Solar PV system for heating water

[vtmaps]

I might go for option 2 and check for further info on it in the forum.

Check carefully... there are a number of issues with using and switching DC current. 

Regarding dgd's option 3:  Some folks buy a dedicated, cheap non-sinewave inverter to heat water with AC.  Others use their main household inverter to heat the water.

--vtMaps

[asdex]

Thanks, yes I look at using a SSR.
Just wanted to ask, I'm using the calculator to check if the cable I'm going to use is big enough.
The rated PV array current is 17.22 amps and the battery charging current is 66.7 amps both a 24 volt system (nominal battery voltage). Which one would I use to calculate using cable size and distance to get voltage drop and current carrying ability?
Also what would be the voltage in the cable to the controller? VMP or VOC?
Thanks,

[dgd]

Thanks very much. I might go for option 2 and check for further info on it in the forum.
I'll use just a couple of say 60A/H batteries in series with little draw off so they will be fully charged nearly all the time. The controller should sense this and the output from the panels will be diverted most of the time to a 24v dc element.
Thanks,.

So little draw off from the low capacity batteries then why bother having them and the controller? Just wire up the solar panels in series and then to an element via a breaker.  High DC voltage and smaller current means smaller cable/lower cable costs.

When the fire with wetback takes over water heating then rewire the PVs to your existing solar electric system, that is where a Classic controller would be most useful.

Or what about just increasing the size of your solar electric system by adding panels and then using a Classic to divert excess power, after batteries are charged, to the water cyclinder. Ideal for summer time when there tends to be excess power available.
then in those cold months with fire on the extra PVs would be good for getting some decent power into your main batteries.

dgd

[asdex]

So little draw off from the low capacity batteries then why bother having them and the controller? Just wire up the solar panels in series and then to an element via a breaker.  High DC voltage and smaller current means smaller cable/lower cable costs.

Ok thanks, I think this would be a nice simple option. My only thought and the complicating factor was that an MPPT controller in the system would be of benefit by improving the solar panel output. Some say 15-20% more.
If this isn't the case then I'll go ahead and wire direct via a SSR controlled by the thermostat.
Cheers,

[dgd]

Ok thanks, I think this would be a nice simple option. My only thought and the complicating factor was that an MPPT controller in the system would be of benefit by improving the solar panel output. Some say 15-20% more.
If this isn't the case then I'll go ahead and wire direct via a SSR controlled by the thermostat.
Cheers,

The mppt controller would not offer any advantage as the PVs produce the same power whether connected to the controller or direct to the heating element.
However, on your existing PV/turbine system the Classic would certainly be an improvement 

SSR such as Crydom D2D20, 200V 20A (Ebay). Look it up on interweb and you will see a wiring diagram with snubber diode. Can also get a schottky diode suitable for this on eebay.. you will probably have to power the control signal inputs to the SSR from your existing battery via thermostat switch and breaker or fuse...
Hope this is useful.. 
dgd

ps where are you in NZ?

[vtmaps]

Or what about just increasing the size of your solar electric system by adding panels and then using a Classic to divert excess power, after batteries are charged, to the water cyclinder. Ideal for summer time when there tends to be excess power available.
then in those cold months with fire on the extra PVs would be good for getting some decent power into your main batteries.

That's what I suggested in Reply #1 in this thread.

As far as direct connection of solar panels to a heating element:  not very efficient... the heating element cannot adjust its resistance to find the max power point.

--vtMaps

[dgd]

Or what about just increasing the size of your solar electric system by adding panels and then using a Classic to divert excess power, after batteries are charged, to the water cyclinder...

That's what I suggested in Reply #1 in this thread.

yes, I still think this is the best way to go 

As far as direct connection of solar panels to a heating element:  not very efficient... the heating element cannot adjust its resistance to find the max power point.

Good point. Matching the element to the available PV wattage at near MPV is probably the most efficient way to use PVs direct connected to an element.  Otherwise it can be inefficient.

In my diversion system, where some of my PV strings connect to a 1Kw element, I see almost max power from the panels to the element when batteries are in float and its a nice bright day.
dgd

[vtmaps]

As far as direct connection of solar panels to a heating element:  not very efficient... the heating element cannot adjust its resistance to find the max power point.

Here's a good explanation by Bill at NAWS of direct PV to heating element:
http://www.wind-sun.com/ForumVB/showthread.php?p=169845#post169845

So, you can see that your resistive heater on a solar panel is only "optimum" very near rated Voltage*Current. And with solar panels that have variable output current during the day, the power collected will fall with the square in the drop of current. And give you optimum power for only a couple of hours in the middle of the day (with good weather, a tracking array, etc.).

--vtMaps

[dgd]

I had looked at that NAWS thread before but had decided it did not give me any new insight in to how I could improve the potential efficiency of my water heating. A good and informative read though..

