When it comes to using active solar heating systems with a conventional home heating system, adding on to a solar domestic water heating system can be a viable solution—but the costs and benefits should be carefully considered.
Drainback and antifreeze solar water heating systems are easily adapted as an add-on home heating system—an additional one or two solar collectors serve the home’s space-heating load. Balance-of-system components needed include a larger (or second) storage tank, pump, control, and heat exchanger. (Locations in which simpler direct forced-circulation SWH systems or passive water heaters usually have little or no need for space heating.)
To understand the economics of each system, the local winter solar resource and length of the winter heating season need to be examined. A common SWH system has an 80-gallon storage tank with two 4- by 8-foot collectors. Such a system might have an installed cost of $8,000.
Adding a third collector for space heating and replacing the tank with a 120-gallon tank brings the cost to $11,000. The first two–collector system cost $4,000 per collector, and the third collector adds $3,000. Let’s assume that the heating season of the installation is six months, thus, the third collector is only used only half of the year.
However, because we’re installing this system in a location with a lower winter solar resource (where half of the yearly average resource is available), the value of the third collector doesn’t stack up to the first two. The math shows that the third collector gets only one-quarter of the useful production of the first two (0.5 for half the resource × 0.5 for half the year = 0.25). To have the same benefit-to-cost ratio of the original system, the add-on in this simple scenario would need to cost $1,000—not $3,000.
But the economics of the system change with location. Let’s look at the add-on system scenarios in four different regions of the United States (see the “Add-On Heating System Economics” table). The Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors (bit.ly/SolarData) provides the solar resource and winter length information needed to evaluate active solar heating systems.
The heating season length is estimated using the heating degree days (HDD) per month in the solar radiation data. In the cities selected, the heating season varies from seven to nine months if we discount any month with fewer than 100 HDDs. (This is reasonable, since a month that measures fewer than 100 HDDs translates to about 3 HDD per day, for which even a small amount of passive solar gain will meet this need. If not, that’s what sweaters are made for.)
The average solar resource is listed as “kWh per square meter” (aka peak sun-hours) per day for five different tilt angles. The data is given with a monthly and cumulative average. To compute the solar resource for the heating season only, the heating season months’ resources are added and sum is divided by the number of months.
We used the “latitude plus 15° tilt” row for this evaluation, since, in most of the United States, a higher tilt angle gives better winter performance. However, in a few areas, such as the Pacific Northwest, heating season average peak sun-hours per month is slightly better at a tilt angle equal to the location’s latitude. This is due to the extremely cloudy winters.
In all cases, during the coldest three months of the year the system benefits from having a greater tilt angle. The latitude plus 15° angle also provides a hedge against the possibility of summer overheating. Note that the “Add-On Heating System Economics” table contains the data gleaned from the U.S. Department of Energy’s Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors (aka the “Redbook”) and has a multiplier to represent the relative benefit-to-cost ratio of additional collectors required for space heating only.