Lesson 18: Energy Efficiency PDF

Lesson 18: Energy Efficiency Overview The energy flow diagram for the U.S. discussed in Lesson 5 showed that roughly two thirds of the nation's primary energy consumption is lost as wasted energy. If this energy consumption could be made more efficient, the U.S. would be able to use less energy for...

Lesson 18: Energy Efficiency Overview The energy flow diagram for the U.S. discussed in Lesson 5 showed that roughly two thirds of the nation's primary energy consumption is lost as wasted energy. If this energy consumption could be made more efficient, the U.S. would be able to use less energy for the same amount of energy services with less greenhouse gas emissions and/or use the saved energy to make these services even more widespread for people across all economic levels. One target that could yield among the greatest amount of energy savings through efficiency improvements is buildings, the main energy sink for the residential and commercial sectors. This lesson begins by reviewing how buildings lose/gain heat to/from the outside. Heating and cooling technologies are then reviewed, along with the principal metric used to estimate a building's heating and cooling needs, the degree day. This is followed by a review of the potential energy savings for the U.S. if all residential, commercial and even industrial buildings were upfitted with the latest energy efficient building materials and appliances. And while the savings are huge, the lesson ends by pointing out that the impact of efficiency in reducing energy use has limitations, and in fact, can counterintuitively lead to even greater energy use for certain technologies, such as lighting. Lecture Outline 1. The energy use by both the residential and commercial sectors is primarily for buildings a. When combined, buildings account for 40% of US primary energy consumption and 70% of the nation’s electricity use b. Biggest energy draws for buildings is space conditioning; both heating and cooling 전도 대류 2. Building conductive and convective heat losses a. Buildings lose or gain heat from outdoors by conduction and convection b. Conduction i. Heat moves through building walls, windows, floor and roof by transfer of  internal energy between matter 1. There is no macroscopic movement of energy 2. Only energy transfer at the atomic and subatomic levels ii. Conductive heat loss is a function of 1. Building area 2. Building material thermal conductance 3. Time, and 4. The difference between the inside and outside temperatures iii. Inverse of thermal conductance, U, is thermal resistance or R 1. The higher the R rating of building material, the more insulating it is 2. The higher U, the more easily the building lets in or out heat iv. Outside-inside temperature difference key 1. When outside temperatures mild, not much inside-outside difference, so minimal heat transfer 2. When outside temperatures hot or cold relative to inside, heat gain or loss inside building is high c. Conduction i. This involves heat loss or gain due to physical movement of air out of or into building ii. Dependent on… 1. Interior volume 2. Inside-outside temperature difference 3. Number of times air is cycled through the building to keep the air fresh and cut down on airborne disease a. This variable is quite important b. Ventilation needed to for healthy interior enviro  c. In commercial buildings, ventilation – not heat loss due to it but energy use to drive it – is the third greatest source of energy consumption 3. Basic heating and cooling technologies a. Heating i. Furnace ii. Electric heater b. Cooling i. A/C 1. Operation is reverse of a heat engine, such as in a power plant 2. Accomplished using a refrigerant a. E.g., pure ammonia i. Boils at -28°F or -33°C b. Refrigerant is pumped into a warm area as a liquid i. E.g., the inside of a house or the inside of a fridge c. Absorbs heat in the warm area, turning into a gas d. Then compressed and pumped outside i. Either outdoors for say a home or the back of the appliance for a fridge e. The pressurized gas releases heat to the outside enviro and cools f. It is then passed through an expansion tube back into the enclosed area, which rapidly cools it back into a liquid g. Cycle is repeated 3. Only main energy draw is the electricity needed to pump and compress the refrigerant a. This energy draw is less than the heat removed from the  enclosed space b. As a result, the energy removed is less than the energy expended c. Efficiency is still not greater than one; it’s just that the phase changes of the fluid is doing most of the work 4. Estimating building energy use using degree days a. Recall that the difference between temperatures inside and outside a building is a major factor in unwanted heat gain or loss via both conduction and convection i. The smaller this difference, the smaller the inside-outside heat exchange ii. As a rule of thumb, when the outside temperature is 65°F, there is no building heat loss or gain 1. This is because an additional heat source is heat emanating from appliances and people 2. When this extra heat source is added to 65°F, inside temperature rises to ~72°F b. At temperatures >65°F or