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SolOpt: A Novel Approach to Solar Rooftop
Optimization
Lars Lisell Ian Metzger Jesse Dean
National Renewable Energy Laboratory National Renewable Energy Laboratory National Renewable Energy Laboratory
Golden, CO, USA Golden, CO, USA Golden, CO, USA
ABSTRACT
Traditionally, photovoltaic (PV) and solar hot water (SHW) technologies have been designed with separate design tools,
making it difficult to determine the appropriate mix of PV and SHW. A new tool developed at the National Renewable Energy
Laboratory changes how the analysis is conducted through an integrated approach based on the life cycle cost effectiveness
of each system. With 10 inputs, a person with only basic knowledge of the building can simulate energy production from PV
and SHW, and predict the optimal size of each system. The user can also select from 5 optimization criteria: Greenhouse
Gas Reduction, Net-Present Value, Renewable Energy Production, Levelized Cost of Energy, and Discounted Payback
Period. SolOpt provides unique analysis capabilities not currently available in any other software programs. The
optimization procedure and tool comparison study results with industry accepted tools for both SHW and PV are presented.
INTRODUCTION
As organizations throughout the United States look for ways to cost effectively deploy solar technologies throughout
their entire building stock, a need arises to develop a standardized solar rooftop optimization tool. Traditionally, photovoltaic
(PV) and solar hot water (SHW) systems have been designed with separate design tools, making it difficult to determine the
appropriate mix of the two technologies. Without a tool that optimizes the two systems on a single building through a single
modeling program, building planners are forced to make less-than-optimal solar installation decisions. Because
meteorological characteristics and incentives vary significantly from one region of the country to the next, it is becoming ever
more important to optimize rooftops for the synergistic application of each solar technology.
A new tool developed at the National Renewable Energy Laboratory (NREL) addresses these issues through the use
of a new modeling program that optimizes the roof space of a facility for PV and SHW systems. The tool was designed with
the intention of being as user friendly as possible without sacrificing calculation robustness. The tool is set up with a variety
of default and auto-sizing features that allow the user to get started with an analysis even if they do not have a technical
background. The tool also has an advanced inputs capability that allows users of a more technical nature to modify detailed,
built-in inputs. Starting with 10 inputs, the user will be prompted to enter information on the building location, energy usage,
building type, energy rates, and existing building system type. SolOpt then uses built-in data to optimize the roof area
allocations and determine the economics associated with each system.
Lars Lisell, Ian Metzger, and Jesse Dean are Engineers at the National Renewable Energy Laboratory (NREL) in the Integrated
Applications Center, Golden CO. NREL is a national laboratory of the U.S. Department of Energy Office of Energy Efficiency and
Renewable Energy Operated by the Alliance for Sustainable Energy, LLC
METHODOLOGY
The goal of the analysis is to determine the best way to utilize the available space between SHW and PV at a
specific site. For a basic analysis, there are 10 pieces of information that are critical.
User Inputs
The first input into the program is the location. SolOpt uses TMY3 datasets to access location specific weather files.
There are currently datasets available for 1,454 locations. [1] The next user input is a total area that can be utilized for a solar
system, which is used to set the capacity limits for the two solar systems. The third user input is the facility square footage.
Building square footage and building type information is used to develop the domestic hot water (DHW) energy intensity
(gallons/ft2) and draw profile. DHW energy intensity and DHW draw profiles were developed for 9 facility types using
default data from the energy modeling program eQUEST™.[2] These DHW loads are used to size the SHW system and
analyze the total energy benefit. The next user input is the building tax status. This information is used to determine the tax
appetite of the end user and the user’s ability to capture the federal investment tax credits. The next user input is the current
DHW system type. This field is used to characterize the existing fuel source and system efficiency. There are 11 options for
existing DHW system types. Once the current DHW system is selected, the user must enter the utility rates for the facility.
The electric rate input is currently a blended electric rate and is intended to include demand and energy use charges. The
annual consumption for the facility is also entered and is used to limit the amount of energy the building can receive for a
solar power system. The utility rates are used to calculate the economics of each system. Along with these basic inputs, there
are 45 additional advanced inputs that can be accessed by the user who has further knowledge of solar power systems. The
default values for the advanced inputs where chosen based on industry standard designs.
