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					Energy Conversion and Management 45 (2004) 3075–3092
www.elsevier.com/locate/enconman

Environmental benefits of domestic solar energy systems
Soteris A. Kalogirou
*

Department of Mechanical Engineering, Higher Technical Institute, P.O. Box 20423, Nicosia 2152, Cyprus Received 1 May 2003; received in revised form 10 October 2003; accepted 26 December 2003 Available online 27 February 2004

Abstract All nations of the world depend on fossil fuels for their energy needs. However the obligation to reduce CO2 and other gaseous emissions in order to be in conformity with the Kyoto agreement is the reason behind which countries turn to non-polluting renewable energy sources. In this paper the pollution caused by the burning of fossil fuels is initially presented followed by a study on the environmental protection offered by the two most widely used renewable energy systems, i.e. solar water heating and solar space heating. The results presented in this paper show that by using solar energy, considerable amounts of greenhouse polluting gasses are avoided. For the case of a domestic water heating system, the saving, compared to a conventional system, is about 80% with electricity or Diesel backup and is about 75% with both electricity and Diesel backup. In the case of space heating and hot water system the saving is about 40%. It should be noted, however, that in the latter, much greater quantities of pollutant gasses are avoided. Additionally, all systems investigated give positive and very promising financial characteristics. With respect to life cycle assessment of the systems, the energy spent for manufacture and installation of the solar systems is recouped in about 1.2 years, whereas the payback time with respect to emissions produced from the embodied energy required for the manufacture and installation of the systems varies from a few months to 9.5 years according to the fuel and the particular pollutant considered. Moreover, due to the higher solar contribution, solar water heating systems have much shorter payback times than solar space heating systems. It can, therefore, be concluded that solar energy systems offer significant protection to the environment and should be employed whenever possible in order to achieve a sustainable future. Ó 2004 Elsevier Ltd. All rights reserved.
Keywords: Solar water heating; Solar space heating; Environmental impact; Embodied energy

*

Tel.: +357-22-406-466; fax: +357-22-494-953. E-mail address: skalogir@spidernet.com.cy (S.A. Kalogirou).

0196-8904/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2003.12.019

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1. Introduction Energy is considered a prime agent in the generation of wealth and a significant factor in economic development. The importance of energy in economic development is recognized universally, and historical data verify that there is a strong relationship between the availability of energy and economic activity. Although in the early seventies, after the oil crises, the concern was on the cost of energy, during the past two decades, the risk and reality of environmental degradation have become more apparent. The growing evidence of environmental problems is due to a combination of several factors, since the environmental impact of human activities has grown dramatically. This is due to the increase of the world population, energy consumption and industrial activities. Achieving solutions to the environmental problems that humanity faces today requires long term potential actions for sustainable development. In this respect, renewable energy resources appear to be one of the most efficient and effective solutions. A few years ago, most environmental analysis and legal control instruments concentrated on conventional pollutants, such as sulphur dioxide (SO2 ), nitrogen oxides (NOx ), particulates and carbon monoxide (CO). Recently however, environmental concern has extended to the control of hazardous air pollutants, which are usually toxic chemical substances that are harmful even in small doses, as well as to other globally significant pollutants such as carbon dioxide (CO2 ). Additionally, developments in industrial processes and structures have led to new environmental problems. A detailed description of these gaseous and particulate pollutants and their impacts on the environment and human life is presented by Dincer [1,2]. One of the most widely accepted definitions of sustainable development is: ‘‘development that meets the needs of the present without compromising the ability of future generations to meet their own needs’’. There are many factors that can help to achieve sustainable development. Today, one of the main factors that must be considered in discussions of sustainable development is energy, and one of the most important issues is the requirement for a supply of energy that is fully sustainable [3,4]. A secure supply of energy is generally agreed to be a necessary but not a sufficient requirement for development within a society. Furthermore, for a sustainable development within a society, it is required that a sustainable supply of energy and effective and efficient utilization of energy resources be secured. Such a supply in the long term should be readily available at reasonable cost, be sustainable and be able to be utilized for all the required tasks without causing negative societal impacts. This is why there is a close connection between renewable sources of energy and sustainable development. Today the world daily oil consumption is 76 million barrels. Despite the well known consequences of fossil fuel combustion on the environment, this is expected to increase to 123 million barrels per day by the year 2025 [5]. In developed countries, energy consumption in the building sector represents a major part of the total energy budget. In the European Union, this is approximately equal to 40% of the total energy consumption [6]. Most of this amount is spent for hot water production and space heating. One way to reduce this amount of energy is to employ solar energy. The principal objective of this paper is to discuss the environmental impacts of energy utilization and to analyze the environmental benefits resulting from the use of solar water and space heating systems. Additionally, the amount of pollution saved because of the use of solar energy against the pollution caused for the manufacture of the systems is examined.

