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The performance of lithium-ion batteries is closely related to temperature, and much attention has been paid to their thermal safety. With the increasing application of the lithium-ion battery, higher requirements are put forward for battery thermal management systems. Compared with other cooling methods, liquid cooling is an efficient cooling method, which can control the maximum temperature and maximum temperature difference of the battery within an acceptable range. This article reviews the latest research in liquid cooling battery thermal management systems from the perspective of indirect and direct liquid cooling. Firstly, different coolants are compared. The indirect liquid cooling part analyzes the advantages and disadvantages of different liquid channels and system structures. Direct cooling summarizes the different systems' differences in cooling effectiveness and energy consumption. Then, the combination of liquid cooling, air cooling, phase change materials, and heat pipes is examined. Later, the connection between the cooling and heating functions in the liquid thermal management system is considered. In addition, from a safety perspective, it is found that liquid cooling can effectively manage thermal runaway. Finally, some problems are put forward, and a summary and outlook are given. [ABSTRACT FROM AUTHOR]

Electric vehicles (EVs) offer a potential solution to face the global energy crisis and climate change issues in the transportation sector. Currently, lithium-ion (Li-ion) batteries have gained popularity as a source of energy in EVs, owing to several benefits including higher power density. To compete with internal combustion (IC) engine vehicles, the capacity of Li-ion batteries is continuously increasing to improve the efficiency and reliability of EVs. The performance characteristics and safe operations of Li-ion batteries depend on their operating temperature which demands the effective thermal management of Li-ion batteries. The commercially employed cooling strategies have several obstructions to enable the desired thermal management of high-power density batteries with allowable maximum temperature and symmetrical temperature distribution. The efforts are striving in the direction of searching for advanced cooling strategies which could eliminate the limitations of current cooling strategies and be employed in next-generation battery thermal management systems. The present review summarizes numerous research studies that explore advanced cooling strategies for battery thermal management in EVs. Research studies on phase change material cooling and direct liquid cooling for battery thermal management are comprehensively reviewed over the time period of 2018–2023. This review discusses the various experimental and numerical works executed to date on battery thermal management based on the aforementioned cooling strategies. Considering the practical feasibility and drawbacks of phase change material cooling, the focus of the present review is tilted toward the explanation of current research works on direct liquid cooling as an emerging battery thermal management technique. Direct liquid cooling has the potential to achieve the desired battery performance under normal as well as extreme operating conditions. However, extensive research still needs to be executed to commercialize direct liquid cooling as an advanced battery thermal management technique in EVs. The present review would be referred to as one that gives concrete direction in the search for a suitable advanced cooling strategy for battery thermal management in the next generation of EVs. [ABSTRACT FROM AUTHOR]

As data centers increasingly become the backbone of the digital age, managing their substantial energy consumption and mitigating heat generation are paramount. This paper focuses on direct liquid cooling as a transformative technology for enhancing energy efficiency and operational safety in high-density computing environments. Significant advancements and persisting challenges in this field have been identified by analyzing various direct liquid cooling methodologies including immersion, spray/jet, and microchannel cooling. Comparative analysis of these systems reveals their potential to substantially lower thermal resistances and improve energy utilization. Despite these advancements, the research highlights persistent challenges such as integration complexities and scalability issues. Recommendations for future research directions to improve the efficiency and ability of direct liquid cooling applications in data centers were concluded, emphasizing the need for user-friendly and cost-effective solutions, comprehensive guidelines for cooling method selection, and hybrid cooling systems. It also recommends advanced energy management strategies such as real-time power adjustment that dynamically matches energy supply with computational demand to optimize efficiency. These contributions underscore the importance of advancing data center cooling technologies to meet future demands. • A comprehensive review of data center direct liquid cooling advances. • The ranges of heat flux, PUE, and cost are compared for different technologies. • Comprehensive guidelines for cooling method selection are presented. • The main limitation of direct liquid cooling systems lies in complex integration. • Coupling direct liquid cooling with tactics like waste heat recovery is promising. [ABSTRACT FROM AUTHOR]

