PV panels connected to hot water cylinder element

[asdex]

Hi, just new here and becoming interested in using solar power.
We have a small house and some land off the grid with a wind generator and 3 x 75w solar panels feeding into batteries. We don't use much electricity, just LED lighting, radio and laptop. We have a further two 70 w panels for a 12 v water pump.
There are two wood fires, one inside for the house and one out for water heating. I want to get rid of this one and will replace the inside fire with a wood fired oven with wetback. I would like to get 6-8 300 w PV panels and connect them to a new hot water cylinder.
Any information on how to connect would be very useful. I am an electrician but not to experienced with PV, controllers, and the like. I'm happy to run 240-300 volt DC around the place and I will need to use contactors and relays suitable for DC. One of the biggest advantages I can see in using PV panels is they aren't restricted to just heating water. In winter with the wetback running I can hopefully use the panels to supplement the battery panels. Also as we only go the the property at this stage in weekends and holidays, I won't have to worry about freezing and overheating.
Ideally I can stay with 48 volts maximum so I can use an MPPT controller in the system when charging the batteries.
Thanks,
asdex

[inetdog]

Hi, just new here and becoming interested in using solar power.
We have a small house and some land off the grid with a wind generator and 3 x 75w solar panels feeding into batteries. We don't use much electricity, just LED lighting, radio and laptop. We have a further two 70 w panels for a 12 v water pump.
There are two wood fires, one inside for the house and one out for water heating. I want to get rid of this one and will replace the inside fire with a wood fired oven with wetback. I would like to get 6-8 300 w PV panels and connect them to a new hot water cylinder.
Any information on how to connect would be very useful. I am an electrician but not to experienced with PV, controllers, and the like. I'm happy to run 240-300 volt DC around the place and I will need to use contactors and relays suitable for DC. One of the biggest advantages I can see in using PV panels is they aren't restricted to just heating water. In winter with the wetback running I can hopefully use the panels to supplement the battery panels. Also as we only go the the property at this stage in weekends and holidays, I won't have to worry about freezing and overheating.
Ideally I can stay with 48 volts maximum so I can use an MPPT controller in the system when charging the batteries.
Thanks,
asdex

The idea of using solar PV to heat water via resistance is not particularly good, especially for an off grid system where the water heating power will come from the batteries much of the time. The biggest problem is that the heating element has a constant resistance and so needs to be driven from a constant voltage and current or from a source that supplies current and voltage proportionally. But solar panels produce a variable current depending on the amount of light but deliver their highest power at an almost constant voltage independent of light level.
You would be better off with solar thermal panels than with PV.

[asdex]

Ok thanks for that. I wasn't planning to have the batteries in the circuit but feed directly to the element. I have seen some information where a person did have three elements of different power (resistance) and a switching circuit to cope with three general voltages. That's possible but a bit complex. Also could an MTTP controller be used to help with delivering a constant voltage?
Cheers,

[SunEagle]

I have a solar thermal water heating system and it does a great job. The panel is 4 ft x 10 ft and feeds an 80 gallon tank. There is a small controller that will open valves to allow circulation to keep the tank water at about 140 F. There is also a small electric immersion heater for days when I don't get enough sunlight to do the work.

Certainly the control system would be run from batteries but a backup electric heating element could quickly drain your batteries. So you may have to decide on warm water or no electric power.

You are headed in the right direction by first reducing your electric footprint to a modest level. That will help keep the size of your electrical battery system down. Using wind and solar pv for electric generation along with solar thermal for heating is a nice package. Oh and don't forget about getting one or two fossil fuel generators for those long periods of little sunlight.

[J.P.M.]

Hi, just new here and becoming interested in using solar power.
We have a small house and some land off the grid with a wind generator and 3 x 75w solar panels feeding into batteries. We don't use much electricity, just LED lighting, radio and laptop. We have a further two 70 w panels for a 12 v water pump.
There are two wood fires, one inside for the house and one out for water heating. I want to get rid of this one and will replace the inside fire with a wood fired oven with wetback. I would like to get 6-8 300 w PV panels and connect them to a new hot water cylinder.
Any information on how to connect would be very useful. I am an electrician but not to experienced with PV, controllers, and the like. I'm happy to run 240-300 volt DC around the place and I will need to use contactors and relays suitable for DC. One of the biggest advantages I can see in using PV panels is they aren't restricted to just heating water. In winter with the wetback running I can hopefully use the panels to supplement the battery panels. Also as we only go the the property at this stage in weekends and holidays, I won't have to worry about freezing and overheating.
Ideally I can stay with 48 volts maximum so I can use an MPPT controller in the system when charging the batteries.
Thanks,
asdex

Depending on your climate and inclinations, a batch heater , also sometimes called "breadbox heaters" because of the common appearance of the relatively easy to build homemade types may be a consideration. Such a device, depending on your skills (you sound more self sufficient than most), may be easier and less expensive to build, simpler to operate and perhaps less maintenance. It may likely be a bit more efficient as well. Freezing may eliminate this as an option however. It's just not sexy and spiffy like PV. The old admonition "KISS" comes to mind - no offense intended.

[asdex]

Hi thanks for that info, yes I had to Google a batch heater and use to have something like this. We turned on the hose and pushed the near boiling water out for dishes and washing.
This site looks interesting and there is a couple of videos on Youtube: http://techluck.com/
I might follow that up. I know I can take the straight forward option and buy a vacuum tube thermal solar water heating system and sort out all the various problems like overheating and freeing but in winter is will be $3500 doing nothing while the wetback is in use which will supply all our hot water needs.
We are alos on rain water only so are reasonable frugal with water use. I'm just installing a compost toilet so we aren't flushing water away.
Oh and we are about 45deg South.
Cheers,