Renewable Energy - Exam Notes

Section I

1. (a) Relationship between GHI, DNI, and DHI

Definitions:

• Direct Normal Irradiance (DNI): Solar radiation received directly by a panel perpendicular to the sun.

• Diffuse Horizontal Irradiance (DHI): Solar radiation received per unit area by a surface, scattered by molecules and particles in the atmosphere (illumination from clouds and the blue sky).

Relation:

GHI = DHI + DNI * cos(\theta)

Where \theta is the solar zenith angle.

1. (b) Sustainable Farm House

• (i) Highly efficient solar panel: Monocrystalline panels are highly efficient.

• (ii) Combine farming with PV: Agrovoltaics combines energy generation and agriculture on the same land.

• (iii) Buildings constructed using PV panels: Building Integrated Photo Voltaic (BIPV) systems use solar PV panels to construct the building itself.

• (iv) Additional solar panel on the roof-top which moves in the direction of the sun: A solar panel that moves in the direction of the sun is called a solar tracker.

• (v) Configuration: Standalone or off-grid system because there is no access to electricity.

Standalone solar photovoltaic system:

• Consists of a photovoltaic array, charge regulator, battery, inverter, DC load center, and AC load center.

• The DC output of the PV array is connected to the battery through the charge regulator unit.

• The charge regulator consists of a blocking diode in series with the photovoltaic array. It prevents the battery from being discharged through the PV array at night when there is no sunshine.

• The blocking diode also prevents overcharging and discharging of the battery and also protects the battery from short circuits.

• The battery gets charged from the DC output of the PV array.

• Battery output can be directly connected to the DC loads.

• For AC loads, the battery output is first connected to an inverter which converts DC to AC and then it is connected to the AC load center.

1. (c) Solar Panel Specifications

• Voc – 43.42 V

• Isc – 4.48V

• Vmp - 35.82 V

• Imp - 4.26 A

Calculations:

• Solar panel max power: Vmp \times Imp = 35.82 \times 4.2 = 150 Wp

• Peak capacity of the solar panel at 5 hours Effective sunshine hours is 150 \times 5 = 750 watts

2. (a) Comparison of Roof Mounted and Ground Mounted Solar PV Systems

2. (b) Difference between Solar Power Conditioning Unit (PCU) and Solar Inverter

Solar Power Conditioning unit (PCU)

• Integrated system consisting of a solar charge controller, inverter and a Grid charger.

• Provides the facility to charge the battery bank through either a Solar or Grid/DG set.

• Continuously monitors the state of battery voltage, solar power output and the load.

• If the battery voltage goes below a set level, the PCU will automatically transfer the load to the Grid/DG power and also charge simultaneously.

• The PCU always gives preference to the solar power and will use Grid/DG power only when the solar power / battery charger is unable to meet the load requirement

Solar Inverter

• Solar inverter has priority for solar charging similar to PCU. However during solar day, battery will be charged through solar but in presence of mains the load will be fed through mains through bypass mode.

• Solar energy stored in battery can be utilized when AC mains is absent (in Inverter mode).

• As long as the mains is present solar power is kept stored.

2. (c) Charge Controller Recommendation

Recommend MPPT charge controller for a 3kW rooftop PV system with a 24V battery.

• MPPT (Maximum Power Point Tracking) measures the Vmp voltage of the panel and down-converts the PV voltage to the battery voltage.

• It raises the current to utilize more of the available power from the panel.

Charge Controller Current Calculation:

Charge Controller Current = Total solar panel power / Battery bank Voltage

Charge Controller Current = \frac{3000}{24} = 125 A

125 A would be the better rating of the charge controller for 3kw PV system.

2. (d) Advantages and Disadvantages of Lithium-ion Batteries in Solar Photovoltaic Systems

Advantages

• Require almost no regular maintenance

• Higher battery energy density

• Longer life cycle (at least 10 years)

• Higher depth of discharge

Disadvantages

• More expensive compared to other energy storage technologies

• Higher chance of catching fire due to thermal runaway(if not installed properly)

Section -02

3. (a) PV Water Pumping System Design

• Daily water requirement: 7000 litres (7 m3/day)

• Depth: 42m

• Elevation: 10 m

• Standing water level: 30 m

• Drawdown: 2 m Water density: 1000 kg/m3

• Acceleration due to gravity (g): 9.8 m/s2

• The peak power rating of the solar module: 310 WP

• Operating factor: 0.8.

