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. Here, 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. To understand why this tradeoff is advantageous, we first introduce a framework for evaluating storage technologies that treats round trip efficiency (RTE) as a variable, in addition to cost per unit energy stored (CPE) and cost per unit power (CPP). It is from this perspective that the TEGS-MPV concept offers a compelling economic proposition.
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