1、12.1CHAPTER 12DISTRICT HEATING AND COOLINGEconomic Considerations 12.2CENTRAL PLANT . 12.5Heating and Cooling Production. 12.5Distribution Design Considerations 12.9DISTRIBUTION SYSTEM 12.10Hydraulic Considerations 12.10Thermal Considerations. 12.12Methods of Heat Transfer Analysis 12.13Expansion Pr
2、ovisions. 12.22Distribution System Construction 12.23CONSUMER INTERCONNECTIONS . 12.32Components 12.33Heating Connections 12.35Chilled-Water Connections 12.37Temperature Differential Control 12.38Metering . 12.38Operation and Maintenance 12.39ISTRICT heating and cooling (DHC) or district energy (DE)
3、Ddistributes thermal energy from a central source to residential,commercial, and/or industrial consumers for use in space heating,cooling, water heating, and/or process heating. The energy is dis-tributed by steam or hot- or chilled-water lines. Thus, thermalenergy comes from a distribution medium r
4、ather than being gener-ated on site at each facility.Whether the system is a public utility or user owned, such as amultibuilding campus, it has economic and environmental benefitsdepending somewhat on the particular application. Political fea-sibility must be considered, particularly if a municipal
5、ity or gov-ernmental body is considering a DHC installation. Historically,successful DHC systems have had the political backing and supportof the community. IDEA (2008) has many applicable suggestions indeveloping and designing district cooling systems.ApplicabilityDistrict heating and cooling syste
6、ms are best used in marketswhere (1) the thermal load density is high and (2) the annual loadfactor is high. A high load density is needed to cover the capitalinvestment for the transmission and distribution system, which usu-ally constitutes most of the capital cost for the overall system, oftenran
7、ging from 50 to 75% of the total cost for district heating systems(normally lower for district cooling applications).The annual load factor is important because the total system iscapital intensive. These factors make district heating and cooling sys-tems most attractive in serving (1) industrial co
8、mplexes, (2) denselypopulated urban areas, and (3) high-density building clusters withhigh thermal loads. Low-density residential areas have usually notbeen attractive markets for district heating, although there havebeen many successful applications in Scandinavia. District heatingis best suited to
9、 areas with a high building and population densityin relatively cold climates. District cooling applies in most areasthat have appreciable concentrations of cooling loads, usually asso-ciated with taller buildings.ComponentsDistrict heating and cooling systems consist of three primarycomponents: the
10、 central plant, the distribution network, and the con-sumer systems (Figure 1).The central source or production plant may be any type ofboiler, a refuse incinerator, a geothermal source, solar energy, orthermal energy developed as a by-product of electrical generation.The last approach, called combi
11、ned heat and power (CHP), has ahigh energy utilization efficiency; see Chapter 7 for information onCHP.Chilled water can be produced by Absorption refrigeration machinesElectric-driven compression equipment (reciprocating, rotaryscrew or centrifugal chillers) Gas/steam turbine- or engine-driven comp
12、ression equipmentCombination of mechanically driven systems and thermal-energy-driven absorption systems The second component is the distribution or piping network thatconveys the energy. The piping is often the most expensive portion ofa district heating or cooling system. The piping usually consis
13、ts of acombination of preinsulated and field-insulated pipe in both concretetunnel and direct burial applications. These networks require sub-stantial permitting and coordinating with nonusers of the system forright-of-way if not on the owners property. Because the initial cost ishigh in distributio
14、n systems, it is important to optimize its use.The third component is the consumer system, which includesin-building equipment. When steam is supplied, it may be (1) useddirectly for heating; (2) directed through a pressure-reducing stationfor use in low-pressure (0 to 15 psig) steam space heating,
15、servicewater heating, humidification, and absorption cooling; or (3) passedthrough a steam-to-water heat exchanger. When hot or chilled wateris supplied, it may be used directly by the building HVAC systemsor indirectly where isolated by a heat exchanger (see the section onConsumer Interconnections)
16、.Environmental BenefitsEmissions from central plants are easier to control than thosefrom individual plants and, in aggregate, are lower because of higherquality of equipment, seasonal efficiencies and level of maintenance,diversity of loads, and lower system heat loss. Although todayspolitical clim
17、ate may not perceive new coal plants as desirable, exist-ing central plants that burn coal can economically remove noxiousThe preparation of this chapter is assigned to TC 6.2, District Energy.Fig. 1 Major Components of District Heating System12.2 2012 ASHRAE HandbookHVAC Systems and Equipment sulfu
