Energy performance optimization of radiant slab cooling using building simulation and field measurements

Few field studies of energy performance of radiant cooling systems have been undertaken. A recently constructed 17,500 m² building with a multi-floor radiant slab cooling system in the tower was investigated through simulation calibrated with measured building energy use and meteorological data. For the very cold, dry region where the building was located, it was found that a typical floor of the tower would have had 30% lower annual energy use with a conventional variable air volume system than with the as-built radiant cooling-variable air volume combination. This was due to (1) simultaneous heating and cooling by the existing radiant cooling and air systems, (2) the large amount of free cooling possible in this climate, and (3) suboptimal control settings. If these issues were remedied and combined with improved envelope and a dedicated outdoor air system with exhaust air heat recovery, a typical floor could achieve annual energy use 80% lower than a typical floor of the existing building HVAC system. This shows that radiant thermal control can make a significant contribution to energy-efficiency, but only if the building design and operating practices complement the strengths of the radiant system.     

Experiences with a gas driven,desiccant assisted air conditioning system with geothermal energy for an office building

Thermal driven desiccant assisted air conditioning systems make use of waste heat to dehumidify humid outside air in a desiccant wheel. Within the scope of a research project, an investigation of a desiccant assisted air conditioning system was carried out, and a demonstration plant was built in an office building in Hamburg, Germany. The HVAC system consists of a small CHP-plant, a desiccant assisted ventilation system and an earth energy system (borehole heat exchangers) for cooling instead of an electric driven compression chiller. The radiant floor heating system of the building is used for cooling. In this paper, measurement results and investigations of performance, energy demand and operating costs will be presented. It was found that considerable primary energy savings can be achieved (70%) using desiccant air conditioning with borehole heat exchangers. But even if electric chiller is used, savings of 30% in primary energy can be accomplished. Starting costs for the demonstration plant were not higher than for a conventional system, but running costs could be reduced drastically.     

Energy simulation in the variable refrigerant flow air-conditioning system under cooling conditions

As a high-efficiency air-conditioning scheme, the variable refrigerant flow (VRF) air-conditioning system is finding its way in office buildings. However, there is no well-known energy simulation software available so far which can be used for the energy analysis of VRF. Based on the generic dynamic building energy simulation environment, EnergyPlus, a new VRF module is developed and the energy usage of the VRF system is investigated. This paper compares the energy consumption of the VRF system with that of two conventional air-conditioning systems, namely, variable air volume (VAV) system as well as fan-coil plus fresh air (FPFA) system. A generic office building is used to accommodate the different types of heating, ventilating, and air-conditioning (HVAC) systems. The work focuses on the energy consumption of the VRF system in the office buildings and helps the designer's evaluation and decision-making on the HVAC systems in the early stages of building design. Simulation results show that the energy-saving potentials of the VRF system are expected to achieve 22.2% and 11.7%, compared with the VAV system and the FPFA system, respectively. Energy-usage breakdown for the end-users in various systems is also presented.     

夏热冬暖地区超高层办公楼大温差空调设计

南宁市位于夏热冬暖地区。本文以该市的一个城市综合体项目为例,重点介绍了其中的超高层办公楼的大温差空调系统设计。文章结合了对该项目的空调冷热源设计,空调水系统竖向分区,大温差小流量下风机盘管修正以及新风系统分区设计等的分析,探讨了超高层大温差空调设计需要关注的问题,提供了一些设计思路。     

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.     

An exergy application for analysis of buildings and HVAC systems

The paper presents an extended method for exergy analysis of buildings and Heating Ventilation Air Conditioning (HVAC) systems, according to an energy demand build-up model from the building side to the energy supply side. The HVAC systems comprise a thermal energy emission and control system, a thermal distribution system, an electricity distribution system and an energy conversion system. Energy and exergy that are required by a building and a HVAC system are posed into the external part and classified by different forms of energy carriers. The external part is out of the boundary of the study. The method is illustrated with an office building equipped with low-temperature heating and high-temperature cooling systems situated in the Netherlands. Thermal exergy and thermal energy demands of the building and thermal energy and thermal exergy losses occurring in the HVAC systems are discussed. The building and the HVAC systems to be considered meet standard Dutch energy performance regulations. Nevertheless their overall exergy efficiencies are low in both cases (17.15% and 6.81% subsequently). The exergy analysis also pinpoints that the thermal energy emission and control system and the energy conversion system are the main causes of the exergy inefficiencies in the heating and cooling cases, respectively.     

