Battery Backup or not??

vidhan

Aloha All!

This is my first post, so please excuse me if it's not in the right forum.

I'm a building contractor in Hawaii. As part of a large addition and renovation, we (my crew and my electrician) will be installing a grid-tied system in a home where the current electrical usage has averaged 51KW/day. The owners would like to offset part of this with PV. They don't like the idea that they would be without power if the grid went down, and so someone suggested to them a system with battery back-up that would use 64 SolarWorld SW175 modules and 24 Discovere AGM 6v 390 amp-hour batteries.

I am wondering if the battery back-up makes sense from a practical standpoint. It seems to me that, with the inefficiencies involved in the battery back-up and the additional costs for batteries, charge-controller etc., they would be better off installing a propane-driven generator for the few times when the power would be out.

Any comments on this??

Also, does anyone out there know, given their current usage figures, how long those batteries might provide power for? I am thinking that we might need a generator anyway just to keep the batteries from being too deeply discharged!

Thanks for reading this.

Windsun

Re: Battery Backup or not??

In a really practical sense, battery backup for such systems is usually a poor choice, unless they are losoing grid power quite often and have critical systems they have to keep powered up.

The size of the battery system you are talking about would provide about 10-15 hours backup power at most, and would add several thousand dollars to the cost. And they would have to be replaced in 6-8 years.

niel

Re: Battery Backup or not??

firstly, let me say that, yes, a generator might be good to have on hand, but when the power is out at a gas station you won't be pumping any gas as the pumps need power to run.
secondly, this sounds like one of those prepackaged deals and one size does not fit all. i assume that 51kw/day means 51kwh (kilowatt hours). know that you never want to deplete your batteries below 50% except if it's an emergency because of battery life issues. that means 102kwh in batteries/day and i recommend doubling it for another days use so now you have 204kwh in batteries. add to that some charging losses, resistance losses, and inverter losses and that may be around 25% more for 255kwh in batteries. i'd guess the batteries to be wired in a 48v configuration and if they aren't in that prepackaged deal then definitely don't go with that deal. 255,000w/48v=5,312.5ah. yeowwww.
this is quite huge. in a case like this, it is better to use the grid-tie arrangement for better efficiency without the battery backup. you could allow for a small battery backup for certain key things to run briefly for say 8 hours or so to put off the worst of any outages. this would be a seperate system from the grid-tie only arrangement and this one can still be grid-tie, but with a smaller battery backup and smaller number of pvs. things like the pool and air conditioning are high power suckers and these types of things aren't good to run on a backups system as it makes it quite large and expensive.
if you are going with the entire grid-tie with battery arrangement the batteries are undersized as it is about 56kwh and is good for 1/2 day at the rates you use power. the pvs seem ok at 11.2kw and will cover all of your electrical needs barring cloudy days. i'm guessing you'd get 5-6 hours solid sun there nearly year round and after losses seems about right. doubling the speced 56kwh battery bank with those pvs and your usage would be in order to allow for a full days backup and the charge percentage with the system you speced would be around 26% and doubling the battery bank would cause a charge % of 13%.
if you don't need to have solar supply all of the power daily and only want a part of it, i'd suggest cutting the pvs in half to use with that size of a battery bank you specified. others may say even smaller to use with the generator as backup(noisy though). too many variables as this can be configured in many ways to work for them most of the time. the more you want, the more it costs.
i also thought i'd add that conservation efforts need to be taken by them to cut back their usage because it's easier to save a watt than produce it.

vidhan

Re: Battery Backup or not??

Thank you both for your input.

Windsun, I believe the extra cost of the battery back up is about $15,000 in batteries, charge controller and odds&ends. I am thinking to tell my clients they would be better off spending the money on something else and having a more efficient PV system.

Niel, I anticipate that once we have added another 1200 sq ft. to this house that their electric usage will be even greater than 51kwh/day. I don't want to go with fewer panels. Then again I don't want to buy four times the number of batteries.

You lost me regarding charge %-I should have told you I'm a newbie when it comes to solar. Can you explain?

Thanks again.

BB.

Re: Battery Backup or not??

Regarding the xx% charging of batteries...

Typically, the recommendation is the charging device (solar panels, AC charger, genset, etc.) should be around 5% to 13% of the battery bank's 20 hour rating... (20 hour rating is the current which a fully 100% charged battery can output for 20 hours, to 0% state of charge)...

Say you have an 800 AH rated battery bank, then the charging current should be around 5%*800AH=40 amps to 13%*800AH=104 amps.

Below 5%, for a flooded cell battery, you can't bubble the electrolyte to properly mix it... Above 13%, you may overheat the battery with too much current (certain types of batteries can go below or above those ratings--but you have to check with the manufacturer's recommendations).

