Comparative Environmental andEconomic Analysis of Conventionaland Nanofluid Solar Hot WaterTechnologiesT O D D P . O T A N I C A R † A N DJ A Y S . G O L D E N * , ‡ , §

National Center of Excellence for SMART Innovations for UrbanClimate & Energy, Department of Mechanical & AerospaceEngineering, Department of Civil, Environmental & SustainableEngineering, and School of Sustainability, Arizona StateUniversity, P.O. Box 875502, Tempe, Arizona 85287-5502

Received January 5, 2009. Revised manuscript receivedJune 3, 2009. Accepted June 9, 2009.

This study compares environmental and economic impactsof using nanofluids to enhance solar collector efficiency ascompared to conventional solar collectors for domestic hot watersystems. Results show that for the current cost of nanoparticlesthe nanofluid based solar collector has a slightly longerpayback period but at the end of its useful life has the sameeconomic savings as a conventional solar collector. The nanofluidbased collector has a lower embodied energy (∼9%) andapproximately 3% higher levels of pollution offsets thana conventional collector. In addition if 50% penetration ofresidential nanofluid based solar collector systems for hot waterheating could be achieved in Phoenix, Arizona over 1 millionmetric tons of CO2 would be offset per year.

IntroductionThe conventional solar hot water heater is a well establishedtechnology but recently this technology has been combinedwith the emerging technologies of nanofluids and liquid-nanoparticle suspensions to create a new class of nanofluid-based solar collectors (1, 2). From a technical perspectivethese collectors are have been shown numerically (2) andexperimentally (1) to have improved efficiencies over con-ventional solar collectors but the overall economic andenvironmental impacts of the technology in contrast withconventional solar collectors has not yet been addressed.

Domestic solar hot water heaters for residential use inthe Phoenix metropolitan area, as well as throughout thestate of Arizona, are eligible for tax rebates at the state andfederal level, in addition to incentive programs through localutilities. The local utilities offer a one-time incentive rangingfrom $0.50 to $0.75 per kWh saved in the first year (3, 4), withthe state of Arizona offering a one time tax rebate of 25% ofthe installed cost up to $1,000 and the federal governmentoffering a tax rebate for 30% of the cost up to $2,000 (5). Inaddition to the economic incentives available for the instal-lation of domestic solar hot water heaters the usage of solarenergy to offset the energy required to heat water for

residential use, representing 12.5% of a home’s energy usage(6), will result in reduced greenhouse gas and smog-formingemissions from the combustion of conventional fossil fuels.Finally, the state of Arizona has adopted a renewable portfoliostandard requiring 15% of energy generated by 2025 to comefrom renewable sources, with 30% of this coming fromdistributed, of which 50% must be to residential installations(5).

Life cycle assessment (LCA) is an accepted methodologythat can be used to assess economic and environmentalimpacts of products. Recently many researchers have usedLCA methodologies to evaluate the economic and environ-mental impact of solar hot water heating systems (7-10). Allof these studies have focused on utilizing solar hot waterheaters in European countries, with most focusing only onthe environmental aspect (8-10) and very limited prior worksexamining both the economic and environmental impacts(7). In addition, all of these studies focus on collector systemsthat are currently in domestic use. This study focuses onsupporting an LCA, by confining our efforts to focus on theenergy and selected pollutant savings, of two different solarhot water heaters: a conventional flat plate collector and anew nanofluid-based direct-absorption collector. The eco-nomic and environmental impacts are compared for the twocollectors operated within the Phoenix metropolitan area.

