5. Solar Photovoltaic System

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1 5. Solar Photovoltaic System 5.1 Introduction Harnessing of non polluting renewable energy resources to control green house gases is receiving impetus from the government of India. Under the JNNSM, rooftop solar PV systems can avail several benefits and subsidies. This will help the solar photovoltaic device systems for power generation, deployed in the various parts in the country for electrification where the grid connectivity is either not feasible or not cost effective as also some times in conjunction with diesel based generating stations in isolated places. With the downward trend in the cost of solar energy and appreciation for the need for development of solar power, many solar power projects have recently been implemented within the country. A significant part of the large potential of solar energy in the country could be developed by promoting grid connected solar photovoltaic power systems of varying sizes as per the need and affordability coupled with ensuring adequate return on investment. In this chapter, the details of the proposed rooftop solar photovoltaic system to be installed in the IIT Roorkee campus are presented along with the anticipated benefits. The Case for Solar Photovoltaic: Solar photovoltaic systems are highly reliable and are very easy to install and maintain. Using solar PV systems as sources of electricity would provide a significant savings in the energy consumption of the campus Advantages of Solar PV systems Electricity produced by solar cells is clean and silent. Because they do not use fuel other than sunshine, PV systems do not release any harmful air or water pollution into the environment, deplete natural resources, or endanger animal or human health. Photovoltaic systems are quiet and visually unobtrusive. Small-scale solar plants can take advantage of unused space on rooftops of existing buildings. PV cells were originally developed for use in space, where repair is extremely expensive, if not impossible. PV still powers nearly every satellite circling the earth because it operates reliably for long periods of time with virtually no maintenance. Solar energy is a locally available renewable resource. It does not need to be imported from other regions of the country or across the world. This reduces environmental impacts associated with transportation and also reduces our dependence on imported oil. And, unlike fuels that are mined and harvested, when we use solar energy to produce electricity we do not deplete or alter the resource. Solarification of IIT Roorkee Campus Page 68

2 A PV system can be constructed to any size based on energy requirements. Furthermore, the owner of a PV system can enlarge or move it if his or her energy needs change Disadvantages of Solar PV systems Some toxic chemicals, like cadmium and arsenic, are used in the PV production process. These environmental impacts are minor and can be easily controlled through recycling and proper disposal. Solar energy is somewhat more expensive to produce than conventional sources of energy due in part to the cost of manufacturing PV devices and in part to the conversion efficiencies of the equipment. As the conversion efficiencies continue to increase and the manufacturing costs continue to come down, PV will become increasingly cost competitive with conventional fuels. Solar power is a variable energy source, with energy production dependent on the sun. Solar facilities may produce no power at all some of the time, which could lead to an energy shortage if too much of a region's power come from solar power. 5.2 Site description Given the aforementioned benefits of installation of Solar PV systems, it is proposed to install solar PV arrays on the rooftops of academic departments and a few central buildings in the campus. These buildings have regular grid electricity supply and in the unlikely event of a grid failure, backup DG sets are present in some of these buildings. The proposed sites for installation are mentioned below along with their electrical load: S.No. Department Electrical consumption (kwh p.a.) 1 AHEC 3,00,803 3 Biotechnology 1,90,693 4 Civil 5,50,866 5 Chemistry 2,02,334 6 Chemical 2,89,226 7 DOMS 1,94,984 8 Earthquake 3,56,318 9 Earth Science 2,92, ICC 4,96, WRDM 2,57, E&C 6,05, HS 68, Maths & Physics 5,04, IIC 2,22,698 Solarification of IIT Roorkee Campus Page 69

3 18 Mechanical 4,79, Library 1,66, New LH 4,79, Others NA Total 56,56,160 Table5.1: Electricity consumption at Departments and Centres of IIT Roorkee These buildings represent the ideal site for installation of solar PV systems on campus. The reasons are manifold. The electricity requirement of these buildings peaks in the afternoon owing to usage of air-conditioners, computers, water coolers etc and it is during this time that the performance of the PV system is optimum (due to maximum insolation). Also, the need for electricity is negligible in the nights; hence the disadvantage of not having solar energy at night is irrelevant. It is not prudent to use ground level land for installation of these systems as the insolation received would be low. Hence the rooftop has been proposed as the area for installation. Given their large electricity requirements and by virtue of having large, shade free roofs, these buildings represent the ideal sites to install solar photovoltaic systems on the campus. 5.3 Technology description of proposed system Fig. 5.1: A 270W polycrystalline module 5.3.1: The Modules The solar PV system shall be designed with either mono/ poly crystalline silicon modules or using thin film photovoltaic cells or any other superior technology having higher efficiency. Solarification of IIT Roorkee Campus Page 70

