Most of the energy required by residential buildings is for low temperature heat. Often this demand is met by the use of high quality energy sources such as fossil fuels and electricity. The energy analysis alone does not take into account the quality of energy and so it hides the real potential for the so-called energy savings. Thus the concept of exergy must be involved.
In particular the low-exergy approach shows that the energy with low exergy level has to be supplied with energy with similar exergy content. Consequently, the problem is shifted to find energy sources suitable for this purpose. The electricity can be exploited with low exergy losses with high-COP heat pumps. Instead the use of fossil fuels for thermal purposes should be avoided.
One of the possibilities is the energy cascade. In this solution the energy flow is used several times, despite its quality decreases with each step, before being discharged into the environment. Thus the use of the district heating from cogeneration for the space heating seems to be an interesting choice. The issues connected to the cogeneration exploitation force to shift the boundary layers of the analysis from the building to the community level.
In this work different solutions to meet the electricity and heat demands of a community of houses have been compared. The aim is to show that the results obtained from the low-ex approach allow an effective reduction of fuel consumption through a more rational use of the resources. The comparison was addressed initially in steady-state case and then with the use of the software HOMER also for the dynamic case.
To achieve reliable results, this required that the annual profiles for thermal and electrical demand of the reference community are modelled to represent the actual loads as closely as possible. Finally, for each case taken into account, the total consumption, the energy and the exergy efficiencies have been calculated.
Source: KTH
Author: Prandin, Mosè
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