AnalysisThe life cycle assessment is an effective methodology forevaluating various parts of a systems impact on the environ-ment from its initial resource allocation to its disposal/reuseafter the consumer use phase. This study is a confined LCAthat focuses on the major components of manufacturing,based on embodied energy, and operation of the solarcollector. The manufacturing of the collector and the supportstructure represent more than 70% of the embodied energyof the system (10), for this reason the manufacturing energywas focused on while the distribution, maintenance, anddisposal phases of the collectors were not taken intoconsideration. The analysis was done with the collector areaas the functional unit and set equal for both collectors. Toperform both the economic and environmental analysis thethermal performance of the two collectors first had to beevaluated for the Phoenix, Arizona area. The typical homein Phoenix is a 3-person family dwelling (11) that uses 188L (50 gallons) of hot water daily (6) from a mix of electric andnatural gas water heaters, 60% and 40%, respectively (12).The average hot water temperature delivered is 59.4 °C (139°F) with the typical load profile outlined in Figure 1 (13).

A conventional solar collector works by absorbing solarenergy on a black plate, typically copper, which is coupledto copper tubing carrying thermal fluid that removes theheat from the plate and in turn heats the water. The copperplate is surrounded by insulation to minimize heat losses viaconduction, while the glazing, usually a highly transmissiveglass clover, is used to minimize the convective losses whileallowing the maximum amount of solar energy to be incidenton the black plate surface. The nanofluid-based directabsorption collector eliminates the need for a copper backplate and tubing, and thus losses from transferring heat fromplate to fluid, by using the nanofluid to directly absorb theincident radiation. Schematics for both types of collectorsincluding the materials used are presented in Figure 2.

As can be seen in Figure 2 the nanofluid-based solarcollector simplifies the design of the system and eliminatesa large portion of copper, which is subsequently replaced by

* Corresponding author e-mail: Jay.Golden@asu.edu.† National Center of Excellence for SMART Innovations for Urban

Climate&Energy,DepartmentofMechanical&AerospaceEngineering.‡ Department of Civil, Environmental & Sustainable Engineering.§ School of Sustainability.

Environ. Sci. Technol. 2009, 43, 6082–6087

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additional glass and steel. The nanofluid is composed ofgraphite nanoparticles suspended at 0.1% volume fractionin the thermal fluid, here a mixture of water and propyleneglycol to prevent freezing.

Thermal Performance. The thermal performance of asolar collector for domestic hot water heating is based on theamount of energy that can be offset by the installation of asolar system, which is determined from the solar collectorefficiency and solar insolation for a given location. Theamount of useful energy gained from a solar collector canthen be determined based on the collector efficiency fromeq 1.

where η is the collector efficiency, Gt the solar insolation,and A is the area. The collector efficiency is based onexperimental data that is a function of the solar insolationand the ambient temperature. For a typical conventionalcollector available in the southwestern U.S., the SunEarthEmpire EC-24, which employs “a moderately selectiveabsorber”, an efficiency equation shown below is used (14).

Since the nanofluid-based collector is still entirely experi-mental and therefore not certified by the Solar Rating andCertification Corporation (SRCC) there is no efficiency curve

available. Based on the experimental work (1) a 3.5%improvement in efficiency is expected over conventionaltechnology, therefore eq 2 becomes the following for ananofluid direct absorption collector.

This is then used to evaluate the auxiliary energy requiredbeing supplied from the conventional electric or natural gaswater heater.

where Qload is the energy required by the daily hot waterload, and Qenv is the loss to the environment. Theseparameters are all evaluated for the ambient weather dataof Phoenix given in Table 1. The inlet water temperature isbased on data for Thessaloniki, Greece which has ambientweather conditions similare to those of Phoenix.

With the auxiliary energy determined the solar fractioncan also be calculated, representing the percentage of energythat is saved due to the use of the solar system, from eq 5.

Results of the thermal analysis form the basis for theeconomic as well as the environmental analysis.

FIGURE 1. Hot water daily consumption profile (13).

FIGURE 2. Conventional solar collector (top) and nanofluid-based direct absorption collector (bottom).

