Energy systems should be analysed according to principles of sustainable development. The optimality of technical systems usually is evaluated by only technical and economical criteria. But the estimation of these criteria is not comprehensive enough in the case when system life cycle is much longer than the period of reliable economical prognosis. In this paper the criteria set of energy system evaluation and optimisation was expanded by energy and ecological standpoints and a new multiple criteria indicator (3E factor ‐ Energy, Economy, Ecology) was introduced. The application of this factor was demonstrated by two examples: optimisation of the district heating network pipeline replacement periodicity and optimisation of the thermal insulation of the single family house.

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Abstract. Phase Change Materials (PCMs) are utilized in different engineering fields, such as thermal storage of building structures, such as domestic hot water, heating and cooling systems, electronic products, drying technology, waste heat recovery, refrigeration and cold storage, and solar cookers. The biggest technological problems for the implementation of low and high temperatures PCM are the insufficient thermal conductivity of the available phase change materials, compatibility with the container material, cycling stability. In order to evaluate efficient and cost effective operation of thermal energy storage, different design concepts for PCM integration into heat exchanger/ accumulator should be investigated.

One of the most important issues in the implementation of energy saving measures is a proper evaluation of possible energy savings. If energy savings are overestimated in an energy audit (EA), a situation may occur where after the renovation, the implemented energy saving measures is not paid back or their pay back time is longer than expected before the funds (mostly loaned) were invested. To avoid such a situation, the procedure of a building's energy audit should involve the correction of calculated heat balance according to its real heat consumption, recalculated to the nonnative conditions. In manipulating different building parameters of not exact meanings, it is necessary to attain that the building's heat demand before renovation (theoretically calculated by the same methodology and parameters as demand after renovation) would be as close as possible to the actual heat consumption, recalculated under normative conditions. Only after such a fitting of the actual and theoretical heat demands (decomposition of building heat balance and correction of its components) is it possible to have a reasonable assessment of the feasible heat savings, after the implementation of heat saving measures. The aim of this work is to disclose some peculiarities of building heat balance that is used for energy audit calculations of existing buildings, to offer a calculation methodology evaluating these peculiarities, to show evidently the importance of the investigated problem.

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