P V dump loads

[petect]

As I start to  design my “perfect” p v  system, I find that planning for panel output vs. storage vs. the best use of a dump load is kind of like trying to put together a jigsaw puzzle made of jello.

For now, I'm trying to firm up my knowledge of dump loads, and I'd be grateful to anyone willing to share what has worked for him or her.

I currently have an electric water heater, so water heating (preheating) is one obvious candidate. I don't like the idea of running a water heater off the main system inverter, and think a cheap inverter and transformer (  if there are such things  ) could work to produce the sloppy 120 or 240v needed for a resistive load.

Of course, putting the raw excess  d.c  coming off the panels would be even better.

Any thoughts on doing something like this, OR other dump loads would be greatly appreciated.

Pete

[madlabs]

I assume when you say dump load you mean a way to use excess power after the battery bank is full. Solar doesn't need a dump load in the same sense as wind or hydro.

Anyway, one thing I have done (for a buddy) is hack the controls of a chest freezer and lower the temp. when there is surplus power. Then when there isn't excess power, go back to the regular setting. Makes it run way less at night. I like this scheme because it actually helps the battery bank get less discharged overnight, making it last that much longer. As soon as I finish my power upgrade and have an inverter and system that can handle a chest freezer (this spring I hope) I'm going to do the same thing.

The thing is it takes a lot of juice to heat much water to a useful temp.. But why not use your main inverter? Adding a second one and an xfrmr is just going to waste more juice.

Right now all I am doing is doing garden watering in the afternoon after there is surplus power.

Jonathan

[hydrosun]

Many off grid systems use a dc heating element in the hot water tank. If you have a 48 volt system you may be able to use a stock heating element have to use a dc relay if you want a thermostat. If you give more details on your plans I can give more advice.

Madlabs. How did you hack the freezer thermostat? Is it the digital type? I have done a hack on a heat pump thermostat (thermistor) by adding a resistor in parralel to change the apparent temperature. Is that similar to what you are doing with the freezer?
Chris

[thirteen]

Have you checked out the cost of installing an undemand water heater. I plumbed in and old water tank in line with my on demand water heater. I heat with wood.  But I took the insulation off of the old water heater. It gets up to around 65 during the day so my fuel usage is less once I start to use it. Plus I did not have to take it out for it is framed into the wall. All 4 sides. (not my doings). I put one in where I lived before and it cut my heating costs by 15% Just an idea to play with.  13   

[petect]

Thanks guys!

Jonathan
Yes, could you give some more info on your freezer hack? I assumed that the thermostats on my freezers are mechanical, but maybe not - I'll have to check.

Chris
Running excess 48V dc into a water preheat tank ( maybe the main water heater) seems like a good idea. I looked for parts for dc water heaters on RV web sites and didn't find any. Do you know of a source? Or do you mean to run the 48v dc into a 115V element, getting the lower temp output?

thirteen

Do you mean electric?  I only anticipate having around 1500 Watts of panels. I don't think that would do much with the tankless water heaters I have seen - several KW. Unless there are much smaller units on the market that I don't know about.


Pete

[Bruce S]

Pete;
 You might have a look/read at this post by Operhouse he did a great write up, even with code included.
http://www.fieldlines.com/index.php/topic,146985.msg1011361.html#msg1011361

[madlabs]

I wanted to leave the original controller intact, so what I did was to use a relay at the compressor. So in an un-energized state the stock controller is in charge. When in surplus power mode my controller is in charge. This way if my controller fails you don't wind up with rotten food.

I used a PIC 12F675 and a LM34 temp. sensor for my controller. It looks to see if the float pin on the charge controller (midnite classic 150) is high and if so takes over. Simple and cheap.

Jonathan

[thirteen]

 Pettect; I should have said I have an propane water heater here and one in my other house I had. The flow of water starts the heater working no electricity needed except for the water pump. The elect ones use to much energy.    13 

[OperaHouse]

I don't understand the need for a transformer except if you wanted to make a 120V inverter maybe 60V.  I heat hot water right off PV with a power point PWM controlled by an ARDUINO.  Have about 35V into a 120V 2000W element.  That heats up water with only about 1KW a day.

I also run a chest freezer and store cold.

