Reverse Carnot cycle heat pumps

An exergy analysis of any prior art heat pump highlights the existence of cycles more or less different from an ideal Carnot cycle and of included processes that are high exergy consumers. Most of these pumps operate in subcritical mode, following an inverted Rankine cycle (Fig.1A). In this mode, the vapor pressure at the compressor outlet must be equal to that in the condenser, which can lead to their temperatures much higher than the condensation temperature. Therefore, the cooling of these vapors after entering the condenser is a heat transfer process with large temperature differences. Another exergy-devouring process is the isoenthalpic expansion of the liquid at the entrance to the evaporator. These processes make the COP of such a heat pump approximately two times lower than that of an ideal inverted Carnot cycle. However, HP with PCM continues to dominate the HP market, due to the fact that the isothermal evaporation and condensation processes occur at much higher speeds compared to gas-gas or gas-liquid heat transfer through the walls of prior art heat exchangers.

         The isothermalizer is the device that can radically change this situation. First, due to the fact that both the compression and the expansion of the working gas take place at a constant temperature (by changing in real time the speed of the moving components that influence the heat transfer), the principle of minimum entropy generation is satisfied (with the achievement of the maximum exergy efficiency corresponding to the respective difference between the temperature of the gas and that of its ambient environment). Second, due to the fact that no phase changes occur, the use of exergy-consuming expansion valves is not necessary. The absence of condensation/evaporation processes is compensated by the introduction of the deformable thermal sponge which, unlike the non-deformable thermal sponge used in the case of “almost” isothermal compressors in the state of the art, maintains its large heat transfer surface throughout the isothermal transformation. When this surface is very large (a condition easily achieved in the case of large isothermalizers) high piston speeds and therefore very high power densities can be achieved

          Other advantages of isothermalizers, which recommend them for the construction of heat pumps Fig.1B (and of external combustion heat engines):

- they can operate with any substance that maintains its gaseous state throughout the entire working range, consequently gases such as helium, oxygen, nitrogen, argon, neon, carbon dioxide, etc. can be used, cheap, non-toxic, non-flammable natural refrigerants, without harmful effects on the atmospheric environment

- they maintain their energy and exergy efficiency even at very high temperature differences between the gas and the casing

- the speed of the processes is easily controlled by the control processor provided, by appropriately modifying the isothermal speeds, also maintaining control over the value of energy efficiency and optimal power (the Curzon-Ahlborn efficiency)

- they can be made in a wide range of constructive and dimensional configurations: with solid, liquid, or gaseous piston, rotary

          Therefore, isothermalizers can be the main element of extremely efficient and versatile heat pumps, usable in the most varied domestic or industrial applications, in extremely varied temperature and composition ranges. There are two application areas in which gas heat pumps are clearly superior to phase change heat pumps:

1. the area of ​​small temperature differences between source and sink (for example, pumps for air conditioning systems). These small differences allow the control processor to accelerate the piston speed at the beginning of the compression phase, or at the end of the expansion phase, enough so that in this short time interval, the thermodynamic transformation approaches an isentropic one, and the cycle becomes Carnot type. In this way, an increase in cycle efficiency is achieved. On the other hand, using antifreeze as a heat transfer fluid, the application area can drop far enough below 00 C. Another great advantage of this type of heat pump is that it can use even the air to be conditioned as a working gas. This allows the use of a device for destroying pathogens by simply calcining them, in a simple thermodynamic device, without the use of chemical or electrical processes and without additional energy consumption

2. the field of very large temperature differences (industrial field). In this field, Carnot cycles are not indicated. Instead, Stirling or Ericsson cycles can be used, which have the same energy efficiency (the exergetic one being lower) With the help of the isothermalizer, Stirling or Ericsson cycles can also be achieved, with high exergetic efficiency, using phase change refrigerants