Heat%20pumps

1
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.

2
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 modern thermoelectric cooling/heating
• Modern devices use semi-conductors (more
  efficient).
• Not efficient enough for large scale heating or
  cooling
• Creates a lot of excess heat

3
Peltier Effect

• Right hand junction is heated-electrical energy
  is converted to heat
• Left hand junction is cooled-heat energy is
  converted to electrical energy.

4
Peltier effect
5
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?

6
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

7
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.

8
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

9
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

10
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