Comparative Environmental and Economic Analysis of Conventional and Nanofluid Solar Hot Water Technologies

Transcription

1 Environ. Sci. Technol. 2009, 43, Comparative Environmental and Economic Analysis of Conventional and Nanofluid Solar Hot Water Technologies TODD P. OTANICAR AND JAY S. GOLDEN*,, National Center of Excellence for SMART Innovations for Urban Climate & Energy, Department of Mechanical & Aerospace Engineering, Department of Civil, Environmental & Sustainable Engineering, and School of Sustainability, Arizona State University, P.O. Box , Tempe, Arizona Received January 5, Revised manuscript received June 3, Accepted June 9, This study compares environmental and economic impacts of using nanofluids to enhance solar collector efficiency as compared to conventional solar collectors for domestic hot water systems. Results show that for the current cost of nanoparticles the nanofluid based solar collector has a slightly longer payback period but at the end of its useful life has the same economic savings as a conventional solar collector. The nanofluid based collector has a lower ( 9%) and approximately 3% higher levels of pollution offsets than a conventional collector. In addition if 50% penetration of residential nanofluid based solar collector systems for hot water heating could be achieved in Phoenix, Arizona over 1 million metric tons of CO 2 would be offset per year. Introduction The conventional solar hot water heater is a well established technology but recently this technology has been combined with the emerging technologies of nanofluids and liquidnanoparticle suspensions to create a new class of nanofluidbased solar collectors (1, 2). From a technical perspective these collectors are have been shown numerically (2) and experimentally (1) to have improved efficiencies over conventional solar collectors but the overall economic and environmental impacts of the technology in contrast with conventional solar collectors has not yet been addressed. Domestic solar hot water heaters for residential use in the Phoenix metropolitan area, as well as throughout the state of Arizona, are eligible for tax rebates at the state and federal level, in addition to incentive programs through local utilities. The local utilities offer a one-time incentive ranging from $0.50 to $0.75 per kwh saved in the first year (3, 4), with the state of Arizona offering a one time tax rebate of 25% of the installed cost up to $1,000 and the federal government offering a tax rebate for 30% of the cost up to $2,000 (5). In addition to the economic incentives available for the installation of domestic solar hot water heaters the usage of solar energy to offset the energy required to heat water for * Corresponding author 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. residential use, representing 12.5% of a home s energy usage (6), will result in reduced greenhouse gas and smog-forming emissions from the combustion of conventional fossil fuels. Finally, the state of Arizona has adopted a renewable portfolio standard requiring 15% of energy generated by 2025 to come from renewable sources, with 30% of this coming from distributed, of which 50% must be to residential installations (5). Life cycle assessment (LCA) is an accepted methodology that can be used to assess economic and environmental impacts of products. Recently many researchers have used LCA methodologies to evaluate the economic and environmental impact of solar hot water heating systems (7-10). All of these studies have focused on utilizing solar hot water heaters in European countries, with most focusing only on the environmental aspect (8-10) and very limited prior works examining both the economic and environmental impacts (7). In addition, all of these studies focus on collector systems that are currently in domestic use. This study focuses on supporting an LCA, by confining our efforts to focus on the energy and selected pollutant savings, of two different solar hot water heaters: a conventional flat plate collector and a new nanofluid-based direct-absorption collector. The economic and environmental impacts are compared for the two collectors operated within the Phoenix metropolitan area. Analysis The life cycle assessment is an effective methodology for evaluating various parts of a systems impact on the environment from its initial resource allocation to its disposal/reuse after the consumer use phase. This study is a confined LCA that focuses on the major components of manufacturing, based on, and operation of the solar collector. The manufacturing of the collector and the support structure represent more than 70% of the of the system (10), for this reason the manufacturing energy was focused on while the distribution, maintenance, and disposal phases of the collectors were not taken into consideration. The analysis was done with the collector area as the functional unit and set equal for both collectors. To perform both the economic and environmental analysis the thermal performance of the two collectors first had to be evaluated for the Phoenix, Arizona area. The typical home in Phoenix is a 3-person family dwelling (11) that uses 188 L (50 gallons) of hot water daily (6) from a mix of electric and natural 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 solar energy on a black plate, typically copper, which is coupled to copper tubing carrying thermal fluid that removes the heat from the plate and in turn heats the water. The copper plate is surrounded by insulation to minimize heat losses via conduction, while the glazing, usually a highly transmissive glass clover, is used to minimize the convective losses while allowing the maximum amount of solar energy to be incident on the black plate surface. The nanofluid-based direct absorption collector eliminates the need for a copper back plate and tubing, and thus losses from transferring heat from plate to fluid, by using the nanofluid to directly absorb the incident radiation. Schematics for both types of collectors including the materials used are presented in Figure 2. As can be seen in Figure 2 the nanofluid-based solar collector simplifies the design of the system and eliminates a large portion of copper, which is subsequently replaced by ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 15, /es900031j CCC: $ American Chemical Society Published on Web 06/23/2009

