ASHRAE Journal - October 2019 - 66

COLUMN ENGINEER'S NOTEBOOK

in urban areas. The equipment and installation
TABLE 1 Energy storage characteristics of gasoline compared with lithium ion batteries.
costs and land-use easement costs for the underGASOLINE
LITHIUM ION BATTERY
ground 300 kV transmission lines from remote
Energy Density
12.9 kWh/kg
0.25 kWh/kg
power stations were significantly greater than the
Round-Trip Efficiency
One-Way
85% Two-Way
underground gas pipelines. Co-location of the
End-Use Conversion Efficiency
20%
93%
power generation with district thermal plants and
Lifespan 1 year (Tank 20 Years)
500 Cycles
sewage treatment plants, allowed recovered heat
from power generation to be utilized for generating
Speed of Charge
4,500 kW (Gas Pump)
7 - 250 kW
district cooling and domestic hot water, while sewInput/Output Form
Liquid-Heat/Kinetic
Electric-Electric/Kinetic
age treatment plant effluent could be used for coolMining, Electric Sourcing
Environmental Overhead CO2 Generation; Drilling
ing tower makeup.
Transport includes moving energy from its initial
how much end-use activity can occur between "refuelsource to its ultimate end use. Along the way, convering" connections? Energy storage technologies have a
sions may be necessary to traverse obstacles and connumber of important metrics, including:
duits specific to certain segments of that path. To some
* Energy density;
extent transport issues are related to the energy density
* Round Trip Storage efficiency (ratio of net energy
issue discussed below in Energy Storage. Efficient trans- extracted from medium to the gross amount of energy
port of energy over anything beyond a short distance
required to recharge to same level);
requires a certain minimum energy density. A good
* Initial conversion efficiency;
example of this issue is the utilization of low-grade
* Lifespan (degradation rate per recharging cycle);
waste heat (<150°F [<66°C]) from cogeneration. Unless
* Speed of charge/discharge;
the cogeneration plant is located immediately adja* Storage energy input/output form; and
cent to the thermal consumer, transport energy will
* Environmental overhead for storage medium.
be greater than the value of the recycled "waste" heat.
Evaluation of storage technologies should account
Similarly, utilization of agricultural waste for cellulosic
for all of these factors. The comparison of gasoline to
ethanol production requires location of the ethanol gen- lithium ion batteries for automobiles, shown in Table 1,
erator immediately adjacent to the source of agricultural shows the characteristics of each technology.
waste. The energy density of the waste is so low that the
Currently, high density forms of energy storage have
energy required for transport over more than a short
significant disadvantages in other categories. Processed
distance will be greater than the energy recovered from
nuclear fuel, currently, the densest available storage
the waste.
medium, requires enormous infrastructure for conversion. Nuclear fueled iPods will not be available in the
Storage Efficiency
near future. Hydrocarbon fuels also have a very high
Energy Storage
energy density but have significant environmental
Just as there are spatial discrepancies between energy
disadvantages.
sources and end uses, so, also, are there temporal disPerhaps the greatest opportunity for reducing the
crepancies. Temporal discrepancies may involve disenvironmental impact of our energy habit lies with
continuity between supply rate and consumption rate
electricity storage. Increasing percentages of renewable
as often happens with renewable sources or may involve energy in the electrical supply grid increase the need
connection discontinuity between stationary sources
for energy storage. Rapid weather changes quickly alter
and mobile uses such as automobiles. The amount of
solar or wind production, requiring rapid response from
energy storage required depends upon the desired max- backup generation sources to match the changes from
imum aggregate deviation of the supply source from the the renewable assets. The nimblest load tracking genend-use consumption. In the case of a closed-loop, con- eration technologies, gas turbines, sometimes cannot
nected system with a random supply source, how much
match the fluctuations of the renewable source, resultusage overage must be accommodated before the source ing in poor power quality, voltage and frequency varia"catches up"? For an open loop or disconnected system,
tions. Utility scale electrical storage, with high charge/
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