Electric panel resistive element water heating

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Re: Electric panel resistive element water heating

Postby APR » Sun Jul 24, 2016 9:44 am

davidg wrote:.... measure the voltage being input into the element, then you can use the very simple math of ohms law again V x I = P to verify it.


I have decided to trial using spare solar panels I have to directly power an immersion element to heat hot water, and have been evaluating the panel arrangement and element voltage and wattage I would use. My HWS is 250 litres, is powered by a 3,600 watt 240v element, and typically uses between 5 and 8kWh a day during winter, although when the washing machine does a couple of cycles this can increase by a kWh or 2.

Considering I want this system to be able to heat water reasonably effectively in winter, I am looking at a fair bit more panel power than the element will consume. After doing a few sums I have decided on a 240v element of 3,000 watts, and feeding the element with 2 parallel strings of 5 series 250w panels. The resistance of a 3,000 watt element is around 19.2 ohms.

The MPPT voltage of the paralleled 5 panel strings will be 152.5 volts, and at this voltage a 19.2 ohm resistance element will draw 7.94 amps and produce slightly over 1200 watts of heat energy. Thus, around 4 amps will be required from each string of panels at the maximum power point voltage of the panels. I expect to be able to achieve 4 amps from each north facing panel string on most winter days for a period of around 6 hours a day, although a couple of days of cloud will still be an issue.

6 hours a day of 1,200 watts of heat energy, plus a kWh or so of heat energy outside the 6 hours, should provide adequate energy to heat the water required on most winter's days, and summer should be more than OK.

Even though I am using more panel power than the element requires, power to the element will to a large extent be self regulating. While STC open circuit panel voltage is around 188 volts, at this voltage the panels will not produce current. Panel specs suggest that on a high insolation day at STC with the element drawing 4 amps from each string, the panel string voltage could rise to slightly over 36 volts, or 180 volts for each string, however, because of increased panel temperature in summer I don't expect to see that level of voltage rise in summer when the panel strings are each supplying 4 amps of current. Below is a graph of the wattage I expect to see through the element as panel amps rises with increased sunshine. Element wattage should never exceed around 1,400 watts, as panel amps drops away as the panel voltage progressively exceeds the maximum power point voltage.

Image
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Re: Electric panel resistive element water heating

Postby australsolarier » Sun Jul 24, 2016 2:15 pm

i think my preferred option would be load diversion. either have a second cheap inverter or a large single inverter. it can be programmed via to battery voltage. when the battery is full it turns on and after some discharge off again. it pulses, turning on and off every minute or so depending how it is programmed. so if you added extra panels for hot water heater, when the tank is hot enough, you can still use the extra power for other things.
and yes the heat pump is preferred, particularly if you could hack into the electronics to steer the load parallel with the battery voltage and then no pulsing. there are already 48v heat pumps on the market that do that although for space heating. but the principle is the same.
but!!!! not matter what, in winter whilst overcast a backup is still needed. the last 10% is double the cost of the the first 90%.
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Re: Electric panel resistive element water heating

Postby APR » Sun Jul 24, 2016 2:41 pm

australsolarier wrote:i think my preferred option would be load diversion. either have a second cheap inverter or a large single inverter.

At present I am using a power diverter that sends excess solar, that would otherwise go to the grid, to the HWS. The problem is that on many winter days I have not had enough excess solar generation to keep the HWS adequately hot.

I already have the 10 extra panels, and they can not be permanently connected to the inverter without potentially overloading the inverter in summer. I would have to parallel panels on one MPPT, and the amps from paralleled panels exceeds the recommended max amperage into the MPPT.

I have previously considered paralleling my 2 existing equal length strings on one MPPT during winter when the amps are down, and adding the 10 extra panels I have to the freed up MPPT. I am having these extra panels added to the northern roof and wired so that at the inverter I will be able to series the 2 strings of 5 panels that I am proposing to use for the HWS and feed them into an MPPT on my existing inverter. The panels should be installed on the roof by Tuesday afternoon.
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Re: Electric panel resistive element water heating

Postby 34by151 » Sun Jul 24, 2016 6:00 pm

APR wrote:My HWS is 250 litres, is powered by a 3,600 watt 240v element, and typically uses between 5 and 8kWh a day during winter


Make sure the thermostat connected to the PV is set to the highest setting you can get away with and lower the grid connected thermostat low as you can and still have hot water.

The thermostat should handle the voltages but consider switching the DC with a Solid state relay or a contractor to prevent arcing. The techluck controller chops the DC via an IGBT to prevent this
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Re: Electric panel resistive element water heating

Postby APR » Sun Jul 24, 2016 9:23 pm

34by151 wrote: but consider switching the DC with a Solid state relay or a contractor to prevent arcing.

