Coil Unit Research Articles

Experimental investigation on the energy storage/discharge performance of xylitol in a compact spiral coil heat exchanger

The main challenge ahead of widespread application of renewable energy sources is their availability intermittence preventing continuous power supply. In order to circumvent the temporal mismatch between their supply and demand, thermal energy storage systems can be utilized. In this study, a compact spiral coil thermal storage unit was investigated to analyze the impact of operating parameters such as inlet temperature and flow rate of the heat transfer fluid and explain the physics of heat transfer during the phase change process of xylitol. A concentric double spiral coil was inserted into a storage unit to ensure an improved heat transfer performance. Using experimental data, average temperature variation, heat storage/discharge rate and liquid fraction of xylitol in the storage unit were calculated. It was found that when the PCM melted, the lower density liquid PCM created buoyancy forces resulting in natural convection. On the other hand, discharging process was mainly governed by conduction. In the present storage unit, xylitol stored 450 kJ of heat in 35 min for an inlet temperature and flow rate of 130 °C and 2.5 lpm during charging and discharged 345 kJ in 50 min at an inlet temperature and flow rate of 45 °C and 2.5 lpm. The outcomes of this analysis are expected to be greatly applicable for the design of phase change material based spiral coil units to be used for instance in solar heating systems.

Performance of a cold storage air-cooled heat pump system with phase change materials for space cooling

This paper studies the performance of a cold storage heat pump system integrated with phase change materials (PCMs) for space cooling. An air-cooled heat pump unit comprising of two constant rotation speed compressors is selected. The PCM is an organic material that is commercially available, and has a phase change temperature of 10–12 °C. A coupled computational heat transfer model of the heat pump unit, fan coil units, and a cold storage tank filled with PCM slabs is developed. The simulations are performed under the climatic condition of Fuzhou, China. The operational behaviors of the cold charging and discharging processes of the system are analyzed. Furthermore, the overall electrical energy and electricity charge consumptions at various indoor temperature set points are compared. The results indicate that, the electrical energy consumption of the cold storage heat pump system increases as the indoor temperature increases. However, there is very little difference in the electrical energy consumption of the conventional heat pump system and the cold storage system. While under the demand tariff, the electricity charge saving ratio of the cold storage system over the conventional system is 9.07%-11.28%. In general, as the indoor temperature set point reduces, the electricity charge saving ratio increases. Moreover, the effect of cold charging temperature is also discussed. It is found that higher cold charging temperature consumes less electrical energy, however, takes longer to store the same amount of cold, and the differences relies on the required amount of cold.

Thermal Efficiency Enhancement of an R-32 Air Conditioner and the Ultrasonic Wave Technique

This work presents a technique to improve the thermal performance of an R-32 air conditioner by using ultrasonic waves. Two 1-TR air conditioners — one with and one without 40[Formula: see text]kHz/50[Formula: see text]W ultrasonic generators — were tested under controlled conditions in a 3.6[Formula: see text]m [Formula: see text][Formula: see text]m [Formula: see text][Formula: see text]m room. The air temperature and humidity conditions entering the condenser and evaporator coils were controlled under 12 conditions to mimic the climate of Thailand. The testing results indicated that the average energy efficiency ratio (EER) of the modified ultrasonic R-32 air conditioner unit was 3.685[Formula: see text]kWth/kWe and the normal R-32 unit without the ultrasonic generators was 3.375[Formula: see text]kWth/kWe. The testing conditions also indicated that the cooling efficiency of the modified system could be increased by approximately 7.69%. Moreover, the convective heat transfer coefficient of the modified ultrasonic R-32 unit was approximately 17.36% higher than that of the normal R-32 unit without the ultrasonic generators because the ultrasonic waves could increase the turbulence in the refrigerant flow and decrease the condensed water at the fan coil unit. In terms of economic results, the payback period of the modified unit based on the climate of Thailand ranged from 0.8–1.2 y, leading to a savings in power consumption of approximately 400–620[Formula: see text]kWh/y.

Development of an optimal control method of chilled water temperature for constant-speed air-cooled water chiller air conditioning systems

Constant-speed air-cooled water chillers (CAWCs) are typically on–off controlled in accordance with a fixed set point of chilled water return temperature (Twr), which seriously restricts their efficiency during part load conditions. To improve the performance of CAWC air conditioning (A/C) systems, this paper developed an optimal control method to reset the Twr of CAWCs real time. Firstly, a simulation platform was established based on an actual office building with a CAWC A/C system using fan coil units (FCUs) as terminal devices in Beijing. Secondly, a dataset of optimal values of Twr at different part load conditions was calculated by simulation. Thirdly, to enable the practical use of the dataset, a general regression neural network (GRNN) model was built to predict the optimal Twr value according to three ambient parameters and one operation parameter. Finally, a GRNN based optimal control method was developed for the CAWC/FCU A/C system. The numerical results demonstrated that the proposed method can achieve a good control over indoor thermal environment, and lead to a reduced energy use for the A/C system by 11.0% over the cooling season. This study provided a promising control method of chilled water temperature for CAWC/FCU A/C systems in dry climates.

