Solar Air Heating Project Analysis - Speaker's notes
SLIDE 1: Solar Air Heating Project Analysis
This is the Solar Air Heating Project Analysis Training Module of the RETScreen Clean Energy Project Analysis Course. Here, we discuss the use of solar energy to heat air by systems such as the one integrated into the wall of this large industrial building in Quebec, Canada.
SLIDE 2: Objectives
This module has three objectives. These are first, to review the basics of solar air heating, or SAH, systems; second, to illustrate key considerations for solar air heating project analysis; and third, to introduce the RETScreen Solar Air Heating Project Model.
SLIDE 3: What do SAH systems provide?
Solar air heating systems use solar energy to heat air. This warmed air can be used for building ventilation or for processes, such as drying, that require warm air. Since the sun does not shine brightly all the time, solar air heating systems typically supply only a portion of the energy required to heat ventilation or process air. In so doing, they reduce the consumption of conventional energy, such as natural gas or diesel fuel, and generate significant cost savings. In the top photo on this slide, students from a school in Yellowknife stand beside a new solar air heating system that reduces their school's reliance on diesel fuel, represented by the drums behind them.
The benefits of solar air heating go beyond hot air, however. Solar air heating systems function as weather cladding for the building, so conventional cladding is not needed where the solar system is installed. The bottom photo on this slide shows a solar air heating collector up close. It is standard steel weather cladding that has been painted a dark colour and perforated with tiny regularly spaced holes. Any water that enters through the holes runs down the inside of the cladding and falls out the bottom. There is no glazing, and the collector is simple and robust.
The solar air heating system incorporates an airspace between the cladding and the rest of the wall. This airspace provides extra insulation for the building. Furthermore, since this airspace collects the solar-heated air by drawing air through the perforations in the cladding, heat losses through the building wall are recuperated rather than lost to the environment. Thus, a SAH system reduces heat losses through the wall on which it is installed.
In industrial buildings with large volumes, warm air often pools near the ceiling, with layers of successively cooler air found below it. When such stratification occurs, heat is lost through the building roof at the same time that people at floor level are cold. A solar air heating system can make use of this hot air pooled near the ceiling and diminish stratification.
Solar air heating reduces the costs associated with supplying more fresh air to a building. This encourages building operators to improve building air quality.
Finally, many buildings suffer from negative air pressure; that is, the ventilation system exhausts more air than is brought in through inlets. This results in the infiltration of cold air as well as annoying air currents through doorways and corridors. As a side benefit, solar air heating can eliminate this problem.
SLIDE 4: SAH System Operation
The operation of a solar air heating system is very simple, yet it converts 50 to 80% of the sun's incident energy into useful heat. First, let's look at the components of a solar air heating system.
A solar air heating system consists of relatively few components. There is a dark, perforated collector, often mounted as weather cladding on a wall. The collector is steel or aluminum, has no glass cover, and is making it inexpensive and robust. Behind it, there is an airspace that permits air to flow up to a canopy, which draws air evenly from across the wall. A fan is responsible for creating the air flow, and ducting transports the air around the building.
Now let's examine how these components work, step-by-step.
First, sunlight strikes the solar air heating collector which, being a dark color, absorbs the radiation and warms up. Air in the vicinity of the collector is also heated.
Second, a fan draws air from the outside through the perforations in the collector, into the
airspace and into the canopy. Note that the heat in the air in the vicinity of the collector, which would otherwise be lost to convection, is drawn into the building. In addition, the collector temperature stays relatively close to the ambient air temperature, lowering heat losses further. These are the principal reasons that this unglazed collector is able to operate so efficiently.
Third, controls regulate the temperature of the air entering the building. These controls may either adjust the mix of recirculated and solar-heated air or modulate the output of an auxiliary heater, typically powered by gas, diesel, or electricity.
Fourth, the air is distributed throughout the building by ducts.
Note that heat escaping through the building wall, indicated by the number 5 in this diagram, enters the air that is drawn back into the building. This reduces heat losses through the wall by about 50% in most applications.
In buildings with stratification, the temperature of the air distributed around the ceiling can be cooler than the desired interior air temperature. When this cool air mixes with the hot upper layers of air, the resulting mix will be at the desired temperature. This makes use of heat that would otherwise be lost through the ceiling or through roof-mounted exhaust vents.
Finally, a bypass damper on the canopy permits unheated air to be drawn into the ventilation system during summer, when heating is not required.
SLIDE 5: Commercial/Residential SAH Systems
Two types of ventilation systems are commonly found in commercial and residential buildings. Apartment buildings and schools tend to have dedicated ventilation systems, which function independently of the building heating and cooling system and are responsible only for refreshing the building air. Other buildings may combine the function of ventilation with that of heating and cooling. Ducting distributes heated or cooled air, of which 10 to 20% is fresh. Solar air heating can be used with either type of system.
In commercial and residential buildings, solar air heating often requires no additional fans or ducting. The canopy above the solar collector is ducted right into the conventional fresh air intake. A constant flow of air is drawn through the collector. This raises the temperature of the air entering the intake such that less conventional heating is needed to raise the air to a comfortable temperature. SAH thus entails low cost equipment and materials that are easily integrated into conventional ventilation systems.
A commercial or residential solar air heating system is not intended to perform destratification; the air in these buildings is normally well-mixed and ceilings are low, so stratification is rarely a problem.
