Near-isothermal Compression Research Articles

Development of a near-isothermal transcritical CO2 compression system with a liquid piston compressor

Compressors are critical components in vapor compression cycle systems, significantly contributing to energy consumption. As global demand for HVAC&R systems rises, enhancing compressor efficiency becomes increasingly vital. This paper introduces a novel liquid piston compressor integrated with a gas cooler for the transcritical CO2 refrigeration cycle. The liquid piston enables CO2 refrigerant compression within various types of heat exchangers, facilitating the transfer of compression heat to the heat transfer fluid. By releasing significant heat, this compressor allows for removing or downsizing the traditional gas cooler in the refrigeration system. This paper presents the experimental performance of the first near-isothermal compressor utilizing a liquid piston in a vapor compression cycle. The critical parameters affecting heat transfer are analyzed by using a 1-D simulation model to achieve a near-isothermal compression process. The results show that the developed prototype successfully reduced the compression temperature increase from 95 K to 10 K, achieving 90 % isothermal efficiency. Moreover, the 1-D simulation results suggest the smaller internal diameter tubes benefit the isothermal efficiency the most.

Design, analysis and optimisation of a novel adiabatic-isothermal CAES system with coupled stepped phase change energy storage unit

Compressed air energy storage (CAES) technology is one of the important technologies to address the instability of renewable energy sources. To further make full use of the system heat of compression and reduce the problem of energy grade dissipation inside the accumulator, this paper proposes a novel CAES system coupled with a graded phase change thermal energy storage unit (SP-TES) and near isothermal compression of a liquid piston. A combination of adiabatic compression and near-isothermal compression is used to reduce the generation of compression heat, while the graded storage of compression heat through SP-TES reduces the reduction of energy grade. The design of the proposed SP-TES is carried out through the binary eutectic theory, and a one-dimensional transient entransy dissipation model, a thermodynamic model and an economic model are developed to analyze the 4E performance (energy, exergy, entransy, and exergoeconomic) of the proposed SP-TES. Further, the effect patterns of key node parameters on the proposed system are investigated by developing system related models and the system is optimized with multiple objectives from the point of view of economic and thermodynamic performances. The results show that the 4E performance of SP-TES is in between that of high and low temperature single-stage accumulators, and that SP-TES combines the advantages of higher economic performance of single-stage low-temperature accumulators and higher thermal performance of single-stage high-temperature accumulators. The SP-TES with a temperature range of 350 K-451 K is gradually stabilized with the number of charging and discharging, and the efficiency of the system in the stabilization stage is 73.24 %. The SP-TES with a temperature range of 407 K-440 K has the best economic performance with exergoeconomic parameter of 360.05 kJ/USD. The thermal storage performance and power output of the SP-TES decreases with the increase in the number of stages. The multi-objective optimization results show that the new CAES system efficiency is 63.03 % and the design is optimal when the unit energy cost is 0.2217 $/kWh.

Experimental investigation on the performance of compressed air energy storage using spray-based heat transfer

Near-isothermal compression and expansion may be accomplished by injecting water droplets into the air during the process to increase the overall efficiency. However, little is known about the relationship between spray system parameters and compressed air energy storage (CAES). Furthermore, the experiments about compressed air energy storage using spray-based heat transfer have not been investigated. The aim of this paper is to study the relationship between the performance of CAES and the spray system parameters by experimentally. The parameters including the spray closing time, the spray opening time, and the nozzle diameter are discussed. Results show that under the same operating conditions, the maximum air pressure in compression chamber reach to constant value when the spray closing time is 0.6 s; and spraying water mist within 0.6-1.2 s has no heat exchange effect on the air in the cylinder. During the compression process, the smaller the nozzle diameter is, the higher maximum pressure in compression chamber is. During the expansion process, if we ignore the energy consumption of spray system, the larger the nozzle diameter is, the more the expansion output is. Further investigation is recommended to optimize spray parameters based on different CAES systems.

