In the answers to this question, it is said that
The de-icing system on most turbine aircraft (including MD-82 involved in that accident) uses bleed air from the engines, that is it extracts some air from behind the (low pressure stage of the) compressor. This air is therefore not ejected from the nozzle and not producing thrust, so the thrust is reduced.
My question is:
Why is bleed air taken from some stage of the compressor used? Why not e.g. exhaust gas?
For de-icing, the temperature of the used medium must be above 0°C (melting ice), while the environmental temperature usually is far below. So the engine has to invest energy to generate bleed air with reasonable temperature, the air cools down to still >0°C during de-icing, and is then vented out to the environment. There, it expands and cools down far below the environmental temperature. This expansion means that energy is wasted, and also, when the bleed air is cooled down / expanded before inserting it into the de-icing system (I don't know if this is done), energy would be lost. In addition, air/pressure is lost in the compressor stage, which makes the combustion less effective. (Again, I don't know how much air is taken, and how big the effect is)
On the other side, the exhaust gas of the engine is very hot due the combustion and could be used without the need of extra compression. So, using some exhaust gas, one would not waste so much energy. If it's too dirty, heat exchangers could be used to heat fresh air.
I would think of this reasons
- Bleed air is anyway used for many purposes in an aircraft, so this is more economic than a completely separated system
- De-icing is not used for long time during flight, so again no need for a dedicated system
EDIT:
I'd like to expand the question to explain what makes me curious. In the comment, the correctness of this sentence is challenged:
There, it expands and cools down far below the environmental temperature.
This is a simple thermodynamic process. The air is compressed adiabatically, i.e. without adding heat to it. The heat comes from the thermal energy of the air, now also compressed to a lower volume. De-icing cools down the air, and when releasing the air to the environment, it expands to the orignal pressure. As thermal energy has been removed during de-icing, the temperature drops below environmental temperature.
Here is the math behind:
The relation of pressure and temperature in this case is:
$$ p_1^{1-\gamma}\cdot T_1^\gamma=p_2^{1-\gamma}\cdot T_2^\gamma\qquad \gamma \approx 1.4$$
Let's assume a pilot switches on de-icing during flight at 11km altitude. There, environmental pressure is 0.25bar (atmospheres) and temperature is -50°C (223K). It was also said here in the answers that it's possible that the bleed air has about 200°C (473K). The formula now gives a bleed air pressure of 3.47 bar, so a pressure ratio of about 14. The air is now cooled down while the pressure is maintained by the engine. I assume de-icing will be effective for bleed air temperatures above 0°C. So if the air is released at this temperature, the temperature will fall down to -144°C (128K). Another number: If releasing at 100°C, the temperature will drop to -97°C (175K).
(Of course, the air will mix with the environmental air immediately)
In principle, one can play with the numbers, increase/decrease altitude / temperatures and discuss how adiabatically this (de)compression processes are.
Anyway, this is a big air conditioner, using the thermal energy for de-icing and wasting the cooled air. If one only needs hot air, something coming from the exhaust system would always be more efficient.
This is not really efficient. May be the bleed air behind the de-icing system can still be used for other purposes, as it still has the pressure?