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Passive Solar Heating in Built Environment

Glossary Solar architecture The deliberate use of solar energy by means of the building architecture, thereby reducing purchased energy dependence while enhancing the quality of enclosed space. - Passive Not requiring actions to achieve a desired goal. In the case of passive solar energy use, solar energy is captured and distributed in a building without machinery by using the physics of conduction, free convection, and radiation. - Direct gain The direct gain of heat within a building by sunlight entering through glazed openings in the enclosure, which then traps and stores the heat. - Indirect gain Solar energy absorbed in some fashion on or in walls or roofs and converted to heat. This heat either remains entrapped in the building envelope to reduce building heat losses, or it is transferred into the building by conduction or convection. There may be a delay between the time when sunlight is absorbed and when heat penetrates into the enclosed volume. - Isolated gain Solar energy absorbed outside the insulated building envelope and then transported by free convection to the enclosed volume. - Solar air system Type of isolated gain system where heat from the collector transported to the point of use or storage by air (verses water in active thermal systems). - Hybrid solar system A passive system assisted by a small fan to increase system efficiency, possibly PV-powered. The energy ratio of heat output to electrical input can easily exceed 20:1.

Definition and Importance of Passive Solar Heating

Passive solar heating is the use of solar energy to heat a building without mechanical or electrical energy. The architecture and construction capture, store, and distribute the sun's energy. Every building with windows exposed to the sun is passively heated, but heat losses may exceed the solar gains. Accordingly, if the passive heat gain is to reduce heating costs, the system heat losses must be minimized. Ideally, the concept includes mass to store daytime solar heat for nights, increasing the usability of the gains. Finally, the heating system must shut off when solar heating achieves the desired room temperature. Two constraints on passive solar use are glare control and shading during non-heating months.

Maximizing usable passive solar gains is an important design aspect, but often designers focus only on minimizing heat losses. Taking the finance world as an analogy, no one will accumulate wealth through savings alone, income must be maximized and wisely invested. So, not only reducing heat loss is essential to low energy architecture, maximizing solar gains is important, as it has been over the history of building.

An often cited early example of solar design awareness is the "Megaron House" described by Socrates in the year 400 B.C. Numerous other examples can be found, i.e., the New England "salt box" of the seventeenth century or Swiss farm houses of the eighteenth century. In the twentieth century, the term "solar house " became popular and following the first oil shock of 1973, the term "passive solar buildings " was coined. In all these examples, the basic principles are the same; maximize the south exposure of a building to capture as much solar heat as possible and insulate the enclosure well to keep the heat in.

Passive solar building design is an important means for slowing climate change by reducing the burning of fossil fuels. It is not, however, a least first-cost way to build. Larger, better insulating windows or opaque collector constructions cost more than conventional constructions. Three arguments justifying this investment are:
  • Long-term (>10 years) good return on the investment
  • Economy and security as future fossil fuel prices increase and supply subject to interruptions
  • Living qualities of passive solar buildings flooded with light and natural warmth (Fig. 1)
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Passive Solar Heating in Built Environment. Figure 1 A living room flooded with sunlight from large south-facing windows (photo source: robert.hastings@aeu.ch)

Introduction

Concepts

Buildings that consume less fossil fuel are "nice to have" today, but will be essential in the future. Since buildings are long-term investments, they must be built or rebuilt looking to the future.

Heating is a major use of energy in buildings and dependence on fossil fuels for heating can be dramatically reduced through two strategies: reducing heat loses and increasing the use of solar heat. Logically, a combination of these two strategies is desirable.

Solar energy can be used by passive or active means:
  • Passive solar use does not rely on mechanical components to capture, store, and distribute the heat, the building construction fulfills these functions.
  • Active solar use typically involves a remote solar collector and a pump or fan to transfer the heat to storage and from storage to point of use.
A low energy building must lose as little heat as possible, hence the importance given to insulation, air tightness, and heat recovery. An example design standard promoting extreme energy conservation is the "Passive House Standard" [1]. To meet this standard in Europe, three requirements must be fulfilled:
  • The annual heating requirement must be less than 15 kWh/(m2a) or maximum heating power 10 W/m2a based on the net heated floor area.
  • The combined primary energy consumption for heating, hot water and household electricity may not exceed 120 kWh/(m2a).
  • The air leakage of the enclosure tested under a pressured difference of 50 Pa (n50) may not exceed 0.6 house air volumes per hour.

