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4. Solar Water Heating System 4.1 Introduction:
A solar water heating system (SWHS) is a device that makes available the thermal energy of the incident solar radiation for use in various applications by heating the water. The SWHS consist of thermal collectors, interconnecting pipelines, the water tanks and the water which gets circulated in the system. Solar radiation incident on the collector heats up the tubes thereby transferring the heat energy to the water flowing through it. The performance of the SWHS largely depends upon the collectors efficiency at capturing the incident solar radiation and transferring it to the water. With todays SWHS water can be heated up to temperatures of 60 to 70 degree Celsius. Heated water is collected in a tank insulated to prevent heat loss. Circulation of water from the tank through the collector and back to the tank continues automatically due to the thermo siphon principle. The amount of hot water a solar water heater produces depends on the type and size of the system, the amount of sun available at the site, proper installation, and the tilt angle and orientation of the collectors. The hot water generated finds many end-use applications in domestic, commercial and industrial sectors. Solar water heaters can be either active or passive. An active system uses an electric pump to circulate the heat-transfer fluid; a passive system has no pump.
For IIT Roorkee campus, solar water heaters are being considered for all possible installations in Bhawans (hostels), faculty residences, guest houses and department tea rooms. This will ensure all round usage of solar energy and maximum savings. In this chapter the details of the technology used as well as the results of the techno-commercial analysis are presented.
Fig 4.1: Shows percentage global share of solar water heater of different countries.
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Typical solar water heating system can save up to 1500 units of electrify every year for every 100 l of solar water heating capacity
Coal has Carbon Emission Factor 26.8 tons of carbon/Tera joules Very low pay back period 1-1.5 yrs Long life 15-20 yrs Very less maintenance cost Zero running cost
4.3 Benefits of Solar Water Heaters: There are many benefits to owning a solar water heater, and number one is economics. Solar water heater economics compare quite favorably with those of electric water heaters, while the economics aren't quite so attractive when compared with those of gas water heaters. Heating water with the sun also means long-term benefits, such as being cushioned from future fuel shortages and price increases, and environmental benefits. 4.3.1 Economic Benefits Many home builders choose electric water heaters because they are easy to install and relatively inexpensive to purchase. However, research shows that an average household with an electric water heater spends about 25% of its home energy costs on heating water. It makes economic sense to think beyond the initial purchase price and consider lifetime energy costs, or how much you will spend on energy to use the appliance over its lifetime. It found that solar water heaters offered the largest potential savings compared to electric heating, with solar water-heater owners saving as much as 50% to 85% annually on their utility bills over the cost of electric water heating. However, at the current low prices of natural gas, solar water heaters cannot compete with natural gas water heaters in most parts of the country except in new home construction. Although you will still save energy costs with a solar water heater because you won't be buying natural gas, it won't be economical. Paybacks vary widely, but one can expect a simple payback of 3 to 8 years on a well-designed and properly installed solar water heater. (Simple payback is the length of time required to recover your investment through reduced or avoided energy costs.) One can expect shorter paybacks in areas with higher energy costs. After the payback period, the savings are accrued over the life of the system, which ranges from 15 to 40 years, depending on the system and how well it is maintained. Under the JNNSM, subsidies up to a maximum of 60% may be obtained on a solar water heating system.
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4.3.2 Tax Incentives and Rebates In India some States offer subsidies on domestic as well as commercial solar water heating systems installations. Government of India offers 100% depreciation claim in the first year itself on installation of commercial solar water heating systems. 4.3.3 Long-Term Benefits Solar water heaters offer long-term benefits that go beyond simple economics. In addition to having free hot water after the system has paid for itself in reduced utility bills, the family will be cushioned from future fuel shortages and price increases. The National Remodelers Association reports that adding a solar water heater to an existing home raises the resale value of the home by the entire cost of the system. 4.3.4 Environmental Benefits Solar water heaters do not pollute. By investing in one, there is avoiding of carbon dioxide, nitrogen oxides, sulfur dioxide, and the other air pollution and wastes created when the utility generates power or one burns fuel to heat your household water. When a solar water heater replaces an electric water heater, the electricity displaced over 20 years represents more than 50 tons of avoided carbon dioxide emissions alone. Carbon dioxide traps heat in the upper atmosphere, thus contributing to the "greenhouse effect." 4.4 A Bright Future A solar water heater is a long-term investment that will save money and energy for many years. Like other renewable energy systems, solar water heaters minimize the environmental effects of enjoying a comfortable, modern lifestyle. In addition, they provide insurance against energy price increases, help reduce our dependence on foreign oil, and are investments in everyone's future. 4.5 General Requirements It is necessary to have solar water heating system with right technologies, features and capacity to ensure long term reliable and smooth operation of the system. Installation of solar water heating in the right manner, suitable for the specific site conditions is very important for the optimal performance of the system. In case large system located on the roof top of a building, provision of a lighting arrester must be made in case it is not already provided. The name plates should be easily visible to the installers. Safety instructions must be provided along with the system to ensure installation safety at site.
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4.6 Additional System requirements
The following additional system technology features may help in the selection of a solar water heating system with appropriate technology, suitable for the size and site conditions.
4.6.1 Thermo siphon system: For a small application of up to 3000 liters capacity user may prefer the thermo siphon system for its simplicity and ease of operation. In such cases the source of the cold water must be placed at least 7 feet above the terrace level for size up to 500 liters. For large tank sizes, the height requirement may go up to 10 feet or higher where the solar water heating system is installed.
4.6.2 Forced circulation system: For systems of size larger than 3000 liters per day the customer may choose the forced circulation system. This system may also be used for smaller than 3000 liters/day capacity where the thermo siphon system cannot be used due to limitation of the height of the cold water tank.
