I have read this article, Which says that conversion efficiency of the LED have exceeded 100%. The results are published in Physical Review Letters
In their experiments, the researchers reduced the LED’s input power to just 30 picowatts and measured an output of 69 picowatts of light - an efficiency of 230%.
How is this possible, is it not the violation of conservation of energy or I am missing something.
Edit: I missed that the LED worked as TE cooler and absorb heat from atmosphere to convert it to light. But still it is overwhelming observation for me.
Is it really possible that a device use thermal energy to produce light.
Answer
The device is apparently working as a heat pump, for which I give a brief theoretical analysis here.
In the example given, the $P_h=69{\rm pW}$ light output comprises the $W=30{\rm pW}$ input by the researchers together with $P_c=39{\rm pW}$ of heat that was formerly in the chip.
We can model the process by ideal heat pumping as follows. Heat drawn from the chip will lead to a drop in the chip's entropy of $\Delta S_c = -\frac{P_c}{T_c}$, and the light and output to the ambient World increases the entropy of the latter by $\Delta S_w = \frac{W + P_c}{T_h}$, where $T_h$ is the effective temperature of the light (measuring the latter's degree of thermalization together with its optical grasp). Since the light ends up in the environment, its effective temperature is ambient or greater.
The total entropy change of the World is then
$$\frac{W}{T_h} + P_c\,\left(\frac{1}{T_h} - \frac{1}{T_c}\right)$$
We know that $T_h>T_c$ because the effective "exhaust" temperature is at least ambient and $T_c$ must wind up less than this, because heat is being pumped out. So the second term with the brackets is negative: this means we must supply enough work $W$ to at least make the quantity positive. So the device can very plausibly (and probably does) work as claimed and comply with both the first and second laws of thermodynamics.
No comments:
Post a Comment