Solar Domestic Hot Water Heating Systems. Design, Installation and Maintenance

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1 Solar Domestic Hot Water Heating Systems Design, Installation and Maintenance Presented by: Christopher A. Homola, PE

2 A Brief History of Solar Water Heating Solar water heating has been around for many years because it is the easiest way to use the sun to save energy and money. One of the earliest documented cases of solar energy use involved pioneers moving west after the Civil War. They would place a cooking pot filled with cold water in the sun all day to have heated water in the evening. The first solar water heater that resembles the concept still in use today was a metal tank that was painted black and placed on the roof where it was tilted toward the sun. The concept worked, but it usually took all day for the water to heat, then, as soon as the sun went down, it cooled off quickly because the tank was not insulated.

3 A Brief History of the American Solar Water Heating Industry 1890 to 1930's - the California Era The first commercial solar water heater was introduced by Clarence Kemp in the 1890's in California. For a $25 investment, people could save about $9 a year in coal costs. It was a simple batch type solar water heater that combined storage and collector in one box. The first thermosyphon systems with the tank on the roof and the collector below were invented, patented, and marketed in California in the 1920's by William Bailey. One of the largest commercial systems in California was installed for a resort in Death Valley. Natural gas was discovered in Southern California and cheap natural gas, aggressively marketed by utility companies, ended the solar water heating market. Patents were sold to a Florida company, owned by HM Carruthers in 1923 and the solar hot water industry began in the coastal cities of central Florida and southern Florida.

4 1930's to the South Florida Era Floridians purchased or shipped to the Caribbean more than 100,000 thermosyphon water heaters between 1930 and 1954 when the industry collapsed. During the second World War (1942 to 1945) copper was reserved for the military and the solar industry was not able to make solar collectors. After the war, the Florida industry boomed again for about six years. Half of Miami homes had solar water heaters with over 80% of new homes having them installed. In the early 1950's electricity became cheap in Florida and utility companies gave away electric water heaters in an effort to eliminate the solar water heating industry. By 1973, there were only two full-time solar water heating companies left in the United States both operating out of Miami, Florida.

5 1973 to Oil Embargo and Tax Credits The oil embargo of 1973 resulted in a rise in fuel prices. A few companies started experimenting with solar water heaters and designing systems but there were really no national solar collector manufacturers with widespread distribution until the late seventies. The federal government sponsored a few HUD Grants for domestic solar water heaters in the period just before the start of the 40% Federal tax rebate in The tax credit era, 1979 to 1986, started a nationwide boon in solar hot water systems that resulted in hundreds of manufacturers and thousands of contractors and distributors starting new businesses.

6 Equipment has improved since the 1980 s. Improvements were precipitated by both certification design review and experienced installers. Today, more than 1.2 million buildings have solar water heating systems in the United States. Japan has nearly 1.5 million buildings with solar water heating. In Israel, 30 percent of the buildings use solarheated water. Greece and Australia are also leading users of solar energy. There is still a lot of room for expansion in the solar energy industry. There are no geographical constraints. For colder climates, manufacturers have designed systems that protect components from freezing conditions. Wherever the sun shines, solar water heating systems can work. The designs may be different from the early solar pioneers, but the concept is the same.

7 Environmental Benefits Solar water heaters do not pollute. Solar water heaters help to avoid carbon dioxide, nitrogen oxides, sulfur dioxide, and the other air pollution and wastes created when the local utility generates power or fuel is burned to heat domestic 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.

8 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, owners could be cushioned from future fuel shortages and price increases. Solar water heaters can assist in reducing this country's dependence on foreign oil. It is estimated that adding a solar water heater to an existing home raises the resale value of the home by the entire cost of the system. Homeowners may be able to recoup their entire investment they sell their home.

9 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. The Florida Solar Energy Center studied the potential savings to Florida homeowners of common water heating systems compared with electric water heaters. It found that solar water heaters offered the largest potential savings, with solar water heater owners saving as much as 50% to 85% annually on their utility bills over the cost of electric water heating.