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2. Environmental impact of conventional energy sources Pollution depends on energy consumption. There are a large number of factors that are significant in the determination of the future level of energy consumption and production. Such factors include population growth, economic performance, consumer tastes and technological developments. Furthermore, governmental policies concerning energy and developments in the world energy markets will certainly play a key role in the future level and pattern of energy production and consumption [7]. During the last two decades, environmental considerations have been given increasing attention by energy industries and the public, and the concept that due to irrational use, consumers share responsibility for pollution and its cost has been increasingly accepted. In some cases, the prices of some energy resources have increased over the last one to two decades in order to account for environmental costs [7]. Another parameter to be considered is world population. This is expected to double by the middle of this century, and as economic development will certainly continue to grow, the global demand for energy is expected to increase. At the same time, concern regarding energy related environmental pollution will increase. Problems associated with energy supply and use are related not only to global warming but also to other environmental impacts such as air pollution, acid precipitation, ozone depletion, forest destruction and emission of radioactive substances [7]. Today, much evidence exists which suggests the future of our planet and of the generations to come will be negatively impacted if humans keep degrading the environment. A summary of the pollutants and their environmental impact is tabulated in Table 1 [8]. The three major environmental problems that are internationally known are analyzed in more detail below. 2.1. Acid rain This is a form of pollution in which SO2 and NOx produced by the combustion of fossil fuels, such as industrial boilers and transportation vehicles, are transported over great distances through the atmosphere and deposited via precipitation on the earth, causing damage to ecosystems that
Table 1 Main gaseous pollutants and their impact on the environment [8] Gaseous pollutant Carbon monoxide (CO) and carbon dioxide (CO2 ) Methane (CH4 ) Nitric oxide (NO) and nitrogen dioxide (NO2 ) Nitrous oxide (N2 O) Sulfur dioxide (SO2 ) Chlorofluorocarbons (CFCs) Ozone (O3 ) Greenhouse effect + + Ozone depletion +/) +/) +/) + + ) + + +/) + + + + Acid rain Smog

Note: + stands for positive contribution, ) stands for variation with conditions and chemistry.

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are exceedingly vulnerable to excessive acidity. Therefore, it is obvious that the solution to the issue of acid rain deposition requires an appropriate control of SO2 and NOx pollutants. These pollutants cause both regional and transboundary problems of acid precipitation. Recently, attention is also given to other substances, such as volatile organic compounds (VOCs), chlorides, ozone and trace metals that may participate in a complex set of chemical transformations in the atmosphere, resulting in acid precipitation and the formation of other regional air pollutants. A number of evidences that show the damages of acid precipitation are reported by Dincer and Rosen [4]. It is well known that some energy related activities are major sources of acid precipitation. Additionally, VOCs are generated by a variety of sources and comprise a large number of diverse compounds. Obviously, the more energy we spend the more we contribute to acid precipitation, and therefore, the easiest way to reduce acid precipitation is by reducing energy consumption. Other possible methods to reduce this type of pollution include the cleaning of coal before combustion, with respect to electricity generation, and the use of three way catalytic converters, with respect to transport vehicles [7]. 2.2. Ozone layer depletion The ozone present in the stratosphere at altitudes between 12 and 25 km plays a natural equilibrium maintaining role for the earth through absorption of ultraviolet (UV) radiation (240– 320 nm) and absorption of infrared radiation [1]. A global environmental problem is the depletion of the stratospheric ozone layer, which is caused by the emissions of CFCs, halons (chlorinated and brominated organic compounds) and NOx . Ozone depletion can lead to increased levels of damaging UV radiation reaching the ground, causing increased rates of skin cancer and eye damage to humans and is harmful to many biological species. It should be noted that energy related activities are only partially (directly or indirectly) responsible for the emissions which lead to stratospheric ozone depletion. The most significant factor in ozone depletion has been CFCs, which are mainly used in air conditioning and refrigerating equipment as refrigerants, and NOx emissions, which are produced by fossil fuel and biomass combustion processes, natural denitrification and nitrogen fertilizers. In 1998, the size of the ozone hole over Antarctica was 25 million km2 . It was about 3 million km2 in 1993 [5]. Researchers expect the Antarctic ozone hole to remain severe in the next 10–20 years, followed by a period of slow healing. Full recovery is predicted to occur in 2050; however, the rate of recovery is affected by climate change [7]. 2.3. Global climate change The term greenhouse effect has generally been used for the role of the whole atmosphere (mainly water vapour and clouds) in keeping the surface of the earth warm. Recently however, it has been increasingly associated with the contribution of CO2 , which is estimated to contribute about 50% to the anthropogenic greenhouse effect. Additionally, several other gasses, such as CH4 , CFCs, halons, N2 O, ozone and peroxyacetylnitrate (also called greenhouse gasses), produced by industrial and domestic activities can also contribute to this effect, resulting in a rise of the earthÕs temperature [7]. Increasing atmospheric concentrations of greenhouse gasses increase the amount

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of heat trapped (or decrease the heat radiated from the earthÕs surface), thereby raising the surface temperature of the earth. According to Colonbo [9], the earthÕs surface temperature has increased by about 0.6 °C over the last century, and as a consequence, the sea level is estimated to have risen by perhaps 20 cm. These changes can have a wide range of effects on human activities all over the world. The role of various greenhouse gasses is summarized in Ref. [4]. Humans contribute through many of their economic and other activities to the increase in the atmospheric concentrations of various greenhouse gasses. For example, CO2 releases from fossil fuel combustion, methane emissions from increased human activity and CFC releases all contribute to the greenhouse effect. Scientists today agree that there is a cause and effect relationship between the emissions of greenhouse gasses and global warming. Furthermore, predictions show that if atmospheric concentrations of greenhouse gasses, mainly due to fossil fuels combustion, continue to increase at the present rates, the earthÕs temperature may increase by another 2–4 °C in the next century. If this prediction is realized, the sea level could rise by between 30 and 60 cm before the end of this century [9]. The impacts of such sea level increase could easily be understood and include flooding of coastal settlements, displacement of fertile zones for agriculture toward higher latitudes and decrease in the availability of fresh water for irrigation and other essential uses. Thus, such consequences could put in danger the survival of entire populations.

