A Comparative Study of Solar Heating Options for Domestic Hot Water in Pennsylvania

A Comparative Study of Solar Heating Options
for Domestic Hot Water in Pennsylvania
Abstract
Solar water heating is an increasing interest to Pennsylvanian homeowners because of rising energy
prices, the green energy movement, and the high energy demand of hot water. Conventionally, solar
water heating uses thermal collectors which transfer energy from the sun straight to the hot water
tank. However, plummeting PV prices and the spread of net-metering and heat pump technology is
causing many to wonder whether PV is yet cost effective against solar thermal. This study attempts
to answer this question by analyzing the energy supply and demand from various solar setups. A
steady-state model of a typical basement was used to determine the effect of a heat pump on home
heating, concluding that with the addition of insulation, the GE GeoSpring heat pump performance
is reduced from 2.4 to 2. This small change is significant to the yearly energy required to heat the
water. A financial model was developed to compare these systems over a 30 year life. Replacement
water heaters and inverters were required after their own 15 year life. Results show that the solar
thermal remains cost effective over photovoltaic; at least until PV installation can drop below
$4.2/W. An alternative PV system is proposed and will be further studied alongside the
development of a dynamic basement model.

Introduction
Growing interest in sustainable housing is causing many Pennsylvanian homeowners to consider
small scale solar power as an option for their domestic water heating. According to the National
Renewable Energy Lab water heating can account for 18-20% of a household’s energy demand1
and
without conservation, upwards of $750/year. The potential for impact is high as many homeowners
expect rising electricity prices.
Currently, the common approach to solar water heating is by thermal collectors. These systems
pump water through a solar collector (typical flat plate configuration: copper pipe on top of an
insulation layer and under a solar absorption surface) and into a preheat tank. The system is usually a
closed loop with a heat exchanger to protect the pipes from freezing and calcium buildup. The
conventional hot water heater draws from the preheat tank instead of the water main before it mixes
with cold water to reach the desired temperature, saving electrical energy by using the preheated
water. Solar thermal is popular due to its low maintenance and simple design, which drastically
reduces installation costs.
However, interest in photovoltaic systems is cropping up because of the dramatically reduced cost
of PV modules2
. So much so that some are wondering whether solar thermal is dead3
. Photovoltaic
hot water heating works differently than the thermal alternative. It works by a ‘net-zero’ approach to
electricity. The conventional water heater functions separately from the PV array, drawing energy as
usual from the grid. The photovoltaic system delivers it energy onto the grid as well, thus reducing
the net purchased electricity. So PV solar water heating is not exactly a direct approach.
But this indirect approach has advantages; PV systems: cannot freeze or overheat (and work better
in the cold); are generally smaller; have few (if any) moving parts; and are productive even when hot
water is not needed. This last point is most notable in daily supply and demand: homeowners are
away at work and school when the sun offers greater irradiance (midday); and most families have
some of the highest demand in early morning, well before the sun has reached its peak.

A net-zero approach (PV) rather than a conservation approach (thermal) may provide significant
cost advantages. Regardless, a homeowner should consider a heat pump before any solar installation.
These devices work like a refrigerator in reverse. A heat pump compresses warm air and draws off
the heat energy (air warms when it is compressed) with a heat exchanger, adding the energy to the
water. By extracting energy from the air, efficiencies over 100% can be achieved. A photovoltaic
system coupled with a heat pump may be a viable solution for the Pennsylvanian homeowner
looking for a green energy option. Given this option, is there are better alternative for hot water
heating than solar thermal? Does the Pennsylvanian have to deal with all the disadvantages of solar
thermal in order to ‘go green’?

