By Paul Shippee

There are two main types of solar heating systems for homes and buildings: passive systems and active systems. Passive systems utilize some of the actual building components, such as glass walls and thick slabs, to collect solar heat during the day and store it for nighttime use. Active systems typically use a number of flat plate solar collectors, exterior to the building, through which air or water (plain or mixed with antifreeze) is circulated via fans or pumps when the sun is shining. As the solar heat is collected it is circulated to a tank of water or a bin of rocks (heat storage) to be re-circulated to rooms in the house as needed.

Rather than looking at solar heating options as a simple choice between passive and active systems, it might be interesting to consider combining them in creative ways that enhance the performance of both. There are many reasons for combining active and passive solar heating systems in hybrid fashion. For example, house orientation, views, noise, street traffic, aesthetics, or privacy may be reasons to steer your home design away from using a south-facing wall filled up with glass. Perhaps you have purchased a hundred-year-old home with a warm-air furnace that can be retrofitted with active air collectors that utilize the old brick walls and the existing furnace fan for heat storage and distribution.

Sometimes the creative design of a high-performance solar home can be derived by thinking first about some available heat distribution methods and options, and there are many. In this way consideration of temperature, comfort, and efficiency can begin a design process that leads naturally into architectural choices. Heat distribution, by the way, means delivering stored solar heat where and when it is needed.

How do you design a solar heating system that collects enough heat during a sunny winter day to get you through a cold night and some cloudy days, without overheating the rooms during the sunny daytime? Typically, a large tank of water or a rock bin for active systems has been used. These require higher temperatures to function well (120F-180F). Massive walls for passive systems have been used with relative success in various passive configurations utilizing much lower temperatures (70F-100F). These walls can be charged with solar heat by direct gain and by warm air convection, then give back their stored heat slowly by low temperature infrared radiation as rooms cool down at night. Some provision for moving warm air into the cooler north areas of the house by free convection during sunny days can also be employed. Mild temperature swings in dwellings have been shown to be healthy for the occupants.

One way I have found to combine some of these complex features of solar design actually results in a robust, simple, and efficient system – an elegant solution. This system pumps solar heated water from flat plate collectors on the roof – in a drain-back configuration – directly into a thick floor of adobe or concrete. If designed correctly, this system can eliminate the large tank of water, and the high temperatures required, and can boost the efficiency of the solar collection by ten or twenty percent due to lower operating temperatures. Then passive solar heat streaming through south windows can also heat mass walls properly sized.

When large areas, such as floors and walls, can be employed to store and distribute solar heat, then the temperatures involved throughout the system can be low, i.e., not much above room temperature. Flat plate collectors love to operate at low temperatures. Reason: they do not lose much heat to the cold surrounding environment when operating. This means they deliver more of the sun’s available energy for use in the house. These low operating temperatures are a “perfect match” for radiant floor distribution of house heat because floors are quite large in area. In heat transfer, large areas mean lower temperature differences are needed to deliver the same amount of heat. And people often report high satisfaction with warm floors in winter.

Large distribution areas, like walls and floors — which are in every house– mean high-temperature heat is not needed. These low-temperature high-mass systems have worked well when balanced with good design practice. They are in fact, a key to approaching zero-energy solar homes in a cost-effective manner. Ideally, this hybrid design, combining active and passive solar heating technologies, can eliminate that large propane tank you see sitting in yards all over town and country. No backup system – how “green” can you get? Communities that value relocalization will most likely want to learn sustainable practices in both energy and food growing. To learn more about passive solar design, thermal storage options, tax credits, window insulation, drain-back systems and solar heating workshops visit www.crestonesolarschool.com