4 Three key elements in a solar cell form the basis of their manufacturing technology. The first is the semiconductor, which absorbs light and converts it into electron-hole pairs. The second is the semiconductor junction, which separates the photo-generated carriers (electrons and holes), and the third is the contacts on the front and back of the cell that allow the current to flow to the external circuit. The two main categories of technology are defined by the choice of the semiconductor: either crystalline silicon in a wafer form or thin films of other materials. Given the advantages possessed by a crystalline system (described in chapter), polycrystalline modules have been proposed for installation. The proposed system is a grid tie system without any battery backup. Due to the regular nature of the grid supply and the availability of DG set for backup, a battery bank is not required. This results in lower capital costs better economic viability of the project. A grid interactive roof top solar PV system comprises the following equipment: 1) SPV Power Source 2) Inverter (PCU) 3) Mounting Structure 4) AC and DC Cables Earthing equipment /material Junction Boxes or combiners Instruments and protection equipments (disconnect switches and fuses) The components are shown diagrammatically via the wiring schematic below: Fig 5.2: Line diagram of battery-less grid tie solar PV system Solarification of IIT Roorkee Campus Page 71

5 Photovoltaic solar system use the light available from the sun to generate electricity and feed this into the main electricity grid or load. The amount of power produced is roughly proportional to the intensity and the angle of the light reaching them. PV panels produce maximum output when able to 'track' the sun's movements during the day and the various seasons. However, these tracking mechanisms are expensive; they increase the cost of the system significantly and disproportionate to the gain accrued. Moving parts increase the maintenance hassles of such a system. Hence, a fixed solar PV system without tracking is proposed to be used in the rooftops of building within the campus. Figure 5.3: An example of a 2 axis tracking system The power generating capacity of a photovoltaic system is denoted in Kilowatt peak (measured at standard test conditions of insolation 1000 W per m2). Solar photovoltaic modules can combined in various ways depending upon the requirements of the voltage and power output to be taken from the solar plant Inverter i. The inverter converts the DC power of the array into AC. ii. The output of the inverter synchronizes automatically its AC output to the exact AC voltage and frequency of the grid Protection and Controls: 1. Inverter is provided with islanding protection to isolate it from the grid in case of no supply, under voltage and over voltage conditions so that in no case there is any Solarification of IIT Roorkee Campus Page 72

6 chance of accident. In case of a power cut, the PV module too ceases to provide any power. In such a case, the backup DG set is utilized. 2. In addition to above, PV systems is provided with adequate rating fuses, fuses on inverter input side (DC) as well as output side (AC) side for overload and short circuit protection and disconnecting switches to isolate the DC and AC system during maintenance. 5.4 Methodology The survey was carried out on the rooftops of the buildings listed previously. Roofs with free, unused and unshaded areas were considered for solar PV installation. Shading is extremely detrimental to the operation of a solar PV system, as described below Shading effects on Solar PV systems From case studies it is known that PV systems with the same nominal power generate quite different energy yields due to different shading patterns. The common problems are: 1. Reduction of power output: As the insolation is reduced by shading we get a reduced photo current. As cells are in series connection, the current for all the cells is reduced. 2. Thermal stress on the module: Depending on the level of shading, the PV generator circuit and the load the voltage of shaded cells might reverse. In this case they operate in the blocking state as a resistive load. The losses in the individual cell can increase the cell temperature dramatically and overheating might occur. In order to overcome some of the problems related to shading, by-pass-diodes are connected parallel to a number of solar cells. Under normal operating conditions the diodes are blocked compared to the voltage generated by the cells. When shading occurs the reversal of the voltage can be observed in that specific section and now the by-pass diode in parallel will conduct the current. The results are: The current of the unshaded section flows through the by-pass diode and the power/voltage characteristic shows a second local maximum The shaded cell is only loaded with that fraction of power produced by the remaining unshaded cells of that section The use of by-pass diodes results in some drawbacks as well: Higher cost for the module production and assembly problems of the by-pass diodes Losses in the by-pass diode due to shading Matching problems between the solar inverter and the photovoltaic generator because of the second local power maximum might not be included in the range of operation of the inverter. Solarification of IIT Roorkee Campus Page 73

