1、128 2008 ASHRAE ABSTRACTThis article compares four heating and cooling systems.The systems are: a high efficiency furnace and electric airconditioner; a ground source heat pump; an absorption airconditioner and direct heating; and a thermally driven heatpump; the last two systems use solar thermal e
2、nergy andbackup non-renewable energy. A comprehensive program wasdeveloped that predicted the entire life cycle cost, energyusage, exergetic efficiency, and exergy destruction, of all foursystems operating in the same home figuratively placed in thecities of Louisville, KY; Houston, TX; Minneapolis,
3、 MN;Sacramento, CA; and Phoenix, AZ. The results showed that thevertical ground source heat pump always paid back in theshortest time, between 4-15 years in all five cities compared tothe furnace and air conditioner system. The economic pay backperiod was the shortest between 4-7 years in the cities
4、 of Louis-ville, Minneapolis, and Phoenix, which have larger heatingand/or cooling requirements. The thermally driven heat pump,which largely used renewable energy, had equal or greaterexergetic efficiency than the ground source heat pump in eachcity, while the furnace and air conditioner always had
5、 thelowest exergetic efficiency.INTRODUCTIONToday people are becoming increasingly concerned aboutthe diminishing fossil fuel resources, energy costs, pollution,and climate change. In residential buildings, most energy isused in heating, cooling, and hot water; therefore, by deter-mining the most ef
6、ficient heating, cooling, and hot watersystems, energy consumption will be decreased in residentialbuildings. This article presents a program that determines theenergy used and exergy destroyed in conventional heating,cooling and hot water systems and compares this to the energyused and exergy destr
7、oyed in new proposed systems thatobtain a portion of energy needed from renewable energy fromthermal solar collectors. This article also presents a life cyclecost analysis of the conventional and newer systems to deter-mine if the newer systems are currently cost effective, and ifnot what factors su
8、ch as utility costs or costs of thermal solarcollectors need to change to make the newer systems costeffective.The systems studied are fully described in the sectiontitled “Case Studies” and the systems include condensingfurnaces, ground source heat pumps, absorption air condition-ing, and thermally
9、 driven heat pumps. Because the systemsinclude the devices listed above, previous studies that involvethese heating and cooling devices are presented. A condensingfurnace is typical natural gas furnace except that it cools theflue gases down so low that some of the water vaporcondenses, and uses the
10、 heat from this process to add to theheating of the home. Wright et al. (1984) give details of proto-type and performance data of gas furnaces using plastic heatexchangers, which are less expensive and more corrosionresistant than steel heat exchangers, to condense a portion ofthe flue gases exhaust
11、ed from the combustion process; theprototypes exhibited thermal efficiencies of 92%. Cohen et al.(1991) studied the effect of condensing furnaces that replacedtraditional furnaces in three US cities and found that thereplacements saved 31-41 GJ/yr and were cost-effective.Zogou and Stamatelos (1998)
12、demonstrated that ground-source heat pumps performed better than air-source heatpumps in all climates. Shonder et al. (2002) also showed thatthere were savings from retrofitting air-source heat pumpswith ground-source heat pumps.Investigation of Optimal Heating and Cooling Systems in Residential Bui
13、ldingsAngela L. Bolling James A. Mathias, PhD, PEAssociate Member ASHRAEAngela L. Bolling is a masters student and James A. Mathias is an assistant professor in the Department of Mechanical Engineering andEnergy Processes, Southern Illinois University, Carbondale, IL.NY-08-0182008, American Society
14、of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 114, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permissi
15、on.ASHRAE Transactions 129Absorption chillers utilize a refrigerant-absorbent binarymixture that is environmentally benign to cool the home.Florides et al. (2002) used computer modeling of a domestic-size absorption air conditioner powered by solar energy todetermine the energy costs associated with
16、 cooling by theabsorption process plus heating by a boiler. Life cycle cost anal-ysis showed that the unit was not economically feasible at thattime. Priedeman and Christensen (1999) suggested that absorp-tion air conditioners may offer lower energy costs than electricair conditioners when natural g
17、as rates are favorable comparedto electric rates. Sumathy et al. (2000) reported on a lithiumbromide absorption chiller that was developed and tested inChina and powered by low-grade thermal energy between 60C(140F) and 75C (167F). This low-grade energy allowed foravailable solar energy to be an opt
