An analysis of stochastic properties of room temperature, cooling load and air-conditioning systems

Exact prediction of the heating and cooling load, proper sizing of the HVAC systems and the optimal control of the HVAC systems are essential to minimize energy consumption. Since external climates which are the main factors affecting cooling loads fluctuate randomly, calculation of the cooling load must take account of the stochastic nature of these processes.

This paper proposes methods of thermal calculation which give the density function of the HVAC system load considering the stochastic properties of the external climates. Simple HVAC systems are analysed for illustration of the applicability of these methods to many practical problems.     

暖通空调系统的环境影响价值工程

本文将暖通空调系统所营造的建筑室内舒适环境与暖通空调系统耗能引起的温室气体排放相结合,提出了一个新的评价概念——暖通空调系统的环境影响价值工程。本文在对此概念给予理论定义的同时,讨论了其量化计算方法。并在此基础上,以重庆市一酒店建筑为例,进行了实例计算分析。暖通空调系统的环境影响价值工程作为一个性能化评价方法,将暖通空调系统的价值、环境功能和环境成本作为一个整体同时来考虑,充分体现了建筑节能与室内环境改善的统一,旨在对暖通空调系统给予科学合理的评判。     

Energetic, economic and environmental performance of a solar-thermal-assisted HVAC system

A solar-assisted HVAC system was retrofitted in 2006-2009 onto an earlier (1980) energy-efficient building. A hybrid system of flat plate and vacuum tube solar collectors heats water in a large hot storage tank that is delivered to an absorption chiller in the cooling season or directly to heating coils in the heating season. Large chilled water storage tanks are charged off-peak and discharged during the day, cooling the building in parallel with the chiller. Measurements of the seasonal performance of the system are presented. Good overall agreement between actual measurements and earlier numerical modeling results is reported for our system, with one notable discrepancy attributable to the operation of the air terminal units, which requires tuning. In cold seasons, solar thermal energy can easily displace a large fraction of traditional heating sources. In the cooling season, the conversion of heat to cooling capacity incurs several parasitic losses, which if not accounted for properly in the design stage, have the capacity to completely offset any advantage gained from the solar system. The economics of building-scale solar thermal systems are strongly dependent on the cost of energy, and electricity in particular. The economics are favorable where electricity costs are high, and vice-versa.     

建筑环境设计模拟分析软件DeST第9讲冷热源与水系统模拟分析(下)

冷热源系统是整个集中空调系统的核心,它决定了系统能否保障用户的冷热需求,是投资的主要部分,也是能源消耗的主要部分。同时冷热源产生的冷热量主要依靠水系统输配到各末端用户中去。因此冷热源与水系统的联合优化设计是整个空调系统设计过程中至关重要的环节。综述了目前冷热源与水系统联合模拟的现状,详细阐述了DeST模拟软件中冷源和水系统模拟所采用的的模型与模拟方法。通过介绍一个设计实例,指出了冷热源与水系统全工况模拟的应用范围和实际意义。     

Influence of building parameters and HVAC systems coupling on building energy performance

The performance of different HVAC systems varies when coupled with different buildings. This paper examines the relationship between building heating and cooling load and subsequent energy consumption with different HVAC systems. Two common HVAC systems in use throughout the UK office building stock, variable air volume (VAV) system and fan coil (FC) with dedicated outside air system, have been coupled with a typical narrow plan office building with and without daylight control and for both cellular and open plan.The results presented in this paper clearly indicate that it is not possible to form a reliable judgment about building energy performance based only on building heating and cooling loads. For the two investigated systems, variable air volume system and fan coil with dedicated outside air system, the difference between system demand and building demand varied from over −40% to almost +30% for cooling and between −20% and +15% for heating. If a heat recovery unit is used, the difference in heating performance is even greater, rising to −70%.     