The escalating demand for computational power and data storage has led to a substantial increase in the number and scale of data centers (DCs) worldwide. Due to characteristics of high heat flux and reliable operational security, there is an urgent need for innovative solutions to solve the problem of DC cooling and waste heat recovery. Direct liquid cooling technology is one of the most promising energy-saving cooling technologies due to its advantages of high cooling efficiency, low noise, and reduction of hot spots. Waste heat recovery is also one of the effective ways to improve energy efficiency and reduce carbon emissions due to higher coolant temperature. This study provides a comprehensive review of direct liquid cooling and waste heat recovery technologies for DCs that have been published in high level journals in recent years. Herein, the primary innovation and contribution of this review paper is its comprehensive coverage of advanced/emerging cooling technologies for DCs including immersion cooling and spray cooling, and its broad overview of global trends in performance, energy consumption, power density, and PUE of related systems. In addition, this review addresses the issue of heat recovery and reuse in DCs with an overview of common waste heat recovery technologies such as heating, district heating networks, cooling supply and organic Rankine cycle (ORC) are summarized and then compared and analyzed their application scenarios, advantages and disadvantages. Through the comprehensive review of cutting-edge technologies and methodologies for direct liquid cooling and waste heat recovery, this review aims to provide a thorough understanding of current landscape and future direction of sustainable and high-performance DC solutions. [Display omitted] • The systematic research and analysis of direct cooling and waste heat recovery are elaborated. • Simulation, and experimental studies of immersion cooling and spray cooling are summarized. • Comparison of energy saving potential and PUE of DC direct liquid cooling are presented. • Regard various applications to conduct a comprehensive review of waste heat recovery in DCs. • The existing challenges and future work opportunities are discussed. [ABSTRACT FROM AUTHOR]

The performance of lithium-ion batteries is closely related to temperature, and much attention has been paid to their thermal safety. With the increasing application of the lithium-ion battery, higher requirements are put forward for battery thermal management systems. Compared with other cooling methods, liquid cooling is an efficient cooling method, which can control the maximum temperature and maximum temperature difference of the battery within an acceptable range. This article reviews the latest research in liquid cooling battery thermal management systems from the perspective of indirect and direct liquid cooling. Firstly, different coolants are compared. The indirect liquid cooling part analyzes the advantages and disadvantages of different liquid channels and system structures. Direct cooling summarizes the different systems’ differences in cooling effectiveness and energy consumption. Then, the combination of liquid cooling, air cooling, phase change materials, and heat pipes is examined. Later, the connection between the cooling and heating functions in the liquid thermal management system is considered. In addition, from a safety perspective, it is found that liquid cooling can effectively manage thermal runaway. Finally, some problems are put forward, and a summary and outlook are given.

Electric vehicles (EVs) offer a potential solution to face the global energy crisis and climate change issues in the transportation sector. Currently, lithium-ion (Li-ion) batteries have gained popularity as a source of energy in EVs, owing to several benefits including higher power density. To compete with internal combustion (IC) engine vehicles, the capacity of Li-ion batteries is continuously increasing to improve the efficiency and reliability of EVs. The performance characteristics and safe operations of Li-ion batteries depend on their operating temperature which demands the effective thermal management of Li-ion batteries. The commercially employed cooling strategies have several obstructions to enable the desired thermal management of high-power density batteries with allowable maximum temperature and symmetrical temperature distribution. The efforts are striving in the direction of searching for advanced cooling strategies which could eliminate the limitations of current cooling strategies and be employed in next-generation battery thermal management systems. The present review summarizes numerous research studies that explore advanced cooling strategies for battery thermal management in EVs. Research studies on phase change material cooling and direct liquid cooling for battery thermal management are comprehensively reviewed over the time period of 2018–2023. This review discusses the various experimental and numerical works executed to date on battery thermal management based on the aforementioned cooling strategies. Considering the practical feasibility and drawbacks of phase change material cooling, the focus of the present review is tilted toward the explanation of current research works on direct liquid cooling as an emerging battery thermal management technique. Direct liquid cooling has the potential to achieve the desired battery performance under normal as well as extreme operating conditions. However, extensive research still needs to be executed to commercialize direct liquid cooling as an advanced battery thermal management technique in EVs. The present review would be referred to as one that gives concrete direction in the search for a suitable advanced cooling strategy for battery thermal management in the next generation of EVs.