18、imal energy source to powerthe absorption unit. Unfortunately, the performance of devicewas low when operating from low-grade energy, and reporteda cooling COP between 0.31 to 0.39.Santoso (1989) described a Rankine cycle engine-drivenheat pump system where a compressor and pump werepowered by a Ran
19、kine cycle, instead of electricity. A computerprogram was developed that simulated the conditions at vari-ous locations and determined the mechanical efficiency andcoefficient of performance of the system at different operatingconditions. The energy source for the boiler in the Rankinecycle could be
20、 natural gas or low grade energy such as exhaustor thermal solar energy. The boiler operating temperatureranged from 36.7C (98 F) to 61.1C (142F). Using R-22 forthe vapor compression cycle and R-113 for the Rankine cycle,the coefficients of performance for heating and cooling werefound to be as good
21、 as or better than other common systems.The objective of the study and paper is to examine fourheating and cooling systems operating in the same home figu-ratively placed in five different climate regions in the UnitedStates. The program developed evaluated these four systemsusing data entered by th
22、e user and then determined the energyefficiency, exergy efficiency, and cost effectiveness of eachdesign in each region. The systems studied will be presentedin detail in Case Studies section and were (1) a condensing gasfurnace with a high efficiency electric air conditioner andnatural gas hot wate
23、r heater; (2) a vertical ground source elec-tric heat pump which provides some hot water and has electricbackup furnace and hot water; (3) solar collectors that providethermal energy for an absorption air conditioner and directheating with natural gas backup; and (4) solar collectors thatprovide the
24、rmal energy to a thermally driven ground sourceheat pump with electric backup heat pump and hot waterheater. These systems were chosen because the furnace andAC system is the most commonly used system, and theground source heat pump is highly efficient and currentlyavailable. The other systems were
25、chosen because they userenewable thermal solar energy and the current status of thesesystems is that all the equipment exists for the absorptionsystem, and the thermally driven heat pump is currently underdevelopment. The study and the paper are original in thatprevious researchers have analyzed som
26、e of these systems butthese four systems have never been studied simultaneouslyand two of the four systems studied are likely the most prom-ising for utilizing renewable energy in a residence. In addi-tion, the study determines the energy efficiency, exergyefficiency, and cost effectiveness of newer
27、 systems comparedto traditional systems and determines if it is currently viable toinvest in these systems and if not how conditions need tochange to encourage investment in efficient systems thatpartly use renewable energy. Furthermore, the study examineshow exergy efficiency and destruction correl
28、ates to energyefficiency and cost effectiveness in residential heating andcooling systems which may encourage the use of exergy anal-ysis in determining optimized systems.ANALYSISHeat TransferA commercial program was used to determine the heatingand cooling required for the home in the five cities o
29、f Louis-ville, KY; Phoenix, AZ; Sacramento, CA; Minneapolis, MN;and Houston, TX. The commercial program accounted for theenergy transferred by heat to or from the building due tooutside temperature, sunlight, equipment (electrical appli-ances, such as stoves, dishwashers, computers, etc.), air infil
30、-tration, and people. The program also accounted for energytransfer due to latent energy such as moisture evaporation.Heat transfer was calculated by determining the thermalconduction resistances of the various building materials of thehome and the thermal resistances associated with convectionand s
31、umming up these thermal resistances in parallel or seriesas appropriate. Equations 1 and 2 show resistances in conduc-tion and convection, respectively.(1)(2)Summing up resistances in series is done by directlyadding up each resistance; summing up resistances in parallel,such as a wood stud and insu
32、lation in a wall, is shown in Equa-tion 3.(3)The total thermal resistance of each of the walls wascalculated in Equation 4 by accounting for inside and outsideconvection resistances and conduction resistances of sheet-rock, insulation, wood, sheeting, and siding.RcondLkA-=Rconv1hA-=1Rparallel-1Rwood
33、-1Rinsulation-+=130 ASHRAE Transactions(4)The thermal resistance of the floor, ceiling, and windowsare shown in Equations 5 through 7. The outside convectiveresistance above the ceiling in the attic was different thanoutside the siding because it is a ventilated attic but notexposed to the outside a
