Industrial excess heat for district heating - Comparison of potentials from top-down and bottom-up studies for energy-intensive process industries

The threat about increased global warming and the over-use of the Earth’s resources makes it important to reduce fuel usage. One way to reduce fuel usage is to be more energy efficient and a way to be more energy efficient is to use industrial excess heat as a source of district heating. District heating from excess heat is delivered by Swedish industrial plants today. There have been studies trying to evaluate the potential for further production of district heating from excess heat at a national level in Sweden. In one major study, ratios were calculated for different industrial sectors that propose estimates for excess heat delivery to district heating networks per unit of fuel usage at the industrial plant. In this study five energy analyses of different energy-intensive industrial plants have been evaluated. The potential for producing district heating from excess heat using pinch analysis and the current potential for producing district heating based on heat flows in process stream coolers have been calculated. These results have been compared to each other and with the results using the ratios described above for specific industrial sectors. In the pinch analysis, the impact of different values of the minimum temperature difference allowed in the heat exchangers (∆Tmin) on the potential for export of excess heat have been evaluated. An increase in ∆Tmin results in an increase in the minimum cooling demand. Generally, one would expect that an increased cooling demand would increase the possibility to produce district heating. However, this is not the case for all of the industrial plants studied in this report. Two of the five studied industrial plants show an opposite behaviour. An increase in ∆Tmin does increase the cooling demand but the potential for producing district heating is however decreased. This makes it difficult to define a minimum amount of excess heat for district heating, which could be regarded as “true” excess heat, for these plants. For the other plants studied, an increased value of ∆Tmin leads to an increased potential for district heating delivery. These plants clearly illustrate the trade-off between increased energy efficiency at the plant, leading to decreased internal fuel usage, and delivery of excess heat as district heating. For these plants it would be possible to define a minimum amount of “true” excess heat for district heating. However, it should be noted that the theoretical potential for export of excess heat depends on the selected value of ∆Tmin, thus it is difficult to provide a general definition of “true” excess heat. The potentials estimated using GCC curves from pinch analysis, available heat in existing process coolers, as well as estimates based on plant fuel usage differ to different extents for the different plants, but are on the same order of magnitude. The results based on GCC curves and available heat in process coolers are in most cases very sensitive to the assumed value of district heating supply temperature