Energy and Human Ambitions on a Finite Planet, 2021a

6 Putting Thermal <strong>Energy</strong> to Work 99
mode. 54 Houses equipped with electric heat pumps can typically be run
for both cooling <strong>and</strong> heating applications, making them a versatile <strong>and</strong>
efficient solution for moving thermal energy in or out of a house.
Heat pumps leveraging the moderate-temperature ground just below
the surface as the external thermal bath are called “geothermal” heat
pumps, but have nothing to do with geothermal energy (as a source).
Compared to heat pumps accessing more extreme outside air temperatures,
geothermal heat pumps benefit from a smaller ΔT, <strong>and</strong> thus
operate at higher efficiency.
54: EER <strong>and</strong> HSPF numbers are “inflated”
by a factor of 1/0.293 ≈ 3.41 compared to
COP due to the unfortunate choice of units
for EER <strong>and</strong> HSPF.
6.6 Upshot on Thermal <strong>Energy</strong>
Sometimes we just want heat. Cooking, home heating, <strong>and</strong> materials
processing all need direct heat. Burning fossil fuels, firewood, biofuels,
extracting geothermal energy, or simply letting the sun warm our houses
all directly utilize thermal energy. Specific heat capacity tells us how
much thermal energy is needed to change something’s temperature,
using 1,000 J/kg/ ◦ C as a rough guess if lacking more specific information.
55 We also saw how to estimate home heating dem<strong>and</strong> using a
metric of heat loss rate, such as 200 W/ ◦ C.
But it turns out that we use heat for much more than this. 84% of our
electricity is produced by heat engines, using heat flow to drive a turbine
to turn a generator. The maximum efficiency a heat engine can achieve
is set by limits on entropy <strong>and</strong> amounts to ε max < ΔT/T h , although in
practice we tend to be a factor of two or more short of the thermodynamic
limit. 56 In any case, thermal energy plays a giant role in how we run our
society.
Heat pumps are like heat engines in reverse: driving a flow of thermal
energy against the natural hot-to-cold direction by putting in work. Any
refrigeration or cooling system is likely to use this approach. 57 Because
heat pumps only need to move thermal energy, each Joule they move can
require a small fraction of a Joule to accomplish, making them extremely
clever <strong>and</strong> efficient devices.
55: Or we frequently use water’s value at
4,184 J/kg/ ◦ C, connected to the definition
of a kcal.
56: Typical efficiencies are 20% for cars <strong>and</strong>
35% for power plants—compared to 60%
theoretical.
57: A notable exception is evaporative cooling.
6.7 Problems
1. How many Joules does it take to heat your body up by 1 ◦ C
if your (water-dominated) mass has a specific heat capacity of
3,500 J/kg/ ◦ C?
58: We only consider the air for this problem,
<strong>and</strong> ignore other objects—including
2. How long will it take a space heater to heat the air 58 in an empty
walls <strong>and</strong> furniture—that would add substantially
to the time required in real life.
room by 10 ◦ C if the room has a floor area of 10 m 2 <strong>and</strong> a height of
2.5 m <strong>and</strong> the space heater is rated at 1,500 W? Air has a density 59 59: Use density to get at the mass of air.
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.