An integrated model is constructed for a Li‐ion battery module composed of cylindrical cells by coupling individual first‐order equivalent circuit models (ECMs) with a 3D heat transfer model, also considering the fluid flow dynamics of the applied cooling liquid, and bench‐marked against experimental data. This model simulates a representative unit of the battery module with direct liquid cooling in a parallel configuration. Instead of assigning specific values to the featured parameters involved in the ECMs, they are here defined as 4D arrays. This makes it possible to simultaneously consider the effect of the state of charge, current rate, and temperature on the battery dynamics, making the model more adaptive, versatile, and connectable to the battery cell electrochemistry. According to the simulation results, the model employing state‐dependent battery properties fits better with the experimental cooling results. Additionally, the temperature uniformity of the module with a parallel cooling configuration is improved compared to a serial configuration. However, the increase of the absolute core temperature cannot be directly controlled by the surface cooling due to the slow heat transport rate across the battery material. The simulations also provide directions for the modification of module design, to the potential benefit of battery pack developers. [ABSTRACT FROM AUTHOR]

In this work, a numerical optimisation process is applied to improve the fluid dynamical aspect of an innovative direct liquid cooling strategy for lithium-ion–based HEV/EV. First, the thermofluidic numerical model of the battery cell defined by means of CFD computational tools was validated with experimental tests. Then, a comparison between different flow patterns was developed to analyse the influence of the fluid distribution geometry. Finally, a parametric multi-objective optimisation process was implemented arranged by a two-level full factorial design. Considering as input variables the height of the fluid, the number of cooling channels, the number of distributors, and the flow rate, the optimal relationship between the thermal performance of the battery cell, the volumetric energy density of the system, and the power consumption of the strategy was obtained. As a result, the energy density of the system was maximised, and the power consumption was reduced while keeping the cell temperature within the optimal range. [ABSTRACT FROM AUTHOR]

High charge/discharge rates and high energy density require a greater cooling power and a more compact structure for battery thermal management systems. The Immersion cooling (direct liquid cooling) system reduces the thermal resistance between the cooling medium and the battery and greatly enhances the cooling effect of the system. However, the high viscosity and low specific heat capacity of dielectric fluid limit the cooling effect of immersion cooling. This study presents an immersion cooling system that uses water as the cooling medium. In this system, a special seal structure was designed to prevent contact between water and the battery’s electrodes. The cooling effect of the system on the battery pack was numerically studied. Even if the battery pack is discharged at 3 C rate, a small water flow rate (200 ml/min) can ensure that the maximum temperature of the battery pack falls below 50°C. However, a good cooling capacity will increase the temperature difference of the battery pack. The temperature difference of the battery pack is difficult to reduce to 5°C until the water flow rate exceeds 1,000 ml/min. Adding a buffer structure at the inlet/outlet can be reduced the negative effects of the turbulent flow and then improve the temperature uniformity of the battery pack. These findings provide a better understanding of the influencing factors of the water immersion cooling system and can help to design a better immersion cooling system.

We formulate a modeling and control framework aimed at direct liquid cooling of data servers. In our application scenario, the server’s heat load is rejected into a liquid cooling circuit that extends to individual chips. We start with a comprehensive discussion of our modeling derivations. We then show how to dynamically provision the coolant, while 1) regulating the temperatures of any self-heating components within the safe operational envelope; 2) minimizing the coolant supply cost; and 3) increasing the server outflow temperature (a key performance objective toward heat recovery systems). We confirm experimentally the benefits of the proposed controlled cooling strategy over several realistic scenarios corresponding to different inlet coolant temperatures and computational loads. [ABSTRACT FROM AUTHOR]

In this paper, a direct liquid cooling method is proposed for a radial-flux permanent-magnet motor. To demonstrate the feasibility of the cooling method, a test motor with a rated output of 205 kW was designed, constructed, and tested in an actual vehicle application, an electric city bus. The energy consumption tests were conducted by applying a heavy-duty chassis dynamometer capable of simulating the inertia, weight, and road loads that the buses are subjected to in the normal on-road operation. The electricity consumption on the real bus route of the Espoo line 11 in Finland was 0.61 kWh/km. The test results of the cooling solution show that the motor is capable of meeting the most challenging requirements of the load cycle even with a full payload. The highest winding temperature rise in the test driving cycles was only 26 °C, which proves the effectiveness of direct-liquid-cooled coils in a vehicle motor.