A new three-dimensional CFD model for efficiency optimisation of fluid-to-air multi-fin heat exchanger

New simulation and experimental results have been obtained and are presented for a multi-tube fin heat exchanger unit, from which semi-analytical correlations for the Fanning friction and Colburn factors were developed. The multi-tube and fin heat exchanger represents the main component of the Fan Coil Unit, an essential component of HVAC systems used for domestic and commercial heating and cooling. Improving the efficiency of the heat exchanger typically comes at the expense of higher pressure drops or costlier materials and production costs. Here, an experimental setup was designed and constructed to evaluate the thermal performance of such a heat exchanger. Geometrical modifications were explored for thermal performance enhancement. Furthermore, full three-dimensional CFD case studies of the heat exchanger were investigated to examine the effect of the geometrical features on the air side of the heat exchanger to study the effect of fin spacing, transverse and longitudinal pitches. The CFD model developed was first globally validated against experimental results. The model results were used to predict the Fanning and Colburn factors and the local fin efficiency based on the carefully selected geometric parameters. The data obtained was utilised to develop two new semi-analytical models for the Fanning and Colburn friction factors which were well within ±10% error bands and showed strong correlation coefficients of more than 98 and 97% respectively.

Model predictive control of building energy systems with thermal energy storage in response to occupancy variations and time-variant electricity prices

Developing effective operational strategies for commercial buildings is a high priority as the global community seeks to reduce energy usage and greenhouse gas emissions. This study aims to validate the feasibility of a model predictive control (MPC) strategy for commercial building in response to occupancy variations and time-variant electricity prices in comparison to a conventional rule-based control (RBC) strategy. The building energy system included an air-cooled chiller, stratified chilled water thermal energy storage, two fan coil units, three heat exchangers, and five pumps. The optimal operations of the chiller and storage system were determined with the goal of minimizing the operating cost while maintaining the zone temperature at a cooling set point temperature during cooling operation hours. Artificial neural network was utilized as prediction models and metaheuristics algorithm was employed as optimization solver to construct a reliable and computationally manageable MPC controller. The simulation was performed for four days during the cooling season with a confirmed optimal prediction time horizon of 24 h and a control timestep of 1 h intervals. In conclusion, MPC reduced the total operating cost by 3.4% compared to the RBC, which prioritized the storage system operation to manage the thermal load.

Feasibility analysis of a modular uniform magnetic field coil.

In this paper, a feasibility analysis of a modular uniform magnetic field coil is provided. The modular uniform magnetic field coil consists of n2 square coil units. By adjusting the number of coil units or the current in the coil units, different sizes and patterns of uniform magnetic field coils can be easily formed to meet different experimental requirements. An analytical model of the modular uniform magnetic field coil considering the gap between coil units is first developed based on the Biot-Savart law and the superposition principle, and the analytical model is verified by the 3D finite element method. Then, the characteristics of the modular uniform magnetic field coil are analyzed and compared with the traditional Helmholtz coil, demonstrating the advantages of the proposed magnetic field coil. These advantages precisely verify the original intention of the modular uniform magnetic field coil proposed in this paper. Finally, a robustness analysis of the modular magnetic field coil is performed, including the effects of gap between coil units and assembly misalignment on the performance of the modular uniform magnetic field coil.

Operation strategy of a solar-gas fired single/double effect absorption chiller for space cooling in Indonesia

