Grid or Batteries?

evilgenius · Posted: Mon Dec 10, 2007 7:30 am Post subject: Grid or Batteries?

I would like to know from the members which they think makes more sense going forward, using renewable energy to sell excess to the grid or to store it for your own future use. I was in B&Q (Guildford) the other day and saw a wind turbine for sale for about 1,800 pounds. It seemed a good deal until I looked a little closer and realized that it didn't have any kind of an energy storage system attached. They didn't include an option for a storage system either. They wanted you to sell what power you didn't use to the grid.

I'm a bit of a curmudgeon, so I want a battery system in order to achieve energy security for myself, but I might be over conservative in this matter. Maybe selling is the best option.

By the way, does anybody know at what rate you will be paid or credited if you do sell? If you are under a punitive two tier rate structure does the energy you sell count against a particular tier or can the company you deal with apply it arbitrarily?

WisJim · Posted: Mon Dec 10, 2007 9:36 am Post subject: Re: Grid or Batteries?

Batteries add a significant amount of cost to the system, and reduce efficiency. If you want to be cost effective and have grid power available, a batteryless system makes sense. But, I have the grid available, but I also have a large battery bank, adequate for 3 days or more. I will admit it is not really cost effective, but is certainly nice when we have had a storm that took out the grid in the neighborhood for 3 or 4 days. We lived a normal life while neighbors were running generators, loosing freezers full of food, hauling water, etc.

The rate and method depend on the country and state you are in, and the power company that you are dealing with. Even in the USA, with laws that are supposed to make it uniform throughout the country, there is a lot of variation.

frankthetank · Posted: Mon Dec 10, 2007 12:00 pm Post subject: Re: Grid or Batteries?

Jim-

In your experience how long does a battery last (i'm guessing you are using 6volt golf cart type batteries?). Do you use an inverter or just use 12volt or something?
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WatchfulEye · Tar Sands Joined: Jun 07, 2007 Posts: 54

Batteries are a bit of a liability - so unless you really need them, you're best off avoiding them.

They are energy inefficient, expensive, bulky, potentially hazardous, and require regular maintenance and/or replacement. Batteries can be installed and used safely and reliably, but if you have grid power available, they are not cost effective (except in specific cases, e.g. to maintain power to critical equipment) and if backup power is required, then a petrol/diesel generator would be a far better option even with fuel prices twice or three times the current UK pump price.

If you want to set up with personal renewable energy, then it's relatively straight-forward to get set up:

1) You tell your electricity supplier that you want to connect a grid-connected wind generator and they will then sort out getting permission for you to connect it
2) Once you have permission, you plug the generator in, or you get an electrician to hook it up.

Whenever your generator runs, it will deliver power to your fusebox, which will be distributed around your home. E.g. if your appliances are using 1000 W and your generator is producing 500 W, only 500 W will need to come from the grid, and your meter will read 500 W.

The problem here is that if your home is using 500W and your generator generating 1000W, 500W will be sent out to the grid, but a normal electricity will not record it (because they do not allow selling of electricity).

If you use a lot of electricity, and have a small generator like the B&Q one, then this may not be too much of a loss. But, if you use only a little power, or have a big generator, this can be a problem.

You need to negotiate with your electricity supplier, about how to sell your energy.

Typically, there are 2 ways of doing it:
1. You pay normal price for all electricity that you import. You get a credit (usually about 1/2 retail price) for any energy you generate, whether you use it or not.

The advantage of this, is that it only requires installation of a meter on your generator, so is relatively straightforward and cheap to arrange. Great deal if your generator supplements your usage, but you lose out if you generate more than you use and actually need to sell.

2. You obtain a 'net metering' agreement. This requires upgrading your electricity meter to distinguish between imported and exported energy and installing a generator meter.

This will get you the best price as any power that you use yourself, obtains full credit, and any power that you sell gets a good price (usually 75 - 100% of the retail price) and you get to keep the subsidies yourself.

The disadvantage is that your supplier needs to arrange upgrade of the metering, and do all the associated paperwork in sorting you out with multiple meters, etc. plus you have to pay for all the extra meters (up to £250) - so this can take weeks or months to organise.

gnm · Posted: Mon Dec 10, 2007 3:04 pm Post subject: Re: Grid or Batteries?

Batteries are problematic and expensive, but there are many good solutions available which are less trouble than the old golf cart type FLA (flooded lead acid).

I would suggest that anyone who is serious about solar PV and wants to be truly independent of the utilities (or is a doomer) look into the varieties of AGM sealed batteries available for their system size. AGM's currently offer good bang/buck ratios and are WAY less trouble than FLA's. Single AGM's come in many sizes and can be grouped in series and/or parallel to suit just about any size need. If you really have a large load and want to run the whole house I would suggest skipping straight to forklift type AGM batteries.

Frank, how long a given battery will last depends on how big your load is, how big the battery is, and how long it has been in service. If you mean lifespan in years then it would depend on the type of battery, how often and how deeply it is discharged, and how good the maintenance has been on it.

Generally speaking you want to avoid discharging your battery bank more than 20% for regular use. To size a battery bank you need to add all the household loads up (time and wattage). The occasional 50% discharge will not significantly harm it as long as that is not a daily occurrence. So if you are looking at say a large 12V AGM (think looks like a bus battery - roughly 3 car batteries in size) it will have about 220Ah (amp hours) at 12V available. You really want to stay within the top 50Ah or so of this. So, 50Ah at 12V is 600Watt hours. Translation: You can run a 27inch tube TV (~250watts) for 2-3 hours or a 100watt light bulb for 6 hours without hurting this battery. If you tank the battery you could keep the load up until the inverter hit its low voltage disconnect or about 24 hours on that 100 watt lightbulb.

