Environmental Engineering Reference
In-Depth Information
10.3.2 High pressure gas
The most common form of high pressure gas energy storage is compressed natural
gas (CNG), and sometimes liquefied natural gas (LNG). Extracted from under-
ground reservoirs, natural gas is a fossil fuel composed mainly of methane along
with other hydrocarbons such as propane, butane and inert gases (carbon dioxide,
helium and nitrogen amongst others). Natural gas is used today as feedstock for
alternative fuel vehicles, where it is stored on-board in pressure cylinders rated
3,000-5,000 psi. Discussion of raising the storage pressure to 10,000 psi is under
investigation. LNG is more difficult because it requires cryogenic temperatures of
259
F in order to be liquid. LNG must be stored in Dewar canisters (thermos
bottles).
ICEs modified for CNG use typically have upgraded fuel delivery systems,
heated pressure reduction valves and carburetion systems. CNG provides quicker
cold starts because it vaporizes readily and burns with lower emissions. It also has a
higher octane number than gasoline, so it's less prone to knocking. The fill time for
a CNG vehicle is from 2 to 5 min. On the downside, CNG does not have the energy
density of gasoline, so the vehicle range is substantially less. Natural gas is one of
the fuels in the US that is subject to the 1988 Alternative Motor Fuels Act (AFMA),
public law 100-494 meant to encourage more widespread use of alcohol and natural
gas as transportation fuels.
Propane or LPG is available today as a by-product of NG processing and from
crude oil refining. The only grade of LPG available for transportation use is HD-5,
approximately 95% propane and 5% butanes. HD-5 can be burned in alternative
fuel vehicle (AFVs) in much the same manner as CNG, but without the need of
high pressure containment. Propane fuel has a high octane number, and potentially
smog free and excellent cold starting performance. Because propane has so many
agricultural and home heating uses, it may be in short supply in some regions and
during cold weather. There are roughly 3.5 M propane fuelled vehicles in operation
worldwide, most in SI engine applications.
Table 10.11 expands on Table 10.1 and summarizes these energy storage
mediums of practical transportation system use.
In comparison to liquid fossil fuels, alternative energy storage mediums are
presently two orders of magnitude lower in gravimetric energy density. Recent high
energy power capacitors such as the ultra- or super-capacitor have very modest
energy density but very high pulse power capability. Pulse power capability is very
necessary in hybrid propulsion systems. A subsequent section will describe ultra-
capacitors in further detail.
10.4 Flywheel systems
Flywheel energy storage and attitude control for spacecraft are under development
using brushless dc motor (BDCM) technology with IGBT power switches and
PWM control [30]. The BDCM is a permanent magnet synchronous machine
(PMSM) ring wound having four poles, capable of 41-53 krpm at 6.5 kW power,
