Search

Near-zero energy buildings (nZEBs) are considered as an effective solution to mitigating CO 2 emissions and reducing the energy usage in the building sector. A proper sizing of the nZEB systems (e.g. HVAC systems, energy supply systems, energy storage systems, etc.) is essential for achieving the desired annual energy balance, thermal comfort, and grid independence. Two significant factors affecting the sizing of nZEB systems are the uncertainties confronted by the building usage condition and weather condition, and the degradation effects in nZEB system components. The former factor has been studied by many researchers; however, the impact of degradation is still neglected in most studies. Degradation is prevalent in energy components of nZEB and inevitably leads to the deterioration of nZEB life-cycle performance. As a result, neglecting the degradation effects may lead to a system design which can only achieve the desired performance at the beginning several years. This paper, therefore, proposes a life-cycle performance analysis (LCPA) method for investigating the impact of degradation on the longitudinal performance of the nZEBs. The method not only integrates the uncertainties in predicting building thermal load and weather condition, but also considers the degradation in the nZEB systems. Based on the proposed LCPA method, a two-stage method is proposed to improve the sizing of the nZEB systems. The study can improve the designers’ understanding of the components’ degradation impacts and the proposed method is effective in the life-cycle performance analysis and improvements of nZEBs. It is the first time that the impacts of degradation and uncertainties on nZEB LCP are analysed. Case studies show that an nZEB might not fulfil its definition at all after some years due to component degradation, while the proposed two-stage design method can effectively alleviate this problem. [ABSTRACT FROM AUTHOR]

Net zero energy building (NZEB) is widely considered an effective solution to the increasing energy and environmental problems. Focusing on performance optimizations at single-building-level, conventional control strategies have not considered collaborations among NZEBs (such as renewable energy sharing) and thus optimal results may not be able to be achieved at a higher level, i.e. building-group-level. Few studies have evaluated the building-group-level performance of NZEBs using non-collaborative controls and researchers are still unclear about the performance improvement potentials if collaborative controls are implemented. Considering economic cost, load matching and grid interaction, this study aims to evaluate the performance of a group of NZEBs in which conventional controls are used. Meanwhile, associated performance improvement potentials are analyzed at building-group-level. The study results show that significant performance improvement potentials exist if NZEB collaborations are enabled. The study results also analyze the benefits of the NZEB collaborations in the three different aspects. To improve NZEB performance at building-group-level, new collaborative controls need to be developed to replace the conventional ones. [ABSTRACT FROM AUTHOR]

Real-time optimization (RTO) has been developed to improve the cost and/or energy efficiency of complex heating, ventilation and air-conditioning (HVAC) systems. In current literature, almost all of the developed real-time optimization methods belong to the type of time-driven optimization, in which the action of optimization is triggered by “time”. As optimization should be done when the system operating conditions experience a change that is large enough to cause current operational setting not optimal any more, optimization strategies should recognize ‘significant’ changes and perform optimization when necessary. Since the time-driven optimization is a periodic mechanism in nature while those ‘significant’ changes may not be periodic, the time-driven optimization cannot capture ‘significant’ changes and do the optimization promptly. Therefore, this paper proposes an event-driven optimization (EDO) for complex HVAC systems, the key idea of which is to use “event” rather than “time” to trigger the action of optimization. A systematic event-driven optimization method is illustrated, where its main tasks, including event definition and event identification, will be discussed. The proposed method will be compared with a conventional time-driven method using case studies, through which the main advantages of the proposed method will be identified. [ABSTRACT FROM AUTHOR]

This paper presents a method which combines the bilinear control technique with a set-point reset technique to control the space temperature when its cooling is provided by a GSHP equipped with an on/off capacity control, aiming to (i) improve the robustness of the space temperature control by taking account of load uncertainty; and (ii) to reduce the frequency of the GSHP on/off cycling by using the indoor space temperature set-point reset. When the GSHP is on, a smaller space temperature set point is used; while the GSHP is off, a larger set point is used. The proposed control was tested on a simulation platform, which consists of a ground source heat exchanger, a GSHP, an air-handling unit (AHU) and a middle-sized room. The test results show that the proposed control method is able to achieve a good control performance and has the potential to be applied to a real GSHP integrated air conditioning system to improve the robustness of space temperature control and simultaneously reduce the on/off frequency of the GSHP. [ABSTRACT FROM AUTHOR]

