OTEC Pump Motor Cooling Using Sub-Cooled Working Fluid
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Solution Overview
Problem
Conventional OTEC systems face challenges in efficiently cooling working fluid pump motors due to the lack of suitable COTS pumps for water submergence, leading to heat dissipation issues within pressure vessels, which complicates the cooling process and increases failure potential with corrosive seawater use.
Innovation Solution
A cooling system that utilizes sub-cooled working liquid from the pump outlet to directly or indirectly cool the motor via heat exchange with a secondary fluid, either air or a secondary liquid, within the pressure vessel, allowing for efficient heat dissipation and reducing the load on the evaporator by reintroducing de-subcooled working fluid into the system.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the pump is located below the water line inside a pressure vessel, then the pump is protected from ocean environment and pressure differences, but heat dissipation becomes complicated and motor cooling becomes insufficient
Solution Approach 1:
The system separates the cooling function into a distinct cooling loop that draws working fluid from the condenser outlet and routes it through a dedicated cooling coil positioned adjacent to the motor. This segmentation allows the motor cooling function to be independently optimized without affecting the pump's pressure vessel protection.
Solution Approach 2:
A cooling coil acts as an intermediary heat exchange surface between the working fluid and the motor. The cooling coil transfers heat from the motor to the working fluid, enabling indirect cooling while maintaining the motor's sealed environment within the pressure vessel.
2Temperature
If sea water is used for cooling the motor through a heat exchanger, then cooling capacity is increased, but system complexity increases and failure potential increases due to additional components and corrosion
Solution Approach 1:
The working fluid serves multiple functions: it is pumped through the evaporator for heat absorption, condensed in the condenser for heat rejection, and then reused as the cooling medium for motor cooling. This multi-functionality eliminates the need for separate cooling systems and reduces overall system complexity.
Solution Approach 2:
The system uses its own working fluid to cool the motor, rather than requiring external sea water cooling systems. The working fluid circulates through the cooling coil, absorbing heat from the motor, and returns to the condenser to reject the absorbed heat, creating a self-contained cooling loop.
3Device complexity
If the motor inefficiency heat is not extracted, then the system is simpler, but the motor temperature exceeds the maximum allowable temperature
Solution Approach 1:
The motor cooling function is merged with the existing working fluid circulation system. The cooling coil is integrated into the pressure vessel, and the working fluid flows continuously through it, combining the heat rejection and motor cooling functions into a single integrated system.
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
This approach enhances heat dissipation efficiency, reduces the risk of failure by minimizing additional components and corrosive exposure, and improves the overall energy conversion efficiency by recycling heat energy back into the system.
Implementation Method 1
direct forced convection cooling
Implementation Method 2
indirect cooling can include either working fluid to air heat exchange or working fluid to secondary liquid, such as fresh water, heat exchange
Data Source
AI summary
A cooling system and process in an OTEC system are described where the sub-cooled working liquid from the working fluid pump outlet is used to cool the working fluid pump motor, either directly or indirectly via heat exchange with a secondary fluid. The heat from the motor that is being rejected into the working fluid just prior to the working fluid flowing to the evaporator helps to alleviate heat duty in the evaporator meaning more potential for the evaporator to create energy. Also, because two-phase evaporators, such as those in an OTEC system, are less efficient than single-phase heat exchangers at single-phase heating, this pre-heating of the working fluid will help the evaporator performance substantially.