Storing thermal energy by thermally stratifying the hot fluid layer on top of a cold fluid layer in a single tank packed with granite particles and then filling the volume between the particles with a fluid such as molten salt or oil, can reduce the cost of thermal storage because large portion of the expensive molten salt or oil is replaced with lower cost granite and a single tank is used. This storage system is commonly referred to as dual-media thermocline thermal energy storage. However, two technology barriers have prevented the use of this dual-media thermocline thermal storage system.
- First, and most importantly, the degradation (widening of the transition zone) of the thermocline1 resulting from periodic charge and discharge reduces the availability of thermal storage capacity (and /or requires periodic maintenance to restore the thermocline). Simulations have shown that the storage capacity can degrade to 40% of the designed capacity after ten charge / discharge cycles, as shown in the figure 1 on the left.
- Second, thermal ratcheting2 resulting from settling of the solid media due to cyclic expansion on heating and contraction on cooling, causes high stresses on the tank walls.
Terrafore’s invented a method to eliminate the degradation of thermocline by using TerraKline™ technology in which the flow rate from the storage tank is actively manipulated using a model predictive controller and by modifying the design of tank. Using simulation models, Terrafore showed that TerraKline™ technology can solve the degradation of the thermocline. F igure 1 &2 below show the thermal profiles of fluid inside the tank without and with the TerraKline technology. Figures 1&2. Thermocline profiles at end of charge (green) and end of discharge (red) for 10 cycles without TerraKline technology ( figure 1 left) and with TerraKline (figure 2 right)
The second barrier – that of thermal ratcheting with rock/granite solid media is being addressed by others by using sloped walls and screens and meshes to create layers within the tank.
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.
1 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. This grows to 40% to 50% upon partial charge and discharge. This growth of thermocline is referred to as the degradation of the Thermocline. Useful stored capacity is approximately (=1 – Height of thermocline).
2 Thermal ratcheting – 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.