Introduction TO energy Engineering 2080 Solution

 

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Energy Engineering Exam Solutions

Question 1

a) How can Nepal diversify its energy sources to ensure a more secure and sustainable energy supply? [7]

Nepal can diversify its energy sources by investing in a mix of renewable and non-renewable energy resources. The primary strategies include:

• Hydroelectric Power: Nepal has significant hydroelectric potential due to its numerous rivers. Developing small, medium, and large-scale hydroelectric projects can provide a stable energy supply.
• Solar Energy: With abundant sunlight, solar power can be harnessed using photovoltaic (PV) panels. Solar farms and rooftop solar installations can contribute to the energy mix.
• Wind Energy: Certain regions in Nepal have suitable wind conditions for generating wind power. Installing wind turbines in these areas can help diversify energy sources.
• Biogas and Biomass: Utilizing agricultural waste, animal manure, and organic waste to produce biogas and biomass energy can provide rural areas with sustainable energy.
• Geothermal Energy: Exploring geothermal energy potential in tectonically active areas can add to the energy mix.
• Energy Efficiency and Conservation: Implementing energy-efficient technologies and promoting energy conservation practices can reduce overall energy demand.
• Policy and Investment: Government policies and incentives to encourage private sector investment in renewable energy projects and infrastructure development are crucial.

b) What are climate change models, and how are they used to predict future climate patterns and impacts? What are some of the most commonly used climate change models? [5+3]

Climate Change Models: Climate change models are complex mathematical representations of the Earth's climate system. They simulate interactions between the atmosphere, oceans, land surface, and ice. These models use physical, chemical, and biological principles to predict future climate patterns based on different greenhouse gas emission scenarios.

• General Circulation Models (GCMs): These models simulate the global climate system, including the circulation of the atmosphere and oceans. They provide detailed projections of temperature, precipitation, and other climate variables.
• Earth System Models (ESMs): ESMs are advanced versions of GCMs that include additional components such as the carbon cycle, biogeochemistry, and dynamic vegetation. They provide a more comprehensive understanding of climate change impacts.
• Regional Climate Models (RCMs): RCMs provide high-resolution climate projections for specific regions. They are used to assess local climate change impacts and inform adaptation strategies.

OR

What are the key impacts of climate change on building design and how are architects and engineers adapting to these changes? [4+4]

Key Impacts of Climate Change on Building Design:

• Increased Temperature: Higher temperatures increase the demand for cooling and can affect indoor comfort levels. Buildings need better insulation and cooling systems to maintain a comfortable indoor environment.
• Extreme Weather Events: Increased frequency and intensity of storms, floods, and heatwaves require buildings to be more resilient. Structures must be designed to withstand these events.
• Rising Sea Levels: Coastal buildings are at risk of flooding and erosion. Adaptation measures include elevating buildings, using flood-resistant materials, and implementing effective drainage systems.
• Energy Efficiency: Climate change necessitates the reduction of greenhouse gas emissions. Buildings need to incorporate energy-efficient designs, use renewable energy sources, and reduce overall energy consumption.

Adaptations by Architects and Engineers:

• Green Roofs and Walls: These features reduce heat absorption, improve insulation, and provide natural cooling.
• Improved Insulation and Ventilation: Enhancing insulation and incorporating natural ventilation reduces the need for artificial heating and cooling, lowering energy consumption.
• Water-Efficient Systems: Implementing water-efficient fixtures and rainwater harvesting systems helps manage water resources more sustainably.
• Resilient Materials and Design: Using materials that can withstand extreme weather conditions and designing structures to be more resilient to natural disasters.