Predicted roundtrip efficiency for compressed air energy storage using spray-based heat transfer

Compressed air energy storage (CAES) is a low-cost, long-duration storage option under research development. Several studies suggest that near-isothermal compression may be achieved by injecting water droplets into the air during the process to increase the overall efficiency. However, little is known about the thermal-fluid mechanisms and the controlling nondimensional parameters of the expansion process, which has previously been assumed to mirror the compression process. Furthermore, the isothermal round-trip efficiency and the impact of spray-based CAES have not been investigated. This study uses a validated 1-D model for compression and expansion with spray injection to complete a parametric analysis to analyze the thermal-fluid time-dependent physics and resulting roundtrip isothermal efficiency of a CAES system. Comparing the results for compression and expansion simulations, compression is found to have a higher isothermal efficiency than expansion for the same set up. The polytropic index for both compression and expansion tends to decrease and approach the ideal isothermal limit as nondimensional mass loading increases and as nondimensional Crowe number (ratio of thermal response time to domain time) decreases. As such, the highest efficiency designs are those with slow compression speeds and high spray flow rates to achieve high mass loading and those with small droplets to achieve low Crowe numbers—as long as spray work is neglected. If spray work is included, the optimum spray conditions shift to those with lager drop sizes. For example, roundtrip isothermal efficiency peaks around 95 % at a mass loading of 14 and at Crowe numbers <0.1 with a pressure ratio of ten. The results indicate that near-isothermal CAES compression and expansion is possible but that spray work should be included for significant mass loadings (e.g. greater than unity). Further investigation is recommended to consider effects of multi-dimensionality, turbulence, wall-interactions, and droplet dynamics.

Thermodynamic analysis of a hybrid system combining compressed air energy storage and pressurized water thermal energy storage

Large-scale electrical energy storage is an urgent requirement currently. This paper presents a hybrid system integrating compressed air energy storage (CAES) with pressurized water thermal energy storage (PWTES). The open type isothermal compressed air energy storage (OI-CAES) device is applied to the CAES subsystem to achieve near-isothermal compression of air. Meanwhile, the heat storage capacity of liquid water is improved by pressurization in the PWTES subsystem. Electricity is converted directly to heat through joule-resistive heating to improve the temperature of pressurized water. Moreover, the thermodynamic model of the hybrid system and the transient mathematical model of the OI-CAES in the four stages of intake, compression, exhaust and expansion are developed in this paper. The effects of design parameters on the working process of OI-CAES with and without spraying are the focus of the study. Furthermore, three different cases of the hybrid system are analyzed. The results show that the increase in tank height or tank volume is beneficial to restrain the variation of air temperature in the operation of OI-CAES when there is spraying, and the fluctuation range of the air temperature with spraying can be controlled within 25.7 K. When the tank volume is 5 m3 and the flow rate of the pump unit is 10 m3/h, the maximum temperature rise of the air in the compression process of OI-CAES can be managed within 30.5 K and 11.5 K respectively when there is no spraying and spraying. When the water in the PWTES subsystem supplies thermal energy, the efficiency of the CAES subsystem of the hybrid system can achieve 91.9 % with the energy storage density being 2.28 kWh/m3. When the hybrid system provides thermal energy through heat transfer, the energy efficiency of the hybrid system is expected to reach 65.6 %. With the supply of thermal energy, the flow rate of heating water can be flexibly adjusted and the heating water supply temperature is able to meet 363.16 K. Without the supply of thermal energy, the electrical efficiency of the hybrid system can reach 53.7 %.

Influence of Rotational Speed on Isothermal Piston Compression System

An isothermal piston is a device that can achieve near-isothermal compression by enhancing the heat transfer area with a porous media. However, flow resistance between the porous media and the liquid is introduced, which cannot be neglected at a high operational speed. Thus, the influence of rotational speed on the isothermal piston compression system is analyzed in this study. A flow resistance mathematical model is established based on the face-centered cubic structure hypothesis. The energy conservation rate and efficiency of the isothermal piston are defined. The effect of rotational speed on resistance is discussed, and a comprehensive energy conservation performance assessment of the isothermal piston is analyzed. The results show that the increasing rate of the resistance work increases significantly proportional to the rotational speed, and the proportion of resistance work in the total work increases gradually and sharply. The total work including compression and resistance cannot be larger than the compression work under adiabatic conditions. The maximum rotational speed is 650 rpm.