• Pump efficiency: around 30 %

• Mismatch factor: 0.85

• Effective sunshine hours = 5

Steps:

1. Daily water requirement = 7000 litres = 7 m3 /day

2. Total vertical lift = Elevation + Standing Water Level + Drawdown

   Total vertical lift = 10 m + 30 m + 42m = 42 m

   Frictional loss = 5 % of the total vertical lift = 42 × 0.05 = 2.1 m

   Total Dynamic Head (TDH) = Total vertical lift + Frictional loss

   Total Dynamic Head (TDH) = 42 m + 2.1 m = 44.1m

3. Hydraulic energy required = Mass × g × TDH

   Hydraulic energy required = Density × Volume × g × TDH
   Hydraulic\ energy\ required = 1000 \frac{kg}{m^3} \times 7 \frac{m^3}{day} \times 9.8 \frac{m}{s^2} \times 44.1 m

   (multiply by 1/3600 to convert seconds in hours)

   Hydraulic energy required= 840.35 Wh/day

4. Calculate the solar radiation available at the site.

   Solar radiation available at the site (No. of hours of peak sunshine per day) = 5.0h/day (1000 W/m2 equivalent)

5. Calculate the size and number of PV modules required, the motor rating, its efficiency, and losses.

   Total wattage of PV panel = \frac{Total \ hydraulic \ energy}{(No.of \ hours \ of \ peak \ sunshine \ per \ day \times Pump \ efficiency \times Mismatch \ factor \times Operating\ factor )}

   Total wattage of PV panel = \frac{840.35}{5.0 \times 0.3 \times 0.85 \times 0.8} = 823.87 W

   No. of PV panels required of 80 Wp = \frac{823.87}{80} = 10.3 = (11 \ round \ figure)

   Power rating of the DC motor = \frac{823.87}{746}

== 1. 1 HP motor

Power rating of the DC motor = 1.1 HP motor

3. (b) PV System Design Considerations

1. Load bearing capacity of roof

2. The load of the structure including PV and the supporting structure varies from 30 \frac{kg}{m^2} to 60 \frac{kg}{m^2}

3. As per MNRE technical specification the total load of the structure should be less than 60 \frac{kg}{m^2}

4. For a large system a suitable walk-way will be required for maintenance purpose.

5. Operating temperature affects performance. Therefore PV arrays should be installed in such way that there sufficient air flow/ventilation for cooling.

6. High wind pressure can damage the structure and modules. Therefore mounting structure should be opted such a way that there is minimum wind pressure.

7. High humidity and salty atmosphere can corrode the structure and the extreme levels in the site should be known.

8. Lightning strikes can damage the electrical equipment and sometimes the modules. Therefore lightning vulnerability in the site should be evaluated.

3. (c) PV performance standards

IEC 61215 and IEC 61646 are used for

• Diagnostic: Visual inspection, Hot spot.

• Electrical: Insulation resistance, Wet leakage current Performance test: Pmax at STC and Pmax at low irradiance.

• Thermal test: Bypass diode test, Hot spot. Irradiance: Outdoor exposure, UV exposure, Light soaking.

• Environmental test: Effect of Temperature, Humidity, Damp Mechanical: Mechanical load, Robustness of terminations, Hail impact.

4. (a) Anti-islanding techniques in Solar PV system connected to grid

A. Passive methods

Based on monitoring characteristic parameters in the point of common coupling (PCC)

B. Active methods

Introduce deliberated changes or disturbances to the AC output .Certain parameters are monitored at the PCC in order to detect if the generator is functioning in island-mode or grid-connected mode.

