1、7.1CHAPTER 7 COMBINED HEAT AND POWER SYSTEMSCHP System Concepts . 7.3Performance Parameters 7.5Fuel-to-Power Components . 7.9Thermal-To-Power Components 7.24Thermal-to-Thermal Components 7.32Electrical Generators and Components. 7.39System Design 7.41Codes and Installation . 7.47Economic Feasibility
2、 7.48OMBINED heat and power (CHP) is the simultaneous produc-Ction of electrical or mechanical energy (power) and usefulthermal energy from a single energy source. By capturing and usingthe recovered heat energy from an effluent stream that would other-wise be rejected to the environment, CHP (or co
3、generation) systemscan operate at utilization efficiencies greater than those achievedwhen heat and power are produced in separate processes, thus con-tributing to sustainable building solutions.Recovered thermal energy from fuel used in reciprocatingengines, steam or combustion turbines (including
4、microturbines,which are typically less than 500 kW in capacity), Stirling engines,or fuel cells can be used in the following applications: Direct heating: exhaust gases or coolant fluids used directly fordrying processes, to drive an exhaust-fired absorption chiller, toregenerate desiccant materials
5、 in a dehumidifier, or to operate abottoming cycleIndirect heating: exhaust gases or coolant fluids used to heat a sec-ondary fluid (e.g., steam or hot water) for devices, to generatepower, or to power various thermally activated technologiesLatent heat: extracting the latent heat of condensation fr
6、om arecovered flow of steam when the load served allows condensa-tion (e.g., a steam-to-water exchanger) instead of rejecting thelatent heat to a cooling tower (e.g., a full condensing turbine witha cooling tower)There are many potential applications, including base-loadpower, peaking power where on
7、-site power generation (distributedgeneration) is used to reduce the demand or high on-peak energycharges imposed by the electric energy supplier, back-up power,remote power, power quality, and CHP, providing both electricityand thermal needs to the site. Usually, customers own the small-scale, on-s
8、ite power generators, but third parties may own andoperate the equipment. Table 1 provides an overview of typicalapplications, technologies and uses of distributed generation(DG) and CHP systems.On-site CHP systems are small compared to typical central sta-tion power plants. DG systems are inherentl
9、y modular, which makesdistributed power highly flexible and able to provide power whereand when it is needed. DG and CHP systems can offer significantbenefits, depending on location, rate structures, and application.Typical advantages of an on-site CHP plant include improved powerreliability and qua
10、lity, reduced energy costs, increased predictabil-ity of energy costs, lowered financial risk, use of renewable energysources, reduced emissions, and faster response to new powerdemands because capacity additions can be made more quickly.CHP system efficiency is not as simple as adding outputs anddi
11、viding by fuel inputs. Nevertheless, using what is normally wasteexhaust heat yields overall efficiencies (O) of 50 to 70% or more(for a definition of overall efficiency, see the section on Perfor-mance Parameters).CHP can operate on a topping, bottoming, or combined cycle.Figure 1 shows an example
12、of topping and bottoming configura-tions. In a topping cycle, energy from the fuel generates shaft orelectric power first, and thermal energy from the exiting stream isrecovered for other applications such as process heat for cooling orheating systems. In a bottoming cycle, shaft or electric power i
13、sgenerated last from thermal energy left over after higher-level ther-mal energy has been used to satisfy thermal loads. A typical toppingcycle recovers heat from operation of a prime mover and uses thisthermal energy for the process (cooling and/or heating). A bottom-ing cycle recovers heat from th
14、e process to generate power. A com-bined cycle uses thermal output from a prime mover to generateadditional shaft power (e.g., combustion turbine exhaust generatessteam for a steam turbine generator).Grid-isolated CHP systems, in which electrical output is used onsite to satisfy all site power and t
15、hermal requirements, are referredto as total energy systems. Grid-parallel CHP systems, which areactively tied to the utility grid, can, on a contractual or tariff basis,exchange power with or reduce load on (thus reducing capacitydemand) the public utility. This may eliminate or lessen the need for
16、redundant on-site back-up generating capacity and allows operationat maximum thermal efficiency when satisfying the facilitys ther-mal load; this may produce more electric power than the facilityneeds.The preparation of this chapter is assigned to TC 1.10, CogenerationSystems. Fig. 1 CHP Cycles7.2 2
17、012 ASHRAE HandbookHVAC Systems and Equipment (SI)CHP feasibility and design depend on the magnitude, duration,and coincidence of electrical and thermal loads, as well as on theselection of the prime mover, waste heat recovery system, and ther-mally activated technologies. Integrating design of the
