I Kraftringens geotermianläggning cirkulerar köldmediet R134a som är en fluorerande växthusgas med relativt hög GWP (Global Warming Potential). När den nya F-gasförordningen trädde i kraft 2015 resulterade det i att priserna ökade markant på de köldmedierna med hög GWP. Kraftringen vill därför undersöka vilka konverteringsalternativ det finns till R134a, dels för att minska på kostnaderna men också för att minimera verksamhetens klimatpåverkan.   Olika köldmedier har jämförts med avseende på hur de påverkar anläggningens prestanda. Det har också undersökts vilka köldmedier som ligger inom rätt tryck med hänsyn till vad kompressorn och anläggningens rör är konstruerade för. Undersökningen har gjorts genom teoretiska beräkningar som är grundade på uppmätta värden från när värmepumparna går på maximal belastning. Vid urvalet av köldmedier har det utgåtts från samma förångningstemperatur och kondenseringstemperatur som vid mätningen.   I undersökningen fann vi två köldmedier, R152a och R450A, som skulle fungera som konverteringsalternativ i avseende att de ligger inom rätt tryck och temperatur. Båda alternativen har lägre GWP och en högre värmefaktor än nuvarande köldmediet R134a. Dock är R152a brandfarligt och har högre säkerhetsklass än de två andra medierna. / In the geothermal plant of 'Kraftringen' the refrigerant R134a, which is a fluorescent greenhouse gas with relatively high GWP (Global Warming Potential), circulates. When the new F-Gas regulation was introduced in 2015, it resulted in a significant price increase for high-pressure GWP refrigerants. 'Kraftringen' therefore wants to investigate which conversion options there are for R134a, partly to reduce costs, but also to minimize the climate impact of the business.   Different refrigerants have been compared with regard as to how they affect the plants performance. Moreover, it has been investigated which refrigerants are in the correct pressure range with regard to what the plants compressor and pipes are designed for. The survey has been done by performing theoretical calculations based on measured values from when the heat pumps operate at maximum load. In the refrigerant selection process, the same evaporation and condensation temperature as during the measuring was used.   In the study we found two refrigerants, R152a and R450A, which would serve as conversion options in terms of them being in the correct pressure and temperature range. Both options have lower GWP and a higher heat factor than the current refrigerant R134a. However, R152a is flammable and has a higher degree of safety than the other two refrigerants.

Two-phase heat transfer in mini/micro-channels is capable of meeting the high cooling demands of modern high heat flux applications. The phase change process ensures better temperature uniformity and control for local hot spots. Furthermore, these compact channels could be helpful in reducing the required charge and material inventories.Environmental concerns—mainly ozone depletion and global warming—have instigated a search for new alternatives in refrigeration industry. While new compounds are being developed to address stringent legislative demands, natural alternatives are also coming into prominence. A limited number of investigators have reported on thermal performance of such alternatives. The current study is therefore focused on saturated flow boiling heat transfer, pressure drop and dryout characteristics for three low global warming potential (GWP) refrigerants (R152a, R600a and R1234yf) in a vertical mini-channel.In this study experiments were carried out by uniformly heating a test section (stainless steel tube with 1.60 mm inside diameter and 245 mm heated length) at 27 and 32 oC saturation temperature with 50-500 kg/m2s mass velocities. The effects of various parameters of interest (like heat flux, mass flux, system pressure, vapor quality, operating media) on flow boiling heat transfer, frictional pressure drop and dryout characteristics were recorded. R134a, which has been widely used in several applications, is utilized as a reference case for comparison of thermal performance in this study.Experimental results for saturated boiling heat transfer showed strong influence of heat flux and system pressure with insignificant contributions from mass flux and vapor quality. Two phase frictional pressure drop increased with mass flux, vapor quality and with reduced operating pressure. The dryout heat flux remained unaffected with variation in saturation temperature, critical vapor quality in most cases was about 85%. The experimental results (boiling heat transfer, two-phase pressure drop and dryout heat flux) were compared with well-known macro and micro-scale correlations from the literature. / <p>QC 20141124</p>

<p dir="ltr">Energy consciousness initiatives have seen a recent uptick to curb the ever growing concerns of global warming. Heat Pumps are a crucial piece of technology for these efforts, as they consume lower energy than the requirement they satisfy and are typically used for refrigeration and HVAC systems. Hybrid Heat Pump Water Heater (HPWH) technologies have seen increased adoption, and the improvement of these technologies could pay dividends in the long run. </p><p dir="ltr">This project explores the optimal design space of HPWHs within the context of the Department of Energy Guidelines for their performance rating and compares several up and coming refrigerants with lower GWP than the current market dominant refrigerant, R-134a, to provide consistent performance with improvements on the environmental front along with potential cost improvements on the manufacturing front. For this purpose, Dymola, a simulation software that employs the Modelica language for modeling complex dynamic systems, is employed to study the transient behavior of a market example Heat Pump Water Heater. </p><p dir="ltr">The results of these simulations were validated using experimental data gathered in the laboratory using relevant instrumentation on the physical device and manufacture specified performance ratings to compare the validity of the simulation results. The results of the study indicated the presence of a multi-dimensional design space with a defined set of possible combinations for device implementation. Within that feasible region, there exist multiple trajectories of iso-preference which alter the overall device performance, and the careful study of these parameters and their implications on the device performance can lead to a more robust design pathway for future improvements of the device. The work also contextualizes these improvements by quantifying the relative importance of different parameters upon the final performance of the device, showing how to identify which parameters to focus on when embarking upon an improvement journey. Additionally, preliminarily ideal specifications for the device operation under different refrigerants studied were also identified to provide similar or better performance to the current device. </p><p dir="ltr">The study showed that when matching mass flux rates, R-152a, R-290, and R-600a outperform R-134a in terms of expected COP. Of the 3, only R-290 uses a smaller compressor size than the baseline R-134a cycle for achieving the required heating capacity. The other refrigerants studied do not improve upon the COP of the cycle, but do have benefits over R-134a in terms of their respective GWPs. </p><p dir="ltr">The results suggest that with the considered alterations, R-290 systems within the current charge restrictions (<150g) can be developed and achieve the same heating performance with slight improvements on COP and therefore potentially UEF values. </p><p dir="ltr">The study also shows that all refrigerants considered could achieve the required heating capacity with a considerably downsized condenser and appropriately reduced subcooling. It highlighted the trends being consistent across refrigerants and implemented a final alternative refrigerant through the identified optimization steps to arrive at a new configuration without revalidating the trends, showing that newer optimal configurations could be identified with minimal time spent in the simulation environment. </p><p dir="ltr">Finally, the study explored alternative control possibilities by way of overheating the water beyond its required setpoint and enabling a control based mixing at the outlet to reduce the energized time of the device and leveraging the exceptional insulation capabilities for thermal storage.</p>