Solar Water Heating

SOLAR HEATING

By Aruliya Keerthi S (CB105PE006) Gokulnath P (CB105PE015) Karthikeyan G (CB105PE023) Sneha R (CB105PE035)

Guided by Dr. Sriram Devanathan. Department of Polymer Engineering, Amrita School of Engineering.

Amrita Institute of Technology Amrita Vishwa Vidyapeetham Ettimadai, Coimbatore – 641 105. MARCH 2007

Eyes, though not ours, shall see Sky-high a signal flame, The sun returned to power above A world but not the same - CECIL DAY LEWIS

INTRODUCTION Strictly speaking, all forms of energy are derived from the sun. However, our most common form of energy fossil fuels – received their solar input eons ago and has changed their characteristics so that they are now in a highly concentrated form. Since it is apparent that these stored, concentrated energy forms are now being used at such a rapid rate that they will be depleted in the not too distant future, we must begin to supply a large portion of our energy needs not from stored, but from incoming solar energy as soon as possible. This report describes Solar Water Heating Systems that have been used in many parts of the world for decades. Water heating is one of the simplest applications of solar heat in building and one of the least expensive. Therefore it is also the most common application. Solar water heating systems have excellent life cycle cost characteristics, since water heating loads are rather uniform year round compared to that for all solar space heating loads and the payback in fuel savings is therefore accelerated compared to that for all solar space heating systems. This report also compares Solar water heating system with Standard water heating System, and shows which is better and finally concludes what type of solar water heating system used in our college and why its opted rather than other solar water heating systems and standard water heaters. HISTORICAL PERSPECTIVE The first person known to have used the sun’s energy on a large scale is Archimedes, who reputedly set fire to an attacking Roman fleet at Syracuse in 212 B.C. “by means of a burning glass composed of small square mirrors moving every way upon hinges … so as to reduce [the Roman fleet] to ashes at the distance of a bowshot.” Serious studies of the sun and its potential began in the seventeenth century, when Galileo and Lavoisier utilized the sun in their research. By 1930 Robert Goddard had applied for five patents on various solar devices to be used in his project to send a rocket to the moon. By the1920s and 1930s practical use was being made of the sun’s energy in California for solar service hot-water heaters. The first building to be practically heated with converted solar service hot-water heaters was constructed at the Massachusetts Institute of Technology in 1938. The success of solar cells in powering service modules of the National Aeronautics and Space Administration (NASA) in terrestrial orbit and lunar excursions led some engineers to propose other uses for solar energy in the space program. CONVERSION OF SOLAR ENERGY TO HEAT Solar energy is transmitted from the sun through space to the earth by electromagnetic radiation. It must be converted to heat before it can be used in practical heating or cooling systems. Since solar energy is relatively dilute when it reaches the earth, a system used to convert it to heat on a practical scale must be relatively large. Solar energy Collectors, the devices used to convert the sun’s radiation to heat, which raises the temperature of the absorbing material. Part of this energy is then removed from the absorbing surface by means of a heat transfer fluid, which may be either liquid or gaseous.

Hence Solar Collectors are used in Solar Hot Water Systems, which use the sun's energy either to heat water directly or to heat a fluid such as antifreeze that indirectly heats the water through a heat exchanger. Solar-heated water is then stored for use as needed. A conventional water heater provides any additional heating that might be necessary. The heat exchanger keeps two different liquids physically separated but allows heat energy to pass between them. In a solar hot water system, tubes carry hot antifreeze in and out of a reservoir filled with water. The detailed explanation is given in Solar Collector. SOLAR COLLECTORS A Solar Collector is a device for extracting the energy of the sun directly into a more usable or storable form. The energy in sunlight is in the form of electromagnetic radiation from the infrared (long) to the ultraviolet (short) wavelengths. The solar energy striking the earth's surface at any one time depends on weather conditions, as well as location and orientation of the surface, but overall, it averages about 1000 watts per square meter on a clear day with the surface directly perpendicular to the sun's rays. A solar thermal collector that stores heat energy is called a "batch" type system. Other types of solar thermal collectors do not store energy but instead use fluid circulation (usually water or an antifreeze solution) to transfer the heat for direct use or storage in an insulated reservoir. Water/glycol has a high thermal capacity and is therefore convenient to handle. The direct radiation is captured using a dark colored surface which absorbs the radiation as heat and conducts it to the transfer fluid. Metal makes a good thermal conductor, especially copper and aluminium. In high performance collectors, a "selective surface" is used in which the collector surface is coated with a material having properties of high-absorption and low-emissive. The selective surface reduces heat-loss caused by infrared radiant emission from the collector to ambient. Another method of reducing radiant heat-loss employs a transparent window such as clear UV stabilized plastic or Low-emissive glass plate. Again, Low-E materials are the most effective, particularly the type optimized for solar gain. Borosilicate glass or "Pyrex" has lowemissive properties, which may be useful, particularly for solar cooking applications.