3. Renewable energy technologies Renewable energy technologies produce marketable energy by converting natural phenomena into useful forms of energy. These technologies use the sunÕs energy and its direct and indirect effects on the earth (solar radiation, wind and falling water), gravitational forces (tides) and the heat of the earthÕs core (geothermal) as the resources from which energy is produced. These resources have massive energy potential, however, they are generally diffuse and not fully accessible, and most of them are intermittent and have distinct regional variabilities. These characteristics give rise to difficult, but solvable, technical and economical challenges. Nowadays, significant progress is made by improving the collection and conversion efficiencies, lowering the initial and maintenance costs and increasing the reliability and applicability. Several potential solutions to the current environmental problems associated with harmful pollutant emissions from the burning of fossil fuels have evolved, including renewable energy and energy conservation technologies. Many countries consider today solar, wind and other renewable energy technologies as the key to a clean energy future. Worldwide research and development in the field of renewable energy resources and systems have been conducted during the last two decades. Energy conversion systems that are based on renewable energy technologies appear to be cost effective compared to the projected high cost of oil. Examples are given in Refs. [10,11]. Furthermore, renewable energy systems can have a beneficial impact on the environmental, economic and political issues of the world. The benefits arising from the installation and operation of renewable energy systems can be distinguished into three categories: energy saving, generation of new working posts and decrease of environmental pollution [12]. The energy saving benefit derives from the reduction in consumption of electricity and/or Diesel that are used conventionally to provide energy. This benefit can be directly translated into

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monetary units according to the corresponding production or avoided capital expenditure for the purchase of imported fossil fuels. Additionally, another point that makes countries interested is the economic development potential of exporting renewable energy technology expertise. One area that seems to be of considerable importance in many countries is the ability of renewable energy technologies to generate jobs as a means of economic development to a country. The penetration of a new technology leads to the development of new production activities, contributing to the production, market distribution and operation of the pertinent equipment. Specifically in the case of solar energy, the collector job creation mainly relates to the construction and installation of the collectors. The latter is a decentralized process, since it requires the installation of equipment in every building or every individual consumer. The most important benefit of renewable energy systems is the decrease of environmental pollution. This is achieved by reduction of the air emissions due to the substitution of electricity and conventional fuels. The most important effects of air pollutants on the human and natural environment are their impact on public health, on agriculture, on buildings and historical monuments and on forests and ecosystems [12]. It is relatively simple to measure the financial impact of these effects when they relate to tradable goods, such as agricultural crops, however, when it comes to non-tradable goods, like human health and ecosystems, things become more complicated. It should be noted that the level of environmental impact and, therefore, the social pollution cost largely depends on the geographical location of the emission sources. Contrary to the conventional air pollutants, the social cost of CO2 does not vary with the geographical characteristics of the source, as each unit of CO2 contributes equally to the climate change thread and the resulting cost [12]. In this paper, emphasis is given to solar energy systems and in particular, to solar water heating and space heating systems. These are very popular systems used extensively in many countries with good sunshine potential, such as the Mediterranean countries.

4. Solar systems considered Two types of solar systems are considered in this study; a solar water heating (SWH) system and a solar space and water heating (SSWH) system. Flat plate collectors are used in both systems, which are, by far, the most used type of collectors. The instantaneous efficiency of the collector considered is given by the equation:    2 DT DT À 0:06 ; ð1Þ n ¼ 0:792 À 6:65 I I where DT is the temperature difference between the collector inlet (Ti ) and ambient (Ta ) temperatures, i.e. DT ¼ ðTi À Ta Þ and I is the global solar radiation. A schematic diagram of the solar water heating system is shown in Fig. 1 and a schematic of the solar central heating and water heating system is shown in Fig. 2. In both cases, active solar systems are used, i.e. a pump is employed to transfer the solar thermal energy to storage, which is operated by means of a differential thermostat. The thermostat compares the temperature at the outlet of the solar collectors and the storage tank and whenever the collector temperature is higher than the storage temperature by more than 8 °C, the pump is

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Hot water Mixing supply Power supply
device

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Relief valve Collector array 2-panels
Storage tank (160 l)

Auxiliary heater

Differential thermostat

Burner

Solar pump

Make-up water

Fig. 1. Schematic diagram of the solar water heating (SWH) system.

Relief valve Collector array 10-panels
150 liter

Hot water Mixing supply
device

Auxiliary heater

Burner Differential thermostat Storage tank (1500 l) House heating system

Solar pump

Make-up water

Fig. 2. Schematic diagram of the solar heating and water heating (SSWH) system.

switched on. It switches off whenever this difference is lower than 4 °C. The storage tank is well insulated to reduce thermal losses to the environment and is equipped with heat exchangers for both the solar system and the auxiliary system. In the case of SWH systems, the auxiliary can vary from electricity, Diesel or both, whereas in the case of SSWH systems, only Diesel is considered. In the cases where Diesel is considered, this is used in a central heating boiler, which supplies the energy for the heating needs of a house and is not used only as the solar system backup. No details of the house heating system are given in Fig. 2. What is of interest to note is that the water from the hot water storage tank is mixed with that returning from the heating appliances to provide the required temperature. The building considered has an insulated roof and walls and has a floor area of 150 m2 , and its solar system satisfies, in addition to the heating load (part), the hot water requirements of a 4 persons family. As can be seen from Fig. 2, a secondary hot water cylinder is used in the heating and hot water system installed within the primary tank (tank-in-tank system). The hot water storage tank of the system has a capacity of 1500 l, 150 l of which are used for hot water. The specifications of the various components of the two solar systems are shown in Table 2.

