No abstract

According to the official statistical reports, gas-fired boiler units still remain to be one of the main equipment types for meeting the space heating and daily hot water demand of the residential dwellings across the European Union. Due to the prevalence of the natural gas grid and performance stability, gas-fired boilers are considered to remain as one of the standard energy sources. On the other hand, even though gas-fired water heating technology is a well-known concept, existing numerical models found in the literature are often case-specific with poor reusability mostly reflected in fitted efficiencies. Algorithms behind these models usually require the input of large amount of hardly attainable design characteristics of the units. In this paper, a modelling method for acquiring the performance of a heating gas-fired condensing boiler unit will be shown. The model is based on the limited input data available in the official characteristics of the units issued by the relevant manufacturers. The simulations are programmed by using the programming language Modelica and the software tool Dymola. The model is based on the fixed natural gas intake which combusts into a stable mixture of the combustion gases that further heat the circulating water. During the heat transfer process inside the condensing boilers there is a possibility for condensate formation out of the water vapour of the combustion gases which increases the efficiency of the unit. The formation of condensate, however, is depending on the return water temperature of the unit which has to be lower than the dew point temperature of the combustion gasses. The goal of this research is to determine how accurate can performance indicators of gas-fired boilers be attained with the use of a limited amount of available input data together with clearly defined assumptions that follow the modelling methodology.

Residential buildings claim a significant share of the total energy use worldwide. In order to have more realistic energy performance predictions, increased attention is paid to the analysis of the building’s energy use through comprehensive, transient detailed numerical simulations. In this article, the self-consumption and self-sufficiency values of three detached residential buildings are assessed through numerical models made in the programming language Modelica and software tool Dymola. The three buildings have the same structure and different space heating energy demands of 15 kWh/m2year, 30 kWh/m2year and 45 kWh/m2year. The energy use of the buildings coincides with the occupancy profile where domestic hot water use dominates over the space heating demand provided by an air to water heat pump. The discrepancy between renewable energy production and energy consumption is mitigated by means of thermal load shifting and electrical energy storage. In this research, the self-consumption and self-sufficiency of the studied buildings have been analysed as a function of the economically favourable energy storage sizing. For the use of an electrical battery with the installed capacity of 2.5 kWh and thermal energy storage of 250 l, the self-sufficiency results to be 40%, 38.5% and 37% for the three buildings respectively at the specific simulated energy demand conditions.

In the line of measures aiming to reduce the greenhouse emissions and the total energy use of residential buildings, all new buildings built from the year 2020 within the European Union must meet the requirements of so-called Nearly Zero Energy Buildings (NZEB). An NZEB represents a building with high energy performance and significant coverage of its energy demand by utilizing renewable energy sources through on-site energy generation. A self-sustained building represents a building, which is only using energy generated on site to meet its energy requirements. In this article, self-consumption and self-sufficiency of an NZEB are assessed through numerical, transient detailed models made in programming language Modelica and simulated in the software tool Dymola. The two-floor residential building is modelled as a dwelling with the space heating energy demand of 30 kWh/m²year. The energy use of the building coincides with the occupancy profile of a family with four members with the dominating domestic hot water use over the space heating need. The space heating comfort conditions are attained with the low-temperature underfloor heating system. The domestic hot water demand is provided through a 300 l water tank. Both systems are coupled to an air to water heat pump. The aim of the research is to investigate the potential of increasing the self-consumption and self-sufficiency values by using thermal load shifting and electrical energy storage in order to compensate for the direct unbalance in optimal energy generation and energy use. For aspirations in achieving a complete self-sustainable dwelling, different sizes of energy storage and photovoltaic panels are used for the analysed case study conditions. However, special attention is paid to the available south roof space of the dwelling. For the use of 28 m² of optimally positioned photovoltaic panels, the studied building achieves 49.1% of self-consumption and 27.2% self-sufficiency without any use of energy storages. With the electrical battery of 20 kWh and thermal energy storage of 1000 l, the building can meet self-consumption of 89.8% and self-sufficiency of 49.8% for specific boundary conditions.