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Nowadays, the threat of possible effects of global warming has to be taken seriously. The substantial part of overall greenhouse gases emission is produced by the buildings' maintenance needs- burning fossil fuels for heating, or using vast amounts of electricity on air conditioning. These emissions can be significantly decreased by using passive solar design strategies for heating/cooling the buildings.
The aim of this project is to research some of the passive solar concepts, calculate their performances in different conditions and therefore evaluate their suitability for domestic and non-domestic buildings. The final aim of the project is creating an ArchiCAD library of passive solar systems that can be customised for different climatic conditions and easily integrated into the projects.
We can define comfort as the sensation of complete physical and mental well-being (Goulding etal, 1992). It is a very personal experience, dependant on one's particular biological, emotional and physical characteristics. If a group of people is subjected to the same climatic conditions, individual members' comfort ranges might differ, which makes the designer's control of the comfort quite limited. Nonetheless, he/she must aim to provide optimal thermal comfort for the group as a whole, i.e. he or she must provide conditions under which most of the people in the group feel comfortable.
Achievement of thermal comfort is especially important in passive solar buildings. The way in which the sun's energy is collected, stored and distributed deeply affects the comfort of the occupants.
The variables affecting comfort can be divided into personal factors (e.g. activity and clothing) and environmental factors (e.g. air temperature, mean radiant temperature, air velocity and air humidity). The second group depends directly on the building's design and its heating and cooling systems.
PMV and PPD
The designer can estimate how hot or cold a certain environment will feel by calculating the Predicted Mean Vote (PMV). It is an index based on Fanger's comfort equation which predicts the mean value of the votes of a large group of people exposed to the same environment on the seven-point thermal sensation scale:
+1 slightly warm
-1 slightly cool
Predicted Mean Vote index takes into account clothing and activity level as well.
Figure2.1. PMV comfort chart (Source: http://personal.cityu.edu.hk).
Since people in the group are different, the votes tend to be scattered around the mean value. Therefore, it is useful to know what percentage of people in a group is likely to feel uncomfortably hot or cold in a certain environment. This can be figured out by using the Predicted Percentage of Dissatisfied index.
Figure2.2. PMV-PPD relationship (Source: http://personal.cityu.edu.hk).
There is a direct relationship between the PMV and PPD. It is recommended that the PPD be <10%. This means that -0.5 < PMV < +0.5
A building is always designed to be a shelter for its occupants; it is a system of components meant to mediate the external conditions into a comfortable environment (Moore, 1993).
Generally, there are five main climates on the earth: cold/boreal (the winter is a dominant season and concerns for heating and heat storage are predominant), temperate (almost equally harsh summer and winter conditions divided by milder transitional periods), hot/arid (very high summer temperatures with significant variation with dry conditions throughout entire year), warm/humid (stable conditions with high humidity all year long) and hot with seasonal humidity (high temperatures are dominant, but there is a humid season with stable temperatures and a dry season with great temperature fluctuations).
The main factors that influence any climate are: solar radiation, air temperature, humidity and precipitation, wind speed and direction and clearness of the sky. The data for specific countries/latitudes can be found in CIBSE Guide J.
Figure2.3.CIBSE Guide J: Daily mean irradiance data for 40Â°N latitude (March 29th)
Figure2.4.CIBSE Guide J: Daily mean irradiance data for 40Â°N latitude (June 21st)
Figure2.5.CIBSE Guide J: Daily mean irradiance data for 40Â°N latitude (December 4th)
The complementing figures are sun path diagrams, giving the sun's position (altitude and azimuth) at a given date and time (Brown, DeKay, 2001). The date lines are intersected by hour lines. The lines "radiating" from the centre indicate sun's azimuth at 15Â° intervals, and the concentric lines represent sun's altitude. The sun path diagrams are created for different latitudes. Using both irradiance tables and sun path diagrams, we can get a better idea about the solar geometry and energy.
Figure2.6. Sun path for diagram 40Â°N latitude on March 29th ( Source: Weather Tool).
Figure2.7. Sun path diagram for 40Â°N latitude on June 21st (Source: Weather Tool).
Figure2.8. Sun path diagram for 40Â°N latitude on December 4th ( Source: Weather Tool).
