As the cost of renewable energy falls below fossil fuels, the most important challenge to enable widespread sustainable power generation has become making renewables dispatchable. Low cost energy storage can provide this dispatchability, but there is no clear technology that can meet the need. Pumped hydroelectric and compressed air storage have low costs, but they are geographically constrained. Similarly, lithium-ion batteries are becoming ubiquitous, but even their lower bounding asymptote cost is too high to enable cost-competitive dispatchable renewables. In this project, we introduce a concept based on thermal energy grid storage (TEGS) using a multijunction photovoltaic heat engine (MPV) with promising initial experimental results that could meet the low cost required to enable cost competitive dispatchable renewables. The approach exploits an important tradeoff between the accession of an extremely low cost per unit energy stored, by storing heat instead of electricity directly, while paying the penalty of a lower round trip efficiency.
The new TEGS-MPV system concept and consists of a low-cost thermal storage fluid, nominally 553 metallurgical grade (98.5% pure) silicon. The liquid Si is stored in a “cold” tank, nominally at 1900°C, in the discharged state. To charge the system, the 1900°C Si is pumped, using an all graphite seal-less sump pump, through a series of pipes that are externally irradiated by graphite heaters that draw electricity from the grid. In this heater sub-system, the temperature of the Si is nominally raised to ~2400°C as it is pumped into the “hot” tank, where it is stored. In this process, excess electricity from the grid is stored as sensible heat in the liquid over a 500°C temperature difference (1900- 2400°C). When electricity is desired, the 2400°C Si is pumped out of the hot tank and through a MPV power cycle. The MPV power cycle is envisioned to consist of an array of graphite pipes that are covered in tungsten (W) foil. The W foil acts as a lower vapor pressure barrier between the graphite pipes and the MPV cells, which are mounted to an actively cooled block that keeps their temperature near the ambient temperature (i.e., ~30°C). The W foil, therefore, serves as a photon emitter, almost identical to an incandescent lightbulb that emits light to the MPV cells, which subsequently convert a fraction of it to electricity. As the Si passes through the graphite piping network it cools down to ~1900°C, as energy is extracted and converted to electricity, at which point it is returned to the “cold” tank to await later recharging. This new approach has several noteworthy benefits including the ability to reach > 50% RTE with a CPP < $0.5 W-e, and the potential to offer load following capabilities to grid operators. These benefits strongly suggest that if realized the TEGS-MPV approach could be one of the few grid storage approaches that are inexpensive enough to enable the eventual 100% penetration of renewables onto the grid.