1,720,968 research outputs found
Efficacy of coupling heat recovery ventilation and fan coil systems in improving the indoor air quality and thermal comfort condition
Full text linkMechanical Ventilation with Heat Recovery (MVHR) systems are gaining increasing interest in buildings with low energy demand, for improvement of the Indoor Air Quality (IAQ) and reduction of the ventilation energy loss. In retrofitted buildings, MVHRs are often integrated with an additional air heater to cover space heating demand. Hence, evaluation of the interactions between MVHR and heat emitter, and their effects on indoor airflow characteristics is of significant importance. The present study aims to investigate effects of a combined MVHR-fan-coil system in heating mode on IAQ and thermal comfort parameters inside a retrofitted room, by means of a computational fluid dynamic (CFD) code. The proposed CFD model is validated by comparing the numerical results with experimental data. The results yielded by numerical simulations allow evaluating the indoor environmental quality characteristics as well as addressing the MVHR and fan coil interactions. The results indicate that the airflow discharged from the fan coil could have a significant impact on the age of the air; while it provides a desirable thermal comfort condition within the room, it may hinder to some extent delivery of the fresh air to the occupied zone due to creation of counterflow fields. Furthermore, it is shown that although increasing the fan speed (ON mode) would slightly enhance the air change efficiency, the OFF mode yields not only a better distribution of the fresh air but also a higher ventilation efficiency than when fan coil operates
Thermal performance of the vertical ground heat exchanger with a novel elliptical single U-tube
No full textGround-Coupled Heat Pumps (GCHPs) are known as a promising technology for building climatization and domestic hot water production. The main concern to employ these systems in urban areas is their high drilling and initial cost. This cost could be rebated by lowering the borehole thermal resistance and by shortening the borehole depth. In the present paper, a novel and simple design of the Ground Heat Exchanger (GHE), namely the vertical GHE with an elliptical U-tube, is proposed to improve thermal performance of GCHP systems. To prove that; a series of 3D finite element simulations are performed to study the thermal comportment of elliptical U-tubes and to compare their performance with typical single U-tubes. A dimensionless shape factor γ is introduced to evaluate effect of the elliptical U-tube geometry on thermal performance of the GHE. Furthermore, influential parameters on thermal performance of elliptical U-tubes are investigated and the decrement percentage of the borehole thermal resistance, with reference to the typical U-tube, are compared for each parameter. The results show that elliptical U-tubes could enhance heat transfer to significant extent and could decrease the borehole thermal resistance by more than 17 %, compared with typical single U-tubes. It is shown that the higher value of the shape factor γ leads to the better heat transfer enhancement. It is concluded that elliptical U-tubes are able to decrease the borehole thermal resistance and to boost the coefficient of performance (COP) of GCHP systems
Nanofluid suspensions as heat carrier fluids in single U-tube borehole heat exchangers
Full text linkThe borehole heat exchanger (BHE) is a critical component to improve energy efficiency and decreasing environmental impact of ground-source heat pump systems. The lower thermal resistance of the BHE results in the better thermal performance and/or in the lower required borehole length. In the present study, effects of employing a nanofluid suspension as a heat carrier fluid on the borehole thermal resistance are examined. A 3D transient finite element code is adopted to evaluate thermal comportment of nanofluids with various concentrations in single U-tube borehole heat exchangers and to compare their performance with the conventional circuit fluid. The results show, in presence of nanoparticles, the borehole thermal resistance is reduced to some extent and the BHE renders a better thermal performance. It is also revealed that employing nanoparticle fractions between 0.5% and 2 % are advantageous in order to have an optimal decrement percentage of the thermal resistance
Energetic Optimisation of the Domestic Hot Water System in a Residential Building by Means of Dynamic Simulations
No full textThe present study deals with the energetic optimisation of Domestic Hot Water (DHW) system in a residential building located in Catania, Italy. Each dwelling is equipped with a specific decentralised tank with an internal heat exchanger which is connected to a 2-pipe hot water network system for tank charging. The technical water is produced by an Electrical Heat Pump (EHP) coupled to a central storage tank. The energy performance analysis of the DHW model is evaluated by means of dynamic simulations under three different scenarios of charging the decentralised storage tanks by circulating pump unit: Pump activated during daytime, activated twice a day, and activated three times per day. The results obtained allow an evaluation of the DHW consumption profile, temperature variation in central storage and decentralised tanks, and the annual electrical/thermal energy analysis. The results indicate that the activation of the circulating pump during the day leads to an achievement of the highest amount of thermal energy, as well as having minimum temperature oscillation in both central storage and decentralised tanks. However, these advantages are at the cost of consuming much more electrical energy by the heat pump and up to 29 % higher emissions of CO2. The best scenario in terms of energy-saving and CO2 emission is the case in which the circulating pump works twice a day, consuming annually 5,832 kWh less electrical energy, compared to the case of an activated pump during the day
