Passive Solar Heating Project Analysis - Speaker's notes
SLIDE 1: Passive Solar Heating Project Analysis
This is the Passive Solar Heating Project Analysis Training Module of the RETScreen Clean Energy Project Analysis Course. Here, we discuss the heating of buildings using the solar gains available through high performance windows. This house in France, with a prominent solarium, makes use of passive solar heating.
SLIDE 2: Objectives
This module has three objectives. These are first, to review the basics of passive solar heating (PSH) systems; second, to illustrate key considerations in passive solar heating project analysis; and third, to introduce the RETScreen PSH Project Model.
SLIDE 3: What does PSH provide?
By making use of solar energy admitted through windows, passive solar heating reduces the conventional energy required to heat a building. Compared to random orientation of conventional double glazed windows, careful placement of the windows and selection of the window technology can reduce a building's requirement for space heating by 20 to 50%.
In addition to this reduction in the required conventional heating energy, passive solar heating improves occupant comfort. Passive solar heating generally involves the use of high performance windows that reduce the heat losses through the window. As a result, during cold weather the temperature of the interior surface of the glass is warmer than that of conventional windows. This has two beneficial effects: occupants feel more comfortable, since they are not losing heat by radiation to large cold surfaces, and condensation on the interior of the window is reduced or eliminated.
Furthermore, the use of windows with good thermal performance can permit the building designer to make better use of daylight. Heat losses through conventional windows are often so large that their size and placement must be restricted; this restriction is relaxed somewhat when better windows are used.
Better windows and passive solar heating design considerations can also reduce the energy required for building cooling during summertime. Careful design can ensure that windows are shaded during summer and thus, the solar gains causing building overheating are minimized. When the air is warmer outside than in, better window technology reduces heat gains through the window by conduction, although this tends to be a minor benefit in climates requiring heating.
By reducing peak cooling requirements in summer, shading and other passive solar heating techniques can sometimes permit a smaller conventional cooling plant to be used. In the heating season, on the other hand, with warmer temperatures of interior window surfaces, perimeter heating can often be eliminated. These reductions in initial costs for conventional heating and cooling can offset a portion or even all of the additional costs associated with high performance windows.
SLIDE 4: Principles of Operation of PSH
Let's examine the principles of passive solar heating design by way of a comparison with a conventional building. In the summer, the windows of the conventional building admit sunlight that heats the building. Either this heat must be removed from the building by good ventilation or air conditioning or the building becomes uncomfortably warm. In winter, sunlight also enters the building, but over the course of the day, heat losses through the windows to the cold outside environment far outweigh the solar gains. The building will have a tendency to be cold, especially at night, and the heating system will consume a lot of energy.
In comparison, the building with passive solar heating uses less energy to maintain a comfortable interior environment year-round. It does this in three ways. First, high performance windows lose less heat during winter due to their low thermal conductivity. Second, during summer, equator-facing windows are shaded, in this case by an overhang. Solar gains are thereby reduced during those times of the year when they would cause overheating. Since the sun is lower in the sky during winter, the overhang does not shade the windows when solar gains are beneficial. Third, the interior building materials store heat from solar gains during the day and release it at night. When the equator-facing window area is not overly large, conventional North American lightweight construction of wood or steel frame walls with gypsum board is sufficient. Heavy materials such as brick and ceramic tiles permit use of a larger equator-facing window area without daytime overheating.
SLIDE 5: Advanced Window Technologies
Over the last thirty years, commercial window technology has greatly improved. Advanced window technologies incorporate a number of innovations that reduce the rate at which heat will escape from the building while still admitting much of the incident solar radiation.
One of the oldest and best known innovations is the use of double and triple glazing. The gap between the parallel panes of glass creates a barrier to heat loss: it acts as a layer of thermal insulation. This gap can be made an even better barrier to heat loss by filling it with argon, krypton, or another gas that has a lower thermal conductivity than air. Another innovation has been the use of low-emittance films and coatings. Although glass does not transmit infrared radiation - that is, heat is not radiated across it - a warm pane of glass will lose heat by emitting infra-red radiation. Low emittance, or low-e, treatments reduce these losses.
Heat losses occur not just through the glazing but also across the frame and the spacer separating the panes of glass. In the past, these have often been made of aluminum, an excellent conductor of heat. Now much more attention is being paid to the design of the frame and window spacer and these losses are being minimized. Insulating materials such as wood and vinyl are used in the frames, and the assembly that separates the panes contains an insulating spacer that reduces heat conduction.
The effects of these measures can be seen in the table and figures on this slide. These compare the performance of six different windows in terms of their overall thermal conductivity, expressed as a U-value, and the fraction of incident solar energy that passes through the window and becomes heat gain, expressed as the solar heat gain coefficient. Smaller U-values reduce heat losses and larger solar heat gain coefficients increase heat gains. For each window, two values of the U-value and the solar heat gain coefficient are given: one for the center of the glazing, which largely eliminates the effect of the frame and the spacer, and another for the whole window, including the frame. The frame and spacer have a detrimental effect on both the U-value and the solar heat gain coefficient. In general, low U-values are more important than high solar heat gain coefficients, because heat losses occur all the time, even at night, while heat gains are limited to those times when the sun is shining through the window.
The entry at the bottom of the table is for a simple single pane of glass with aluminum frame. Considering the window as a whole, the U-value is over 7 W/m² per degree Celsius, potentially resulting in enormous heat losses, but this window transmits about 75% of the solar energy incident upon it. Simply adding a second pane of glass to create an insulated glazing unit, shown in the second to last entry in the table, halves the Uvalue of the window while only decreasing the solar heat gain coefficient by 10 percentage points. This is already an enormous advance. An incremental improvement to the U-value of this window can be made by changing to a wooden frame, as seen in the next entry up.
