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

Effectiveness measured by the Coefficient of Performance C.O.P. = T h /(T h -T c ) This comes from the Carnot Efficiency As the outside air gets colder, T h -T c 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 centers- cannot 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 24- inch 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 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 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=3.00x10 8 m/s) and h is Planck’s constant (h= × m 2 kg / s)