Elsevier

Renewable Energy

Volume 33, Issue 1, January 2008, Pages 155-161
Renewable Energy

Technical Note
Effect of solar radiation correlations on system sizing: PV pumping case

https://doi.org/10.1016/j.renene.2007.01.005Get rights and content

Abstract

The precise determination of solar radiation intensity is important for accurate system sizing. For PV pumping system case, the issue becomes more important since inaccurate solar radiation data nonlinearly reflected to the calculations. Inappropriate solar radiation model selection for a specific design site is a major cause for inaccurate system sizing. The differences in power output of a PV pumping system due to using some solar radiation correlations are examined here. The results are then compared with those of long-term solar radiation measurements. It is reported that the system sizing with the measurements and the most appropriate correlation for the application site significantly differ since the errors in predicting solar radiation data are nonlinearly propagated to the system power output.

Introduction

Estimation of long-term performance of a solar energy application requires having accurate information on the instantaneous solar radiation data since radiation intensity dynamically varies with both the space and the time. Mathematical correlations or meteorological measurements are used for obtaining such data. The best radiation information for a location can be obtained by experimental measurements. The use of mathematical correlations are however more common due to lack of enough number of solar radiation stations around the world. The correlations are also preferred due to being directly compatible with computer simulations. Several proposed mathematical correlations or models with exponential, polynomial or sinusoidal characters exist in the literature to obtain instantaneous radiation data [1], [2]. These correlations generally use the monthly mean daily data as an input to calculate the instantaneous values [3], [4]. The successes of most models are found to be limited by comparing hourly values of measurements with those obtained by the models [5], [6]. The moderate level of deviations (i.e. within 10%) may not usually have large influences when designing solar systems with linear output (i.e. solar water heaters). However, this level of deviations in solar radiation values can become critical when designing solar systems composed of highly complex components (i.e. photovoltaic powered systems).

We here consider a directly coupled photovoltaic water pump system (DC-PVPS) and examine absolute differences in its power outputs by using measured and calculated solar radiation data. The basic components of the system are schematically shown in Fig. 1. The PV array generates sufficient electrical power from the sunlight to operate the DC motor, which converts the electrical energy into the mechanical energy and drives the pump. The mechanical energy is then converted into the hydraulic energy by the pump to draw water from the well. This type of arrangement is known as DC-PVPS since PV array is directly connected to the DC motor–pump [7].

The major difficulty in the analysis of a DC-PVPS is that the volumetric rate of pumped water is dependent on many factors, which can be described by highly nonlinear equations [8]. These factors can be classified as meteorological parameters (radiation intensity and ambient temperature) of the site, PV array specifications (IV output, area, conversion efficiency, and slope), and DC motor–pump–hydraulic system characteristics (IV output of the motor–pump assembly, static and dynamic head of pipeline), as noted by Firatoglu and Yesilata [9]. The operating points depend on the current–voltage (IV) characteristics of both the motor–pump assembly and the PV array. IV characteristics of the PV array vary nonlinearly with solar radiation, ambient temperature, and the current drawn by the DC motor.

In this study, we have developed an algorithm to demonstrate the possible errors in power output, originated by using an empirical model rather than the long-term radiation measurements, when designing a DC-PVPS. The long-term (20 years between 1985 and 2004) solar radiation measurements on hourly basis for the design site (Sanliurfa, Turkey) are used to obtain monthly averaged data for each hour of the day. Three accepted solar radiation correlations given in the next section are considered for comparison. As a first step, the most appropriate correlation for the design site among these three is found. The deviations on PV power output of the DC-PVPS due to using measured and calculated data with the selected correlation are then determined.

Section snippets

Selection procedure for the correlation

The schematic of the algorithm used in this research is shown in Fig. 2. One of the main steps in the algorithm is the long-term data averaging. We have constructed a large electronic database for global solar radiation on horizontal surface (H) and ambient temperature (Ta) measured hour-by-hour between years of 1985 and 2004 and then found the corresponding values with the procedure given in the next section. The analysis presented here is made for the months in summer season. The monthly mean

Characterization of the PV pumping system

The main object in designing a PV pumping system (PVPS) is that the required pumping power should be generated by a minimum number of PV panels in order to improve the cost effectiveness [13], [14]. The DC-PVPS considered here has been previously optimized for the design site by using the long-term measured weather data [9]. The technical data for the components of the selected system is given in Appendix A. The multi-step optimization procedure used in our earlier work [9] provided the system

Comparisons of power outputs

The power outputs of the selected DC-PVPS are compared below by using the LJ correlation and measured solar radiation data. In calculation of the total incoming hourly solar radiation data on tilted PV panels surface (shown with Ht here), a routine procedure described in several references [1], [16], [17], [18] is used. The calculation is based on the knowledge of hourly direct and diffuse radiations on horizontal surface (shown with Hb and Hd, respectively) and the tilt-angle (α). The

Conclusions

In this study, we have developed an algorithm to demonstrate nonlinear propagation of the errors in power output of a DC-PVPS due to using the empirical correlation instead of the long-term solar radiation measurements. The main conclusions that can be drawn from this work are summarized as follows:

  • (i)

    The most appropriate correlation for the design site, among the three accepted solar radiation correlations of Lui and Jordan (LJ), Collares-Pereira and Rabl, and Kilic, is found to be LJ

Cited by (0)

View full text