10 Economic Benefits Continued A solar hot water heater heats the same amount of water for a fraction of the cost. A solar hot water heating system s performance is dependent on the intensity of the sun in its location. The initial expense of installing a solar hot water heater ($3500 to $5500) tends to be greater than installing an electric ($450 to $650) or gas ($750 to $1000) water heater. The costs vary from region to region. Depending on the price of fuel sources, the solar water heater can be more economical over the lifetime of the system than heating water with electricity, fuel oil, propane, or even natural gas because the fuel (sunshine) is free.

11 Economic Benefits Continued 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 on a dollar for dollar basis. Paybacks vary widely, but you can expect a simple payback of 4 to 8 years on a well designed and properly installed solar water heater. You can expect shorter paybacks in areas with higher energy costs. After the payback period, you accrue the savings over the life of the system, which ranges from 15 to 40 years, depending on the system and how well it is maintained.

12 Economic Benefits Continued You can determine the simple payback of a solar water heater by first determining the net cost of the system. Net costs include the total installed cost less any tax incentives or utility rebates. After you calculate the net cost of the system, calculate the annual fuel savings and divide the net investment by this number to determine the simple payback. An example: Your total utility bill averages $160 per month and your water heating costs are average (25% of your total utility costs) at $40 per month. If you purchase a solar water heater for $2,000 that provides an average of 60% of your hot water each year, that system will save you $24 per month ($40 x 0.60 = $24) or $288 per year (12 x $24 = $288). This system has a simple payback of less than 7 years ($2,000 $288 = 6.9).

13 For the remainder of the life of the solar water heater, 60% of the hot water will be free, saving $288 each year. You will need to account for some operation and maintenance costs, which are estimated at $25 to $30 a year. This is primarily to have the system checked every 3 years. If you are building a new home or refinancing your present home to do a major renovation, the economics are even more attractive. The cost of including the price of a solar water heater in a new 30 year mortgage is usually between $13 and $20 per month. The portion of the federal income tax deduction for mortgage interest attributable to the solar system reduces that amount by about $3 to $5 per month. If your fuel savings are more than $15 per month, the investment in the solar water heater is profitable immediately.

14 Peak Power Benefit A typical residential solar water heating system (SWHS) for a family of four delivers 4 kilowatts of electrical equivalent thermal power when under full sun and when the temperature of the water in the storage tank is about the same as the air temperature. Such a system typically has about 64 square feet of solar collector surface area and produces approximately the same peak power as 400 square feet of photovoltaic panels.

15 Production Capacity Benefit Ratings of collectors and systems, along with other information specific to the local area, can be used to calculate the specific reduction in a utility s peak demand. On average, for every solar water heating system that is installed, 0.5 kilowatts of peak demand is deferred from a utility s load.

16 Energy Production Benefit Because peak performance occurs infrequently, a more realistic indication of solar thermal system performance is the rated daily energy output of the collectors or system. Using this method, a typical solar water heating system contributes 7 to 10 kilowatt-hours per day, depending on the solar resource and type of collector. Electric water heating for residential applications typically consumes about 12 kilowatt-hours per day, depending on ground water temperature. Annual site-specific energy savings for domestic water heating systems are available at for all systems certified by the Solar Rating and Certification Corporation (SRCC). Using this data, a typical solar water heating system produces about 3,400 kilowatt-hours per year, depending on local conditions and type of collector.

17 What Influences the Amount of Solar Radiation? Atmosphere Angle of Incidence Geography Latitude and Season Air Pollution and Natural Haze

18 Atmosphere The atmosphere absorbs certain wavelengths of light more than others. The exact spectral distribution of light reaching the earth's surface depends on how much atmosphere the light passes through, as well as the humidity of the atmosphere. In the morning and evening, the sun is low in the sky and light waves pass through more atmosphere than at noon. The winter sunlight also passes through more atmosphere versus summer. In addition, different latitudes on the earth have different average thicknesses of atmosphere that sunlight must penetrate. The figure below illustrates the atmospheric effects on solar energy reaching the earth. Clouds, smoke and dust reflect some solar insolation back up into the atmosphere, allowing less solar energy to fall on a terrestrial object. These conditions also diffuse or scatter the amount of solar energy that does pass through.

