Hybrid energy storage systems combine grid-connected electricity generation and electricity storage, and there are a wide variety of hybrid storage systems, such as:
- Compressed Air Energy Storage (CAES) uses exhaust heat from a combustion turbine to increase the efficiency of power extraction from high pressure air stored underground in caverns.
- Gas turbine/battery hybrids use a battery for fast response while the turbine starts up, to allow a peaking plant to offer spinning reserve capability without consuming fuel.
- Liquid Air Combined Cycle™ (LACC) cools air to cryogenic temperature so it can be stored at atmospheric pressure in above-ground tanks. LACC avoids CAES’s need for underground storage caverns, but also uses combustion turbine exhaust heat to increase the efficiency of power extraction. Efficiency is then further enhanced by using the cryogenic air as the heat sink for part of the power cycle.
- Liquid Salt Combined Cycle™ (LSSC) integrates electrically- heated thermal energy storage with combustion turbine exhaust heat to boost power output and fuel efficiency, while also using the exhaust heat to boost storage efficiency.
In transportation, hybrid vehicles remove range anxiety and cost compared to pure electric vehicles and improve fuel economy and emissions compared to gasoline powered vehicles.
Likewise, a hybrid storage system has more benefits than can be obtained from stand-alone generation and storage. Pintail Power hybrid technologies deliver better performance, more flexible operation, and lower capital and operating costs by creating synergies that work together holistically:
- Higher thermodynamic efficiency reduces fuel and electrical costs.
- Lower capital cost results from efficiency and allows smaller, less-expensive equipment.
- Bulk storage provides large capacity in a compact space close to the need.
- Smaller footprint reduces land and construction costs and makes more sites available.
- Exhaust heat enables use of safe, long-lifetime and non-toxic storage media, reducing O&M costs.
- Thermal storage provides operating flexibility and reduces fuel consumption.
- Lower technical risk results from integrating proven equipment and operated at conservative conditions, which are made economical by hybrid synergy.
New metrics are being developed by professional associations to support advances in storage technology. The key performance metrics of Energy Storage Systems (ESS) defined in the ASME PTC-53 Performance Test Code (currently available in draft form) are:
- Discharge Power Output, for example MegaWatts (MW)
- Discharge Energy, for example MegaWatt-hours (MWh)
- Charge Energy, for example Megawatt-hours (MWh)
- Storage Efficiency.
Hybrid storage technologies use two energy inputs – electricity during charging and fuel or waste heat during discharging and can discharge more electric energy than was stored. Since an efficiency greater than 100% is potentially confusing, PTC-53 follows the practice used in conventional power generation and expresses the efficiency as the energy input per unit of electrical energy output.
- Fuel Heat Rate, expressed as Btu/kWh, is the ratio of fuel energy consumed per electricity produced. This is the customary efficiency for thermal power plants; for pure electric storage systems, like Pumped Hydro or batteries, this is zero.
- Electrical Rate, expressed as kWh/kWh, is the ratio of electrical energy consumed per unit of electricity produced (the inverse of Round-Trip Efficiency used for single input storage systems).
Marginal Cost of Energy
These performance metrics are convenient for calculating the marginal cost of energy (MCOE) discharged from storage:
MCOE = Fuel Heat Rate * Fuel Cost + Electrical Rate * Charge Energy Cost
In an energy market, units will dispatch when the difference between the price paid for energy (the Locational Marginal Price) and the cost of producing electricity is positive. Conventionally, this is indicated by the Spark Spread, which refers to fuel ignition.
Spark Spread = Locational Marginal Price-Fuel Heat Rate * Fuel Cost
Hybrid storage units have an additional factor – the cost of stored energy – which determines whether the energy can be profitably discharged or should remain in storage (parked):
Park Spread = Locational Marginal Price-
[Fuel Heat Rate * Fuel Cost + Electrical Rate * Charge Energy Cost]
Accordingly, it is desirable to have both a low Electrical Rate, so less stored energy must be purchased, and a Charge Energy Cost. Since the energy price is volatile in markets, it is likewise desirable to buy as much of it ‘on sale’ as possible. This leads to the next figure of merit, the Time Rate which indicateshow long it takes to charge the system:
TimeRate = (Charge Time)/(Discharge Time)= Electrical Rate * (Charge Power)/(Discharge Power)