PERFORMANCE PARAMETERS OF AN OFF-GRID ...

33rd European Photovoltaic Solar Energy Conference and Exhibition

PERFORMANCE PARAMETERS OF AN OFF-GRID BUILDING INTEGRATED PHOTOVOLTAIC SYSTEM IN SOUTH AFRICA Edson L. Meyer1, Carine L. Buma2, Raymond Taziwa3 University of Fort Hare, Institute of Technology 1 Fort Hare Drive, 3rd Floor Chemistry Building, Alice 5700, South Africa [email protected], [email protected], [email protected] ABSTRACT: Less than 4% of South Africa’s energy is being generated from renewable energy sources. With South Africa’s vast solar resource, a lot more can be done to nurture the dependence on renewable sources of energy such as solar energy. The use of off-grid photovoltaic systems is one major way of reaching out to the rural and isolated communities inaccessible to the utility grid. Monitoring the performance of such systems is the key to ensuring growth and development in the sector. The performance of a stand-alone photovoltaic system is not as straight forward as for a grid-connected system. The performance of a typical off-grid building integrated photovoltaic system was monitored for a period of seven months. The work demonstrated that the system performed better in winter than in the summer. It was observed that the average reference yield, array yield, final yield, array capture loss and system losses for the system were of 4.5, 2.5, 1.1, 1.9 and 1.4 h/d respectively. Average array, inverter and system efficiencies were 10%, 78% and 8% respectively. Also, the system recorded an average performance ratio of 27%. Keywords: Off-grid system, building integrated photovoltaic system, performance ratio.

1

INTRODUCTION

common in stand-alone PV systems [4]. The PR being a dimensionless quantity that indicates the overall effect of losses on the rated output, does not represent the amount of energy produced because a system with low PR in a high solar resource location might produce more energy than a system with a high PR in a low solar resource location. Nonetheless, for any given system, location and time, if a change in component or design increases the performance ratio, the final yield increases accordingly. The performance ratio can identify the existence of a problem, but not the cause. To identify the cause of the existing problem, a research at the site is needed. The performance analysis of stand-alone PV systems is not as straight forward as it is for grid connected systems. A poor performance does not necessarily mean that the system encounters technical problems but can also be the consequence of a bad matching between the production and the load [5]. The IEA PVPS task II experts collected operational data for a set of PV systems amongst which were over thirty stand-alone PV systems. The reason was to access their performance over time, and create a database. The results showed that the standalone systems presented a wide range of PR values which did not reflect the proper operation of the system from a technical view point. The typical range of PR values obtained was between 0.1 and 0.7. Over 80% of the PR was between 0.3 and 0.4 while just 10% ranged between the 0.6 and 0.7. An oversized stand-alone PV system usually faces frequent array disconnection in order to prevent the batteries from overcharging. This tendency has a negative effect on the value of the PR for the system. The aim of this work is to evaluate the performance of a 3.8 kWp stand-alone building integrated photovoltaic system that was installed at the Fort Hare Institute of Technology, South Africa. The results obtained from this study will create awareness of the potential of such a system being used to curb the problem of energy poverty and energy insecurity in various parts of the world, especially in less developed countries in the African continent, more particularly in remote rural communities.

The global demand for energy has skyrocketed greatly over the last few decades principally due to industrial development and population growth. South Africa is not left out in this massive increase in the demand for energy. The major concern is that over 90% of South Africa’s energy is being generated from coal [1], with less than 4% coming from renewable sources. It is worth noting that this country is endowed with a very rich solar resource. On average, it receives annual 24hours global solar irradiance levels of about 220 W/m2 compared with about 150 W/m2 for parts of the USA, and 100 W/m2 for Europe and the United [2]. The advancement of the photovoltaic (PV) technology in the country is one major way of offsetting this gross dependence on unclean sources of energy. However, compared to other developed countries, South Africa’s PV market is still in the early stages. One of the major ways of ensuring a brighter future for this technology in the country is through accurate and continuous performance monitoring of existing PV systems on the field. Off-grid PV systems are one of the major applications of PV systems. This is very important in that it can be used to power locations inaccessible to the utility grid and for individuals wishing to get some degree of energy independence. This technology is equally very important to most remote communities in Africa where connecting to the national utility grid is still far-fetched. An even more innovative advancement in the PV technology is the building integrated photovoltaic (BIPV) system. However, after over three decades of existence, this technology has successfully claimed just about 1% of the total PV installations worldwide. Very few of such systems have been reported in South Africa. Continuous research is required in this sector in order to promote its growth. One of the most important quantities usually determined when evaluating the overall performance of a PV system is the performance ratio (PR). Several studies have demonstrated the continuous improvement in PV system PR over the years. It rose from 0.5 to 0.75 in the late 1980s and 0.65 to 0.7 in the 1990s, to more than 0.8 currently [3]. However, PR values as low as 0.35 are still

