The air is being heated by industrial process, which can't be modified. Basically I need to keep the air at 25C, as it may go much higher than that, if not dissipated. Sadly, in my case the ambient air is also 25C.
It was pointed out already in this topic, but not very clear. So I will try to explain to future readers why this is so difficult.
This is a book I have read for the basics (1k+ pages):
https://www.amazon.com/Thermodynamics-Engineering-Yunus-Cengel-Dr/dp/0073398179It's available from TPB, of course. Really good and easy to follow book, I highly recommend purchasing it if you need it for actual learning, not just one evening like I did.
What I was trying to achieve is known as Rankine cycle:
https://en.wikipedia.org/wiki/Rankine_cycleAfter reading some more about thermodynamics, here are some basics. Basically you have a heat source (25C in my case), where your refrigerant is vaporized, then you have a turbine which generates electricity, after which your refrigerant drops in temperature and pressure and can be condensed back in to liquid form and pumped back for re-heating. The main issue is the condenser, of course, which will have to dissipate a LOT of energy to environment for this to work. In my case, since ambient temperature is also 25C, I would have to use another heat pump to extract heat from condenser and move it somewhere else, like back in to hot side... More on that later.
The problem is, energy can only by taken from a difference between hot and cold sides. Max theoretical efficiency of thermal engine is: ? = 1-Tout/Tin. T is in Kelvin.
This very well applies to steam engines and also to internal combustion engine. Let me explain, why efficiency of internal combustion engine is so low... Better yet, let's explore steam turbines in power plants. For example, max temperature of steam can be 565C (due to various technical limitations), and lowest possible condenser temperature is around 30C (if a lake or river is used to dump the excess heat). So max theoretical efficiency of such turbine is:
? = 1 - (303K/838K) = 63.8 %. That sounds not too bad, but in reality your steam turbine will not run at 30C... So you end up much lower, usually around 40% for single stage, in large scale, or closer to theoretical max if multiple turbine stages are used.
Now, let's calculate what we get at 25C. Let's say we use low temperature refrigerant, like R134A, and also let's assume we will be able to move the condenser down to -10C... So the max theoretical efficiency will be:
? = 1 - 263K/298K = 11.7 %
Here's an interesting simulation of Rankine cycle. Note the numbers.
So basically I feed 20kW heat in, take my 2.35 kW out, and dump the remaining 17.65 kW to somewhere.
2.35 kW "for free" sounds not too bad, but wait, there's more.
Remember, I assumed we will be able to keep the condenser at -10C. To do that, I would need another heat pump to take that 17.65 kW, up the temperature to at least [ambient+10C] and dump in to air. A COP of a typical REAL heat pump with this temperature difference (25+10+10) will be maybe up to 3. Sooo... To extract that 17.65kW, I would need to feed another 9kW or electricity to run the heat pump.
In the end I would get:
+2.35 kW from turbine
-9 kW to run heat pump
Which equals a loss of 6.65 kW.
So there you have it, it's a russian business.
Here's an example of industrial power plant, running from geothermal energy at 105C:
https://youtu.be/RAT6LK3aLNM?t=1m43sThey are proud to have efficiency of 10.9%. In their case, the cold side is likely around 30C, being dumped either to ambient air (or nearby lake), or another, much colder water bead underneath. Not sure how mother Earth feels about it though. Should be a warm furry feeling for sure.
On a further note on heat pumps.
Typical COP of an existing heat pumps varies between 2-4, depending on temperature difference and construction.
A theoretical COP, however, is much more optimistic and is determined by this equation, and in my case would be:
COPmax = Tcold / (Thot-Tcold) = 263K / (308-263K) = 5.84
So even if I would be able to achieve maximum theoretical efficiency of heat pump, I would still consume more energy to run it, than produced by the turbine.
Basically I have answered my own question and hopefully provided some basic information for someone else, googling for this in the future.