A major jump in performance occurs with the addition of a low e coating, the substitution of argon for air as the fill gas, and the replacement of the aluminum spacer with an insulating material. This reduces heat losses by 30 to 40% but results in only a 15% reduction in the solar heat gain coefficient. The U-value of this window is actually better than that of a triple glazed window without low-e coating, argon filler, and insulating spacer; on the other hand, the solar heat gain coefficient is slightly higher with the triple glazed window, so the two can be considered roughly comparable in performance.
Combining triple glazing, low-e, argon fill, and insulating spacer results in a very high performance window, as seen in the first entry of the table. The U-value has been reduced by 85%, but the reduction in solar heat gain coefficient is only 45%. Much sunlight still passes through the window to warm the interior, but heat losses are drastically reduced.
SLIDE 6: Shading and Thermal Mass
When passive solar heating involves an increase in the glazed area on a building, the energy required for summer cooling may also increase. Furthermore, there may be an uncomfortable diurnal cycle to the temperature with the building overheating during the day. Fortunately, careful design and the use of the shading and thermal mass canalleviate or eliminate these drawbacks.
There are various measures for shading, but all have the same objective: to prevent sunlight from entering the window during the summer, when heating is undesirable, while not obstructing sunlight during the heating season. The overhang is a simple and very effective shading measure for equator-facing windows. In summer, the sun is high in the sky and the overhang shades the window below it. In winter, the sun is low in the sky, and sunlight passes under the overhang and through the window. Deciduous trees, which lose their leaves during winter, can also be used, but they must be fairly large trees before they are effective. The shadows cast by nearby buildings and other structures may also be beneficial. Shading devices in or around the window, such as screens, blinds, awnings, and shutters constitute another approach, but may prevent the occupant from seeing through the window. There exist motorized shading devices with automatic controllers that adjust the shading to suit the conditions of temperature and irradiance, but these are expensive and may be prone to failure.
Orienting the building such that it has a large equator-facing wall in which to place the windows will facilitate passive solar heating. These equator-facing windows are much easier to shade, with overhangs for example, than east or west-facing windows. Windows in the western façade should generally be avoided, since they admit light in the afternoon, when the building is unlikely to need heat. Sloped equator facing windows, including some skylights, also tend to cause overheating. Windows oriented directly away from the equator result in little useful solar energy gain, but can provide even daylight without glare.
When equator-facing glazing is less than roughly 10% of the heated floor area of the building, conventional materials used in the interior of the building will typically have sufficient thermal mass to store the heat gained on a winter day and thus prevent overheating. When the glazing exceeds this threshold, the building interior should include additional thermal mass that can absorb heat and release it at night. This thermal mass may take the form of double layers of gypsum board in the walls and ceilings, ceramic floor tiles, concrete, or brick.
Passive solar heating can be combined with active heat distribution systems to transport the gains at the equator-facing wall to the rest of the building. Fans or operation of the conventional air distribution system can also help reduce overheating of the area near the window.
SLIDE 7: Solar Resource vs. Requirement for Space Heating
Passive solar heating requires that sunshine be available at those times of the year when building heating occurs. Let's examine the average solar energy incident on vertical, equator-facing windows at four cities around the world. For each figure on this slide, the numbers along the horizontal axis correspond to the month of the year, with January being 1 and December being 12. The vertical axis indicates the average daily solar energy incident on the window during the month, in peak sun hours per day. This unit, equivalent to 1 kWh/m² per day, is roughly equal to the sunshine that would fall on a 1 m² surface between noon and 1 pm on a clear day, with the surface oriented towards the sun. The months that are shaded blue have an average temperature of 10ºC or less, and roughly represent the heating season.
In Iqaluit, Nunavut, Canada, temperatures are below 10ºC year-round. Despite the Arctic location, there is a reasonable solar resource during winter, and a very strong resource in February, March, April, and May, when heating will be required. In Moscow, Russia, the winter resource is weaker, due to the cloudier climate, but late winter and early spring are quite sunny, and temperatures are still low. In Buffalo, USA, at 43º north, and Lanzhou, China, at 36º north, there is a good solar resource throughout the six months of the winter heating season.
These graphs hint at the potential for passive solar heating and reflect the fact that vertical equator-facing windows are much better oriented to catch winter sunshine than horizontal or near-horizontal surfaces.
SLIDE 8: Example of PSH Costs & Savings
The costs and the energy savings associated with passive solar heating will vary from location to location and project to project. To give an idea of its potential, let's look at the case of a typical single family dwelling in Canada. First, we'll examine the additional costs associated with building the house using high performance windows instead of conventional windows. Then we'll look at the annual savings in the heating energy bill for the house.
As our base case window technology, we'll consider fixed double glazed windows with wood frames but aluminum spacers, air fill, and no low-e treatment. You will recall, from the fifth slide in this presentation, that this type of window technology is already much more energy efficient than single glazing. The installed cost of this type of window is around $250/m² of window.
This window's performance can be improved, as seen previously, by adding a low-e treatment, substituting argon gas for air, and replacing the aluminum spacer with an insulating material. Each of these improvements adds to the cost of the window, but only incrementally. Including all three will raise the window cost by around $35 to $40/m². Adding a third pane of glass will require a substantially greater investment, resulting in a cost of around $340/m². In short, depending on the nature of the improvements, the cost of the windows will increase by 5 to 35%, or $400 to $2,000 for all the windows in a typical house.
