Dump loads for Renewables.

[Ivanvet]

Hi everybody. Long text ahead (you've been warned).
Since I can’t do any boat related projects while recovering from surgery, I’ve decided to take advantage of this time off to learn about what to do with energy excess from renewable energies. I’m not near ready to change to lithium yet, but when I do, I will most likely redesign all our system including charging methods and increasing our power available from solar and wind.

I’ve spent the last week watching and reading hundreds of videos and articles about this subject, and today I went down to a rabbit hole that made me say, aha! I thought I would spend some time making diagrams and share them here, not to ‘explain how to do it’ (I’m just learning) but just to open a discussion and see how people are using dump loads from excess solar to heat water, etc…

I’m pretty basic when it comes to understanding electrical systems so please feel free to correct any of the assumptions made in the diagrams. Just be civil.

The diagrams are conceptual, and only include the positive side of the charging loads (no negative, no loads, no battery size, etc…)

Let’s start with the current system in my boat to heat water: Water heater with a single coil (to use engine heat when motoring) and an A/C heater to heat water only when connected to shore power.



I understand that when there is a wind generator on board, and batteries are fully charged, you can stop the windgen by using a brake (and then tie the blades). It is also recommended to use a dump load to protect the generator: Batteries reach float voltage, the controller from the windgen tells a relay to close (an automatic switch), and excess energy goes to a dump load. This load is typically a resistor that dissipates heat.



Because this seems to be a tad wasteful, some people use a heat element (DC powered) to heat water:



There seem to be a bunch of users on off the grid houses doing this (youtube is your friend here). I’ve seen that some of the residential water heaters have more than one heat element, and one of them can be replaced with a DC type, or add multiple elements. I don’t know whether this is a possibility in my marine water heater (isotemp). Instead, I can imagine adding an accumulator tank before the water heater, to which a DC heat element (or multiple) can be added.



If space is of the essence (when is it not in a boat), perhaps you can replace the AC element on the current water heater with a DC element, and then, after the tank, a tankless electric water heater can be added. The latter will only be used with AC (shore power only), but you still have the ability of heating water when motoring.




Since I expect most of my power to come from solar, I did some reading on how this applies to PV energy. Here is when I came into some trouble understanding the process. If I read correctly, solar doesn’t ‘need’ to dump load, as opposed to wind generators. MPPT controllers stop pumping energy to the batteries when they reach the specific voltage and the solar panels don’t get damaged. However, it is still a good idea to use excess power when available. Victron MPPT controllers have load output but not the ability of using dumping loads, at least at this point. https://community.victronenergy.com/...dump-load.html
One option is to use a controller that can use solar +/- wind +/- water like for example, the outback controller. However, it seems like if you connect your solar panels to this controller and add a dump/diversion load, the controller works more as PWM and not as an MPPT, which means that you are not using the panels as efficiently. Can someone verify is this is true?

Even if some of the solar and wind can be damped into the water heater, it seems like the batteries may suffer consequences long term. Essentially, since the relays switch on and off as battery voltage increases and decreases, the batteries have these microcycles that can affect their life. Again, can someone verify if this is true? And if it is, how big of a deal is it? This guy explains it relatively well.

https://www.youtube.com/watch?v=A9Meq8wLbME

So, as I kept reading about this, I came across some new devices that seem interesting. Instead of using relays that open/close based on changes in battery voltage, there are these frequency controlled switches that open/close automatically based on changes in frequency. Frequency Controlled Switches | Wind & Sun
These are connected to the inverter, let’s say a Victron multiplus. When there is energy excess, from solar ( I guess also from wind +/- hydrogenerator), the inverter detects a shift in frequency. This closes the frequency controlled switched, which is connected to a load, in this case a heat element in the water heater, and voila, hot water.



In this method, Would the batteries still go through microcycles like with the other example? If not, it seems like this would solve the problem of mycrocycles, plus we get to use MPPTs for solar.

