Is Solar Still Active? Water Heating and Other Solar Thermal Applications
Those of us who were promoting solar in the late ’70s just knew that by the turn of the century solar would be a standard component of building design and a significant fraction of our national energy mix. The solar water heating industry would be burgeoning. Vast arrays of concentrating collectors in the desert Southwest would be generating electricity safely and affordably. Passive solar energy would be a standard part of building design. And photovoltaic modules would be cost-effectively powering homes and utility grids. The reality, of course, hasn’t lived up to this rosy vision. Indeed, we are nowhere near meeting projections from the 1970s of solar energy market penetration. One reason for this shortcoming is that while many of us solar advocates were singing the praises of solar energy—a supply-side answer to energy needs—others (fortunately) were approaching the issue from a different angle: touting demand-side solutions to our energy needs. Reducing demand for energy is almost always more cost effective than supplying it, no matter what our energy sources might be. Reduced demand also makes it more feasible to meet all or most of the remaining load with solar and other renewables.
This article provides an overview of one area of solar energy utilization: active solar or solar thermal, including solar water heating, high-temperature solar thermal, and transpired solar collector systems for ventilation-air preheating. Solar thermal has been out of the spotlight in recent years, while photovoltaic (PV) power generation and—to a lesser extent—passive solar heating garner most of the attention. Yet a lot is going on in the solar thermal arena, and there are some very exciting systems and applications that developers, designers, and builders should be aware of.
Those of us who were promoting solar in the late ’70s just
knew that by the turn of the century solar would be a standard component of building design and a significant fraction of our national energy mix. The solar water heating industry would be burgeoning. Vast arrays of concentrating collectors in the desert Southwest would be generating electricity safely and affordably. Passive solar energy would be a standard part of building design. And photovoltaic modules would be cost-effectively powering homes and utility grids.
The reality, of course, hasn’t lived up to this rosy vision. Indeed, we are nowhere near meeting projections from the 1970s of solar energy market penetration. One reason for this shortcoming is that while many of us solar advocates were singing the praises of solar energy—a
supply-side answer to energy needs—others (fortunately) were approaching the issue from a different angle: touting
demand-side solutions to our energy needs. Reducing
demand for energy is almost always more cost effective than supplying it, no matter what our energy sources might be. Reduced demand also makes it more feasible to meet all or most of the remaining load with solar and other renewables.
This article provides an overview of one area of solar energy utilization: active solar or
solar thermal, including solar water heating, high-temperature solar thermal, and transpired solar collector systems for ventilation-air preheating. Solar thermal has been out of the spotlight in recent years, while photovoltaic (PV) power generation and—to a lesser extent—passive solar heating garner most of the attention. Yet a lot is going on in the solar thermal arena, and there are some very exciting systems and applications that developers, designers, and builders should be aware of.
Defining Solar Thermal
Solar thermal energy systems collect energy from the sun and convert it into useable heat. The heat can be used for water heating, space heating, ventilation air preheating, process heat in industry, cooling (through absorption chillers), and electricity generation (usually through steam turbines). Solar thermal systems generally have solar collectors that are separate from the heat storage component and the energy end-use location. This separation of function is what generally differentiates
solar thermal or
active solar systems from
passive solar design, in which the building itself serves as the solar collector and heat storage/distribution system.
History of Solar Thermal
Efforts to make use of solar thermal energy have a rich history that is well told in
A Golden Thread by Ken Butti and John Perlin (1980, Cheshire Books). In the early 1500s, Leonardo da Vinci sketched concentrating collectors that would track the sun and focus sunlight onto a central receiver, and he began building such a collector in 1515. During the Industrial Revolution of the late 1800s and early 1900s, a number of well-known inventors built and successfully demonstrated solar-powered steam engines. In 1901, for example, the English-born inventor Aubrey Eneas built a 33-foot-diameter (10 m) concentrating collector that fueled a 15-horsepower (11 kW) pump to irrigate 300 acres (120 ha) of farmland with 1,400 gallons of water per minute (90 l/s).
