EESC 340 Energy Resources Assessment # 2

Question 1

Describe the components that go into calculating Levelized Cost of Energy.
What is one cost that LCOE does not include?

Answer 1

Components:
* Construction
* Financing
* Operation and Maintenance
* Fuel Costs

-> Most calculations don’t consider energy storage and backup costs
-> Does not take into account the energy source’s effect on society, such as overall environmental impact.

Question 2

What are the 3 interconnects in the North American electricity grid?
Discuss what happened in the 2021 Texas Grid Failure and give at least two actions that can be taken to prevent it in the future.

Answer 2

• 3 Interconnects:
  -> Western Interconnect
  -> Eastern Interconnect
  -> Electricity reliability council of Texas Interconnection.

-> Severe winter storm and cold weather causes a record high demand for electricity. State’s power grid was not winterized, equipment failed and power supply was limited.
-> ERCOT initiated power cut to millions of customers to maintain grid stability.

• What can be done:
  -> winterize equipment,
  -> Interconnection with neighboring grids

Question 3

Explain why perpendicular rays provide more energy to the solar collector than rays at an angle.

Answer 3

• At noon, over the equator, the sun’s rays are directly overhead (90 degrees). Therefore the intensity of the sunlight stays constant as it strikes Earth.
• But most of the time, the rays of the sun hit the Earth at an angle. This spreads the beam out over a larger area.
• The surface area increases which decreases the intensity that’s reaching the solar panel.

Question 4

How is the best angle for the tilt (elevation) of solar panels determined?
Why is that the best value for maximizing annual electricity production?

Answer 4

• A surface tilted at the same angle as the Latitude will be perpendicular to the sun’s rays at mid-day on the spring and autumn equinox.
  ->This optimizes annual power.
• A more horizontal tile is better for summer
  (arc of sun is higher)
• A more vertical tilt is better for the winter
• (arc of sun is lower).

Question 5

What is the best azimuth for solar panels in the
Northern Hemisphere?
Southern Hemisphere?

Answer 5

• Azimuth Northern Hemisphere: 180 degree south
• Azimuth Southern Hemisphere: 180 degree north

Question 6

Here are Elevation & Azimuth values for solar panels in Fredericksburg.
What season(s) is the elevation optimized for?
What time of day (am, midday, pm) is the azimuth optimized for?

Elevation: 38°, Azimuth: 180°
Elevation: 30°, Azimuth: 170°
Elevation: 45°, Azimuth: 190°

Answer 6

• Elevation: 38°, Azimuth: 180°:
  Season: optimal for annual power
  Time of Day: standard direction/midday
• Elevation: 30°, Azimuth: 170°
  Season: Summer (tilt less than latitude)
  Time of Day: morning (Azimuth less than 180)
• Elevation: 45°, Azimuth: 190°
  Season: Winter (tilt more than latitude)
  Time of Day: afternoon (Azimuth more than 180)

Question 7

How would you distinguish a
solar thermal collector from a PV panel?

Answer 7

• A solar thermal system has visible water supply pipes and/or glass tubes.
• The sun’s heat is used to heat water
• No electricity is produced.

Question 8

Sketch and label the components and water flow path of an active solar thermal system
(regular or thermosiphon). Denote which areas have hot water and which have cold water.
Include these components:
- Collector
- Hot and Cold pipes
- Pump (if present)
- Storage tank
- Heat exchanger

Answer 8

• Storage tank:
  The system starts with water in the storage tank, which is considered the “cold” water source.
• Pump:
  A pump activates the water flow, drawing water from the storage tank through the “cold” pipe towards the solar collector.
• Collector:
  The water travels through the collector panel, where it absorbs heat from the sun’s radiation.
• Hot pipe:
  Once heated in the collector, the water flows back through the “hot” pipe to the storage tank.
• Heat exchanger (if applicable):
  In an indirect system, the heated water from the collector may first pass through a heat exchanger where it transfers its heat to the household water supply, ensuring the potable water never directly touches the collector loop.
• Back to storage tank:
  The now heated water (or the heated household water in an indirect system) is then returned to the storage tank, ready to be used when needed.

Question 9

Why is a heat exchanger used in active solar thermal systems?

Answer 9

• Solar water heating systems use heat exchangers to transfer solar energy absorbed in solar collectors to potable (drinkable) water.
• It uses a heat-transfer fluid that circulates through the solar collector, absorbs heat, and then flows through a heat exchanger to transfer its heat to potable water in a storage tank.
• Heat-transfer fluids, such as propylene glycol antifreeze, protect the solar collector from freezing in cold weather.

