Researchers from Finland's Aalto University have conducted experimental and numerical evaluations of a Stirling engine-based Carnot battery (SECB) prototype that uses sand as the thermal energy storage (TES) material. SECB is a system that uses a Carnot battery to store electricity as heat, then uses a Stirling engine to convert the stored heat back into electricity.
A Stirling engine is a closed-cycle regenerative heat engine with a permanent gaseous working fluid such as gas or air. It generates mechanical motion from the heat-driven compression and expansion of the fluid – using a heat transfer fluid to meet demand.
“Theoretical and numerical studies predict high efficiencies but lack experimental validation, while experimental Stirling-based Carnot batteries rely almost exclusively on costly metal-based phase change material (PCMs),” said the researchers. “Although sand has been identified as a promising alternative thermal storage medium, its performance within a complete, integrated Carnot battery system has not yet been experimentally demonstrated.”
The prototype consisted of a 0.2 m³ insulated thermal storage tank filled with brown silica sand with a grain size of 0.6–2 mm as the thermal energy storage medium. The sand had a specific heat capacity of 703 J/(kg·K), a thermal conductivity of 0.2–0.7 W/(m·K), and a bulk density of 1,800 kg/m³.
Image: Aalto University, Journal of Energy Storage, CC BY 4.0
Ten electric heater elements provided charging, each rated at 3 kW and 230 V. Heat transfer from the sand to the engine occurred through a copper block measuring 0.14 m × 0.225 m × 0.225 m, connected to two copper plates of 0.65 m × 0.65 m × 0.01 m. The system used a 1 kWe and 26%-efficient commercial Microgen free-piston Stirling engine manufactured by Microgen Engine Corporation.
The researchers tested the prototype under two operating conditions, with an engine head temperature set point of 300 C or 350 C.
Image: Aalto University, Journal of Energy Storage, CC BY 4.0
In addition, the researchers also developed a three-dimensional numerical model in COMSOL Multiphysics environment to simulate the thermal behavior and performance of the SECB. The simulation was first validated against the experimental cases of 300 C and 350 C. It was then extended to investigate higher operating temperatures of 400 C and 500 C, which could not be tested experimentally.
“Increasing the engine's head set-point temperature from 300 C to 350 C elevated the peak electrical power from about 500 W to 690 W and extended the discharge period from roughly 9 h to 14 h,” the results showed. “Despite this, the round-trip efficiency remained relatively low: 4.4–5.9% at 300 C and 6.8–8.3% at 350 C. These results show clear temperature-driven performance gains but also indicate that significant design and integration improvements are required before this approach can reach competitiveness for electricity-to-electricity storage.”
According to energy balance results, losses outside the conversion stage dominated the outcome. For a 300 C cycle, 53 kWh of electrical input produced 2.33 kWh of electrical output, with 16.54 kWh rejected via the cooling loop and 34.13 kWh lost to the surroundings or through structural heat paths. In addition, the low effective thermal conductivity of the sand bed limited the heat flux to the engine head, leading to temperature dips and intermittent operation.
“The numerical model suggests that higher round-trip efficiencies between 19.1% and 31.6% for 300 C to 500 C are achievable if heat leakage and boundary losses are minimized,” concluded the team. “These figures should serve as design targets and sensitivity guides rather than actual performance benchmarks, especially for operations above 400 C, which cannot be experimentally validated with the current prototype.”
The system was presented in “Stirling engine-based Carnot battery with sand as heat storage medium: 1 kWe prototype,” published in the Journal of Energy Storage.
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