18、projectselectrical and thermal requirements with the CHP plant is requiredfor optimum economic performance. Matching the CHP plantsthermal/electric ratio with that of the building load is required foroptimum economic benefit. The basic components of the CHPplant are the (1) prime mover and its fuel
19、supply system, (2) gen-erator and accessories, including interconnection and protectionsystems, (3) waste heat recovery system, (4) thermally activatedtechnologies, (5) control system, (6) electrical and thermal trans-mission and distribution systems, and (7) connections to mechani-cal and electrica
20、l services.This chapter describes the increasing role of CHP in sustainabledesign strategies, presents typical system designs, provides meansand methods to understand system performance, and describes primemovers, such as reciprocating and Stirling engines, combustion andsteam turbines, and fuel cel
21、ls, and their characteristics for varioususes. It also describes thermally activated technologies (TAT) such asheat recovery, absorption chillers, steam turbine-driven chillers, anddesiccant dehumidifiers, as well as organic Rankine cycle (ORC)machines for waste heat recovery. Related issues, such a
22、s fuels,lubricants, instruments, noise, vibration, emissions, and mainte-nance, are discussed for each type of prime mover. Siting, inter-connection, installation, and operation issues are also discussed.Thermal distribution systems are presented in Chapters 12 and 13.TERMINOLOGYAvoided cost. Increm
23、ental cost for the electric utility to generateor purchase electricity that is avoided through provision or purchaseof power from a CHP facility.Back-up power. Electric energy available from or to an electricutility during an outage to replace energy ordinarily generated bythe CHP plant.Base load. M
24、inimum electric or thermal load generated or sup-plied over one or more periods.Black start. A start-up of an off-line, idle, non-spinning gener-ation source without the electric utility.Bottoming cycle. CHP facility in which energy put into the sys-tem is first applied to another thermal energy pro
25、cess; rejected heatfrom the process is then used for power production.Capacity. Load for which an apparatus is rated (electrical gen-erator or thermal system) at specific conditions.Capacity credits. Value included in the utilitys rate for purchas-ing energy, based on the savings accrued through red
26、uction or post-ponement of new capacity resulting from purchasing electrical orthermal from cogenerators.Capacity factor. Ratio of the actual annual output to the ratedoutput over a specified time period.Coefficient of performance (COP). Refrigeration or refrigera-tion plus thermal output energy div
27、ided by the energy input to theabsorption device, refrigeration compressor, or steam turbine. Seealso Combined heat and power (CHP).Combined heat and power (CHP). Simultaneous production ofelectrical or mechanical energy and useful thermal energy from asingle energy stream.Coproduction. Conversion o
28、f energy from a fuel (possiblyincluding solid or other wastes) into shaft power (which may beused to generate electricity) and a second or additional useful formof energy. The process generally entails a series of topping or bot-toming cycles to generate shaft power and/or useful thermal output.CHP
29、is a form of coproduction.Demand. Rate at which electric energy is delivered at a giveninstant or averaged over any designated time, generally over aperiod of less than 1 h.Annual demand. Greatest of all demands that occur during a pre-scribed demand interval billing cycle in a calendar year.Billing
30、 demand. Demand on which customer billing is based, asspecified in a rate schedule or contract. It can be based on thecontract year, a contract minimum, or a previous maximum,and is not necessarily based on the actual measured demandof the billing period.Coincident demand. Sum of two or more demands
31、 occurring inthe same demand interval.Peak demand. Demand at the instant of greatest load.Demand charge. Specified charge for electrical capacity on thebasis of billing demand.Demand factor. Average demand over specific period dividedby peak demand over the same period (e.g., monthly demand factor,a
32、nnual demand factor).Demand-side management (DSM). Process of managing theconsumption of energy, generally to optimize available and plannedgeneration resources.Economic coefficient of performance (ECOP). Output energyin terms of economic costs divided by input fuel in terms of energypurchased or pr
33、oduced, expressed in consistent units (e.g., kW) foreach energy stream.Efficiency. See the section on Performance Parameters.Energy charge. Portion of the billed charge for electric servicebased on electric energy (kilowatt-hours) supplied, as contrastedwith the demand charge.Energy Information Admi