In 1891, Clarence Kemp of Baltimore patented and began selling a solar collector for heating water. It had black tanks behind glass, and water was heated and stored in the collector. Available in eight sizes—from 35 gallons to 700 gallons (130 l to 2,650 l)—these were the first commercially marketed solar water heaters. When brought to California in 1895, Climax sales really took off; in 1900, sales in Southern California exceeded 1,600 units. The Day and Night solar water heater, introduced in California in 1909, separated the storage and collection function—the storage tank was situated above the flat-plate solar collectors and solar-heated water circulated into the tank through a process called
thermosiphoning (see below). In Florida, as many as ten companies, beginning with the Solar Water Heater Company, produced tens of thousands of solar water heaters from the early 1920s until 1941. On both coasts, the solar water heating industry ended with the outbreak of World War II, when resources and manufacturing capacity were diverted to the war effort.
Solar Thermal Today
The renaissance for solar thermal began in 1973 with the Arab Oil Embargo. Industrialized nations were suddenly crippled by a cartel of oil-producing nations, and attention turned again to the sun. In the United States, numerous research and demonstration programs were launched. Companies sprang up to capitalize on this interest. State and federal governments developed incentives, such as tax credits and low-interest loans, to spur the implementation of solar technologies.
Unfortunately, growth was too rapid, dependence on artificial market forces (such as tax credits) too great, and some technologies too complex or not well tested. When oil prices began dropping in the 1980s and as political support for solar disappeared with the election of President Reagan, the solar industry shrank dramatically. From hundreds of companies making solar water heaters and active solar heating systems in the early 1980s, only a few dozen remain today. At the large-system end of the scale, the Luz Corporation, which had built more that 350 megawatts of solar thermal electricity generation capacity in the Mojave Desert, went bankrupt (though the system lives on).
Despite the relatively low market demand, solar thermal technology is performing extremely well. Costs are lower, operation simpler, performance better, and durability greater than during the heyday of solar.
Solar Water Heating
Solar water heating systems run the gamut from very simple systems for swimming pools to large, sophisticated systems serving laundromats and industrial users. Solar water heating systems are commonly categorized as low-temperature or medium-temperature. Low-temperature systems generate temperatures up to 110°F (43°C); medium-temperature from 110°F to 180°F (43°C to 82°C). High-temperature solar collector systems, which generate temperatures over 180°F (82°C), are rarely used for water heating.
Checklist:Solar Thermal Equipment Manufacturers
Most low-temperature systems are used for pool heating, which represents by far the largest and most mature segment of the solar thermal industry. Of 8.1 million square feet (750,000 m2) in total solar collector shipments in 1997, low-temperature collectors for pool heating accounted for 93%, or 7.5 million square feet (700,000 m2). Pool heating systems are generally very simple: an unglazed polymer collector (black rubber or plastic absorbers with integral tubing) installed on a roof close to the pool, with a simple pump that circulates pool water through the collector and back into the pool. This relatively inexpensive system can easily extend the swimming season by several months. During months when freezing might occur, these systems are generally drained, though the polymer can usually survive some freezing without significant damage. Engineer Craig Christensen of the National Renewable Energy Laboratory (NREL) describes solar pool heaters as being fully accepted in the marketplace.
Medium-temperature solar water heating
To heat water for use in buildings, more sophisticated
medium-temperature systems are generally used. While these systems have the greatest potential for displacing large quantities of fossil fuel and electrical energy consumption, sales have unfortunately been declining. 1997 shipments of medium-temperature collectors totaled just 606,000 square feet (56,000 m
2), down 23% from 1996 and 39% from 1991.
There are a number of generic system types among medium-temperature solar water heaters. The simplest is the
integral collector storage (ICS) system, sometimes referred to as a
batch solar water heater. This is a passive system that requires no pumps or controls. The water is heated where it is stored (see figure). In the available systems, large-diameter copper pipes coated with a selective absorber surface are set in an insulated box behind high-transmissivity (low-iron) glass. Sun Systems, Inc. of Scottsdale, Arizona produces an ICS system designed for incorporation into roofs during construction—the glass cover plate of the collector ends up relatively flush with the roof surface, so the installed system looks just like a skylight from the exterior. The builder cost for this system is often less than $2,000. ICS systems do have some drawbacks, though: they are more prone to freezing in cold climates; they are often less suitable for retrofit applications because of the difficulty of integrating the large and heavy collector system into the roof structure; and it is inherently risky to have such a large volume of pressurized water up near the roof of a house (a leak can do a great deal of damage!).
