Solar powered hot tub

[bernhard]

G'day,

In order to lower my electricity bill, I want to use the power of the sun to heat my hot tub. The setup is the following:

A 100W solar panel is driving a raspberry pico microcontroller which is measuring the temperature of the water in the hot tub. If the water is below 40C=105F it will start a 70W pump. Water will be pumped through a hot water collector and outputting warm water into the hot tub. The microcontroller will stop as soon as the target temperature is reached. I want to connect this directly, meaning I do not need a battery because if there is no sun, there is no need for pumping water anyway.

My setup is the following:

Solar Panel 100W 18V connected to a 8-40V/12V 10A DC-DC Converter. Now I get out nice 12V. Those are connected to a 12V/5V DC-DC Converter for the microcontroller. The pump is connected via a 10A relay of the microcontroller to the 12V. I am not going to go into detail for the microcontroller and all the sensors, display etc.

What do you think about this setup? Am I missing something? Is there something I could do smarter? I also thought about a charge controller which has also a load output and comes with a USB plug. That would be much easier for the microcontroller.

Looking forward for your thoughts,
Bernhard

 

[efficientPV]

The power converter is goofy and won't add anything. It will actually produce less power. A controller??????? At 70W this will never shut off. If you can actually program, use that controller to make a power point controller for the panels instead of wasting power with direct connect.

 

[bernhard]

The power converter is goofy and won't add anything. It will actually produce less power. A controller??????? At 70W this will never shut off. If you can actually program, use that controller to make a power point controller for the panels instead of wasting power with direct connect.

I am afraid but I dont quite understand what you mean. The pump will switch off when the temperature is reached. The microcontroller is taking care of this.

 

[BentleyJ]

The setup as you describe it sounds simple yet functional. The regulated DCDC converter gives you good stable 12V to the pump. I'm wondering what happens to the 5V output from the secondary DCDC converter when the solar panel voltage drops below 8V and the primary DCDC converter stops making power? Seems like the RPI is going to shut down in an improper manner. I know they are somewhat tolerant of being shut down by simply turning the power off (I'm using one with a PiCAN Duo to capture CAN Bus data from a BMS and transmit it via Ethernet with a remote desktop connection.) but realistically it seems like it would be a good idea to set up a 5V power supply with a small battery that will act sort of like a UPS during the night time. Then the next day get charged up when the primary and secondary DCDC converters fire up.

 

[bernhard]

Thanks for your reply! I think when the voltage drops below 8V, then there is little sun out, the pump can be off anyway, and so can the microcontroller. i dont know how the raspberry pico will cope with the constant on and off with every cloud. I think it will be fine though

 

[Don B. Cilly]

I would add a small battery anyway. It will basically act as a big capacitor, smoothing things out.
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[TorC]

I'd include temp sense of the panel, too. Both for start and cutoff regulation. Maybe not for your pump size, but I have seen small DC pumps for hot water circulation on a tiny panel run when there isn't enough heat and cool the water down. A temp sense or three in the panels to figure out when there is heat to add doesn't at much complexity for a significant improvement in performance.