On the other hand, this investment in improved windows will typically reduce the requirement for space heating by 20 to 50%. If a natural gas heating system is used, and the cost of natural gas is 25 ¢/m³, this will result in annual cost savings of $150 to $380. If more expensive fuels, like oil or electricity, are used, savings will be higher: for electricity at 6 ¢/kWh, for example, annual savings will be $270 to $680. It should be noted that while the implied simple payback periods may not be especially quick, the savings continue to mount over the life of the window, and the improved windows may thus be a very profitable investment for the occupant or owner.
SLIDE 9: Passive Solar Heating Project Considerations
The ideal time to consider an investment in passive solar heating is when planning new building construction. Freed from the constraints of existing building layout, the designer can orient the building so that it has a façade facing the equator, populate that façade with an appropriate number of high performance windows, include overhangs or other shading devices to avoid overheating in summer, and avoid west-facing windows. Having maximized passive solar heating, the designer can then evaluate whether perimeter heating can be eliminated and the extent to which the size of the cooling plant may be reduced. Unlike in new construction, an existing building with heating and cooling systems already installed cannot benefit from the potential reductions in capital costs possible with passive solar heating.
If the windows of an existing building need replacement, high performance windows can be cost-effective compared with conventional double glazed windows. On the other hand, the energy benefit associated with high performance windows rarely justifies replacing conventional double glazed windows that are still in good condition.
Passive solar heating is most cost-effective when the heating energy demand of the building is high compared to its cooling demand. Both climate and the type of building determine whether this is the case. Cold and moderately cold climates are the most promising. Low-rise residential construction tends to make more sense than commercial and industrial buildings, where internal heat gains may be very high, decreasing the heating energy demand. On the other hand, such buildings may require perimeter heating even when the building's net heat load is zero or negative; if high performance windows obviate the need for this perimeter heating, they may be very cost-effective.
The windows should always be considered in conjunction with the rest of the building envelope. It makes little sense to install high performance windows in a poorly insulated wall; similarly, a highly insulated wall should be matched with high performance windows. The envelope is only as a good as its weakest element.
SLIDE 10: Low Energy Buildings - Examples: Canada and USA
Passive solar heating and other improvements to the thermal performance of the building envelope have been incorporated into many very conventional-looking buildings throughout the world. In fact, many standard building practices of today would have been considered low-energy techniques thirty years ago. Nevertheless, there is still room for cost-effective improvements. Even when investments in the building envelope, and passive solar heating in particular, are not strictly cost-effective, they are still included in many buildings for other reasons. Better windows improve occupant comfort and result in better sound abatement. They are also a logical choice for those who appreciate quality in their living and working environment, and for those who feel strongly about their obligations to the natural environment.
The photo on the left shows the top two storeys of a three-storey housing unit in Massachusetts, USA. Notice the good shading provided by tall trees. With advanced windows and a ground-source heat pump, this building's purchase energy requirement was reduced by 50%. On the right is the Waterloo Green Home in Ontario, Canada. Notice the significant overhang immediately above the windows, the deciduous trees, and the photovoltaic and solar hot water system on the roof.
SLIDE 11: Self-sufficient Solar Houses - Examples: Germany and Lesotho
The previous slide demonstrated that passive solar heating can reduce the heating requirements of fairly conventional buildings. It is also an essential element in buildings that are completely energy self-sufficient, that is, that rely only on solar energy. Such buildings usually include larger glazed areas, to generate higher solar gains, more thermal mass, to store heat, and control of air distribution, to transport gains around the building. These techniques permit solar energy to entirely meet all space heating needs, even in relatively cold environments. Advanced windows are a critical component of these buildings. They allow more flexible placement of windows and, in some cases, can even generate net solar gains from diffuse radiation.
Self-sufficient houses are rarely cost-effective in cold climates, especially at high latitudes where there is little solar energy available in winter. Nevertheless, they demonstrate the potential for new technologies and underline the shortcomings of conventional construction. For example, the photo on the right shows the Freiburg solar home, an extremely advanced experimental house built in Germany in 1992 by the Fraunhofer Institute for solar energy systems. All of its energy requirements are met by solar energy. The curved, south facing façade is a combination of transparent glass and transparent insulation. Photovoltaic modules are seen on the roof.
It is easier to construct a self-sufficient solar building in cold climates at low to moderate latitudes. The photo on the left, for example, shows a solar Rondavel in Thaba-Tseka, Lesotho. The house is at an altitude of 1,800 m, where night-time winter temperatures are well below freezing. The translucent corrugated fiberglass houses a solarium where plants are grown. At night, the occupants open their doors to the solarium to permit warm air to enter their dwelling. A thermosyphon solar hot water system and a photovoltaic system are installed on the roof.
SLIDE 12: RETScreen Passive Solar Heating Project Model
The RETScreen Passive Solar Heating Project Model is a simple but very useful tool for the preliminary investigation of the technical and financial feasibility of passive solar heating. For a building anywhere in the world, it can provide an analysis of the impact of high performance windows on heating and cooling energy requirements, life-cycle costs, and greenhouse gas emissions reductions. The model can be applied to low-rise residential and small commercial buildings in a heating dominated climate. It accounts for solar gains and thermal losses through the windows as well as the average effects of shading. The tool can also be used to investigate the effect of changing the orientation of the building and modifying the glazed area on each façade.
RETScreen requires only minimal input data from the user. For outdoor temperature and solar resource, it requires only monthly average values. These data are available for many locations around the world in a built-in weather database. It is, in general, much easier to procure monthly average values than the hourly data required by many simulations. This facilitates quick analyses of different scenarios.
While the RETScreen model can be applied to many buildings, it has several limitations. It cannot be applied to sloped windows, but rather assumes windows on four vertical walls that are at right angles to each other. It does not account for the instantaneous, as opposed to average, effects of shading, nor does it permit the user to vary the level or storage of solar energy in the building mass.