By raising the temperature of the air entering the conventional ventilation system, the SAH system permits the energy-saving economizer cycle to use more fresh air, which may improve indoor air quality. The economizer cycle is an operating strategy for using fresh air to cool a building when the outdoor air temperature is below the temperature of the air being drawn from within the building. This saves energy compared with operating a mechanical cooling system. When the air temperature is low outside, the economizer cycle mixes fresh air and recirculated building air to achieve the desired air temperature.
The lower the outside air temperature, the less fresh air is used. Since the solar air heating system raises the temperature of the outside air, it increases the amount of fresh air that is used by the economizer cycle.
SLIDE 6: Industrial SAH Systems
Industrial solar air heating systems are typically used in large volume buildings such as factories and warehouses. They are different from commercial and residential solar air heating systems in three ways.
First, an industrial solar air heating system includes dedicated ducting for distributing the solar-heated air around the ceiling; commercial and residential solar air heating systems entail no ducting beyond what is normally found in standard HVAC systems. The industrial system uses flexible fabric ducting with perforations to release air along the length of the duct.
Second, the air flow through the collector varies, in contrast to the constant flow through the collector of a commercial or residential system. In the industrial system, a constant speed fan is combined with a recirculation damper system. The flow rate through the distribution ducts is constant, but the portion of this flow that is cool air drawn in through the solar air collector and the portion of this flow that is warm air drawn from within the building is continuously varied to achieve a specified mixed air temperature.
Third, the specified mixed air temperature is usually in the range of 15 to 18ºC. This is cooler than the desired building air temperature; when this cool air is injected into the warm air near the building ceiling, the mixed air will be at the desired building temperature. Being cooler, this air will descend towards the floor. This achieves destratification and reduces the conventional heating required by the building.
SLIDE 7: SAH System for Process Heat
The processing of certain materials, both in industry and agriculture, requires large quantities of warm or hot air. Solar air heating can provide some of the energy for this. In these systems, the solar collector is mounted on any convenient surface that is welloriented to catch the sun. Low pitch roofs are used in some tropical countries, for example. The output of the collector is ducted directly to the process. The temperature of the process air is controlled by the addition of heat from a conventional source or the mixing of air streams by a damper. The inclusion of a conventional heater permits the system to operate even when there is little or no sunlight; its fuel consumption is reduced, however, compared to the situation of it having to provide all the heat.
One successful application of solar air heating is crop drying, as seen in this photo of a tea drying operation in West Java, Indonesia. Drying of crops requires large quantities of warm, but not hot air: the aim is to dessicate, not bake. Solar air heating systems, which raise the temperature of the ambient air by a few tens of degrees Celsius at most, match this requirement.
Solar air heating systems can also preheat air destined for industrial processes. These processes may demand higher temperatures than a solar air heating system can provide, but the solar collector can reduce the heat that must be added by conventional heaters, thus saving energy.
SLIDE 8: Solar Resource vs. Demand for Ventilation Heat
Solar air heating only makes sense if the heat is generated when there is a need for it. This has led some people to dismiss its utility for building ventilation: they assume that heating is needed in winter, and that there is no sunshine available during this season.
Let's see whether this is the case. Each of the graphs on this slide shows the situation at one of five cities. For all but Jakarta, the graphs indicate the solar energy available to a unit area of south-facing vertical collector during each of the twelve months of the year. The vertical scale expresses the solar energy in terms of the average number of peak sun hours per day, a unit equivalent to the energy received by a surface in bright sunshine for one hour. Along the horizontal axis, the twelve bars represent the twelve months of the year, starting with January as month number 1.
To investigate how the solar resource corresponds to the demand for solar air heating, the months of the year having an average air temperature less than 10ºC are shaded blue. During these months, it is likely that many buildings will need some ventilation heating.
Let's start at Iqaluit, in Canada's Nunavut Territory. Being close to the Arctic Circle, average temperatures are low year round, and thus all months are shaded blue. But Iqaluit is a remarkably cloudless place, and with the sun being so low in the sky due to the northern location, the vertical solar collector receives a great deal of solar energy. This is especially true in March, April, and May, when temperatures in Iqaluit are still very cold. This demonstrates that solar air heating can be very well-suited to communities in the far north.
Moscow, Russia, about 10º further south than Iqaluit, is a much cloudier place, and the solar resource is not as strong. Nor is the heating season as long. There is some solar energy available throughout the winter, however.
Buffalo, New York, USA, another ten or so degrees further south, experiences a continental climate. It needs ventilation heating about six months of the year as a result. And note, the vertical collector performs better during winter than summer, when the sun's rays are not oriented towards the collector. These tendencies are even more pronounced at Lanzhou, China, at 36º north. Despite the low latitude, the site experiences a relatively cold winter due to the Asian continental climate, during which there is a strong solar resource. So solar air heating is promising for mid-latitude locations as well.
The fifth graph is for Jakarta, Indonesia, virtually on the equator. No ventilation heating is needed, of course, but solar air heating systems are used for crop drying. Here the vertical axis shows the solar energy available to a horizontal, roof-mounted collector, rather than a collector mounted on a vertical wall. The solar resource barely varies from one month to the next, and is always strong.
SLIDE 9: SAH System Costs and Savings
Purchasing and installing a solar air heating system incurs certain costs and generates certain benefits. These costs and benefits can be expressed, in a rudimentary analysis, per square meter of collector. Solar air heating systems require little or no maintenance, so their costs are predominantly upfront. On the other hand, they generate annual savings through reduced expenditures for fuel.