Building signatures: A holistic approach to the evaluation of heating and cooling concepts

A sustainable and environmentally responsible building concept aims at a high workplace comfort, a significantly reduced heating and cooling demand, a high-efficient plant system, and the use of renewable energy sources to condition the built environment. This paper presents a comprehensive analysis of the heating and cooling concepts of 11 low-energy buildings in terms of energy use, efficiency and occupant thermal comfort. All buildings investigated employ environmental energy sources and sinks – such as the ground, ground water, rainwater and the ambient air – in combination with thermo-active building systems. A limited primary energy use of about 100 kWh_prim/(m²_neta) as a target for the complete building service technology (HVAC and lighting) was postulated for all buildings presented. With respect to this premise, a comprehensive long-term monitoring in high time resolution was carried out for two to five years, with an accompanying commissioning of the building performance. Measurements include the useful heating and cooling energy use, auxiliary energy use for the hydraulic system, as well as end and primary energy use, occupant thermal comfort and local meteorological conditions. A new methodology is proposed for a holistic approach to the evaluation of heating and cooling concepts, which not only considers the occupants thermal comfort, but also the useful energy consumption and the efficiency of the generation, distribution and delivery of heating and cooling energy.     

HVAC system optimization for energy management by evolutionary programming

Energy management of heating, ventilating and air-conditioning (HVAC) systems is a primary concern in building projects, since the energy consumption in electricity has the highest percentage in HVAC among all building services installations and electric appliances. Without sacrifice of thermal comfort, to reset the suitable operating parameters, such as the chilled water temperature and supply air temperature, would have energy saving with immediate effect. For the typical commercial building projects, it is not difficult to acquire the reference settings for efficient operation. However, for some special projects, due to the specific design and control of the HVAC system, conventional settings may not be necessarily energy-efficient in daily operation.In this paper, the simulation-optimization approach was proposed for the effective energy management of HVAC system. Due to the complicated interrelationship of the entire HVAC system, which commonly includes the water side and air side systems, it is necessary to suggest optimum settings for different operations in response to the dynamic cooling loads and changing weather conditions throughout a year. A metaheuristic simulation–EP (evolutionary programming) coupling approach was developed using evolutionary programming, which can effectively handle the discrete, non-linear and highly constrained optimization problems, such as those related to HVAC systems. The effectiveness of this simulation–EP coupling suite was demonstrated through the establishment of a monthly optimum reset scheme for both the chilled water and supply air temperatures of the HVAC installations of a local project. This reset scheme would have a saving potential of about 7% as compared to the existing operational settings, without any extra cost.     

Indoor cooling/heating load analysis based on coupled simulation of convection,radiation and HVAC control

A computational fluid dynamics (CFD) simulation for analyzing indoor cooling/heating load is presented in this study. It is coupled with a radiative heat transfer simulation and heating, ventilating, and air-conditioning (HVAC) controlling system in a room. This new method feeds back the outputs of the HVAC system control to the input boundary conditions of the CFD, and this method includes a human model to evaluate the thermal environment. It would be used to analyze the heating/cooling loads of different HVAC systems under the condition of the same human thermal sensation (e.g. PMV, operative temperature, etc.) even though the temperature and air-velocity distribution in the room are different from each other.To examine the performance of the new method, a cooling load and a thermal environment within a semi-enclosed space, which opens into an atrium space, is analyzed under the steady-state conditions during the summer season. This method is able to analyze the indoor cooling load with changes of target thermal environments of a room and/or changing clothing conditions of occupants considering the temperature and air-velocity distribution in the room. In this paper, two types of HVAC system are compared; i.e. radiation-panel system and all-air cooling system. The radiation-panel cooling system is found to be more energy efficient for cooling the semi-enclosed space. Changes of the level of thermal environment reduce cooling load effectively in case of the all-air cooling system while the radiation-panel system does not reduce cooling load even though the targeted thermal condition is relaxed. Energy saving effect is expected by easing the clothing conditions of occupants. In this study, the reducing effect of cooling load is quantitatively evaluated with clothing conditions also.     