Techno-economic analysis of a novel retrofit solution for the domestic hot water system: A comparative study
Full text linkThe retrofit solution for domestic hot water (DHW) system in existing buildings requires to ensure the long-term energy security and efficiency as well as to minimise occupants’ disturbance, construction works and installation costs. In this regard, the present study performs a techno-economic evaluation on a novel retrofit solution for DHW production in a pilot building. The proposed solution appoints a substantial role to the thermal energy storage through a 2-pipe hot water network utilisable for both DHW and heating purposes. The first storage level is provided by a centralised buffer storage supplied by a PV-BESS-driven heat pump while the second level consists of decentralised modular tanks installed in each dwelling for the production and storage of hot water. Firstly, experimental thermal performance of the proposed decentralised storages is investigated. By developing a dynamic simulation code, the energy efficiency of the proposed solution is compared to that of the existing system in the pilot building as well to that of a typical centralised system as a benchmark solution. Finally, economic analysis of the retrofit solution is performed to address capital expenditures of the system, including purchasing and installation costs, as well as its life cycle cost (LCC). The obtained results indicate that the proposed system reduces the annual energy consumption for DHW production more than 7,200 kWh, with respect to the existing DHW system. Furthermore, it is shown that, in the proposed system, the fraction of thermal loss from piping network decreases by 31.5%, compared to a typical DHW centralised system. Economic assessment of the proposed solution implies that this system, in terms of both mechanical and electrical components, requires 13.7% lower initial investment than a typical centralised system. However, the cost of control systems in this system is higher since it is inherently a control-based system
Numerical study on indoor environmental quality in a room equipped with a combined hrv and radiator system
Full text linkHeat recovery ventilation (HRV) systems can be integrated with an additional air heater in buildings with low energy demand in order to cover space heating demand. The employment of coupled HRV-heater systems is, therefore, gaining increasing interest for the improvement of the indoor environmental quality (IEQ), as well as the reduction of ventilation energy loss. The present paper analyses the efficacy of a HRV system, coupled with a low-temperature radiator, in satisfying the IEQ indices inside a retrofitted dormitory room. A computational fluid dynamic (CFD) model based on the finite volume method is established to investigate IEQ characteristics including indoor air quality and thermal comfort condition. The presented CFD code provides a practical tool for a comprehensive investigation of the IEQ indices in spaces employing a coupled HVAC system. In an analysis of indoor air quality, parameters such as age of the air, air change efficiency, and ventilation efficiency in removal of gaseous contaminants, namely VOCs and CO2, are evaluated. The results obtained by the numerical model allow addressing the interaction between HRV and radiator systems and its effects on airflow field. The results show the decrease of the indoor operative temperature with increment of the supply air flow rate, which is mainly due to the decreased thermal efficiency of the HRV system. The obtained results indicate that, while higher ventilation rates can significantly decrease the age of the air and gaseous contaminants level, at the same time, it would cause a local discomfort in some parts of the room
A Multi-criteria Optimization Framework for the Residential Hot Water Network Emphasizing on the Role of Control Strategy
No full textCutting-edge technologies and optimization frameworks for energy efficiency enhancement of the entire domestic hot water (DHW) chain are crucial to fulfill the ambitious goals of the future building regulations. In this context, the present study establishes a multi-objective optimization framework for the DHW network in a typical residential building, in which the hot water is supplied by a PV-BESS driven air source heat pump system relying on the thermal energy storage (TES) to decouple energy production and demand. Emphasizing on the role of in-building control strategies and user behavior, the optimization algorithm employs the response surface methodology (RSM) with central composite design (CCD). It seeks to simultaneously minimize the total energy use for DHW production and total heat loss from the DHW network, while maximizing the temperature of delivered hot water to users as well as the TES mean temperature. To examine interactions in components of the DHW network, dynamic simulations are carried out by developing a TRNSYS model coupled to a MATLAB code. The latter generates the hourly DHW consumption profiles using Gaussian distribution. It is shown that the developed optimization framework strikes a balance between conflictive design factors to meet the targets of multi-criteria optimization. The variable TES set-point is found to be the most influential factor in terms of providing hot water at a higher temperature to users. Furthermore, adjusting the activation time (and flow rate) of recirculating loop and the TES charging time slots in accordance with the user behavior (draw-off) and peak consumption timespans demonstrate a significant impact on minimizing either the total energy use or thermal loss
