Magnus Hultén

Opening dimensions of variation: An empirical study of learning in a web-based discussion

The discussions held in specifying an automatic vehicle as a project in a distance work-related course are in focus for an analysis of learning in networked discussions. Learning is characterized as coming to experience things in distinctly new ways, in keeping with the underlying phenomenographic research approach for the study. Group discussions are seen as one feature in the experience of, and the formation of, the context for learning, in an experiential interpretation of activity systems. The analysis has led to the identification of pivotal contributions to discussions that can be said to afford learning within the group, in the sense of opening dimensions of variation around critical features of the task. This is developed to produce a taxonomy of contributions, with participatory, factual, reflective and learning contributions, suggested to be necessary, though not sufficient, conditions for learning. The taxonomy is illustrated and discussed, as are its implications for tutoring in networked courses of this type.

The compression/absorption heat pump cycle—conceptual design improvements and comparisons with the compression cycle

Performance improvement of an industrial single-stage compression/absorption heat pump (CAHP) using an ammonia/water mixture as the working fluid has been studied theoretically. By allowing a higher absorber pressure (40 bar) than the highest design pressure of todays screw compressors (25 bar), higher COPs could be obtained. Longer falling-film tubes in the vertical shell-and-tube absorber and desorber also increased the COP. These two modifications together increased the COP of the CAHP by 10%. The improved design has a lower optimal absorber glide (temperature difference due to composition change in absorber) and reduced solution heat exchanger sizes. The study was performed with a constant total area. Furthermore, the CAHP performance was studied for five heating cases. Its performance was compared to that of a two-stage compression heat pump (CHP) using isobutane as working fluid, on the basis of approximately equal investment cost. It could be concluded that only heating cases where both the sink and the source temperature changes are high (>20 K) give superior performance for the CAHP.