2

SYSTEM DESCRIPTION This off-grid BIPV system is located at the

2450

33rd European Photovoltaic Solar Energy Conference and Exhibition

University of Fort Hare, Alice, South Africa.32° 47' 0" South, 26° 50' 0" East. The system has a peak capacity of 3.8 kW. The SANYO HIT (Hetero-junction with Intrinsic Thin layer) PV modules used here have a high capacity to frame area ratio, with a conversion efficiency of 16.1%. These PV modules have been integrated to form part of the roof of a passive zero energy solar building. The twenty HIT PV modules occupy a total roof area of 24 m2, at an inclination of 25o. A Victron Energy MultiPlus5048 bidirectional inverter is used in this system which converts the DC energy produced by the panels to AC energy needed by the loads. The inverter has a nominal output power of 5 kVA at 25oC. There is also a FLEXmax 80 MPPT charge controller. This regulator ensures peak performance from the PV panels, by letting them deliver their maximum power at any given instance. The charge controller also protects the battery by controlling its charging current and voltage, thereby prolonging the battery life span. A C/100 900 Ah rated capacity flooded lead acid battery is used in this system to store energy during excess generation. The battery bank is a 48 V system. Also, there is a data acquisition system composed of sensors, transducers and data loggers for measuring and recording electrical system parameters and the meteorological parameters affecting system performance. Below is a schematic representation of the house containing the BIPV modules and the balance of system components.

between February and August, 2017. This data was used to evaluate the performance of this off-grid BIPV system based on certain normalized system performance parameters. This was done based on the IS/IEC 61724, 1998 standards. These performance parameters are discussed below; The yields have units of kWh/day/kWp (or h/day) and indicate the amount of time during which the array would be required to operate at the array’s rated output power, Po to provide a particular quantity [6]. 3.1 Reference yield (YR) The reference yield is the theoretically possible energy output of the PV plant. It is defined as the anticipated output from the same system with nominal efficiency determined under STC of PV modules. In other words, the reference yield is the number of peak sun-hours. It is the ratio of the total in-plane solar radiation (Hid) [kWh/m2], to the reference irradiance (1 kW/m2) at standard test conditions. H id YR  (1) G STC

3.2 Array yield (YA) The array yield represents the number of hours per day that the array would need to operate at its nominal or peak power to contribute the same quantity of energy to the system as that actually measured in practice. It is the ratio of the daily energy output of the PV array (EA) to the peak power of the installed PV array. YA 

EA Po

(2)

3.3 Final yield (YF) The final yield is the usable portion (used by the load, EL) of the energy derived from the PV modules (Epv). In a standalone system, the energy stored in the battery bank (Ebat) is equally taken into consideration. The final yield is obtained from; EPV YF  (3) Po

E pv 

Figure 1: Schematic representation of the off-grid BIPV system components 3

EL Ebat   1  EA   

(4)

3.4 Performance ratio (PR) The performance ratio is one of the most important parameters to consider when evaluating the performance of a PV system. It is the ratio of the actual energy output to the theoretically possible energy output. It is independent of the orientation of the PV plant and the incident solar radiation on the PV plant. This is why the performance ratio can be used to compare PV plants at different locations all over the world, or for performance comparison of a particular system over the years. It is the ratio of utilizable AC electricity to the amount of energy which could be generated in case modules were operated under STC continuously and without any further losses in the system [7]. Y PR  F (5) YR

METHODOLOGY

This system is made up of sensors and data loggers for measuring and recording both electrical and meteorological parameters of the system. The electrical parameters include the current and voltage flowing from and into each balance of system component. The meteorological parameters include; solar irradiance on the plane of array, ambient temperature, cell temperature, relative humidity, wind speed and direction. There are two data loggers used for measuring the electrical and meteorological parameters respectively. The electrical data is sampled every 30 seconds and averaged over five minute intervals. Likewise, the meteorological data is sampled every minute and averaged over thirty minute intervals. Data was collected for a period of seven months;

