Solar Architecture in Energy Engineering

Deﬁnition: Solar Architecture represents the conﬂuence of the two disciplines of energy engineering and architecture. The concept of Solar Architecture deﬁnes a decision-making process to select, design, deploy, and operate solar energy-enabled solutions for environments where solar energy resources are part of the energy mix. The principles of Solar Architecture include maximizing solar energy harvesting from solution’s surfaces with a positive balance of energy, carbon, and cost provided by the solution. Solar Architecture application selection is built on two major cornerstones, features and groups, deﬁning the best options in energy engineering of a solar solution. Solar surfaces are key to solar architecture. They are the “heart”, and balance-of-system components are the “muscles” of solar solutions. Addressing energy losses in photovoltaic, solar to thermal, and solar to chemical energy conversion allows for increasing energy harvesting yield. Life Cycle Assessment and solar energy harvesting methodologies based on solar surface characteristics deﬁne Solar Architecture Balance. This balance allows for deﬁning energy, carbon, and cost return on investment for solar solutions and selecting the best solution for related assets/environment.


Introduction
This Solar Architecture guide describes the practice of efficiently and productively harnessing the main source of external energy supplying our planet, solar energy. Solar energy is vital to life on Earth, accessible to everybody, and transformable to other key sources of energy such as electrical, thermal or chemical.
To harvest solar energy, sunlight must strike a surface. We are surrounded by these surfaces in the built and natural environment.
Optimized "solar surfaces" are always well organized. In the natural world, photosynthetic plants are the most sophisticated practitioners of solar architecture. In the built environment, there are more challenges in achieving both practical and beautiful solutions. As Vitruvius famously said, one must strive to achieve "firmness, commodity, and delight".
Energy engineers know that "solar surfaces" are essential to energy harvesting but not the only part of it. Solar energy harvesting is provided by a "system" in which solar surfaces are the core. Depending on the nature of an engineering solution, the balance-of-system components may differ from each other and vary from application to application.
This guide describes how Solar Architecture in Energy Engineering enables solar energy harvesting.

History
Energy engineering is one of the emerging engineering disciplines, based on energy sustainability and involving energy efficiency and clean energy concepts for and approaches to any environment.
We believe that energy engineers were among the first to recognize the value and importance of energy in our environments, and our ability to use and leverage energy in

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Selecting and comparing solution options for energy resources; • Choosing the best option in terms of energy resources, their use, cost, and carbon footprint; and • Deploying and maintaining the use of the chosen energy resource mix.
Sustainable energy engineering practice: • Selects and compares solution options for energy resources where renewable energy is a growing and preferably-a dominating resource in the energy mix; and • Minimizes the use, cost, and carbon footprint of the energy mix.
Solar energy engineering practice is focused on: • Selecting and comparing solution options for solar energy resources; and • Deploying and maintaining the use of solar energy resources.
Sustainable Architecture: Sustainable architecture minimizes harmful impacts to communities and the ecosystem at large through "improved efficiency and moderation in the use of materials and energy" [11]. Solar Architecture is a sustainable system architecture addressing solar energy harvesting capability embedded in solar solutions. Solar Architecture Balance: Solar architecture balance is a set of Solar Architecture metrics. It includes: • Energy balance-the difference between solar energy harvested by the solution (the energy absorbed by the solution's solar harvesting surfaces over its life cycle) and energy embedded in a solution; • Carbon balance-the difference between carbon saved (the carbon dioxide not emitted by a solution or an environment the solution is a part of due to the solution's operations during its life cycle) and carbon embedded in the solution; and • Cost balance-the difference between the saved cost of energy (the cost that would be paid to a utility/energy service provider should the solution not use solar energy harvested by the solution in its operations over the life cycle) and the cost of the solution components and operations.

Solar Architecture Table
Based on application features and groups for a solution, the best solution options are selected for energy engineering. The Solar Architecture Table reviews and considers possible options in selecting and engineering solutions. A table template defining the features of a solution to be reviewed at a high level is shown in Table 1. Based on application features and groups for a solution, the best solution options are selected for energy engineering. The Solar Architecture Table reviews and considers possible options in selecting and engineering solutions. A table template defining the features of a solution to be reviewed at a high level is shown in Table 1. The Solar Architecture Table may also be used for solar harvesting to add new solution categories and applications in energy engineering and provide solution selection and design.
Examples of solar solution options are shown in Figure 1.

Solar Architecture Table
Based on application features and groups for a solution, the best solution options are selected for energy engineering. The Solar Architecture Table reviews and considers possible options in selecting and engineering solutions. A table template defining the features of a solution to be reviewed at a high level is shown in Table 1. The Solar Architecture Table may also be used for solar harvesting to add new solution categories and applications in energy engineering and provide solution selection and design.
Examples of solar solution options are shown in Figure 1.

Solar Architecture Table
Based on application features and groups for a solution, the best solution options are selected for energy engineering. The Solar Architecture Table reviews and considers possible options in selecting and engineering solutions. A table template defining the features of a solution to be reviewed at a high level is shown in Table 1. The Solar Architecture Table may also be used for solar harvesting to add new solution categories and applications in energy engineering and provide solution selection and design.
Examples of solar solution options are shown in Figure 1.

Solar Architecture Table
Based on application features and groups for a solution, the best solution options are selected for energy engineering. The Solar Architecture Table reviews and considers possible options in selecting and engineering solutions. A table template defining the features of a solution to be reviewed at a high level is shown in Table 1. The Solar Architecture Table may also be used for solar harvesting to add new solution categories and applications in energy engineering and provide solution selection and design.
Examples of solar solution options are shown in Figure 1.

Solar Architecture Table
Based on application features and groups for a solution, the best solution options are selected for energy engineering. The Solar Architecture Table reviews and considers possible options in selecting and engineering solutions. A table template defining the features of a solution to be reviewed at a high level is shown in Table 1. Examples of solar solution options are shown in Figure 1. Examples of solar solution options are shown in Figure 1. Examples of solar solution options are shown in Figure 1. Examples of solar solution options are shown in Figure 1. Examples of solar solution options are shown in Figure 1.

