Solar thermal technologies convert the sun's radiant energy into heat to warm up water or air. A solar collector, placed outside in an area exposed to the sun, absorbs sunlight and radiates heat -- in the process heating water, air or a heat transfer fluid that circulates through it. Solar-heated water can be used to provide domestic hot water, heated pool water, space heating or cooling, or to produce steam for running an industrial-sized power generator. Solar-heated air can also be used for space heating. There are many applications that save users money while reducing pollution.
The most common solar thermal use is for domestic water needs. Solar water heating can be done efficiently in almost any climate. Already over 200,000 commercial buildings, 1.5 million homes and a growing number of indoor and outdoor pools heat their water with the sun. Solar water heaters can typically handle 40 to 80 percent of annual water heating needs. The remaining demand is usually covered by a conventional heater acting as backup. According to the Interstate Renewable Energy Council, residential solar water heaters in the Sun Belt save enough money in averted fuel costs to pay for themselves in 3-8 years, depending on the amount of water used, electric utility rates and the specific geographic location.
HOW A SOLAR WATER HEATER WORKS
A solar hot water heater has three main components: a collector, a storage tank and a system to circulate fluids to and from the collector. Even though collectors are mounted outdoors, solar water heaters can be used year-round in any climate. Often in colder climates, a fluid with a lower freezing point than water, for example propylene glycol, is heated in the collector. When it reaches the appropriate temperature, it is directed to the storage tank where its heat is exchanged with stored water. These are called closed-loop systems. In climates where the temperature rarely drops below freezing, water can be heated in the collector and used directly. These are called open-loop systems.
In warmer climates, users have the option of using active or passive systems. Passive systems are less expensive to buy and maintain because they use gravity, water pressure or the tendency of hot water to rise, instead of pumps, controls, sensors or other mechanical parts, to circulate water. There are two types of passive systems. In integral collector/ storage systems (also called "breadbox" or "batch" water heaters), the collector doubles as a storage tank. Gravity moves the water from the roof into the building. In thermosyphon systems, a separate storage tank is located above the collector.
In colder climates, users often choose active systems. An active system uses an electric pump to supply fluid to the collector and to circulate the heated fluid to a storage tank or point of use. In buildings where hot water is only needed during sunshine hours, a photovoltaic panel can be used to supply electricity for the pump. Often a temperature sensing system is used to turn the pump on when the temperature on the roof is warmer than the water in the storage tank.
There are three basic types of solar thermal collectors. Flat-plate collectors, the most common, are shallow, rectangular, insulated boxes made of rubber, plastic, metal or wood. Glazed flat-plate collectors feature a transparent top and a dark-colored base absorber, often copper or aluminum. Tubes carrying either water or a heat transfer fluid run just above the plate. Glazed flat-plate collectors are often used for domestic hot water needs. Unglazed flat-plate collectors are non-transparent and don't utilize a metal absorber. They are less expensive than glazed collectors and are often used for heating swimming pools. In evacuated-tube collectors, the fluid-carrying tube is typically encased in a larger tube in which a vacuum has been created. The vacuum between tubes increases the inner tube's insulation, decreasing heat loss before the water or fluid is delivered to its end use and increasing the collector's efficiency. Lastly, parabolic trough collectors use long U-shaped mirrors to focus sunlight onto a fluid-filled tube that runs along the middle of the arc. Parabolic trough collectors are the most efficient, but unlike flat-plate and evacuated-tube collectors, they can only use direct light. Parabolic trough collectors are often used in structures with large hot water demands, such as prisons and hospitals.
Since it is inefficient to store hot water for several days, it often makes sense to use an electrical or gas- powered heater as a backup. Without a backup system, clouds or extra demand may cause shortages. Combining a conventional hot water heater with a solar hot water collector is easy because the solar collector becomes an extension of the pipes leading to the storage tank.
WHERE ARE SOLAR WATER HEATERS BEING USED?
Solar water heating systems are used in a variety of settings. In residential buildings, including public housing complexes and dormitories, they supply hot water for showers, cleaning and laundry services. In commercial buildings they can provide hot water for cafeterias and recreation facilities. Hospitals, prisons and animal shelters, which use large amounts of hot water, are also good settings for solar water heaters. Solar-heated swimming pools are becoming increasingly common because they have the shortest payback period of solar water heating applications.