Production Calculation
The production calculation for each of the solar technology types are calculated by running different ratios of the
available area through the calculation process. The method used involves dividing the area into 50 parcels, which range from
0% to 100% of the total usable size in 2% intervals. System production is then calculated for each of the 50 parcel sizes for
both PV and SHW. The production values are calculated using a TMY3 [3] data file that the user selects.
PV Production. The PV system production is calculated using panel properties from a database of panel types and
manufacturers. This subset of panel options was taken from the System Advisor Model [4] database of panels. The user can
select from any of the panels that are currently listed. The production calculation algorithms are based on the simplified
efficiency model [5]. The PV module output is based on the temperature-adjusted efficiency of the module. The cell
temperature of the module is calculated on an hourly basis, and the efficiency of the module is adjusted accordingly [6]. The
PV production calculation also includes the inverter efficiency curve based on a standard inverter efficiency curve. The
following results are reported to the user for PV systems: Optimal PV System Size (ft2), Optimal PV System Capacity
(kilowatt [kW]), PV Capacity Factor (%), Number of PV Panels, Installed Cost of PV ($), and Annual System Production
(kilowatt-hour [kWh]). The production calculation assumes the system is a grid-connected PV system, with all of the power
produced by the system either used in the building or fed back into the grid.
SHW Production. The SHW system is modeled as an indirect active system with a wrap-around heat exchanger.
The storage tank is assumed to be a solar preheat tank that supplies the DHW system with preheated water. The SHW panel
properties are defaulted to a 4ft x 8ft flat plate collector. Additional SHW panel properties for various panel types can be
found on the Solar Rating and Certificate Corporation (SRCC) website [7], and the user can enter this information into
SolOpt to conduct calculations with various panel types. The amount of heat transfer from the working fluid to the tank is
calculated through an hourly heat exchanger effectiveness calculation. The heat exchanger properties are defaulted as U-
value of 128 watts per degree Celsius (W/C) [8] and a glycol flow rate of 0.2 kilograms per second (kg/s) per solar collector.
[9] The working fluid is assumed to be 34% glycol and 66% water for the calculation. The SHW system calculations are
based on the methods outlined in the “Solar Engineering of Thermal Processes” text [10]. The hourly calculation was
performed by setting up a system of equations and allowing the system to iterate until the tank temperature converged upon a
point within 0.001% convergence. The initial tank temperature is arrived at by giving the tank an initial warm-up period of 2
months. This ensures that the yearly production is representative of the system production over the life of the project. The
amount of auxiliary energy needed to bring the water up to the DHW set point is calculated based on existing DHW system
efficiency. The following results are reported to the user for SHW systems: Optimal SHW System Size (ft2), Optimal SHW
System Capacity (kW), SHW Solar Fraction (%), SHW Capacity Factor (%), Installed Cost SHW($), Number of SHW
Panels, and Annual System Production (kWh/yr). The production values for each of the 50 system sizes for SHW and PV are
stored in arrays to be used when calculating the economics associated with each system.
Financial Calculation
Four different system analysis types can be considered when running SolOpt. The system analysis type selected will
determine which financial calculations are conducted. The user can choose to analyze PV only, SHW only, PV and SHW fill
roof, or PV and SHW of any size. These options give the user the flexibility to conduct various scenarios depending on the
type of facility that is being analyzed, and the intention of the user. The following financial calculations are conducted for
each analysis: Cost Savings, Total System Cost, Simple Payback Period, Discounted Payback Period, Net Present Value, Life
Cycle Cost, Cost of Business as Usual, Savings to Investment Ratio, Internal Rate of Return, Levelized Cost of Energy, and
Greenhouse Gas Reduction. The financial analysis includes a project lifetime cash flow that is adjusted based on user inputs
of system degradation, escalation rates, inflation rates, discount rates, and equipment replacement costs. The equipment
replacement cost term allows the user to place a fixed cost at some point in the project life. This could be used to simulate an
inverter replacement or pump replacement. The financial calculations also have the ability to limit the amount of cost savings
of a system based on net metering limits. If the user defines a net metering limit of 100%, the system will not receive cost
benefit from producing more energy than the user enters in the annual consumption fields. The different system analysis
types are explained below.
