Tania Ullah

An assessment of efficient water heating options for an all-electric single family residence in a mixed-humid climate

An evaluation of a variety of efficient water heating strategies for an all-electric single family home located in a mixed-humid climate is conducted using numerical modeling. The strategies considered include various combinations of solar thermal, heat pump, and electric resistance water heaters. The numerical model used in the study is first validated against a year of field data obtained on a dual-tank system with a solar thermal preheat tank feeding a heat pump water heater that serves as a backup. Modeling results show that this configuration is the most efficient of the systems studied over the course of a year, with a system coefficient of performance (COPsys) of 2.87. The heat pump water heater alone results in a COPsys of 1.9, while the baseline resistance water heater has a COPsys of 0.95. Impacts on space conditioning are also investigated by considering the extra energy consumption required of the air source heat pump to remove or add heat from the conditioned space by the water heating system. A modified COPsys that incorporates the heat pump energy consumption shows a significant drop in efficiency for the dual tank configuration since the heat pump water heater draws the most heat from the space in the heating season while the high temperatures in the solar storage tank during the cooling season result in an added heat load to the space. Despite this degradation in the COPsys, the combination of the solar thermal preheat tank and the heat pump water heater is the most efficient option even when considering the impacts on space conditioning.

Deployment of a load simulator in simulating residential household appliances

Real-time electricity monitoring in residences is a growing market, with a number of systems being commercialized that provide feedback to occupants on electrical power and energy consumption. These systems are also useful for research purposes, by better quantifying occupant energy usage for input into whole building energy models. Different monitoring systems, however, may perform better in certain circumstances than in others, so this work discusses efforts to create residential electrical energy emulators that can be used to evaluate energy monitoring systems. Our research focuses on the efficacy of tools available to mimic individual electrical loads in residential buildings. In our study, appliances are simulated using typical load profiles, with programmable power and power factor. The advantage of the load simulator is that it provides a variety of fully controllable loads for experimentation. By applying this method, building energy analysis research will have more flexibility to mimic real household appliances and electrical loads than is currently available.

Detailed energy model of the National Institute of Standards and Technology Net-Zero Energy Residential Test Facility: Development, modification, and validation

The National Institute of Standards and Technology Net-Zero Energy Residential Test Facility is a highly instrumented, highly configurable, single-family, net-zero energy house occupied by a virtual family of four. A detailed transient model of the Net-Zero Energy Residential Test Facility and the accompanying mechanical equipment was created using information available before construction; the model incorporated building geometric details and construction material properties, as well as manufacturers’ specifications for HVAC, water heating, solar photovoltaic and other equipment. This model represents the typical design paradigm, where actual building performance and detailed equipment operation are not known. This original model underpredicted the measured annual energy consumption by 13.8%. The measured data were used to understand and correct the sources of error at the component level; modifications to the heating, ventilation, and air-conditioning system, interior thermal capacitance, and domestic hot water system improved the energy consumption prediction to within 1.6% of measured data. The differences between the original and modified models are useful for understanding the sources, magnitudes, and possible corrections to errors in energy models for high-efficiency residences. The modified model will be used in future studies of alternative energy system configurations and control strategies, contributing to cost-effective and optimum design of net-zero energy houses in America.