A passive solar building is not defined to this extent; it simply describes a structure in which the designer deliberately maximized using solar energy passively. So, in fact, a passive house can also be a passive solar house and indeed, in the planning recommendation for a passive house, using passive solar energy is encouraged and credited.

Three passive solar heating concepts were defined after the first oil shock of 1973 and are still useful today: MediaObjects/978-1-4419-0851-3_15_Part_Fig27-372_HTML.jpg

Direct gain : Windows capture the sun in a well-insulated building; interior construction mass stores the potentially excess daytime gains into the night; and some form of shading provides comfort during non-heating seasons. Direct gain is the oldest and still most cost-effective concept, given its potential to also enhancing the quality of life in buildings. MediaObjects/978-1-4419-0851-3_15_Part_Fig28-372_HTML.jpg

Indirect gain : The building envelope captures solar heat, which is then conducted and/or convected to the building interior, possibly with a time delay of up to 8 h. Alternatively, the goal may be simply to capture enough solar heat in the envelope construction to eliminate heat losses from the building much of the time, i.e., a dynamic U-value over the heating season approaching zero. Indirect gain systems nicely compliment direct gain systems. MediaObjects/978-1-4419-0851-3_15_Part_Fig29-372_HTML.jpg

Isolated gain passive : Solar energy is converted to heat outside the insulated building envelope and then deliverd to the building interior or storage. This can be by gravity-driven convection, or with the help of a small fan (a "hybrid" system). While not purely passive, hybrid systems are reported here because the proportion of delivered heat to electrical energy is so small. MediaObjects/978-1-4419-0851-3_15_Part_Fig30-372_HTML.jpg

A sunspace or attached greenhouse with controlled opening to the building is also an isolated gain system. Isolated gain systems are the most complex and expensive, but offer the most control of when and how much solar heat is delivered into the building.

It can be useful to consider passive solar heating opportunities by building types and climates. Note, that in this section, locations north of the equator are assumed. South of the equator, north orientations have priority.

Building Types

Buildings where heating loads dominate over cooling loads are the obvious candidates for passive solar design, i.e., residential buildings and small commercial or institutional buildings. Three factors are decisive here:
  • As the ratio of enclosing surface to enclosed volume increases, heat loss increases, so a solar heating can be more beneficial.
  • As the density of heat production from people or appliances increases, the usefulness of solar heat decreases.
  • Direct solar gains in the form of heat and light are a combined asset, i.e., for hospitals, old age homes, and schools as well as residences. (Examples and Design insights for passive solar use in commercial and institutional buildings were researched and documented in an IEA project [2].

The energy optimization of a building must balance passive solar and daylight benefits against mechanical cooling and electric lighting energy demands, both of which have very high primary energy factors.

Climates

Northern climates such as Scandinavia would seem to pose a problem for passive solar use. Winter days are short, the sun is weak, and the sun path is at a very low angle. This means, however, that windows or vertical collection surfaces intercept the sun at a more direct angle. Furthermore, the heating season is very long, extending from early autumn to late spring. Before 21 September or after 21 March, heating is still needed, when days are longer than in southern latitudes. Passive solar concepts must maximize the usefulness of spring and autumn solar heating, while minimizing mid-winter heat losses.

Temperate climates are the ideal situation for passive solar buildings. Not just sunny temperate, but also overcast temperate climates. This has become possible with the development of very high-performance glass (U-value < 1.0 W/m2K). Consider the example of diffuse solar radiation at only 100 W/m2 for 6 h and an ambient temperature of 5°C. The solar gains through a glass with a g-value of 0.5 (admitting 50% of the solar radiation) will offset the 24 h heat losses of a glass with a U glass of 0.8 W/m2K. If the sun shines with more intensity or more hours, it is a passive solar winner. Because temperate climates often have hot summers, the concept must also include shading.

Mild climates offer a challenge: to achieve zero-heating energy buildings by combining passive solar design and conservation without degrading summer comfort. This is at least as challenging as achieving net-zero-energy buildings . The latter achieve a net zero balance by taking a credit from the summer electrical output of a large PV-roof (multiplied by a high primary energy factor) against the energy deficit in winter, which must somehow be met. Passive solar heating of a highly insulated building can answer part of the "somehow" question.