First, the benefits. At most installations, a square meter of collector will generate between 1 and 3 GJ of useable heat per year. This solar heat reduces the need for conventional heating, usually by natural gas, diesel fuel, or electricity. The price per unit of each of these conventional energy sources will vary by location and with time. Where it is available, natural gas, for example, will generally cost between 17 and 45 ¢/m³.
Taking 2 GJ/m² per year as the output of a typical solar air heating system and making some reasonable assumptions about the efficiency of conventional heaters, the annual cost savings of a square meter of solar collector can be calculated. The graph at bottom shows that a square meter of collector saves about $10 to $30/yr when it reduces natural gas consumption, $20 to $45/yr when reducing diesel fuel consumption, and $30 to $65/yr when reducing electricity consumption.
Now, let's turn to the costs of this square meter of collector. Purchase and installation of the collector will vary between $100 and $250/m², depending on location, size of collector, and the particularities of the building. Modifications to the ventilation system are rarely required, but in some cases can add up to $100/m² of collector. So the total cost of the collector is $100 to $350/m². But note that the collector supplants regular cladding, and therefore the material and labour costs of regular cladding should be subtracted from this total cost. These savings will be one third to one half of that of the purchase and installation cost of the collector. The net solar system cost will range from $50 to $225/m² of collector, with most installations falling in the lower end of this range.
A comparison of these initial costs with the annual savings reveals typical simple payback periods of two to five years. Furthermore, the solar air heating system will last for decades, and continue to generate savings year upon year even after it has paid back its initial costs.
SLIDE 10: Solar Air Heating Project Considerations
Solar air heating is most cost-effective when employed in new construction. This ensures that the collector will displace some form of regular building cladding, reducing the net cost of the solar system. In addition, the building ventilation system will be designed and situated so as to facilitate integration of the solar collector, avoiding additional ducting and fans.
The next most cost-effective application of SAH is retrofitting motivated by the need to renovate or repair an existing exterior wall, improve interior air quality, or eliminate negative air pressure problems. Here the solar system will benefit from the cladding credit, but may require minor modifications to the existing ventilation system. When the cost of energy is high, solar air heating can be financially attractive as a retrofit or a source of process heat, solely due to its energy benefits.
Black will absorb more of the sun's energy than other colors, but that does not always make it the best choice for a solar collector. Most dark colors can convert 80 to 95% of the sun's incident energy into heat, so changing from black to another dark color will reduce the collector output by 15% at most. As a result, architectural considerations are often more important than the small improvement in performance that a black collector can provide.
When buildings are not occupied, the need for ventilation and solar air heating is decreased. This makes solar air heating more competitive when buildings are occupied all day and on weekends and holidays.
It is easiest to install a solar collector on a wall that has no windows or doors punched through it, but where necessary, these can be accommodated.
Solar air heating systems add little or nothing to maintenance costs. The summer bypass damper requires little attention and can be treated in the same way as other dampers in the ventilation system. Building ventilation fans need the same maintenance regardless of whether they draw air through a solar collector or a regular intake. The steel collector has no maintenance requirements above those of the steel cladding it replaces, and can be repainted if necessary. Dirt does not significantly lower the efficiency of the collector. Pollen, dust, and snow do not clog the collector's perforations, and the flow rate per unit area of collector is too low to draw them towards the wall. The flow of warm air desiccates the space behind the collector, making it an inhospitable environment for insects. These are all important considerations to building owners and designers contemplating the use of solar air heating.
SLIDE 11: Ventilation Air Heating Systems - Examples: Canada and USA
To date, the principal application of solar air heating has been the heating of building ventilation air. The solar collector reduces the cost of heating fresh air, making it a costeffective solution to interior air quality problems. The photo on the bottom left of this slide shows a portable classroom that had serious air quality problems; heating additional fresh air for the classroom by conventional means would have been very expensive, but a solar collector eliminated the problem and made use of the sunshine that was available during the hours when the classroom was occupied.
Heating fresh air with solar energy is especially attractive for industrial buildings that require large volumes of outdoor air to replace air exhausted from painting, welding, automotive fabrication, or other manufacturing processes. But solar air heating is also used with commercial and residential buildings. The photo on the right side of this slide shows an apartment building in Ontario, Canada, with a solar air heating system installed in a dark vertical band on the near corner of the building.
Solar collectors can be as small as a few square meters, such as collectors for detached homes, or as enormous as the industrial system in Quebec, Canada, shown on the first and last slides of this presentation. This 10,000 m² collector, the largest solar air heating system in the world, is found on the walls of an aircraft sub-assembly manufacturing plant. The area of this collector is equivalent to that of a building wall 3 stories high and 1 km long.
In the northern hemisphere, solar air heating systems function best when placed on a south-facing wall, although east and west-facing walls will also collect a reasonable amount of solar energy. The cost of additional ducting will be avoided if the intake for the building ventilation system is located near the wall on which the solar collector is installed.
Solar air heating used in new construction, renovations of existing walls, or in response to air quality and negative pressure problems will often have a simple payback period of two to five years. Industrial systems are often the most cost-effective. The center photo on this slide shows an industrial system in Connecticut, USA. Note that the collector is brown, not black, and accommodates numerous doors and windows.
SLIDE 12: Process Heat Systems - Example: Indonesia
Solar air heating can provide warm process air. This has been harnessed for crop drying and preheating of air for industrial processes.
These systems generally maintain a constant flow of air through the collector, as in commercial or residential SAH systems, and utilize very simple controls. A conventional heater adds more heat if the temperature at the outlet of the solar collector is too low.