SLIDE 13: RETScreen PSH Energy Calculation
The RETScreen Passive Solar Heating Energy calculation determines the impact of improved windows over the period of a year. It does two calculations, one for the heating energy savings and one for the cooling energy savings. Here we provide an overview of these calculations; for more information see the RETScreen Engineering and Cases Textbook, available online and free-of-charge.
The first step in these calculations is the adjustment of the window properties based on their dimensions. Window properties, such as the U-value and solar heat gain coefficients vary, for a given window construction, with the size of the windows. RETScreen has a database of windows; if the user specifies a window size different from the tested window in the database, RETScreen adjusts the window properties to account for the actual size and shape of the window.
Next, RETScreen calculates the heating energy demand for each month based on the difference between the heating setpoint temperature, assumed to be 21ºC, and the average outside air temperature. For the base case, typical house heat loss coefficients are used to correlate the heat loss to the difference between the outside and the inside air temperatures. Then, for the proposed case, the heat loss coefficient is adjusted to reflect the net impact of changing from the base case windows to the proposed case windows. The adjusted heat loss coefficient is used to determine the heating energy demand for the proposed case. Note that although the use of typical house heat loss coefficients is not particularly accurate, the error thus induced is largely cancelled out by taking the difference between the base case and the proposed case heating energy demand.
The building internal gains are assumed to be the same in both the base case and the proposed case, and constant year-round. The user specifies a daily value and RETScreen finds the equivalent monthly internal heat gain.
Now RETScreen calculates the usable solar gains during the heating season for both the base and the proposed case. First, it calculates the solar energy incident on each of the four façade orientations during each month. Then, it calculates, based on the window's solar heat gain coefficient, the fraction of this solar energy that will enter the building. Then, it determines what fraction of this admitted solar energy will reduce heating demand without causing overheating. A function, whose shape is determined by the thermal mass of the building, as specified by the user, and an assumed maximum permissible interior air temperature swing of 5.5ºC, is used to estimate this fraction based on the ratio of monthly admitted solar energy to the net monthly heating demand.
Then, for each month, the heating energy savings are calculated as the difference between the energy required to heat the building in the base case and the proposed case. The conventional heating energy required is the heating energy demand minus the internal gains minus the usable solar gains. This is summed over all months in which heating is required to find the annual heating energy savings.
A very similar set of calculations is done for the cooling energy savings. First, the cooling demand is calculated based on a cooling setpoint temperature of 25ºC. Then, the increase in the required cooling energy due to the changes in window size, orientation, and properties is calculated for each month. Here, thermal conduction through the windows is considered negligible; only non-usable solar gains are included. That is, the function that was used to define the fraction of the admitted solar energy that will reduce heating demand without causing overheating is now used to calculate that fraction that will cause overheating. The change in cooling energy demand is summed over all months to find the annual cooling energy savings. The heating energy savings and the cooling energy savings are combined to find the overall energy savings.
Finally, the change in the peak heating and cooling load is determined. The peak heating load is found from the product of the building heat loss coefficient and the difference between the indoor air temperature (assumed to be 21ºC) and the heating design temperature, or minimum outdoor temperature likely to be encountered. The peak cooling load is calculated similarly, but with the indoor temperature assumed to be 25ºC and the effects of the maximum solar gains added in. Dehumidification loads are not considered as they are assumed to be equal in both the base case and the proposed case.
SLIDE 14: Example Validation of the RETScreen PSH Project Model
The accuracy of the RETScreen Passive Solar Heating Project Model was validated in two ways. First, the RETScreen model was compared to HOT2-XP, a residential energy analysis simulation package. A typical Canadian house was assumed, and then the effect of upgrading the windows was assessed. The base case windows were double glazed with air fill, metal spacer, and wood frames. The proposed case windows were double glazed with argon fill, low-e coating, and fibreglass frame. RETScreen's estimate of the reduction in the heating energy demand was only 18% lower than that made by HOT2-XP.
In the second evaluation, RETScreen's ranking of the annual energy savings achieved by eight different windows was compared to that predicted by the Energy Rating (ER) Method. The ER Method is a Canadian standard that was developed based on hourly energy simulations. The baseline was considered to be a low performance conventional double glazed window. As can be seen in the figure, RETScreen's predictions matched those of the Energy Rating Method very closely.
SLIDE 15: Conclusions
Passive solar heating refers to a suite of building technologies and design techniques that maximize the space heating achieved by solar energy admitted through windows. These include optimal orientation of the building and its windows, energy efficient windows, shading measures to prevent summertime overheating, and appropriate levels of thermal mass for heat storage within the building. When constructing a new building or replacing old windows, using high performance windows in place of conventional double glazed windows will greatly improve the performance of the building envelope. Thus, a relatively minor additional investment in the windows will pay long-term dividends through reduced heating and cooling costs.
The RETScreen software helps determine the additional costs and benefits associated with such a window upgrade. For the level of thermal mass associated with typical light residential construction, it determines the effect of building orientation, window orientation, window size, and window technology on the solar gains admitted to a building. It determines the effect of the window technology on heat losses from the building, and also estimates the impact of shading on the building cooling energy demand. Annual energy savings are calculated on the basis of monthly calculations using average values for outdoor temperature and solar irradiance. Using a minimum of input data, RETScreen can provide accuracy comparable to hourly simulation, and thereby significantly reduce the cost of conducting preliminary feasibility studies of passive solar heating projects.
SLIDE 16: Questions?
This is the end of the Passive Solar Heating Project Analysis Course Training Module in the RETScreen International Clean Energy Project Analysis Course.