This company (MY-PV) has multiple devices that seem to be relatively new and are all based on the similar principle (frequency shift relays connected to the inverter to use solar for heating water)

https://www.youtube.com/watch?v=PGrnW0O95XQ

Some of them, like the MY-PV AC ELWA-F, is a heat element ready to be ‘plugged into an AC outlet, but ‘DC-solar-ready’.
The one I foresee using in my system is the A-Thor (explained in the video). And with this, I came to this ‘final diagram’:



I assume you would still need to protect the batteries from high voltage (from the charging devices). You can see in the diagram a High-voltage-disconnect battery protect, just in case. Would this affect the ability of the Multiplus to do its ‘dump load’ function? The yellow shaded part would be how the power would flow from the charging sources through the inverter into the heating element.
I also added a resistor in the wind gen, in case there is so much energy that the water gets to the specified temperature, shutting down the dumping load function, and using the resistor to dissipate heat from the win-gen.
Keep in mind that the alternator circuit has been left out, as well as BMS, monitors loads and such, just to simplify.

Perhaps some of you have already implemented some of these systems, in which case I would love to see diagrams to understand this better. Feel free to fill any gaps on my knowledge. Just to minimize any thread drifts, I’m not planning on DIY-ing my system, I just want to understand how it works or the new ways of doing these things.

Blimey this was a long one.

Fair winds and hot water.

[john61ct]

Check out Electrodacus, FOSS solar / BMS project.

There's an option for resistance heating of the living space, seem to recall not just using "excess" production, but may well have good logic controls for that aspect.

[Matt Johnson]

If the idea is to dump extra solar and wind into heating the water tank, I'd first take a look at your real world usages and power generated and see just how much extra you really have. I think most full time cruisers overestimate this number and would have little extra at the end of the day to heat their water tank. But you don't have a boat listed in your bio, so you maybe putting a huge solar array on a large cat and it might work for you.

I, along with just about everyone setting up solar and wind on a cruising boat, thought of doing something similar, but after looking at the numbers, and now experience with a real world system, I see that heating via extra wind/solar energy wouldn't do much at all. Realistically, we have had our wind gen (D400) go into dump mode so little, and it creates so little power when it does, that trying to get it to heat a water tank is a pretty useless exercise.



Matt

[Ivanvet]

Originally Posted by john61ct

Check out Electrodacus, FOSS solar / BMS project.

Brilliant. I'll take a look at this BMSs. Thanks!

Originally Posted by funjohnson

If the idea is to dump extra solar and wind into heating the water tank, I'd first take a look at your real world usages and power generated and see just how much extra you really have. I think most full time cruisers overestimate this number and would have little extra at the end of the day to heat their water tank.
But you don't have a boat listed in your bio, so you maybe putting a huge solar array on a large cat and it might work for you.

Yes, this makes sense. We have a beneteau 473, with currently 440W of solar, but this situation is irrelevant at this point and different from when we go full time cruising (currently, it's just seasonal). It was more of an exercise about understanding how people do it so I can include it into my design.

I, along with just about everyone setting up solar and wind on a cruising boat, thought of doing something similar, but after looking at the numbers, and now experience with a real world system, I see that heating via extra wind/solar energy wouldn't do much at all. Realistically, we have had our wind gen (D400) go into dump mode so little, and it creates so little power when it does, that trying to get it to heat a water tank is a pretty useless exercise.

This is good to know. I guess there are situations where it's not worth the expense and complexity for just a small amount of power.

Thanks for the insight. Love the videos. Keep at it!

[john61ct]

Yes it usually is not worth worrying about "wasting" the excess panel production.

A few hundred Wh certainly won't raise temperature of a tank of water significantly.

[warrior 90]

There is another situation where I really would like to have this feature. I always thought that would be a great way to protect a LifePo4 battery from over charging. By that I mean just totally disconnecting the battery once charged to limit and not reconnecting until the battery is at some ca.0,2-0,5v lower. I think that makes perfect sense in bigger solar installations like on CATS that at some point of the day have more energy then they need. To make this more safe another good dump or diversion load would be also a dehumidifier as there will be never too much use of that on a boat.
I would even do this manually if I have no other choice.

I mentioned this here on several occasions but there was never any response.

[john61ct]

Just take your LFP offline whenever you like, that goal is not much related to implementing a load dump, long as you leave a lead batt for the alternator.

Unlike a wind gennie, solar does not "need" its surplus power to go anywhere, the SC only produces what is "pulled" from it.