Question 2

a) Derive the equation for turbine and heat exchangers with the help of steady energy equation by stating all their assumptions. [7]

Steady Flow Energy Equation: The steady flow energy equation for a control volume is given by: \[ \dot{Q} - \dot{W} = \dot{m} (h_2 - h_1 + \frac{V_2^2 - V_1^2}{2} + g(z_2 - z_1)) \] For Turbines:

• Heat transfer \(\dot{Q} = 0\)
• Change in kinetic energy and potential energy is negligible
• Work output \(\dot{W} = \dot{m} (h_1 - h_2)\)

The equation simplifies to: \[ \dot{W} = \dot{m} (h_1 - h_2) \] For Heat Exchangers:

• No work is done, \(\dot{W} = 0\)
• Heat transfer is significant
• Heat exchanged \(\dot{Q} = \dot{m} (h_2 - h_1)\)

The equation simplifies to: \[ \dot{Q} = \dot{m} (h_2 - h_1)

b) The inner surface of a 200 cm thick and 500 cm x 500 cm wall (K = 10 W/mK) is at 400°C. The outer surface dissipates heat by combined convection and radiation to the ambient air at 27°C. If the wall surface has the emissivity of 0.85 and the convection heat transfer coefficient between outer wall surface and ambient air is 20W/m²K, determine the outer surface temperature of the wall. [8]

To determine the outer surface temperature, \(T_s\), we need to set up the heat transfer equations. The total heat transfer rate \(Q\) is given by the sum of convective and radiative heat transfers. \[ Q = k \cdot A \cdot \frac{(T_{in} - T_s)}{d} = h \cdot A \cdot (T_s - T_{ambient}) + \epsilon \cdot \sigma \cdot A \cdot (T_s^4 - T_{ambient}^4) \] Where:

• \(k = 10 \, W/mK\)
• \(A = 500 \times 500 \, cm^2 = 25 \, m^2\)
• \(T_{in} = 400°C\)
• \(T_{ambient} = 27°C\)
• \(d = 200 \, cm = 2 \, m\)
• \(h = 20 \, W/m^2K\)
• \(\epsilon = 0.85\)
• \(\sigma = 5.67 \times 10^{-8} \, W/m^2K^4\)

Solving these equations will give the outer surface temperature \(T_s\). The detailed calculations involve solving a fourth-order polynomial, which typically requires numerical methods or iterative approaches.

Question 3

a) What is the principle of solar photovoltaic power generation? What are the main elements of a PV system? Describe all the components. [2+6]

Principle of Solar Photovoltaic Power Generation: Solar photovoltaic (PV) power generation is based on the photovoltaic effect, where sunlight is converted directly into electricity using semiconductor materials. When photons from sunlight strike the semiconductor material (usually silicon), they excite electrons, creating electron-hole pairs. These free electrons are then captured and directed to flow as an electric current through an external circuit. Main Elements of a PV System:

• Solar Panels: Comprised of multiple solar cells, they convert sunlight into direct current (DC) electricity.
• Inverter: Converts the DC electricity generated by the solar panels into alternating current (AC) electricity, which is used by most household appliances and can be fed into the grid.
• Battery Storage: Stores excess electricity generated during the day for use at night or during cloudy periods.
• Charge Controller: Regulates the voltage and current coming from the solar panels to prevent overcharging and damage to the batteries.
• Mounting Structures: Supports and secures the solar panels at the optimal angle and orientation to maximize sunlight exposure.
• Cables and Wiring: Connects all the components of the PV system, ensuring efficient and safe transmission of electricity.
• Monitoring System: Tracks the performance and output of the PV system, providing data on energy production and system status.

a) Calculate the annual energy output of the PV power plant which consists of 30 modules located at a site having an average daily global radiation of 4.1 kWh/m² and an average ambient temperature of 35°C. The specification of a PV module under standard testing conditions are:

Parameter	Value
Number of cells	36
Max power	150 W
Short circuit current	9.2 A
Open circuit voltage	20.1 V
Max power current	8.8 A
Max power voltage	17.1 V

Solution To calculate the annual energy output of the PV power plant, we need to first calculate the daily energy output and then multiply it by 365 (days per year). The daily energy output of a single PV module can be calculated using the following formula:

