BRAE 348 Final Practice Problems
The amount of energy used for an activity depends on the energy required for that activity and the frequency of that activity. What are the two approaches listed from the book that address the efforts of energy conservation?
1. Lifestyle change 2. Technical fix
If the world use of oil is about 76 MBPD, how many years would you expect this resource to last given the proven world reserves are 1486 x 109 barrels (bbl)?
53.6 bbl Refer to example problem 1.1 in text Proven reserves / daily use / 365 day per year Make sure units cancel; consumption is in days while question asks for years. The correct answer is: 53.6
In 2013 fossil fuels made up about what % of global energy use?
80%
In 2013 domestic energy production supplied what fraction of the nations demand?
85% Nowadays, this number is about 90%.
What things can be done to reduce the rate of heat transfer through a single pane window plus storm window?
About 35% of the energy requirements for an insulated home are a result of heat losses through the windows. The correct answers are: Use a window shade, Insulating drapes plus valance, low-e coatings
Categorize the following units as those of work or power. BTU: Kilowatt-hr: Joules: ft-lb: Calorie: Hp (horsepower): Watt: ft-lb/min:
BTU → energy Kilowatt-hr (kw-hr) → energy Joules → energy ft-Lb → energy Calorie → energy Hp (horse power) → power Watts → power ft-lb/min → power
A recent wind farm project in Idaho uses 7 turbines of 4 MW rating; 27 million kW-hr per year is expected from this site. What is the capacity factor for this project as a PERCENT (i.e., energy output divided by the maximum output on an annual basis)? (answer as %)
Basically this question is asking what percent of the time are we running this turbine at 100% capacity equivalent. If we have X turbines rated at y MW * 24 hrs per day we will get some number of KW-hrs per day, * 365 to get to Kw-hr per year. This is the maximum possible. We take the actual Kw-hrs of generation and divide that by the max possible to get the capacity factor. The correct answer is: 11.0
Newton's First Law of Motion states that: An object continues in a state of rest or motion with a constant velocity unless compelled to change by an unbalanced (net) outside force. Then why do objects stop after we push them (like rolling a ball on the floor)?
Because of the force of friction.
A (natural) gas-fired turbine generator for producing electricity has an efficiency of about 50%. How many cubic feet would be needed to produce 1000 MW of electricity for 1.0 year? Answer will be in cubic feet.
Because the conversion is 50%, you will need 2 times the amount of natural gas due to energy lost in the form of waste heat. We also need to remember there are 1000 Btus per cubic foot of natural gas. We also need to remember there are 3413 Btu / Kw-hr. Start with the 1000 MW convert that to Kw-hrs per year; Multiply by 24 hrs per day and by 365 days per year and 1000 Kw per MW; Now we have Kw-hr/year. Now divide by the efficiency (50% or 0.5). Now convert Kw-hr / year to Btu per year by multiplying by the 3413. Lastly divide by 1000 Btu / cubic foot and you have found out the required cubic feet. The correct answer is: 5.98e10 ft^3
If an electric clothes dryer has a power rating of 5198 watts and is used for 2 hour per day, how much money can be saved over 29 days by using the backyard clothes dryer (hang dry on a clothes line). Assume electricity costs 12 cents per kw-hr. (answer should be is dollars $)
Calculate the Kw-hrs of savings and multiply that by the cost. dryer watts * hr/day * days = watt-hrs * 1/1000 = Kw-hr * cost = money saved The correct answer is: $36.18
How does energy conservation help you as an individual or us as a society?
Conservation technologies are cost effective alternatives to the development of additional supply technologies, Conservation will gain time for the possible development of inexhaustible resources such as solar, Conservation will reduce the pollution of our environment, Conservation technologies can be put to use more quickly than we can increase supplies, Conservation will save you money
Classify the following principal energy sources as RENEWABLE or NON-RENEWABLE. Corn ethanol: Biomass: Natural gas: Nuclear: Solar: Oil: Coal:
Corn ethanol: renewable Biomass: renewable Natural gas: non-renewable (fossil fuel) Nuclear: non-renewable Solar: renewable Oil: non-renewable (fossil fuel) Coal: non-renewable (fossil fuel)
If all of our needs for oil were to be provided by that available from the estimated reserves located in the Arctic National Wildlife Reserve, 5.1 billion barrels, how long (years) would that supply last using the current consumption, 19.2 million barrels per day (MBPD)? Give answer in YEARS.