Question 10

Explain how Heat Pipes take advantage of latent heat of condensation to transfer heat energy to the water tank.
Include an explanation of what latent heat is.

Answer 10

• Latent Heat = Energy released when a vapor turns back into a liquid (change it’s phase/physical state), which occurs within the condenser, transferring a large amount of heat without a temperature change.
• When a substance changes phase, it releases or absorbs heat as the molecules rearrange.

Question 11

Explain why the quality of the glazing is so important in a solar thermal system
(What two key roles does it play?)

Answer 11

• Good glass passes light but traps heat.
• The incoming solar radiation (short wavelength) gets absorbed into materials and reemitted as long wavelengths (aka heat) but the glass won’t let the heat out, which is perfect for a solar thermal collector.
  -> Glass lets light through, but not heat. Radiation bounces back as infrared (cannot pass thru glass)

Question 12

Name and describe the 3 types of heat movement. Provide one means of reducing heat loss from each type.

Answer 12

1. Convection: heat currents due to warm fluids becoming less dense.
   Use a heavy gas between glass panes (Argon) to prevent loss.
2. Conduction: heat moving through a thermally conductive material.
   Use materials w/small pockets or trapped air.
3. Radiation: the transfer of heat through electromagnetic waves. This means that heat is transferred through space w/o the need for any medium or substance.
   Use low-E coatings.

Question 13

Describe three actions that passive solar design can use to heat and cool a building.

Answer 13

• South facing windows
• Multi-paneled glass w/inert gas filler and low emissivity
• Thick concrete floor for thermal mass
• avoid over-shading. Make sure mid-winter sun can reach into living spaces

Question 14

What is a “dopant” and why are they used in PV cells?

Answer 14

• Doping is the intentional introduction of a foreign substance into semiconductor crystals.
  -> They successfully improved PV efficiency in silicon lattices

Question 15

What is a semiconductor and why are they used in PV cells?
(your explanation should discuss electrons)

Answer 15

Semiconductors:
- don’t conduct under normal conditions
- in-betweeen insulator and conductor
- typically half full valence shell
- likes to form crystalline structures that share those outer electrons
- everyone sharing their outer electrons with their 4 neighbors gives them all a full shell

• When a little energy is added, semi-conductors convert from an insulator to a conductor.
  The other electrons are moved to the conduction band.
  Some semiconductors (Si) have a band gap that matches the energy provided by photons coming from the sun.
  That means, sunlight can nudge electrons into the conduction band.

Question 16

What is a “band gap”?
Why does the band gap of certain semiconductors make them so good to use in solar cells?

Answer 16

Band gap= the amount of energy that it takes to bump an electron from the valence band into the conduction band.

Some semiconductors (Si) have a band gap that matches the energy provided by photons coming from the sun.
That means, sunlight can nudge electrons into the conduction band.

Question 17

What is a “multijunction” PV cell and why does it increase efficiency?

Answer 17

• A multijunction photovoltaic (PV) cell is a solar cell that’s made up of multiple layers of different semiconductor materials, each with a different bandgap.
  This allows the cell to absorb a wider range of the solar spectrum than a single-junction cell, which only has one layer of material.

Question 18

Know the steps (including sketches) by which a PV cell generates electricity.

Your answer should include:
dopants, n/p types, holes, electrons, diffusion, depletion layer, and electrical potential).
(Will give you a step and then ask you to describe what comes next, and why that is important.)

Answer 18

• Doped semiconductors cause an area of charge depletion (“depletion zone”) to be formed at the spot where they join (the “P/N junction”)
• Electrons that are bumped loose by sunlight hitting the depletion zone are quickly whisked away (by the pull of opposing charges) to accumulate on one side of the junction. The same thing happens with positive holes on the other side.
• This accumulation of opposite charges on either side of the junction is “electric potential”, aka voltage. Connecting a wire from one side to the other allows electrons to flow…electricity!

Question 19

Draw a graph that contains two I/V Curves, one for full sun and one for partly cloudy conditions.
(Label the axes, including units) and the Maximum Power Point (MPP).

Alternatively, I may give you a graph and ask you to identify I sc (short circuit current), V oc (open circuit voltage), and MPP (Max Power Point. Why is finding MPP important?

Answer 19

• Power is the product of amps and volts. There is a spot on the graph where A*V reaches a maximum. That is the load at which the panel is producing the greatest power - at that light level. That spot is called the Maximum Power point.
• But that maximum power point changes as the sunlight intensity changes (which is all the time!)
• Somehow, we need to keep changing the load on a cell as the sunlight changes, so that we create just the right resistance to keep it at its maximum power point with an MPPT charge controller.