Daily energy output (Wh) = Peak sun hours (PSH) x Module efficiency x Module area x Irradiance

Given the average daily global radiation of 4.1 kWh/m², we can assume the peak sun hours (PSH) to be around 5 hours (a reasonable value for a location with an average daily global radiation of 4.1 kWh/m²). The module efficiency can be calculated using the following formula:

Module efficiency (%) = (Max power / (Short circuit current x Open circuit voltage)) x 100

Module efficiency (%) = (150 W / (9.2 A x 20.1 V)) x 100 ≈ 18.2% The module area can be calculated using the following formula:

Module area (m²) = Max power / (Irradiance x Module efficiency)

Module area (m²) = 150 W / (4.1 kWh/m² x 0.182) ≈ 2.04 m² Now, we can calculate the daily energy output of a single PV module:

Daily energy output (Wh) = 5 PSH x 0.182 x 2.04 m² x 4.1 kWh/m² ≈ 745 Wh

Since there are 30 modules, the total daily energy output is:

Total daily energy output (Wh) = 745 Wh x 30 ≈ 22,350 Wh

The annual energy output is:

Annual energy output (Wh) = 22,350 Wh x 365 ≈ 8,156,250 Wh or 8.16 MWh

Question 4

a) With the help of neat schematic diagram explain about anerobic digestion. [7]

Anaerobic Digestion: Anaerobic digestion is a biological process where microorganisms break down organic matter in the absence of oxygen. This process produces biogas (a mixture of methane and carbon dioxide) and digestate (a nutrient-rich substance). Stages of Anaerobic Digestion:

• Hydrolysis: Complex organic molecules (carbohydrates, proteins, fats) are broken down into simpler molecules (sugars, amino acids, fatty acids).
• Acidogenesis: Simple molecules are converted into volatile fatty acids, alcohols, hydrogen, and carbon dioxide.
• Acetogenesis: Volatile fatty acids and alcohols are further converted into acetic acid, hydrogen, and carbon dioxide.
• Methanogenesis: Acetic acid, hydrogen, and carbon dioxide are converted into methane and carbon dioxide by methanogenic bacteria.

Components of an Anaerobic Digestion System:

• Feedstock storage
• Digester tank
• Gas storage
• Biogas utilization system
• Digestate storage

b) The farmer and his 10 family members reside in a rural area and possess a livestock of 73 cows, 220 hens, and 80 sheep. Given the absence of other energy sources, the farmer intends to construct a biodigester to fulfill the energy requirements of his family. Additionally, he plans to set up a dual fuel engine-driven generator with 73% efficiency for the generator and 27% thermal efficiency for the engine. At present, they require five 100 W lamps for lighting, which are used for 6 hours daily. The family also wishes to utilize the biodigester for cooking purposes and run a 3 hp pump. Moreover, with the surplus energy, they intend to power one television and a few computers, each consuming 300 W, for 4 hours daily. [8]

Annual Energy Output of PV Power Plant

Given:

• Number of modules: 30
• Average daily global radiation: 4.1 kWh/m²
• Average ambient temperature: 35°C
• Module specifications:
• Number of cells: 36
• Max power: 150 W
• Short circuit current: 9.2 A
• Open circuit voltage: 20.1 V
• Max power current: 8.8 A
• Max power voltage: 17.1 V

Solution:

Step 1: Calculate the daily energy output per module

The daily energy output per module can be calculated using the following formula:

E_daily = G * η * A

where E_daily is the daily energy output per module, G is the average daily global radiation, η is the efficiency of the module, and A is the area of the module.

