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Article Excerpt
BACKGROUND
Combined heat and power (CHP) systems offer great promise for alleviating some of the looming problems of increased energy demands and peak power issues arising from deregulation of the electric market, petroleum shortages, and the drive for better energy efficiency. For over a decade, the Department of Energy (DOE) as well as several state agencies (e.g., California Energy Commission [CEC] under its PIER program and New York State Energy Research and Development Authority [NYSERDA]) have actively funded projects involving the use of CHP plants for industrial and commercial/ institutional buildings.
The application of distributed generation (DG) technologies to building systems has, until recently, been limited to specific back-up generation roles for high risk and high priority applications such as hospitals, telecommunications switching centers, certain government facilities, etc. For these facilities, backup generation, in instances where grid supplied electric energy was not available even for brief time periods, was either mandated by law or necessitated by business needs. The high cost of installing, maintaining, and operating a backup generation system relative to grid-supplied energy was outweighed by the need for increased reliability of electric service. If controlled properly by providing additional on-site generation capacity, DG promises to better utilize the available power generation and transmission equipment of both regulated and deregulated states. In order to accelerate the adoption of CHP technologies, the DOE provided seed money for the development of several regional CHP regional application centers and also funded national laboratories, such as Oak Ridge National Laboratory (ORNL), which developed a facility for testing CHP equipment and systems under a variety of operating conditions.
The 2003 power outage in the Northeastern United States, transmission and rate crises in the last few years in California and elsewhere, and concerns over electric grid susceptibility to disruptions from natural and man-made causes have driven many customer groups (residential, industrial, and commercial) to seek alternatives to conventional, central power station-supplied electric energy. New technology options for DG are rapidly becoming commercially available. Microturbine, fuel cell, and combined heat and power system technologies have received considerable attention as options for backup, supplemental, and primary electric energy sources. These new technologies are in addition to existing technologies that include diesel or natural gas-powered internal combustion engine-driven generators, as well as steam-driven generators.
To date, much of the effort on the applications side appears geared toward industrial CHP systems and involves evaluating the benefits of CHP plants and developing methods for their proper siting and design. In the case of BCHP systems for commercial/institutional buildings involving multiple prime movers, chillers, and boilers, the large variability in thermal and electric loads requires a more careful design process. Further, equipment scheduling and control for BCHP systems needs to be performed in a more sophisticated manner than it is in industrial CHP plants. The benefits of using BCHP systems are (1) increased overall energy use efficiency, (2) higher system reliability (fewer outages), (3) greater stability against volatile real-time price (RTP) fluctuations, and (4) reduced environmental emissions.
CHP systems have been constructed for large campus or municipal applications for decades (Ryan 2002) and they can be found in all economic sectors of the world (Caton and Turner 1997). CHP has also been the topic of several publications, including a comprehensive book by Petchers (2003) and handbooks by Orlando (1996) and Borbely and Kreider (2001). Absorption cooling technologies have also reached a respectable level of maturity and acceptance (Sweetser et al. 2000) and studies such as Grossman and Rasson (2003) describe how to use them to advantage in CHP plants.
Increasingly, building owners and the design community are beginning to recognize the value of electric service reliability. Heightened sensitivity to power supply interruptions and their cost implications has created interest in seeking alternatives to grid-supplied electric energy. More and more businesses are considering and installing DG technologies. However, current costs of DG technologies place a premium on their cost-effective design and operation as well as their integration and interaction with the electric energy-consuming systems within a building system.
It is only in the last 15 years, with the advent of micro-turbines, that interest in using CHP systems in building applications has increased (Cowie et al. 2002). Several articles (e.g., Ryan 2002, 2004; Zogg et al. 2005) have appeared demonstrating, by means of case studies in different geographic locations and varying cost ratios of electricity and gas, that BCHP systems make economic sense. Cowie et al. (2002) point out three main differences between energy requirements for commercial sites and those for industrial sites: (1) total energy demand in commercial sites is lower, (2) total energy demand for commercial sites fluctuates more during the day and year, and (3) the ratio of thermal to electric demand for commercial sites is much smaller.
The use of BCHP systems is especially relevant at this time because of its relation to changes in the regulation of the electric utility industry and the price volatility in energy markets. With a more open retail energy market, there is enormous pressure for customers to seek options to manage their loads in order to strengthen their bargaining position with power marketers. It is sometimes suggested that the early agreements are based on market power, market presence, and the need to gain market experience. As deregulated utility markets evolve, there is a need for improved engineering tools to identify building on-site generation opportunities and to provide guidance in their operations, which will ultimately add sound engineering analysis and decrease risk for everyone involved in the transaction.
Problem Statement
The objective of this research was to study the problem of proper scheduling of equipment in BCHP systems for commercial/institutional buildings. This was achieved by defining a number of scenarios on which to perform the optimization and, towards this end, a rational methodology of identifying a small number of scenarios is crucial. The scenarios involved (1) selection of representative building types and geographic climates, (2) defining representative electric utility dynamic rate schedules, (3) performing careful design and sizing of the BCHP systems and equipment, (4) using a detailed simulation program to generate hourly building loads, and (5) identifying a small set of days over which to perform the evaluations. This paper describes the methodology adopted and the test matrix identified, while a companion paper (Reddy and Maor n.d.) reports on the simulation and optimization results related to the cost penalties of operating the BCHP systems in a near-optimal as opposed to optimal manner.
Identification of Scenarios
As a starting point, LeMar's (2002) study provides information on market assessment for BCHP systems. One notes that common prime mover sizes are 30 kW-1.0 MW; common building types are offices, schools, hospitals, retail hotels, and colleges; and favorable regions are Pacific, Mid-Atlantic, New England, South Atlantic, and East-North Central. A more careful approach was followed in order to determine the BCHP scenario matrix. It involved the following tasks: (1) identification of the prime mover fuel, (2) identification of building type (sector) and building size, (3) identification of climatic region, (4) identification of type and size range of the prime mover, (5) definition of auxiliary BCHP equipment and primary cooling and heating systems, and (6) selection of the electrical and thermal rate signals. The selection and development of the scenarios matrix included existing as well as...
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