3.5 Losses The losses in a PV system may be caused by high

2451

33rd European Photovoltaic Solar Energy Conference and Exhibition

array temperatures, incomplete utilization of the irradiation, improper system sizing and system component inefficiencies or failures [8]. Energy losses indicate the amount of time during which the array would be required to operate at its nominal power in order to produce power for the losses. The losses are in the two parts: system losses (LS) and array capture losses (LC). The capture loss is the difference between the reference yield and the array yield. It is caused by incomplete utilization of the available radiation. The system loss is the difference between the array yield and final yield. It mainly comes from the inverter in converting the DC power to AC [9]. and from losses in the other balance of system components. (6) L  Y Y (7)

at 5 PM (0.45 kW). These peaks fall within the utility grid’s peak demand periods (between 7 am and 9 am in the morning and between 5pm and 9pm in the evening). At night, from 7 PM to 3 AM, the average demand is 0.26 kW. The figure reveals that the highest consumption occurs during the working days or weekdays (Monday to Friday) with an average consumption of 0.31 kW, followed by the Sunday consumption of 0.23 kW and finally, a Saturday consumption of 0.22 kW. It can equally be observed that peak irradiation occurs at about midday when the load demand is least. Thus, this system is not being utilized at its full capacity. If this system was connected to the grid to feed in this surplus energy, a lot of energy and monetary savings will be generated. It is recommended that the occupants of this house make an effort to rearrange their consumption pattern by using energy intensive appliances at about midday when the PV generation is highest.

3.6 Efficiencies The efficiencies involved in a PV system are in two major levels; at the level of the array and the whole system in general. The system efficiency embodies the efficiencies of the balance of system components and the PV array. Given that the major component where losses are incurred is the inverter, the system efficiency is simply the product of the array efficiency and the inverter efficiency.

4.2 Solar radiation and ambient temperature variation The performance of a PV system is influenced by so many factors amongst which are meteorological factors. The principal meteorological factors affecting the power output and efficiency of the system are ambient temperature and solar radiation. Figure 3 below shows the variation of solar radiation (average number of peak sun hours per day) and ambient temperature for the various months considered for this study.

c

R

A

Ls  YA  YF

array 

EA Aarea  Gid

sys   A inv 4

(8)

(9)

RESULTS AND DISCUSSIONS

4.1 Load profile for the building powered by the off-grid BIPV system During the analysis period, the house powered by this PV system was occupied by two adults and three kids.

Figure 3: Average solar radiation and ambient temperature profiles for the analysis period. It can be observed that the highest radiation occurs in March, with an average of 5.9 peak sun hours per day while the least radiation occurs in May, with an average daily radiation of 3.9 peak sun hours per day. The temperature variation follows a similar pattern. However, the highest average daily temperature is noticed in March (22.0oC) while the least is observed in July (12.1oC). From these profiles, the PV system is expected to have the best performance in July, given that it is the month with the least temperature and considerably high levels of solar radiation.

Figure 2: Load profile for the house powered by the offgrid BIPV system

4.3 Yield factors for the system As earlier mentioned, the yields indicate the amount of time during which the array would be required to operate at its rated output power to provide a particular quantity. Figure 4 below shows the relationship between the reference, array and final yields for this system for the various months considered for the study.

Figure 2 above presents the load profile for an average weekday, Saturday and Sunday. An average demand profile for the whole period is also illustrated. Furthermore, the figure also shows the variation of solar irradiance at various hours of the day. It can be observed that the consumption pattern for the weekend is slightly different from the weekday pattern. On average, the morning demand peak occurs at 7 AM (0.35 kW) while the evening demand peak occurs

2452

33rd European Photovoltaic Solar Energy Conference and Exhibition

and array losses results in the reference yield. So, the full length of each bar represents the amount of energy this system would have produced at standard test conditions and at the arrays rated efficiency, in the absence of losses. Furthermore, it can be noticed that the system encounters a lot of capture losses. Given that this system has been in existence for about eight years, there are signs of degradation observed on the surface of some modules. This is principally module browning caused by encapsulant delamination. Previous studies on the modules reveals a drop in the short circuit current of the modules. This effect contributes greatly to the high capture losses. Another reason for the high capture losses is the huge difference between the load consumption and the PV generation potential. There is a lot of energy which is not tapped because the battery bank gets fully charged and the load is demanding very minimal energy when the PV is still at its peak generating capacity. Additionally, it can be observed that most of the capture losses occur in the summer months of February March and April (an average of 3 h/d) when temperatures are still very high. During the winter months (between May and August), the capture loss is greatly reduced as temperatures have become quite low (an average of 1.4 h/d). High operating temperatures reduce the output voltage and thus the power output of the modules. On average, the system recorded capture losses of 1.9 h/d and system losses of 1.4 h/d.

