Evolution of energy standards led to high-performance buildings requiring very low energy for their operation. Occupancy is the variable with the greatest impact on nearly Zero Energy Building (nZEB) ...energy performance because both constructive and technical characteristics have been improved over time. Occupants influence energy use in buildings as they contribute to internal gains, interact with systems and modify indoor conditions with their behaviour. Assumptions about occupancy schedules are usually adopted in energy models for compliance calculation and when experimental data are not available. These theoretical profiles might be far from real conditions and frequently generate a mismatch between expected and actual performance. The present work analyses six months of monitored data from an nZEB in Denmark. A simulation model is used to analyse the effect of three different occupancy profiles on the final energy use: the 'Compliance profile', defined on the basis of regulations, the 'Standard profile', built on average data obtained from surveys, and the 'Actual profile', customized on measured data from the actual building case. Significant differences are detected in the three different occupancy profiles as well as in the results achieved by applying the three occupancy models in performance prediction.
The thermal performances of building components require an accurate evaluation of the heat transfer processes on the internal and external surfaces. These evaluations involve the convective and the ...radiative heat transfer coefficients, which in actual conditions vary dynamically. Nevertheless, international standards indicate combined and steady heat transfer coefficients, especially in building energy performance calculations. In this paper, an experimental campaign carried out on a real scale measurement station available at the University of Calabria (Italy), has allowed for comparing the actual data with those calculated by a numerical code based on the Finite Difference Method (FDM), implementing the usual equations governing the heat transfer processes. The code is generalizable for every enclosure and does not require excessive computational effort. By assuming dynamic and separated heat transfer coefficients first, and successively steady and combined values, a comparison in terms of surface temperatures and transmitted thermal fluxes was conducted using the developed code on different building components, having different dispersing and thermal mass features. Regarding the measured values, the provided results showed a large discrepancy when steady and combined heat transfer coefficients are adopted, especially for the inner surfaces. Dynamic and separated heat transfer coefficients, instead, allow for meeting experimental data quite well, independently from dispersing and thermal mass characteristics. Furthermore, in simulation tools, a large discrepancy appears both in terms of energy needs and thermal comfort conditions on the experimental set-up, when simulated with steady convective heat transfer coefficients.
•Experimental values were compared with modeled surface temperatures and thermal fluxes.•McAdams correlations are suitable for the internal convective heat transfer coefficients.•The radiance method is robust for evaluating internal radiative exchanges.•Isothermal components are indicative of reliable results.•Steady convective heat transfer coefficients produce in TRNSYS a worsening of the results.
•A set of parameters for a complete thermal characterization of PCM layers are defined.•Various PCM layers with different melting temperatures are considered.•The analysis regard a continental and a ...mediterranean climate.•Dynamic parameters are related to the latent storage efficiency.•Phase change in the 35% of the layer thickness is sufficient to reach high thermal performances.
The objective of the research is the definition of a new set of parameters to evaluate the effective dynamic thermal behavior of a layer subject to phase change (PCM) that, for the effect of non-sinusoidal periodic boundary conditions, characterizing the external walls of air-conditioned buildings, give rise to the formation of one or more melting or solidification bi-phase interfaces. Such bi-phase interfaces originate on the boundary surfaces, or are always present and fluctuate within the layer. Defined parameters are to be used for the thermal design of innovative walls containing a PCM layer, targeting the reduction of power peaks entering the environment, in order to reduce the energy requirements and even to improve the indoor thermal comfort.
The study has been developed by a finite difference numeric calculation model, which explicitly determines, the number and the position of the bi-phase interfaces that originate in the layer and the temperature and the heat flux fields.
The methodology developed allowed us to determine the dynamic characteristics, for each month of the year, of PCM layers with different melting temperatures and thermophysical properties and subject to climatic conditions of two locations, one with a continental climate and the second one with a Mediterranean climate. In particular, it was found that all defined dynamic parameters, irrespective of locality and of PCM type, are related to the latent storage efficiency and, furthermore, some calculation correlations between the dynamic parameters were obtained.
Finally, the results show that it was sufficient to reach the phase change in a portion of the layer of about 35% to obtain excellent dynamic thermal performance.