As it heats up, thermal losses from the collector itself will reduce its efficiency, resulting in increased radiation, primarily infrared. This is countered in two ways. First, a glass plate is placed above the collector plate which will trap the radiated heat within the airspace below it. This exploits the so-called greenhouse effect, which is in this case a property of the glass: it readily transmits solar radiation in the visible and ultraviolet spectrum, but does not transmit the lower frequency infrared re-radiation very well. The glass plate also traps air in the space, thus reducing

heat losses by convection. The collector housing is also insulated below and laterally to reduce its heat loss. The second way efficiency is improved is by cooling the absorber plate. This is done by ensuring that the coldest available heat transfer fluid is circulated through the absorber, and with a sufficient flow rate. The fluid carries away the absorbed heat, thus cooling the absorber. The warmed fluid leaving the collector is either directly stored, or else passes through a heat exchanger to warm another tank of water, or is used to heat a building directly. The temperature differential across an efficient solar collector is usually only 10 or 20°C. While a large differential may seem impressive, it is in fact an indication of a less efficient design. Solar collectors can be mounted on a roof but need to face the sun, so a north-facing roof in the southern hemisphere and a south-facing roof in the northern hemisphere are ideal. Collectors are usually also angled to suit the latitude of the location. Where sunshine is readily available, a 2 to 10 square meters array will provide all the hot water heating required for a typical family house. Such systems are a key feature of sustainable housing, since water and space heating is usually the largest single consumer of energy in households. There are basically three types of thermal solar collectors: • • •

Flat-plate Collectors Evacuated tube Collectors Concentrating Collectors

Flat-Plate collectors or Nonconcentrating collectors intercept solar radiation on a metal or glass absorber plate from which heat is transferred and used in the thermal application. It comprise of an insulated, weatherproof box containing a dark absorber plate under one or more transparent or translucent covers. Water or heat conducting fluid passes through pipes located below the absorber plate. As the fluid flows through the pipes it is heated. Since the temperature of the absorber plate is greater than that of the environment, unrecoverable heat losses occur from the entire absorbing surface of the collector to the environment. Consequently, 100 percent collector efficiency cannot be realized in practice. This style of collector, although inferior in many ways to evacuated tube collectors, is still the most common type of collector in many countries. Evacuated Tube solar water heaters are made up of rows of parallel, glass tubes. There are several types of evacuated tubes (sometimes also referred to as Solar Tubes). Type 1 (Glass-Glass) tubes consist of two glass tubes, which are fused together at one end. The inner tube is coated with a selective surface that absorbs solar energy well but inhibits radiative heat loss. The air is withdrawn ("evacuated") from the space between the two glass tubes to form a vacuum, which eliminates conductive and convective heat loss. These tubes perform very well in overcast conditions as well as low temperatures. Because the tube is 100% glass, the problem with loss of vacuum due to a broken seal is greatly minimized. Glass-glass solar tubes may be used in a number of different ways, including direct flow, heat pipe, or U pipe configuration. Type 2 (Glass-Metal) tubes consist of a single glass tube. Inside the tube is a flat or curved aluminium plate, which is attached to a copper heat pipe or water flow pipe. The aluminium plate is generally coated with Tinox, or similar selective coating. These types of tubes are very efficient but can have problems relating to loss of vacuum. This is primarily due to the fact that their seal is glass to metal. Glass-glass tubes although not quite as efficient glass-metal tubes are generally more reliable and much cheaper. Type 3 (Glass-glass - water flow path) tubes incorporate a water flow path into the tube itself. The problem with these tubes is that if a tube is ever damaged water will pour from the collector onto the roof and the collector must be "shut-down" until the tube is replaced.