2006年全国暖通空调制冷学术年会综述

概要介绍了年会盛况，报道了各专题讨论会的主题，以附录介绍了低温地板辐射供暖技术，供热节能技术，安全、健康、节能的通风技术，暖通空调能源的合理利用，建筑节能——标准、设计与实践，温湿度独立控制空调系统，水系统变流量，蓄冷空调，区域供冷，地源热泵技术，地源热泵技术与工程应用，能耗模拟与系统控制，建筑环境仿真等13个专题讨论会的交流小结摘要。     

Utilization of wind energy in space heating and cooling with hybrid HVAC systems and heat pumps

Approximately one-third of the primary energy resources are consumed in space heating, cooling, and air-conditioning with a very low exergetic efficiency. The depleting nature of primary energy resources, negative environmental impact of fossil fuels and low exergetic efficiencies obtained in conventional space heating and cooling are the main incentives for developing alternative heating, ventilating, and air-conditioning (HVAC) techniques which can employ low density and interrupted energy sources. In this respect, in spite of difficulties primarily encountered in coupling wind energy with conventional space heating and cooling equipment, wind energy seems to be an exciting alternative provided that synectic combinations are pursued and applied. In this paper, a new wind turbine coupled hybrid HVAC system is presented, which consists of an optimum combination of convective and radiant heating and cooling systems with in-space thermal energy storage. A design case for a single family home is presented. In this study a 6 kW(e) wind turbine drives a ground source heat pump (GSHP) which is coupled to a hybrid HVAC system to satisfy the thermal loads of a 100 m² home. In this example, sensible heating and cooling loads are satisfied by the high mass radiant floor which matches the daily peak demand and the available peak wind energy. Latent heating and cooling loads, along with ventilation requirements are satisfied by a forced-air system. Variable radiant and convective split type of control is implemented, and both systems are served by the same GSHP which also satisfies the domestic hot water (DHW) demand.     

Efficiently produced heat and cold is squandered by inappropriate control strategies: A case study

The large thermal time constant of thermally activated building systems (TABS) hampers communication between the system's production and emission. Therefore, conventional building control strategies, typically using room temperature feedback, are unadapted to control thermal comfort efficiently. In this paper, measurement data and simulation results reveal that unadapted TABS control has a dramatic impact on overall energy performance. Measurements in a TABS building with room temperature feedback show the HVAC system switching between heating and cooling in a very short time frame. A simplified, generic room model is used to simulate, understand and evaluate this behaviour. For room temperature feedback control, only 45% of the cold and 15% of the heat produced actually controls room temperature. The remainder is stored in the TABS and exchanged between the heating and cooling system. Enlarging the heating-to-cooling set point band improves this ratio, while maintaining thermal comfort. On the other hand, night time operation control of the circulation pump, adapted to the TABS thermal time constant, eliminates this ‘unused’ energy completely. In this case, however, even with perfect heat gain forecasts, it is difficult to avoid room temperatures dropping below thermal comfort limits during initial office hours. Hence, a supplementary air-conditioning system seems inevitable.     

Coupled simulation of convicton,radiation, and HVAC control for attaining a given PMV value

A new computational fluid dynamics (CFD) simulation for designing indoor climates is presented in this study. It is coupled with a radiative heat transfer simulation and heating, ventilating, and air-conditioning (HVAC) control system in a room. This new method can feed back the outputs of the CFD to the input conditions for controlling the HVAC system, and includes a human model to evaluate the thermal environment. It can be used to analyze the conditions of the HVAC system (e.g. temperature of supply air, surface temperature of radiation panel, etc.) and the heating/cooling loads of different HVAC systems under the condition of the same human thermal sensation (e.g. PMV, operative temperature, etc.) To examine the performance of the new method, a thermal environment within a semi-enclosed space which opens into an atrium space is analyzed under steady-state conditions in the summer season. Using this method, the most energy efficient HVAC system can be chosen under the same PMV value. In this paper, two types of HVAC system are compared: one is a radiation-panel system and the other is an all-air cooling system. The radiation-panel cooling is found to be more energy efficient for cooling the semi-enclosed space in this study.     

