Dedicated Outdoor Air System Research Articles

Indoor environment evaluation of a Dedicated Outdoor Air System with ceiling fans in the tropics – A thermal manikin study

Energy savings of the Air-Conditioning (AC) system is crucial in achieving energy conservation in buildings in the tropical region. A Dedicated Outdoor Air System with ceiling fans (DOAS-CF) aims to achieve energy savings arising from elevated room air temperature and minimizing the air-conditioning requirements. The objective of this study is to investigate the cooling effect produced by the DOAS-CF system at various locations in an AC room using a thermal manikin. The sedentary thermal manikin was set at three different locations such as under the ceiling fan, outside the fan blades and the center of the room at temperatures of 24 °C, 27 °C and 30 °C and at 60%RH with air velocities ranging from 0.02 m/s to 2.26 m/s at the height of 1.1 m. Two ceiling fans formed circulation airflows around each fan. High air speeds and low turbulence intensity were measured right under the ceiling fan and near the floor while low air speeds and high turbulence intensity were found at other locations. The cooling effect at the whole body of −3.3 °C was higher under the fan than that of −1.3 °C at other locations. The cooling effect was affected by temperature, location and fan speed mode significantly. The maximum cooling effect by manikin based equivalent temperature was higher than that by SET model due to an evaluation of the local cooling effect on each body part. The knowledge of the cooling effect is important and useful for a mechanical engineer to design the thermal environment produced by the DOAS-CF system.

Thermal simulation of a DOAS+CRCP air conditioning system coupled with a floor containing a phase change material (PCM)

The phase change materials(PCMs) is an excellent means for cooling application in buildings to stabilize the indoor air temperature variation due to solar and internal gains and therefore reducing the size, operating time and stress of mechanical systems. This wok studies the use of radiant systems for discharging heat stored in a PCM: a PCM is incorporated into the floor, and a Ceiling Radiant Cooling Panel(CRCP) combines with Dedicated Outdoor Air System(DOAS) is used as the energy discharge system. The advantages and disadvantages of this configuration in terms of cooling energy demands and thermal comforts of occupants are analyzed using the simulation software TRNSYS. This simulation reveals that this configuration can significantly improve the degrees of occupant comfort and reduce the cooling energy demand.

Calibrated simulation analysis for integration of evaporative cooling and radiant cooling system for different Indian climatic zones

Radiant cooling system (RCS) has proven to be an energy efficient system for meeting a building's cooling requirements. RCS is energy efficient and provides better thermal comfort compared to conventional all-air heating ventilation air conditioning (HVAC) system. To further improve the efficiency of RCS, a parallel evaporative cooling system (cooling tower) is coupled with RCS and analyzed for different climatic condition using calibrated model. A statistical analysis of weather files (based on wet bulb temperature), was used to identify the availability of useful water for cooling tower integration with RCS. A comprehensive simulation feasibility study of the application of cooling tower in RCS was performed for different cities to cover every climatic zones of India. It was found that in summer, the wet bulb temperature (WBT) of different climatic zones, except warm-humid, is suitable for the integration of cooling tower with RCS. An experimental setup was designed and developed for integration of parallel chiller and cooling tower with panel-based RCS for cooling and a dedicated outdoor air system (DOAS) for dehumidification and ventilation. Experiments were conducted for chiller and cooling tower operated RCS in Composite climate of Jaipur, Rajasthan, India. Building Energy Models (BEM) were developed for both the chiller-operated RCS and cooling tower-operated RCS in EnergyPlus and calibrated with the measured data. Using the calibrated models, performance of the system was analyzed for different climatic zones of India. A chiller-operated RCS was considered as a baseline to compare the annual energy saving potential and monthly performance of the cooling tower integrated RCS. In the cooling tower–operated RCS, a total annual savings of 7% in hot and dry climates, 11% in composite climates and 20% in temperate climates was achieved compared to the chiller-operated RCS.