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Table 2 Specifications of the solar systems considered Parameter Type of system Collector area (m2 ) Collector slope (°) Storage capacity (l) Auxiliary capacity (kW) Heat exchanger Heat exchanger area (m2 ) House area (m2 ) Hot water demand (lt) House construction Domestic hot water system Active 3.8 (2-panels) 40 160 3 Internal 3.6 – 120 (4 persons) – Space heating + hot water system Active 19 (10-panels) 31 1500 (150 l for SWH) 7 Internal 10 150 120 (4 persons) Insulated walls and roof

Traditional hot water systems comprise a hot water cylinder powered either by electricity or by Diesel oil through the central heating boiler. Therefore, the extra equipment required for the solar system are the solar collectors, piping to connect the collectors with the storage tank and differential thermostat. Central heating systems are usually of the direct type, i.e. there is no storage. Therefore, the extra equipment required by the solar system are the solar collectors, storage tank, piping and controls.

5. Thermal and economic analysis of solar systems All systems are simulated with the Polysun [13] program (version 3.3.5g) with the weather conditions of Nicosia, Cyprus. The monthly solar radiation and mean ambient air temperature for Nicosia, as derived from the typical meteorological year [14], are shown in Table 3. The program provides dynamical annual simulations of solar thermal systems and helps to optimize them. It operates with dynamic time steps from 1 s to 1 h, thus simulation can be more stable and exact. The program is user friendly and the graphic user interface permits a comfortable and clear input of all system parameters. All aspects of the simulation are based on physical models that work without empirical correlation terms. In addition, the program performs economic viability analysis and ecological balance, which includes emissions from the eight most significant greenhouse gasses, thus the emissions of systems working only with conventional fuel and systems employing solar energy can be compared. The program Polysun was validated by Gantner [15] and it was found that the mean accuracy of the program is within 5–10%. The optimum slope of the solar collectors, shown in Table 2, was calculated with a special routine of the Polysun program. Three types of solar water heating systems were considered, one with electric heating backup, one with a combination of electricity and boiler backup and one with only a boiler backup. In houses where a central heating system exists, the two last options are preferred as the price of Diesel is much lower than that of electricity and the owners prefer to use their central heating boiler to produce hot water, as a solar system backup, irrespective of the requirement for heating. The second case where both electricity and boiler are used is a case that occurs

S.A. Kalogirou / Energy Conversion and Management 45 (2004) 3075–3092 Table 3 Mean monthly solar radiation and ambient air temperature for Nicosia Month January February March April May June July August September October November December Year Horizontal global radiation (kW h/m2 ) 74.8 97.9 145.3 172.3 212.9 227.4 229.0 206.9 168.9 134.7 95.8 70.0 1840.1

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Mean ambient air temperature (°C) 10.7 11.3 13.4 16.7 19.8 23.3 25.9 26.5 24.7 21.0 16.4 12.5 18.5

frequently when in intermediate periods with no heating requirement, the users may prefer to use electricity as a backup instead of using the central heating boiler. The annual energy balance and the monthly solar contribution of the systems considered are shown in Tables 4 and 5, respectively. As can be seen from Table 5, all variations of the domestic hot water systems considered cover all the requirements during the summer time and a large percentage during the winter time. The annual figure is also high. In the case of space heating, as the system is oversized for the case of summer where no heating load is required, the solar contribution is 100%, i.e. all the hot water needs are covered by solar energy. In this case, a large number of collectors can be discontinued or shaded to avoid overheating. It should be noted that in domestic hot water systems, by adjusting slightly the consumption profile, contributions of 100% could be obtained in the months from May to October, which is what actually happens in practice. The program, however, considers a standard consumption throughout all months and that is why values slightly below 100% are

Table 4 Annual energy balance of the systems considered Parameter Solar water heating (SWH) Electricity backup Solar system yield (kW h) Total auxiliary energy (kW h) Hot water demand (kW h) Space heating demand (kW h) Solar fraction 2046.4 269.3 1780.0 – 88.4 Electricity and Diesel backup 2054.7 211.4 (Diesel) 40.9 (electricity) 1780.0 – 89.1 Diesel backup 2063.1 238.5 1780.0 – 89.6 Space heating + SWH Diesel backup 6205.1 4616.0 2034.3 6768.4 57.3

Note: Solar fraction ¼ solar yield/(solar yield + auxiliary).

