of solar power we should be fine.

As an example if we had gone with measured Uniconn ampage at 0.7 amps...the solar array would have only required 73 watts of power, so we should be good stead.


Here is an example of a power calculation done for an earlier project in Venezuela
Where multiple items were going to be powered by the system a higher safety margin would be required with the multiple items being used


Item   Description   Voltage    Power   Current
Max   Hours per day   AH/DAY
1   DTS 320   1 channel   24   25   1.0   24   25
2   Scada pack   24   0.4   0.017   24   0.4
3   12-24V transformer   24       0.1   24   2.4
4   Radio   24   0.25   0.015   24   0.5
5   Chiller   24   25   1.0   6   6.25
6   Radio Transmitting   24   30   1.25   8   10
7   Gauges    24       0.06   24   1.44
    3.5       47.99

A Total of 47.99 amps required per day
      47.99 / 4.5(Max power hours per day) =10.66 amps / Hour required

      2 off 150-watt panels will produce;   
      150W / 24V X 2 =12.5 AH
      12.5 amps X 4.5 Hrs =  56.25 amps as a nominal current produced
   
     

     




Batteries


     
Battery bank sizing can be one of the more complex and important calculations in your system design. If the battery bank is oversized, you risk not being able to keep it fully charged; if the battery bank is sized too small, you won't be able to run your intended loads for as long as you'd planned

Never an easy thing to specify as there are so many types, makes and technologies available
     
So we'll just start with the basics.
     
We want to do two things, have the system operate every day, keeping the battery in good   condition, and have a system reserve for those days with no sunlight due to storms or sand.

So whichever type of battery you choose you need to find out the discharge capacity the battery prefers and its ultimate discharge level before it gets damaged.

All batteries have a shorter life span the deeper the discharge , so a nominal percentage for the average AGM is <30% discharge on a regular basis.

Temperature too effects battery life and efficiency, a 10% margin is again added for overly hot or cold condition's

The battery reserve needs to be considered too for those weather anomalies and sand storms that for us were a concern

4 days no sun is a pretty standard battery reserve for remote SCADA or acquisition systems and often specified in solar powered projects

You can then use this data to size your system










So going back to our original Uniconn system requirements and adding in our preferred battery state.

Using a 2 days reserve as we are in a desert situation rather than temperate zone
In Normal operating the battery only discharges by 30%
and a 10% reduction in efficiency for winter or high temperatures
     

      36 AH/Day x 2 days x 1.3(Normal discharge) x 1.1(Reduction)
= 103 amp hour battery


      The system installed housed 2 off 12V 105AH batteries wired in series
producing 24Volts
     
     
The supplier however used a different method of calculating the battery size
With a selection of factors

Battery aging factor
Low temperature capacity compensation factor
Design margin
Minimum state of charge at the end of autonomy period
      Battery Inefficiency factor

They had the same battery size in the end but is listed as three days reserve in the datasheets.


thats my rough way of doing it- there are many ways !

[OzarkBrandon]

Everything running would be 110 volt. Figured separate inverter and charger so I could turn off the inverter when I am not there.  As small as I think the system would be, I would guess 12 volt, but maybe that is irrelevant.

[MountainDon]

Figured separate inverter and charger so I could turn off the inverter when I am not there.

The charger that is in an inverter/charger unit is only for charging from a generator or other 120 VAC source.

The charger that is used to use the DC power from a PV panel(s) is called a charge controller and is a separate thing.

~~~~~

I'd buy a PV panel or two to keep the batteries charged while away, but would plan on using a generator for the heavy recharging.

To keep the batteries charged while you are away and the system is shut down will only require a very small panel.

Depending on a generator to do the bulk of the battery charging has a few drawbacks. A generator can be useful for a bulk charge, getting the batteries up to around 80& charged. After that is is wasteful of fuel as the actual charge current into the batteries is slowed down. The final charge to 100% takes a while and uses much fuel. That final charge is best done with solar but will require more than just a small panel.

FWIW, if you are going to be grid tied in the end I would skip the batteries and solar. Or maybe have a small battery setup (a couple golf cart batteries) with a small inverter and rely on the generator for major power. A small (15 - 25 watt PV panel with a cheapy charge controller to maintain the charge while absent. A small battery bank can easily supply power for LED lights, TV, small stuff. Run the gen for microwave and larger stuff. IMO, that may be more cost efficient IF in the end the grid is tied in.

~~~~

[OzarkBrandon]

Thanks MountainDon, I think that is sound advice.  At some point it would be grid tied, but I still wouldn't mind maintaining a small PV array and battery bank for emergency backup power.  That sounds good in theory, but I don't want to over spend on controllers, inverters, and then never really use them.

So, if I just go with an inexpensive inverter, and maybe spend a little more on panels (maybe 400 watts or so) to do all the recharging.  I would still use the generator for big stuff like you suggest. Would it be bad for the batteries to run down over the weekend (no less than 50%), and then slowly recharge them over the course of the week when the inverter is off and their is no external drain.  I wouldn't be using the system every weekend, but it would always have at least a week to recharge.

[MountainDon]

Would it be bad for the batteries to run down over the weekend (no less than 50%), and then slowly recharge them over the course of the week ...

It would not be optimal.  The reason is it is not good for a lead acid battery to be left in a lower state of charge for extended periods. More than a day is extended to me; but I am conservative and cautious. Some articles I have read indicate it takes a longer period, weeks, for the sulfation to turn "hard". The less than full state of charge can allow the sulfation to harden. Once that occurs it is irreversible, or at best difficult to break down.

In reality the discharge down to 50% may do more harm (shorten the battery life) than taking a few days to get the charge back to full. I'm not sure.

BTW, the sulfation issue is one big reason I do not like the idea of using a generator as the main power source to charge batteries. Seldom will the generator be run long enough to completely recharge the batteries to where they pass through bulk, into absorb and then into float. If generators must be used to recharge they are best used early in the day; a small PV array can then bring the batteries through into float. It's very good for the batteries to reach float daily.

Panel capacity required varies tremendously with location. I am very fortunate in NM. Others farther north or in areas with lots of clouds need more panels. Not sure what your location is like. There are solar insolation maps that can help as a guide.

And I agree that a backup battery system is very nice to have when grid tied. Also does your power utility offer a net buy back plan for grid ties? Many do, but they also have very specific requirements for equipment.

[OzarkBrandon]

I do recall seeing that net metering is available, but I didn't dig into the details as I didn't imagine I'd have a big enough system to make it worth the expense of tying it to the grid.  Thanks for the advice Mountain Don, that was very helpful.  Now I just need to get that 33acre parcel purchased!

[hpinson]

A bit off topic, but I ran into this this other day - a power consumption chart of LCD and Plasma TVs.

http://www.rtings.com/info/lcd-vs-led-vs-plasma/power-consumption-and-electricity-cost

[OzarkBrandon]

That was pretty awesome.  I think I'll have to buy a LED TV when the time comes.