On April 26, the Tucson Solar Potluck will be held in the desert north of Tucson. It is the 32nd straight year for the gathering of solar enthusiasts, a streak that some believe qualifies it as the longest continuously held solar event in the U.S.
Passive Solar Energy - The Starting Point
The sun’s energy is an incredible bounty. The energy contained in solar rays make their way through our filtering atmosphere and is critical to life on this planet...
and is fundamental to human survival. It can also provide for our comfort.
The use of the sun’s power in solar energy design is usually identified in 2 contexts - Passive Solar - that which uses natural processes without mechanical equipment and additional electrical or gas energy to operate, and Active Solar - that which uses nature’s resources with the inclusion of mechanical equipment and hardware driven by electricity and gas.
All solar design starts from a simple base - Passive Solar First.
What can be achieved by using all of the natural resources available to meet specific needs? This is the basic question and tenet of Passive Solar applications whether it be applied to heating and cooling a building, lighting, heating water, cooking, etc. Passive solar applies both to buildings and equipment.
Sound fundamentals of good passive applications and integration can beneficial and are directly related to active solar equipment use and implementation:
by meeting needs with no mechanical equipment dependent on external energy incorporation,
in improving conditions which reduce the amount and size of equipment required to meet needs,
by improving the conditions for active solar equipment applications, and
in minimizing the commensurate costs that accompany the purchase and use of any equipment, solar or non-solar.
In short , Passive solar design and applications is the base which sets the conditions for effective active solar incorporation and use.
Passive and Active solar applications should be considered as elements of the same palette - sort of the one-two punch of living with the sun,
and the mutuality is undeniable. Both rely on the same design considerations of orientation, access to the sun, behavior of materials, and appropriate use of site resources, and vary only in the inclusion external energy of electricity and gas. Most active system guidelines even point out starting with passive considerations first.
Besides providing for direct meeting of needs, Passive Solar design is a primary basis for enhancing the quality of active solar systems. Passive solar actions can result in the reduction of quantity of equipment needed to meet a particular task. For example, daylight is an available resource to meet illumination needs. Good day lighting design of buildings uses that resource effectively, and reduces the need and cost of daytime artificial lighting and equipment.
The beginning point consideration is at the end use side of things whether using traditional equipment or using solar equipment. Considerable savings can be gained in applying natural energy actions to reduce the cost of both supplying equipment as well as running and maintaining it. Quite simply, the less work that needs to be accomplished by equipment, the less amount of equipment is needed, and the less it needs to run when used - this all translates to less cost for purchase of the equipment, and less on-going cost for running and maintaining. Passive solar applications mitigate the quantity of active solar equipment needed, and resulting the tandem of both is optimal. Information about the sun and how to use it effectively is common in both applications.
Traditionally, the term Passive Solar has been identified with heating and cooling of buildings, but it has a broader context and application. There is, of course passive solar heating and cooling of buildings. There is also passive solar water heating, solar cooking, natural lighting, passive solar heating of pools, and even passive solar devices which move things - equipment, air, etc. Even the process of direct conversion of sunlight to electricity can be considered a “passive” action since it occurs through the appropriate use and placement of materials and capitalizes on the behavior of the combinations created, without infusing man-made energy sources and machines to make it work.
Knowledge and understanding of natural processes is the heart of Passive Solar. Knowledge about the composition, attributes and behavior of sunlight and heat; the behavior of heat flow; the behavior and capacities of materials, both in nature and man-made; the sun’s annual, seasonal, and daily movement; diurnal and seasonal temperatures and conditions; human sensory response and comfort ; the patterns of nature and of people; and the physiology and psychology of the interaction between people and Nature, all are applied to effective solar application and utilization.
NATURE’S CONTRIBUTION - a gift that also keeps us on our toes
intense heat, cold) The conditions that nature provides, in the form of climate, is variable. Cold in the winter, hot in the summer, nice other times of the year. Arizona climate covers the entire spectrum with extremes at the desert and mountain locations. Simultaneously, nature also provides the tools for mitigation of the extreme conditions. Sunlight and materials for a warming system; breezes, water, earth, gravity, and materials for a cooling system. It is the application of these resources into a system that addresses conditions that makes passive, and active, solar so effective.