A solar-gas fired absorption cooling system has been installed and tested in a real environment at the University of Indonesia, Depok, Indonesia. The cooling system provides chilled water to the building of the Mechanical Research Center of the university. This system has a unique single/double-effect water/Lithium Bromide absorption chiller with a nominal cooling capacity of 239 kW. In addition, the system consists of evacuated tube solar collectors (~181 m2 total aperture area) and fan coil units installed in the building. The absorption chiller is driven by (i) hot water (75 – 90 °C) produced by the solar collectors which supply heat to the single-effect generator and (ii) by a direct gas-fired burner at the double-effect high-temperature generator when it is needed. The driving heat supply from the gas-burner is 113.2 kW and 93.3 kW (at 90 °C) from the solar collector field. This study focuses on the control strategy of the solar-gas fired absorption chiller system based on operational data obtained from the field test carried out in an Asian tropical climate, particularly Indonesia. The proposed and implemented control strategy consists of the control of both external and internal operation variables. The external control strategy includes the control of three water flow loops (hot water, cooling water, and chilled water) that are controlled according to the cooling load of the building and weather conditions (i.e., solar irradiation and ambient temperature). The internal operation variable (total solution flow rate) and gas flow rate are adjusted according to the required cooling capacity. The field tests of thermal and electrical COP are around 0.9–1.1 and 4.5–5.5, respectively. The field test results demonstrate the feasibility of the implemented control strategy for the optimum and safe operation of the system in an Indonesian climate, with the possibility of adapting to other similar Asian tropical climates.

A Case Study Based Approach for Remote Fault Detection Using Multi-Level Machine Learning in A Smart Building

Due to the increased awareness of issues ranging from green initiatives, sustainability, and occupant well-being, buildings are becoming smarter, but with smart requirements come increasing complexity and monitoring, ultimately carried out by humans. Building heating ventilation and air-conditioning (HVAC) units are one of the major units that consume large percentages of a building’s energy, for example through their involvement in space heating and cooling, the greatest energy consumption in buildings. By monitoring such components effectively, the entire energy demand in buildings can be substantially decreased. Due to the complex nature of building management systems (BMS), many simultaneous anomalous behaviour warnings are not manageable in a timely manner; thus, many energy related problems are left unmanaged, which causes unnecessary energy wastage and deteriorates equipment’s lifespan. This study proposes a machine learning based multi-level automatic fault detection system (MLe-AFD) focusing on remote HVAC fan coil unit (FCU) behaviour analysis. The proposed method employs sequential two-stage clustering to identify the abnormal behaviour of FCU. The model’s performance is validated by implementing well-known statistical measures and further cross-validated via expert building engineering knowledge. The method was experimented on a commercial building based in central London, U.K., as a case study and allows remotely identifying three types of FCU faults appropriately and informing building management staff proactively when they occur; this way, the energy expenditure can be further optimized.

Effects of air inlet or outlet position of a fan coil unit ventilation system on smoke movement and fire severity

Air conditioning engineers design the inlet or outlet position of a fan coil unit (FCU) system according to thermal comfort and cost. However, the effect of the position on fire growth has not been considered. In this study, the effect of the relative inlet and outlet positions was investigated. Thirty cases were simulated using fire dynamics simulator (FDS). The simulation results revealed that the air inlet and outlet positions influence the smoke movement, doorway temperature and time to flashover. The air inlet is best installed near the door because the downward flowing cold air can cool the hot smoke exiting from the door. The air outlet positioned far from the doorway efficiently enables the smoky air to move outward, while it positioned near the doorway exhausts some fresh air. Furthermore, the air outlet position has a larger influence on flashover than the air inlet position. The air outlet immediately exhausts the smoke.

Investigating the performance of a thermal energy storage unit with paraffin as phase change material, targeting buildings’ cooling needs: an experimental approach

Abstract For the reduction of the electricity cost for cooling buildings in hot climate zones, thermal energy storage at low temperature using phase change materials is a potential solution. The storage medium can be cooled during periods of low electricity cost and then absorb the cooling load any time of the day. In this study, an experimental cold thermal energy storage system using organic phase change materials (A9 and A14) with melting temperature of 9 or 14 °C is investigated. The system is designed for real application conditions coupled with a heat pump which cools down the organic material through the circulation of water in the pipes and fan coil units which dissipate their thermal load in the initially cooled storage medium. It comprises of a thermal energy storage tank, inside which is positioned a staggered heat exchanger with finned tubes immersed in phase change material. The results from the experiments yielded shorter process duration (for both melting and solidification) and higher heat transfer rate when the feed rate was higher. Moreover, the temperature of the water entering and exiting the tank is more than 5 °C for A9 organic material for more than half the capacity of the tank thus, using A9, the fan coil units can operate efficiently for a long time. The heat transfer rate exceeds 5 kW for 32 and 24 min using A9 and A14 respectively and the thermal load that the tank can absorb from the fan coils is above 6 kWh per tank. During the heat pump operation, A9 results to a more constant water outlet temperature over the time compared to A14 which in addition is not affected significantly by the water flow rate. Moreover, the staggered heat exchanger transfers less than 10 kW of heat when the water outlet temperature ranges between 5 and 10°C, a power value which can easily be provided by a typical commercial heat pump.

