Storing thermal energy using a thermal-stratification method can reduce the cost of thermal storage. This method is commonly referred to as <em>dual-media thermocline thermal energy storage</em>.
Terrafore’s TerraKline thermal storage solution uses a single tank packed with granite particles and the volume between the particles filled with a fluid such as molten salt or oil. It involves using the concept of thermocline which is created when heat from the thermal charge results in a hot fluid layer on top of a cold layer.
TerraKine Brings Down Thermal Storage Cost
TerraKline storage system, as opposed to currently-used systems, brings down the cost of storage significantly because it uses a single tank instead of two and replaces the expensive molten oil or salt with granite.
Major Technology Barriers to Dual-Media Thermal Storage Systems
TerraKline also offers a solution to one major technology barrier that has prevented the wide-spread use of dual-media thermocline thermal energy storage systems.
- Eliminates Thermocline Degradation*
The widening of the transition zone between the hot and cold layers, a result of the periodic charge and discharge cycles, reduces the thermal storage capability and requires periodic maintenance to restore the thermocline.
* In a perfect thermocline system, the height of the hot layer on top and the cold layer at the bottom is zero. In a nominal thermocline design this height is typically 10% to 15% of the height of the tank. With partial charge and discharge, this grows to 40% to 50%. This growth of thermocline is referred to as the degradation of the thermocline. Useful stored capacity is approximately (=1 – Height of thermocline).
TerraKline™ technology actively controls the flow rate from the storage tank by using a model predictive controller and by modifying the design of tank. Using simulation models, Terrafore has shown how TerraKline™ technology can solve the degradation of the thermocline. Figures 1 and 2 show the thermal profiles of fluid inside the tank without and with the TerraKline technology.
- Dealing with Thermal Ratcheting *
The other barrier, thermal ratcheting with rock/granite solid media is addressed by others, by using sloped walls and screens and meshes to create layers within the tank.
*In a dual media storage, when the tank is discharged or cooled, the solid media contracts and settles in the tank; and when this is followed by charging with high temperature fluid, the media expands exerting pressure on the tank walls.
Solving these barriers will reduce the cost of thermal energy storage (TES) by 33%, when compared to a conventional two-tank TES system. Current two-tank sensible heat TES systems cost about $28 per kWh. Thermocline TES is estimated to cost about $19 per kWh.
Terrafore is open to working with CSP developers to demonstrate this technology on a pilot scale.