Solar underground heating cables how to question

I'm looking to place an underground heating cable/fan set up in my greenhouse. Total cable length about 220 feet.

Would appreciate any ideas on how best to do this. It would be a direct system with a switch to go heating cable or fan with no storage batteries or inverter if feasible.

Thank you

Hi Lorne. Interesting project. (I want a heated greenhouse too BTW.)

I'd like to know more about what you are trying to accomplish. There are a lot of ways to heat a greenhouse.

Offhand, I suspect it would take a very large solar generation capacity to make this work via heating cables. It's possible that the cost just won't make sense. What solar capacity do you currently have? Are you running a large surplus at some times of the day?

Currently I have no solar system. I'm thinking it should not require too large a system as low amp heating cable does not require more than a 15 amp 110v to run. I may have problems to get the full length to heat up. Just assuming mostly, hoping an expert will comment.

PV solar calculations are full of trap doors. It usually boils down to to "you need more wattage than you think." My thoughts FWIW:

For 110 VAC heating cables that put out enough heat to keep pipes from freezing, the power consumption seems to be 210 watts for 30 ft. That's 7 watts per foot. 7 w/ft x 220 ft = 1540 Watts.

110 VAC heating cables designed for warming interior floors draw about 3.7 watts/foot. 3.7 w/ft x 220 ft = 814 watts. Heat output is correspondingly less, of course.

For a 12 VDC direct-wired application:

You will have more loss in your transmission cables.

I found one 12 VDC freeze prevention heating cable (expensive, industrial grade) that consumes 5 watts/ft. 5 x 220 = 1100 watts.

But here's the trap door: PV solar panels only operate at full capacity in full sun. Planning has to account for averages, including worst case scenarios. That depends on your normal weather. I'm guessing it may take up to 50% more wattage to get useful results.

To me, this is looking pretty spendy, and there are probably more trap doors to watch for. With solar, the devil is in the details.

My 2 cents.

[Edited for typos]

As mentioned above using solar directly for heating purposes for large areas is usually a non starter. First off you need heat in the winter when the daylight hours are shorter and therefor arrays produce less total watts.  

How much heat/btu does your greenhouse need per day? E.g 48,000WHr
How much heat does your solar panel produce per day? E.g 500W x 4Hr = 2,000WHr
With the above example the solar panel will provide less than 5% of the energy needed.
Is it worth all the effort/resources for that limited gain. Could you maybe exhaust your furnace/dryer into the 'attached greenhouse'. Spend the money on insulating the north-wall.

Now lets say the number add up and it is the best use of your resources.
I recommend using silicone insulated wires. Silicone insulation has good oil and solvent resistance, and good corona, and ozone and weathering resistance, has a higher temperature rater(more amperage), and a low halogen-toxin content.

I recommend adding a single super cheap battery and a low voltage cutoff switch for the fan/cable-heater, each for about $30. We don't care about the storage capacity(amp-hours) of the battery so we are not limited by the amount of cycles. And because the battery is in the greenhouse we are not worried about efficiency. Without the battery we dont get max powerpoint and solar panels don't deliver their max energy. We would also have to setup a complicated logic board for 9AM when the solar panels are only ouputting 20W vs at 12-noon when they outputting 250W each.

I'm not trying to heat the greenhouse with it. Just keeping the soil a little warmer. I have a wood stove for the heat. Otherwise the greenhouse is warm but the plants feet are cold.

Using the wood stove to heat water and then sending the pex piping thru the soil seems like another good solution to send more heat to the roots vs just the leaves

S Bengi makes an excellent suggestion. Since you already have a heat source, maybe the solution is to redirect it to where it's needed.

To add perspective 1 lb of wood has roughly 7000btu in it or roughly 2300watts or 2.3kW of heat energy. In the darkest part of the winter in Nova Scotia you could expect an average of 1.5 hrs of sun. So each kW of installed solar 0anels can be expected to produce as much heat as 1lb of firewood.
Cheers,  David

Thank you all for your input.
To me heating water and pumping it through pipes requires too much attention and will also be fairly costly. The solar idea is for set and forget, heating in winter, cooling in summer. Panels running direct energy to an a/b switch for either heating cables or cooling fan. No controllers or inverters is my way of thinking.

Lets see the summer cooling fan side of this project.
How much power will the fans be rated for? What voltage will the fans operate at?
Are you thinking about a bi-metal switch that auto-turns on at a set temperate or just a switch you flip on 'once peach tree has leaved out/May/etc'
How big is the greenhouse, what the cfm airflow that you looking for? Would the fan focus on rpm or blade size to reach the cfm/airflow target?

