Solar water heating systems in India

Solar water heating systems in India

Technology Review Solar water heating systems in India Anand Doraswami The Indian Government has been trying to develop and promote the use of solar ...

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Technology Review

Solar water heating systems in India Anand Doraswami The Indian Government has been trying to develop and promote the use of solar thermal energy through solar water heating systems for about a decade. Efforts in this direction had been going on on a smaller scale for at least five years before that. Various designs and combinations of materials have been tried out. Solar water heating systems, both domestic and non-domestic (commercial or industrial), are fairly well-established in the marketplace as a viable alternative to the use of non-renewable energy options for water heating. This review traces the evolution of the technology during the last decade and a half.

An industrial solar water heating system

THE DEVELOPMENT of solar thermal energy in India has now been going on for more than 15 years. Organised efforts by India’s central (i.e., federal) government to promote the use of solar thermal energy, particularly in solar water heating systems, began in 1984, when the Solar Thermal Extension Programme (STEP) was established under the Department of Non-conventional Energy Sources (DNES), now the Ministry of Non-conventional Energy Sources (MNES). During the last 15 years the design of solar water heaters (SWHs) has undergone extensive development. Simultaneously, public awareness of the importance and utility of harnessing solar thermal energy for heating water has been growing. Initially, the potential user had to be

urged to install a SWH by incentives such as a government subsidy, but the technology is now proven and ready to take off. Despite the tapering-off and removal of subsidies, the installation of SWH systems has not diminished but is showing signs of steadily, though not dramatically, rising. This is true of both domestic (small) and industrial or commercial (large) SWHs. 1. The basic design A SWH consists essentially of a collector for solar radiation connected to a storage tank by pipes. The ‘‘flat plate’’ collector is the part which actually traps solar radiation as thermal energy and becomes heated as a consequence. For the water in the storage tank to receive this heat, it must be

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circulated by means of the pipes in and out of the collector by a natural or forced process. Obviously, the efficiency of the system will depend on the efficiency with which the collector traps solar energy, the efficiency of the heat transfer to the water circulating into the collector, and the efficiency with which both collector and storage tank are insulated from heat losses. A standard design for a domestic SWH is shown in Fig. 1A, with an exploded view of the collector in Fig. 1B. As the water in the riser tubes gets heated by ‘‘insolation’’ (the absorption of solar radiation), it flows upwards into the upper header tube and from there into the top of the insulated storage tank, pushing down the colder water at the bottom of the tank through the connecting pipe into the lower header tube. This natural flow of water by a form of convection or thermosyphon process ensures that heat keeps flowing from the fins of the absorber (which has a black coating for better absorption of radiation) into the water contained in the risers. For large systems the length of the pipes connecting the collectors to the tank or tanks becomes so great that the pressure drop across the system makes it impossible to use the thermosyphon process, so that a small electrical pump has to be provided to maintain a forced flow system. The collector is mounted on a horizontal surface, usually a roof, at a slope 10 degrees greater than the local latitude so that it receives optimum incident solar radiation during the winter months when the hours of sunlight are least. Both the absorber and the storage tank must be well insulated, the former so that the solar radiation received goes entirely into heating the water and the latter so that heat loss from the hot water during both day and night is minimised. The glass cover of the collector ensures that radiation is received by the absorber but very little is lost by it (since glass is opaque to the far infrared radiation emitted by a warm body, the principle of the greenhouse). The performance of the system depends on the choice of materials, the important factors being the thermal conductance of the May 1994

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Fig 1A. Domestic solar water heating system

absorber tubes and fins, the efficacy of the bonding between tubes and fins, the quality of the insulation, and the corrosion resistance of the entire system, bearing in mind that some parts are exposed to water (of variable purity) at moderately high temperatures and most of the others have to withstand sun and humid air. It is relevant to point out here that water supply in India, even in urban areas, is far from free of suspended matter, even grit. Besides, it can, especially if it is taken from an open well or tubewell, contain a significant proportion of dissolved salts. For a large (non-domestic) SWH system, the design is complicated by such factors as the need for several collectors connected together to the same storage tank, whose capacity, however, need not be equal to the heating capacity per day of the system, since the water is drawn off all round the day. Beyond a capacity of 3,000 litres per day (LPD), the array of collectors required and the length of pipes needed to serve them cause the thermosyphon process to fail, as was noted above, and electrical pumping is needed to maintain the circulation of the water. 2. Evolution of absorbers Wide variations in the choice of materials as well as departures, sometimes radical, from the basic design have been tried during the past 15 years. These variations have by trial resulted in optimal choices for the materials and design so as to offer the 52