Qu ) η · GtA (1)

η ) 0.702 - 3.2828 × (Ti - Ta)/Gt - 0.0099 ×(Ti - Ta)2/Gt (2)

η ) 0.737 - 3.2828 × (Ti - Ta)/Gt - 0.0099 ×(Ti - Ta)2/Gt (3)

Qaux ) Qload - (Qu - Qenv) (4)

f )Qload - Qaux

Qload(5)

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Economic Analysis. The results of the thermal perfor-mance give the baseline energy that still needs to be suppliedby an electric or natural gas hot water system to meet thehot water demand. In this model all of the capital costs arepaid in the first year and no consideration of financing forthe system is considered. The capital costs for solar collectorsare usually written as the combination of the area basedcosts and the area independent costs.

where Cf is the area independent costs, and CA is the areadependent costs. The area independent cost for both solarcollectors is taken to be $200 (7). The area dependent costsare estimated to be $180/m2 based on a solar collectormanufacturing survey (17). Because of the nature of thenanofluid solar collector, which eliminates a large portionof the copper used in the system, the area based cost isreduced based on a scaling of the overall percentage weightof the collector that uses copper. This cost is offset by thecost of the graphite nanoparticles, currently $3/g (18). Themaintenance cost (Cm) is taken to be 1% of the capital costwith a 1% increase yearly for the system lifetime of 15 years(7). The nanofluid collector is expected to have the samelifetime as the conventional solar collector since it utilizesthe same material technologies as a conventional collector.Although the lifetime is expected to stay the same as aconventional collector, included in the analysis is theadditional cost and embodied energy of having to refillthe collector with a new nanofluid solution once during thelifetime. The total cost is then:

To determine the amount saved due to the operation of thesolar collectors, the energy flow per day is used in conjunctionwith the local electricity rates based in dollars per kWh (4)and natural gas rates in dollars per therm (19).

Environmental Analysis. A major reason for switching toa solar collection system to heat water is the reduction inemissions resulting from the combustion of fossil fuels togenerate the energy to heat the water. Using the results ofthe thermal performance analysis, which gives the energyflows on a daily and monthly basis, the amount of emissionsoffset by the use of a solar collector can be determined basedon the amount of energy offset (Qload-Qaux) and the emissionsprofile for electricity generation for Arizona.

where A is the amount of pollutant (in kg) per MJ of energygenerated, B is the percentage of energy generated from aspecific fuel type, j is the fuel type, and x is the pollutanttype. The distribution of electricity from various fuel typesand the key pollutants generated for the state of Arizona isshown in Table 2.

In addition to this consideration a complete environ-mental analysis should also include the impact on variousenvironment receptors from the additional phases of dis-tribution, maintenance, and postconsumer use. One impactcategory to evaluate during each of these phases is to considerthe embodied energy in each phase. Tsillingiridis et al. (9)recently showed that the majority of the solar collectorembodied energy comes from the manufacturing operation.Based on these results only the manufacturing process isaddressed in the embodied energy analysis the other phaseswill be discussed later. To assess the embodied energy ofeach solar collector type the main materials and materialmass must be known as well as the embodied energyassociated with each of these materials. This study utilizesthe embodied energy index created by Alcorn (21) whichassesses the embodied energy in various common construc-tion materials, and these indices include the acquisition andtransport of the materials. The amount of materials in theconventional collector is based on the specification providedby SunEarth for the Emprie collector (22). Similarly theamount of materials for the nanofluid-based collector isdetermined by starting with the conventional collector andredesigning the system to accommodate working as a directabsorption collector as shown in Figure 2. For the nanofluid-based collector the embodied energy of the nanoparticles isalso considered based on an average of results for energyrequirements for carbon nanoparticle production (23). Withthe amount of each component material determined andthe embodied energy index, the embodied energy and thusthe emissions from the manufacturing of the collectors (fromeq 8, where Qload-Qaux is replaced with the embodied energy)can be determined. The offset damage costs are calculatedbased on the damage cost factors for the three main pollutantsstudied: CO2, NOx, and SOx (24). These offset damage costsare not factored into the economic analysis as they are notcosts directly applicable to the collector owner.