The cost of these systems is rarely offset by a cladding credit, and they must be justified on their energy benefits alone. High energy costs make them more financially attractive.
Crop drying is a good application of solar air heating, especially when the alternative is expensive diesel fuel, such as in the Indonesian tea drying shelter shown here. The capital cost of the solar air heating system is most easily justified if the system can be used throughout the year. This favours crops that are grown and harvested year-round. Failing this, crops that are harvested outside of a monsoon or cloudy season are preferable.
SLIDE 13: RETScreen Solar Air Heating Project Model
The RETScreen Solar Air Heating Project Model is a simple but very useful tool for the preliminary investigation of the technical and financial feasibility of solar air heating projects. For an installation anywhere in the world, it can provide an analysis of the energy production, life-cycle costs, and greenhouse gas emissions reductions. The solar system can be used for ventilation or process air heating, and RETScreen calculates the reduction in building heat loss through the wall on which the collector is installed. For industrial buildings, it also calculates the destratification benefit.
To conduct this analysis, the user provides the site's average temperature, average daily solar radiation on a horizontal surface, and average wind speed for each month. The software includes a large database of solar radiation, temperature, and wind data from around the world. These monthly data are much more easily obtained and manipulated than the 8,760 values needed for an hour-by-hour simulation.
RETScreen models the operation of the Solarwall technology developed by Conserval, and is not meant for use with other solar air heating technologies. It also does not model the operation of advanced heat recovery ventilation systems, either in combination with or as a competitor to solar air heating. As a final limitation, it assumes that a building ventilation system is balanced - that is, that the system draws in as much air as it exhausts - which is not the case for all buildings.
SLIDE 14: RETScreen SAH Energy Calculation
The RETScreen Solar Air Heating Energy calculation determines the performance of the SAH system over the period of a year, using monthly solar radiation, temperature, and wind speed data. The calculation is based on monthly average values, and does not involve a detailed, hour-by-hour simulation of the operation of either the solar system or the ventilation system. Here, we provide an overview of this calculation; for more information, see the RETScreen Engineering and Cases Textbook, available online and free-of-charge.
RETScreen performs three calculations: one for the useful energy collected from the sun, a second for the reduction in the heat losses through the wall that is covered by the collector, and a third for the energy savings associated with destratification. For an industrial ventilation system, RETScreen sums all three savings; for a commercial or residential system, RETScreen calculates and sums the first two; and for process heat, RETScreen calculates only the solar energy savings.
The determination of these solar energy savings involves several steps. First, the solar energy available in the plane of the collector is calculated using the solar energy available on a horizontal surface. Second, based on variables such as the rate of airflow through the collector and the wind speed, RETScreen estimates the collector efficiency. Third, RETScreen uses the efficiency and the available solar energy to calculate the increment by which the solar collector will raise the temperature of the air that passes through it. Recognizing that high outside air temperatures and strong sunshine may lead the solar collector to generate heat that is not required by the building or process, RETScreen estimates the portion of each month's available solar energy that will actually be used.
In commercial, residential, and process heat systems, the airflow through the collector is constant, and the determination of the solar energy savings is straightforward. In industrial systems, in contrast, the rate of airflow through the collector is continuously varied by a mixing damper in response to the temperature rise through the collector. The collector efficiency, being a function of the rate of airflow, varies in consequence. But this very collector efficiency is a key determinant of the temperature rise through the collector. These variables must therefore be solved by iteration so RETScreen loops through the second and third steps in this calculation three times.
SLIDE 15: Example Validation of the RETScreen SAH Project Model
The RETScreen software has been validated in a number of ways. In one validation exercise, RETScreen was compared with Natural Resources Canada's SWift software, a sophisticated hourly simulation tool for solar air heating that is formulated on basic thermodynamics rather than empirical relations.
The basis for comparison was an imaginary 1,200 m² building with a black solar air heating collector and a 4,000 L/s flow through the ventilation system. The size of the solar collector varied with the design objective; for high efficiency it was 100 m². Meteorological data from Toronto, Ontario, Canada and Winnipeg, Manitoba, Canada were used. Three combinations of ventilation and solar system were considered: a commercial system with a design objective of high efficiency and two industrial systems, one with a design objective of high efficiency and the other for high temperature rise.
The two tools agreed to within 10% in five out the six cases examined. RETScreen did not appear to systematically over or underestimate the system performance on an annual basis. This suggests that RETScreen can be virtually as accurate as hourly simulation, and is sufficiently accurate for pre-feasibility purposes.
SLIDE 16: Conclusions
Solar air heating is a cost-effective technology for commercial, residential, and industrial building ventilation and for agricultural and industrial process heat. In many climates, a strong solar resource is available during the seasons that ventilation air heating is required.
Solar air heating collectors do not just provide energy benefits - they also serve as weather cladding. SAH systems feed into the intake of conventional building ventilation systems. They are most financially attractive in new construction and retrofit applications aimed at repairing an existing wall, improving air quality, or balancing the ventilation system.
The RETScreen software calculates the energy produced by a SAH system, the collector efficiency, and the temperature rise across the collector. The energy calculation includes benefits due to destratification and the recovery of heat loss across the collector-covered wall of the building. Using monthly climate data but achieving accuracy comparable to hourly simulations, RETScreen can significantly reduce the cost of conducting preliminary feasibility studies of solar air heating projects.
SLIDE 17: Questions?
This is the end of the Solar Air Heating Project Analysis Training Module in the RETScreen International Clean Energy Project Analysis Course.
