ASHRAE Journal - July 2020 - 33

TECHNICAL FEATURE

FIGURE 1 Energy in everything above absolute zero.

Steam Condenses (1,526)

1,500

Solid
°R 0 400
°F -460

450

i
Typ

ify

mid

per

atu

hu
De

Tem

144 Matches Takes 1 lb
of Water from 32°F Ice
to 32°F Water

One Match

Tem
cal

ure

144 Matches

cal

Ice Melts (202)

at
per

A.
One Match Takes
1 lb of Water from
32°F to 33°F

Ofﬁ

144 Btu/lb

One British thermal unit (Btu) is the amount of energy that will raise the temperature of 1 lb
of water by 1°F.

Typ
i

Btu/lb

Liqu

Water Freezes (346)

300
200

Vapor
-1,000 Btu/lb

Water Boils (526)

500
400

FIGURE 2 Water's phase change (Btu).

500
550
32 44 72

600

650

700
212

H2O at 1 atm.

Water goes through a major phase change, storing lots of energy, at almost exactly the
best temperature for human comfort conditioning.

558°R (98.6°F or 37°C), we like our buildings at 530°R
(70°F or 21°C), we supply liquid to cool buildings at
about 500°R, or 40°F (5°C). It's hard to imagine "cold"
40°F (5°C) water as having a lot of energy in it, but at
500°F (277°C) above absolute zero, it clearly does. "Cold"
and "warm" are relative. Figure 1 illustrates two types
of energy in materials, sensible and latent. Sensible
energy is the energy absorbed or released by a material as it changes temperature. In the case of H2O it is
about 0.5 Btu/lb (1.2 J/g) in the solid phase and 1.0 Btu/lb
(2.3 J/g) in the liquid phase. Latent energy is the energy
needed to change the phase of a material (solid to liquid
or liquid to gas) without a change in temperature.
A more accurate naming of "heat pumps" is "energy
pumps," because they extract energy from one temperature level and elevate (pump) it to a higher temperature level. And like water, energy naturally only
ﬂows "downhill" from a higher temperature to a lower
temperature.

Big Chillers are Big Heat Pumps
Traditional heat pumps are reversible, so the heat
exchangers can be used as condensers or evaporators,
depending on which way you want the energy to go.
Chillers are heat pumps; the only fundamental difference is that they only pump the energy in one direction, from an evaporator to condenser. As an example,
in a heat pump used to heat a building, energy can be

pumped from an outdoor airside evaporator to waterside condenser; to cool the space, this is reversed, so
energy can be pumped from the indoor waterside evaporator to the outdoor condenser. The energy is pumped
into or out of the space depending on the need.

Thermal Energy Storage
Society has stored thermal energy extensively for
thousands of years. Many HVAC systems use ice or
chilled water storage to cool buildings, but is freezing a tank of water really storing energy? The accurate
answer is actually no. It's the reverse. To demonstrate,
let's go back to basics. In Figure 2a you see 1 lb (0.5 kg) of
liquid water at 32°F (0°C) and a wooden match burning beneath it.
It turns out that a wooden match, when burned completely, releases about 1 Btu (1055 J) worth of energy,
so, by deﬁnition, the heat from one match can raise
the temperature of a pound of water by 1°F (0.6°C). So,
what happens when one has ice at 32°F (0°C)? H2O goes
through a phase change at this temperature. In Figure 2b,
you see the same 1 lb (0.5 kg) of H2O, but in solid phase,
so imagine 16 one-ounce (28 g) ice cubes at 32°F (0°C).
If you burn the 144 matches, and all that energy goes
into the ice, all the ice will melt. So as odd as it sounds,
melting ice is actually absorbing, and storing, massive
amounts of energy. Figure 3 shows the latent energy storage capacity of a typical thermal storage tank based on
water's 144 Btu/lb (335 J/g).
Changing ice to water is not the only way to store thermal energy in association with heat pumps. The most
common way is to store it in the ground, with either
deep vertical pipes in wells or horizontal ﬁelds of buried pipe used to convey the stored heat from above- to
J U LY 2 0 2 0

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