Transcritical CO2 power systems are being investigated for site independent electro-thermal energy storage (ETES). The storage plant uses electrical energy with a standard vapor-compression heat pump/refrigeration cycle to store thermal energy as hot water and ice over a period of approximately 8 hours during low power demand. The power cycle is then reversed and operated as a simple Rankine cycle to produce ∼100 MWe for about 4.5 hours during peak demand. During the power generation cycle the storage plant uses the heat stored in the hot water tanks, together with ice melting, plus ambient heat rejection for the heat sink. For 100 MWe class power plants, the round trip efficiency is estimated to be up to 60%. CO2 was selected as the working fluid because it improves the ability of the plant to operate with high reversibility. In addition, it is compact and can operate below the freezing point of water.
This report describes the major control characteristics of the plant, together with methods, tools, and results of the model. Because the plant is nearly “closed”, it must operate only by consuming electrical energy during the charging cycle and by producing electrical energy plus some waste heat during the discharge cycle. All other heat transfer processes occurs solely within the storage plant itself and consists of either heating or cooling water and by making or melting ice. For the plant to operate continuously, both the water thermal storage and ice storage must be returned to their initial conditions after every 24 hour period. Otherwise, small changes in the thermal environment during waste heat rejection or performance variations of internal components will cause the storage system to drift from its designed operating temperature, pressure and energy storage capability, challenging its ability to operate.
The control concept for the storage plant addresses both the operation of the plant during charging and discharging. It also addresses strategies for control during off-design situations or due to disturbances such as load following or changes in ambient heat rejection conditions. The process simulations described in the paper include models for the main physical components of the plant including the turbomachinery, the heat exchanger network, states of charge of the cold and hot storage, and CO2 inventory.