Methods
Seven configurations were considered, which cover all system combinations given two major
choices: solar thermal, PV, or no solar (base case); and electric or heat pump water heater. The
seventh set-up is a direct-connected PV ‘overheat’ system which strips the DC-AC inverter and grid
connection from the conventional PV design. This final system is functionally similar to the solar
thermal model. Simplified schematics of each configuration are shown in Figures 1 – 7 below.
Figure 1: Conventional Electric Water Heater Figure 2: Heat Pump Water
Heater
Figure 3: Electric Water Heater w/ Solar Thermal

Figure 4: Electric Water Heater w/ Solar PV
Figure 5: Heat Pump Water Heater w/ Solar Thermal

Figure 6: Heat Pump Water Heater w/ Solar PV
Figure 7: Hybrid Water Heater w/ Solar PV Overheat

This economic analysis requires three major considerations for each system: 1. Hot water
consumption (volume and energy demand); 2. Solar utility and capture; and 3. Basement
thermodynamics concerning heat pump efficiency. Component costs are given throughout the
report and summarized in Table 2 on page 9.
1.0 Hot Water Consumption
1.1 Volume
Each system was analyzed according to a set of assumptions about the user. This study is concerned
with a single family household (3-4 persons) in Philadelphia, Pennsylvania, in a house of
approximately 2000 square feet. Larger or smaller systems have different economies of scale,
rendering such systems outside the scope of this study. Philadelphia was chosen because of its high
electricity prices, its solar utility, and its neighborhood density. Geographical location will affect the
economics of this study due to electricity price, solar utility, and component costs.
According to energy.gov, the average American household uses 64 gallons of hot water per day4
.
However, more conservative estimates have been made under the assumption that a family
considering a solar installation will already have cut their hot water demand. Shown in Table 1
below, a daily draw profile was developed using a sizing model from energy.gov, resulting in a daily
use of 57 gallons.
Operation Gal/use Qty Peak Hour
(gal)
Qty Daily Use
(gal)
Showers 10 2 20 4 40
Dishes 6 1 6 1 6
Hand Washing 2 2 4 2 4
Clothes Washer 7 1 7 1 7
Total 37 gallons 57 gallons
Table 1. Sizing a New Hot Water Heater5

1.2 Energy
A daily energy demand was calculated based off the 57 gallon daily water draw. Assuming constant a
specific heat of . k kg and a temperature rise from 50 °F to 120 °F (10 °C to 48.9°C) the
daily energy required by the household is 9.77 kWh. The comparative nature of this study allows for
the assumption of a constant daily water draw over the entire year. Given 9.77 kWh/day, a yearly
energy demand is calculated at 3565 kWh. This number will help size and compare the solar systems
and calculate energy cost.
( )
1.3 Water Heater Selection
Electric and heat pump hot water heaters were required for price and energy rating. A 50-gallon tank
was sought to match the first hour draw rating of 37 gallon. A 12 year lifetime warranty was chosen
for the 30 year analysis, needing replacement once over the scope of the study (assuming a 15 year
life). The electric and heat pump water heaters chosen are: the $458 Whirlpool 50-gal (Model#:
ES50R123-45D) and the $999 GE GeoSpring 50-gal (Model#: GEH50DEEDSR) respectively. The
Whirlpool model has an energy factor of .93 compared to 2.4 of the GE heat pump.
Figure 8: Whirlpool’s 50-gal Electric Water Heater (left) and
GE’s GeoSpring 50-gal Heat Pump Water Heater (right)