Solar Architecture Table
Based on application features and groups for a solution, the best solution options are selected for energy engineering. The Solar Architecture Table reviews and considers possible options in selecting and engineering solutions. A table template defining the features of a solution to be reviewed at a high level is shown in Table 1. Examples of solar solution options are shown in Figure 1.

Solar Architecture Table
Based on application features and groups for a solution, the best solution options are selected for energy engineering. The Solar Architecture Table reviews and considers possible options in selecting and engineering solutions. A table template defining the features of a solution to be reviewed at a high level is shown in Table 1. Examples of solar solution options are shown in Figure 1.
The Solar Architecture Table may also be used for solar harvesting to add new solution categories and applications in energy engineering and provide solution selection and design.
Examples of solar solution options are shown in Figure 1.
o Marine vehicles (e.g., solar ferries) and infrastructure (e.g., solar buoys) o Aerial vehicles (e.g., solar-powered planes) and infrastructure (e.g., solar run-  An example of the Solar Architecture Table defining and comparing solution options is shown in Appendix A for the solar buildings/external envelope applications group.

Solar Harvesting Surface Methodologies
Solar Harvesting Surface Methodologies include the means to define and manage An example of the Solar Architecture Table defining and comparing solution options is shown in Appendix A for the solar buildings/external envelope applications group.

Solar Harvesting Surface Methodologies
Solar Harvesting Surface Methodologies include the means to define and manage solar harvesting surfaces, understand solar irradiation on these surfaces, and access solar irradiation data for these surfaces.

Energy Harvesting Surfaces-The Key to Solar Architecture
The key to solar architecture is surface. To better appreciate energy engineering, we can paraphrase the statement of David Leatherbarrow and Mohsen Mostafavi in their "Surface Architecture" publication [12]: "The properties of an object's surface absorbing sunlight . . . are not merely superficial; they construct the spatial effects by which solar architecture communicates. Through its surfaces an object declares both its autonomy and its ability to be energized".
To define any object in energy engineering from a Solar Architecture standpoint, the following statements and definitions are important.

DIVERSITY OF SURFACES
Each object has a number of surfaces, from one (e.g., simple PV module) to many (e.g., walls and roofs of a building)-see Figure 2. For non-linear (curved) surfaces, the tilts and azimuths of ent.
• Every point on the surface has its specific geographic coordin longitude).

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Any surface has latitude and longitude ranges.

SPATIAL DIMENSIONS
• Any surface in 3D space can be described by two spatial properties: tilt and azimuth.
For linear surfaces (planes), tilt and azimuth of all the points on the surface is the same (Figure 3). For non-linear (curved) surfaces, the tilts and azimuths of points may be different.
• Every point on the surface has its specific geographic coordinates (e.g., latitude and longitude). • Any surface has latitude and longitude ranges.
o For linear surfaces (planes), tilt and azimuth of all the points on the surface is the same (Figure 3). o For non-linear (curved) surfaces, the tilts and azimuths of points may be different.
• Every point on the surface has its specific geographic coordinates (e.g., latitude and longitude). • Any surface has latitude and longitude ranges.

CONNECTION WITH THE SUN: STATIC OR DYNAMIC
The tilt and azimuth of the Sun in relation to a surface point are changing through time.
Any surface of the object may be static (or stationary) or dynamic. If the surface is static, the surface points have tilt and azimuth defined and not changing. This means that for every surface point, the difference between the tilt and azimuth of the point and the relative tilt and azimuth of the Sun are known at any moment.
If the surface is dynamic, the tilt and azimuth of any point on this surface will be changing depending on the move of this surface, and the difference between the tilt and azimuth of the point and the relative tilt and azimuth of the Sun have to be calculated for any moment.

OBSTRUCTION OF SUNLIGHT AND SOLAR ENVELOPE
Sunlight incident on a surface may be obstructed by surrounding objects (e.g., a tree, a building, a hill, or a moving truck), which reduces the ability to harvest energy.
The shading effect depends on the nature of the obstructing objects and is generally proportional to the altitude angle to the top of the object and spread of the azimuth angle

CONNECTION WITH THE SUN: STATIC OR DYNAMIC
The tilt and azimuth of the Sun in relation to a surface point are changing through time. Any surface of the object may be static (or stationary) or dynamic. If the surface is static, the surface points have tilt and azimuth defined and not changing. This means that for every surface point, the difference between the tilt and azimuth of the point and the relative tilt and azimuth of the Sun are known at any moment.
If the surface is dynamic, the tilt and azimuth of any point on this surface will be changing depending on the move of this surface, and the difference between the tilt and azimuth of the point and the relative tilt and azimuth of the Sun have to be calculated for any moment.

OBSTRUCTION OF SUNLIGHT AND SOLAR ENVELOPE
Sunlight incident on a surface may be obstructed by surrounding objects (e.g., a tree, a building, a hill, or a moving truck), which reduces the ability to harvest energy.
The shading effect depends on the nature of the obstructing objects and is generally proportional to the altitude angle to the top of the object and spread of the azimuth angle of the solar path behind the object in relation to the surface point. Note that everything below the surface plane does not produce shading of direct sunlight.
Access to sunlight and the nature of obstruction of sunlight for a set of surfaces are well defined by Ralph Knowles in his solar envelope concepts [13][14][15][16] (see Figure 4). "The solar envelope is a construct of space and time: the physical boundaries of surrounding properties and the period of their assured access to sunshine. These two measures, when combined, determine the envelope's final size and shape" [17].
Encyclopedia 2022, 2, FOR PEER REVIEW 9 of the solar path behind the object in relation to the surface point. Note that everything below the surface plane does not produce shading of direct sunlight. Access to sunlight and the nature of obstruction of sunlight for a set of surfaces are well defined by Ralph Knowles in his solar envelope concepts [13][14][15][16] (see Figure 4). "The solar envelope is a construct of space and time: the physical boundaries of surrounding properties and the period of their assured access to sunshine. These two measures, when combined, determine the envelope's final size and shape" [17].

