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Abstract
Climate change requires cleaner, sustainable energy systems, a goal underscored by the Paris Agreement’s 2 K temperature increase limit. The industrial sector uses large amounts of energy, which requires strict focus if these climate targets are to be met. Industry was responsible for 25 % of the final energy consumption in the European Union, which is used mainly for process heating. Consequently, it is essential to explore more efficient and environmentally friendly methods of supplying usable heat and cold for industrial purposes. Substantial amounts of lowgrade excess heat remain untapped in various industrial processes. Heat pumps, which recover and upgrade energy from low temperature sources, are ideal for replacing fossil fuel based heating. However, commercially available heat pumps struggle to supply heat above 100 °C, while industries such as food, paper, and chemical production require up to 250 °C. The most efficient application of heat pumps is difficult to identify because it depends on both the heat pump design and the temperature levels at which it operates. Hydrofluoroolefins (HFOs) are notable high temperature heat pump working fluids, but their potential release of trifluoroacetic acid (TFA) is concerning. The new observations make it preferable to exclude HFOs as the working fluid if similar performance can be achieved by natural working fluids.
This thesis analysed ways to ensure highly efficient and future-proof installations of heat pumps at everincreasing temperature levels and consisted of two parts.
The first part analysed the impact of supply temperatures and the simultaneity of sink and source on heat pump performance. The levelized cost of heat (LCOH) dependedmainly on COP and electricity prices at a high number of operating hours, with minimal impact of initial investment. A 10 °C change in the delivery temperature affects LCOH by 1e/MWh to 4e/MWh due to the impact on COP. The potential gain in efficiency of employing heat pumps as combined cooling and heating systems over separate heat pumps and refrigeration cycles in industrial settings was studied showing savings of 12 % to 34 %, predominantly influenced by COP and fan performance. The feasibility required the heating demand to be between one and ten times the cooling demand. The second part evaluated a multitude of working fluid and heat pump designs. A portfolio of natural working fluids was proposed to deliver heat up to 250 °C. R718 performed well at source temperatures above 80 °C, while hydrocarbons performed well in a wide temperature range when incorporating a suction line heat exchanger. R717 and R744 showed promise in transcritical cycles with high sink temperature glides. Standards and regulations are in place for the building of safe and legal heat pumps using flammable or toxic working fluids, but the standards depend on the application and safety requirements. However, multiple working fluids showed similar COPs, making room for other considerations. An exergy analysis showed that compressor efficiency is crucial, contributing up to 45 % to total exergetic destruction. Cascade heat exchangers should be designed with low pinch temperature difference, while expanders showed potential only in transcritical cycles. Up to 40 % of total exergy destruction was attributed to the mismatch of the working fluid with the sink and the source, a more critical factor at low temperature lift and large sink and source glides. Recovering excess heat through the use of multiple evaporator pressure levels showed promise for improving the COP of heat pumps under conditions with large source temperature glide, and heat pumps with multiple sink pressure levels showed high potential at large sink temperature glides.
The thesis developed future-proof heat pumps that ensure high performance in various applications with natural working fluids. Key boundary conditions for optimal economic potential were identified, and future work recommendations were made to further leverage heat pumps to achieve a sustainable industrial sector society.
This thesis analysed ways to ensure highly efficient and future-proof installations of heat pumps at everincreasing temperature levels and consisted of two parts.
The first part analysed the impact of supply temperatures and the simultaneity of sink and source on heat pump performance. The levelized cost of heat (LCOH) dependedmainly on COP and electricity prices at a high number of operating hours, with minimal impact of initial investment. A 10 °C change in the delivery temperature affects LCOH by 1e/MWh to 4e/MWh due to the impact on COP. The potential gain in efficiency of employing heat pumps as combined cooling and heating systems over separate heat pumps and refrigeration cycles in industrial settings was studied showing savings of 12 % to 34 %, predominantly influenced by COP and fan performance. The feasibility required the heating demand to be between one and ten times the cooling demand. The second part evaluated a multitude of working fluid and heat pump designs. A portfolio of natural working fluids was proposed to deliver heat up to 250 °C. R718 performed well at source temperatures above 80 °C, while hydrocarbons performed well in a wide temperature range when incorporating a suction line heat exchanger. R717 and R744 showed promise in transcritical cycles with high sink temperature glides. Standards and regulations are in place for the building of safe and legal heat pumps using flammable or toxic working fluids, but the standards depend on the application and safety requirements. However, multiple working fluids showed similar COPs, making room for other considerations. An exergy analysis showed that compressor efficiency is crucial, contributing up to 45 % to total exergetic destruction. Cascade heat exchangers should be designed with low pinch temperature difference, while expanders showed potential only in transcritical cycles. Up to 40 % of total exergy destruction was attributed to the mismatch of the working fluid with the sink and the source, a more critical factor at low temperature lift and large sink and source glides. Recovering excess heat through the use of multiple evaporator pressure levels showed promise for improving the COP of heat pumps under conditions with large source temperature glide, and heat pumps with multiple sink pressure levels showed high potential at large sink temperature glides.
The thesis developed future-proof heat pumps that ensure high performance in various applications with natural working fluids. Key boundary conditions for optimal economic potential were identified, and future work recommendations were made to further leverage heat pumps to achieve a sustainable industrial sector society.
Original language | English |
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Place of Publication | Kgs. Lyngby |
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Publisher | Technical University of Denmark |
Number of pages | 409 |
Publication status | Published - 2024 |
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Dive into the research topics of 'High Temperature Heat Pumps: Integration, Design, and Cycle Selection for Industrial Applications'. Together they form a unique fingerprint.Projects
- 1 Finished
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Novel high-temperature heat pump solutions for industrial applications
Andersen, M. P. (PhD Student), Markussen, W. B. (Supervisor), Zühlsdorf, B. (Supervisor), Jensen, J. K. (Main Supervisor) & Stathopoulos, P. (Examiner)
01/05/2021 → 14/01/2025
Project: PhD