This is the Passive Solar Heating Project Analysis Training Module of the RETScreen Clean Energy Project Analysis Course. Here, we discuss the heating of buildings using the solar gains available through high performance windows. This house in France, with a prominent solarium, makes use of passive solar heating.
SLIDE 2: Objectives
This module has three objectives. These are first, to review the basics of passive solar heating (PSH) systems; second, to illustrate key considerations in passive solar heating project analysis; and third, to introduce the RETScreen PSH Project Model.
SLIDE 3: What does PSH provide?
By making use of solar energy admitted through windows, passive solar heating reduces the conventional energy required to heat a building. Compared to random orientation of conventional double glazed windows, careful placement of the windows and selection of the window technology can reduce a building's requirement for space heating by 20 to 50%.
In addition to this reduction in the required conventional heating energy, passive solar heating improves occupant comfort. Passive solar heating generally involves the use of high performance windows that reduce the heat losses through the window. As a result, during cold weather the temperature of the interior surface of the glass is warmer than that of conventional windows. This has two beneficial effects: occupants feel more comfortable, since they are not losing heat by radiation to large cold surfaces, and condensation on the interior of the window is reduced or eliminated.
Furthermore, the use of windows with good thermal performance can permit the building designer to make better use of daylight. Heat losses through conventional windows are often so large that their size and placement must be restricted; this restriction is relaxed somewhat when better windows are used.
Better windows and passive solar heating design considerations can also reduce the energy required for building cooling during summertime. Careful design can ensure that windows are shaded during summer and thus, the solar gains causing building overheating are minimized. When the air is warmer outside than in, better window technology reduces heat gains through the window by conduction, although this tends to be a minor benefit in climates requiring heating.
By reducing peak cooling requirements in summer, shading and other passive solar heating techniques can sometimes permit a smaller conventional cooling plant to be used. In the heating season, on the other hand, with warmer temperatures of interior window surfaces, perimeter heating can often be eliminated. These reductions in initial costs for conventional heating and cooling can offset a portion or even all of the additional costs associated with high performance windows.
SLIDE 4: Principles of Operation of PSH
Let's examine the principles of passive solar heating design by way of a comparison with a conventional building. In the summer, the windows of the conventional building admit sunlight that heats the building. Either this heat must be removed from the building by good ventilation or air conditioning or the building becomes uncomfortably warm. In winter, sunlight also enters the building, but over the course of the day, heat losses through the windows to the cold outside environment far outweigh the solar gains. The building will have a tendency to be cold, especially at night, and the heating system will consume a lot of energy.
In comparison, the building with passive solar heating uses less energy to maintain a comfortable interior environment year-round. It does this in three ways. First, high performance windows lose less heat during winter due to their low thermal conductivity. Second, during summer, equator-facing windows are shaded, in this case by an overhang. Solar gains are thereby reduced during those times of the year when they would cause overheating. Since the sun is lower in the sky during winter, the overhang does not shade the windows when solar gains are beneficial. Third, the interior building materials store heat from solar gains during the day and release it at night. When the equator-facing window area is not overly large, conventional North American lightweight construction of wood or steel frame walls with gypsum board is sufficient. Heavy materials such as brick and ceramic tiles permit use of a larger equator-facing window area without daytime overheating.
SLIDE 5: Advanced Window Technologies
Over the last thirty years, commercial window technology has greatly improved. Advanced window technologies incorporate a number of innovations that reduce the rate at which heat will escape from the building while still admitting much of the incident solar radiation.
One of the oldest and best known innovations is the use of double and triple glazing. The gap between the parallel panes of glass creates a barrier to heat loss: it acts as a layer of thermal insulation. This gap can be made an even better barrier to heat loss by filling it with argon, krypton, or another gas that has a lower thermal conductivity than air. Another innovation has been the use of low-emittance films and coatings. Although glass does not transmit infrared radiation - that is, heat is not radiated across it - a warm pane of glass will lose heat by emitting infra-red radiation. Low emittance, or low-e, treatments reduce these losses.
Heat losses occur not just through the glazing but also across the frame and the spacer separating the panes of glass. In the past, these have often been made of aluminum, an excellent conductor of heat. Now much more attention is being paid to the design of the frame and window spacer and these losses are being minimized. Insulating materials such as wood and vinyl are used in the frames, and the assembly that separates the panes contains an insulating spacer that reduces heat conduction.
The effects of these measures can be seen in the table and figures on this slide. These compare the performance of six different windows in terms of their overall thermal conductivity, expressed as a U-value, and the fraction of incident solar energy that passes through the window and becomes heat gain, expressed as the solar heat gain coefficient. Smaller U-values reduce heat losses and larger solar heat gain coefficients increase heat gains. For each window, two values of the U-value and the solar heat gain coefficient are given: one for the center of the glazing, which largely eliminates the effect of the frame and the spacer, and another for the whole window, including the frame. The frame and spacer have a detrimental effect on both the U-value and the solar heat gain coefficient. In general, low U-values are more important than high solar heat gain coefficients, because heat losses occur all the time, even at night, while heat gains are limited to those times when the sun is shining through the window.
The entry at the bottom of the table is for a simple single pane of glass with aluminum frame. Considering the window as a whole, the U-value is over 7 W/m² per degree Celsius, potentially resulting in enormous heat losses, but this window transmits about 75% of the solar energy incident upon it. Simply adding a second pane of glass to create an insulated glazing unit, shown in the second to last entry in the table, halves the Uvalue of the window while only decreasing the solar heat gain coefficient by 10 percentage points. This is already an enormous advance. An incremental improvement to the U-value of this window can be made by changing to a wooden frame, as seen in the next entry up.
