04-07-2014, 04:28 PM
Sizing and Best Management of Stand Alone Hybrid PV-Wind System using logistical model
Sizing and Best Management of Stand.doc (Size: 399.5 KB / Downloads: 12)
Abstract
Wind and photovoltaic generators are the most promising approach to supply electrical energy as stand-alone systems for remote areas. Sizing of these supplies for reliable operation and low cost is a very important issue. A sizing and best management technique for a PV-wind hybrid system with batteries is proposed in this report, where the best size for every component of the system will be optimized according to the weather conditions and the load profile. The average hourly values for wind speed and solar radiation for two regions in Egypt have been used in the design for the systems, along with different expected load profiles. A logistical model is also developed for battery operation, according to the power balance between the generators and loads used in the software, to anticipate performances for the different systems according to the different weather conditions. The program will output the performance of the system during the year, the total cost of the system and the best size for the PV-generator, wind generator and battery capacity.
Keywords: Wind, photovoltaic, battery, sizing.
INTRODUCTION
Wind power and photovoltaic hybrid stand-alone systems are the most promising way to handle the electrification requirements of numerous isolated consumers worldwide owing to the distribution of these sources of energy across the earth. To install a PV-wind system with battery storage in a remote location, where it is inconvenient or expensive to use the conventional grid to supply electricity, is more economical and competitive than building a utility grid. Compared with the utility-supplied electrical energy, the efficiency of a PV-wind hybrid system is lower and the cost per kWh is more expensive. Nevertheless, owing to the increase in conventional fuel costs, the PV and wind stand-alone systems would be comparatively economical for electrification in rural areas (Mortensen, 2003). The basic approaches to generalized renewable energy usage are to improve the technology level to offer higher efficiency of PV and wind power generators and to propose a better design for the system. Many researchers have considered the sizing of the hybrid systems and different aspects have been defined, and to increase the availability of the system many have used conventional energy sources, generators and battery storage (Borowy, 1994), (Nehrir & Lameres, 2000), (Bonanno, Consoli, Lombardo, & Raciti, 1997). It has been demonstrated that hybrid energy systems (renewable coupled with conventional energy sources) can significantly reduce the total lifecycle cost of stand-alone power supplies in many off-grid situations, and it is certainly difficult in many rural areas to obtain a fuel supply. In the work proposed here the software will design the system without using any conventional energy source. A sizing and best management computer program will be developed to give the best sizes and management of the solar generator, the wind generator and the batteries of the hybrid system. A simple diagram shows the different components and the energy flow of the hybrid PV-wind with battery system is shown in Fig.1. The sizing and best management of the different components of the system are dependent on the weather data, which are the solar radiation, wind speed and air temperature, Furthermore, the cost ($/kW) of the wind turbine, solar panels, power electronic components and batteries, and also the profile of the demand kW power (Kellogg, Nehrir, Venkataramanan, & Gerez, 1998), (M, Agbossou, & Hamelin, 2003). The high reliance of the design on the weather conditions demands that weather data should be monitored for their effect on the management of the system. In some regions, for example, the wind generators may be not economical for use and the main source of the system is the PV generators owing to the low average wind speed during the year and the comparatively high solar power. Otherwise, we may not be able use solar generators in some regions owing to the low average irradiance and the relatively low average period of sunshine, so the usage of wind generators is more economical. The annual average hourly weather data will be provided to the developed software computer program and processed to produce the hourly average generated power. The weather data will be monitored for two regions in Egypt: the first is Shikh-Zwaid in Sinai in the north-east and the second region is in Hergada in the south-east, on the Red Sea.
The wind generator model
The wind-generator is a wind turbine connected to an electrical AC generator and an electrical rectifier, as shown in Fig.1. The electrical power supplied to the system is shown in Eq. 7, which takes into account the generator efficiency (ηgw), and the controller MPPT efficiency.
THE SIZING AND BEST MANAGEMENT ALGORITHM
In this section an iterative algorithm is described for determining the wind generator size, the PV generator size and the battery capacity for a hybrid system. The sizing program will make several iterations beginning from zero size for every component and increase the size by adjustable steps, and use the weather conditions to process the general performance of the system during the year. At every step, the autonomy (A) and the total cost of the system are calculated and according to these two factors the best sizing is chosen.
The system will be tested for different load profiles, as shown in Fig. 3, where there will be 3 profiles; The first profile is for housing application (profile 1), the second profile is for water pumping system (day pumping only) (profile 2), and The third profile is for water pumping plus housing application (profile 3). The simulation will be run for different weather conditions in two different regions.
The first region (A): Shikh-zwaid in the north-east of Egypt in Sinai. The data are collected by a weather station owned by the Desert Research Centre (DRC). The wind speed is very low; the average wind speed is 2.6 m/s and the average solar radiation during the day being 470 W/m2, which is normally high. The second region (B): this region is in the south-east of Egypt at Hergada. Here both the solar radiation and wind speed are high, the average wind speed being 6.4 m/s and the average solar radiation 480 W/m2 during the day. Fig. 4 shows the difference between the wind speeds in the two regions. The program will run with the values shown in Table 1 are taken into account in the sizing process; these values are according to the universal prices and data sheets of the components in the last year.
RESULTS AND DISCUSSION
Table 2 summarises the results obtained in the last section where the table shows a sizing of the PV generator, the wind generator and the batteries, together with the total cost of the system according to each profile and region. The two regions have different weather conditions: in the first region the solar radiation is very high and the wind speed is relatively low, while in the second region both the solar radiation and the wind speed are high throughout the year. The effect of the weather conditions on the design of the system different components can be seen from the sizing software output.
The following can be concluded from the sizing process:
The system mostly chooses a hybrid system of wind and PV if both solar radiation and temperature are available in the site. That because the PV and the wind sources are mostly cumulative as sources of energy because during the day the PV energy is more available than the wind energy while at night the wind energy is more available. Wind energy is also more available in the winter than solar energy. Owing to this cumulative advantage, hybrid systems are generally more economical
CONCLUSION
This paper has proposed the sizing and best management software of photovoltaic-wind hybrid energy systems with battery storage based on logistical model. Software has been used to size different profiles for two different regions in Egypt. One region contains high solar energy but wind energy is low, and the other region contains both high solar and wind energy. The following can be concluded from the design process:
-The hybrid PV-wind systems are more economical and reduce the need for high battery storage capacities.
-The load profiles affect the sizing of the different components of the hybrid system and for accurate sizing the load profile must be taken into account.
-The weather data are a mean variable affecting the design of the hybrid PV-wind system, and for accurate sizing the weather data must be processed for a long period.
-The distribution in the daylight hours and the lower cost of the wind generator make it a more economical source of energy even in regions with relatively low wind speed.
-The existence of batteries for storage of energy is very important in remote regions owing to the unpredictable weather conditions.
In future work a more general cost formula will be proposed and the reliability of the system will be calculated. The program will also be generalized to design the different hybrid systems, including the conventional energy source generators.