learn from historical data and complexity of the ANN-
based methods to specify numerous model parameters
require a hybrid of different methods to resolve the short-
comings of the ind ividual forecasting techniques. A hybrid
of ARMA and a time delay neural network (TDNN) was
proposed in Wu and Chan (2011) which demonstrated
more accurate results than each method separately. How-
ever, the feasibility of the proposed method for short-term
solar forecasting is not justified due to its high-
computational complexity. Voyant et al. (2012) used a
combination of ARMA and ANN with numerical weather
prediction (NWP) to forecast the hourly mean global hor-
izontal irradiance. Self-organizing map (SOM) has been
also used in hybrid methods for pattern recognition and
classification of the input data (Chen et al., 2011; Yang
et al., 2014). Meteorological predictions for relative
humidity, solar irradiance and temperature were utilized
in Chen et al. (2011) to forecast the solar power genera-
tion. However, inaccuracies of the meteorological fore-
casts degrade the quality of the solar power prediction.
Yang et al. (2014) developed a hy brid approach
combining a SOM, a learning vector quantization (LVQ)
network, the SVR and fuzzy inference methods for a
day-ahead PV power prediction. Numerical results
demonstrated that the proposed hybrid approach per-
forms better than the traditional ANN and the simple
SVR methods individually. However, SOM has some lim-
itations for non-convex or discontinuous input distribu-
tions. In addition, non-winning neurons do not
participate in the learning phase which reduces the quality
of the map and accuracy of the forecast. Game theories
have been used for SOM training to resolve these prob-
lems and globally optimize the map (Herbert and Yao,
2005; Neme et al., 2005 ). Herbert and Yao (2005) pro-
posed a training algorithm that considers the neurons as
players with strategy sets and utility functions. The algo-
rithm was used to improve the quality of the map, but
no justification was provided by either numerical results
or experiments. In addition, the algorithm defines the
player’s utility function based on all players’ actions and
requires solving the game for all iterations which increases
its computational complexity. Neme et al. (2005) proposed
different strategies to enable the winning neuron to select
between its neighbors for the weight adaption. This
reduces the complexity of the original SOM algorithm
where the weight vectors of all neighboring neurons are
updated. However, the proposed strategies were non-
cooperative in topographic map formation without mini -
mizing the error measure. Engelbrecht (2007) proposed a
hybrid model of self-organizing maps (SOM), support vec-
tor regression (SVR) and particle swarm optimization
(PSO) to forecast hourly global solar radiation. SOM
algorithm was applied in the first step to divide the entire
input space into several disjointed regions or clusters. SVR
models were then used to model each cluster for detecting
characteristic correlation between the predicted and the
actual values. In order to deal with the volatility of SVR
with different parameters, PSO algorithm was used to
improve the forecasting performance of the SVR models.
However, the SOM algorithms may converge to non-
optimal clustering results depending on the initialization
and learning rate considered for the algorithm. In addi-
tion, neighborhood violations occur if the output space
topology does not match with the data shape.
The amoun t of solar radiation that reaches the ground is
extremely variable because of the apparent motion of the
sun and changing conditions of the atmosphere, which
make its forecasting complicated. The chaotic nature of
the solar data disrupts the neural network learning process
and presents the forecasting results with high errors (Di
Piazza et al., 2011). An improved version of SOM algo-
rithm is proposed in this paper, to group the solar time ser-
ies data into clusters to better characterize its irregular
features. This technique allows handling groups of data
separately, whic h by identifying anomalies and irregular
patterns as well as neglecting outliers, provides a better
understanding of the collected information. This leads to
more appropriate learning for neural networks and
improves the accuracy of the forecasts.
This paper develops a hybrid solar radiation forecasting
based on a novel clustering algorithm. Game-theoretic con-
cepts with new strategies are used in conjunction with SOM
to provide a modified GTSOM clustering. The proposed
clustering method increases the non-winning neurons’ par-
ticipation in the learning and enables their competition
with the winning neurons. This provides a better clustering
performance as compared to the original SOM where the
dead neurons have negligible chances to obtain input
patterns. Unlike the original SOM which defines the
neighborhood ba sed on the neurons’ distances in the two-
dimensional lattice, NG is combined with CHL to define
the neighborhood based on the neurons’ distances in the
input space. This speeds up the learning and enhances
the mapping quality. Wavelet decomposition and time
series analysis are used to preprocess the solar radiation
data. The time series analysis develops the structure of
the training and forecasting for BNNs. The proposed
GTSOM groups the pre-pr ocessed solar radiation datasets
into a number of clusters determined by the elbow method.
A cluster selection technique is proposed to select the clus-
ter that is used for the solar foreca sting of each individual
hour. Temperat ure, wind speed and wind direction data are
also included in the inputs to the BNN whose outputs
provide the solar radiation forecasts.
The rest of the paper is organized as follows. The orig-
inal SOM algorithm and game-theoretic concepts are
explained in Section 2. This section also describes the pro-
posed GTSOM clustering and the hybrid solar radiation
forecasting methods. Section 3 provides a case study where
the accuracy results are calculated for the hybrid forecast-
ing with different clustering algorithms including the pro-
posed GTSOM method and the existing K-means, the
original SOM and NG clustering. Conclusions are given
in Section 4.
1372 M. Ghayekhloo et al. / Solar Energy 122 (2015) 1371–1383