Results and DiscussionThe comparative analysis above addresses the thermalperformance, economic impact, and environmental impactfor conventional and nanofluid-based solar collectors. Thethermal performance is the foundation for the rest of thestudy as it provides the monthly variation in energy neededto supplement the solar collectors. This energy flow whencompared with the base load forms the basis for determiningcost and pollutant reductions from the collector operation.The results of the thermal analysis are shown in Figure 3,including the base load and auxiliary energy flows.

From Figure 3 it can be seen that the nanofluid-basedsolar collector provides a higher solar fraction and lower

TABLE 1. Ambient Weather Data for the City of Phoenix

month

solar insolation(kWh/m2/day)

(15)

ambienttemperature

(°C) (16)

watertemperature

(°C) (9)

January 5.0 13.0 8.2February 5.7 15.7 7.9March 6.6 17.3 9.2April 7.4 23.7 12.8May 7.7 27.3 16.8June 7.6 34.0 20.2July 6.9 35.6 21.5August 7.0 33.8 22.8September 6.9 30.4 22.1October 6.5 24.8 19.4November 5.8 18.0 15.7December 5.0 11.7 11.0

Cs ) CAA + Cf (6)

C ) Cs + Cm (7)

Px,offset ) ∑j)1

N

Aj,xBj,x(Qload - Qaux) (8)

TABLE 2. Electricity Generation by Fuel Type and PrimaryEmissions Mix for Arizona (20)

fuel

% ofelectricitygenerated

carbondioxide,

CO2 (kg/MJ)

sulfuroxides,

SOx (kg/MJ)

nitrogenoxides,

NOx (kg/MJ)

coal 38.70% 0.274 0.00031 0.0005petroleum 0.10% 0.220 0 0natural gas 31.50% 0.113 0 0.00003nuclear 23.00% 0 0 0other (includes

hydro andrenewables)

6.70% 0 0 0

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auxiliary energy requirement than a conventional collectorfor most months. During the summer months, May-September, both collectors provide all the energy needed tomeet the demand load. The demand load is also significantlyreduced in the summer months due to the increasedincoming water temperatures. The annual solar fraction forthe conventional collector and nanofluid-based solar col-lector is 80% and 85%, respectively.

With the thermal analysis the capital costs, maintenancecosts, and offset fuel savings cost can all be calculated basedon the current prices for electricity and natural gas as shownin Table 3. Due to the current market cost of the nanoparticlesat $3/gram, the nanofluid-based solar collector has anadditional $120 in capital cost and an additional $20 inmaintenance costs. Because of the increased efficiency andhigher annual solar fraction of the nanofluid-based solarcollector, the fuel cost savings per year, for both electricityand natural gas, is greater than that of the conventional solarcollector.

The payback period for the conventional collector is lessthan the nanofluid collector primarily due to the increasedcapital cost of the nanoparticles needed for the nanofluid-based collector. The payback period for both collectors issignificantly longer when a natural gas water heater is useddue to the reduced cost of the fuel although data for thePhoenix area and state show that the majority of utilitycustomers (60%) use electric water heaters over natural gaswater heaters (3) due to limited natural gas infrastructurewithin the state. The total life cycle savings, when usingelectricity, are $3,358 and $3,352 for the conventional solarand nanofluid-based solar collector respectively. When usingnatural gas for a fuel the savings are reduced substantiallyto $511 and $413 for the conventional solar and nanofluid-based solar collector, respectively. The results show that froman economic perspective there is very little difference betweenthe two types of solar collectors, although both options canprovide substantial savings over the life of the collector. In

addition, any drop in the price of nanoparticles, which is tobe expected as they become more widely used and produced,would result in further savings with the nanofluid based solarcollector.

The two collectors shown in Figure 2 differ in some of themajor materials used in their construction, which has animpact both on the collector weight and embodied energy.Table 4 presents the amount of materials used in the twocollectors as well as the embodied energy for each materialand the collector total.

FIGURE 3. Monthly energy flow and solar fraction (X-Electric water heater, triangles: nanofluid solar collector, squares: conventionalsolar collector).

