Hot water heater for my solar powered house
Hot water heater for my solar powered house
(OP)
My solar powered house generates power to spare, so I thought rather than chainsawing dead logs, my time would be better spent developing a control system for my immersion heater, which is a 3 kW 240V single phase element in the bottom of the hot water tank. My inverter is only 3 kW, so I'm not real keen on running it flat out when the immersion is on, which is likely to be one or two hours a day typically.
So, how about fitting a 240 to 120V transformer to it, thereby reducing the current draw by 75%, and then running the thing for longer? That way the instantaneous power consumption is well inside the inverter's specs, and I can match the time when it is on to the sun a bit better, so less stress on the battery.
Assuming that's OK I still ideally need a way of controlling it depending on battery SoC and time of day and panel power, but for the foreseeable future a timer will do.
Incidentally I'm more or less wedded to 240V power to the house, the solar sheds are 50m from the house, so running a DC line is a PITA.
So, how about fitting a 240 to 120V transformer to it, thereby reducing the current draw by 75%, and then running the thing for longer? That way the instantaneous power consumption is well inside the inverter's specs, and I can match the time when it is on to the sun a bit better, so less stress on the battery.
Assuming that's OK I still ideally need a way of controlling it depending on battery SoC and time of day and panel power, but for the foreseeable future a timer will do.
Incidentally I'm more or less wedded to 240V power to the house, the solar sheds are 50m from the house, so running a DC line is a PITA.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?





RE: Hot water heater for my solar powered house
You may want to fit a two stage thermostat, so that if the solar energy input during heavy usage times is not enough, the 240 Volt grid supply will kick in. You can play with the setting of the second stage to optimize the operation.
Have you considered a solar water heater, or is all available roof space covered with PV panels?
Sounds like an interesting project.
BTW, no problem with a transformer, but consider an auto transformer or an auto transformer connection on a conventional transformer. It will be half the rating and almost half the size, weight and cost.
That takes care of water temperature. On the other side, matching to the available power, I would monitor the load on the inverter.
Install a CT on the incoming line to the house. Drop the CT secondary current across a resistor to develop a voltage and use a voltage monitor relay. Make to connection for the heater upstream of the CT so that the CT doesn't see the heater current.
When a heavy load cuts in while the heater is on, you will have some milli-seconds of overload until the relay and contactor drop off the heater. Make sure that the inverter can take the momentary overload.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Hot water heater for my solar powered house
So, my conclusion is that such schemes require a 2nd hot water tank in series to be used as a pre-heater. Otherwise the human factors make the control scheme 'impossible' or 'unacceptable'.
Electric Hot Water tanks in Canada usually have two heaters. One at the bottom used in the expected manner, and another one at the top (near the outlet) used for quicker recovery. One could rewire the heaters in series to reduce load, but one would still have to include the thermostats.
Somewhere I saw a table of elements of various power ratings. It seemed that one could replace the elements with higher power versions (and rewire appropriately), but also go in the other direction towards lower power.
RE: Hot water heater for my solar powered house
I do have some solar hot water panels more or less ready to fit, but installing them is a lot harder than a bit of reconfiguration of existing systems.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Hot water heater for my solar powered house
But being I only have two panels, it can't heat much water during the day.
RE: Hot water heater for my solar powered house
RE: Hot water heater for my solar powered house
RE: Hot water heater for my solar powered house
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Hot water heater for my solar powered house
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Hot water heater for my solar powered house
The system should obviously still have a thermostat to limit the max temperature.
RE: Hot water heater for my solar powered house
Maybe that's why my system has a PLC.
RE: Hot water heater for my solar powered house
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Hot water heater for my solar powered house
Living off-grid on solar is challenging. Your tank should be super well insulated. I suspect it is not if there is a wood boiler hooked to it.
Propane is another option to consider.
RE: Hot water heater for my solar powered house
That approach also allows you to select the power that you need to match the heat you need to pump into the water to get the temperature rise you want in the time that you have excess electrical power available.
RE: Hot water heater for my solar powered house
RE: Hot water heater for my solar powered house
The upside of this is that with a 6A current draw I can use a domestic 10A timer, and I'm not dilly dallying around with plumbing or high power electronics.
I agree about the insulation, the tank itself is not too bad (if i find one i'll fit another jacket) but all the pipes need a 1" foam cover.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Hot water heater for my solar powered house
How do you handle overheating with an insulated tank, Greg?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Hot water heater for my solar powered house
http://www.rheem.com.au/Assets/1628/1/OwnersGuidea...
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Hot water heater for my solar powered house
http://www.ebay.com.au/itm/Automatic-Electronic-Co...
but I need to check the head, 30 psi is 2 atmospheres, 20m, might be a bit shy on that
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Hot water heater for my solar powered house
RE: Hot water heater for my solar powered house
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Hot water heater for my solar powered house
Many installations over here tend to use the electric heating as a supplementary source with the primary heat source being a water-water heat exchanger from the boiler. Most have a thermostat sensing tank wall temperature plus the heater's own on-board protective cut-out. Bill might have identified a possible failure mode, although I haven't heard of it actually happening.
RE: Hot water heater for my solar powered house
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Hot water heater for my solar powered house
I haven't put the low cold water pressure cutout in yet.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Hot water heater for my solar powered house
As mentioned, hot water tanks have run-dry protection by virtue of design: cold inlet at the bottom, hot outlet at the top, and are fitted with a combined temperature + pressure relief valve.
RE: Hot water heater for my solar powered house
That is a great suggestion, and a great objection. It's too late, the job is done my way!
I suspect this weekend will be occupied wrapping the hot water tank in more insulation. Of course by murphy's law last week was the week the sun went out, so my testing took the batteries right down, from a low base. So I've run it this week at just 2 hours per day, by my calculations that is going to give us a tankful of tepid water by Friday, I'll see how happy the batteries are with that. This is an incentive to bring my 12V system on line, as detailed in the other thread.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?