Direct normal irradiance forecasting and its application to concentrated solar thermal output forecasting – A review Edward W. Law a,⇑ , Abhnil A. Prasad a , Merlinde Kay a , Robert A. Taylor a,b a School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, New South Wales 2052, Australia b School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney, New South Wales 2052, Australia Received 7 April 2014; received in revised form 6 July 2014; accepted 9 July 2014 Communicated by: Associate Editor Jan Kleissl Abstract Solar irradiance forecasting can reduce the uncertainty of solar power plant output caused by solar irradiance intermittency. Concen- trated solar thermal (CST) plants generate electricity from the direct normal irradiance (DNI) component of solar irradiance. Different forecasting methods have been recommended for a range of forecast horizons relevant to electricity generation. High DNI forecast accu- racy is important for achieving accurate forecasts of CST plant output which are shown to increase CST plant profitability. This paper reviews the DNI forecast accuracy of numerical weather prediction models, time series analysis methods, cloud motion vectors, and hybrid methods. The results of the reviewed papers are summarised to identify the best DNI forecast accuracy for particular forecast horizons. The application of DNI forecasts to operate CST plants is also briefly reviewed. This paper found that additional research is required for time series analysis methods to corroborate current results and for satellite- based cloud motion vectors to establish DNI forecast accuracy. It was also concluded that future research should use the same error metrics to report results to facilitate fair comparison of DNI forecast accuracy from different studies. In addition, the creation of a com- mon high quality DNI data set to evaluate all forecasting methods would also help to verify best forecast accuracy. The review of DNI forecasting for CST plants found that using accurate 2-day ahead DNI forecasts can increase revenue and decrease penalty costs. Future research should investigate benefits from using short-term DNI forecasts from the intra-hour forecast horizon up to the 6-h forecast hori- zon to determine CST plant operation. Another aspect to research is to determine whether the benefit of DNI forecasts for a CST plant is affected by different regulations in different electricity markets. Ó 2014 Elsevier Ltd. All rights reserved. Keywords: DNI forecasting; Numerical weather prediction; Time series analysis; Cloud motion vector; Forecast accuracy; Concentrated solar thermal power 1. Introduction Solar energy is a renewable resource that has established itself in both small-scale and large-scale electricity genera- tion. Electricity can be generated from solar irradiance by either non-concentrated photovoltaic (PV) modules or by concentrated solar thermal (CST). PV output is calculated from global irradiance on the plane of the PV modules which can be derived from global horizontal irradiance (GHI) (Lorenz et al., 2009, 2011). GHI consists of diffuse irradiance and direct normal irradiance (DNI) compo- nents. In contrast to PV, CST output calculations only use the DNI component because diffuse irradiance cannot http://dx.doi.org/10.1016/j.solener.2014.07.008 0038-092X/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Address: School of Photovoltaic and Renew- able Energy Engineering, Tyree Energy Technologies Building, University of New South Wales, Sydney, New South Wales 2052, Australia. E-mail address: edward.law@unsw.edu.au (E.W. Law). www.elsevier.com/locate/solener Available online at www.sciencedirect.com ScienceDirect Solar Energy 108 (2014) 287–307