(b) Importance of earthing of solar PV system

Explain the significance and methods of grounding solar PV systems for safety and protection against electrical faults.

4. (c) Design of 100 KW grid connected solar PV system

• Solar panel to be used Wp : 80 wp

• Vn: 17.6 V

• Im: 4.55 A

• System efficiency: 0.8

• Inverter efficiency: 95%

1. Inverter selection

   Size of the inverter for the system = \frac{(Total \ power \times 1.25)}{0.8 \times inverter \ efficiency}

   Size of the inverter for the system =\frac{(100 \times 1.25 )}{0.8 \times 0.95}

== 164.47 KVA. Therefore 160 KVA 240 V DC inverter is chosen

2. Module sizing and selection

   • Size of the PV system is 100 Kw

   • System Voltage 24 V

   Number of solar modules required = \frac{100 \times 1000}{80} = 1250 \ Panels

   • Number of solar modules required 1250 of 80 W.

   • To match inverter voltage, no of panels in series = \frac{240 V}{17.6 V} = 13.63=14

     Number of Strings connected in parallel are \frac{1250}{14}=89.28= 90

Bill of materials for 100kW Solar PV System

Approximate cost of the 100kw PV solar system is 62,30,000

Payback period

Average cost of state electricity supply in India, Indian rupees per kilowatt hour 6.15

Total initial investment of the 100Kw plant is 62,30, 000

Yearly returns is 400 X 365 X 6.15 = 897900 rs

So, payback period will be \frac{67,30,000}{897900} = 7.5 \ years

Calculate Carbon footprint

Total Coal Saved(1KG/unit) = 400 kg/day

Total Water Saved 3.3 Litres/Unit=\3.3 \times 400=1320

Lts/day

Burning 1 kg of coal will produce about 3.3 kg of CO2. So 400 kg X 3.3 =1320 \frac{kg}{day}

of carbon foot will be saved.

SECTION – 03

5. (a) Standard Operating Procedure (SOP) for O&M of a PV project

• The timely and regular cleaning of solar cells and PV panels

• Regular maintenance of all thermal-based components

• Servicing of HT side equipment on an annual basis

• Testing and upkeep of circuits

• Tracing of IV curves and thermal imaging

• Measure of earth resistance value

• Retro-commissioning (fine-tuning )

• Management of warranties

• System checks pertaining to data acquisition, etc.

5. (b) Communication protocol for SCADA in a 2 GW solar plant

Recommend MODBUS protocol for communication used in SCADA

• MODBUS allows communication between several devices connected to the network through RTUs in SCADA.

• In this protocol, the master may initiate a MODBUS command to activate the connected element / device. The command contains the MODBUS address of the device. Only the indented device will act on receiving the command though other devices receive them. The MODBUS command ensures that the command is received unchanged through an inbuilt checking information.

• Easy installation, configuration, and use

• Open specifications that do not require hardware constraints.

• Reliable communication between automation devices Interoperability between devices from different manufacturers.

• The use of the Modbus RTU protocol is compatible with many other industrial automation products: PLCs, temperature controllers, operator panels, data loggers, etc., which can easily communicate with a common supervisor.

Hardware

SCADA mainly consists of RTUs, Central processor and associated memory, Communications and Other equipment, etc.

Remote Terminal Units (RTU)

The RTU provides an interface to the field analog and digital sensors situated at each remote site.

RTU Hardware:

The RTU Hardware consists of inputs, outputs, memory and communications.

Inputs

• Analog inputs (AI)

• Counter inputs (CI)

• Digital inputs (DI)

Outputs

• Analog outputs (AO)

• Digital outputs (DO)

Memory

• Static and Dynamic Memory

Software functions

• Real time operating system.

• Driver for the communications system, which is the link to the SCADA Master.

• Device drivers for the I/O system connecting to the field devices.

• SCADA applications like scanning of inputs, processing and storing of data, responding to requests from the SCADA master over the communications network.

