Carnot Battery & PTES Lab

Model grid-scale Pumped Thermal Electricity Storage (PTES), thermodynamic heat pump charging, molten salt and packed-bed rock thermal reservoirs, expansion turbine power generation, round-trip efficiency (RTE), and 30-year LCOS pro-forma.

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Industry Presets:
Net LCOS
$0.0
per MWh discharged
Round-Trip Eff (RTE)
0.0%
AC-to-AC electrical
30-Yr Project NPV
$0.0M
@ 7.0% WACC discount
Unlevered IRR
0.0%
Equity/project return
Simple Payback
0.0 Yrs
Breakeven crossover
CO2 Abated
0.0k t/yr
Peakers displaced
Plant & Thermodynamic Controls
1. Power & Storage Sizing
100 MW
10 h
300
2. Thermal Reservoir & Cycle Physics
°C
°C
%
%
%
%/d
3. Capital Costs & Market Revenue
0%
Discounts power island & BOP by reusing existing steam turbine, generator, and switchyard.
Levelized Cost of Storage (LCOS) Breakdown
Carnot Battery Thermodynamic Closed Loop Schematic
Round-Trip Efficiency vs. Hot Reservoir Temperature
30-Year Project Cash Flow & Breakeven Payback
Technical & Financial Pro-Forma Summary
Thermodynamic & Energy Metrics
Discharge Power Rating: 100 MW
Storage Duration: 10 Hours
Storage Energy Capacity: 1,000 MWh_e (2,100 MWh_th)
Charging Input per Cycle: 1,610 MWh_e
Annual Discharged Energy: 300,000 MWh/yr
Carnot Thermodynamic Limits: 70.8% (HE) / 1.41 (HP COP)
Capital & Operational Finance
Gross Facility Capex: $137,500,000
IRA Section 48 ITC Credit: $41,250,000 (30%)
Net Initial Capex: $96,250,000
Unit Capital Cost: $96/kWh_e ($962/kW)
Annual Fixed O&M: $2,200,000
Annual Variable O&M: $1,050,000
Annual Charging Power Cost: $15,480,000
Annual Gross Operating Revenue: $58,500,000
Net Annual Cash Flow: $39,770,000/yr
5x5 Sensitivity Matrix: Off-Peak Charging vs. Peak Arbitrage Selling Price

This matrix evaluates the dual sensitivity of Net LCOS ($/MWh) and 30-Year Project Net Present Value ($M) across varying off-peak charging electricity costs and wholesale peak discharge arbitrage prices.

Off-Peak Charge \ Peak Arbitrage $120 / MWh $150 / MWh $180 / MWh $210 / MWh $240 / MWh
Engineering Guide: Carnot Battery & PTES Principles
1. The Carnot Battery Concept & Thermodynamic Decoupling

A Carnot Battery (officially codified by the International Energy Agency under IEA Annex 36) is an energy storage system that stores electricity as thermal exergy. During charging, low-cost or surplus renewable electricity powers an industrial heat pump that elevates thermal energy into an insulated hot reservoir (e.g. molten solar salt at 565°C or packed basalt gravel at 650°C), while rejecting or chilling a cold reservoir (-30°C to 20°C). During discharge, the stored thermal gradient drives a heat engine (closed Brayton turbine, Rankine steam turbine, or transcritical CO2 cycle) to generate dispatchable AC electricity.

Unlike electrochemical batteries where power (MW) and capacity (MWh) are tightly coupled inside each cell, Carnot batteries completely decouple power and energy. The power rating is governed by turbomachinery, while storage duration is scaled simply by expanding insulated steel tanks filled with cheap, earth-abundant materials ($15–$30/kWh_th). This makes PTES extraordinarily competitive for long-duration storage (10 to 24+ hours).

2. Coal-to-Carnot Repurposing & Capital Efficiency

Thousands of gigawatts of legacy coal and fossil thermal power stations are being decommissioned globally. A major commercial advantage of Carnot batteries is the ability to perform a Coal-to-Carnot retrofit. By replacing only the polluting coal-fired boiler with a high-temperature heat pump and molten salt storage island, project developers can reuse up to 60% of the existing facility assets:

  • Turbine & Synchronous Generator: Avoids purchasing a new power island.
  • Grid Interconnection Switchyard: Retains valuable transmission interconnection rights and transformers without waiting years in utility interconnection queues.
  • Cooling Towers & Water Permits: Reuses existing condenser heat sinks and water rights.
  • Energy Community Bonuses: Qualifies for up to 40% Investment Tax Credits (ITC) under the US Inflation Reduction Act (IRA Section 48).
Formative Assessment: Carnot Battery & PTES Mastery
1. Why does a Carnot battery achieve significantly lower levelized storage costs (LCOS) than lithium-ion for 16-hour duration storage?
2. What thermodynamic relationship sets the fundamental upper limit on the round-trip electrical efficiency (RTE) of a Carnot battery?
3. In a "Coal-to-Carnot" power plant conversion, which key capital components are preserved to save 40% to 60% of power island costs?
4. Which of the following is a major environmental and operational benefit of Carnot batteries compared to chemical lithium-ion systems?
Frequently Asked Questions

A Carnot battery transforms electrical power into thermal energy using a high-temperature heat pump, stores that heat and cold in insulated hot (molten salt, liquid metal, or volcanic gravel) and cold (chilled brine or ice) thermal reservoirs, and subsequently discharges electricity back to the grid using a forward thermodynamic heat engine (such as a closed Brayton, steam Rankine, or transcritical CO2 turbine).

Commercial and pilot Carnot batteries achieve electrical round-trip efficiencies between 52% and 68%. The net RTE is determined by the maximum cycle temperature difference (Carnot limit), the isentropic efficiency of the compressors and expanders (88% to 93%), heat exchanger thermal effectiveness, and auxiliary pumping loads.

PTES completely decouples power capacity (turbomachinery in $/kW) from energy capacity (storage tanks in $/kWh). Adding extra storage hours (10 to 24+ hours) only requires purchasing cheap earth-abundant storage media like molten nitrate salt ($25/kWh) or crushed basalt rock ($15/kWh), rather than buying expensive electrochemical battery cells, while operating for 30+ years with zero calendar or cyclic degradation.

Repurposing retiring coal and gas power plants allows the developer to retain the existing steam turbine, synchronous generator, high-voltage transformer, condenser, and grid interconnection switchyard. This avoids 40% to 60% of the power island capital cost and qualifies the facility for additional federal energy community tax credit bonuses.

The primary media are molten nitrate salts (Solar Salt: 60% NaNO3 / 40% KNO3, operating up to 565°C), packed beds of high-density volcanic basalt or quartzite gravel (operating up to 650°C to 1000°C with air or argon), refractory firebricks, and pressurized hot water for lower-temperature Rankine cycles.
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