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 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
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.
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