MGA Advantages

  WHY ARE MISCIBILITY GAP ALLOYS A SUPERIOR TECHNOLOGY?

The industry standard thermal storage material is a mixture of NaNO3 and KNO3 salt (thermal salt) which is kept liquid all the time. Heat is stored as temperature rise in the liquid i.e. the latent heat of melting is not used. The blue rectangle in the figure below (adapted from Reed et al., 2017) illustrates the temperature range (width) and energy density (height) of molten salt storage in a trough solar concentrating power plant, the most commonly used commercial implementation of thermal storage.

A large amount of energy (up to 24%) is wasted in keeping the salt molten and pumping it through complex heat exchangers.  The infrastructure for doing this is very expensive as the salt has a very low thermal conductivity (<0.7W/mK) and regular system maintenance is required.  There is a risk that a system failure may allow the salt to freeze in which case it is likely to take months to melt it again and re-start the system.  

By comparison, the two newest high temperature MGA systems are also illustrated on the graph above (orange and red rectangles). The far greater energy density and suitablility for Rankine cycle steam turbines is clear. Data for some more established MGA systems are shown in the table below.

As recently published at the World Renewable Energy Congress 2017, MGA materials have many advantages for practical thermal storage systems including:

1. Externally the material remains and behaves as a solid meaning: 

• the storage unit can be modular “blocks” shaped for convenience with integrated heat transfer    tubing to convey the working fluid; 
• no movement (convection, pumping etc.) of the storage material is required, greatly reducing    infrastructure, maintenance costs and plant footprint.

2. Thermal conductivity is 50 - 200 times greater than the majority of installed thermal storage materials. Consequently:

• heat transfer within the block is very rapid leading to relatively uniform temperature distribution;
• the storage block can be directly heated from heat sources such as a Concentrated Solar Power (CSP) receiver; and
• heat transfer infrastructure costs can be further reduced as large tube spacing (0.5 - 2 m) can be used to transfer heat into working fluids such as steam to operate a turbine/generator.

3. Energy density is in the moderate to high range, which means:

• further reduction in the plant footprint with associated cost savings; and
• reduced socio-environmental impact.  

4. There are good opportunities for re-use and re-cycle:

• modular MGA thermal storage blocks can be re-configured if they become redundant in one application and 
• MGA, being composed of immiscible metals, can be readily separated by melting and recycled at the end of life.

5. Thermodynamically stable MGA are expected to function for decades with little maintenance.