Peltier cooler-based air cooler
In this video I tried a new gadget. This gadget is a heat sink “channel” that has three TEC12706 Peltier coolers on two of its sides and has a fan on one of its end which pushes air through the heat sink channel. This is sort of an air cooler with a nominal cooling performance of 420 W. However, the real cooling performance is much closer to 200 W in “normal conditions”. In this test, I want to see what kind of outlet temperature can be sustained with different configurations.
The system
So, as you could also see in the video, first I tried to cool down the heat sink without any airflow through it. This is important because if there is no airflow, the cold side of the Peltier coolers do not experience a lot of thermal load. This means that the Qc (heat transferred from the cold side to the hot side) is low which also allows the DT (temperature difference between the cold side and the hot side of the Peltier cooler) to be high. So, while the hot side was around 33°C, the cold side/heatsink could go down to -12°C. This is roughly 40°C temperature difference. It is worth to notice that I measured the water temperature and not the hot side directly, so the real hot side temperature would be somewhere around 50°C, so the DT would be closer to 60°C as well.
As I started to supply higher and higher voltages to the fan on the heatsink, more and more heat was introduced to the cold side of the Peltier coolers. This increased the Qc (more heat was pumped to the hot side) and decreased the DT, therefore the temperature difference between the two sides started to drop. With the highest airflow (fan is at 12 V), the temperature difference was about 45°C. Still, the outlet temperature was around 20°C which is nice. But one has to stay close to the outlet of the heatsink to notice the cold breeze.
All in all, this system is a fun experiment, but I don’t think that it can replace a normal air conditioner. First of all, this is very inefficient. Furthermore, the airflow and the outlet air temperature is nowhere near that of those mobile AC units. It is like comparing a hair dryer with a space heater. The hair dryer produces a nice airflow and high temperature if you are close to it while a space heater generates a large amount of heat that you can feel across the room. Same here: this Peltier cooler device generates a nice cold breeze with a noticeable airflow if you are close to it, but its performance cannot really be noticed on a larger scale.
Finally, the biggest issue with this system is that it is expensive. The building cost is is about $300 with all the parts considered and it gives you about 2-300 W real cooling power. So let’s say, 1 Watt of cooling power cost you $1. Meanwhile, $300 can buy you a fully assembled, tested and safe(!) mobile air conditioner with 2000 W cooling power. So, each Watt of cooling power only cost you $0.15. Furthermore, this device was consuming 470 W, so let’s say roughly half of it was converted into cooling power and the rest of it was dumped as useless heat. A portable AC on the other hand has over 2-3 COP.
Additional thoughts
In the above two images I did some simple chart readings. On the first picture I tried to estimate the cooling power based on the data (voltage, current, temperatures) I knew. On the second picture I suggested some other parameters which could improve the cooling performance.
First Image:
1; We know that the supply voltage was 10 V, because we measured it, so we draw a horizontal line at 10 V on the Voltage-DT chart.
2-3; Either we know that the supplied current was around 3.5 A because we measured it or we know that the DT was around 45°C because we measured the hot side and the cold side temperature. Either way, we draw a vertical line at DT = 45°C and we draw a diagonal line at 3.5 A. We see that they meet roughly at the same spot, so the assumption that we need to shift the measured water temperature by +20°C to get a more realistic hot side temperature was good enough.
4; We go to the Qc-DT chart and draw a vertical line at DT = 45°C.
5; Then, we draw a diagonal line at 3.5 A.
6; We draw a horizontal line which starts at their intersection and ends at the Y axis. The intersection of this horizontal line and the Y axis will show us the achieved cooling power.
7; The cooling power with the above conditions is 15 W per Peltier cooler. We used 6 of it, so the total cooling power is 90 W.
We used 300 W and we got 90 W cooling power, so the efficiency of the whole system (pumps, fans…etc. too!) is about 30%.
Second Image:
We want to improve the above achieved 80 W cooling power. How should we do that? There are 2 main things we can do and they are also somewhat related to each other. We can lower the DT and the input voltage. If we decrease the input voltage, the current also drops (Ohm’s law!) and this also generates less Joule heat. Obviously this puts less load on the cooling system of the hot side, therefore the DT can be changed. Also, we can play with the airflow which would result in different cold side temperatures.
1; Let’s say we want to run the Peltiers at 8 V, so let’s draw a horizontal line at 8 V on the Voltage-DT chart.
2; Let’s say we managed to get DT = 35°C, so let’s draw a vertical line at DT = 35°C.
3; The intersection of these two lines show the current which is I = 3 A.
4; Now, we go to the Qc-DT chart, draw a vertical line at DT = 35°C and see where it intersects the I = 3 A line.
5; Then, we draw a horizontal line from this intersection towards the Y axis.
6; We see that Qc = 20 W.
7; We have 6 Peltier coolers: 6x20 W = 120 W
In this case, the estimated consumption of the system would be around 240 W, so the efficiency of the whole system would be around 50%. But keep in mind that this is still only 120 W cooling power which is about 1/10th of the smaller portable ACs.
Conclusions:
Cool the hot side as good as possible!
Use low voltage/current and more Peltier coolers. Peltier coolers -due to the Joule heating- are inefficient at higher currents. The higher the current, the more detrimental the Joule effect is on the cooling performance.