Most other types of medium-temperature solar water heating systems rely on one or more flat-plate collectors or evacuated-tube collectors. A typical flat-plate collector includes a low-iron glass cover plate; a copper or aluminum absorber plate with a selective absorber surface that absorbs a very high percentage of the sunlight striking it but restricts the re-radiation of long-wavelength heat radiation; copper tubing bonded to the absorber plate; and an insulated, sealed box.
An alternative to the standard flat-plate collector is an evacuated-tube collector. The solar absorber consists of a selective-surface metal fin and copper tubing, but this is sealed within a glass tube in which a vacuum is drawn. The vacuum reduces conductive heat loss back out of the collector to almost zero, allowing higher collection efficiency. There are at least two manufacturers of evacuated-tube collectors with a presence in the U.S. One of these, Thermomax, uses heat-pipe technology. In these systems, a phase-change fluid—like the refrigerant used in air conditioners—is used in the evacuated tube. When heated, the fluid is vaporized and rises to the top of the tube, where it condenses at the manifold, transferring heat to the water. Most evacuated-tube systems are fully modular. If one tube fails, it can easily be replaced without scrapping the entire collector.
An innovative type of solar collector has been introduced by Solel Solar Systems, Ltd. in Israel and will soon be sold in the U.S. by Duke Solar. Known as a compound parabolic concentrator (CPC) collector, it has copper tubing coated with a selective surface, but rather than attaching these to a flat absorber plate, a parabolic-trough reflector is positioned behind each section of tubing to reflect sunlight into the tubes. Instead of 40% efficiency (typical with standard flat-plate collectors), this CPC collector achieves about 60% efficiency, according to Duke Solar vice president Gilbert Cohen.
Flat plate, evacuated-tube, and CPC collectors can be used in at least four different system configurations: active closed-loop, draindown, drainback, and thermosiphoning. The active closed-loop system (see figure below) has an antifreeze heat transfer fluid in the collector all the time. When the collector warms up, a controller turns on a pump that circulates this heat transfer fluid through the collector and through a heat exchanger (usually in a separate tank) in the basement or wherever the conventional water heater is located. Another controller and pump is usually used to transfer heat to the conventional water heater tank. Sometimes the controller circulating the antifreeze through the collector is replaced by a PV-powered pump—when the sun is shining the pump turns on; a separate controller is not required and the system is simpler. Another increasingly common modification is relying on natural thermosiphoning to transfer heat from the solar tank into the conventional water heater tank.
drainback system is similar, except that antifreeze solution is not kept in the collector all the time. Rather, the fluid (antifreeze or water) drains back to a small tank at night or when the system is not operating. This solves the problem of damaging the antifreeze (usually nontoxic propylene glycol) if the collectors are to stagnate because either the pump or controller fails. As with the active closed-loop system, a PV-powered pump can be used. Heat is transferred to the conventional water heater tank as in the system above.
Another option, the
draindown system, is making a somewhat surprising reappearance. Draindown systems lost favor during the 1970s and ’80s because they were prone to freezing problems, but one of the most trusted names in the solar water heating industry—Heliotrope General, which has produced controllers since 1974 —has introduced a simple draindown system that carries a ten-year guarantee against freezing. This system, called the Solar Sidebar™, includes a third-party-manufactured flat plate collector, a PV-powered pump, and connections to retrofit a conventional electric water heater for solar. Potable water is drawn out of the bottom of the electric water heater through the drain valve, circulated through the collector, and the solar-heated water returned to the center of the tank via a dip-tube. Either just the lower element in the electric water heater can be turned off or, if there is enough sun (during the summer, typically), both the upper and lower elements can be turned off.
Flat-plate collectors can also be used with passive systems that require no pumps or controls, as happens with a
thermosiphon system (see figure above). Here, the solar collector is positioned
below the storage tank. As water in the collector heats up, it naturally rises, driving a convective loop. Hot water enters the top of the storage tank, and cold water from the bottom of the tank flows back into the collector. This thermosiphoning gradually heats water in the tank. Because this circulation is slow, the water in the tank remains stratified with the hot water at the top and cooler water below. The challenges with thermosiphoning systems include finding a place for the tank and, if potable water is used in the collector, freeze protection. In cold climates, some designers and builders incorporate thermosiphoning systems into sunspaces, allowing the sunspace to provide freeze-protection.
