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5. A Critique & Analysis of Four Major Distributed Generation Technologies*

5.1 Introduction

Distributed generation (DG) is not a new concept. Large scale centralized power systems were created based on an 'economy of scale' model and currently dominate the United Stated power grid. Unfortunately, recent problems with the transmission system, the grid, and the dynamic fuel market have created a need to stabilize, distribute, and equalize grid power flow. While some DG technologies have not commercially or economically matured, the authors believe that DG technologies will become a reality in the next few decades. In this module, the authors analyze and rank four small-scale DG technologies, evaluating the technologies' current status and potential status. This ranking provides the basis of comparison among the technologies as practical solutions to the woes of the United States power grid. Distributed generation technologies potentially hold the key to unlocking current grid power flow stabilization, distribution, and equalization problems.

Scope of This Document

This study compares distributed generation in three technology groups: internal combustion engines (ICE), gas turbines (two different types), and one type of fuel cell (solid oxide fuel cells). In order to maximize applicability, we have focused our analysis on non-site specific DG technologies. So for example, we have not compared photovoltaic and wind turbine technologies to the previously mentioned technologies. Technologies such as photovoltaics and wind power are dependent upon usable sunlight and average wind speed respectively which vary from location to location. Further, these require a large amount of area per generated kW[1]. Another location specific example is landfill DG technologes. A landfill gas production facility requires a large landfill and a specific location. It must be noted that the authors of this paper feel these location restricted technologies should be implemented as they become technologically and economically viable. However, we consider the aforementioned to be very specific centralized generation technologies that cannot be realized everywhere. While it is possible to put photovoltaic arrays on top of every home, gas station and large corporation, current power densities preclude profitability and practicality. Likewise, operating a two hundred foot tall wind turbine in the heart of "the Windy City" (Chicago) is not a practical engineering solution to our power demands. In this module, we have limited our scope to non-site specific practical technologies. It is our hope that by limiting our scope to these technologies we have opened up more possibilites in more places.

Distributed Generation Facility Sizing

There are three basic methods for determining the generating capacity of a DG system, base, intermediate, and peak. A general overview of storage and load growth can be found at Chapter 2.5 Storage. For the base method, the capacity of the facility is defined as the average base power required by the load. In this case, the DG system effectively generates maximum power all the time. The end user will buy power from the grid system any time the base power of the DG system is exceeded (peak power). For the intermediate method, the capacity is defined as the mean power required. During peak power demand, the electricity is obtained from the grid, and conversely, during low power demand, the electricity is sold back to the grid, thus maintaining full utilization of the facility. The third method is peak capacity. The DG system operates at a base level at all times, but has an auxiliary power supply that only provides power during peak power requirements. This type of system can theoretically be grid independent because of the auxiliary power capacity [1].

We propose a slightly different approach to ensure higher power quality (PQ) among demanding customers. In our system, the DG facility is sized for the peak power demand of the high PQ customers. Should the grid feed be lost, the DG station would shed non-critical loads freeing capacity to cover the mission critical load. Under normal operation, the critical load's peak power would still be sourced from the grid and the DG system would return to the base case generating facility.   This type of toggling facility would increase the security of power system through the use of DG technologies while providing alternative forms of energy during non-critical times.

Penetration Depth

Penetration depth, described in a very simplistic fashion, is the amount of power the DG system provides to the load in relation to the capacity of the feeder line (as known as the transmission line). The penetration depth is determined by many factors, including but not limited to: distance to substation, DG induced frequency fluctuations and damaging harmonics, recloser/relay location, feeder line size and type of DG system. For example, the harmonics created by a line-commutated based DG system will significantly limit penetration depth as compared to the same system with a pulse width modulator (PWM) inverter. Here, the line-commutated inverter is dependent on the feeder line for conversion from DC to AC power and ,therefore , it is dependent on the feeder line harmonics. However, the PWM inverter produces nearly sinusoidal output at minimum current harmonics.

The detailed calculation of penetration depth is outside of the scope of the paper, but is directly related to the size of the DG system and the feeder line capacity. For this paper, we will assume a typical 13.8kV feeder line with an 11MVA capacity. The typical feeder capacity should be large enough to mitigate any complications caused by a 5MW DG system in normal grid connected operations [1]. A 5MW DG system is sufficiently large to be credible as defined in the distributed generation facility sizing section. A system of this size can carry a peak load and perform as a toggling facility. This size also serves to minimize the distance to the load, reduce transmission losses and improve reliability. Hence, the penetration depth will be maximized in these terms.

The load density (power requirements) of the given area further dictates the proximity to the DG system. It follows that in a large city, skyscraper and large building basements provide suitable locations for enough DG systems to meet the city's energy needs. Likewise, in smaller cities and communities demand far less energy and one or two DG systems located within the community could provide ample power. Large-scale industrial and commercial complexes would be outfitted with multiple 5MW, or larger units if the base load demand warrants extra capacity.

According to a model like this, each DG system will support a defined power 'borough' or electrical community. Potentially, smaller communities' DG systems could be less than 5MW. Further, the possibility of household power generation on the 1-2kW level and commercial power generation on the level of 10-100kW could be realized. Presently, this is not a viable solution as there are many unresolved system control, cost, and technology maturation issues. This paper will discuss distributed generation technologies on the 5MW level and on a limited basis include generation technologies down to the 250kW level.

Ranking system

We describe our ranking parameters below. The ranking system is broken down into several different areas of interest. There are 4 technologies being considered and thus we have ranked them from 1 to 4, where 1 is the highest and 4 is the lowest rank.

Technology Status This ranking is based on the status of the technology being described. This includes a brief history with regard to when the technology was developed, global market penetration and current competitive status with other DG technologies.

Overall Efficiency This ranking is based on electrical, fuel and overall efficiency. The efficiency is very important for reasons of conserving natural resources. Higher efficiency technologies will have lower fuel costs per kW because less fuel will need to be purchased to generate the same amount of power.

Environmental and Noise Considerations This ranking is based on technology location restrictions based on pollution and noise levels. Obvious reasons dictate the public's acceptance (and subsequent ranking) level of DG systems when implemented near population centers in lieu of centralized generation. This section also takes into account pollution impact on a global level.

Device Cost Per kW As suggested, it is the cost of the given technology per kW or monetary efficiency. Lower cost solutions will enable faster implementation and increased acceptance to investors.

Physical Size The physical size determines the amount of area a technology will occupy. Obviously a higher power density is desired because the DG system will occupy less real estate and will be able to be implemented in more locations. Smaller DG stations will also be less unsightly to residents at some potential locations.

Intended Application The intended application as defined in the scope is on the minimum level of 250kW to maximum level of 5MW. This analysis compares how well the technology performs in terms of power generation to the other technologies.

Potential For Improvement Realizing that ICEs are the oldest technology followed by CTs, MTs and fuel cells, this section compares the technologies' potential for improvement. Basically, this section determines the best of these technologies suited for DG when figured on a long time frame.

*This module was originally written as a manuscript by Joel Gouker and Michael Schenck.

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