A numerical study of daytime passive radiative coolers for space cooling in buildings

A passive daytime radiative cooler is made of a sky facing surface which can preserve the indoor air temperature below ambient without energy consumption by simultaneously reflecting solar radiation and emitting thermal radiation to the universe through the atmospheric window located between 8–13 μm of the electromagnetic spectrum. After the first demonstration of radiative cooling under direct sunlight, a solar mirror coated with a mid-infrared (MIR) emissive thin film has become the standard device architecture. This study firstly reviews recent developments in daytime passive radiative cooling, followed by describing the development of an energy balance mathematical model to study the potential application of passive radiative coolers in HVAC systems of buildings. Some micro-channels are fabricated on the back side of the passive radiative cooler, allowing fluid to flow in an isolated loop such that the coolant can be chilled and transported to the demand side for spacing cooling. This leads to the partial replacement of conventional vapor compression refrigeration by the radiative cooling panel. Considering the steady state energy balance within the radiative cooling panel integrated HVAC systems, the cooling performance and indoor air temperature are evaluated by numerical analysis. A 100 m2 passive radiative cooling panel could chill water for the cooling of air, reducing indoor air temperature by 10 °C, equivalent to a net cooling power of 1600 W. This study suggests that the proposed passive radiative cooling system should be used to pre-cool the ambient hot air such that the overall energy consumption of a traditional air-conditioning system can be reduced. The findings promise the application of passive daytime radiative cooling in building HVAC systems.     

Development of system concepts for improving the performance of a waste heat district heating network with exergy analysis

The building sector is responsible for a great share of the final energy demand and national CO₂ emissions in countries like Germany. Nowadays, low quality thermal energy demands in buildings are mainly satisfied with high-quality sources (e.g. natural gas fired in condensing boilers). Exergy analysis, pursuing a matching in the quality level of energy supplied and demanded, pinpoints the great necessity of substituting high-quality fossil fuels by other low quality energy flows, such as waste heat. In this paper a small district heating system in Kassel (Germany) is taken as a case study. Results from preliminary steady-state and dynamic energy and exergy analysis of the system are presented and strategies for improving the performance of waste-heat based district heating systems are derived. Results show that lowering supply temperatures from 95 to 57.7 °C increases the final exergy efficiency of the systems from 32% to 39.3%. Similarly, reducing return temperatures to the district heating network from 40.8 to 37.7 °C increases the exergy performance in 3.7%. In turn, the energy performance of all systems studied is nearly the same. This paper shows clearly the added value of exergy analysis for characterising and improving the performance of district heating systems.     

Design optimisation for a low energy home in Sydney

A simple model for a detached house in Sydney is optimised using a building energy simulation program to reduce the annual heating and cooling requirement to the point where a heating and cooling system is no longer necessary. A net present cost analysis, including the construction cost, the HVAC capital cost, and the electricity cost for space heating and cooling, is used to conduct the optimisation. The insulation thickness of the walls and roof, the window type, the thickness of an internal thermal mass wall, and the night ventilation air change rate were varied in the optimisation. Results for the best performing optimisation cases are presented and compared with the net present cost for a similar house, designed to meet the BASIX energy efficiency requirement for New South Wales. The best performing optimisation cases are shown to have a lower net present cost than the equivalent BASIX compliant house. With a view towards creating an operationally zero energy house, an optimisation case with a very low space energy requirement is selected and a photovoltaic system is sized to cover the remaining household electricity consumption over the course of a year.     