The sun’s available energy varies in amount and impact through the year. The amount and intensity of energy from the sun that impacts the earth is affected by the composition of the earth’s atmosphere, and the angle of the solar radiation waves. The more dense the atmosphere, whether by clouds or smog, the less solar energy reaches the ground. Additionally the more directly perpendicular the sun is to the earth’s surface, the more concentrated the energy is in a given area and the more intense its impact. The highest capitalization of solar radiation for heat is when surfaces are perpendicular to the sun, allowing the most density of radiation at a given point.
THE BUILT ENVIRONMENT IN TUNE WITH NATURE
PASSIVE SOLAR ENERGY - PRELUDE TO SOLAR EQUIPMENT CONSIDERATION
There are a number of passive energy fundamentals which can be considered in reducing the amount of equipment and/or its’ operation.
ORIENTATION - It’s a necessary thing...
Like all direct solar applications, capturing the sun as a resource is as simple as providing for its clear path to where it can do its work - be it heating water, cooking food, or warming a space. Orientation is a fundamental concept of solar use for passive, and active, systems -
|Orientation and a direct relationship with the sun is the first rule of solar energy use when trying to capitalize on its heat providing attributes. The sun’s traverses the sky every day -|
In the winter it is a low and short path and in the summer a long and high path - and even though the sun’s location is constantly changing, it is a predictable path that can be used in incorporating the sun’s energy to meet needs, and to exclude when we want to minimize the same. Applied knowledge of both the sun’s movement, position at any given time, and time of the year, as well as impact in the form of radiation (solar incidence) , enables us to take advantage of these attributes to meet needs, and to make use of our buildings and our equipment more effective and efficient.
Proper orientation is critical to optimizing the solar resource. A properly oriented building can optimize solar gain for human comfort heating, and with proper shape and overhangs can minimize summertime overheating. Likewise, properly oriented solar equipment, be it a solar water heating panel or a photovoltaic electricity generating panel, will have optimum production and minimal negative impact. Using natural winter heating and minimizing summer heat impacts reduces the size of heating and air conditioning equipment as well as the energy, traditional or photovoltaic, needed to provide energy to these systems. Additionally, proper orientation allows for full benefit of the operation of electric solar panels and maximizing solar water heating operations.
|Proper building orientation also eases the integration of active solar equipment into the building form and shape, and mitigates the conflicts of solar installations contested in numerous subdivisions regulations.|
Proper orientation with direct exposure to the south, is best for passive solar heating of building spaces as well as the operation of solar equipment (water heaters, pv panels, cookers, pool panels, etc.). slight adjustments to the east or west of south allow for earlier or later use of the sun's energy, or for mitigating it.
FORM - It’s a right thing
For this reason roof design is important re: slopes and orientations to the sun’s path. (slide 28) Equally important is the integration of passive solar strategies to building additions such as thermal chimneys to accelerate cross ventilation, cooling towers, and north facing clerestories which incorporate hot water and pv panels on their back sides.
LOCATION - It’s the effective thing
Additionally, ancillary spaces located to the perimeter east and west sides provide a thermal barrier zone to the habitable spaces, thereby reducing the heating and cooling loads to an easily manageable level. Proper orientation and spatial design allows for optimum use of the sun for providing thermal comfort in both winter and summer, and reduces the amount of heating and cooling equipment, as well as the energy required to run it. Additionally, proper location can reduce the amount of solar equipment needed in a radiant floor system and/or for pv systems which provide power to air conditioning and heating machinery.
