Simulation comparison of VAV and VRF air conditioning systems in an existing building for the cooling season

Performance of two widely used air conditioning (AC) systems, variable air volume (VAV) and variable refrigerant flow (VRF), in an existing office building environment under the same indoor and outdoor conditions for an entire cooling season is simulated by using two validated respective models and compared. It was observed that the indoor temperatures could not be maintained properly at the set temperature by the VAV no-reheat boxes. However, it could be maintained by the VAV boxes with reheat with a significant energy consumption penalty. It was found that the secondary components (indoor and ventilation units) of the VRF AC system promised 38.0-83.4% energy-saving potential depending on the system configuration, indoor and outdoor conditions, when compared to the secondary components (heaters and the supply fan) of the VAV AC system. Overall, it was found that the VRF AC system promised 27.1-57.9% energy-saving potentials depending on the system configuration, indoor and outdoor conditions, when compared to the VAV AC system.     

The effect of heat-pipe air-handling coil on energy consumption in central air-conditioning system

A study was carried out to investigate the effect of heat-pipe air-handling coil on energy consumption in a central air-conditioning system with return air. Taking an office building as an example, the study shows that compared with conventional central air-conditioning system with return air, the heat-pipe air-conditioning system can save cooling and reheating energy. In the usual range of 22–26 °C indoor design temperature and 50% relative humidity, the RES (rate of energy saving) in this office building investigated is 23.5–25.7% for cooling load and 38.1–40.9% for total energy consumption. The RES of the heat-pipe air-conditioning system increases with the increase of indoor design temperature and the decrease of indoor relative humidity. The influence of indoor relative humidity on RES is much greater than the influence of the indoor design temperature. The study indicates that a central air-conditioning system can significantly reduce its energy consumption and improve both the indoor thermal comfort and air quality when a heat-pipe air-handling coil is employed in the air-conditioning process.     

Field performance measurements of a heat pump desiccant unit in heating and humidification mode

In this study, a novel self-regenerating electric vapor compression heat pump desiccant (HPD) unit operated in the heating and humidification mode during the winter season is introduced. The HPD unit was installed in an office suite for the field test. The performance of the HPD unit and the provided indoor conditions were measured over a wide range of operating conditions. The target indoor humidity ratio was set to 4.4 g/kg, which is the minimum required indoor humidity ratio for a comfortable indoor environment indicated in the ASHRAE winter thermal comfort zone. The seasonal comparison revealed that even though 77.7% of all outdoor humidity ratio data was lower than 4.4 g/kg, 78.2% and 85.8% of all the indoor humidity ratio data of each room were found to be higher than 4.4 g/kg. In addition, due to the significant sensible capacity of the HPD unit, the indoor temperatures could be maintained within 20-25 °C. These results prove that the HPD unit not only properly humidifies the indoors without using any additional water source, like the conventional humidifier, but also helps to keep the indoor temperature at the desired temperature levels.     

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)).     

办公建筑变风量分区空调系统设计问题探讨

使用DeST软件对办公建筑两个空调方案进行了模拟分析,指出供暖期内外区合用的变风量空调系统内区房间室温过高,能耗增加,外区房间的室温基本满足设计要求;内外区独立设置的变风量空调系统各房间室温能满足设计要求,且能耗较低。对内外区合用的变风量空调系统提出了改进建议。     

变风量空调系统全工况性能模拟

介绍了变风量空调系统软件模拟的方法,该软件可用于模拟采用定静压控制、变静压控制和总风量控制的变风量空调系统的性能和能耗。利用该软件对香港地区典型办公楼的空调系统进行了模拟,结果表明,当采用变静压控制时,全年风机能耗可比传统的定静压控制方法减少13％;在负荷较大的时候,风机节能不明显,而负荷较小的时候,节能非常显著。     

Source apportionment of volatile organic compounds in Hong Kong homes

Indoor volatile organic compound (VOC) data obtained in 100 Hong Kong homes were analyzed to investigate the nature of emission sources and their contributions to indoor concentrations. A principal component analysis (PCA) showed that off-gassing of building materials, household products, painted wood products, room freshener, mothballs and consumer products were the major sources of VOCs in Hong Kong homes. The source apportionments were then evaluated by using an absolute principal component scores (APCS) technique combined with multiple linear regressions. The results indicated that 76.5 ± 1% (average ± standard error) of the total VOC emissions in Hong Kong homes attributes to the off-gassing of building materials, followed by the room freshener (8 ± 4%), household products (6 ± 2%), mothballs (5 ± 3%) and painted wood products (4 ± 2%). Analysis on the source strength in the monitored homes revealed that although six indoor sources were identified and quantified in the Hong Kong homes, only some homes were responsible for the elevated concentrations of target VOCs emitted from these sources. The findings provide us the mechanism of reducing levels of indoor VOCs and ultimately lead to cost effective reduction in population exposures.     