But if you think the utility value of putting the excess production to use, is greater than the time cost and added complexity involved then go for it!

ideally posting your build details and results for other like-minded members to learn from.

[Dsanduril]

The problem with dump loads in our environment is that dump loads really need to be variable loads. This is why heaters are used, they can be made to use only "surplus" energy. But even that is difficult. In the end you want to keep the battery charging running to completion, feed all your standing loads, and then use any excess capacity into your dump load. But that excess could be 100W now, 200W in a minute from now, and 50W another two minutes later. The logic and controls required to match that excess to a dump, while not doing things like microcycling your batteries, is not simple. Most appliances that you would consider for dump (such as the dehumidifier mentioned above) need a constant power level, and dump power is intrinsically not steady/constant.

[john61ct]

Originally Posted by Dsanduril

you want to keep the battery charging running to completion, feed all your standing loads, and then use any excess capacity into your dump load. But that excess could be 100W now, 200W in a minute from now, and 50W another two minutes later. The logic and controls required to match that excess to a dump, while not doing things like microcycling your batteries, is not simple.

Yes, why I reco those wanting to dive down that rabbit hole check out

Originally Posted by john61ct

Electrodacus, FOSS solar / BMS project.

There's an option for resistance heating of the living space, seem to recall not just using "excess" production, but may well have good logic controls for that aspect.

Heating your home from PV output is a pretty extreme goal, but many say he's a pretty smart fellow.

Integration with BMS functionality seems a solid approach?

[Dsanduril]

If you read about his resistance heating approach, he uses multiple discrete heating elements and switches on/off 1, or 2, or 3... depending on how much excess is available. So he has minimum step levels and multiple heating elements. Which works in a larger installation, but has some complexity that may not be suitable on a 1kW boat system. How easy is it to put 3 or 6 or 8 25/50W elements into your water heater?

[john61ct]

Yah heating water just seems like busywork to me.

Variable-speed freezer compressor driving eutectic holding plates?

In addition to the evaporator one(s) powered from storage.

Fundamentally feels to me like a solution in search of a problem.

[warrior 90]

I have no building details but I am very interested in the details of a posible concept.

[QUOTE=john61ct;3035367]
But if you think the utility value of putting the excess production to use, is greater than the time cost and added complexity involved then go for it!

1.) We fight for every little bit of more efficiency from our solar panels and windgen for the limited space on an average boat.
2.) In case of heating water.... not all temperature stages cost the same amount of energy. If water is preheated it will lower run time from cooling system on engine/ Gen, or use of other energy like propane or even from the battery to get to desired temperature.
3.) So now there is a third reason with microcycling that I was not even aware of.

Originally Posted by Dsanduril

The problem with dump loads in our environment is that dump loads really need to be variable loads. This is why heaters are used, they can be made to use only "surplus" energy. But even that is difficult. In the end you want to keep the battery charging running to completion, feed all your standing loads, and then use any excess capacity into your dump load. But that excess could be 100W now, 200W in a minute from now, and 50W another two minutes later. The logic and controls required to match that excess to a dump, while not doing things like microcycling your batteries, is not simple. Most appliances that you would consider for dump (such as the dehumidifier mentioned above) need a constant power level, and dump power is intrinsically not steady/constant.

Wouldn´t it be just a lot easier to calculate a bigger water heating element to absorb the full capacity of the Solar/wind capacity in the moment the battery is fully charged ?
Yes, then the usual consumers will draw from the battery but there is no extreme micro cycling close to the upper limit as pointed out. Of course with this concept it needs to be determined how far the consumers are allowed to discharge the battery.
Problem I see....what happens once the water get´s to 100ºC. Of course with space heating this problem won´t exist.

Just my thoughts and considerations on different concepts.

This topic came up a while ago on.....
http://www.cruisersforum.com/forums/...-221917-4.html

in relation to this product https://www.green-yachting.online/
and the YT video of installation installation........

https://www.youtube.com/watch?v=x3iC...ature=youtu.be

He promised an up date but unfortunately no further news on the topic till now.