Strengths and Weaknesses

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Living quality: daylight and naturally warmth from the sun's warmth.

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Security: in the event of energy supply interruptions.

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Costs: only the marginal costs of added aperture area, be it collector or window area and mass, must be amortized by energy savings.

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Return on investment: As energy prices rise, return on the investment in passive solar measures increasingly attractive.

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Low maintenance: There are no maintenance costs for pumps or fans.

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All of these factors can positively affect resale value of the property.

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Operation: often passive solar use requires active occupants adjusting sun-shading or opening windows or vents.

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Poorly designed or incorrectly used passive solar buildings may use more energy than conventional buildings. Informed design, strict quality control, and intelligent operation are essential.

Road Map to This Section

A historic review of passive solar design shows how this approach has developed in parallel with technological developments of building components. It is instructive to examine which concepts came into existence, evolved, flourished, or died out. This may save reinventing a broken wheel, or ideas might arise for new variations or concepts.

Principles and applications review different approaches to passively capturing, storing, and using solar energy to heat buildings.

Direct, indirect, and isolated gain concepts are reported in detail.

Finally, the past, present, and future of passive solar heating are discussed in the context of expected energy supply developments, demographics, and increasingly well-insulated and automated buildings.

History

Concepts for passive solar heating date back millennia. Materials and components were very primitive by today's standards, but comfort expectations were also much lower. A net solar gain is possible even with single glazing if the required room temperature is only 16°C. The twentieth century saw dramatic developments in material science and production techniques, e.g., in glass production. The evolution in the last century has been equally spectacular. Single glazing at the beginning of the twentieth century (U = 5.8 W/m2K), evolved to fused double glazing in the 1950s (U = 2.8 W/m2K). Insulating glazing (U = 1.2 W/m2K) in the 1990s is now available in triple glazing (U = 0.5 W/m2K), or more than a factor 10 better than window glazing a century ago.

As a result, some concepts, which earlier proved ineffective for a given climate or building type, may indeed be effective today and should be "rediscovered." Following is a short-time journey through the evolution of passive solar heating.

Ancient times

The most often cited example of awareness of passive solar use is a concept house, the "Megaron House " (Fig. 2) described by Socrates (469-397 B.C.). He expressed the following thoughts: "Doesn't the sun shine into houses facing south in winter, whereas in summer the sun wanders over us and the roof so that we have shade? Because this is comfortable, then south-oriented rooms should be built higher in order not to shut out the sun, whereas the north rooms should be lower because of the cold north wind." This was the logic for this funnel-shaped house concept, opening in plan and section to the south. A roofed porch admitted sunlight into the main room in winter but shaded it in summer. A room to the north served as both storage and as a buffer from the north exposure.
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Passive Solar Heating in Built Environment. Figure 2 The Megaron House concept described by Socrates

1600-1900

New England Salt Box : A classic passive solar house form appeared between 1650 and 1830 in New England, the "Salt Box" (Fig. 3). Its name is derived from the shape of boxes used to store salt at that time. Initially, the house form came about when an addition was made to the rear and the roof slope carried down from the two-story main house. Typically, the addition incorporated a kitchen with its own fireplace, a pantry, and a room for child birth or nursing the ill [3]. The main chimney rose inside the house to keep its heat inside. Also, very practical are the double-hung windows. The sashes were hung on ropes with counterweights of iron or bricks in a cavity of the window frame. The upper sash could be lowered and/or the lower sash raised independently. The height difference between the upper and lower openings induced air circulation.
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Passive Solar Heating in Built Environment. Figure 3 A New England "salt box" house with large south façade and long protecting roof to the north (photo source: robert.hastings@aeu.ch)
Swiss Appenzell House : The Swiss Appenzell houses from the eighteenth to early nineteenth century had facades with many window bands protected by a projecting roof at each storey (Fig. 4). This afforded summer shading and weather protection for the windows. The curved white under surfaces captured and deflected additional daylight down to the windows.
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Passive Solar Heating in Built Environment. Figure 4 An Appenzell house with large window area protected by multiple roof projections (photo: robert.hastings@aeu.ch)

