# Using excess power to heat the shed containing my storage tank

Registered Users Posts: 13
Hi All,

I just installed a solar pump setup with a CU200 and a Grundfos float switch. I'm thinking I would like to use excess power to heat the room that the above ground, 1500 gal poly tank is in. I've heavily insulated the small room that it's in but might as well see if I can use that wasted power to generate heat. My thought was a simple dump load but I'm not sure how to set that up so that the pump takes priority over the dump (load). Be advised that I know just enough about solar to be dangerous so I might require additional explanations for complex solutions. Thank you in advance for your insight!

## Comments

• Super Moderators, Administrators Posts: 31,684 admin
Re: Using excess power to heat the shed containing my storage tank

The easiest/most reliable... Some MPPT solar charge controllers have an output that can be configured to turn on if the batteries are charging or in "float" (Outback). Midnite has a "waste not" mode that can better control a heating element to match available "excess power" to the load.

Now--The practical question (sorry--the "engineer" in me). If you have a well insulated building and an uninsulated 1,500 gallon tank. To raise the temperature of that tank 10 degree F... The math (yes, there will be math):
• 1,500 gallons of water * 8.33 lbs per gallon * 10 degree rise = 124,950 BTU
• 124,950 BTU * 1/3.413 BTU per WH = 36,610 Watt*Hours = 36.6 kWH

That is a lot of power to raise the temperature of 1,500 gallons of water 10 degrees F. And to get a 70F room, you would need to raise the water temperature by 20-30F (depending on well water temperature in your region).

Practically speaking, you will really only have success in heating the room if the tank is well insulated. Otherwise, the temperature of the room will be set by the average temperature of the water in the thank. You could also end up with mold/mildew problems if the room is "humid" and the tank starts sweating (even if you fiberglass the tank, the tank will still sweat unless you have some sort of vapor barrier or venting around the tank.

Given that the water in the tank is constantly being refilled by the well--You now have (most likely) an additional cooling source in the flow from the well.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Banned Posts: 17,615 ✭✭
Re: Using excess power to heat the shed containing my storage tank

Direct-from-solar pump so there are no batteries or charge controller to work with.

You would have to "piggyback" the pump control switch so that when it demands "ON" it would also activate a relay to switch out the accessory load (as these controls are not double-throw).

Another problem is that DC is difficult to work with for thermostatic control as most T-stats are not meant to handle the arcing potential. You would wand a T-stat to prevent overheating of at least the element. Selection of components will be highly dependent on what array you have and what Voltage/current it produces.
• Registered Users Posts: 13
Re: Using excess power to heat the shed containing my storage tank

Thanks for the feedback. The intent is to prevent the tank/lines from freezing during the winter nights when temperatures usually range from single digits to the negative teens. So the water (and tank) would actually be my main source of heat. I'll be heavily insulating the lines but I'm worried about where the water exits the tank through the bung. I'll probably end up insulating the whole tank at some point, for that reason. I'll take your word on the math, it's definitely eye opening as to how much power would be required. Good things to think about regarding the sweating/mildew.

My pump only runs about 5-10% of the time and it seems a shame to not do anything with the excess power. Heat for the winter makes the most sense but I wasn't sure how feasible/affordable it would be. Based on both your comments, I'm thinking it's not what I'm looking for. I might eventually get a different controller and a battery or two. Setup some LED lights, maybe a camera.
• Solar Expert Posts: 5,911 ✭✭✭✭✭
Re: Using excess power to heat the shed containing my storage tank
opensky wrote: »
Thanks for the feedback. The intent is to prevent the tank/lines from freezing during the winter nights when temperatures usually range from single digits to the negative teens. So the water (and tank) would actually be my main source of heat. I'll be heavily insulating the lines but I'm worried about where the water exits the tank through the bung. I'll probably end up insulating the whole tank at some point, for that reason. I'll take your word on the math, it's definitely eye opening as to how much power would be required. Good things to think about regarding the sweating/mildew.

My pump only runs about 5-10% of the time and it seems a shame to not do anything with the excess power. Heat for the winter makes the most sense but I wasn't sure how feasible/affordable it would be. Based on both your comments, I'm thinking it's not what I'm looking for. I might eventually get a different controller and a battery or two. Setup some LED lights, maybe a camera.

Do what your grandfather might do. Build a Low Thermal Mass Sunspace on a south wall. Use 2 small fans one high and one low and a fan controller like the Ranco ETC. Gramps might use a thermosiphon instead of fans but they are much easier to design with. Probably less than \$500 for the Sunspace. You would have to do something else for periods of low sun if unattended, but maybe not if the mass of the water will hold. You would be amazed at what can be done. The build it solar website has many ways to do this inexpensively.

I do not like depending on diversion loads for an offgrid power system. If I were to heat water with DC solar, I would rather have it's own source of PV and not risk the home power to water heating. Especially for someone new to this !!!!! Keep it simple and redundant.
"we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
http://members.sti.net/offgridsolar/
E-mail [email protected]

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