ASHRAE Journal - June 2021 - 50

TECHNICAL FEATURE
Analysis
In the five years prior to starting the
NPE timeline (2009 to 2014) the total
energy use averaged 18 MWh per year,
with 80% of that from thermal energy
(Et) as LP consumption. The estimate
is based on monthly meter readings
of kWhEe and LP refill volume*
(91.5 kBtu/gallon ≈ 27 kWh/gallon
[25.5 MJ/L = 7 kWh/L]6). The start
of using the GSHP resulted in an
approximately 47% reduction
in the total energy use
(Figure 3a) in 2015 compared
to the average of five years
prior; while the electricity
(Ee) net use increased by
60% (Figure 3a), there was
80% reduction of LP use
(Figure 3a). A local LP price
increase occurred in 2014
due to a shortage in supply,
resulting in the cost spike
shown in Figure 3b.
The purple bar with
dashed contour in Figure
3a corresponds to the LP
A.
35
30
25
20
15
10
5
-5
TABLE 1 Summary of investments.
ITEM
Installation
Equipment
Well Drilling: Four,
140 ft Wells
Wisconsin Focus on
Energy Program
30% Federal Tax
Credit
Total
GROUND SOURCE
HEAT PUMP
$5,638
$7,685
$9,260
PV PHASE 1 PV PHASE 2
$22,000 $ 13,000
-
-
- ($2,400)
($6,189)
$16,394
($5,880)
$13,720
FIGURE 3 Yearly outlook: a) energy; b) energy cost from external suppliers.
B.
Ee Net Use
Solar Offset
HPWH/D
Energy Demand
Et Use
GSHP
Total Energy Input
~85%
(Total Energy Input)
~47%
(Demand)
-
-
($1,560)
($3,432)
$8,008
HEAT PUMP
WATER HEATER
$170
$1,371
-
-
-
$1,541
HEAT PUMP
DRYER
TOTAL
- $ 40,808
$1,405 $ 10,461
$9,260
-
- ($3,960)
$1,405
- ($15,501)
$41,068
~
60% (Ee Use)
~
80% (Et Use)
3.0
2.5
2.0
1.5
1.0
0.05
0.0
-0.5
Cost Ee Use
Cost Et Use
Total Cost
~85%
consumption reduction due to the GSHP. In Figure 3a, the
yellow bars with dashed contours represent the amount
of electrical load offset by the solar PV panels, while the
blue bars with solid contours represent grid electricity
use. Starting in 2016, the first set of PV panels was sufficient
to produce more electricity than the house consumed,
and surplus was returned to the grid. The total
solar generation is the sum of the yellow and blue bars
starting in 2016.
The total energy use-represented by the solid, dark
blue line in Figure 3a-was relatively constant between
2014 and 2019. The solid red line in Figure 3a corresponds
to the net energy input (electricity + LP) from suppliers;
in 2019, it became negative-i.e., achieved NPE-with the
addition of the second set of PV panels. Finally, despite
the relatively small LP gas use in 2019, the HPWH and
dryer eliminated the need for fossil fuel from 2020
onward.†
The energy costs fell by more than 80%, from an
average of $1,500/year to less than $200 in 2019. The
surplus from the PV panels in 2019 was equivalent to a
$100 credit, with a surplus in energy costs being generated
in 2020 onward. More than 85% energy and cost
consumption reductions were achieved after the NPE
timeline began in 2019 (Figure 3). Although not captured
in Figure 3, between mid-2019 and mid-2020, when the
home was fully electric, the net positive generation was
3,156 kWh-26% greater than 2019 alone.
*The LP tank did not need to be refilled monthly. The assumption used is for uniform consumption, and the amount attributed to each
month is proportional to the number of days of each month in-between refills.
†Despite the fireplace's negligible use throughout the years, in 2019 approximately 0.15 cord of ash, maple and oak woods were used
for supplemental heating. The estimated energy use due to firewood burning was approximately 2 MMBtu (2.1 GJ) or 600 kWh. This
amount is equivalent to 5% of total energy demand and 30% of the LP use in 2019. This value is not accounted for in Figure 3, due to
high uncertainty in its estimate, and it is possible that the home was NPE by a smaller margin.
50
ASHRAE JOURNAL ashrae.o rg
J U N E 2021
Energy (MWh)
Energy Cost ($ in Thousands)
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
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ASHRAE Journal - June 2021

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