Backup power DC/inverter approach

[Bill Arden]

I've pushed 12V car alternators up to 48 volts by removing the core regulator and driving it externally, but that is not something I would rely on.

From a technology point of view, it's easier (probably cheaper when using surplus) to use a rotary converter for large motor starting. The rotary converter also gives you sine wave three phase power.

The variable speed alternator/inverter approach is probably the most efficient when you are operating at low loads most of the time.

--
Some other batteries to think about.
http://en.wikipedia.org/wiki/Vanadium_redox_battery
http://en.wikipedia.org/wiki/Nickel-iron_battery

 

[Bob NH]

Yes the battery, if used, will be carefully selected.

But what I need now is a DC alternator, 48 VDC, maybe 8 - 12 KW with good surge characteristics for motor starting, with engine speed dependent on actual load. Using propane or capable of being converted. This is the purpose of the post. The variable speed engine characteristic will probably make surge response less effective than a constant speed generator.

A larger version of the Honda inverter based 6500 (5 KW) would be considered.

A process that I have found useful in designing a system is the following:

1. Make a list of all of the requirements.
2. Create a system design concept that meets the needs described by those requirements.
3. Analyze that design (performance, cost, fuel economies, deficiencies, . . . )
4. Create one or more additional system designs that improve in some way on the original design.
5. Analyze and compare the designs to see where you can select features or improve on the designs to achieve the system with optimum cost and performance.

I offer/suggest a baseline concept for your consideration:

1. A battery system with inverter that will provide enough power to start the largest motor while running your basic household needs.
2. A load-shedding system that will temporarily shed non-essential loads if there is not enough capacity to start a large motor.
3. Battery capacity somewhere in the range of 5 to 10 kWHr when fully charged.
4. A generator that can charge the battery system while operating near the maximum fuel efficiency point, and which can be shut off at intervals to save fuel. The kW of the generator might be in the range of 3 times your average kW over a typical day.

Determine the sizes and cost of the elements of the system, then see where improvements can be made. Maybe you will find that large savings can be made if you adjust the requirements a bit.

Then try to find areas where you can make improvements in cost or performance.

Now compare the parameters of that system with what you already have in mind. How does it compare on cost, performance, fuel consumption, and whatever else is important to you.

Now you have a basis for picking the best systm to meet your needs.

 

[Sponsor]

 

[Alternety]

This is what I am trying to synthesize in the 12 KW range.http://www.hayesequipment.com/eu6500is.htm

Two of them would give me the total KW but unless both are running won't fix peak loads. Using this unit on propane would mean converting the carburetor - hence a voided warranty.

Lets just take 10 KW of battery capacity. They should not be discharged past 50% of capacity. So 20 KWhr of battery@ 48 V is about 420 Ahr batteries. Just for argument lets say my average load is Just a quick look I found a 6 V 546 Ahr battery for $845 X 8 for 48 V, we have $6760 +shipping. Plus shipping and a battery box, ventilation, cables, battery charging control, etc. 48 V is a good place. Inverters are readily available and keeps wire losses/sizes down.

Inverter costs will exist with or without batteries. Another quick price is $1,855 for 3500 W X3 = 10,500. And I still don't have the generator. Assembled DC generators in the necessary range are fairly rare from what I have been able to find. Some welding units are out there. They are noisy and I suspect they have minimal voltage regulation.

So I am looking at 10 -20 K without a generator for the two approaches. Obviously many permutations are possible. I think the Honda generator is in the $5 K range. I have not seen one locally and they can't have prices in ads. @ of them would fix it. The load most likely to cause a problem (ignoring any future elevator) is the water pump. Although the freezer and refrigerator starting simultaneously could do almost as poorly. I could make that wait until a second generator came online. But, there is no reasonable way (and esthetically acceptable to my wife) to get suitable control wires from the pump to the generator. There is really no good way to get to batteries either. I would probably have to build the battery box in what is going to be a shop. A gas vent from there would be near impossible.

I really have spent a lot of thought on permutations. Simplest to install, probably second cheapest, comes back to A DC generator with inverter. Or of course just put in the Kohler backup unit. The Kohler is by far the easiest to install and control. It is also the simplest in terms of things to break, and the least expensive to buy. And with the Kohler, I don't have to be there when power goes out. With the other approaches I have to build controls to do that. Then when I die, my wife won't be able to get it fixed. It is not simple.