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Solar PV direct to heating element.

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Started 8 years ago by Marty50
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Marty50
Member

Anyone able to give advice on using solar PV to heat a resistive heating element?

I want to heat an 200 watt electric element in a device I am building. I figured I would work with the KISS principle and run the PV directly to the heater.

I got a bit sidetracked by looking at maxi-misers but they are designed to deal with inductive loads such as the high start currents of electric motors. I am not concerned with reduced power at the beginning and start of each day as I will be running mostly during the hottest part of the day.

Any advice from someone experienced welcome.

https://www.facebook.com/photo.php?v=472756472830418&notif_t=video_processed

Posted Thursday 9 Jan 2014 @ 2:16:19 pm from IP #
Benny
Member

Marty - I presume its 200W at 240V ? If so then R = Vsq/P = 288ohm. At 5A this would have 1440V across it and if you draw this line on the chart above you will see the operating point for a direct connection would be very low down - nowhere near the MPPT. The panel would be on the const V part of the curve - at about 35V, with I = 35/288 = 0.12A and so power would be 4W.
If its a 12V 200W element then R = 0.72ohm. At 5A V = 3.6V. Drawing this line on the chart gives a very steep line, so we'd be on the const I part, with I 8.6A (at 1000W/sqm), V = 6.2V and P = 53W.
Max power (at 1000W/sqm) occurs at about 8.2A, 28V, so R would need to be 3.4ohm. At lower insolation this would operate on the constant current part of the curves.
I'm also a semi retired electronic engineer - in Maylands not far from you. My projects pale in comparison to yours though (-;

Posted Thursday 9 Jan 2014 @ 11:44:17 pm from IP #
Bushwalker
Member

There was a report in Renew a year or so ago of a direct connect between solar PV and resistive HWS. The owner had tried solar hws with poor performance but the direct connect did the trick. I think it was in the south of NZ, perhaps Dunedin.

Are old versions of Renew online?

BW

Posted Friday 10 Jan 2014 @ 12:15:20 am from IP #
Marty50
Member

Yes that’s correct. I have all my old copies of Renew.

Michael Laba in Issue 121 apparently used a direct connection but the article also featured and appeared to recommend a device called a Liberty Box. When I read the comments on their web site most were very derogatory about the product.
Michael’s house is also featured here.

http://www.odt.co.nz/lifestyle/home-garden/192543/power-people

On another thread I posted in Benny suggested I needed to make sure the impedance of the heating element was a match for the power curve of the PV I am proposing to use and that because it may be difficult to get that right that doing it dynamically through the use of MPPT may be the way to go.

Posted Friday 10 Jan 2014 @ 7:17:46 am from IP #
sun2steam
Member

You could have a look at these Power Optimizers: http://www.ecodirect.com/Solar-Power-Optimizers-s/277.htm.
These modules are designed to have the PV module always running at the optimum power point in the curve. However I don't know what the output specification is in case the modules are not connected into an inverter.

But it may be even simpler and cheaper and more straight forward to use a micro inverter per panel which converts the voltage to 220V and use a 220V heating element. Inverters have an internal design to always use the panel at its maximum power point. And the efficiency is around 98%.

Posted Friday 10 Jan 2014 @ 7:43:30 am from IP #
Benny
Member

I wonder how an inverter would handle direct connection to a load rather than to a network which is already at 240V ? If the panel is generating at least 200W and the load is 200W then there would be no problem. What happens when the panel is only generating 100W say - as it would do for much of the day unless it was tracking. The heater is a simple resistor, so if it has 240V across it then it must be conducting 240V / 288ohm = 0.84A and using 200W. So what does the inverter do ? Only put out 0.42A in which case the output voltage will not be 240V but 0.42 * 288 = 120V ? Can a "simple" inverter do this ?
I think a MP transfer device is a bit smarter than this (not that I have any direct knowledge). The output is regulated/modulated somehow to ensure that the output power matches the input power. When I studied simple dc-dc converters this was done by pulse width modulation. If you have a nominal 12V output voltage and give it only a 50% duty cycle the filter it heavily, the average output voltage will be 6V.
If you have a device which outputs a nominal 240V ac but you only let it output for say 50% of the time, ie say each alternate cycle, then the load will only dissipate half the full power, 100W instead of 200W and as in the above example the power in and out will be balanced. The "device" (MPT) has energy storage in terms of capacitors and inductors to enable it to output 240V ac in short bursts to keep the load happy as well as the PV panel operating ot the MPPT.
Marty - I think you are also going to have issues with this driving your fridge. Even if its a 200W element and you have a 245W panel - except for a short time near mid-day the panel will be outputting less than 200W and so the heater element will not get to the required temperature to make the refrigeration cycle operate. Isn't this what you found when trying to use the SHWS hot water to make it run ? If you had a battery being charged by the PV AND the fridge only needs to run say 20% of the time to hold temperature, then the battery could ensure that the heater runs at full power/temp when its needed.
What is also probably needed is an "energy" balance rather than all this talk about power. How many kWh does the fridge need per day to stay cold ? If its 1kWh say, then you need enough panels to generate this. A 250W panel should give 1kWh/day on average but again it will rely on a battery to store the excess during the day and give it out when the sun is gone. I suppose this is where eutectic(?) fridges are handy - just keep freezing water while there is power and store the "coolth" energy in the ice to keep the fridge cold overnight.