HOW COST-EFFECTIVE ARE THEY?
Many factors affect how much money a solar water heater will save, including local utility rates, incident solar radiation levels and water usage. But despite the variables involved, the life-cycle cost of solar-heated water is generally much lower than using electricity. Without factoring in maintenance, the cost of generating solar-heated water is still greater than the cost of natural gas, but is as low as half the price of buying electricity and about the same as bottled propane gas, according to the American Solar Energy Society. When all life-cycle costs are included, solar water heaters fare even better in comparison to electric and propane gas water heaters, and begin to catch up to natural gas. In addition, solar water heaters require less maintenance and result in lower cooling costs in buildings which would have to counteract unwanted heat emitted from indoor water boilers if an electrical or gas system was used.
Also, if solar hot water is factored in at the beginning, it can enable the facility manager to purchase a smaller conventional hot water heater than would otherwise be required, and only use it as a backup.
Recent advances in technology are partly responsible for the cost-effectiveness of solar water heaters. Between 1980 and 1990, the costs of flat-plate collectors fell 30 percent as a result of improved designs and manufacturing techniques. Presently, installed flat-plate systems cost $40-65 per square foot of collector area.
Solar water heaters are also very reliable, lasting 15-20 years. According to the Solar Energy Industries Association, surveys taken over the last ten years have found that more than 94 percent of solar water heater owners believed their system was a wise investment.
Some solar water heating systems are particularly affordable. Solar-heated swimming pools, which require relatively small elevations in water temperature and do not require a storage tank, have an average payback period of three years, according to the Solar Energy Research and Education Foundation (see Swimming In Energy Savings, p. 19). Passive solar water heating systems are the least expensive solar water heaters because they use the least equipment and are virtually maintenance-free. As with photovoltaic systems, any solar water heating system becomes more cost-effective when the real costs of using polluting energy are factored into cost comparisons.
FINDING A RELIABLE SYSTEM
One of the best ways to determine how well a solar collector or solar water heater will perform is to consult the Solar Ratings and Certification Corporation (SRCC). The SRCC is a nonprofit council directed by state government, solar industry and consumer group representatives. SRCC verifies manufacturers' claims by testing and certifying the performance, durability and safety of the majority of major manufacturers' solar hot water collectors and systems. These findings are published in SRCC's certified products directory, available for $25. For More Info: Solar Thermal Resources, p. 22.
Solar heated pools are also cost-effective because:
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* Heated pool water is usually between 76 and 80 degrees Fahrenheit; thus
collectors need only to provide a relatively small elevation in water
temperature. * There is often no need for an expensive storage tank -- the pool itself serves that function. * The solar water heating system can use the pool's filtration pump to move water through the collectors, minimizing the necessity for additional equipment.
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The Department of Energy's Reduce Swimming Pool Energy Costs (RESPEC) Program helps potential users estimate the costs, savings and payback of using a solar pool heating system. These estimates can be used, as they were last year in Springfield, Oregon and in Washington, DC, to qualify a project for utility-sponsored energy conservation rebate programs. RESPEC offers free analyses over the phone, and free copies of the Energy Smart software used to project savings based on local variables. To obtain a phone analysis, Energy Smart software or more information about RESPEC, contact the DOE's Energy Efficiency and Renewable Energy Clearinghouse (see Additional Resources, p. 28).
The solar system heats the Beck Center's domestic hot water supply as well as its year-round Olympic-size swimming pool. It provides nearly 100 percent of the energy needed to heat the pool in the summer and early fall. The pool is normally kept at 85 degrees Fahrenheit, a temperature slightly warmer than many pools since the Center specializes in classes for younger children and elderly persons. The solar heating system offers an increased level of comfort for these people, since it adds energy to the pool gradually, without the large temperature swings that often result from using a conventional backup heater.
The company that installed the system, Industrial Solar Technology (IST) Corporation, assumes all responsibility for maintenance. According to IST, the only upkeep required is a monthly cleaning of the collectors, at a total cost of $600/year. The pool's staff need only record the output of the Btu meter, which measures the amount of energy produced by the collectors.