PV Only and SHW Only Cases. The PV only and SHW only cases are intended for users that only want to consider
one of the technologies available in SolOpt. Some examples of this could include facilities that only have use for one of the
energy types (i.e., no hot water usage) or facilities that already have renewable energy available from other sources (i.e.,
electricity from a hydro plant). If the PV only case is selected, all of the financial calculations are performed with the PV
system production, PV operations and maintenance costs, and PV system costs. If SHW only is selected, all of the financial
calculations are performed with the SHW system production, SHW operations and maintenance costs, and SHW system
costs. For these two analysis types, the financials are calculated for each of the 50 parcel sizes discussed earlier, from 0% to
100% of the roof area.
PV & SHW Fill Roof. The PV and SHW fill roof case is intended for use on buildings with relatively low roof-to-floor
areas or facilities with high energy intensities that can utilize all of the power that can be produced with a rooftop system. For
most multistory facilities, this will be the most applicable case. The vast majority of commercial buildings have a larger
energy consumption than can be generated with a solar power rooftop system. For the PV and SHW fill roof case, the
financial calculations are performed with the PV+SHW system production, PV+SHW operations and maintenance, and
PV+SHW system cost. For this analysis type, the the financials are calculated slightly differently than in the PV only and
SHW only cases. For this analysis, the roof is fully occupied for each calculation. For this analysis, the financials are
calculated for each of the 50 parcel sizes discussed earlier, moving from 100% PV to 100% SHW, with the available roof
area shifting from PV to SHW with each of the 50 steps.
PV and SHW Any Size. The PV and SHW are intended for use on buildings anticipated to require only a portion of
the roof to supply the entire building energy load. This would also be a useful analysis to investigate if there is a system that
requires less than the full roof size that has better economics than a system that utilizes the full roof size. This could
potentially happen as a result of incentive caps, net metering limits, and other factors. For the PV and SHW any size case, the
financial calculations are performed with the PV+SHW system production, PV+SHW operations and maintenance, and
PV+SHW system cost. For this analysis type, the financials are calculated slightly differently than in the last two cases. For
this analysis, all combinations of the 50 different system sizes for each technology are considered. This equates to 1,326
different system configurations. A financial calculation is conducted for each of these system configurations. By using this
method, the tool will find the most appropriate system configuration whether it is a full roof, an empty roof, or some
combination of systems that leaves the roof partially filled.
OPTIMIZATION ALGORITHMS
The optimization algorithm for SolOpt allows the user to select from 5 optimization criteria options. The 5 options
are: Maximize Net Present Value (NPV), Maximize Greenhouse Gas Reduction, Maximize System Energy Production,
Minimize Discounted Payback Period (DPP), and Minimize Levelized Cost of Energy. These criteria were selected in order
to allow the tool to be effective across a wide range of project goals. Once the optimization criteria is selected, the tool finds
the system configuration that meets the criteria. This is accomplished by cycling through the system configuration
possibilities, calculating the optimization criteria parameter, and determining if it is a minimum or maximum. When all of the
system configurations have been cycled through, the minimum/maximum is printed to the output table with all of the outputs
of the specific system. An example output plot can be seen below in Figure 1.
Figure 1 The figure illustrates a typical simulation result plot for a PV & SHW fill roof case. The optimization
criteria that is selected is Maximize Greenhouse Gas Reduction. The output for this analysis recommends
20% of the roof be filled with SHW and 80% of the roof be filled with PV.
UNIQUE CHARACTERISTICS
SolOpt introduces several unique characteristics into its simulation methodology. These unique characteristics separate
SolOpt from other simulation tools. This tool is built from an assessor’s standpoint, making it simple and easy to use for
those who are conducting solar assessments and feasibility studies. SolOpt brings together PV and SHW technologies and
analyzes the combined system, including production, financial, and design components, to produce a comprehensive solution
for using available space most effectively. Some of the unique characteristics compliment the usability of the tool and others
increase the technical integrity.
Integrated System and Financial Analysis
Most current simulation tools consider one technology at a time and simulate a single production and financial analysis
based on a user input capacity. SolOpt incorporates a unique method to accommodate a more integrated approach that
common to those conducting assessments. Usable area is often the most critical input and the tool determines the most
effective solution for the area based on the 5 different optimization criteria. This allows users to consider more possibilities
and optimize the usage of the available area. SolOpt removes the “either, or” mindset from solar technologies and analyzes an
integrated system.