Question 20

Sketch the layout of a ______________ PV system showing the following components in the correct order
(I will choose one: Grid-tied, Off-grid, or Hybrid).

• Optimizer
  -Inverter
• MPPT Charge Controller
• PV panels
• Sun
  -Electric Grid (ie the power utility)
• Battery
• House (ie electrical devices)

Answer 20

• Grid-tied: Sun, panel with max power point tracker optimizer, DC to AC inverter, house. Or inverter directly to the grid. Grid supplies house.
• Off grid: Sun, solar panel, MPPT charge controller, household battery, DC to AC inverter, house.
• Hybrid: sun, solar panel, MPPT charge controller, household battery then DC to AC inverter. Or MPPT controller directly to inverter when battery is fully charged, house, grid. Excess power goes directly to the grid, grid provides power when sun and battery are insufficient.

Question 21

Explain “Net Metering” and why it is so important for residential solar applications.

Answer 21

• Net metering is a billing mechanism that credits solar energy system owners for the electricity they add to the grid
• It allows them to use their generated power whenever they need it, not just when it’s produced

Question 22

Why would a homeowner choose to implement a HYBRID system? Under what circumstances would it benefit them?

Answer 22

The hybrid system includes a battery. You won’t have nighttime energy if there’s no battery and
you subject your appliances to variable energy which can damage them.

Question 23

Consider a solar PV setup where the array is connected to an inverter which is connected to the house. There is no battery and no grid connection. What are two problems with the design?

Answer 23

Power fluctuations due to inconsistent sunlight, leading to potential disruptions in electricity supply to the house, and the inability to utilize solar power when electricity demand is low, resulting in wasted energy.

Question 24

Describe the causes of wind at the global scale and the local scale.

Answer 24

• On a global scale, wind is primarily caused by uneven heating of the Earth’s surface by the sun, leading to the formation of high and low pressure areas
  Rotation of the Earth (Coriolis effect) influences the direction of these wind patterns
• At a local scale, wind is driven by smaller variations in temperature and pressure due to geographical features like mountains, bodies of water, and land surfaces, creating localized wind patterns like sea breezes and land breezes.

Question 25

Identify the area in the continental US that has the highest ONSHORE wind power potential.
Identify the areas in the continental US that have the highest OFFSHORE wind power potential.

Answer 25

• The Central and Midwest regions of the US have the highest onshore wind power potential because of the consistent and high wind speeds in these areas.
• East coast, West coast, Gulf area, Hawaii have the best Offshore wind potentials.

Question 26

What is the primary factor that has driven the rapid increase in our ability to capture wind power?

Answer 26

• Significant decrease in cost in wind turbine technology. Wind turbine designs have evolved to have larger blades and taller towers, allowing them to capture more wind energy from higher altitudes.
• As the wind energy industry has grown, production costs have decreased.

Question 27

Give two reasons why winds are higher at higher altitudes.

Answer 27

• Less surface friction:
  As you move higher in the atmosphere, there are fewer obstacles like trees and buildings to slow down the air, resulting in less friction and faster wind speeds.
• Increased pressure gradients at higher altitudes, leading to stronger winds.
• Air density decreases with altitude

Question 28

In the wind power equation, which parameter has an outsized effect? How does that affect how wind design turbines?

Answer 28

Velocity gets cubed. Velocity increases with elevation. Taller turbines are being built.

Question 29

What is an airfoil?

Answer 29

A structure with curved surfaces, such as an airplane wing or the cross-sectional shape of wind turbine blades, designed to provide the best ration of lift and drag when they move through the air.

Question 30

Explain the two means by which lift is generated. Be specific

Answer 30

Lift is caused by two main factors:
- Having an “angle of attack” on an airfoil forces air downward. For every action there is an “equal and opposite” reaction. The reaction is the upward force on the wing.

• High pressure on the bottom of the wing seeks to move toward Low pressure on the top.

Question 31

What is “apparent wind” and why is it relevant to turbine operations?

Answer 31

• For startup, the “angle of attack” on turbine blades will be FLAT, like an airplane, as the wind is coming from directly in front, and this will generate an upward lift.
• But, as the blade begins to spin, the wing experiences wind from the rotation itself
• So, the wing now experiences an “APPARENT” wind that is the combination of the two wind sources.
• The turbine blade will be PIVOTED by computer control to maintain its “angle of attack” to the apparent wind, which maximizes efficiency

Question 32

Why are turbine blades twisted?

Answer 32

The closer to the tip of the blade you get, the faster the blade is moving through the air and so the greater the apparent wind angle is.
The blade is built with a twist along its lengths to change the angle of attack, and is therefore built with a twist along its length.