Concentrating collectors for are usually parabolic troughs that use mirrored surfaces to concentrate the sun's energy on an absorber tube (called a receiver) containing a heat-transfer fluid, or the water itself. This type of solar collector is generally only used for commercial power production applications, because very high temperatures can be achieved. It is however reliant on direct sunlight and therefore does not perform well in overcast conditions. SOLAR HEATING SYSTEMS The solar heating system consists of the collector described above; a heat transfer circuit that includes the fluid and the means to circulate it; and a storage system including a heat exchanger (if the fluid circulating through the collector is not the same liquid being used to heat the object of the system). The system may or may not include secondary distribution of heat among different storage reservoirs or users of the heat. The system can be used in a variety of ways, including warming domestic hot water, heating swimming pools, heating water for a radiator or floor-coil heating circuit, heating an industrial dryer, or providing input energy for a cooling system, among others. The heat is normally stored in insulated storage tanks full of water. Heat storage is usually intended to cover a day or two's requirements, but other concepts exist including seasonal storage (where summer solar energy is used for winter heating by just raising the temperature by a few degrees of several million liters of water.) Types of Solar Water Heating Systems Solar water heating systems can be either active or passive. An active system uses an electric pump to circulate the fluid through the collector; a passive system has no pump and relies on thermo-siphoning to circulate water. 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, installation angle and orientation. SWHS are also characterized as open loop (also called "direct") or closed loop (also called "indirect"). An open-loop system circulates household (potable) water through the collector. A closed-loop system uses a heat-transfer fluid (water or diluted antifreeze) to collect heat and a heat exchanger to transfer the heat to the household water. A disadvantage of closed looped system is that efficiency is lost during the heat exchange process. 1. Active Systems Active systems use electric pumps, valves, and controllers to circulate water or other heattransfer fluids through the collectors. They are usually more expensive than passive systems but generally more efficient. Active systems are often easier to retrofit than passive systems because their storage tanks do not need to be installed above or close to the collectors. If installed using a PV panel to operate the pump, an active system can operate even during a power outage. Open-Loop Active Systems or Direct Systems use pumps to circulate household potable water through the collectors. This design is efficient and lowers operating costs but is not appropriate if water is hard or acidic because scale and corrosion will gradually disable the system. Open-loop active systems are popular in regions that do not experiences subzero temperatures. Flat plate open-loop systems should never be installed in climates that experience sustained periods of subzero temperatures. Closed-Loop Active Systems or Indirect Systems pump heat-transfer fluids (usually a glycolwater antifreeze mixture) through the solar water heater. Heat exchangers transfer the heat from the fluid to the water that is stored in tanks. Double-walled heat exchangers or twin coil solar tanks prevent contamination of household water. Some standards require double walls when the heat-transfer fluid is anything other than household water. Closed-loop glycol systems are popular in areas subject to extended subzero temperatures because they offer good freeze protection. However, glycol antifreeze systems are more expensive to purchase and install and the glycol

must be checked each year and changed every few years, depending on glycol quality and system temperatures. Drainback systems use water as the heat-transfer fluid in the collector loop. A pump circulates the water through the solar water heater. When the pump is turned off, the solar water heater drains of water, which ensures freeze protection and also allows the system to turn off if the water in the storage tank becomes too hot. A problem with drainback systems is that the solar water heater installation and plumbing must be carefully positioned to allow complete drainage. The pump must also have sufficient head pressure to pump the water up to the collector each time the pump starts. Electricity usage is therefore slightly higher than a sealed closed or open loop. These indirect systems work well in colder climates because they minimize the chance that frozen water in some part of the system could damage the solar water heater.

2. Passive Systems Passive systems move household water or a heat-transfer fluid through the system without pumps. Passive systems have the advantage that electricity outage and electric pump breakdown are not issues. This makes passive systems generally more reliable, easier to maintain, and possibly longer lasting than active systems. Passive systems are often less expensive than active systems, but are also generally less efficient due to slower water flow rates through the system. Thermosiphon Systems relies on warm water rising, a phenomenon known as natural convection, to circulate water through the solar absorber and to the tank. In this type of installation, the tank must be located above the absorber tubes/panel. As water in the absorber heats, it becomes lighter and naturally rises into the tank above. Meanwhile, cooler water in the tank flows downwards into the absorber, thus causing circulation throughout the system. This system is widely used with both flat plate and evacuated tube absorbers. The disadvantages of

this design are the poor aesthetics of having a large tank on the roof and the issues with structural integrity of the roof. Often the roof must be reinforced to cope with the weight of the tank. Batch Heaters are simple passive system consisting of one or more storage tanks placed in an insulated box that has a glazed side facing the sun. Batch heaters are inexpensive and have few components, but only perform well in summer when the weather is warm. Evacuated tube solar collectors are now an affordable and much more efficient alternative to either batch or flat plate collectors.

