Table of Content
Why is a heat pump "above 120 °C" needed?
In processes such as food drying, pulp bleaching, and synthetic fiber setting, the most common heat transfer medium is saturated steam at 4–8 bar (125–170 °C). This type of steam mostly comes from gas-fired or oil-fired boilers, with an efficiency of approximately 80–85 %, and emits nearly 2 tons of CO₂ per ton of steam. With both carbon costs and gas costs rising, manufacturers urgently need solutions to simultaneously reduce energy costs and carbon emissions.
Traditional heat pumps can only provide hot water at ≤ 90 °C and cannot directly replace high-temperature steam. It wasn't until the last decade, with the commercialization of new refrigerants and two-stage compression technology, that high-temperature heat pumps (HTHPs) emerged, capable of providing stable heating temperatures of 150–165 °C. Therefore, the emergence of HTHPs is not simply about "making heat pumps hotter," but rather a result of energy policies, carbon tax pressures, and the reuse of process waste heat. It unlocks the low-to-medium grade waste heat that was previously discarded into cooling towers or chimneys within factories, boosting this "free heat source" to a usable temperature, creating a positive cycle that saves money and reduces carbon emissions.
Basic physics and sources of efficiency of heat pumps
Carnot cycle and COP
A heat pump is essentially a reverse-cycle refrigeration system that "transfers heat from the cold end to the hot end".
Its theoretically optimal efficiency is determined by the Carnot cycle:
COPCarnot=Thot / (Thot-Tcold)
- ThotTcold Measured in absolute temperature (K).
- The smaller the difference (ΔT) between the two, the smaller the denominator, and the higher the theoretical COP.
- Real-world unit COP ≈ 0.45–0.55 × COPCarnotDue to irreversible losses caused by compressors, throttling, oil separation, heat exchange, etc.
Why can high-temperature heat pumps save more energy than boilers?
Boilers are primary energy conversion systems that convert the chemical energy of fossil fuels into heat "in one go," achieving a thermal efficiency of only about 85 %.
Heat pumps, on the other hand, "transfer heat": electricity only drives the compressor to heat low-grade waste heat. If COP = 2.5, it means that for every 1 kWh of electricity consumed, 2.5 kWh of heat can be output, with an equivalent thermal efficiency of 250 %. Even considering the efficiency of the power generation end, high-temperature heat pumps still have an energy advantage of 1.8–2 times.
Evolution and Bottleneck Breakthrough of High-Temperature Heat Pump Technology
| Development stage | Refrigerant Representative | upper limit of heating | Key Challenges | Latest solution |
|---|---|---|---|---|
| Before 2010 | R134a, ammonia | 90 °C | Low critical temperature of refrigerant | Hot water is available only. |
| 2015 | HFO-1234ze, R245fa | 115 °C | Increased compression ratio | Dual compression enabled |
| 2020 | HCFO-1224yd(Z) HFO-1336mzz(E/Z) | 150 °C | Refrigerant stability, Lubricant temperature resistance | High-temperature lubricating oil and oil separator upgrade |
| 2024+ | Supercritical CO₂ | 180 °C (Experiment) | System pressure > 10 MPa | High-strength steel/electronic expansion valve |
The two-stage compression/inter-jet architecture can split the total compression ratio into two stages, significantly reducing the exhaust temperature and mechanical load of a single stage; in conjunction with an intercooler, it can cool the mid-stage refrigerant and improve the overall COP.
ΔT–COP Relationship and Simplified Estimation
Quickly extract COP from images
Assuming the waste heat source is 60 °C, and saturated hot water at 140 °C needs to be supplied, ΔT = 80 K.
The measured table shows:
- ΔT = 60 K → COP ≈ 3.1
- ΔT = 80 K → COP ≈ 2.6
- ΔT = 110 K → COP ≈ 2.2
Rule of thumb: For every 10 K increase, COP decreases by approximately 0.4–0.5. Therefore, if your process only requires 135 °C, there's no need to pursue 150 °C—reducing COP by 15 K can save an additional 0.6 COP, immediately shortening the payback period.
Trial calculation: Food drying line
- Evaporator end: Waste heat exhaust 65 °C (3 t/h)
- Demand side: Hot air 140 °C (100 kW)
- ΔT = 75 K, COP ≈ 2.7 (from table)
- With 5,000 hours of operation per year, the power consumption is 100 kW / 2.7 = 37 kW → 185 MWh per year.
- Switching to a gas-fired boiler (efficiency 85 %) requires 100 kW / 0.85 ≈ 118 kW of gas → annual gas consumption of 590 MWh (equivalent to 580,000 Nm³). The cost of solar energy alone can be reduced by 35–40 %.
Energy/Carbon Emissions Calculation: Heat Pumps vs. Steam Boilers
| index | Current status: Gas-fired boiler | HTHP Case Study | Saves (%) |
|---|---|---|---|
| Annual primary energy | 700,000 Nm³ of natural gas | 1.3 GWh of electricity | — |
| Energy costs* | NT$ 20.6 million | NT$ 11.7 million | ▼ 43% |
| CO₂ emissions | 1400T | 650T | ▼ 54% |
| Annual carbon cost (300 yuan/ton) | NT$ 4.2 million | NT$ 1.95 million | ▼ 54% |
| Maintenance fee | 中 | 低 | — |
| Investment recovery period | — | 3.8 years | — |
In 2025, industrial electricity will cost 2.7 yuan/kWh, natural gas 11 yuan/Nm³, and carbon cost is assumed to be 300 yuan/t CO₂.
With the combined advantages of energy saving and carbon reduction, the investment payback period is typically less than 4 years; if local governments provide energy-saving subsidies, this period can be shortened by another 0.5–1 year.
Three major industry cases
Food Baking
- Background: The cookie baking room originally used 130 °C steam hot air.
- Import: 120 °C heat pump hot water + plate heat exchanger.
- Results: Annual energy consumption reduced by 35 %, baking temperature stability ±1 °C, and product moisture content error reduced by 40 %.
Pulp bleaching
- Background: Black liquor (95 °C) was lost to the cooling tower before bleaching.
- Import: 95 → 155 °C dual-stage heat pump.
- Results: Black liquor recovery was 80 %, cooking tower capacity increased by 8 %, and NaOH consumption decreased by 5 %.
Chemical fiber shaping
- Background: 170 °C saturated steam + heat exchanger heating, the heat exchanger is prone to scale buildup and corrosion.
- Import: 160 °C hot air heat pump, air is blown directly into the shaping chamber.
- Results: Thermal efficiency increased by 28 %, heat exchanger life increased from 2 to 5 years, and downtime for maintenance was significantly reduced due to the absence of condensate corrosion.
From Today's Pain Points to Long-Term Competitiveness
While high-temperature steam boilers are economically mature, they struggle to meet net-zero emissions and carbon cost pressures. The value of high-temperature heat pumps lies in "upgrading previously discarded waste heat," leveraging 2-3 units of usable heat with 1 unit of electricity. A detailed evaluation is warranted if the process heat demand exceeds 4000 h/year and there is ≥ 50 °C of waste heat on-site.
Next step:
- Data collection: Measure waste heat temperature/flow rate and annual operating hours.
- Estimate COP: Use the ΔT–COP experience table or vendor software.
- Calculate ROI by including electricity price, fuel price, and carbon cost.
Master these three steps, and you can quickly determine whether a high-temperature heat pump is suitable for your production line, and even lock in the carbon reduction benefits for your company before 2030!