• Some method may exist to allow the user applications to be configured in the RTU. This may be simple parameter setting, enabling or disabling specific I/O's or it may represent a complete user programming environment.

• Some RTU's may have a file system with support for file downloads. This supports user programs, and configuration files.

5. (c) Objectives of using SCADA

• Monitoring : Continuous monitoring of the parameters of voltage , current, etc..

• Measurement: Measurement of variables for processing.

• Data Acquisition: Frequent acquisition of data from RTUs and Data Loggers / Phasor data Concentrators (PDC)..

• Data Communication: Transmission and receiving of large amounts of data from field to control centres.

• Control: Online real time control for closed loop and open loop processes.

• Automation: Automatic tasks of switching of transmission lines, CBs, etc.

6. (a) Electricity needed to charge Tesla model 3

• Tesla model 3 electric car has 50kWh battery capacity and 350 kms range.

• Estimating the amount of Electricity needed to fully charge this vehicle.

• Assuming that the owner of the vehicle travels 2000 kms /month,calculate the number of solar panels needed.

Missing data can be suitably assumed.

• Distance travelled per month = 2000 kms.

• Tesla 3 can drive 350 kms on a single charge.

Number of full charges needed per month can be calculated as\frac{2000}{350} = 5.71 approx 6 charges \ per \ month

• Therefore energy needed per month is,

• 50 kWh battery x 6 charges per month = 300 kWh needed per month

• Energy needed per day:

• \frac{300}{30}= 10 \ kWh \ needed \ per \ day

• Total wattage of solar panels = \frac{10}{5.5 \times 0.78}=2.33 \ kW

• (effective Sunshine hours \times panel efficiency)

• Choosing 400W or 0.4 kW solar panel, the number of solar panels needed will be,

• \frac{2.33}{.4}= 5.82 \ approx 6 solar panels

• Rounding up, 6 solar panels of 400W each are needed to charge the Tesla 3 with the long- range battery.

6. (b) Necessity of renewable sources of electricity to charge Electric vehicle

• Electric vehicles are more efficient than combustion engine cars because they do not have tail pipe emissions.

• Electric vehicles have very little emissions even when charged from a fossil fuel electricity grid.

• If non-renewable resources are used to charge the EV, it requires additional investment on the power grid and also pollution. So renewable source is used for EV charging that eliminates the additional investment on the grid.

• If solar panels are used to charge the batteries and for charging EVs, then additional demand caused by the charging stations on the grid can be reduced. The energy stored in the batteries can also reduce the demand on the power station during times of peak demand.

• By implementing renewable energy in EV infrastructure, pollution caused by greenhouse gas emissions from both the vehicles and the power plants can be reduced.

6. (c) Difference between PLC and RTU

PLC (programmable logic controller)

• PLC is a small industrial computer which originally replaced relay logic.

• It had inputs and outputs similar to those an RTU has.

• It contained a program which executed a loop, scanning the inputs and taking actions based on these inputs.

• Originally the PLC had no communications capability, but they began to be used in situations where communications was a desirable feature. Communications modules were developed for PLC's, supporting ethernet (for use in distributed control systems) and the Modbus communications protocol for use over dedicated (wire) links.

• As time goes on we will see PLC's support more sophisticated communications protocols

RTU

• RTU's have always been used in situations where the communications are more difficult, and the RTU's strength was its ability to handle difficult communications.

• RTU's originally had poor programmability in comparison to PLC's. As time has gone on, the programmability of the RTU has increased.

Section = 04

7. (a) Difference between type A and type B wind turbine generators

Limited variable speed generator type A

• Squirrel cage Induction Generator (SCIG) is used.

• It has an inherent torque–speed curve that fits the WPP application quite naturally.

• Its mechanical simplicity, robust construction and relatively lower cost have made it quite popular.

• Whenever the torque varies, the speed of the SCIG increases or decreases only a little, thereby resulting in less tear and wear of the gearbox. . This is the most important reason for using induction generator rather than a synchronous generator in a WPP.

