H2Tech - Q3 2021 - 37

HYDROGEN STORAGE
startup. The resulting storage capacity required is on the order
of 0.5 Bft3
-4 Bft3 (14.2 MMm3
increased to whatever a cycling
plan may require, assuming
that the economics
support the additional capital
expense.
Round trip efficiency.
-113.3 MMm3), which is well
within the cavern feasibility range of most salt deposits.
With additional electrolysis capacity, cycling time can be
(e.g., has a more flexible duty cycle), giving it a more competitive
round-trip efficiency for needle peakshaving.
Also, round-trip efficiency is primarily dependent on
TABLE 2 also illustrates the
weakness of HES that its
low round-trip efficiency
makes HES uncompetitive
with batteries for shortduration
service (< 10 hr).
Existing battery technologies
are very efficient (75%-90%) across their design basis
duty cycles of 4 hr/d-8 hr/d, depending on technology and
configuration. Batteries are increasingly less efficient as the
duration of storage period increases beyond 8 hr-10 hr due
to self-discharge over time, degradation of performance with
use, and depending on ambient temperature conditions. The
economic round-trip efficiency of batteries decreases further
if the economic discharge period is less than the discharge
duty cycle.
H2
energy content of H2
efficiency remains constant indefinitely. The only loss of efficiency
with time is due to volumetric losses due micro-permeation
of H2
in storage does not degrade with time.
Also, since the energy content of H2
does not change with time, its energy
efficiency remains constant indefinitely.
H2
choice of combustion technology. TABLE 3 compares roundtrip
efficiency for the component
technologies in
various equipment combinations.
Note that the
increase in round-trip efficiency
of the waste heat
from the generation cycle
can be captured and used
beneficially (e.g., combined
heat and power, or " CHP " ).
Preliminary studies of
the levelized cost of energy
(power), or LCOE,9
show
the comparative cost impact of round-trip efficiency, using fuel
cell generation, vs. duration of storage (FIG. 9). H2
fuel cells,
coupled with geologic storage, have a flat to slightly declining
levelized cost of energy from 0 d-7 d. Batteries, due to self-discharge,
are advantaged below a storage duration of 13 hr. Over
13 hr, H2
technologies are increasingly favored.9
Cycling limitations. The rate of HES cycling is constrained
by the rate at which H2
in storage does not degrade with time. Also, since the
does not change with time, its energy
through containment materials over long time
periods (months to years). Depending on generation technology
choice, the resource can be rapidly ramped up and down
TABLE 2. Seasonal operating scenario
Injection/generation periods
Spring injection
Days
92
Summer dispatch
Power generation
Injection
Fall injection
Winter dispatch
Power generation
Injection
Ratio of charging to dispatch
Total energy shifted (generated)/yr
Total energy consumed (load)/yr
H2 storage capacity
(Bft3
at 60°F - 1 atm)
Round trip (charge/discharge) efficiencyb
122
122
61
90
90
Hr/d
24
4.5
14
5.5
24
4
20
4.15
18,522 MWh
85,008 MWhr
65,000 kg H2
(0.027 Bft3
)
21.8%
inventory can be replaced
(e.g., charging), the variable cost of generation equipment
maintenance affected by starts and stops, and the ramping
characteristics of the generation technology.
Ramping characteristics vary with electrolysis technology
and standby condition. PEM electrolyzers can reach full power
in 1 sec ( " hot standby " ) to 5 min (from off condition). Other
electrolysis technologies, which operate at higher temperatures
and pressures, can take up to 30 min to reach full power.
20.8 MWea
2,156 hr
501 hr
10,410 MWhr
Standby
1,540 hr
390 hr
8,112 MWhr
47.3 MWe
2,208 hr
55,862 MWhr
550 hr
23,523 MWhr
1,708 hr
43,212 MWhr
Standby
1,464 hr
37,039 MWhr
360 hr
18,330 MWhr
1,800 hr
45,540 MWh
7.9
41,854 MWh
181,653 MWh
2,408,000 kg H2
(1 Bft3
)
23%
a Variance in charging and discharging due to use of standard containerized equipment packages vs. field-erected facilities for smaller sizes
b Assumes simple-cycle gas turbine; combined-cycle and use of larger units would increase efficiency by 5%-32%
H2Tech | Q3 2021 37
236.5 MWe
2,208 hr
248,842 MWhr
550 hr
101,601 MWhr
1,708 hr
192,491 MWhr
Standby
1,464 hr
164,992 MWhr
360 hr
79,100 MWhr
1,800 hr
202,860 MWh
7.9
180,772 MWh
809,185 MWh
9,632,000 kg H2
(4 Bft3
)
22.3%

H2Tech - Q3 2021

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