This is the Solar Air Heating Project Analysis Training Module of the RETScreen Clean Energy Project Analysis Course. Here, we discuss the use of solar energy to heat air by systems such as the one integrated into the wall of this large industrial building in Quebec, Canada.
SLIDE 2: Objectives
This module has three objectives. These are first, to review the basics of solar air heating, or SAH, systems; second, to illustrate key considerations for solar air heating project analysis; and third, to introduce the RETScreen Solar Air Heating Project Model.
SLIDE 3: What do SAH systems provide?
Solar air heating systems use solar energy to heat air. This warmed air can be used for building ventilation or for processes, such as drying, that require warm air. Since the sun does not shine brightly all the time, solar air heating systems typically supply only a portion of the energy required to heat ventilation or process air. In so doing, they reduce the consumption of conventional energy, such as natural gas or diesel fuel, and generate significant cost savings. In the top photo on this slide, students from a school in Yellowknife stand beside a new solar air heating system that reduces their school's reliance on diesel fuel, represented by the drums behind them.
The benefits of solar air heating go beyond hot air, however. Solar air heating systems function as weather cladding for the building, so conventional cladding is not needed where the solar system is installed. The bottom photo on this slide shows a solar air heating collector up close. It is standard steel weather cladding that has been painted a dark colour and perforated with tiny regularly spaced holes. Any water that enters through the holes runs down the inside of the cladding and falls out the bottom. There is no glazing, and the collector is simple and robust.
The solar air heating system incorporates an airspace between the cladding and the rest of the wall. This airspace provides extra insulation for the building. Furthermore, since this airspace collects the solar-heated air by drawing air through the perforations in the cladding, heat losses through the building wall are recuperated rather than lost to the environment. Thus, a SAH system reduces heat losses through the wall on which it is installed.
In industrial buildings with large volumes, warm air often pools near the ceiling, with layers of successively cooler air found below it. When such stratification occurs, heat is lost through the building roof at the same time that people at floor level are cold. A solar air heating system can make use of this hot air pooled near the ceiling and diminish stratification.
Solar air heating reduces the costs associated with supplying more fresh air to a building. This encourages building operators to improve building air quality.
Finally, many buildings suffer from negative air pressure; that is, the ventilation system exhausts more air than is brought in through inlets. This results in the infiltration of cold air as well as annoying air currents through doorways and corridors. As a side benefit, solar air heating can eliminate this problem.
SLIDE 4: SAH System Operation
The operation of a solar air heating system is very simple, yet it converts 50 to 80% of the sun's incident energy into useful heat. First, let's look at the components of a solar air heating system.
A solar air heating system consists of relatively few components. There is a dark, perforated collector, often mounted as weather cladding on a wall. The collector is steel or aluminum, has no glass cover, and is making it inexpensive and robust. Behind it, there is an airspace that permits air to flow up to a canopy, which draws air evenly from across the wall. A fan is responsible for creating the air flow, and ducting transports the air around the building.
Now let's examine how these components work, step-by-step.
First, sunlight strikes the solar air heating collector which, being a dark color, absorbs the radiation and warms up. Air in the vicinity of the collector is also heated.
Second, a fan draws air from the outside through the perforations in the collector, into the
airspace and into the canopy. Note that the heat in the air in the vicinity of the collector, which would otherwise be lost to convection, is drawn into the building. In addition, the collector temperature stays relatively close to the ambient air temperature, lowering heat losses further. These are the principal reasons that this unglazed collector is able to operate so efficiently.
Third, controls regulate the temperature of the air entering the building. These controls may either adjust the mix of recirculated and solar-heated air or modulate the output of an auxiliary heater, typically powered by gas, diesel, or electricity.
Fourth, the air is distributed throughout the building by ducts.
Note that heat escaping through the building wall, indicated by the number 5 in this diagram, enters the air that is drawn back into the building. This reduces heat losses through the wall by about 50% in most applications.
In buildings with stratification, the temperature of the air distributed around the ceiling can be cooler than the desired interior air temperature. When this cool air mixes with the hot upper layers of air, the resulting mix will be at the desired temperature. This makes use of heat that would otherwise be lost through the ceiling or through roof-mounted exhaust vents.
Finally, a bypass damper on the canopy permits unheated air to be drawn into the ventilation system during summer, when heating is not required.
SLIDE 5: Commercial/Residential SAH Systems
Two types of ventilation systems are commonly found in commercial and residential buildings. Apartment buildings and schools tend to have dedicated ventilation systems, which function independently of the building heating and cooling system and are responsible only for refreshing the building air. Other buildings may combine the function of ventilation with that of heating and cooling. Ducting distributes heated or cooled air, of which 10 to 20% is fresh. Solar air heating can be used with either type of system.
In commercial and residential buildings, solar air heating often requires no additional fans or ducting. The canopy above the solar collector is ducted right into the conventional fresh air intake. A constant flow of air is drawn through the collector. This raises the temperature of the air entering the intake such that less conventional heating is needed to raise the air to a comfortable temperature. SAH thus entails low cost equipment and materials that are easily integrated into conventional ventilation systems.
A commercial or residential solar air heating system is not intended to perform destratification; the air in these buildings is normally well-mixed and ceilings are low, so stratification is rarely a problem.
