# Spatial and Temporal Wind Power Forecasting by Case-Based Reasoning Using Big-Data

^{*}

## Abstract

**:**

## 1. Introduction

^{2}(i.e., 7.6 km × 7.6 km), which could be not suitable for accurately describing local wind dynamics in complex regions. Moreover, they require very large computational resources and complex, time-consuming solution algorithms, which make complex their deployment in a real grid operation scenario.

## 2. The Role of Physical Downscaling Models in Wind Power Forecasting

## 3. Wind Power Forecasting by Case Based Reasoning

**X**

_{B}be the input matrix storing n boundary conditions, each of them described by a vector of m components, representing the values of the weather variables on a spatial mesh with large resolution; and let

**Y**

_{B}be the output matrix storing the corresponding n downscaled solutions obtained by the CFD solver, each of them described by a vector of r components, representing the wind speed components in the points of interest, i.e., the wind turbines locations. These input/output matrixes represent the knowledge base of the CBR process, since they allow computing the downscaled solution for a boundary condition described by the query vector

**x**

_{q}according to the following procedure:

- Compute the distance between the query point
**x**_{q}and each vector of the input matrix**X**_{B}:$${d}_{j}=\sqrt{{\displaystyle \sum}_{i=1}^{m}{\left({\mathit{x}}_{q}\left(i\right)-{\mathit{X}}_{B}\left(j,i\right)\right)}^{2}}\text{\hspace{1em}}\forall j\in [1,n]$$ - Compute the similarity degree between the query and the stored vectors:$${w}_{j}=\frac{\underset{i}{\mathrm{max}}({d}_{i})}{{d}_{j}}\forall j\in [1,n]$$
- The downscaled solution ${\widehat{\mathit{y}}}_{q}$ for the query vector can be approximated by processing the downscaled solutions corresponding to the “most similar input vectors”, here referred as the neighbors, which can be identified ordering the input vectors according to their similarity degrees. To this aim, the following naïve approach can be applied:$${\widehat{\mathit{y}}}_{q}\left(i\right)=\frac{{{\displaystyle \sum}}_{j\in N}{\mathit{Y}}_{B}\left(j,i\right)\times {w}_{j}}{{{\displaystyle \sum}}_{j\in N}{w}_{j}}\text{}\forall i\in [1,r]$$

**X**

_{B}and

**Y**

_{B}. In fact, these matrixes are characterized by a large number of both rows and columns, depending on the number of available input/output patterns (e.g., order of several hundreds), and the spatial resolution of the environmental variables profile (e.g., order of several thousands), respectively.

## 4. Enabling Methodologies for Feature Extraction from Massive Wind Data

#### 4.1. PCA: Principal Component Analysis

- ${\overline{\mathit{X}}}_{B}$ and ${\overline{\mathit{Y}}}_{B}$ are the center of the matrixes ${\mathit{X}}_{B}$ and ${\mathit{Y}}_{B}$, respectively;
- ${\mathit{X}}_{Bs}$ and ${\mathit{Y}}_{Bs}$, whose dimensions are $[{n}_{PCx},\text{}n]$ and $[{n}_{PCy},\text{}n]$ ${(\mathrm{with}\text{}n}_{PCx}\ll {m,\text{}n}_{PCy}\ll r$), are the score matrixes;
- $\mathit{P}$ and $\mathit{Q}$, whose dimensions are $[n{,\text{}n}_{PCx}]$ and $[n{,\text{}n}_{PCy}]$, respectively, are the loadings matrixes;
- ${\mathit{\u03f5}}_{x}$ and ${\mathit{\u03f5}}_{y}$ are the error matrixes.

**P**and

**Q**, respectively, and f is the number of principal components, which can be selected by applying the methodologies described in [16].

#### 4.2. Partial Last Square Regression

**Y**as follows:

_{B}#### 4.3. Proposed Method

**X**

_{B}and

**Y**

_{B}, which are processed by a feature extraction technique based on PCA or PLSR. The corresponding reduced matrixes,

**X**

_{Bs}and

**Y**

_{Bs}, are stored in the database of the historical physical downscaling solutions, and used to infer the solutions for the query vectors. The overall forecasting process is summarized in Figure 3.

## 5. Experimental Results

#### 5.1. PCA Results

#### 5.2. PLSR Results

## 6. Conclusions

## Author Contributions

## Conflicts of Interest

## List of Symbols

x_{q} | query vector |

X_{B} | the input matrix storing n boundary conditions |

Y_{B} | output matrix storing the downscaled solutions obtained by using of the CFD solver |

${\mathit{d}}_{\mathit{j}}$ | distance between the query point x_{q} and each vector of the input matrix X_{B} |

${\mathit{w}}_{\mathit{j}}$ | similarity degree between the query and the stored vectors |