Multi-objective study on an innovative system for domestic hot water production: A pilot building in Southern Europe
Full text linkThe present study deals with a multi-objective analysis of an innovative decentralised system to produce and store domestic hot water (DHW), emphasising on the combined effects of the technological aspect, control strategy and user's behaviour. The proposed system, by relying on thermal energy storage, decouples energy production and demand while shaves peaks in the energy demand and, at the same time, provides more autonomy to users through local storages. To identify subtle interactions in components of DHW system, dynamic simulations are carried out by establishing a coupled TRNSYS-MATLAB code, calibrated and validated by experimental measurements. The energy analysis implies that the proposed system cuts the required annual electrical energy in half, of which up to 82% of needed primary energy is supplied from renewable sources, compared to previous electrical-decentralised system. The optimisation of the results through applying control strategies indicates that adopting a three-time charging scheme is advantageous in terms of providing a more stable temperature profile as well as a higher hot water temperature. Compared to an available-by-demand operation, this scheme reduces the required total annual electricity by 5.2 % and enhances total thermal loss from components up to 4.0%. Furthermore, a sensitivity analysis on the results emphasises the striking role of the user behaviour in electrical energy consumption either via draw-off temperature or adjusting the pre-defined temperature for activation of the built-in auxiliary heater
The role of near-wall downdraught and asymmetric temperature distribution in dispersion of respiratory aerosols in radiant floor heating systems
No full textIn radiant floor heating (RFH) systems, the downdraught due to the asymmetric temperature distribution causes descending airflows towards the floor surface, where the warm air adjacent to the RFH system tends to be driven upwards by the buoyancy force. Hence, this conflict creates a zone that prevents the warm air to ascend affecting the streamlines of the indoor air. The present study aims to investigate the integrated effects of this phenomenon at different levels of the RFH temperature and thermal transmittance on the behaviour of indoor airborne particles (PMs2.5). In this context, a Eulerian-Lagrangian computational fluid dynamic (CFD) code is established, validated against experimental data, to address the dispersion and deposition patterns of PMs2.5. To generate different levels of the downdraught and non-uniform temperature distribution, five scenarios are considered regarding different thermal transmittance (U-value) levels, assessed for four RFH temperatures. Firstly, by introducing the near-wall and zonal spaces, the dispersion of particles in each scenario is evaluated. Then, the role of RFH system temperature in conjunction with each scenario is investigated. Finally, simple correlations are proposed allowing for fast evaluation of the decay rate coefficient of PMs2.5. According to the obtained results, the asymmetric temperature distribution causes a striking disparity in the zonal concentration of suspended PMs2.5, i.e., up to 32 % difference in the number of particles between quarters. It is shown that a faster decay rate of PMs2.5 is associated with a larger value of the characteristic temperature difference and the Rayleigh number (Ra). For a given RFH temperature, thermal performance improvement of the envelope reduces the number of deposited particles on the ceiling surface, whereas it gives a boost to the number of particles adhering to the floor. A sensitivity analysis on the results revealed that a 1 °C increment in the RFH temperature leads to an 8.4 % reduction on average in the number of suspended PMs2.5 in the breathing zone, regardless of the level of thermal transmittance from surrounding walls
CFD Analysis of the Impact of Building Shape on Natural Ventilation Effectiveness in High-Rise Buildings
No full textWith the increasing global emphasis on sustainability and high energy efficiency in buildings, natural ventilation is increasingly recognized as an effective passive strategy for reducing the cooling energy demand of buildings while ensuring indoor comfort and IAQ. Given its unstable nature and high dependence on building form, understanding and predicting how various aspects of building design impact natural ventilation effectiveness within the building becomes crucial to benefit it fully. This paper combines CFD analysis with building energy simulation to evaluate the effect of building shape on the cross-ventilation cooling potential of a real high-rise building in India. A comparative analysis of two distinct units with different shapes was conducted to evaluate the better shape for enhancing natural ventilation performance and thermal comfort. The results suggest proper orientation to prevailing winds, sizing and positioning of openings, optimization of indoor sections and interconnection between spaces are crucial in improving the effectiveness of wind-driven cross-ventilation. The unit with a higher ratio of inlet to outlet area (2:1 ratio) on the windward and leeward facades achieved a higher air exchange rate and average air velocity, ensuring an average 0.3 °C and a maximum up to 1.07 °C reduction in operative indoor temperature. The research outcomes provide valuable insights to optimize the building shape of high-rise buildings improving natural ventilation
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