Calculate the efficiency of the module:

η = (Max power / (Short circuit current * Open circuit voltage)) * (Max power voltage / Open circuit voltage)

η = (150 W / (9.2 A * 20.1 V)) * (17.1 V / 20.1 V) ≈ 0.145 or 14.5%

Calculate the area of the module:

A = (Max power / η) / G

A = (150 W / 0.145) / 4.1 kWh/m² ≈ 2.55 m²

Calculate the daily energy output per module:

E_daily = 4.1 kWh/m² * 0.145 * 2.55 m² ≈ 2.43 kWh/day

Step 2: Calculate the daily energy output of the PV power plant

The daily energy output of the PV power plant can be calculated by multiplying the daily energy output per module by the number of modules:

E_daily_plant = E_daily * Number of modules

E_daily_plant = 2.43 kWh/day * 30 ≈ 72.9 kWh/day

Step 3: Calculate the annual energy output of the PV power plant

The annual energy output of the PV power plant can be calculated by multiplying the daily energy output by 365 (days per year):

E_annual = E_daily_plant * 365

E_annual = 72.9 kWh/day * 365 ≈ 26,553.5 kWh/year

Therefore, the annual energy output of the PV power plant is approximately 26,553.5 kWh/year.

i) The volume of digester with the retention time of 22 days

ii) The number of computers that can be operated with the energy, fulfilling all the desired needs

Take the following data:

S.N.	Raw Material	Production rate (kg/day/head)	Gas Yield (m³/kg)	Solid content (%)
1	Poultry Manure	0.06	0.49	65
2	Cow Dung	11	0.34	18
3	Sheep Manure	0.75	0.55	35

Mix slurry has a density of 1085 kg/m³. Heating value of biomass = 23 MJ/m³. Biogas required for cooking is 0.227m³/person/day.

Solution:

Volume of the Digester:

• Total livestock = 73 cows + 220 hens + 80 sheep
• Using the production rate and gas yield: \[ \text{Gas production} = (73 \times 11 \times 0.34) + (220 \times 0.06 \times 0.49) + (80 \times 0.75 \times 0.55) \, m³/day \]
• Summing these values will give the total daily gas production. The volume of the digester can then be calculated using the retention time of 22 days: \[ \text{Volume} = \text{Daily gas production} \times 22 \, days \]

Number of Computers that can be Operated:

• Calculate the total biogas energy available after fulfilling the energy needs for lighting and cooking.
• Compute the surplus energy available for operating the computers.
• Determine the number of computers that can be operated based on their energy consumption. \[ \text{Number of computers} = \frac{\text{Surplus energy}}{\text{Energy consumption per computer}} \]

Question 5

a) Using a clear and concise schematic diagram, outline the operation of a wind-solar hybrid system while enumerating all its components. [8]

Wind-Solar Hybrid System: A wind-solar hybrid system combines wind turbines and solar panels to generate electricity. The components include:

• Wind Turbines: Convert wind energy into electrical energy.
• Solar Panels: Convert solar energy into electrical energy.
• Inverter: Converts DC output from solar panels and wind turbines into AC power.
• Battery Storage: Stores excess energy generated for use during periods of low wind or sunlight.
• Charge Controller: Regulates the charging of batteries to prevent overcharging.
• Control System: Manages the integration of wind and solar power, ensuring efficient and reliable operation.
• Load: The electrical devices and appliances powered by the hybrid system.

b) Let's suppose we have a stream that possesses a useful vertical distance of 28000 mm and a flow rate of 1152000 liters per day. Considering an efficiency of 72 % for the hydro plant, how much power can it potentially generate? Additionally, what would be the annual energy output of the hydro plant? Lastly, how many individuals can be sustained by this energy, assuming an average consumption of around 1.3 MWh per person? [7]

Power Generation Calculation:

• Vertical Distance (Head), \(H = 28000 \, mm = 28 \, m\)
• Flow Rate, \(Q = 1152000 \, liters/day = 1152 \, m³/day = \frac{1152}{86400} \, m³/s\)
• Efficiency, \(\eta = 72\%\)
• Potential Power: \[ P = \eta \times \rho \times g \times H \times Q \] Where: \(\rho = 1000 \, kg/m³\) (Density of Water)\\ \(g = 9.81 \, m/s²\) (Acceleration due to Gravity)
• Substitute the values to find \(P\)

