Urbanization and economic development are leading to a rapid rise in energy demand in urban areas in our country leading to enhanced Green House Gas (GHG) emissions. Many cities around the world are setting targets and introducing polices for promoting renewable energy and reducing GHG emissions. Ministry of New and Renewable Energy (MNRE), Government of India has taken initiatives to develop green campuses under ‘Development of Solar Cities’ programme which aims at minimum 10% reduction in projected demand of conventional energy at the end of five years. The forward step in this respect is to utilize the background of educational institutes for renewable energy utilization.
The major requirement of electrical power in educational institutes is at day time, as major work of teaching-learning is carried out at day time. That is the plus point for the use of solar energy. Hybrid systems such as Solar-Wind, Solar-Diesel, and Solar-Biomass may be also beneficial set up depending on the geographical condition. But the solar energy is the most commonly available source, and it’s economical with many factors. Factors may include easy erection, instant generation, easy repairs, tailor-made projects and tie grid projects etc.
Major power requirement in an educational institute is for lighting load. It includes lamps, fans, computers etc. There are power equipments in institutes such as air-conditioners, projectors, heaters etc. As in wiring system separate wiring path is provided for light and power circuit, it is easy to equip light circuit with solar power system.
At present there is limitation on solar energy production due to the space availability for solar panel erection. The shadow-free area required for installation of a rooftop solar PV system is about 12 m2 per kW (kilowatt).
Rooftop available is having its own limitation due to the load bearing capacity of roof. Fixing of panels to the normal direction of incident radiations i.e. placement of solar panels at proper tilt angle may be a major problem. The minimum clearance required for cleaning and servicing of the panels is 0.6 metre from the parapet wall and in between rows of panels. In between the rows of solar panels sufficient gap needs to be provided to avoid the shading of a row by an adjacent row. Placing the panels on ground, disturb the playgrounds, space for cultural activities and garden. So due this limitation of solar PV (Photovoltaic) generation, light circuits can be easily fed with solar power.
Components of Solar PV System
Below is shown a block diagram of basic PV system. It can be used directly for DC load with the help of charge controller. With the help of battery and inverter same system can be used for serving AC load.
TYPICAL PV STAND-ALONE AC SYSTEM…
The key component of solar PV system is the solar panel. The cost of solar panels in entire solar PV system is near about 50% of entire system. The Maximum Power Point Tracking (MPPT) charge controller is having major role for the increased efficiency of solar PV system output. Inverters are used to convert DC power into AC power. Industrial flexible cables are used as it is open to weather. It may be a armored or not depending on its use. Infrastructure for panel is another expenditure. Its erection is a typical task with reference to tilt angle facing to south-east or south-west. Below is given the general cost analysis (without battery) for 1kW system.
Factors Affecting PV Output
Energy efficiency factors must be carefully considered while designing any solar PV systems to get the best out of your efforts and investment. Following are six important considerations for efficient power output from PV system:
- Cable Thickness:Normally in PV system DC voltages is 12V, 24V or 48V. For the same wattage much higher currents are involved in the PV systems. This brings into picture resistance losses in the wiring. So higher cross sectional area cables are used.
- Temperature:Solar cells perform better in cold rather than in hot climate and as things stand, panels are rated at 25°C which can be significantly different from the real outdoor situation.
- Shading:Ideally solar panels should be located such that there will never be shadows on them because a shadow on even a small part of the panel can have a surprisingly large effect on the output.
- Charge Controller:An inherent characteristic of solar silicon cells is that the current produced by a particular light level is virtually constant up to a certain voltage (about 0.5V for silicon) and then drops off abruptly. MPPT (Maximum Power Point Tracking) charge controller tries keeping the panel at its maximum voltage and simultaneously produces the voltage required by the battery.
- Inverter Efficiency:When the solar PV system is catering to the needs of the AC loads, an inverter is needed. Although inverters come with wide ranging efficiencies, typically affordable solar inverters are 80 to 90% efficient.
- Battery Efficiency:Whenever backup is required batteries are needed for charge storage. Lead acid batteries are most commonly used. All batteries discharge less than what go into them; the efficiency depends on the battery design and quality of construction of cell of batteries.
Case Study: For the study of output power from solar system, system erected by Shri Shivaji College, Akola (MS) is studied. The college is multi faculty discipline with arts, commerce and science. Solar system is off grid connected (PCU: Power Conditioning Unit) of 6kVA capacity. The system was commissioned on 20th Feb, 2016 in college. Solar power is given to two administrative offices and to two laboratories. Total connected load of offices is 2 kW and that of laboratories is 1.8 kW. Connected area lighting load operating at night is 500 Watt. Normal working of college is from 7.30am to 6.00pm. Preferences of operation of PCU for power supply is first solar, second grid and last battery. Load is adjusted up to the 75% capacity of inverter.
Fig. 2: HOMER Model of Solar System…
Fig. 3: PCU with Battery in Control Room…
Fig. 4: LCD Display for Solar System Readings…
Solar Resource Inputs
Akola is a hot place in the state of Maharashtra. As compared to wind energy potential, solar energy is available in ample quantity, throughout the year. HOMER model for solar radiation and radiation incident on PV array in the D-map format is shown above (Fig. 2).
