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 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 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)