An electro-hydrogen cogeneration system combining compressed air energy storage and methanol cracking reaction

In order to solve the problems of insufficient utilization of compression heat in compressed air energy storage (CAES) system and the need for supplementary heat in methanol cracking reaction (MCR) for hydrogen production, an electro-hydrogen cogeneration system combining CAES and MCR was proposed in this study. The energy storage module of this system consists of two parts: adiabatic compression and near-isothermal compression. The heat generated by adiabatic compression is used to preheat methanol, and carbon monoxide generated by methanol cracking is used to supplement the heat of the cracker and the energy release module of the CAES system, respectively. The proposed system makes full use of low-grade compression heat while consuming the generated carbon monoxide to output hydrogen to the outside. By constructing a systematic thermodynamic model of the system, the effects of the key parameters on the system performance were investigated in depth. The results show that the energy efficiency and exergy efficiency of the system can reach 85.71 % and 80.94 %, respectively. When the suction pressure of the water-air coexisting tank is increased from 5 atm to 20 atm, the air storage mass increases by 33.96 %. At the cracking temperature of 708 K, the energy storage density can reach 12.37 kWh/m3, the hydrogen production mass is 3113.38 kg, and the relative energy saving rate is 54.38 %. The system performance can be improved by increasing the turbine inlet temperature of the first section and decreasing the turbine pressure ratio of the first section while keeping the total pressure ratio constant. The combustion chamber, cracking reactor and heat exchanger are the main factors that cause the exergy loss, accounting for 46.54 %, 30.16 % and 6.67 % of the total, respectively.

Energy distributing and thermodynamic characteristics of a coupling near-isothermal compressed air energy storage system

Intermittent renewable energy generation systems bring serious adverse impacts to the stable operation of the grid. In this regard, large-scale compressed air energy storage (CAES) systems with the potential to serve as long-term could be a solution for their optimal utilization. However, in a conventional CAES, most of the electricity is transformed into heat which passed into heat exchange mediums leads to serious heat transfer losses. In this study, a novel CAES system employing a Kalina cycle to effectively utilize the pre-compression heat in a near-isothermal compressed air process, is proposed and its holistic dynamic model is developed and presented for a deeper understanding of performance. The results showed that a near-isothermal compression undertakes the responsibilities of storing pressure potential energy, and the adiabatic pre-compression process helping in raising the inlet pressure of the liquid piston is primarily for storing thermal energy. In addition, the Kalina cycle working in the adiabatic pre-compression process achieves a 2.7 % improvement in the electrical efficiency. The thermal energy storage temperature was highlighted as the key factor in the energy allocation among the Kalina cycle, the thermal energy storage component, and the energy releasing sub-system in the proposed system. Through a systematic analysis of the proposed system performance, the highest electrical efficiency attained was 65.1 % with a 353 K storage temperature.

Techno-economic analysis of offshore isothermal compressed air energy storage in saline aquifers co-located with wind power

Offshore wind power projects are increasingly attractive in many regions even though capacity is impacted by intermittency as it is with other renewable power sources. We examine balancing the intermittency with an Offshore Compressed Air Energy Storage (OCAES) system that combines near-isothermal compression and expansion processes via water spray injection with air storage in saline aquifers. Spray injection maintains the air at nearly constant temperatures to improve round-trip efficiency, and saline aquifers are abundant in near-shore environments at suitable depths. This techno-economic analysis estimates the efficiency, cost, and value of OCAES, and demonstrates it in the context of the Atlantic coast of the United States, for a wind lease near Virginia. The round-trip efficiency of the OCAES system is projected using a thermal fluid process model that accounts for machinery performance as well as geophysical subsurface characteristics and gradients. Cost estimates are based on combining axial gas turbine technology with water spray injection retrofits and drilling experience from the oil and gas industry. Value to the electric grid is quantified with a price-taker dispatch model that optimizes the value of delivered electricity. The study analyzed power capacities from 10 to 390 MW, and our results show that for the geophysical conditions considered, a 200 MW OCAES system is expected to have a round-trip efficiency of 77% and a capital cost of $1457/kW. When paired with a 500 MW wind farm, OCAES is able to increase revenue from $0.031/kWh, without storage, to $0.048/kWh. We also show that a 350 MW OCAES system with 168 hours of storage is able to make the wind farm power output constant with a levelized cost of electricity (LCOE) of $0.22/kWh, 81% less than with 10-hour lithium-ion battery technology.