Implicit Method for Calculating SHW Tank Temperature
An analytical model based on the methods described in the Solar Engineering of Thermal Processes textbook by Duffie
and Beckman was used for the SHW analysis. The SHW tank temperature equation is solved implicitly for every time step in
an hourly calculation, resulting in an indeterminate tank temperature equation listed below in Equation 1. The equation is
indeterminate because the useful solar energy, tank skin losses, and fluid characteristics are functions of the tank temperature.
Therefore, it requires iterative methods to solve for tank temperature within a specified allowable error. The allowable error
was selected at 0.001%. However, it was found that increasing the allowable error can significantly reduce simulation time
without significant effects on the solution.
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Anti-scalding SHW Mixing Valve
An anti-scolding mixing valve was included in the analytical model of the SHW system. This allows the SHW storage
tank to exceed the service hot water set-point. The solar hot water system can therefore absorb more solar energy than if it
was limited by the set-point. The SHW storage tank is typically limited to a maximum temperature of 180oF. In practice, if
SHW storage tank temperature exceeds the service hot water set-point, it is mixed with mains water to achieve the desired
temperature. This is accomplished using the First Law of Thermodynamics and several conditional statements to enable the
control algorithms for this device.
SHW Dynamic Fluid Characteristics and Heat Exchanger Effectiveness
Fluid thermal characteristics for the different water streams and water-glycol solution are calculated at each time step. A
regression analysis was completed for the thermal dependence of various fluid characteristics including specific heat and
density. Since different parts of the system are operating at different temperatures (storage tank, mains, SHW loop, service
hot water), capturing the changes to fluid characteristics due to the thermal effects on density and specific heat becomes a
significant influence on the accuracy of the solution.
The thermal dependencies of the water-glycol solution also affect the effectiveness of the heat exchanger because of the
specific heat fluctuations. Therefore, the heat exchanger effectiveness is calculated for each time step to account for the
variations in the temperature of water-glycol solution. The heat exchanger effectiveness equation can be seen below in
Equation 2. In a sensitivity analysis, heat exchanger effectiveness was found to be highly influential on simulation results,
and using a static effectiveness could introduce up to a 5% error.
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Automatic SHW System Pipe and Pump Sizing
SolOpt optimizes roof area for the best mix of PV and SHW; therefore, system sizes are constantly changing throughout
the simulation. To account for changing system sizes, some design features of SHW systems are automated, such as pipe
size, pump size, and solar storage tank capacity. A regression analysis on how pipe and pump size change with system size
was conducted to approximate the relationship between pipe and pump sizes and collector area. Table 1 describes this
relationship [9]. This unique characteristic of SolOpt provides optimal system design components as part of the simulation
results.
Table 1. General Pump Requirements for Drainback Systems Using Water or 33-40% Glycol/Water Mixture
Collector Area (ft2)
Pipe Size (Inches)
Pump Size (HP)
0-96
¾
1/25
96-135
7/8
1/12
135-280
1 1/8
1/8
280-360
1 ¼
1/6
360-480
1 ½
1/6
Prebuilt Hot Water Draw Profiles and Building Size Multipliers
SolOpt includes prebuilt, hot water draw profiles and hot water use multipliers based on building size and type. These
two features simplify the amount of information needed to run a simulation. The hot water draw profiles account for time of
day usage and the hot water use multipliers estimate the total hot water load. The draw profiles and the hot water usage
intensity were developed using the default building models in eQUEST™. This data is used to develop the hourly hot water
draw based on the building type selected by the user. The building specific data can be seen below in Figure 3 and Figure 4.
Automatic Panel Spacing to Avoid Self-Shading
SolOpt automatically takes panel spacing into account by derating the usable area based on panel tilt. The system is
assumed to be designed to have no shading on the winter solstice at noon. Sun angles are calculated for this time to quantify
length of a panel’s shadow. Then the percentage of area increase per panel, due to shading, is used to derate the usable area.
This allows the simulation to take into account system layout effects without burdening the tool user. Based on these system
layout characteristics and panel characteristics, the panel power density is updated. This feature represents the give-and-take
between using a panel tilt to increase energy production and using a flat mount to maximize power densities.
