Economic valuation of energy storage coupled with photovoltaics : current technologies and future projections

Abstract

A practical framework for the economic valuation of current energy storage systems coupled with photovoltaic (PV) systems is presented. The solar-with-storage system's operation is optimized for two different rate schedules: (1) Time-of-use (TOU) for residential systems, and (2) Real-time wholesale rates for centralized generators. Nine storage technologies are considered for PV coupling, including six different battery chemistries, hydrogen electrolysis with a fuel cell, compressed air, and pumped-hydro energy storage. In addition, these technologies are assessed in the capacity of enabling a solar energy generator to provide a set service requirement. Concentrating solar thermal power (CSTP) with thermal storage is presented as a comparison for this final baseload scenario. Some general insights were gained during the analysis of these technologies. It was discovered that there is a minimum power rating threshold for storage systems in a residential TOU market that is required to capture most of the benefits. This is about 1.5 kW for a 2 kWP residential PV system. It was found that roundtrip efficiency is extremely important for both TOU and real-time markets, but low self-discharge rates are even more critical in real-time rate schedules. It was also estimated that large storage systems for centralized generation would capture the most revenue with a power rating twice that of the storage capacity (2 hours of discharge). However, due to cost limitations, actual optimal ratios were calculated to be about 3 to 7 hours of discharge for operation in a real-time market. None of the current technologies considered are able to economically meet the requirements for a residential TOU rate schedule; and only CSTP with thermal storage, pumped-hydro, and potentially compressed air storage are able to offer value in a centralized real-time market or a baseload scenario. Recommendations for future research and development (R&D) on the various storage technologies are given. For many of the electrochemical batteries, the key focus areas include cycle lifetime as well as energy and power costs. Roundtrip efficiency was identified as the weak-point of hydrogen systems; the energy cost of lithium-ion batteries was found to be prohibitively expensive for energy arbitrage applications; and the balance of system (BOS) and power costs were identified as the main focus areas for the larger pumped-hydro and compressed air storage systems.