
36 ISE Magazine | www.iise.org/ISEmagazine
The potential energy savings of heat reuse
liquids (NGL) ethane, propane, butane
and pentanes from the natural gas stream
is a main process prior to transportation
to consumers. It leaves only methane in
the natural gas stream as a requirement
to what is called “pipeline quality.” The
separated natural gas liquids have a high
monetary value when sold separately and
used as energy sources, raw material for
petrochemical products and in oil en-
hanced recovery – a process that injects
water or gas into depleted oil reservoirs
to increase pressure and extract more oil.
Cooling the natural gas stream is a
requirement for the separation of NGL.
This is conventionally done using com-
pression chillers that produce the re-
quired cooling through a compressor that
exerts mechanical work on the refriger-
ant (Freon); the cooling cycle of com-
pression chillers is akin to the air condi-
tioning cooling process. The compressor
consumes a large quantity of electricity,
making this process expensive.
To save energy consumption in the
cooling process, absorption chillers have
been used. Waste heat recovered from
a gas turbine exhaust is used to operate
these chillers instead of the compressor
used in compression chillers. Since there
is no compressor in the cooling cycle of
the absorption chiller, only a pump to
circulate the refrigerant, electricity con-
sumption is one-tenth that of the com-
pression chiller.
There are two kinds of absorption
chillers: one uses water and lithium bro-
mide (H²O-LiBr), the other water and
ammonia. For the former, water is used as
a refrigerant and lithium bromide as the
absorbent; ammonia is the refrigerant and
water is the absorbent for the latter. The
working principles of absorption chiller
are simple: The evaporator allows the re-
frigerant to evaporate, due to the pressure
difference between the evaporator and
condenser, and to be absorbed by the ab-
sorbent, a process that extracts heat from
the chilled water loop. The combined
fluids then go to the generator, which is
heated by the waste heat stream, driving
the refrigerant back out of the absorbent.
FIGURE 1
Heat pump cuts energy loss
Using a fossil-fuel fired boiler to generate 1 million BTU of thermal energy or a heat pump
with a source of waste heat can produce the same output.
Data centers a viable source of reusable heat
One major source of wasted heat occurs in data centers, where 98% of energy is lost in
the form of low-grade heat. As a result, some companies are looking at ways to reuse a
portion of this energy by transforming the wasted heat into usable energy.
Some examples cited in a recent Data Center Knowledge article include IBM in
Switzerland reusing such heat to warm a swimming pool; Finnish data centers by
Yandex and Academica using heat to warm 500 to 1,000 residents’ homes; and
Amazon reusing its data center heat for a Seattle biosphere project.
The two main challenges are converting low temperature heat to useful energy, then
finding a way to transport it effectively. For the former, highly efficiently heat pumps have
been used to take waste heat that originally measures between 80 and 95 degrees F
(28 C) and increase it to 130-160 F. To transport it, waste heat can be transferred to a
liquid medium and used in industrial process or district heating needs.
Capturing waste heat for such use, however, is not easy or inexpensive, and involves
insulated ducting or plumbing. As a result, much of the heat reuse from data centers
thus far involve venting it to a nearby greenhouse or other building in its pure form.
Yet waste heat can also serve as a source of income to help defray such expenses.
A sizable data center (1.2 megawatt) that sells its waste heat could bring in as much as
$350,000 per year. And there are the environmental benefits. A data center of that size
could save nearly 6,000 metro tons of CO2 per year through such recycling.
CyrusOne recently partnered with the municipality of Haarlem, The Netherlands,
and a local business park to research capturing waste heat from its Amsterdam data
center to help heat 15,000 homes. The process involves using heat pumps and
refrigerant gas to capture and transfer the heat, allowing the municipality to cut back
on petroleum fuel use.
“It is crucial that we build data centers that work in a way that is compatible with
a sustainable future,” said Matt Pullen, CyrusOne’s executive vice president and
managing director of Europe.
Sources: www.datacenterknowledge.com, www.datacenterdynamics.com