ASHRAE Journal - July 2020 - 38

TECHNICAL FEATURE

Peak Energy Demand Reductions
For the past 30-plus years, thermal storage used for
cooling buildings have been sold mainly to provide
electric load flexibility to reduce summer peak electric
demand, saving money (and source energy and possibly
site energy) on electric bills. With the advent of electrification, winter morning warm-up will likely, if it isn't
already, become a troublesome (peak) period for many
electric grids. Having stored energy available could
greatly reduce the amount of energy demand, compared
to electric resistance heat, during morning warm-ups
with heating COPs that are above 5.

A Clean Grid Needs Energy Storage in Buildings
The need for vast quantities of energy storage is pretty
clear with a future grid based mainly on intermittent
renewable resources. If you think about it, fossil fuels
are forms of stored energy, which has made it easy to
dispatch the energy when needed. Since almost no
energy storage exists on the grid now, vast amounts of
energy storage of all types will be needed to integrate
large-scale renewables.
A recent WoodMackenzie2 report says that by 2025
over eight gigawatts of energy storage will be installed
and 25% of that will be "distributed" on the building's
side of the electric meter. Two types of energy storage
will likely end up in buildings: electric batteries and
thermal storage. With the current costs of thermal storage being a quarter to a third the price of electric batteries, a good storage strategy for the customer will be
to have electric batteries meet electric loads and thermal storage meet thermal loads. With the availability of
thermal storage, using an electric battery to run a compressor to instantaneously cool something is a waste of
money, energy and resources. Store the thermal energy
instead.

TABLE 1 Heating energy costs.a
ENERGY SOURCE

UNITS

QUANTITY

APPROXIMATE
COSTS

Natural Gas (Boiler)

Therms ($1.26)

12.5 Therms

$15

Fuel Oil (Boiler)

Gallons ($2.98)

9.0 Gal

$26

Steam (Con Ed)

Mlb ($40.00)

0.833 Mlb

$35

Electricity
(Resistance)

kWh
($0.25 Incl. Demand)

292 kWh

$73

Electricity
(Storage Source HP)

kWh
($0.25 Incl. Demand)

51 kWh

$13

aPrices

are for general references only. Taken from different online sources (Bls.gov,
Nyserda.com, ConEd.com) for New York 2018.

is without the defrost cycle factored in). The storage
source heat pump system would have a constant source
temperature of 32°F (0°C), no matter what the outside conditions are, so the efficiency (COP > 5) and the
capacity of the chiller (heat pump) would be consistent
and stay high.

Reduces Outdoor Footprint for Air-Source Heat Pumps
Large outdoor air-source heat pumps need lots of
room and lots of heat exchange coils for air to circulate
so energy can be extracted or rejected to the ambient
conditions. All these outdoor coils are exposed to dirt,
dust and snow, which can play a major role in performance, especially snow in cold conditions. In contrast,
thermal storage has massive amounts of heat exchange
surface area in direct contact with the storage source
(water) within the tank. No additional outdoor space is
needed for the system other than the existing cooling
towers, and no heat exchanger surface area is exposed to
fouling in outdoor conditions.

Comparable Costs for Heating
Raises "Source" Energy Temperatures During Extreme Conditions
For decades, conventional air-source heat pumps
have been used successfully for heating in the appropriate climate zones. These zones are mainly the
temperate zones, because in cold climates the heat
pumps drop dramatically in efficiency and capacity.
An air-source heat pump running when it is 40°F (5°C)
outside may have a COP of ~4.4 at 100% of its nominal capacity. That same heat pump with 14°F (−10°C)
air has a ~COP of 2.8 and 40% less capacity (and this
38

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Table 1 is a rough estimate of the costs in New York
City for different types of energy sources based
on 1 million Btu (1 GJ) delivered and used in the
building. Steam and electric resistance are very
expensive methods, especially when compared to the
direct combustion of fossil fuels at the site. However,
because of the high COP of the SSHP system, the cost
of heating is comparable or possibly lower than fossil
fuels, while meeting the goal of electrification of the
building.

http://www.Bls.gov http://www.Nyserda.com http://www.ConEd.com http://ashrae.org

ASHRAE Journal - July 2020

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