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Table 5 Monthly solar contribution of the systems considered Month Solar water heating (SWH) Electricity backup January February March April May June July August September October November December Year 61.1 75.8 86.5 90.8 96.0 96.5 97.0 98.8 98.5 94.9 87.3 72.0 88.4 Electricity + Diesel backup 61.8 76.5 88.6 90.3 96.9 96.5 97.0 98.8 98.5 95.0 89.5 73.5 89.1 Diesel backup 61.7 76.5 88.7 91.2 98.2 97.3 98.5 99.4 99.9 94.9 89.0 74.5 89.6 Space heating + SWH Diesel backup 22.9 36.0 60.0 86.5 100.0 100.0 100.0 100.0 100.0 100.0 72.6 32.0 57.3

Note: All values are expressed in percentage.

given. All the systems presented here are optimized with respect to their economic viability, i.e. lowest pay back time. The results of the economic analysis are shown in Table 6. These were obtained by using the current fuel and electricity rates, a 20 years period [16] and a market discount rate of 4%. No subsidies were considered. As can be seen, in all cases, the solar systems give much lower specific energy costs than conventional systems, and the pay back times are reasonable especially in the cases of solar water heating (SWH) systems. It should be noted that the cost of the boiler and other necessary auxiliary equipment is not taken into account in the economic analysis, i.e. only the cost of the extra equipment required for the solar installation is considered.

Table 6 Results of the economic analysis Parameter System (backup fuel) SWH (electricity) Total system cost (solar) Annual fuel savings (C£) Pay-back time (years) Energy costs Solar energy only (C£/kW h) Solar + conventional (C£/kW h) Conventional (C£/kW h) Note: 1C£ ¼ 1.84US$ (January 2003). 550 95 4.2 0.0253 0.0332 0.105 SWH (electricity + Diesel) 550 71 5.6 0.0252 0.0301 0.0739 SWH (Diesel) 550 74 5.4 0.0251 0.0293 0.0739 Space heating + SWH (Diesel) 2690 228 9.8 0.0361 0.0623 0.0864

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6. Environmental benefits of solar energy systems To investigate the environmental benefit of utilizing solar energy instead of conventional sources of energy, the different emissions resulting from the solar system operation are estimated and compared to those of a conventional fuel system. These are given directly from the software employed, which is unique in this respect [13]. The emissions reported are those that are responsible for the most important environmental problems as outlined in the previous sections. The environmental interventions are expressed in physical units of the emitted substances per year. The quantities of the emissions depend on the solar collector size and the required auxiliary energy and are compared to a non-solar system, which is using conventional fuel. The environmental analysis of the above systems, which includes the different pollutants as calculated by the program is tabulated in Tables 7–10. In the tables, the eight most important greenhouse gasses are compared. For the case of electricity backup (Table 7), Polysun considers a mixture of European power stations (coal based, nuclear, hydroelectric, etc.) in order to estimate the emissions of the conventional system [13]. As can be seen in all cases, by using solar energy instead of conventional fuel, a very large amount of pollutants are avoided. In the case of SWH systems, the amount of emissions depends on the type of fuel used as auxiliary. The percentage saving obtained in the cases where electricity or Diesel backup is used is about 80% whereas in the case that both electricity and Diesel are employed, this is 74.2%. It should be noted, however, that the quantities of emissions in all these cases are completely different, and the proximity of the percentage numbers obtained is due to the generation efficiency of each system. Electrical energy is produced at a maximum efficiency of about 35%, whereas in the case of Diesel backup, a boiler efficiency of 85% is considered. The size of SWH system considered is the usual type encountered in Cyprus but operated in thermosyphon mode, i.e., without pump and differential thermostat. Cyprus began manufacturing solar water heaters in the early sixties. Today, more than 93% of all houses have solar water heating systems installed and operating. The total number of systems is equal to 190,000 units. In fact, the number of units in operation today corresponds to one heater for every 3.7 people in the island, which is a
Table 7 Environmental impact of the SWH system with electricity backup Emissions Carbon dioxide (CO2 ) Carbon monoxide (CO) Nitrogen oxides (NOx ) Nitrous oxide (N2 O) Methane (CH4 ) Hydrocarbons Sulfur dioxide (SO2 ) Dust Savings in GHG Units tons/year g/year g/year g/year g/year g/year g/year g/year % Conventional 1.982 496 74 7 12 50 743 248 – Solar system 0.40 100 15 2 3 10 150 50 – Savings (%) 79.8 79.8 79.8 79.8 79.8 79.8 79.8 79.8 79.8 Equation y y y y y y y y – ¼ 0:0009x þ 0:1635 ¼ 0:2173x þ 40:86 ¼ 0:0328x þ 5:9127 ¼ 0:0026x þ 1:3075 ¼ 0:0054x þ 0:9851 ¼ 0:0219x þ 4:0365 ¼ 0:3266x þ 61:042 ¼ 0:1093x þ 20:182 R2 -value 0.9994 0.9995 0.9996 0.9892 0.9988 0.9992 0.9994 0.9992 –

Note: In equations, y represents pollutant emission and x represents system annual energy requirements in kW h.

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Table 8 Environmental impact of the SWH system with both electricity and Diesel backup Emissions Carbon dioxide (CO2 ) Carbon monoxide (CO) Nitrogen oxides (NOx ) Nitrous oxide (N2 O) Methane (CH4 ) Hydrocarbons Sulfur dioxide (SO2 ) Dust Savings in GHG Units tons/year g/year g/year g/year g/year g/year g/year g/year % Conventional 0.964 796 728 7 15 60 770 90 – Solar system 0.299 280 228 1 3 11 148 51 – Savings (%) 69.0 64.9 68.7 80.2 81.5 81.9 80.9 44.1 74.2