ENERGY SINK AND THERMAL MASS
Holding, discharging, or transmitting solar energy imposed on a surface depends on the thermal mass the surface is a part of or connected to, making it an energy sink.
Thermal mass allows for keeping energy in the form of heat or chill, providing "inertia" against temperature fluctuations [18].
The impact of thermal mass of the surface is important for passive or active heating and cooling.

Solar Irradiation on Surfaces: Direct and Diffuse
Solar energy at any object's surface is provided by two key irradiation components: direct and diffuse irradiation. The "direct" irradiation component directly reaches a sur- ENERGY SINK AND THERMAL MASS Holding, discharging, or transmitting solar energy imposed on a surface depends on the thermal mass the surface is a part of or connected to, making it an energy sink.
Thermal mass allows for keeping energy in the form of heat or chill, providing "inertia" against temperature fluctuations [18].
The impact of thermal mass of the surface is important for passive or active heating and cooling.

Solar Irradiation on Surfaces: Direct and Diffuse
Solar energy at any object's surface is provided by two key irradiation components: direct and diffuse irradiation. The "direct" irradiation component directly reaches a surface. The "diffuse" irradiation component is scattered by the particles in the atmosphere and is non-directional. The total of these two components reaching the same surface is called global irradiation [19].
The object's surface at each of its surface points absorbs and/or reflects solar energy. Total solar energy absorbed by the surface is a spatial integral of energy absorbed by its surface points.
Solar irradiance refers to the rate of energy received from the Sun per surface unit: W/sq.m. Integrating it spatially over a surface will give solar irradiance for the surface in units of power (e.g., Watts). Integrating it temporally over a period of time provides solar irradiation in units of energy (e.g., Wh or Joules). This spatio-temporal integration is used to define the characteristics of an object or solution's Solar Architecture.

Access to Energy Harvesting Data: Solar Resource Maps
Data on solar irradiation necessary for solar solutions are represented by solar maps. These maps with dedicated spatial resolution may present daily, monthly, and annual solar irradiation averages.
These maps are often presented online in web mapping applications built in Geographic Information System (GIS) environments; these web applications also define spatial and temporal resolution of the maps presented.
Many countries today have public maps and web mapping applications issued at the federal level and covering the whole jurisdiction. These maps are built on solar radiation and meteorological data collected over a long period of time. An example is the National Solar Radiation Data Base (NSRDB) [20]. An open dataset, the current NSRDB provides solar irradiance at a 4 km horizontal resolution for each 30 min interval computed by the National Renewable Energy Laboratory's (NREL's) Physical Solar Model. The data can be freely accessed via NREL's web link [21].
Often, solar resource maps with higher resolution are required for regional (such as state or provincial maps) and municipal (urban solar maps) applications (see Figure 5). These maps may be also available and free to use.
Encyclopedia 2022, 2, FOR PEER REVIEW Solar Radiation Data Base (NSRDB) [20]. An open dataset, the current NSRDB prov solar irradiance at a 4 km horizontal resolution for each 30 min interval computed b National Renewable Energy Laboratory's (NREL's) Physical Solar Model. The data ca freely accessed via NREL's web link [21].
Often, solar resource maps with higher resolution are required for regional (suc state or provincial maps) and municipal (urban solar maps) applications (see Figur These maps may be also available and free to use. Depending on solar solution applications, solar maps at the community and site may require higher spatial and temporal resolution and display additional propertie quired for solar engineering (e.g., global horizontal, direct normal and diffuse irradia specific tilt and azimuth, etc.)-see Figure 6. Depending on solar solution applications, solar maps at the community and site level may require higher spatial and temporal resolution and display additional properties required for solar engineering (e.g., global horizontal, direct normal and diffuse irradiation, specific tilt and azimuth, etc.)-see Figure 6. Depending on solar solution applications, solar maps at the commun may require higher spatial and temporal resolution and display addition quired for solar engineering (e.g., global horizontal, direct normal and di specific tilt and azimuth, etc.)-see Figure 6. To inform architects, energy engineers, and their clients on the use o the derivatives of solar resource maps defining solar energy harvesting b are added to solar maps. As an example, photovoltaic potential maps units) are provided.
An example of access to solar resource and solar harvesting techno tovoltaic potential and solar resource maps of Canada" web mapping To inform architects, energy engineers, and their clients on the use of solar solutions, the derivatives of solar resource maps defining solar energy harvesting by solar solutions are added to solar maps. As an example, photovoltaic potential maps (in kWh/kW peak units) are provided.
An example of access to solar resource and solar harvesting technology is the "Photovoltaic potential and solar resource maps of Canada" web mapping application [22]. This application gives estimates of photovoltaic potential (in kWh/kW peak ) and of the mean daily global irradiation per surface unit for any location in Canada on a 2 km grid.
Another example of access to solar resource and photovoltaic power potential is the Global Solar Atlas provided by the World Bank Group to support the scale-up of solar power in its client countries [23].
An important step in understanding solar resource and its energy generation potential in communities was presented by urban solar maps. These maps describe solar generation potential at building roof resolution and the efficiency of solar technologies used [24].
The initial generation of solar urban maps was based on the analysis of 2D aerial images available from open sources such as Google Maps. These two-dimensional images did not allow for collecting correct data on roof slopes nor provide information on building walls. Additionally, these maps did not allow for automated reconstruction of 3D environments to consider roof-specific obstructions to solar radiation. However, these maps presented important spatial information on the distribution of solar generation potential in communities.
The next generation of urban maps was driven by advances in aerial photo imagery. Better image resolution as well as the use of oblique photography which showed buildings and houses from all sides allowed for manual reconstruction of building structure in 3D. Still, the technology did not allow for automated creation of a 3D model of the city/community and automated processing of obstructions data.
The third and most advanced urban solar mapping was based on airborne LiDAR (Light Detection and Ranging) technology for optical remote sensing. LiDAR measures properties of scattered light to find range and/or other information of a distant target.
Airborne LiDAR data are used to reconstruct a 3D urban environment using digital elevation models of urban areas with high spatial resolution. Using LiDAR-based surface models accounts for surface geometry and allows for precise determination of the impact of obstructions to solar irradiation [25] (see Figure 7).
Better image resolution as well as the use of oblique photography which showed build ings and houses from all sides allowed for manual reconstruction of building structure in 3D. Still, the technology did not allow for automated creation of a 3D model of th city/community and automated processing of obstructions data.
The third and most advanced urban solar mapping was based on airborne LiDAR (Light Detection and Ranging) technology for optical remote sensing. LiDAR measure properties of scattered light to find range and/or other information of a distant target Airborne LiDAR data are used to reconstruct a 3D urban environment using digital ele vation models of urban areas with high spatial resolution. Using LiDAR-based surfac models accounts for surface geometry and allows for precise determination of the impac of obstructions to solar irradiation [25] (see Figure 7).