A major jump in performance occurs with the addition of a low e coating, the substitution of argon for air as the fill gas, and the replacement of the aluminum spacer with an insulating material. This reduces heat losses by 30 to 40% but results in only a 15% reduction in the solar heat gain coefficient. The U-value of this window is actually better than that of a triple glazed window without low-e coating, argon filler, and insulating spacer; on the other hand, the solar heat gain coefficient is slightly higher with the triple glazed window, so the two can be considered roughly comparable in performance.
Combining triple glazing, low-e, argon fill, and insulating spacer results in a very high performance window, as seen in the first entry of the table. The U-value has been reduced by 85%, but the reduction in solar heat gain coefficient is only 45%. Much sunlight still passes through the window to warm the interior, but heat losses are drastically reduced.
SLIDE 6: Shading and Thermal Mass
When passive solar heating involves an increase in the glazed area on a building, the energy required for summer cooling may also increase. Furthermore, there may be an uncomfortable diurnal cycle to the temperature with the building overheating during the day. Fortunately, careful design and the use of the shading and thermal mass canalleviate or eliminate these drawbacks.
There are various measures for shading, but all have the same objective: to prevent sunlight from entering the window during the summer, when heating is undesirable, while not obstructing sunlight during the heating season. The overhang is a simple and very effective shading measure for equator-facing windows. In summer, the sun is high in the sky and the overhang shades the window below it. In winter, the sun is low in the sky, and sunlight passes under the overhang and through the window. Deciduous trees, which lose their leaves during winter, can also be used, but they must be fairly large trees before they are effective. The shadows cast by nearby buildings and other structures may also be beneficial. Shading devices in or around the window, such as screens, blinds, awnings, and shutters constitute another approach, but may prevent the occupant from seeing through the window. There exist motorized shading devices with automatic controllers that adjust the shading to suit the conditions of temperature and irradiance, but these are expensive and may be prone to failure.
Orienting the building such that it has a large equator-facing wall in which to place the windows will facilitate passive solar heating. These equator-facing windows are much easier to shade, with overhangs for example, than east or west-facing windows. Windows in the western façade should generally be avoided, since they admit light in the afternoon, when the building is unlikely to need heat. Sloped equator facing windows, including some skylights, also tend to cause overheating. Windows oriented directly away from the equator result in little useful solar energy gain, but can provide even daylight without glare.
When equator-facing glazing is less than roughly 10% of the heated floor area of the building, conventional materials used in the interior of the building will typically have sufficient thermal mass to store the heat gained on a winter day and thus prevent overheating. When the glazing exceeds this threshold, the building interior should include additional thermal mass that can absorb heat and release it at night. This thermal mass may take the form of double layers of gypsum board in the walls and ceilings, ceramic floor tiles, concrete, or brick.
Passive solar heating can be combined with active heat distribution systems to transport the gains at the equator-facing wall to the rest of the building. Fans or operation of the conventional air distribution system can also help reduce overheating of the area near the window.
SLIDE 7: Solar Resource vs. Requirement for Space Heating
Passive solar heating requires that sunshine be available at those times of the year when building heating occurs. Let's examine the average solar energy incident on vertical, equator-facing windows at four cities around the world. For each figure on this slide, the numbers along the horizontal axis correspond to the month of the year, with January being 1 and December being 12. The vertical axis indicates the average daily solar energy incident on the window during the month, in peak sun hours per day. This unit, equivalent to 1 kWh/m² per day, is roughly equal to the sunshine that would fall on a 1 m² surface between noon and 1 pm on a clear day, with the surface oriented towards the sun. The months that are shaded blue have an average temperature of 10ºC or less, and roughly represent the heating season.
In Iqaluit, Nunavut, Canada, temperatures are below 10ºC year-round. Despite the Arctic location, there is a reasonable solar resource during winter, and a very strong resource in February, March, April, and May, when heating will be required. In Moscow, Russia, the winter resource is weaker, due to the cloudier climate, but late winter and early spring are quite sunny, and temperatures are still low. In Buffalo, USA, at 43º north, and Lanzhou, China, at 36º north, there is a good solar resource throughout the six months of the winter heating season.
These graphs hint at the potential for passive solar heating and reflect the fact that vertical equator-facing windows are much better oriented to catch winter sunshine than horizontal or near-horizontal surfaces.
SLIDE 8: Example of PSH Costs & Savings
The costs and the energy savings associated with passive solar heating will vary from location to location and project to project. To give an idea of its potential, let's look at the case of a typical single family dwelling in Canada. First, we'll examine the additional costs associated with building the house using high performance windows instead of conventional windows. Then we'll look at the annual savings in the heating energy bill for the house.
As our base case window technology, we'll consider fixed double glazed windows with wood frames but aluminum spacers, air fill, and no low-e treatment. You will recall, from the fifth slide in this presentation, that this type of window technology is already much more energy efficient than single glazing. The installed cost of this type of window is around $250/m² of window.
This window's performance can be improved, as seen previously, by adding a low-e treatment, substituting argon gas for air, and replacing the aluminum spacer with an insulating material. Each of these improvements adds to the cost of the window, but only incrementally. Including all three will raise the window cost by around $35 to $40/m². Adding a third pane of glass will require a substantially greater investment, resulting in a cost of around $340/m². In short, depending on the nature of the improvements, the cost of the windows will increase by 5 to 35%, or $400 to $2,000 for all the windows in a typical house.
