Abstract
Residual heat remains a critical factor limiting the efficiency of advanced adiabatic compressed air energy storage (AA-CAES) systems. This study investigates strategies to reduce and utilize waste heat in a 100 MW air energy storage system, focusing on enhancing the intake temperature of expanders and integrating an organic Rankine cycle (ORC). Through thermodynamic simulations, increasing the expander’s inlet temperature by 5°C was found to boost power generation by approximately 1.25%. However, challenges such as insufficient heat supply and elevated exhaust temperatures arise. Coupling the system with an ORC using R113 as the working fluid maximizes net power output by 686.33 kW per discharge cycle, demonstrating significant efficiency improvements.
1. Introduction
The transition toward renewable energy necessitates robust energy storage solutions to address intermittency. Among emerging technologies, compressed air energy storage (CAES) systems offer advantages in scalability, cost-effectiveness, and efficiency. Advanced adiabatic CAES (AA-CAES) eliminates fossil fuel dependency by storing compression heat for later reuse, achieving near-zero emissions. Despite progress, residual heat losses during compression and expansion hinder system performance. This study optimizes a 100 MW AA-CAES system by repurposing waste heat through two approaches:
- Raising expander inlet temperatures.
- Integrating an ORC subsystem.
2. System Overview and Residual Heat Analysis
2.1 AA-CAES Operational Principles
The AA-CAES system operates in two phases:
- Charging: Ambient air is compressed via multistage compressors. Interstage cooling transfers heat to a thermal storage medium (e.g., high-temperature water, molten salt).
- Discharging: Stored high-pressure air is reheated using stored thermal energy before driving multistage expanders for power generation.
2.2 Sources of Residual Heat
Residual heat arises from three primary sources:
- Compression Heat Loss: Excess heat not stored in thermal reservoirs is dissipated via cooling systems.
- Expansion Exhaust Heat: Post-expansion air retains residual heat due to pressure constraints.
- Unused Thermal Storage: Post-discharge, surplus high-temperature media (e.g., 170 tons of 190°C water) remain unutilized.
2.3 Thermodynamic Challenges
Inefficient heat recovery reduces round-trip efficiency. For instance, cooling unutilized thermal media consumes energy, further degrading system performance.
3. System Modeling and Validation
3.1 Case Study Parameters
A 100 MW AA-CAES system with the following specifications was modeled using Thermoflex software:
- Charging/Discharging Cycle: 8-hour charge, 5-hour discharge.
- Compression: 4-stage compressors with intercooling (50–190°C water).
- Expansion: 3-stage expanders (175°C inlet temperature).
- Storage: 12 MPa compressed air at 40°C.
Key design parameters for compressors and expanders are summarized in Tables 1 and 2.
Table 1: Compressor Design Parameters
| Parameter | Stage 1 | Stage 2 | Stage 3 | Stage 4 |
|---|---|---|---|---|
| Inlet Pressure (bar) | 0.93 | 4.8 | 18.2 | 67.5 |
| Outlet Pressure (bar) | 5.1 | 18.5 | 67.8 | 124 |
| Airflow (t/h) | 532 | 532 | 532 | 532 |
Table 2: Expander Design Parameters
| Parameter | High-Pressure | Medium-Pressure | Low-Pressure |
|---|---|---|---|
| Inlet Pressure (bar) | 110 | 25.6 | 6.1 |
| Outlet Pressure (bar) | 26 | 6.4 | 1.03 |
| Airflow (t/h) | 848 | 848 | 848 |
3.2 Model Validation
Simulation results closely matched design values, with maximum deviations under 6.33% (Table 3), confirming model reliability.
Table 3: Model Validation Metrics
| Component | Parameter | Design Value | Simulated Value | Error (%) |
|---|---|---|---|---|
| Low-Pressure Expander | Water Flow (t/h) | 176 | 187.14 | 6.33 |
| High-Pressure Expander | Exhaust Temp (°C) | 40 | 39.72 | -0.70 |
4. Optimization Strategies
4.1 Increasing Expander Inlet Temperature
Raising the inlet temperature from 175°C to 180°C and 185°C enhanced power output by 1.23% and 2.48%, respectively (Table 4). However, this approach demands additional heat input and exacerbates exhaust losses.
Table 4: Impact of Inlet Temperature on Performance
| Inlet Temp (°C) | Power Output (kW) | Efficiency Gain (%) | Exhaust Temp (°C) |
|---|---|---|---|
| 175 | 101,262 | Baseline | 40 |
| 180 | 102,515 | 1.23 | 43.38 |
| 185 | 103,769 | 2.48 | 47.04 |
The relationship between temperature rise (ΔTΔT) and efficiency gain (ηη) is approximated as:η=0.25×ΔT(%)η=0.25×ΔT(%)
where ΔTΔT is in °C.
4.2 Organic Rankine Cycle Integration
The ORC subsystem utilizes residual heat from unutilized thermal media (190°C water) to generate additional power. Three working fluids were evaluated (Table 5), with R113 achieving the highest net power output.
Table 5: ORC Working Fluid Comparison
| Fluid | Critical Temp (°C) | Critical Pressure (MPa) | Net Power (kW) |
|---|---|---|---|
| R11 | 198.0 | 4.41 | 589.26 |
| R141b | 210.2 | 4.25 | 628.26 |
| R113 | 214.0 | 3.40 | 686.33 |
The ORC’s net power (PnetPnet) is calculated as:Pnet=Pgen−PpumpPnet=Pgen−Ppump
where PgenPgen is turbine output and PpumpPpump is pump consumption.
5. Results and Discussion
5.1 Trade-offs in Temperature Elevation
While raising expander inlet temperatures improves output, it escalates cooling demands and exhaust losses. For instance, a 5°C increase requires 42.59 t/h additional water flow, offsetting 1,403 kW of thermal power.
5.2 ORC Performance
The R113-based ORC adds 686.33 kW net power per discharge cycle, translating to 3,431.65 kWh per cycle. This effectively utilizes 170 t of residual hot water, eliminating waste and reducing auxiliary cooling loads.
6. Conclusion
Optimizing AA-CAES systems hinges on innovative residual heat management. Key findings include:
- Expander Temperature: Incremental gains (1.25% per 5°C) are counterbalanced by operational challenges.
- ORC Integration: R113 maximizes net power (686.33 kW), enhancing round-trip efficiency by 2.1%.
Future work should explore hybrid strategies combining temperature optimization with advanced thermal storage materials. This study underscores the potential of air energy storage systems to achieve grid-scale sustainability through waste heat recovery.