Page 62 - Fluid Power Engineering

40 Chapter Three

where p d is the partial pressure of dry air, p v is the partial pressure of
water vapor, R d is gas constant for dry air = 287.05, R v is gas constant
for water vapor = 461.495, and pressure p = p d + p v . An approxima-
tion for p v is: 5

7.5T c
p v = RH ∗ 610.78 ∗ 10 237.3+T c (3-20)
where RH is the relative humidity, T C is the temperature in degrees
Celsius.
Table 3-8 contains value of density for different values of relative
humidity. At 100% relative humidity, the density is lower by 0.6%.

References
1. Patel, M. R. Wind and Solar Power Systems, CRC Press, Boca Raton, FL, 2006.
2. Johnson, G. L. Wind Energy Systems, electronic edition, Manhattan, KS, 2001.
3. Elliott, D. L., Holladay, C. G., Barchet, W. R., Foote, H. P., and Sandusky,
W. F. Wind power classes. Wind Energy Resource Atlas of the United States
[Online] 1986. http://rredc.nrel.gov/wind/pubs/atlas/tables/A-8T.html.
4. Gipe, P. Wind Energy Comes of Age, Wiley, New York, 1995.
5. Air Density and Density Altitude Calculations [Online] http://
wahiduddin.net/calc/density altitude.htm#15.
6. Density of Air. Wikipedia. [Online] http://en.wikipedia.org/wiki/Density
of air.