Self-tuning dynamic models of HVAC system components

A great majority of modern buildings are equipped with Energy Management and Control Systems (EMCS) which monitor and collect operating data from different components of heating ventilating and air conditioning (HVAC) systems. Models derived and tuned by using the collected data can be incorporated into the EMCS for online prediction of the system performance. To that end, HVAC component models with self-tuning parameters were developed and validated in this paper. The model parameters were tuned online by using a genetic algorithm which minimizes the error between measured and estimated performance data. The developed models included: a zone temperature model, return air enthalpy/humidity and CO₂ concentration models, a cooling and heating coil model, and a fan model. The study also includes tools for estimating the thermal and ventilation loads. The models were validated against real data gathered from an existing HVAC system. The validation results show that the component models augmented with an online parameter tuner, significantly improved the accuracy of predicted outputs. The use of such models offers several advantages such as designing better real-time control, optimization of overall system performance, and online fault detection.     

Optimal supply air temperature with respect to energy use in a variable air volume system

In a variable air volume (VAV) system with 100% outdoor air, the cooling need in the building is satisfied with a certain air flow at a certain supply air temperature. To minimize the system energy use, an optimal supply air temperature can be set dependent on the load, specific fan power (SFP), chiller coefficient of performance, outdoor temperature and the outdoor relative humidity. The theory for an optimal supply air temperature is presented and the heating, ventilation and air-conditioning (HVAC) energy use is calculated depending on supply air temperature control strategy, average U-value of the building envelope and two outdoor climates. The analyses show that controlling the supply air temperature optimally results in a significantly lower HVAC energy use than with a constant supply air temperature. The optimal average U-value of the building envelope is in practise mostly zero.     

Performance of temperature and humidity independent control air-conditioning system in an office building

The temperature and humidity independent control (THIC) system, which controls indoor temperature and moisture separately, may be an attractive alternative to existing conventional HVAC systems for its prominent improvement on the overall system performance and utilization of low grade energy resources. In order to verify the effectiveness of THIC system, a pilot project has been implemented in an office building in Shenzhen, China. In the system, liquid desiccant fresh air handling units driven by heat pumps are utilized to remove the entire latent load of outdoor air supplied for the whole building, and chilled water at the temperature of 17.5 °C from chiller is pumped and distributed into dry fan coil units and radiant panels to control indoor temperature. This paper presents the results of field test of the system, which shows that the system can provide a comfortable indoor environment even in very hot and humid weather. The COP of the entire THIC system can reach 4.0. According to the energy usage data recorded from the year 2009, the energy consumption of the THIC system in the tested office building was 32.2 kWh/(m² yr), which demonstrates magnificent energy-saving potential compared with the conventional air-conditioning system (around 49 kWh/(m² yr)).     

Primary energy and comfort performance of ventilation assisted thermo-active building systems in continental climates

This article presents a simulation study comparing the primary energy and comfort performance of ventilation assisted thermo-active building systems (TABS) relative to a conventional all-air (VAV) system in a compact office building featuring good thermal envelope performance, heat recovery, and solar gain control for the continental climate of Omaha, Nebraska with pronounced heating and cooling periods. TABS heating is accomplished using a geothermal heat pump and TABS cooling using a geothermal heat exchanger without an additional vapor compression cycle required. It was found that the coordination of the TABS and VAV systems is crucial, i.e., supply air temperature and active layer temperature setpoints and reset schedules greatly affect the performance of the overall system. The small contribution of TABS in the heating case shows the need for the adaptation of the ventilation system configuration to the TABS system. Annual cooling energy demand for the ventilation assisted TABS is higher than for the pure VAV system, which is due to lower occupied period room operative temperatures and thus a higher comfort provided. While a 4% useful energy penalty for the combined TABS/VAV was recorded, the VAV case requires 20% more delivered energy than the TABS case because of the displacement of compressor driven coil loads with low-exergy cooling through the ground heat exchanger in the TABS case. A primary energy intensity of 189 kWh/m² a was recorded for the TABS case; in contrast, the conventional all-air (VAV) equipped building incurs a primary energy intensity of 229 kWh/m²a, which represents a penalty of 20%. Clear advantages of the TABS approach can be observed with respect to thermal comfort: during summer cooling periods, the mean radiant temperature of the TABS case is on average 2 K below that of the VAV case. Moreover, the VAV system is associated with a fairly constant predicted mean vote (PMV) value of 0.75, which is quite warm, while the TABS equipped system reveals an average of 0.56, which results in only 12% instead of 17% of people dissatisfied. Based on these results, ventilation assisted thermo-active cooling systems appear to be a very promising alternative to conventional all-air systems offering both significant primary energy as well as thermal comfort advantages provided the TABS is mated with low-exergy heating and cooling sources.     

