Modified mechanism of a Stirling engine for use with solar power generators.

Dehelean, Nicolae ; Ciupe, Valentin ; Maniu, Inocentiu 等

1. INTRODUCTION

Solar energy has to become a practical alternative to fossil fuels and humans must find out other efficient ways to convert solar radiation into electricity, other synthetic fuels and heat. The need for better conversion technologies is a driving force behind many recent developments in biology, materials, mechanical engineering, electronics and opto-electronics.

The Sun delivers to Earth [10.sup.17] joules of energy in one second. Earth's ultimate recoverable resource of oil, estimated at 3 trillion barrels, contains 1.7 x [10.sup.22] joules of energy, which the Sun supplies to Earth in 1.5 days. The amount of energy humans use annually, about 4.6 x [10.sup.20] joules, is delivered to Earth by the Sun in one hour.

The Sun continuously delivers to Earth, 1.2 x [10.sup.5] terawatts, so this issue surpasses every other energy source, renewable or nonrenewable. It dramatically exceeds the rate at which human civilization produces and uses energy, currently about 13 TW.

The main purpose of this paper is to find out an answer to the question: "How to convert more efficiently the energy that comes from the Sun?"

There is an enormous gap between the potential of solar energy and our conversion capacity. Solar electricity accounts for a minuscule 0.015% of world electricity production, and solar heat for 0.3% of global heating of space and water. Biomass produced by natural photosynthesis is by far the largest use of solar energy; its combustion or gasification accounts for about 11% of human energy needs. The use of biomass as fuel is limited by the production capacity of the available land and water.

When thinking about converting heat from the sun in either electricity or mechanical work, the most viable solution that comes in mind is to use a concentrator that collects sun radiation over a larger area and focuses it onto a small spot, where a conversion "machine" or "device" is placed. Here the choice is limited to steam generators for use with turbines or steam engines, and to Stirling type engines.

[FIGURE 1 OMITTED]

Figure 1 presents such a concentrator dish having a Stirling engine placed in the focal plane (Wikipedia, 2008). This approach gives better performance over steam use but it is still relatively low on efficiency.

The paper proposes a technical solution meant to improve the heat transfer or p-V diagram for a modified version of an alfa-type Stirling engine.

2. THE CLASSICAL STIRLING ENGINE

Stirling engines are classified into three distinct types, shown in Figure 2. The Alpha type engine relies on interconnecting the power pistons of two cylinders to move the working gas, with the cylinders held at different temperatures. The Beta and Gamma type Stirling engines use a displacer piston to move the working gas back and forth between hot and cold heat exchangers in the same cylinder (Wikipedia, 2008).

The ideal cycle of any Stirling engine is the goal for every technical solution. This diagram is represented and marked in Figure 3. Overimposed is the real cycle of an alfa type engine. By looking at the two diagrams one can easily spot a problem in the design of these engines (Karlsruhe Uni, 2000).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The solutions found for enhancing the behavior of the engine, from the mechanism point of view, usually rely on the modification of the piston driving mechanisms. A few methods for approaching the ideal cycle are presented in Figure 4 showing a modified scoth yoke mechanism, a spatial cam with a special profile and profiled slider mechanism (Youtube, 2008). These approaches have the disadvantage of sliding contacts and points of increased acceleration on the sliding profile, drastically reducing the reliability of the solution.

3. THE MODIFIED MECHANISM

In order to overcome the above stated problems, a new mechanical structure was proposed for an alfa-type Stirling engine and it works as follows:

The two cylinders are moved by two crank and connecting rod mechanisms. The space law movement is sinusoidal for both pistons. The expansion cylinder needs a movement law with two stop points to improve the p-V diagram of the engine. We consider being an improvement to obtain a new diagram that seems like the Carnot one as near as it is possible. In order to improve the efficiency of the engine we have to increase the area between the superior and inferior curve, considering the ideal diagram in in Figure 5 (Herzog, 2005). This is possible by increasing the enthalpy difference between admission and expansion into the expansion cylinder.

It starts in 1 point of the diagram by an isothermal compression stroke (process 1 [right arrow] 2). This phase takes place by removing heat, Q12, from the working fluid in order to keep the temperature at a constant low value, TC. This is followed by an isochoric compression (process 2 [right arrow] 3). The later takes place because heat of the amount Q23 is added to the working fluid. Expansion (process 3 [right arrow] 4) takes place at constant temperature, TH. In order to keep the temperature constant, heat Q34 has to be added to the working fluid. Finally, the pressure is lowered at constant volume by removing heat, Q41, from the working fluid to close the cycle.

In order to achieve this, we need to replace the crank and connecting rod mechanism with another one that is able to perform a space function with two stop points.

This mechanism is presented in Figure 6 and is essentially a crank-rod mechanism coupled to a swivel mechanism for the secondary piston. The motion law is greatly enhanced (not over the entire cycle) and is constructed with 5th class joints only.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Figure 7 pesents three positions of the simulated mechanism (Kanagawa, 2005) in order to observe the functioning of the two pistons and their movement laws. The first position represents the starting point of the first stop point for the cold piston and this is held for about 120 degrees, until the torque generated by the hot piston drops with the cranck angle. The second stop point is on a smaller portion of the compression stroke (3rd position). This setup greatly enhances the torque of the engine by allowing the gas in the hot cylinder to expand fully and give more energy to the engine shaft. The diameters of the two cylinders differ so that they have the same displacement despite their different strokes.

4. FUTURE RESEARCH

The proposed solution regarding the modified mechanism must be further optimized and tested in real life conditions. This presumes building a working model of the engine that can be run with different heat sources. The engine must be built to accept a wide variety of working gases (air, hydrogen, carbon dioxide or methane). Performance analysis must be conducted on the modified engine in order to the optimal solution. Also comparative analysis between simulated performance and real engine performance has to be conducted.

The practical implementation of such an engine should have positive impact over power generation and could easily replace portable electrical generators used in remote locations, running alternatively on solar power or a wide variety of fuels.

As a drawback of this engine is the difficult self-starting problem and, if higher efficiency is required (hydrogen as the working gas) maintaining quantity and pressure inside cylinders require additional costs and system complications.

5. CONCLUSION

Using clean, renewable energy is a very appealing thing. But the goal is to convert this energy as efficiently as possible, taking into account both the initial cost of the conversion unit and the reliability issues. The proposed mechanism is presumed to give a boost in performance while maintaining a simple and cost effective technical solution.

6. REFERENCES

Herzog, Z. (2005). The ideal Stirling Cycle and Heat Load on the Regenerator, Available from: http://mac6.ma.psu.edu/ stirling/ideal_stirling_cycle/index.html Accessed: 200805-07

Kanagawa, P.E.C. (2005). Linkage mechanism simulator, Available from: http://www.edu-ctr.pref.kanagawa.jp/ LinkWeb/index_e.htm Accessed: 2008-05-16

Karlsruhe University, Institut fur Kolbenmaschinen (2000). Investigation of concepts for high power Stirling engines, Available from: http://www-ifkm.mach.uni-karlsruhe.de/ Html-e/Project/Stirling/stirling.html Accessed: 2008-05-07

Wikipedia (2008). Stirling engine, Available from: http://en. wikipedia.org/wiki/Stirling_engine Accessed: 2008-04-23

Youtube (2008). Ideal cycle Stirling engines, Available from: http://www.youtube.com/results?search_query=ideal+cycle +stirling+engines&search_type=&aq=f Accessed: 200805-28