MATERIALS - It’s the smart thing:
All solar heating and cooling systems are based on the ability to gather and store solar energy within a material for a period of time. This is accomplished by using a material which will hold heat until it is needed for heating, or capturing heat that will be dispelled at a later time. Solar water heaters use water. Solar buildings use their own structure - floors, walls, even roofs. Of course, some materials are better for this purpose than others. Glass, wood, and insulation are not good holders of heat. More dense materials like earthen materials (adobe, stone, brick, etc) and man-made materials like concrete are very good. This attribute is called thermal mass.
Heating application of thermal mass is to select material(s) that will absorb heat from solar exposure during the day, hold that heat for a time during non-solar periods, then give it up as conditions warrant. The same action can be incorporated for building cooling. As an area heats up, heat can be absorbed into the thermal mass material in the walls, floor or ceiling, like a thermal sponge, then held until evening time where effective cooling practices using cross ventilation, night sky radiation and even whole house mechanical ventilation (remember - nighttime electricity rates are lower than daytime). This action is based upon fundamental principles of thermal transfer.
Nature is always seeking to even out things so if there is a “difference”, there is a natural action which moves to make every thing the same or to even out. In the case of heat transfer, heat migrates to cold, so in a building, hot walls will radiate into cold spaces, and conversely, hot, summertime spaces will have their heat migrate to cooler walls. An analogy that has been used is one of 2 buckets of water - 1 full and one empty. If they are placed at an equal height, and there is a connection between the 2, below the water line of the first, water will flow to the second until there is an equal amount in the second where all action will stop because a balance has
Passive solar buildings utilize the very fabric of the building as part of the comfort system for heating and cooling, and the addition of active solar system for running naturally heated or cooled water to through a thermal mass wall, floor, or roof structure enhances the performance of the system by the additional thermal mass capacity and heat capture/transfer attributes of the
WINDOWS - It’s the clear thing.
One of the major design considerations affecting a building’s energy consumption is the location and size of windows. Windows are the weakest point of the building envelope and usually the leakiest when it comes to energy, both in terms of losing heat in the winter, and gaining heat in the summer. A square foot of glass will lose 12 time more energy than a wood wall with insulation. As a rule, windows should be located primarily on the south side where they can be used as part of the heating system, as well as provide for natural lighting. That is the side where the sun is!. East and west sides of desert buildings should have minimal or no windows since these are the 2 worst exposures for early morning and late afternoon summer sun. East windows allow for early summer morning heat up and west windows allow for late afternoon negative impacts.
Solar windows should be sized in accordance with the heating and cooling performance of the building. Typical oversizing to have the “feel of the great outdoors” is not an optimal situation when it comes to solar design.
Clerestorey windows are a design tool for getting sunlight benefits (heating and lighting) to areas not able to be located at the south face. Clerestories also provide a mechanism for diffusing the direct impact of sunlight and moderating glare. Additionally, operable clerestorey windows are a good device for house ventilation cooling in the summer.
Reverse clerestories, those opening to the north can be a benefit in desert conditions . Facing north, they provide even natural light to interiors and their angled backs can be a perfect mounting structure and angle for solar equipment like photovoltaic and solar water heating panels.
THERMAL DECOMPRESSION - It’s the healthy thing
A building trying to maintain a comfortable internal temperature will always be in conflict with the temperatures adjacent to the exterior. Heat always moves to cold - in the winter , interior warmth is moving toward the exterior cold. In the summer, the external heat is trying to move to the interior coolness. In both situations, the greater the difference in temperature between inside and outside conditions, the faster the movement of heat and the greater the amount of heat moved, and the more equipment is required to mitigate conditions.
In temperate times when inside and outside are at or near the same temperatures, there is minimal movement and therefore minimal need for equipment. Add to this the fact that sudden and abrupt changes in temperature are not positive to the human body which has to react rapidly to changed conditions, and good passive site planning of thermal decompression is important for not only comfort, but for health.
Thermal decompression simply means that there is layering of vegetation, landscape features, and built elements that gradually temper the environment to a point where the temperatures adjacent to the building are much closer to its internal temperature. This decompression approach establishes a condition where the difference between the internal temperature and the temperature on the building skin are much closer, so less heat is gained (or lost) and less mitigating equipment, and commensurate energy, is required.