Divide the total reserves in billions of barrels by the daily consumption in millions of barrels per day, divide by 365 days per year. Make sure you convert billions to millions. The correct answer is: 0.73 years
What happens to the kinetic energy of a car as it rolls up an incline and stops? (Remember the seven forms of energy) Describe the energy transformation that occurs.
Energy is transformed and must be accounted for. Energy is transformed from kinetic (moving) to potential (due to elevation), but some is lost due to friction as heat or thermal energy. The correct answer is: Kinetic energy (KE) is transformed to Potential energy (PE) and Thermal energy (TE)
True or false? You can cool a kitchen by leaving the refrigerator door open.
False Near the outside of the door may feel cooler, however, all of the heat that was taken out of the refrigerator is expelled on the backside of the refrigerator. In addition there is electrical energy used to run the refrigerator and that is transformed into heat so actually the kitchen will not get cooled off.
A wall is made up of four elements, use table 5.2 to calculate the R-value. 0.5 inches of wood siding (lapped) 1 inch of plywood 3.5 inches of fiberglass insulation 1 inch of sheetrock
Find the R-value for each material and add them together. Remember it is linear, so if you have 1 inch of plywood but the table has a value for 0.5 inches of plywood then you need to double the table R-value. Wood (0.81) + Plywood (2*0.62) + Fiberglass (10.9) + Sheetrock (2*0.45) = Rtotal = 13.85 The correct answer is: 13.85
What is the rate of emissions in Kg/hr of particulates from a power plant with an electrical output of 1000 MW that burns coal with an ash content of 1.9 %? This power plant has installed an electrostatic precipitator that removes 94 % of the ash. Remember to use 9000 tons/day coal to get 1000 MW.
For this problem we start with 9000 tons per day and multiply by the ash content. Then we need to convert the tons per day to Kg per hour and then multiple by (100%-efficiency of the precipitator). This will give us the emission rate of ash to the environment in Kg/hr. The correct answer is: 389 kg/hr
A 83 W light bulb is accidentally left on for 3 days in a basement. If electricity costs 10 cents per kilowatt hour (¢/kw-hr), how much did this oversight cost?
For this question you need to find the energy used and multiply that by the cost of the energy. You are given the power of the bulb in watts, then multiply by the time it was used, this gives you energy (make sure units match, use hours, not days). Now you multiply the energy times the rate and you will find the money wasted for leaving the light on. The correct answer is: 60 cents
A Boeing 777 airplane flying New York to San Francisco (3,000 miles) consumes 13131 gallons of jet fuel. If the plane is full with 385 passengers, then how much energy (Kw-hr) is used for this trip per passenger? (assume energy content for jet fuel is the same as that of gasoline)
Given the gallons consumed in total, you need to divide by the number of passengers. Assume jet fuel has the same energy content as gasoline, 1.25 x 10^5 BTU per gallon you can now convert to Kw-hr. Also need to convert from BTU to Kw-hr (Table 3.4 pg 86) The correct answer is: 1239 Kw-hr
If a 1825 megawatt power plant is shut down for one day, what will be the loss in revenues if the utility could sell its electricity at 0.13 dollars per kw-hr?
Given the megawatts of the plant, multiply by 1000 to get to Kw then multiply by 24 hrs per day to get to Kw-hr. Now we know the Kw-hr lost due to the shut down and we multiply that by the value per kw-hr; this give a number in cents or $ per day. The correct answer is: 5,694,000 Dollars
Suppose you left a 80 watt light bulb on continuously for 34 days. If the electricity generation and transmission is 30% efficient, how much chemical energy (in JOULES) was wasted at the power plant for this oversight? (Remember Joules are Energy, Watts are Power; Power is the rate of using energy)
Given the watts of the light bulb and the period of time we can calculate the Energy (joules). We must also account for the efficiency. So we need to use more total energy than what is transformed to electricity. We need to divide the useful energy out by the efficiency to calculate the total energy. Total energy in = Useful energy out / efficiency. Also need to convert from Kw-hr to Joules (Table 3.4 pg 86) The correct answer is: 785,536,000 Joules
A skateboard with a mass of 4 kg is moving at a speed of 8 meters per second on a level surface. It encounters a hill and rolls up until it stops. Ignoring friction, to what vertical height does it rise (in meters)?