Passive solar water heaters don't have electrically powered components, they are generally more reliable, easier to maintain and sometimes last longer than active systems. Pros and Cons of Solar Water Heating Systems Pros: Solar hot water systems have many advantages. First, they are nonpolluting. Solar water heaters are fueled by the sun, a renewable energy source that emits none of the greenhouse gases that contribute to global warming. Second, solar water heaters save energy. In fact, by cutting down on conventional water heating, some estimates show solar water heating systems can reduce the use of electricity to heat water by as much as 80%. Using the sun instead of conventional fuels to heat water also conserves non-renewable energy sources like oil, coal and natural gas for other uses. Third, solar water heaters save consumers money. Research shows that an average household with an electric water heater spends about 25% of its home energy costs on heating water. Using solar water heaters, which use free solar energy, can save hundreds of dollars a year. When electricity rates increase, the savings increase. The average solar water heating system pays for itself over the long run, usually in four to eight years. Adding a solar water heater to an existing home can also raise its resale value. Depending on where you live, installing a solar hot water heater may allow you to take advantage of state and local government tax incentives and rebates. Some electric utilities also offer rebates. Cons: Depending on where you live and what kind of system you choose, solar hot water systems can present some challenges. For example, to take advantage of solar water heating, an unshaded, south-facing location is necessary. Areas with hard or acidic water are not prime locations for some active solar water heating systems. Hard or acidic water tends to corrode systems that circulate water. Direct systems should never be installed in climates that experience freezing temperatures for long periods. And because they require parts that run on electricity, active solar hot water systems will not function during power outages. Passive solar hot water systems also have some potential disadvantages. For example, simple batch heater systems that use storage tanks housed in insulated boxes to heat water need a roof or other structure strong enough to support them. Special building regulations in areas where there is earthquake or hurricane danger also may limit the weight or type of equipment that can be placed on a roof. Even though solar hot water heating systems pay for themselves over time, the up-front purchase and installation costs, usually between $1000 and $3000, are higher than those of conventional electric or gas water heaters. Applications Using the sun to heat water is not a new idea. Solar energy has been used for years to heat water for homes and businesses. In some countries, such as Israel, solar hot water systems are mandatory for residential use. In the United States, as far back as the turn of the 20th century, solar hot water systems were common in southern California.

Today, solar heated water systems provide hot water for everything from park bathrooms and single-family homes to hospitals and prisons. Large facilities generally require more complicated active solar hot water systems. Active systems require electric pumps, valves and other equipment. Some homes and smaller facilities in remote areas can use passive solar hot water systems, which don't require electricity to function. For example, the federal government uses solar systems to heat water in many buildings in its national parks. Solar water heaters also can be used to heat swimming pools, hot tubs, and spas. More than a million solar pool heaters have been installed in the United States alone. Swimming-pool solar water-heaters are quite simple in design. The pool's existing filtration system pumps water through simple solar heat collectors, usually made of black plastic or rubber, and the heated water then goes directly into the pool. GENERALISED PROCESS FLOW CHART OF SOLAR WATER HEATING

WATER HEATING LOADS Solar water heating energy demands can be calculated precisely if the quantity of hot water required is known. The amount of energy needed to heat water is the product of the volume of water, its density, its specific heat, and the required temperature increase. Expressed in equation form, the amount of heat required to heat water is given by Qhw = V (ρc) (Tset – Tsource)  (1) in which Qhw is the energy requirement per day, V is the volume of water required per day, ρ is the density, c is the specific heat, Tset is the thermostat set point and desired delivery temperature, and Tsource is the temperature of the inlet water from the city water mains or a well.