Modeling the effect of hygroscopic curtains on relative humidity for spaces air conditioned by DX split air conditioning system

The use of hygroscopic materials for moisture buffering is a passive way to moderate the variation of indoor humidity. Through absorption and desorption, surface materials in the indoor environment, such as curtains, carpets and wall paper, are able to dampen the moisture variations. The moisture buffering capacity of these materials may be used to improve the relative humidity of the indoor environment at reduced energy costs.The objectives of this paper are threefold. The first objective is to derive a theoretical model for the transient moisture transfer between a curtain system and the indoor air for the case where the curtain is placed in front of a wall. The second objective is to conduct experiments inside environmental chambers to validate the theoretical model and to test the ability of curtains to moderate indoor humidity. It is shown that the experimental results for the curtain moisture uptake and the relative humidity inside the chamber compared well with the model simulation results. The third and final objective is to test and evaluate the model under “real environment conditions” for a case study of a hygroscopic cotton curtain, placed in a “typical” office space in the city of Beirut with an area of 25 m² that uses direct expansion (DX) air conditioning system. It is found that hygroscopic curtains maintain humidity of less than 65% during part load operation compared to the upper limit of 70% relative humidity when no curtain is used. On the other hand, it is found that the energy use, as determined by the daily electrical power consumption of the DX system, is almost the same for the two cases, (with and without a curtain), where approximately 20 kWh of energy input is required 13 kWh of sensible energy and 7 kWh of latent energy.     

对热舒适、空气感觉质量及能耗的模拟研究

室内空调设计温度和新风量对热舒适，室内空气质量及能耗量有重要影响，然而对它们之间相互关系进行研究的文献却较少。通过计算机模拟空调系统在7种室内设计温度和7种新风量条件下的运行情况，得到不同的设计条件组合对热舒适、人体感觉空气质量及建筑能耗量的影响。基于这项分析，提出了此办公建筑合理的室内设计温度和新风量取值。     

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.     

A comparative study of different control strategies for indoor air humidity

There has been a rising concern in controlling the high indoor humidity of hot and humid countries. When an air-conditioned space experiences only part of its design heat load, its humidity tends to rise as a result of the air-conditioning system trying to control the indoor temperature by reducing its cooling capacity. In this study, the part-load dehumidification performances of three temperature control strategies are compared, namely, chilled water flow control, bypass air control and the variable air volume control. Coil simulations are employed to study the part-load performance of these control strategies. The coil model has been validated with experimental data to within ±6.5%. The relative humidity of a space depends of factors such as design sensible heat factor of the space, temperature control strategy employed and load condition experienced by the space. Simulation results have indicated that chilled water control strategy results in the highest indoor humidity throughout the range of conditions studied while variable air volume system provides highly effective dehumidification performance of the cooling coil. Bypass air control appears to be a good option for adoption as it is able to provide an acceptable humidity over a wide range of load conditions without having to affect the air movement within the space.     

Determination of indoor humidity profile using a whole-building hygrothermal model

During the design of a new building or retrofitting of an existing one, it is important to reliably assess the indoor humidity levels of the building as it can potentially affect the building envelope durability, occupants’ comfort and health risks associated with mould growth. Simplistic assumptions of indoor humidity profiles, which ignore the dynamic coupling of the indoor environment and building enclosure, may lead to inaccurate conclusions about the indoor environment and moisture performance of the building enclosure. In this paper, a whole-building hygrothermal model called HAMFitPlus, which takes into account the dynamic interactions between building envelope components, mechanical systems and indoor heat and moisture generation mechanisms, is used to assess the indoor humidity condition of an existing occupied house. HAMFitPlus is developed on SimuLink development platform and integrates COMSOL multiphysics with MatLab. The basic input parameters of the model are discussed in detail, and its simulation results are presented. In general, the HAMFitPlus simulation results are in good agreement with the measured data.     

北京南银大厦空调设计

概要介绍了采用ＶＡＶ空调方式的该办公楼空调设计,指出变风量空调系统具有使用标准高、运行节能、便于维护管理和房间装修分隔等特点,认为空调设计控制方式选择和施工安装质量优劣是决定ＶＡＶ系统能否正常运行的关键因素,而ＤＤＣ系统是变风量控制系统的基础。总结了设计经验教训,对存在的新风量不平衡、噪声偏大等问题进行了剖析。     

A model to study the effects of different control strategies on space humidity during part-load conditions

Many air conditioning systems have small moisture removal capacities and are not equipped to maintain space humidity under part-load conditions particularly during hot and humid periods. They are able to provide desired temperature control but humidity. The primary objective of this work is to identify control strategies that can be used to prevent significant indoor humidity degradation during part-load conditions. These control strategies are chilled water flow control, bypass air control, variable air volume control, run-around coil control and low face velocity/high coolant velocity control. Coil simulations have been employed to study the part-load performance of these control strategies. The coil model compares favourably with experimental data to within ±6.5%. Simulation examples are conducted for each control strategy under varying part-load conditions. Results from the coil model have indicated that some strategies are more effective than others in sustaining acceptable indoor humidity under part-load conditions. For instance, chilled water control strategy has been observed to produce highest indoor humidity throughout the range of conditions studied while variable air volume system provides highly effective dehumidification performance of the cooling coil. In addition, higher ventilation rates have been observed to increase the space humidity during part-load conditions.     