2 FIGURE 1. Hot water daily consumption profile (13). FIGURE 2. Conventional solar collector (top) and nanofluid-based direct absorption collector (bottom). additional glass and steel. The nanofluid is composed of graphite nanoparticles suspended at 0.1% volume fraction in the thermal fluid, here a mixture of water and propylene glycol to prevent freezing. Thermal Performance. The thermal performance of a solar collector for domestic hot water heating is based on the amount of energy that can be offset by the installation of a solar system, which is determined from the solar collector efficiency and solar insolation for a given location. The amount of useful energy gained from a solar collector can then be determined based on the collector efficiency from eq 1. Q u ) η G t A (1) where η is the collector efficiency, G t the solar insolation, and A is the area. The collector efficiency is based on experimental data that is a function of the solar insolation and the ambient temperature. For a typical conventional collector available in the southwestern U.S., the SunEarth Empire EC-24, which employs a moderately selective absorber, an efficiency equation shown below is used (14). η ) (T i - T a )/G t (T i - T a ) 2 /G t (2) Since the nanofluid-based collector is still entirely experimental and therefore not certified by the Solar Rating and Certification Corporation (SRCC) there is no efficiency curve available. Based on the experimental work (1) a 3.5% improvement in efficiency is expected over conventional technology, therefore eq 2 becomes the following for a nanofluid direct absorption collector. η ) (T i - T a )/G t (T i - T a ) 2 /G t (3) This is then used to evaluate the auxiliary energy required being supplied from the conventional electric or natural gas water heater. Q aux ) Q load - (Q u - Q env ) (4) where Q load is the energy required by the daily hot water load, and Q env is the loss to the environment. These parameters are all evaluated for the ambient weather data of Phoenix given in Table 1. The inlet water temperature is based on data for Thessaloniki, Greece which has ambient weather conditions similare to those of Phoenix. With the auxiliary energy determined the solar fraction can also be calculated, representing the percentage of energy that is saved due to the use of the solar system, from eq 5. f ) Q load - Q aux Q load (5) Results of the thermal analysis form the basis for the economic as well as the environmental analysis. VOL. 43, NO. 15, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

3 TABLE 1. Ambient Weather Data for the City of Phoenix month solar insolation (kwh/m 2 /day) (15) ambient temperature ( C) (16) water temperature ( C) (9) January February March April May June July August September October November December Economic Analysis. The results of the thermal performance give the baseline energy that still needs to be supplied by an electric or natural gas hot water system to meet the hot water demand. In this model all of the capital costs are paid in the first year and no consideration of financing for the system is considered. The capital costs for solar collectors are usually written as the combination of the area based costs and the area independent costs. C s ) C A A + C f (6) where C f is the area independent costs, and C A is the area dependent costs. The area independent cost for both solar collectors is taken to be $200 (7). The area dependent costs are estimated to be $180/m 2 based on a solar collector manufacturing survey (17). Because of the nature of the nanofluid solar collector, which eliminates a large portion of the copper used in the system, the area based cost is reduced based on a scaling of the overall percentage weight of the collector that uses copper. This cost is offset by the cost of the graphite nanoparticles, currently $3/g (18). The maintenance cost (C m ) is taken to be 1% of the capital cost with a 1% increase yearly for the system lifetime of 15 years (7). The nanofluid collector is expected to have the same lifetime as the conventional solar collector since it utilizes the same material technologies as a conventional collector. Although the lifetime is expected to stay the same as a conventional collector, included in the analysis is the additional cost and of having to refill the collector with a new nanofluid solution once during the lifetime. The total cost is then: C ) C s + C m (7) To determine the amount saved due to the operation of the solar collectors, the energy flow per day is used in conjunction with 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 to a solar collection system to heat water is the reduction in emissions resulting from the combustion of fossil fuels to generate the energy to heat the water. Using the results of the thermal performance analysis, which gives the energy flows on a daily and monthly basis, the amount of emissions offset by the use of a solar collector can be determined based on the amount of energy offset (Q load - Q aux ) and the emissions profile for electricity generation for Arizona. N P x,offset ) j)1 A j,x B j,x (Q load - Q aux ) (8) TABLE 2. Electricity Generation by Fuel Type and Primary Emissions Mix for Arizona (20) fuel %of electricity generated where A is the amount of pollutant (in kg) per MJ of energy generated, B is the percentage of energy generated from a specific fuel type, j is the fuel type, and x is the pollutant type. The distribution of electricity from various fuel types and the key pollutants generated for the state of Arizona is shown in Table 2. In addition to this consideration a complete environmental analysis should also include the impact on various environment receptors from the additional phases of distribution, maintenance, and postconsumer use. One impact category to evaluate during each of these phases is to consider the in each phase. Tsillingiridis et al. (9) recently showed that the majority of the solar collector comes from the manufacturing operation. Based on these results only the manufacturing process is addressed in the analysis the other phases will be discussed later. To assess the of each solar collector type the main materials and material mass must be known as well as the associated with each of these materials. This study utilizes the index created by Alcorn (21) which assesses the in various common construction materials, and these indices include the acquisition and transport of the materials. The amount of materials in the conventional collector is based on the specification provided by SunEarth for the Emprie collector (22). Similarly the amount of materials for the nanofluid-based collector is determined by starting with the conventional collector and redesigning the system to accommodate working as a direct absorption collector as shown in Figure 2. For the nanofluidbased collector the of the nanoparticles is also considered based on an average of results for energy requirements for carbon nanoparticle production (23). With the amount of each component material determined and the index, the and thus the emissions from the manufacturing of the collectors (from eq 8, where Q load - Q aux is replaced with the ) can be determined. The offset damage costs are calculated based on the damage cost factors for the three main pollutants studied: CO 2,NO x, and SO x (24). These offset damage costs are not factored into the economic analysis as they are not costs directly applicable to the collector owner. Results and Discussion carbon dioxide, CO 2 (kg/mj) sulfur oxides, SO x (kg/mj) nitrogen oxides, NO x (kg/mj) coal 38.70% petroleum 0.10% natural gas 31.50% nuclear 23.00% other (includes hydro and renewables) 6.70% The comparative analysis above addresses the thermal performance, economic impact, and environmental impact for conventional and nanofluid-based solar collectors. The thermal performance is the foundation for the rest of the study as it provides the monthly variation in energy needed to supplement the solar collectors. This energy flow when compared with the base load forms the basis for determining cost 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-based solar collector provides a higher solar fraction and lower ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 15, 2009

