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Hot water heater for my solar powered house
2

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.

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

Hi Greg. Is 750 Watts during the hours available enough to heat the incoming water and is your tank large enough to supply 24 hours of water use?
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

Do you have alternate (e.g. commercial) power connected to the hot water tank? Or will it be strictly solar-powered? If there are other sources of power, and on the assumption that the residents would want hot water on tap at any time, the primary hot water tank should (normally) always be full of hot water. In such cases, it's a poor load dump for excess solar-power because the water inside is already hot. It shouldn't accept the power because the thermostat should have already opened (or would open almost immediately). Obviously you wouldn't want to bypass the thermostat for all the obvious reasons.

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

(OP)
The main source of hot water at the moment is the insulated large tank,say 300 litres, heated via the back boiler of the woodstove. There is no external power connection to the site. Bear in mind this is primarily a means of heating the water 'for free', it is not the main system. The occupants can have cold showers if they can't be bothered to keep the stove going. OK in practice that means I keep the stove going at the moment.


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

My solar panels (for heating water), use a hot-water tank for storage, and that is the pre-heater for my hot-water heater, which also heats my 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

Are you up north - where winter heat requirements far exceed spring-summer-fall A/C requirements?

RE: Hot water heater for my solar powered house

I was thinking if you have the two element heater as mentioned above, hook the lower unit to your 120 volt system with a fairly high thermostat setting, say 125-130 F and leave the top element connected to the 220 mains with the themostat set 10-15 F lower. That way most of the daily heating would occur on the solar, with the mains as backup at a high enough temp to be considered hot water. Once rigged, additional insulation blanket on the heater and insulating the incoming and outgoin piping for at least 4 feet would help.

RE: Hot water heater for my solar powered house

(OP)
No I'm down near the Antartic, we get a few 40-44 degree C days in summer, and the occasional frost in winter. A/C is not a requirement. However it would be nice to have warm water without the stove in summer.

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

Use some solar generated electricity to run a small pump to move the water through a black pipe left out in the sun. In direct sunlight on a hot day, your tank will soon be full of hot water. You can arrange things so that the pump stops pumping when the sun isn't sufficiently bright to heat the water.

The system should obviously still have a thermostat to limit the max temperature.

RE: Hot water heater for my solar powered house

I don't know about there, but here if the water gets too hot the over tempeture valve will open, and it may not shut off before you lose all your hot water.

Maybe that's why my system has a PLC.

RE: Hot water heater for my solar powered house

(OP)
Ve1BLL- I already have thermal solar panels waiting to be fitted, hot hoses are fine, but require just as much plumbing. cranky - that is what they call a first world problem, I'm more likely to get hit by lightning.

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've seen immersion heaters as small as 150 watts with a 1/4" NPT thread for RV water heaters, so you can find any wattage you like. Changing the heater is probably easier than wiring a transformer, which you would consume some energy all the time unless you switch it off with the heater. I've used a diode to reduce heater power but I'm not sure how your inverter would like that.
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

I agree with changing the heater element rather than jamming in a transformer.

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

The normal AC system really doesn't like diodes used to limit the power to something; I can't imagine that an inverter would put up with such an insult.

RE: Hot water heater for my solar powered house

(OP)
The obvious advantage in a transformer is all I have to do is buy the damn thing and plug it in, since 220->110 is commonly available up to 2 kW for builders to use American tools. In fact that is what I've ordered. I could get the transformer itself for 20 bucks plus shipping, or an end user item for 100 delivered. I went the latter.

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

When I was growing up, wood burning stoves with water jackets to heat water were common. The tanks were always bare. Even with the BTUs radiated from the bare tank, they would occasionally overheat and blow live steam out the safety valve.
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

(OP)
There's a very annoying pressure valve on the side of Australian hot water tanks, the usual failure mode is that after giving it the annual check it refuses to reseal and so your hot water dribbles away until you replace it, or hit it hard with a hammer to reseat it, aka the rental property solution. However the intention is that it'll depressurise the system.

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

(OP)
So now you've got me thinking again, the cold water supply is from a 1000 gallon header which I refill from the 40 ton rainwater tank at fairly random intervals. I don't want to have the element on when the cold water service fails. There is an overtemperature cutout but it would be nice to shut it off if there is no cold water pressure. That's easy

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

It should not be possible to drain the tank through the plumbing. Water will not come out unless water goes in. Reseating can be an issue, but the most common reason for a dribbling TP valve is having a closed hot water loop, which does not allow for the expansion of water as it heats.

RE: Hot water heater for my solar powered house

The common hot water tank thermostat up here has a high temperature cutout but it only responds to water temperature. If the tank runs dry, there is no water to conduct the heat from the dry element to the thermostat and the element may burn up without the thermostat responding.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Hot water heater for my solar powered house

On British tanks the cold inlet is located at the bottom, and the hot draw-off is at the top. Other than a gross failure of the tank or pipework it should be impossible to have the tank run dry. Mains-pressurised tanks are typically equipped with a combined temperature and pressure relief valve, for example http://www.advancedwater.co.uk/header-520-475-TPR2.... If they run at about 50% - 60% of relief pressure they are ok, but much higher and they get wire-draw problems on the valve seat given enough time. For those who dislike dribbling showers it becomes a knife-edge adjustment between maximising shower pressure without the TPV cracking open.

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

(OP)
30 bucks seems like cheap insurance to me, especially since I can install the sensor in the pantry which doubles as the internal services hub, rather than yet another journey into the roof space.

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

(OP)
The beta test worked well, pulled 3.3A at 240V, or 16A from my 400Ah battery strings, so nice and gentle. I wasn't game to leave it running unattended, this weekend I'll run it for 8 hours with multiple on/off cycles. If that works OK then we go live next week

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

Greg: a diode will reduce your power draw to half, for what? Two dollars? Way cheaper than a transformer, WAY easier than replacing the heating element, dead easy to wire, and a resistance heater couldn't care less about running on half-wave rectified DC versus AC. Want the original power back out of your heater for some peak hot water demand situation? No problem- short the diode. Used that trick many times in past, but only on heaters fed from the mains. Your inverter might not like the load variation every half cycle...

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

(OP)
"a diode...Your inverter might not like the load variation every half cycle"

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?

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