In this work, a numerical optimisation process is applied to improve the fluid dynamical aspect of an innovative direct liquid cooling strategy for lithium-ion–based HEV/EV. First, the thermofluidic numerical model of the battery cell defined by means of CFD computational tools was validated with experimental tests. Then, a comparison between different flow patterns was developed to analyse the influence of the fluid distribution geometry. Finally, a parametric multi-objective optimisation process was implemented arranged by a two-level full factorial design. Considering as input variables the height of the fluid, the number of cooling channels, the number of distributors, and the flow rate, the optimal relationship between the thermal performance of the battery cell, the volumetric energy density of the system, and the power consumption of the strategy was obtained. As a result, the energy density of the system was maximised, and the power consumption was reduced while keeping the cell temperature within the optimal range.

Direct liquid cooling enables the development of IC, especially processors in servers, toward higher density, which demands more efficient means of heat removal. To exploit the capability, the control system that dynamically balances between cooling power and energy consumption is critical to operational safety and cost. This article presents the design, implementation, and characteristic of a dynamic control system for server processor direct liquid cooling in a high-density data center. The controller was trained and deployed in an online–offline structure. The loss function to train this model was built based on the model predictive control (MPC) principle. To simplify the complex physical model, a hybrid model with a nonlinear steady-state term and a linearized transient term was extracted. We used both averaged overheat and ratio of pumping power consumption to evaluate the performance of controllers. Benchmark tests on open- and closed-loop controllers were conducted. The results showed the advantages of the deep explicit MPC (DEMPC) in terms of cooling effectiveness and energy efficiency. The normalized pumping energy consumption of DEMPC reached $2.3\times 10^{-4}$ with the mean overheat level of −0.96 °C and the overheat time percentage of 6.5%. Comparing with open-loop control that has similar normalized energy costs, the mean overheat level was reduced by 1.2 °C and the time percentage of overheat was reduced by eight times. Meanwhile, normalized energy consumption was reduced by 21.7% than closed-loop proportional control who has a similar average overheat level and overheat time percentage. [ABSTRACT FROM AUTHOR]

With the rapid growth of information technology (IT), the requirement of higher computing performance results in a much higher power density in data center server. A Si microfluid cooler with jet-slot array has been developed for direct liquid cooling of a server processor in a data center to handle increased heating power and maintain stable and uniform thermal performance. To mitigate the energy cost of the cooling system, the developed liquid cooling solution can enable aggressive heat removal capability with a low power consumption requirement. A stacked Si cooler of three plates and four different layer structures has been designed, fabricated, and tested. The novel heat sink includes jet-slot array, drain-slot array, and multiple pin fins. By overcoming the negative cross-flow effect, a high heat transfer rate has been obtained, covering the whole cooling area. With a volume flow rate of 0.5 L/min, 150-W chip heating power can be dissipated while maintaining the maximum chip temperature rise of ~15 °C. A chip temperature variation of 5% has been realized, suggesting much desirable cooling uniformity for the whole chip. With the Si structure, this liquid cooling solution can be directly implemented onto the chip, and the thermal path can be sharply shortened, which enable a decreased thermal resistance between the cold plate and the heat source. Great agreement has been obtained between the experimental results and the simulation results. The developed direct liquid solution with Si cooler shows guarantee to enable the potential capability of future advanced server processors. [ABSTRACT FROM AUTHOR]

• Two jet impingement devices for direct body liquid cooling are proposed. • The lowest total thermal resistance of 0.041 k/W is achieved. • Thermal model for the body cooling methods is established. Thermal management of high-power electronic devices has become the bottleneck that restricts the working performance. Taking fully advantage of the cooling potential is urgently required. However, most existing direct liquid cooling methods only consider the cooling of the top surface of the electronic devices/chips, causing insufficient utilization of exposed chip surfaces. In this paper, a jet impingement body cooling device (JIBC) and a channel/jet impingement hybrid body cooling (HBC) device with micro-nozzles using the traditional mechanical machining method were developed. The body cooling devices provide high efficient cooling to the top surface and the side surfaces of the chip. The cooling performance were tested by experiments and it is found to be better than the traditional direct liquid cooling method. The total thermal resistance of 0.041 k/W, which can be compared to the thermal resistance of the thermal interface material, was achieved by the JIBC device when the flow rate is 1800 mL/min. The thermal model for the developed cooling methods was established to predict the chip temperature and characterize the heat transfer mechanism. It is found that the critical nozzle diameter exists, which determines which one of the two developed body cooling device has the better cooling performance. The proposed thermal model is found to be necessary for the quick design and optimal design of the body cooling device for high power and high heat flux electronics. [ABSTRACT FROM AUTHOR]

