bReeze (version 0.3-2)

profile: Calculate wind profile

Description

Calculates a wind profile, using on the Hellman exponential law.

Usage

profile(mast, v.set, dir.set, num.sectors=12, 
  method=c("hellman", "loglm", "fixed"), alpha=NULL,
  subset, digits=3, print=TRUE)
pro(mast, v.set, dir.set, num.sectors=12, 
  method=c("hellman", "loglm", "fixed"), alpha=NULL,
  subset, digits=3, print=TRUE)

Arguments

mast

Met mast object created by createMast.

v.set

Set(s) to be used for wind speed, specified as set number or set name. The first given dataset is used as reference height. If one single dataset is given, the same Hellman exponent is assumed for each sector and can be specified using alpha

dir.set

Set to be used for wind direction, specified as set number or set name.

num.sectors

Number of wind direction sectors as integer value greater 1. Default is 12.

method

Method to be used for the calculation. One of "hellman", "loglm" or "fixed" (see Details section). Optional argument: "fixed" is used if v.set is a single dataset, "hellman" is

alpha

Hellman exponent -- one general exponent or a vector of exponents (one per sector). Optional and only used if the choosen method is "fixed". If alpha is NULL, 0.2 is used as default.

subset

Optional start and end time stamp for a data subset, as string vector c(start, end). The time stamps format shall follow the rules of ISO 8601 international standard, e.g. "2012-08-08 22:55:00".

digits

Number of decimal places to be used for results as numeric value. Default is 3.

print

If TRUE, results are printed directly.

Value

• Returns a list of:
• profile
  Data frame containing alpha and reference wind speed for each direction sector.
• h.ref
  Reference height of the profile (the height of the first given dataset in v.set).

encoding

UTF-8

Details

The average wind speed as a function of height above ground gives a site's wind profile. For reasons of cost-efficiency met mast heights are usually below hub heights of modern wind turbines, thus measured wind speeds must be extrapolated by based wind profile. A common method is the Hellman exponential law or power law ("hellman"), defined as: $$\frac{v_2}{v_1} = \left(\frac{h_2}{h_1} \right)^\alpha$$ where $v_2$ is the wind speed at height $h_2$, $v_1$ is the wind speed at height $h_1$ and $\alpha$ is the Hellman exponent (also power law exponent or shear exponent).

To calculate $\alpha$, profile uses the inverted equation as: $$\alpha = \frac{\log \left(\frac{v_2}{v_1} \right)}{\log \left(\frac{h_2}{h_1} \right)}$$

If data is available for two or more heights, a log-linear model fit can be used for the computation of $\alpha$. In this case the data is logarithmized to allow for fitting a linear model. The models slope is then used as $\alpha$. Please note: depending on the data this method might result in negative $\alpha$.

If the wind speed is only known for one height, $\alpha$ must be estimated. $\alpha$ depends on various issues, including roughness and terrain of the site. Some empirical values for temperate climates are:

ll{ Site conditions $\alpha$ Open water 0.08--0.15 Flat terrain, open land cover 0.16--0.22 Complex terrain with mixed or continuous forest 0.25--0.40 Exposed ridgetops, open land cover 0.10--0.14 Sloping terrain with drainage flows 0.10--0.15 }

References

Bañuelos-Ruedas, F., Camacho, C.A., Rios-Marcuello, S. (2011) Methodologies Used in the Extrapolation of Wind Speed Data at Different Heights and Its Impact in the Wind Energy Resource Assessment in a Region. In: Gastón O. Suvire (Ed.), Wind Farm -- Technical Regulations, Potential Estimation and Siting Assessment, Chapter 4, p. 97--114, InTech

Brower, M., Marcus, M., Taylor, M., Bernadett, D., Filippelli, M., Beaucage, P., Hale, E., Elsholz, K., Doane, J., Eberhard, M., Tensen, J., Ryan, D. (2010) Wind Resource Assessment Handbook. http://www.renewablenrgsystems.com/TechSupport/~/media/Files/PDFs/wind_resource_handbook.ashx

International Electrotechnical Commission (2005) IEC 61400-12 Wind Turbines -- Part 12-1: Power Performance Measurements of Electricity Producing Wind Turbines. IEC Standard

See Also

createMast, plotProfile, printObject

Examples

# load and prepare data
data(winddata)
set40 <- createSet(height=40, v.avg=winddata[,2], dir.avg=winddata[,14])
set30 <- createSet(height=30, v.avg=winddata[,6], dir.avg=winddata[,16])
set20 <- createSet(height=20, v.avg=winddata[,10], v.std=winddata[,13])
ts <- formatTS(time.stamp=winddata[,1])
neubuerg <- createMast(time.stamp=ts, set40, set30, set20)
neubuerg <- clean(mast=neubuerg)

# create profile based on one height
profile(mast=neubuerg, v.set=1, dir.set=1)	# default alpha=0.2
profile(mast=neubuerg, v.set=1, dir.set=1, alpha=0.15)

# calculate profile based on two heights
profile(mast=neubuerg, v.set=c(1,2), dir.set=1)
profile(mast=neubuerg, v.set=c(1,3), dir.set=1)
# same as above
profile(mast=neubuerg, v.set=c("set1", "set3"), dir.set="set1")

# calculate profile based on three heights
profile(mast=neubuerg, v.set=c(1,2,3), dir.set=1)

# change the method used for computation
# note: negative alphas!
profile(mast=neubuerg, v.set=c(1,2), dir.set=1, method="loglm")

# change number of direction sectors
profile(mast=neubuerg, v.set=c(1,2), dir.set=1, num.sectors=8)

# data subsets
profile(mast=neubuerg, v.set=1, dir.set=1, 
  subset=c("2009-12-01 00:00:00", "2009-12-31 23:50:00"))
profile(mast=neubuerg, v.set=c(1,2), dir.set=1, 
  subset=c("2010-01-01 00:00:00", NA)) # just 'start' time stamp
profile(mast=neubuerg, v.set=c(1:3), dir.set=1, 
  subset=c(NA, "2009-12-31 23:50:00")) # just 'end' time stamp

# change number of digits and hide results
profile(mast=neubuerg, v.set=1, dir.set=1, digits=2)
profile(mast=neubuerg, v.set=1, dir.set=1, print=FALSE)