I have ordered one of these plus a heatsink to go with it...
http://www.ebay.com.au/itm/171327333120

...but I also have a Tyco EVC500 relay I could use in this application. I have not used a DC to DC SSR before, so I try the SSR first.
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Re: Electric panel resistive element water heating

Postby APR » Mon Jul 25, 2016 10:20 am

I have a solar bloke on the roof at the moment installing 10 X 250w panels. They are going on landscape as they will not fit in portrait orientation.
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Re: Electric panel resistive element water heating

Postby 34by151 » Mon Jul 25, 2016 2:00 pm

Sounds good, what are you using for the MPPT as it sounds like your not going to use the controller I have

Cheers
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Re: Electric panel resistive element water heating

Postby APR » Tue Jul 26, 2016 9:43 am

34by151 wrote:what are you using for the MPPT as it sounds like your not going to use the controller I have


The principle I am working on is that the more parallel strings I have, the less need there will be to wring every ounce of power from the panels. If I only had one string of 5 x 250w panels and needed roughly 7.5kWh a day from the panels, I would need 6 hours a day with the string needing to be continuously producing 8 amps at MPPT voltage, excluding any energy from the time periods each side of that 6 hours.

If I parallel 2 strings of 5 panels in series, each string only has to produce 4 amps at the MPPT voltage for both strings to produce 7.5kWh in 6 hours.

Taking this further, with 3 strings of 5 panels in series, each string only has to produce 2.66 amps at the MPPT voltage for 3 strings to produce 7.5kWh in 6 hours.

Panels are pretty much constant current devices below the MPPT voltage for a given level of insolation, and so while loading the panels pulls the voltage down, the current will pretty much stay the same. If the strings of panels, when loaded by the immersion element, are able to maintain the MPPT voltage, or above it, I don't feel an MPPT controller will result in extra energy to the element unless it can boost the output voltage from the panels. I recognise that in the morning and afternoon when there is low insolation, an MPPT controller would be a benefit, and I may still go with a controller.

I have been in communication with Barbara at Easy Warm in NZ...

http://www.easywarm.co.nz/

... about their HotPV inverter, and am waiting on a price (which will be telephone numbers I am sure). Their device is a modified inverter that is designed to optimise the panel energy to maximise energy through the element. I do believe this unit will be well outside what I am prepared to spend on a controller.

The panels I am having installed (the rails went up yesterday, the panels are going up today) could be fed straight into into an MPPT on my inverter in winter time, and my current panels paralleled and fed the second MPPT. During summer I regularly see 10 amp from each string from cloud effect, and to parallel my current strings would see over 20 amp going into an MPPT that is speced at 15 amp input.

I started a thread about wanting to parallel panels and switch panel strings in and out automatically, and I still haven't given this concept away yet.

wind-solar-misc/topic6660.html

Connecting the new string to my existing inverter would yield far more energy from the panels during winter, when I don't need to disconnect panels because of high amperage, than I will get from attaching the panels directly to a HWS element.

If I was to be connecting the panels to the inverter I would rather them on the western roof, as the extra generation in the late afternoon would be useful, if I am attaching the panels directly to the HWS I want them on the northern roof where they will give higher amperage during winter through the middle of the day.

To help my frame of mind, the temp/pressure relief valve on my HWS started dribbling late yesterday (that's when I noticed it), and overnight the top of tank temperature went from mid 60's celsius when I noticed the valve dribbling to 32 degree C this morning.
2 x Goodwe GW5048D-ES Hybrid inverters, 13kW panels, 20kWh LiFePO4
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Re: Electric panel resistive element water heating

Postby 34by151 » Tue Jul 26, 2016 6:03 pm

It does sound expensive.

From memory the total for the one I used was just under $1000
That includes the controllers, isolators and box/heatsink
Not including the cables, plugs, realy as I had them already
Panels were free

Re the Panel switching take a look at the unit from Latronics
http://www.latronics.com.au/products/sa ... fer-switch

Sounds like what your looking for

Cheers
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Re: Electric panel resistive element water heating

Postby APR » Tue Jul 26, 2016 6:56 pm

34by151 wrote:Re the Panel switching take a look at the unit from Latronics
http://www.latronics.com.au/products/sa ... fer-switch
Sounds like what your looking for

Now that is interesting. Thanks for the link.
2 x Goodwe GW5048D-ES Hybrid inverters, 13kW panels, 20kWh LiFePO4
Mk2 PV Router feeding excess solar generation to a 3.6kW 250 ltr HWS
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