A combined experimental and modelling investigation of an overground compressed-air energy storage system with a reversible liquid-piston gas compressor/expander

We consider a small-scale overground compressed-air energy storage (CAES) system intended for use in micro-grid power networks. This work goes beyond previous efforts in the literature by developing and showing results from a first-of-a-kind small-scale (20kWh) near-isothermal CAES system employing a novel, reversible liquid-piston gas compressor and expander (LPGC/E). Additionally, we extend our study to assessments, for the first time, of the economic and environmental characteristics of these small-scale overground CAES systems through a combination of experimental, thermodynamic, technoeconomic and environmental analyses. Five system configurations are considered: (1) CAESbase, which is the base-case system; (2) CAESplate, in which parallel plates are inserted into the LPGC/E as a heat exchanger for achieving near-isothermal compression and expansion; (3) CAESPCM, in which a phase change material (PCM) is employed to store thermal energy from the compressed air during charging that is later recovered during discharge; (4) CAESPCM&plate, which is a combination of the CAESplate and CAESPCM arrangements; and (5) CAESheater, in which a heater is utilised instead of the PCM to preheat the compressed air during discharge. Data for the validation of a computational design tool based on which the assessments were performed were obtained from a prototype of the CAESbase system. Results show that the CAESPCM&plate system exhibits the highest roundtrip efficiency of 63% and the shortest payback period of 7 years; the latter with the inclusion of governmental incentives and an electricity smart export guarantee (SEG) support rate of 5.5 p/kWh (6.8 ¢/kWh). The CAESPCM&plate system is found to be cost-effective even without incentives, with a payback period of 10 years. This system is also associated with 71 tonnes of fuel consumption savings and reduced CO2 emissions amounting to 51 tonnes over a lifetime of 20 years.

Effect of Integrating Metal Wire Mesh with Spray Injection for Liquid Piston Gas Compression

Heat transfer enhancement techniques used in liquid piston gas compression can contribute to improving the efficiency of compressed air energy storage systems by achieving a near-isothermal compression process. This work examines the effectiveness of a simultaneous use of two proven heat transfer enhancement techniques, metal wire mesh inserts and spray injection methods, in liquid piston gas compression. By varying the dimension of the inserts and the pressure of the spray, a comparative study was performed to explore the plausibility of additional improvement. The addition of an insert can help abating the temperature rise when the insert does not take much space or when the spray flowrate is low. At higher pressure, however, the addition of spacious inserts can lead to less efficient temperature abatement. This is because inserts can distract the free-fall of droplets and hinder their speed. In order to analytically account for the compromised cooling effects of droplets, Reynolds number, Nusselt number, and heat transfer coefficients of droplets are estimated under the test conditions. Reynolds number of a free-falling droplet can be more than 1000 times that of a stationary droplet, which results in 3.95 to 4.22 times differences in heat transfer coefficients.

A Novel Isothermal Compression Method for Energy Conservation in Fluid Power Systems.

