Flat-plate solar heat collectors are most often mounted facing the sun at some fixed tilt and orientation. The same is true for evacuated tube collectors. The tilt angle must be chosen to allow the collector good exposure to the most useful altitude angles of the sun, while the orientation is chosen to provide good exposure to the solar azimuth angles (the sun’s path from east to west). For most common heating loads in the northern latitudes, the best orientation angle is true south and small variations to the east or west (plus or minus 10 degrees or so) will produce only a minor impact on the annual performance. The tilt angle, however, can be altered to produce widely different annual results, and so it is important to understand the consequences when choosing a fixed tilt.

Monthly solar climate data

To understand the solar heating climate at any location, it is helpful to inspect it month by month. A good resource for this kind of climate data is the National Renewable Energy Laboratory (NREL) website. Their “Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors” contains a handy summary for each of 239 locations across the U.S.  Even though it is out of print, it can be obtained free on the their website at www.rredc.nrel.gov/solar/pubs/redbook/ in PDF format as well as ASCII Data Files (comma delimited) that can be copied into spreadsheets for solar calculations. 

Each data page describes the location, lists the average solar radiation values month by month for flat-plate and concentrating collectors, and gives average climatic conditions such as temperatures, heating degree days and humidity. Data are presented by NREL for five tilt angles from the horizontal: 0 degree, latitude minus 15 degrees, , latitude plus 15 degrees, and 90 degrees. Data for a tilt of 0degree (referred to as global horizontal solar radiation) show how much solar radiation is received by a horizontal surface such as a solar swimming pool cover.

The Solar Data Manual states that the maximum yearly solar radiation can be achieved using a tilt angle approximately equal to a site’s latitude, and to optimize performance in the winter, the collector can be tilted 15 degrees greater than the latitude. To optimize performance in the summer, the collector can be tilted 15 degrees less than the latitude. Data for a tilt of 90 degrees apply to collectors mounted vertically (e.g. on south-facing walls primarily for winter space heating) and also apply to south-facing windows for passive solar heated designs.

Consider the heat load profile

Residential domestic hot water (DHW) can often be characterized as a constant load each month when the residence is continuously occupied. For example, if 80 gallons per day are consumed with a 70 degree temperature rise required from the water heater, then around 46,000 BTUs are needed per day with little variation from month to month. This idealized hot water load is shown on Figure 21-1 as a horizontal line that does not vary from month to month for a sample retrofit project near Sterling, Virginia.

The residential space heating load is another matter. For example, a 2000-square foot house near Sterling may require 400 BTUs per hour per degree F of difference (delta T) between indoors and outdoors plus some extra heat loss due to air infiltration. So, the heat load is far from constant over the year, rising dramatically in winter and dropping to nothing but the water heater load in summer. Everybody in the heating industry knows this, of course, but when you graph it (as shown in Figure 21-2), you begin to see the picture from the solar heating perspective. The space heating load is not a straight line like the DHW, but wildly fluctuating over the year with a shape not unlike the Big Dipper” without the handle. And, to make things even more challenging, if you really choose the wrong collector tilt, the need for space heat can be perfectly out of phase with the solar heat available, off target by six months as seen with the horizontal collector. (By the way, most common skylights act like horizontal solar collectors.)

Matching the heat load to the solar heat source

The NREL solar climate data described above has been plotted on the Figures 21-1 & 21-2 for our sample project in Sterling, Virginia (except that I have omitted latitude minus 15 degrees and added a steep tilt of 72 degrees). These five curves show how much solar heat is available on a typical day each month at different tilts for solar collectors of a reasonable size for the heating job. This can be compared to the heat load on a typical day each month, plotted as a red line marked with circles.

In the case of a simple solar water heater in Figure 21-1 the comparison is straightforward. The typical daily DHW heating load is seen as a horizontal line from month to month. The collector tilt that provides solar heat that most closely follows the load is the winner. Just as NREL predicts, the Latitude plus 15 degrees collector tilt tends to match the load better in both winter and summer, providing a better match than the latitude tilt curve which produces more excess heat in summer and a bit less heat in winter. In this example, the 64square feet of collectors could be installed with a 100 – 120gallon DHW storage tank to absorb the extra heat in summer while providing 80 gallons of hot water to the faucets each day.

The same choices are compared in Figure 21-2, but this time applied to a much larger collector array (320 square feet) which is big enough to make a significant contribution to both the space heating and the water heating load for this house. Notice that the familiar horizontal line from the DHW is visible only in summer as a constant load. The area above this line and below the solar collector tilt line is extra summer heat, not really needed for water heating, but available for summer pool heating if a pool is nearby. In our sample project in Sterling, there is no pool, so the design becomes a balancing act of reducing the summer excess heat while maintaining the solar heat available in winter as much as possible. The data in Figure 21-2 shows that the solar collector tilt that best fits the “dipper” shape of the annual heating load is the vertical collectors (90 degree tilt). But when you look at the solar heat available in the spring, the latitude plus 15 degrees tilt works much better. Perhaps the happy medium is around 72 degrees, where heat is not cut off too drastically in the spring, but does cut off pretty well in the summer.

A perfect match is not always easy or practical, which is why solar heating systems are commonly designed to take up a significant amount of the annual heating load, but not all of it. In this example, it looks like 320 square feet of collectors may provide over half the annual heating load. The size of the solar heat contribution shown here is typical of solar heating retrofits, but keep in mind this is only one example. Solar heating systems can be designed for higher or lower solar contributions depending on climate, economics, heat storage schemes, controls and many other factors. 

Practical considerations

When solar tax credits or other incentives are available, this may alter the economics in favor of more collectors. When more collectors are deemed to be affordable, the designer must take more care to avoid overheating during the warm seasons. See earlier issues of this column for previous discussions about designing to prevent overheating. 

Some vacuum-tube collectors have a tilt limit, and may not function properly if they are tilted too close to vertical or too close to horizontal. Consult the manufacturer and the installation manuals to be sure the solar collectors are right for the job.

Snow drainage takes longer at a shallow tilt, and this can be a problem in some installations when collectors become covered with snow with no practical way to clean them off.

Drain-back collectors must be installed so they drain properly by gravity, so keep in mind that creative piping and unconventional tilts may interfere with that. 

When a steep tilt is required, a low profile collector may be preferable to overcome the aesthetic issue of free-standing tall collectors. For example, most manufacturers offer a lower-profile panel that can be mounted with a free-standing height of less than six feet. Some can be mounted to be less than four feet high. But be careful. The proper collector for this job must be manufactured with internal horizontal headers and other structural support to handle the ‘landscape’ orientation. Not all solar heat collectors are intended to be mounted the low way and may not function correctly or may sustain damage when mounted in the wrong orientation. 

Bristol Stickney has been designing, manufacturing, repairing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed Mechanical Contractor in New Mexico. He is the Chief Technical Officer for SolarLogic LLC in Santa Fe, N.M., where he is involved in development of solar heating control systems and design tools for solar heating professionals (visit www.solarlogicllc.com for more information.)