A second thermal demand is present for solar water heating systems. It is the amount of heat lost from the water heating tank and the recirculation system, if one is used. (A recirculation system is used in large buildings to ensure that hot water is present continuously at all hot water outlets.) This parasitic heat loss can consume 20 to 25 percent of the fuel used in a hot water installation over the course of a year. The tank and piping loss is given by the thermal conductance of the insulation Uhw multiplied by its surface area Ahw and by the temperature difference the water and the surroundings. In simplified form, it can be expressed by Qstandby = Uhw Ahw (Tset – Ta) Nh  (2) in which Ta is the ambient temperature in the vicinity of the water heating and delivery piping if a recirculation loop is used and Nh is the number of hours of use per day, usually 24. It may be necessary to express the right hand side of eq. 2 as two terms, one accounting for tank losses and the second for recirculation loop losses, if their ambient temperatures Ta are different. If the ambient temperatures are the same, the UA product in eq. 2 can be calculated to include both tank and pipe heat loss area and thermal conductance. The total water heating demand is expressed by the sum of eq. 1 and eq. 2. SOLAR WATER HEATING SYSTEM SIZING The size of the water heating systems is an economical question based on a trade-off of solar energy cost versus backup energy cost. The system size is optimized in most cases, to provide something less than 100 percentage of the annual heating requirement. It has been found that size of all components in the solar system can be related to the collector area. Sizing rules for Domestic Water Heating Systems The following rules have been developed from the component tests in situ as well as detailed computer simulations. Conclusions of both types of studies agree with the following results. •

Collector area: determined by economic analysis [approx. 1 ft2 / (gal day)]

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Storage: 1.5 to 2 gal / ft2c

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Water equivalent collector flow rate: 0.025 gal / (min ft2c)

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Storage flow rate: 0.03 to 0.04 gal / (min ft2c) (indirect systems)

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Heat exchanger: 0.05 to 0.1 ft2hx / ft2c (indirect systems)

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Collector tilt: latitude 5°

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Expansion tank volume: 12 % of collector fluid loop (indirect systems)

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Heat rejecter capacity: equivalent to peak collection rate possible under clear sky conditions at heat rejection specified temperature.

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Collector turn-on t: 15 to 20 °F

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Controller turn-off t: 3 to 5 °F

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System operating pressure: to provide 3 lb / in2 gauge at topmost collector manifold.

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Storage tank insulation: R-25 to R-30

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Mixing valve set point: 120 to 140 °F

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Pipe diameter: to maintain fluid velocity below 6 ft/s and above 2 ft/s.

DESIGN OF A SOLAR WATER HEATING SYSTEM Solar water heating systems should be designed to minimize life-cycle cost. It is never costeffective to design a system to provide 100% of the load with solar because of the excessive investment in collector area and storage volume. Minimize life-cycle cost by designing a system that meets 100% of the load on the sunniest day of the year. Such a system will usually produce about 70% of the annual load. Other design consideration includes maintenance, freeze protection, overheating protection, aesthetics of the collector mount, and orientation. Also, utility rebate programs may impose additional design requirements. For example, a solar water heating system must meet 90% of the load in order to qualify for Hawaiian Electric Company rebates. Steps in designing a solar water heating system include:

1. Proper location of the solar collectors. 2. Protect against freezing. 3. Provide a tempering valve and bypass capability. 4. Provide periodic maintenance for all systems. SOLAR WATER HEATER VS STANDARD WATER HEATER SOLAR WATER HEATER

STANDARD WATER HEATER

FREE energy from sun

COSTLY gas or electric

Annual operating cost: $50

Annual operating cost: $500+

Storage Capacity: 80-120 gal

Storage Capacity: 40-50 gal

Life expectancy: 15-30 years

Life expectancy: 8-12 years

Lifetime operating cost: $1,000

Lifetime operating cost: $10,000

Does NOT pollute environment

Depletes fossil fuels

Increases equity in your home

No added value to your home

25% return on your investment

No return on utility payments

Protection from future increases

At mercy of utilities/government

BONUS: Hot water during blackout!

COLD showers, laundry, dishes?

Ultimately, the above tabular column shows Solar Water Heating System is better than the Standard Water Heating System. Over all, only one criterion should be noted, solar water heater is expensive during installation. CONCLUSION This report gives enough required information about the Solar Water Heating techniques and explains its types and its mechanism of heating water. And also provides the information about its specifications, requirements, pros and cons, there by it is easy to find the suitable Solar Water Heater for suitable location. As per the project, our college uses Thermosiphon Water Heating System, because of its high efficiency, less expensive, no requirement of electrical power and other advantages; it is very much suitable for large populated areas like hostels, prisons, hotels, hospitals etc.

REFERENCES • •

Solar Energy – Fundamentals, Design, Modeling and Applications – 2002, by G. N Tiwari Solar Heating and Cooling - Active and Passive design – Second Edition - 1982, by Jan F. Kreider, Frank Kreith