Can't decide on 24v or 48v for new build?

[machinist]

I have 64 1"x 2" x 1/2" ne42 magnets, 20 lbs of 14 gage coil wire, and a fairly well equiped home machine shop. I have a modified 10' hawt flying in my back yard now with a 12v Hugh Piggott axial flux alternater on it that I use to heat my wife's greenhouse.
I would like to have a battery bank to support the greenhouse and have some reserve in case of grid power outage,(which is more frequent now). I'm swinging a 11' blade diameter now, but I'm limited to a maximum of 12' because of city regs and tower hiegth.
Any input would be appreciated.
thanks
i_r_machinist 

[DanG]

Just getting the power down the tower, forgetting charger and inverter efficiency...

If you're at 5% losses using 12V down-the-tower to go to 24 halves your current line losses, to 48 halves that again.

Example: if at 5% line losses now, reusing your drop lines...

if 1kwh daily harvested (avg) = 365000 watt hours yearly - at 12V 18.25kwh lost, at 24V 9.13kwh lost, at 48V 4.6kwh lost.

At those numbers 48V is two weeks free wind power over 12V and five days free over 24V.

I went 48V, still don't have a complete system up, but paid myself back with banked kilowatt hours to cover any hassle

[Flux]

At 12 ft you should be able to manage well enough with 24v or 48v. Battery cost for the same energy store will be similar. Quite a lot depends on your other factors and how you use the heat. If you heat directly at 24 or 48v then much will depend on the cost of heaters.

If you use an inverter then for heating a cheap msw inverter will be fine and you will be able to get much cheaper ones for 24v.

System losses depend a lot on the distance from the mill to the battery, if it is a very long run then 48v will be better, but otherwise you may be as well off at 24v. If you design for an efficient alternator and low line loss you will stall and you will have to add the loss somewhere.

I haven't even looked at your magnet and wire situation to see how that fits in, it may be the deciding factor if your wire size suits one voltage much better than the other.

Normally I would say 48v but in this case I think you are perfectly within the scope of 24v and you need to look at other factors such as the cost of switchgear, that tends to rise very steeply beyond about 30v as that is the point where sustained arcs tend to dictate that you use better equipment.

I think you are right to move away from 12v but either alternative should be fine.

Flux

[rossw]

Just getting the power down the tower, forgetting charger and inverter efficiency...

If you're at 5% losses using 12V down-the-tower to go to 24 halves your current line losses, to 48 halves that again.

It's worse than that, Dang. For a given *power*, the cable losses are *squared*.

Plucking numbers to make calculations easy:

480 watts is 10A at 48V.  20A at 24V.  40A at 12V. (I agree the current halves with doubling the volts)

Copper loss is I^2 * R, so if you have 0.1 ohms resistance:
At 10A thats 100*.1 = 10 watts loss.
At 20A thats 400*.1 = 40 watts loss.
At 40A thats 1600*.1 = 160 watts loss.

[machinist]

Sorry for the late reply. I had to go roof a house in San Angelo, Texas. Wind blew a bunch of shingles off.
So....
24v= less loss than 12v, more loss than 48v, cheaper inverters
48v= less loss than 24v, more expensive inverters, can take the power longer distances

30' pole height, 25' to greenhouse. I have the bridge rectifiers in the greenhouse. Is this a problem? Should they be up at the alternator?

thanks
i_r_machinist

[Flux]

No problem with the rectifiers in the greenhouse, if you already have the cable then the decision is made. If you were starting from scratch and you could t lerate the rectifier up on the turbine where it is difficult to deal with then you could have saved one cable.

The loss is the same for the same size cable with 3 wires for 3 phase or 2 wires for dc. ( If heat was an issue then the 3 phase case runs cooler as you have 3 wires dissipating the heat instead of 2 but cable heat is not an issue).

55ft is not a long run for 24v and if your cables were sized for 12v you will be well in for 24v.

I think you have summed up your options correctly.

Flux

[jlt]

Small wind power say !2 ft is not going to heat much of a green house.A better use be for a pump to move water from  solar collectors.12'x12' collector would make ten times more heat.It would heat 200 gal. of water on a sunny day. wind turbine only about 10 gallons.But solar collectors are not much fun to see working.Sorry to be off topic.but I have a 12 ft running and the 1000watt dump load has never made much heat.

[Flux]

This is way off topic but I tend to agree that a 10ft mill won't do much heating unless it is on a very windy site.

This is a tricky area, solar collectors are fine but make the heat when it's not needed, it's quite a challenge to store much of this heat for use when it is wanted.

