BRAE 348 Midterm 1

The cost of running a set of 12 -75 watt light bulbs for 18 hours, with the cost of electricity at $0.15 per kWhr, is approximately:

$2.43 12 (0.075 kW)(18 hr) =16.2 kWhr ($.15)= $2.43

Be able to mathematically define a calorie 1 cal= 1 Btu=

(1 cal = the amount of heat added to 1 g of water to raise it 1C) (1 BTU = the amount of heat added to 1 Lb of water to raise it 1F) (1 BTU = 252 cal = 1055J)

carnot cycle efficiency

(Th-Tc)/Th in Kelvin

An engine performs 3595 joules of work in 21 seconds. What is its power output in horsepower?

0.23 A watt is a Joule per second... so: 3595 joules/ 21 seconds/ 746 watts= .23 hp 1 hp = 746 watts

How long, in hours, will it take to heat 45 gallons of water from 52 to 152 degrees F with a 12 KW immersion heater (assuming no energy is lost to the environment)?

0.91 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

Two energy conservation factors

1) lifestyle change 2) technological fix

Suppose the solar radiation is 858W/m^2, and you can collect 24 % (efficiency) of the energy that falls on the reflecting surface of a solar hot dog cooker. If you need 257 watts for the cooker, what is the minimum collector area required?

1.2 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

typical solar insolation under ideal conditions:

1000W/M^2

A Boeing 777 airplane flying New York to San Francisco (3,000 miles) consumes 10200 gallons of jet fuel. If the plane is full with 362 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)

1024 Kw-h 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)

If the energy conversion efficiencies in a 3 step process are 90% for the first step, 70% for the second, and 25% for the third, what is the overall efficiency?

16% multiple all the % together: .9 x .7 x .25= .1575

How many pounds of coal have the same heating value as 17 gallons of gasoline?

169 lbs 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 Gal of gasoline * 1.24x10^5 BTU/gal * 1 Lb of coal/1.25x10^4 BTU

US is only 4.5% of the population; it produces ____% of the worlds energy, but consumes ___% of the worlds energy.

17% 20%

In 2008 the US used 99.9 Quadrillion Btu of energy which is about ___% of the total world energy consumption?

20 %

f your instructor weighs 200 pounds, what is his weight in Newtons and mass in Kg?

200lbs/2.2lbs/kg= 90 kg 9.8 x 90 kg= 890 Newtons

According to the US EIA (Energy Information Administration), China's energy use will be double to that of the US in the year ____

2040

If a 774 megawatt power plant is shut down for one day, what will be the loss in revenues if the utility could sell its electricity at 0.11 dollars per kw-hr?

2043360 Dollars 774 megawatt (1000 kW/megawatt) (24 hrs) (0.11 dollars)

A household furnace has an output of 105939 BTU / Hr. What size electrical heating unit (in KW) would be needed to replace this? (think of unit conversions table 3.4 page 86)

31 Kw 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

What is the potential energy of a 70 kg person sitting on a ladder 5 m off of the ground? ANSWER in Joules (J)

3430 joules PE = m * g * h

How much of world's energy is US and China?

40%

A 61 W light bulb is accidentally left on for 2 days in a basement. If electricity costs 14 cents per kilowatt hour (¢/kw-hr), how much did this oversight cost?

41 cents 61 W=0.061 kW 2 days=48 jrs 0.061kW(48hrs)(.14 cents)=41 cents

A U.S Army infantryman goes on patrol and has to climb a 5000ft mountain behind his basecamp in Afghanistan. He weights 200 lbs and carries 75lbs of equipment with him. Determine about how many Calories he will burn to move taht weight to that height.

453.75 Calories 275 lbs x (5000 ft) =1375000 ft-lb (0.33 calories/1 ft-lb)(1 Calorie/1000 calories)=453. 75 Calories

A skateboard with a mass of 2 kg is moving at a speed of 10 m/s on a level surface. It encounters a hill and rolls up until its stops. Ignoring friction, to what vertical height does it rise?

5.1 m 1/2(mass)(velocity^2)=(mass)(9.8m/s)(h) solve for h

If an electric clothes dryer has a power rating of 5716 watts and is used for 2 hour per day, how much money can be saved over 32 days by using the backyard clothes dryer (hang dry on a clothes line). Assume electricity costs 14 cents per kw-hr. (answer should be is dollars $)

51.22 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

If the world use of oil is about 81 MBPD, how many years would you expect this resource to last given the proven world reserves are 1540 x 109 barrels (bbl)?

52.1 years 1540 x 109 / 81,000,000 per day/ 365 days per year

A house in St. Cloud Minnesota in January needs 39907.1 BTU/hr space heating. What fraction of this requirement can be met by a flat plate collector (FPC) of area 724 ft^2? Assume that the collector is tilted at an angle equal to the latitude and the system efficiency is 51.2 %. Use tables in appendix D. (Answer will be in percent) (Keep you units straight)

55 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

For a fossil fuel electrical generating plant, 10000 Btus of chemical energy input into the plant will result in about how many Btus of waste heat dumped into the environment?

6000 Btu *2/3 normally is waste heat

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)

6500 BTUs 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

If 88 Btu of heat energy are added to 8 Lb of water at 55 degrees F, what will be the final temperature of the water (in degrees F)?

66 Use the equation Q = m c (t2 - t1). We are given Q (in Btu), m (the Lb of water), c (for water = 1 Btu / Lb / deg F) and we know t1 (given initial temperature of the water). Plug in all the information and solve for t2.

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 26 degrees C) and rejected as heat to the Hudson River (assume 5 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)

7 % 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.