19 Angle of Incidence The sun s electromagnetic energy travels in a straight line. The angle at which these rays fall on an object is called the angle of incidence. A flat surface receives more solar energy when the angle of incidence is closer to zero (i.e. perpendicular) and therefore receives significantly less in early morning and late evening. Because the angle of incidence is so large in the morning and evening on earth, about six hours of usable solar energy is available daily. This is called the solar window.

20 Absorptance vs. Reflectance Certain materials absorb more insolation than others. More absorptive materials are generally dark with a matte finish, while more-reflective materials are generally lighter colored with a smooth or shiny finish. The materials used to absorb the sun's energy are selected for their ability to absorb a high percentage of energy and to reflect a minimum amount of energy. The solar collector's absorber and absorber coating efficiency are determined by the rate of absorption versus the rate of reflectance. This in turn, affects the absorber and absorber coating's ability to retain heat and minimize emissivity and reradiation. High absorptivity and low reflectivity improves the potential for collecting solar energy.

21 Collecting and Converting Solar Energy Solar collectors capture the sun s electromagnetic energy and convert it to heat energy. The efficiency of a solar collector depends not only on its materials and design but also on its size, orientation and tilt. Available solar energy is at its maximum at noon, when the sun is at its highest point in its daily arc across the sky. The sun's daily motion across the sky has an impact on any solar collector's efficiency and performance in the following ways. 1.Since the angle of incidence of the solar energy measured from the normal (right angle) surface of the receiving surface changes throughout the day, solar power is lower at dawn and dusk. In reality, there are only about 6 hours of maximum energy available daily. 2.The total energy received by a fixed surface during a given period depends on its orientation and tilt and varies with weather conditions, time of day and season.

22 Insolation Insolation is the amount of the sun s electromagnetic energy that falls on any given object. Simply put, when we are talking about solar radiation, we are referring to insolation. In Florida (at about sea level), an object will receive a maximum of around 300 Btu/ft 2 hr (about 90 watts/ft 2 or 950 watts/meter 2 ) at high noon on a horizontal surface under clear skies on June 21 (the day of the summer equinox).

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24 PV Solar Radiation (Flat Plate, Facing South, Latitude Tilt) Static Maps These maps provide monthly average daily total solar resource information on grid cells of approximately 40 km by 40 km in size. The insolation values represent the resource available to a flat plate collector, such as a photovoltaic panel, oriented due south at an angle from horizontal to equal to the latitude of the collector location. Resource: National Renewable Energy Laboratory

25 Optimum Performance Considerations Optimum Tilt: To latitude for greatest performance or up to latitude minus 5degrees. Optimum Summer Load: Latitude minus 15 degrees (e.g. solar air conditioning). Optimum Winter Load: Latitude plus 15 degrees (e.g. solar space heating). Optimum Azimuth: Toward the equator (e.g. Facing south in northern hemisphere).

26 Figure 1. Sun Path Diagrams for 28º N. Latitude Seasonal Variations The dome of the sky and the sun s path at various times of the year are shown in Figure 1.

27 Figure 2a And 2b. Collected Energy Varies with Time of Year And Tilt For many solar applications, we want maximum annual energy harvest. For others, maximum winter energy (or summer energy) collection is important. To orient the flat-plate collector properly, the application must be considered, since different angles will be best for each different application.