Comparison of the performance of two different DOAS configurations involving conventional and renewable energies

Spanish building regulations have increased the ventilation rates creating humidity control problems. Dedicated outdoor air systems (DOASs) including a desiccant wheel can dry the ventilation air so that the cooling units are relieved of the latent loads carried by the ventilation air and those generated inside the building. The purpose of this study was to compare the performance of two different DOAS configurations in terms of primary energy consumption under different Spanish climates. The DOAS comprising a desiccant wheel and a conventional refrigeration system was called DOAS-A. The DOAS comprising a desiccant wheel regenerated by solar air collectors and an indirect evaporative cooler was called DOAS-B. The performance of both DOASs was simulated with TRNSYS, and the models were validated with data from experimental setups built at the Miguel Hernández University of Elche (Spain) and at the Technical University of Cartagena (Spain). It was found that the DOAS-B configuration consumed less primary energy than the DOAS-A configuration for commercial use, that is, when the latent load coincides with the supply of solar radiation. These savings reached 68.3% in Seville owing to its high level of solar radiation. It was also found that the net sensible load supplied by the ventilation air to the conditioned space was negative for all climates. This sensible cooling capacity was shown to be greater at lower outdoor dry-bulb temperature and lower latent load.

Exergy Analyses and Modelling of a Novel Extra-Low Temperature Dedicated Outdoor Air System

A novel dedicated outdoor air system (DOAS) comprising a multi-stage direct expansion coil to produce extra-low temperature (XT) outdoor air to handle the entire space cooling demand has been confirmed more efficient than conventional systems. To further enhance the performance of XT-DOAS, the optimum number of cooling stages and treated outdoor air temperature need to be determined. This process requires the development of a coil performance model that takes into account the extra-high and extra-low entering air temperatures at the first and the last cooling stages. In this study, factory test data and field measurement data were used to develop such a performance model. Different statistical analyses were employed to validate the developed model. Based on the developed model, energy and exergy analyses were conducted to evaluate use of XT-DOAS for space cooling of a typical office building in Hong Kong. EnergyPlus was employed for the energy analysis. The laws of thermodynamics were used for the exergy analysis. Their combined results indicate that for better energy efficiency and performance for air-conditioning of office buildings in subtropical region, the optimum configuration for XT-DOAS is two cooling stages with a treated outdoor air temperature of 7 °C. The model developed and the energy and exergy analyses described will contribute significantly to future research in this area.

Analysis of Energy Saving Potential in High-Performance Building Technologies under Korean Climatic Conditions

This study aims to suggest a basis for the selection of technologies for developing high-performance buildings to reduce energy consumption and greenhouse gas emissions. Energy-saving technologies comprising 15 cases were categorized into passive, active, and renewable energy systems. EnergyPlus v8.8.0 was used to analyze the contribution of each technology in reducing the primary energy consumptions and CO2 emissions in the Korean climate. The primary energy consumptions of the base model were 464.1 and 485.1 kWh/m²a in the Incheon and Jeju region, respectively, and the CO2 emissions were 83.4 and 87.4 kgCO2/m²a, respectively. Each technology (cases 1–15) provided different energy-saving contributions in the Korean climate depending on their characteristics. The heating, cooling, and other energy-saving contributions of each technology indicate that their saving rates can be used when selecting suitable technologies during the cooling and heating seasons. Case 15 (active chilled beam with dedicated outdoor air system + ground source heat pump) showed the highest energy saving rate. In case 15, the Incheon and Jeju models were reduced by 189.4 (59.2%) and 206.2 kWh/m²a (57.4%) compared to the base case, respectively, and the CO2 emissions were reduced by up to 32.7 (60.8%) and 35.6 kgCO2/m²a (59.3%), respectively.

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.

Evaluation on the Indoor Thermal Environment and Cooling Operation Characteristics of Thermally Activated Building System integrated with Dedicated Outdoor Air System during Cooling Operation in Hot and Humid Climate of Seoul

Evaluation on the Indoor Thermal Environment and Cooling Operation Characteristics of Thermally Activated Building System integrated with Dedicated Outdoor Air System during Cooling Operation in Hot and Humid Climate of Seoul

Model calibration of a variable refrigerant flow system with a dedicated outdoor air system: A case study

With increased use of variable refrigerant flow (VRF) systems in the U.S. building sector, interests in capability and rationality of various building energy modeling tools to simulate VRF systems are rising. This paper presents the detailed procedures for model calibration of a VRF system with a dedicated outdoor air system (DOAS) by comparing to detailed measured data from an occupancy emulated small office building. The building energy model is first developed based on as-built drawings, and building and system characteristics available. The whole building energy modeling tool used for the study is U.S. DOE’s EnergyPlus version 8.1. The initial model is, then, calibrated with the hourly measured data from the target building and VRF-DOAS system. In a detailed calibration procedures of the VRF-DOAS, the original EnergyPlus source code is modified to enable the modeling of the specific VRF-DOAS installed in the building. After a proper calibration during cooling and heating seasons, the VRF-DOAS model can reasonably predict the performance of the actual VRF-DOAS system based on the criteria from ASHRAE Guideline 14-2014. The calibration results show that hourly CV-RMSE and NMBE would be 15.7% and 3.8%, respectively, which is deemed to be calibrated. The whole-building energy usage after calibration of the VRF-DOAS model is 1.9% (78.8kWh) lower than that of the measurements during comparison period.