user the best possible combination of efficiency, long-lasting service and economy. As a general rule of thumb, a domestic SWH should aim to heat 100 LPD (or if the user so prefers, say in the case of a large family, 150 or 200 LPD) of water to about 60 or 70 deg C during the course of a fairly sunny day, when the cold water temperature is around 25 deg C. Jyoti Ltd., a major manufacturer of industrial boilers and heating systems located at Vadodara in Gujarat state, was the first private or public organisation to show an interest in SWHs. Beginning in the late 1970s with absorber tubes and fins made of galvanised iron (GI), i.e., hot-dip galvanised mild steel (MS), it established a market all over India for both domestic and industrial SWH systems. Later, to offer a choice of higher efficiency with a lighter and less cumbersome system, it introduced copper absorbers (fins and tubes) which facilitated the reduction of the thickness of the material used owing to the superior thermal conductivity of copper. Meanwhile, in May 1978, the Karnataka State Council for Science and Technology, a state governmentsponsored body, began experimenting with solar thermal energy in sericulture. Since in some processes of sericulture, an important industry in Karnataka, hot water is used, and the state enjoys sunny weather for a large part of the year, SWHs were tried out for generating hot water. SWHs procured from both Jyoti Ltd. and a

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small-scale manufacturer were installed. It was discovered that when the absorber was made of GI, the process of welding the tubes to the fins can create conditions for corrosion. Since two or more different metals are involved (GI being iron plated with zinc) and the process of welding could lead to exposure of more than one metal, it creates electrochemical cells (the electrolyte being produced by the action of atmospheric moisture) which cause the galvanising to corrode with time along the weld. It was thought that an alternative design might overcome this problem: instead of having a serrated (grooved) sheet for the fins, with the riser tubes welded into the grooves, the absorber could be made from two serrated MS sheets clamped together. Semi-circular (or, as was actually done in practice, semi-hexagonal) grooves in the two sheets would be matched together to form the riser and header tubes, yielding the high conduction afforded by welded tubes and fins but without the risk of corrosion along the welds (which could otherwise be minimised only by using clamps, and hence sacrificing conduction, instead of welding). The absorber assembly was held together firmly by crimping its four sides and by riveting the two sheets between the serrations, where they were in

Fig 1B . Flat plate collector

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contact. A small-scale manufacturer was selected to fabricate a few collectors to the specifications of this design, and the SWH units were installed in the silk filature at Kanakapura near Bangalore. Although the design avoided the problem of choosing between poor contact (and hence poor conduction) and damage to anti-corrosive treatment (and hence shortened life) due to welding, it was unsuccessful. In practice it was found that the system had low tolerance for inaccuracy in holding the two formed MS sheets together. Water could leak out of the tubes into the space between the two layers constituting the fins and thereby reduce both the efficiency and the trouble-free life of the system. The experiment thus returned to the original design of Jyoti Ltd. The Government of India (i.e., the DNES) formally introduced STEP in 1984, using models of SWHs that had been experimented with at some of the five Indian Institutes of Technology, the DNES’s solar energy centre at Gurgaon near Delhi, and some other scientific institutions. About 100 approved manufacturers choosing from a few design specifications were licensed to sell SWHs all over the country. Jyoti Ltd. was the leader in terms of turnover; among the important large-scale manufacturers were Bharat Heavy Electricals Limited (BHEL), a public sector company, and Best and Crompton in the private sector. In an effort to promote the installation of SWHs, 75 to 100% subsidies were offered to certain categories of users. Government departments were the major ‘‘beneficiaries’’ of 100% subsidised systems, most of which lay idle for want of appreciation of the value of a SWH that had cost the user nothing. Such users (or non-users) did not care about the performance of the system. One state government enterprise began selling units made of MS collectors, which corroded and started leaking on rooftops within months. The DNES had approved only copper-copper (i.e., copper for both fins and tubes) and hot-dip galvanised MS absorbers, but the quality of the supposed galvanising could not be

Components of Surja 1. Cold water overhead tank 3. G.I. collector or absorber plate 5. G.I. hot water outer collector box 7. G.I. hot water outer storage tank 9. Insulation material 11. Hot water outlet

2. 4. 6. 8. 10.