[OperaHouse]

I was on a library short use computer and only had a few minutes to answer.  Both options would be a great dump.
However, if you don't add some smarts to them like an ARDUINO the execution will stink.  With a freezer the manual
temperature control can be set to zero F and the dump control can be set to -20 or so.  That should get it through
the night.  Just add a relay between the manual control and the compressor.  When the inverter turns on, it will power
the relay and transfer the power.  The control should always be looking to see if the fridge is running.  Transfer
should not take place if the compressor is currently running ot it has been on in the last eight minutes. Hot starts
should be prevented.  It is best to operate the compressor for timed periods like five minutes.  No cooling happens
in the first two minutes. After five minutes, look at the battery voltage and decide if you wish to continue.  The
controller will know the daylight hours and you could add an additional relay to lower the temp on cloudy days to
prevent numerous wild cycling of temps.

I was given two ten gallon water heaters.  The 2000W 120V element was about 7 ohms ( a 5400W 240V element is about
10 ohms).  I used an Arduino with a FET and capacitor bank to keep the power point at about 35V.  at 10am and 4pm,
this more than doubled the power going into the heater vs a direct connect.  This reliably gave me showers each daY.
Next year I will run a higher string voltage and two water heaters in series.  When the first heater reaches temp the
second will turn on.  The 490hz PWM of the controller makes driving the FET easy.  I avoid duty cycles under 10% and
over 90%.  In such cases I either run the FET fully on or off in bursts.  This eliminates FET heating.

[petect]

Thanks again to ALL !
Pete

[cardamon]

There are many different ways.  I use morningstar diversion controllers into a 35 gallon water heater that has 12V elements.  This works well and is very straightforward.  I would say the disadvantages are it may be more expensive than some sort of homebrew setup, and its noisy - the elements buzz under PWM.  Another good way is to use a midnite classic.  You would get MPPT, the ability to run high voltage from the array, and diversion all in one package (it has outputs that can open/close under a variety of conditions and people hook these up to a relay that sends power to the elements.  I thing the aux can even be programmed to PWM which could drive a SSR)  How much excess wattage  would you have under best conditions?

[hydrosun]

A 120 volt heating element will worl on a 48vdc system with just under  1/4 of the rated wattage. But you need to bypass or use a dc relay for the thermostat. So  a 2000 watt 120 volt element will run at 500 watts at 58 vdc.  You can parallel two for 1000 watts in a larger tank. Not a fast heater but perfect for small hydro systems. Solar systems may need to heat faster for the shorter surplus time. So then regular elements wirh an inverter. 4500 wattt 240 element will be 1100 watts wirh 120 volts.
Chris

[trbarto]

My advice is don't use the water heater...its not as obvious as you might be thinking...I have a solar/wind system and use the hot water tank as a means to dump if the grid goes out...

The problem is when you dump, you don't ONLY dump excess wind power - you are dumping your entire battery bank to your dump load from your load controller/charge controller.

This is nuts - but I've not found any charge controller that will dump only the excess wind load....and therefore figure the same for solar tho most ch controllers simply shut down the array or portions of the array during similar events.

T

[Mary B]

Morningstar relay driver, monitor battery state, only dump down to X voltage turn at at Y. What I am going to do to feed my fridge and freezer. Switch the DC through a high current FET switch to the inverter and have a transfer switch on the circuit feeding both.

[Simen]

My advice is don't use the water heater...its not as obvious as you might be thinking...I have a solar/wind system and use the hot water tank as a means to dump if the grid goes out...

The problem is when you dump, you don't ONLY dump excess wind power - you are dumping your entire battery bank to your dump load from your load controller/charge controller.

This is nuts - but I've not found any charge controller that will dump only the excess wind load....and therefore figure the same for solar tho most ch controllers simply shut down the array or portions of the array during similar events.

Diversion type controllers; like the Morningstar TS series dump only excess power not needed to maintain the current charging cycle. Simpler dump load controllers - like Ghurd's controller, does turns the dump fully on at a set voltage, but turns it off when a lower set voltage has reached. (For example on at 14.4V, and off at 14.1V)

Sounds like you are describing either a solar controller, or a load controller; either can not be used with wind...

[JimB]

I made a simple/cheap dump load for my RE system using glow plugs from a VW diesel. They were installed in a pipe fitting manifold that works on a thermosiphon on the side of my preheater tank. I made a small box with fuses , switches and an amp gauge for a car. The load can be varied using the switches.

[Bruce S]

JimB;
Being a MB diesel kind of guy, I would very much like to see a pic of your setup.
Would you happen to have a pic you can upload for all the see and enjoy?
Cheers
Bruce S