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.     

Mathematical model of particle penetration through smooth/rough building envelop leakages

People in China spend about 85% of their time indoor, of which residential building accounts for about 50%. Furthermore, most residential dwellings do not have mechanical ventilation systems, and air infiltrates during heating and cooling seasons. Consequently, human inhalation exposure to particles of outdoor origin depends substantially on the degree to which particles can penetrate the building envelope and remain suspended in indoor air. In this paper, the impact factors of particle penetration are analyzed, and a mathematical model is developed to simulate particle penetration through cracks with rough/smooth inner surfaces. For smooth inner surfaces, the results from the mathematical model show good agreement with the experimental data; for penetration through actual building envelop, the results from the rough surface model shows better agreement than the smooth surface model. It is observed that the roughness of the cracks plays a more important role in influencing deposition of particles with diameter d < 0.04 μm. The roughness mainly influences the Brownian diffusion mechanism of particles deposition. Discrepancies exist between two models and experimental data for particles with diameter ranging from 0.075 to 0.54 μm, which implies that inertia interception caused by roughness plays a more important role in enhancing deposition for particles in this range. The thermal property of the building shell is not an effective factor at removing infiltrated particles.     

Microclimate and ventilation in Estonian and Finnish dairy buildings

A series of ventilation, thermal and indoor air quality measurements were performed in 14 different dairy buildings in Estonia and Finland. The number of animals in the buildings varied from 30 to 600. Measurements were made all year round with ambient temperatures ranging between −40 °C and +30 °C. The results showed that microclimatic conditions in the dairy buildings were affected by the design of the building, outside temperature, wind, ventilation and manure handling method. The average inside air concentration of carbon dioxide was 950 ppm, ammonia 5 ppm, methane 48 ppm, relative humidity 70% and inside air velocity was 0.2 m/s. Although occasionally exceeded, the ventilation and average indoor air quality in the dairy buildings were mainly within the recommended limits.     

香港地区办公楼变风量空调系统节能分析

利用能耗模拟软件HTB2／BECON对香港地区两栋办公大楼能耗进行了定量分析，比较了CAV与VAV系统的风机能耗与总能耗，预测了两个系统全年的能耗与VAV系统的节能情况。     

Impact of wind-driven rain on historic brick wall buildings in a moderately cold and humid climate: Numerical analyses of mould growth risk, indoor climate and energy consumption

This paper gives an onset to whole building hygrothermal modelling in which the interaction between interior and exterior climates via building enclosures is simulated under a moderately cold and humid climate. The focus is particularly on the impact of wind-driven rain (WDR) on the hygrothermal response, mould growth at interior wall surfaces, indoor climate and energy consumption. First the WDR load on the facades of a 4 m × 4 m × 10 m tower is determined. Then the hygrothermal behaviour of the brick walls is analysed on a horizontal slice through the tower. The simulations demonstrate that the impact of WDR loads on the moisture contents in the walls is much larger near the edges of the walls than at the centre. The obtained relative humidity and temperature at the interior wall surfaces are combined with isopleths of generalised spore germination time of fungus mould. The results show that WDR loads can have a significant impact on mould growth especially at the edges of the walls. Finally, for the case analysed, the WDR load causes a significant increase of indoor relative humidity and energy consumption for heating.     

Assessment of the thermal environment effects on human comfort and health for the development of novel air conditioning system in tropical regions

This research shows the result of a brainstorming by medical experts in the first ranking university medical school and hospital of Thailand. It was based on Delphi technique. The objective of this research was to study both direct and indirect effects of humidity and temperature on human health in air-conditioned buildings in Thailand. Afterwards, the result was used to design and develop split type air conditioner (conventional air conditioner) which could control relative humidity and temperature with precision air conditioning system to comply with the climate and the suitability of the people living in Thailand building. The result of operation with precision inverter air conditioning system showed that the temperature inside the room changed from the default value around ±0.2 °C (Case 1) and around ±0.35 °C (Case 2) and it could control relative humidity as a desired condition between 50-60% (both cases) which was the appropriate range for Thai climate. Moreover, energy consumption of precision inverter air conditioning system was still less than conventional air conditioning system for about 7.5%. This research could provide people living in Thailand air conditioned building with human thermal comfort and health.