IEEE Power & Energy Magazine - January/February 2017 - 23

january/february 2017

the gas grid, or used as fuel or feedstock. Excess wind
energy that would otherwise be curtailed can be used to
run an electrolyzer and produce hydrogen. The resulting
hydrogen can be used as transportation fuel or industry
feedstock. Alternatively, the hydrogen can be fed directly
into the gas system or processed to synthetic natural gas.
The admissible hydrogen concentration for direct injection into the gas grid is limited mainly by gas combustion
equipment because the different combustion properties of
hydrogen lead to flame speeds and reactivity, while hydrogen embrittles the pipeline. This HES enables storage of
excess renewable electricity in a gaseous fuel, thus providing access to the vast storage capabilities of the gas infrastructure. The gas network offers storage capabilities over
all time frames, from daily cycling as line pack to interseasonal storage underground, and is thus much more flexible
than other storage technologies. Therefore, this HES provides considerable operational flexibility, but its economic
potential depends largely on the price spread between wind
energy and hydrogen, as well as between hydrogen and natural gas. Also, the electrolyzer's cost itself makes deployment today prohibitive. A few pilot plants have been built
in Germany since 2013 with support from both industry
and government. The Danish system operator expects to
rely on electrolyzer systems by 2030 to provide flexibility.

Benefits of Wide-Scale HES Deployment

Conventional:
Single-Input,
Single-Output
Energy Converters

Output Side

Input Side

The economic flexibility resulting from wide-scale HES
deployment is manifest by an increase in resilience. An
indication of resilience of a certain system is typically the
total price increase of electric and natural gas services
nation- or continent-wide after a disturbance or disruption.
A lower price increase indicates a higher level of resilience.
As shown in Figure 7, HES deployment increases the link
density of the overall networked system, better delivering

Output Side

low grade, ranging from 30 to 38 °C, and can be extracted from a wide range of processes including heat
pumps and CHPs. This sets AD apart compared to processing biomass via gasification or pyrolysis because
these latter two methods require high-quality heat.
✔✔ Second, many attractive regions for wind-energy
development (for example, in the United States, the
Midwestern north-south "belt" from about Wyoming
on the western side to Illinois on the eastern side) are
also highly agricultural with a diversity of biomass
feedstock including animal waste, grass and maize
silage, and grains (e.g., wheat and triticale). Thus, as
wind and solar penetrations grow, so will these regions' need for flexibility, a need that could be met
by HESs through a biomass resource indigenous to
the region. Although there are over 11,000 AD facilities in Europe and 2,100 in the United States, the
potential for agricultural biomass digestion remains
underutilized. Most U.S. facilities (about 1,880) are
associated with wastewater treatment plants or landfill gas projects; only 247 are on farms and thus make
use of agricultural biomass.
✔✔ Third, if the input feedstock decomposed naturally, undergoing the same biological process as in AD, then it
would emit methane directly into the atmosphere. Considering that the global warming potential of methane
is at least 21 times higher than the CO2 released if AD
is used, then AD operation can represent a significant
reduction of greenhouse gas emissions.
✔✔ Finally, investment in AD provides an effective hedge
against long-term natural gas price volatility, a manifestation of the economic flexibility of the HES.
Another example of an HES is a hybrid of heat pumps/gas
boilers that uses both gas and electricity to supply residential heat. The smart integration of such heaters enables shifts
in real time between the different fuels to respond to system
conditions. For example, the hybrid heaters can switch from
gas to electricity for generating heat during times of excess
renewable electricity on the power grid; conversely, at times
of peak electricity demand, they can switch from electricity
to gas. Wide-scale deployment minimizes electricity capacity
expansion compared to single-fuel heat pump deployment and
reduces up-front costs for consumers because the expensive
heat pump can be downscaled. Such deployment also supports
decarbonization and reduces natural gas dependency compared to single-fuel gas boilers. Technical flexibility is limited
by consumer comfort, which depends on personal preferences
as well as on building properties. An investment study of this
technology for Ireland indicates that its deployment is costeffective and enables system-wide cost reductions compared
to boiler- or heat pump-only deployment.
A third HES example is based on electrolyzers, which
are fueled by electricity and produce synthetic natural gas
and hydrogen. These outputs can be stored locally, fed into

Input Side

✔✔ First, although AD requires heat, the necessary heat is

HES:
Multiple-Input,
Multiple-Output
Energy Converters

figure 7. The impact of HESs on network link density.
ieee power & energy magazine

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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - January/February 2017

IEEE Power & Energy Magazine - January/February 2017 - Cover1
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IEEE Power & Energy Magazine - January/February 2017 - Cover3
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