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
Quote from: asdex on December 15, 2013, 02:38:32 AM
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
Quote from: vtmaps on December 15, 2013, 04:30:58 AMThe 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!
Hi Wayne,
Is there a way to limit voltage on your Tristar controller such as a Classic ?
Quote from: waynefromnscanada on December 15, 2013, 09:51:33 AM
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...
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.
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,
Quote from: zoneblue on December 15, 2013, 01:28:55 PM
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.
Quote from: zoneblue on December 15, 2013, 01:28:55 PM
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
Quote from: asdex on December 15, 2013, 02:11:10 PM
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.
Quote
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
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.
Quote from: zoneblue on December 15, 2013, 06:16:18 PM
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...
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,
NAWS Northern Arizona Wind and Sun .... http://www.solar-electric.com/
Quote from: asdex on December 15, 2013, 06:41:53 PM
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
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,.
Quote from: asdex on December 15, 2013, 09:02:52 PM
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
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,
Quote from: asdex on December 15, 2013, 09:02:52 PM
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
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,
Quote from: asdex on December 16, 2013, 11:53:04 PM
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?
Quote from: dgd on December 16, 2013, 10:33:32 PM
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
Quote from: vtmaps on December 17, 2013, 12:31:50 AM
Quote from: dgd on December 16, 2013, 10:33:32 PM
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 :)
Quote
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
Quote from: vtmaps on December 17, 2013, 12:31:50 AM
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
QuoteSo, 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
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
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,
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,
Quote from: asdex on December 17, 2013, 06:36:43 PM
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
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.
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
Quote from: dgd on December 17, 2013, 08:55:44 PM
Quote from: asdex on December 17, 2013, 06:36:43 PM
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
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.
Thanks, yes all this is pretty new to me even though I have had solar panels and regulators on a bus we lived in and then a yacht, it was much simpler. We still use those original Solarex panels. 83 watt and $990 each in 1996. The six 320w panels will be nearly 2kw. I have alot (40 meters) of 16 mm two core TPS I hope to use. I will prefer to site them nearer the house if I can but will have to convince the team of the benefits. (wife thinks they might not be aesthetically acceptable)
But as you say, I'll need to work out the system in total before preceding to do the install.
Cheers,
My neighbor uses the dump load to trip a solid state relay when battery voltage exceeds 54 (48 volt nominal system). He has an electric water heater with 110 volt elements, which run 400 watts plus at 48 vdc. I want to mirror that, and since my water heater just died, I'm not too far away.
I also have a summer direct type M copper heating array for water, and a wood burner for when nothing else works. I'm going to relocate the coil next summer, as i relocate my panels for wind protection.
Quote from: asdex on December 15, 2013, 02:38:32 AM
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
Hi, thanks for your reply. Just waiting for my panels to arrive in a couple off weeks then will look at element voltage and get the Midnite 150 controller.
I hope do do the same as the neighbor you mentioned but in reverse so temperature will operate the relay rather than voltage.
I think I'll connect them 3x2 to give 17.22A and 111VMP and change my 12volt battery bank to 24 volts.
Cheers,
Would someone that is using low voltage water heater elements please point me to the source of these. I am using propane flash boiler heater now but would like to add small pre heater to use excess solar power.
tnx. td
Missouri wind and solar amongst others with Ebay stores.
dgd
dgd,
Thanks, I will check that out. I do have excess solar power at least 9 months a year and do not like lugging propane bottles to my house.
td
Quote from: asdex on December 25, 2013, 11:10:00 PM
Hi, thanks for your reply. Just waiting for my panels to arrive in a couple off weeks then will look at element voltage and get the Midnite 150 controller.
I hope do do the same as the neighbor you mentioned but in reverse so temperature will operate the relay rather than voltage.
I think I'll connect them 3x2 to give 17.22A and 111VMP and change my 12volt battery bank to 24 volts.
Cheers,
By doing it the way he does, it is only excess power that heats water. His primary use of PV power is to charge his batteries and run his fridge/freezer/etc...just like my setup. I'm currently just building a small wood fire when I want hot water...and I keep a pan on a small propane heater in my bathroom for quick washup warm water.
Looking to use a 250 litre hot water electric heater.
using 200 litre per day and 45'c temp rise (15 to 60'c) will need 10.7kWh of energy (2.6KW array for 4 peak sun hours)
using 10 250W PVs (Vmpp 30 volts) will give 2.5kW @ 300 volts DC, water heater uses 2.4kW heating element, almost a good match.
use solid state relay & use thermostat from water heater to control.
All I need to do is wait for PVs to get under 60cents per watt, to make it cost effective with heat pump or solar thermal water heater.
Aprox cost $3k with 250 litre storage tank & 10 250W PVs ??
My panels will cost $3000 plus element and SSR and should be comparable with a thermal system. Probably a bit cheaper even if I need to buy a PWM controller to divert to load. With the added advantage of being able to produce electricity if needed, I'm hoping the system will be worth the effort.
We only use about 50 to 80 lts max and I plan to supplement the hot water with some basic water heating devices like solar shower and passive heating systems (copper tubing in glass front box connected to tap)
I have gone for the standard 180 lt cylinder with a wrap to reduce loses.
Looking at using a MPPT type DC circuit, for PVs to charge a capacitor bank & dump to heater element, problems if not using MPPT circuit, PV array and heater element will have a very small window of max power transfer.
Aim is to transfer kWh from PV array to heater element.
Looking to set up a system if total installed cost can be under $2.5-3k
Quote from: phonetic on February 16, 2014, 02:51:58 AM
Looking at using a MPPT type DC circuit, for PVs to charge a capacitor bank & dump to heater element, problems if not using MPPT circuit, PV array and heater element will have a very small window of max transfer.
Aim is to transfer kWh from PV array to heater element.
Looking to set up a system if total installed cost can be under $2.5-3k
Sounds interesting. Capacitor? what type and size?
What about just connecting PVs direct to water heating element?
dgd
Good example: PV array gets 25% of nominal isolation ie 25% of Impp....as power is proportional to the square of voltage/current for resistance load..will only yield 1/8 of mpp power, but if the array charges a capacitor bank, when capacitor bank gets charged to Vmpp.. switched to load, the power duty cycle is approx 25% rather than 12.5%
I hope my ramblings makes sense..maybe the midnite guru's can offer a better example ?
One other option to consider is one I'm about to do at home. I have swapped the 220 elements from my electric hot water heater for 2000 watt, 110 elements. I have manually connected them directly to my 48 volt battery bank, and they each pull just over 300 watts of 48 volt DC. Once I get the solid state relays in place, I will automate this at home and never lose another watt. :)