Annual Energy Output:

• Energy Output per day = \(P \times 24 \, hours\)
• Annual Energy Output = Energy Output per day \(\times 365 \, days\)

Number of Individuals Sustained:

• Average Consumption = 1.3 MWh/person
• Number of Individuals = \(\frac{\text{Annual Energy Output}}{1.3 \, MWh}\)

Question 6

a) What do you mean by thermal comfort based on ASHRAE standard? Explain the factors that indicates thermal comfort. [2+5]

Thermal Comfort: Thermal comfort is defined by ASHRAE as the condition of mind that expresses satisfaction with the thermal environment. It involves the perception of the environment as neither too hot nor too cold. Factors Indicating Thermal Comfort:

• Air Temperature: The ambient temperature of the air surrounding the occupants.
• Radiant Temperature: The temperature of the surrounding surfaces that can affect the heat exchange by radiation.
• Humidity: The amount of moisture in the air, which affects the ability to evaporate sweat and cool the body.
• Air Velocity: The speed of air movement, which can enhance heat loss through convection and evaporation.
• Metabolic Rate: The level of physical activity, which influences the amount of heat produced by the body.
• Clothing Insulation: The thermal resistance of the clothing worn by the occupants, which affects heat exchange with the environment.

b) Moist air exists at 40°C dry-bulb temperature, 20°C thermodynamic wet-bulb temperature, and 101.325 kPa pressure. Determine the humidity ratio, enthalpy, dew-point temperature, relative humidity, and specific volume. [8]

Given:

• Dry-bulb temperature (T_db) = 40°C
• Thermodynamic wet-bulb temperature (T_wb) = 20°C
• Pressure (P) = 101.325 kPa

Humidity Ratio (W):

• Use the psychrometric equations to find the humidity ratio.

Enthalpy (h):

• Calculate using the specific enthalpy equations for moist air.

Dew-Point Temperature (T_dp):

• Determine from the saturation temperature corresponding to the partial pressure of water vapor.

Relative Humidity (RH):

• Calculate using the psychrometric chart or equations.

Specific Volume (v):

• Determine from the psychrometric chart or using specific volume equations for moist air.

Question 7

Write short notes on: (Answer any two) [5 X 2]

• a) Environmental aspects of energy
• b) Energy conversion potential
• c) Refrigeration Cycle

a) Environmental Aspects of Energy:

• Energy production and consumption have significant environmental impacts, including greenhouse gas emissions, air and water pollution, and habitat destruction.
• Transitioning to renewable energy sources like wind, solar, and hydro can reduce environmental impacts.
• Energy efficiency and conservation measures can minimize the negative effects of energy use.
• Environmental regulations and policies play a crucial role in managing the impact of energy production and use.

b) Energy Conversion Potential:

• Energy conversion involves transforming energy from one form to another, such as converting chemical energy in fuels to electrical energy in power plants.
• The efficiency of energy conversion processes is critical for maximizing output and minimizing waste.
• Advanced technologies, such as combined cycle power plants and renewable energy systems, offer higher energy conversion potentials.
• Research and development are focused on improving conversion efficiencies and integrating multiple energy sources.

c) Refrigeration Cycle:

• The refrigeration cycle is a process used to transfer heat from a low-temperature space to a high-temperature space, commonly used in refrigerators and air conditioners.
• The cycle involves four main stages: compression, condensation, expansion, and evaporation.
• In the compression stage, the refrigerant gas is compressed, increasing its pressure and temperature.
• During condensation, the high-pressure gas releases heat and condenses into a liquid.
• In the expansion stage, the liquid refrigerant expands, reducing its pressure and temperature.
• Finally, during evaporation, the low-pressure liquid absorbs heat and evaporates, cooling the surrounding environment.

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Energy EngineeringIntroduction to energy engineering

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