Conventional Variable Air Volume Research Articles

Energy saving of vacuum-based membrane dehumidification with indirect evaporative cooling in a low sensible-heat-ratio building

In hot and humid climates, dehumidification generally consumes more energy than cooling, and conventional mechanical vapor compression system is not efficient for dehumidification. Vacuum-based membrane dehumidification system have been developed as a more environmentally friendly alternative for dehumidification due to non-refrigerant system and isothermal dehumidification process. In this study, a novel hybrid air conditioning system that combines vacuum-based membrane dehumidification with an indirect evaporative cooler for pre-cooling is proposed to achieve efficient air conditioning in building with low sensible heat ratio. The energy performance of the proposed system is investigated by detailed thermodynamic simulations and compared with conventional variable air volume (VAV) system and indirect evaporative pre-coolers combined VAV system. The simulation results show that when the average sensible heat ratio of the space is 0.73 and the COP of the vacuum-based membrane dehumidifier is 1.0, the proposed system can reduce energy consumption by 16.1 % compared to conventional VAV system and by 6.4 % compared to indirect evaporative pre-cooler combined VAV system during the cooling season. The energy performance of the proposed system is significantly affected by the sensible heat ratio of the building and the energy efficiency of the vacuum-based membrane dehumidifier. When the COP of the vacuum-based membrane dehumidifier reaches 2.0, the proposed system can achieve an energy savings of 48.2 % compared to conventional VAV system.

A novel variable refrigerant flow system with solar regeneration-based desiccant-assisted ventilation

Variable refrigerant flow (VRF) based air-conditioning systems provide better thermal comfort with respect to the conventional all-air systems involving either constant air volume (CAV) or variable air volume (VAV) technique. In this study, a novel VRF strategy involving potential of improved indoor air quality has been proposed by integrating solar energy with desiccant-assisted dedicated outdoor air system (DOAS). DOAS is responsible for ventilation and humidity control. An indirect evaporative cooling arrangement is incorporated in the path of supply air from desiccant for reducing the load on the VRF system. Using Energyplus simulations, thermal and electrical energy characteristics of the proposed system have been investigated for three climatic conditions. A comparative assessment of the novel system has been made against the conventional VRF and all-air VAV air-conditioning system. Results show that for warm-humid climate, electrical energy saving potential of the proposed solar energy-assisted VRF system is around 23.9 % and 9.5 % than the conventional all-air VAV and VRF systems, respectively. Under composite climate the saving offered by the proposed system with respect to VAV and VRF is determined as 13.8 % and 9.4 %, respectively. Finally, the energy saving obtained with solar energy based VRF for the hot-dry climate is 17.5 % and 11.6 % against conventional all-air VAV and VRF systems, respectively. Excellent solar fraction is observed for composite and hot-dry climates, whereas for warm-humid climate, it is determined as 0.97. A comparative analysis with conventional cooling coil assisted DOAS-VRF system also shows energy savings by the proposed system.

Investigating the flexibility of a novel multi-zone air heating and ventilation system using model predictive control

In forced air heating systems, there are basically two approaches to set the temperature of each zone within a multi-zone building: regulating the supply airflow rate and regulating the supply air temperature (or a combination of these two parameters). Conventional variable air volume (VAV) systems usually maintain the supply air temperature constant within all rooms and regulate only the supply airflow rate to the room. Recently, a novel multi-zone air heating and ventilation (MZHV) system has been developed, which is capable of regulating both the supply airflow rate and the supply air temperature to the rooms independently of each other. This paper investigates the flexibility of the novel MZHV system and compares it with the conventional VAV systems. The MZHV system as well as a multi-zone case study building are modeled in a high-fidelity simulation environment (IDA ICE). Instead of traditional rule-based control schemes, a model predictive control (MPC) scheme is developed to control real-time power consumption of the multi-zone building. For this purpose, a second-order state space model is identified for each zone and calibrated against the results from the high fidelity simulations. MPC is implemented in MATLAB environment by using the second-order state space models. Two co-simulations are performed to compare the flexibility of the novel MZHV system with a conventional VAV system. The results show that the MZHV system provides a significant flexibility in terms of the energy usage, without affecting the comfort levels. This flexibility can for example be exploited to reduce the overall energy usage or provide ancillary services for a smart electricity grid.