34、ir.Rfloor= Rconv,in+ Rfloor(5)Rceiling= Rconv,in+ Rsheetrock+ Rinsulation+ Rconv,out(6)Rwindow= Rconv,in+ Rwindow+ Rconv,out(7)After the resistances were calculated the amount of heattransferred through each wall was calculated by Equation 8.This same form of equation was used to determine the heatt
35、ransfer through the windows, ceiling, and floor. As seen, theheat transferred varies with respect to outside temperature,therefore the outside temperature was divided into discreteranges and the heat transfer was calculated for each range ofoutside temperature. (8)The heating needed to maintain the
36、temperature of thehome was calculated from the heat leaving the home throughthe walls, ceiling, floor, windows, and air infiltration, as shownin Equation 9.(9)Equation 10 determined the total energy required to besupplied to the heating system, which was calculated by multi-plying the heat loss occu
37、rring from the home at a given outsidetemperature range by the amount of time (bin) the outsidetemperature exists in that range, and divided by the efficiencyof the system, which also may vary by outside temperature. (10)The calculation to determine the amount of coolingneeded for the home was done
38、the same way except that thehome gained energy through the walls, ceiling, floor, win-dows; in addition, sunlight and the energy produced by peopleand appliances were included as energy gained by the home.The amount of non-renewable energy needed by systemsthat largely use thermal solar collectors f
39、or heating and cool-ing was determined by first obtaining the amount of solarenergy available each month in each of the five cities. Theamount of solar energy available was based on the amount ofdaylight and cloud cover of each city. The cities choseninclude a large variety of latitudes and cloud co
40、ver, thereforethe amount of sunlight at different latitudes and climates wereaccounted for in the simulation. After this, it was determinedhow much of the available solar energy was collected, andincorporating the efficiency of each system that uses renew-able energy, it was determined how much heat
41、ing, cooling, andhot water was produced. If the system using renewable energydid not provide enough heating and cooling it was determinedhow much non-renewable energy was needed to meet therequirements by incorporating the efficiency of the systemusing non-renewable energy. The size and number of so
42、larcollectors of the absorption system were chosen such thatrenewable energy provided much of the cooling in the summertime, between 50 to 90% except in Sacramento, CA where100% was provided, and a noticeable or significant amount ofheating in the winter, between 30 to 70%. The thermally drivenheat
43、pump was sized to provide between 15 to 50% of coolingand 30 to 45% of heating except in Phoenix, AZ where 80%of heating was provided. The same size units were used in alllocations because they are the smallest units available but lesssolar collectors were needed in Phoenix, AZ and Houston, TXto ope
44、rate these units as there is more sunlight available inthese climates. Less solar collectors is evident by the lowercost of solar collectors shown in the Results and DiscussionSection. If the thermal solar collectors provided all the heatingand cooling the excess solar energy collected was considere
45、dto be discarded and not used in other months. The power usedby the pumps and controls of the thermal solar collectors weresmall and therefore neglected in the analysis.EconomicsThe economics assumed that all systems lasted 20 yearsand that there was no value at the end of the 20 years. In addi-tion
46、, it was assumed that the price of each system did not varythroughout the country, thereby removing the effect of differ-ent costs of living. The economics, purchase and operatingcosts, of each system were determined by first obtaining theaverage cost of natural gas and electricity of each state of
47、thecorresponding cities that were studied as shown in Table 1(EIA 2006). The energy obtained from 1 ft3of natural gaswas determined to be 1027 Btu/ft3. The escalation rates, ER,of natural gas and electricity, all with respect to 2006 dollars,were also obtained (EIA 2006), which stated that naturalga
48、s and electricity will escalate, in decimal form, at 0.003and 0.002, respectively.RwallRconv in,Rsheetrock+=11Rwood-1Rinsulation-+- RsheetingRsidingRconvout,+QwallTinToutRwall-=QheatingQwallQceilingQfloorQwindowsQair+ +=QsystembinQheating-=Table 1. Average 2006 Utility Rates of Selected StatesCA KY
49、MN AZ TXNatural GasRate ($/GJ) 12.5 16.8 12.3 16.2 13.2($/MBtu) 11.89 15.97 11.70 15.39 12.59ElectricityRate ($/kWh) 0.1436 0.0686 0.0872 0.0928 0.1258ASHRAE Transactions 131The interest rate, after removing the effect of inflation,which was used in conjunction with the additional cost orsavings of each of the newer systems, compared to the furnaceand air condit