Table 9 Environmental impact of the SWH system with Diesel backup Emissions Carbon dioxide (CO2 ) Carbon monoxide (CO) Nitrogen oxides (NOx ) Nitrous oxide (N2 O) Methane (CH4 ) Hydrocarbons Sulfur dioxide (SO2 ) Dust Savings in GHG Units tons/year g/year g/year g/year g/year g/year g/year g/year % Conventional 0.766 1615 1615 7 15 62 775 136 – Solar system 0.259 363 324 1 3 11 145 52 – Savings (%) 66.3 77.5 80.0 80.7 82.8 82.8 81.3 61.7 80.0 Equation y y y y y y y y – ¼ 0:0004x þ 0:1662 ¼ 635:34 lnðxÞ À 307:7 ¼ 626:96 lnðxÞ À 3065:2 ¼ 0:0033x þ 0:1759 ¼ 0:0069x þ 1:1017 ¼ 0:0267x þ 4:4069 ¼ 0:3231x þ 67:577 ¼ 61:172 lnðxÞ À 278:73 R2 -value 0.9970 0.9973 0.9971 0.9999 0.9962 0.9999 0.9987 0.9964 –

Note: In equations, y represents pollutant emission and x represents system annual energy requirements in kW h.

world record [17]. Therefore, for the Cyprus case, if the above numbers are considered, one can understand the magnitude of the environmental pollution reduction per year, just for water heating. A smaller percentage saving is obtained in the case of space heating but the absolute quantities of emissions saved are much bigger. This leads to the conclusion that solar space heating should also be promoted as much as possible. It is believed that similar results can be obtained for other countries with a good solar resource. In the above tables, regression equations are given to enable the interested reader to estimate the emission from the systems according to the auxiliary energy required. These were obtained by running the program Polysun with a number of collector areas and recording the emissions. As the accuracy of these equations, represented by the R2 -value, is quite acceptable, these equations can be used with confidence to estimate the emissions of systems of similar configuration. The R2 value is known as the coefficient of determination. It indicates how closely the values estimated correspond to the actual data. An exception to the above is the case where both electricity and Diesel are considered (Table 8), as the program can give emissions for only one fuel at a time. In this case, the emissions are estimated by the ratio of the quantity of each fuel requirement as given in Table 4.

S.A. Kalogirou / Energy Conversion and Management 45 (2004) 3075–3092 Table 10 Environmental impact of the SSWH system with Diesel backup Emissions Carbon dioxide (CO2 ) Carbon monoxide (CO) Nitrogen oxides (NOx ) Nitrous oxide (N2 O) Methane (CH4 ) Hydrocarbons Sulfur dioxide (SO2 ) Dust Savings in GHG Units tons/year g/year g/year g/year g/year g/year g/year g/year % Conventional 3.406 3168 3168 32 69 276 3445 203 – Solar system 1.481 2384 2378 14 30 118 1476 194 – Savings (%) 56.5 24.7 24.9 57.1 57.2 57.2 57.1 4.6 39.5 Equation y y y y y y y y – ¼ 0:0003x ¼ 0:1293x þ 1787:1 ¼ 0:1273x þ 1789:7 ¼ 0:03x ¼ 0:0064x ¼ 0:0256x ¼ 0:3205x ¼ 0:0028x þ 181:07

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R2 -value 0.9999 1.0 1.0 0.9999 0.9994 1.0 1.0 0.9992 –

Note: In equations, y represents pollutant emission and x represents system annual energy requirements in kW h.

Electricity Dust Sulfur dioxide (SO2) Hydrocarbons Methane (CH4) Nitrous oxide (N2O) Nitrogen oxides (NOx) Carbon monoxide (CO) Carbon dioxide (CO2) 0 20

Electricity+diesel

Diesel

40 60 Percentage savings

80

100

Fig. 3. Comparison of the percentage emissions of the three SWH systems considered.

A comparison of the percentage savings of emissions obtained from the three solar water heating systems is shown in Fig. 3. As can be seen, considerable amounts of polluting gases are saved by all types of systems considered, which implies that the solar water heating is environmentally friendly irrespective of the backup fuel used or the mode of operation.

7. Pollution created from solar systems The negative environmental impact of solar energy systems includes land displacement and possible air and water pollution resulting from manufacturing, normal maintenance operations and demolition of the systems. However, land use is not a problem when collectors are mounted on the roof of a building, the maintenance required is minimal and the pollution caused by demolition is not greater than the pollution caused from demolition of a conventional system of the same capacity. The pollution created for the manufacture of the solar collectors is estimated

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by calculating the embodied energy invested in the manufacture and assembly of the collectors and estimating the pollution produced by this energy. Initially, the embodied energy of one solar collector panel, 1.9 m2 in area is determined. This is the same collector considered in the performance analysis of the systems. The analysis is based on the primary and intermediate embodied energy of the components and materials as illustrated in Fig. 4. In the present analysis, no allowance is made for unit packing, transportation and maintenance, as these have insignificant contributions compared to the total. The total embodied energy required to produce a complete flat plate collector is calculated using the primary and intermediate production stages. The primary stage is established from an assessment of the various materials used and their corresponding mass. Using the embodied energy index (MJ/kg) defined in Ref. [18], the material embodied energy content within the unit is determined. Table 11 summarizes the unit materials used and lists their corresponding mass and embodied energy content. The total embodied energy content for the production of one flat plate collector panel is calculated at 3540 MJ. This comprises the primary embodied energy of materials and the intermediate embodied energy, i.e. the amount of energy used in the production and assembly of the component parts during the construction stage and was determined through a stage by stage appraisal of the power sources used. Inherent within this intermediate stage is the fabrication of purchased components, like screws, glass and insulation. An analysis of the embodied energy content of a complete solar hot water system is shown in Table 12 and for the complete solar central heating system in Table 13. It should be noted that only the extra components of the solar system are considered in this analysis, as the other components are standard and are also present in the conventional system. Here, the objective is to compare the pollution created for the manufacture and installation of the solar systems against its

PRIMARY PRODUCTION Paint, sealant, coatings Primary raw materials extraction and production Glass Copper pipes

INTERMEDIATE PRODUCTION

Absorber Copper Sheet Insulation Casing Galvanized sheet Packing Transportation Installation Maintenance Demolition Disposal/recycling

Fig. 4. Factors considered in the calculation of embodied energy of a flat plate collector.