Reducing carbon emissions is an urgent problem around the world while facing the energy and environmental crises. Whatever progress has been made in renewable energy research, efforts made to energy-saving technology is always necessary. The energy consumption from fluid power systems of industrial processes is considerable, especially for pneumatic systems. A novel isothermal compression method was proposed to lower the energy consumption of compressors. A porous medium was introduced to compose an isothermal piston. The porous medium was located beneath a conventional piston, and gradually immerged into the liquid during compression. The compression heat was absorbed by the porous medium, and finally conducted with the liquid at the chamber bottom. The heat transfer can be significantly enhanced due to the large surface area of the porous medium. As the liquid has a large heat capacity, the liquid temperature can maintain constant through circulation outside. This create near-isothermal compression, which minimizes energy loss in the form of heat, which cannot be recovered. There will be mass loss of the air due to dissolution and leakage. Therefore, the dissolution and leakage amount of gas are compensated for in this method. Gas is dissolved into liquid with the pressure increasing, which leads to mass loss of the gas. With a pressure ratio of 4:1 and a rotational speed of 100 rpm, the isothermal piston decreased the energy consumption by 45% over the conventional reciprocation piston. This gain was accomplished by increasing the heat transfer during the gas compression by increasing the surface area to volume ratio in the compression chamber. Frictional forces between the porous medium and liquid was presented. Work to overcome the frictional forces is negligible (0.21% of the total compression work) under the current operating condition.

Efficiency improvement of liquid piston compressor using metal wire mesh for near-isothermal compressed air energy storage application

Intermittent nature of power from renewable energy resources demands a large-scale energy storage system for their optimal utilization. Compressed air energy storage systems have the potential to serve as long-term large-scale energy storage systems. Efficient compressors are needed to realize a high storage efficiency with compressed air energy storage systems. Liquid piston compressor is highly effective in achieving efficient near-isothermal compression. The compression efficiency of the liquid piston can be improved with the use of heat transfer enhancement mechanism inside the compression chamber. In this study, a novel heat transfer enhancement technique using metal wire mesh is experimentally tested in a liquid piston compressor to improve compression efficiency. Metal wire meshes of aluminum and copper materials and different wire diameters along with various stroke times of compression are considered in the experimental design. A distinctive Archimedean spiral form of metal wire mesh is considered to facilitate heat transfer in the axial and radial direction inside the compression chamber. Experiments are conducted for the compression of air from the atmospheric pressure to about 280 kPa pressure at various stroke times of compression. Results show that the peak air temperature was reduced by 26–33 K with the use of metal wire mesh inside the liquid piston compressor. Both the materials are observed to be equally effective for temperature abatement. The use of metal wire mesh in liquid piston shifts the compression process towards the near-isothermal conditions. Furthermore, the isothermal efficiency of compression is evaluated to assess the potential for efficiency improvement with this technique. The metal wire mesh was observed to improve the isothermal efficiency of compression to 88–90% from the base efficiency of 82–84%. A 6–8% improvement in efficiency was observed at faster compression strokes signifying the efficacy of metal wire mesh to accomplish an efficient compression with a high power density.

Experimental investigation of water spray injection in liquid piston for near-isothermal compression

Near-isothermal compression is desired to achieve high efficiency in many compressor applications. Low heat transfer characteristic of conventional compressors is a major bottleneck in attaining a near-isothermal compression. A high heat transfer rate is possible with an injection of a large number of water droplets using a spray nozzle inside the compression chamber. In this paper, the effectiveness of spray injection to achieve near-isothermal compression is investigated experimentally in a liquid piston compressor for a compression ratio of about 2.5. Parametric investigations are performed by varying injection pressures of spray from 10 psi (69 kPa) to 70 psi (483 kPa), using different spray nozzle angles (60°, 90°, and 120°), and by changing the stroke time of compression. It is observed that water spray injection is highly effective in abating the air temperature rise during the compression process. The pressure-volume plots indicate a significant reduction in the compression work, and they approach near-isothermal compression with spray at higher injection pressures. The isothermal efficiency of compression consistently increases with an increased injection pressure of spray and reaches up to 95% at the highest injection pressure studied (70 psi). Furthermore, the spray nozzle angle marginally affected the isothermal efficiency with a 1–4% improvement with the use of a 60° nozzle angle over a 120° spray angle at all injection pressures. Also, comparable isothermal efficiencies are observed for compression with different stroke times between 3 and 5 s especially at higher injection pressures which highlight the efficacy of spray injection in attaining a high power-density along with high efficiency. Overall, with an optimized spray design, water spray injection can achieve a highly efficient near-isothermal compression in liquid piston.