If this is mains back up and it is not off for too long then heaters from batteries may hold things in check long enough especially if producing minimum heat just to keep plants going.

Normally for heating, batteries are best avoided, but in this case the problems of storing direct heat are similar to the solar collector and storing the basic greenhouse heat so it may be more practical to have a large battery bank and hope it is not called for too long or too often.

Flux

[DanG]

Maybe the greenhouse heat part of wind power is to run the controls and pumps/fans for (propane, diesel, kerosene, wood, coal) heat?

I hesitate posting this part but here goes.. cheaper inverters?

Remember you get what you pay for. I'm not trying to make a jab at anyone's flavor-of-the-month inverter/charger brand, just sharing advice that might get overlooked. The practice of buying cheap & buying often has been discouraged in these forums for a long time.

There are just less 2nd & 3rd tier technology & component-quality inverters available in 48V, before any modified-sine or pure-sine decisions. The top quality UL or CSA or Eu CE listed** option-rich inverters usually have nearly identical prices at 24/48V input voltages. (+/- 5%?)

Take the time to draft out your needs and ask questions!

[ChrisOlson]

Sorry to be off topic.but I have a 12 ft running and the 1000watt dump load has never made much heat.

We heat all our water with electric because my wife refuses to have any gas appliances in the house.  We use 4.6 kWh per day, on average, for our hot water needs.  My 12 foot turbine, plus the solar panels, makes 14.8 kWh per day averaged out over the long term, about half coming from the turbine and the other half from the solar panels over the course of a year.

We use about 25 gallons of hot water a day, and heat it from well temperature to 125 degrees with the inverters driving the elements at 2 kW each.  So I would say a 12 foot turbine is definitely capable of heating more than 10 gallons of water per day.  I never got no hot water either when I used DC power to drive the elements.  But driving them with the inverters at the full rated capacity of the element seems to be a lot more efficient than using low voltage DC power, even with the losses in the inverters factored in.

It takes .166 kWh to heat every gallon of water 60 degrees F.  So even a very poor performing 12 foot turbine that only makes 4 kWh a day should be able to heat 25 gallons of water.
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Chris

[machinist]

Doesn't take much heat for a greenhouse in this part of Texas. I think we had less than 20 nights below freezing, and 3 days where it did get above that during the daytime, and this was a bad winter. It has been extremely windy here. To the point where I've tightened my guy wires.
Thanks
i_r_machinist

[ChrisOlson]

Doesn't take much heat for a greenhouse in this part of Texas. I think we had less than 20 nights below freezing, and 3 days where it did get above that during the daytime, and this was a bad winter. It has been extremely windy here. To the point where I've tightened my guy wires.

I maybe missed it, but what sort of heaters are you running?
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Chris

[machinist]

I have about a foot long hot water heater element in an open steel box with lava rocks. Direct short from the rectifiers. Small fan circulates air. Not exactly rocket science but for a quicky fix it did the trick. On the coldest nights I did start an electric heater from grid power.

Chris,
I've read several posts here referencing your designs. Do you have a web site or photos anywhere? What are you doing different?
Thanks
 

[ChrisOlson]

I have about a foot long hot water heater element in an open steel box with lava rocks. Direct short from the rectifiers. Small fan circulates air. Not exactly rocket science but for a quicky fix it did the trick. On the coldest nights I did start an electric heater from grid power.
I've read several posts here referencing your designs. Do you have a web site or photos anywhere? What are you doing different?

Indeed.  You get 3400 BTU/kWh and being you're not clamping the voltage with a battery I'm guessing it probably runs pretty good.  If it works and keeps the greenhouse above freezing, and does the job, then it works despite the doubters that say, "It just CAN'T work".

Unfortunately I do everything different and none of my stuff is supposed to work either.  But it does     It's a long story so check your PM's and I can sort of answer your questions there.
--
Chris

[Watt]

Sorry for the late reply. I had to go roof a house in San Angelo, Texas. Wind blew a bunch of shingles off.
So....
24v= less loss than 12v, more loss than 48v, cheaper inverters
48v= less loss than 24v, more expensive inverters, can take the power longer distances

30' pole height, 25' to greenhouse. I have the bridge rectifiers in the greenhouse. Is this a problem? Should they be up at the alternator?

thanks
i_r_machinist

Sorry for the thread high jack... Where do you live?  I have to wish I would have helped you with that roof.  I live just northwest of San Angelo  ~15 miles.  Darn the luck.  Anyway, back to your topic.