KE = 1/2 M V^2 PE = mgh If all the energy gets transferred from KE to PE, then you can set the equations equal to each other and solve for H (height) Height will be in meters so use metric units and g = 9.8 m/s/s The correct answer is: 3.3 meters
A house in St. Cloud Minnesota in January needs 54442.3 BTU/hr space heating. What fraction of this requirement can be met by a flat plate collector (FPC) of area 804 ft^2? Assume that the collector is tilted at an angle equal to the latitude and the system efficiency is 47.7 %. Use tables in appendix D. (Answer will be in percent)
Look up in appendix D for St.Cloud Minnesota and you should find 1410 Btu/ft^2/day Use Q = I * Eff * A We are given I, Eff, Area solve for Q. Then compare the heat provided by the FPC to the required heat for space heating needs. Calculated Q / required Q * 100% = percent of needed remember the 1410 is per day and the answer wants per hour The correct answer is: 41
How many Kg per year of carbon monoxide (CO) would your car emit to the atmosphere if it just conformed to the emissions standards for automobiles? (Assume you drive 10,000 miles per year)
Refer to table 8.7 from chapter 8 Standard of 3.0 grams per mile The correct answer is: 30 kg/yr
About how many BTUs of waste heat are dumped into the environment by a fossil fueled electrical generating plant that uses 10,000 BTUs of chemical energy during combustion? (Assume 35% efficiency)
Remember Efficiency = Useful energy out / Total energy in. And Total energy = Useful energy + wasted energy. Assume efficiency is 35% from chapter 3 conversion of fossil fuel to electricity. 35% = Useful energy/ 10,000 BTU; Useful energy = 3500 BTU Wasted energy = 10,000 BTU - 3500 BTU = 6,500 BTU The correct answer is: 6500
A stream flowing at a rate of 18 kg/sec has a vertical drop of 5 meters. What is the maximum power that can be obtained from this stream? (in Watts) Assume maximum efficiency for hydro power is 80% (acceleration due to gravity is 9.8 m/s/s)
Remember Power = Energy / time; Also remember this stream has potential energy. So power = Energy / time Max efficiency of hydro power is about 80% Energy = force * distance ==== mass * acceleration * distance Power = mass * acceleration * distance / time * Efficiency The correct answer is: 706
Match the following to either Primary Recovery, Secondary Recovery or Enhanced Oil Recovery. Pumps out from 15% - 35% of the original oil Pumps out up to 15% of the original oil Water injection into the oil reservoir to increase pressure and output Pumps out from 35% - 45% of the original oil Inject steam or CO2 into the reservoir Mainly just pumping can get the oil out
The correct answer is: -Pumps out from 15% - 35% of the original oil → Secondary Recovery -Pumps out up to 15% of the original oil → Primary Recovery -Water injection into the oil reservoir to increase pressure and output → Secondary Recovery -Pumps out from 35% - 45% of the original oil → Enhanced Oil Recovery -Inject steam or CO2 into the reservoir → Enhanced Oil Recovery -Mainly just pumping can get the oil out → Primary Recovery
Ash particles associated with the burning of coal and oil can be anywhere between 0.01 and 100 microns in size. Particulates are removed in such devices as settling chambers, cyclone collectors, electrostatic precipitators, filters and / or scrubbers. Please match the pollution control systems to the following conditions. There is a best answer although there may be other good answers. -Removes about 50% of particles smaller than 1 micron -Usually the first device used to treat effluent gases -Removes up to 99% of particles greater than 5 microns -Uses dispersion as the primary removal technique -Used for sulfur dioxide (a gas) removal -Removal of 99.9% of particles greater than 0.1 micron
The correct answer is: -Removes about 50% of particles smaller than 1 micron → Electrostatic precipitator -Usually the first device used to treat effluent gases → Settling Chamber -Removes up to 99% of particles greater than 5 microns → Cyclone collector -Uses dispersion as the primary removal technique → Tall stacks -Used for sulfur dioxide (a gas) removal → Scrubber -Removal of 99.9% of particles greater than 0.1 micron → Fabric filter
