Bottoming Cycle Heat Recovery for Closed Cycle Engine Efficiency
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Solution Overview
Problem
Closed cycle engines, such as Stirling engines, face inefficiencies due to inefficient combustion, heat exchange, heat losses, non-ideal working fluids, friction, and other imperfections, limiting their power output and efficiency, particularly in achieving high power density and portability.
Innovation Solution
A closed cycle engine system incorporating a bottoming-cycle loop with a chiller loop, constant density heat exchanger, and expansion device to optimize heat transfer and power generation, using a working fluid like supercritical carbon dioxide, and incorporating pulse converters to smooth fluid flow and enhance thermal energy extraction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If closed cycle engines use conventional heat exchange and power generation methods, then the system structure is simpler, but power generation efficiency and power density are limited
Solution Approach 1:
The system divides the power generation process into two separate cycles: a top cycle (closed cycle engine) and a bottoming cycle (chiller loop with expansion device). This segmentation allows each cycle to be optimized independently, with the top cycle handling high-temperature heat input and the bottoming cycle recovering waste heat from the cold side, thereby increasing overall power generation efficiency without requiring complete redesign of a single complex system
Solution Approach 2:
The bottoming cycle is nested within the overall system by utilizing the cold side heat exchanger of the top cycle as its heat input source. The chiller loop's working fluid absorbs heat from the top cycle's cold side, and the expanded fluid is then used to drive the expansion device for additional power generation. This nesting allows waste heat recovery while maintaining a compact integrated structure
2Power
If closed cycle engines increase power output, then power generation capacity improves, but power density and portability are compromised
Solution Approach 1:
The bottoming cycle operates continuously to recover waste heat from the top cycle's cold side, converting previously wasted thermal energy into additional mechanical work through the expansion device. This continuous utilization of waste heat increases total power output without requiring proportional increases in system mass, thereby improving power density
Solution Approach 2:
The system employs supercritical carbon dioxide as the working fluid in the bottoming cycle, which allows for compact heat exchanger design and high-density fluid storage. The constant density heat exchanger maintains high pressure and temperature parameters during heat addition, enabling efficient energy transfer in a compact volume and improving overall power density
3Use of energy by moving object
If closed cycle engines operate at higher efficiency, then energy utilization improves, but heat losses to environment increase due to larger heat exchange surfaces
Solution Approach 1:
The system converts the previously wasted heat from the cold side heat exchanger into useful work by implementing the bottoming cycle. The chiller loop captures this waste heat and uses it to drive the expansion device, transforming an energy loss into an additional power source. This approach increases overall energy utilization efficiency while the compact supercritical CO2 system minimizes additional heat loss surfaces
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system achieves improved power generation efficiency and power density, reducing emissions and transmission losses, while maintaining portability and scalability, with potential thermal efficiencies up to 80% and power densities exceeding 51 kW/m³.
Implementation Method 1
a cold side heat exchanger positioned along the bottoming-cycle loop in fluid communication with the pump and positioned in a heat exchange relationship with the cold side of the closed cycle engine
Implementation Method 2
a constant density heat exchanger positioned along the bottoming-cycle loop and downstream of the cold side heat exchanger, wherein the constant density heat exchanger is operable to hold a volume of the working fluid flowing therethrough at constant density during heat application via a heat source such that a temperature and a pressure of the volume of the working fluid is increased
Implementation Method 3
an expansion device in fluid communication with the constant density heat exchanger, the expansion device operable to extract thermal energy from the working fluid to produce work
Implementation Method 4
a third heat exchanger positioned along the bottoming-cycle loop and having an inlet and an outlet, the inlet of the third heat exchanger in fluid communication with the expansion device and the outlet of the third heat exchanger in fluid communication with the pump, wherein the third heat exchanger is operable to decrease the working fluid to a third temperature that is less than the first temperature
Implementation Method 5
a pump positioned along the bottoming-cycle loop and operable to move a working fluid along the bottoming-cycle loop
Data Source
AI summary
Systems and methods for converting energy are provided. In one aspect, the system includes a closed cycle engine defining a cold side. The system also includes a bottoming-cycle loop. A pump is operable to move a working fluid along the bottoming-cycle loop. A cold side heat exchanger is positioned along the bottoming-cycle loop in a heat exchange relationship with the cold side of the closed cycle engine. A constant density heat exchanger is positioned along the bottoming-cycle loop downstream of the cold side heat exchanger and upstream of an expansion device. The constant density heat exchanger is operable to hold a volume of the working fluid flowing therethrough at constant density while increasing, via a heat source, the temperature and pressure of the working fluid. The expansion device receives the working fluid at elevated temperature and pressure and extracts thermal energy from the working fluid to produce work.