A bridged loop gap resonator (BLGR) for small animal imaging by 1.5 T MRI systems.

A bridged loop gap resonator (BLGR) was developed as a transmit and receive coil for a mobile insert to be used for small animal proton imaging by 1.5 T MRI devices. The insert system has its own gradient system, radio frequency (RF) transmit and receive coil, and control and signal processing unit. The reflection S11 and transmission S21 parameters, quality factor (Q), sensitivity, signal to noise ratio (SNR), and maps of the static (B0) and RF (B1) magnetic flux densities were measured. The RF coil was developed starting from a loop gap resonator (LGR) for a balanced LGR and a shielded balanced LGR for a shielded bridged balanced LGR. The purpose of developing this device is to minimize the influence of the sample and surroundings on the RF coil parameters. The final design of the BLGR does not require retuning after a sample change. A 3D image of a mouse in formalin was acquired with a fast low angle shot (FLASH) MRI sequence. The SNR was calculated from one FLASH image. The signal for SNR calculation was acquired from a gadolinium-doped water sample and the noise from the air outside of the sample. This article verifies that the BLGR is viable for small animal nuclear magnetic resonance imaging at 1.5 T and is independent of sample size and material.

SoftCharge: Software Defined Multi-Device Wireless Charging Over Large Surfaces

This paper proposes a method for converting a large existing surface into a programmable wireless charger, capable of distributing energy efficiently at multiple locations on demand and charge different types of devices. The key innovation here is to combine magnetic resonance-based energy transfer with the so called concept of `energy hopping' across wireless inter-connected coils, where the magnetic fields are carefully shaped on the fly. The overall framework, called SoftCharge, has three components: (i) energy tiles (ETs), which are individual programmable coil units that can be attached underneath an existing table with AC mains supply only to the master tile, (ii) energy shaping algorithm executed by the master tile, that shapes the flow of energy over tiles through real-time impedance adjustment combined with selective power blocking, creating optimal energy paths to specific tiles where a device needs to be charged, and (iii) a resonance sensing architecture design and method executed in each given tile that detects the type of device to be charged and its location without any direct feedback. We build a prototype of energy tiles and provide experimental results on SoftCharge charging multiple COTS devices like mobile phones, laptops, tablets, and drones, resulting in a maximum charging rate of 23 W up-to 20 cm over a larger surface.

Improved operating strategy for air-conditioning systems based on the indoor occupancy rate

The energy efficiency of an air-conditioning system is reduced when operating under low cooling load, e.g., when few occupants remain in the building, leading to extra energy consumption. This paper proposes an operating strategy for air-conditioning systems based on the indoor occupancy rate (R), where the start-stop is adjusted according to R while still satisfying indoor environment quality (IEQ) requirements. The dedicated outdoor air system (DOAS) is simultaneously used to feed in fresh air and combined with indoor fan coil units to independently control humidity and temperature. An office building air-conditioning system was tested with the improved operating strategy applied. The test results show that the energy efficiency ratio (EER) of the cooling plant reached 4.2, a 7.2% improvement over the same system operating under previous conditions. The IEQ in the test building was also significantly improved. Because the air-conditioning system did not operate at low energy efficiency, 15% of the total energy consumption of the system was saved.

Evaluation of thermal imbalance of ground source heat pump systems in residential buildings in China

The thermal comfort level of Chinese residents in the hot-summer and cold-winter zone is increasing with economic development in recent years. More district heating and cooling systems were installed in these areas. Supported by the government, ground source heat pump (GSHP) is used as a common environmentally friendly district heating and cooling system in many residential communities. However, the evaluation of the real performance of GSHP is limited, especially limited understanding of the common issue of thermal imbalance of GSHP systems over several years—the mismatch between heat extraction from and rejection to the ground. Performance of GSHP systems can deteriorate over years if the thermal imbalance is significant and not addressed. For GSHP in the hot-summer and cold-winter climate zone, there are few thermal imbalance studies based on real field data and modeling. To address this gap, this paper proposes an integrated simulation model to evaluate thermal imbalance of GSHP systems considering key influencing factors including occupant behavior and heat pump performance. A case study was conducted to simulate and analyze the actual operation of GSHP in Chinese residential buildings. The indoor comfort temperature setting and flexibility of residents’ control on indoor terminal air-conditioning units determine the actual heating and cooling demand in residential buildings, which resulted in differences in thermal imbalance for GSHP systems. It was found that GSHP systems with radiant floor only for heating lead to the most severe thermal imbalance, while GSHP systems with fan coil units or radiant ceiling for cooling and/or heating achieved much better thermal balance. The proposed thermal imbalance ratio is an important indicator that can be used to evaluate thermal imbalance of GSHP systems and thus inform their design and operation for energy efficiency.

