Abstract--Environmentally friendly technologies such as photovoltaics and fuel cells are DC sources. In the current power infrastructure, this necessitates converting the power supplied by these devices into AC for transmission and distribution which adds losses and complexity. The amount of DC loads in our buildings is ever-increasing with computers, monitors, and other electronics entering our workplaces and homes. This forces another conversion of the AC power to DC, adding further losses and complexity. This paper proposes the use of a DC distribution system. In this study, an equivalent AC and DC distribution system are compared in terms of efficiency. Index Terms-DC power systems, power system modeling, power distribution, losses I. INTRODUCTION ncreasing demand and environmental concerns have forced engineers to focus on designing power systems with both high efficiency and green technologies. Green technologies are those that conserve natural resources such as fossil fuels while reducing the human impact on the environment through a reduction in pollution [1]. The most well-known green technologies include photovoltaics and wind turbines. Although fuel cells are not considered a green technology, fuel cells have low emissions compared to other forms of energy and are deemed more environmentally friendly. Unfortunately, the prevailing power system infrastructures are based on alternating current (AC) while two of the leading environmentally friendly energies, fuel cells and photovoltaics, produce direct current (DC). Currently, power system infrastructures that wish to incorporate fuel cells and photovoltaics must first convert the DC power produced by these energy sources to AC. This adds complexity and reduces efficiency of the power system due to the need of a power converter. Furthermore, an ever increasing number of DC consuming devices such as computers, televisions, and monitors are being incorporated into our buildings. The power supplied to these devices must be converted again from AC back to DC adding further losses and complexity to the power system. Instead of using multiple converters to convert DC to AC and then AC to DC, the power system could solely be based on DC. This would eliminate the need for two sets of converters for each DC load, reducing the cost, complexity, and possibly increasing the efficiency. However, a definitive analysis on a DC distribution system is needed to determine the net benefits of eliminating the converters. In this paper, a large steady state analysis of an existing AC grid is constructed along with a DC counterpart. These models are compared in terms of efficiency. II. BACKGROUND Since the development of electricity, AC has been depicted as the better choice for power transmission and distribution. However, Thomas Edison one of the founders of electricity supported the use of DC. No method at that time existed for boosting and controlling DC voltage at the load, so that transmission of DC power from generation to load resulted in a large amount of losses and voltage variations at the different load locations. To resolve this issue, Westinghouse proposed AC distribution. Nikola Tesla had only recently at that time developed the transformer which had the capability of boosting voltage in AC. This allowed for efficient transmission of power from one location to another resulting in a complete transformation of the power systems to AC [2]. Although many things have changed since the invention of electricity, AC is still the fundamental power type of our power infrastructure. However, due to the development of power converters and DC energy sources, interest in DC has returned. Several studies have investigated the use of a DC distribution system. In [3], a small-localized DC distribution system for building loads is investigated. This power system is supplied by a DC distributed energy source for the DC loads and has a separate AC grid connection for the AC loads. The author relates that this methodology leads to a higher efficiency compared to a system solely based on AC through avoiding the use of the rectifier. The author notes that power rectifiers have a relatively low efficiency compared to inverters and DC-DC converters. In [4], a DC zonal distribution system for a Navy ship is investigated to provide electrical isolation, reduce cost, and increase stability. Essentially each zone has a separate distribution system providing protection to the overall ship power systems when an attack has occurred. Due to the need of multiple levels of DC voltage, a DC system was deemed superior to a AC system in terms of efficiency and cost. The AC system would need an inverter and then DC-DC converter for each DC bus voltage level, while a DC system would only implement DC-DC converters. AC vs. DC Distribution: A Loss Comparison Michael Starke 1 , Student Member, IEEE, Leon M. Tolbert 1,2 , Senior Member, IEEE, Burak Ozpineci 2 , Senior Member, IEEE, 1 University of Tennessee, 2 Oak Ridge National Laboratory I 978-1-4244-1904-3/08/$25.00 ©2008 IEEE