- Armin Hafner
Our main objectives are to develop components required for cost-effective implementation of efficient systems for heat pumping and conversion. The focus is on heat exchangers, compressors and work recovery.
To achieve these goals, we develop methods and tools required for designing components. Focus is given to cycles with natural working fluid mixtures, thermodynamic properties, system optimization, and experimental development. The research area also performs design, support integration and maintains flexible component test facilities for the HighEFFLab infrastructure.
RA2.1 Heat Exchangers
In order to realise useful heat recovery from off-gas cooling, it is important to get the size of the large heat recovery heat exchanger (HRHE) to an acceptable level. Another issue is that the off-gas contains particles. The target is to design a heat exchanger that will have low velocities at the inlet, to avoid particles entering further into the heat exchanger – but for good heat transfer properties, the gas should accelerate once inside.
In 2020, we worked in collaboration with WP4.2 (Surplus heat recovery) in HighEFF, which has given us the opportunity to study such a HRHE by extending the general heat exchanger modelling framework to create a simulation model that allows the geometry to be adapted to the changes in the off-gas conditions. Research on how to obtain the optimal variable geometry will continue in 2021.
Much of the work in WP2.1 is to develop models and methodology to study the simultaneous effects of process and heat exchanger geometry optimisation. The methodology depends on the underlying physical descriptions being valid over the operating range and on the models including the effects of all the geometry parameters subjected to optimisation. The underlying models are developed by laboratory measurements, but to be able to "measure" over a wider range of geometries these can be supplemented with Computational Fluid Dynamics (CFD). In 2020, we established a link between the optimisation models and a CFD model, so that we can numerically verify the result. This led to a paper where we compared CFD results with literature data from experiments, and with results from the developed heat exchanger optimisation model using a traditional tube-in-fin heat exchanger as case.
RA 2.2 Work Recovery & Compressors
The R&D focus is tuned towards compressors and technologies for expansion work recovery. The application of environmentally friendly and future proof natural refrigerants is a novelty of the research conducted by FME HighEFF. While the experimental method investigation on the compressors was delayed due to covid-19, the modelling achieved a major step.
New compressors are under development having an extended operation envelope e.g.: towards higher temperatures. These compressors are paving the way for compact and reliable industrial high temperature heat pumps (HTHP) as applied in WP 3.2 (HTHP, Cooling and Drying). HTHP’s delivering process heat up to 200°C will work with renewable energy and may cover up to 37% of all industrial process heat.
In WP 2.2 (Work Recovery & Compressors), a 3-dimensional dynamic ring plate valve model for reciprocating compressors, as applied in refrigeration, heat pump and HTHP applications, was developed. The model investigates the three-dimensional dynamics of the ring plate discharge valve, using loosely coupled computational fluid dynamics (CFD) and finite element element method (FEM). The model was further compared to measurement results giving an insight into key elements of the compressor valve dynamics. The dynamic motion of the valve ring is controlled by the combined actions of forces resulting from applied pressure, spring reaction, impact with the valve's plate and cage, and damping eff ects. Future studies of these phenomena in greater depth will allow to understand the wearing of the valve and enable the compressor manufacturers to build more efficient and reliable compressors.
EXPANSION WORK RECOVERY
The implementation of ejectors for expansion work recovery allows to increase efficiency and make environmentally friendly refrigeration systems more competitive. Investigations of the real system performance of an ejector-supported supermarket refrigeration system have been performed. 3 months of various system operation modes were analysed, concluding with an energy saving potential of about 10% when utilising the ejector technology in a real supermarket. Further improvements were identified, which have to do with heat recovery of the CO2 based refrigeration system. The largest benefits of the investigated sytems are seen in warmer climates, such as southern Europe or India.