Passive heating concepts.
There are various concepts used for increasing the heat gain due to solar radiation into a building, two main strategies are direct gain and indirect gain concept (Sodha etal, 1986).
Direct gain method is a straightforward concept- the sunlight is admitted through a window or a glazed wall (south-facing), heating up the walls, floors, objects- and consequently the air in the living space. The heavy bare floor is usually used as a heat storage device, but the temperature fluctuations are nonetheless large.
Indirect gain method reduces the swings in the room temperature occurring in direct gain method by introducing a heat storage element between the direct solar radiation and the occupied space. Thermal storage element absorbs the sun's energy at its surface; then the sun's heat is transferred into the room in from a thermal storage element as a result of convection and radiation. The following concepts based on thermal storage wall have been forwarded for the indirect gain passive heating:
This report will be focusing on the chosen indirect gain strategies.
A massive thermal (masonry or concrete) wall, facing south (on the northern hemisphere), is usually blackened to maximise heat gain and glazed. It greatly reduces the temperature fluctuations in the living space throughout the day. The south facing wall collects, stores and transfers the heat inside the building. Double glazing minimises the heat losses. Blackened, south-facing wall is heated up by the sun throughout the day. The air between the glazing and thermal storage wall gets heated up and enters the living space through the vents in the massive wall. The heat input can be adjusted by operating the dampers controlling the air flow. During off-sunshine hours the glazing is covered with insulation, and the dampers are closed in order to reduce the heat losses.
Figure3.1. Indirect gain method: typical arrangement for Trombe wall (Sodha et al, 1986).
The thermal storage wall made up of drums filled with water is usually referred to as water wall. It is generally more effective in reducing the temperature variations, but in this case, there is less lag between minimum/maximum of solar radiation and heat flux into the living space.
Figure.3.2. Water wall (Sodha et al, 1986.).
This concept merges the direct and indirect gain methods. There is a thermal storage wall on the south side of the living space; and there is a second space enclosed by the glazing, attached to the mass wall.
Figure3.3. Sunspace (Sodha et al, 1986).
This glazed enclosure is called sunspace, and it receives heat via direct gain, while the room is heated up by indirect gain, through the mass wall. The sunspace's performance can be significantly improved by installing a moving insulation over its walls.
Passive cooling by ventilation- solar chimney
In its basic form, the solar chimney is simply a black-paintedÂ chimney. During the day solar energy heats the chimney and the air within it, buoyancy forces due to temperature difference help induce an upward flow along the plate (Goulding et al, 1992).
There isÂ suction created at the chimney's base, which can be used to ventilate and cool the spaces below. In most parts of the world it is easier to wind-reliant cross ventilation,Â but in hot climates on windless days a solar chimney is the only passive system that can provide cooling.
The solar chimney has to be higher than the roof level, and is usually constructed on the south-facing wall (northern hemisphere). The amount of heat absorbed can be increased by glazing a surface facing the sun (http://en.wikipedia.org).Thermal storage material is usually used on the other side.
Figure4.1. Solar chimney. (based on: D. J. Harris, N. Helwig, 2007).
Methods of calculating the thermal performance of passive heating systems.
There are various mathematical techniques for sizing and optimising the passive houses. We can divide the mathematical models developed so far into three categories (Sodha et al, 1986).
These models are commonly used to find out the average heat requirements of a building. They are used in the project's planning stage. Typical examples of approximate methods are the steady state method and the degree day method.
In these models, we express the thermal relationship of the building in terms of a correlation coefficient, which expresses the sun's energy fraction with its heating requirements. These methods can take various factors into account, like a building's orientation and the sizing of the thermal storage mass.
These methods are based on the solution of the heat conduction equation with certain boundary conditions applied. These are quite complex structures necessitating the use of powerful computers. However, in certain cases it is possible to use smaller computers to perform the necessary computations. Analytical models take various parameters and construction variations into account.
This paper focuses on the steady state model, which is one of the approximate methods.
Steady state method
For calculating the average heating load the average of the inside and the ambient conditions can be taken into account and the assumptions about the duration of the heating season and outdoor conditions can be made. In addition, assuming that the system is in steady state implies that the temperature does not change with time. The heat flux for the desired indoor temperature tb is calculated knowing the overall heat loss coefficient.
