Waste heat is often created as a byproduct of industrial processes where flowing streams of high-temperature liquids, gases, or fluids must be exhausted into the environment or removed in some way in an effort to maintain the operating temperatures of the industrial process equipment. Some industrial processes utilize heat exchanger devices to capture and recycle waste heat back into the process via other process streams. However, the capturing and recycling of waste heat is generally infeasible by industrial processes that utilize high temperatures or have insufficient mass flow or other unfavorable conditions.
Waste heat may be converted into useful energy by a variety of turbine generator or heat engine systems that employ thermodynamic methods, such as Rankine cycles, that are typically steam-based processes that recover and utilize waste heat to generate steam for driving a turbine or other expander connected to a generator. An organic Rankine cycle utilizes a lower boiling-point working fluid, instead of water, during a traditional Rankine cycle. Exemplary lower boiling-point working fluids include hydrocarbons, such as light hydrocarbons (e.g., propane or butane) and halogenated hydrocarbon, such as hydrochlorofluorocarbons (HCFCs) or hydrofluorocarbons (HFCs) (e.g., R245fa).
In addition, the turbines and pumps utilized in turbine generator systems are susceptible to fail due to over-pressurization, as well as, under-pressurization within the fluid systems, especially near the inlets and outlets of the turbines and pumps. If the system inlet pressure decreases to a level in which the working fluid loses energy, then a system pump may be catastrophically damaged by way of cavitation. Generally, once the system pressure becomes uncontrollable, control of the system temperature is also lost. Therefore, the turbines and pumps may also be susceptible to fail due to thermal shock when exposed to substantial and imminent temperature differentials. Such rapid change of temperature generally occurs when the turbine or pump is exposed to a supercritical working fluid. The thermal shock may cause valves, blades, and other parts to crack and result in catastrophic damage to the unit.
A turbine-driven pump, such as a turbopump, may be utilized in an advanced Rankine cycle. Generally, the manner in which the turbine-driven pump is controlled may be quite relevant to the operation and efficiency of the overall thermal cycle process. The control of the turbine-driven pump is often not precise enough to achieve the most efficient or maximum operating conditions without damaging the turbine-driven pump. Also, to increase the efficiency of the overall thermal cycle, the turbine-driven pump may achieve self-sustained operation during the start-up process and maintain such self-sustained operation during the thermal cycle. However, the turbine-driven pump often over pressurizes or under pressurizes segments of the working fluid circuit when attempting to obtain or maintain self-sustained operation, which in turn, may lead to the damaging of the turbomachinery or other components within the system.
Therefore, there is a need for a heat engine system and a method for activating and sustaining a turbopump within the heat engine system, whereby the turbopump achieves self-sustained operation in a supercritical cycle without over pressurizing the working fluid circuit during a start-up process and maintains self-sustained operation while maximizing the efficiency of the heat engine system to generate energy.