Model TerraKline - Dual-Media Thermocline Thermal Energy Storage
Storing thermal energy using a thermal-stratification method can reduce the cost of thermal storage. This method is commonly referred to as dual-media thermocline thermal energy storage. 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.
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