Cold water inlet Glass cover Thermosyphon hose pipe G.I. hot water inner storage tank Outer lid

Fig. 2 . Sectional view of collector and storage system Surja -- Solar water heating system

checked since the absorber has to be coated black. Within a year or so, GI was banned and only copper was permitted. Large manufacturers who could maintain quality standards with GI were also forced to restrict themselves to copper. However, non-standard designs and specifications were tried out from time to time. In 1987, Soladur Systems of Bangalore, a small manufacturer, was permitted to make a system in which the entire collector was a single piece, a polypropylene sheet having 2.5 mm capillary hollows running along its length as risers, with plastic header tubes welded to it at the two ends. This had the advantage of needing no insulation and hence no enclosing box. However, because of the absence of a glass cover, heat could be conducted away by air circulation and the greenhouse effect was also lost. Hence the system was limited to relatively lower temperatures (50-55 deg C at the end of the day, below 50 deg C, after overnight storage, in the morning, when hot water is mostly drawn from domestic heating systems for baths). Besides, the imported absorber sheet was welded to the header tubes locally, and the technology of plastic welding proved problematic. Welding gave way and leaks developed, and repairs could not be carried out promptly. Another problem was the need for a

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mechanical screen filter to keep suspended impurities in the water out of the capillaries. Frequent regular cleaning of the filter to avoid the circulation being constricted proved a nuisance. Consequently, the use of this design was stopped in 1991-92. Another unusual design, a low-cost one called Surja (shown in Fig. 2), was developed by the KSCST in 1986-87. It dispensed with the tubular ‘‘ladder’’ welded to the sheet forming the fins; the absorber was just a closed GI box, making for very efficient heat transfer to the water. The collector area, reduced to 1 sqm from the usual 2 sqm, still yielded a temperature of 50 deg C for 100 LPD of water. Costs for the storage tank and the two covers (of the collector and the tank) were cut by using GI for the tank and resorting to ferrocement construction of the covers in situ. The cost was about 40 per cent of that for the conventional design. However, the system was limited by the low temperature it yielded. The collector size could not be the usual 2 sqm because a GI box of that size could bulge and break at the ends. Two 1-sqm collectors, on the other hand, would raise the cost to about 80% of the conventional design’s. The in situ construction made it difficult to replace defective parts and follow-up service could not be offered as no reliable manufacturers May 1994

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took up production of the design. Besides, since the design was not approved by the DNES, the absence of a subsidy made it unattractive to users. Of about 50 systems installed in and around Bangalore, 15 or so are still working. Many of the rest developed rust on collector or tank and consequent leaks, leading to failure. Another unusual system, the Sigma solar water heater (see Fig. 3) designed by A.R. Shivakumar of the KSCST, was a single-piece singlepass SWH. This was the only nonstandard design approved by the MNES. In this, the absorber is a single copper tube bent in a sinusoidal shape and attached to an aluminium sheet by means of GI or copper wire. A thermostatic (bellow) valve operating at 60 deg C ± 2 deg C lets water pass from the absorber to the storage tank only when it attains the desired temperature. A flat storage tank made of stainless steel (SS) sheet, placed below the absorber and insulated with simple locally available material like hay or coir, was found adequate, the overnight drop in temperature being comparable to that in a conventional system (around 5 deg C, which is also the permissible drop according to the limits prescribed). The tank did not need to be placed at a higher level than the collector because water did not circulate back to the collector from it. The absorber, valve and tank were enclosed in an aluminium box

with an open top that received a plain sheet glass to allow sunlight to pass. The single-piece construction of the collector and storage tank allowed for easy installation and maintenance and the cold water source (another storage tank) head needed for this system was less than half that for the usual design (1m instead of 2.5m) since the hot water tank was not placed at a higher level than the collector. Since the absorber was a single tube without any brazing or welding, there was no scope for leakage. In fact except for the SS inner tank, there was no welding or brazing. Thus fabrication was easy, with cost around 60% of that for a conventional system. The user could be sure of getting water at a fixed temperature of 60 deg C; only the quantity would be reduced on a cloudy day, or even fall to zero on a rainy day if the water in the absorber tube failed to reach 60 deg C. As the quantity of hot water collected in the tank, and not its temperature, was variable, an electrical back-up heater to make up for insufficient heating on a cloudy day could not be installed in the tank. Besides, the thermostatic valve, because of operating in hot water, was prone to deposition of salts, necessitating replacement in 1824 months depending on the hardness of the water. Over the years the standard design shown in Fig. 1 has been stabilised.