Solar Energy Conversion and Losses Reduction Methodologies
Energy conversion performance is a critical factor defining energy harvesting in a solar solution. The key indicator defining energy harvesting losses in the solution is th

Solar Energy Conversion and Losses Reduction Methodologies
Energy conversion performance is a critical factor defining energy harvesting in a solar solution. The key indicator defining energy harvesting losses in the solution is the Conversion Performance Ratio (PR). It is defined as the ratio between energy input P input and energy output (energy yield) P output of the solution after harvesting losses have been deducted: PR = P output /P input . PR is also called Total Derate Factor D total as it is defined by the total loss in energy harvesting L total : PR = D total = 1 − L total.
Losses in the three major areas of solar energy harvesting-photovoltaic, solar to thermal, and solar to chemical energy conversion-and related technological innovation opportunities focused on reducing the losses and increasing the energy harvesting yield are briefly described below.

Photovoltaic Conversion
PV conversion is used in a very broad spectrum of applications ranging from milliwatts to gigawatts in conversion power. Core conversion components include solar cells/modules/arrays, DC/AC inverters, and/or DC/DC converters.
PV conversion losses include optical losses (e.g., incidence angle modifier factor, soiling loss factor), PV conversion losses (e.g., cell efficiency, cell degradation loss, module quality loss, mismatch loss), and DC-related losses (e.g., DC wiring loss, inverter efficiency). Power grid-related applications also include AC-related losses (e.g., AC wiring loss, system auxiliary losses, transmission losses, and system unavailability).
Technology innovation opportunities with PV include new developments in power conversion efficiency of solar cells.
Research and development of high-efficiency crystalline Si solar cells addressed new cell structures. Examples include passivated emitter rear cell, tunnel oxide passivated contact solar cell, interdigitated back contact cell, heterojunction with intrinsic thin-layer cell, and heterojunction solar cells with interdigitated back contacts [26].
New achievements were demonstrated with thin-film solar cells. Commercially produced copper-indium-gallium-selenide (CIGS) cells that have high potential in flexible or multijunction PV applications demonstrated 16.7% efficiency [27].
Perovskite and organic materials achieving efficiencies of conventional silicon solar cells. Recent work demonstrated a power conversion efficiency of 23.6%, approaching that of conventional silicon solar cells. This technological breakthrough paved the way for flexible, light-weight, low-cost, and ultra-thin photovoltaic cells for wide-ranging applications [28].
Other technological advances addressed operating characteristics of solar cells, such as operating temperature essential for photovoltaic conversion. For example, the rapid development of radiation cooling technology brought opportunities for radiation cooling of solar cells [29].

Solar to Thermal Energy Conversion
Solar thermal solutions mostly include concentrating solar power (CSP) generation and solar water/air heating applications. All these applications experience optical and thermal losses in radiative, convective, and conductive heat transfer.
In central receiver-based CSP plants, solar irradiation is concentrated on a tower mounted receiver by the use of large mirrors (heliostats). Heat transport fluid flowing through the receiver tubes is heated up by absorbing the incident energy on the receiver and is used to produce steam which drives a turbine. Energy losses determine receiver efficiency and CSP generation [30].
In collector-based CSP plants with linear Fresnel or parabolic trough collectors, heat transfer fluid is brought to a power generation unit and a thermal energy storage unit. Energy losses determine collector, power generation, and thermal energy storage efficiency [31,32].
Technology innovation opportunities with CSP applications include new developments in particle technologies. Particle-based CSP (Generation 3) will enable higher temperatures (>700 • C) with direct storage for next generation, dispatchable, concentrating solar power, process heating, thermochemistry, and solar fuels production [33,34]. The particle receiver system uses solid particles (ceramic or sand) that are heated directly as they fall through a beam of concentrated sunlight. Solar water heating is often used for urban and rural applications. The core application components include flat-plate solar collectors (flat-plate or vacuum tube), solar hot water storage tank, water heater, and flow pipe network. Energy losses occur due to heat transfer in collectors [35,36] and to solar hot water recirculation.
Technology innovation opportunities with solar water heating address solar hot water recirculation automation, specifically in multi-unit residential buildings [37].

Solar to Chemical Energy Conversion
Solar chemical solutions cover chemical, electrochemical, and photoelectrochemical and thermochemical areas converting solar energy into hydrogen or other hydrocarbon products such as energy storage media. Photosynthesis is the most sophisticated system of solar-to-chemical energy conversion developed by nature; this solar chemical conversion is currently used for engineered photosynthesis [38].
Technology innovation opportunities are currently focused on efficient and economically attractive thermochemical energy storage (TCES) systems at high temperatures with long-term durability and performance stability [39,40].
Technology advancements in the solar thermochemical area using concentrating solar thermal for industrial decarbonization enable CSP with thermal energy storage to be integrated with high-temperature process technologies to produce economically important products, such as steel, cement, ammonia, and other chemicals and fuels, e.g., used for the decarbonization of the transportation sector [41].
In the engineered photosynthesis area, technology innovation addresses the development of more efficient photosynthesis, along with sustainable and climate-resilient cropping systems to improve both energy and crop yields and crop nutritional value [42].