The issue, for me anyhow, is that effective water heating, after battery is charged, really depends on having enough excess power. That means PVs working at their most efficient, mpv and high wattage. That means good bright sunshine day with many hours clear skies.
Matching the element wattage to the available max PV power, or somewhat under it, is essential.

When weather conditions are not optimal and still the batteries get to float then with perhaps little excess power its never going to be a good scenario for water heating. No matter how you do the sums if you put less than about 60% of the elements power rating into it then water heating becomes either impossible or very very slow.

If water heating was my main priority then I would go for those evacuated tube water heating panels.

dgd

[asdex]

Thanks, there must be some way to do it efficiently. I can spend the same money on a vacuum tube thermal system but it will sit there for a large part of the year doing nothing. We go away a lot too so will  have problems with over temperature and freezing. At least with PV I can use it to charge batteries in winter when my existing panels are making the best of the low light. We are in Central North Otago.
Cheers,

[asdex]

Ok, I think I'll go ahead with the direct to element. I'll get a 24 volt element I think and switch to a Midnite 150 once the water gets hot.
Cheers,

[dgd]

Ok, I think I'll go ahead with the direct to element. I'll get a 24 volt element I think and switch to a Midnite 150 once the water gets hot.
Cheers,

I see you have a sig line now with details of your RE system.
Although this is getting away from the topic I see you use 12v 880Ah bank, it would appear you have 6v 220Ah golf cart type batteries in four parallel strings of two.
There is a lot of discussion on this forum about battery configurations and I see how changing to 24v would be easy enough for you to do. That may also improve the battery life reducing to 2 string of 4 batteries. The ideal would be one string but that would need reconfig to a 48V system, one string of eight batteries.
These six new PVs you are getting would setup nicely as 3 strings of two into classic to charge 24v or 2 strings of 3 to charge 48v.
With the 3 in series that would be about 120V DC which you could feed direct to a 2kw 120V element then when water hot enough to switch the PVs to Classic using a decent DC ssr such as Crydom D2D40 (200V at 40A).
This could be a completely automatic system if the water temp thermostat controls the ssr.

Just some thoughts...

dgd

[zoneblue]

In real life a mppt controller gains maybe 10% efficiency over a PWM controller, mostly because the battery forces a placement on the IV curve well below the max power point. With panel direct you dont get that as bad. After max power the curve falls off really quick to Isc, where in a design like this your element could be running at.

If we are concerned about every last drop, a bigger problem is finding a big enough low voltage element. I havent seen any bigger than 1200W.

And then,  of course once the water is hot, the thermostat kicks off and all  discussions about efficency become a lot more immaterial. Therefore efficiency to my mind isnt really the primary criteria here.

If you really dont need the power for household use, then wired direct to panel is by far the simpliest, cheapest, most effective route.

But im not sure about the cost benefit of pure PV, youd have to run the numbers, cf evacuated tube. The real value of PV hot water is the use of surplus power, that otherwise wouldve been wasted. CBA numbers for that look pretty darn spectacular. Almost free.

[asdex]

This sounds like good advice thanks. Once I have the panels I'll look at how to connect them depending on how far away I site then from the controller/batteries/element. One site is 35 meters away so may need to set up the panels for 48 volts but i don't want to go higher than 24 volts for the battery bank as 48 volt converters, inverters etc are expensive while 24volt equipment is easy to find.
Thanks

Ok, I think I'll go ahead with the direct to element. I'll get a 24 volt element I think and switch to a Midnite 150 once the water gets hot.
Cheers,

I see you have a sig line now with details of your RE system.
Although this is getting away from the topic I see you use 12v 880Ah bank, it would appear you have 6v 220Ah golf cart type batteries in four parallel strings of two.
There is a lot of discussion on this forum about battery configurations and I see how changing to 24v would be easy enough for you to do. That may also improve the battery life reducing to 2 string of 4 batteries. The ideal would be one string but that would need reconfig to a 48V system, one string of eight batteries.
These six new PVs you are getting would setup nicely as 3 strings of two into classic to charge 24v or 2 strings of 3 to charge 48v.
With the 3 in series that would be about 120V DC which you could feed direct to a 2kw 120V element then when water hot enough to switch the PVs to Classic using a decent DC ssr such as Crydom D2D40 (200V at 40A).
This could be a completely automatic system if the water temp thermostat controls the ssr.

Just some thoughts...

dgd

[zoneblue]

I suggest you go back to basics and do a design for the core system. eg:

- 35m at 44v/3kW sounds like a LOT of copper/v drop
- battery voltage cf system size
- pv string config cf controller compat

Theres no substitute for extensive reading where the basics are concerned. Dont even think about making any important decisions until you have a good grasp of it, otherwise youll just be flailing around spending money on things you cant use.