Outdoor Air System Research Articles

Optimum regeneration temperature of a desiccant solution in a packaged liquid desiccant-assisted air conditioning unit

Recently, a package unit of liquid desiccant and an indirect and a direct evaporative cooling-assisted 100% outdoor air system (LD-IDECOAS) has been proposed. In this system, the amount of heating energy consumed to regenerate the desiccant solution is significant and should be reduced to achieve better energy conservation than that of a conventional air handling unit. The purpose of this research is to empirically investigate the optimum regeneration temperature of a lithium chloride (LiCl) desiccant solution, balancing the regeneration performance and solution heating energy consumption in the LD-IDECOAS package unit. The adiabatic cross-flow regenerator was tested in a test facility at desiccant solution temperatures of 50–90 °C. The measurement results were compared with the predicted data in the regenerator to assess the reliability of the experimental results. By balancing the regeneration performance with the solution heating energy consumption, the recommended temperature to regenerate the desiccant solution was 65 °C.

Modeling and experimental investigation of an advanced direct-expansion outdoor air dehumidification system

Dedicated outdoor air system is drawing much attention in air-conditioning applications. However, the cooling and dehumidification process of supply air in the dedicated outdoor air system is energy-intensive. In this study, we proposed a highly energy efficient dedicated outdoor air system using novel vapor-compression refrigeration cycle which is featured by a fine combination of two-stage direct-expansion dehumidification, subcooled liquid-to-air reheating, and exhaust air heat recovery. The novel system was optimally designed and analyzed with a detailed mathematical model. A prototype was also made and tested. Test results showed that the energy efficiency of the proposed system reaches 5.42 under rated cooling conditions and 26% higher than the conventional one. The model accuracy was also validated with the measured data. Moreover, energy efficiency improvement from feature technologies was numerically analyzed. Control strategy for key operating parameters was developed and illustrated. System performance under varying operating conditions was evaluated as well.

Thermoelectric Module Integrated Fuel Cell in a Liquid Desiccant-Assisted Air-Conditioning System

This study aims to estimate the energy performance of a liquid desiccant and evaporative cooling-assisted 100% outdoor air system (LD-IDECOAS) combined with a thermoelectric module integrated proton exchange membrane fuel cell (TEM-PEMFC). During the cooling season, recovered heat from the PEMFC was reclaimed to heat a weak desiccant solution and the generated electricity was used to operate the LD-IDECOAS. The TEM was operated as an auxiliary heater for heating the weak desiccant solution. In the off-cooling season, the PEMFC was operated to generate electricity and the recovered heat was also used to generate electricity using TEMs. In this study, a detailed energy simulation model was developed to estimate the energy savings potentials of the proposed system compared with the conventional LD-IDECOAS that uses a gas boiler and grid power without TEM-PEMFC. The result shows that TEMs can operate with a mean coefficient of performance of 2.0 when utilized for auxiliary heater in the cooling season. In addition, TEMs generate additional electricity with a mean power generation efficiency of 0.9%. Finally, the proposed system can save the 10.6% of annual primary energy compared with the conventional LD-IDECOAS. Therefore, the advantages of using TEM-PEMFC as heating and energy harvesting components were verified.

Analysis of a DOAS operation in different Spanish climates using an experimentally validated TRNSYS model

Current ventilation airflow rates imposed by building regulations eventually force HVAC systems to remove high latent loads. Dedicated outdoor air systems (DOASs) equipped with a desiccant wheel, are an alternative to conventional refrigeration equipment for humidity control in buildings. This paper studies the operation of a DOAS intended to remove the ventilation air and space latent load in different Spanish climates during the summer season. The model developed in TRNSYS was validated with experimental data using the root mean standard error (RMSE). Maximum RMSE of 0.94 °C and 0.472 g/kg were obtained for the temperatures and humidity ratios of the process and regeneration airflows. The RMSE for the compressor power input was of 52.2 W. The results obtained with the developed model showed that the DOAS exhibited better performance in humid climates with moderate temperatures (Bilbao and Barcelona). It was also found that its application in dry climates (Madrid) was not appropriate, and that a high outdoor temperature lowered the DOAS efficiency when comparing cities with similar latent loads (Bilbao and Sevilla) or when comparing residential and commercial schedules.

Energy saving potential of an independent dedicated outdoor air system integrated with thermal energy storage for a childcare center

This study investigated the thermal performance of an independent dedicated outdoor air system integrated with thermal energy storage (TES) used in a childcare center located in the eco-friendly energy town of Jincheon. During the cooling season, chilled water from the TES served buildings based on the use of a thermal network. To thermally charge the TES, a ground source heat pump was operated during off-peak periods (i.e., overnight). Based on the monitoring data for the childcare center and building leakage test results, the required sensible and latent cooling loads were estimated through simulations. The potential for operational energy savings in the proposed system was investigated and compared with the existing heat pump system, which was integrated with the TES and a dehumidifier. It was found that the proposed system could save up to 38% of the operating energy consumption compared with existing systems.

