10 th EUROPEAN CONFERENCE ON COAL RESEARCH AND ITS APPLICATIONS: 10 th ECCRIA OPTIMAL FUNCTIONING PARAMETERS FOR A STIRLING ENGINE HEATER R. GHEITH 2,3, * F. ALOUI 1,2 and S. BEN NASRALLAH 3 1 Université de Valenciennes et du Hainaut-Cambrésis, ENSIAME, TEMPO (EA 4542) - DF2T Valenciennes France 2 GEPEA (UMR 6144), École des Mines de Nantes, DSEE, 4 rue Alfred Kastler - BP20722, 44307 Nantes Cedex 03 France 3 Université de Monastir, Laboratoire LESTE, ENIM, Monastir - Tunisia * E-mail : Fethi.Aloui@univ-valenciennes.fr
OUTLINE I. Introduction Obectives of the study II. Experimental device III. Some experimental results VI. Conclusions
1 Invention Robert Stirling Stirling engine (1816) This Stirling engine is: - A quiet engine (no vibrations because no internal combustion), - Ecological engine (closed gas circuit), - Any heating source can be used to heat the working fluid, - Its efficiency is more important than that of an internal combustion engine (between 30 and 40%)
The different Stirling engine configurations 2 Regenerator Cooling Heating Regenerator Heating Alpha type Cooling Regenerator Heating Cooling Beta type Gamma type
The Stirling Cycle 3 12 : Isochoric heating 23 : Isothermal expansion P 2 Q 12 >0 1 a Q 41 <0 b d Q 23 >0 c 3 4 Q 34 <0 34 : Isochoric cooling V 1 V 2 V 41 : Isothermal compression
Example of some applications using a Stirling engine 4 Submarine domain Space domain (salellites) Stirling engine dish Stirling engine - generator Micro-cogenerator system Liquefaction of gases (receipt machines) Exchanger Gas valves Mean burner Stirling engine From heating system Natural gas Electricity To heating system Electricity
GLOBAL THERMODYNAMIC MODELS 5 Cooler k Heater h D k r h l Compression Space c Regenerator r Expansion Space e Isothermal model Adiabatic model Quasi-steady model - Thermal losses Adiabatic model + - Mechanical losses - Real gas assumption
Obectives of the study: 6 Optimization of the heat transfer inside the Stirling engine in order to increase its global efficiency, by: - Studying heat transfers in: - The regenerator (porous medium), - The expansion room (hot source), - The compression room (cold source). - Seeing the effect of porous medium on the efficiency, the heating and cooling temperatures.
7 Operation of Stirling engine Hot source T ch Q 2 > 0 Driven machine Q 1 < 0 Cold source T fr W < 0 The amount of heat supplied by the electrical heating (electrical resistance) 2 ' cal The amount of heat actually received by the air Q U I dt Q2 th The indicated work W méca Mechanical work effectively recovered on the brake shaft W ' W ' W ' W Q.. 2 glob Q2 ' W Q2 Q2 ' méca th cal
8 Operation of Stirling engine Q 1 > 0 Q 2 < 0 Stirling Cold source S Heating source S 2 1 at T at T 2 1 Machine W < 0 U W Q Q2 Q1 Q2 S S T T First law of thermodynamics on a cycle 0 1 Second law of thermodynamics on a cycle 0 W Efficiency 1 Q 1 1 2 c
9 The Gamma type Stirling engine Compression Space Regenerator Cooler Crank-Rod System Heater Heating system Expansion Space
10 Alternateur Transmission belt Oscillant plate P Comp. Water output T cold Force transducer Porous media 8 thermocouples 4 in each side Compression space Regenerator Expanxion Space Heating system T E-input P Det. T hot TR 1 TR 2 TR 3 TR 4 T E-input. Water input TR 5 TR 6 TR 7 TR 8 crank angle transducer TR 5 TR 6 TR 7 TR 8 Cold working fluid Porous media (Régénérateur) Cold working fluid Dissipation energy system TR 1 TR 2 TR 3 TR 4 Hot working fluid Hot working fluid
p 4 p p C T V T x T C x t p C 1 x T V t T.. ' '.. air air 11
13 The heater exchange Composed of: - 20 curved pipes (internal diameter: 1cm) - 20 tubes (length 0.50m each one) - 3 thermocouples located: inside the heater, outside the heat and in heating system.