This paper presents the design and analysis of fractional-slot concentrated-windings for use with direct liquid cooling. To achieve considerably higher torque density than state of the art of electrical machines, recently patented cast coils that allow slot fill factors up to 90% are considered as basis for further enhancement. The sensitivity of large conductors to high losses due to current displacement as well as the implementation of direct liquid cooling of every single conductor is discussed. Two innovative coil designs are developed and compared, in consideration of current displacement using finite element methods, with basic trapezoidal cast coils. The heat dissipation capacities of the new coil designs are tested analytically as well as by measurement. Additionally, different cooling mediums are compared for their suitability for direct liquid cooling of coils. The proposed coil designs lead to possible current densities of 100Amm2 and to a reduction of the additional losses to about 50%. [ABSTRACT FROM AUTHOR]

Water-cooled micro/minichannel heat sinks are an important component in managing the temperature of electronic components, particularly where high density of heat rejection is required. This study examines the potential to decrease the thermal resistance and enhance convective heat transfer of a serpentine heat exchanger by introducing chevron fins which create secondary flow paths. This novel design is found to significantly reduce both the pressure drop across the heat exchanger and the total thermal resistance by up to 60% and 10%, respectively, and enhance the average Nusselt number by 15%. A three-dimensional conjugate heat transfer model is developed and validated against experimental measurements, before being used to carry out a parametric study involving the chevron oblique angle, secondary channel width and heat flux. The design of the serpentine minichannel with chevron fins is then optimised in terms of the minichannel width, minichannel number and chevron oblique angle. A 50 point Optimal Latin Hypercubes Design of Experiment is constructed within the design variable space, using a permutation genetic algorithm, and accurate metamodels built using Radial Basis Functions. A Pareto front is constructed which enables designers to explore appropriate compromises between designs with low pressure drop and those with low thermal resistance. [ABSTRACT FROM AUTHOR]

The demand for alternative fueling methods to reduce the need for fossil fuels is not limited to the electrification of ground vehicles. More-electric and all-electric aircraft pose challenges, with extensive requirements in terms of power density, efficiency, safety, and environmental sustainability. This paper focuses on electrical machines and their components, especially for high-power applications like the main propulsion. The electrical machine is evaluated from different aspects, followed by a closer look at the components and materials to determine the suitability of the current standard materials and advanced technologies. Furthermore, the mechanical and thermal aspects are reviewed, including new and innovative concepts for the cooling of windings and for the use of additive manufacturing. Aircraft have special demands regarding weight and installation space. Following recent developments and looking ahead to the future, the need and the possibilities for light and efficient electrical machines are addressed. All of the approaches and developments presented lead to a better understanding of the challenges to be expected and highlight the upcoming opportunities in electrical machine design for the use of electric motors and generators in future aircraft. Several prototypes of electrical machines for smaller aircraft already exist, such as the electric drive of the Siemens powered Extra 330LE. The focus of this paper is to provide an overview of current technical possibilities and technical interrelations of high performance electric drives for aviation. A 1 MW drive is exemplified to present the possibilities for future drives for airplanes carrying a larger number of passengers. All presented techniques can also be applied to other drive power classes. [ABSTRACT FROM AUTHOR]

The demand for alternative fueling methods to reduce the need for fossil fuels is not limited to the electrification of ground vehicles. More-electric and all-electric aircraft pose challenges, with extensive requirements in terms of power density, efficiency, safety, and environmental sustainability. This paper focuses on electrical machines and their components, especially for high-power applications like the main propulsion. The electrical machine is evaluated from different aspects, followed by a closer look at the components and materials to determine the suitability of the current standard materials and advanced technologies. Furthermore, the mechanical and thermal aspects are reviewed, including new and innovative concepts for the cooling of windings and for the use of additive manufacturing. Aircraft have special demands regarding weight and installation space. Following recent developments and looking ahead to the future, the need and the possibilities for light and efficient electrical machines are addressed. All of the approaches and developments presented lead to a better understanding of the challenges to be expected and highlight the upcoming opportunities in electrical machine design for the use of electric motors and generators in future aircraft. Several prototypes of electrical machines for smaller aircraft already exist, such as the electric drive of the Siemens powered Extra 330LE. The focus of this paper is to provide an overview of current technical possibilities and technical interrelations of high performance electric drives for aviation. A 1 MW drive is exemplified to present the possibilities for future drives for airplanes carrying a larger number of passengers. All presented techniques can also be applied to other drive power classes.