2.0 Solar Utility and Capture
2.1 System Size and Cost
A solar system which will provide for the energy needs for a family was sized according to a typical
solar day. In Philadelphia, the sun, on average, shines for 11 hours each day6
. In sizing a solar array,
we assume that five to six of those hours can be considered full-sun hours during which any solar
array will operate at full capacity (provided clear skies and no shading)7
. Requiring 9.77 kWh/day of
heat, dividing this by the expected sun hours/day yields the estimated size of the solar array. This
five to six hour, full sun range comes to 1.95 kW and 1.63 kW, respectively.
( )
( )
(kW)
SunPower’s SPR-X20-445-COM8
photovoltaic panel was chosen. This panel operates with 20.58%
efficiency at 444.58 W DC. It is 2.162 m2
. Four of these panels put the system size at 1.778 kW,
within the sought after range. The SunPower panel has a warranty ensuring 95% performance for
the first five years and a .4% degradation for the next twenty. According to SolarToday’s 20 State
of the Industry report, silicon modules are averaging at $0.85/W wholesale9
. This study takes the
price per watt to be $1 considering the small size of these systems.
The Chromagen CR-130-A-SP glazed flat plate collector was chosen (SRCC #2009059D10
). Thermal
collectors do not operate with constant efficiency and depend largely on the temperature rise sought
after. The panel is 2.96 m2
. Two panels were required to be comparable to the PV output. The
System Advisor Model (SAM) used in this study rated the two panel array at 3.75 kW.
The modules are but one of many costs associated with a solar array. Component costs and financial
considerations are shown in Table 2 on the next page, including the water heaters and electricity
price which have already been stated above.

a
Values were generated from the System Advisor Model and consultations with Andy Lau
b
www.lowes.com (price likely to change)
Component
Cost, $/unit Quantity Total Cost
Photovoltaic Thermal Photovoltaic Thermal Photovoltaic Thermal
Modules $445 $1100 4 2
Inverter $37811
― 2 ― ―
Preheat Tank ― $448 ― 1 ―
Balance of Systema
$1500 $1500
Installationa
$960 $960
Margin/Overhead $252712
―
Indirect Capital $160813
$306
Maintenancea
$35/year $21/year14
―
Investment Tax
Credit15
30% of
total cost
$2739 ―
Capacity Incentive15
$0.75/W $1334 ―
Total Cost of Solar
Installation16
$5.13/W $5414
Down Paymenta
$2500
Loan Rate 7%
Discount Rate 2%
Sales Tax 6%
Electricity Price $0.1567/kWh17
GE GeoSpringb
$699 (w/ $300 rebate)
Whirlpool Electricb
$448
Lowes Installationb
$80
Table 2. Cost of Components and Project Financing

2.2 Solar Output
Solar installations rely heavily on sun position and weather which were both modeled with the
System Advisor Model (SAM) by the National Renewable Energy Lab (NREL). While SAM does
financial calculations, we were merely concerned with energy output. This study assumes zero
shading for the sake of simplicity.
Energy calculations in SAM show the PV system annual output at 2,428 kWh, a 15.6% capacity
factor. The thermal system was first analyzed with one panel (1.875 kW); however, the 7.4% capacity
factor (and thus the total annual energy) was lower than expected because the efficiency is so
dependent on the local temperature. Adding a second panel set the annual output at 2,454
kWh/year, comparable to the photovoltaic system. The yearly output for the PV and Thermal
panels are 2,428 and 2,454 kWh respectively.

3.0 Thermodynamic Modeling of a Generic, 900 Sq. Ft Basement
A thermodynamic model of a basement was developed to answer questions regarding the efficiency
of the heat pump. Figures 9-11 below show the effect of a water heater on the basement
temperature. In Figure 9, no water heater is present, yet an energy flow from the main house to the
basement is present. The addition of the electric water heater (Figure 10) adds an energy flow into
the basement from tank losses, reducing the heat flow from the upstairs. Finally, in Figure 11, the
heat pump water heater draws heat out of the basement in addition to the similar tank losses of its
electric counterpart. Because of this heat draw from the basement air, the heat transfer through the
basement ceiling may be affected enough to effectively offset some of the energy savings of the heat
pump. The model was developed to quantify this effect and possibly alter the rated energy factor.
Figure 9: Basement Base Case
Figure 10: Electric Water Heater Figure 11: Heat Pump Water Heater