Suppose you want to lift 500 gallons of water to a storage tank at a height of 10 meters in a period of 5 hours using a solar panel and pump system like the one in activity. Determine the power rating of the pump (in Watts) and calculate the wattage of the solar panel required if the pump has an efficiency of 65%. Potential answers: a. 1) 10 Watt pump; 2) 16 Watt solar panel b. 1) 50 Watt pump; 2) 75 Watt solar panel c. 1) 2.3 Watt pump; 2) 1.5 Watt solar panel d. 1) 10 Watt pump; 2) 6.5 Watt solar panel e. 1) 50 Watt pump; 2) 33 Watt solar panel f. 1) 2.3 Watt pump; 2) 3.5 Watt solar panel
The correct answer is: 1) 10 Watt pump; 2) 16 Watt solar panel
Why do you expect photochemical smog to be a greater problem in a city like LA than in a heavy industrialized city like Pittsburgh?
The correct answers are: -LA has more sunlight, hydrocarbons present from rush hour traffic -Reaction with NOx from automobile combustion process
Consider an old house (from 1858) with walls constructed of 2 inch hardwood planks, 0.5 inch lapped wood siding with 0.5 inch sheetrock on the inside. The size of the house is 45 ft wide, 27 ft long and 10 ft tall. Assume a flat roof covered with 1 inch plywood, asbestos shingles and 3.5 inches of fiberglass insulation. Assume 25% of the walls are covered with single pane windows. This old has is very leaky and has 3 exchanges per hour (k value). Assume no losses through the floor. The outside temperature is 47 degrees F cooler than the inside temperature. Considering only conductive heating losses, calculate the heating needs in BTU/hr.
The total heat loss from a house is the sum of the conductive losses and the infiltrative losses. Here we are only calculating the conductive losses. Use Q/t = 1/R * Area * Delta T; Q/t (total) = (1/R*A*Delta T) of the walls + (1/R*A*Delta T) of the ceiling+ (1/R*A*Delta T) of the windows You need to find the Q/t for the walls, the Q/t for the ceiling and the Q/t for the windows separately and then add them together. You must calculate the R value for the walls separately from the R value of the ceiling and the R value of the windows. Keep them separate. Also calculate the areas separately. There are 4 walls so find the area of each one and then add them together to get the total area. Area = length * height or width * height. The area of the ceiling is the length * width. Remember the wall area must be multiplied by 0.75 because the walls are only 75% of the space as the other 25% is taken up by windows. The wall area is multiplied by .25 or 25% to find the area of the windows. The correct answer is: 4.0*10^4
Consider an old house (from 1858) with walls constructed of 2 inch hardwood planks, 0.5 inch lapped wood siding with 0.5 inch sheetrock on the inside. The size of the house is 37 ft wide, 26 ft long and 10 ft tall. Assume a flat roof covered with 1 inch plywood, asbestos shingles and 3.5 inches of fiberglass insulation. Assume 25% of the walls are covered with single pane windows. This old has is very leaky and has 3 exchanges per hour (k value). Assume no losses through the floor. The outside temperature is 44 degrees F cooler than the inside temperature. Considering only infiltration heating losses, calculate the heating needs in BTU/hr.
The total heat loss from a house is the sum of the conductive losses and the infiltrative losses. Here we are only calculating the infiltration losses. We use the equation from chapter 5, page 134 Q/t = 0.018 * Volume * K * delta T 0.018 is a constant and units are Btu / ft^3 / F Volume is the volume of the space or length * width * height K is the number of exchanges, new house have a K value of 0.5 - 1.5 old houses may have a K value of 4 - 6. This is the total volume exchanges per hour The correct answer is: 2.3*10^4
Electrical power plants dissipate about 2/3 of their input energy into the environment, primarily in the form of hot water at about 20C (35F) above ambient temperatures. What would be some of the challenges in using this "waste heat" for the heating of buildings near the power plant?
