Inverter question / Solar System - Check My Work?

[DIY Guy]

Ok, so I am trying to hook up my hot tub to a solar panel system. Therefore, I need a 120/240 volt, and at least 2000 watt inverter. However, all of such inverters that I can find are pure sine wave inverters (as opposed to modified sine wave) and thusly are in excess of $2000. Is it possible to get a hold of a modified sine wave inverter that is 120/240 volts, or at least something more reasonably priced?

 

[donkey]

http://www.jaycar.com.au/productView.asp?ID=MI5160&CATID=8&form=CAT&SUBCATID=680
dunno how many watts you wanted... but there's a start... the price is australian hope that doesn't deter you. if you have to convert to another country just try googling the product

 

[DIY Guy]

Well, as far as I'm aware I need around 2000 watts at least and I think everything I could get from Australia Europe etc will be 230 volt, but being in the states I need 120/240.

 

[donkey]

in that case ebay or catch of the day sites or others similiar are your friend

 

[DIY Guy]

Is it possible to use a smaller inverter (120vac output), then use a step-up transformer to get to the 120/240v? In cursory searching I can't seem to find any trasformers with a 120/240 output (although there were some that are labeled output: 120V-220V-230V-240V and I'm not sure what exactly that means, i.e. if you could use that for a 120/240v plug or not...)

 

[DIY Guy]

New Solution! Stacking 120v inverters! Now if only someone could explain how in the world to do that...

 

[Resqueline]

What do you mean by 120/240? It doesn't make sense to me. Either you run it on 120 or you run it on 240. I can't see the need for both voltages at the same time.

One other thing is that if I were to heat water I'd not use precious electricity straight from photovoltaic panels (~15% efficiency) into a resistive heater element.
I'd rather use fluid solar heat collectors (~55% efficiency), and/or run a heat-pump with the electricity (~15% * 3 = ~45% efficiency).

 

[DIY Guy]

I'm powering a pre-existing hot tub that requires both 120 and 240 in a 4 wire system. I don't want to delve into essentially building my own by isolating my own heating and pumping systems etc.

 

[DIY Guy]

Solar System - Check My Work?

Ok, so I think I have a completed design for the solar system to power my 120/240v 1400w hot tub 24/7. However, I'm not exactly the most experienced of electricians, and I wouldn't mind having someone make sure my diagrams aren't going to lead me down a path of miserable failure. I've done a lot of my research online over the past few days and I'm not entirely sure I trust that all the math I've been given is 100% correct, especially since I'm trying to splice multiple sets of instructions for each section together. So anyways, would someone be willing to check over my system as a whole to make sure I didn't skip anything, and my math, to make sure I have the right specs for each unit?
Attatched should be a .png with the diagram and specs. Here in CO, we supposedly get about 5hr of direct sunlight a day.
THANKS A MIL GUYS!

 

[DIY Guy]

Specifically, am I right that it should be a 7 ohm resistor, because that seems really small for such a system... And if anyone has suggestions for cheap 12vdc - 120vac inverters, it would be much appreciated!

 

[daddles]

Jeez, from the title, I thought I was going to read about how you developed a star and a bunch of planets...

I think you'll find running the system 24/7 is going to be difficult -- for the same reason there are no solar-powered flashlights.

It's not clear to me what you're trying to accomplish -- do you want this solar system to be supplying 1400 W of power when the sun is shining to heat your hot tub? That's quite a bit of power and, assuming you have enough solar panels, you might want to use it to also provide some electricity for the house or even sell some back to the utility.

I think a solar hot water heating unit would be a heck of a lot cheaper...

 

[DIY Guy]

Yeah, I realized too late that PV System might have been better...

Anywho, the hope is that there will be enough battery backup to last all night as well.

As far as I understand, the hot tub requires 1400W of power every hour to run (as it is rated at 1400W, I believe that is what this translates to).

According to some calculations I got off line, to supply 2kW per hour you only need one 20W solar panel. Now I agree, this doensn't seem like much, but a whole array of panels for one appliance also seems ridiculous.

It doensn't seem to me like providing for this one appliance would leave room for providing back to the house, as the system is supposed to be designed to be just good enough for the hot tub, and not have much extra power. Plus, I don't have a grid tie inverter in my system, just normal 120v stacked modified sine wave inverters going to a sub panel.

While I agree that directly solar heating the water would be cheaper, I think it would be a lot simpler and more asthetically pleasing to run the whole hot tub as is off of a panel, that way I don't have to deal with separating the systems, having hoses running to and from my hot tub, plus having to have both solar heating and solar power for the hot water and pump, filtration, jets, led screen etc.