Is it possible to just install 'big' bi-metalic windows that open automatically and cools down the greenhouse?

How much you plan on spending for the solar panel and support and to clean off the snow on the solar panel and also on the switch and heating wire. What gauge wire will you use (this is dependent on the output of the solar array)


Heating with Pex Piping Cost only $300 (more like $500 once the misc things start getting added)
500ft Flexible PEX piping $70
https://www.homedepot.com/p/Apollo-1-2-in-x-500-ft-Blue-PEX-Pipe-APPB50012/302341821
Pump $100
https://www.supplyhouse.com/Grundfos-59896341-UPS15-58FC-3-Speed-Circulator-Pump-1-25-HP-115-volt-4701000-p
Coil of copper piping/heat exchanger $100
https://www.homedepot.com/b/Plumbing-Pipe-Fittings-Pipe-Copper-Pipe/Coil/N-5yc1vZ1z18i44Z1z1b3xh?
Temperature and Pressure Relief Valve $15x2 (I would get an additional pressure relief valve)
https://www.homedepot.com/p/Cash-Acme-3-4-in-Bronze-NCLX-Temperature-and-Pressure-Relief-Valve-with-1-1-4-in-Shank-MNPT-Inlet-x-FNPT-Outlet-23576-0150/203164746

You know, Lorne, when I try out a new system I often do a pilot project. It's a test on a small scale, to see if it's worth doing on a larger scale and figure out the bugs.

Sometimes it works out and sometimes it doesn't. It's always worth trying, because I get an education in the school of hard knocks without investing big chunk of money. I guess I have a lot of ideas that seem really good on paper, but don't always work out in the real world. That's life.

Anyway, I wonder if that approach could work for you. Thoughts?

My two cents also vote for the hot water pipes in the ground. The water can be heated via woodstove, Gas tank type or on demand hot water heaters.  I know a man that lives here in AK that grows competition pumpkins (ie way over 1000 pounds) His green house that he grows the pumpkins in is not just a raised bed, but a raised GH.  Whole thing is close to 2 feet higher than the surrounding ground.  He uses a natural gas water heater and pumps the water through PEX tubing.  This allows him to start the GH in March and is growing pumpkins by April in a controlled climate that is at least 3 or 4 climate zones further south than we live.  Now, you don't have to go to those lengths, but burying Pex tubing about a foot deep and heating the water with either a woodstove or gas water heater and pumping it through the tubing will probably do more good than trying to heat the soil with the electric cables.  If you use raised beds in the GH then you would only need to run the tubing in the beds where you will be planting.

If you want to incorporate solar electric heat it gets easy if you run pex and water. There are 24 volt dc rated heating elements that could take the feed from a 60 cell panel when it was available....

I would like to hear more information about this using 36 volt heating cables!
After hearing about Finland new sand battery it got me searching more with thermals mass which many greenhouses are starting to use.
I have a small greenhouse and I’m planning on digging a 6x8 area down 2 feet, foam board to bottom and sides, lay 8in of sand, lay heat cables, lay 6in sand, lay 4in pipe with manifold, lay another 6in sand then top with stone pavers.
The heat cables will be connected to solar and the manifold will pump air with a fan.
I’m not sure if I’m correct but with 36 volt heating cable puts out 7.5 watts per meter length so a 15 meter long heat cord should use 113 watts total.
I know that the solar panel will not fully provide the wattage it rated for so my question is if I should go with a larger panel wattage like 200 watts?

Jim,
Welcome to permies.  The graphene cables are now very effective but a little dear at present.
What ever the wattage of a solar cell, plan for up to 50% efficiency.  both heat and light will dictate this.  There are good tables out there so I will not dwell on this aspect.
The other thing you would need is a maximiser so that the power switched on at the correct wattage in the morning and off once the wattage drops.

For what you are trying to do, I suggest that you use a rocket mass heater for green houses with underground poly tubing ( a 1" garden hose works just as well.) pull the heat out of the thermal mass with an inline 12 volt fan.  It is easier to suck (creating negative pressure) than blow the air around.  The alternative is to use a liquid with an inline pump.  If your pipes are about 3' in the ground,you will have a heat battery.  The fan or pump will work on a small solar cell and maximiser.  Alternately, you can have a battery and timer so the system works over night.  From my experience with using heated fluid, which worked very well using an old solar hot water system. Keep it simple.  With water, you can also use the thermosyphon to move the fluid. out at the top and in at the bottom.  Above all. each component used can break so keep it simple.
Let us know how you go with your project

Would earth tubes do what you are wanting to do?