Thermo-Economic Potential of Carnot Batteries for the Waste Heat Recovery of Liquid-Cooled Data Centers with Different Combinations of Heat Pumps and Organic Rankine Cycles
Energies (MDPI AG) · Xiaoyu Zhou, Xinxing Lin et al.
The paper proposes a Carnot battery for liquid-cooled data centres: a heat pump upgrades and stores data-centre waste heat (charging), and an organic Rankine cycle (ORC) later turns it back into electricity (discharging).
CC BY 4.0 — share with attribution
On this page
Overview
To fully recover the abundant waste heat of liquid-cooled data centres and reduce operating cost, a Carnot battery consisting of a heat pump (HP) and ORC is proposed. Because HPs and ORCs have different cycle states, four subcritical and transcritical HP/ORC combinations are compared, working fluids are screened, and thermo-economic models are built for round-trip efficiency (RTE), exergy efficiency and specific cost. Design conditions assume liquid-cooled data-centre coolant entering the HP evaporator at 70 °C and leaving at 60 °C (0.1 MPa), with an 8 h charging time and 30 °C ambient.
R134a and R290 were the optimum fluids for subcritical and transcritical heat pumps, and R245fa and R143a for subcritical and transcritical ORCs. Under design conditions, the subcritical HP/transcritical ORC system achieved the highest RTE of 76% and exergy efficiency of 39%, and its RTE can exceed 100% at a 14 °C Condenser2 outlet temperature. System cost is about 767-796 USD/kWh depending on the cycle combination. HP evaporating temperature has the greatest effect on RTE (mean 1.8%/°C), and exergy efficiency rises 0.9% per 1 °C of ORC evaporating temperature.
After optimisation (Condenser2 outlet 30 °C), the subcritical HP/subcritical ORC reached 83% RTE, but cost rose to 1125 USD/kWh, an increase of 328 USD/kWh. The authors cite PFAS concerns over fluorinated coolants such as Fluorinert as a deployment challenge, along with space constraints and charge/discharge control.
Limits: a simulation-based thermo-economic model; RTE can exceed 100% when the ORC condenses cold, but specific cost is steep. Design conditions assume 70 °C coolant outlet, so the concept is specific to liquid-cooled data centres and does not apply to air-cooled halls.
Key points
- Design conditions: liquid-cooled data-centre coolant entering the HP evaporator at 70 °C and leaving at 60 °C (0.1 MPa), with an 8 h charging time and 30 °C ambient.
- R134a and R290 were the optimum fluids for subcritical and transcritical heat pumps, and R245fa and R143a for subcritical and transcritical ORCs.
- Under design conditions, the subcritical HP/transcritical ORC system achieved the highest RTE of 76% and exergy efficiency of 39%, and its RTE can exceed 100% at a 14 °C Condenser2 outlet temperature.
- System cost is about 767-796 USD/kWh depending on the cycle combination.
- HP evaporating temperature has the greatest effect on RTE (mean 1.8%/°C); exergy efficiency rises 0.9% per 1 °C of ORC evaporating temperature.
- After optimisation (Condenser2 outlet 30 °C), the subcritical HP/subcritical ORC reached 83% RTE, but cost rose to 1125 USD/kWh, an increase of 328 USD/kWh.
- The authors cite PFAS concerns over fluorinated coolants such as Fluorinert as a deployment challenge, along with space constraints and charge/discharge control.
Using it in design
- When evaluating turning liquid-cooled data-centre waste heat into electricity, weigh RTE (76-83%) against specific cost (767-1125 USD/kWh); higher RTE can come with a cost increase.
- Make the 60-70 °C coolant outlet requirement and the finding that HP evaporating temperature affects RTE most (1.8%/°C) design inputs.
- Add PFAS risk for fluorinated coolants, space and charge/discharge control to the feasibility study.
Monthly newsletter · free
Keep your reading list current
Once a month: new library sources, changes to Turkish regulations and upcoming data centre events.
SoSo CAD
Compare the cost of design decisions
Evaluate equipment costs, energy expenses and total cost of ownership (TCO) using your inputs and assumptions in SoSo CAD.
Request a demo
Who should read it
Energy engineers and researchers evaluating waste-heat-to-electricity and thermal storage options for liquid-cooled data centres.
