ENVS 350 MidTerm Exam

This exam consists of 12 short/medium answer questions. Questions are either worth 10, 15 or 20 points. There are a total of 160 points available on the exam.

Write Legibly and Carefully - Sloppily prepared exams will receive a point deduction. Take your time on this exam, there is no reason to hurry through it.

In all of the questions below, please confine your answers to the space that is provided for that question. For any numerical question, be sure to show your work, don't just write down an answer.



10 Point Questions

  1. Explain how levelized costs are determined and what some of the uncertainies are in producing a reliable estimate:

    Answered well

    Need to discuss and define

    • capital costs
    • fixed costs
    • variable costs

    Product is then metered at relevant rate over the lifetime of the facility

    Uncertainties include:

    • unreliable estimates of the lifetime
    • usually underestimates of the variable costs (price/distribution of fuel)



  2. Explain how a basic electrical generator works to produce AC electricity.

    Answered with much variation: Note that Magnets don't have "charge" and that AC means alternating current, not Air Conditioning.

    Need to discuss

    • stationary magnets
    • rotating coil or loop of wire between the stationary magnets produces a continuously variable voltage (see diagram in web pages)
    • induces an alternating current of electricity
    • faster the crank turns, the more electricity is generated


  3. Nameplate Capacity vs Net Generation.

    Answered unnecessarily poorly

      Nameplate Capacity: The amount of continuously produced power under ideal conditions (e.g. the wind is blowing all of the time)

      Net Generation: The amount of power that is actually produced, usually quoted as an annual average.



  4. Describe the basic operation of a Photovolatic (PV) cell and what limits its overall operating efficiency.

    Answered unnecessarily poorly

    • incoming solar photons have sufficient energy to move electrons from the valence band to the conductor band in some material
    • once in the conductor band the material the electrons flow through the material (i.e. a current is generated)
    • silicon is the material of choice due to its abundance
    • as the material heats up collisions between the free charge and the silicon nuclei in the lattice increase and so the internal resistance of the material increases and its ability to carry a current decreases


  5. Describe how a Solar Thermal Electricty Facilty works and is able to deliver electricty 24 hours a day. What are the physical limits to this kind of facility.

    Answered well

    • focus heliostats to a container of molten salts located on a central tower
    • molten salts heat up to 500-1000 K and retain that heat for 24 hours
    • Heat exchange with water makes steam
    • Higher capacity towers require more heliostats and eventually the heliostats get too far from the tower to focus the sunlight.


  6. Explain why wind produced electricity is projected to have the lowest levelized cost of any renewable energy technology. What aspects of this technology might conspire against achieving this low cost?

    Answered reasonably (but see below)

    Need to discuss

    • capital costs for wind are moderate
    • fixed costs are very low
    • variable costs are really non-existant
    • MW footprint on the land is increasing: scaleable technology!

    Conspire:

    • future price of components (steel towers, etc) may significantly increase
    • Cost of transmission lines infrastructure may defeat low levelized costs. (this was left out by too many)


    15 Point Questions

  7. Explain the factors that go into determining an exponential depletion timescale and how it is that thsi time scale is not ery sensitive to the vaule of R (the total resource available)

    Answered: mostly generically and not specifically.

      Need to know three parameters:

      1. The exponential rate of resource consumption (k)
      2. The total Resource available (R)
      3. The current rate of resource usage (ro)
      Those integrate together into an equation for an exponential exhaustion time:

      Te = 1/k * ln (Rk/ro +1)

      which many of you memorized but didn't understand its meaning.

      Mathematically, because 1/k is outside the Ln term, it dominates as the exhaustion timescale depends directly on 1/k but only depends on R logarthmically.

      Hence k, the consumption rate, dominates the calculation of Te as changes in R only show through as LN (R).

      Hardly anyone said the above.

      The best qualitative version of the above is something like this

      Resource production is described by a Bell curve - this means you can calculate the depletion using the initial consumption and the growth rate.

