ASHRAE Journal - October 2021 - 14

TECHNICAL FEATURE
Another major benefit of zone-level electric resistance
heating coils is that they eliminate parasitic pipe heat
losses inherent to all water-based designs. Preliminary
research4 indicates these losses can be as large as the
amount of heat needed for space conditioning.
Both electric resistance design strategies are, however,
limited by thermodynamics to a peak COPh of 1. Even in
states like California, which generates much of its electricity
from zero-carbon wind, solar and hydro plants,
the grid is not low-carbon in the early morning when
heating systems peak. Resistance heating options are
therefore likely to remain worse than natural gas boilers
on a carbon basis in at least the near term in most parts
of the country after accounting for generation, transmission
and distribution losses.
Electric resistance options can additionally present
new challenges to electrical engineers by making buildings
winter-peaking instead of summer-peaking. This
is particularly an issue in cold climates, but winterpeaking
can also occur with electric resistance heating
options in mild west coast climates. Not only will winterpeaking
increase building electrical service sizes vs. current
practice, but the entire utility distribution system
would have to be up-sized at considerable expense.5
Code compliance can also be an issue with electric
resistance heating systems. ASHRAE Standard 90.12019's
Energy Cost Budget Method, for instance, allows
electric resistance heat but puts the proposed design
up against a fan-powered box system baseline with
zero reheat. California Title 24 prescriptively prohibits
electric resistance with few exceptions and does not
include electric resistance heat in any of its performance
method baseline system types.
Heat Recovery Chillers
Another alternative is to use heat recovery chillers
that can provide high-efficiency simultaneous heating
and cooling when concurrent heating and cooling loads
exist. In most applications this condition does not occur
when heating loads are at their highest. When heating
loads are high (e.g., on a cold winter day or during
morning warm-up), there is typically little or no cooling
load because cold ventilation outdoor air provides all the
cooling needed. The time-dependency issue with heat
recovery chillers is sometimes addressed with geothermal
heat exchange systems, wherein heat absorbed from
the building in warm summer weather is rejected to the
14
ASHRAE JOURNAL ashrae.o rg O CTO B E R 2021
earth, and the heat needed to warm the building in cold
winter weather is extracted from the earth. However,
geothermal bore fields for large buildings are extremely
expensive to install, especially when site limitations
require deep bores, and are prone to performance degradation
over time when the heating and cooling loads
are not well balanced.
Summary
The current market presents owners with two mediocre
options for all-electric heating and cooling systems:
either accept the large space requirements and high first
costs inherent to ASHPs, or select an electric resistance
option that increases energy cost and may yield worse
carbon performance than a natural gas boiler plant for
the foreseeable future while electricity is still primarily
generated from fossil fuels. Current applications with
heat recovery chillers are limited or are prohibitively
expensive when coupled with geothermal systems.
The Solution
The key to solving these issues, as indicated by
MacCracken,6 is coupling thermal energy storage (TES)
with heat recovery. TES has long been used as an HVAC
strategy for peak shifting, primarily as a cost-saving
strategy through reduced demand and peak utility
charges, but rarely as an energy recovery mechanism.
Multiple versions of thermal energy storage systems
exist, including:
* Condenser water (CW) storage (stratified and unstratified)
*
Hot water (HW) storage;
* Chilled water (CHW) storage;
* Ice storage; and
* Phase-change material (PCM) storage;
Combining TES with energy recovery leads to the concept
of time-independent energy recovery (TIER), an allelectric
central plant design that improves on the existing
alternatives for large commercial and mixed-use
buildings with respect to energy efficiency, cost effectiveness,
equipment spatial requirements and support
of grid-interactive efficient building (GEB) initiatives.
All TIER plants have three components in common: a
TES component, an energy recovery component (heat
recovery chillers) and a trim heat source component
(usually ASHPs, but these can be electric boilers in
cold climates or where roof space is limited). When
http://ashrae.org
ASHRAE Journal - October 2021

Table of Contents for the Digital Edition of ASHRAE Journal - October 2021