Document contents
- 1. Introduction (1.1 Background, 1.2 Waste Heat Recovery of DCs, 1.3 Energy Storage, 1.4 Purpose of This Study)
- 2. System Description
- 3. Model Development (3.1 Energy Model, 3.2 Exergy Model, 3.3 Economic Model, 3.4 Performance Evaluation Indexes)
- 4. Working Conditions and Working Fluids (4.1 Working Conditions, 4.2 Selection of Working Fluids)
- 5. System Optimization
- 6. Results and Discussion (6.1 Design Performance for Different Working Fluid Combinations)
- 7. Conclusions
Read the document
carnot-battery-liquid-cooled-dc-heat-2025.pdf
CC BY 4.0 — share with attribution
The document is provided unaltered because its licence permits redistribution.
Publication details
- Authors
- Xiaoyu Zhou; Xinxing Lin; Wen Su; Ruochen Ding; Yaran Liang
- Publisher
- Energies (MDPI AG)
- Year
- 2025
- Volume
- 18
- Issue
- 6
- Pages
- 1556
- Document language
- English
- Source type
- Scientific
- Access
- Open licence — downloadable
- Licence
- CC BY 4.0 — share with attribution
- Licence evidence
- www.mdpi.com
Original licence note
PDF p.1 states the article is open access under the Creative Commons Attribution (CC BY 4.0) licence; MDPI publishes all articles under CC BY 4.0.
Citation
Xiaoyu Zhou; Xinxing Lin; Wen Su; Ruochen Ding; Yaran Liang (2025). Thermo-Economic Potential of Carnot Batteries for the Waste Heat Recovery of Liquid-Cooled Data Centers with Different Combinations of Heat Pumps and Organic Rankine Cycles. Energies (MDPI AG). 18(6), 1556. https://doi.org/10.3390/en18061556
Events on this subject
- Data Centre World Frankfurt
- SC26 – International Conference for High Performance Computing, Networking, Storage and Analysis
- iMasons Climate Accord Summit
All content and rights belong to the respective authors and publishers. SoSo CAD provides only the citation, its own summary and a link to the source, and is not responsible for the sources' content.
Related sources
All sources on this topic
Energies (MDPI AG) 2021
Techno-Economic Analysis of Waste Heat Utilization in Data Centers: Application of Absorption Chiller Systems
Evaluate an absorption chiller driven by roughly 70 °C water from liquid-cooled racks to cut compression chiller load; calculate the hybrid saving from the two COPs with the closed-form formula.
- These savings correspond to annual reductions of 3068 and 9208 tons CO2-eq respectively.
National Academy of Engineering, The Bridge vol. 56 no. 1 2026
Avoiding Waste Heat through AI Infrastructure Thermal Integration
In the heat-reuse calculator, take coolant return temperature as the key input.
- About 70-80% of data centre heat could be recovered.
National Laboratory of the Rockies (formerly NREL) 2026
High-Performance Computing Data Center Heat Capture and Reuse (ESIF)
In the heat-reuse module, support on-site end uses (low-temperature terminals, snow melt).
Nature (Springer Science and Business Media LLC) 2025
Using life cycle assessment to drive innovation for sustainable cool clouds
Add a cooling-technology comparison calculator that computes annual energy as IT MW x load factor x PUE x 8760 and applies the paper's Table 3 deltas (PUE -3%/-6%, IT capacity +3%/+6%) and Table 4 savings ranges as tagged defaults.
- Switching to 100% renewable electricity cut GHG by 85-90%, energy demand by 6-7% and blue water by 55-85%; server embodied carbon became more significant.
Leibniz Supercomputing Centre 2025
HPC and AI in Supercomputing Centres: Green Today, Greener Tomorrow (SCA25 Workshop on Energy Efficiency, LRZ)
Let liquid-cooled design templates allow facility supply temperatures up to 45 C with dry-cooler-only heat rejection, and show the resulting PUE.
- Systems span from SuperMUC (2012) to SuperMUC-NG Phase 2.
Open Compute Project Foundation 2025
System Integration of a Computing Cluster, a Heat Recovery System and an Adsorption Chiller for Cold Production (OCP EMEA Summit 2025, Future Technologies Symposium)
In heat-reuse modelling, allow an adsorption-chiller sink with a minimum drive temperature (around 65 C).
- Related standard: ISO/IEC 30134-6 (ERF).