Posted Friday 10 Jan 2014 @ 8:56:43 am from IP #
Marty50
Member

Benny, I agree about the battery and have that option available.

http://renew.org.au/photovoltaics/making-my-home-free-from-the-grid/

But my battery banks are used for other things so I was trying to keep the cooler self-contained apart from its reliance on hot water. The problem I am predicting with the hot water is that when I ran a test on the fridge mechanism using a 240 volt element connected to the mains the measuring probe in the chimney sat at 108 deg C. On that test I produced a container of ice in the freezer.

I will conduct my first run with the water only and see how it performs at below 100 deg C. The solar PV idea is only to top up a bit more energy whilst the water is very hot and see if I can get it up to its 108 deg C.

I’m going to run some tests on the weekend with a new test device I purchased. I will try several elements direct off several PV and even try an MPPT controller with a small battery.

I have been looking up sun2steam’s suggestions and found some maximizers and micro-inverters for sale in Oz. If I can get a good price I may make a purchase and do some additional tests.

Thanks guys for all the suggestions. They are much appreciated.

Posted Friday 10 Jan 2014 @ 9:48:46 am from IP #
Marty50
Member

A bit of searching on my chinese supplier's list and I came up with this.

http://www.aliexpress.com/item/2013-New-Arrival-water-proof-IP65-LED-display-wide-voltage-input-22-50V-260W-pure-sine/775583922.html

This would handle the 250 watt PV easily and directly supply the 220 watt 240 volt AC heating element as suggested by sun2steam.

Posted Friday 10 Jan 2014 @ 1:34:41 pm from IP #
Benny
Member

Careful Marty. Note that its a grid tie inverter - ie it will be expecting an external 240V ac supply to sync to. Will it not output anything if its just connected to the heater load ?
Even for a non-grid-tie inverter, what would they do if not supplied with enough power to run the load ?

Posted Saturday 11 Jan 2014 @ 1:51:39 am from IP #
Marty50
Member

Yes. I just realized that Bernie after I posted here and shot off an email last night asking if it can run without sync. They haven't got back yet.

Posted Saturday 11 Jan 2014 @ 1:55:14 am from IP #
Marty50
Member

I may go back to DC and consider this instead. I have both 240 volt AC and 12 volt DC elements. Need to check ratings.

http://www.ebay.com.au/itm/310503597300?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

Posted Saturday 11 Jan 2014 @ 1:58:43 am from IP #
Benny
Member

A little off topic Marty but this is an interesting read ....
http://outbackpower.com/forum/viewtopic.php?f=29&t=2865&hilit=electron&sid=d91ecc2527631518a85cd6a2b3c89f1a&start=25

Talks about an offgrid system which has a separate output that can drive a resistive load (water heater) when the batteries reach float mode. The excess power from the panels is directed to the heater via a PWM SSR. In particular look on pg 2 and the posts from crewzer.

Posted Saturday 11 Jan 2014 @ 2:14:48 am from IP #
Marty50
Member

Running tests on my front drive with a 12 volt 200 watt PV I had laying around.

Posted Saturday 11 Jan 2014 @ 9:51:44 am from IP #
Marty50
Member

I put a few videos like this up on my Facebook page of the quick tests I conducted.

https://www.facebook.com/photo.php?v=473445756094823&set=vb.107425999363469&type=2&theater

Posted Saturday 11 Jan 2014 @ 9:57:46 am from IP #
TanksAndPanels
Member

This thing was mentioned in the new products section of the latest edition of Renew. Is that what you want ?

http://commodoreaustralia.com.au/products-page/solar-hot-water/solar-hot-water-booster/

Posted Monday 13 Jan 2014 @ 4:47:04 am from IP #
Lance
Key Master

And there are others as well, such as http://www.presolarnet.com/products/liberty_box.htm (possibly bad feedback on this one?) and http://europe.refusol.com/accessories/ae-pv-heatingsolution/

You can also get 12 and 24V water heater elements, although they may not fit Aussie tanks, such as http://www.ebay.com.au/itm/301035958162 (those guys have other interesting things).