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.

Applicability of thermoelectric heat pump in a dedicated outdoor air system

The main objective of this research is to propose a dedicated outdoor air system assisted by a thermoelectric heat pump, and to evaluate the operating energy performance of the proposed system compared with a reference system. The proposed dedicated outdoor air system consists of an enthalpy wheel, a desiccant wheel, and the thermoelectric heat pump to condition the introduced outdoor air to provide required ventilation air to the conditioned zone at neutral temperature, while the reference system consists of a desiccant wheel, a sensible heat exchanger, a cooling coil, and a regeneration air heater. To develop an empirical model for the thermoelectric heat pump applicable to the energy simulation of the proposed system, a thermoelectric heat pump prototype was built, and its operating data were collected in a fully controlled laboratory environment. By integrating the existing models of each system component used in both dedicated outdoor air systems, detailed energy simulations were performed to compare the energy performance of the proposed dedicated outdoor air system with the reference system. The results showed that the proposed system assisted by the thermoelectric heat pump consumed 23% more operating energy than the reference system because of the lower coefficient of performance of the thermoelectric heat pump compared with the conventional vapor compression system. For comparable energy performance, the proposed dedicated outdoor air system should have heat exchange effectiveness values greater than 49% at least at both the process and secondary air channels of the thermoelectric heat pump, and the figure of merit of the thermoelectric modules used in the thermoelectric heat pump should be over 1.35.

A field study on indoor environment quality of Chinese inpatient buildings in a hot and humid region

In this research, objective physical measurements and subjective questionnaire surveys are used to investigate the indoor environment quality of Chinese inpatient buildings. The relative humidity in the inpatient buildings reaches 65%–75% during summer, resulting in the regular appearance of microbial growth on indoor surfaces. The average outdoor air change rate measured through the CO2 concentration decay method in the sampled inpatient rooms is 1.1 h−1, which is 45% below the standard threshold. The CO2 concentration in over 99% of the functional spaces is below the threshold of 1000 ppm. However, the dissatisfaction rate of the air freshness is higher than 25%, owing to the characteristics of healthcare activities. Insufficient fresh air volume and high supply air humidity ratio of the outdoor air system result in the inadequate dehumidification capacity and the over-humid environment in the inpatient buildings. From the perspective of indoor TVOC and PM2.5 concentration, a hospitable IAQ is achieved in the inpatient buildings. In the nurse unit, the illumination levels in public areas, such as patient corridors and nurse stations, are inadequate. The average noise levels (A) in the inpatient rooms and nurse stations are 50.7 and 61.6 dB, respectively, which exceeds the Chinese standard. According to the subjective survey, the dissatisfaction rates of overall IEQ in the summer for patients and visitors are 7.9% and 10.4%, respectively, while for staff it is 34.8%. Statistical analysis reveals that the satisfaction levels of the patient with the IEQ are higher than that of the visitor and staff.

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.

A gradient-based adaptive balancing method for dedicated outdoor air system

This paper presents an online air balancing method applied in dedicated outdoor air systems, named as gradient-based online adaptive balancing (GOAB) method. The GOAB method derives the damper adjustment rule based on the gradient of the objective function that quantifies the discrepancy between actual flow rate and the set point in normalized form. An adaptive mechanism is applied to capture the change of Jacobian matrix during the balancing process. The optimum direction of damper angle increment is determined based on the null space of the gradient vector to speed up the convergence and ensure at least one damper to be fully open. Compared with the existing methods, the proposed GOAB method offers the following advantages:1.) The method can be performed on a duct system in operation, which provides possibility to re-balance the system under dynamic working conditions; 2.) The energy loss on dampers is minimized without compromising the balancing accuracy. Simulations are performed in MATLAB to investigate several model parameters and provide guidance for the parameter design. Experiments are conducted on a 5-ternimal duct system to validate the proposed method under various design flow requirements. The results show that the maximum absolute percentage error can be controlled within 4.3% among all test cases. This method improves the energy efficiency of the duct system and speeds up the convergence rate towards air balance to save time and cost.