Figure 4: Variation of yield factors for this system For each month, the reference yield is highest, followed by the array yield and then the final yield. The reference yield depicts the number of peak sun hours for a particular month. It represents the amount of energy which would be produced if the system operated at STC conditions and at the rated efficiency. This is highest in February, with a total number of 5.4 peak sun hours per day. The least value occurs in May (3.9 peak sun hours per day). Given that this is an off-grid system with a battery back-up and a charge controller, only the amount of energy needed to charge the battery bank and to feed the loads can be used irrespective of the available energy from the sun. This partly explains the huge difference between the reference yield and the array yield. Moreover, the inefficiencies involved in the conversion of the available solar radiation to electricity also contributes to this difference. There is an average of 2.1 kWh per day of energy difference between the available solar radiation and the amount of energy tapped by the MPPT charge controller. This is termed the capture loss as illustrated on Figure 5 below. Furthermore, of the energy that is tapped by the charge controller to feed the battery bank and the loads, part of it is lost before if finally gets to the loads. This is as a result of energy losses in the various system components such as the charge controller, the battery bank, the inverter and other wiring losses. This explains the gap between the array yield and the final yield in Figure 4 above. This difference is termed the system loss. It is equally depicted on Figure 5 below. The system recorded an average reference yield, array yield and final yield of 4.5 h/d, 2.5 h/d and 1.1 h/d respectively.

4.4 Performance ratio of the system As earlier mentioned, performance ratio is a globally accepted parameter used to compare similar PV systems at various locations on the globe. A low value for performance ratio in an off-grid PV system does not necessarily imply poor system performance or technical problems. It is equally an indication of poor matching between the PV generator and the electrical appliances powered by the system. An oversized system incurs losses because the PV system is not being used at its full potential. Figure 6 below illustrates the changes in the performance ratio for this system at various months of the year. The figure also reveals the dependence of the performance ratio on temperature.

Figure 6: Variation of performance ratio and ambient temperature for various months of the year As shown on Figure 6 above, this system shows a relatively low performance ratio compared to other standalone PV systems in different parts of the world. It however falls within the range of the PR values obtained by the IEA PVPS task II experts. They carried out a survey on about thirty off-grid PV systems and

Figure 5: An illustration of the two major types of losses incurred in this system Figure 5 above illustrates the major losses incurred in the system. A combination of the final yield, system loss

2453

33rd European Photovoltaic Solar Energy Conference and Exhibition

discovered that over 80% of the PR values ranged between 0.3 and 0.4 while just 10% ranged between 0.6 and 0.7. The months with the highest average daily performance ratio are May and August, while the performance ratio is least in February (a daily average of 14%). However, the highest temperature is in March and the least is in July. This system recorded an average performance ratio or 27%. It is well known that the performance of a PV system is adversely affected by high temperatures. The fact the month with the highest performance ratio does not exactly correspond to the month with the least ambient temperature justifies the fact that the performance ratio of this off-grid system is also affected by other factors, load matching being one of the key factors. Figure 7 below illustrates the correlation between the performance ratio for this system and the energy demand. It shows a linear relationship with a correlation coefficient of 0.63. This goes to justify the fact that the performance ratio for an off-grid system is highly dependent on its load consumption.

7%, 68% and 5% respectively occur in February (summer). Similarly, the highest array, inverter and system efficiencies of 12%, 81% and 10% respectively occur in May (winter). Figure 8 below represents the correlation between array conversion efficiency and the operating array temperature.

Figure 8: Correlation between efficiency and module temperature

array

conversion

Figure 8 above represents a negative linear relationship between the array efficiency and the operating array temperature with a correlation coefficient of -0.63. 4.6 Inverter efficiency The inverter is one of those components whose efficiency is highly dependent on load demand. This is because being a stand-alone PV system, the input power into the inverter is determined by the demand from the loads. In the absence of any demand, little or no energy is fed to the inverter. Figure 9 below reveals the relationship between the inverter efficiency and the normalised input power into the inverter. The normalised input power was calculated as the inverter’s input power as a percentage of its rated capacity.

Figure 7: Correlation between performance ratio and energy demand 4.5 System efficiency The overall efficiency of the system is equally an important parameter used in describing the performance of a PV system. It encompasses the efficiencies of the various balance of system components used in the system. The system efficiency shown on Figure 8 below is a product of the array efficiency and the inverter efficiency. The inverter efficiency was used because it is one of the major balance of system component where losses are incurred. The array efficiency accounts for the percentage of incident radiation converted to electricity by the modules.