Life Cycle Assessment Methodologies for Circular Economy
The Life Cycle Assessment (LCA) methodologies referred to in this guide are inspired by the "cradle-to-cradle" approach in the circular economy [43].
The International Organization for Standardization (ISO) 14040 Environmental Management standard series are recommended for selecting methodologies for Life Cycle Assessment in energy engineering practice [44,45].

Life Cycle Energy Assessment
Life Cycle Energy Assessment involves all energy use components of a solution in its life cycle. These life cycle energy components include embodied energy (energy content of all the materials used in solution including manufacturing and processing, transportation, delivery, and installation), operating energy (energy required for operations, maintenance, and upgrades), and waste disposal energy (energy required to demolish solution and to transport the material to landfill sites and/or recycling plants).

Life Cycle Carbon Assessment
Life Cycle Carbon Assessment involves all energy use components of a solution in its life cycle, and carbon cycle support in product eco-design and operations.
The life cycle carbon components include embodied carbon (energy content of all the materials used in solution, including manufacturing and processing, transportation, delivery, and installation), operating carbon (required for operations, maintenance, and upgrades), and recycling.

Life Cycle Cost Assessment
Life Cycle Cost Assessment involves all the components and operations costs of a solution in its life cycle. These life cycle costs include components' embodied energy cost (cost of all the materials used in solution including manufacturing and processing, transportation, delivery, and installation), operating costs (including costs required for operations, maintenance, and upgrades), and recycling/waste disposal costs (costs required to reuse, repair, refurbish and recycle, and to demolish and transport the material to landfill sites and/or recycling plants).

Expected Results and Outcomes
The results and outcomes referred to in this guide are focused on the metrics enabling decision making in selecting and engineering solar solutions.
These metrics define how much energy and carbon were used by the solution options in their life cycles, and what were the costs during this period. The metrics also define how much solar energy was harvested over the life cycle, and what were the carbon and cost savings brought by the solution due to solar energy harvesting. Finally, these metrics define Solar Architecture Balance for each of the reviewed solution options as the difference between energy, carbon, and cost used and saved by these options.

Solar Architecture Ratios
Solutions' Solar Architecture ratios define relationships between energy, carbon, and cost in the solution's life cycle.
There are two groups of the Solar Architecture ratios: • Those using Solar Architecture Balance, and based on energy, carbon, and cost; and • Those using Solar Architecture categories such as life cycle use and energy harvesting, and based on sub-categories such as embodied, operating and waste energy, carbon, and cost. Solar Architecture Balance ratios and related units are defined as follows: • Energy-to-Carbon Ratio, kWh/kg CO 2 • Energy Cost Ratio, kWh/$ • Carbon Cost Ratio, kg CO 2 /$ These ratios allow for comparing solar solutions and support the decision-making process in the initial stages of energy engineering.
Life Cycle Assessment sub-category ratios are defined as follows: • Embodied-to-operating, embodied-to-waste, and operating-to-waste ratios; and • Embodied-to-life cycle use, operating-to-life cycle use, and waste-to-life cycle use ratios within the life cycle use category.
Solar energy harvesting sub-categories are defined as follows: • PV to Solar Thermal, PV to Solar Chemical, Solar Thermal to Solar Chemical; and • PV to Energy Harvesting, Solar Thermal to Energy Harvesting, Solar Chemical to Energy Harvesting.

Solar Architecture Scorecards
A Solar Architecture Scorecard is shown in Table 2. Examples of solar solution options are shown in Figure 1.
Raw Material Extraction 6 (e.g., solar ferries) and infrastructure (e.g., solar buoys) .g., solar-powered planes) and infrastructure (e.g., solar runsigns, terminals, etc. Examples of solar solution options are shown in Figure 1. Manufacturing and Processing 6 (e.g., solar ferries) and infrastructure (e.g., solar buoys) .g., solar-powered planes) and infrastructure (e.g., solar runsigns, terminals, etc. Examples of solar solution options are shown in Figure   Encyclopedia 2022 Examples of solar solution options are shown in Figure   Encyclopedia 2022 Examples of solar solution options are shown in Figure 1. Transportation and Delivery 6 (e.g., solar ferries) and infrastructure (e.g., solar buoys) .g., solar-powered planes) and infrastructure (e.g., solar runsigns, terminals, etc. Examples of solar solution options are shown in Figure   Encyclopedia 2022 Examples of solar solution options are shown in Figure 1. Usage and Retail 6 (e.g., solar ferries) and infrastructure (e.g., solar buoys) .g., solar-powered planes) and infrastructure (e.g., solar runsigns, terminals, etc. Examples of solar solution options are shown in Figure   Encyclopedia 2022 Examples of solar solution options are shown in Figure   Encyclopedia 2022 The Solar Architecture Table may also be used for solar harvesting to ad tion categories and applications in energy engineering and provide solution s design.
Examples of solar solution options are shown in Figure 1.