Common passive heating concepts performance (Sodha et al, 1986) :
Trombe wall (without vents).
For this type of wall the average heat flux coming into the room to be maintained at a temperature tb, is calculated by the following expression:
Q- heat flux coming into the room (W/m2)
ho- external wall surface heat transfer coefficient (W/m2Â°C)
hsi- internal wall surface heat transfer coefficient (W/m2Â°C)
k- thermal conductivity (W/mÂ°C)
L- wall's thickness (m)
Î±- absorption (dimensionless)
Ï„- transmittance (dimensionless)
Å¡- mean solar radiation (W/m2)
ta- ambient temperature (Â°C)
tb- desired internal temperature (Â°C)
In case of the water wall, the above equation is written as:
h'1- external drum surface heat transfer coefficient (W/m2Â°C)
h'2- internal drum surface heat transfer coefficient (W/m2Â°C)
For a sunspace, we need to calculate the overall heat transfer coefficient to obtain the net heat flux into the room.
hm- overall heat transfer coefficient, sunspace to ambient (W/m2Â°C)
hTS- heat transfer coefficient between the wall and the sunspace (W/m2Â°C)
U- overall thermal transmittance (dimensionless)
Night heat losses are calculated by substituting the mean solar radiation value with 0 and inserting suitable design temperatures.
To calculate the net heat loss, the value of night heat loss has to be added to the value of the heat flux coming into the building.
Passive cooling- solar chimney (wbdg.org)
We can assess the thermal performance of the cooling system by calculating the volume flow rate that it provides. Calculating the exact solar chimney performance is complex and requires CFD simulations. For the purpose of this paper, the basic stack equation is used:
Q- volume flow rate (m3/s)
Cd- discharge coefficient
Outlet area (m2)
g- acceleration due to gravity (9.81 m/s2)
h- the vertical distance between the inlet and outlet (m)
ti- temperature at the inlet (Â°C)
to- temperature at the outlet (Â°C)
To improve passive systems performance, night insulation can be used in order to decrease night heat losses. In that case, night loss equations take the form of:
LK- thickness of the insulation (m)
K- insulation's thermal conductivity (W/mÂ°C)
A spreadsheet has been created for calculating the net heat flux coming into the building.
Correlation methods (Sodha et al., 1986)
Correlation methods express the results using correlating parameters. As a rule, two techniques are used for calculating the passive solar systems performance:
the solar load ratio design method and
the load collector ratio
Solar Load Ratio
The solar load ratio method uses monthly weather data to predict the buildings' thermal performance. The prediction of monthly thermal performance using this method leads to relatively high standard errors (+-8%). However, the standard error in annual building thermal performance prediction is limited to +-3%, which is deemed adequate for design purposes.
This technique is used to estimate the performance of passive heating systems that have a mass wall. Detailed hourly simulations of four systems were carried out for 29 locations and the results were plotted in terms of the ratio of the absorbed solar energy to loads. The simulated system included a solid storage wall and a water wall each with and without night insulation (Sodha et al., 1986). The basic parameters of the systems tested are given in the Table 6.1 below.
room temperature range
night insulation resistance (where used)
night insulation time
5 p.m. to 8 a.m.
wall to room conductance
5.68 W/m2 Â°C
storage wall thermal conductivity
K= 1.73 W/m Â°C
storage wall vhermal capacitance
Ïc=2 .0 MJ/m3 Â°C
double glazed (normal transmittance 0.747)
building thermal mass other than storage wall negligible
storage wall has vents with backdraught dampers
Table 61. Parameters and Characteristics of Systems Simulated in the Development of the SLR Design Method. (Sodha et al., 1986 after Balcomb and McFarland, 1978)
The solar load ratio can be defined as the ratio of monthly solar energy absorbed on the storage wall surface to the monthly total building load including the losses through the wall when no solar gains occur. S is the mean daily solar energy absorbed by the wall. It is calculated from the incident radiation on a certain orientation. Multiplying that by the area of the receiver (Ar) and the number of days in the month (nd) the average monthly solar gain is obtained.
For all other components apart from the mass wall, loads are obtained by using the standard methods (degree day or direct calculations). As no solar gains occur, the mass wall loses energy. Therefore, an equivalent load is added to the loss due to the remaining building components.
