Lesson 3
Answers

1. How does our system perform on an annual basis?

Answer: A system with two AE-40 collectors with an 80 gallon storage tank has a solar fraction of 68% and an efficiency of 30%. A system with two AE-40 collectors and a 120-gallon storage tank has a solar fraction of 71% and an efficiency of 32%. So for the modest increase in cost of the larger tank, we get more energy and higher efficiency. The other benefit of a larger storage tank is there is less chance of the system overheating in the summertime.

2. Using the data for Wilkes Barre in the table provided, what is the percent difference in the yearly average daily solar insolation incident on a surface facing true south (azimuth angle 1800) with a tilt of 25 degrees versus that with a 55 degree?

Answer: 5%

For 25 degree tilt versus a 40 degree tilted surface?

Answer: Zero

3. What is the percent difference between the yearly average value for a surface tilted at 25 degrees facing true south versus the same surface, same tilt but with an azimuth angle of 210 degrees?

Answer: 1.7%

For surfaces with 40 and 55 degree tilts?

Answer: 2.4% and 2.5%

4. What is the percent difference between the yearly average value for a surface tilted at 25 degrees facing true south versus the same surface, same tilt with an azimuth angle of 270 degrees?

Answer: 14.3%

For surfaces with 40 and 55 degree tilts?

Answer: 19.6% and 22.4%

5. Given the percent differences shown in question #3 which tilt angle is more reasonable to accept if you had no choice but to install a solar system with an azimuth angle of 270 degrees? Please explain your answer.

Answer: The 25 degree tilt angle because the percent difference between true south azimuth and 270 degree is the lesser of the three options (i.e. the net amount solar insolation incident on the 25 degree surface is larger).

6. If you lived in Wilkes Barre and wanted to maximize the collection of solar insolation in the winter, what tilt and azimuth angles would you mount the solar collectors?

Answer: 55 degree tilt and 180 degree azimuth angles

Conversely, if you wanted to maximize the summer solar collection, what tilt and azimuth angles would you mount the solar collectors?

Answer: 25 degree tilt and 180 degree azimuth angles

7. In the solar system sizing example, the total daily heat energy demand for 80 gallons of hot water was calculated at 45,081 Btus. What would the total heat energy demand be for 80 gallons with the hot water temperature set at 1400F with the same cold water temperature?

Answer: 58,952 Btus

8. What would the auxiliary energy demand be for 80 gallons of hot water with the hot water temperature set at 1200F and a solar water heating system providing 1000F water to the cold water inlet of the conventional domestic hot water heater? Assume the heat losses for the 120 degree set temperature from the conventional heater when making the calculation.

Answer: 15,358 Btus (13,211 Btus to heat the water from 100 to 1200F plus the 2,147 Btus of heat losses in the conventional hot water heater.)