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
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
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
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
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
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.
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
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.