Multi-sensor information fusion based control for VAV systems using thermal comfort constraints

The conventional control methods of variable air volume (VAV) air conditioning systems usually assume that the indoor air is well mixed, and consider each building zone as one node with homogeneous temperature distribution. The average temperature is subsequently used as the controlled parameter in the VAV cascade control process, which might cause uneven temperature distribution and unsatisfactory thermal comfort. This paper presents a coupled simulation of computational fluid dynamics (CFD) and building energy simulation (BES) for the VAV system in an office building located in Shanghai for the purpose of simulating the building, the VAV control system, and indoor thermal environment simultaneously. An external interface is developed to integrate the CFD and BES models based on quasi-dynamic coupling approach. Based upon the developed co-simulation platform, the novel VAV control method is further proposed by fusing information from multiple sensors. By adding two temperature sensors to constrain the thermal comfort of the occupied zone, the supply air temperature setpoint of the VAV terminal unit can be reset in real time. The novel control method is embedded into the co-simulation platform and compared with the conventional VAV control approach. The results illustrate that the temperature distribution under the proposed method is more uniform. At most times of the typical test day, the air diffusion performance indexes (ADPIs) for the proposed method are above 80%, while the ADPIs for the conventional control method are between 60% and 80%. Due to multi-sensor information fusion, the proposed VAV control approach has better ability to ensure the indoor thermal comfort.

Energy impact of vacuum-based membrane dehumidification in building air-conditioning applications

The energy and operating performances of a vacuum membrane dehumidification system were theoretically investigated for building air conditioning applications. The dehumidification effectiveness and coefficient of performance (COP) were adopted as performance indices. A one-dimensional mass-transfer model was derived to analyze the effect of each operating parameter on dehumidification performance. The data predicted by the model agreed well with measured results, with an error bound of 20%. Parametric analysis results indicated that the vacuum side pressure had the most significant impact on the energy and dehumidification performances. It was also observed that the membrane module length plays an important role in the module design. Based on the analysis results, a configuration and operating strategy of a vacuum membrane dehumidifier-assisted variable air volume air conditioning system were suggested. Subsequently, the proposed system was evaluated by comparing its energy and operating performances with those of the conventional variable air volume air system via energy simulations. The results indicated that the total primary energy consumption from the proposed system was 8.2% lower than that of the conventional system. However, the fan energy consumption in the proposed system was 2.5 times higher than that of the conventional system owing to the high pressure drop of membrane module. In addition, the proposed system consumed 7.7% more primary energy for dehumidification and sensible cooling process because of its lower COP. To achieve comparable energy performance, further research on the optimized design of a membrane module and optimized operation of a vacuum pump in the proposed system are necessary.

Probabilistic assessment of overcooling risk for a novel extra-low temperature dedicated outdoor air system for Hong Kong office buildings

Overcooling causes thermal discomfort and wastes energy. Effectiveness of a novel dedicated outdoor air system (DOAS) consisting of a multi-stage direct expansion coil to generate extra-low temperature (XT) outdoor air to handle the entire space cooling demand for better energy and humidity control has been confirmed in previous studies. In these studies, the terminal system was assumed using variable air volume (VAV) without reheating. The conventional VAV system without reheating has overcooling problem. This is associated with uncertain fluctuations of operating characteristics. However, virtually no study has been done to investigate its overcooling risk, and how it is compared with XT-DOAS. In this study, a rigorous probabilistic approach is proposed to account for the uncertain fluctuations of operating characteristics to assess their overcooling risks. Monte Carlo simulations with 10,000 interactions was used for risk analysis. A morphing method was employed to develop 10,000 weather files that could give an accurate prediction of future variations in yearly weather conditions. Reliable datasets were used to develop the probability distribution functions to account for the daily variations in use patterns. The most probable operating characteristics were input to EnergyPlus for hour-by-hour simulations. The annual cooling load profile formulated by the probabilistic approach was successfully validated by in-situ measurements. The probabilistic assessment results show that XT-DOAS has far less overcooling hours and lower long-term percentage of dissatisfied than the conventional VAV system. The results also indicate that the proposed probabilistic approach is useful for a robust system performance evaluation.