Waste Disposal
Encyclopedia 2022, 2, FOR PEER REVIEW 16 Specific LCA characteristics below may be defined per unit of solution surface exposed to the Sun: specific energy, kWh/m 2 ; specific carbon, kg CO2/m 2 ; specific cost, $/m 2 . Other specific Solar Architecture characteristics are defined by specific energy, carbon, and cost to solar irradiation on the solution surface(s) over the period of use (in kWh/m 2 ): specific energy, %; specific carbon, kg CO2/kWh; specific cost, $/kWh. Additional metrics such as levelized cost of energy [46] may also be added to solution characteristics.
Existing and in-development LCA reviews for photovoltaic, solar thermal, and solar  Specific LCA characteristics below may be defined per unit of solution surface exposed to the Sun: specific energy, kWh/m 2 ; specific carbon, kg CO2/m 2 ; specific cost, $/m 2 . Other specific Solar Architecture characteristics are defined by specific energy, carbon, and cost to solar irradiation on the solution surface(s) over the period of use (in kWh/m 2 ): specific energy, %; specific carbon, kg CO2/kWh; specific cost, $/kWh. Additional metrics such as levelized cost of energy [46] may also be added to solution characteristics.
Existing and in-development LCA reviews for photovoltaic, solar thermal, and solar  Specific LCA characteristics below may be defined per unit of solution surface exposed to the Sun: specific energy, kWh/m 2 ; specific carbon, kg CO2/m 2 ; specific cost, $/m 2 . Other specific Solar Architecture characteristics are defined by specific energy, carbon, and cost to solar irradiation on the solution surface(s) over the period of use (in kWh/m 2 ): specific energy, %; specific carbon, kg CO2/kWh; specific cost, $/kWh. Additional metrics such as levelized cost of energy [46] may also be added to solution characteristics.
Existing and in-development LCA reviews for photovoltaic, solar thermal, and solar  Specific LCA characteristics below may be defined per unit of solution surface exposed to the Sun: specific energy, kWh/m 2 ; specific carbon, kg CO2/m 2 ; specific cost, $/m 2 . Other specific Solar Architecture characteristics are defined by specific energy, carbon, and cost to solar irradiation on the solution surface(s) over the period of use (in kWh/m 2 ): specific energy, %; specific carbon, kg CO2/kWh; specific cost, $/kWh. Additional metrics such as levelized cost of energy [46] may also be added to solution characteristics.
Existing and in-development LCA reviews for photovoltaic, solar thermal, and solar  Specific LCA characteristics below may be defined per unit of solution surface exposed to the Sun: specific energy, kWh/m 2 ; specific carbon, kg CO2/m 2 ; specific cost, $/m 2 . Other specific Solar Architecture characteristics are defined by specific energy, carbon, and cost to solar irradiation on the solution surface(s) over the period of use (in kWh/m 2 ): specific energy, %; specific carbon, kg CO2/kWh; specific cost, $/kWh. Additional metrics such as levelized cost of energy [46] may also be added to solution characteristics.
Existing and in-development LCA reviews for photovoltaic, solar thermal, and solar  Specific LCA characteristics below may be defined per unit of solution surface exposed to the Sun: specific energy, kWh/m 2 ; specific carbon, kg CO2/m 2 ; specific cost, $/m 2 . Other specific Solar Architecture characteristics are defined by specific energy, carbon, and cost to solar irradiation on the solution surface(s) over the period of use (in kWh/m 2 ): specific energy, %; specific carbon, kg CO2/kWh; specific cost, $/kWh. Additional metrics such as levelized cost of energy [46] may also be added to solution characteristics.
Existing and in-development LCA reviews for photovoltaic, solar thermal, and solar  Specific LCA characteristics below may be defined per unit of solution surface exposed to the Sun: specific energy, kWh/m 2 ; specific carbon, kg CO2/m 2 ; specific cost, $/m 2 . Other specific Solar Architecture characteristics are defined by specific energy, carbon, and cost to solar irradiation on the solution surface(s) over the period of use (in kWh/m 2 ): specific energy, %; specific carbon, kg CO2/kWh; specific cost, $/kWh. Additional metrics such as levelized cost of energy [46] may also be added to solution characteristics.
Existing and in-development LCA reviews for photovoltaic, solar thermal, and solar  Specific LCA characteristics below may be defined per unit of solution surface exposed to the Sun: specific energy, kWh/m 2 ; specific carbon, kg CO2/m 2 ; specific cost, $/m 2 . Other specific Solar Architecture characteristics are defined by specific energy, carbon, and cost to solar irradiation on the solution surface(s) over the period of use (in kWh/m 2 ): specific energy, %; specific carbon, kg CO2/kWh; specific cost, $/kWh. Additional metrics such as levelized cost of energy [46] may also be added to solution characteristics.
Existing and in-development LCA reviews for photovoltaic, solar thermal, and solar

Solar Thermal
Encyclopedia 2022, 2, FOR PEER REVIEW 16 Specific LCA characteristics below may be defined per unit of solution surface exposed to the Sun: specific energy, kWh/m 2 ; specific carbon, kg CO2/m 2 ; specific cost, $/m 2 . Other specific Solar Architecture characteristics are defined by specific energy, carbon, and cost to solar irradiation on the solution surface(s) over the period of use (in kWh/m 2 ): specific energy, %; specific carbon, kg CO2/kWh; specific cost, $/kWh. Additional metrics such as levelized cost of energy [46] may also be added to solution characteristics.
Existing and in-development LCA reviews for photovoltaic, solar thermal, and solar  Specific LCA characteristics below may be defined per unit of solution surface exposed to the Sun: specific energy, kWh/m 2 ; specific carbon, kg CO2/m 2 ; specific cost, $/m 2 . Other specific Solar Architecture characteristics are defined by specific energy, carbon, and cost to solar irradiation on the solution surface(s) over the period of use (in kWh/m 2 ): specific energy, %; specific carbon, kg CO2/kWh; specific cost, $/kWh. Additional metrics such as levelized cost of energy [46] may also be added to solution characteristics.
Existing and in-development LCA reviews for photovoltaic, solar thermal, and solar  Specific LCA characteristics below may be defined per unit of solution surface exposed to the Sun: specific energy, kWh/m 2 ; specific carbon, kg CO2/m 2 ; specific cost, $/m 2 . Other specific Solar Architecture characteristics are defined by specific energy, carbon, and cost to solar irradiation on the solution surface(s) over the period of use (in kWh/m 2 ): specific energy, %; specific carbon, kg CO2/kWh; specific cost, $/kWh. Additional metrics such as levelized cost of energy [46] may also be added to solution characteristics.
Existing and in-development LCA reviews for photovoltaic, solar thermal, and solar  Specific LCA characteristics below may be defined per unit of solution surface exposed to the Sun: specific energy, kWh/m 2 ; specific carbon, kg CO2/m 2 ; specific cost, $/m 2 . Other specific Solar Architecture characteristics are defined by specific energy, carbon, and cost to solar irradiation on the solution surface(s) over the period of use (in kWh/m 2 ): specific energy, %; specific carbon, kg CO2/kWh; specific cost, $/kWh. Additional metrics such as levelized cost of energy [46] may also be added to solution characteristics.
Existing and in-development LCA reviews for photovoltaic, solar thermal, and solar