18、r emissions, where removal with individual combustors itwould not be cost effective. Furthermore, solid-fuel boilers also canburn other waste products, including biomass fuels. Similarly, thethermal energy and gaseous waste from municipal wastes can pro-vide an environmentally sound system. Cogenera
19、tion of heat andelectric power allows for combined efficiencies of energy use thatgreatly reduce emissions, decrease energy used, and allow for fuelflexibility. In addition, refrigerants can be monitored and controlledmore readily in a central plant. Where site conditions allow, remotelocation of th
20、e plant reduces many of the concerns with use of anymore hazardous compounds such as ammonia for cooling systems.ECONOMIC CONSIDERATIONSConsumer EconomicsA district heating and cooling system offers many economic ben-efits. Even though the basic costs are still borne by the districtenergy provider (
21、central plant owner/operator), the customer alsobenefits from a large, centralized systems economies of scale.Operating Personnel. One of the primary advantages to a build-ing owner is that operating personnel for the HVAC system can bereduced or eliminated. Most municipal codes require operatingeng
22、ineers to be on site when high-pressure boilers are in operation.Some older systems require trained operating personnel to be in theboiler/mechanical room at all times. When thermal energy isbrought into the building as a utility, depending on the sophistica-tion of the building HVAC controls, there
23、 may be opportunity toreduce or eliminate operating personnel.Insurance. Both property and liability insurance costs are sig-nificantly reduced with the elimination of a boiler in the mechanicalroom, because risk of a fire or accident is reduced.Usable Space. Usable space in the building increases w
24、hen aboiler and/or chiller and related equipment are no longer necessary.The noise associated with such in-building equipment is also elim-inated. Although this space usually cannot be converted into primeoffice space, it does provide the opportunity for increased storage orfor conversion to other u
25、ses.Equipment Maintenance. With less mechanical equipment,there is proportionately less equipment maintenance, resulting inless expense and a reduced maintenance staff.Higher Thermal Efficiency. A larger central plant can achievehigher thermal and emission efficiencies than the sum of severalsmaller
26、 plants. Partial-load performance of central plants may bemore efficient than that of many isolated, small systems because thelarger plant can operate one or more capacity modules as the com-bined load requires and can modulate output. Furthermore, centralplants take advantage of the fact that build
27、ings that have differentfunctions and their loads do not peak at the same time, but are diver-sified. With this system diversity, less equipment must be energizedto serve the peak system load. Also, central plants generally haveefficient base-load units and less costly peaking equipment for use inex
28、treme loads or emergencies. Thornton et al. (2008) found that,although actual operating data on in-building cooling plants arescarce, the limited data available indicate that in-building systemsoperate at an average efficiency of 1.2 kW/ton (2.9 COP). Erpelding(2007) found that central district cool
29、ing plants can have efficienciesof 0.85 kW/ton (4.1 COP) under less-than-optimal design/operation.Thus, the efficiency of chilled-water generation in a district plantmight be estimated to be approximately 40% of an in-building chillerplant. However, IDEA (2008) suggested much greater efficiencyimpro
30、vements relative to air-cooled, in-building cooling systems:approximately 1.65 kW/ton (2.1 COP) for air-cooled, in-buildingsystems versus approximately 0.70 kW/ton (5.0 COP) for electri-cally driven district cooling with thermal storage, an efficiencyincrease of nearly 140% for district cooling. Whe
31、n strict regulationsmust be met, additional pollution control equipment is also moreeconomical for larger plants than smaller plants. Cogeneration ofheat and electric power results in much higher overall efficienciesthan are possible from separate heat and power plants.Lower Utility Rates. Buildings
32、 that connect to district energysystems, specifically for cooling, have flatter electrical load profilesonce the chilled-water generation equipment is removed from themetered equipment. Flatter electrical profiles are usually conduciveto lower electrical rates, because the peaks are removed. Also, t
33、hefact that the building has no cooling towers drastically reduces thewater and sewer utility bills (i.e., no makeup water and blowdownrequirements). Similar benefits exist for gas usage or steam blow-down and makeup needs.Reliability. DE systems are typically highly reliable and rarelyhave outages.