By raising the temperature of the air entering the conventional ventilation system, the SAH system permits the energy-saving economizer cycle to use more fresh air, which may improve indoor air quality. The economizer cycle is an operating strategy for using fresh air to cool a building when the outdoor air temperature is below the temperature of the air being drawn from within the building. This saves energy compared with operating a mechanical cooling system. When the air temperature is low outside, the economizer cycle mixes fresh air and recirculated building air to achieve the desired air temperature.
The lower the outside air temperature, the less fresh air is used. Since the solar air heating system raises the temperature of the outside air, it increases the amount of fresh air that is used by the economizer cycle.
SLIDE 6: Industrial SAH Systems
Industrial solar air heating systems are typically used in large volume buildings such as factories and warehouses. They are different from commercial and residential solar air heating systems in three ways.
First, an industrial solar air heating system includes dedicated ducting for distributing the solar-heated air around the ceiling; commercial and residential solar air heating systems entail no ducting beyond what is normally found in standard HVAC systems. The industrial system uses flexible fabric ducting with perforations to release air along the length of the duct.
Second, the air flow through the collector varies, in contrast to the constant flow through the collector of a commercial or residential system. In the industrial system, a constant speed fan is combined with a recirculation damper system. The flow rate through the distribution ducts is constant, but the portion of this flow that is cool air drawn in through the solar air collector and the portion of this flow that is warm air drawn from within the building is continuously varied to achieve a specified mixed air temperature.
Third, the specified mixed air temperature is usually in the range of 15 to 18ºC. This is cooler than the desired building air temperature; when this cool air is injected into the warm air near the building ceiling, the mixed air will be at the desired building temperature. Being cooler, this air will descend towards the floor. This achieves destratification and reduces the conventional heating required by the building.
SLIDE 7: SAH System for Process Heat
The processing of certain materials, both in industry and agriculture, requires large quantities of warm or hot air. Solar air heating can provide some of the energy for this. In these systems, the solar collector is mounted on any convenient surface that is welloriented to catch the sun. Low pitch roofs are used in some tropical countries, for example. The output of the collector is ducted directly to the process. The temperature of the process air is controlled by the addition of heat from a conventional source or the mixing of air streams by a damper. The inclusion of a conventional heater permits the system to operate even when there is little or no sunlight; its fuel consumption is reduced, however, compared to the situation of it having to provide all the heat.
One successful application of solar air heating is crop drying, as seen in this photo of a tea drying operation in West Java, Indonesia. Drying of crops requires large quantities of warm, but not hot air: the aim is to dessicate, not bake. Solar air heating systems, which raise the temperature of the ambient air by a few tens of degrees Celsius at most, match this requirement.
Solar air heating systems can also preheat air destined for industrial processes. These processes may demand higher temperatures than a solar air heating system can provide, but the solar collector can reduce the heat that must be added by conventional heaters, thus saving energy.
SLIDE 8: Solar Resource vs. Demand for Ventilation Heat
Solar air heating only makes sense if the heat is generated when there is a need for it. This has led some people to dismiss its utility for building ventilation: they assume that heating is needed in winter, and that there is no sunshine available during this season.
Let's see whether this is the case. Each of the graphs on this slide shows the situation at one of five cities. For all but Jakarta, the graphs indicate the solar energy available to a unit area of south-facing vertical collector during each of the twelve months of the year. The vertical scale expresses the solar energy in terms of the average number of peak sun hours per day, a unit equivalent to the energy received by a surface in bright sunshine for one hour. Along the horizontal axis, the twelve bars represent the twelve months of the year, starting with January as month number 1.
To investigate how the solar resource corresponds to the demand for solar air heating, the months of the year having an average air temperature less than 10ºC are shaded blue. During these months, it is likely that many buildings will need some ventilation heating.
Let's start at Iqaluit, in Canada's Nunavut Territory. Being close to the Arctic Circle, average temperatures are low year round, and thus all months are shaded blue. But Iqaluit is a remarkably cloudless place, and with the sun being so low in the sky due to the northern location, the vertical solar collector receives a great deal of solar energy. This is especially true in March, April, and May, when temperatures in Iqaluit are still very cold. This demonstrates that solar air heating can be very well-suited to communities in the far north.
Moscow, Russia, about 10º further south than Iqaluit, is a much cloudier place, and the solar resource is not as strong. Nor is the heating season as long. There is some solar energy available throughout the winter, however.
Buffalo, New York, USA, another ten or so degrees further south, experiences a continental climate. It needs ventilation heating about six months of the year as a result. And note, the vertical collector performs better during winter than summer, when the sun's rays are not oriented towards the collector. These tendencies are even more pronounced at Lanzhou, China, at 36º north. Despite the low latitude, the site experiences a relatively cold winter due to the Asian continental climate, during which there is a strong solar resource. So solar air heating is promising for mid-latitude locations as well.
The fifth graph is for Jakarta, Indonesia, virtually on the equator. No ventilation heating is needed, of course, but solar air heating systems are used for crop drying. Here the vertical axis shows the solar energy available to a horizontal, roof-mounted collector, rather than a collector mounted on a vertical wall. The solar resource barely varies from one month to the next, and is always strong.
SLIDE 9: SAH System Costs and Savings
Purchasing and installing a solar air heating system incurs certain costs and generates certain benefits. These costs and benefits can be expressed, in a rudimentary analysis, per square meter of collector. Solar air heating systems require little or no maintenance, so their costs are predominantly upfront. On the other hand, they generate annual savings through reduced expenditures for fuel.