It just seems less complicated to run the hot tub as a whole as it was meant to, just from a PV system instead of the grid, as opposed to essentially building my own hot tub from scratch making each system, and probably ending up with a hot tub that is neither as good looking, nor as comfortable as one factory made.

 

[Resqueline]

Ok so you're referring to a 120/240V 4-wire three-phase high-leg delta-connected system, where the midpoint of one phase winding is grounded. (Only found in the US.)
So in essence you required a 2kVA 3-phase inverter. I can't imagine those coming cheaply no matter what. You might want to establish if the hot tub requires all voltages.
Edit: looking at your diagram (after I merged the two threads) it seems you didn't require the third phase after all, but your solution won't work wired like that no matter what.
Apart from not understanding power systems you seem to have some serious misconceptions about the power involved: 1.4kW * 24h = 33.6kWh
Add inverter & battery recharging losses and the need to replenish all that power in 5 hours and you end up needing a 10kW PV array.
Besides you'll need a 10 times larger battery bank. 12V * 300Ah = 3.6kWh which will only run the tub for something like 2 hours.
A 20W panel (1.2A) is hardly good for supplying reading lights in a camper, and what's with the 25A charger & 7 ohm resistor?

 

[DIY Guy]

You see, I thought I must have misunderstood something in all the calculations I was trying to follow. Well dang it!
The charge controller is there to make sure you don't overcharge the batteries, so it will dump to the resistor once the batteries are full (which due to the miscalculations, they probably never would be from a 20W panel)
Anyways, I'm pretty sure a 10kW array is rather out of the question price wise (and would the over 400A charge controller needed to go with that even exist commercially?), so it looks like I'm back to straight up grid power from the utility company!
Oh well, thanks for making sure I didn't spend the $1000 to fail miserably and have nothing but a hot tub that wont run, and a bunch of dead batteries! Seeing as I have no clue what the picture attached to your reply even means, it's probably best I not dive straight in to this anyways eh?

 

[Resqueline]

Hehe, yes, I guess a little more research is in order..
The 25A charge controller is 20 times overkill with regards to the 20W panel, but still 30 times underkill with regards to the 10kW array you'd need.
But at the kW levels one usually connects panels in series anyways to get a higher voltage and lower, manageable current levels.
Solar panels don't require a dump load btw., they're just disconnected from the batteries when they're full.
Windmills often require dump loads though, in order to not spin out of control and generate unmanageable high voltages.
The picture I attached is an illustration of the strange (to me) US 3-phase transformers used to deliver both 120V bi-phase and 240V 3-phase.
High-power loads are only connected to A-B-C and not to N (or ground). Ordinary lower-powered loads are connected to either A-N or C-N.

 

[DIY Guy]

That makes, sense; I had forgoten about that. Initially I was thinking of using a turbine, but decided to switch, and didn't think about not requiring a dump load.

Anyways, pricing out doing this with real numbers is getting above $40000, which is more than a bit out of my price range, so I think I'm abandoning the project all together.

 

[DIY Guy]

On looking back over the math, I'm not sure you're right that it takes over 30kWh to power my hot tub all day. No disrespect of course, but that would come out to over 1000kWh a month, and my entire house last month only used 855kWh, so I'm not sure how that could possibly be correct...

 

[Resqueline]

Well, the 1400W heater elements may not be on 100% of the time all the time. The actual power use will depend on how well it's insulated, if the cover is off etc..
You'll have to connect it through a kWh meter of its own to determine the average power it consumes.

 

[davenn]

SNIP

and my entire house last month only used 855kWh, so I'm not sure how that could possibly be correct...

ONLY used in a month !!! ?? farout! what are you using all the power for... major manufacturing at home ?

I could run my home for 10 years with that much power surely you number is incorrect ?
if that is a correct number, then its time you need to assess what you are using it all on and start to look at ways of cutting it back and saving some money that way

I just got my latest power bill in the post this last week for 3, yes Three, months I used 21 kWh which cost me AU$330

It bounces around the 18 - 22 kWh / 3 months depending on how much heating in winter or aircon in summer

cheers
Dave

 

[donkey]

with regards to this project i am offering an alternative that people have offered already, use a solar hot water system. this will lower the heating bill of the water going into the hot tub. the hot tub will still heat it and you may need a plumber to hook up, i forget what its called but it stops 100 degree celcius water from entering(thats boiling temp), then you will use less electricity in heating. the pumps and other things in the hot tub would still run of mains but you might find the cost savings there to be well worth it