IEEE Power & Energy Magazine - January/February 2017 - 22

table 2. Different time constants in electricity and gas balancing.
Issue

Electricity System

Gas System

Balancing requirement

Need to maintain system frequency within
strict limits in real time

Maintenance of operating pressures within a certain
range due to line-pack capability

Balancing process

Close to real time (<1 s) power balance

Cumulative energy deviation over balancing time
frame or day

Balancing time frame

Focus on immediate action in the last
minutes/hour before real time

Focus on delayed actions (ex-post), typically
≥2 h

Adapted from DNV GL.

referred to as a multiple-input poly-generation conversion
system; a multiple-input,multiple-output conversion system;
energy hubs; or a hybrid energy-conversion system (HES).
The flexibility can be utilized for the electricity system, the
natural gas system, or both. Two kinds of flexibility can be
distinguished for HESs.
✔✔ Operational flexibility enables meeting highly variable net loads or maximizing operation at steady
state of certain HES appliances to minimize wear
and tear.
✔✔ Economic flexibility enables arbitrage between input
resources and output services, i.e., utilizing least-price
input resources while providing highest-price output services, subject to contractual and physical constraints. A traditional single-input, single-output power
plant may provide significant operational flexibility,
but it would not have economic flexibility.

HES. However, a large variety of alternative HES designs are
conceivable through the combining of different inputs (electricity, heat, fuels, and/or biomass) and outputs (electricity,
heating and cooling services, water, hydrogen, transportation
fuels, and/or commodity chemicals). The flexibility benefits
of HES deployment are exemplified here based on three different HES designs: 1) an advanced HES based on anaerobic
digestion (AD), 2) hybrid residential heaters, and 3) windelectrolyzer systems.
An advanced HES can be conceived around AD. As
illustrated in Figure 6, such an HES has three energy inputs (natural gas, biomass, and electricity), three energy
output services (biomethane, electricity, and cooling/heating), and three storage devices (biomass, heat, and biogas).
Additionally, it contains two heat sources: the combined
heat and power (CHP) unit and the low-temperature geothermal system with heat pump. The CHP and heat pump
serve both the anaerobic digester and the district energy
system. AD utilizes low-grade heat to support the digesExamples of Advanced
tion of organic materials (e.g., wasted food, plant clippings,
HES Designs
A combined heat-and-power, or cogeneration, plant fueled by animal manure, sewage) to produce biogas. The biogas can be
natural gas and biogas is a familiar design that exemplifies an used directly to fuel the CHP, it can be stored, or it can be
cleaned and upgraded before its
injection into a natural gas pipeline system. The district energy
system distributes heat obtained
Biomethane
Cleaning/
Biogas
from the CHP and heat pump
Injected
Upgrading
Storage
into Pipeline
systems to the demand; it could
also provide cooling, if an abNatural Gas
Combined
sorption chiller is included.
Biogas
Heat and
Electricity
Biomass
The heat storage (or accumuAnaerobic
Power
Mixing
Digester
lator) facility and the heat pump
provide that the HES meets heatHeat
ing and cooling demand while the
Heat
Heat
Heat
CHP meets the flexibility requireStorage
Heat
Biomass
ments from the electricity system;
Storage
Cooling
Low-Temp
alternatively (or additionally), the
District Energy
Services
Geothermal w/
heating demand may be controlled,
System
Electricity
Heating
Heat Pump
reducing the need for the heat pump
Services
or the heat storage.
The integration of AD into this
figure 6. An advanced HES design based on an anaerobic digester (three inputs and
HES
is motivated in four ways.
three outputs).
22

ieee power & energy magazine

january/february 2017

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