Solar Chemical
Encyclopedia 2022, 2, FOR PEER REVIEW 16 Specific LCA characteristics below may be defined per unit of solution surface exposed to the Sun: specific energy, kWh/m 2 ; specific carbon, kg CO2/m 2 ; specific cost, $/m 2 . Other specific Solar Architecture characteristics are defined by specific energy, carbon, and cost to solar irradiation on the solution surface(s) over the period of use (in kWh/m 2 ): specific energy, %; specific carbon, kg CO2/kWh; specific cost, $/kWh. Additional metrics such as levelized cost of energy [46] may also be added to solution characteristics.
Existing and in-development LCA reviews for photovoltaic, solar thermal, and solar  Specific LCA characteristics below may be defined per unit of solution surface exposed to the Sun: specific energy, kWh/m 2 ; specific carbon, kg CO2/m 2 ; specific cost, $/m 2 . Other specific Solar Architecture characteristics are defined by specific energy, carbon, and cost to solar irradiation on the solution surface(s) over the period of use (in kWh/m 2 ): specific energy, %; specific carbon, kg CO2/kWh; specific cost, $/kWh. Additional metrics such as levelized cost of energy [46] may also be added to solution characteristics.
Existing and in-development LCA reviews for photovoltaic, solar thermal, and solar  Specific LCA characteristics below may be defined per unit of solution surface exposed to the Sun: specific energy, kWh/m 2 ; specific carbon, kg CO2/m 2 ; specific cost, $/m 2 . Other specific Solar Architecture characteristics are defined by specific energy, carbon, and cost to solar irradiation on the solution surface(s) over the period of use (in kWh/m 2 ): specific energy, %; specific carbon, kg CO2/kWh; specific cost, $/kWh. Additional metrics such as levelized cost of energy [46] may also be added to solution characteristics.
Existing and in-development LCA reviews for photovoltaic, solar thermal, and solar  Specific LCA characteristics below may be defined per unit of solution surface exposed to the Sun: specific energy, kWh/m 2 ; specific carbon, kg CO2/m 2 ; specific cost, $/m 2 . Other specific Solar Architecture characteristics are defined by specific energy, carbon, and cost to solar irradiation on the solution surface(s) over the period of use (in kWh/m 2 ): specific energy, %; specific carbon, kg CO2/kWh; specific cost, $/kWh. Additional metrics such as levelized cost of energy [46] may also be added to solution characteristics.
Existing and in-development LCA reviews for photovoltaic, solar thermal, and solar

Solar Architecture Balance
Solar Architecture Balance defines positive or negative values showing how any solution meets the Solar Architecture Principles over its period of use.
Specific LCA characteristics below may be defined per unit of solution surface exposed to the Sun: specific energy, kWh/m 2 ; specific carbon, kg CO 2 /m 2 ; specific cost, $/m 2 . Other specific Solar Architecture characteristics are defined by specific energy, carbon, and cost to solar irradiation on the solution surface(s) over the period of use (in kWh/m 2 ): specific energy, %; specific carbon, kg CO 2 /kWh; specific cost, $/kWh. Additional metrics such as levelized cost of energy [46] may also be added to solution characteristics.
In PV conversion, the example LCA reviews present a holistic evaluation of different types of PV from traditional silicon-based to innovative non-silicon-based, such as organic PV [47,48]. It also involves concentrating photovoltaic (CPV) cells and modules [49]. Assessment is provided for very large-scale PV systems working in different environments, specifically operating in desert areas [50]. Comparison is made of the energy and environmental profile of PV modules (thin-film and crystalline) and solar thermal collectors (flat plate and vacuum tube) [51], and the LCA of selective flat-plate coatings is explored [52].
In concentrating solar thermal conversion, examples of parabolic trough collector and power tower LCA are presented [53], and key design alternatives for power tower based concentrating solar power systems are reviewed [54]. A comparison of a solar thermal power plant with parabolic trough collectors and a photovoltaic plant with a single-axis tracking system LCA is shown [55].
In solar chemical conversion, Life Cycle Assessments of a concentrating solar power plant in tower configuration with and without thermal energy storage [56] and thermochemical energy storage integration concept examples are presented [57]. An LCA review of microalgal and lignocellulosic bioenergy products from thermochemical processes is also shown as an important example [58]. Overall, environmental and circular economy implications of solar energy in decarbonized grids are summarized [59].

Energy Return on Investment
Energy return on investment (EROI) is the ratio of energy delivered from a particular energy resource to energy required to deliver that energy [60].
When the EROI of a source of energy is less than or equal to one, that energy source becomes a net "energy sink" and can no longer be used as a source of energy.
In Solar Architecture practice, "Energy Delivered" is the energy absorbed by a solution's solar harvesting surfaces over its life cycle, and "Energy Required to Deliver that Energy" is the energy used to manufacture and maintain the solutions' components of and embedded technologies in these surfaces over this life cycle.
In an example of Solar Architecture EROI for a stand-alone solar photovoltaic system solution, "energy delivered" is energy generated by a stand-alone solar photovoltaic system over its life cycle (e.g., 25 years or more), and "energy required to deliver that energy" is the energy needed to manufacture, transport, install, and maintain a set of solar PV modules, the mounting system, cables, inverters, transformer, monitoring equipment, and other balance-of-system equipment used over this period of time.