Think about how would you use the waste heat that is captured in the cooling water from a power plant. The temperature gradient is very low, 20C, so it is a lower quality of heat. Think of building the piping system to get this warm water to the buildings and then back to the power plants. This would be very expensive to implement and maintain. The correct answers are: Capital costs would be high to build the piping network to get the warm water to the buildings and back to the power plant, The quality of warm water is low due to the low delta temperature (between the building and the water). It leads to very low efficiency, Operating costs might be high because you would have to move a very large volume of water through the pipes to get enough heat from the warm water.
True or false? According to the US EIA (Energy Information Administration), China's energy consumption has surpassed that of the US.
True China surpassed the US in energy consumption in the year 2010 and is estimated to be twice that of the US in 2040. Chapter 1 power point presentation.
True or false? Heat sources at high temperatures are more useful than those at low temperatures because an engine becomes more efficient as the delta T is larger (the change in temperature is larger).
True The change in temperature is the driving force, sources with high temperatures have the potential for greater change in temperature. Think of the maximum efficiency equation (heat in - heat out) / (heat in).
How long, in hours, will it take to heat 39 gallons of water from 65 to 133 degrees F with a 24 KW immersion heater (assuming no energy is lost to the environment)?
Use Q = m c (t2-t1) to solve for the energy required to heat the water. Then you need to convert Q into Kw-hr and divide by the size of the heater, in KW. This leaves you with hours. Remember KW is power and power is the energy/time or the rate at which energy is used. You can get m by multiplying the gallons by 8.3 Lb/gallon; c = 1 Btu/Lb/deg F; t2 and t1 are given. There are 3413 Btu / Kw-hr The correct answer is: 0.27 hours
A 40.8 kg meteor is moving in outer space. If a 7.5 N force is applied opposite the direction of motion, what is the deceleration (in outer space we assume no friction)?
Use equation: Force = mass * acceleration. In this case we are given the mass and the force, so we must solve for the deceleration (or negative acceleration). acceleration = Force / mass The correct answer is: 0.18 m/s^2
Suppose the solar radiation is 887W/m^2, and you can collect 16 % (efficiency) of the energy that falls on the reflecting surface of a solar hot dog cooker. If you need 223 watts for the cooker, what is the minimum collector area required?
Use the equation Q = I * Eff * Area (page 182) -I = insolation -Here Q = energy required for the cooking (watts = energy/time) -Eff = efficiency -Area = size of the surface The correct answer is: 1.6
How many Btus of heat energy are needed to raise the temperature of 15 gallons of water from 45 degrees F to 130 degrees F?
Use the equation Q = m c (t2-t1). Must convert the gallons of water to pounds. 1 gallon = 8.3 Lb. c = 1 Btu / Lb / deg F; you are given t2 and t1. Plug in and solve. The correct answer is: 1.0583 *10^4 BTU
What is the rate of heat gain in a small hut kept at 71 degrees F when the outside temperature is 92 degrees F? Assume the hut is 10 ft wide by 11 ft long by 6 ft tall and the five exterior surfaces are made of 1 in softwood. (neglect losses through the floor, but not the ceiling)
Use the equation Q/t = 1/R * A * delta T. The area must be calculated by adding the area of each of the four walls and the area of the ceiling. This gives the total area of the hut that will experience conductive heat gains. The area of each wall is calculated by multiplying the length X height for two walls, the width X height for two walls and the length X width for the ceiling. Delta T is the temperature difference between the inside and outside temperature The R value is found in table 5.2 and is 1.25 ft^2-hr-F / Btu The correct answer is: 6100
Given a wall is made up of four elements, find the heat loss rate through the wall per square foot when the inside temp is 67 degrees F and the outside temp is 15 degrees F. (units will be BTU/hr/ft^2) 0.5 in wood siding 0.5 inch plywood sheeting 3.5 inch fiberglass 0.5 inch sheetrock
Use the equation Q/t = 1/R * A * delta T. However, because we are not given an area, we divide both sides of the equation by Area. So use Q/(t*A) = 1/R * delta T. -Q/t = heat flow rate -A = area -R = R value found by adding each individual R value from Table 5.2 for each component of the wall (should be 12.78 for this problem) -delta T = temperature difference between inside and outside Q = t*1/R*A*deltaT Q/t/A = 1/R*deltaT Q/t/A = (1/12.78)(52 degreesF) Q/t/A = 4.068 BTU/hr-ft^2 The correct answer is: 4.1 BTU/hr/ft^2
What is the potential energy of a 69 kg person sitting on a ladder 4 m off of the ground?