Energy performance of
solar water heaters
According to Craig Christensen at NREL, the typical efficiency of most flat-plate solar water heating systems is about 40%. In data presented at the recent American Solar Energy Society conference in Portland, Maine, he showed that this efficiency is relatively constant no matter where in the U.S. the system is located. For the actual water heating performance (Btus or kWh delivered), in addition to considering efficiency, we also need to consider available solar radiation and the incoming cold-water temperature. Sunny climates are better for solar water heating, but colder incoming water temperatures in the north can sometimes result in those systems delivering
more energy than comparable systems in the south.
Solar collector performance is also affected by the orientation and tilt—though not as much as one might suppose. The best collector performance occurs when the collectors are facing due south and are tilted at an angle close to the latitude (i.e., 40° tilt for 40° north latitude). However, in most of the U.S. there is less than 10% loss in performance with the collector tilt anywhere from 15° to 60° (pitch of 2:12 to 8:12) and any orientation within 45° of true south. The good news from this information is that collectors work just fine when surface-mounted on a pitched roof. We don’t need to put up with the visual pollution of wildly pitched collector arrays that extend up from the roof or at an oblique angle.
As for how much energy a solar water heater will provide (or save), that depends on how large the collector area is, what the hot-water usage characteristics are, and what percentage of total annual water heating load you want to provide with solar. How to calculate that performance is beyond the scope of this article. The bottom line, according to Tim Merrigan of NREL, is that a solar water heating system cost-effectively sized for the climate where it is installed will generate a remarkably consistent 2,800 to 3,200 kWh (9.5 to 11 million Btu) of energy per year. In a cold, cloudy climate a larger, more expensive system will be installed, but the incoming cold-water temperature will likely be lower so the collector can operate more efficiently. The net result is that the larger solar water heating system in the north will save about as much energy as the smaller system in the Sun Belt.
In the table below, we can see how this energy savings—rounded to 3,000 kWh (10.2 million Btu) per year—equates to dollar savings (as well as reductions in CO2 emissions) in selected states.
The least expensive solar water heating systems today are probably ICS systems that are incorporated into new houses during construction. Sun Systems, Inc. claims a contractor cost as low as $1,500 for their system. Conventional flat-plate collector systems typically cost $2,500 to $3,500 installed, and some are considerably more expensive than that.
A $3,000 system in New York that is installed to offset electricity and produces 3,000 kWh per year would have a simple payback of 7.4 years —simple payback does not take into account a discount rate or inflation ($3,000 ÷ $408). Compared with natural gas water heating, that same system would have a payback of 18.6 years, and in Colorado the payback could be more than 37 years! Even the $1,500 system in Colorado would have a payback of over 18 years compared with natural gas.
Clearly, solar water heating systems are not cost effective everywhere. Solar advocates in the 1970s were basing their projections for widespread implementation of solar thermal in large part on expectations that conventional energy costs would rise significantly. That hasn’t happened. If society at some point decides to factor the “societal costs” into the pricing of conventional energy sources, or if prices rise through a combination of increasing demand and/or diminishing supply, solar will do much better. With concern over global warming and recent studies about petroleum supply, either of those possibilities is very real.
Solar water heating trends
A few of the trends we are seeing today in the solar water heating industry are as follows:
Larger collectors—In flat-plate collector systems, there is a trend toward single, larger collectors in place of arrays with two or more collectors. 4’ x 10’ (1.2 m x 3 m) collectors are common today. Sun Trapper Solar Systems, Inc. of San Antonio, Texas has patented a design for fabricating solar collectors on-site for commercial systems in which a single collector can be up to 100 feet (30.5 m) long.
Installation of collectors on the roof plane—Better understanding of flat-plate collector performance for water heating argues for simple surface-mounting of collectors on pitched roofs. There are only minimal losses in performance (compared with optimizing the tilt), and the collectors are less likely to clash with the building’s architecture.
Use of PV-powered pumps—Complex failure-prone controllers are being eliminated in favor of PV-powered pumps that operate whenever there is sufficient sunshine. Proportional control is even possible with some pumps—the more sunlight, the more electrical output from the PV panel and the greater the output of the pump.