This study assumes a house to have an unconditioned, 30 feet by 30 feet, basement with electric
baseboard heat in the upstairs. A generic basement size was chosen to remain relevant to various
size basements and the electric baseboard heat was chosen as a worst case scenario. The 2001
Ashrae Handbook18
was consulted to develop a working, steady-state model for a first law
calculation of the basement heating load. The general procedure outlined in Chapter 28.7 was
followed. Walls, floor, ceiling, and hot water tank are modeled by the equation . To
account for tank losses, a tank insulation rating of R8 was assumed. Finally, a mathematical model
for the COP of the GE Geospring Heat Pump Water Heater was developed by the Bonneville
Power Administration (BPA) in the “Interim Report and Preliminary Assessment of GE GeoSpring
Heat Pump Water Heater”19
. Using the BPA’s COP(Tdb, Twb) function and the heat needed to be
delivered to the hot water, the heat draw from the basement air was calculated according to ̇
( ). Accounting for the heat pump draw, tank losses, and all the walls of the basement,
the model was complete for steady state analysis and calibrated assuming the no tank conditions in
Figure 9.

Results and Discussion
The basement heating model was developed to answer two questions: how does the basement
temperature affect heat pump performance and how does a heat pump affect a home energy
balance? Considering the basement model detailed above, a third question of insulation was raised.
Given the option of insulating the basement ceiling from the floor above, what approach would
yield the greater energy savings? Figure 12 shows the effect of insulation on basement temperature.
The purple and green lines, representing the uninsulated ceiling, show smaller variation in
temperature than the insulated alternatives. At an R-value of 23.41, insulated ceiling option is more
so affected by outside air temperature.
Considering a basic knowledge of heat pump efficiency, we expected a higher COP from the
uninsulated case and thus greater energy savings. Figure 13, supports this assumption; in the
uninsulated case, the COP is higher for all outside air temperatures, converging at some point
beyond 70 °F. However, in the uninsulated, heat pump case much of the basement air was heated
through the ceiling, energy for which the homeowner has already paid. The net effect of the heat
pump is shown in dollar value in Figure 14. While the COP of a heat pump is greater without
30.0
40.0
50.0
60.0
70.0
10 30 50 70
BasementTemperature(F)
Outside Air Temperature (F)
Variation in Basement Temperature as Affected by
Ceiling Insulation and Water Heater Type
Insulated, Conv WH
Insulated, HP WH
Uninsulated, Conv WH
Uninsulated, HP WH
Figure 12: Variation in Basement Temperature as Affected by Ceiling Insulation and Water Heater Type

insulation, the net savings is better with insulation than without. With this understanding, and
insulation recommended, the COP for the insulated heat pump case was averaged over the given
temperature range. Thus reducing the GE Heat Pump energy factor from 2.4 to 2.0.
Figure 13: Heat Pump COP as a Function of Outside Air Temperature
Figure 14: Relative Electricity Use of HPWH vs. Conventional

A financial model was developed to compare the seven systems over a 30 year lifetime. Major
considerations include: water heater and energy cost; solar installation cost and energy savings;
maintenance cost; and interest due over a 15 year loan. Financing options such as second mortgage
or tax free interest payments were not considered for the sake of simplicity in comparison. Under
‘Life Cycle Cost’, Table 3 below shows the resultant cost of each system after 30 years. Grouped
pairs show a comparison, holding one part constant. The two non-solar cases show the hot water
energy cost after 30 years; the conventional electric water heater incurs $20,000 in cost after 30 years,
a strong incentive for change. Also shown, in the rightmost column, is a ‘System Payback’ period
which compares each system to the first: the conventional hot water heater without solar. Note the
‘No Solar, Heat Pump’ option, a small investment to a more efficient water heater will pay for itself
in one year.
For the solar systems (still in Table 3), a solar installation without a heat pump is still cost effective
over the 30 year lifespan but requires a longer investment period from the homeowner (19 or 24
years before system payback). However, coupled with a heat pump, the entire system becomes cost
effective over just an electric water heater after 2-3 years. This optimal design cuts electricity costs by
combining solar energy and a high efficiency water heater.
No Solar, Electric (20,584.41)$ 0 0
No Solar, Heat Pump (10,800.26)$ 0 1
Thermal, Electric (17,208.21)$ 18 19
PV, Electric (19,103.65)$ 22 24
Thermal, Heat Pump (7,663.98)$ 19 3
PV, Heat Pump (9,599.81)$ 23 2
PV, Heat Pump (Simple) (7,663.98)$ 16 2
4.42$ /W
Heat Pump, PV
Overheat
(8,279.35)$ 18 2
Solar
Payback
(Years)
System
Payback
(Years)
System
(Solar, Water Heater)
Life Cycle
Cost
Table 3. Life Cycle Cost