무선전력 전송효율 음영영역 제거와 최대효율 전송을 위한 3D 스캐닝 MR-WPT 시스템

This paper proposes a 3D scanning magnetic resonance wireless power transfer system to eliminate dead-zones and improve transmission efficiency. This system consists of an oscillator, power amplifier, microcontroller unit, personal computer, pan-tilt motor, transceiver coil, rectifier, and wireless communication units. This system provides the maximum voltage to the receiver within a short timeframe using the zone-based maximal transmission efficiency searching process algorithm. From the measurement results, it can be observed that the time to find the maximum transmission efficiency with this algorithm is 11 times faster than that of the full scan by 5° and that the position error of maximum transmission efficiency is less significant.

Natural heat transfer air-conditioning terminal device and its system configuration for ultra-low energy buildings

In consideration of the lowered heating and cooling load of ultra-low energy buildings, a natural heat transfer air-conditioning terminal device (NHTACTD) is presented by the authors. The terminal device is able to undertake heating, cooling and moisture load of a room according to the inlet water temperature. Its comparative advantages are pointed out by comparing it with radiators, fan coil units, chilled beams and radiant heating and cooling terminals. After the actual thermal properties of the NHTACTD are provided, three air-conditioning system configuration schemes based on the NHTACTDs are presented: (1) In the NHTACTDs plus fresh air system, the NHTACTDs and the fresh air handling unit undertake all the air-conditioning load of the system together, and it is used where high indoor air quality is demanded; (2) In the scheme of the air-conditioning system including the NHTACTDs only, all the air-conditioning load of the system has to be undertaken by the terminals alone and it is used where high indoor air quality is not rigidly demanded. (3) In the scheme of the air-conditioning system including both the NHTACTDs and radiant panels, the NHTACTDs undertake all the moisture load of the system, and the remaining sensible cooling load is undertaken by the radiant panels. The scheme can be used where there is large sensible cooling load while high indoor air quality is not rigidly demanded. Then an ultra-low energy residential building is taken as a case project to elaborate the design method of an air-conditioning configuration scheme based on the terminal device with the help of psychrometric chart.

Magnetic field optimization for high-positioning-tolerant wireless charging platforms

This paper proposes an omnidirectional high-positioning-tolerant wireless power transfer (WPT) platform using an array of large-scale coil units with moderate overlaps. A generalized magnetic field (MF) design and an optimization methodology for the coil layout are introduced based on analytical solutions of the MF distribution in 3D space. An MF density distribution matrix is derived, and basic evaluation criteria including both the MF uniformity and the MF intensity are quantitatively analyzed. Using a synthesized figure of merit (FoM) as an objective function, a sequential quadratic programming (SQP) algorithm is adopted to obtain the optimal parameters of the coil array. The maximal intensity and uniformity of the induced MF are achieved simultaneously. Both theoretical and experimental results demonstrate equal and stable system performance when a power receiver is placed arbitrarily on the platform.

Performance test & analysis on fan coil unit and improved induction air unit system

Induction air unit was improved by decreasing the induction ratio and adding the rectifier. Performance test on fan coil unit and improved induction air unit system was carried in the model houses. Test results showed: (1) Induction air unit system could meet the temperature requirement of room. It had a strong dehumidifying ability and much uniformity air velocity in the range of comfortable. (2) Induction air unit system had little temperature gradient and difference. (3) The unit was energy efficient combined with the extensive air handling unit and medium temperature chiller. Improved induction air unit would be used in the high quality air conditioning spaces.

Auditing on Energy Consumption and Thermal Load Calculation of a Building and Optimized Duct Design to Attain Uniform Cooling

Heating ventilation and air conditioning (hvac) main goal is to provide thermal comfort and acceptable air quality inside rooms. In large buildings huge amount of energy is wasted due to its weak thermal performance and moderate hvac system. Heat load calculation is a basic step to be considered during designing a hvac system for a building. During heat load calculation weather conditions and occupants of a room are to be considered. Auditing is done on power consumption of a building when two different hvac system are used for distribution of conditioned air and also design a duct. In these paper fan coil unit and air handling unit are compared with each other and investigated how much power is consumed while using each system separately. When fan coil unit is connected with a duct the power consumed by a building is 32.75 kw/hr and when air handling unit is connected with a duct the power consumed is 41 kw/hr. Thus fan coil unit is preferred over air handling unit because it consumes less power and according to that duct is designed using designing tool to attain uniform cooling.