Finally, third party investors paid for the solar technology, and they are recouping their investment through the steady sale of energy to the City of Aurora. The payback period (the point at which the savings exceeds the purchase cost) was 8-10 years. The pool, then, is a winning situation for all parties, especially Aurora. The City didn't pay for the installation or manufacturing costs, it doesn't incur additional maintenance fees, it saves money, and it reduces its consumption of fossil fuels.
For More Info: Rich Abrahams, Recreation Director, Paul Beck Recreation Center, 800 Telluride, Aurora, CO 80011; (303) 695-7200. Randy Gee, IST Corp., 4420 McIntyre Street, Golden,CO 80403; (303) 279-8107.
In 1986, in Adams County, Colorado, a solar water heater manufacturer, the state energy office and a collection of "green investors" combined funds to finance a solar hot water collector system at the Adams County jail. Rather than owning and operating the system, the jail managers pay the investors for the hot water at 70 percent of what they would pay for natural gas. While the jail saves several thousand dollars in energy and maintenance costs each year, most of the investors have earned their investment back within five years.
The Adams County Detention Facility solar water heater supplies about 46 percent of the hot water needed for personal hygiene, kitchen and laundry -- about 19,200 gallons per day. 7,800 square feet of parabolic trough collectors warm a heat transfer fluid which heats potable water in a 5,000 gallon thermal energy storage tank. The system supplies 1,400 million Btus annually. A natural gas-fired boiler acts as a backup. The cost of installation in 1993 dollars was approximately $28 per square foot.
For More Info: Marc Roper, Colorado Office of Energy Conservation, 1675 Broadway #1300, Denver, CO 80202; (303) 620-4292; Fax: (303) 620-4288; E-mail: mroper@csn.net.
The solar water heating system annually supplies 6,000 million Btus of energy. The system has been so successful that the Department of Corrections is presently initiating new private investor-owned solar heating projects at several of its other facilities.
For More Info: Harry Franey, Energy Manager, CA Department of Corrections, PO Box 942883, Sacramento, CA 94283; (916) 327-1134.
Privately-owned systems are not necessary for a solar water heating project to be affordable. The following is one of many examples of public facilities that have used solar water heating --and saved money -- without private investment.
In 1982, about 4,000 square feet of closed-loop propylene glycol solar collectors were installed on the Massachusetts State Transportation Building in Boston. The system cost $250,000 to purchase and install, and saves $26,280 per year in avoided electricity costs. At this rate, the solar water heating system paid for itself in 9.5 years and has been earning net savings for the Transportation office since 1993. The closed-loop propylene glycol system enables the solar water heaters to operate year-round, even though the outside temperature remains below freezing for extended periods of time. The collectors supply 83 percent of the building's annual domestic hot water needs, offsetting approximately 5,800 gallons of oil annually.
For More Info: Martha Goldsmith, Director, Office of Leasing, Commonwealth of Massachusetts, 1 Ashburton Place, 15th Floor, Boston, MA 02108; (617) 727-8000.
A transpired solar collector is a dark-colored, perforated, metal panel that mounts on the exterior of a building's south-facing wall [See Illustration]. A gap is left between the panel and the interior wall. Ventilation fans at the top of the resulting cavity suck air through the tiny holes in the metal, into the air cavity, and up toward the top of the panel. The air is heated by its proximity to the hot metal. When it reaches the ventilation fans, the now-heated air is circulated through the building's air ducts. A transpired collector can be formed to fit with most types of conventional building facades.
The transpired collector is capable of reducing annual heating costs $1-3 per square foot. With these savings, newly constructed transpired collectors pay for themselves in an average of three years. A retrofit typically pays for itself in six to seven years. Two reasons for these savings are the collector's ability to increase the temperature of incoming air by as much 54 degrees F, and its ability to counter air stratification which wastes energy when warm air is trapped near the ceiling. The transpired collector also creates energy savings by dramatically decreasing the amount of heat lost through the wall inside the metal panel shell, giving the inner wall a resistance to heat transmission of R-50, an exceptionally high insulation rating. The heat that escapes through the inner wall is transferred to the fresh air moving up the pocket and into the heating ducts. In the process, virtually no heat is lost. Overall, this process enables building operators to insulate their buildings well without sacrificing high indoor air quality. This effect also minimizes the need for auxiliary heating once the sun goes down since heat collected during the day can be retained throughout the night. The same effect enables heat gathered on a sunny day to warm a building on later days if the weather turns cloudy.