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...     

办公楼建筑空调方案的选择

通过分析对比的方法，综述了目前常用于办公楼空调各种方案的优缺点。通过一个实例的具体设计，指出ＶＡＶ系统是将成为办公楼空调方案首选。     

The impact of indoor thermal conditions,system controls and building types on the building energy demand

It is possible to evaluate the energy demand as well as the parameters related to indoor thermal comfort through building energy simulation tools. Since energy demand for heating and cooling is directly affected by the required level of thermal comfort, the investigation of the mutual relationship between thermal comfort and energy demand (and therefore operating costs) is of the foremost importance both to define the benchmarks for energy service contracts and to calibrate the energy labelling according to European Directive 2002/92/CE. The connection between indoor thermal comfort conditions and energy demand for both heating and cooling has been analyzed in this work with reference to a set of validation tests (office buildings) derived from a European draft standard. Once a range of required acceptable indoor operative temperatures had been fixed in accordance with Fanger's theory (e.g. −0.5 < PMV < −0.5), the effective hourly comfort conditions and the energy consumptions were estimated through dynamic simulations. The same approach was then used to quantify the energy demand when the range of acceptable indoor operative temperatures was fixed in accordance with de Dear's adaptive comfort theory.     

Double cooling coil model for non-linear HVAC system using RLF method

The purpose of heating, ventilating and air conditioning (HVAC) system is to provide and maintain a comfortable indoor temperature and humidity. The objective of this work is to model building structure, including equipments of HVAC system. The hybrid HVAC model is built with physical and empirical functions of thermal inertia quantity. Physical laws are used to build the sub-model for subsystems that have low thermal inertia while the empirical method is used to build the sub-model for subsystems with high thermal inertia. The residential load factor (RLF) is modeled by residential heat balance (RHB). RLF is required to calculate a cooling/heating load depending upon the indoor/outdoor temperature. The transparency, functionality of indoor/outdoor temperatures and simplicity of RLF makes it suitable for modeling. Furthermore, the parameters of the model can be calculated differently from room to room and are appropriate for variable air volume (VAV) factor. Nowadays, a VAV system is universally accepted as means of achieving both energy efficiency and comfortable building environment. In this research work, a pre-cooling coil is added to humidify the incoming air, which controls the humidity more efficiently inside conditioned space. The model presented here is verified with both theoretical and numerical methods.     

The impact of air filter pressure drop on the performance of typical air-conditioning systems

Filters are used in heating, ventilation, and air-conditioning (HVAC) systems for both commercial and residential buildings to protect the equipment and improve indoor air quality in conditioned spaces. Although there are many benefits of using the air filter in an air-conditioning system, the resistance associated with it can increase fan energy use and may adversely affect air-conditioning system performance and efficiency. The paper explores the impact of air filtration on energy consumption for a typical air-conditioning (AC) system with constant- or variable-speed fan. A whole building simulation model is used to simulate the annual energy consumption for various air-conditioning system capacities, different levels of filter cleanliness, and various filter minimum efficiency reporting values (MERV). The results indicate that with a constant-speed fan, the cooling energy use increases as the filter gets dirty over time and the energy use in the fan may increase but this depends heavily on the investigated fan performance curve. With a variable-speed fan, the fan energy use increases with a dirty filter but the cooling and heating energy uses are slightly affected. The fan energy use rise due to the dirty filter depends mainly on air system capacities, filter MERV ratings, and the degree of the filter cleanliness.     