PASSIVE SOLAR APPLICATIONS - It’s first thing
Natural Lighting -
However, addition of a solar electricity generation system (photovoltaics) allows for the capture of daytime sunlight and its transformation into electricity, which can then be stored and used in the night. Add to this the use of efficient fixtures and systems, and costs in both resources and dollars are further reduced.
Water Heating -
Batch or Integrated Collector/Storage (ICS) System
Simply water in a tank within a container and exposed directly to the sun. This is the basis of batch/ breadbox, systems which combine collection, heating, and storage of water into a single component. Direct heating of the storage tank or tanks, makes this system compact, simple, and effective. These units are called a passive systems because they do not rely on equipment to make them function. When hot water is removed, it is replaced by an equal amount of "new" water.
The "batch" approach has been used for quite some time and improvements in design have enhanced their effectiveness in increasing water heating capabilities. Newer systems use a number of small-diameter connected storage tanks connected to expose more of the water surface to the sunlight, heating the water at a faster rate. In some cases reflectors are integrated, bouncing more of the sun's rays onto the water tank, and when the sun falls, the reflectors, made of highly insulating material, fold over the glazing to insulate the tank. Some systems use evacuated glass tubes (like a thermos bottle) around the collector to keep heat loss to a minimum. Thermosyphon Systems.
There are basic elements of passive energy buildings which use the form and materials to provide comfort. Some of these are applicable to solar equipment design and use, even to the point where there is solar equipment which are passive in their operation - i.e. thermal energy flows in the system naturally. Solar water heating is one type of equipment that can be a passive solar piece of equipment. A “batch” water heater and a thermosiphon water heater can be considered passive solar equipment - since they do not rely on out side energy source to make them function. Of course, when talking about passive and active solar, optimum conditions and control occur best when these two are coupled.
Basics of passive applications are rooted in dealing with the sun (exposure to and capture of the sun’s energy when we want heat; protection from the sun when we want cooling), the materials used (for effective capture, storage, and use), and natural processes of physics for both). Every passive system for solar heating, whether it is heating water, or heating a building, requires exposure to the sunlight and trapping it - this is done by glazing - windows for a building and glass covers for solar panels. Every passive system is dependent upon materials which will absorb the sun’s heat, store a good quantity of it and easily distribute it. In a building, the effective material can be the structure itself, in the form of thermal mass.
Thermal mass is characterized by those dense materials like concrete and earthen materials, and also by an extremely good material - water. These materials can readily absorb solar radiation, hold its warmth, and easily and evenly give it up to adjacent spaces.
Generally there are 3 passive heating building concepts - Direct Gain, Indirect Gain and Isolated Gain These concepts have inherent within them cooling strategies and applications as well.
Direct Gain -
Simply stated, sunlight comes directly through windows into the space to be heated.
The building materials struck by the sunlight are thermal mass materials - concrete/tile floor, masonry walls, or even strategically placed containers of water.
Building windows act in exactly the same way as solar panel glazing - they let the sunlight (short wave radiation) in and inhibit heat (long wave radiation) from escape. Direct gain design system is always working, letting in not only direct sunlight but also the diffuse light of cloudy days, and the intense light of summer.
Like any system, optimization is the goal - so the building eaves and overhangs become a designed-in optimizing element - summertime conditions, when heating is not required, are mitigated by keeping the sunlight off of the windows via the overhang, while in the winter, the sun is much lower in the sky and can easily skirt under the building’s brow.
Heating is quite simple in this approach - sunlight, absorbed by the thermal mass materials, solid and/or liquid, is stored as heat. When the space cools in the evening, the heat migrates to the cooling spaces directly (radiation) or by air movement across the surface of the material (convection). For this approach, a careful consideration of the site, solar energy availability, and seasonal conditions, are all necessary to determine the appropriate amount of windows and thermal mass. Too many windows in an Arizona desert setting will result in a human cooker; too few windows in a Rim setting will result in not enough capture.