4 FIGURE 3. Monthly energy flow and solar fraction (X-Electric water heater, triangles: nanofluid solar collector, squares: conventional solar collector). auxiliary energy requirement than a conventional collector for most months. During the summer months, May- September, both collectors provide all the energy needed to meet the demand load. The demand load is also significantly reduced in the summer months due to the increased incoming water temperatures. The annual solar fraction for the conventional collector and nanofluid-based solar collector is 80% and 85%, respectively. With the thermal analysis the capital costs, maintenance costs, and offset fuel savings cost can all be calculated based on the current prices for electricity and natural gas as shown in Table 3. Due to the current market cost of the nanoparticles at $3/gram, the nanofluid-based solar collector has an additional $120 in capital cost and an additional $20 in maintenance costs. Because of the increased efficiency and higher annual solar fraction of the nanofluid-based solar collector, the fuel cost savings per year, for both electricity and natural gas, is greater than that of the conventional solar collector. The payback period for the conventional collector is less than the nanofluid collector primarily due to the increased capital cost of the nanoparticles needed for the nanofluidbased collector. The payback period for both collectors is significantly longer when a natural gas water heater is used due to the reduced cost of the fuel although data for the Phoenix area and state show that the majority of utility customers (60%) use electric water heaters over natural gas water heaters (3) due to limited natural gas infrastructure within the state. The total life cycle savings, when using electricity, are $3,358 and $3,352 for the conventional solar and nanofluid-based solar collector respectively. When using natural gas for a fuel the savings are reduced substantially to $511 and $413 for the conventional solar and nanofluidbased solar collector, respectively. The results show that from an economic perspective there is very little difference between the two types of solar collectors, although both options can provide substantial savings over the life of the collector. In TABLE 3. Economic Comparisons for Conventional and Nanofluid-Based Solar Collectors conventional solar collector ($) nanofluid solar collector ($) capital costs independent costs area based costs nanoparticles total capital (one time cost) total maintenance (for 15 year life) total costs electricity cost savings per year years until electricity savings ) costs natural gas cost savings per year years until natural gas savings ) costs electricity price November-March (per kwh) May-October (per kwh) daily service charge gas price rate (per therm) monthly service charge addition, any drop in the price of nanoparticles, which is to be expected as they become more widely used and produced, would result in further savings with the nanofluid based solar collector. The two collectors shown in Figure 2 differ in some of the major materials used in their construction, which has an impact both on the collector weight and. Table 4 presents the amount of materials used in the two collectors as well as the for each material and the collector total. VOL. 43, NO. 15, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