What is the rate of heat gain in a small hut kept at 75 degrees F when the outside temperature is 104 degrees F? Assume the hut is 10 ft wide by 10 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)

7900 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

If a 71 kg sprinter running at 13 m/sec could convert all of her kinetic energy into upward motion, how high could she jump? This should be in METERS.

8.6 meters 1/2*m*vel2 = m*g*h

Heat loss through windows is substantial. What percent savings will be gained by covering a double pane window (0.5 inch air space) with a 2 inch sheet of rigid polystyrene board? (hint compare the difference in R values for the two conditions)

82 First calculate the R value for the window alone (table 5.2) - 1.72 ft^2-hr-F / BTU Then the new R value for the window plus the 2 inch board = 4.0 ft^2-hr-F / BTU per inch and we have two inches so the R value becomes 8.0 ft^2-hr-F / BTU + 1.72 ft^2-hr-F / BTU = 9.72 You can find the percent savings by looking at the R values: % savings = (New R value - Old R value) / (New R value) * 100% Or (9.72 - 1.72) / (9.72) * 100%

In 2013 domestic energy production supplied what fraction of the nations demand?

85%

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 answer is: Use a window shade, Insulating drapes plus valance, low-e coatings

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?

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.

most electricity comes from

Coal

3 forms of heat transfer:

Conduction, Convection, Radiation

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

forms of radiant solar energy

Domestic Hot Water

efficiency of Solar

Electric output/solar input

Vapor to a liquid

Heat Energy Liberated

According to the book, what is the definition of exponential growth?

Increase in growth is proportional to the amount present

What are three fundamentals of a passive solar house?

Insulation, south facing windows, thermal storage

Units of Energy:

Joules

7 forms of Energy: (Mr Chens)

Mechanical Radiant Chemical Heat Electrical Nuclear Sound

Color absorbs heat most and least?

Most=black Least=silver or white

Force

Newton, Lb

Which of the following is/are a unit of Force? a. kg b. N c. Lb d. m/s

Newtons and pounds

Renewable or Non renewable: Coal

Non

Renewable or Non renewable: Natural gas

Non

Renewable or Non renewable: Oil

Non

Renewable or Non renewable: nuclear

Non

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 38 ft wide, 28 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 {exchanges} exchanges per hour (k value). Assume no losses through the floor. The outside temperature is 42 degrees F cooler than the inside temperature. Considering only conductive heating losses, calculate the heating needs in BTU/hr.

The correct answer is: 3.3e4 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.

Calculate the annual conductive heat loss (Btu) for a house in Buffalo, New York, with 7,000 degree days per year with the following specifications: All exterior surfaces have an R value of 16 ft^2-hr-F/Btu, 30 ft length, 52 ft width and 10 ft height. (do not include the floor in your area calculations)

The correct answer is: 3.4e7 Use the degree day equation: Q total = sum(1/R*A) * 24hr/day* degree days (Chap 5 pg 135) R value is given, for all surfaces Area must be calculated for the four walls and the ceiling All else is given

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 43 ft wide, 27 ft long and 12 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 46 degrees F cooler than the inside temperature. Considering only infiltration heating losses, calculate the heating needs in BTU/hr.

The correct answer is: 3.5e4 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

What methods can be used to reduce infiltration in a house?

This represents the air that leaks in or out of a house. Reduce this leakage and you will reduce infiltration. Weatherstripping doors, Caulking doors and windows, Sealing exterior electrical plates

Power units:

Watts, Ft-lbs per second

Best season for a vertical south facing wall:

Winter

Why is a fishnet type undershirt good to wear in cold weather?

air gets trapped in the small hole and insulates

Three things in a solar heating system:

collection component, storage component, distribution component

Pyronameter lab:

converted mV to W/M^2

Is this a unit of work or power? joules

energy

Is this a unit of work or power? BTU

energy

Is this a unit of work or power? Calorie

energy

Is this a unit of work or power? ft-lb

energy

Is this a unit of work or power? kilowatt-hr

energy

Power is defined as?

energy used divided by time

T/F You can cool a kitchen by leaving the refrigerator door open.

false

weight in lbs=

force

Work done=

force x distance

On a windy day Infiltration_____ on a house.

increases

How does each of the following impact solar photovoltaics? Primarily impacts the current

irradiance

which of the following is/are units of energy? a. watt b. ft lb/s c. joule d. BTU e. kw-hr

joule BTU kw-hr

weight in kilograms=

mass

3 advantages of using an LED bulb over a fluorescent bulb that helps to offset higher investment cost:

more efficient longer lifespan less impact on the environment

Is nuclear a renewable source of energy?

no

Which of the following sources of energy is/are NOT renewable? a. vegetable oil b. nuclear c. biomass d. coal

nuclear and coal

How does each of the following impact solar photovoltaics? Impacts the overall W/M^2 produced

panel orientation

Is this a unit of work or power? ft-lb/min

power

Is this a unit of work or power? horse power

power

Is this a unit of work or power? watts

power

List four arguments for Energy conservation.

reduces pollution saves consumers money can be implemented now prolongs life of finite resources

Renewable or Non renewable: Biomass

renewable

Renewable or Non renewable: Corn Ethanol

renewable

Renewable or Non renewable: solar

renewable

How does each of the following impact solar photovoltaics? Primarily impacts the voltage

temperature

Define Sustainability:

the ability of our generation to meet its needs w/o compromising the ability of future generations to meet their needs

T/F First Law of Thermodynamics states: The change in energy of a system is equal to the heat added and the work done to the system W + Q = ΔKE + ΔPE + ΔTE

true

T/F 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).

During what season will more insolation be delivered to a vertical south facing window on a clear day

winter

Power=

work done/time