Methylpolysiloxane mixtures as a heat-carrier fluid
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Solution Overview
Problem
Methylpolysiloxane mixtures used in Concentrated Solar Power (CSP) plants face issues with low viscosity at low temperatures, critical point below operating temperatures, and high vapor pressure, leading to inefficiencies and potential system failure due to increased pumping power requirements and seal swelling.
Innovation Solution
Development of methylpolysiloxane mixtures with a higher molar M:D ratio (1:15.5 to 1:30) and a higher fraction of cyclic dimethylpolysiloxanes (10-95 wt%), which maintain low viscosity and have their critical point above 425°C, ensuring long-term stability and reduced vapor pressure, thereby addressing the inefficiencies and operational challenges in CSP plants.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If low-viscosity methylpolysiloxane mixtures are used as heat transfer oil, then the oil flows easily and pumping power is reduced, but the critical point falls below operating temperature and vapor pressure increases
Solution Approach 1:
The invention changes the molecular parameters of the methylpolysiloxane by controlling the M:D ratio (terminal Me3Si groups to Me2SiO units) to be at least 1:2.5, and specifically 1:3 to 1:10. This parameter adjustment modifies the equilibrium composition during thermal cycling, ensuring that the critical point remains above operating temperature (450-500°C) while maintaining low viscosity through controlled molecular weight distribution and cyclic compound content (10-40 mass%).
Solution Approach 2:
The invention creates a composite methylpolysiloxane system comprising multiple components: linear methylpolysiloxanes (formula I), cyclic methylpolysiloxanes (formula II), and optionally T-units and Q-units. This composite structure leverages the complementary properties of each component - linear chains provide flexibility and low viscosity, while cyclic structures contribute to thermal stability and elevated critical point, achieving both low pumping power and high operational reliability.
2Use of energy by moving object
If the M:D ratio is increased to reduce viscosity, then the mixture becomes less viscous and easier to pump, but the critical point decreases and vapor pressure increases
Solution Approach 1:
The invention optimizes the M:D ratio within the specific range of 1:2.5 to 1:10 (preferably 1:3 to 1:10) to achieve the desired balance. This controlled parameter change ensures sufficient low-molecular-weight species for low viscosity while preventing excessive formation of volatile components that would lower the critical point. The presence of T-units (RSiO3/2) and Q-units (SiO4/2) further modulates this balance by affecting molecular weight and intermolecular forces.
Solution Approach 2:
The invention introduces T-units and Q-units as intermediary structural elements that mediate between the conflicting requirements of low viscosity and high critical point. These units act as molecular bridges that connect lower molecular weight segments while contributing to overall molecular weight and thermal stability, thereby maintaining low viscosity through flexible chain structures while elevating the critical point through increased molecular complexity and intermolecular interactions.
3Temperature
If high molecular mass methylpolysiloxanes are used to raise the critical point, then the critical point increases above operating temperature, but the viscosity increases and pumping power requirements increase
Solution Approach 1:
The invention controls the molecular mass distribution by limiting the degree of polymerization in linear methylpolysiloxanes (x ≤ 80 in formula I) while maintaining a high M:D ratio. This parameter control ensures that molecular mass is sufficient to elevate the critical point above operating temperature but not so high as to cause excessive viscosity. The equilibrium process during thermal cycling further adjusts the molecular weight distribution, breaking down excessively long chains while reforming stable cyclic structures.
Solution Approach 2:
The invention exploits the dynamic equilibrium between linear and cyclic methylpolysiloxanes during thermal cycling. The mixture continuously undergoes equilibration reactions where linear chains break and reform, and cyclic structures open and close, adapting the molecular weight distribution to operating conditions. This dynamic behavior ensures that the effective molecular weight remains optimized for both critical point elevation and viscosity control throughout the operating temperature range.
4Stress or pressure
If low-boiling cyclic compounds are added to ensure high operating pressure, then the operating pressure is maintained, but the critical point falls below operating temperature and vapor pressure increases
Solution Approach 1:
The invention changes the parameter of cyclic compound selection from low-boiling species (D3, D4) to higher-boiling species (D5, D6, D7, D8, D9, D10). This parameter change in the cyclic compound distribution raises the overall boiling point and critical point of the mixture above operating temperature while still providing sufficient vapor pressure for heat transfer function. The cyclic compounds serve as stable high-temperature components that maintain pressure without becoming excessive vapor hazards.
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 proposed methylpolysiloxane mixtures exhibit significantly lower viscosity and vapor pressure, maintaining stability and efficiency even at high temperatures, ensuring reliable operation and reduced pumping power requirements, while maintaining subcritical conditions within the operating temperature range.
Implementation Method 1
Under thermal load, methylpolysiloxanes undergo a rearrangement: they equilibrate. Independently of the initial composition, the result is a methylpolysiloxane mixture of linear methylpolysiloxanes (Si2, Si3, Si4, etc.) and cyclic dimethylpolysiloxanes (D3, D4, D5, etc.) which is in thermal thermodynamic equilibrium.
Implementation Method 2
Low-viscosity mixtures of linear and cyclic methylpolysiloxanes are currently used as the heat transfer oil (e.g. Helisol® 5A) in CSP plants (CSP=Concentrated Solar Power, solar thermal power station with ray concentration).
Implementation Method 3
The mixture reaches the critical point only at temperatures above 400° C. The measurements also show that the methylpolysiloxane mixtures of the invention are likewise subcritical in the region of the operating temperature.
Data Source
AI summary
A methylpolysiloxane mixture along with uses and methods for operating a solar thermal power station (or CSP plant) utilizing the same. The use for the methylpolysiloxane mixture includes providing a mixture (a) wherein the methylpolysiloxane mixture includes a linear methylpolysiloxanes MDxM, wherein x is an integer with 0≤x≤100, and wherein the mixtures have a molar M:D ratio of 1:15.5 to 1:30; or (b) wherein the methylpolysiloxane mixture includes a linear methylpolysiloxanes MDxM, wherein x is an integer with 0≤x≤80 and cyclic dimethylpolysiloxanes Dy where y is an integer≥3, wherein the sum of the fractions of all cyclic dimethylpolysiloxanes Dy is 10-95 wt %, and wherein the mixtures have a molar M:D ratio of 1:10.5 to 1:30. The methylpolysiloxane mixture is used as a heat transfer fluid in a CSP plant with operating temperatures in a range of 300 to 500° C.