Commercial Solar Thermal installations require engineering

This is a good article I found about commercial application of Solar Thermal system.

Large Thermal Arrays: Location, Layout and Piping
By Chuck Marken
Oct/Nov 2008 (Issue 1.1): View Table of Contents | Download Issue
PDF Version (564 KB)

Large solar water heaters have design and installation considerations that are often inconsequential in smaller residential systems. Since the number of variables and the financial investment are scaled up in commercial systems, the collectors, racks, layout, piping and pipe insulation all need more careful design consideration. The better the upfront design work, the faster the system installation will go, and the more optimal the system performance will be.

COLLECTOR CHOICES
Flat plate or evacuated tubes are the two collector options. Climate and the end use application are considerations in the selection. Installers typically chose flat plate collectors for lower temperature applications. For higher temperature jobs in colder climates, specifying evacuated tubes can increase performance. Large jobs in the past have been overwhelmingly flat plates, primarily due to a lower cost giving flat plate collectors a market edge. Since flat plate collectors contain significant amounts of copper compared to tube collectors, this edge is slowly diminishing in the US with the recent price hikes in copper products.

The "bigger is better" rule applies when choosing a specific collector model for a commercial system. Less racking and fewer penetrations and piping joints make for less work, fewer mistakes and an earlier happy hour. Larger collectors also cost slightly less per square foot. Collectors of 40 to 50 square feet are as big as they get, and they should be the choice for commercial projects. Larger collectors are uncommon, because a two- or three-person crew has a tough time positioning them on the roof due to their weight. When it comes to weight, evacuated tube collectors have a niche in residential work, since one person can accomplish a roof installation with the modular tube design. This is not usually an issue with larger jobs, since a crane is generally used for the lifting.

WHY THE ROOF?
Collectors do not necessarily have to be on the roof, but that is where they end up more than 90% of the time in commercial installations. Space permitting, a ground-mounted system can actually be a better choice because of easier and safer installation logistics. Real estate is valuable, though, and businesses are often reluctant to give it up. Roof space is usually less prized in comparison. Except for very mild climates in the subtropics and tropics, commercial scale designs are configured as closed loop, antifreeze or drainback systems. There are a few related design constraints with ground mounts to consider. Freeze protection strategies typically require a groundmounted system to rely on an antifreeze rather than drainback approach, unless an underground thermal storage tank is planned. The piping and pipe insulation need to be buried in most cases when ground mounting the system. Finally, collector shading can make siting ground-mounted thermal arrays difficult or impossible.

A large, wall-mounted collector array is a possibility, but that is as uncommon as a smile on a building inspector. Shading, adequate space, building orientation and cosmetic appearance are all obstacles to wall-mount systems. In very rare cases wall-mount systems have enhanced a building’s appearance by configuring the collectors as awnings. A south facing wall mount, if at all possible, is actually preferable to a roof mount, however, due to easier installation and future roof maintenance issues.

ROW RULES
For roof-mounted collectors, the roof ’s available south facing surface area and design details—hip roofs or valleys, for example—will often dictate the optimum layout of a collector array. Depending on the building, other considerations definitely come into play. In addition to dealing with constraints existing on any roof, a large collector layout has limits on the number of collectors in a row.

The Active Solar Heating Systems Design Manual recommends a maximum of eight solar collectors per row. This twenty-year old manual was sponsored by the federal government and published by the American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE). With a few exceptions in small details, it is still the best guide around for commercial scale thermal systems. A PDF version of the manual is available for free download at the cyber home of the Solar Rating and Certification Corporation (solar-rating.org/commercial/guidelines.htm).

Eight collectors per row is a little conservative for smaller collectors but probably valid for 4 x 10 foot ones. The limitation arises from ensuring an equal flow through all the collectors. I have seen many systems with ten in a row that worked great for decades. A larger factor limiting the flow is the header size. A system comprised of collectors with 1-inch headers will have only slight differences in flow with up to ten collectors in parallel. But another limitation—the expansion and contraction of the collector headers—makes rows of more than ten collectors, or 40 feet, inadvisable.

ORIENTATION AND TILT
The same orientation rules apply for large and small systems. Collectors should face within 30º of true south for best performance. For year-round, optimum production of heat, a tilt angle equal to the latitude of the installation is best. However, if a system is designed to produce more than about 60% to 70% of the annual demand load, a latitude tilt angle will produce excess heat in the summer.

The overheating that can result from a poorly designed, large antifreeze system will be an ongoing maintenance problem. The largest flat plate solar water heating system in the world, the $30 million ground-mounted Packerland Solar System in Green Bay, Wisconsin, was crippled by overheating during the summer. Half of the original 10,000 4 x 8 collectors had to be removed to keep the 330,000-gallon storage tank from rupturing.

