Thermodynamic And Experimental Study On Minto Solar Heat Engine

Requirement for energy is growing rapidly with the continuous progress of human civilization and the continuous improvement of people’s living standards. Development of renewable energy is of great significance because of existence of threat of conventional energy exhaustion. Heat source from solar collectors and other low-quality heat are good complementary and alternative with a variety of sources and a huge amount. But there are still some difficulties in production of mechanical energy with them. Minto solar heat engine is a device that coverts solar energy or other waste heat into mechanical energy. The engine with characteristic of low speed and high torque can be driven by a small temperature difference. But low performance hindered the development and application of the heat engine.There are few theoretical studies on the Minto solar heat engine. Only one paper presented its thermal efficiency formula which is misused sometimes in the later studies. Theoretical study of Minto solar heat engine is not perfect yet. The aims of this paper are to discuss the problems existing, and to make some attempts in analysis, calculation, and improvement of performance of Minto solar heat engine.The main contents of this paper include theoretical, numerical simulation and experimental study of Minto solar heat engine.First of all, energy convention and distribution absorbed from heat source is analysis in order to analyze the reasons for the low efficiency of original Minto engine. Then an improved Minto engine model with structural improvements is presented. Additional internal adiabatic wall and adiabatic piston is used to divide working fluid into two parts:one small part that has phase change and product useful work can absorb energy from heat source; and the other major part which is on transmission and weight function and doesn’t have phase change is insulated from heat source in order to avoid energy waste and improve the efficiency and power greatly.In the theoretical analysis part, based on the previous assumption and analysis, both sensible heat method and enthalpy method thermal efficiency formula applies to both original and improved Minto aolsr heat engine are presented in order to simplify calculation and avoid being misused. The thermal efficiency formula of improved Minto engine is simplified when temperature difference of working fluid is small, then conclusion that its thermal efficiency is close to Carnot maximum efficiency is made. According to the two typical applications that solar collector and solar concentrator, both power formula for original and improved Minto solar heat engine are presented that applies to constant temperature heat sources and to constant heat flux density, which fill the gaps of theoretical power ayalysis. In the improved Minto engine, a conclusion is made that the maximum power appears when working temperature difference is about half of that of low and high temperature heat sources.Using numerical simulation methods with independent programming,6materials are chosen to estimate the performance of both original and improved Minto solar heat engine under conditions of constant temperature heat sources and to constant heat flux density. The thermophysical properties of working fluid are calculated by the principle of Corresponding States. Simulation results show that the thermal efficiency and power of original Minto engine is low under both conditions, and impacted greatly by thermophysical properties of working fluid. Under condition of constant temperature heat sources, maximum power appears when working temperature difference is a bit less than half of that of heat sources. Under condition of constant heat flux density, temperatures of working fluid are high, which is practical difference. The thermal efficiency and power of improved Minto engine is high under both conditions, and impacted less by thermophysical properties of working fluid. Under condition of constant temperature heat sources, maximum power appears when working temperature difference is a bit more than half of that of heat sources. Under condition of constant heat flux density, temperatures of working fluid are low, which is in line with the actual conditions of application. Under condition of constant temperature heat sources, thermal efficiency of both original and improved Minto engine is impacted less by different heat transfer coefficient, and higher heat transfer coefficients will significantly improve the power of heat engine.A two-meter in diameter original Minto solar heat engine with acetone as working fluid is build in order to test the performance. Dynamic temperature and pressure data is recorded when the temperature of high temperature source is40℃,50℃, and60℃. A phenomenon has been observed first time that pressure is consistent with temperature at first and then inconsistent. Qualitative and quantitative analysis of the process of the change is promoted. Different parts of heat absorption of this experiment are calculated. All above may provide some suggestions for further design and optimization of Minto solar heat engine.In summary, this paper focuses on the theoretical research on the thermal efficiency and power of Minto solar heat engine. This paper presented the model of improved Minto solar heat engine, some results of targeted simulation study, and experimental analysis based on dynamic process. It may provide some suggestions on development and optimization of Minto engine, especially in solar energy applications.