In Ocean Thermal Energy Conversion systems (OTEC), the production of electric energy is achieved in tropical areas exploiting the warmer surface seawater as hot source for the vaporization of a ...specific working fluid, and the coldest water pumped from the depths for its condensation. These systems are very expensive due to both the required large infrastructures and the limited values of thermal efficiency. To reduce the payback period of investment costs, greater values of thermal efficiency are recommended and they could be obtained by an augment of the temperature difference between the two heat sources. In this paper the possibility of using Liquid Natural Gas (LNG) stored at atmospheric pressure and temperature of 113K in regasification terminals is investigated. With reference to data concerning an LNG regasification plant designed for Gioia Tauro (Southern Italy), the energy performances of a novel OTEC system which operates through two thermodynamic cycles in cascade, using pure ammonia and LNG as working fluids, were evaluated. The proposed plant allows an increase of the electric power recovering the LNG exergy during the gasification process and, at the same time, guarantees the complete gasification of LNG for the immission in the gas pipeline network.
•LNG to decrease the cold source temperature in OTEC plants has been investigated.•Recovery of energy from LNG vaporization has been studied.•Correspondent improvements of OTEC thermal efficiency have been quantified.•Two cycles in cascade have been proposed to promote LNG vaporization.•Preliminary economical and environmental analysis has been carried out.
The authors propose a methodology to schematize correctly the capacitive effects in the transmission of heat in the multilayered walls of buildings.
An analytical study is presented related to a ...steady periodic regime allowing consideration of three external loads acting singularly or simultaneously: air temperature, apparent sky temperature and incident solar irradiation.
Such a study is applied in the case of four traditional types of wall (A – brick wall, B – hollow wall, C – polarized brick wall, D – prefabricated wall).
The expression of the oscillating heat flux, which penetrates the internal environment, and the conductive heat flux which penetrates the wall in contact with the external air, was obtained by means of the electrical analogy and the resolution of the equivalent circuit. It is demonstrated that the nondimensional periodic global transmittance, the ratio between the heat flux which is transferred to the indoor environment and the external heat flux, with the plant turned on, is the most suitable nondimensional parameter for the dynamic analysis of the walls. This parameter allows for the evaluation of all the typical dynamic quantities for the complete description of the thermal behavior of the walls.
•The thermal behavior of building walls by the harmonic method is analyzed.•Three forcings for the thermal dynamic characterization of the walls are considered.•New methodology and parameters for the dynamic analysis of the walls are formulated.•The parameters are applied to compare the thermal performance of different walls.
Dynamic thermal characteristics, for each month of the year, of PCM layers with different melting temperatures and thermophysical properties, in a steady periodic regime, were determined (Mazzeo et ...al., 2017 1). The layer is subjected to climatic conditions characterizing two locations, one with a continental climate and the second one with a Mediterranean climate. This data article provides detailed numerical data, as a function of the latent storage efficiency, including monthly average daily values: of the latent energy fraction, of the decrement factors of the temperature, of the heat flux and of the energy, and of the time lags of the maximum and minimum peaks of the temperature and of the heat flux.
•Dynamic behaviour of building walls subjected to sinusoidal and actual loadings.•The joint action of more temperature and heat flux loadings has been considered.•Dynamic parameters were defined by ...the internal and external fluctuating heat flux.•Use of the Total Harmonic Distortion to determine the number of harmonics required.•Study of the influence of external and internal loadings on dynamic parameters.
The dynamic behaviour of opaque components of the building envelope in steady periodic regime is investigated using parameters defined by the fluctuating heat flux that is transferred in the wall. The use of the heat flux allows for the joint action of the loadings that characterise both the outdoor environment and the indoor air-conditioned environment to be taken into account.
The analysis was developed in sinusoidal conditions to determine the frequency response of the wall and in non-sinusoidal conditions to identify the actual dynamic behaviour of the wall. The use of non-dimensional periodic thermal transmittance is proposed for the sinusoidal analysis in order to evaluate the decrement factor and the time lag that the heat flux undergoes in crossing the wall as well as the efficiency of heat storage.
In the presence of non-sinusoidal loadings, the identification of the dynamic behaviour of the wall is obtained using several dynamic parameters: the decrement factor in terms of energy, defined as the ratio between the energy in a semi-period entering and exiting the wall; the decrement factor and the time lag in terms of heat flux, considering the maximum peak and the minimum peak. These parameters allow for the identification of how the form of the heat flux trend crossing the wall is modified.
The number of harmonics to be considered for an accurate representation of heat fluxes is determined by means of the introduction of the Total Harmonic Distortion (THD), which quantifies the distortion of a non-sinusoidal periodic trend compared to a sinusoidal trend.
The methodology developed was used to evaluate the influence of external and internal loadings on the dynamic characteristics of two commonly used walls on a monthly and seasonal basis. The external loadings were changed considering two climatically different locations and different orientations of the walls; the internal loadings were changed by varying the operating mode of the plant and the shortwave radiative heat fluxes contributions on the inner surface.
