Badrul H. Chowdhury

Electrical & Computer Engineering Department

University of Missouri-Rolla

Rolla, MO 65409

The goal of distributed generation (DG) is to put generation as close to the point of consumption as possible. Distributed generation typically includes microturbines, advanced engines, wind power, photovoltaic power, and fuel cells. Typical sizes range from 5 kW to 500 kW connected at low voltage networks to 2 MW to 10 MW connected at medium voltage networks. Applications of DG are: standby/back-up power, base load power, combined heat and power (CHP), power quality, ancillary services, transmission and distribution (T&D) support, and micro-grid applications. Among the benefits of DG are their modularity, lower first-time costs, faster payback, lower emissions, lower T&D costs, higher efficiency, and improved reliability and power quality.

Microturbines operates on basically the same principles as traditional gas turbines (GT). However, the compressor and generator are typically driven at high speeds, such as 70,000 to 120,000 rev/min. The generator produces high-frequency AC power that is converted to 50/60Hz by power electronics.

Fuel cells converts hydrogen or a hydrogen rich gas directly to electricity. Fuel, primarily hydrogen, enters the anode and air (oxygen) enters the cathode. The fuel and oxygen are separated into ions and electrons. Ions are conducted through the electrolyte while the electrons flow through the anode and the cathode via an external circuit. Fuel cells are characterized by the type of electrolyte used, for example alkaline, proton exchange membrane, phosphoric acid, molten carbonate or solid oxide. Depending on the electrolyte, the fuel cell operates between 80 and 1000°C. Ignoring this produced heat, fuel cell efficiency can range between 35–65%. Utilizing the produced heat can raise the efficiency to over 80%. Hybrid systems, wherein fuel cells provide the hot-gas flow needed to drive a GT, can generate additional electricity with efficiency of the total process reaching above 60 percent, even at sizes of less than 1 MW. With larger systems, in the 100-400-MW range, efficiency can be above 75 percent overall.

Typical wind power systems range from 30 kW for individual units to 50 MW for wind farms. The rotor construction is either variable blade angle or non-variable. Either synchronous or induction generators are used to convert the mechanical energy to electrical energy. The market for wind power is growing annually at 40%. An ORNL study found that it is possible to integrate new wind resources on the order of 50 to 100 MW to supply local load in many areas, without the need for significant upgrades to the transmission system.

Conversion of solar energy directly to electrical energy is viewed as a highly desirable, non-polluting form of generation. However, the main difficulty is the high cost of photovoltaic systems, US$ 6000/kW being a typical figure. Additionally, the power output is directly proportional to the surface area of the cells, and footprint sizes are hence relatively large (0.02 kW/m²). Typical applications of photovoltaic cells is still in the below 10 kW range mainly used on building rooftops or for remote power systems.

Use of diesel and petrol engines to provide standby power for commercial and small industrial customers is not new. Engines operating on natural gas have recently been developed. Typical capacities are 50 kW to 6 MW, and footprints are in the order of 50 kW/m². Disadvantages of combustion engines are pollution (both emissions and noise) and relatively high maintenance and operating costs. Reciprocating engines fueled by natural gas or other gaseous fuels reduce harmful emissions to an acceptable range of 0.0015 to 0.037 lb/kWh. A research program recently approved by the Department of Energy - the Advanced Reciprocating Engine System (ARES) - expects to increase the efficiency of this technology by 50 percent.

Distributed storage can play a multi-functional role in the electricity grid to manage resources effectively. In combination with renewable resources, energy storage can increase the value of photovoltaic (PV) and wind-generated electricity, by making supply coincident with periods of peak consumer demand. Strategically placed storage systems can increase the utilization of existing T&D equipment and defer or eliminate the need for costly T&D additions. Energy storage can also provide premium services, including (a) power quality for sags or surges lasting less than 5 seconds, (b) uninterruptible power supply for outages lasting about 10 minutes, and (c) peak demand reduction.

There are many that believe that numerous distributed generators might adversely impact system stability and reliability. Without a centralized means for controlling the amount of DG injection into the grid, there may be problems with synchronization, post-disturbance reconnection, voltage regulation, and frequency control. There are others who believe that numerous distributed generators might also adversely impact harmonic injection, and thus, the quality of power. On the other hand, unless new incentives are given to DG companies, it might be too expensive for them to enter the market in large quantities. It is likely that large generation companies will continue to exercise market power. Price signals are one mechanism that might be used to coordinate the operation of the power system in the emerging competitive market. Since DG start-up times are fast, they can respond to price signals effectively. Evaluation of transmission line pricing is difficult for DG though. Transmission line charges are not involved if DG is used locally. However, DG leads to lower transmission losses even if used locally. Sometimes DG can be located far from load centers (e.g. wind and PV). Thus, distance-related transmission line charges would make DG more costly to reach customers in urban areas.

Utilities operating wind power plants connected to weak, isolated grids may have difficulty maintaining normal system frequency and thus create flicker problems.

Net metering, green pricing programs and state-mandated Renewable Portfolio Standards (RPS) will continue to help increase market penetration of renewables. Net metering, where utilities bill only the net consumption or generation of customers with small generating facilities, creates a market for DG with very small capacities. This program has been established by law and regulations in 23 states. Connecticut, Massachusetts, Maine, Nevada, and Arizona have already introduced the RPS wherein a quota is set for the state’s electricity generation which must come from solar, wind, sustainable biomass, landfill gas, or fuel cells.

DG holds the potential to significantly alter the design and operation of the power system and the nature of the electric utility industry. The high cost per kW of upgrading a T&D system creates one of the most cost-effective opportunities for DG applications. Other benefits are improved reliability and power quality. However, a number of issues will likely determining the rate and scope of implementation of DR. Regulatory, economic and institutional issues will play major roles in this determination. Laws and regulations still favor central station power plants. Air quality regulations still favor market power held by holders of emission reduction credits. At present, there are hardly any standardized and streamlined permitting processes for DG existing among local and state agencies.

For DG to increase market share, safe and reliable interfaces must be created. Prices of fuel cells, PV and wind power must also continue to fall. If the industry meets these challenges, DG may become a major player in the newly-restructured electric utility industry.