TABLE 3. Economic Comparisons for Conventional andNanofluid-Based Solar Collectors

conventionalsolar

collector ($)

nanofluidsolar

collector ($)

capital costsindependent costs 200.00 200.00area based costs 397.80 327.80nanoparticles 188.79

total capital (one time cost) 597.80 716.59total maintenance (for 15

year life)96.23 115.35

total costs 694.03 831.94

electricity cost savings per year 270.13 278.95years until electricity savings ) costs 2.57 2.98natural gas cost savings per year 80.37 83.02years until natural gas savings ) costs 8.64 10.02

electricity priceNovember-March (per kWh) 0.08 0.08May-October (per kWh) 0.09 0.09daily service charge 0.25 0.25

gas pricerate (per therm) 0.74 0.74monthly service charge 9.70 9.70

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As shown in Table 4 the reduction in copper material inthe nanofluid based solar collector, even when supplementedwith nanoparticles, glass, and steel, still results in a reductionof over 200 MJ when compared to the conventional collector.The additional glass and steel, though, results in an 11 kgweight increase for the nanofluid-based solar collector.

The results from Table 5 represent all of the embodiedenergy from the manufacturing and installation phases,assuming the installation is 3% of the total system energy(7), of the solar collectors including the frame (7).

With the results of the thermal analysis and the embodiedenergy assessment the pollution created in the manufacturingof the collector as well as the pollution offset (savings) by theoperation of the collector can be examined. The results ofthis analysis are presented in Table 6, with the savings basedon a 60/40 distribution of electric/natural gas backup.

As shown in Table 6, the manufacturing of the nanofluid-based solar collector results in 34 kg less CO2 emissions whileoperationally it saves 50 kg/year when compared to aconventional solar collector. The differences between the

remaining emissions, SOx and NOx, are of a much smallermagnitude. Over the 15-year expected lifetime of the solarcollectors, the nanofluid-based solar collector would offsetmore than 740 kg of CO2 in comparison to a conventionalcollector and 23 000 kg of CO2 when compared to traditionalwater heaters.

Finally, using the results of Spadaro and Rabl (24) theoffset damage costs from the pollution savings of thecollectors can be established. The total damage cost avoidedper year is $88 and $91 for the conventional and nanofluid-based solar collectors, respectively.

Table 7 shows that the avoided damage cost per year is$3 higher with the nanofluid-based solar collector repre-senting a total damage cost avoided over 15 years of morethan $1,300. This savings is not directly related to the installerof the solar collector, the utility, or the state and federalgovernment but distributed throughout all of these. If thissavings was completely attributed to the government it wouldrepresent 3 times the amount invested through the rebatesystem. The damage costs capture the impact of the pollutants

TABLE 4. Embodied Energy Comparisons for Conventional and Nanofluid-Based Solar Collectors

conventional solar collector nanofluid-based solar collector

descriptionembodied energy

index (MJ/kg) mass (kg)embodied energy

content (MJ) mass (kg) embodied energy (MJ)

insulationpolyester 53.7 1.74 93.22 1.74 93.22fiberglass 30.3 3.26 98.75 3.26 98.75

glass 15.9 14.20 225.76 28.40 451.52copper pipe

collector 70.6 4.97 350.72 0.00 0.00manifold 70.6 3.48 245.57 3.48 245.57

aluminum extrusion 201.0 0.56 111.58 0.56 111.58aluminum backplate 199.0 2.12 421.75 2.12 421.75steel backplate 34.8 0.00 0.00 5.97 207.65sealant 87.0 0.70 60.90 0.70 60.90black paint 90.4 0.30 27.12 0.30 27.12casing paint 90.4 0.90 81.36 0.90 81.36screws 34.8 0.00 0.04 0.00 0.04copper absorber 70.6 4.05 285.80 0.00 0.00nanoparticles 246.8 0.00 0.00 0.06 15.55thermal fluid 17.0 5.84 99.28 5.84 99.28conversion rate (27%) 567.50 516.86