A combined system of chilled ceiling,displacement ventilation and desiccant dehumidification

Different types of heating, ventilation, and air-conditioning (HVAC) systems consume different amounts of energy yet they deliver similar levels of acceptable indoor air quality (IAQ) and thermal comfort. It is desirable to provide buildings with an optimal HVAC system to create the best IAQ and thermal comfort with minimum energy consumption. In this paper, a combined system of chilled ceiling, displacement ventilation and desiccant dehumidification is designed and applied for space conditioning in a hot and humid climate. IAQ, thermal comfort, and energy saving potential of the combined system are estimated using a mathematical model of the system described in this paper. To confirm the feasibility of the combined system in a hot and humid climate, like China, and to evaluate the system performance, the mathematical model simulates an office building in Beijing and estimates IAQ, thermal comfort and energy consumption. We conclude that in comparison with a conventional all-air system the combined system saves 8.2% of total primary energy consumption in addition to achieving better IAQ and thermal comfort. Chilled ceiling, displacement ventilation and desiccant dehumidification respond consistently to cooling source demand and complement each other on indoor comfort and air quality. It is feasible to combine the three technologies for space conditioning of office building in a hot and humid climate.     

Adaptive HVAC zone modeling for sustainable buildings

Control of energy flows within a building is critical to achieving optimal performance of heating, ventilation and air-conditioning (HVAC) systems. To design optimal HVAC control strategies, a dynamic model of the HVAC system - particularly the building zones that it services - is essential. As analysis of building energy consumption is facilitated by the accurate prediction of indoor environmental conditions, techniques that dynamically model HVAC zones are crucial, and as such, is an active area of research. This paper focuses on real-time HVAC zone model fitting and prediction techniques based on physical principles, as well as the use of genetic algorithms for optimization. The proposed approach is validated by comparing real-time HVAC zone model fitting and prediction against the corresponding experimental measurements. In addition, comparison with prediction results using an algorithm based on feedback-delayed Kalman filters has demonstrated the superiority of the proposed approach in terms of prediction accuracy.     

Numerical simulation and performance assessment of an absorption solar air-conditioning system coupled with an office building

To minimize environmental impact and CO₂ production associated with air-conditioning, it is reasonable to evaluate the prospects of a clean energy source. Solar energy, via thermal collectors can provide a part of the heating needs. Moreover, it can drive absorption chiller in order to satisfy the cooling needs of buildings. The objective of the work is to evaluate accurately the energy consumption of an air-conditioning system including a solar driven absorption chiller. The complete simulation environment includes the absorption chiller itself, the cooling tower, the solar collectors field, heater, storage devices, pumps, heating-cooling distribution, emission system and building. A decrease of primary energy consumption of 22% for heating and cooling is reached when using a solar air-conditioning system instead of classical heating and cooling devices. The modelling of each subsystem is detailed. TRNSYS software modular approach provides the possibility to model and simulate this complete system.     

夏热冬冷地区办公建筑降低供暖空调能耗的技术路径

以上海地区某办公建筑为例,基于EnergyPlus能耗模拟,探讨了围护结构性能提升和暖通空调系统优化这2条节能技术路径对夏热冬冷地区办公建筑降低供暖空调全年能耗的有效性.结果 表明:围护结构性能提升的节能潜力较小,经济性较差;单纯提高围护结构保温隔热性能并不能保证降低建筑年耗冷量,应综合分析全年供热供冷能耗确定围护结构...     

Measuring residential duct efficiency with the short-term coheat test methodology

Assessing the thermal efficiency of a forced-air distribution system is difficult, in large part because of interactions between energy loss mechanisms and other building characteristics. This paper describes short-term coheating, a method of measuring the thermal efficiency of residential heating and cooling distribution systems in situ, and presents the results of a series of studies that utilized the short-term coheat methodology. Short-term coheat tests were conducted in 53 residential buildings including both site-built and manufactured housing. The magnitude of the distribution efficiency, defined as the ratio of the energy required to heat the building if there were no duct losses to the actual heating energy required, ranged from less than 50% for homes with disconnected ducts to more than 90% for well sealed and insulated systems. Duct retrofits were also performed at 20 of the test sites and, following the retrofits, on average, the homes required 16–17% less heating energy. These results show that residential distribution system losses can be responsible for substantial energy loss and that duct retrofits are a viable energy conservation strategy for homes with distribution systems located outside of the conditioned space.