Applicability and energy saving potential of thermoelectric radiant panels in high-speed train cabins

This paper proposes a dedicated outdoor air system (DOAS) with thermoelectric radiant panels (TERPs) for high-speed train cabins that prevents air quality problems and reduces operation energy consumption. The DOAS supplies cooled and dehumidified outdoor air for ventilation and the removal of latent loads. The cooling and heating loads can be accommodated using TERPs, which are suitable for use in train cabins owing to their simple structure. Moreover, the air flow at the exterior surface during train running can be used to remove rejected heat from the TERP. In this study, detailed energy simulations were conducted to evaluate the energy saving potential of the proposed system compared to the conventional variable air volume system. The results indicated that the proposed system can save 32.6% of the annual operation energy. In addition, the TERP system showed an annual COP exceeding 2.2 during 75% of its operational time.

Indoor Air Quality Enhancement Performance of Liquid Desiccant and Evaporative Cooling-Assisted Air Conditioning Systems

The main objective of this study is to investigate the indoor air quality enhancement performance of two different liquid desiccant and evaporative cooling-assisted air conditioning systems, such as the variable air volume (VAV) system with the desiccant-enhanced evaporative (DEVap) cooler, and the liquid desiccant system with an indirect and direct evaporative cooling-assisted 100% outdoor air system (LD-IDECOAS), compared with the conventional VAV system. The transient simulations of concentration variations of carbon dioxide (CO2), coarse particles, and fine particles (PM10 and PM2.5) in a model office space served by each system were performed using validated system models that were found in the literature. Based on the hourly thermal loads of the model space predicted by the TRNSYS 18 program, each air conditioning system was operated virtually using a commercial equation solver program (EES). The results indicated that the LD-IDECOAS provided the lowest annual indoor CO2 concentration among all the systems considered in this research, while the VAV system with DEVap cooler exceeded the threshold concentration (i.e., 1000 ppm) during the cooling season (i.e., July, August, and September). For the indoor particulate contaminant concentrations, both liquid desiccant and evaporative cooling-assisted air conditioning systems indicated lower indoor PM2.5 and PM10 concentrations compared with the reference system. The LD-IDECOAS and the VAV with a DEVap cooler demonstrated 33.3% and 23.5% lower annual accumulated indoor PM10 concentrations than the reference system, respectively. Similarly, the annual accumulated indoor PM2.5 concentration was reduced by 16% using the LD-IDECOAS and 17.1% using the VAV with DEVap cooler.

Energy savings potential of passive chilled beams vs air systems in various US climatic zones with different system configurations

The energy savings potential of passive chilled beams in various climatic zones is analyzed in this study. A passive chilled beam model, developed based on full-scale experiments, is used as a system module in a whole building simulation tool to account for the convective and radiative effects of passive chilled beams. The model was validated with measurements from a field study in an open plan office equipped with multiple passive chilled beams. In addition, a parallel field study in an adjacent identical office space equipped with an air (VAV) system was conducted to compare the resulting energy consumption with the two systems.To further study the energy savings potential and indoor thermal conditions with passive chilled beams, four different chilled beam system configurations, combined with a parallel variable air volume system, were studied using the whole building model in different US climatic zones. The results showed that using a dedicated outdoor air system, or a separate chiller for the passive chilled beams, or a desiccant wheel after the cooling coil may result in significant relative energy savings depending on the location. The last option proved to be the most efficient, providing up to 12% for hot and humid climates and up to 20% for hot and dry climates –compared to a conventional variable air volume system. Finally, the radiative-to-total cooling effect of passive chilled beams varied between 7% and 8% and has only a small impact on energy savings and indoor thermal conditions.

Heating energy performance and part load ratio characteristics of boiler staging in an office building

Commercial buildings account for significant portions of the total building energy in South Korea and thus a variety of research on the boiler operation related to heating energy in office buildings has been carried out thus far. However, most of the researches have been conducted on the boiler itself, i.e., the part load ratio characteristics and the corresponding gas energy consumption patterns are not analyzed in the existing studies. In this study, the part load ratio and the operating characteristics of gas boiler have been analyzed within an office building equipped with the conventional variable air volume system. In addition, the gas consumption among different boiler staging schemes has been comparatively analyzed. As a result, significant portions of total operating hours, heating load and energy consumption has been found to be in a part load ratio range of 0 through 40% and thus energy consumption is significantly affected by boiler efficiency at low part load conditions. This suggests that boiler operation at the part load is an important factor in commercial buildings. In addition, utilizing sequential boiler staging scheme can save a gas usage of about 7%. For annual heating energy saving, applying the sequential control boiler with a 3:7 proportion staging is considered to be the optimal control algorithm for maximum efficiency of boilers.