Figure 9: Variation of inverter efficiency with normalized inverter input power at various temperatures Also, this calculation was done for ambient temperatures above 25oC and below 25oC given that the inverter is rated at 25oC. From Figure 9, it can be observed that the efficiency of the inverter increases with an increase in the normalized input power into the inverter up to when the inverter is operated at about 15% of its rated power. The efficiency then remains fairly constant and then starts dropping slightly when operated above 50% of its rated

Figure 7: Variation of array, inverter and system efficiency for various months of the year The least array, inverter and system efficiencies of

2454

33rd European Photovoltaic Solar Energy Conference and Exhibition

capacity. This low efficiency at low load can be associated with the switching losses at low input power. The input capacitance of the MOSFETS which are the switching devices in the inverter, consume some power whether there is output power from the inverter or not. Furthermore, at very high input power into the inverter, the inverter efficiency drops due to resistive losses which increase as output current increases. In addition, the temperature rise of the electronic devices in the inverter that accompany high output power may cause the inverter to reduce its output power and hence, its efficiency. This is called temperature derating. Also, it can be observed from Figure 9 that the higher inverter efficiency also corresponds to periods when ambient temperature is lower than 25oC, and the lower ambient temperatures highly contribute to the high efficiency values of the inverter. 5

[3] A. Woyte, M. Richter, D. Moser, S. Mau, N. Reich, U. Jahn, Monitoring of photovoltaic systems: good practices and systematic analysis. European photovoltaic solar energy conference proceedings. Paris, France (2013). [4] C. Iskander and E. Scerri, Performance and cost evaluation of a stand-alone photovoltaic system in Malta. Renewable energy journal. Vol. 8 Issues 1–4, May–August (1996) 437-440. [5] D. Mayer and M. Heidenreich, Performance analysis of stand-alone Photovoltaic systems from a rational use of energy point of view. International energy agency photovoltaic power system program. Austria (2003). [6] IS/IEC 61724, Performance monitoring – guidelines for measurement, data exchange and analysis. Indian standards (1998). [7] W.G. Van Sark, N.H. Reich, B. Muller, A review of PV performance ratio development. Proceedings of world renewable energy forum conference Vol. 6 (2012). [8] W. Xinfang, L. Yongsheng, X. Juan, L.Wei, S. Xiaodong, D. Wenlong, Z. Chunjiang, Z. Yunbo, P. Lin, L. Jia, Monitoring the performance of the building attached photovoltaic (BAPV) system in Shanghai. Journal of energy and buildings. Vol. 88 (2015) 174-182. [9] E. Kymakis, S. Kalykakis, T. M. Papazoglou, Performance analysis of a grid connected Photovoltaic Park on the island of Crete. Journal of energy conversion and management, Vol. 50 (2009) 433-438.

CONCLUSIONS

It is recommended that off-gird PV systems be used only is locations inaccessible to the grid. In locations where the PV system is installed close to the utility grid, the system should be connected to the utility grid to feed in the surplus energy that is otherwise wasted due to poor system sizing, which is of course difficult to avoid. Furthermore, the study shows that temperature is a very important parameter affecting the performance of a PV system. The system performed better in the winter months than in the summer months even though high irradiation values were recorded in the summer months. The performance ratio ranged from 14% in February to 30% in August, with a summer average of 22.3% and a winter average of 28.3%. Furthermore, the summer values for the array efficiency, inverter efficiency and system efficiency were 8%, 72% and 6.3% as opposed to the winter values of 11.25%, 79.25% and 9.25% respectively. This system encounters a lot of array losses (1.9 h/d on average) principally due to the mismatch between the array capacity and the load consumption. It is recommended that energy intensive appliances be used at about midday when PV generation is maximum, so as to minimize the wasted PV generation potential. 6

ACKNOWLEDGMENTS

The authors wish to acknowledge the Department of Science and Technology, the National Research Foundation and GMRDC for their financial support. Also, the technical support of Mr Melvin Adonis and Mr Vincent Schoemann is much appreciated. 7

REFERENCES

[1] D.V. Brits and R. Zietsman. The Feasibility of Building Integrated Photovoltaic Systems for Single Residential Buildings in the Western Cape, South Africa. Proceedings of the 5th built environment conference (2010). [2] C.L Buma, E.L Meyer, R Taziwa, Energy Management in the balance of system components in a stand-alone building integrated photovoltaic system in Alice. Proceedings of the South African universities power engineering conference (2017).

2455