ASHRAE Journal - September 2021 - 16

TECHNICAL FEATURE
Scenario 2: Low-Carbon Grid
In many states, the grid is currently a mix of carbonbased
electricity and renewables. In this scenario, even
if there is no gas consumption in the building, a building
will never be truly carbon neutral without further
interventions. Several other options can achieve carbon
neutrality in this case. A building owner can choose to
either install on-site renewables to offset energy use,
join a CCA or similar consumer choice programs that
source carbon-free electricity, directly purchase renewable
energy (available to large customers in deregulated
markets) or purchase RECs.
Scenario 3: Carbon-Free Grid
Net zero energy buildings that use photovoltaics to
offset all electrical energy consumption can easily
decarbonize if all of the building systems are electric.
Unfortunately, not all buildings can generate enough
on-site electricity due to building height, roof capacity,
roof orientation or other constraints. Luckily many electric
grids are moving in the direction of providing 100%
carbon-free electricity.
Coupled with this and all-electric building systems,
buildings are considered to be carbon-free. The grids
of the nine states, several large utilities and scores of
municipalities that have pledged to eliminate their carbon
emissions from electricity generation by 2050 in
the last five years will use a combination of solar, hydro,
wind, geothermal and, when necessary, nuclear energy
coupled with batteries and thermal storage to meet electricity
demand during the day and night. As mentioned,
several utilities now offer options for 100% renewable or
carbon-free electricity at slightly higher rates.
Electric Heating Systems
Heat Pumps for Space Heating
Current typical HVAC systems can be replaced with
alternative systems that use heat pumps for heating
instead of gas. Heat pumps use the refrigeration cycle to
transfer heat energy from one space to another. Similar
to a refrigerator, home air-conditioning unit and car airconditioning
unit, a heat pump uses a compressor and
refrigerant to absorb heat from a cold space and release
it to a warmer one. In some cases boilers are replaced
with heat pumps. But the most cost-effective option
is where a compressor system is designed to provide
both heating and cooling, as is the case with variable
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refrigerant flow (VRF) systems. In these systems a
reversing valve changes the direction of refrigerant flow
to enable heating and cooling in the building.
The market for heat pumps is growing in the U.S., and
new options are coming on the market. Many of the new
heat pump systems are coming from Asia and Europe.
The quality and number of options available has grown,
and the price is coming down. In most cases the conversion
to heat pump systems will occur when the building
is built or when the building goes through a major
renovation in which much of the HVAC equipment is
replaced. With most mechanical equipment lasting 20 to
30 years,9 major HVAC system renovations are more like
new construction. The suggestions below apply to new
construction, major renovations and retrofits.
Commercial Buildings
VAV Reheat | In a typical variable air volume (VAV) reheat
system in a large building, hot water is provided by a
central gas boiler. Air-source heat pumps are the most
common alternative to boilers. Ground-source heat
pumps are sometimes used. In some cases electric
resistance reheat is used but can result in higher overall
energy costs.
Variable Refrigerant Flow | Variable Refrigerant Flow
(VRF) systems have gained traction in recent years given
the capital cost savings of using the same compressor
for both heating and cooling instead of a separate boiler
and cooling compressor. The design is also compact and
allows for simplicity of operation. They also offer heat
recovery options and can provide simultaneous heating
and cooling. The more efficient VRF systems also
include variable speed fans and compressors for additional
energy savings. VRF systems do require a separate
system for ventilation. Dedicated outdoor air systems
(DOAS)10 and their associated benefits couple well with
VRF systems.
Single Zone Packaged Unit | Many commercial buildings,
particularly retail, use single zone packaged units with
natural gas heating and compressor-based cooling.
These same packaged units are available in a heat pump
configuration in which the cooling compressor coupled
with reversing valves is able to produce the required
heating. This system is another opportunity for a simple
switch from gas heating to heat pump heating. Due to
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ASHRAE Journal - September 2021

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