This gives us the total heating load of the building. The average heat loss coefficients for collector storage walls that were used by Balcomb and McFarland (1978) are given in the Table 6.3. Loads are calculated by using the equation:
The solar load ratio (Ar.S.nd'/LOAD) is estimated for each month and correlated with the solar heating fraction (SHF) which is defined as (Sodha et al., 1986)
type of wall
Heat loss coefficient through double-glazed wall. (W/m2 Â°C)
No night insulation
With r9 night insulation (R=1.6 m2 Â°C/W)
450 mm concrete wall
Table 6.2. Average loss coefficients for collector storage walls for calculating loads in the SLR method (Sodha et al., 1986)
Solar heating fraction is dependent on the passive solar system used and it is estimated from the graph developed by for the specific system.
The auxiliary energy requirement per month is (1-SHF) LOAD. The auxiliary energy needed per year is therefore the sum of all the monthly auxiliary energy requirements.
The above-mentioned graph has been developed using the results obtained from a system described by the parameters from the Table 6.2 and plotting tem with the weather data of previously mentioned 29 locations. The curves had been fitted to the data in order to reduce the errors in annual SHF. It should be noted that simulation's results can vary considerably for individual months. Only annual results from the solar load ratio technique are to be deemed significant (Sodha et al., 1986)
Technically, with this method the assumption is made, that it is possible to express the monthly solar heating as the unique function of solar load ratio, which is independent of either time of the year or location. Using the results of hourly simulation analyses Balcomb and McFarland, 1978 derived a plot between the monthly SHF and monthly SLR. Based on this data, the following function forms were chosen for the relationship between SHF and SLR
At SLR=R', SHF=SLR. The values of the correlation coefficients between SHF and SLR (R', a1, a2, a3, a4 and Ïƒ) are given in the table 6.4.
Load Collector Ratio.
Similarly as in the previous method, one needs to calculate the building loss coefficient UA first, excluding the storage wall. Later, the building Load Collector Ratio is calculated. Load Collector ratio (LCR) is defined (Balcomb and McFarland, 1978) as:
The solar wall collector area involves the glazed area only. The values of SHF, matched to this value of the LCR, are given in the tables provided by Balcomb and McFarland (1978) for passive solar systems in different latitudes. That includes Trombe wall (TW), Trombe wall with night insulation (TWNI), direct gain (DG), direct gain with night insulation (DGNI),water wall (WW), and water wall with night insulation (WWNI). The values of LCR for various solar heating fractions for North European climates are given by Milborn (1980), and they are a bit different from the previous ones.
The following correlations have been suggested for SHF and the LCR. The monthly performance was assumed satisfying for the following correlation
The above formula needs the extra condition that . X is, quoting Sodha "the generalized solar load ratio":
LCRs represents the LCR of the net glazed area of the storage wall. The parameter H is dimensionless, and it is concluded from the correlation process. S is the monthly solar radiation absorbed by the building per unit area. Some of the values of the correlation coefficients (A, B, C, D, G and H) for different systems described above are given in Table 6.7. For direct gain H=0 and for other systems B= 1, G=0 and R'= -9.
Thermal Time constant method
Boundary conditions are constantly changing for most of the thermal structures. Because of that, the steady methods can only provide the approximate results. However, the precise analysis of heat transfer in unstable conditions is intricate. Nevertheless, an estimation of the time determined response of the thermal structure can be assumed if the temperature at any instant is taken as constant throughout the structure.
If that temperature changes in time dt by a small amount dÎ¸, the change in internal energy equals the overall heat flow rate across the boundary:
Giving the solution
Where (Ïc V/h'A) is the time constant of the structure. Its value indicates the time taken for the difference between the structure temperature and the ambient temperature to achieve 36.8% of the original temperature difference.
The parameter (h'L/K) is known as Biot modulus (Bi). If its value is less than 0.1, the error introduced by the presumption that the temperature is constant at any instant will be less than 5%. The time constant (Ïc V/h'A) for a layered structure is
The periodic solutions.