One more thing to consider - grid intertie can be rather paperwork and red tape intensive.

hope that helps....

-G
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jbeckton · Expert Joined: Jan 05, 2007 Posts: 1920

Like others have said, storing the energy when the grid is stable is inefficient. If you are going to have a surplus of power I suggest you look into selling it back to the grid. Are you really going to use all of the power generated in the middle of the night?

Here in the states, they are considering converting the standard meters to "smart meters" which will charge you the current price per kWh, not just add it all up for a flat fee. This means that the power generated in the late afternoon will be the most expensive to buy and the most profitable to sell. The idea is to help with demand side management (DSM) and reduce peak load. This is great for those with solar panels because this (late afternoon) is when they have excess power to sell.

You might want to look into what hours of the day you will be generating power (when the wind is statistically stronger) and compare that to what times of the day you use power.

Anyone have information on this or does it vary by region?

If you find that you are not using power at peak times you should look into selling to power back or adjusting your usage habits to take full advantage of the power you generate.
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WisJim · Posted: Tue Dec 11, 2007 10:42 am Post subject: Re: Grid or Batteries?

To answer some questions about my system:
currently my wind generator and battery and inverter are a 24 voltd DC system. The battery is a 1500+ amp hour set made up of 12- 2 volt cells, like a forklift battery, and weighs over a ton (12 cells at about 200 pounds each). They cost over $4000 almost 10 years ago, and would cost more than that to replace today.

I expect them to last 20 to 25 years, based on my experience with previous sets of batteries. A typical user with his first set of golf cart type batteries, however, is lucky to get more than 2 or 3 years use out of them because most people don't take proper care of their batteries, and that type of battery doesn't have the potential life of the better quality and heavier and more expensive batteries. Golf cart type batteries are a good first battery, however, as they are cheap to replace as a "learner" set.

gnm · Posted: Tue Dec 11, 2007 11:15 am Post subject: Re: Grid or Batteries?

Nice setup WisJim!

I started with a bank of about ~1000Ah made up of Trojan T105's - a good quality FLA. I found the maintenance on those to be a little irritating but not as irritating as the little bubbles and vapors which corrode any uncoated metal connections and other things in near the battery bank. Those lasted almost 8 years and I switched them out for 12V concorde sun-xtender 2120 AGM's. I love these things. you could eat lunch off them and they don't corrode connections. I should mention that many/most? forklift type cells are 2V AGM's.

-G
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PeakOiler · Posted: Tue Dec 11, 2007 11:18 am Post subject: Re: Grid or Batteries?

The last time I checked with the local utility company, they require a US$2 million liability insurance policy to net meter.

I didn't bother to ask my insurance company what the premium would be for such a policy.

Guess it's batteries for me.
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frankthetank · Posted: Tue Dec 11, 2007 1:35 pm Post subject: Re: Grid or Batteries?

My friend did some work for the local phone company and they didn't use the lead acid type, i think it was zinc or something. This was for those little buildings you see out in the country (backup power). Not sure the voltage, but i think he said they were really old.

Thats a very nice setup and i would never have believed u could keep a battery going for 20+years!
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"Oil is going up because we use too much oil, and the capacity to replace reserves is dwindling"
-President Bush 11/07/07

Mechler · Posted: Tue Dec 11, 2007 1:43 pm Post subject: Re: Grid or Batteries?

I've been looking into the forklift batteries because, as long as you can move 'em, they seem the most suitable for a full-power back-up system. They are marketed as lasting over 20 years. WisJim, you mentioned taking proper care of the batteries - with these forklift types, can you tell me some of the details? Thanks.
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gnm · Posted: Tue Dec 11, 2007 3:49 pm Post subject: Re: Grid or Batteries?

Mechler, my neighbor (1/4 mile away) who is completely off grid purchased a 36V forklift battery at salvage for about $300 (lead salvage) - We took each cell out of the steel frame (they are about 180lbs each) and tested them individually. We found that only 3 of the cells were bad. He has been running this for a couple of years now quite successfully. Since he only runs 12V on the DC side and a 12V DC inverter for the AC side we were able to put together 2 strings at 12V. These are AGM batteries and the only maintenance needed is not running them too far below 20% of rated capacity. Most of the newer forklift batteries I have seen are AGM's.

-G
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All right, you primitive screw-heads, listen up!

WisJim · Posted: Wed Dec 12, 2007 8:49 am Post subject: Re: Grid or Batteries?

WisJim · Posted: Wed Dec 12, 2007 9:01 am Post subject: Re: Grid or Batteries?

Some one asked about taking care of batteries. Two things of utmost importance are to NEVER let the electrolyte level drop below the top of the plates, in a flooded lead acid battery. Never discharge the battery below 50% of its capacity, or whatever the recommended maximum discharge of the battery is. There are numerous books, articles, and websites with more info and more detail.
http://www.backwoodssolar.com/
http://www.windsun.com/Batteries/Battery_FAQ.htm#Industrial%20deep%20cycle%20batteries
Using a battery monitor such as a Tri-Metric enables you to keep better track of the actual charging and discharging of your battery and can make a significant difference in battery life.

PeakOiler · Posted: Wed Dec 12, 2007 8:44 pm Post subject: Re: Grid or Batteries?