The ambitious DOE SunShot cost target ($0.06/kWh) for concentrated solar power (CSP) requires innovative concepts in the collector, receiver, and power cycle subsystems, as well as in thermal energy storage (TES).
For the TES, one innovative approach is to recycle waste from metallurgic industry, called slags, as low-cost high-temperature thermal energy storage material. The slags are all the non-metallic parts of cast iron which naturally rises up by lower density at the surface of the fusion in the furnace. Once cooled down some ceramic can be obtained mainly composed of oxides of calcium, silicon, iron, and aluminum. These ceramics are widely available in USA, about 120 sites in 32 States and are sold at a very low average price of $5.37/ton. The US production of iron and steel slag was estimated at 19.7 million tons in 2003 which guarantees a huge availability of material.
In this paper, electric arc furnace (EAF) slags from steelmaking industry, also called “black slags”, were characterized in the range of temperatures of concentrated solar power. The raw material is thermo-chemically stable up to 1100 °C and presents a low cost per unit thermal energy stored ($0.21/kWht for ΔT = 100 °C) and a suitable heat capacity per unit volume of material (63 kWht/m3for ΔT = 100°C). These properties should enable the development of new TES systems that could achieve the TES targets of the SunShot (temperature above 600 °C, installed cost below $15/kWht, and heat capacity ≥25 kWht/m3). The detailed experimental results are presented in the paper.
After its characterization, the material has been shaped in form of plates and thermally cycled in a TES system using hot-air as heat transfer fluid. Several cycles of charge and discharged were performed successfully and the concept was validated at laboratory scale. Apart from availability, low-cost, and promising thermal properties, the use of slag promotes the conservation of natural resources and is a noble solution to decrease the cost and to develop sustainable TES systems.
An industrial-scale air-ceramic horizontal packed-bed thermal energy storage (Eco-Stock®) has been designed and built by Eco-Tech Ceram and tested during an experimental campaign of 500h. The goal is to provide experimental data and analysis of a horizontal and containerized packed bed TES at high temperature, with performance indicators specific to waste heat recovery. A single charge-discharge at 525°C and 3 cycles at 500°C were carried out (300 kW Th for charge, 350 kW Th for discharge). The unit was able to store up to 1.9 MWh Th in a TES system (1.7 × 1.7 × 3.08 m 3 ) composed by 16-ton of bauxite ceramic media. The packed bed was able to valorize up to 90% of the heat source, which demonstrates the system ability to recover waste heat up to 525°C at industrial scale. No channeling effect and moderate radial thermal gradient were detected, even though the tank had a horizontal geometry. The plug and play TES presents a non-negligible part of its energy stored in the insulant, which can be recovered during discharge. To take into account such phenomenon, a one-dimension 3 temperature model is proposed. The model fitted well with experimental results, with a root mean standard deviation of the model below 20°C for the temperature profiles.
The influence of solar constraints is investigated on the Eco-Stock®, a commercial packed bed developed by the company Eco-Tech Ceram (1.9 MWhth at 525°C). The first test takes into account the simulated solar power variation along the day. Power variation has a minor impact on the packed bed performance, with a 3% decrease on the yield at 0.8 of cutoff temperature. Numerical results match the experiment, proving the model to handle properly power variation. The second test takes into account intermittencies, due to potential clouds. The stand-by phases had a major impact, reducing the yield by 13% at 0.8 of cutoff temperature, due to heat losses and both axial and radial stratification.
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