Use of thermosiphoning—Instead of a pump to transfer heat from the heat exchanger to the storage tank, more systems are relying on passive thermosiphoning. This approach is simpler and less prone to failure.
Continued innovation—Despite relatively small markets for solar water heating, we continue to see exciting technological innovation, such as Solel Solar Systems’ compound parabolic concentrator and Heliotrope General’s Solar Sidebar.
Emphasis on the Sun Belt—Most of the action with solar water heating is currently in the Sun Belt, where population growth is most rapid and where good solar performance can be realized from very simple, straightforward solar water heating systems.
Most solar collectors and solar collector systems being sold in the U.S. carry certification from the Solar Rating and Certification Corporation (SRCC). SRCC was founded in 1980 as a nonprofit organization to develop and implement a national certification program for solar equipment. Solar collectors are rated according to the OG-100 standard, and complete solar water heating systems are rated according to the OG-300 standard. Full information about these standards is available on the SRCC Web site (see listings at the end of this article).
High-Temperature Solar Thermal
As noted earlier, low-temperature and medium-temperature solar collector systems are generally used for water heating. High-temperature solar collector systems, which concentrate sunlight to produce temperatures up to 1,450°F (800°C) or even higher, are used to generate electricity, produce process heat for industry, or operate thermally driven equipment such as absorption chillers.
Many people had given high-temperature solar thermal up for dead after the shining star of the industry, Luz Corporation, went bankrupt in 1991. Luz had built nine Solar Electric Generating System (SEGS) plants with a total capacity of 354 megawatts (MW) in the California desert between 1984 and 1991. These use long parabolic trough collectors that track the sun and reflect sunlight onto black pipes in highly insulating glass vacuum tubes. A special heat transfer fluid is pumped through the collectors, where it is heated to about 750°F (400°C). This hot fluid, in turn, boils water to generate high-pressure steam and drive a steam turbine. Natural gas is used as a supplemental heat source in the SEGS plants (up to 25% of total energy input), which helps the plants generate electricity during peak demand periods when electricity is worth more. Because
thermal energy is generated in a concentrating solar system instead of electricity directly, this energy can be stored on-site until it is needed.
The original SEGS developer went bankrupt both because of changes in the investment tax credit in California and because they were overextended with debt—not because of their technology. Even with Luz out of the picture and SEGS operations handled by other companies, the plants have been operated with very high reliability. In fact, there was a 30%
increase in output between 1992 and 1998, according to Craig Tyner of Sandia Laboratories in Albuquerque, New Mexico. Operation and maintenance costs have dropped from 4¢ to 2¢ per kWh output, and the levelized cost of energy has dropped from 23¢ to 14¢ per kWh. If similarly sized plants were built today, according to Tyner, they would be producing power at 10¢ to 12¢ per kWh. This cost of power is significantly lower than today’s cheapest PV power.
The U.S. Department of Energy, through Sandia National Laboratory and the National Renewable Energy Laboratory, is continuing to research and promote high-temperature solar thermal power systems. In addition to parabolic trough collectors, which track the sun on one axis, researchers are working on two-axis parabolic dish collectors. Sandia has demonstrated 25 kW systems that generate temperatures of 1450°F (800°C) and utilize Stirling engines to produce electricity. Commercial installations have been proposed for Egypt, Mexico, Morocco, and Crete, according to Tyner.
One of the most exciting recent developments in the high-temperature solar thermal arena is the formation of Duke Solar Corporation, a collaboration of Duke Energy Corporation (itself a subsidiary of the large utility company Duke Power), the architecture firm Innovative Design, and the Israeli company Solel Solar Systems, Ltd., which purchased the assets of the Luz Corporation when it folded and has been further developing high-temperature (and now medium-temperature) solar thermal technologies. Duke Solar is trying to commercialize a next-generation concentrating collector known as the Power Roof™. Like the SEGS systems in California, the Duke Solar design makes use of parabolic trough collectors that focus light onto pipes insulated by vacuum tubes. Unlike the Luz system, however, the trough collectors do not track the sun as it moves across the sky. Rather, the vacuum tube moves to track the focal point of the light as the sun’s angle changes throughout the day. A second, much smaller parabolic reflector inside the vacuum tube and behind the absorber pipe makes this system work. (This is the same compound parabolic concentrator—CPC—idea used in Solel’s medium-temperature collectors, described above.) This modification means that the large tracking motors required with tracking collectors can be replaced with much smaller motors that simply shift the absorber pipe’s position.