34、nistration (EIA). Independentagency in the U.S. Department of Energy that develops surveys, col-lects energy data, and analyzes and models energy issues.Electric tariff. Statement of electrical rate and terms and condi-tions governing its application.Grid. System of interconnected distribution and t
35、ransmissionlines, substations, and generating plants of one or more utilities.Table 1 Applications and Markets for DG/CHP SystemsDG TechnologiesStandby PowerBase-Load Power OnlyDemand Response PeakingCustomer Peak ShavingPremium PowerUtility Grid Support CHP Applicable Market SectorsReciprocating en
36、gines: 50 kW to 16 MWXXXXXXXCommercial buildings, institutional, industrial, utility grid (larger units), waste fuelsGas turbines: 500 kW to 50 MWXXX Large commercial, institutional, industrial, utility grid, waste fuelsSteam turbines: 500 kW to 100 MWXInstitutional buildings/campuses, industrial, w
37、aste fuelsMicroturbines: 30 to 500 kWX XXXXXXCommercial buildings, light industrial, waste fuelsFuel cells: 5 kW to 2 MW XXResidential, commercial, light industrialSource: Adapted from NREL (2003).Combined Heat and Power Systems 7.3Grid interconnection. System that manages the flow of powerand serve
38、s as communication, control, and safety gateway betweena CHP plant and an electric utilitys distribution network.Harmonics. Wave forms with frequencies that are multiples ofthe fundamental (60 or 50 Hz) wave. The combination of harmonicsand the fundamental wave causes a nonsinusoidal, periodic wave.
39、Harmonics in power systems result from nonlinear effects. Typi-cally associated with rectifiers and inverters, arc furnaces, arc weld-ers, and transformer magnetizing current. Both voltage and currentharmonics occur.Heat rate. Measure of generating station thermal efficiency,generally expressed in J
40、J or Mg of steamkWh.Heating value. Energy content in a fuel that is available as usefulheat. The higher heating value (HHV) includes the energy neededto vaporize water formed during combustion, whereas the lowerheating value (LHV) deducts this energy because it does not con-tribute to useful work.In
41、termediate load. Range from base electric or thermal load toa point between base load and peak.Interruptible power. Electric energy supplied by an electricutility subject to interruption by the electric utility under specifiedconditions.Isolated plant. A CHP plant not connected to the electric utili
42、tygrid.Load factor. Load served by a system over a designated perioddivided by system capacity over the same period.Off-peak. Periods when power demands are below average; forelectric utilities, generally nights and weekends; for gas utilities,summer months.Peak load. Maximum electric or thermal loa
43、d in a stated periodof time.Peak shaving. Reduction of peak power demand using on-sitepower generation, thermally activated technologies, or other load-shifting device.Point of common coupling. Point where a CHP system is con-nected to the local grid.Power factor. Ratio of real power (kW) to apparen
44、t power (kVA)for any load and time; generally expressed as a decimal.Premium power. High reliability power supply and/or high volt-age/current power quality. Reactive power. Reactive power exists in all alternating current(ac) power systems as current leads or lags voltages; inadequatereactive power
45、 reserves can contribute to voltage sags or even volt-age collapse. CHP and/or on-site power systems can provide reac-tive power support where they are connected to the grid.Selective energy systems. Form of CHP in which part, but notall, of the sites electrical needs are met solely with on-site gen
46、era-tion, with additional electricity purchased from a utility as needed.Shaft efficiency. Prime movers shaft energy output divided byits energy input, in equivalent, consistent units. For a steam turbine,input can be the thermal value of the steam or the fuel value requiredto produce the steam. For
47、 a fuel-fired prime mover, it is the fuelinput to the prime mover.Standby power. Electric energy supplied by a cogenerator orother on-site generator during a grid outage. Supplemental thermal. Heat required when recovered engineheat is insufficient to meet thermal demands.Supplemental firing. Inject
48、ion and combustion of additionalfuel into an exhaust gas stream to raise its energy content (heat).Transmission and distribution (T this isthe critical factor in economic feasibility. Therefore, the primemovers thermal/electric ratio and load must be analyzed as a firststep towards making the best c
49、hoice. Maximizing efficiency is gen-erally not as important as thermal and electric use.CHP paralleled with the utility grid can operate at peak effi-ciency if (1) the electric generator can be sized to meet the valley ofthe thermal load profile, operate at a base electrical load (100% fullload) at all times, and purchase the balance of the sites electricneeds from the utility; or (2) the electric generators are sized for100% of the sites electrical demand and recovered heat can be fullyused at that condition, with additional thermal demands met by sup-plementary means and exce
