Adjustment of Heat Flux parameters (Offset and Rms), Mast vs Wind Farm

[carlos.pinto]

Hello. I’m analyzing a site in East Africa in a generally flat coastal area.

The mast is located near the sea at about 150m distance, measuring height 30m. The wind farm is located inland, with 1 row aligned parallel along the coast line, at about 2km distance, hub height 80m. Winds prevail from sea and are perpendicular to the wind farm. The distance between mast site and the wind farm central point is 5km.

We are considering correcting WASP’s heat flux parameters resulting from mesoscale simulation as it improves the wind shear at mast site. The problem here is that there is a big difference between the mast location (near the sea) and the wind farm (2km inland). For this reason we have different heat fluxes for the mast and the wind farm location.

Which would be the advisable procedure?
1. Use only the heat flux for the mast site?
2. Use the heat flux for the wind farm location?
3. Use the heat flux of the mast site for the ATLAS and the heat flux of the wind farm location for AEP calculations?

The difference in AEP is +1.2% for the 2nd option and -1.2% for the 3rd option, when compared to the 1st option.

Thanks in advance.

[Mark Kelly]

Hello, generally the WAsP heat flux parameters do not correspond to physically measured fluxes (although for very coarsely averaged mesoscale output they might begin to converge in some cases). It is not clear how you are using the mesoscale simulation output--do you mean that you are adjusting WAsP's offset heatflux based on the mesoscale shear output, or what? I can respond further based on your response.
--Mark

[carlos.pinto]

Hi. We had success in doing this adjustment in specific projects with good results, but all masts were located inside the project area (procedure 1 was used), while in this project the situation is different.
Without going into much detail, we are able to get directly from the mesoscale simulation heat flux series. Then we perform some statistical analysis for different thermal stability ranges and afterwards we select the most likely heat flux offset and RMS values for land and water, taking in consideration the wind regime that WASP is prepared to work.
For the mast site, adjusting the heat flux parameters helps WASP modeled wind shear to be closer to measured wind shear.
--Carlos

[Mark Kelly]

Hello Carlos, it is still not clear what you are doing--you write that you "perform some statistical analysis for different thermal stability ranges and afterwards we select the most likely heat flux offset and RMS values for land and water, taking in consideration the wind regime that WASP is prepared to work"; if by "most likely fluxes" you mean the ones that correspond to desired shear statistics for given stability ranges, then you might be able to improve results some by using option 3. Again, since the offset heat flux in particular may not directly correspond to the meso- fluxes, it is the shears that are most important; maybe you get some improved (e.g. seasonal) results by breaking the meso fluxes into groups and matching shears that way.
However, because you have 2 rather different wind climates, depending on how you use the meso data (it is not quite clear), the uncertainty of 'bending' the Wind-Atlas method (perturbed geostrophic drag law) in this way is likely to be comparable to (or maybe larger) than the 2.4% spread that you quote.
In other words, WAsP was not really intended to be used this way; unfortunately the new stability model implementation is not released.