Most important to this study is the comparison between solar options. When given the choice
between a thermal or photovoltaic installation our study finds that, over a thirty year lifespan, solar
thermal remains the better option. With both water heater options considered, the thermal system is
more cost effective by $1900 over the photovoltaic system. Close behind, while still under
development, the PV Overheat model is more cost effective than the grid tie. The PV Overheat
system is attractive but a cheap, variable-resistance heating element is commercially unavailable.
However, some external factors may add unforeseen costs since this model has yet to be studied
extensively.

Conclusions
This study set out to determine if photovoltaic was cheaper than thermal for domestic hot water.
After analysis of water heater options, a basement heating model, and the solar systems themselves
we have determined:
 The solar thermal system is more cost effective over the photovoltaic system by $1,900.
 A solar thermal coupled with a heat pump, which will recapture some losses, will pay for
itself (compared to mere electricity) in two years.
 All solar systems are economical feasible under testing conditions (i.e. better than having no
solar whatsoever in both conventional electric and heat pump cases)
 The price of photovoltaic installation in PA would have to be $4.2/W before the grid-tie,
‘net-zero’ approach would be advantageous.
 PV installer margin and overhead costs could be drastically reduced for the DIY homeowner
which account for 27% of cost.
 The major inhibitor to the photovoltaic installation seems to be government regulation on
permitting and grid connection ($1600, 17% of total cost).
 The PV-Overheat system would be attractive considering its similarity to solar thermal and
simplicity in design.
The PV Overheat system is in fact so similar to the thermal design that many Do-It-Yourselfers may
soon find this to be a go to model, provided a cheap, variable-resistance heating element comes onto
the market.
There are many externalities to this study that may be reason enough for a closer look. I believe our
basement heating model, while accurate under its assumptions, is inconclusive in its results. Many
questions have yet to be answered and certain assumptions may be too bold. For example, we have
assumed that 100% of the heat energy from the thermal array will be added to the water, not
accounting for preheat tank losses or their effect on heat pump COP. The list below shows factors
excluded from this study that could contribute to error (in order from most probable to least
probable).

 Effect of the preheat tank on the heat pump COP in both the solar thermal and PV overheat
systems
 Efficiency of heat transfer from preheat tank to main water heater
 Effect of outside temperature above 70°F and how the heat pump would affect home cooling
 Effect of unconsidered heat sources in basement (natural gas furnace)
 Dynamic modeling of the basement
 Periods of increased or decreased use (extended stay guests, seasonal travel)
Future Work
Extension of this study to a dynamic basement model would allow for more conclusive results and a
larger picture view of energy use in the home. With a more developed thermodynamic model,
studies of prototype systems could refine the economic model to a greater degree, especially the PV-
Overheat system as a variable-resistance heating element is currently unavailable.
This study was analyzed for Philadelphia, PA but could be easily expanded to other locations; a solar
installer with a similar financial model could use this study as a tool for determining the best solar
water-heating option on a case-by-case basis.
Acknowledgements
I would like to thank Chris Hill, co-researcher on this project, for his knowledge of the System
Advisor Model and his passionate interest in solar technology.
And a special thanks to Andrew Lau of The Pennsylvania State University’s Center for Sustainability
for his direction, wisdom, and knowledge throughout this study.