28 Actual Collector Orientation Possibilities

29 Collector Orientation Collectors work best when facing due south. If roof lines or other factors dictate different orientations, a small penalty will be paid, as shown in Figure 3. As an example: for an orientation 20 degrees east or west of due south, we must increase the collector area to 1.06 times the size needed with due south orientation (dashed line on Figure 3) to achieve the same energy output. The orientation angle away from due south is called the azimuth and, in the Northern Hemisphere, is plus if the collector faces toward the east and minus if toward the west. Figure 3. Glazed Collector Orientations

30 Tilt Angle The best tilt angle will vary not only with the collector s geographical location but also with seasonal function. Solar water heating systems are designed to provide heat year-round. In general: A)Mounting at an angle equal to the latitude works best for yearround energy use. B)Latitude minus 15 degrees mounting is best for summer energy collection. C)Latitude plus 15 degrees mounting is best for winter energy collection.

31 Various Collector Tilt Angles

32 Solar Water Heating System Basics Solar water heating systems include storage tanks and solar collectors. There are two types of solar water heating systems: Active, which have circulating pumps and controls, and Passive, which don t. Most solar water heaters require a well insulated storage tank. Solar storage tanks have an additional outlet and inlet connected to and from the solar collector. In two tank systems, the solar water heater preheats water before it enters the conventional water heater. In one tank systems, the back up heater is combined with the solar storage in one tank.

33 Electric Back-Up Solar systems with single tanks are designed to encourage temperature stratification so that when water is drawn for service, it is supplied from the hottest stratum in the tank (i.e. top of tank). While a solar system tank in the United States normally contains a heating element, the element is deliberately located in the upper third of the tank. The electric element functions as back-up when solar energy is not available or when hot water demand exceeds the solar-heated supply.

34 Natural Gas Back-Up Natural gas back-up systems may use passive (thermosyphon or integral collector system) solar preheating plumbed in series for proper operation. Or two separate tanks may be used for active solar systems with natural gas back-up heating systems. The solar storage tank is piped in series to the auxiliary tank sending the hottest solar preheated water to the gas back-up tank.

35 Solar Collectors Four types of solar collectors are used for residential applications: Flat plate collector Integral collector storage systems Batch system Evacuated tube solar collectors

36 Flat Plate Collector Flat plate collectors are designed to heat water to medium temperatures (approximately 140 degrees Fahrenheit).

37 Flat plate collectors typically include the following components: 1.Enclosure: A box or frame that holds all the components together. 2.Glazing: A transparent cover over the enclosure that allows the sun s rays to pass through to the absorber. Most glazing is glass but some designs use clear plastic. 3.Glazing Frame: Attaches the glazing to the enclosure. Glazing gaskets prevent leakage around the glazing frame and allow for contraction and expansion. 4.Insulation: Material between the absorber and the surfaces it touches that blocks heat loss by conduction thereby reducing the heat loss from the collector enclosure. 5.Absorber: A flat, usually metal surface inside the enclosure that, because of its physical properties, can absorb and transfer high levels of solar energy. 6.Flow Tubes: Highly conductive metal tubes across the absorber through which fluid flows, transferring heat from the absorber to the fluid.

38 Integral Collector Storage (ICS) Systems In other solar water heating systems the collector and storage tank are separate components. In an integral collector storage (ICS) system, both collection and solar storage are combined within a single unit. Most ICS systems store potable water inside several tanks within the collector unit. The entire unit is exposed to solar energy throughout the day. The resulting water is drawn off either directly to the service location or as replacement hot water to an auxiliary storage tank as water is drawn for use. Cutaway of an ICS system

39 Batch System Batch solar water heater

40 The simplest of all solar water heating systems is a batch system. It is simply one or several storage tanks coated with black, solar-absorbing material in an enclosure with glazing across the top and insulation around the other sides. It is the simplest solar system to make. When exposed to sun during the day, the tank transfers the heat it absorbs to the water it holds. The heated water can be drawn directly from the tank or it can replace hot water that is drawn from an interior tank inside the building.

41 Evacuated Tube Solar Collectors This type of system features parallel rows of transparent glass tubes. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin s coating absorbs solar energy but inhibits radiative heat loss. These collectors are used more frequently for commercial applications.