A Novel Design of Triple-Hybrid Absorption Radiant Building Cooling System With Desiccant Dehumidification

In air-conditioning, strategy of decoupling cooling and ventilation tasks has stimulated considerable interest in radiant cooling systems with dedicated outdoor air system (DOAS). In view of this, current paper presents a simulation study to describe energy saving potential of a solar, biogas, and electric heater powered hybrid vapor absorption chiller (VAC) based radiant cooling system with desiccant-coupled DOAS. A medium office building under warm and humid climatic condition is considered. To investigate the system under different operational strategies, energyplus simulations are done. In this study, a novel design involving solar collectors and biogas fired boiler is proposed for VAC and desiccant regeneration. Three systems are compared in terms of total electric energy consumption: conventional vapor compression chiller (VCC) based radiant cooling system with conventional VCC-DOAS, hybrid VAC-based radiant cooling system with conventional VCC-DOAS, and hybrid VAC-based radiant cooling system with desiccant-assisted VCC-DOAS. The hybrid VAC radiant cooling system and desiccant-assisted VCC-DOAS yields in 9.1% lesser energy consumption than that of the VAC radiant cooling system with conventional VCC-DOAS. Results also show that up to 13.2% energy savings can be ensured through triple-hybrid VAC radiant cooling system and desiccant-assisted VCC-DOAS as compared to that of the conventional VCC-based radiant system. The return on investment is observed to be 14.59 yr for the proposed system.

Full-scale operation of a novel two-pipe active beam system for simultaneous heating and cooling of office buildings

This paper presents an investigation on the operation of a novel active beam system installed in an office building located in Jönköping, Sweden. The system consists of two parts: a dedicated outdoor air system (DOAS) to satisfy latent loads and ventilation requirements, and a water circuit to meet sensible heating and cooling loads. The novelty of the system is in relation to the water circuit, which is able to provide simultaneous heating and cooling through a single water loop that is near the room temperature. The energy performance of the system is currently being monitored through a number of sensors placed along the water circuit. Relevant physical parameters are being measured and data are available through a monitoring system. A preliminary analysis shows that the system is performing as designed. Results are shown for a typical week in winter, spring and summer. In particular, the supply water temperature in the circuit was between 20°C (in summer) and 23.2°C (in winter). The maximum supply/return temperature difference was found in summer and it assumed a value of 1.5 K. It is noticed that in spring supply and return water temperatures almost overlap.

Energy-saving potential of dedicated outdoor-air system assisted by vacuum-based membrane dehumidifier

The proposed research presents a dedicated outdoor-air system (DOAS) integrated with a vacuumbased membrane dehumidifier (VMD). The primary objective of this study was to evaluate the energy-saving potential of the proposed VMD–DOAS combination. VMD–DOAS comprised a membrane-energy exchanger (MEE), dew-point indirect evaporative cooler (DP-IEC), and VMD. VMD possessed a characteristic by virtue of which the dehumidification process was isothermal; i.e., no temperature change was observed during the VMD process. While VMD served to control the dry-air supply, the required target temperature (i.e., 17 °C) was maintained via DP-IEC operation. The remaining sensible heat of the conditioned zone was controlled by the ceiling radiant cooling panel (CRCP). The load of the conditioned zone was driven by TRANSYS 18, and an engineering equation solver (EES) was used for evaluating the energy-saving potential of the proposed system with CRCP by comparing it against the variable-air-volume (VAV) system. Results of this study demonstrated that the proposed DOAS with CRCP consumed 37% less operating energy compared to the VAV system. This observed energy-saving potential of the proposed system was driven by reducing the dehumidification load and subsequent energy recovery by MEE.

Thermal Zoning Based on Design Cooling Loads: Methodology and Simulation Case Study for a DOAS with Local Recirculating Units

A framework for creating thermal zones in a building for effective and efficient heating ventilation and air conditioning (HVAC) system design is introduced here. This method is based on simple “sort and eliminate” schemes and requires design cooling loads of conditioned spaces obtained from load calculation tools as primary input. The developed methodology is applied for creating thermal zones, determining corresponding supply conditions and ascertaining sizing of a dedicated outdoor air system (DOAS) with local recirculating units. A simulation study on a prototype-building model shows that a DOAS coupled with zoned recirculating systems that serve distinct thermal zones in a building (zoned model) perform comparatively better in controlling both space temperatures and humidity without significantly compromising HVAC energy and chiller loads than un-zoned HVAC systems serving the whole building as a single thermal block (un-zoned model). The consistency in the performance of zoned HVAC systems is verified by applying three different simulation weather files for New Delhi. Better performance along with logical and computational simplicity makes this design procedure a good alternative to traditional methodologies.

