Solar Power Systems Energy Production The power produced by a PV generator system depends on a range of factors which need to be examined when the system is designed: it is also useful to assess the accuracy of simplified treatments where these factors are ignored or neglected. Such analysis is conveniently carried out by looking at the total annual energy produced by the solar home generation. A recent comprehensive study has identified seven factors influencing the annual performance of PV modules [l]and a brief summary of thc main conclusions follows. Cumulative solar irradiance. Long-term irradiance profiles depend on surface orientation and possibly tracking. This factor depends on the location and varies between a reduction by about 25% for a vertical surface to over 30%increase for two axis tracking, in comparison with a latitude-tilt fixed system. The effect of module orientation is considered further in Section 5, and is analysed in detail in Part I. Module power rating at standard test conditions. Analysis of several PV technologies has shown that for the same power rating all technologies were equivalent in terms of the expected annual energy production within 5%calculation error. Operating temperature. Analysis of various technologies and sites shows that the annual production can be reduced due to the operating temperature by a factor between 2 and lo%,depending on the module design, wind speed, mounting technique and ambient temperature. The effect of operating temperature is discussed quantitatively in Section 4. Maximum power point voltage dependence on irradiance level. a-Si and CdTe modules tend to have a value of the maximum power point voltage larger at low irradiance levels than at the standard 1sun conditions. This fact can result in an additional 10%increase in annual energy production. Soiling. Soiling may account for up to a 10%of reduction of the annual energy production. Variation in solar spectrum. It is found that the effects of the hourly variation of the solar spectrum almost cancel out in a yearly basis. Amorphous silicon technology has the highest sensitivity to this effect, but the observed changes usually remain under 3%. Optical losses when the sun is at a high angle of incidence (AOZ). The optical losses are due to the increased reflectance of the cover glass of the PV modules for A01 greater than approximately 60".However, the effect on a long term basis is relatively small (typically under 5%) although it may have larger effect on a seasonal basis (close to 10% for a vertical inclination). 2 Peak Solar Hours: Concept, Definition and Illustration The initial approximate analysis and design of a PV solar system is usually based on Peak Solar Hours (PSH): a convenient definition of the equivalent of one day.
llla-1 -Energy Production by a PV Array 5 19 This concept is particularly useful for the first-order sizing of flat-plate (non-concentrating) arrays which operate under global radiation (see IIIa-3). The magnitude of Peak Solar Hours is equal to the length of an equivalent day with a constant irradiance equal to the 1sun intensity (1kW/m2),resulting in the same value of the daily radiation. This parameter has units of time and when given in hours, it has the same numerical value as the total daily radiation in kWh/m2-day. Accordingly, the total generation of a PV array exposed to solar radiation a wholeyear canbe estimatedas,
365
EA = (PSH),Po
i= 1
where (PSH),.is the value of the parameter PSH for day i and Po is the nominal array power under standard or reference conditions. For arrays operating at the maximum power point, the normalised instantaneous power output PA/P,,, depends on temperature and irradiance, and Equation (1)is therefore only an approximation. A better estimate of the annual generation can be obtained by a model which uses actual values of the ambient temperature, and estimates the cell operating temperature using the NOCT concept (see Section 4)[3] and the cell efficiency temperature coefficient [2].Figure 1 shows the error involved in the use of Equation (1)rather than by taking into account the full temperature and irradiance dependence of the array power output. As can be seen, the average annual errors lie between -2% and -15%. similar to the experimentally observed values discussed in Section 1. Clearly, the error is larger in locations where the yearly radiation and average temperature are higher. Figure 1 Error involved in the use ofEquation (1) to calculate the annual energy output by a PV array at
several locations worldwide. Numbers by locations indicate the latitude. Source of the radiation and temperaturedata: Meteonormprogram 1111.
520 Practical Handbook of Photovoltaics: Fundamentals and Applirations
3 Nominal Array Power Results in Figure 1are specific for arrays operating at the maximum power point This usually includes all systems with an inverter and larger DC systems, with power rating in excess of 1kW or so. In these systems, the voltage is controlled by the maximum power point tracker (MPPT) to follow the optimum maximum power point voltage V,,,. The situation is somewhat different in smaller off grid solar systems where the array voltage is controlled by the load typically consisting of a charge regulator and a battery (Figure 2). Here, the array delivers power at a voltage close to the battery voltage Vbat and the reduction in array output shown in Figure 1 is compensated by a lower nominal array power instead of P,,,,,. The design of a standard PV module consisting of 36 crystalline silicon cells has evolved from the need to charge a 12V battery. In practical usage, the module then operates in the linear part of its I-V characteristic and supplies approximately the same current Isc as at short circuit. The power PAdelivered by the array to the battery and load in parallel connection is then which takes into account the actual bias point of the module and the instantaneous value Vbat of the battery voltage. The voltage drop at the blocking diode which is not included in Equation (2) is discussed in more detail in Section 7. Losses in the charge regulator can often be neglected as most modern regulators have switches in place of diodes and hence the voltage drop is small. The average value of PA -the effective module power rating -is often approximated by nominally setting Vbatequal to 12 V: Equation (3) -valid for a system without maximum power point tracking -is a useful approximation for the analysis of the long-term energy balance in stand alone systems, and for the development of sizing procedures, as discussed in Chapters IIIa-2 and -3. The accuracy of Equation (2) in a specific application can be verified for the actual daily load profile with the help of detailed system models. As a practical example, Solar panels with manufacturers rating of 85 W,, I,, = 5 A and maximum power point values V, = 18V, I,,, = 4.72 A produces P,8= 12 x 5 = 60 W which is a reduction of about 2 9%on P,,, at STC. 4 Temperature Dependence of Array Power Output The principal effect of temperature on the PV array output comes from the temperature dependence of the open-circuit voltage (see Figure 4) which can be described by Hebe Solar.
|