42 Evacuated-tube collectors generally have a smaller solar collecting surface because this surface must be encased by an evacuated glass tube. They are designed to deliver higher temperatures (approximately 300 degrees Fahrenheit). The tubes themselves comprise the following elements: 1.Highly tempered glass vacuum tubes, which function as both glazing and insulation. 2.An absorber surface inside the vacuum tube. The absorber is surrounded by a vacuum that greatly reduces the heat loss.

43 Active Solar Water Heating Systems There are two Solar Water Heating System types: Active and Passive There are two types of Active Solar Water Heating Systems: Direct Circulation Systems Indirect Circulation Systems

44 Direct Circulation Systems Pump circulates domestic water through the collector(s) and into the building. This type of system works well in climates where it rarely freezes.

45 Direct Pumped System

46 Direct System with Photovoltaic Powered Pump

47 Direct System with Automatic Drain-down system configuration

48 The direct pumped system has one or more solar energy collectors installed on the roof and a storage tank located somewhere within the building. A pump circulates the water from the tank up to the collector and back again. This is called a direct (or open loop) system because the sun s heat is transferred directly to the potable water circulating through the collector and storage tank. Neither an anti freeze nor heat exchanger is involved. This system has a differential controller that senses temperature differences between water leaving the solar collector and the coldest water in the storage tank. When the water in the collector is about F warmer than the water in the storage tank, the pump is turned on by the controller. When the temperature difference drops to about 3 5 F, the pump is turned off. In this way, the water always gains heat from the collector when the pump operates. A flush type freeze protection valve installed near the collector provides freeze protection. Whenever temperatures approach freezing, the valve opens to let warm water flow through the collector. The collector should always allow for manual draining by closing the isolation valves (located above the storage tank) and opening the drain valves. Automatic recirculation is another means of freeze protection. When the water in the collector reaches a temperature near freezing, the controller turns the pump on for a few minutes to warm the collector with water from the storage tank.

49 Direct System Advantages Service water used directly from collector loop. No heat exchanger more efficient heat transfer to storage. Circulation pump (if needed) needs only to overcome friction losses system pressurized.

50 Direct System Disadvantages Quality of service water must be good to prevent corrosion, scale or deposits in components. Freeze protection depends on mechanical valves. Recommended in climates with minimal/no freeze potential, and good water quality.

51 Indirect Circulation Systems Pump circulates a non freezing, heat transfer fluid through the collector(s) and a heat exchanger. This heats the water that then flows into the home. This type of system works well in climates prone to freezing temperatures.

52 Indirect Pumped System Using Anti Freeze Solution

53 This system design is common in northern climates, where freezing weather occurs more frequently. An anti freeze solution circulates through the collector, and a heat exchanger transfers the heat from the anti freeze solution to the storage tank water. When toxic heat exchanger fluids are used, a double walled exchanger is required. Generally, if the heat exchanger is installed in the storage tank, it should be located in the lower half of the tank. A heat transfer solution is pumped through the collector in a closed loop. The loop includes the collector, connecting piping, the pump, an expansion tank and a heat exchanger. A heat exchanger coil in the lower half of the storage tank transfers heat from the heat transfer solution to the potable water in the solar storage tank. An alternative of this design is to wrap the heat exchanger around the tank. This keeps it from contact with the potable water. The differential controller, in conjunction with the collector and tank sensors, determines when the pump should be activated to direct the heat transfer fluid through the collector. The photovoltaic panel located on the roof supplies the power to operate the circulating pump.

54 Indirect Pumped System Using Anti Freeze Solution and Wrap Around Heat Exchanger

55 A fail safe method of ensuring that collectors and collector loop piping never freeze is to remove all the water from the collectors and piping when the system is not collecting heat. This is a major feature of the drain back system. Freeze protection is provided when the system is in the drain mode. Water in the collectors and exposed piping drains into the insulated drain back reservoir tank each time the circulating pump shuts off. A slight tilt of the collectors is required in order to allow complete drainage. A sight glass attached to the drain back reservoir tank shows when the reservoir tank is full and the collector has been drained. In this particular system, distilled water is recommended to be used as the collector loop fluid transfer solution. Using distilled water increases the heat transfer characteristics and prevents possible mineral buildup of the transfer solution. When the sun shines again, the circulating pump is activated by a differential controller. Water is pumped from the reservoir to the collectors, allowing heat to be collected. The water stored in the reservoir tank circulates in a closed loop through the collectors and a heat exchanger at the bottom of the storage tank. The heat exchanger transfers heat from the collector loop fluid to the potable water located in the storage tank.