This system has worked effectively in Arizona designs, as well as that sunniest of place of Liverpool, England.
For effective cooling, such as the desert setting, Direct Gain Avoidance is the rule, BUT the thermal mass of the building can still be used in the cooling cycle. The materials, by nature of their thermal mass attribute, remain cool (or can be cooled during nighttime conditions - naturally by cross ventilation, or mechanical y by lowest cost energy driven fans). This coolness allows their absorption of unwanted heat in the building - acting as a sort of thermal sponge, moving heat away from people and holding to the evening, where cross ventilation or even whole house fans can dispose of the captured heat. Control of Direct Gain systems is done with the addition of movable insulation, either on the exterior or with interior blinds, and cross ventilation planning with placement of low wall vents on the cool side of the building, and high wall vents on the warm side of the building.
Indirect Gain -
Indirect gain is an “next step” of a Direct Gain system. Sunlight penetrates south facing windows, then strikes thermal mass located behind the window and between the sun and living space. There are basically three types of indirect gain systems, each defined by where the thermal mass is located. The three strategies are:
Thermal Wall and Plenum
Thermal Wall and Plenum -
South facing windows front a thermal mass wall of masonry, and/or water, placed directly behind to create a vertical plenum or chase. The sun side of the thermal wall is typically dark to capture more of the solar spectrum. This mass absorbs, stores and distributes heat while acting as a buffer to the interior spaces, and moderates temperature changes and provides for extended use of thermal gain well into the evening. Sunlight passes through the glass and converts to heat energy as it impacts the thermal mass and is absorbed, slowly saturating or moving through the mass until it radiates into the living space - the wall is a delayed action radiator.
At the same time the air trapped between the windows and the mass heats up, and the addition of vents at the top and bottom of the wall allow for direct passive heating. Warmed between the glass and the wall spills into the living space through the opened upper vent and since Nature abhors a vacuum, cooler room area enters the plenum through the bottom vent, and is heated by the sun warmed wall, rises, spills into the room and is replaced by cooler air again, and this natural convection process continues as long as there is sunlight. There are a number of examples of this application - the Trombe wall which uses masonry, earthen materials like, and water like Steve Baer’s water barrel. Variations in thermal mass wall materials vary from commercial water tubes to incorporation of stone.
The Baer barrel wall installation provides for optimizing the heating capabilities as well as cooling of the adjacent spaces with the addition of movable insulating panels. During Winter conditions, Insulating panels are moved to allow for solar access to heat the water barrels, then at night the insulating panels are raised to cover the glazing and the barrels radiate their warmth to the space. In the summer heat, the insulation is raised and the barrels, with their cool water, act as absorbers, pulling unwanted heat from the spaces. At nightfall, the insulation is lowered, and the barrels give up their stored heat to the exterior by radiation and convection. the water, now cooled, is ready to act as a cooling absorber the next day.
Sunspaces are a combination of Direct Gain and Thermal Wall systems, utilizing both approaches in tandem with a dedicated Direct Gain area (Sunspace) adjacent (fronting) the living space, with a Thermal Wall placed between the two. The Sunspace, has extensive south glazing and large daily fluctuations, while the adjacent living space is protected from these fluctuations by the Thermal Wall separating the spaces. Vents or operable doors and windows in the Thermal Wall allow warmed Sunspace heat to circulate to adjacent living spaces by natural convective actions during the day, and radiate the absorbed Sunspace heat to the living spaces in the evening. An additional usable area, Sunspaces are often used as solar greenhouses. Temperature control is best achieved with operable venting windows and cross ventilation.
Sunspaces - green houses:
Thermal Roof -
The thermal Roof approach places thermal mass on the roof rather than at a wall, and is very effective as both a passive heating and cooling strategy. The system is both a radiator and an absorber and replaces standard heating and cooling mechanical systems and the inherent ductwork distribution system. Using water as thermal mass, roof ponds are constructed directly on top of heat conducting ceilings of metal pans or metal decking so there is direct thermal transfer. Movable insulation is placed above the ponds to facilitate better retention of heat in the winter and to prevent absorption of external heat in the summer.