Analysis of thermo-active foundations for office buildings

Ground source heat pumps (GSHPs) have been widely used to heat and cool buildings. However, the installation costs of GSHPs with vertical boreholes can be significant and may result in reducing their cost-effectiveness relative to other conventional heating and cooling systems. As alternatives to GSHPs, thermos-active foundations (TAFs) have lower installation costs but similar thermal performance. This paper provides a simplified analysis approach using the G-function method specific to TAF systems. In particular, G-functions have been determined with various thermal pile shapes to predict the short-term and the long-term performance of TAFs. As part of analysis outlined in the paper, the G-functions are integrated in a whole-building simulation tool to compare the energy efficiency and cost effectiveness of TAFs and GSHPs to conventional VAV systems for office buildings in three US climates. It is found that TAFs have slightly better energy efficiency but are significantly more cost effective than both GSHPs and variable air volume (VAV) systems in all the US climates considered in the analysis.

Energy Performance of Liquid Desiccant and Evaporative Cooling-Assisted 100% Outdoor Air Systems under Various Climatic Conditions

The main purpose of this study is to evaluate the applicability of a liquid desiccant and evaporative cooling-assisted 100% outdoor air system (LD-IDECOAS) in six typical cities in China. The six cities are located in different climatic zones in China and are selected because they are comparable owing to the outdoor air conditions. Many studies have shown that the annual operating energy consumption of LD-IDECOAS is nearly half compared with the conventional variable air volume (VAV) system. Because the climate characteristics of the six selected cities were different, the appropriate mode of operation of the LD-IDECAOS was applied to each studied city, and energy simulations were performed. Based on the design conditions of each region, the required cooling and heating loads were calculated for office buildings using transient system simulations (TRNSYS) 17, and the performance of the LD-IDECOAS and its energy consumption were simulated with a commercial engineering equation solver (EES) program. Depending on the climate characteristics of each city, adequate modifications were evaluated with simulations in terms of energy consumption. The proposed system was compared with the VAV system and the evaporative cooling assisted 100% outdoor air-conditioning system (IDECOAS) for detailed simulation results in the effort to evaluate the energy-saving potential. Finally, the results show that the proposed system saves considerable energy over conventional VAV systems and, in summer, the applications save even more energy than IDECOAS. However, there is a slight difference between the different geographical regions in terms of the annual operating energy consumption. In summary, the proposed system can yield significant energy-saving benefits in hot and humid regions whereas, in dry regions, the proposed system is more applicable in the summer. Consequently, LD-IDECOAS can be adopted for different climatic zones as a heating, ventilating, and air-conditioning (HVAC) system by introducing 100% outdoor air.

Energy saving potential of thermoelectric radiant cooling panels with a dedicated outdoor air system

The objective of this study is to investigate the energy saving potential of a dedicated outdoor air system (DOAS) with a thermoelectric module radiant cooling panel (TEM-RCP) by comparing it with those of both a conventional variable air volume (VAV) system and a DOAS with hydraulic ceiling radiant cooling panels (CRCP) via a detailed energy simulation. The theoretical and empirical models for each component were used to estimate the energy performance of a VAV system and a DOAS with CRCP. Additionally, the TEM-RCP was analyzed by using the developed simulation models based on the semi-black box model. Hourly simulations were conducted to measure the annual energy performance. The results indicated that the DOAS with a TEM-RCP consumed 40.7% less operating energy annually when compared with that of the conventional VAV system while the proposed system exhibited only 1.0% more annual energy consumption when compared with the DOAS with hydraulic CRCP given the relatively low dimensionless figure of merit (ZT) of the TEMs (i.e., 0.73) that is used in the TEM-RCP. However, the average coefficient of performance (COP) of the TEM-RCP for cooling was 8.3, which is significantly higher than that of the conventional CRCP (4.6) during the operation period. This is because the TEM-RCP uses a cooling tower for heat removal, and its cooling water temperature is sufficiently low without requiring additional cooling of the TEM except during summer. Therefore, TEM maintains radiant panel temperature with low electric energy consumption. The study also indicated that if the ZT value of TEMs exceeded 0.75, then the proposed system exhibited lower annual energy consumption when compared to that of the DOAS with CRCP.