A tilt angle of latitude plus 15° will help limit overheating during the summer in most climates, although the system will produce fewer kWhs of energy per year. An array tilted to latitude plus 15º will produce about 8% to 10% more energy in the winter when the sun’s intensity and availability is less and mitigate overheating in the summer. In most of the US, a latitude plus 15º angle is the only tilt that can produce close to 100% load displacement without excess summer heating. In some northern coastal areas, winter solar irradiance is so intermittent and limited that the increased tilt angle would be of no value in winter; it would simply reduce annual heat production.

MINIMIZE SHADING
Large thermal arrays will often require more than one row of collectors. In addition to the usual shady characters, like trees and nearby buildings, one row of collectors can shade another. Rows must be spaced adequately to prevent this from happening. The height of each row and the latitude of the installation will determine row spacing. How low the sun is in the sky at the winter solstice is dependent on latitude.

An easy way to determine the angle of the sun at the solstice at any latitude is a simple math calculation. Subtracting the latitude from 90 gives the angle of the sun at the equinoxes. To find the angle of the sun above the horizon at noon on the winter solstice, subtract another 24 (the earth’s tilt is 23.45, so 24 has a tiny fudge factor). Adding 23.5 gives the angle at the summer solstice. Placing a row far enough behind the one in front to account for the winter solstice angle is acceptable for no shading at noon. There will still be partial shading before 11am and after 1pm. If you have enough roof space and want to eliminate all shading, subtracting an additional 10°–15°— depending on latitude—from the winter solstice angle will give no row-to-row shading anywhere in the US during each day’s solar window.

This angle calculation is accurate only for collectors oriented true south. If the building roof will not accommodate a true south orientation, you should get familiar with the formula for minimum spacing distance on page 3-23 of the ASHRAE design manual:

D = DC + DS = (L)(cosΣ) + [(L)(sinΣ)(cos∂) / (tan 0)]

If that formula gives you a headache like it does me, maybe it is time to hire a professional engineer. If you cannot space rows of thermal collectors to avoid all inter-row shading, it is not catastrophic, as it would be with a PV system. Spacing the rows based on the angle of the solstice at noon will result in minimal shading: the bottom few inches of the collectors will be shaded part of the day in December and January. Since these are the two months with the least intense sunlight, the resulting reduction in year-round heat production will be less than 1%.

PIPING SELECTION, SIZING & LAYOUT
Type M (red stripe) copper tubing ranging from 0.5 to 1 inch is commonly used in residential piping for all but the largest systems. Commercial piping frequently specifies the slightly thicker walled Type L (blue stripe) tubing. Type M is made only in the hard variety of tubing available in 20-foot lengths. The thicker walled Type L and Type K (green stripe), the thickest tubing, are available in the hard variety and also in soft rolls of 60 and 100 feet for tubing up to 1 inch. 1.25- to 2-inch tubing is made in 40- to 60-foot rolls. Soft tubing is usually used only in new construction under concrete slabs. However, in retrofit work, many installers find it handy to have a roll on the truck to snake through hard to reach, concealed spaces. The outside diameter (OD) of each type of tubing is the same and called the nominal pipe size. The inside diameter (ID) varies with the difference in the wall thickness. Type M tubing will carry a slightly higher volume than the thicker walled tubes.

All piping has frictional head loss. This resistance to flow is similar to electrical resistance in wiring. The relation is inverse: the larger the pipe, the lower the resistance. In addition to the size relationship, changes in direction also increase frictional head loss. Piping needs to be large enough to ensure that frictional head loss is not a factor in lowering system performance.

Collectors in a given array should be piped in parallel. Series piping configurations elevate the temperature in the second and subsequent collectors located downstream. This will result in a loss of efficiency in the downstream collectors due to increased heat loss. Piping the collectors in reverse return will help to balance the flow. With reverse return piping, the closest collector to the cold inlet pipe is farthest from the hot outlet. Finally, large systems should use collectors that have internal headers. This allows parallel piping with the use of couplings or unions.

The piping layout needs extra attention in drainback systems to establish a proper slope for collector drainage. A minimum slope of 0.25 inch per foot is required. The minimum required slope of a large drainback design is easier to configure if the rows of collectors are mounted at different levels. If the piping is designed to slope to the south for drainage, placing the northern rows at higher levels can allow narrower row spacing and result in more collectors in the available roof space. Sometimes mounting northern rows of collectors higher than the southern row is the only way to get enough collectors on a roof without any row shading problems.

In Illustration 2 ( See Below), the sizing example is a layout of 32 collectors in four rows with industry standard 1-inch headers. In order to supply the collectors with adequate flow, the supply piping needs to be equal to the four sets of headers. Table 2 gives the cross sectional areas and volume of common Type L tubing sizes.

