Lecture XIII

Physics 367

Conservation: An Important Energy Source



Many of us think conservation means ``doing without''. This is a misconception! Conservation measures help us

do more with the same amount of energy
do the same amount at a lower cost

Or

achieve a similar or better benefit at smaller energy cost

This is something everyone can relate to. The 55 mph speed limit was a conservation measure! If we could convince ourselves that 55 mph was as good as 65 mph or higher, then we would force automakers to produce slower more efficient cars (Europe) or greatly increase taxes on faster cars.

SUV's are proof we are not there yet. Car makers surcharged SUVs $15,000; they still sold!

Why conservation?

America imports ~ half of its oil.
Over half of our balance of payments deficit is due to money we pay for oil.
Deficits are partly responsible for loss of markets to China, Japan and Europe.
Oil addiction costs money we spend billions on military presence in the Persian Gulf because we, Europe, and Japan buy Arab oil.

Oil and coal are resources for energy.

Conservation allows us to save some of these resources until later.

Conservation can reduce the building of new power plants. We would have had to have twice as many power plants we have if Americans energy use patterns before 1973 had not changed as a result of the Arab oil embargo.

Every power plant saved means:

less pollution we have to live with.
large amounts of oil and coal saved
smaller greenhouse warming.

Savings in Illumination

5-10% of all energy generated in the US is spent on lighting. This is a large number. Estimates indicate that half of the lighting electricity in North America could be saved if the most efficient measures were used.

What can you do?

Turn out the lights when you leave a room for ____ min?

Use fluorescent bulbs
Incandescent bulbs contain a tungsten filament in a vacuum. A current through the filament heats it red hot and it emits light. A 100W bulb puts 95% of its power into heat and 5% into light. In fluorescent bulbs, a mercury gas excited by an electric discharge emits UV. A phosphor coating on the tube absorbs UV and emits Visible light. In a fluorescent bulb 80% of power goes into heat, 20% into light.

 Use reflecting fixtures - this can double available light

What else can you think of?

Savings in Appliances

Refrigerators and freezers are on all the time, and they last typically 20 years. Even small savings per refrigerator can generate large savings when considering the number of households.
   Efficiency
Appliance Lifetime (yr) Average New Best
Refrigeratora 19 1,134 979 515
Room Air Conditionerb - 7.2 8.1 NA
Central Air Conditionerc 14 7.9 9.0 16.9
Heat Pumpc 14 8.4 8.7 16.4
Electric Water Heaterd 13 0.82 0.84 3.5
Gas Water Heaterd 13 0.49 0.51 0.76
Gas Furnacee 23 0.66 0.75 0.97
Oil Furnacee 23 0.76 0.80 0.95

Savings in Heating and Air Conditioning

Space heating and air conditioning account for 20% of US energy use.
Space heating, water heating, refrigeration and cooking account for 95% of commercial energy use.
Space heating, water heating, refrigeration and cooking combined account for 85% of residential energy use.
Second law efficiencies are still very low! Why?
Use % US fuel 2nd law eff
Space heat 18% 0.06
Water heat 4% 0.03
Air conditioning 2.5% 0.05
Refrigeration 2% 0.04

Energy savings in the home

Heat leaks out of homes through walls (17%), roof (12%), doors, and windows via heat transfer: conduction, convection, and radiation of energy.

Conduction of thermal energy can take place when heat transfer causes atoms at the hot end of a solid to move more agitatedly about in their positions. Since the atoms are in contact with nearby atoms, they [in time] transfer kinetic energy to those nearby atoms. They in turn affect atoms farther away from the hot area. Eventually, there will be a gradual decrease in temperature of the solid from the hot to the cold end.

Insulation keeps the indoor temperature higher than the outside temperature at a lower energy cost. The R-value of insulation (dx/k) tells how effective it is at doing this. Windows leak more heat than walls because the layer of glass is thinner than the walls, and the R-value of glass is low. The transfer of heat through windows takes place by conduction, convection, and radiation.

Super insulated homes can cut heating costs by a factor of 2!

