Liquid Salt Combined Cycle
Liquid Salt Combined Cycle
Pintail Power’s patented Liquid Salt Combined Cycle™ (LSCC) technology transforms existing thermal generation assets into a renewables storage solution. LSCC technology provides cost-effective bulk energy storage in a compact footprint to provide low-carbon dispatchable power for utility grids, microgrids, islands, and facilities. LSCC integrates proven utility equipment. Thermal energy storage is integrated with combustion and steam turbines in a novel way to deliver hybrid synergies:
- Fuel efficiency for low-carbon power,
- Operational flexibility,
- Low capital cost with long lifetime,
- Low environmental impact by using a non-flammable, non-toxic, and environmentally benign storage medium.
What is it?
The Liquid Salt Combined Cycle adds thermal energy storage and re-arranges the typical combined cycle in these significant ways:
- We add an electrically heated thermal energy storage system, shown in orange in the diagram below, that uses a liquid salt as a storage and heat transfer medium.
- We move the steam evaporator outside the exhaust gas path, using stored energy to boil water, and replace the evaporator with addition superheater and economizer tubes.
This novel arrangement removes heat transfer constraints to increase steam flow 250%, to boost the power from the steam cycle. The stored energy provides operating flexibility beyond absorbing over-generation and providing long discharge duration.
How is it different?
Unlike a conventional combined cycle, the stored energy permits the evaporator to operate independently of the combustion turbine. This enables fast startup by pre-heating the steam piping, establishing condenser vacuum, and spinning the steam turbine so it is ready be rapidly loaded in coordination with the combustion turbine.
Conventional practice to boost power (supplemental firing using a duct burner) worsens the fuel heat rate and increases emissions. In contrast, LSCCs’ has an exceptionally low fuel Heat Rate, less than 5000 Btu/kWh LHV, thanks to the large increase in steam flow, which is due to the synergistic use of stored energy for evaporation and exhaust energy for sensible heating.
To be sure, the LSCC displaces fuel by using stored electricity, which is often in excess and is low cost. This in turn reduces the marginal cost of energy.
How does it work?
The LSCC charges from the grid when electricity prices are low due to renewable over-generation, and discharges when power is needed.
Charging. Liquid salt stored in the cold tank at about 250°C (482°F) is pumped through an electric heater which heats it to 427°C (800°F) for storage in the hot tank. At this temperature, low-cost carbon steel is suitable for vessels and piping. Medium voltage heaters, such as from Chromalox, are economical and efficient, respond quickly to variations in renewable generation, and are infinitely controllable.
Storage. The salt is held in insulated storage tanks, with heat losses typically less than 1°C per day. This type of tank has been used for decades by the Concentrating Solar Power (CSP) industry in the U.S., Spain, and North Africa to provide thousands of Megawatt-hours of energy storage.
LSCC typically uses a low freezing point (142°C/288°F) eutectic salt such as Hitec® salt from Coastal Chemical, to provide months of standby without risk of freezing in the tanks. This also reduces the need to operate heat tracing on piping and valves.
Discharging. Hot liquid salt is pumped from the Hot Tank, through a Steam Generator, and then to the Cold Tank. On the steam side, feedwater is pre-heated by exhaust gas, evaporated in the Steam Generator using stored energy, and then superheated by exhaust gas. Superheated steam produces power in the steam turbine generator, condenses and is recycled as feedwater. In parallel, a combustion turbine produces power and exhaust gas, which is used as described above.
|Combustion Turbine||Simple Cycle||Liquid Salt Combined Cycle™|
|Manufacturer & Model||Gross Power (MW)||Fuel Heat Rate (Btu/kWh)||Power (MW)||Fuel Heat Rate|
Power Heat Rate
|Siemens Trent WLE||66.0||8150||113.5||4,740||0.93||72%||42%|
|GE LM6000 SPRINT||50.0||8,392||91.6||4,642||0.95||83%||45%|
|BH Nova LT16||16.2||9,456||33.0||4,647||0.91||104%||51%|
|Solar Taurus 65||6.5||10,375||13.3||4,880||0.96||104%||53%|
LSCC indicative performance is for single-pressure non-reheat, steam cycle with air cooled condenser, net of plant loads; ISO ambient, 60 Hz, natural gas fuel at Lower Heating Value.
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).