Experimental study of heat transfer enhancement in liquid piston compressor using aqueous foam

Efficiency of gas compression can be significantly improved by achieving isothermal compression. A high heat transfer rate in the compression chamber is desired to achieve the isothermal compression process. A large surface area and a high heat transfer coefficient of aqueous foam can be used to achieve a significantly high heat transfer rate in the compression chamber. In this study, a novel heat transfer enhancement technique using aqueous foam is investigated in a compressor for achieving near-isothermal compression. Experiments are performed with the use of aqueous foam generated inside a liquid piston compressor. The volume of aqueous foam in the compression chamber, the air flow rate for foam generation, and various foam generator designs are considered in this parametric investigation. It is observed that the use of aqueous foam in the compression chamber is highly effective in reducing air temperature during the compression process. A higher volume of aqueous foam in the compression chamber leads to a significant increment in isothermal efficiency, however, with higher variability. The higher variability in efficiency is due to the higher cyclic variation of the temperature profiles during compression. A compression chamber completely filled with aqueous foam shows a 4–8% improvement in the efficiency for a compression ratio of 2.5. Moreover, several foam generator designs were tested to identify if there is any dependency of cyclic variability on foam generator design parameters. The results show some promise on optimizing the design to reduce the variability. Overall, the heat transfer enhancement using aqueous foam is effective in achieving an isothermal efficiency up to 92% compared to 86% for the no-foam case in a liquid piston compressor.

Experimental investigation of heat transfer in liquid piston compressor

The use of liquid pistons is a promising approach for attaining efficient near-isothermal compression. One of the key factors affecting the efficiency of a liquid piston compressor is heat transfer. Understanding the heat transfer mechanism during compression is crucial for the design and development of an efficient liquid piston compressor. In this paper, heat transfer in the liquid piston compressor is studied experimentally for air compression. An analytical model is presented based on a thermal resistance circuit. Experiments are performed using compression chambers of different materials for a compression ratio of 2.05–2.35 with various stroke times of compression. It is observed that the rate of heat transfer increases with faster stroke time of compression. However, a faster compression process requires a higher compression work and results in a higher air temperature. The convective heat transfer coefficient of air decreases rapidly as compression proceeds and approaches a steady value towards the end of compression. Thermal resistance analysis for compression with different chamber materials indicates that convective thermal resistance of air has a significant contribution in the total thermal resistance. During the initial phase of compression, the high conductivity of the chamber material helps improve the overall heat transfer coefficient; however, it has a marginal effect during the later phase of compression. An isothermal compression efficiency of 84–86% is observed with the liquid piston.

Semi-empirical modeling and analysis of oil flooded R410A scroll compressors with liquid injection for use in vapor compression systems

Oil flooding is a technique that can be utilized in compression systems to achieve near-isothermal compression. This can lead to a boost in system efficiency and a reduction in compressor power consumption. In this paper a semi-empirical model for oil flooded compressors using liquid injection was developed. The model is validated with experimental data and integrated into a thermodynamic model of a vapor compression system with oil flooding and regeneration. The performance of the heat pump system is predicted and the semi-empirical model is used to identify and estimate the magnitude of the irreversibilities during the compression process. A method for generalizing the model for different working fluids is also presented. Using this model, design recommendations are made to improve the efficiency of the studied liquid flooded compressors for heat pump applications.