(3)
Total _ Area
TOOL COMPARISON STUDY
In a technical comparison against the System Advisor Model the tool was observed to agree within 7% for all
portions of the production curve (see the plot below in Figure 2). The comparison was conducted using similar system
configurations, and run using the same hot water draw profile. This comparison was conducted using the parametric category
in the Solar Advisor Model, an industry accepted and validated solar calculation tool.
FUTURE IMPROVEMENTS
NREL is currently developing a plan to incorporate the mathematical solver and unique features of SolOpt into one of
NREL’s Web-based solar analysis tools, In My Backyard (IMBY) [11]. IMBY is a solar simulation tool designed to
introduce homeowners and business owners to the potential benefits of renewable energy. Its main purpose is to provide an
easy-to-use interface to estimate the hour-by-hour amount of electricity produced by a PV system over a year. IMBY
provides a map-based interface and allows a user to specify an address at which to place a PV system. The map centers itself
on that address and the user may draw a potential PV system anywhere on the map. After the user has drawn a system,
several default values are used to populate information about the PV system’s configuration. These values are the size, derate,
tilt, and azimuth of the PV system. The size represents the DC rating of the system. NREL is currently working on adding
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more technologies to IMBY, including a number of SHW systems. The map-based interface and future inclusion of SHW
systems make IMBY a natural match for SolOpt. Users will have the ability to simply draw a rectangle where they want to
locate the system and optimize the use of both technologies. The incorporation into IMBY will also provide the benefit of
automatically linking to weather data, utility rates, and incentives.
Figure 2 This plot depicts the production values reported by the two tools. The SAM plot has 7% error bars.
Figure 3 The 9 different draws that are used based on the building type. The ‘Other’ category refers to the
‘Unknown, Custom or Mixed Use’ draw profile in eQUEST™.
Figure 4 This figure outlines the different hot water use intensities of the different building types included in SolOpt.
These are combined with the draw profile values to generate the hourly hot water draw. The ‘Other’
category refers to the ‘Unknown, Custom or Mixed Use’ hot water draw in eQUEST™.
NOMENCLATURE
Ts = SHW Tank Temperature
Ts
-= SHW Tank Temperature of previous time step
Tm = Mains Water Temperature
Qu = Useful Solar Energy
Qtankloss = Skin losses from SHW Tank
Drawh = Hot Water Draw Flow Rate
CPs = Specific Heat of Water in the SHW Tank
CPm = Specific Heat of Water from the Mains
ps = Density of Water in SHW Tank
Vols = Volume of Water in SHW Tank
ε = Heat Exchanger Effectiveness
UAhx = Heat Exchanger UA-value
m = Mass Flow Rate of Water-Glycol Solution
CP = Specific Heat of Water-Glycol Solution
αS = Sun Altitude Angle on Winter Equinox at Noon
REFERENCES
[1] Wilcox, S., and Marion W., 2005, “Users Manual for TMY3 Data Sets,” NREL/TP-581-43156
[2] eQUEST Building Energy Simulation Tool. http://doe2.com/equest/
[3] ASME ES2009-90461. 2009. Analysis of Web-Based Solar Photovoltaic Mapping Tools. San Francisco: American Society of Mechanical Engineers.
[4] System Advisor Model. https://www.nrel.gov/analysis/sam/
[5] Menicucci, D.F. “Photovoltaic Array Performance Simulation Models.” Photovoltaics and Insolation Measurements Workshop, Vail, CO, U.S.A., June
30 – July 3, 1985. Albuquerque, NM: Sandia National Laboratories, 1985.
[6] Fuentes, M. K. “A Simplified Thermal Model of Photovoltaic Modules.” SAND85-0330 Albuquerque, NM: Sandia National Laboratories, 1985.
[7] Solar Rating and Certification Corporation. http://www.solar-rating.org/ratings/ratings.htm
[8] "Rheem Consumer Website -Water Heating -Solar Water Heaters." Message to the author. 17 Sept. 2010. E-mail.
[9] Lane, Tom. Solar Hot Water Systems. Florida: Energy Conservation Services. 2004.
[10] Duffie, J.A.; Beckman, W.A. Solar Engineering of Thermal Processes. 3rd edition. New Jersey: John Wiley & Sons Inc. 2006
[11] In My Backyard (IMBY) http://www.nrel.gov/eis/imby/