total 42.10 2669.34 53.32 2431.14

TABLE 5. Embodied Energy Comparisons for Conventional and Nanofluid-Based Solar Collectors for Total Installation

conventional solar collector nanofluid-based solar collector

descriptionembodied energy

index (MJ/kg)mass(kg)

embodied energycontent (MJ)

mass(kg)

embodiedenergy (MJ)

collector 42.10 2669.34 53.32 2431.14copper pipe 70.6 3.80 268.28 3.80 268.28pipe insulation 120.0 1.00 120.00 1.00 120.00steel frame 34.8 30.00 1044.00 30.00 1044.00

total 4101.62 2.12 3863.42installation (3% of total) 123.05 115.90

grand total 76.90 4224.67 90.24 3979.32

TABLE 6. Embodied Energy Emissions from a Solar Collector and Consumer Phase Operational Energy

emissionspollution from solar collector embodied energy savings of solar collector

conventional (kg) nanofluid-based (kg) conventional (kg/year) nanofluid-based (kg/year)

carbon dioxide (CO2) 599.77 564.94 1500.89 1550.33sulfur oxides (SOx) 0.51 0.48 0.83 0.85nitrogen oxides (NOx) 0.84 0.79 1.53 1.58

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from the operation and manufacturing of the collector butone important environmental consideration that is notincluded in any of the economic or environmental impactsis the nanoparticles. Much work has been devoted tounderstanding the impact of nanoparticles on the environ-ment, especially ecological systems and human health(25-28). Many of these studies focus on a specific particleand health/environmental impact due to the complex natureregarding their potential impact. In addition nearly all ofthese studies focus on nanoparticles that are not suspendedin the liquid, which greatly reduces the risk for inhalation,but could present a potential problem if introduced into thewater cycle. More recent work has shown that carbon particlessuspended in aqueous solutions form large aggregates whichwould settle out of solution which could offset exposure risks(29).

Due to its large number of homes (1,375,000 (11)) andplentiful solar radiation (15) Phoenix, AZ is an ideal place forutilizing solar thermal energy for domestic hot water ap-plications. Based on incentive data for one of the region’stwo public utilities, Salt River Project (SRP), the number ofsolar collectors for hot water heating is increasing from 51installed in 2005 to 260 in 2007. Based on these increasingbut still relatively small numbers, we would expect that lessthan 5% of households in the area have solar collectorsinstalled. If 50% penetration of nanofluid-based solar col-lectors could be achieved in Phoenix, AZ over 1,000,000 tonsof CO2, 500 t of SOx, and 1000 t of NOx could be offset fromthe use of solar energy instead of electricity and natural gasto heat water per year. Additionally, based on the ratio ofoffset damage costs to government rebate incentive, currentlyat a level of 3, it is demonstrated that additional governmentincentives could still be provided without losing money onthe investment but potentially leading to increased penetra-tion of solar collectors in Phoenix, Arizona.

AcknowledgmentsWe thank Mr. Tom Hines of Arizona Public Service and Mr.Don Pelley of Salt River Project for their assistance inobtaining data on solar collector installations in the Phoenixregion as well as the distribution on conventional hot waterheaters. This work was partially supported by the NationalCenter of Excellence on SMART Innovations for UrbanClimate + Energy (www.asuSMART.org).

Literature Cited(1) Otanicar, T.; Phelan, P. E.; Rosengarten, G.; Prasher, R. S.

Experimental testing and modeling of a micro solar thermalcollector with direct absorption nanofluids. In Proceedings ofthe Inaugural US-EU China Thermophysics Conference; Beijing,China, 2009.

(2) Tyagi, H.; Phelan, P. E.; Prasher, R. S. Predicted Efficiency of aNano-Fluid Based Direct Absorption Solar Receiver. In ASME1st International Conference on Energy Sustainability; LongBeach, CA, 2007.