Posted Monday 13 Jan 2014 @ 6:31:08 am from IP #
Marty50
Member

Cooler box mounted on wall ready for plumbing to hot water. Will need to get back to the auxiliary solar PV contribution. Thanks to all those that have provided advice.

Posted Tuesday 14 Jan 2014 @ 11:47:07 pm from IP #
Marty50
Member

I had previously spoken with the commodoreaustralia person and he refered me to Matthew in the USA, the mob that apparently make the device advertised. I exchanged a couple of emails but never got to find out the principle of his device. And he seemed more interested in advising me that solar hot water systems weren't very good, even vacuum tubes, totally the opposite of my own experience.

The essential purpose of my building this is to cool down my hot water tank whenever it overheats on hot summer days. Cold air off it to cool my home will be a bonus. Its not actually meant to replace my other cooling but merely supplement it. Even if it only drops the temperature a couple of degrees I will consider it a success.

Posted Wednesday 15 Jan 2014 @ 3:42:45 pm from IP #
Marty50
Member

Lance,

I did look up the Liberty Box and it seems to have received a lot of negative criticism on their website. I can't tell how it works or what it will do from the blurb they have on there but they do seem to be shipping a lot of them out.

Posted Wednesday 15 Jan 2014 @ 3:45:29 pm from IP #
Marty50
Member

Plumber comes tomorrow to hook the hot water up.

Posted Monday 27 Jan 2014 @ 2:15:10 am from IP #
franks
Member

Discussion using standard electric hot water storage tank & PV powered.

Example add 250lt electric water storage tank (2.4kW element)
2.5kW PV array. (10 x250W)
as part of AC coupled stand alone system, water tank element operates between 10am to 3pm each day via time switch, with manual boost if needed.

large stand alone setups with 7kW+ arrays would be a good candidate

Can pick up 250ltr electric storage tanks for $700 & 2.5kW PV array $2.2k, makes a good renewable hot water system with out all the mucking around with roof mounted solar collectors, plumbing, pumps & controllers.

here a discusssion on WP forums

http://forums.whirlpool.net.au/forum-replies.cfm?t=2190151&p=3

Posted Monday 27 Jan 2014 @ 5:46:14 am from IP #
Wayneconnor
Member

Anyone tried one of these yet?

http://techluck.com

Wayne

Posted Thursday 13 Mar 2014 @ 2:17:07 am from IP #
Marty50
Member

I contacted the importer and he didn't seem to know how it worked. I was wondering about getting the ATA kit and building it but never got around to it. I got sidetracked importing and fitting a 24 volt DC evap cooler from China. I'm back onto my absorption system now.

Posted Saturday 12 Apr 2014 @ 6:22:01 am from IP #
tom.scott
Member

I have had a PV Direct system running at my house for 4 months.
Details here, http://pvdirect.wordpress.com/

Posted Monday 21 Jul 2014 @ 9:11:46 am from IP #
darrynm
Member

Looks great Tom. Approximately how much power / heat are you getting from the panels?

Posted Monday 21 Jul 2014 @ 11:40:08 am from IP #
Marty50
Member

Tom,

I am very interested as I haven't sorted my problem yet despite conducting some quite extensive experiments with a 24 volt 180 watt PV and a 12 volt 240 watt PV connected to several different configurations of heating elements.

Both my 120 watt 12 volt element and my 220 watt 12 volt element were no way near the resistance of 40 ohms you quoted. Far far lower in fact. Perhaps this is because you element was intended for 240 volts but I would have thought it would be much lower?

When you say the Sharp PV had a maximum current of 2amps was that the short circuit current they were rated at? If that is the case then wouldn’t they deliver less than that across a 40 ohm load?

Assuming they can deliver the full 2 amps into your 40 ohm load by my calculation power would be P=i*i*R or 2 amps * 2 amps * 40 ohms = 160 watts.

Alternately if you were measuring 90 volts across your 40 ohm load then P = v*v/R or (70 volts * 70 volts)/ 40 ohms = 122.5 watts.

I saw that you had a power measuring device across the setup. What sort of power reading did you get on that?

I will see if I can find the results of my tests and publish them here.

Posted Monday 21 Jul 2014 @ 12:52:14 pm from IP #

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