Natural Working Fluids
Hydrocarbons as a 4th generation of working fluids has the capability to be utilised in many different applications, but in order to do so, hydrocarbon charge needs to be minimised to ensure safety. Internally enhanced tubes offer a possible solution, wherein the reduced volume minimises the charge. The critical design parameters such as pressure drop and heat transfer coefficient are unknown in these conditions. Experimental tests were performed at NTNU to measure these values in smooth and internally enhanced tubes. The results provide reliable prediction methods for design of heat exchangers. In the next steps, these predictive methods will be used for designing a heat exchanger in a predefined system.
Dry ice sublimation
We are developing a refrigeration system with heat absorption below –56˚C. The challenge is that CO2 exists in an equilibrium between dry ice and gas at temperatures below -56 ˚C. Thus, in order to realize a refrigeration system with heat absorption below this temperature, one needs to develop a heat exchanger concept that enables sublimation of dry ice and to understand the phenomena related to this.
A tapered evaporator/sublimator with a swirl promoter, which induces a swirling flow of solid (dry ice)-gas two-phase flow, was designed and constructed and installed into a CO2 ultra-low temperature cascade refrigeration system. By means of the heat transfer of solid (dry ice) -gas two-phase flow is investigated. Based on the measurement of heat transfer characteristics, it was verified that the CO2 refrigeration system can operate continuously and stably without dry ice blockage in the evaporator/ sublimator. It was understood that dry ice particles are uniformly distributed along the inner wall of the evaporator/sublimator by installing the swirl promoter, where the heat transfer coefficient is largely improved.
LT-Compressor replacement ejector
Decreasing the complexity of R744 commercial refrigeration systems for supermarkets could substantially reduce their cost, promoting worldwide spreading. Typically, the share of the low-temperature (LT) load in the total load for supermarkets is low. Nevertheless, significant investments must be made to cover the LT demand, including dedicated compressors, inverter, desuperheater, and oil management equipment.
A new concept development based on high-pressure lift ejector to support LT evaporators reduces the system cost by replacing the whole LT compressor section with a fixed geometry ejector. In addition, elimination of the compressor(s) allows overfeeding operation of LT evaporators.
Propane evaporation in smooth and microfinned tubes
Utilize and adapt an in-house heat transfer test facility (inside small diameter tubes) to perform experimental investigations with hydrocarbons and mixtures. Experimental campaigns for maintaining and adapting specially designed in-house tools, for analysing and developing correlations for hydrocarbons and their mixtures.
A new test section has been developed in order to measure the heat transfer coefficient and pressure drop of hydrocarbons in small diameter tubes. Following a study of different methods, a model was analysed and then a prototype built. The preliminary tests were positive and will be the basis for the final test section.
The work on humid gas in 2019 includes the modelling of two heat transfer scenarios using the thermo- dynamic model for humid gas implemented in 2018. The first scenario is to simulate the heat transfer of humid gas on a cold flat plate, where the calculation of heat transfer is based on the traditional method for dehumidification process. The second scenario is to simulate a tube-intube heat exchanger with humid gas flowing in the inner tube and cooling water flowing in the annulus. The method treats the humid gas as normal mixtures and uses the thermodynamic libraries for mixtures.
It requires further validation of the results based on experimental data, which is not available at present. Regarding the future work on modelling, it is suggested to find a suitable case relevant to the practical applications, such as the flue gases containing Sulphur components. The differences between using the dehumidification method and method of general mixtures should be further investigated.
The work related to heat exchanger (HX) for cold thermal energy storage (CTES), applicable for display cabinets in supermarkets continues, with focus on increased performance of display cabinets and power peak shaving. Experience has been gained on how the derivative free optimisation framework NOMAD can be implemented for process and HX optimisation. For simple Rankine cycles the gradient based method was compared to NOMAD in terms of time consumption and robustness.
The work on 3D-printing continues to streamline the process from HX geometry optimisation, on-screen visualization, 3D-editing and finally 3D-printing. The main outcome was a visualization scripting with Python toward Paraview, a well-known visualization toolkit for CFD.
HighEFF Lab – Heat Exchanger Laboratory was designed. The two test rigs cover: HX prototype test rig up to 40 kWth and a small-scale heat transfer coefficient and pressure drop test rig.
