Genuine question. When I look up options for heating greenhouses its either propane, kerosene or electricity. So if you dont want to use fossil energy for any reason, why not use peltier elements which would boil down to resistive heating and moving heat from the outside to the inside?
Is it because they dont last in the weather? Or because they require a DC source? (couldnt you simply move the PSU inside the greenhouse and use any conversion losses as additional heating?) Or maybe its because you would have to possibly cut a hole in your greenhouse walls so that the peltiers cold side is actually on the outside?
Peltier elements suck at heating and cooling.
The problem is efficiency. Of all methods of heating and cooling, Peltier elements are the least efficient at both. The main benefits to Peltier elements are they’re very compact, very simple, and very cheap. In some applications, these benefits are enough to warrant the trade in efficiency. In a greenhouse, these benefits are either not very helpful or completely invalid.
Your money and energy are more efficiently invested in other methods.
For a truly comprehensive explanation from someone much more knowledgeable than I am, refer to the Technology Connections video on the topic.
Google “Peltier vs resistive heater” and you should find your answer.
That doesnt really answer my question. my understanding is that if I have a resisitve heater that takes 100W of power and a peltier that also takes 100W of power, the peltier will actually give me 100W + whatever heat it moves from one side to the other. So if my goal is heat, then a peltier would give me more for the same amount of electricity, no?
I’m no expert, but from what I have been reading. It seems that it is difficult to design a heatsink that is able to dissipate the heat quickly enough before the entire device heats up and causes the cool side to warm and efficiency to drop.
i thought about using a cpu heatsink and fan. those are designed to dissipate 100+ W of heat.
You’re not going to be able to heat a greenhouse in temperatures where frost is an issue with only 100w (~341btu) of energy.
that number was just an example, of course you could scale things up as needed (multiple elements with multiple heatsinks for example)
That understanding is mostly correct, but when you crunch the numbers, the “ + whatever heat it moves from one side to the other”-component of the equation tends to be small in practice relative to the input energy, such that you wind up very close to resistive heating. At best, you’ll get a COP around 1.5.
That means you pay a lot of overhead for marginal efficiency gains, and if you have clean electricity to run your heat pump, you already have clean electricity to run resistive heat.
yeah marginal gains are probably the actual reason
No, you’re using 100w to move 100w. You can’t ever have more efficiency than you put in.
What would you be cooling to get the heat? If it’s cold outside at night, there’s nothing go take heat from either. You would need a way to store it during the hot day.
its a heatpump, of couse I will have more Watts in heat on one side than i put in as electricity
This is not correct.
Heat pumps are capable of being more than “100% efficient” because they are tapping into an external heat source, the electricity that is being used to operate the device is not the entire sum of the energy available to the device.
Once you step out to the greater picture of the electricity + the available environmental energy, then it returns to equilibrium, but when you’re only calculating wattage and the electricity required to operate it, then you can get up to four times the amount of heat output per watt of electricity used with a properly designed system compared to a resistive element.
@viertesauge
They’re inefficient and costly compared to simple resistive heat. If you need to heat inexpensively and simply, resistive heating elements are impossible to beat. If you need to heat (and cool) efficiently, then a refrigerant based heat pump can’t be beat.Don’t let the “heat pump” label of peltier units fool you. These aren’t like the heat pumps that use phase change of a refrigerant to be efficient. They’re just heat pumps because they cool one side while heating the other (and can reverse that effect). A phase change heat pump is far more efficient.
The only practical applications of peltier systems are where space is an important factor and solid state operation is a deciding advantage (though fans are often integrated in systems not using heat pipes or liquid heat transfer).
Okay cost may be one factor, but what about them is inefficient (compared to a resisitve heater not a heatpump with compressor and everything) if the goal is heat? my understanding is that whatever power they take will be turned into heat anyways plus they move some from the outside.
@viertesauge
Using the same amount of energy input, a resistive heater will output more heat.but any electrical device is a resistive heater, no? this one just happens to move heat aswell
Key factors reducing Peltier efficiency include:
Thermal Leakage: Heat naturally flows back through the device, counteracting the pumping effect.
High Electrical Resistance: Significant energy is wasted as internal heat rather than being used for the Peltier effect. (This is essentially production of resistive heat, bit it’s not where you want it)
*Compressor-based systems can achieve COPs of 3 to 6 (300–600% efficiency) by moving large amounts of heat with minimal energy, whereas Peltier devices require horrendous amounts of current to move even small amounts of heat. This is because the thermoelectric materials (bismuth alloys) have inherently low electrical conductivity, causing efficiency to drop sharply as the temperature difference across the device increases. Leading us to the next problem…
Temperature Sensitivity: Efficiency declines rapidly as the difference between the hot and cold sides grows, making them unsuitable for large temperature differentials.
Heat Dissipation: Effective operation requires complex heat sinks to remove waste heat from the hot side, adding system complexity and loss.
Peltier plates are one of those things that always seem so cool and useful until you do the math.
I absolutely hate those tiny fridges that everyone assumes must use a tiny amount of electricity when in reality, they use more than a full size chest freezer.
Thermoelectric generators can be nice, though, in the right application. I like how they are used in the biolite stoves to drive a fan that improves combustion. It’s a nice feedback loop.
thanks for the wall lol. yeah like another user and I said in my other comment the actual reason is probably marginal gains even if you can get it to work.
heat dissipation is solvable with a cpu heatsink and temperature sensitivity probably isnt that relevant in this application if all you want i to protect the greenhouse from frost for example, but thermal leaking might be a problem.
It’s possible future advances in materials science (the conducting alloys and heatsinks) could increase the efficiency but those are the major barriers in my non-expert opinion.
Commercial Bi₂Te₃ modules sit around ZT ≈ 1.0–1.2 at room temperature. Active research is pushing this closer to 2, but not at practical temperatures yet. (ZT is not the same as COP but it does directly relate to it for the purposes here. Even at ZT = 2, the COP gain for a heating application is modest because the device still must reject the pumped heat plus all Joule losses on the hot side.)
A mini fridge is like $120 and would give ~300 watts of heating on 100 watts of energy.
A peltier would be like $80 (including power supply and fan. It would use 100 watts of energy and give 150watts of heating.
thats true but the frigde pobably isnt designed for being in the cold and weather all the time (although you might be able to modify it so that it could). plus the compressor would run all the time since its constantly trying to cool the outside (previously inside of the fridge). at that point you are better off installing a regular compressor heatpump. what is appealing about peltiers to me is that they have no moving parts and no nasty refrigerants that could leak. I wasnt really asking about compressor heatpumps vs peltiers anyways but about resistive heaters vs peltiers.
The math holds the same.
And what do you think a heat pump is? It’s the exact same thing as a fridge, with a compressor, evaporator and condenser.
when did i say that a compressor fridge isnt a compressor heatpump,lol
maybe try understanding a comment before you go on a downvoting spree. thanks for adding nothing to the discussion.
