ENVS 350 MidTerm Exam

This exam consists of 13 short/medium answer questions. Questions are either worth 10, 15 or 20 points. There are a total of 175 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.



Note: All Underlined words were important terms to use in your answer to a specific question. Most answers did not include these terms. Since you had a study question list in advance, you did have adequate preperation to proudce more exact answers than most of you did.

There were lost of "mercy" pionts given on answers which were more generic than specific. Don't come looking for more.

In many cases, the second half of the posed question was not answered much at all.

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. The SunShine MoonBeam all-electric roadster has a battery pack with 36 KWH of energy storage. The battery back has a 75% charging efficiency. Approximately how long does it take to fully charge this battery if SunShine MoonBeam has a 220 Volt 30 Amp circuit in her garage.?

      1. You have to deliver 36/.75 = 48 KWH to fully charge the battery.

      2. P = VxI = 220x30 = 6.6 KWH

      3. 48/6.6 = 7.2 hours.



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

    Answered Variably.

    • 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! suprisingly, most people left this out of their answer but this is fundamental!


    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 so even if R is really 2R the resource is just exhausted in the next doubling time!


  8. Explain some of the pros and cons associated with different devices used to turn wave energy into electricity. If you were to advise the adoption of one particular wave power technology which would you choose and why?

    This one ws answered well.

    Devices:

    • OWC
    • TAPCHAN
    • Power Buoys
    • Sea Snakes
    • Wave Dragons
    • Underwater rollers

    Most Viable

    • snakes or dragons make the most sense

  9. What are some of the components of the wind energy supply chain and which part of that supply chain is likely to limit the rate at which wind farms can be deployed?

    Again answered more generically than specifically

    Essential Elements of Supply Chain

    • Towers
    • Blades
    • Hub/Nacelle
    • Generators
    • GearBoxes

    Note: Transmission is not part of the supply chain!

    All of the above have various supply chain limitations but as stressed in class, Gearboxes are the most complicated part of the chain as they need to be assembled from many parts and need to be thoroughly tested before deployment. Most answers to this question left out Gearboxes entirely.

    Second most limiting factor is turbine blade production.

  10. By the end of 2011 assume there is 40 GW of wind nameplate capacity installed in the US and that our total nameplate capacity is 1.4 TW. Assume that US Electrical consumption will increase at the rate of 3% per year and that wind energy can continue to be built out at a rate of 10% per year. Based on these trajectories, estimate when wind energy will reach 50% of our nameplate capacity?

    This one was answered reasonably well.

    Use doubling times to estimate.

    What do we now?

    1. US Nameplate will double in 70/3 = 23 years

    2. Wind will double in 70/10 = 7 years.

    Solve each independently.

    So take US Namplate

    • 1.4 in 2011
    • 2.8 in 2034
    • 5.6 in 2057

    When will wind be about 1/2 of this - I don't know, let's find out.

    • 40 GW in 2011 (not 1/2)
    • 80 GW in 2018 (not 1/2)
    • 160 GW in 2025 (not 1/2)
    • 320 GW in 2032 (compare that to the 2.8 TW above not 1/2
    • 640 GW in 2039 (not 1/2)
    • 1.3 TW in 2046 (okay, this is about 1/2 of what nameplate was in 2034 so we are getting close)
    • 2.6 TW in 2053 (okay, that's about 1/2 of US in 2057) we are done



  11. 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 correspondingly 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

  12. 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?


  13. 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
    • Similar idealogical goals between "sustainabily and green" and Mumford's Neotechnic Era Purification
    • 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.
    • Congress has no committment to long term projects
    • An Obama Unit has already been spent and little of it went is this direction