Work recovery & Compressors
Design and testing of Multi-ejector expansion modules continue. These devices are substituting standard high-pressure electronic expansion valves in refrigeration systems.
Two expansion devices designed for R744 (CO2) refrigeration systems (capillary tube and fixed geometry ejector) were experimentally tested to determine performance in different ambient conditions. Tests and numerical investigations concluded that ambient conditions did not significantly affect the operation of the capillary tube and the R744 ejector.
Three approaches to perform modelling of piston compressor processes have been developed: 1D model; 3D CFD model; and 3D FEM impact model. These modelling approached give complementary insights into the piston compressor processes and how to optimize compressors for increased efficiency and reliability. A measurement campaign has been performed with the prototype compressor for butane. The prototype compressor shows high isentropic efficiencies in the order to 0.70 to 0.78, nearly constant over pressure ratio investigated. The volumetric efficiency declined from 0.89 to 0.75 with increasing pressure ratios.
Natural Working fluids
The investigations on hydrocarbon heat transfer and pressured drop in small pipes continues. Calibration was done against correlation for single phase flow in vapor form; a 5.6 % deviation for the pressure drop; 1.6 % for heat transfer coefficient. For fluid visualization, a high speed camera will be installed in 2019.
A library for general humid gas "hglib" was built based on "hxlib" library. hglib is connected to THERMOPACK, enabling to calculate humid gas as real gas. The EOS, e.g. PR, EOCGC, GERG2008 have been tested with the reference result by hxlib at low pressure, and EOCGC, GERG2008 perform well. Available methods for calculating the viscosity of a hydrocarbon mixture at high pressures and temperatures were investigated. A sufficiently general correlation is difficult. The TRAPP method is quite accurate but tends to overpredict the viscosity. The CS2 model has a similar accuracy, but underpredicts the results. For both models the density calculations needs to be as accurate as possible. Refprop results are generally better than TRAPP and CS2 with some exception
A heat exchanger for cold thermal energy storage (CTES), applicable for display cabinets in supermarkets, has been developed and implemented into a dedicated CO2 laboratory refrigeration cycle. The CTES will increase the performance of display cabinets and enable power peak shaving.
A new laboratory rig, financed by the infrastructure project HighEFFlab, for testing of heat exchangers used in low temperature heat to power applications is under planning. A pinch point analysis of relevant hydrocarbon working fluids with low ODP and GWP has been performed to investigate which working fluids are the most interesting for testing in the laboratory rig and the optimal operating conditions for these.
Work recovery & Compressors:
Design and testing of Multi-ejector expansion modules, intended as a substitute for standard high-pressure electronic expansion valves, for performance mapping of four ejector cartridges is completed. Results shows that four ejector geometries perform their vapour compression duties efficiently within the assumed range of operating conditions, offering the highest ever recorded values of ejector efficiency up to 36.8%. the liquid compression reduces the effectiveness significantly, where the highest recorded recovery values of ejector efficiency were equal to 12.6%.
The propane - butane High Temperature Heat Pump test facility has been installed successfully and delivering heat sink outlet temperature of 116 °C. The R600 compressor from DORIN is functioning within expectation with a total compressor efficiency of 71%.
Natural Working fluids:
In the evaporator/sublimator process of CO2 (R744) ultra-low temperature cascade heat pump system, it is known, as a problem, that dry-ice blockage makes the system operation fail. The design of an expanding channel for the evaporation/sublimation process is one solution to solve the problem. In order to give better understanding to heat transfer process in the refrigeration system, particular attention is focused for CO2 dry-ice solid-gas two-phase flow to obtain the effectivity of heat transfer and to verify the flow-phenomena where a visualization test apparatus is placed horizontally. Development of a mathematical model for baseline and the enhanced systems for the next generation of refrigeration systems for chilling of fish in fishing vessel. By applying adapted ejector technology and parallel compression, these units can be applied globally with a significant energy saving compared to traditional system layouts.