Limited variable speed generator Type B

• Wound rotor induction generator (WRIG) that operates in a narrow range speed variation (upto 10%–16% above the synchronous speed) is used.

• The mechanical construction of the WRIG stator is similar to that of the SCIG. However, the rotor circuit has copper windings that are not shorted by the end rings, but are star connected to the external variable resistors through slip rings and brushes.

• The resistors lower the voltage to the required value by dropping some of the supply voltage across it. Modern Type-B WPP has done away with slip rings by mounting rotating variable resistors and the optically controlled (so-called opti-slip or flexi-slip) control circuitry is mounted on the generator rotor shaft itself whereby, the heat that is generated in the resistors is dissipated.

7. (b) Calculate the power developed and speed.

• Rotor efficiency =40%

• Generator efficiency =70 %

• Rotor swept area =2.11 m2

• Wind speed = 8.6 m/s

• Tip Speed Ratio =7

• Radius of rotor = 0.82 m

Given:

• Cp = 0.4

• N =0.7

• V = 8.6m/s

• A = 2.11 m2

• TSR = 7

• R = 0.82

P(w) = 0.6 \times Cp \times N \times A \times V^3

P(w) = 0.6 \times 0.4 \times 0.7 \times 2.11 \times (8.6 \times 8.6 \times 8.6)

P(w) =225.43 watts

Rpm = \frac{V \times TSR \times 60}{6.28 \times R}

Rpm = \frac{8.6 \times 7 \times 60}{6.28 \times 0.82}

Rpm =701.4 rpm

8. (a) Methods of producing bio ethanol.

a) From first generation Biomass

• The first generation bioethanols refer to the ethanols that have been derived from the food- based feedstocks, such as starch, sugar, etc

• These fermentable sugars are extracted by grinding or crushing followed by fermentation to ethanol.

b) From non-feed stocks

• The second generation bioethanols are those produced from non-food feedstocks.

• The non-food based feedstocks used for production of second generation ethanol comprises of dedicated energy crops (e.g., switchgrass,) and agricultural and wood residues (e.g., woodchips, sugarcane bagasse, and sawdust).

• Cellulosic biomass is first pre-treated for microbial fermentation to produce ethanol.

c) From Algae biomass

• The third-generation bioethanol is obtained from the algal biomass. Algae can be used to produce a variety of biofuels such as hydrogen, diesel, isobutene, and ethanol.

• Microalgae are unicellular organisms that are either autotrophic (produce their own food) or heterotrophic and can grow in diverse environment.

• Autotrophic algae use sunlight and atmospheric CO2 to form carbohydrates such as starch and cellulose via photosynthesis.

• On the other hand, heterotrophic algae species can utilize small organic carbon compounds that are turned into protein, and oils.

• Ethanol production from algal starch is like conversion processes of starch or sugars to ethanol (plant source).

• Further, CO2 produced in industrial flue gases can be used to produce algal biomass. Waste water from industrial and domestic sewage can also be used for the cultivation of algal biomass.

• Cultivation of microalgae through open ponds is economical but has inherent disadvantages such as low productivity, water loss, low CO2 utilization, and high affinity to be contaminated by other algal strains.

• The disadvantages of open ponds led to development of closed photobioreactors, which facilitate higher productivity, less contamination, and less water loss.

8. (b) Technologies available to generate electricity using garbage

• The segregated wet garbage (food waste) is brought to the plant site in bins and containers. It is loaded on a sorting platform and residual plastic, metal; glass and other non-biodegradable items are further segregated.

• The waste is loaded into a Waste Crusher along with water, which is mounted on the mixer platform.

• The food waste slurry mixed with hot water is directly charged into the Primary digester. This digester serves mainly as hydrolysis cum acidification tank for the treatment of suspended solids. For breaking slag compressed air is used for agitation of slurry. Compressed air will also helping aeration since bacteria involved in this tank are aerobic in nature.