The differences of solar intensity as well as the external temperature may be presented as a Fourier series with a frequency Ï‰, viz.
f(t) can also be written as
Since the building is exposed to solar radiation and the ambient temperature, varying periodically for a number of years, the temperature inside the room shows variation corresponding to the variations of the input functions (solar insolation and ambient temperature), and the solution of the heat conduction equation <6.72> can be expressed as
If the transverse dimensions of the building element are much greater than the characteristic length Lc= (K/Ï‰Ïc)1/2, the assumption of one-dimensional heat flow leads to only marginal inaccuracies.
Substitution of eq. 6.76 into equation 6.72 and equating the coefficient of equal powers of exp(inÏ‰t), one gets
Solution of eqs. 6.77 and their substitution into equation 6.76 yields
And the constants A0, A1, Î»n and Î»1n have to be determined from the appropriate boundary conditions for the system to be studied.
Example calculations using the spreadsheet.
Let's consider a site in Naples, Italy (40.85N, 14.30E) on December 4th. The average daily temperature in December in Naples is 10Â°C and the night temperature is 6Â°C (http://www.bbc.co.uk). South facing wall receives daily mean solar radiation of 216 W/m2 (direct beam + diffuse). We can take the desired indoor temperature as 18Â°C. There is optional night insulation (5 cm thick) with thermal conductivity of 0.025 W/mÂ°C.
For Trombe wall:
Heat flux into the building
Qh= 27.436 W/m2
Qn= -19.908 W/m2
With night insulation
Qins= -4.610 W/m2
Net heat flux into the building:
Qnet= 7.528 W/m2
With night insulation
For water wall:
Heat flux into the building:
With night insulation
Net heat flux into the building:
With night insulation
Heat flux into the building
Qh= 36.802 W/m2
With night insulation
Net heat flux into the building:
Qnet= 22.957 W/m2
With night insulation
It can be observed that when there is no insulation used, sunspace is best suited for given conditions, as it will give the highest value of heat flux into the building from all three systems. Also, the sunspace has the smallest performance difference between insulated and non-insulated conditions. However, if the night insulation is to be used, the water wall gives the best performance in given conditions.
Introducing Geometric description language (GDL):
GDL is a scripting language of ArchiCAD library parts, used to produce 3D form. The 2D and 3D features are connected (Nicholson-Cole, 2004 ). However, it also has other possible uses, e.g. creating "intelligent" objects, such as windows and doors that would "know" how to cut holes in the walls. It allows to list parameters which can be changed by the user to adjust an object for a specific project. It is possible to customise the object according to manufacturer's specifications, which helps in design process.
A library of four GDL objects has been created as a part of this report (please find the enclosed CD). The objects are scripted in a way that enables the user to view the thermal performance of the passive solar systems discussed above, using the steady state method of calculation. The variables edited by the user include the mean solar radiation, the presence of night insulation or lack thereof, external and desired internal temperature, as well as the thickness of the glazing, the size of the air gap and more. The purpose of creating that library was to provide working design objects for ArchiCAD, as well as a quick and easy way of calculating the thermal performance of the passive solar systems. All of the objects have been developed using ArchiCAD 13 version, and they can be placed in a 2D working environment.
The GDL model of Trombe wall consists of a solid wall created with CPRISM_ command with the possibility to change its length, thickness and height within the specified ranges. A blackened layer has been added on the external side of the wall. The interior material can be specified by the user by changing the correct parameter. The vents have been cut through the wall (CUTPOLY) and the dampers have been added. The dampers can be changed to either open or closed position. The air gap and glazing thicknesses can also be changed within the range specified in the Master Script. All the lengths and thicknesses in the parameters section are expressed in millimetres, while the corresponding values in 2D, 3D and Master Script are expressed in meters.
The other part of the parameters section deals with calculating the net heat flux for Trombe wall. It acts as a spreadsheet where the data is specified by the user (within the reasonable range of values described in the Master Script. There is also an option to add/remove night insulation. If the night insulation is set to "off", the script will automatically lock the parameter specifying the thickness of night insulation. The parameters representing the heat flux values stay locked at all time, as no input from the user is needed.