5 TABLE 4. Embodied Energy Comparisons for Conventional and Nanofluid-Based Solar Collectors description index (MJ/kg) conventional solar collector mass (kg) nanofluid-based solar collector content (MJ) mass (kg) (MJ) insulation polyester fiberglass glass copper pipe collector manifold aluminum extrusion aluminum backplate steel backplate sealant black paint casing paint screws copper absorber nanoparticles thermal fluid conversion rate (27%) total TABLE 5. Embodied Energy Comparisons for Conventional and Nanofluid-Based Solar Collectors for Total Installation description index (MJ/kg) conventional solar collector mass (kg) content (MJ) nanofluid-based solar collector mass (kg) embodied energy (MJ) collector copper pipe pipe insulation steel frame total installation (3% of total) grand total TABLE 6. Embodied Energy Emissions from a Solar Collector and Consumer Phase Operational Energy emissions pollution from solar collector savings of solar collector conventional (kg) nanofluid-based (kg) conventional (kg/year) nanofluid-based (kg/year) carbon dioxide (CO 2 ) sulfur oxides (SO x ) nitrogen oxides (NO x ) As shown in Table 4 the reduction in copper material in the nanofluid based solar collector, even when supplemented with nanoparticles, glass, and steel, still results in a reduction of over 200 MJ when compared to the conventional collector. The additional glass and steel, though, results in an 11 kg weight increase for the nanofluid-based solar collector. The results from Table 5 represent all of the embodied energy 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 embodied energy assessment the pollution created in the manufacturing of the collector as well as the pollution offset (savings) by the operation of the collector can be examined. The results of this analysis are presented in Table 6, with the savings based on a 60/40 distribution of electric/natural gas backup. As shown in Table 6, the manufacturing of the nanofluidbased solar collector results in 34 kg less CO 2 emissions while operationally it saves 50 kg/year when compared to a conventional solar collector. The differences between the remaining emissions, SO x and NO x, are of a much smaller magnitude. Over the 15-year expected lifetime of the solar collectors, the nanofluid-based solar collector would offset more than 740 kg of CO 2 in comparison to a conventional collector and kg of CO 2 when compared to traditional water heaters. Finally, using the results of Spadaro and Rabl (24) the offset damage costs from the pollution savings of the collectors can be established. The total damage cost avoided per year is $88 and $91 for the conventional and nanofluidbased solar collectors, respectively. Table 7 shows that the avoided damage cost per year is $3 higher with the nanofluid-based solar collector representing a total damage cost avoided over 15 years of more than $1,300. This savings is not directly related to the installer of the solar collector, the utility, or the state and federal government but distributed throughout all of these. If this savings was completely attributed to the government it would represent 3 times the amount invested through the rebate system. The damage costs capture the impact of the pollutants ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 15, 2009

6 TABLE 7. Yearly Avoided Damage Costs for Conventional and Nanofluid-Based Solar Collectors from the operation and manufacturing of the collector but one important environmental consideration that is not included in any of the economic or environmental impacts is the nanoparticles. Much work has been devoted to understanding the impact of nanoparticles on the environment, especially ecological systems and human health (25-28). Many of these studies focus on a specific particle and health/environmental impact due to the complex nature regarding their potential impact. In addition nearly all of these studies focus on nanoparticles that are not suspended in the liquid, which greatly reduces the risk for inhalation, but could present a potential problem if introduced into the water cycle. More recent work has shown that carbon particles suspended in aqueous solutions form large aggregates which would settle out of solution which could offset exposure risks (29). Due to its large number of homes (1,375,000 (11)) and plentiful solar radiation (15) Phoenix, AZ is an ideal place for utilizing solar thermal energy for domestic hot water applications. Based on incentive data for one of the region s two public utilities, Salt River Project (SRP), the number of solar collectors for hot water heating is increasing from 51 installed in 2005 to 260 in Based on these increasing but still relatively small numbers, we would expect that less than 5% of households in the area have solar collectors installed. If 50% penetration of nanofluid-based solar collectors could be achieved in Phoenix, AZ over 1,000,000 tons of CO 2,500tofSO x, and 1000 t of NO x could be offset from the use of solar energy instead of electricity and natural gas to heat water per year. Additionally, based on the ratio of offset damage costs to government rebate incentive, currently at a level of 3, it is demonstrated that additional government incentives could still be provided without losing money on the investment but potentially leading to increased penetration of solar collectors in Phoenix, Arizona. Acknowledgments We thank Mr. Tom Hines of Arizona Public Service and Mr. Don Pelley of Salt River Project for their assistance in obtaining data on solar collector installations in the Phoenix region as well as the distribution on conventional hot water heaters. This work was partially supported by the National Center of Excellence on SMART Innovations for Urban Climate + Energy ( Literature Cited cost ($/kg) conventional solar collector damage costs avoided ($) nanofluid based solar collector carbon dioxide (CO 2 ) sulfur oxides (SO x ) nitrogen oxides (NO x ) total (1) Otanicar, T.; Phelan, P. E.; Rosengarten, G.; Prasher, R. S. 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