A cloudy day, however, need not be a complete loss for buildings equipped with a transpired collector because the metal panel is able to utilize diffuse light, which comprises about 25 percent of annual solar radiation. Ironically, snow actually boosts the transpired collector's heating performance because snow ground-cover can reflect up to 70 percent additional solar radiation onto the panel, enabling it to absorb more heat.
Award-Winning Solar Collector Heats Air, Lowers Costs
Over the last few years, a new, award-winning "transpired" solar collector has helped industrial, school and residential buildings lower energy expenses by preheating ventilation air twice as efficiently as any other active solar heater. In the process it has made the 1994 R&D Top 100, earned a rating in the top two percent of all energy innovations from the US Department of Energy and was included in the "Best of What's New" from Popular Science magazine. What may be most promising about this invention is that it has proved itself in the cloudier areas of the northern United States and Canada.
A transpired solar collector -- also known as a "solar wall" -- is a dark-colored, perforated, metal panel that mounts on the exterior of a building's south-facing wall [see Illustration]. A gap is left between the panel and the interior wall. Ventilation fans at the top of the resulting cavity suck air through the tiny holes in the metal, into the air cavity, and up toward the top of the panel. The air is heated by its proximity to the hot metal. When it reaches the ventilation fans, the now-heated air is circulated through the building's air ducts. A transpired collector can be formed to fit most types of conventional building facades.
The "solar wall" is capable of reducing annual heating costs $1-3 per square foot. With these savings, newly constructed transpired collectors pay for themselves in an average of three years. A retrofit typically has a 6-7 year payback period. Two reasons for these savings are the collector's ability to increase the temperature of incoming air by as much 54 degrees Fahrenheit and to counter air stratification which wastes energy when warm air is trapped near the ceiling. The transpired collector also saves energy by dramatically decreasing the amount of heat lost through the wall inside the metal panel shell, giving the inner wall a resistance to heat transmission of R-50, an exceptionally high insulation rating. The heat that escapes through the inner wall is transferred to the fresh air moving up the pocket and into the heating ducts. In the process, virtually no heat is lost. Overall, this technology enables building operators to insulate their buildings well without sacrificing indoor air quality. The "solar wall" also minimizes the need for auxiliary heating once the sun goes down since heat collected during the day is retained throughout the night. Similarly, heat gathered on a sunny day can warm a building on later days if the weather turns cloudy.
Even a cloudy day is not a complete loss for buildings equipped with a transpired collector because the metal panel can utilize diffuse light, which comprises about 25 percent of annual solar radiation. Ironically, snow actually boosts the transpired collector's heating performance because snow ground-cover can reflect up to 70 percent additional solar radiation onto the panel, enabling it to absorb more heat.
The transpired collector's usefulness is not limited to cold, snowy climates. Just as it lowers a building's heating costs, it can also lower cooling costs. Unlike a typical wall, a wall outfitted with a transpired solar collector will not bring in unwanted outside heat. During hotter months, ventilation fans draw in heated air through the metal panel. But instead of funnelling it into the building, bypass dampers selectively whisk the hot air outside by capitalizing on the tendency of hot air to rise. As a result, virtually no hot air has a chance to come in contact with, or radiate through, the inner wall.
One reason why the transpired collector can deliver these benefits at a manageable cost is its efficiency. Tests by the National Renewable Energy Laboratory (NREL) and the Canadian National Solar Test Facility have found that the collector can absorb and utilize up to 80 percent of the available solar energy it comes in contact with. In contrast, common solar air heaters which use glazed, flat-plate collectors are only 35-40 percent efficient, according to NREL's Chuck Kutscher. The cost of the transpired collector is also about half the cost of glazed air heaters. Because the only moving parts are ventilation fans, no additional fossil fuels are necessary and the system is virtually maintenance-free.
The transpired collector can be effective in any building that requires, or could benefit from, heated ventilation or makeup air. Industrial, commercial and institutional buildings that have substantial ventilation requirements are ideally suited for this technology. Apartment buildings, buildings with roof fans and penthouses, hospitals, schools, government and military buildings, and new air-tight homes can also benefit from a transpired collector.