Study of an integrated personalized ventilation and local fan-induced active chilled beam air conditioning system in hot and humid climate

A novel integrated personalized ventilation and local fan-induced active chilled beam (PV-ACB) air conditioning system is proposed and analyzed through energy and Computational Fluid Dynamics (CFD) simulation methods. Energy performance and indoor environmental conditions of this system are investigated via Integrated Environmental Solutions (IES) and ANSYS Fluent respectively. Energy simulation results illustrate that PV-ACB system, compared with the conventional variable air volume (VAV) system, could achieve 16% energy savings at 100% peak cooling load. A maximum 42% energy savings is obtained at 25% part cooling load, indicating the superior energy saving potential under part load conditions. Vertical thermal stratification is observed in CFD simulation results, 23–24 °C at the occupant level (0–2 m) and 26–27 °C in the upper zone (2–4 m). Horizontally, temperature around occupants is 22–23 °C, lower than the temperature of 26–27 °C in other areas, demonstrating the horizontal temperature difference caused by the conditioned outdoor air from personalized ventilation. Both energy and CFD simulation results illustrate that this novel PV-ACB air conditioning system is able to achieve considerable energy savings, superior occupant thermal comfort and enhanced indoor air quality (IAQ).

Using a chain recooling system on buildings in hot and humid climates

In hot and humid climates, warm outside air with rich moisture content impacts several aspects of building operation. To meet the ventilation requirement while maintaining indoor relative humidity (RH) at the desired level, dehumidification is usually required. This is typically done by cooling the air to 55 °F (12.8 °C). However, if the zone cooling load is low, then inefficient reheat may be necessary to maintain the indoor temperature set-point. To achieve better system efficiency, a new HVAC system concept called the "Chain Recooling System" is proposed. In this paper, it is shown that compared to a conventional Single Duct Variable Air Volume (SDVAV) system, the proposed system has the potential to provide better indoor air quality, eliminate the reheat requirement, and reduce fan energy consumption. To demonstrate the advantages of the proposed system, a building on the Texas A&M University campus in College Station, TX, is selected as a case study. It is projected that the HVAC system related energy consumption would be reduced by 15.2% and the average contaminant level would be reduced by 27% if the SDVAV system were replaced by the CRS without sacrificing comfort in the individual zones.

Comparison of Different Solar-Assisted Air Conditioning Systems for Australian Office Buildings

This study has investigated the feasibility of three different solar-assisted air conditioning systems for typical medium-sized office buildings in all eight Australian capital cities using the whole building energy simulation software EnergyPlus. The studied solar cooling systems include: solar desiccant-evaporative cooling (SDEC) system, hybrid solar desiccant-compression cooling (SDCC) system, and solar absorption cooling (SAC) system. A referenced conventional vapor compression variable-air-volume (VAV) system has also been investigated for comparison purpose. The technical, environmental, and economic performances of each solar cooling system have been evaluated in terms of solar fraction (SF), system coefficient of performance (COP), annual HVAC (heating, ventilation, and air conditioning) electricity consumption, annual CO2 emissions reduction, payback period (PBP), and net present value (NPV). The results demonstrate that the SDEC system consumes the least energy in Brisbane and Darwin, achieving 56.9% and 82.1% annual energy savings, respectively, compared to the conventional VAV system, while for the other six cities, the SAC system is the most energy efficient. However, from both energy and economic aspects, the SDEC system is more feasible in Adelaide, Brisbane, Darwin, Melbourne, Perth, and Sydney because of high annual SF and COP, low yearly energy consumption, short PBP and positive NPV, while for Canberra and Hobart, although the SAC system achieves considerable energy savings, it is not economically beneficial due to high initial cost. Therefore, the SDEC system is the most economically beneficial for most of Australian cities, especially in hot and humid climates. The SAC system is also energy efficient, but is not as economic as the SDEC system. However, for Canberra and Hobart, reducing initial cost is the key point to achieve economic feasibility of solar cooling applications.