Use the equation for potential energy: PE = mgh m=mass; g=acceleration due to gravity (9.8 m/s/s); h=height In English units the equation is: PE = weight * height The correct answer is: 2705 J
If the solar cells used in a "solar" farm have an efficiency of 12 %, what area of land is needed, in hectares, to produce an output of 904 MW? Assume that the mean insolation is 612 W/m^2. There are 10,000 m^2 per hectare.
Use the equation from chapter 6: Power = Insolation * Efficiency * Area We are given Insolation, Power and Efficiency, solve for area The correct answer is: 1200
What maximum output would you expect from a wind turbine with a blade diameter of 23 ft in a 19 mph wind? ( Answer in Kw )
Use the equation from the book on page 393: P = 2.83 * 10^-4 * D^2 * v^3 ; this gives answer in Kw (use metric system, meter and meters per second ) P = 2.36 * 10^-6 * D^2 * v^3; this gives answer in Kw (use English units, feet and miles per hour) The correct answer is: 8.6
A simple heat engine might make use of the warm air around New York City. Energy could be taken as heat from the atmosphere (assume 31 degrees C) and rejected as heat to the Hudson River (assume 9 degrees C). What is the maximum efficiency of such an engine for the conversion of thermal energy into mechanical energy? (answer will be in percent)
Use the maximum efficiency equation we learned in chapter 4. Maximum Efficiency = (Temp Hot - Temp Cold) / (Temp Hot) * 100% Remember that temperatures must be converted to the absolute scale Kelvin. You must 273 to the degrees C and use that value for each temperature. The correct answer is: 7 %
What is the definition of sustainable?
Utilizing resources today in a way that will allow future generations to meet their needs.
How many pounds of coal have the same heating value as 48 gallons of gasoline?
We need to look up the energy content of coal and gasoline (table 3.4 pg 86) 1 gallon of gasoline = 1.24 x 10^5 BTU 1 ton of coal = 25 x 10^6 BTU OR 1.25 x 10^4 BTU / pound So divide the two and multiply by the number of gallons of gasoline: Gal of gasoline * 1.24x10^5 BTU/gal * 1 Lb of coal/1.25x10^4 BTU The correct answer is: 476 pounds
A household furnace has an output of 185077 BTU / Hr. What size electrical heating unit (in KW) would be needed to replace this? (think of unit conversions table 3.4 page 86)
We need to look up the power conversion from BTU/hr to Kw (table 3.4 pg 86) 1 watt = 3.413 Btu / hr 1 Kw = 1,000 watt So divide the given Btu/hr by 3.413 and then divide by 1,000 to get Kw: furnace output Btu/hr * 1 watt/3.413 Btu/hr * 1 Kw/ 1,000 watt The correct answer is: 54.2 Kw
How much work do you expend (in ft-lb) to move a 407 Lb refrigerator 9 ft across the floor if you are exerting a force of 80 Lb?
Work = Force * Distance We are not lifting the refrigerator up, so we do not care about the weight of the refrigerator (if we lifted the refrigerator then its weight would be the force needed). We only care about the force acting along our path of travel. So we do not use Force = Mass * Acceleration because the force was already given The correct answer is: 720 ft-lb
Why is a fishnet type undershirt good to wear in cold weather?
air gets trapped in the small hole and insulates
What are three types of passive solar homes?
direct gain, indirect gain, attached greenhouse
