Transcript Heat Pumps
Heat Pumps • In a heat engine, heat is converted to mechanical energy by taking advantage of the fact that heat flows from hot to cold. The heat is taken from a source, some of it turned into mechanical energy and the rest sent to a heat sink, which is at a lower T than the source. • Could we reverse this process? Heat pumps • A compressor compresses a gas (Freon) to raise its Temperature and pressure. • It flows through a heat exchanger in which the gas is cooled by room temp air and it condenses. • The heat it gives up in condensing goes to heat the inside air around the heat exchanger. • The gas then passes through a valve to a region of lower pressure where it expands and becomes very cold. • It next passes through a heat exchanger exposed to outside air. The outside air warms the gas and it returns to the compressor and starts the cycle all over again. • Reverse the process for cooling Heat pumps Heat pumps • Effectiveness measured by the Coefficient of Performance • C.O.P. = Th/(Th-Tc) This comes from the Carnot Efficiency • As the outside air gets colder, Th-Tc gets larger to C.O.P decreases. This means heat pumps are less efficient in very cold weather and very cold climates. Usually this occurs when the outside T falls below 15 F. Peltier effect • Peltier was experimenting with electricity • Connected a bismuth and copper wire together and hooked them to a battery. • Found one side became hot and the other cold as the current flowed! • Basis for thermoelectric cooling/heating • Modern devices use semi-conductors (more efficient). • Not efficient enough for large scale heating or cooling Peltier effect Cogeneration • Power plants generate lots of waste heat • Modern coal fired plants convert 38% of the energy in the coal to electricity, the other 62% is waste! • Usually shed off into the environment (air, cooling pond, river, lake etc) • Can have environmental consequences • Can it be put to use? Cogeneration • Problem arises when the power plant is located far away from population centerscannot effectively transport the heat over long distances • In principle, the waste heat could be used to heat a boiler and provide steam for space heating and cooling. • Or it could be recycled to drive turbines to produce additional electricity Types of cogeneration plants • Topping cycle plants - produce electricity from a steam turbine. The exhausted steam is then condensed, and the low temperature heat released from this condensation is utilized for heating. • Bottoming cycle plants- produce high temperature heat for industrial processes, then a waste heat recovery boiler feeds an electrical plant. Need a high initial source of heat-metal manufacturing plants. Examples • The New York City steam system - district heating system which carries steam from central power stations under the streets of Manhattan to heat, cool, or supply power to high rise buildings and businesses. • Another example is in use at the University of Colorado, Boulder - Total efficiency is 70% • Possibility of explosions due to pipe failures exists Example of Explosions • The July 18, 2007 New York City steam explosion sent a geyser of hot steam up from beneath a busy intersection, with a 40-story-high shower of mud and flying debris raining down on the crowded streets of Midtown Manhattan • It was caused by the failure of a Consolidated Edison 24inch underground steam pipe installed in 1924 Possibilities • Outside the U.S., energy recycling is more common. Denmark is probably the most active energy recycler, obtaining about 55% of its energy from cogeneration and waste heat recovery. • In the US about 8% of its electricity is produced via cogeneration Solar Power • Power derived directly from sunlight • Seen elsewhere in nature (plants) • We are tapping electromagnetic energy and want to use it for heating or convert it to a useful form, usually electricity • Renewable-we won’t run out of sunlight (in its current form) for another 4-4.5 billion years Solar Energy • Sun derives its energy from nuclear fusion deep in its core • In the core Hydrogen atoms are combining (fusing) to produce helium and energy. • Physicists refer to this as Hydrogen burning, though be careful, it is not burning in the usual (chemical) sense. • The supply of H in the sun’s core is sufficient to sustain its current rate of H burning for another 4-4.5 billion years Solar Energy • The energy is released in the H burning deep in the sun in the form of photons. • Here we use the particle description of light, where light is considered a packet of energy called a photon. • Photons have energy E=hν or E =hc/λ where ν is the frequency of the light, λ is the wavelength of the light, c is the speed of light (c=3.00x108m/s) and h is Planck’s constant (h=6.626068 × 10-34 m2 kg / s)