First, the benefits. At most installations, a square meter of collector will generate between 1 and 3 GJ of useable heat per year. This solar heat reduces the need for conventional heating, usually by natural gas, diesel fuel, or electricity. The price per unit of each of these conventional energy sources will vary by location and with time. Where it is available, natural gas, for example, will generally cost between 17 and 45 ¢/m³.
Taking 2 GJ/m² per year as the output of a typical solar air heating system and making some reasonable assumptions about the efficiency of conventional heaters, the annual cost savings of a square meter of solar collector can be calculated. The graph at bottom shows that a square meter of collector saves about $10 to $30/yr when it reduces natural gas consumption, $20 to $45/yr when reducing diesel fuel consumption, and $30 to $65/yr when reducing electricity consumption.
Now, let's turn to the costs of this square meter of collector. Purchase and installation of the collector will vary between $100 and $250/m², depending on location, size of collector, and the particularities of the building. Modifications to the ventilation system are rarely required, but in some cases can add up to $100/m² of collector. So the total cost of the collector is $100 to $350/m². But note that the collector supplants regular cladding, and therefore the material and labour costs of regular cladding should be subtracted from this total cost. These savings will be one third to one half of that of the purchase and installation cost of the collector. The net solar system cost will range from $50 to $225/m² of collector, with most installations falling in the lower end of this range.
A comparison of these initial costs with the annual savings reveals typical simple payback periods of two to five years. Furthermore, the solar air heating system will last for decades, and continue to generate savings year upon year even after it has paid back its initial costs.
SLIDE 10: Solar Air Heating Project Considerations
Solar air heating is most cost-effective when employed in new construction. This ensures that the collector will displace some form of regular building cladding, reducing the net cost of the solar system. In addition, the building ventilation system will be designed and situated so as to facilitate integration of the solar collector, avoiding additional ducting and fans.
The next most cost-effective application of SAH is retrofitting motivated by the need to renovate or repair an existing exterior wall, improve interior air quality, or eliminate negative air pressure problems. Here the solar system will benefit from the cladding credit, but may require minor modifications to the existing ventilation system. When the cost of energy is high, solar air heating can be financially attractive as a retrofit or a source of process heat, solely due to its energy benefits.
Black will absorb more of the sun's energy than other colors, but that does not always make it the best choice for a solar collector. Most dark colors can convert 80 to 95% of the sun's incident energy into heat, so changing from black to another dark color will reduce the collector output by 15% at most. As a result, architectural considerations are often more important than the small improvement in performance that a black collector can provide.
When buildings are not occupied, the need for ventilation and solar air heating is decreased. This makes solar air heating more competitive when buildings are occupied all day and on weekends and holidays.
It is easiest to install a solar collector on a wall that has no windows or doors punched through it, but where necessary, these can be accommodated.
Solar air heating systems add little or nothing to maintenance costs. The summer bypass damper requires little attention and can be treated in the same way as other dampers in the ventilation system. Building ventilation fans need the same maintenance regardless of whether they draw air through a solar collector or a regular intake. The steel collector has no maintenance requirements above those of the steel cladding it replaces, and can be repainted if necessary. Dirt does not significantly lower the efficiency of the collector. Pollen, dust, and snow do not clog the collector's perforations, and the flow rate per unit area of collector is too low to draw them towards the wall. The flow of warm air desiccates the space behind the collector, making it an inhospitable environment for insects. These are all important considerations to building owners and designers contemplating the use of solar air heating.
SLIDE 11: Ventilation Air Heating Systems - Examples: Canada and USA
To date, the principal application of solar air heating has been the heating of building ventilation air. The solar collector reduces the cost of heating fresh air, making it a costeffective solution to interior air quality problems. The photo on the bottom left of this slide shows a portable classroom that had serious air quality problems; heating additional fresh air for the classroom by conventional means would have been very expensive, but a solar collector eliminated the problem and made use of the sunshine that was available during the hours when the classroom was occupied.
Heating fresh air with solar energy is especially attractive for industrial buildings that require large volumes of outdoor air to replace air exhausted from painting, welding, automotive fabrication, or other manufacturing processes. But solar air heating is also used with commercial and residential buildings. The photo on the right side of this slide shows an apartment building in Ontario, Canada, with a solar air heating system installed in a dark vertical band on the near corner of the building.
Solar collectors can be as small as a few square meters, such as collectors for detached homes, or as enormous as the industrial system in Quebec, Canada, shown on the first and last slides of this presentation. This 10,000 m² collector, the largest solar air heating system in the world, is found on the walls of an aircraft sub-assembly manufacturing plant. The area of this collector is equivalent to that of a building wall 3 stories high and 1 km long.
In the northern hemisphere, solar air heating systems function best when placed on a south-facing wall, although east and west-facing walls will also collect a reasonable amount of solar energy. The cost of additional ducting will be avoided if the intake for the building ventilation system is located near the wall on which the solar collector is installed.
Solar air heating used in new construction, renovations of existing walls, or in response to air quality and negative pressure problems will often have a simple payback period of two to five years. Industrial systems are often the most cost-effective. The center photo on this slide shows an industrial system in Connecticut, USA. Note that the collector is brown, not black, and accommodates numerous doors and windows.
SLIDE 12: Process Heat Systems - Example: Indonesia
Solar air heating can provide warm process air. This has been harnessed for crop drying and preheating of air for industrial processes.
These systems generally maintain a constant flow of air through the collector, as in commercial or residential SAH systems, and utilize very simple controls. A conventional heater adds more heat if the temperature at the outlet of the solar collector is too low.