Another difference with the Duke Solar system is that its initial applications are likely to be process heat rather than electricity. Gary Bailey of Innovative Design says that the Duke Solar system will power a new generation of absorption chillers for commercial buildings. “It’s going to hit a market that hasn’t been reached,” he said. Chiller manufacturers Trane and Carrier are developing double- and triple-effect chillers that will be able to use high-temperature fluid as the energy source instead of natural gas. Companies that want to get away from fossil fuel dependence for manufacturing will be an ideal market for the Power Roof, according to Bailey.
While active solar space heating systems have generally lost favor since the early 1980s, due to high cost and complexity, there is one notable exception: transpired solar collectors. These are very simple unglazed, perforated, sheet metal wall panels through which ventilation air is drawn. (Note that these systems are not included in the government definition of low-temperature solar collectors, because it is air, not water, that is heated.) During colder months, outside air is brought into a building through the collector system. When there is no heating load, a damper is activated to bring in outside air directly without passing through the Solarwall system.
These systems, developed and patented by the Canadian company Conserval Engineering, Inc. as Solarwall® (see
Vol. 5, No. 1) are very inexpensive and, because of the low temperature of collection, are highly efficient (70% to 80%). To date Conserval has manufactured and sold over 1 million square feet (93,000 m2) of Solarwall, according to company president John Hollick. With a new factory that went on line this July in Buffalo, New York, the cost of the corrugated, perforated panels will be comparable to conventional metal cladding, according to Hollick. The only concern
EBN has heard with this system was fairly high fan noise in a large open warehouse application.
Interestingly, Conserval began business in 1977 producing glazed solar collector systems, but they determined by 1989 that energy performance wasn’t much lower with unglazed collectors, and these systems were a lot less expensive and considerably simpler. The primary application is for commercial and industrial buildings that have significant ventilation loads. In addition, Solarwall systems are being used around the world for agricultural or industrial drying applications: tea in Indonesia, cocoa in Malaysia, cigars in Cuba, and pressure-treated wood in Canada.
Mark Kelley, P.E., of Building Science Engineering in Harvard, Massachusetts, specified a 1,000 sq. ft. (93 m2) Solarwall system for a 65-unit apartment complex in Cambridge. To meet fire code, corridors generally have to be pressurized (to keep smoke out in case of fire). Preheating all this pressurization air is an ideal application for Solarwall, according to Kelley. The only added complication is a damper to change between heating season and cooling season operation. He said that at least in this application, there should be no increase in fan energy consumption or noise as long as the total net area of the Solarwall perforations is at least as large as the damper area. (If the net perforation area of the Solarwall is a lot smaller, air resistance may increase fan energy consumption or noise.)
Typical installed cost for Solarwall ranges from US$6 to $7 per square foot ($65 to $75/m2) for new construction and is about US$10 per square foot ($108/m2) in retrofit applications, according to the company. When Solarwall is used in place of brick facing, there can be a dramatic first-cost savings. For example, in a school in Minneapolis, substituting Solarwall for brick facing
saved $12 per square foot of wall ($129/m2) in construction costs! Hollick expects prices to drop somewhat with the new Buffalo plant on line, because what had been a two-step forming and perforating process is now done in one pass.
Clearly we won’t achieve the market penetration goals for the turn of the century that many of us hoped for 20 years ago. But solar thermal energy systems appear to have a bright future. For pool heating, solar is a no-brainer that extends the swimming season significantly. For water heating in homes and small businesses, solar systems are cheaper, simpler, and more reliable than they’ve ever been. High-temperature solar thermal seems finally to be getting back on track. And ventilation-air preheating with a transpired solar collector is one of the best investments we can make in large buildings with significant heating loads, sometimes with no net increase in cost, or even a reduction in cost, compared with conventional practice. Solar thermal systems of all types are poised to become commonplace, when and if the true cost of fossil fuel use becomes reflected in its price.
(1999, July 1). Is Solar Still Active? Water Heating and Other Solar Thermal Applications. Retrieved from https://www.buildinggreen.com/feature/solar-still-active-water-heating-and-other-solar-thermal-applications