Heat Loss

Convection is the transfer of energy through transfer [or exchange] of actual hot material to a colder region. For example, a gas furnace uses forced convection to bring air heated in the furnace into living areas of the house. In the case of the window cited above, convection can increase the rate of heat transfer out of a window. A layer of air outside the windowpane is heated. If it stayed in place, it would decrease the effective temperature difference, and decrease the rate of heat transfer. Normally, though, since it is warmer than the surrounding air, the warmed air rises. Its place is taken by colder air. This can increase the effective temperature difference between inside and outside. A wind moves air out of the way and can increase the heat transfer rate, too.

Exchange of air in homes accounts for 40% of the heat loss in homes.

Radiation is the transfer of heat by emission of electromagnetic radiation by hotter materials and absorption of electromagnetic radiation by colder materials. Sunlight coming through a window can heat a room; light and infrared radiation escaping through a window cools a room.

Conservation in action

How can you help?

Do not use energy you do not need to use
Have you ever walked into a house during winter that was so hot you wanted to open the windows? That is a waste of precious energy (and money).

Don't overheat or overcool your house.
Get a thermostat that turns your heat down at night and turns it on again in the morning just before you get up. New sensors can sense the temperature in each room of a house and adjust equipment accordingly.

The geometry of your house could save energy
Have overhangs built when you build to keep the sun out in summer but allow the winter sun access. How? Plant trees that shed their leaves in winter allowing sunlight to warm your house, but are in full leaf in summer, intercepting sunlight.

 Turn off the lights if you plan to be out of the room more than a minute.
Replace incandescent bulbs with fluorescent bulbs. Replace old ballast with new efficient ballast. Don't leave the TV on if no one is watching.

Buy energy-efficient appliances when you get new ones.
New refrigerators are being built that use less than half the energy for the same amount of storage than a refrigerator built just two decades ago. Even if the efficient one costs more, you can save money (and energy) by buying efficiency. A typical refrigerator uses about 150 kWh per month. I pay about 10 cents per kWh, so that cost is about $180 per year just to run the refrigerator. A refrigerator may last 20 years. One using half as much energy saves $90 in electricity bills every year. Over the 20 years you have it, it would save $1800 in electricity costs. It would be worth it to spend an extra $100 or $200 to buy the most efficient refrigerator.

Similar things could be said for room air conditioners. The more expensive air conditioner could actually be the cheapest to own over a lifetime of use. A study done some years ago revealed that air conditioners on the market that cooled air the same amount differed in their efficiencies by over a factor of 2. At today's electric rates, this could add up to an extra $80 more spent for electricity every year by the person purchasing the least efficient unit.

New furnaces are much more efficient than older models. If you need to replace your furnace, make sure to buy the most efficient model. This can dramatically reduce winter heating bills. In areas where there is no gas, consider heat pumps. Heat pumps use the energy in outdoor air, add work, and bring higher temperature air into a room. Heat pumps can operate in reverse in summer, air conditioning living spaces. Heat pumps can be the best choice where the difference between outdoor air temperatures and indoor temperatures is relatively low. Ground-water heat pumps are the most efficient types of heat pump.

Finally, consider gas mileage when you replace your car. We are still vulnerable to pressure from oil producing nations. Some experts predict the world to run out of oil by 2050 if nothing is done. By buying an efficient car, you help the environment now and protect yourself in case the price of gas skyrockets again. Some cars sold now get over 50 miles per gallon!

Industry is the largest energy user, and therefore has the largest savings attached. Since 1973, the efficiency of industrial energy use has increased dramatically. There is still a long way to go.

Problem of the day

Suppose a power plant which produces a total of 1000 MW is 35% efficient. Water (rho=1000 kg/m3) passes through a condenser at a rate of 31 m3/s. What temperature increase, in °C, is expected in the water?

So what do we do?

35% efficiency 65% goes into heat! So we have to remove

P = Q/s = 650 MW = 650 x 106 J/s

Now water can absorb

so to remove heat flow water

mass/sec = (1000 kg/m3)(31 m3/s)
= 31x103 kg/s

and