On the other hand, this investment in improved windows will typically reduce the requirement for space heating by 20 to 50%. If a natural gas heating system is used, and the cost of natural gas is 25 ¢/m³, this will result in annual cost savings of $150 to $380. If more expensive fuels, like oil or electricity, are used, savings will be higher: for electricity at 6 ¢/kWh, for example, annual savings will be $270 to $680. It should be noted that while the implied simple payback periods may not be especially quick, the savings continue to mount over the life of the window, and the improved windows may thus be a very profitable investment for the occupant or owner.
SLIDE 9: Passive Solar Heating Project Considerations
The ideal time to consider an investment in passive solar heating is when planning new building construction. Freed from the constraints of existing building layout, the designer can orient the building so that it has a façade facing the equator, populate that façade with an appropriate number of high performance windows, include overhangs or other shading devices to avoid overheating in summer, and avoid west-facing windows. Having maximized passive solar heating, the designer can then evaluate whether perimeter heating can be eliminated and the extent to which the size of the cooling plant may be reduced. Unlike in new construction, an existing building with heating and cooling systems already installed cannot benefit from the potential reductions in capital costs possible with passive solar heating.
If the windows of an existing building need replacement, high performance windows can be cost-effective compared with conventional double glazed windows. On the other hand, the energy benefit associated with high performance windows rarely justifies replacing conventional double glazed windows that are still in good condition.
Passive solar heating is most cost-effective when the heating energy demand of the building is high compared to its cooling demand. Both climate and the type of building determine whether this is the case. Cold and moderately cold climates are the most promising. Low-rise residential construction tends to make more sense than commercial and industrial buildings, where internal heat gains may be very high, decreasing the heating energy demand. On the other hand, such buildings may require perimeter heating even when the building's net heat load is zero or negative; if high performance windows obviate the need for this perimeter heating, they may be very cost-effective.
The windows should always be considered in conjunction with the rest of the building envelope. It makes little sense to install high performance windows in a poorly insulated wall; similarly, a highly insulated wall should be matched with high performance windows. The envelope is only as a good as its weakest element.
SLIDE 10: Low Energy Buildings - Examples: Canada and USA
Passive solar heating and other improvements to the thermal performance of the building envelope have been incorporated into many very conventional-looking buildings throughout the world. In fact, many standard building practices of today would have been considered low-energy techniques thirty years ago. Nevertheless, there is still room for cost-effective improvements. Even when investments in the building envelope, and passive solar heating in particular, are not strictly cost-effective, they are still included in many buildings for other reasons. Better windows improve occupant comfort and result in better sound abatement. They are also a logical choice for those who appreciate quality in their living and working environment, and for those who feel strongly about their obligations to the natural environment.
The photo on the left shows the top two storeys of a three-storey housing unit in Massachusetts, USA. Notice the good shading provided by tall trees. With advanced windows and a ground-source heat pump, this building's purchase energy requirement was reduced by 50%. On the right is the Waterloo Green Home in Ontario, Canada. Notice the significant overhang immediately above the windows, the deciduous trees, and the photovoltaic and solar hot water system on the roof.
SLIDE 11: Self-sufficient Solar Houses - Examples: Germany and Lesotho
The previous slide demonstrated that passive solar heating can reduce the heating requirements of fairly conventional buildings. It is also an essential element in buildings that are completely energy self-sufficient, that is, that rely only on solar energy. Such buildings usually include larger glazed areas, to generate higher solar gains, more thermal mass, to store heat, and control of air distribution, to transport gains around the building. These techniques permit solar energy to entirely meet all space heating needs, even in relatively cold environments. Advanced windows are a critical component of these buildings. They allow more flexible placement of windows and, in some cases, can even generate net solar gains from diffuse radiation.
Self-sufficient houses are rarely cost-effective in cold climates, especially at high latitudes where there is little solar energy available in winter. Nevertheless, they demonstrate the potential for new technologies and underline the shortcomings of conventional construction. For example, the photo on the right shows the Freiburg solar home, an extremely advanced experimental house built in Germany in 1992 by the Fraunhofer Institute for solar energy systems. All of its energy requirements are met by solar energy. The curved, south facing façade is a combination of transparent glass and transparent insulation. Photovoltaic modules are seen on the roof.
It is easier to construct a self-sufficient solar building in cold climates at low to moderate latitudes. The photo on the left, for example, shows a solar Rondavel in Thaba-Tseka, Lesotho. The house is at an altitude of 1,800 m, where night-time winter temperatures are well below freezing. The translucent corrugated fiberglass houses a solarium where plants are grown. At night, the occupants open their doors to the solarium to permit warm air to enter their dwelling. A thermosyphon solar hot water system and a photovoltaic system are installed on the roof.
SLIDE 12: RETScreen Passive Solar Heating Project Model
The RETScreen Passive Solar Heating Project Model is a simple but very useful tool for the preliminary investigation of the technical and financial feasibility of passive solar heating. For a building anywhere in the world, it can provide an analysis of the impact of high performance windows on heating and cooling energy requirements, life-cycle costs, and greenhouse gas emissions reductions. The model can be applied to low-rise residential and small commercial buildings in a heating dominated climate. It accounts for solar gains and thermal losses through the windows as well as the average effects of shading. The tool can also be used to investigate the effect of changing the orientation of the building and modifying the glazed area on each façade.
RETScreen requires only minimal input data from the user. For outdoor temperature and solar resource, it requires only monthly average values. These data are available for many locations around the world in a built-in weather database. It is, in general, much easier to procure monthly average values than the hourly data required by many simulations. This facilitates quick analyses of different scenarios.
While the RETScreen model can be applied to many buildings, it has several limitations. It cannot be applied to sloped windows, but rather assumes windows on four vertical walls that are at right angles to each other. It does not account for the instantaneous, as opposed to average, effects of shading, nor does it permit the user to vary the level or storage of solar energy in the building mass.