Energy performance of GCC-specification LCC optimized dedicated outdoor air system configurations coupled to an air-cooled outdoor unit

Direct expansion (DX) dedicated outdoor air system is shown to be a cost-effective energy efficient complement to ductless variable refrigerant flow and radiant cooling systems by providing conditioned ventilation air at the required set point temperature and relative humidity in an energy efficient manner. We compare five balanced flow dedicated outdoor air system configurations under typical office/lab ventilation loads coupled to an air-cooled outdoor unit for 24 Gulf Cooperation Council (GCC) locations representing GCC climates. All configurations use an enthalpy recovery wheel (ERW) to remove heat and moisture from intake air—before it reaches the active DX dehumidification element—by passive heat and mass transfer to the exhaust stream. It is found that of two run-around heat exchanger configurations, using a rotary heat wheel across the evaporator is a more efficient configuration than placing the heat wheel (HW) between the supply and return air streams. Furthermore, adding a subcooling coil for reheat in parallel to the HW results in higher efficiency than adding the coil in series with the HW. The Life cycle cost optimized ERW and HW effectiveness for the configuration with HW between supply and return air was found to be 0.86 while the ranges for run-around HW configurations with subcooling/reheat were found to be 0.8–0.85 for ERW and 0.74-0.82 for HW in GCC climates with significant dehumidification loads. Thus, the evaporator run-around HW reduces the volume, mass and cost of the DOAS unit while providing higher overall efficiency.

A frost-free dedicated outdoor air system with exhaust air heat recovery

DOAS (dedicated outdoor air system) has drawn much attention recently. However, it is quite energy-intensive resulted from the big temperature and/or humidity difference between outdoor air and supply air. This paper proposed a new DOAS with exhaust air heat recovery. In cooling mode, there are one evaporator and three condensers in parallel. The outdoor air is cooled and dehumidified by the evaporator. Besides the outdoor condenser, another two are used for supply air reheat and exhaust air heat recovery. In heating mode, the reheat coil is off and two evaporators work for exhaust air heat recovery and ambient heat absorption, respectively. A pressure regulating valve equipped on the indoor evaporator keeps proper evaporating temperature for a frost-free operation. A validated model is applied to fulfill the system design and performance prediction. The results showed that the system cooling COP reaches 3.3 at ambient temperature 35°C/28°C and the heating COP achieves 4.8 at ambient temperature 7°C/6°C, respectively. In winter the proposed system can maintain the desired supply temperature and keep both evaporators frost-free (ambient temperature −20°C to 10°C). The proposed system can efficiently recover waste energy from exhaust air in the whole year and has the extensive applicability, especially in the cold climate where considerable energy savings can be found.

Experimental Validation of the Simulation Model of a DOAS Equipped with a Desiccant Wheel and a Vapor Compression Refrigeration System

A dedicated outdoor air system (DOAS) can be designed to supply 100% of the outside air and meet the latent load of the room with dry air. The objectives of this study were to develop a model of a DOAS equipped with a desiccant wheel and a vapor-compression refrigeration system, build a prototype, validate the model with experimental data, and gain knowledge about the system operation. The test facility was designed with the desiccant wheel downstream of the cooling coil to take advantage of the operating principles of cooling coils and desiccants. A model of the DOAS was developed in the TRNSYS environment. The root mean standard error (RMSE) was used for model validation by comparing the measured air and refrigerant properties with the corresponding calculated values. The results obtained with the developed model showed that the DOAS was able to maintain an indoor humidity ratio depending on outdoor conditions. Laboratory tests were also used to investigate the effect of changes in the regeneration air temperature and the process airflow rate on the process air humidity ratio at the outlet of the wheel. The results are consistent with the technical literature.

