ENERGY BALANCE OF STIRLING ENGINE CYCLE

An energy balance of a Stirling engine cycle can be do through an indicator diagram or trend pressure inside a Stirling engine which is calculated. Results of this energy balance are an input heat, a rejection heat, a regenerated heat, an internal work of the cycle and an internal thermal efficiency.

This article describes analytical calculation through equations, which are derived from the equations of the first law of thermodynamic. Through these derived equations can be calculated energy flows and theirs connection with others quantities.

Assumptions of solving

The equations of this energy balance are true under certain simplifying assumptions. Calculated results through these equation can vary from reality with increasing of differences between simplified and real processes: (1) Working gas is ideal gas. – (2) Stirling engine is perfect sealed. – (3) There is no pressure loss, pressure of working gas is same in whole working volume. (4) Regenerator has perfect thermal isolation. – (5) Pressure as function of angle of rotation (p(φ)), working volume on hot side as function of angle of rotation (VT(φ)) and working volume on cold side as function of angle of rotation (VS(φ)) are known. – (6) Stirling engine cycle is steady (the same cycle repeats).

Internal work of Stirling engine

The internal work of the engine is function the pressure and the working volume:

Energy balance

Inside the Stirling engine is transformed heat to work. For this transformation are true the rules of heat cycle, it is means, only a piece of the heat input to the engine is transformed to work and others the heat is necessary rejected from the engine. For input and output of the heat to/from the engine between the working gas and a heat transfer surface must be a temperature gradient. This temperature gradient is vary during one cycle, because working gas temperature is changed also (see chapter 34. Temperature change of working gas inside Stirling engine). Therefore in all internal sections of the engine the heat is inputted and outputted to/from working gas during one cycle. It is evident (from the principle of the Stirling engine) the heat balance during one cycle of the hot side is positive (the heat is inputted to the working gas) and the heat balance during one cycle of the cold side is negative (the heat is outputted from the working gas), the heat balance of the regenerator during one cycle must be neutral:

The heat ΔI inputs through the cycle on the hot side and it increases of internal energy of the working fluid and other part of the cycle outputts on the cold side from the engine (through the temperature gradient between the hot and cold side). This heat decreasing a dimension capacity-rating of the heat transfer surface and a thermal efficiency of the cycle. On the other hand the heat ΔI develops the temperature gradient between the hot and the cold side even in for case of the regenerator with small capacity or for case it is not installation inside the engine. For cases isothermal processes on the hot and the cold side (e.g. Schmidt theory) must be the change of the enthalpy ΔI equal zero.

The internal work of the engine can be measured indirectly, e.g. by indication of the pressure of the working gas or by measuring work shaft (in this case is necessary to know mechanical losses of the engine). The internal work of the engine can be approximately calculated of the pressure trend by the procedure which is shown in article Stirling engine cycle (calculation without losses) or in article Losses in Stirling engines (calculation with losses).

Values of heats Q_T and Q_S can to get from a measuring of heat flow to/from the engine or it can be approximately calculated through an estimate the internal thermal efficiency:

Internal efficiency of Stirling engine

The internal thermal efficiency of the Stirling engine is ratio between its the internal work A and heat, which inputted to the working gas from a surroundings during one cycle Q_D* respectively this definition is the same as definition of efficiency of heat cycle, because inside engine is performed complet cycle. For the conditions, which are shown on List 1 can be descripted equality of heat flows:

The internal thermal efficiency can be estimated through similarities of the Stirling engines similar construction:

Regenerated heat inside regenerator

The regenerated heat inside the regenerator can be calculated from a function which describes an amount of a heat transfered inside the regenerator of the working gas from a start of the cycle to any point of the cycle. The regenerated heat is equal of a difference between the maximum and the minimum of this function:

The enthalpy of the working gas inside the hot or the cold side in case isothermal processes is not changed respectively in Equation 5 is true I_T,x=0; I_T,x=0.

The amount of the heat Q_Reg can be calculated approximately if the amount of the regenerated heat Q_Reg is much more than the amount of the heat ΔI:

During the solving of previous problem was neglected an influence of the change of enthalpy of the working gas on the hot and cold side respectively the heat ΔI. For actually case the influence of the change enthalpy is increased with decrease of the temperature ration τ respectively the ration between the heat Q_Reg and the heat Q_D significantly decreasing and this procedure of the calculate used in Problem 3 increases its inaccuracy. This fact must be taken into account when designing the size of the regenerator:

Entropy of working gas

An equation of a change of specific entropy of the working gas can be derived from First law of thermodynamics for closed system:

Through last Equation can be constructed a T-s diagram of the cycle. From T-s diagram of cycle can be identified losses of the cycle. In praxis can be measured perfectly only pressure of the working gas as function of angle of rotation. Exact amount of the working gas inside of the engine and its exact mean temperature of the working gas is impossible to measure. Therefore is constructed Θ-s diagram*, where Θ is ratio between the mean temperature of working gas inside of the engine and maximum temperature of the working gas inside of the engine. The Θ-s diagram can be constructed only from the measured pressure.

Θ-s diagram, which is designed from a measurement, can show losses and weaknesses of the engine:

Amount of working gas inside engine

The amount of the working gas inside of the Stirling engine can be computed by the equation of state [2, p. 67 (cz)] for each working volumes. The amount of the working gas is computed for a known state of the working gas. If real mean temperature of the working gas inside individual volumes is not known, then can be use of computed of temperature change of the working gas inside Stirling engine:

References

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