Thermal energy storage (TES), together with control strategies, plays an increasingly important role in expanding the use of renewables and shifting peak energy demand in buildings. Different control strategies have been developed for the integration of TES into building-related systems, mainly including building envelopes, HVAC systems and hot water tanks (HTWs). A systematic survey of control strategies using TES in the building-related systems is still lacking. This paper presents a comprehensive review of these control strategies. It provides a summary of the applied strategies and makes recommendations for future studies. Considering that control techniques serve as a basis for the implementation of control strategies, typical control techniques utilized in the building systems, as well as their strengths and weaknesses associated with the application, are also introduced to help users gain a better understanding. [ABSTRACT FROM AUTHOR]

Chiller sequencing control is an essential function for a multiple-chiller plant that switches on and off chillers in terms of building instantaneous cooing load. It significantly affects both indoor temperature control (hence indoor thermal comfort) and building energy consumption. Various chiller sequencing controls have been developed and implemented in practice, and all of them switch on or off chillers according to a direct or an indirect indicator of the building instantaneous cooling load. Potential uncertainties in the direct or indirect indicator may cause the sequencing control misbehave and deteriorate the control and energy performance of the chiller plant. Until now, there is no any systematic study to investigate those uncertainties. This paper, therefore, proposes such a study. Four typical chiller sequencing controls are considered, including total cooling load-based sequencing control, return water temperature-based sequencing control, bypass flow-based sequencing control, and direct power-based sequencing control. Their uncertainty sources are identified and grouped into different categorizes. In order to facilitate the uncertainty modelling and analysis, all of those uncertainties are shifted to the load indicator of the corresponding sequencing control. Case studies are presented to show that using the proposed method of uncertainty shift and modelling the impacts of the uncertainties on the sequencing controls can be easily identified and analysed. [ABSTRACT FROM AUTHOR]

Highlights: [•] A demand-based supervisory temperature control for a large-scale room is proposed. [•] Uneven distribution of load in a large-scale room is investigated. [•] A large-scale room is divided into several zones for demand-based control design. [•] Primary/secondary VAV boxes are defined and used for demand-based control design. [•] Wireless sensor network is employed to measure temperature at breathing level. [Copyright &y& Elsevier]

Highlights: [•] Four building load prediction methods are compared. [•] Methods are compared in terms of accuracy, general applicability and adaptability. [•] Results are useful for researchers in the area of building energy efficiency. [Copyright &y& Elsevier]

As a type of superfine thermal insulations, vacuum insulation panels (VIPs) have already been extensively applied in insulating refrigerators, cold storages and building envelopes. However, their thermal insulation performance becomes poor with the increase of internal pressure, which results in a short service life of 5–25 years for commonly used VIPs. To enhance their thermal performance and prolong their service life, VIPs with fiber felt/silica aerogel composite cores were prepared. The microstructure and thermal conductivity of the as-prepared core material were experimentally investigated. Based on the experimental results, theoretical models for predicting the thermal performance and service life of VIPs were firstly developed. By using the models the effects of aerogel density (50–200 kg m −3 ) and fiber content (0–20 vol.%) on the thermal performance and the service life of VIPs with aerogel composite cores were studied. The results indicated that a minimum insulation thickness with 5.6 mm was obtained by optimizing the fiber content for a maximum U -value of 0.6 W m −2 K −1 in accordance with the building efficiency standard of the cold area in China of the energy efficiency of public buildings (GB 50189-2015). Finally, the simultaneous effects of the aerogel density and fiber content on service life were further explored by using the service life contour. A long service life with over 50 years was achieved for VIPs with aerogel composite cores, which could promote their applications in building thermal insulation for energy efficiency. [ABSTRACT FROM AUTHOR]

