The multiplier applies to the value of solar home heating collectors when compared to collectors used exclusively for domestic solar water heating—a 12-month job. To calculate the multiplier, first divide the heating season solar resource by the annual resource. Then, multiply that value by the length of the heating season divided by 12 months. For Seattle, the math would be 2.8 ÷ 3.5 = 0.8; 0.8 × (8 ÷ 12) = 0.53. The multipliers are somewhat misleading in that the average heating season sun-hours are higher than the sun-hours of the midwinter months. This is particularly true in the case of Seattle, which has only 1.6 average daily sun-hours when November, December, and January are factored in together. Essentially, there is almost no solar resource to harvest and the multiplier would fall to 0.31 for those months, close to the 0.25 of the first simple scenario outlined at the beginning of the article. Contrast this with Alamosa, Colorado, where these three months average 5.9 sun-hours, and the multiplier is 0.73.
The cost of the extra installed collectors, the value of the fuel displaced, and available incentives will also affect the relative value of a solar space-heating system. Having a summer application for the extra collectors—such as heating a pool or hot tub—makes the extra collectors as valuable as the domestic solar water heating system if the cost is the same per collector.
Even larger space-heating systems with massive storage are influenced by the diminished heating season resource and limited months of use, but many of these systems have lower costs per collector due to their economy of scale. Solar space-heating systems (and solar domestic water heating) are usually designed with a day-to-day cycle of energy gathering, storage, and distribution. A design that calls for more heat storage than the collectors can supply in a day relies on banking the energy in thermal mass. This semi-seasonal storage strategy could increase the benefit-to-cost ratio of solar heating systems.
Simple, lower-cost air collectors can have a reduced cost per collector. The collectors are less expensive per square foot and installations are simple compared to liquid collector heating systems. These simple systems can be wall-mounted vertically. In areas with snowfall, wall-mounted air collectors further benefit from the reflected winter sunlight.
PV systems can also be used for heating. Just a few years ago, there was no question that solar thermal collectors were more economical than a PV system for heating domestic water or a home. The ratio of benefit-to-cost value of the two technologies was about 10:1 or more in favor of SWH collectors over PV modules. This has changed in the last decade—PV prices have come down dramatically, and the cost of SWH installations has increased due to a more than eightfold copper cost increase. A grid-tied PV system, which produces energy year-round that can be “banked” by the utility, is a better value than an active solar space heating system in locations with significantly reduced winter solar resources.
Chuck Marken is a Home Power contributing editor and solar energy trainer/instructor. Chuck’s 30-plus-year career as a solar installer included installing, repairing, or altering more than 5,000 solar energy systems, which included domestic water, pool, and space heating systems, and on- and off-grid PV systems. Chuck has taught classes and workshops for Solar Energy International, Sandia National Labs, and numerous other educational and nonprofit organizations in the United States.