Some customers want what they want... If you can talk with them and find out their minimum needs (enough for the fridge, some lights, and other emergency requirements), a minimum sized emergency solar off-grid system will work nicely. If they need to run 100% of their loads for X days--and it is a rare event--a dedicated genset and appropriate fuel supply may be a better solution. And use the money saved (hopefully) to install a nice Grid Tied Solar system (green and cost control--depending on the price of power in your region).

There are also nice hybrid Grid Tied / Off Grid systems (like the Xantrex XW)... They behave like grid tied when power is good, and have battery backup for when grid goes down. Again--sizing the system for the smallest emergency loads would be a big help.

If you need a better estimate on costs--let us know which city is nearest the solar calculator's reference sites. If the loads are seasonal (high when the sun is

-Bill

niel

Re: Battery Backup or not??

the charge % is just the amount of charge current going into a battery in conjunction with its amphour (ah) capacity. most standard lead acid batteries should fall in the range of 5-13%, but as i stated, the battery bank you speced would windup with a 26% rate of charge, which is a very fast rate. this can be utilized if 1 the battery manufacturer says you can or 2 there is a draw or load on the pvs enough to take away from the high rate of charge. i am thinking the agms would most likely do fine with that high rate, but i can't say that the manufacturer oks it.
as to the customer, i'd recommend a smaller battery backed grid-tie for only vital things such as some lights, the refrigerator, security systems, etc. and the rest can be the more efficient grid-tie only that would not do anything during an outage, but would send power to the grid better than the battery backed gt system would. with the battery backed grid-tie you can still sell some of that power(or offset your power usage) and have something to automatically kick in should the power go down. generators can do this too, but are noisy and must have a supply of fuel. (gas goes bad in time) when you're away it's nice to know your food won't go bad for short outages and if longer outages occur this affords the time to get gasoline and setup the generator.
in any case this will be a big and expensive project with much work to do even in the planning stages of it. let us know what they elect to do and inform us how well things go when it's completed. of course you can still ask questions in the meantime.

crewzer

Re: Battery Backup or not??

Vidhan,

Good Day from Arlington, Washington, where I’m visiting OutBack’s company HQ. We manufacturer battery-based utility-interactive (BBUI) inverters, and we believe our product is well-suited to load offset and backup applications such as you are considering.

See: http://www.outbackpower.com/products/sinewave_inverter/grid_tie/
and: http://www.outbackpower.com/pdf/brochures/Grid_Interactive_Systems_2.pdf

The cost for a battery bank to backup a full home load for a long period of time can indeed be a sizeable expense. However, the size of the array and the size of the battery bank need not be an “all-or-nothing” proposition. Instead, identifying key loads, sizing a battery bank to realistic requirements, and a bit more operational understanding may point to a broad and cost effective solution range.

For example, a home with a 51 kWh/day energy load may only require 5 kWh/day to back up “essential” loads via sub-panel. Such loads might include a fridge, compact fluorescent lights (CFLs), a computer network, and an entertainment system. The attached picture provides a graphic overview of a typical application.

A 48 V x 250 Ah AGM battery bank (four each size 8D batteries) could meet this net energy requirement for 24 hours without discharging the battery bank below 50% and with virtually no maintenance. Sunny weather during a blackout would extend this autonomy by powering loads and/or charging the batteries during the day.

A set of four Concorde 8D AGM batteries would cost $2,127.15 from NAWS, this site’s host. That’s not a trivial amount. However, it needs to be compared against the cost, maintenance requirements, fuel, and noise for a generator backup. Alternately, a smaller bank to support a smaller load and/or shorter outage would cost less. One report I just read indicated that 99.6% of U.S. utility outages that last over three minutes also last less than 2 hours (ref: “Handbook of Pwer Quality”; A. Baggini).

It’s my view that battery efficiency should not be a major decision factor. Our system operates by keeping the batteries at/near their target float voltage, which requires little charge current. Similarly, since the batteries are kept at float and are rarely cycled, they should last for a decade ore more. Exide claims 20 year float service for their Absolyte AGM batteries.

In short, a well thought-out BBUI system can serve a home well without a huge battery expense or significant maintenance commitment. Such a system will rarely actually “sell” back to the grid, but it will offset (reduce) overall energy consumption, and, when (not if) the grid fails, it will provide quiet battery backup to essential loads for a customer-determined period of time.

Another consideration might be to invest in conservation, aka reduction in energy consumption. In general, it can often be cheaper to purchase new energy efficient appliances (i.e., Energy Star fridge, HE washer, compact fluorescent lamps, set-back thermostat, etc.) and install a smaller PV system than it would be to spec a PV system to run older and less efficient loads.

For example, my “new” 22 cf fridge consumes an average of 1.2 kWh/day and cost $700. The old fridge was rated at 2.3 kWh/day. It would have cost a lot more than $700 to increase the size of my PV system just to run the old fridge.

HTH,
Jim / crewzer