• The secondary digester is designed in such a way that after the system reaches equilibrium in initial 4-5 days, Main digester tank serves as a methane fermentation tank and BOD reduction takes place here.

• The treated overflow from this digester is connected to the manure pits. This manure can be supplied to farmers at the rate of Rs.4-5 per Kg. Alternatively municipal gardens and local gardens can be assured of regular manure from this biogas plant.

8. (c) Role of Biogas in clean energy

• Biogas systems rely on the natural interaction between microorganisms and organic wastes

• The gas is distributed through a network of pipes and is used for cooking and heating.

• the creation free methane-rich fertiliser as a natural by-product. This can either be sold to make an income or used on farm land to improve crop yields.

SECTION = 05

9. (a) Concepts and Advantages of Agrovoltaics

• Agrovoltaics is a combination of solar panels and agricultural land.

Advantages of Agro-Voltaics

• The main benefit of agrovoltaics is that it reduces greenhouse gas emissions from the agricultural sector.

• The dual use of land for both agriculture and for energy relieves pressure on ecosystems and biodiversity, which are affected when cultivation areas are expanded.

• Studies estimate that the electricity generated by solar panels increases the economic value of agrovoltaic farms by more than 30 %, as it improves land-use efficiency and yields, as can be seen in the infographic. This is especially true in warmer areas, where the shade can protect crops by reducing temperatures and preventing excessive evaporation.

9. (b) Battery energy storage system grid applications

1. Electricity can be purchased and stored during off-load period when the cost is low and used or sold during peak load period for higher rate.

2. Reduces the need to buy new generator to meet additional load demand.

3. Helps to maintain grid frequency.

4. Helps to maintain reserve capacity which is used at the time of grid failure.

5. Used in black start restoration, to start the generators when there is total failure of power in the generating station.

6. Helps to maintain good power quality by maintaining the frequency, voltage and power factor at required value.

9. (c) Hydrogen production and cost effective methods

• Polymer Electrolyte Membrane Electrolysers.

• Alkaline Electrolysers.
  Carbon monoxide is reacted with water to produce additional hydrogen. This method is the cheapest, most efficient, and most common. Natural gas reforming using steam accounts for the majority of hydrogen produced

9. (d) Green hydrogen and black hydrogen

Green Hydrogen

• Green hydrogen is produced through water electrolysis process by employing renewable electricity.

• The reason it is called green is that there is no CO2 emission during the production process.

• Water electrolysis is a process which uses electricity to decompose water into hydrogen gas and oxygen.

Brown Hydrogen

• Black or brown hydrogen is produced from coal.

• It is a very polluting process, and CO2 and carbon monoxide are produced as by-products and released to the atmosphere.

• The gasification of coal is a method used to produce hydrogen.

10. (a) 6Rs of sustainability

• Rethink: Do we make too many products? Design in a way that considers people and the environment.

• Refuse: Don’t use a material or buy a product if you don’t need it or if it’s bad for people or the environment.

• Reduce: Cut down the amount of material and energy you use as much as you can.

• Reuse: Use a product to make something else with all or parts of it.

• Recycle: Reprocess a material or product and make something else.

• Repair: When a product breaks down or doesn’t work properly, fix it.

10. (b) Concept and benefits of microgrids

Microgrids are local power grids that can be operated independently.

BENEFITS:

• Microgrids can switch away from the main grid and continue to provide power during emergencies like these. This process is known as ‘islanding’.

• Microgrids can also provide power in remote places that have no access to electricity.

• As they can generate and store their own energy, microgrids increasingly use renewable energy – like solar panels, wind turbines and batteries, this more microgrids would help reduce greenhouse gas emissions.

10. (C) Difference between Net zero and Zero

Net Zero

The term net zero means achieving a balance between the carbon emitted into the atmosphere, and the carbon removed from it.

Zero

Zero can be achieved only by stopping the carbon emitting sources and using eco friendly sources in the place of carbon emitting sources. Like electric vehicles in place of petrol or diesel vehicles.