Using Peltier modules as heaters with geothermal ground loop

[azaxi]

Hi everyone, I would like to discuss efficiency of Peltier modules.

Please tell me if I am talking nonsense, I never worked with Peltier modules, my understanding is theoretical.

Would like to do a project with garden bed heated with geothermal loop buried 1.5m deep and array of Peltier modules (say TEC1-12710) with cold side connected to the loop and warm side to large (massive) heatsink/radiator, warm side should never go more than a degree above ambient.

According to datasheet, when working optimally, Peltiers should achieve CoP>3 at DT<10'C. An edge case when CoP is zero, they should still be able to get base electric heat with some losses to ground loop coolant through internal conduction, but at that point I will stop circulating the coolant so losses should be small.

To construct 200W heater and be able to use most optimal point with CoP>3, only <20W of heat per element can be transfered, so I would need 10 elements, right?

How realistic is CoP>3 ?

 

[Diver300]

If the manufacturer is publishing performance curves like that, they are probably about right.

Cop of 3 with a DT = 10 °C is far lower than you can get from a proper heat pump, but Peltier cells are small and cheap, so they have their uses.

If you are running at Qc of 20 W you are pumping 20 W from the cold side, and the data sheet seems to say that you will need about 2 A at about 3 V to make that happen.

20 W would cost about £32 in electricity if you used a resistor. Your Peltier element is £8 from Amazon, and there will be pipework as well, so it will make sense to have a system like that if it is run for a long time.

 

[azaxi]

Cop of 3 with a DT = 10 °C is far lower than you can get from a proper heat pump, but Peltier cells are small and cheap, so they have their uses.

If you are running at Qc of 20 W you are pumping 20 W from the cold side, and the data sheet seems to say that you will need about 2 A at about 3 V to make that happen.

Average Qc+Pel=200W in January/February then dropping and almost no heating needed May-Oct so say heating season average 100W x 24hrs/d x 30days/m x 7months = ~500kWh/y. Residential price here is $0.15/kWh and cost of operating electric heater would be $75 Canadian per year.

With average CoP=2.5 of Peltiers, geothermal ground loop will save about $54/yr in electricity.

Considering amount of labor and materials to build this, this project makes no economical sense. But it is proof of concept, then I will build larger greenhouse and at that point will use compressor heat pump. I only chose Peltier elements, because there are no tiny heat pumps and Peltiers and heat exchange accesories are readily available.

 

[Pommie]

My experience of peltier devices is,
They heat much better than cool due to the electrical energy put in turns to heat.
They act as a generator opposing the applied voltage. When ΔT = 0 then max current can flow because no back EMF generated. At some value of ΔT no further change in ΔT) can take place.

Good luck with your project.

Mike.

 

[azaxi]

Thanks guys, I am still digging little by little, but should be installing the pipes soon perhaps tomorrow. I will get back to this thread when its cold enough to see some meaningful results.