ORC Low-Temperature Waste Heat Power Generation: Turning Waste Heat into Electricity
For low-temperature waste heat with flue gas temperatures of 150–300°C, the conventional steam Rankine cycle is difficult to utilize efficiently due to water temperature limitations. The Organic Rankine Cycle (ORC) uses a low-boiling-point organic working fluid instead of water, enabling efficient power generation even at lower temperatures, and provides a "turning waste into treasure" technological pathway for large amounts of low-grade industrial waste heat.
I. ORC vs. Conventional Steam Cycle
| Comparison Item | Steam Rankine Cycle | Organic Rankine Cycle (ORC) |
|---|---|---|
| Applicable heat source temperature | >300°C | 80~350°C |
| Working fluid | Water | R245fa/R1233zd/pentane, etc. |
| System pressure | High pressure (>3 MPa) | Medium-low pressure (0.5~2.5 MPa) |
| Expander type | Steam turbine | Screw/turbine expander |
| System complexity | Complex (multi-stage extraction regeneration) | Relatively simple (mainly single-stage) |
| Power generation efficiency (150°C heat source) | Cannot operate | 6%~10% |
| Power generation efficiency (250°C heat source) | approx. 15%~18% | 11%~15% |
II. ORC Working-Fluid Selection Principles
The working fluid is the core of ORC technology. Selecting a suitable working fluid requires comprehensive consideration of:
- Thermodynamic performance: High critical temperature and suitable boiling point, enabling the cycle to operate efficiently at the heat source temperature
- Safety: Non-toxic or low-toxicity, low flammability (GWP value as low as possible)
- Environmental friendliness: Zero ODP (ozone depletion potential), low GWP (global warming potential)
Comparison of commonly used working fluids:
| Working Fluid | Critical Temp. (°C) | Applicable Heat Source (°C) | GWP | Characteristics |
|---|---|---|---|---|
| R245fa | 154 | 100~180 | 1030 | Most mature, but with high GWP |
| R1233zd(E) | 165 | 110~200 | 1 | New-generation environmentally friendly working fluid |
| Cyclopentane | 239 | 150~280 | 0 | Natural working fluid, flammable |
| R1336mzz(Z) | 171 | 120~210 | 9 | Non-flammable, new-generation choice |
III. ORC System Economic Analysis
Taking a 1 MW-class ORC power generation unit as an example:
- Equipment investment: approx. 8–12 million RMB (including flue gas heat exchanger + ORC main unit + cooling system + grid connection)
- Heat source: flue gas at 250°C, flow rate 40,000 Nm³/h, recovering approx. 2.5 MW of heat
- Power generation capacity: 2.5 MW × 12% ≈ 300 kW
- Annual power generation: 300 kW × 8,000 h = 2.4 million kWh
- Annual electricity revenue: 2.4 million kWh × 0.7 RMB/kWh = 1.68 million RMB
- Payback period: 8–12 million ÷ 1.68 million ≈ 4.8–7.1 years
Note: If national energy-saving subsidies (approx. 15%–20% of investment) are available, the payback period can be shortened to 4–5 years. The designed equipment service life is over 20 years.