4.6.3 Solar water heater with heat exchanger: In places where the water quality is not suitable for direct use in the solar collector o in cold regions where the water in the collector may freeze in the night a solar water heater system with indirect heating is required.
For thermo siphon system the heat exchangers shall always be in the hot water storage tank. Whereas the forced flow systems the heat exchanger may be located inside the hot water storage tank or outside the tank, depending upon design.
4.6.4 Flat plate collector: A flat plate collector collects solar radiation and converts solar energy into heat for heating water. The flat plate collector should be reliable and durable with a useful life of 15 years. The main characteristics of this collector should be:
Resistance to environmental conditions ( marine environment, rain, dust, hail etc) Resistance to large variations from any part of the system Stable and durable Easy to install Efficient in energy conversion
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Fig 4.2: A rooftop solar water heating system installation
The main components of the flat plate collector are important to meet the above characteristics.
The casing contains all the components of the collector and protects them from environmental impact. The casing also makes the collector sturdy and stable. The material used for the casing must be resistant to corrosion.
The seal is made out of elastic material to prevent leakage and ingress of rain water into the collector. The sealing material must withstand high temperature variations and UV radiation to ensure long life of 15 years or more.
3) Transparent cover
The transparent cover should be mad of toughened glass with high transmission coefficient (close to 1). This also protects the component inside the collector from environmental impact.
4) Thermal insulation
The thermal insulation reduces undesirable heat loss from the back and sides of the collector area. The insulations must be able to withstand the maximum temperature of the absorber plate.
5) Absorber plate
The absorber plate absorbs the solar energy and converts it into heat energy in the fluid. The absorber is made out of high conductivity material like copper with a selective coating on it for maximum absorption of solar radiation and minimum emission of infrared radiation
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The fluid that flows through the collector shall collect the heat for useful applications. The transfer of heat shall takes place mainly through conduction and convection process. Therefore, the tubes should be made out of a high conductivity material like copper.
4.6.5 Evacuated tube type collector:
The evacuated tube collector uses solar energy to heat fluid inside the tube through absorption of radiation, but reduces the loss of heat to the atmosphere due to the vacuum inside the tube. An evacuated tube has different subcategories based on the material used and application requirement. The life of the evacuated tube varies from 5 years to 15 years. The main characteristics of the evacuated tube collector should be:
The capacity to endure environmental conditions (rain, dust, etc. and in some cases marine environment)
The capability of enduring large variations in temperature Resistance to leakage from any part of the system Stability and durability Easy to install Efficient in energy conversion
The characteristics are required to be fulfilled by the collector in the existing MNRE standard.
Fig 4.3: Evacuated tube collectors installed on roof
The main component of the ETC should meet the above characteristics.
1) Glass tube:
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The glass tube is formed by fusing two co-axial tubes at both the ends. The air between the two glass tubes in evacuated to create a vacuum, which works as an insulation. The outer surface of the inner tube in the ETC forms the collector area.
2) Absorber coating:
An absorber coating shall be selectively applied on the outer walls of the inner tube to absorb the solar radiation to collect energy and to convert light energy into heat energy. The selective absorption coating has an absorption co-efficient of 0.94 or more and emissivity of 0.12 or less. The coating should not deteriorate during the life of the system.
The gasket between the tank and the ETC or between the manifold and the ETC is crucial for the trouble free performance of the water heating system with the ETC. The gasket should be capable of sustaining the temperature and pressure encountered. This should fit tightly above the tube to avoid any leakage. The life of the gasket should be equal to the life of the entire system.
4.7 Technology of Choice: The Evacuated tube collector (ETC)
Solar water heaters are available in two different technologies: the flat plate collector technology and the evacuated tube collector technology. The ETC is the late stand most efficient solar thermal technology and is fast replacing FPC as the technology for solar thermal applications. The evacuated glass tubes are the most crucial component of solar collectors. High quality of tube results in higher efficiency through greater heat absorption and more consistent performance, ensuring having hot water even during non-sunny days.
Thermo siphon Systems A thermo siphon system relies on warm water rising, a phenomenon known as natural convection, to circulate water through the collectors and to the tank. In this type of installation, the tank must be above the collector. As water in the collector heats, it becomes lighter and rises naturally into the tank above. Meanwhile, cooler water in the tank flows down pipes to the bottom of the collector, causing circulation throughout the system. The storage tank is attached to the top of the collector so that thermo siphoning can occur. These systems are reliable and relatively inexpensive but require careful planning in new construction because the water tanks are heavy. They can be freeze-proofed by circulating an antifreeze solution through a heat exchanger in a closed loop to heat the household water.
4.8 Site description
We are basically considering Bhawan for installing solar water heating system in IIT Roorkee campus. Each bhawan has ample space on the roof for installation of solar collectors. Even if there is shadow its working is not affected to greater extent
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There are 11 bhawans in IIT Roorkee. Out of these, 9 are for boys and 2 for girls. All bhawans have capacities ranging from 500 to 800 seats. The total consumption of hot water (liters/day) is tabulated below:
S.No Bhawan Name
Hot water requirement (LPD)
1 Azad 499 9980 2 Cautley 931 18620 3 Ganga 567 11340 4 Govind 606 12120 5 Jawahar 642 12840 6 Radhakrishnan 600 12000 7 Rajendra 800 16000 8 Ravindra 509 10180 9 Sarojini 251 5020
Total 5405 108100 Table 4.1: Bathroom users in bhawans and capacity required
As from the table above we can see that there is huge consumption of hot water so by implementing solar water heating system we can reduce electricity consumptio...