Energy Conversion

Quasi-isentropic heat exchangers for energy conversion systems

Energy conversion in power plant


Energy conversion systems and entropy

In energy conversion systems, energy is transferred from one form to another form, for instance from chemical (fuels) to electrical or mechanical, or the other way around. Very often this is done by using heat as intermediate energy form.
 

Reversibility

Carnot indicated years ago that the degree of reversibility is linked to the efficiency of the energy conversion systems to go from the one form to the other.


Entropy

Entropy is a state variable, or function of state, that makes it able to quantify the quality of the process involved. Moreover, the Second Law of Thermodynamics states that entropy is always increasing, and by minimizing this increase, the efficiency of energy conversion systems can be maximized.


Quasi-isentropic heat exchangers

BOSAL ECI develops and produces highly efficient, compact, quasi-isentropic heat exchangers for energy conversion systems.

These heat exchangers function between two flows with high inlet-temperature differences (for instance 1000°C and 20°C), but enable a heat transfer within the heat exchanger with small temperature differences, resulting in a high effectiveness value. In turn, this results in a minimal entropy increase.

The change in entropy is defined as the ratio between the amount of heat and the absolute temperature (the amount of heat divided by the absolute temperature):

Formule 1.JPG


In this way, the Hot Flow reduces in entropy when it passes the heat exchanger going from the Hot Inlet Temperature to the Hot Outlet Temperature and releasing an amount of heat Q. The amount of heat Q can be sliced into small amounts of dq. Each dq will reduce in the hot flow the entropy with the value:

Formule 2.JPG


On the Cold Flow side, the entropy will increase because of the added heat. The amount of heat added to the Cold Flow equals the amount of heat extracted from the Hot Flow (First Law of Thermodynamics). The units are properly insulated.

Formule 3.JPG


However, the entropy in the Cold Flow will increase more than the reduction of entropy in the Hot Flow. This is according the Second Law of Thermodynamics. The reason is that the amount of heat dq is divided by a lower absolute temperature (TCold < THot):

Formule 4.JPG



By integrating over the complete amount of exchanged heat, the total entropy increase is achieved:

Formule 5.JPG


To present this in a graphical way, the temperature of the Cold Flow and the Hot Flow as a function of exchanged heat is of importance.

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BOSAL Energy Conversion Industry (ECI) has 20 years of experience in the development, testing and production of high-power heat exchangers. ECI belongs to the global BOSAL group which was founded in 1923 in the Netherlands. With 5,500 employees, 5 R&D centres, 34 factories and 18 distribution centres, it is one of the main global players in emission control and heat exchangers for energy conversion systems.

BOSAL-quasi-isentropic counterflow heat exchanger

Such a heat exchanger is characterized by a counter-flow pattern with high degree of flow uniformity, intensive heat transfer to ensure locally low thermal resistance between the two flows, in combination with a thin, stainless-steel-foil, heat-exchanging surface with high amount of alloys to guarantee low heat transfer between Hot-In and Cold-In areas.

The heat exchange takes place with small temperature differences, as seen in the graphs.

Increase in entropy with BOSAL quasi-isentropic heat exchanger

ECI 57-2.png

To represent graphically the summation of the entropy differences, the reciprocal values of the absolute temperatures (1/T) are plotted. The result of the integral representing the increase in entropy is the surface between the two curves.

 

Co-flow heat exchanger

As a comparison, a co-flow heat exchanger is represented with the same amount of heat-exchanged power. In co-flow configuration, both flows exchange heat immediately downstream of the entrance at higher temperature difference. This is shown clearly in the graphs

Increase in entropy with co-flow heat exchanger

ECI 57-3.png

Also here, the summation of the entropy differences represented graphically by means of the reciprocal values of the absolute temperatures (1/T) shows a significant larger surface between the two curves. Co-flow heat exchangers even as cross-flow heat exchangers are not quasi-isentropic heat exchangers and can therefore not reach the excellent performances of the BOSAL-quasi-isentropic heat exchanger