Fig. 3. Sigma solar water heater

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The only essential modification to this set-up in the case of large systems is, as already noted, the need for an electric pump to maintain forced circulation. The pump acts on the principle of differential temperature control (DTC), i.e., it switches on as soon as the temperature sensors in the absorber and the tank detect a difference of 5 deg C between the former and the latter. (If the single-pass Sigma system is used in a large SWH, the pump is activated by fixed temperature control or FTC, i.e., as soon as the thermostat detects that the absorber has attained the desired temperature.) As we have seen, the MNES had withheld approval from all systems not having copper tubes and fins. This was a regime based on specifying materials rigidly. It tended to shift the emphasis away from performance, to which indeed it did not refer. However, owing to pressure from all sides to permit the use of alternative specifications and allow the user to exercise a choice based on his own priorities of cost and efficiency, which would shift the emphasis back to where it belongs, on performance, the MNES allowed the use of five different design specifications for the absorber from 1990-91. These were: 1. copper tubes and copper fins; 2. copper tubes and aluminium fins; 3. GI tubes and GI fins; 4. MS tubes and MS fins; and 5. polypropylene (with capillaries). Meanwhile the (central) government’s share and stake, in the form of subsidy, in the system cost has progressively decreased from 30% to nil for domestic systems and 75100% to nil for non-domestic systems. Theoretically, at least, in the current situation the user can choose the system that offers him the best in terms of price and efficient service. There has been a new quality consciousness among manufacturers and MS collectors have shown improvement, although copper tubes and fins command the highest preference. Although there is a wider choice of materials, it is pertinent to mention that the MNES and the Bureau of Indian Standards (BIS, formerly the Indian Standards Institution or ISI), whose May 1994

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ISI certification carries some weight with buyers of manufactured products, have laid down minimum specifications for the gauge of the materials used (which varies according to the material, being the greatest in the case of lower conductance materials) and for the performance of the system. A system conforming to MNES or BIS standards or, better still, certified as performing according to the standards, would carry greater credibility with the potential buyer. 3. Other aspects of SWH technology Parallel to the evolution in the technology of absorber materials, there has been an evolution in the standards followed in other parts of the SWH such as the storage tank, the insulation of the absorber, the collector box and the cover. Most important among these is the coating on the absorber, which determines how much of the incident solar radiation is absorbed and how much is radiated away when there is no sunlight (as in the night). Earlier systems used ordinary black paint, but it later became clear that for applications above 70 deg C it would not do as the risk of its peeling off with time was great. ‘‘Selective’’ electrochemical coating had to be specified. Three alternative specifications are now approved: 1. Selective coating with an optical composite of chromium and chromium oxide, the so-called black chrome coating; 2. Dull black paint – black, mat finish stoving paint that withstands 175 deg C without melting or burning; and 3. (Electrostatic) powder coating, oven-seasoned at 250 deg C (this has recently come into use). For systems generating above 70 deg C, the specifications permit only selective coating. In all cases, the coating should have a dull black finish with an absorptivity of not less than 0.92 for solar radiation and an emissivity of less than 0.2 for the infrared radiation given off by the absorber in the usual temperature range in which it operates. (This is the origin of the term ‘‘selective’’, which

means that the coating is highly absorptive in the visible and near infrared range, in which most of the solar radiation is concentrated, while it is has low absorptivity and emissivity in the far infrared range, in which a moderately warm body emits most of its radiation.) For selective coating, which is the most efficient and also most durable, the majority of manufacturers still prefer to use the continuous process technology on copper sheets and tubes. The sheets and tubes are later machined into fins and tubes of the required size and welded to fabricate absorbers. The process technology has been imported from Canada and is being used by a single li- Domestic censee, from whom the manufacturers purchase coated sheets and tubes. However, a batch process called NALSUN, developed by the Materials Division of the National Aerospace Laboratories (NAL), has been slowly gaining acceptance and it has now been granted patents in the USA and some other developed countries besides India. In fact, NALSUN was developed about 10 years ago at a time when selective coating was not being executed in India, before the continuous process know-how was imported. In addition to eight Indian makers of SWHs having taken licences for NALSUN, some foreign manufacturers have also shown interest in using it. The main advantage of NALSUN, which achieves its results because of using different catalysts for the electrodeposition of the black chrome, is that it reduces the current densities needed for the electrolysis to a small fraction of that used in the standard process. It also has the advantage of not needing refrigeration of the bath, which can be kept at room temperature. The consequent reduction in the consumption of electrical energy for the coating process is dramatic, and the cost of the coating is cut from