1900-1950s

In The year 1927 saw a breakthrough in glass production. Using the Penn vernon Drawing Machine, glass was pulled through rollers in a new process implemented by PPG Industries. For the first time, large sheets of glass could be produced. This opened exciting new architectural possibilities, but with large heat losses and comfort problems. With the introduction of insulating glass by LOF, it was possible to have large window areas and net solar heat gains. Architects played with the design opportunities this new technology offered. Researchers quantified how long a room could be kept warm by what outside conditions. The press publicized what was then possible with new solar houses. Solar buildings were a mainstream topic. An example of such architecture is the work of the Architect George Fred Keck. Figure 5 shows the living room with a stone floor and fireplace to absorb the sunlight flooding in from the full southwest front of windows [4]. This house, built for Dr. and Mrs. Hugh Duncan of Flossmore, IL, United States, was monitored by two researchers. The performance was surprisingly good. One winter day in 1941, when the ambient temperature was −20°C, the heating system shut off by 08:30 h and stayed off until 20:30 h [5].
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Passive Solar Heating in Built Environment. Figure 5 Direct gain maximized in the Duncan House (picture by permission of Pilkington, North America, Inc.)

1960s

Oil was plentiful and cheap, everyone was happy, renewable energy was not a topic of any popular importance and very little happened.

1970-1980s

In 1973, an oil embargo imposed on the United States led to a crisis of historic proportions. Americans can react astonishingly effectively and quickly to a crisis and this was the case then: "overnight," a national program to reduce foreign oil dependency was initiated. The Energy Research and Development Administration (ERDA) was activated on 19 January 1975 and the Solar Energy Research Institute (SERI) in Golden CO was founded. The department of Housing and Urban Development (HUD) held a national competition with grants for building the solar houses . Many built projects were monitored by national laboratories and published [6], as, for example, the Balcomb house shown in Fig. 6. In 1977, the first National Passive Solar conference was held [7] and in subsequent years, each conference drew over a 1,000 enthusiasts.
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Passive Solar Heating in Built Environment. Figure 6 The Balcomb house in New Mexico (photo source: robert.hastings@aeu.ch)

Passive solar use was a major topic of the American Solar Energy Society (ASES), a national organization linked with the International Solar Energy Society (ISES). These were the boom years for passive solar buildings. Research and demonstration projects were well funded at the federal and state levels. Atomic physicists "saw the light" and became solar building physicists at renowned national laboratories, including Los Alamos, Lawrence Berkeley, Brookhaven, and the National Bureau of Standards. Exemplary demonstration projects were sponsored by the Tennessee Valley Authority (TVA), an enormous interstate electrical utility. Regional solar energy centers oversaw the evaluation and publicizing of countless solar buildings.

To help energy consultants, researchers, and academics analyze concepts, complex computer models were developed. These could quantify the dynamics of solar and heating input, heat storage, and building heat losses. Auxiliary heat demand and comfort performance were reported on an hourly basis. These tools were, however, difficult to use. Input was cumbersome and errors easily made. Computers in the 1970s still had to be "spoken to" via punched cards. The input was in rows of numbers, separated by spaces or commas punched into cards. Examples of programs include DEROB, NBSLD and BLAST, and later, DOE2. To provide design consultants (designers couldn't compute) with calculation tools, two approaches were followed:
  • Gigantic data bases were computed using research computer models for all thinkable design variations, and then clever nomographs generated. The Passive Solar Handbook by Doug Balcomb and R. Jones is a classic example [8].
  • Simplified calculation tools were programmed, such as SERIRES (later called SUNREL) and CALPAS. These were small enough to run on the first versions of portable computers ("mini" or "midi" computers).

The goal was to learn how sensitive performance was to a given parameter. To demonstrate how terrific a design was, it was useful to compare it to a conventional builder house of the time. For this purpose three reference houses were defined, based on statistics from the national home builders association (NHBA). The reference designs were published by the National Bureau of Standards (NBS, today NIST) [9].

To be sure, the computer models were telling the truth, measurement data from components and even whole buildings were essential. Test cabins for monitoring systems became a common sight at many national research facilities. Figure 7 shows a test house with an interchangeable modular south façade and clerestory windows sun lighting the north rooms.
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Passive Solar Heating in Built Environment. Figure 7 NBS (NIST) test house 1980, Gaithersburg, MD (architect and photo source: robert.hastings@aeu.ch)

Meanwhile at universities, architecture schools continued to teach Le Corbusier as the model for good design. Energy and solar use were not significant design issues, with the exceptions of a small but growing number of architecture and engineering professors. They were the authors of some superb text books, which clearly presented passive solar design principles. Some examples are a passive solar textbook for architects [10], a guide for adapting solar concepts to regional climates and constructions across the whole continent [11], and guidelines for window design strategies to conserve energy [12].

