Omar Abdelaziz

A method for modeling household occupant behavior to simulate residential energy consumption

This paper presents a statistical method for modeling the behavior of household occupants to estimate residential energy consumption. Using data gathered by the U.S. Census Bureau in the American Time Use Survey (ATUS), actions carried out by survey respondents are categorized into ten distinct activities. These activities are defined to correspond to the major energy consuming loads commonly found within the residential sector. Next, time varying minute resolution Markov chain based statistical models of different occupant types are developed. Using these behavioral models, individual occupants are simulated to show how an occupant interacts with the major residential energy consuming loads throughout the day. From these simulations, the minimum number of occupants, and consequently the minimum number of multiple occupant households, needing to be simulated to produce a statistically accurate representation of aggregate residential behavior can be determined. Finally, future work will involve the use of these occupant models along side residential load models to produce a high-resolution energy consumption profile and estimate the potential for demand response from residential loads.

Approximation-Assisted Optimization for Novel Compact Heat Exchanger Designs

This paper describes a systematic approach for the use of an approximation-assisted optimization technique in the design of a novel heat exchanger (HX) with a high degree of model fidelity and a low computational cost. Design optimization is performed using multiobjective optimization, and high model fidelity is achieved by using a multiscale HX simulation tool. An approximation technique was used to provide computational savings in order to ensure computational feasibility. This technique was applied to different HX design problems. It was observed that for a 1 kW (3412 Btu/h) heating coil with pressure drop constraints of 100 Pa (0.4 in. water) on the air side and 1000 Pa (0.15 psi) on the water side there is a potential for a more than 61% reduction in the HX volume and an 84% reduction in the HX material when the proposed geometry is compared with the current state-of-the-art technology. The proposed approximation approach produced significant computational savings. The approximation-assisted optimization platform shows great potential for examining new and existing designs and providing a greater understanding of design limitations.

A MATLAB based occupant driven dynamic model for predicting residential power demand

This paper presents a MATLAB based dynamic model for predicting residential power demand. Markov chain based occupant behavior models developed using data gathered by the U.S. Census Bureau in the American Time Use Survey (ATUS) are used in conjunction with models of the most common residential loads to predict residential power demand on a one-second time scale. First, the methods utilized for the modeling of each residential load are presented. Next, an explanation of how these load models are combined with occupant behavior models to predict residential power demand is given. Simulation results showing the overall contribution of each load to the overall residential sector power demand are shown for both winter and summer cases. Finally, future work will involve the use of this high-resolution dynamic residential model to estimate the potential for demand response from residential loads.

Investigation of the stability of paraffin-exfoliated graphite nanoplatelet composites for latent heat thermal storage systems

Organic materials, such as paraffin wax, are sought as stable and environmentally friendly phase change materials (PCM) for thermal energy storage, but they suffer from low thermal conductivity which limits the rate at which thermal energy flows into and out of the material. A common method to improve the PCM thermal behavior is through loading with high thermal conductivity particulate fillers. However, the stability of these composites in the molten state is a concern as settling of the fillers will change the effective thermal conductivity. In this work, we investigate the stability of wax loaded with exfoliated graphite nanoplatelets either of 1 μm (xGnP-1) or 15 μm (xGnP-15) diameter. The effect of dispersants, oxidation of the wax, viscosity of the wax, mixing time, and hydrocarbon chain length on stability is reported. It was found that the addition of octadecylphosphonic acid (ODPA) is an effective dispersant for xGnP in paraffin and microcrystalline wax. In addition, mixing time, viscosity, and oxidation of the wax influence stability in the molten state. Overall, it was found that a mixing time of 24 hours for xGnP-15 along with ODPA mixed in a high viscosity, oxidized microcrystalline wax results in composite PCM systems with the greatest stability determined at 80 °C in the molten state.

Alternative Refrigerant Evaluation for High-Ambient-Temperature Environments: R-22 and R-410A Alternatives for Mini-Split Air Conditioners

The Oak Ridge National Laboratory (ORNL) High-Ambient Temperature Testing Program for Low-GWP Refrigerants aims to develop an understanding of the performance of low-Global Warming Potential (low-GWP) alternatives to Hydrochlorofluorocarbon (HCFC) and Hydrofluorocarbon (HFC) refrigerants in mini-split air conditioners under high ambient temperature conditions. This interim working paper describes the parties involved, the alternative refrigerants selection process, the test procedures, and the preliminary results.

A dynamic simulation tool for estimating demand response potential from residential loads

This paper presents a MATLAB based dynamic simulation tool for estimating demand response potential from residential loads. First, a review of residential demand response strategies is conducted. Next, the modeling approach used during the development of this tool is described. Markov chain based occupant behavior models constructed using data gathered by the U.S. Census Bureau in the American Time Use Survey (ATUS) are used in conjunction with models of the most common residential loads to predict the dynamic changes in residential power demand on a one-minute time scale. Separate control schemes are used along with these models to simulate different demand response strategies. Finally, simulation results showing the benefits and trade-offs associated with residential demand response programs are presented. Future work will involve using this tool to examine specific utility areas and the development of real-time pricing and incentive program components.