Electronics driven at high currents may experience local hot spots, which may cause thermal degradation or even catastrophic failures. This common problem occurs at lightemitting diode (LED) chips and it is not easily observed by end-users. Driving over 700 mA over a I mm² chip is expected to generate local temperature gradients. In addition, bonding failures at manufacturing or during operation (cracks, delamination, etc.) may also lead to local hot spots. Therefore, possible hot spots over an LED chip have turned attention to direct cooling with dielectric liquids comprises the current study. Computational and experimental studies have been performed to understand the impact of conduction and alternatively convection with various dielectric fluids to abate local hot spots in a multichip LED light engine. To capture the local temperature distributions over the LED light engine with a dome in the domain especially over the LED chip; first, computational models have been built with a commercial computational fluid dynamics (CFD) software. Later, attention has been turned into experimental validation by using a multichip high brightness LED (IIB LED) light engine. An optothermal evaluation has been made at single and multiphase heat transfer modes with dielectric fluids (LS5252, IIFE7000, and silicone oil, etc.) to compare with a series of CFD models and experimental studies. While multiphase liquid-cooled LED system has a better cooling performance but lower optical extraction, single-phase liquid-cooled LED system has shown a reasonable thermal performance with a 15% enhancement at light extraction. [ABSTRACT FROM AUTHOR]

• A numerical analysis is performed for direct liquid cooling of lithium-ion batteries using different dielectric fluids. • Study and compared the thermal performance of three different dielectric fluids including mineral oil, deionised water, and one engineered fluid. • The temperature rise is limited to below 3 °C for 1c- discharge by using deionised water at all mass flow rates, where oil-based fluids are delivering better performance only at higher mass flow rates of 0.05 kg/s. • The mineral oil and engineered fluids deliver almost an equal thermal performance, however, the latter consumed 76.43% less power at a mass flow rate of 0.05 kg/s. Lithium-ion battery (LIB) cells are responsible for powering most electric vehicles. LIB is still a superior battery available in the market because of its high energy density, specific power, and long cycle life. However, LIB comes with the challenges like thermal management as it is highly sensitive to temperature. Amongst different cooling methods, direct liquid cooling, also known as immersion cooling, can deliver a high cooling rate mainly because of its complete contact with the heat source. The single-phase liquid immersion with dielectric fluids (DELC) offers safety and cooling performance with lower parasitic power consumption and space requirements. This research involves studying and comparing different DELC's for the direct cooling of lithium-ion batteries. A numerical analysis of the 4S1P arrangement of LIB cells with direct cooling is conducted with three different DELC's including deionised water, mineral oil, and an engineered fluid. The transient behaviour of the battery module for various mass flow rates of DELC and at 1C,2C,3C-discharge rates are examined. All DELC maintains an excellent temperature homogeneity within the individual cells and LIB cells. The DELC with higher specific heat and thermal conductivity is suitable for cooling the LIB cells during high discharge conditions. However, all the dielectric fluids studied here effectively limit the temperature rise below 5 °C at 2-C discharging operation when the mass flow rate is increased to 0.05 kg/s. This improvement in thermal performance comes at the expense of extra power consumption. Even though both DELC-2 and DELC -3 delivered almost similar temperature rise values of 6.1, 5.2 °C, respectively during 2C discharging operation, the latter consumed 76.43% less power at a mass flow rate of 0.05 kg/s. For this reason, engineered fluids with lower viscosity values can be preferred over mineral oils. The deionised water is more effective for limiting the temperature rise below 2.2 °C for 3C discharging with the least parasitic power consumption of 0.52 mW at a mass flow rate of 0.05 kg/s. A variable cyclic load matching HWFET-driving cycle has been applied to the battery pack with all three DELCs, and all three fluids limited temperature rise below 1 °C. [ABSTRACT FROM AUTHOR]