Came across a FB discussion talking about heating a greenhouse using Peltier units. I've no experience of using these so cant really say if thats feasible (does sound rather ambitious) but i did think maybe a seed propagator could work using them (much smaller area to heat).
An Arduino will be used to sense temperature and switch the units on or off - but not power I'm thinking.
The things can be purchased cheaply from ebay but I've on idea what to use to power them. could anyone advice what i should use to power 'one' or multiples - a link would be nice.
Any thoughts on using them appreciated
Model: TEC1-12706
Size: 40mm x 40mm x 4mm
Operates from 0~15.2V DC and 0~6A
Operates Temperature: -30¡æ to 70¡æ
Max power consumption: 60 Watts
If you only need heating, just use resistors. For seed propagation, you can get nice flat "silicone" heaters that will give you good control over a square foot or so. They are often used for 3D printers so there's a lot of them available on eBay or Amazon. Add a thermistor to an analog input on the Arduino, a big MOSFET and a sufficient power supply and the coding is easy.
If you want to use the cooling abilities of Peltiers, then it's another question.
The Silicone Rubber Heater mats, didn't know you could buy such a thing and is an alternative.
However I've become interested these Peltier modules, the idea that the heat is free is interesting, just dont know the economies of the things, do you actually get more heat out than you would with say a Silicone Heater mats for the energy you use to power them - no idea how to work that out.
So if i was to buy a Peltier module (they are cheap on ebay) what power supply would i need
Possibly I'll decide the Silicone Heater is a better way of doing it but I'd like to have a play first
Peltier elements have a cold and a hot side. Without thermal isolation of both sides you have nothing but a heated resistor. In your case you the cold side must be connected to the environment of your greenhouse, using e.g. a heat pipe. All that becomes so expensive, that you'll be better off with heater mats.
Peltier elements are useful for cooling, when the heat can be removed from the hot side by a heat sink and optional fan, or the cold side is in a closed refrigerator box.
With Peltiers, the key issue is how much heat do you want to add or remove.
Peltiers are very inefficient , COP of about 0.4 which means that they consume lots of power for not much return.
If all you need is heating, then resistors are a much better item , as the small amount of additional heat you will get from a peltier will be offset by the higher costs of the fans and the thermal insulation between the hot and cold sides.
If you keep the cold side of the Peltier below ambient temperature then it will absorb heat and this is added to the heat output on the hot side. It can exceed 100% efficiency.
Doing this in practice is surprisingly difficult. It's best to use a heater to heat. The gain in efficiency is not worth the cost and complexity of the Peltier.
IGraham:
However I've become interested these Peltier modules, the idea that the heat is free is interesting, just dont know the economies of the things, do you actually get more heat out than you would with say a Silicone Heater mats for the energy you use to power them - no idea how to work that out.
You have got the wrong idea that the heat is free, it certainly isn't.
When used for cooling, then it could be said that "heat" is a waste by-product that you need to get rid off, but it is not free - you are paying to throw it away.
Quotes from that link:
Its main disadvantage is high cost and poor power efficiency
Thermoelectric junctions are about 4 times less efficient in refrigeration applications than conventional means (they offer around 10–15% efficiency of the ideal Carnot cycle refrigerator, compared with 40–60% achieved by conventional compression cycle systems.
Yes the heat is free. You don't get a lot and it would take some serious engineering to get 101% efficiency with a Peltier but it is certainly possible. A "solar boosted" hot water system uses a heat pump (also known as a refrigerator) to extract heat from the environment and heat water with greater efficiency than a normal electric water heater.
A Peltier needs lots of amps at 12v or so. A switchmode power supply is essential. $10-$20 should get a bare frame supply that can be put into a box.