Farshid Bagheri

Modelling and optimal energy-saving control of automotive air-conditioning and refrigeration systems

Air-conditioning and refrigeration systems are extensively adopted in homes, industry and vehicles. An important step in achieving a better performance and a higher energy efficiency for air-conditioning and refrigeration systems is a control-based model and a suitable control strategy. As a result, a dynamic model based on the moving-boundary and lumped-parameter method is developed in this paper. Unlike existing models, the proposed model lumps the effects of the fins into two equivalent parameters without adding any complexity and considers the effect produced by the superheated section of the condenser, resulting in a model that is not only simpler but also more accurate than the existing models. In addition, a model predictive controller is designed on the basis of the proposed model to enhance the energy efficiency of the air-conditioning and refrigeration systems. Simulations and experimental results are presented to demonstrate the accuracy of the model. The experiments show that an energy saving of about 8% can be achieved by using the proposed model predictive controller compared with the conventional on–off controller under the examined scenario. The better performance of the proposed controller requires electrification of the automotive air-conditioning and refrigeration systems so as to eliminate the idling caused by running the air-conditioning and refrigeration systems when a vehicle stops.

A Resistance–Capacitance Model for Real-Time Calculation of Cooling Load in HVAC-R Systems

Simulating the real-time thermal behavior of rooms subject to air conditioning and refrigeration is a key to cooling load c alculations. A well-established Resistance-Capacitance (RC) model is employed that utilizes a representative network of electric resistors and capacitors to simulate the thermal behavior of such systems. A freezer room of a restaurant is studied during its operation and temperature measurements are used for model validation. Parametric study is performed on different properties of the system. It is shown that a reduction of 20% in the walls thermal resistivity can increase the energy consumption rate by 15%. The effect of set points on the number of compressor starts/stops is also studied and it is shown that narrow set points can result in a steady temperature pattern in exchange for a high number of compressor starts/stops per hour. The proposed technique provides an effective tool for facilitating the thermal modeling of air conditioned and refrigerated rooms. Using this approach, engineering calculations of cooling load can be performed with outstanding simplicity and accuracy.

A Self-Adjusting Method for Real-Time Calculation of Thermal Loads in HVAC-R Applications

A significant step in the design of heating, ventilating, air conditioning, and refrigeration (HVAC-R) systems is to calculate room thermal loads. The heating/cooling loads encountered by the room often vary dynamically while the common practice in HVAC-R engineering is to calculate the loads for peak conditions and then select the refrigeration system accordingly. In this study, a self-adjusting method is proposed for real-time calculation of thermal loads. The method is based on the heat balance method (HBM) and a data-driven approach is followed. Live temperature measurements and a gradient descent optimization technique are incorporated in the model to adjust the calculations for higher accuracy. Using experimental results, it is shown that the proposed method can estimate the thermal loads with higher accuracy compared to using sheer physical properties of the room in the heat balance calculations, as is often done in design processes. Using the adjusted real-time load estimations in new and existing applications, the system performance can be optimized to provide thermal comfort while consuming less overall energy. [DOI: 10.1115/1.4031018]

Dynamic Heat Transfer Inside Multilayered Packages with Arbitrary Heat Generations

New compact closed-form relationships are developed for calculating 1) the temperature distribution inside multilayered media, 2) the average temperature of each layer, and 3) the interfacial heat flux. As an example, the methodology is applied to a two-concentric-cylinder composite. A detailed parametric study is conducted, and the critical values for the dimensionless parameters are evaluated; beyond these values, the temperature field inside the media is not affected considerably for any combination of other variables. It is shown that there is an optimum angular frequency that maximizes the amplitude of the interfacial heat flux. An independent numerical simulation is also performed using commercially available software ANSYS; the maximum relative difference between the obtained numerical data and the analytical model is less than 2%. Nomenclature A = matrix defined in Eq. (A5) An = constant, Eqs. (A19) and (A20) Cjn = integration coefficient, Eq. (A3) Djn = integration coefficient, Eq. (A3) F = constant, Eq. (A8) Fo = Fourier number, α1t∕x 2 G = arbitrary function of η, Eq. (A11) Gn = constants defined in Eq. (A16) H = constant, Eq. (A11)