Fig. 5: Irradiance Graph of Akola City…
Fig. 6: D-Map of Irradiance Incident on PV Array in Akola City…
PCU (Power Conditioning Unit) is installed in the Power Control Room of College (Fig. 3). The panels are erected on 2nd floor at an angle of 50°C facing to south. Necessary connections of DC output from PV array are done to the input of PCU. The connection is brought by armored DC cable. Following are the components of solar PV system:
- Solar Panel:It is two diode panel and product name is125Wp/12V/SN80. These are of 36 cell structure, connected in series. The entire panel is of 12 Volt, and 125 Watt capacity. For the system 5 strings of 8 series connected panels are used. It is the mandatory design factor as DC input to PCU (Power Conditioning Unit) is 96 Volt. Total numbers of panels used are 40 which is 5000 Wp (Refer opening image).
- G.I. Support to Panels:Panels are arranged on 2nd floor of the college on GI frame structure at an angle of 50°C facing towards south. Sufficient clearance between the panels is kept for the air ventilation.
- Cables & Wires:Single core Cu wire of 10 sq mm flexible, POLYCAB make is used for interconnection of solar panels. DC cable is used of 10 sq mm 2 core Cu Armored for the interconnection of panels output and PCU input. Industrial flexible wire POLYCAB make, of 2 core 10 sq mm is used for connection of protective switches.
- Busbar Box:2 Nos of busbar boxes are used, of 32 Amp rating having HRC fuse of 63 Amp rating.
- HRC Fuse:For the circuit of array of panels, 32 Amp rating HRC fuses (5 Nos.) are used.
- Earthing:Two earthing connections are provided KAPITRODE make, connected with 10 sq mm Cu multi strand wire.
- Lightning Arrestor:One lightning arrestor is provided for protection from lightning.
- DC MCB:Two DC MCB of 2 Pole, 250 Volt, (HAGER make) are connected in DC cable.
- TPN Box:Two TPN boxes 4 way outlet of L&T make is used. It is having 4 Pole MCB of 40Amp (2 Nos.), 4 Pole MCB of 32Amp (3 Nos.) and 4 Pole Isolator of 63 Amp (1 No.) is provided.
- PCU Unit:Power Conditioning Unit (PCU) of Su-Cam make, 6kVA capacity, rated for input supply of 96 DC Volt, is used as a converter.
Expenditure per item incurred for the solar PV system is given below.
Fig. 7: HOMER-Graphical Representation of Cash Flow Summary…
Total life of PV panel is assumed for 25 years and that of PCU is 15 years. Infrastructure and cables/wires are assumed for the life of 10 years. Battery life is for 3 years.
No fuel charges are required for PV power generation. But the charges are incurred to charge batteries at night and to supply grid current to charge batteries at day time, in case the photovoltaic power generation is not sufficient to charge the batteries. It is shown in cash flow diagram (Fig. 8).
Fig. 8: HOMER-Graphical Representation of Cash Flow…
Primary Load Inputs
Below is given the graphs from HOMER model for the day of year and for monthly average of AC primary load. (Refer Figure 9 and Figure 10).
Fig. 9: HOMER- D-Map of Loading Condition…
Fig. 10: HOMER-Graphical Representation of Monthly Average of AC Primary Load…
For the system output power three models are studied. First model is simulink model, which gives theoretical output of designed system.
Second model is HOMER model, which gives predicted output power and cost analysis.
Third model is actual reading from PCU.
- MATLAB Application
6 kVA solar PV system is modeled to get I-V (Current-Voltage) and P-V (Power-Voltage) characteristics at temperature (T) 25°C and irradiance (G) 1000 w/m2. The maximum power (DC) output from the simulation is 6.443e+03 Watt.
Fig. 11: Current- Voltage (I-V) &Power- Voltage (P-V) Characteristics…
- HOMER Application
HOMER is the micropower optimization model, which simplifies the task of evaluating designs of both off-grid and grid-connected power systems for a variety of applications. Basic constraints are introduced to get optimized result.
- PCU Readings
Actual loaded conditions are noted and readings for PV power output and output power of converter are taken. Tabulated chart for reading is given below.
Based on the methods used for output power calculation, that means by MATLAB, HOMER and PCU readings and the results obtained thereon, some notable conclusions can be drawn, which have been stated below:
- Computation of output power by MATLAB-simulation is theoretical. Practically change in temperature and irradiance influence the output power.
2. As HOMER software considers the various constraints for the computation of output power, it gives realistic output power reading. The calculations are very well matched to the actual power output readings given by PCU.
3. HOMER simulates the operation of a system by making energy balance calculations for each of the 8,760 hours in a year. It specify and estimates the cost of installing and operating the system over the lifetime of the project.
4. PCU readings (LCD display) give the actual energy utilized, PV current, PV voltage, output voltage, battery voltage and loading staus.
5. From PCU readings it is evident that output voltage is nearly constant. MPPT charge controller works for this stage.
6. Output power varies with load condition. It is linearly increasing with increase in loading condition.