The operation is quite simple. During wintertime conditions, insulating panels are rolled or pivoted back, exposing water contained in UV inhibiting water beds to the sun. The ponds gather the sun’s warmth and at nightfall, the insulating panels are replaced to contain the gained heat and prevent loss to the cold night air. The heat stored in the bags, warms the supporting metal decking and the entire ceiling is a radiant ceiling throughout the cold winter night. The next morning, the insulating panels are removed when the sun appears and the cycle begins again.
Summer cooling is a reverse process. Ponds, covered during the daytime heat, remain cool and act in concert with the supporting metal ceilings as a thermal sponge absorbing interior heat generated from people, equipment, and infiltration from the outside. At night, panels are removed and the ponds throw off their gathered heat to the night sky by means of radiation, convections, and if wet down, by evaporation.
Roof pond heating and cooling is optimized when all living spaces except the bathrooms and high water use areas are covered by the system. In areas that generate humidity, like a shower, the metal ceilings will tend to “rain” due to the temperature difference of steam vapors and cool metal.
Roof ponds, like Harold Hay’s Skytherm system, have been designed and used in the hot climate of Arizona and New Mexico to the moderate temperatures of the California coast and even planned for the twin cities of Minneapolis/St. Paul.
Isolated Gain -
A hybrid of this system can include moving heated water or air through a radiant floor system where the masonry floor itself acts as the thermal mass storage. This variation can also use cool water to create a “cool” floor by running house supply water, or water from an adjacent pool, through a floor system.
Cool Towers -
A variation to this system is the addition of a south facing thermal chimney to pull cool tower air through the house. Located at an opposite location from the cool tower, the thermal chimney provides an escape vent for interior warm air , which moves more quickly as it get heated and is driven out. This rapid venting has a drawing effect on the cool tower air and it is distributed more extensively through the building. The solar chimney can be set up to become a recirculating air heater during winter conditions.
Natural Cooling -
There are three sources of undesirable heat - direct summer sun solar gains through windows and glazing; heat transmission through the building envelope; and internal heat produced by people, their activities, and their equipment. Direct solar heat gain at windows and glazing can be easily controlled by shading the house - preventing the sun from reaching it (except for good day lighting and operation of solar equipment) and with external shading devices and vegetation as well as thermal insulating shutters. Heat transmission conditions can be nullified by setting up layers of thermal decompression with vegetation, built structure like porches, and water features. While there is not much that can be done to reduce natural heat production by people, equipment heat generation can be impacted by careful selection of energy efficient equipment and by good timing - do the laundry in the evening.
Use of the sun for food preparation is fun, energy saving, and saves money, both in the cooking operation, and in the cooling costs saved when the heat is taken out of the kitchen during the summer. A variety of cooking tools from box cookers to slat faced ovens are available - whether they be commercial products or hand built by the inspiring solar chef.
Passive Solar Energy has many faces and applications and an effective Passive solar building incorporates many of these elements. Natural processes and incorporation of building and site elements to provide for comfort as well as mitigation of untoward conditions are the hallmark of good passive design, and results in establishing a basis for reduction of equipment (solar and otherwise) for achieving comfort, and reduction of equipment purchase and operations costs. Passive solar energy is Direct,
and inherent to the building form, structure, materials and use.
This presentation was constructed by the Arizona Solar Energy Association for the Arizona Solar Center, Inc. under contract with the Arizona Dept. of Commerce Energy Office, funded by the Dept. of Energy Million Solar Roofs program. Materials and information were provided by a number of sources.
Financial support for this presentation has been provided by the Arizona Department of Commerce (Energy Office) and the U.S. Department of Energy through (DOE) Grant No. DE-FG51-01R021250. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the author(s) and do not necessarily reflect the views of the Energy Office or U.S. DOE. The State of Arizona and U.S. DOE assume no liability for damages arising from errors, omissions or representations contained in this presentation.