The Optimization of Hybrid Air Ventilation System Combined with Silica Gel and Thermoelectric Using Monitoring Control

In this paper, the experimental is evaluated temperature, humidity and air pollution from CO<sub>2</sub> by monitoring control of hybrid air ventilation for producing fresh air. The experimental divided into 2 conditions: hybrid air ventilation on-off, recovery silica gel unit. That research consider of a voltage of thermoelectric devices relative with air cooling under thermal comfort zone Thailand climate conditions. In the experimental was designed by the DOAS system (dedicate outdoor air system) concept. Whereas, the hybrid air ventilation was improved by old air ventilation which the hybrid air ventilation combined with silica gel unit and thermoelectric device. Monitoring control is written by Arduino program that is open source for control performance of processes such as sensors, electric current and fan unit. That working is specified temperature and humidity Thailand thermal comfort range. In the part of CO<sub>2</sub> was released rate approximately 900 ppm corresponded with indoor air quality standard (IAQs). The results of experimental suggested that in case of hybrid air ventilation system explained thermal, humidity and CO<sub>2</sub> distribution due to the system was adapted significantly in the comfort zone temperature and humidity about 28 and 58% respectively. However, the CO<sub>2</sub> released into the hybrid air ventilation although control during 900 – 1,000 ppm. High CO<sub>2</sub> concentration in comfort zone used the hybrid air ventilation that decreased CO<sub>2</sub> inlet under ASHRAE 62.2. Therefore, the experimental is an evaluation of monitoring control hybrid air ventilation for management comfort zone. Finally this research concept of the optimization of the hybrid air ventilation combination with thermoelectric devices and silica gel unit.

Energy benefit of a cascade liquid desiccant dehumidification in a desiccant and evaporative cooling-assisted building air-conditioning system

The main objective of this study was to estimate the impact of dehumidification with a cascade liquid desiccant on the primary energy consumption in a liquid desiccant and evaporative cooling-assisted 100% outdoor air system. Detailed energy simulations were performed for two systems serving an identical office space: the conventional liquid desiccant and indirect/direct evaporative cooling-assisted 100% outdoor air system (LD-IDECOAS) (i.e., Case 1) having a single-stage liquid desiccant dehumidification section, and a retrofit case of the LD-IDECOAS having a cascade liquid desiccant section (i.e., Case 2) consisting of a two-stage liquid desiccant dehumidifier. The results showed that Case 2 consumed 12% less primary energy under the peak load condition and 17.4% less primary energy during the cooling season. The overall thermal and primary coefficients of performance of Case 2 were 0.78 and 2.05, respectively, whereas those of Case 1 were 0.65 and 1.45, respectively. In addition, it was also observed that Case 2 could provide lower supply air temperature to the conditioned space compared with Case 1 owing to enhanced evaporative cooling performance, which was mainly caused by enhanced dehumidification in the cascade liquid desiccant section. The lower supply air temperature resulted in reductions in fan and pump energy consumptions during the operation.

Performance investigation of an independent dedicated outdoor air system for energy-plus houses

This study proposes an independent dedicated outdoor air system (IDOAS) for energy-plus houses under the humid subtropical climate located in Daejeon, South Korea. The proposed IDOAS can be used as a stand-alone ventilation and dehumidification system. In order to investigate the energy efficiency of the IDOAS, four different types of systems are simulated including the conventional dehumidifier (i.e., Case 1) and conventional dedicated outdoor air system (DOAS) (i.e., Case 2). The first IDOAS (i.e., Case 3) is designed such that the evaporator of the heat pump is located at the supply air (SA) side and the two separated condensers are located on the SA and exhaust air sides, respectively. The second IDOAS (i.e., Case 4) has a direct evaporative cooler located on the exhaust air side to improve the heat extraction rate. It is shown that the conventional DOAS and IDOAS-2 yielded the highest energy efficient system.

Application of artificial neural networks to the simulation of a Dedicated Outdoor Air System (DOAS)

Tables of performance of installed HVAC (Heating, Ventilation and Air Conditioning) devices are important in the development of consistent building energy audits and appropriate control strategies. However, given the possible complexity of HVAC devices and the need for the deployment to computational environments, tables of performance should be passed in a more complete and flexible format, compared with the current practices in the HVAC sector. In such a context, this paper describes the phases of development and application of Artificial Neural Networks (ANNs) aimed at the assessment of the performance of a Dedicated Outdoor Air System (DOAS). ANNs are well renowned because of their applications in many important fields such as autonomous driving systems, speech recognition, etc. However, they may be used also to calculate the output of complex phenomena (like the ones involved in HVAC components) and are characterized by a very flexible and comprehensive formulation which would be able to adapt to any HVAC component or system. In the frame of this study, three ANNs have been developed and tested, for the full description of the performance of a DOAS. The developed ANNs were trained by means of data coming from a proprietary software. The achieved ANNs showed robust and reliable behavior and ensure high accuracy (mean absolute errors usually below 0.1 K on temperatures and 0.3% on capacity and power) and flexibility. Moreover, in some cases, they may be used also for the identification of anomalous data present among the sets of training and validation data.

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.