SLIDE 13: RETScreen PSH Energy Calculation
The RETScreen Passive Solar Heating Energy calculation determines the impact of improved windows over the period of a year. It does two calculations, one for the heating energy savings and one for the cooling energy savings. Here we provide an overview of these calculations; for more information see the RETScreen Engineering and Cases Textbook, available online and free-of-charge.
The first step in these calculations is the adjustment of the window properties based on their dimensions. Window properties, such as the U-value and solar heat gain coefficients vary, for a given window construction, with the size of the windows. RETScreen has a database of windows; if the user specifies a window size different from the tested window in the database, RETScreen adjusts the window properties to account for the actual size and shape of the window.
Next, RETScreen calculates the heating energy demand for each month based on the difference between the heating setpoint temperature, assumed to be 21ºC, and the average outside air temperature. For the base case, typical house heat loss coefficients are used to correlate the heat loss to the difference between the outside and the inside air temperatures. Then, for the proposed case, the heat loss coefficient is adjusted to reflect the net impact of changing from the base case windows to the proposed case windows. The adjusted heat loss coefficient is used to determine the heating energy demand for the proposed case. Note that although the use of typical house heat loss coefficients is not particularly accurate, the error thus induced is largely cancelled out by taking the difference between the base case and the proposed case heating energy demand.
The building internal gains are assumed to be the same in both the base case and the proposed case, and constant year-round. The user specifies a daily value and RETScreen finds the equivalent monthly internal heat gain.
Now RETScreen calculates the usable solar gains during the heating season for both the base and the proposed case. First, it calculates the solar energy incident on each of the four façade orientations during each month. Then, it calculates, based on the window's solar heat gain coefficient, the fraction of this solar energy that will enter the building. Then, it determines what fraction of this admitted solar energy will reduce heating demand without causing overheating. A function, whose shape is determined by the thermal mass of the building, as specified by the user, and an assumed maximum permissible interior air temperature swing of 5.5ºC, is used to estimate this fraction based on the ratio of monthly admitted solar energy to the net monthly heating demand.
Then, for each month, the heating energy savings are calculated as the difference between the energy required to heat the building in the base case and the proposed case. The conventional heating energy required is the heating energy demand minus the internal gains minus the usable solar gains. This is summed over all months in which heating is required to find the annual heating energy savings.
A very similar set of calculations is done for the cooling energy savings. First, the cooling demand is calculated based on a cooling setpoint temperature of 25ºC. Then, the increase in the required cooling energy due to the changes in window size, orientation, and properties is calculated for each month. Here, thermal conduction through the windows is considered negligible; only non-usable solar gains are included. That is, the function that was used to define the fraction of the admitted solar energy that will reduce heating demand without causing overheating is now used to calculate that fraction that will cause overheating. The change in cooling energy demand is summed over all months to find the annual cooling energy savings. The heating energy savings and the cooling energy savings are combined to find the overall energy savings.
Finally, the change in the peak heating and cooling load is determined. The peak heating load is found from the product of the building heat loss coefficient and the difference between the indoor air temperature (assumed to be 21ºC) and the heating design temperature, or minimum outdoor temperature likely to be encountered. The peak cooling load is calculated similarly, but with the indoor temperature assumed to be 25ºC and the effects of the maximum solar gains added in. Dehumidification loads are not considered as they are assumed to be equal in both the base case and the proposed case.
SLIDE 14: Example Validation of the RETScreen PSH Project Model
The accuracy of the RETScreen Passive Solar Heating Project Model was validated in two ways. First, the RETScreen model was compared to HOT2-XP, a residential energy analysis simulation package. A typical Canadian house was assumed, and then the effect of upgrading the windows was assessed. The base case windows were double glazed with air fill, metal spacer, and wood frames. The proposed case windows were double glazed with argon fill, low-e coating, and fibreglass frame. RETScreen's estimate of the reduction in the heating energy demand was only 18% lower than that made by HOT2-XP.
In the second evaluation, RETScreen's ranking of the annual energy savings achieved by eight different windows was compared to that predicted by the Energy Rating (ER) Method. The ER Method is a Canadian standard that was developed based on hourly energy simulations. The baseline was considered to be a low performance conventional double glazed window. As can be seen in the figure, RETScreen's predictions matched those of the Energy Rating Method very closely.
SLIDE 15: Conclusions
Passive solar heating refers to a suite of building technologies and design techniques that maximize the space heating achieved by solar energy admitted through windows. These include optimal orientation of the building and its windows, energy efficient windows, shading measures to prevent summertime overheating, and appropriate levels of thermal mass for heat storage within the building. When constructing a new building or replacing old windows, using high performance windows in place of conventional double glazed windows will greatly improve the performance of the building envelope. Thus, a relatively minor additional investment in the windows will pay long-term dividends through reduced heating and cooling costs.
The RETScreen software helps determine the additional costs and benefits associated with such a window upgrade. For the level of thermal mass associated with typical light residential construction, it determines the effect of building orientation, window orientation, window size, and window technology on the solar gains admitted to a building. It determines the effect of the window technology on heat losses from the building, and also estimates the impact of shading on the building cooling energy demand. Annual energy savings are calculated on the basis of monthly calculations using average values for outdoor temperature and solar irradiance. Using a minimum of input data, RETScreen can provide accuracy comparable to hourly simulation, and thereby significantly reduce the cost of conducting preliminary feasibility studies of passive solar heating projects.
SLIDE 16: Questions?
This is the end of the Passive Solar Heating Project Analysis Course Training Module in the RETScreen International Clean Energy Project Analysis Course.