S.A. Kalogirou / Energy Conversion and Management 45 (2004) 3075–3092 Table 11 Embodied energy content of one flat plate collector 1.9 m2 in area Description 1.9 · 1 · 0.05 m insulation 1.9 · 1 · 0.005 m glass 2 m, 22 mm copper pipe 20 m, 15 mm copper pipe 2.3 · 1.3 · 0.005 m galvanized steel sheet 6 m sealant Black paint Casing paint 20 No. screws 1.9 · 1 · 0.003 m copper absorber Mass (kg) 6 13.4 2.4 12.4 11.7 0.6 0.3 0.9 0.00125 5 Embodied energy index (MJ/kg) 117 15.9 70.6 70.6 34.8 110 44 44 34.8 70.6 Embodied energy content (MJ) 702 213.3 169.5 875.5 408.4 66 13.2 39.6 Ignored 353 2840.5 284.1 415.4 3540

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Total Add 10% for contingencies Unit manufacture using a net to gross value of conversion rate of 27% Grand total

Table 12 Embodied energy content for the construction and installation of the complete solar hot water system Description 2 No. solar panels 4 m, 22 mm copper pipe 4 m, pipe insulation Steel frame Total Installation Grand total Mass (kg) – 3.8 1 30 Embodied energy index (MJ/kg) – 70.6 120 34.8 Embodied energy content (MJ) 7080 268.3 120 1044 8512.3 187.7 8700

benefits due to the lower emissions realized during the operation of the systems. As can be seen, the total embodied energy for the complete system is 8700 and 82554 MJ for the two types of systems, respectively. For the life cycle assessment of the systems considered, the useful energy supplied by solar energy per year, shown in Table 4, is compared with the total embodied energy of the systems, shown in Tables 12 and 13. As can be seen, the total energy used in the manufacture and installation of the systems is recouped in about 1.2 years for both the solar water heating systems, and solar space heating systems, which is considered as very satisfactory. The emissions created from the total embodied energy for the two types of systems considered are presented in Tables 14 and 15. Additionally, these emissions are compared with the emissions saved because solar energy is used instead of auxiliary energy, according to the type of fuel used in the various cases of solar systems investigated, in order to estimate the payback period for each pollutant. In all cases, the emissions are estimated by considering that all embodied energy was produced from electricity. This is not quite correct, but electricity is chosen, as it is the most

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Table 13 Embodied energy content for the construction and installation of the complete solar central heating system Description 19 No. solar panels 24 m, 22 mm copper pipe 22 m, 28 mm copper pipe 20 m, 35 mm copper pipe 24 m, 22 mm pipe insulation 22 m, 28 mm pipe insulation 20 m, 35 mm pipe insulation Supports Total Installation Grand total Mass (kg) – 22.9 27.5 30.9 6 7.3 7.6 100 Embodied energy index (MJ/kg) – 70.6 70.6 70.6 120 120 120 34.8 Embodied energy content (MJ) 67260 1616.7 1941.5 2181.5 720 876 912 3480 78,987.7 3566.3 82,554

Table 14 Pollution created for the construction and installation of the solar hot water system and payback for the three types of backup fuels considered Emission Pollution created from solar system embodied energy 1.934 tons 483 g 72.5 g 7.3 g 12.1 g 48.3 g 725 g Savings and payback of solar system Electricity 1.582 (1.2) 396 (1.2) 59 (1.2) 5 (1.5) 9 (1.3) 40 (1.2) 593 (1.2) Electricity and Diesel 0.665 (2.9) 516 (0.9) 500 (0.15) 6 (1.2) 12 (1.0) 49 (1.0) 622 (1.2) Diesel 0.517 (3.7) 1252 (0.4) 1291 (0.06) 6 (1.2) 12 (1.0) 51 (0.9) 630 (1.2)

Carbon dioxide (CO2 ) Carbon monoxide (CO) Nitrogen oxides (NOx ) Nitrous oxide (N2 O) Methane (CH4 ) Hydrocarbons Sulfur dioxide (SO2 )

Note: (1) Number in parenthesis represent payback in years. (2) The units of savings are in g/year except carbon dioxide which is tons/year.

Table 15 Pollution created for the construction and installation of the solar central heating system and payback of system with Diesel backup Emission Carbon dioxide (CO2 ) Carbon monoxide (CO) Nitrogen oxides (NOx ) Nitrous oxide (N2 O) Methane (CH4 ) Hydrocarbons Sulfur dioxide (SO2 ) Pollution created from solar system embodied energy 18.34 tons 4586 g 688 g 68.8 g 114.7 g 458.6 g 6880 g Savings and payback of solar system with Diesel backup 1.925 (9.5) 784 (5.8) 790 (0.9) 18 (3.8) 39 (2.9) 158 (2.9) 1969 (3.5)

Note: (1) Number in parenthesis represent payback in years. (2) The units of savings are in g/year except carbon dioxide which is tons/year.