7 5.4.2 Module placement From the above discussion, it is clear that shading should be avoided as far as possible. During the rooftop survey, care has been taken to eliminate areas that have the potential for shading. When eliminating shaded areas, shading from the following sources were considered: trees, structures on the roof and nearby buildings. Once the usable areas were obtained, the plan of the rooftop was drawn which clearly showed the usable rooftop area. Figure 5.4: An example of a rooftop solar PV installation In order to show the placement of modules in the usable area, following standards are used: 1) The module was considered to be polycrystalline modules manufactured by Moser Baer India. The peak power output of the module is 230 Wp. The dimensions of each module are 1*1.7 m, with a spacing of 1.4 m provided to prevent shading between modules. 2) Sufficient space, between 0.5-1m has been provided between the modules and the rooftop boundary walls to allow for sufficient moving space. 3) It is assumed that the inverter and other instrumentation do not occupy usable roof area. Instead they can be placed in the shaded area on in the building electrical control room on the ground 4) In case of certain departments having canteens, some usable roof area is devoted to installation of solar water heating systems. 5) The modules are assumed to be tilted at an angle equal to the latitude of the place in the south direction. In case of Roorkee, the ideal tilt of modules is 30 degrees. 6) The layout of the modules chosen is the one that produces maximum power output Solarification of IIT Roorkee Campus Page 74

8 Figure 5.5: Diagrammatic representation of module orientation and spacing employed The layout maps showing the proposed installation of the modules along with the no. of modules, area available, total peak power output possible are present in the appendix. 5.5 Results The average insolation data for Roorkee, which is assumed to be same as that of New Delhi, is presented in the table below. Since the peak power output of solar modules is delivered at conditions of 1000 W/m2 insolation, the actual power output depends upon the insolation at the place. Month Insolation (kwh/m2/day) Yearly average 6.02 Table 5.2: Long term average insolation data for Roorkee Courtesy PV Watts The average insolation value, W/m2, is considered for evaluating power output of modules. Solarification of IIT Roorkee Campus Page 75

9 5.5.1 Features of some example layout plans 1. DRG C-1: In a few plans such as that of AHEC, a solar water heating system has been considered for installation to supply the hot water needs of the pantry. They are separately shown in the plan and have been placed in a manner which consumes the least rooftop area. 2. DRG C-2: In the layout plan for the institute computer centre, as with many other plans, a lot of area is considered unusable for solar PV installation due to presence of many rooftop structures and a high level of tree shading. 3. DRG C-5: The layout plan for the Lecture Halls 1 and 2 produces low electricity output due to less roof area, but considering the limited requirement of the building, a high percentage of its requirements is produced. 4. DRG D-2: The layout plan for the biotechnology department considers only the new wing, which has a highly usable roof. The adjacent old wing is not considered due to excessive shading present on each rooftop. 5. DRG D-7: The earthquake engineering consists of two separate buildings having shade free rooftops. Harnessing the rooftops of both would lead to a high power output. 6. DRG D-8: For the electrical engineering department, some rooftops do not have direct access via a stairwell, but these rooftops are totally shade free and ideal for solar PV installation. 7. DRG D-11: The department of hydrology rooftop has a number of structures on the roof which make it impossible to consider installation of a large number of modules. Hence relative to its size, only a small portion of the rooftop is occupied with modules. 8. DRG D-13: The Maths and Physics department consists of two academic departments in a single building, with a separate Bose auditorium. Separate solar water heaters have been considered for maths and physics departments. 9. DRG D-16: The WRDM department rooftop has a rooftop structure with high parallel beams running throughout its area. The surface is unshaded but the beams make PV placement impossible on this structure. Solarification of IIT Roorkee Campus Page 76

10 5.5.2 Proposed installation Figure 5.6: Building wise list of proposed modules The total number of photovoltaic modules proposed for installation on building rooftops is Peak Power Output The installed capacity of each site is a given below: Solarification of IIT Roorkee Campus Page 77

11 Figure 5.7: Peak power output of the proposed installation at various departments and centres The net installed capacity is proposed to be kw Annual Generation Considering the DC-to-AC duration factor as 0.77 (see appendix), and the average insolation as 25.1 % of the standard, the actual power output department wise is presented as follows: Solarification of IIT Roorkee Campus Page 78

12 Figure 5.8: Bar chart showing power produced annually by proposed SPV installation The projected annual power output from the rooftop PV system would be 31 lac units and the DC Power Output of the rooftop power plant would be 1812 kw Solarification of IIT Roorkee Campus Page 79

13 5.5.5 Electricity Savings The installation of solar photovoltaic systems has the potential to reduce the energy consumption of the above mentioned buildings significantly. The percentage saving produced by such a system for each building is presented below: Figure 5.9: Table showing percentage savings achievable within buildings Total electricity savings of close to 31 lac units per annum are envisaged with the installation of the system. 5.6 Conclusions From the above discussion and presentation it is amply clear that the installation of the PV rooftop systems would lead to tremendous electricity savings. Not only this, it would be a significant step towards the goal of energy autonomous buildings. With the Government offering subsidy to the tune of 90% on solar PV system, low payback periods would be possible which makes it a very attractive proposition for installation in the building rooftops within the campus. The details of the economic analysis are elaborated in the Economic analysis section of the report. Solarification of IIT Roorkee Campus Page 80