${\widehat{\mathit{y}}}_{\mathit{q}}$ | approximated downscaled solution ${\widehat{\mathit{y}}}_{\mathit{q}}$ for the query vector with CBR |

n | number of downscaled solutions obtained by the CFD solver |

m | number of components of each boundary condition set in X_{B} |

r | number of components of each downscaled solution set in X_{B} |

$\mathit{\beta}$ | regression matrix |

X_{N} | set of nearest neighbors of X_{B} |

Y_{N} | set of nearest neighbor of the Y_{B} correspondent to the X_{N} vectors |

${\overline{\mathit{X}}}_{B}$, ${\overline{\mathit{Y}}}_{B}$ | center of the matrixes X_{B} and Y_{B}, respectively |

X_{Bs}, Y_{Bs} | score matrixes of the matrixes X_{B} and Y_{B}, respectively |

${n}_{\mathit{P}\mathit{C}\mathit{x}}$, ${n}_{\mathit{P}\mathit{C}\mathit{y}}$ | number of principal components of the matrixes X_{B} and Y_{B}, respectively |

$\mathit{P}$, $\mathit{Q}$ | loadings matrixes |

${\mathit{\u03f5}}_{\mathit{x}}$, ${\mathit{\u03f5}}_{\mathit{y}}$ | error matrixes |

$\mathit{\Sigma}$ | covariance matrix |

${\mathit{w}}_{\mathit{k}}$ | example of column vector of loading matrix |

${\mathit{p}}_{\mathit{s}}$, ${\mathit{q}}_{\mathit{s}}$ | the s-th column vectors of loadings matrices P and Q of matrices X_{Bs} and Y_{Bs}, respectively |

${\mathit{\beta}}_{0}$ | intercept matrix |

${\mathit{x}}_{\mathit{q}\mathit{s}}$ | query vector in the new phase space |

${\widehat{\mathit{y}}}_{\mathit{q}\mathit{s}}$ | approximated downscaled solution ${\widehat{\mathit{y}}}_{q}$ for the query vector with CBR in the new phase space |

N | is the set of the neighbors |

${\mathit{k}}_{{\mathit{X}}_{\mathit{B}}},\text{}{\mathit{k}}_{{\mathit{Y}}_{\mathit{B}}}$ | principal components number of matrices X_{B} and Y_{B}, respectively |

RS_{PSLR} | Root Square Ratio |

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**Figure 5.**Comparison between latent variables extracted from CFD solver output (green) and obtained by using of the PCA-based CBR for the 2nd forecasting hour.

**Figure 6.**Comparison between spatial wind speed obtained from CFD solver output (green) and obtained by using of the PCA-CBR for the 2nd hour forecasting.

**Figure 7.**Comparison between spatial power output obtained from CFD solver output (green) and obtained by using of PCA for the 2nd forecasting hour.

**Figure 8.**Comparison between spatial power output obtained from CFD solver output (green) and obtained by using of PCA for the 2nd forecasting hour.

**Figure 13.**Scatter plot observed- fitted response in function of several number of PLS components used in this work.

Case Number | N° xPCs (${\mathit{k}}_{{\mathit{X}}_{\mathit{B}}}$) | N° yPCs (${\mathit{k}}_{{\mathit{Y}}_{\mathit{B}}}$) | Nearest Neighbors (NN) | Normal Root Mean Square Error (NRMSE) | Normal BIAS (NBIAS) | Normal Mean Absolute Error (NMAE) |
---|---|---|---|---|---|---|

1 | 5 | 5 | 3 | 0.5204 | 0.2063 | 0.4439 |

2 | 10 | 12 | 3 | 0.3995 | 0.0166 | 0.3403 |

3 | 5 | 5 | 6 | 0.4195 | 0.2599 | 0.3629 |

4 | 5 | 5 | 3 | 0.2994 | −0.0412 | 0.2366 |

5 | 10 | 12 | 3 | 0.3039 | 0.0864 | 0.2594 |

Case | RMSE on Wind Speed (m/s) | RMSE on Wind Direction (°) | Time Required | N° of PCs | Rsquared Ratio |
---|---|---|---|---|---|

1 | 1.7865 | 25.5 | 6.10 s | 24 | 0.8355 |

2 | 2.1861 | 27.8 | 2.87 s | 12 | 0.7581 |

3 | 1.6119 | 24.0 | 11.39 s | 48 | 0.8538 |

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**MDPI and ACS Style**

De Caro, F.; Vaccaro, A.; Villacci, D. Spatial and Temporal Wind Power Forecasting by Case-Based Reasoning Using Big-Data. *Energies* **2017**, *10*, 252.
https://doi.org/10.3390/en10020252

**AMA Style**

De Caro F, Vaccaro A, Villacci D. Spatial and Temporal Wind Power Forecasting by Case-Based Reasoning Using Big-Data. *Energies*. 2017; 10(2):252.
https://doi.org/10.3390/en10020252

**Chicago/Turabian Style**

De Caro, Fabrizio, Alfredo Vaccaro, and Domenico Villacci. 2017. "Spatial and Temporal Wind Power Forecasting by Case-Based Reasoning Using Big-Data" *Energies* 10, no. 2: 252.
https://doi.org/10.3390/en10020252