designed for solar energy to provide maybe 50% of the design heat load which is sufficient for 70-75% of the heating season. Thus, auxiliary heat is required on 25-30% of winter days only; its installation cost should be low, but its operating cost may be relatively high (for example: electric resistance heat). In a lower latitude, such as 30°N, solar heat should provide maybe 80% of design heat which is sufficient for 85-90% of all winter days. Only a small auxiliary heating system would then be required during 10-15% of the heating days.

At first sight it may appear that solar heating becomes more economical as the location of the house approaches the equator. This is not true for subtropical climates. In Florida, for example, it is doubtful whether the savings in the small winter heating costs can ever be large enough to warrant the capital expenditures for a solar heating system. This is borne out by the analysis of Tybout and Lof (Reference 3). The optimum size of the solar heating system depends on the local winter climate, latitude, ambient air temperatures, cloudless days, and the unit costs of collectors, storage, and auxiliary energy. Sections 4 and 5 illustrate how these factors can be analyzed to achieve maximum overall economy.

The results of many tests (e.g. References 21, 24, 25, 29, 88, 91, 95 of Phase I Report, Reference 1) prove that the most suitable type of solar collector for residential buildings is the fixed, flat plate collector. It consists of a transparent front plate, one or more insulating zones, and an absorbing rear plate (see Reference 1, page 5-27). Thermal insulation is placed behind the absorber to prevent overheating of the building during the day and excessive heat loss at night.

The most favorable orientation for flat plate solar collectors is at an inclination angle to the horizontal equal to the latitude plus 15 degrees (Reference 3). Making the collector vertical deviates

little from this optimum over most of the continental United States, reduces undesirable summer heating, and permits relatively easy incorporation of the collector into the wall of the building. This is shown in greater detail in Report No. NSF/RANN/SE/GI27976/TR72/2, "Architectural Planning and Design Analysis of Energy Conservation in Housing Through Thermal Energy Storage and Solar Heating". Therefore, only vertical solar collectors will be considered in the solar heating systems under discussion in this report.

Southern orientation of the wall containing the solar collector is desirable but not absolutely essential. For example, a vertical collector facing southwest receives almost 80% as much solar energy as a collector of the same size facing south. This fraction is the lowest practical value that could occur because, for a deviation from south greater than 45°, the collector would be shifted to a different wall. Design data for various collector orientations over most geographical locations of the United States during critical winter months are summarized in Figure 2. That figure shows that orientation due south is desirable but by no means mandatory for a solar heated house. In fact, a 25° deviation imposes only a 5% penalty and a 33° deviation a 10% penalty on solar heat availability.

A complete analysis of the variation with azimuth of solar insolation on vertical collectors is given in Report No. NSF/RANN/SE/GI27976/ TR72/18, "Influence of Azimuthal Orientation on Collectible Energy in Vertical Solar Collector Building Walls".

Variation of Solar Insolation on Vertical Walls With Wall Orientation

Between November 21 and January 21 (Latitudes 30 - 45°N)

3-10

4. TECHNICAL AND ECONOMIC TRADE-OFFS

Over most of the geographic area of the U.S., the reliability of sunshine and the space heat requirements are such that solar energy cannot be used as the sole source for comfort heating. To do so would require an inordinately large amount of thermal energy storage which would be used very inefficiently only a few times a year in order to provide heat during a series of successive cloudy days. A stand-by heating system is therefore required.

For the north-eastern part of the country the most economic amount of solar heating provided is, in fact, well below 50% of the total annual heating demand (Reference 3). Thus, one should consider the solar heating system as a fuel saver only. The stand-by system should have the same capacity as a conventional heating system, and the economic trade-off is solely between the acquisition cost of the solar collector/ storage system and the fuel saving between conventional and stand-by systems.

The possibility arises of using a low initial cost, high operating cost stand-by system (like electric resistance baseboard heat) to replace a high initial cost, low operating cost conventional system (like oil). The resultant trade-off becomes obviously more complicated. However, this situation only arises if the bulk of the annual heating demand is provided by solar energy which, according to Reference 3, is only economical in the southwestern United States; at least under today's fuel prices. So far, no work has been done at the University of Pennsylvania on such systems, because this grant addresses itself to housing comfort systems suitable for those regions where the majority of the U.S. population resides.

In order to provide a realistic comparison in fuel economy between a conventional space heating system and a system utilizing solar heat/ thermal energy storage, a planned residential development designed by the Environmental Design Collaborative for Hampton Township, Pennsylvania, a community outside Pittsburgh, was chosen (Figure 3). This very