During this period, there were a few good examples of passive solar innovation in Europe as well. The Michelle-Trombe Wall concept was a notable example (Fig. 8). The original pilot building was constructed at the Centre National de la Rescherche Scientifique (CNRS) in 1967 in the south of France and further developed with a vented version of the wall in 1974 [13].
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Passive Solar Heating in Built Environment. Figure 8 The Michel-Trombe wall house in Odello, FR (photo: robert.hastings@aeu.ch)

1990s

Europeans began to take interest in the American passive solar movement. Many architects and building researchers travelled to the United States to personally visit passive solar houses . Passive houses began to appear across Europe, from Scandinavia to Italy. National research programs investigated how to optimize passive solar concepts to local European climates and constructions. This was essential. Several passive solar buildings did not function as hoped. Europe gets less sun than New Mexico!

During this time, windows were still mostly double glazed or at best triple glazed (U glass = 3.0 or 2.2 W/m2K). Glazing with selective coatings and noble gas fillings were just beginning to enter the market. Accordingly, only windows facing south achieved a net passive solar gain. In northern climates, night insulation of windows was needed for the long dark winters. Several innovative, but expensive roll-down insulating blankets were developed for windows. These largely disappeared from the market as high-performance glazings appeared.

By the end of the 1990s, the growing pains of adapting passive solar architecture to European climates and constructions were over and countless exemplary projects had been built and published. An IEA SHC program searched out and documented exemplary projects [14].

2000

During this period, many conventional passive solar-heated houses were built across Europe. Sunspaces were a favored architectural element. Many houses included active solar systems to heat domestic hot water, with Austria leading in the number of such houses. European architects often succeeded in adapting passive solar house designs into good architecture. An example project from 1992 by a Norwegian architect practicing in Austria, Sture Larsen is shown in Fig. 9 [15]. The exterior of the house is in light, wooden framing, the interior is in massive construction. Solar heated air is circulated through the walls and floors to radiate into the rooms. A sunspace also helps heat the house.
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Passive Solar Heating in Built Environment. Figure 9 An Austrian passive solar house with solar air radiant heating and a sunspace in Nüziders, Vorarlberg (architect and photo source: Sture Larsen, www.solarsen.com)

By the year 2000, a new concept, The Passivhaus (Passive House ) had become well established and on the way to becoming the new mark of excellence in low-energy design. It came out of the PHD work of Wolfgang Feist under his Professor, Bo Adamson in Sweden. To reach this standard requires a highly insulated, thermal bridge-free, and airtight building enclosure. Mechanical ventilation is needed to assure good air quality by such tight construction. Heat from exhaust air is then recovered to preheat incoming air. The ventilation air can be used to deliver the small amount of heating needed (maximum 15 kWh/m2 heated floor area). Obviously, passive solar heating of such houses is also desired, but challenging to dimension because of the small heating load and short heating season.

2010

Today, in the second decade of the new millennium, the term "passive solar heating" is less common. This is paradoxical because with new window frame and glazing systems, highly insulated building envelopes, and sophisticated heating control systems, passive solar gains can cover all heating needs for an extended part of the year in temperate climates. However, the interaction between passive solar gains, internal gains and envelope heat loss needs to be carefully studied to assure comfort and the hope for energy savings.

The former research computer models to study passive solar building concepts, requiring several hours on a main frame computer, today can run in seconds on a laptop. However, today's tools do not consider many of the phenomena the former models did, such as when mass is directly sunlit or only indirectly warmed by room air, or how passive solar heat in south rooms convects to other rooms. This can strongly affect passive solar usability and comfort.