Experimental Investigation on the Effect of Phase Change Materials on Compressed Air Expansion in CAES Plants

The integration of renewable energy in the electrical grid is challenging due to the intermittent and non-programmable generated electric power and to the transmission of peak power levels. Several energy storage technologies have been studied to find a solution to these issues. In particular, compressed air energy storage (CAES) plants work by pumping and storing air into a vessel or in an underground cavern; then when energy is needed, the pressurized air is expanded in an expansion turbine. Several CAES configurations have been proposed: diabatic, adiabatic and isothermal. The isothermal process seems to be the most promising to improve the overall efficiency. It differs from conventional CAES approaches as it employs near-isothermal compression and expansion. Currently, there are no commercial isothermal CAES implementations worldwide, but several methods are under investigation. In this paper, the use of phase change materials (PCM) for isothermal air expansion is discussed. Air expansion tests in presence of PCM were carried out in a high-pressure vessel in order to analyze the effect of PCM on the process. Results show that in presence of PCM near isothermal expansion conditions occur and therefore they affect positively the value of the obtainable expansion work.

Tectonic Implications of Early Paleozoic Metamorphism in the Anakie Inlier, Central Queensland, Australia

Well-defined metamorphic zones are developed in pelitic and psammitic rocks of the Late Neoproterozoic to Cambrian Anakie Metamorphic Group of the Anakie Inlier, central Queensland. They are defined by the incoming of biotite, garnet, and andalusite, with or without staurolite. Mineral assemblages indicate that low pressure–high temperature metamorphism is associated with D1, medium pressure–high temperature metamorphism with D2, and retrograde, low pressure–low temperature metamorphism with D3. A mean b cell parameter of 9.035 obtained from K-white micas in the lowest-grade rocks suggests upper intermediate pressure conditions during D2. The timing of the growth of the index minerals indicates that isotherms retreated to the southwest during D2. Phase diagram calculations for both Al-saturated and Al-undersaturated metapelites containing D1 associations indicate pressure-temperature (P-T) conditions of 0.4 GPa and 560°C. These changed to 0.64–0.79 GPa and 580°–640°C as a result of crustal thickening. The samples thus record a history of heating (D1), followed by near-isothermal compression (D2). This P-T pathway shows that contraction rather than extension, as suggested by some authors, occurred during D2. The contractional event is suggested to have taken place during the Delamerian Orogeny as a result of convergence and collisional processes along the former passive margin of Gondwana.

Pressure, temperature and structural evolution of the Sulitjelma fold-nappe, central Scandinavian Caledonides

Summary Pressure( P )-temperature( T ) paths of regional metamorphism have been constructed for pelitic units of the Sulitjelma fold-nappe (SFN), part of the Köli Division of the Upper Allochthon of the Central Scandes. The SFN comprises the Skaiti Supergroup, a sequence of late Proterozoic supracrustal rocks, intruded by the lower members of the Sulitjelma Ophiolite, which is overlain by an Ordovician to Silurian sedimentary carapace, the Furulund and Sjönstå groups. The P-T paths calculated from garnet zoning modelling are temporally related to texturally distinct periods of garnet growth, which can be correlated with regional deformation episodes. In the Furulund Group: (i) static overgrowth of a planar fabric in garnet cores corresponds to heating and compression to 470°C, 6 kbar; (ii) synkinematic growth corresponds to further heating but with decompression to 500°C, 5 kbar; (iii) static growth of inclusion-free rims occurred during near-isothermal compression to 6 kbar. In the Skaiti Supergroup pelites: (i) static growth of poikiloblastic garnet cores occurred during heating and compression to 500–520°C and 9–10 kbar; and (ii) static growth of inclusion-free rims corresponds to minor heating and significant decompression to 540°C and 7 kbar. Comparison of mesoscale structural features with mineral growth indicates that (i) in both units is related to burial resulting from the closure of the marginal ocean basin floored by the Sulitjelma Ophiolite, while (ii) relates to the emplacement of the SFN onto Baltica, accompanied by overturning of the Ophiolite and its sedimentary cover. In the Furulund Group this occurred during eastward-directed simple shear deformation, giving rise to sheath folds and overturned isograds. In the Skaiti Supergroup there was little deformation during this period suggesting that most of the strain was being taken up by the Furulund Group. The final compressional phase (iii) seen only in the Furulund Group, relates either to the emplacement of the Skaiti Supergroup above the Furulund Group, or to out-of-sequence overthrusting of higher nappes.