A Hybrid Absorption Cycle for Water Heating, Dehumidification, and Evaporative Cooling

In this study, development of a novel system for combined water heating, dehumidification, and space cooling is discussed. The system absorbs water vapor from an air stream into an absorbent. The latent heat of absorption, released into the absorbent, is transferred into the process water that cools the absorbent. The solution is regenerated in the desorber, where it is heated by a heating fluid. The water vapor generated in the desorber is condensed and its heat of phase change is also transferred to the process water. The condensed water is then used in an evaporative cooling process to cool the dehumidified air exiting the absorber. Essentially, this open-absorption cycle collects space sensible heat and transfers it to hot water. Another novel feature of the cycle is recovery of the heat energy from the solution exiting the desorber by heat exchange with process water rather than with the solution exiting the absorber. This approach has enabled heating the process water from an inlet temperature of 15°C to 57°C (conforming to the required DOE building hot water standard) and compact fabrication of the absorber, solution heat exchanger, and desorber in plate and frame configuration. The system under development currently has a water heating capacity of 1.5 kW and a thermal coefficient of performance (COP) of 1.45.© 2015 ASME

Transient Thermofluids Analysis of a Ground-Level Integrated Diverse Energy Storage (GLIDES) System

In this work, a novel Ground-Level Integrated Diverse Energy Storage (GLIDES) system which can store energy via input of electricity or heat and deliver dispatchable electricity is presented [1]. The proposed system is low-cost and hybridizes compressed air and pumped-storage approaches that will allow for the off-peak storage of intermittent renewable energy for use during peak times. A detailed control-volume energy analysis of the system is carried out, yielding a set of coupled differential equations which are discretized using a finite difference scheme and used to model the transient response during charging and discharging. The energy analysis includes coupled heat transfer and pressure drop analysis used to predict system losses for more accurate round trip efficiency (RTE) calculations and specific energy density (ED) predictions. Preliminary analysis of the current prototype indicates an electric-to-electric RTEE of 66% (corresponding to shaft-to-shaft mechanical RTEM of 78%) and ED of 2.5 MJ/m3 of air, given initial air volume and pressure of 2 m3 and 70 bar. The electric power output ranges from a max of 2.5 kW to a min of 1.2 kW and the output current ranges from a max of approximately 21 amps to approximately 10 amps at 120 V, 60 Hz dispatchable electricity, over a period of approximately 50 minutes. Additionally, it is shown that heat transfer enhancement to the point of a 5-fold increase in air heat transfer rates results in a near 5% improvement in RTEE (70% considering all component losses). Additional component efficiency improvements and efficiency gains due to system scale-up could see higher achievable RTEs.Copyright

Auto-calibration and control strategy determination for a variable-speed heat pump water heater using optimization

This article introduces applications of the GenOpt® optimizer coupled with a vapor compression system model for auto-calibration and control strategy determination toward the development of a variable-speed ground-source heat pump water heating unit. The GenOpt® optimizer can be linked with any simulation program using input and output text files. It effectively facilitates optimization runs. Using the proposed GenOpt® wrapper program, objectives can flexibly be defined for optimizations, targets, and constraints. Those functionalities enable running extensive optimization cases for model calibration, configuration design, and control strategy determination. In addition, a methodology is described to improve prediction accuracy using functional calibration curves. Using the calibrated model, control strategies of the ground-source heat pump water heater were investigated, considering multiple control objectives and covering the entire operation range.

A numerical analysis of a magnetocaloric refrigerator with a 16-layer regenerator

A numerical analysis was conducted to study a room temperature magnetocaloric refrigerator with a 16-layer parallel plates active magnetic regenerator (AMR). Sixteen layers of LaFeMnSiH having different Curie temperatures were employed as magnetocaloric material (MCM) in the regenerator. Measured properties data was used. A transient one dimensional (1D) model was employed, in which a unique numerical method was developed to significantly accelerate the simulation speed of the multi-layer AMR system. As a result, the computation speed of a multi-layer AMR case was very close to the single-layer configuration. The performance of the 16-layer AMR system in different frequencies and utilizations has been investigated using this model. To optimize the layer length distribution of the 16-layer MCMs in the regenerator, a set of 137 simulations with different MCM distributions based on the Design of Experiments (DoE) method was conducted and the results were analyzed. The results show that the 16-layer AMR system can operate up to 84% of Carnot cycle COP at a temperature span of 41 K, which cannot be obtained using an AMR with fewer layers. The DoE results indicate that for a 16-layer AMR system, the uniform distribution is very close to the optimized design.