11 tonne petroleum equivalent (TPE), as used in renewable energy, 45.217 GJ. • Finish Reading Chapter 4 and Read Ahead Chapter 5• Homework #5 due today at 5 pm• Homework #6 available and due next Wednesday• Make sure you have Research Project Topic - start finding resources• For Friday’s class, make sure to read the article on the Solar Grand Plan and be prepared for discussionDaily Solar Radiation per Area (Energy / meter2in one day) Daily Solar Radiation per Area (Energy / meter2in one day)Thus, the power in one square meter is:P/Area = (5000 Watt-hours/m2)/24 hours= 208 Watts/m2Boulder ~ 5.0 kWh/m2This is the energy in one square meter during a 24 hour time period.Notice that we had 1400 Watts/m2as the solar constant.•Reduce by factor x2 for atmosphere.•Reduce by factor x3 for 8 hours of sunshine/24 hours.Now we are pretty close. Beware of these big factors!In Boulder, 1000 W/m2in summer at mid-day.300 W/m2in winter at mid-day.Note that this is now the energy / area for a full year !Intensity (energy/time/area) x Time = Energy / Area2Is this a little or a lot of energy? Let’s calculate!Globally people consume 4 x 1020Joules/year (~ 400 QBTU/yr).The surface of the entire earth = 4 π R2= 5 x 1014m2.Assume the year and world average sunshine is 160 Watts/m2Energy/year = (160 W/m2)x(3 x 107seconds/year)x(5x1014m2)Energy/year = 2 x 1024Joules/year! Good news. If we just cover every square inch of the entire earth with solar panels that are 100% efficient, we can grow ourenergy usage by a factor of ~ 5000!Or maybe not?Earlier in the semester we calculated how much of the United State we might need to cover with solar panels to meet our needs. It was a big chunk of real estate.But perhaps solar power can help solve the energy problem, even if it is not a practical complete solution…Start by discussing different forms of “direct solar energy.”Passive Solar HeatingAll houses have some form of passive solar heating.Some houses are specifically designed for it !Note that you also lose heat through windows, so you need to do a careful accounting.An average home uses 106BTU / day for heating in winter.Insulation:very important! Keep heat in in the winter (Chp 7).Collection:getting solar radiation energy into the house (window design and orientation).See example 4.2 in the textbook.Storage:need a “thermal mass” inside the home to regulate temperature. Warm “bricks” during the day, and then they slowly release their stored heat energy at night. Material choice is key. What is good (62 BTU/ft3/degree F).Brick ~ 25, Wood ~ 29, Concrete ~ 20.Things like air and metals are bad.Semi-Active Solar HeatingHow do the numbers work?We will use the Boulder numbers of 200 W/m2averaged over the whole year.Imagine you want to heat 100 gallons of water a day from 50 to 120 degrees Fahrenheit for your household needs.Recalling that there are 8.3 pounds per gallons of water, how large a solar panel do you need on your roof assuming 50% efficiency? Also need 1055 Joules/BTU.Start with how many Joules of energy are needed each day?A) 105JoulesB) 106JoulesC) 107JoulesD) 108JoulesBTUFgallonlbsgallonsEnergy 000,50)70(3.8)100( ≈×⎟⎟⎠⎞⎜⎜⎝⎛×=JoulesBTUJBTUEnergy710511055000,50 ×≈⎟⎠⎞⎜⎝⎛×=x2 eff.3So we need 108Joules of energy incident on our panels per day. Using our value of 200 Watts/m2how large a panel will be a good match to our requirements.A) 1 square meter B) 10 square metersC) 100 square meters D) 1000 square metersEnergyAreaTimeAreaPower=××⎟⎠⎞⎜⎝⎛)()(()JoulesAreaondsmW8210)()sec606024(/200 =××××⎟⎟⎠⎞⎜⎜⎝⎛×××⎟⎠⎞⎜⎝⎛×××≈×××=11128810101010106622130302420010Area2310101001mArea ≈×≈Clicker QuestionSolar Panels of 10 square meters is about 90 square feet.Thus, one panel 9 feet by 10 feet would do the job nicely.Not at all unreasonable….Panels cost ~ $10/ft2, and many $1000’s to set up. Typical is $5000 for a house. This says you might have a 5-10 year payback time.5x107Joules/day Æ 15 kWatt-hour/day. $1.25 per day saved in power costs!4,000 days Æ 10 years!In Winter solar intensity is down by another factor 2-3. You also need some type of backup for sustained poor weather.You will also need storage for nights (and cloudy days).Æ Again moderate storage of heat energy can be achieved with properly insulated materials (e.g. water, bricks,
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