Development and validation of online models with parameter estimation for a building zone with VAV system

The energy consumption by building heating, ventilating, and air conditioning (HVAC) systems has evoked increasing attention to promote energy efficient control and operation of HVAC systems. Application of advanced control and operation strategies requires robust online system models. In this study, online models with parameter estimation for a building zone with a variable air volume system, which is one of the most common HVAC systems, are developed and validated using experimental data. Building zone temperature and zone entering air flow are modeled based on physical rules and only the measurements that are commonly available in a commercial building are used. Various validation experiments were performed using a real-building test facility to examine the prediction accuracies for system outputs. Using the online system models with parameter estimation, the prediction errors for all validation experiments are less than 0.28 °C for temperature outputs, and less than 84.9 m³/h for air flow outputs. The online models can be further used for local and supervisory control, as well as fault detection applications.     

温湿度独立控制空调系统和常规空调系统的性能比较

选择北京和广州相同的办公楼作为比较对象,常规空调系统采用典型的风机盘管+独立新风的形式。应用DeST软件计算了建筑全年逐时冷负荷,计算了两个系统的能效比EER(energy efficiency ratio)和能耗。结果表明,对于北京的办公建筑,温湿度独立控制空调系统全年能耗为14.75k Wh/m2,EER为4.5;常规空调系统全年能耗为18.63k Wh/m2,EER为3.6。对于广州的办公建筑,温湿度独立控制空调系统全年能耗为32.66k Wh/m2,EER为4.5;常规空调系统全年能耗为43.39k Wh/m2,EER为3.4。相比常规空调系统,温湿度独立控制空调系统的节能率在20%～30%。     

某夏季干热地区办公大楼报告厅夏季空调设计

以新疆某办公大楼报告厅空调系统设计为例,分析了夏季干热气候区室外空气设计参数的特殊性。着重介绍了变制冷剂多联机+全热交换系统、风机盘管+新风系统、一级蒸发冷却系统、二级蒸发冷却系统及采用屋顶全新风机组,并结合本项目对这几种不同的空调系统进行分析与比较,确定了适合本项目的空调系统形式。     

Energy conservative building air conditioning system controlled and optimized using fuzzy-genetic algorithm

In this work, the combined effect of the energy conservative variable refrigerant volume (VRV) system and the variable air volume (VAV) system was experimentally investigated using genetic fuzzy optimization method that yielded better thermal comfort, indoor air quality (IAQ) requirements without compromising on the energy savings potential. The proposed system was tested using the demand controlled ventilation (DCV) combined with the economizer cycle ventilation (ECV) techniques and examined for a year-round building air conditioning (A/C) application. The supply air temperature (SAT) set points were varied under three distinct strategies and the optimal solutions obtained for the fuzzy systems designed resulted in an enhanced energy conservative potential. The test results of the proposed system were compared with the conventional fan coil A/C system. Based on the three strategies of the supply air temperature, the proposed system yielded an improved per day energy savings potential of 54% in summer and 61% in winter design conditions. Furthermore, for the strategies considered the proposed system achieved an annual energy conservative potential of 36% and exhibited more possible ways to achieve thermal comfort, IAQ and energy conservation.     

A simulation environment for performance analysis of HVAC systems

Due to the lack of a building simulation program that can simulate in details the combined heat, vapor, and liquid transfer in porous elements and the HVAC (heating, ventilation and air-conditioning) systems, a flexible computational algorithm has been elaborated in order to integrate models for both HVAC systems and multizone hygrothermal building model. In the algorithm, models for the primary system-composed of chiller, cooling tower, primary pumps, and condensation pumps—have been described. For the secondary system, models for the cooling and dehumidifying coil, humidifier, fan, and mixing box have been considered. Those mathematical models have been integrated into the whole-building PowerDomus simulation environment. The simulation environment is presented, and results show the usability aspects of the proposed computer environment by comparing air- and water-cooled equipment.