14 The heater exchange Water exit T Cold P Comp. Series of 8 thermocouples 1 st series of 4 thermocouples Regenerator 2 nd series of 4 thermocouples Compression space T W-entrance Regeneratorr Expansion space T W-Exit P Expansion TR 1 TR 2 TR 3 TR 4 Water input TR 5 TR 6 TR 7 TR 8 Cold working fluid Matrix (regenerator) Cold working fluid Heating T Hot system TR 1 TR 2 TR 3 Pourous media (regenerator) TR 5 TR 6 TR 7 TR 4 TR 8 Hot working fluid Hot working fluid
II. Experimental The cooler exchanger (cold source) 15 Composed of 225 strips (or fins) in the inner cylinder For increasing the heat transfer exchange between the working fluid and water (cold source) Two thermocouples are located at the input and the output of cooling water circuit
16 The regenerator (porous medium) Porous media 8 thermocouples (4 in each side) Cold working fluid Cold working fluid TR 5 TR 6 TR 7 TR 8 Porous medium (Regenerator) TR 1 TR 2 TR 3 TR 4 Hot working fluid Hot working fluid
4 matrices, with differents constituting materials, were used as Stirling engine regenerator: - Stainless Steel matrix - Copper matrix - Aluminum matrix - Monel matrix Properties of the differents used matrices (regenerators) Copper 17 Stainless Steel Aluminum Monel The properties of used regenerator materials for a temperature of 300 C are: Materials with porosity of 90% Stainless Steel (304L) Copper Aluminium Monel 400 Proprieties \ Material Density (kg.m -3 ) 7,850 8,920 2,700 8,800 Specific heat C p (J.kg -1.K -1 ) 477 385 902 430 Thermal conductivity (W.m -1.K -1 ) 26 390 237 21.7 Melting point ( C) 1530 1084 660 1300 T t V T x 1 p T. T T' V '. C p t x. C p x. C 4 p
4 matrices, with differents constituting materials, were used as Stirling engine regenerator: - Stainless Steel matrix - Copper matrix - Aluminum matrix - Monel matrix Properties of the differents used matrices (regenerators) Copper 18 Stainless Steel Aluminum Monel The properties of used regenerator materials for a temperature of 300 C are: Materials with porosity of 90% Stainless Steel (304L) Copper Aluminium Monel 400 Proprieties \ Material Density (kg.m -3 ) 7,850 8,920 2,700 8,800 Specific heat C p (J.kg -1.K -1 ) 477 385 902 430 Thermal conductivity (W.m -1.K -1 ) 26 390 237 21.7 Thermal Diffusivity a (m²/s) 6.94. 10-6 1.14 10-4 9.73 10-5 5.73 10-6 T t V T x a p t x T a. x T' V' a. T. 4
Temperature evolution in each kind of matrix regenerator) Working fluid temperature evolution vs. acquisition time for a copper matrix 19 136.22 244.3 136.2 244.25 The temperature of the working fluid in the regenerator increases in the first half cycle until a maximum value (TR 1 and TR 4 ) TR 1 136.18 136.16 136.14 136.12 TR 4 136.1 mean TR 1 TR 1-4 mean TR 4 244.2 244.15 244.1 244.05 244 136.08 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 243.95 Acquisition time [s] The temperature decreases during the second half cycle until a temperature which is close to that of the cold source Cold working fluid Cold working fluid The Stirling engine regenerator has two roles: TR 1 TR 2 TR 3 Pourous media (regenerator) TR 5 TR 6 TR 7 TR 4 TR 8 - Accumulating heat (ΔTR 1 and ΔTR 4 ) Hot working fluid Hot working fluid - Forming a thermal barrier between both heat and cold sources (ΔTR 1-4 )
20 Temperature evolution in each kind of matrix regenerator) Working fluid gradient temperatures given by the thermocouples TR 1 during one cycle Best Heat accumulator Highest temperature gradient Working fluid gradient temperature between the thermocouples TR 1 and TR 4 The regenerator formed of the material Monel 400 is the best heat accumulator. The stainless steel represents the highest temperature gradient, and the Aluminum the smallest temperature gradient.
21 Influence of each type of matrix on the engine performance versus heating temperature The Stainless steel has the best brake power regardless the heating temperature Brake power [W] 300 250 200 150 100 Brake power = 281 W T H = 500 C T H = 300 C T H = 400 C T H = 500 C The Aluminum regenerator presents the worst brake power regardless the heating temperature Stainless Steel Copper Aluminum Monel The brake power increases with the heating temperature, but at different levels of each kind of regenerator.
Influence of each type of matrix on the engine performance versus charge pressure 22 350 300 P i = 3 bar P i = 5 bar Pi = 8 bar Brake power [W] 250 200 150 100 The Stainless Steel Stanless Steel Copper Aluminum Monel The Aluminum regenerator has the regenerator presents the best brake power worst brake power regardless the initial charge pressure For all experimented regenerators, the brake power increases with the initial charge pressure but at different levels.
Regenerator thermal efficiencies The regenerator thermal efficiency is calculated as the ratio of real heat transferred through the regenerator by the ideal heat, which must be transferred through the regenerator. 23 Efficiencies [%] 0.65 0.6 0.55 0.5 0.45 0.4 Eff Monel Monel Eff Copper Cuivre Eff Steel Inox Eff-A Allum lum 0.35 0.3 0.25 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 Acquisition time fo one cycle [s] The copper presents the best regenerator thermal efficiency and the Monel 400 presents the worst regenerator thermal efficiency.
Regenerator thermal efficiencies The copper oxidizes quickly because of the working fluid (air) which contains about 21% of oxygen. This material oxidation changes the physical properties of the copper, and then leads to bad heat exchanges inside the regenerator. 24 Copper Stainless Steel Monel Aluminum Regenerator matrixes after about 15 hours of use The stainless steel and the Aluminum, used as regenerators, have good thermal efficiencies (about 44%). These two materials do not present a problem of oxidation, but the use of regenerator in Aluminum is limited by its melting temperature.
25 Four regenerator matrices were tested on a Gamma Stirling engine : Stainless steel, Copper, Aluminum and Monel 400 - The Monel matrix presents the best thermal sponge, and the Stainless Steel represents the best thermal barrier between hot and cold heat sources. - The copper regenerator has the best thermal efficiency, but its oxidation decreases extremely the brake power of the Stirling engine. - Using an Experimental Design Approach (OCC or DOE), the impotant parameters of the Stirling engine can be optimised to obtain a good efficiency of this machine.