Can I dump the dump load?

[wiekeith]

I’m considering buying a tow generator or hydrogenerator to supplement the power generated from my solar panels, but note that, as with wind generators, the recommended charge control units are the type that include a dump load to dissipate power not required to charge the batteries. To me that seems an awful waste – why generate power to simply dump it? I’m also unsure what effect a basic two stage charger (which at least one tow generator manufacturer I’ve looked at supplies) would have operating in parallel with the MPPT charge controller fitted to my solar panels.

It occurred to me that I might be able to substitute the dump load with a inverter to power the immersion heater in my calorifier (400 watt), hence heating water rather than wasting the power. I wouldn't expect to have the full 400 watts available, but some heat would be generated from whatever is.

Following on from that thought; what if I were to use an MPPT charge controller with a switchable load output (which most seem to have) and run the inverter from that? The issue I see there though is that the controller will need to be smart enough to prioritise battery charging and only supply what’s ‘left over’ to the load, not just switch off the load when the battery voltage drops below a certain voltage.

So, my questions are, to those more knowledgeable than myself in these matters; would replacing the dump load with an inverter, as described, actually work, and if so, is there an MPPT controller out there that would operate as described? Oh, and of course, what have I overlooked…?

[GordMay]

Originally Posted by wiekeith

... It occurred to me that I might be able to substitute the dump load with a inverter to power the immersion heater in my calorifier (400 watt), hence heating water rather than wasting the power. I wouldn't expect to have the full 400 watts available, but some heat would be generated from whatever is...

In which case, your water heater IS your dump (diversion) load.

[colemj]

Can't you just lift out the hydrogenerator until you need power again? Seems that would also increase your sailing speed.

Mark

[wiekeith]

GordMay, I neglected to mention that the calorifier immersion heater element is rated for 230v shore power, which, if my calculations are correct, makes it about a 135 Ohm load - I think that's far too high to act as a dump load at 12v?

Colemj, that's a very logical suggestion, which I can't argue with . Retrieving a tow-genertor impeller is a bit of a chore though.

Thanks Guys.

[denverd0n]

But you would be running the water heater through an inverter. So if the inverter is intended to convert 12v to 230v then the fact that the final load is 230v is pretty much irrelevant. The generator would be dumping 12v to the inverter from whence it gets converted.

Of course, there are plenty of 12v water heater elements available, also. You could just get one of those and use it directly as your dump load.

[wiekeith]

Yeh, I was thinking along those lines too, but doing the calculations I could see that putting a 135 Ohm load across 12v will only draw 0.09 Amps, and there's not much I can do about that - it's a law of physics. Hence speculating if an inverter would be able to draw the maximum available current from the generator.

A 12v heating element, now that's a thought!

[jtsailjt]

Originally Posted by wiekeith

Yeh, I was thinking along those lines too, but doing the calculations I could see that putting a 135 Ohm load across 12v will only draw 0.09 Amps, and there's not much I can do about that - it's a law of physics. Hence speculating if an inverter would be able to draw the maximum available current from the generator.

A 12v heating element, now that's a thought!

I had a dual water heater element with one element set up for DC and one for AC. It's sold for use with KISS wind generators by SVHotwire.com. The problem is that in the dual element they sell, the AC element is only 500 watts, so it takes forever to heat enough water to take a shower, greatly increasing genset run time. For a boat that spends a lot of time at the dock plugged in, that wouldn't be a problem but I'm almost never at a dock. The DC element was virtually never used since my batteries almost never stay 100% full for any length of time, and if that ever happened I could switch off the windgen if I was aboard. So, I just changed over in the the opposite direction you are headed and now have a coil type dump load and have replaced my dual element with a single 1500W AC element in my water heater.

I hate the idea of wasting any energy aboard a boat too, so I understand where you're coming from on this, but before you make any changes, think it's important to ask yourself how often your batteries will be 100% full and you will simultaneously have a need for more hot water than running your engine has provided you with? If, while sailing only without your engine at all that day, your towed generator powers your autopilot and all the other electrical loads on your boat and still fills your battery bank to capacity, and that situation occurs frequently, then it might be worth switching over to capture the energy you are losing. But with a wind generator and fairly sizable solar array aboard, I never seem to find myself in that situation so no useful energy is being wasted. At least on my boat, it's more a theoretical thing than a reality that ever occurs. So, I'm happier with a traditional dump load that I am quite sure will never even be used. It's just a cheap, simple, insurance policy to protect my batteries in case I'm away from the boat for several days and have left the wind gen switch on and the wind blows hard/long, because that's the only circumstance I can imagine where a dump load might be needed on my boat. I suppose if you have a windvane autopilot that you frequently use while underway and don't use much other electrical equipment, your situation might be different than mine, but making hot water out of this excess energy is only 'saving' energy if more hot water than you already have from running the engine is useful to you.

[noelex 77]

The problem with the water heater and inverter route is that your dump load is 400w (plus inverter inefficiency) or nothing.