Energy Saving Potential of a Thermoelectric Heat Pump-Assisted Liquid Desiccant System in a Dedicated Outdoor Air System

The main objective of this study was to develop a thermoelectric heat pump and liquid desiccant system based on a dedicated outdoor air system (THPLD-DOAS). An internally-cooled and -heated liquid desiccant system was used and a thermoelectric heat pump (THP) served as the desiccant cooling and heating energy source for dehumidification and regeneration of the desiccant solution, respectively. In order to investigate the energy-saving potential of the proposed system, its thermal performance and operating energy consumption during the cooling season were compared to those of a conventional dedicated outdoor air system with a ceiling radiant cooling panel system (DOAS-CRCP). Detailed simulations for each system were conducted under hot and humid climatic conditions. Their thermal performance under various room sensible heat factor (RSHF) conditions was evaluated to observe the energy performance, depending on the dehumidification performance, of the liquid desiccant system integrated with the THP. The results showed that the coefficient of performance (COP) of the THP ranged from 0.8 to 1.2 to maintain a sufficient dehumidification rate. The operating energy of the THPLD of the proposed system was 6.6% to 16.0% less than that of the chiller operating energy of a conventional DOAS. Consequently, the proposed system consumed 0.6–23.5% less operating energy compared to the conventional DOAS.

Performance assessment of radiant cooling system integrated with desiccant assisted DOAS with solar regeneration

The Radiant cooling system integrated with Dedicated Outdoor Air System (DOAS) is a viable substitution for conventional all air system in order to reduce primary energy consumption, as it decouples the cooling and ventilation task. In DOAS major portion of energy is consumed in cooling coil where it dehumidifies the process supply air. This study describes an alternate solution for dehumidification, with the substitution of the desiccant wheel with solar regeneration in place of a chilled water coil based dehumidifier. In this paper, simulations were carried out using EnergyPlus on a reference medium office building to investigate the contribution of solar energy towards the total energy consumption of desiccant assisted DOAS with radiant cooling system. To evaluate the system performance and energy saving potential, desiccant based DOAS is compared with cooling coil assisted DOAS integrated with Radiant cooling system. Simulations were carried out for different solar collector area to evaluate primary energy savings. Results indicate that from 7.4% to 28.6% energy saving (according to different collector area) can be achieved due to the solar regeneration in desiccant assisted DOAS, the impact of different solar collector area on potential of energy savings is also described.

Experimental investigations on a solar assisted liquid desiccant cooling system with indirect contact dehumidifier

Challenges on energy and environmental fronts have initiated momentum in increasing the role of renewable energy in air-conditioning industry. Liquid desiccant cooling systems (LDCS), emerging as promising alternative to conventional vapor compression systems, can run on low grade heat which can easily be drawn from solar energy. In the present study a small capacity liquid desiccant evaporative cooling system for small office application is developed. The system is a dedicated outdoor air system (DOAS). Lithium chloride solution is used as a liquid desiccant. The system consists of a dehumidifier, a regenerator, a regenerative evaporative cooler, heat exchangers (solution-solution, air-water, and solution-water) and non-concentrating solar collectors. The dehumidifier in the system is an indirect contact heat and mass exchanger which eliminates the carryover of desiccant. The major energy consumption in the LDCS is for the regeneration process which is tapped from solar energy. The performance of the overall system is presented in terms of its dehumidification effectiveness, moisture removal rate, cooling capacity and thermal COP.

Accuracy of simplified heating coil models based on manufacturer catalogue data

The overall thermal performance of a heating coil, typical of those used in air conditioning systems, is investigated by two models based on the classical ε–NTU method. The first model, here taken as the reference model, is supported by detailed information about the coil geometry and published correlations for the air and liquid Nusselt numbers. The second model, much simpler, does not require a detailed knowledge of the coil geometry. It is based on mechanistic correlations with unspecified parameters that are calibrated for a particular coil using catalogue information. The purpose of this paper is to compare the prediction ability of both models for a typical HVAC heating coil.The reference model is first validated by comparing its results with those of a certified software provided by a coil manufacturer. The agreement is good in general, but errors are significant when the coil operates at low flow rates. Once validated, the reference model is used to generate two separate sets of coil performance data by systematically varying the mass flow rate and inlet temperatures of both fluids. The first dataset is representative of the conditions typically reported in manufacturer catalogues, while the second dataset includes a wider range of operating conditions that the coil can experience in practice (for example, lower than normal flow rates). The parameters in the non-geometrical model are calibrated using the first dataset. This fitted model is then applied to the second dataset for checking the prediction error. A good agreement is observed between the results, except in some cases where the water flow is laminar or in the transition regime.A practical case study of application of the simplified model for predicting the energy performance of a heating coil is finally conducted. The coil is simulated as part of a dedicated outdoor air system (DOAS) which conditions the ventilation air of a building. The DOAS does not incorporate energy recovery devices. Simulations are performed for the heating season by taking into account the local weather data. The water flow rate varies according to the strategy adopted in controlling the temperature of supply fresh air. The accuracy of different versions of the non-geometrical model is quantified.

