Converting Heat Transfer Fluids into Solar Energy

Paratherm™ receives inquiries and emails from several engineers, mechanical contractors, project managers, process designers, and even homeowners in relation to the use of heat transfer fluids for solar applications.

Domestic Applications

Water/Water-Glycol

Passive Solar

Passive solar systems are the most inexpensive and simplest alternatives that enable direct tapping of solar radiation for practical uses. Such systems rely on natural convection for circulation. In general, in passive solar water heating (SWH) systems, which are the liquid-based passive solar applications most often found, plain water is used as the circulating liquid.

Active Solar

In the case of active (pumped) SWH systems, more often, heat is transferred to the water by a separate loop containing a heat transfer fluid, using a heat exchanger. Typically, a freeze-protected solution of glycol and water, including a corrosion inhibition package, is used as the heat transfer fluid in such systems. Non-aqueous heat transfer fluids, such as Paratherm LR, are also used in these systems.

Power Generation and Thermal Storage

In solar energy generation, heat transfer fluids are used in concentrators such as linear Fresnel, parabolic and trough-based systems. Depending upon the mechanical design of the application and the temperature requirements, several heat transfer media can be used:

• Steam/water is used in turbines for generating electricity
• Molten salts, such as 40% KNO₃ and 60% NaNO₃ (220 °C–600 °C) for thermal storage
• Synthetic/aromatic heat transfer fluids
  • Liquid phase only: Used to supply heat evenly for steam in turbine generators
  • Vapor phase: Used for producing electricity in turbines, analogous to steam but with higher thermal efficiency

Polycrystalline Silicon

To conclude, high-temperature heat transfer fluids are extensively used in the manufacture of polysilicon, the material used to produce photovoltaics (PVs), through a process analogous to that employed during the production of computer chips.

PV solar panels are the solid-state, regular black panels with the bluish tinge, positioned in deserts and on roofs to generate electricity directly from solar radiation. The most familiar example is the solar patio light that absorbs solar energy in the daytime and then automatically lights patios and pathways at night.

The polysilicon process can be performed using Fluid Bed (FB) reactors or the Siemens Reactor, where it is possible to use heat transfer fluids in the purification step to increase the catalytic reaction temperature to transform silicon tetrachloride (STC) into trichlorosilane (TCS). This step is known as the hydrogenation step of the process.

In these units, metallurgical grade silicon (MGS) is purified to produce electronic grade silicon through the hydrogenation of MGS and STC to form TCS. Crystallization and distillation are performed for refining the resulting TCS. Heat transfer fluids are also employed in the distillation process.

The cooling step in the polysilicon synthesis also involves the use of other heat transfer fluids at low temperatures. In this step, it is beneficial to use cryogenic-range heat transfer fluids such as Paratherm CR. The molten silicon that pours from the crucibles into the molds is cooled by the fluid, thereby producing monocrystalline ingots for solar cells and PVs.

Paratherm Fluids for Solar Applications

Paratherm does not produce, support, or distribute glycol-based heat transfer fluids; however, it can recommend suppliers.

This information has been sourced, reviewed and adapted from materials provided by Paratherm.

For more information on this source, please visit Paratherm.