Investigation of Optimum Refrigerant Charge and Fans Speed for a Vehicle Air Conditioning System

In this study, the performance evaluation and optimization of a recently developed battery-powered vehicle air conditioning (BPVAC) system is investigated. A mathematical model is developed to simulate the thermodynamic and heat transfer characteristics of the BPVAC system and calculate the coefficient of performance (COP). Utilizing environmental chambers and a number of measuring equipment, an experimental setup is built to validate the model accuracy and to conduct performance optimization by changing the charge of refrigerant in the system. The model is validated and employed for performance simulation and optimization in a wide range of speed for the evaporator and condenser fans. The modeling results verify that for any operating condition an optimum performance can be achieved by adjusting the speed of condenser and evaporator fans. The optimum refrigerant charge is obtained, and a potential improvement of 10.5% is calculated for the performance of system under ANSI/AHRI 210/240-2008 specifications. [DOI: 10.1115/1.4034852]

Thermodynamic investigation of a typical commercial refrigeration system

In this study, the thermodynamic performance of a typical commercial refrigeration system is investigated and the degradation from design condition is evaluated. A mathematical model is developed to simulate thermal behavior of the refrigeration system and to obtain the coefficient of performance (COP). A number of measuring equipment including thermocouples, pressure transducers, flow meters, and power meters are installed on the refrigeration system to obtain the real-time data for a period of three months. The acquired data is utilized to investigate the thermodynamic behavior of refrigeration system in addition to model validation. A good agreement between the simulation results and the real-time data, with a maximum discrepancy of 9%, is achieved. The results represent a range of 1.1-1.4 for current COP of the refrigeration system under different operating conditions. It is shown that due to degradation, the current COP is lower than design COP by 29%-39% in different duty cycles.

Optimal Time-Varying Heat Transfer in Multilayered Packages With Arbitrary Heat Generations and Contact Resistance

Integrating the cooling systems of power electronics and electric machines (PEEMs) with other existing vehicle thermal management systems is an innovative technology for the next-generation hybrid electric vehicles (HEVs). As such, the reliability of PEEM must be assured under different dynamic duty cycles. Accumulation of excessive heat within the multi-layered packages of PEEMs, due to the thermal contact resistance between the layers and variable temperature of the coolant, is the main challenge that needs to be addressed over a transient thermal duty cycle. Accordingly, a new analytical model is developed to predict transient heat diffusion inside multi-layered composite packages. It is assumed that the composite exchanges heat via convection and radiation mechanisms with the surrounding fluid whose temperature varies arbitrarily over time (thermal duty cycle). As such, a time-dependent conjugate convection and radiation heat transfer is considered for the outer-surface. Moreover, arbitrary heat generation inside the layers and thermal contact resistances between the layers are taken into account. New closed-form relationships are developed to calculate the temperature distribution inside multi-layered media. The present model is used to find an optimum value for the angular frequency of the surrounding fluid temperature to maximize the interfacial heat flux of composite media; up to 10% higher interfacial heat dissipation rate compared to constant fluid-temperature case. An independent numerical simulation is also performed using COMSOL Multiphysics; the maximum relative difference between the obtained Journal of Heat Transfer. Received November 05, 2013; Accepted manuscript posted August 12, 2014. doi:10.1115/1.4028243 Copyright (c) 2014 by ASME Ac ce pt ed M an us cr ip t N ot C op ye di te d Downloaded From: http://fuelcellscience.asmedigitalcollection.asme.org/ on 08/28/2014 Terms of Use: http://asme.org/terms 2 numerical data and the analytical model is less than 6%.