Applying a novel extra-low temperature dedicated outdoor air system for humidity control and energy efficiency

A novel dedicated outdoor air system consisting of a multi-stage direct expansion coil to generate extra-low temperature outdoor air to avoid moisture-related problems and for desirable air conditions and better energy efficiency is proposed for investigation. In this study, the proposed system's (extra-low temperature-dedicated outdoor air system) performance in achieving the desirable air conditions and better energy efficiency objectives is compared with a conventional direct expansion coil system with variable speed compressor and fan used for air conditioning of a typical office building in Hong Kong. Through hour-by-hour simulations, using equipment performance data of a pilot study and realistic building and system characteristics, it was found that the extra-low temperature-dedicated outdoor air system, as compared to the conventional system, could reduce the annual air-conditioning energy consumption by 22.6%, the indoor discomfort hours by 31% and the condensation risk to zero hours. Root-mean-square error and standard deviation analysis of the resultant relative humidity confirmed that the extra-low temperature-dedicated outdoor air system could better achieve the desired relative humidity. The findings of this study confirm that the extra-low temperature-dedicated outdoor air system is better than the conventional system in terms of both energy consumption and the desirable air conditions when used in office buildings in Hong Kong.

Energy benefit of a dedicated outdoor air system over a desiccant-enhanced evaporative air conditioner

The purpose of this study is to comparatively evaluate the energy performances of a dedicated outdoor air system (DOAS) and desiccant-enhanced evaporative air conditioner (DEVap) in building applications. The DOAS effectively accommodates latent cooling loads and some of the sensible cooling loads of the space by introducing cooled and dehumidified ventilation air into a building while integrating a parallel system aimed at reducing the remaining sensible load.The DEVap enhances the energy performance of a variable air volume system by reducing cooling coil loads through preconditioning of the supply air before it reaches a coil. The preconditioning is accomplished by using a liquid desiccant system and dew-point indirect evaporative cooler. In this paper, the operating and annual primary energy consumptions of both the DOAS and DEVap systems are compared based on detailed energy simulations. The results indicated the energy saving potential of DOAS to be greater than that of the DEVap. Specifically, a DOAS with ceiling radiant cooling panels experienced 20% less primary energy consumption compared to a DEVap.

Applying a novel extra-low temperature dedicated outdoor air system in office buildings for energy efficiency and thermal comfort

A novel dedicated outdoor air system consisting of a multi-stage direct expansion coil and a zero-energy heat pipe to generate extra-low temperature outdoor air to avoid moisture-related problems was proposed in this study. The proposed system’s performance in achieving the desirable air conditions and better energy efficiency objectives is compared with a conventional direct expansion system for air-conditioning of a typical office building in Hong Kong based on simulation investigations. The simulations were carried out using equipment performance data of a pilot study, and realistic building and system characteristics. It was found that the proposed system, as compared to the conventional system, could reduce the annual indoor discomfort hours by 69.4%. An energy and exergy analysis was also conducted. It was revealed that the proposed system could reduce the annual air-conditioning energy use by 15.6% and the system exergy loss rate by 13.6%. The associated overall exergy efficiency was also found 18.6% higher. The findings of this study confirm that the proposed system is better than the conventional system in terms of both energy and exergy efficiency and the desirable air conditions.

Model-based multi-objective optimal control of a VRF (variable refrigerant flow) combined system with DOAS (dedicated outdoor air system) using genetic algorithm under heating conditions

A VRF (variable refrigerant flow) combined system adopting a DOAS (dedicated outdoor air system) has been proposed to reduce the total energy consumption while satisfying IAQ (indoor air quality) and THC (thermal and humidity comfort) with minimum outdoor air. The objective of this study is to develop a model-based multi-objective optimal control strategy for the VRF combined system with multi-zone in order to optimize the multi-objective functions of the THC, IAQ, and total energy consumption. The performance of the VRF combined system was evaluated using the EnergyPlus model. The VRF combined system was optimized by GA (genetic algorithm) and RSM (response surface methodology) with the multi-objective functions of the THC, IAQ, and total energy consumption. The proposed multi-objective optimal control strategies (A and B) were compared with the TS (time schedule) strategy and the DCVH (demand controlled ventilation with humidifying). Optimal control strategy B reduced the total energy consumption by 20.4% and increased the ratio of the hours satisfying the extended comfort zone by 19.1% compared to the DCVH strategy.