Simple math shows that one 2-inch tube is slightly smaller than four 1-inch tubes. A pipe run with numerous changes in direction or more than 100 feet in length should be increased to 2.5-inch pipe. The tubing can be reduced to 1.5 inches for the last two rows and 1 inch for the last row. Even though the area of the 2-inch tube is a little too small, the flow will not be affected significantly if the pipe run is fairly straight and less than about 50 feet. In this instance 2-inch tubing would be acceptable and specified by many system designers.

If the pipe is sized correctly and laid out in reverse return, there is rarely a need for flow balancing valves in the system. It is prudent, however, to install a balancing valve at the inlet of each row of collectors. An infrared thermometer is accurate enough to measure the relative difference in temperature to see if any adjustments are needed. A test for flow balance should only be done after the system has been running for about 30 minutes with no shading of any kind. Be certain that the thermometer is pointed to the same spot on the same collector in each row. Any row with a relatively low temperature has too much flow, and it should be restricted in small increments until all the rows have an equal temperature. A couple of degrees difference is acceptable between rows.

PIPE INSULATION
The guide for pipe insulation is the 2006 Uniform Solar Energy Code published by the International Association of Plumbing and Mechanical Officials (IAPMO). For fluid applications under 200ºF and pipe up to 2 inches, the code requires 1 inch of insulation. The thickness is based on insulation with an R factor between 4 and 4.6 per inch. As the application temperature and pipe size increase, the insulation thickness increases. For insulation with higher values than R-4, the wall thickness may be reduced in proportion to increased R factor. To determine the thickness, divide 4.6 by the R factor of the insulation. For example, R-6.2 insulation would be 4.6 / 6.2 = .742. Therefore 0.75 inch of R 6.2 insulation would meet the requirement. Insulation with an R factor of less than 4 requires more insulation in proportion to the difference in R-value. Most brands of high temperature, closed-cell, black pipe insulation have an R factor of 5 to 6; hence the minimum insulation thickness required in many cases is 1 inch.

A suitable weatherproof cover must protect insulation that is exposed to the elements. Closed-cell insulation will deteriorate within a few years if not covered. Paint and paint-like roofing products will last about seven to ten years. Foil faced adhesive tape has a similar lifespan, possibly a little longer. A lifetime insulation cover can be fashioned with architectural grade aluminum. It is available in numerous colors and is easy to bend around insulation without special tools. Smaller sizes of galvanized, round stovepipe ("snaplock") also make a durable weatherproof cover. ABS or PVC pipe can also be used for a long lasting insulation weatherproofing. The straight sections are put in place before the tubing is soldered, and the ells and tees are cut in half and glued back together after the system piping has been leak tested.

Large Projects Call for PE Involvement

Forty collectors is the largest system I have personally designed and installed. A professional engineer (PE), common to the commercial/industrial construction sectors, will typically be called on to design systems of this size and larger. Job size and complexity usually determines if an engineer’s services are required, although some contractors also retain them for higher end residences. PE s must be registered and meet the requirements of each state where they are licensed. On large jobs, engineers are to the respective trades what an architect is to a general contractor—the job boss. A mechanical engineer’s role in commercial scale solar systems is to provide the system design and ensure that the contractor adheres to it.

The engineer’s design will include the drawings and specification schedules that a contractor needs to accomplish the installation, making the contractor’s design responsibilities minimal. The engineer is the responsible party, and the PE stamp, with few exceptions like code violations, can often carry the clout of a building inspector. Contractors give firm bids based on the drawings and specifications provided, and usually the lowest bid wins the contract. The contractor is responsible only for providing the equipment, materials and labor needed to complete the project as designed. Any subsequent changes to the job must be approved by the engineer and are handled with change orders that are often lucrative.

There are many commercial jobs that fall between residential work and large, engineered commercial projects. Retrofit work on existing buildings is the best example and largest market. In many instances large, retrofit solar water heating systems are installed with little or no input from a PE. If the local building department approves the system drawings and issues a permit, the contractor bears the responsibility. The contractor is then liable for system design in addition to the materials and labor provided. In some cases, a building official will request an engineer’s stamp on one or two details of the design that look questionable. In order to get the permit, the contractor or building owner must then hire a PE to approve and stamp the drawing. The details that most commonly require professional engineering services include roof loading, wind loading or collector mount and orientation designs.

NEXT UP
Commercial scale solar thermal systems have additional design considerations related to the equipment in the mechanical room. They are almost all size differences: bigger tanks, more powerful pumps and larger heat exchangers. All of these components need to be sized to match the collector array’s output. In retrofit work, the space available for these components can make or break a job design. We will take a closer look at these issues in a future article.

Chuck Marken / AAA Solar / Albuquerque, NM / solarprofessional.com

References:
Active Solar Heating Systems Design Manual, by American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), 1988, 485 pages, ISBN-0-910110-54-9, $25 PDF / 800.527.4723 / ashrae.org

Copper Tube Handbook, by Copper Development Association, online Technical References / 212.251.7200 / copper.org