The cost of these systems is rarely offset by a cladding credit, and they must be justified on their energy benefits alone. High energy costs make them more financially attractive.
Crop drying is a good application of solar air heating, especially when the alternative is expensive diesel fuel, such as in the Indonesian tea drying shelter shown here. The capital cost of the solar air heating system is most easily justified if the system can be used throughout the year. This favours crops that are grown and harvested year-round. Failing this, crops that are harvested outside of a monsoon or cloudy season are preferable.
SLIDE 13: RETScreen Solar Air Heating Project Model
The RETScreen Solar Air Heating Project Model is a simple but very useful tool for the preliminary investigation of the technical and financial feasibility of solar air heating projects. For an installation anywhere in the world, it can provide an analysis of the energy production, life-cycle costs, and greenhouse gas emissions reductions. The solar system can be used for ventilation or process air heating, and RETScreen calculates the reduction in building heat loss through the wall on which the collector is installed. For industrial buildings, it also calculates the destratification benefit.
To conduct this analysis, the user provides the site's average temperature, average daily solar radiation on a horizontal surface, and average wind speed for each month. The software includes a large database of solar radiation, temperature, and wind data from around the world. These monthly data are much more easily obtained and manipulated than the 8,760 values needed for an hour-by-hour simulation.
RETScreen models the operation of the Solarwall technology developed by Conserval, and is not meant for use with other solar air heating technologies. It also does not model the operation of advanced heat recovery ventilation systems, either in combination with or as a competitor to solar air heating. As a final limitation, it assumes that a building ventilation system is balanced - that is, that the system draws in as much air as it exhausts - which is not the case for all buildings.
SLIDE 14: RETScreen SAH Energy Calculation
The RETScreen Solar Air Heating Energy calculation determines the performance of the SAH system over the period of a year, using monthly solar radiation, temperature, and wind speed data. The calculation is based on monthly average values, and does not involve a detailed, hour-by-hour simulation of the operation of either the solar system or the ventilation system. Here, we provide an overview of this calculation; for more information, see the RETScreen Engineering and Cases Textbook, available online and free-of-charge.
RETScreen performs three calculations: one for the useful energy collected from the sun, a second for the reduction in the heat losses through the wall that is covered by the collector, and a third for the energy savings associated with destratification. For an industrial ventilation system, RETScreen sums all three savings; for a commercial or residential system, RETScreen calculates and sums the first two; and for process heat, RETScreen calculates only the solar energy savings.
The determination of these solar energy savings involves several steps. First, the solar energy available in the plane of the collector is calculated using the solar energy available on a horizontal surface. Second, based on variables such as the rate of airflow through the collector and the wind speed, RETScreen estimates the collector efficiency. Third, RETScreen uses the efficiency and the available solar energy to calculate the increment by which the solar collector will raise the temperature of the air that passes through it. Recognizing that high outside air temperatures and strong sunshine may lead the solar collector to generate heat that is not required by the building or process, RETScreen estimates the portion of each month's available solar energy that will actually be used.
In commercial, residential, and process heat systems, the airflow through the collector is constant, and the determination of the solar energy savings is straightforward. In industrial systems, in contrast, the rate of airflow through the collector is continuously varied by a mixing damper in response to the temperature rise through the collector. The collector efficiency, being a function of the rate of airflow, varies in consequence. But this very collector efficiency is a key determinant of the temperature rise through the collector. These variables must therefore be solved by iteration so RETScreen loops through the second and third steps in this calculation three times.
SLIDE 15: Example Validation of the RETScreen SAH Project Model
The RETScreen software has been validated in a number of ways. In one validation exercise, RETScreen was compared with Natural Resources Canada's SWift software, a sophisticated hourly simulation tool for solar air heating that is formulated on basic thermodynamics rather than empirical relations.
The basis for comparison was an imaginary 1,200 m² building with a black solar air heating collector and a 4,000 L/s flow through the ventilation system. The size of the solar collector varied with the design objective; for high efficiency it was 100 m². Meteorological data from Toronto, Ontario, Canada and Winnipeg, Manitoba, Canada were used. Three combinations of ventilation and solar system were considered: a commercial system with a design objective of high efficiency and two industrial systems, one with a design objective of high efficiency and the other for high temperature rise.
The two tools agreed to within 10% in five out the six cases examined. RETScreen did not appear to systematically over or underestimate the system performance on an annual basis. This suggests that RETScreen can be virtually as accurate as hourly simulation, and is sufficiently accurate for pre-feasibility purposes.
SLIDE 16: Conclusions
Solar air heating is a cost-effective technology for commercial, residential, and industrial building ventilation and for agricultural and industrial process heat. In many climates, a strong solar resource is available during the seasons that ventilation air heating is required.
Solar air heating collectors do not just provide energy benefits - they also serve as weather cladding. SAH systems feed into the intake of conventional building ventilation systems. They are most financially attractive in new construction and retrofit applications aimed at repairing an existing wall, improving air quality, or balancing the ventilation system.
The RETScreen software calculates the energy produced by a SAH system, the collector efficiency, and the temperature rise across the collector. The energy calculation includes benefits due to destratification and the recovery of heat loss across the collector-covered wall of the building. Using monthly climate data but achieving accuracy comparable to hourly simulations, RETScreen can significantly reduce the cost of conducting preliminary feasibility studies of solar air heating projects.
SLIDE 17: Questions?
This is the end of the Solar Air Heating Project Analysis Training Module in the RETScreen International Clean Energy Project Analysis Course.