34、 Mature systems typically have reliability valuesgreater than 99.98%.Higher Level of Control and Monitoring. Typically, the leveland quality of control and monitoring systems in DE systems aresuperior to commercial-grade HVAC systems in the customersbuilding, because the consumers energy is tracked
35、and invoices areprepared using the metered data.Producer EconomicsAvailable Fuels. Smaller heating plants are usually designed forone type of fuel, which is generally gas or oil. Larger facilities canoften be designed for more than one fuel (e.g., gas, biofuels, bio-mass, coal, oil), and may be comb
36、ined with power generation (seeChapter 7 for information on combined heat and power systems) forimproved system efficiency. Often, larger central DHC plants canoperate on less expensive fuels such as coal or municipal refuse.Energy Source Economics. If an existing facility is the energysource, the a
37、vailable temperature and pressure of the thermal fluid ispredetermined. If exhaust steam from an existing electrical generat-ing turbine is used to provide thermal energy, the conditions of thebypass determine the maximum operating pressure and temperatureof the DHC system. A tradeoff analysis must
38、be conducted to deter-mine what percentage of the energy will be diverted for thermal gen-eration and what percentage will be used for electrical generation.Based on the marginal value of energy, it is critical to determine theoperating conditions in the economic analysis.If a new central plant is b
39、eing considered, a decision of whether tocogenerate electrical and thermal energy or to generate thermalenergy only must be made. An example of cogeneration is a diesel ornatural gas engine-driven generator with heat recovery equipment.The engine drives a generator to produce electricity, and heat i
40、srecovered from the exhaust, cooling, and lubrication systems.Recovered heat is used for steam or hot-water heating or even as aheat source for a steam-driven turbine or hot-water absorptionchiller. Other systems may use one of several available steam turbinedesigns for cogeneration. These turbine s
41、ystems combine the ther-mal and electrical output to obtain the maximum amount of availableenergy. Chapter 7 has further information on cogeneration.Selection of temperature and pressure is crucial because it candramatically affect the economic feasibility of a DHC systemdesign. If the temperature a
42、nd/or pressure level chosen is too low, apotential customer base might be eliminated. On the other hand, ifthere is no demand for absorption chillers or high-temperatureindustrial processes, a low-temperature system usually provides thelowest delivered energy cost.The availability and location of fu
43、el sources must also be con-sidered in optimizing the economic design of a DHC system. Forexample, a natural gas boiler might not be feasible where abundantsources of natural gas are not available.Initial Capital Investment. The initial capital investment for aDHC system is usually the major economi
44、c driving force in deter-mining whether there is acceptable payback for implementation.Normally, the initial capital investment includes concept planningand design phases as well as the construction costs of the threemajor system components: (1) thermal energy production plant,District Heating and C
45、ooling 12.3(2) distribution system, and (3) consumer interconnections alsoknown as energy transfer stations (ETSs).Concept Planning. In concept planning phase, many areas aregenerally reviewed, and the technical feasibility of a DHC system isconsidered. This includes master planning and estimating s
46、ystemthermal loads and load growth potential, prospective plant site loca-tions, piping routing, and interconnection or conversion require-ments in the customers building. The overall system concept designusually requires the services of an experienced power plant or DHCengineering firm.Financial Fe
47、asibility. The overall capital and operating costs ofthe DHC system determine the energy rates charged to the prospec-tive customers. These rates must be competitive with the customersalternative HVAC system life-cycle costs, so a detailed analysis ofthis system tailored to the nature of the distric
48、t energy provider isrequired to determine the systems financial feasibility. For example,a municipal or governmental body must consider availability of bondfinancing; if the entity is a private/for-profit organization, then theappropriate discount rates must be considered for the financing.Alternati
49、ve energy choices and fuel flexibility should be reviewed,because potential consumers are often asked to sign long-term con-tracts to justify a DHC system. Fuel flexibility offers the DE providera method to keep generating costs low by being able to react to spikesin fuel costs. The financial analysis must take into account the equip-ments life-cycle costs, including initial construction, operating,maintenance, and replacement, over a system life of at least 50 years.Final Design. This phase is extremely intensive and may takeseveral years to decades to complete the vision of