Carbon Return on Investment
Carbon return on investment (CROI), also sometimes called carbon saved on carbon invested, is the ratio of the amount of carbon saved by using a solution through its life cycle operations to the amount of carbon embedded in the components of the solution.
In Solar Architecture practice, "carbon saved" is the carbon dioxide not emitted by a solution or an environment the solution is a part of due to the solution's operations during its life cycle, compared to status quo scenario of operation, and "carbon invested" is the carbon embedded in the solution components.
In an example of Solar Architecture CROI for a stand-alone solar photovoltaic system solution, "carbon saved" is carbon saved by a stand-alone solar photovoltaic system over its life cycle (e.g., 25 years or more) due to its operations as compared to hosting power utility carbon dioxide emissions. "Carbon invested" is the carbon embedded in PV modules, solar inverters, and other balance-of-system equipment. An example includes carbon dioxide emissions embedded at manufacturing crystalline silicon PV components such as poly-Si, ingots, wafers, cells, and modules [61]. Life cycle greenhouse gas emissions of crystalline silicon photovoltaic electricity generation solutions are adequately presented in this example [62].

Cost Return on Investment
Cost return on investment (ROI) in a solution is a ratio between net income from the solution (over a period) and financial investment in the capital and operation costs of this solution.
In Solar Architecture practice, "net income" is the saved cost of energy that would be paid to a utility/energy service provider should the solution not use the solar energy harvested by the solution in its operations over the life cycle; "investment" is the cost of the solution components in charge of using solar energy in its operations over the life cycle.
In an example of Solar Architecture ROI for a stand-alone solar photovoltaic system solution, "net income" is the saved cost of electricity from a hosting power utility that would be paid to the utility should the solution not use solar energy in its operations over the life cycle. "Investment" is the cost of the solution components (that may include land) purchased to generate solar power in its operations over the life cycle; this includes refurbishing and purchasing additional system components during the PV system life cycle. In this example, solar modules' standard warranty period is 25 years, an average standard warranty period of solar inverters is 10 years, and there may be other balance-of-system components with expected life cycles less than 25 years which will require additional investments during the expected 25-year life cycle period.

Solar Architecture-Decision-Making
A critical step in energy engineering practice driven by Solar Architecture is decision making. This step ensures that the best solar solution is selected for a client.
While clients' needs and strategies for solar energy use are various, the energy engineering objective must match the Solar Architecture Principles: maximize solar energy harvesting on solution's surfaces while ensuring positive energy, carbon, and cost balance in solution's life cycle.
To achieve this objective, a target share of solar energy in the client's energy mix is defined, and comparisons are made on competing applications across different application groups (e.g., rooftop solar vs. carport solar vs. solar garden) or within the same group (e.g., BAPV vs. BIPV or photovoltaic vs. agrivoltaic landscape)-see Appendix A.
The overall result of this step is the decision made on the winning application(s) to be used for design, deployment, and operations in the client's asset portfolio.

Conclusions
Solar Architecture represents the confluence of two disciplines: energy engineering and architecture.
The concept of Solar Architecture defines a decision-making process to select, design, deploy, and operate solar energy-enabled solutions for providing energy resources for environments where solar energy resources are part of the energy mix. It chooses applicable solar solutions, defines solar surfaces and related energy harvesting technologies, assesses energy, carbon, and cost balance over the life cycle of these solutions, and helps making decisions on the best solution options.
The principles of Solar Architecture include maximizing solar energy harvesting from solution's surfaces with a positive balance of energy, carbon, and cost provided by the solution. Solar Architecture application selection is built on two major cornerstones: features and groups. Solar Architecture practice uses application features and groups to define the best options in energy engineering of a solar solution.
Solar surfaces are key to solar architecture. They are the "heart", and balance-of-system components are the "muscles" of the solution.
Understanding and addressing energy losses in the three major areas of solar energy harvesting-photovoltaic, solar to thermal, and solar to chemical energy conversionallows for technological innovation to increase energy harvesting yield.
Life Cycle Assessment and solar energy harvesting methodologies based on solar surface characteristics define Solar Architecture Balance. This balance allows energy engineers to define energy, carbon, and cost return on investment for solar solutions applicable to their asset portfolio and to make decisions on selecting the best solution for the assets/environment.
The evolving integration of the energy engineering and architecture disciplines in Solar Architecture supports a future of energy generation and consumption-both elegant and safe, practical and beautiful. It represents one of the most fundamental aspects of a sustainable future for our world. Acknowledgments: The author is extremely grateful to John Crace for the manuscript concept discussions and adjustments, detailed review, and thoughtful editing. The author is very thankful to Vlad Kostylev, who, over many years, provided expertise in solar energy resource and mapping and solar applications, and greatly assisted collaborative work of the author's team. The author is also much obliged to Vlad Kostylev for his comments and terminology improvements on an earlier version of the manuscript. The author is also grateful to John Harker, who has been supporting the vision expressed in the manuscript and helped improve the manuscript.

Conflicts of Interest:
The author declares no conflict of interest.

Appendix A. Solar Architecture Table Outputs
The guidelines in Appendix A provide recommendations on the Solar Architecture Table outputs supporting energy engineers' decision making on solar solutions.
The three major outputs of the Solar Architecture table are solar surfaces, system components, and energy losses of target solutions. These outputs allow for defining a solution baseline for competing applications and for providing Life Cycle Assessment, solar energy harvesting, and Solar Architecture Balance characteristics. They also allow for addressing harvested energy loss reduction through technological innovations.
An example of the Solar Architecture Table and outputs defining and comparing solution options is shown below.
The major outputs of the Solar Architecture Table: Solar Buildings-External Envelope are indicated as follows.