When the Windsor, Ontario Housing Authority needed to renovate the exterior of its Ouellette Manor Senior Citizens Building in 1993, it installed transpired collector cladding. In addition to improving the appearance of the building, the "solar wall" reduced the amount of energy needed for heating by an estimated 240 kWh per year, saving the Housing Authority $4,860 annually, even though the natural gas it previously used for heating came at the low price of two cents per kWh.
At the 24-story Senior Citizens Building, the transpired collector covers only a small portion of the south-facing wall. The ventilation fans are located on the roof where they continually provide 13,500 cubic feet per minute of fresh air to the building. The six gas-fired heaters that predated the collector act as backup. If the solar-heated air is insufficiently warm, a damper in the collector duct opens, feeding the air to the intake fans of the gas-powered heaters. Otherwise, the solar-heated air bypasses these heaters and directly provides warm air to the building.
At a cost of $30,000 more than conventional wall cladding, the dark brown aluminum, 3,580 ft2 transpired collector will pay for itself in just over six years.
The Ontario Ministry of Housing has so far installed similar transpired collectors on four other apartment buildings.
For More Info: George Robson, Windsor Housing Authority, Box 1330,Windsor, Ontario N9A 6R3, CANADA; (519) 254-1681; Fax: (519) 254-3130.
In February 1996, the Fort Carson military base in Colorado Springs, Colorado completed installation of a transpired collector retrofit on its AVUM Helicopter Hangar. According to the manufacturer's computer simulations, which have so far proved to be accurate within five percentage points, the collector is projected to annually save $14,000 worth of natural gas (3,324 million Btus). This figure includes savings from heating and destratification. The Fort Carson transpired collector is expected to pay for itself in 6-7 years.
The "solar wall" preheats ventilation air that is used to counter diesel fumes from the fuel tanks of the helicopters housed inside. 7,800 ft2 of perforated metal collectors are positioned above the two hangar doors.
The collector was inexpensive enough to be paid for out of the normal construction budget.
For More Info: Steve Snyder, Pollution Prevention Division Chief, HQ Fort Carson Bldg. 302, DECAM, Fort Carson, CO 80913-5000; (719) 526-1684; Fax: (719) 526-1705.
DOE Solar Process Heat Program. The National Renewable Energy Laboratory (NREL) and the Solar Thermal Design Assistance Center provide technical services to prospective and actual users of water heating and other solar thermal systems, including assistance in determining whether "going solar" makes sense economically and technically. Many of these services are free. The program also makes cost-sharing funds available for qualifying new systems and retrofits. For More Info: Russell Hewett, Solar Process Heat Program, NREL, 1617 Cole Blvd., Golden, CO 80401; (303) 384-7463; Fax: (303) 384-7495. Solar Thermal Design Assistance Center; David Menicucci, Sandia National Lab., PO Box 5800, Albuquerque, NM 87185; (505) 844-3077; Fax: (505) 845-9500.
Catalog of Successfully Operating Solar Process Heat Systems, Solar Energy Industries Association, 1995. Directory of the Solar Thermal Industry, Solar Energy Industries Association, updated annually. The catalog presents 30 case studies of successful solar thermal systems currently being used in government and commercial settings for water heating, space cooling and air pre-heating. The publication costs $25. The directory provides information on solar thermal products available from USmanufacturing, engineering and design companies. For More Info: SEIA, Attn: Publications, 122 C Street, NW, 4th Floor, Washington, DC 20001; (202) 383-2600; Fax: (202) 383-2670. Orders also available on SEIA's Web: http://solstice.crest.org/renewables/seia/ catalogue.html.
Directory of SRCC Certified Solar Collector and Water Heating System Ratings, Solar Ratings Certification Corporation, updated annually. The nonprofit Solar Ratings and Certification Corporation (SRCC) tests the performance, durability and safety of a majority of major manufacturers' solar hot water collectors and systems. Their results, including certified efficiency ratings, are published in the SRCC Certified Products Directory. The publication costs $25. For More Info: SRCC, 122 C Street, NW, Washington, DC 20001; (202) 383-2570, Fax: (202) 383-2670.