The energy performance of Ground-Coupled Heat Pump systems (GCHPs) depends heavily on the fluid temperature over the entire time scale inside the ground heat exchangers (GHEs) and the ground temperature profile outside the borehole. These temperatures define not only the coefficient of performance (COP) of GCHPs but also the ground receptivity of the energy demand of buildings. Although GCHPs have been studied for many years, it is still a great challenge to develop a model that can accurately predict both short-term and long-term responses due to the complex borehole configuration and the thermal capacity of grout. A new transient quasi-3D entire time scale line source model is proposed in this paper, which introduces the concept of transient borehole thermal resistance and considers the heat flux profile along the U-pipe as a variable. The proposed model is firstly compared with several existent models, including several traditional line source models and a full scale response model, using the data collected from a reported Sandbox experiment. The comparison study shows that the proposed model is able to predict the temperature with a relative error less than 5%. Then, the outside ground temperature profile that defines the borehole distance is analyzed and compared with the Sandbox experiment result, which shows that the proposed model leads to a maximum relative error being less than 3.85%. Finally, the impact of the heat flux profile along the U-pipe on the ground temperature profile prediction is investigated, which shows that when the heat flux profile along the U-pipe is considered as a variable, the determination of borehole distance will be much more accurate. Therefore, the transient quasi-3D entire time scale line source model is an effective method for the fluid and ground temperature prediction and may offer the theoretical basis for the system control and the borehole distance determination. [ABSTRACT FROM AUTHOR]

Indoor air humidity has a significant impact on indoor air quality, building energy consumption, and equipment performance. Excessively low or high humidity is not good for living and working. Hygroscopic materials can be used to moderate the indoor air humidity level. This paper presents a simplified dynamic moisture transfer model of building wall layer of hygroscopic material, and the parameter identification of the simplified model with genetic algorithm by comparing the frequency characteristics of the simplified model with the theoretic frequency characteristics. The proposed simplified model was validated against the published experimental measurements on the system level. The results show that the predicted moisture flux by the simplified model agrees well with the experimental test. The simplified model was also validated on room level by a common exercise in IEA 41 project. The results show that the simplified model has a good agreement with the analytical solution, within ±2.2% in the case CE1A. In the realistic case CE1B, the model prediction matches the results of these detailed models provided in public literature. The proposed simplified dynamic moisture transfer model can be used in building indoor humidity environment and energy consumption simulation with good accuracy and efficiency. • Developing a simplified dynamic moisture transfer model of building wall layer of hygroscopic material. • Model parameters are identified in the frequency domain by the basic material properties. • Validated against the published experiment measurements as well as by a published benchmark exercise. • Much more accurate than the EMPD model and more efficient than the CFD model. • Can be easily integrated into conventional simulation software, such as Simulink or TRNSYS etc. [ABSTRACT FROM AUTHOR]

As most urban residents spend an average 90% of their time in buildings, it is crucial for building residents to monitor and maintain a comfortable indoor environment and air quality. Controlling the indoor carbon dioxide (CO2) concentration level is the major objective of mechanical ventilation systems to ensure healthy indoor air quality (IAQ). Current methods utilize proportional–integral–derivative (PID) controllers and multi-zone equations to operate ventilation systems in large spaces based on ASHRAE Standard 62. However, this approach suggests to determine the fresh air supply rate primarily based on CO2 level without considering occupancy. This often results in insufficient fresh air or energy waste in unnecessary mechanical operations in unoccupied building zones. Therefore, this study proposes a ventilation strategy based on occupancy profiles that captured by a Wi-Fi probe enabled occupancy sensing system. Based on the detected occupancy profiles, the proposed ventilation strategy and other two conventional strategies were compared in a two-day experiment conducted a multi-zone space. The comparison results in the experiment show that the proposed approach could maintain a CO2 concertation level lower than 1000 ppm for 94% (weekday) and 80% (weekend day) of day time for all zones. At the same time, the proposed strategy saved about 44.26% (weekday) and 55.5% (weekend day) of ventilation energy consumption when compared to the fixed rate ventilation strategy and saved about 23.6% (weekday) and 46.1% (weekend day) of ventilation energy consumption when compared to the multi-zone equation strategy. [ABSTRACT FROM AUTHOR]