[Dsanduril]

IME experience that battery isn't "fully charged" until pretty late in the day. Lots of discussions on that here on CF, and it all depends on what battery chemistry you have. But, if using lead-acid (of any kind) there is a long tail of low charging current required to completely fill the battery. And this is when you generally have "surplus" available. But you still want that high (relatively) voltage-low current tail going to the batteries. (Lithium and other alternate battery chemistries change this picture significantly).

So to effectively use the surplus you need a "backpressure regulator" that will maintain 14.4V (or whatever) on the battery charging side until absorption is complete, then maintain your float voltage after that, all while passing any spare electrons out the other side - at what could be a highly variable rate depending on clouds, your refrigerator turning on, ....

I've been down the same rabbit hole and have not come out the other side with any good, elegant solution (so very interested to see what other responses you get). Our inelegant solution is to use the Mk1 human brain to assess spare capacity and switch variable loads on/off.

[warrior 90]

Originally Posted by Dsanduril

IME experience that battery isn't "fully charged" until pretty late in the day. Lots of discussions on that here on CF, and it all depends on what battery chemistry you have. But, if using lead-acid (of any kind) there is a long tail of low charging current required to completely fill the battery. And this is when you generally have "surplus" available. But you still want that high (relatively) voltage-low current tail going to the batteries. (Lithium and other alternate battery chemistries change this picture significantly).

So to effectively use the surplus you need a "backpressure regulator" that will maintain 14.4V (or whatever) on the battery charging side until absorption is complete, then maintain your float voltage after that, all while passing any spare electrons out the other side - at what could be a highly variable rate depending on clouds, your refrigerator turning on, ....

I've been down the same rabbit hole and have not come out the other side with any good, elegant solution (so very interested to see what other responses you get). Our inelegant solution is to use the Mk1 human brain to assess spare capacity and switch variable loads on/off.

I agree with Your evaluations. There is a significant difference of this concept depending on battery chemistry. Long time until those LA batteries are finally fully charged and floating. However compared with a LifePo4 battery this particular disadvantage is changing as there is a more or less a constant charging rate till full and after that the best is a clean cut. There would be the consumers on one end drawing from the battery and on the other end the water heater with all the capacity of the renewables. Besides the electronic control of the process I think there should be an external manual regulator that allows timing according to actual charge rate and weather conditions. Weather/ clouds can not be predicted. However being on the boat this should not be such a big deal to evaluate according to weather report and daily experience. Let´s asume for a moment there is just a switch connected to a timer that at a certain time of the day disconnects the batteries from charging. With a battery of sufficient size this should not be any issue of running the battery low on the same day. A second switch acts as an override reconnecting the battery back to charging/protect should the state of charge go below a certain level that also can be adjusted.Estimating an estimated system at 500 W at 6 hours per day = 3000 W + what ever the wind gen can bring in. I would estimate a high percentage of days where it should work out very well, especially in the tropics. Is it worth while ?
Well, usually I have to turn on the engine/Gen to get hot water if there is no shore power
A total different situation might be the surplus when required extended motoring with additional more or less stable constant energy from the alternator, where the water is already hot. That is a another big surplus that could be handled with such consumers as a dehumidifier etc. to a simple fixed circuit stabilised by the batteries for the purpose once the battery is full and triggered by the temp sensor in the water heater.
Till here I can see this whole set up with relative simple of the shelf components.

Of course lot´s of variables and speculations and also highly dependent on type of cruising

However I look at this UK company with a 300 W water heater ?
I am sure they did some homework before starting the adventure ?
I am also sure with Lithium batteries the surplus is much higher then with LA. I like the concept because it´s simple . Now it seems there is also a more serious issue with the micro cycling. Don´t like the dancing around close to max charge rate with Lithium batteries. I see the potential.of innovation. So I am open to ideas.

[noelex 77]

Originally Posted by Dsanduril

Our inelegant solution is to use the Mk1 human brain to assess spare capacity and switch variable loads on/off.

This is generally the best solution. KISS.

I don’t think the battery type is very critical. The main requirement is for enough excess energy to make water heating practical, although there are many other discretionary loads, for example fuel polishing, that can run when only a small amount of surplus energy is available.

It water heating is the goal, it is helpful to have a reasonably low wattage water heating element, but this is by no means essential. Our shower heating is relatively high at 1800w (when we are using electricity) although it is an efficienct system so heating time and power consumed is not high.