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- Claesson, Joachim, Docent, 1970-
(författare)
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Värmeövergång och tryckfall vid förångning i små plattvärmeväxlare
- 2001
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Rapport (populärvet., debatt m.m.)abstract
- Detta projekt har behandlat kokning av köldmedier i kompakta lödda plattvärmeväxlare. Experiment har utförts dels på kommersiella produkter och dels på värmeväxlare med speciell utformning. Vid sidan av experimenten har en omfattande litteraturstudie utförts. Plattvärmeväxlare i den internationella litteraturen är sällsynt, speciellt som förångare. Experimenten visade inledningsvis att värmeövergångstalet vid kokning är en funktion av yteffekten, vilket är ett karaktäristiskt drag hos kärlkokning. Detta experiment utfördes med hjälp av termokroma flytande kristaller (TLC). Dessa ändrar färg med temperaturen. Dessutom visades att redan vid måttlig överhettning används stor del av värmeväxlarytan till att överhetta köldmediet. Utifrån detta utvecklades en (iterativ) metod att beräkna prestanda hos en förångare i ett kylsystem med en termostatiskt expansionsventil. Preliminära resultat från denna modell uppvisar god överrensstämmelse med utförda experiment. Visualisering av kokning inuti en plattvärmeväxlare har också utförts. Det visade sig att kokningen skedde med en viss pulsation. En viss intensivare kokning kunde också observeras på en halvan av ytan. Detta kan bero på en viss snedfördelning av flödet inom respektive kanal. Vi kunde inte observera någon ”mist-flow” efter det att ytan blivit torr, d.v.s. inga fria vätskedroppar. Sammanfattningsvis har en god insyn i kokning av köldmedier i plattvärmeväxlare erhållits. Kärlkokning är den dominerande kokregimen och värmeövergångstalen kan predikteras genom att använda en korrelation framtagen för kärlkokning.
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- Fasci, Maria Letizia, et al.
(författare)
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Temperature of energy boreholes accounting for climate change and the built environment - A new model for its estimation
- 2023
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Ingår i: Renewable energy. - : Elsevier BV. - 0960-1481 .- 1879-0682. ; 202, s. 1479-1496
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Tidskriftsartikel (refereegranskat)abstract
- Changes in the ground surface temperature, as it can occur in built-up areas or due to climate change, affect the temperatures of geothermal boreholes. Analytical models for the thermal simulation of boreholes and consid-ering this factor have been proposed. However, they all impose a uniform heat extraction boundary condition along the borehole walls. This boundary condition overestimates the temperature change in the underground caused by the borehole heat extraction and underestimates it in case of rejection. More accurate results are most often obtained by imposing a uniform temperature boundary condition.In this paper, we propose a new model to calculate the boreholes wall temperature taking into account both the heat extractions/rejections from all the boreholes in the area and the change in ground surface temperature. The model is tailored for areas with independent ground source heat pumps and imposes a uniform temperature boundary condition along the borehole walls, overcoming the limitation of the existing models.We apply the new model to a real Swedish neighbourhood and show that existing systems may already be significantly affected by the increased ground surface temperature due to urbanization. We also compare our new model with an existing similar model and show that while the two models provide similar results for smaller areas, their difference tends to be relevant for bigger areas - including the real Swedish neighbourhood analysed -thus making the application of our model important for neighbourhood-and city-scale studies.
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- Fascì, Maria Letizia, 1993-
(författare)
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Thermal Interference Between Neighbouring Ground-Source Heat Pumps : Tools to Calcualte it and Solutions to Limit it
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ground Source Heat Pumps (GSHPs) have emerged as a promising technology for decarbonization of the heating sector. However, their increasing prevalence in densely populated areas may lead to considerable changes in the ground temperature, which can ultimately lead to reduced performance of the systems. Being able to forecast the ground temperature change in areas with a high density of GSHPs is therefore important for a correct management of the geothermal resource and the proper design of the systems. The main scope of this thesis is to propose a new heat transfer model that enable the calculation of the ground temperature development in areas with several GSHP systems connected to vertical borehole heat exchangers. This work resulted in two new models, here referred to as FLSIV and FLSIVTBC, which are built upon the Finite Line Source model, a classic building block for semi-analytical models dedicated to geothermal boreholes. The application of these models to realistic scenarios representative for Stockholm has revealed a potentially significant interference between neighboring geothermal boreholes in densely populated areas. Therefore, a secondary objective of this thesis is to explore possible strategies for limiting this interference. The last section of this thesis investigates the possibility of increasing the borehole length or reducing the thermal load to constrain the ground temperature change, and the possibility of limiting the number of GSHPs in an area to prevent a severe drop of system performance. The results showed that the first two strategies may require a significant change in the operation or design of the systems, while the third strategy may permit a high utilization of GSHPs without modifications of the systems. Keywords: Ground source heat pumps, Thermal interference, Neighboring geothermal borehole, Finite Line Source, Mitigation strategies.
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- Palm, Björn, 1955-, et al.
(författare)
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Effektiva plattvärmeväxlare som förångare i värmepumpar - Vidare studier. Slutrapport
- 2005
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The purpose of the project was to increase the understanding of the flow- and heat transfer phenomena appearing in a brazed plate heat exchanger used as evaporator in a single family heat pump. Several different parameters influence the performance in this case, e.g. inlet vapor quality, heat flux, mass flow, distribution device at inlet and temperature profile on the heat source side. During the project these parameters have been varied and the performance of the heat exchanger investigated. The project has been mainly experimental, but theoretical investigations have been performed, based on the experimental results. One area which was specifically investigated was the influence of the design of the distribution device on the overall performance. Heat exchanges with identical heat transfer surfaces, but with different distribution devices were compared experimentally. Relatively large differences were found, at any given heat load. Depending on the load, different designs of the distributor was found to give the best results. In this report, the results of the project are presented in brief. A more complete description of the results is found in the ph.d. thesis in which the project has resulted. (Joachim Claesson: Thermal and Hydraulic Performance of Plate Heat Exchangers used as Evaporators
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