Energy saving assessment of a desiccant-enhanced evaporative cooling system in variable air volume applications

Abstract Desiccant-enhanced evaporative (DEVap) cooling system have been recently introduced as an alternative to conventional air conditioning systems. This system consists of a liquid desiccant dehumidifier and a dew point evaporative cooler. In this research, workable operation strategies for the DEVap system were suggested and applied to an energy simulation for estimating the energy saving potential of the system over the conventional variable air volume (VAV) system. Four different modes of operation determined according to outdoor air conditions were suggested for annual operation of the DEVap system. Energy simulations for both DEVap and VAV systems were performed for a model office building. The results showed that 2–3% of annual primary energy could be saved by using the DEVap cooling system compared with the conventional VAV system.

Experimental study of the pressure reset control strategy for energy-efficient fan operation: Part 1: Variable air volume ventilation system with dampers

This paper is the first part of a two-part series which investigate the energy saving potentials in a novel mechanical ventilation system by replacing terminal dampers with decentralized fans. In Part 1, a conventional variable air volume (VAV) ventilation system with dampers is studied as a reference for comparison with the novel ventilation system in Part 2. The present study (Part 1) proposes a new method to implement the pressure reset strategy in practice. A common strategy to control the supply fan speed in a VAV system is to install a pressure sensor in the supply duct system. Then the fan speed is controlled such that a constant static pressure is maintained at the sensor location. Further energy saving can be obtained by resetting the pressure setpoint to a lower value at partial load conditions. This strategy is known as pressure reset control or critical zone reset strategy in the literature. The proposed method has been evaluated through an experimental mock-up in a laboratory environment. Experimental results show a minimum reduction of about 20% in fan power demand with applying the reset strategy compared to a constant static pressure setpoint.

Evaluation of energy conservation potential and complete cost-benefit analysis of the slab-integrated radiant cooling system: A Malaysian case study

The energy-efficiency aspects and cost-saving features of various radiant cooling systems have been reported in many previous studies. However, the available literature on the analysis of initial investment and maintenance costs is scanty, especially for the slab-integrated one. A field survey was carried out in a green building in Malaysia that was installed with a combined radiant-convective system. The performance of this multi-floor radiant slab cooling system was studied and the energy conservation potential was evaluated by comparing the data obtained from the building energy management system (BEMS) with the estimated energy consumption of a conventional convective air system. Also, price quotations and service maintenance contracts were obtained for a complete cost-benefit evaluation of both systems. It was found that the radiant slab cooling system was able to reduce 34% of the energy consumption compared to the conventional variable air volume system while providing the same level of air conditioning. Despite its higher initial cost, the payback period for the investment cost is about 2.5 years. These findings are particularly useful for design engineers in the tropics to consider an alternative cooling technology for large office buildings which is more energy efficient and cost effective.

Detailed analysis on part load ratio characteristics and cooling energy saving of chiller staging in an office building

Commercial buildings account for significant portions of the total building energy in Korea and thus a variety of research on the chiller operation was carried out thus far. However, most of the researches were carried out on the chiller itself, i.e., the part load ratio characteristics and the corresponding electricity energy consumption patterns were not analyzed in the existing studies. In this study, the part load ratio and the operating characteristics of vapor compression chiller were analyzed within an office building equipped with the conventional variable air volume system. In addition, the energy consumption among different chiller staging schemes was comparatively analyzed. As a result, significant portions of total operating hours, cooling load and energy consumption turned out to be in the part load ratio range of 0 through 50% and thus the energy consumption was significantly affected by the chiller COP (Coefficient of Performance) at low part load conditions, indicating that the chiller operation at the part load is an important factor in commercial buildings. In addition, utilizing sequential chiller staging scheme can save annual cooling energy usage by more than 10.3% compared to operating a single chiller. For further reduction in cooling energy, applying the sequential control chiller staging during the intermediate season and applying the uniform control chiller staging algorithm during the cooling season is considered to be the optimal control algorithm for maximum efficiency of chillers.