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SolOpt: a novel approach to solar rooftop optimization.

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.

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/[ft.sup.2]) and draw profile. DHW energy intensity and DHW draw profiles were developed for 9 facility types using default data from the energy modeling program eQUES[TM].(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 ([ft.sup.2]), 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 ([ft.sup.2]), 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 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 OMITTED]

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.

[T.sub.s] = [T.sup.-.sub.s] + [([Q.sub.u]) - ([Q.sub.tankloss]) - ([Draw.sub.h]*[Cp.sub.s]*([T.sub.m] + 273.15))/ps*[vol.sub.s]*[Cp.sub.s] * dt (1)

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 180 [degrees]F. 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.

[epsilon] = 1 - [-UA.sub.hx]/[e.sup.mCP] (2)

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 ([ft.sup.2])  Pipe Size (Inches)  Pump Size (HP)

0-96                                        3/4            1/25
96-135                                      7/8            1/12
135-280                                   1 1/8             1/8
280-360                                   1 1/4             1/6
360-480                                   1 1/2             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[TM]. 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.

PV_Area = (tan([[alpha].sub.s)/Cos(tilt - slope)*Tan(tilt - slope) + Tan([alpha].sub.s)) * Total_Area (3)

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 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 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

NOMENCLATURE

[T.sub.s] = SHW Tank Temperature

[T.sup.sub.s] = SHW Tank Temperature of previous time step

[T.sub.m] = Mains Water Temperature

[Q.sub.u] = Useful Solar Energy

[Q.sub.tankloss] = Skin losses from SHW Tank

[Draw.sub.h] = Hot Water Draw Flow Rate

[C.sub.ps] = Specific Heat of Water in the SHW Tank

[C.sub.pm] = Specific Heat of Water from the Mains

[p.sub.s] = Density of Water in SHW Tank

[Vol.sub.s] = Volume of Water in SHW Tank

[epsilon] = Heat Exchanger Effectiveness

[UA.sub.hx] = Heat Exchanger UA-value

m = Mass Flow Rate of Water-Glycol Solution

[C.sub.p] = Specific Heat of Water-Glycol Solution

[q.sub.s] = 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/

Lars Lisell Ian Metzger Jesse Dean Member ASHRAE Member ASHRAE

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
COPYRIGHT 2011 American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc.
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Author:Lisell, Lars; Metzger, Ian; Dean, Jesse
Publication:ASHRAE Transactions
Article Type:Report
Geographic Code:1USA
Date:Jul 1, 2011
Words:3860
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