References
1. Denholm, P. The Technical Potential of Solar Water Heating to Reduce Fossil Fuel Use and Greenhouse Gas
Emissions in the United States. Tech. no. NREL/TP-640-41157. National Renewable Energy
Laboratory, 2007. Web.
2. Mints, Paula. "Solar PV Profit's Last Stand." Renewable Energy World. 20 Mar. 2013. Web. 4 Feb.
2014.
3. Holladay, Martin. "Solar Thermal Is Dead." GreenBuildingAdvisor. N.p., 23 Mar. 2012. Web. 2
Feb. 2014.
4. "New Infographic and Projects to Keep Your Energy Bills Out of Hot Water."Energy.gov. N.p.,
19 Apr. 2013. Web. 3 Feb. 2014.
5. "Sizing a New Hot Water Heater." Department of Energy, n.d. Web. 4 Feb. 2014.
6. "Observed Weather Reports." National Oceanic and Atmospheric Administration, 8 Dec. 2008.
Web. 27 Apr. 2014. <http://www.nws.noaa.gov/climate/index.php?wfo=PHI>.
7. "Average Annual Sunshine in American Cities." N.p., n.d. Web. 28 Apr. 2014.
<http://www.currentresults.com/Weather/US/average-annual-sunshine-by-city.php>.
8. "PV Module SPR-X20-445-COM Details." Solar Hub. Solarnexus, n.d. Web.
<http://www.solarhub.com/solarhub_products/50651-SPR-X20-445-COM-SunPower>.
9. Masia, Seth. "State of the Industry, 2014." Solar Today, 10 Jan. 2104. Web. 28 Apr. 2014.
10. Solar Rating and Certification Corporation. N.p., n.d. Web. 15 Mar. 2014. www.solar-rating.org
11. "SMA Inverters Price Survey – SunnyBoy Inverters." EcoBusinessLinks. N.p., 28 Apr. 2014. Web.
28 Apr. 2014. <http://www.ecobusinesslinks.com/surveys/sma-inverters-price-survey-
sunnyboy-inverters/>.
12. Feldman, David, Barry Friedman, and Robert Margolis. Financing, Overhead, and Profit: An In-Depth
Discussion of Costs Associated with Third-Party Financing of Residential and Commercial Photovoltaic Systems.
Tech. no. NREL/TP-6A20-60401. National Renewable Energy Laboratory, 2013. Web.
13. "The Impact of Local Permitting on the Cost of Solar Power." Sun Run, Jan. 2011. Web. 28
Apr. 2014.
14. Lau, Andrew, and William Aungst. "Solar Water Heating Maintenance: Pennsylvania Survey
Results." Solar '90 (1990): n. pag. Web.
15. “Database of State Incentives for Renewables and Efficiency (DSIRE)”. 15 Mar. 2014
http://www.dsireusa.org/.

16. Cassard, Hannah, P. Denholm, and Sean Ong. Break-even Cost for Residential Solar Water Heating in
the United States: Key Drivers and Sensitivities. Tech. no. NREL/TP-6A20-48986. National
Renewable Energy Laboratory, 2011. Web.
17. Bureau of Labor Statistics. Mid-Atlantic Information Office. Average Energy Prices, Philadelphia-
Wilmington-Atlantic City- March 2014. Web.
18. "Chapter 28: Residential Cooling and Heating Load Calculations." 2001 ASHRAE Handbook:
Fundamentals. I-P ed. Atlanta, GA.: ASHRAE, 2001. 28.7. Print.
19. Larson, Ben. Bonneville Power Administration. "Interim Report and Preliminary Assessment of
GE GeoSpring Heat Pump Water Heater." Web. 28 Apr. 2014.

A Comparative Study of Solar Heating Options for Domestic Hot Water in Pennsylvania

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (12)

Similar to A Comparative Study of Solar Heating Options for Domestic Hot Water in Pennsylvania

Similar to A Comparative Study of Solar Heating Options for Domestic Hot Water in Pennsylvania (20)

A Comparative Study of Solar Heating Options for Domestic Hot Water in Pennsylvania