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solar water heating system

about Rs. 500 (Rs. 30 = $1 approximately) to about a tenth of that if the technology is used in the manufacturer’s plant and not in NAL’s pilot plant. Other parts of the SWH have also shown changes in the materials used for their construction. The collector box, originally made of MS, now offers a choice between MS and aluminium. The glass cover of the absorber has progressed from ordinary glass to toughened glass because of the experience of glass covers being prone to stray damage when exposed on rooftops. The storage tank was also originally made of MS, but currently SS tanks are offered as a better option for corrosion resistance. Insulation materials for the absorber and tank may be either glasswool or synthetic (polyurethane) foam. Polyurethane, which is superior, is a later development; the earlier systems all used glasswool. Although it is not directly concerned with the designing of SWHs, a word about the problem of scaling due to hardness of the water is not out of place here. In areas where the water is quite hard, the absorber tubes may get scaled and consequently narMay 1994

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rowed, or even blocked, with time. In the state of Gujarat, in particular, reduction in the efficiency because of considerable scaling becomes noticeable in four to six years. Several descaling chemicals are available in Gujarat. S.K. Philip, S. Sharma and C.S. Rao at the Sardar Patel Renewable Energy Research Institute, Vallabh Vidyanagar, Gujarat, studied the efficacy of these chemicals when pumped through scaled collectors and found that three of them (all branded products) were quite effective in removing scales (80 to 99% from partially blocked riser and header tubes in 24 hours of circulation). Two of these removed less than 2% of the base metal in the same time, which was considered acceptable. For domestic SWH users, the cost of transporting the collector to the manufacturer’s plant or elsewhere for the descaling process, plus the cost of descaling, could be significant in comparison with the cost of a new absorber minus the scrap value of the metal (about 50%) in the old absorber. Therefore, in most areas where the water is not too hard and hence scaling takes a long time, descaling of domestic SWHs is not a common practice. 4. Government support As we have already seen, the subsidy offered to the user to encourage adoption of SWH technology has been progressively reduced. For domestic systems, from Rs. 3,000 per system (roughly 30% of system cost) between 1984-85, when STEP was introduced, and 1990-91, the subsidy was cut to Rs. 1,000 per sqm of collector area in 1991-92 (which would amount to Rs. 2,000 for the standard 2 sqm collector). In June 1993 it was abolished. For non-domestic systems, it was 75-100% from 1984-85 onwards till it was reduced to 60% in 1988-89, further cut to 30% in 199091 and brought in line with the subsidy for domestic systems in 1991-92. There appears to be no reduction in the offtake of SWHs despite the withdrawal of the subsidy, however, indicating that the potential user’s interest in SWHs may now be intrinsic and independent of any incentive 56

that may be offered. Although prices have not come down (so that they are now higher by at least as much as the withdrawn subsidy), there seems to be greater awareness of technology and the need to offer the user the best value for money. Some manufacturers however still complain of consumer resistance and the need to overcome it indirectly by such means as altering building by-laws to make SWHs compulsory on every new building. 5. Growth of SWH industry Up to the end of 1992, 10,432 domestic and 5,159 industrial SWH installations had been registered with the MNES, although these figures could be low by about 1,000-2,000 because of late reporting. Thus, the figures for the nine months April-December 1992 were reported to be 781 domestic and 330 non-domestic systems, while the full figures for the year 1991-92 (April to March) are about 2,400 and 500 respectively. These compare favourably with 866 domestic and 479 non-domestic systems installed in the first 9 months of 1986-87. Since the number of domestic and non-domestic installations in January 1987 was 1,166 and 1,004 respectively, on an average some 1,500 domestic and 700 non-domestic SWH systems were installed every year from 1987 to 1992. The figures for total capacity in LPD in January 1987 were 131,000 for domestic and 2.155 million for non-domestic systems. (Thus, the average system capacity was about 115 LPD for domestic and 2,150 for non-domestic systems.) Since June 1993, as the subsidy has been withdrawn, it is possible that reporting to the MNES or other governmental agencies taking an interest in SWH installations will remain incomplete, although the MNES still maintains a list of approved manufacturers and should continue to receive figures of installations from them. In the state of Karnataka, according to figures available with the KSCST, which was the agency for certifying installations and sanctioning the MNES subsidy, the number of domestic SWH systems installed each year has risen from 38 in 1985-86 to

