energy costing more than about $6 per million Btu. That is equivalent to about 2ў per (kWh) of electricity, 55ў per gallon of propane, 83ў per gallon of fuel oil, or 60ў per therm of natural gas. Depending on which conventional energy sources are available to them, many Federal facilities are likely to be paying more than $6 per million Btu for their water heating. As can be seen from Table 2 on page 6, national average effective energy costs to Federal facilities are greater than $6 per million Btu (MBtu) for electricity and propane and within possible regional variance for fuel oil and natural gas.
(The Federal government's average cost for energy for buildings in 1995 was $9.95/MBtu.) Parabolic-trough systems benefit greatly from economies of scale and are not as well suited to small projects as nonconcentrating collectors. Typically, 3600 square feet of collector (able to produce about 7500 gal- lons of hot water per day) would be the minimum size for a viable project. Solar water-heating economics are better when there is a relatively constant demand for hot water throughout the week and throughout the year, or higher demand in the summer.
Facilities such as prisons, hospitals, and The Power of the Parabola The parabola is an intriguing geometric shape with important practical uses—includ- ing concentrating sunlight. The curve of a parabola is such that light travelling parallel to the axis of a parabolic mirror will reflect to a single focal point from any place along the curve. Because the sun is so far away, all light coming directly (excludes diffuse) from it is essentially parallel, so if the parabola is facing the sun, the sunlight is con- centrated at the focal point. A parabolic trough extends the parabolic shape to three dimensions along a single direction, creating a focal line along which the absorber tube is run.
Focal point Axis 02279802m barracks that operate 7 days per week and have relatively constant populations throughout the year are therefore often excellent candidates. Clear skies are important for any solar technology but are particularly so for para- bolic troughs because they use only direct radiation. Facilities located within the three East-West Axis Kilowatt-hours per square meter 5 to 6 4 to 5 3 to 4 2 to 3 02279805m Map prepared by the NREL Resource Assessment Program Figure 6. Average daily direct-beam solar radiation for horizontal, east-west single-axis tracking parabolic-trough collector systems.
An east-west axis is better in winter and more even throughout the year than a north-south axis. 7