Solar PV system for heating water

[asdex]

Hi, I'm setting up a solar PV system for heating water. We are off grid and decided we don't want a thermal hot water panel sitting idle for 7-8 months of the year when we heat our hot water with our indoor fire. I'm getting 4-6 320 watt PV panels soon to run a 24 volt system and once I have the details worked out will probibly get a Midnite 150 to gain as much power from the sun as I can then hopefully dump it all to the hot water tank. Anything left will charge the batteries but I already have panels and a wind generator for that.
I'll be following anything about configuring the Midnite Classic and if anyone is heating water with PV panels.
I'm in New Zealand.
Cheer's

[vtmaps]

Anything left will charge the batteries but I already have panels and a wind generator for that.

Welcome to the forum.  Lots of folks are making hot water from PV power.  A heat pump type of water heater is the most efficient way, but there are also good reasons to use a resistive type element to heat the water.

The reason to use a resistive element is that the Classic has some unique features that allow it to draw full power from your PV, even as the battery is in absorb or float.  That means it will send variable power to the resistive element, as the opportunity arises.  (a heat pump does not want a variable, intermittent power source).

You might find that you can add your new panels to your existing array and charge your battery AND make hot water from one array.  The larger array will give you the ability to charge your battery on many more marginal days, especially in the winter when you make hot water from the woodstove.

If you wish to pursue this conversation, I suggest you start a new thread.

--vtMaps

[waynefromnscanada]

The Classic has some unique features that allow it to draw full power from your PV, even as the battery is in absorb or float.  That means it will send variable power to the resistive element, as the opportunity arises.
--vtMaps

WOW I didn't realize that about the Classic! Now my mind is active to the various future possibilities when my Classic finally arrives! For instance this morning after a week of very cold weather my batteries housed in the out building are down to minus 8C, which has driven the absorb voltage to 15.5, extremely near the over-voltage trigger point of my Morningstar inverters!

[SolarMusher]

Hi Wayne,
Is there a way to limit voltage on your Tristar controller such as a Classic ?

[Westbranch]

WOW I didn't realize that about the Classic! Now my mind is active to the various future possibilities when my Classic finally arrives!

Wayne, Don't forget to order the WhizBangJR with that Cl 150!  It adds mega functionality to the Classics... its so small it won't even register when in the box with the Classic...

[zoneblue]

If you are buying panels specifically for the purpose of heating water, by far the simplist approach is to arrange them in strings of 120V, and power US 120V elements direct.

Any other route is either a lot more complex, or expensive. Add a classic, aux 2 waste not, SSR, and enough inverter  capacity to power the element.

Trying to find DC elements that match your panels , tank fitting and voltage is like hens teeth. If you are trying this route, i can give links to the ones ive found.

[asdex]

Hi zoneblue, I have looked at your website previously. Very interesting and and practical ideas. I have used the Niwa calculator for my location as well although haven't figured out what the results mean yet. My budget for panels maxes out at 6 x 320 watt Voc 46.28, Imp/V 37.16. I guess that would work with a 240 volt element but i think the whole system would be more efficient with a MPPT controller?
Cheers,

If you are buying panels specifically for the purpose of heating water, by far the simplist approach is to arrange them in strings of 120V, and power US 120V elements direct.

Any other route is either a lot more complex, or expensive. Add a classic, aux 2 waste not, SSR, and enough inverter  capacity to power the element.

Trying to find DC elements that match your panels , tank fitting and voltage is like hens teeth. If you are trying this route, i can give links to the ones ive found.

[dgd]

If you are buying panels specifically for the purpose of heating water, by far the simplist approach is to arrange them in strings of 120V, and power US 120V elements direct.

Asdex,
This is by far the lowest cost solution. You can get low cost 120V elements from many places on Ebay. The main problem with this method is temperature controlling the water in the tank/cyclinder as thermostats on elements will not survive switching DC current. So you need just a plain element.
How do you do temp control now with your wetback water heater?

dgd

[zoneblue]

Hi zoneblue, I have looked at your website previously. Very interesting and and practical ideas. I have used the Niwa calculator for my location as well although haven't figured out what the results mean yet.

A fellow kiwi. A few of us a long ways from home. Just got my second NAWs order via nzpost youshop. Still worked out a fair bit cheaper than getting from independent solar in auck.

My budget for panels maxes out at 6 x 320 watt Voc 46.28, Imp/V 37.16. I guess that would work with a 240 volt element but i think the whole system would be more efficient with a MPPT controller?
Cheers,

Efficient you say? Well what you gain in mppt , you lose in inverter losses so swings and round abouts, no?

For the thermostat, a SSR or relay and a low voltage signal through the thermostat. You can buy 100A starter solenoids on trademe, for about $20. You used to be able to get SSRs here:

http://www.jaycar.co.nz/productView.asp?ID=SY4086&w=solid+state+relay&form=KEYWORD

[zoneblue]

One other problem is that that the elements are diff phyical sizes. In NZ HWC elements are 1.25 inch BSP, US are 1 inch, europe 1.5 inch. Sigh.

You can get a brass adapter for 1 inch to 1.25 inch from the plumber. Just one more place for leaks to occur.

[Westbranch]

elements are diff phyical sizes. In NZ HWC elements are 1.25 inch BSP, US are 1 inch, europe 1.5 inch. Sigh.

That larger hole would make it a lot easier to clean out the hard water scale that builds up on the tank bottom....when you have really hard water like we do...

[asdex]

Hi, thanks for the information.
Just wondering what a NAW is?
"Just got my second NAWs order via nzpost youshop"
I have been using youshop a bit.
We have a small chip burner with wetback at the moment. I run it for about 4 hours every three days to heat the cylinder. I have just ordered a Scandia Heat n Cook from Bunnings Australia with wetback for out hot water and it has an oven as well.
My latest idea was to run a 24 volt system from the Midnite controller and use a 24 volt DC element which I can get. I can rearrange my battery bank to 24 volts  and charge it with any spare power. Just waiting for some advise from Midnite on the best way for this.
I wouldn't use an inverter. I guess the other option as you say is to string the 6 panels in series though a SSR directly to the element and see what happens.
Cheers,

[Westbranch]

NAWS Northern Arizona Wind and Sun .... http://www.solar-electric.com/

[dgd]

My latest idea was to run a 24 volt system from the Midnite controller and use a 24 volt DC element which I can get. I can rearrange my battery bank to 24 volts  and charge it with any spare power. Just waiting for some advise from Midnite on the best way for this.
I wouldn't use an inverter. I guess the other option as you say is to string the 6 panels in series though a SSR directly to the element and see what happens.
Cheers,

The Classic controller primary task is to control the charging of a battery bank. Once the bank is charged then any spare input power from the source (Pvs, turbine tec) can be diverted to another load such as a water heater.
I don't think the Classic can divert all its incoming power to a load, eg water heater, until some condition is met, ie hot enough, then start the battery charging process.

This power diversion for water heating, after battery charged, can be achieved several ways using the Classic AUX functions
1. use an SSR to divert incoming power from PVs to the element, works at PV voltage
2. use an SSR on battery to divert power to element, works at battery voltage
3. use  an ac  ssr to switch in a 240v element, using AC power from an inverter.

There are many advantages and disadvantages for each method, If you read back through postings on this forum you will see these well explained and details of how to wire them up and use of Classic AUX, SSR types etc..

dgd

[asdex]

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,.