Principles, Applications, and Integration

Principles

Passive solar heating requires glass, frames, seasonal sun shading, mass, and extra planning effort. The economics are clear: energy won is more expensive than the energy saved by adding insulation or eliminating air leakage up to a certain point. However, as the insulation thickness is increased, the marginal energy and economic benefits of the next increment decrease. Further, when conventional thicknesses are exceeded, the costs of anchoring the insulation and detailing jump. In contrast to this, the cost of a larger window is not proportional to the window area increase. Larger windows loose less heat per unit area. There is less perimeter for the glass area, so edge losses are smaller. Also, there are increased benefits such as more daylight, the view outside and sense of well being from being sun-warmed (especially for cats). The challenging questions are therefore, which passive solar heating concept is most effective for a given building type and climate, and how big should the system be?

Applications by Building Types

Well-Suited Building Types

Residences are the most common passive solar application. Detached single-family houses with four outside walls, a roof, and earth contact can best benefit from passive solar gains. Row houses and apartment buildings have many units with only two exposures. If they face east and west, passive solar heating is difficult. One idea would be to install an indirect gain system on the south-facing end walls to compliment morning and afternoon direct gains.

Large buildings suitable for passive solar indirect heating include warehouses, gyms (American), or athletic halls (i.e., tennis halls) [2]. Because a lower air temperature is acceptable or even desired, passive solar gains can make a greater contribution to meeting the heating demand. Heat losses of glazed areas decrease proportionately with less inside to outside temperature difference. Lower required space temperatures increase system efficiency and number of hours when useful passive solar heat can be delivered.

Swimming pool halls are a potentially good building type for passive solar heating because a high air temperature is needed, so there is a very long heating season, well into long day spring and fall seasons. Also, there is a great appeal for the space being sunlit. Direct gain, indirect gain for radiant comfort, and isolated solar air systems for humidity control are possibilities.

Limited Cases

School class rooms have high internal gains and high ventilation requirements. The benefit of passive solar heating occurs primarily during heating season weekends and holidays. At that time, typically there is a temperature set back and temperature swings are tolerated, maximizing the usability of passive solar gains. The obvious choice is direct gain with daylighting within the constraints of glare control, thermal comfort near the windows, and the view out being more interesting than the view to the front. Isolated passive solar heating is another alternative. Figure 10 shows a Swiss school with glazed-in balconies off the classrooms. Sunspace heated air preheats incoming ventilation air over a heat exchanger for the classrooms [16].
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Passive Solar Heating in Built Environment. Figure 10 A Swiss school in Gumpenwiese ZH with sunspaces tied into the ventilation concept [16] (photo: robert.hastings@aeu.ch)

Old-age homes, nursing homes, and hospitals can benefit especially from direct gain with daylighting. Indirect gains systems to supply heat at night or isolated gain systems to preheat ventilation supply air are further possibilities. The occupancy tends to be "24/7" and demand somewhat higher room temperatures, extending the heating season.

Hotels have a transient occupancy. Guest rooms may be vacant with no internal loads for heating, but must be kept at room temperature or, if the thermostat is set back, very quickly warmed up. Accordingly, passive solar gains to maintain a minimal room temperature can be very useful, but mass can slow the heat-up. Daylight and view out may be assets, but overheating is totally unacceptable. Hotels in cities often may have to be isolated from traffic or airport noise. This can be solved with acoustical glazing or by using an indirect or isolated passive solar concept.

New Construction Verses Renovation

New construction should have very low energy demand as a result of very good insulation, mechanical ventilation with heat recovery. They likely will have a sophisticated heat production, delivery, and control system. Therefore, internal heat from occupancy and appliances will maintain the desired room temperature later in autumn and earlier in spring. The design of passive solar heating must consider this shorter heating season. Storing solar heat is very important because the gains can quickly exceed demand and result in overheating. Ideally, the thermal mass should be sunlit directly. Indirect passive solar systems can contribute to helping shorten the heating season.

Renovation is an excellent opportunity to increase passive solar heating. Older buildings have a greater and longer heating demand than well-done new structures. The subject of renovating existing housing was studied in a 4 year project of the Solar Heating and Cooling Program (SHC) of the International Energy Agency (IEA). As part of this work, 60 exemplary projects and an overview with insights were documented in brochures and are available on the Internet (Fig. 11) [17]. Included are apartment buildings, row houses, and single-family houses as well as the special case of historic buildings. The examples come from 10 countries: AT, BE, CA, CH, DE, DK, I, NL, NO, and SE.
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Passive Solar Heating in Built Environment. Figure 11 Renovation with solar and conservation, 60 examples of projects across Europe and Canada [17]