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498 in 1992-93, with a probably incomplete figure of 580 for 10 months of 1993-94 (April 1993 to January 1994). The corresponding figures for non-domestic (industrial/commercial) systems are 73, 129 and 98. The average system capacity broadly agrees with the all-India figures for January 1987. Currently, about 120 manufacturers of SWH systems are on the registered list of the MNES. Some 80 of them have MNES approval to fabricate small systems (of up to 10,000 LPD capacity), another 20 can go up to 25,000 LPD (medium-scale systems) and the rest are permitted to make systems of any size. These manufacturers include all the large-scale industries that went into SWH technology (such as Jyoti Ltd., BHEL, and Best and Crompton mentioned earlier) but the majority of even those SWH manufacturers who are allowed to make large systems are themselves small-scale industries. The reasons for this lie both in the general industrial environment of Indian industry and in specific factors pertaining to the solar thermal energy programme. Foremost come the advantages enjoyed by small-scale industries in India such as lower labour costs and overheads, tax concessions and other economic incentives. Besides, small-scale manufacturers have been able to keep better control of inventories of materials as well as follow up the performance of their systems. As long as the subsidy lasted, they were able to offer the user personal attention in chasing the required papers with the agencies empowered to check on the installation and certify that the user was qualified for a subsidy from the MNES. In a total countrywide market of around 2,000 systems a year of which the bulk are domestic SWHs mostly having a single 2 sqm collector each, large manufacturers did not enjoy any economies of scale that offset the advantages mentioned above for smallscale manufacturers. Consequently even those few large-scale manufacturers that earlier took an interest in SWHs, including the pioneers, have more or less pushed this activity into the background. It was always, necMay 1994

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essarily, no more than a small part of their operations but they are themselves no more than secondary players in the SWH industry. 6. Status and prospects Currently, a 100-LPD SWH costs in the region of Rs. 13,000-16,000, depending partly on the choice of materials used but even more crucially, it seems, on the marketing overheads of the manufacturer and/or his perception of what the buyer is willing to pay. Although, for instance, a costing of basic production costs (exclusive of marketing, after-sales service, etc.) done at KSCST about a year ago put the cost of a SWH with copper tubes-copper fins absorber with black paint and SS storage tank at Rs. 7,700 (this figure has been revised upwards by about Rs. 2,000 very recently) and that of a system using GI or MS at Rs. 5,100, it is not unusual for a manufacturer to charge almost as much for a GI absorber-MS tank system as for one using copper and SS respectively. The best systems, capable of giving trouble-free service for, say 10 years, and heating 100-LPD from 25 deg C to 70 deg C for 300 non-overcast days a year in a relatively dry climate, save about 5.25 kWh a day on those days and hence about 1,600 kWh a year. In monetary

terms the saving may amount to about Rs. 2,000 a year. These figures may be about 50% higher than the actual saving because the system is overdesigned, i.e., it yields more hot water than is needed, especially at certain times of the year. Even with trouble-free service, this does not look attractive if interest lost on capital outlay is calculated at a typical 12% for domestic savings. However, if the likely rate of increase in cost of electricity to the consumer is reckoned equal to inflation (in fact electricity utilities in India will gradually have to phase out subsidies for domestic consumers, thereby raising rates much faster than inflation), a SWH looks much more attractive. Consumer resistance seems to be attributable more to the high initial investment, the need for new plumbing and minor civil engineering work in old buildings and apprehensions about the reliability of the technology than to doubts about long-term economic returns. It is more than likely that the installation of SWH systems will rise more rapidly if the consumer has reliable information about the advantages and risks of various technological options available. Other factors that will certainly affect the growth of SWH usage in India in-

clude, naturally, the potential for bringing down the capital cost through refining the technology and, on the obverse side, the removal of the hidden subsidies (i.e., differential pricing) available to domestic consumers of electricity. There have even been suggestions that electricity suppliers (which are generally stategovernment-owned and hence, for political reasons, compelled to maintain unrealistic tariffs) should offer inducements to consumers who install SWHs, and there is some merit in this argument although it amounts to offering the user one subsidy to balance another rather than abolishing the subsidy that encourages waste.

Anand Doraswami, Associate Editor, Energy for Sustainable Development, Bangalore, India. ACKNOWLEDGEMENT : This review was compiled and written largely through the documentary material and assistance provided by Mr. A.R. Shivakumar, Senior Fellow, Karnataka State Council for Science and Technology, to whom we are indebted. He has also supplied the figures and photographs for this review.

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