The use of electricity has become more and more inclusive in modern times. Electricity is nearly ubiquitous in every day activities. Therefore producing the electricity used by modern societies is a never-ending task. Various forms of conversion are used to convert naturally occurring energy sources into electricity. One naturally occurring energy source is solar energy. Solar energy can be collected and concentrated, and then converted into electrical energy. Specifically, it is generally known in the field how to collect and concentrate solar energy to power various types of heat engines based on generally known power conversion cycles to produce electricity. The various common engines are often categorized according to the various thermodynamic cycles including Stirling cycle engines, Brayton cycle engines, and Rankine cycle engines. Other thermodynamic cycles and engines that implement them exist, and these can use the collected thermal energy, converting it into electrical energy for modern societies. To those skilled in the art, it will be clear that an increase in the efficiency of these processes will decrease the amount of heat resource needed to provide a given level of electrical energy.
A Stirling cycle engine is a thermal energy to mechanical energy conversion device that uses a piston assembly to divide a fixed amount of gas between at least two chambers. The chambers are otherwise connected by a fluid passage equipped with heat source, recuperation, and heat sink heat exchangers. The piston assembly has separate piston heads that act on both chambers simultaneously. As the volume in one chamber is increased, the volume in the other chamber decreases, and vice versa, though not strictly to the same degree since one of the piston heads can have a greater area than the other piston head by design. A movement of the piston assembly in either direction creates an elevation of pressure in the chamber that experiences a decrease in volume while the other chamber that experiences an increase in volume finds its pressure reduced. The pressure differential across the two chambers decelerates the pistons, and causes a flow of gas from one chamber to the other, through the connecting fluid passage with its heat exchangers. The heat exchangers tend to either amplify or attenuate the gas volume flowing through them, depending, respectively, on whether the gas is either heating or cooling as it flows through the fluid passage.
The character of the piston assembly as a finite massive moving object now comes into play according to the laws of motion and momentum. The piston will overshoot the point at which the pressure forces across the piston are in balance. Up to that point, the piston has had an accelerating pressure differential force that charges it with kinetic energy of motion. Once the net forces on the piston balance, the acceleration ceases, but the piston moves on at its maximum speed. Soon the pressure differential reverses and the piston decelerates, transferring its kinetic energy of motion into gas pressure/volume energy in the chamber toward which the piston has been moving up to this point. The increased pressure in the chamber now accelerates the piston in the opposite direction to the point where it reaches its maximum velocity in the opposite direction at the force balance point, and then decelerates as an increasing pressure differential builds in the other chamber. Once again, the piston stops, reverses direction, and repeats the process anew. This is a case of periodic motion as the energy is passed from the form of kinetic energy in the piston assembly to net pressure/volume energy in the chambers.
The periodic motion tends to be damped by small irreversibilities, especially the gas that is pumped back and forth from one chamber to the other through the fluid passage. This is the normal case for a Stirling engine in an isothermal state. However, when it is thermally linked to hot source and cool sink reservoirs at the source and sink heat exchangers respectively, the gas flowing into one of the chambers is heated while the gas flowing into the chamber on the other side is cooled. In this way, a given mass of pressurized cool gas sent to the hot chamber is heated and amplified in volume to a sizable shove. Conversely, a given mass of hot gas leaving the hot side chamber is reduced in volume as it is cooled by passage through the heat exchangers, and the cooled gas push in the cool side chamber is thereby attenuated dramatically due to the reduced volumetric flow of the cooler gas. Thereby, a change in the piston position, and its effects on gas temperature and pressure within the Stirling cycle engine, cause portions of the hot reservoir thermal energy to turn into periodic mechanical piston energy and gas pressure/volume energy, and the remaining thermal energy to flow to the cool reservoir in periodic fashion.
The compressible gas within the two chambers and the piston moving between those chambers form a spring-mass system that exhibit a natural frequency. Similarly, the motion of gas between the two chambers has its own natural frequency of a lower order. The conversion of thermal energy to mechanical within this system would cause such a system have successively higher amplitudes until mechanical interference or some other means of removing the energy appears. For many commercial Stirling cycle heat engine systems, a power piston operating at the same frequency, but out of phase with heat engine piston, is used to remove the excess mechanical energy and convert it into useful work.
One way to produce this energy conversion is to use the time varying position of the power piston to produce a time varying magnetic flux in an electrical conductor, producing thereby, an electromotive potential which can be consumed locally, or remotely over transmission lines, by connection to an electrical appliance such as a motor, battery charger, or heater. Commonly, this is done by using the power piston to drive an alternator mover through a mechanical link. The alternator mover is what converts a time varying position within the alternator into time varying magnetic flux in the alternator electrical conductor(s).
Although many sources can provide the heat to power the Stirling cycle engine, one particular source is solar energy in the form of collimated sunlight. When solar energy is used to drive a Stirling engine, the collimated sunlight is collected, typically by a mirror or mirror array, concentrated, typically by the curvature of the mirror surface or the orientation of the individual mirrors in an array, and absorbed in a small area, typically a cavity absorber. This absorber becomes hot after absorbing the collimated sunlight. The hot absorber is thermally coupled to the source heat exchanger described above.
For solar power systems, the solar energy from the sun is collected and concentrated onto an absorber. The absorbed optical energy provides a source of thermal energy to operate a power conversion cycle or heat engine, such as the Stirling engine. The temperature of the thermal energy at the absorber depends on the concentration ratio, the optical/absorber configuration, and the rate of heat removal to the heat engine and to the environment through losses. The solar energy is generally concentrated into an absorber cavity so that losses are minimized, and the thermal energy is then transferred to the source heat exchanger of a single Stirling cycle engine with minimal temperature loss.
Stirling cycle engines can be designed and tuned for optimal efficiency at various different temperatures for the source heat exchanger. Nevertheless, once a Stirling cycle engine is tuned or optimized for particular operating conditions its efficiency dramatically decreases when these optimum conditions are not maintained. If the concentrated sunlight entering the absorber cavity varies slightly, the efficiency of the single Stirling cycle engine can be compromised. Such variations can occur when only a slight haze or foggy condition exists between the concentrator and the sun. Moreover, time of day and seasonal variations can cause the sunlight to travel through more or less atmosphere and effect the insolation, thereby adversely affecting the concentrated solar power level to a value that is not consistent with operating the Stirling cycle engine at its optimum efficiency.
When the insolation becomes too low, the Stirling engine overcools the thermal cavity. At this point, the temperature of the thermal cavity is below the design temperature of the Stirling engine. This will result in a reduction in the heater head temperature causing the engine to operate at a lower efficiency point. Although, the design of the Stirling engine can be modified by adjusting the stroke length to partially compensate for this, the Stirling engine still may not operate at optimum or designed conditions. Therefore, over a long period of time, this inefficiency can have a significant impact on the life cycle cost of the units of energy produced.
Accordingly, there exists a need for a system that will allow for more efficient conversion of the collected and concentrated solar energy into electrical energy. More specifically, a power conversion system is needed that is flexible enough to allow it to be optimized for various and unique operating conditions so that its overall and long-term operating efficiency increases. Particularly, it is desired that a power conversion system be provided that is able to adapt and alter its operating configuration to optimize the operation of the system over a plurality of insolation levels.
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