ASHRAE Journal - July 2020 - 36

TECHNICAL FEATURE

* Raises source energy temperatures during design
ambient conditions;
* Reduces outdoor footprint for outdoor heat pumps;
and
* Comparable costs for heating.

Electrification
The current natural gas-based system is rather inefficient when compared to heat pumps: gas-fired boilers have a COP of 0.8, whereas the storage source heat
pump (SSHP) system using the phase change of water as
the source will be operating at a COP of above 5. (So, for
1 kWh of electrical energy going to the building, the heat
pump can move four times that amount of energy from
one level to another for a total heating COP of 5). Even
factoring a source energy efficiency of the grid of 38%,
the SSHP will be more than 200% to 250% more efficient
than on-site boilers.

Lower Carbon Emissions
The use of a heat pump whenever simultaneous
heating and cooling is needed should reduce carbon
emissions. With standard emission numbers from
NYC Local Law 97, a heat pump running at a COP of
5.45 has a CO2 emission of 82 lb versus an 80% efficient gas boiler, which emits 370 lb (Figure 7). This is
a 78% reduction in carbon on a source energy basis.
And when the grid is carbon free, the SSHP emissions
go to zero.
A question needing simulation is how much energy
is normally wasted to the atmosphere when there are
afternoon cooling needs on a cold winter day. This
will vary greatly by project, but some things are pretty
clear:
* In the early morning on cold winter days, a lot of
heating is generally needed at a time when there are not
likely to be simultaneous cooling loads in the unoccupied interior zones. So, having stored energy available
at that time would be valuable, especially as we move
toward electrification of entire cities. Fossil fuels are the
"stored energy" of choice at the moment.
* In afternoons, lots of cooling is needed, particularly on the sunny side of an office tower with a glass
façade.
* Finally, lots of usable thermal energy leaving a large
building exists in the winter if all that has to be done to
store it is melt ice. Think of all the energy in the exhaust
36

ASHRAE JOURNAL

ashrae.org

J U LY 2 0 2 0

FIGURE 7 Gas heating vs storage source heat pump.

One Thermal Storage Tank Capable of Holding 2,000,000 Btu

Gas - Boiler
* Assume 80% Combustion Efficiency
and 5% Loss,* Site to Sources, of
Natural Gas
* 2,400,000 Btu delivered to the
Building/0.8/0.95=3.15 Million Btu
* 3.15 Million Btu=3,150
kBtu×0.00005311 tCO2e/
kBtu*=0.167 Tonne CO2=370 lb CO2e

Electric - Storage Source Heat Pump
* 2,400,000 Btu Delivered to the Building
* Cooling COP=12/(kW/ton)/3.412
* Heating COP You Add 1.0 to Cooling COP
* 0.79 kW/ton Chiller Has Cooling COP of
4.45 and Heating COP of 5.45
* 2,400,000 Btu/5.45/3,414 Btu/
kWh=129 kWh
* 129 kWh×0.000288962 tCO2e/kWh*
=0.037 Tonne CO2e=82 lb of CO2e

82/370 = 78% Reduction in CO2e
*tCO2 conversion factors from NY LL97 document

air and used domestic water, which is above 32°F (0°C),
that could be captured by melting ice.
An existing 2.2 million ft2 (204 387 m2) project in New
York City uses 44 thermal storage tanks for cooling the
building year-round, even in the dead of winter. After
a few years of operation, the management1 concluded
that using the thermal storage to cool the building in the
winter is actually less expensive than using their "free
cooling" system. Even on the coldest days, they absorb
all 88 million Btu (93 GJ) of energy (melting ice) that
would have normally been rejected to the atmosphere
with free cooling.
Figure 3 shows the thermal Btu equivalent of those 44
tanks in gallons of fuel oil, therms of natural gas, and
pounds of steam (the approximate equivalent local cost
of each different form of the energy is also included). On
a carbon and cost basis these amounts seem significant
enough not to just waste.

Reduces Domestic Water Use
Whenever a cooling tower runs, it loses water
through evaporation. Looking back at Figure 1, when
water goes from a liquid to a vapor it absorbs about
1,000 Btu/lb (2326 J/g), which lowers the temperature
of the water left behind. Therefore, the 88 million Btu
(93 GJ) in the example (Figure 3) that we did not "free
cool" away in the cooling tower theoretically saved
88,000 lb (39 916 kg) of water or about 10,000 gallons
(37 854 L). While the amount saved will vary depending on the exact wet-bulb temperatures, a green
building can save a significant amount of domestic
water each day.

http://ashrae.org

ASHRAE Journal - July 2020

Table of Contents for the Digital Edition of ASHRAE Journal - July 2020