      The doubling time of consumption occurs independently of R so k must dominate the exhaustion timescale.


  8. Describe how solar concentrator trough farms work and how they differ from the Solar Dish array

    again answered more generically than specifically

    Troughs

    • parabolic mirrors that rotate to track the sun focus sunlight onto a central tube containing heating oil; oil heats to approximatly 400K
    • Hot oil is heat exchanged with water to produce steam in a large steam generator

    Solar Dish

    • Reflector focus sunlight onto a Stirling Engine (e.g. a piston)
    • Piston drives a mechanical generator producing electricity at each dish
    • Unit capacity is 25 KW

    Differences

    • Dishes require more land usage than troughs which can be placed closer together
    • Dishes don't require water!
    • Dishes tend to require more direct sunlight than troughs


  9. Your best friend's eccentric Uncle insists that there are 15,000 Megawatts of wind power available on the Oregon Coast if you simply placed one "commercially available" wind turbine for every 0.5 miles of Oregon coastline. Show whether or not this claim is credible.

    Answered unusually

    Essential Elements:

    • Coastline is 300-400 miles long
    • so that's 600-800 turbines
    • "commercially available turbines" have 1.5--2.5 MW capacity. So at max that's 2.5*800 = 2000 MW; rather far from 15,000 MW (and of course the wind doesn't blow all of the time)

    Some answers estimated the Oregon coast to be 1000,1500 and even 3000 miles long

    Some answers used "commercially available" wind turbines of 20, 25 and 35 MW.

  10. Explain why tranmission lines must operate at fairly high voltage and what infrastructure is used to step down those voltages.

    again answered more generically than specifically

    Need to explicitly discuss the first two items

    • Ohms Law: V = IR

    • Power = VI I2R

    • Power losses through heat dissipation (which was a buzz word phrase that many used without discussing OHMS law at all ) therefore scale as current squared. At constant Power if one increases V one lowers I and correspondly I2 lowers considerably.

    • a network of substations ratchets the high voltage down in stages
    • transformers using variable numbers of coils are the agent which acts to step down the voltage (often not mentioned)

    20 Point Questions

  11. SunShine MoonBeam Questions

    The List is endless:

    1. What is the product lifetime?
    2. How the F*** does this thing actually work?
    3. How does the distibuted network function?
    4. What is the relability factor?
    5. How much land does one unit occupy?
    6. How big is it?
    7. What is the effect on migrating birds?
    8. How fast can you deploy this technology?
    9. What are the levelized costs?
    10. What kinds of materials does it use?
    11. Does it require the use of rare earth materials?
    12. What is the availability of the material?
    13. What is the unit capacity?
    14. How long does it take to produce a unit?
    15. How many people are needed to install one device?
    16. Where can the devices be located?
    17. Can it connect to our existing grid easily?

    18. Did you steal this idea from LOST?


  12. Explain the qualitative parallel between the "energy dilemma" in the 1930s and our current situation as well as quantitative differences in the scale of the problem. What factors may limit our dream of having a rapidly developing green economy.

    Most people got lots of points for whatever they wrote but you got more points if you included some of the following points:

    • Both crises are strongly energy related - running out of fossil energy now; back then wanted a substitute for dirty coal
    • we need large scale renewable energy projects; back then it was hydro
    • we need improved infrastructure
    • we need government initiated large scale wind and solar now that will create jobs/opportunity in a similar way that the hydro and other public works projects (e.g. roads, bridges, etc) did in the 1930s.
    • But the scale is much different now: back then, when completed, hydro projects produced 45% of the nation's electricity and this was done in 10 years!
    • Infrastructure takes a long time to build these days for lots of reason and wages are much higher now than then. Therefore, the equivalent of putting 150,000 people to work on various infrastructure projects in today's terms is more than we can afford with some initial stimulus package.
    • People are collectively stupid.