So if your tow generator is producing 50w extra you end up with the system cycling on and and off. It is OK with a manual system ( turn on the heater for an hour when you know you will have loads of excess powering the future) but it is hard to automate.

If the inverter turns on at say 14.7 v the voltage is likely to rapidly drop when it turns off the voltage will rapidly spike. This makes any control system hard to devise.

With a 12v heating element it can be controlled with a PWM controller. This provides in effect a variable load which can be regulated at a specific voltage.

[Roy M]

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

This is an interesting video which can help the electrically challenged to understand the issues, particularly the scams.

Enjoy.

[wiekeith]

Thanks for all your input guys - it's been very useful and certainly got me thinking.

If my power audit calculations are correct, I will certainly be able to produce more power than required during daylight hours whilst underway. I have wind-vane steering, so no need for autopilot for most of the time, and a water-maker that only takes about 9A and produces 25 lt/hr, so that won't be used too often as it's just for me.

I've received a very comprehensive and detailed response from a manufacturer of a hydrogenerator in the UK, who's enlightened me to the fact that the power applied to the dump load is PWM'd, so an inverter replacing the dump load is out of the question...

So, I guess my course of action is to firstly chose, buy and fit a hydrogenerator; See what power I can generate in reality; consider a 12v heating element if excessive power does prove to be available.

Once again, thanks all.

[sailinglegend]

Originally Posted by jtsailjt

...I hate the idea of wasting any energy aboard a boat too, so I understand where you're coming from on this, but before you make any changes, think it's important to ask yourself how often your batteries will be 100% full.....

Originally Posted by wiekeith

....If my power audit calculations are correct......

This is the big question.

I too never have full batteries when underway, and I have a 400 watt hydrogenerator from DuoGen. This is easy to bring in when the batteries are fully charged. It is also a wind genny and the manufacturers suggest that a charge controller is not needed when you are onboard. Just tie it off when YOU know the batteries are full.

The key to all this is what you call "fully charged". If a little green light comes on or a charge controller drops to float that doesn't mean your batteries are fully charged.

Search this excellent Forum for what "Fully Charged" really means. Most cruising boats I would suggest never get their batteries fully charged, unless they have an extraordinarily large solar array!

[critter]

My apologies for reviving an old thread, but I haven't found out what others did with their dump loads and was looking for some updates on this one.

In the process of installing a D400 windgen. I am in the same camp that I (thanks to two kids aboard) will never have 100% full batteries unless motoring for hours on end. The resistors and wire cage that came with the unit we ordered are ginormous and will take up precious real estate if they are to have any air circulation at all.

So...for those of you who went without the dump load: Did you connect it to ANYTHING at all (water heater element, inverter) or just not use the terminal?

And...for those who ended up keeping the dump load: How hot has it ever gotten? I have the most space to install mine near the aft, but that is also near near the batteries where there may me gassing...should I not be worried about this? Like I said, I doubt these things will ever get used unless I forget to take the generator down during a hurricane, but I'd also rather not start a fire on the boat!

Thanks for your time in advance.

[GILow]

The two dump load resistors on our Aerogen 6 (360 watt max) get hotter than I like to touch and I would suggest they have never had to shed more than about 150 watts while I have owned the boat. Certainly too hot for an enclosed space. Currently they are in a space that would be around 3 or 4 cubic meters of well ventilated space. Much less would feel a bit tight to me.


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[jtsailjt]

I got the resisters and mounted to a board and in a frame facing downward and sitting on top of the 135 gallon aluminum water tank (huge heat sink) that's under the bed in the aft cabin. I used aluminum angle irons riveted into an "S" shape to hold the frame up so the resisters can't actually touch the tank, and there are about 3" of space above the top of the board the resisters are mounted under with our bed above it. The aluminum angle irons that hold the board are prevented from sliding around by that industrial strength clear plastic Velcro type stuff that comes in strips stuck to the tank and the bottom of the outside of the "S" formed by the 2 pieces of aluminum angle iron riveted together.

I eagerly check to see if it's hot whenever the wind blows hard but so far I've never felt any sign of heat. I guess under the right conditions it could become useful, and does add a bit of peace of mind.

[critter]

Thanks for the replies...here is what I ended up doing today:

I had a vent that is under the aft cabin storage and above the battery compartment and decided to use that to my benefit. I ignored the directions (gasp!!!) and flipped the installation of the resistors inside the heat shield so that the heat shield itself acts like a heat sink and the resistors faced the vent. I used poly bushings I already had on hand to put about an inch between the grate and the rear of the plate the resistors are installed on. I then installed a 12v fan to circulate warm air out the vent and into the aft cabin. I will see how this works and have an extra fan I can add to increase the air flow if I have to. Might even put it on the rear of the heat cage to dissipate any heat inside the compartment.

What it's supposed to look like:


View from the front through the vent grate:


Using heat cage as heat sink:


All the wiring is done with the D400 installation...just waiting for the fabricator to finish with the mounting pipe for the generator itself so I can see how this all works out. Fingers Crossed. Thanks for your replies.