One of the pillars of the policies undertaken to achieve climate neutrality by 2050 in the European Union is the decarbonisation of the construction sector. Massive energy renovation of existing ...buildings is expected in the coming years and encouraged by supporting incentives. A considerable amount of resources will be invested in this area and the large-scale interventions will be conveyed by Energy Service Companies that usually employ Energy Performance Contracts for the implementation of the restructuring interventions. This form of contract is generally bound to the achievement of a certain level of energy performance, which ensures an adequate profit margin for the Energy Service Company and the users subscribing the contract. But how reliable is the forecast of energy consumption and to what extent could it constitute a well-founded guarantee based on an Energy Performance Contract? The analysis proposed in this paper aims at investigating in this direction and proposes a design methodology based on Uncertainty and Sensitivity Analysis that can be validly applied to reduce the investment risk. A case study consisting of an Italian social housing complex is considered for energy, economic, and environmental evaluations. The results show that the Uncertainty and Sensitivity Analysis allows for identifying corrective measures at the design stage, making it possible to increase the chances of success of the intervention by 60%, raise the earning potential by 4% compared to the predictions, and obtain an additional reduction in CO2 emissions of 40%. This makes the investment more attractive and safe promoting the participation of Energy Service Companies in the renovation of the existing building stock.
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•Building renovation is crucial for the achievement of carbon neutrality.•Energy Performance Contracts are used to implement building energy redevelopment.•Uncertainty and Sensitivity Analysis validly supports building renovation planning.•Corrective measures that reduce uncertainty allow to increase chances of success.•The Uncertainty and Sensitivity Analysis allows to reduce the investment risks.
•A cost-optimal analysis was carried out for a traditional Italian social housing unit.•A disaggregated analysis showed the weight of different interventions for refurbishment.•Measures on ...technological plants are highly recommended if cooling is required.•Heat pumps integrated with active solar systems are always recommended.•Interventions on the envelope are justified only in warm climates if a cooling plant is adopted.
The renovation of existing buildings represents a major challenge towards the transition to sustainable cities, as these edifices were built according to principles far from the current standards, especially in those urban areas where the poor quality of structures often determines social conditions of hardship and unlivable spaces. For refurbishment planning, suitable tools are required for the choice of more recommended interventions. For this purpose, the cost-optimal analysis represents a reliable way to promote the energy requalification of the existing building stock, identifying the best compromise between the energy demand reduction and the cost of the intervention. In this paper, a cost-optimal analysis was conducted on a reference building (RB) representative of a typical Italian social housing building, widespread in disadvantaged areas, strongly energy-inefficient and requiring urgent renovation. The influence of different energy efficient measures (EEMs) was considered by locating the RB in two different climatic zones, to evaluate how weather data also influence the obtained results. The economic analysis was carried out as a financial projection rather than a macroeconomic evaluation, in order to increase the stakeholder awareness for targeted routes addressed to the achievement of social and environmental advantages, while respecting a sustainable economic frame.
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•Dynamic thermal behaviour of building walls subjected to actual periodic loadings.•Dynamic parameters of wall in terms of energy and of heat flux are defined.•Different solar ...absorption coefficients and orientations of wall are considered.•On the internal surface is present or absent a shortwave radiant field.•Seasonal thermal characteristics for different plant operating regime are provided.
In this work, the dynamic characteristics of the external walls of air-conditioned buildings subject to the joint action of periodic non-sinusoidal external and internal loadings are determined. The dynamic parameters used are the energy decrement factor, which is evaluated by means of the fluctuating heat flux in a semi-period exiting and entering the wall, the decrement factor of the maximum peak and minimum peak of the heat flux in a period and the relative time lags. The fluctuating heat flux in the wall in steady periodic regime conditions is determined with an analytical model obtained by resolving the equivalent electrical circuit. The preceding parameters are used for a study of the influence of solar radiation on the dynamic characteristics of the walls in summer and winter air-conditioning. Solar radiation is considered as operating on the external surface and on the internal surface due to the presence in the indoor environments of a shortwave radiant field. The absorbed solar heat flux by the external surface varies, modifying the solar absorption coefficient and wall orientation. Indoors, we considered a continuous operating regime of the plant and a regime with nocturnal attenuation.
The results obtained, relating to 1152 different boundary conditions, were used for the construction of maps of dynamic characteristics, different on variation of the plant functioning regime and of the shortwave radiant load on the internal surface. The maps show the dependence of the decrement factors and of the time lags on variation of the season, of the external surface absorption coefficient, of the orientation and the type of wall, and of the locations. This representation allows a comparison of the dynamic behaviour of the wall in the two air conditioning seasons and, for defined boundary conditions, to quickly determine the dynamic characteristics.
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