(3) Salt River Project; http://www.srpnet.com (accessed 2008).(4) Arizona Public Service; http://www.aps.com (accessed 2008).(5) Renewable Portfolio Standard; AAC R14-2-1801 et seq.; Arizona

Corporation Commission; Phoenix, AZ, 2006.(6) Buildings Energy Data Book; U.S. Department of Energy:

Washington, DC, 2008.(7) Kalogirou, S. Thermal performance, economic and environ-

mental life cycle analysis of thermosiphon solar water heaters.Sol. Energy 2008, 83 (1), 39-48.

(8) Kalogirou, S. A. Environmental benefits of domestic solar energysystems. Solar Energy 2004, 45, 3075–3092.

(9) Tsillingiridis, G.; Martinopoulos, G.; Kyriakis, N. Life cycleenvironmental impact of a thermosyphonic domestic solar hotwater system in comparison with electrical and gas waterheating. Renewable Energy 2004, 29, 1277–1288.

(10) Ardente, F.; Beccali, G.; Cellura, M.; Lo Brano, V. Life cycleassessment of a solar thermal collector. Renewable Energy 2005,30, 1031–1054.

(11) Selected Housing Characteristics (DP-4), Arizona; U.S. CensusBureau: Washington, DC, 2000.

(12) Email conversations with Don Pelley, Salt River Project, 2008.(13) Much, J. J. Residential Water Heating: Fuel Conservation,

Economics, and Public Policy;R-1498-NSF; RAND Corporation:Santa Monica, CA, 1974.

(14) Solar Rating and Certification Corporation; http://www.solar-rating.org (accessed 2008).

(15) National Solar Radiation Data Base, 1991-2005; NationalRenewable Energy Lab, Department of Energy: Golden, CO,2008.

(16) The Arizona Meteorological Network; http://ag.arizona.edu/azmet/ (accessed 2008).

(17) Annual Solar Collector Manufacturers Survey; EIA-63A; EnergyInformation Administration, Department of Energy: Washing-ton, DC, 2007.

(18) MTI Corporation; http://www.mtixtl.com (accessed 2008).(19) Southwest Gas Corporation; http://www.southwestgas.com

(accessed 2008).(20) Arizona Electricity Profile; Energy Information Administration,

Department of Energy: Washington, DC, November 2007.(21) Alcorn, J. Embodied energy coefficients of building materials;

Centre for Building Performance Research, Victoria Universityof Wellington, New Zealand, 1995.

(22) SunEarth Solar Thermal Products Incorporated; http://www.sunearthinc.com (accessed 2008).

(23) Kushnir, D.; Sanden, B. A. Energy requirements of carbonnanoparticle production. J. Ind. Ecol. 2008, 12 (3), 360–375.

(24) Spadaro, J. V.; Rabl, A. Estimates of Real Damage from AirPollution: Site Dependence and Simple Impact Indices for LCA.Int. J. LCA 1999, 4 (4), 229–243.

(25) Environmental, Health, and Safety Research Needs for EngineeredNanoscale Materials; National Science and Technology Council:Washington, DC, 2006.

(26) Oberdorster, G.; Maynard, A.; Donaldson, K.; Castranova, V.;Fitzpatrick, J.; Ausman, K.; Carter, J.; Karn, B.; Kreyling, W.; Lai,D.; Olin, S.; Monteiro-Riviere, N.; Warheit, D.; Yang, H. Principlesfor characterizing the potential human health effects fromexposure to nanomaterials: Elements of a screening strategy.Particle Fiber Toxicol. 2005, 2, 1–35.

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ES900031J

TABLE 7. Yearly Avoided Damage Costs for Conventional andNanofluid-Based Solar Collectors

damage costsavoided ($)

cost($/kg)

conventionalsolar collector

nanofluid basedsolar collector

carbon dioxide (CO2) 0.03 48.72 50.45sulfur oxides (SOx) 12.13 9.60 9.95nitrogen oxides (NOx) 18.40 27.13 28.12

total 85.45 88.52

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