Environmental Engineering Reference
In-Depth Information
impedance shows the need for standardized specification. In Table 10.10 the spe-
cific energy and power metrics use total cell or module mass versus active material
only mass. This aspect of capacitor specification also requires standardization in
the future so that comparisons are not biased. The final column in Table 10.10
reflects the need for careful selection of the rated current for the device. Note that
the D Cell has a charge and discharge efficiency of 88%, whereas the other devices
have much lower charge and discharge efficiency. Had the rated current been more
clearly identified in the data sheets, the efficiency would be higher than the values
given for maximum discharge current.
The industry must also reach consensus on the specific power metric, P , and
whether to use P ML or P 95 or some other value as done in the Ragone chart in
Figure 10.23. When the matched impedance power is used to determine capacitor
specific power, the value is typically five times higher than the 95% efficiency
discharge power.
10.3 Hydrogen storage
Present fuel cells and internal combustion engines operating off pure hydrogen rely
on pressurized tanks rated from 3,000 to 5,000 psi with talk of moving the upper
bound to 10,000 psi.
Liquefied hydrogen has been proposed as an economical and clean fuel for
transportation systems [27]. In Reference 27, Lawrence W. Jones remarks that the
price of liquid hydrogen in 1975 was equal to the price of gasoline by the litre.
The most promising energy sources on a per unit weight basis are those involving
the lightest elements - in particular, hydrogen. Electrochemical cells based on
lighter metals such as lithium, sodium and zinc are much more energetic than lead-
acid cells. Sodium-sulphur at 240-300 C and lithium chloride operating at 600 C
are two of the highest energy and power electrochemical batteries known. Practical
application of such high temperature flow batteries is problematic not only from a
self-discharge standpoint, but also from the need to maintain the constituents in
liquefied form. Of course, safety is a significant concern in all flow batteries
because of the generally high temperatures involved.
Liquid hydrogen is an ideal energy medium because its only effluent when
combusted or reacted is water vapour. Production of liquid hydrogen would be
most economically accomplished by steam reformation of hydrocarbons. The basic
reaction is
x CH n þ yH 2 O ! z CO 2 þ wH 2
ð 10 : 64 Þ
where the coefficients will balance the chemical reaction equation when the
appropriate hydrocarbon is selected - for example, methane. During processing,
the carbon dioxide would be removed with solvents and the hydrogen would be
liquefied using cryogenic processes. Electrolysis of water to produce hydrogen is
less economical, and with efficient processing may reach 130% the cost of steam
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