56 Indirect System Advantages Freeze protection provided by antifreeze fluid or drainback. Collector/piping protected from aggressive water.

57 Indirect System Disadvantages Must account for reduced heat transfer efficiency through heat exchanger. Added materials = added cost. If not using water, fluids require maintenance. Most designs require added pumping cost.

58 Passive Solar Water Heaters Passive solar water heaters rely on gravity and the tendency for water to naturally circulate as it is heated. Passive solar water heater systems contain no electrical components, are generally more reliable, easier to maintain, and possibly have a longer work life than active solar water heater systems. The two most popular types of passive solar water heater systems are: Integral-Collector Storage (ICS) andthermosyphon systems.

59 Integral Collector Storage System

60 In an integral collector storage system, the hot water storage system is the collector. Cold water flows progressively through the collector where it is heated by the sun. Hot water is drawn from the top, which is the hottest, and replacement water flows into the bottom. This system is simple because pumps and controllers are not required. On demand, cold water from the building flows into the collector and hot water from the collector flows to a standard hot water auxiliary tank within the building. A flush type freeze protection valve is installed in the top piping near the collector. As temperatures near freezing, this valve opens to allow relatively warm water to flow through the collect to prevent freezing. In areas of the country, the thermal mass of the large water volume within the integral collector storage collector provides a means of freeze protection.

61 Thermosyphon System

62 As the sun shines on the collector, the water inside the collector flowtubes is heated. As it heats, this water expands slightly and becomes lighter than the cold water in the solar storage tank mounted above the collector. Gravity then pulls heavier, cold water down from the tank and into the collector inlet. The cold water pushes the heated water through the collector outlet and into the top of the tank, thus heating the water in the tank. In a thermosiphon system there is no need for a circulating pump and controller. Potable water flows directly to the tank on the roof. Solar heated water flows from the rooftop tank to the auxiliary tank installed at ground level whenever water is used with the building. The thermosiphon system features a thermally operated valve that protects the collector from freezing. It also includes isolation valves, which allow the solar system to be manually drained in case of freezing conditions, or to be bypassed completely.

63 Typical Components of a Direct Flat Plate Collector System

64 AIR VENT Allows air that has entered the system to escape, and in turn prevents air locks that would restrict flow of the heat-transfer fluid. An air vent must be positioned vertically and is usually installed at the uppermost part of the system. In active direct systems supplied by pressurized water, an air vent should be installed anywhere air could be trapped in pipes or collectors. Indirect systems that use glycol as the heat-transfer fluid use air vents to remove any dissolved air left in the system after it has been pressurized or charged with the heat-transfer fluid. Once the air has been purged in these indirect systems, the air vent mechanism is manually closed. TEMPERATURE-PRESSURE RELIEF VALVE Protects system components from excessive pressures and temperatures. A pressuretemperature relief valve is always plumbed to the solar storage (as well as auxiliary) tank. In thermosiphon and ICS systems, where the solar tanks are located on a roof, these tanks may also be equipped with a temperature-pressure relief valve since they are in some jurisdictions considered storage vessels. These valves are usually set by the manufacturer at 150 psi and 210 F. Since temperature pressure relief valves open at temperatures below typical collector loop operating conditions, they are not commonly installed in collector loops. PRESSURE RELIEF VALVE Protects components from excessive pressures that may build up in system plumbing. In any system where the collector loop can be isolated from the storage tank, a pressure relief valve must be installed on the collector loop. The pressure rating of the valve (typically 125 psi) must be lower than the pressure rating of all other system components, which it is installed to protect. The pressure relief valve is usually installed at the collector.