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polluting fuel. Therefore, it gives the worst possible results. As can be seen from Tables 14 and 15, the payback periods for the cases investigated vary from a few months to 9.5 years according to the fuel and the particular pollutant considered. It can also be seen from the results presented in Tables 14 and 15 that the payback times for the various pollutants emitted by SWH systems are much lower than those of SSWH systems. This is due to the fact that the solar contribution of SWH systems is much higher than that of SSWH, and thus, higher amounts of pollution gases are avoided (much lower auxiliary is required) because of the utilization of solar energy.

8. Conclusions In the present study, the environmental impact of energy utilization has been investigated and the potential benefits that solar systems offer are discussed in detail. From the analysis presented in this paper, it can be concluded that the environmental impact of any energy system is an important factor, and solar systems have the potential to reduce environmental pollution. Crucial to discussions on prevention of global climate change are thorough evaluations of the costs of reducing emissions. Many countries through several national and international institutes and agencies have started taking actions to reduce (or eliminate) the pollutant emissions and to attain a sustainable supply of energy [7]. One way to achieve this is by using solar energy as much as possible. This is in compliance with the agreement reached in the December 1997 International Kyoto Conference on climate change, where a list of fifteen concrete proposals emerged for the reduction of global greenhouse gas emissions. The list includes, among others, the use of solar energy [7]. Additionally, in this study, the environmental protection offered by the two most widely used renewable energy systems, i.e. solar water heating and solar space heating is presented. The results show that by using solar energy, considerable amounts of greenhouse polluting gasses are saved. For the case of domestic hater heating systems with electricity or Diesel backup, the saving, compared to a conventional system, is about 80%, whereas for the case that both electricity and Diesel backup are used, it is about 75%. For the case of solar space heating and hot water system, the saving is about 40%. It should be noted, however, that in the latter, much greater quantities of pollutant gasses are saved. Additionally, all systems investigated give positive and very promising financial characteristics. With respect to life cycle assessment of the systems, the energy spent for the manufacture and installation of the solar systems is recouped in about 1.2 years, whereas the payback time with respect to emissions produced from the embodied energy required for the manufacture and installation of the systems varies from a few months to 9.5 years according to the fuel and the particular pollutant considered. Moreover, due to the higher solar contribution, solar water heating systems have much shorter payback times than solar space heating systems. It can, therefore, be concluded that solar energy systems are friendlier to the environment and offer significant protection of the environment. The reduction of greenhouse gasses pollution is the main advantage of utilizing solar energy. Therefore, solar energy systems should be employed whenever possible in order to achieve a sustainable future, thus applying the slogan ‘‘THINK GLOBALLY––ACT LOCALLY’’.

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References
[1] Dincer I. Energy and environmental impacts: present and future perspectives. Energy Sources 1998;20(4/5):427–53. [2] Dincer I. Renewable energy, environment and sustainable development. In: Proceedings of the World Renewable Energy Congress, Florence, Italy, 1998. p. 2559–62. [3] Rosen MA. The role of energy efficiency in sustainable development. Technol Soc 1996;15(4):21–6. [4] Dincer I, Rosen MA. A worldwide perspective on energy, environment and sustainable development. Int J Energy Res 1998;22(15):1305–21. [5] www.worldwatch.org. [6] Argiriou A, Klitsikas C, Balaras C, Asimakopoulos D. Active solar space heating of residential buildings in northern Hellas––a case study. Energy Buildings 1997;26(2):215–21. [7] Dincer I. Environmental impacts of energy. Energy Policy 1999;27(14):845–54. [8] Speight JG. Environmental technology handbook. Washington, DC: Taylor and Francis; 1996. [9] Colonbo U. Development and the global environment. In: Hollander JM, editor. The energy-environment connection. Washington: Island Press; 1992. p. 3–14. [10] Kalogirou S, Lloyd S. Use of solar parabolic trough collectors for hot water production in Cyprus––a feasibility study. Renew Energy 1992;2(2):117–24. [11] Kalogirou S, Papamarcou C. Modelling of a thermosyphon solar water heating system and simple model validation. Renew Energy 2000;21(3–4):471–93. [12] Diakoulaki D, Zervos A, Sarafidis J, Mirasgedis S. Cost benefit analysis for solar water heating systems. Energy Convers Manage 2001;42(14):1727–39. [13] Polysun, 2000. UserÕs manual for Polysun 3.3, SPF, Switzerland. [14] Kalogirou S. Generation of typical meteorological year (TMY-2) for Nicosia, Cyprus. Renew Energy 2003;28(15):2317–34. [15] Gantner M. Dynamische Simulation thermischer Solaranlagen. Diploma thesis, Hochschule f€r Technik u Rapperswil (HSR), Switzerland, 2000. [16] Statistical Abstract 2001, Statistical Service, Republic of Cyprus, Report No. 44, 2001. [17] Kalogirou S. 2001. The sun island: solar energy in Cyprus. ReFocus, The International Renewable Energy Magazine of ISES, March 2001. p. 30–2. [18] Alcorn J. Embodied energy coefficients of building materials. Centre for Building Performance Research, Victoria University of Wellington, New Zealand, 1995.


				
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