What is the efficiency of direct electric heating versus water heated via electricity?

Question

I am attempting to keep plants warm in a greenhouse. Therefore, I assume that heating the soil should be most efficient, as that's what plants seem to care about the most (i.e. the temperature of the roots appears to be more important than the temperature of the leaves, though I can't seem to find much evidence around this). If my assumption is true, this is convenient because greenhouses have lousy insulation (and are somewhat difficult to insulate retrospectively), so much of the heat loss happens via the air.

So, which is more efficient for heating the soil?

A) An electric wire element buried in the soil (i.e. a long wire connected directly to A/C power that generates heat).

B) A tank of water containing a heat resistive element and circulation pump with a hose running through the soil.

Intuition may suggest that the pumps are wasting energy, and some heat is lost via evaporation if the tank is not air-tight and insulated, but perhaps the water system is perhaps a little more versatile because you can take energy from other sources (such as the sun) to heat the water. Then again, you could use PV solar energy to power the wire element (though it could take a large array).

I'm attempting to prove which approach is more efficient, and a greenhouse is a difficult environment to control, so I'm struggling to get scientific results. Any insight into which approach generates the most energy loss would be appreciated.

Edit: From the comments, a heat pump sounds like the most efficient option. Caveat: This seems like by far the most expensive in terms of equipment and I'd probably have to buy off the shelf since it's beyond my skill level.

Answer 1

At these temperatures 25ºC (and provided that the outside environment temperature is not subzero or very close to zero ºC), the heat pump would be significantly more economic/efficient.

Option A - wire in the soil.

The main problem with option A is that the resistive elements could warm up the soil, but depending on the moisture of the soil you might have different conductivity coefficients (usually moisture improves conductivity). If the soil is too dry then you might have temperature buildup locally which will not be very good for the plants.

Another problem I forsee, is that if the greenhouse is not insulated then if you put the resistive elements too deep, then approximately half of the energy will be wasted (it will heat up the ground underneath).

Also if you put the wires at shallower depths you may run in problems if you have plants that need digging every year, and you may hurt the plants.

Option B

As mentioned earlier, the heat pump would be the way to go. It is much more efficient (COP could easily reach 4 or higher, i.e. for every electrical kWh it is possible to move about 4 or more kWh).

From the question I was not sure if you were planning to build a open or closed circuit.

IMHO, the closed circuit (i.e. warm the water and circulate it in a closed circuit in buried hoses and then return it - a sort of reverse geothermal installation) would be better off.

If you are planning on an open circuit (i.e. warming up the water and then watering the plants) then the main problems I forsee is with the mass/transfer distribution. I.e. there is a limit on the heat that you may transfer to the plants. So depending on the losses you may be able (or not) to provide enough energy to control the temperature in the greenhouse.

Solar collectors vs PV

Depending on the climate that you are (and more specifically solar irradiance and cloud coverage), it may be more economic to use a system with solar collectors

Figure : Basic solar collector setup eia.gov)

If you are for example located anywhere near the Mediterranean you are probably familiar and it actually makes a lot of sense to have a system where you heat the water when there is sun. If there is no sun you can have a heating element inside it (whether resistive or connected to a heat pump does not make a difference).

The good thing about solar collectors vs PV, is that they use the indirect radiation also (so even if its partially cloudy you can get a preheating), and additionally, the energy conversion is a better than PV system (typically in the order of 60%, while PV is in the order of 20-30% depending on technology).