Similarly as in the above example, the length and thickness of the water wall can be specified. The main difference is changing the object's height. As the wall is made of water drums of 0.2 m height stacked on top of each other, the wall's height can only be a number dividable by 0.2. To prevent the problems the users may encounter in that matter, the height (ZZYZX) parameter has been locked, and the height of the wall is now decided by specifying the number of the water drums that will be stacked together. In the case of the Figure below, the number of drums (NUM) equals nine, which affects the locked ZZYZX parameter. Calculating the net heat flux for the water wall is analogous to the above example of the Trombe wall.
Figure below shows the object settings in the object dialogue box. It can be seen how the parameters work in this environment. The use of bold titles organizes the view and helps to avoid confusion. The object's hotspots are also visible on the preview.
Sunspace/solarium was modelled as a zone 1 of the conservatory, to allow the user to integrate it with a zone 2 of their choice. Like in the case of the Trombe wall, the dimensions of the wall and certain glazing parameters can be changed within the pre-specified range. The heat flux calculations are performed in the exact same way as well.
In the case of the solar chimney, the situation changes slightly. Just like before, there are certain dimensions and materials that can be modified. However, as the solar chimney is needs an inlet cut in the blackened wall, and its height and position must be controlled, as different projects may require different parameters of the opening. Also, it needs to be ensured that the opening cannot be placed above or below the chimney level. Therefore a following condition has been implemented into the script:
IF h <= 0.5 then h = 0.5
IF h >= zzyzx then h = zzyzx-inh
IF inh >= zzyzx then inh = zzyzx-h
Where h is the vertical distance between the inlet and the outlet, inh is the inlet height and zzyzx is the chimney's height. So now the opening will always stay within the chimneys height, and the lowest it can be place is the floor level.
Also, the calculation method and output are different. In the case of the solar chimney the result is the volume flow rate (m3/s). The outlet area parameter is locked, as it is calculated from other variables inserted by the user. Inlet and outlet temperatures also need to satisfy certain conditions, so that the chimney can actually work as a passive ventilation device.
Integration of passive solar systems
When the trend towards using passive solar systems emerged, the term ''integration'' often meant ''invisibility'', as it was recommended to "hide" the presence of solar systems, because they were different from other elements of the building. The situation has now changed. The key to the success of the projects using passive solar systems is that more and more often the architects prefer to use the aesthetic compatibility approach rather than that of invisibility.
When the integrated approach is used, the solar systems become a part of the building's basic design. This is because integrating the solar systems in the building envelope became a necessity if the systems are supposed to be economically reasonable. The solar elements cannot be separate elements that are added after the building's design is completed. They must rather take place of the other building elements, thereby serving dual functions and maybe even reducing total costs. (Hestnes, 1999) However, the passive solar systems can add up to 15% to design and construction costs, but this initial cost provides energy saving in return.
Careful choice of building's orientation, structure and materials of is crucial in order to control the solar heat gains, and therefore reduce the HVAC size. Typically, it could be roof or facade integrations such as wall, balcony or shade of the building. (Chen et al, 2010) Another way of achieving maximum integration is to cover the entire facade with the elements of solar systems. It is an easy solution, and in many cases it may also be the most cost effective one. However, the international architectural trends seem to point away from such homogenous, simple facades. (Hestnes, 1999)The major component of any solar system is the solar collector. They are usually black in order to maximise the radiation's absorption and minimise its emission. Unfortunately, black surfaces are not always considered aesthetically desirable in all cases, e.g. facade integration. Still, passive solar designs are building integrated whereby facades or roofs are part of the heating or cooling system components. This in turn reduces initial cost, as the passive solar systems are much cheaper in construction that the active ones (Chen et al, 2010). In addition, passive solar designs are flexible. Systems such as Trombe wall and solar chimney are able to provide warm air or create cooling effect depend on the climate needs by damper controllers (Chen et al, 2010).
A lot of passive solar designs for heating and cooling have been developed. However, they have their limitations. Passive solar systems may not suffice to provide indoor thermal comfort, particularly in the regions with extreme climatic conditions. Quoting Chen et al. (2010):"The research areas that need to be carried out to improve the existing solar technologies performance and market acceptance are the system efficiency, architectural aesthetic, and cost effectiveness aspects. They in fact have been carried out intensively". Otherwise, research on development of a hybrid system could be an alternative. The solar collector could be used as part of a system which would be providing both active and passive solar benefits, be flexible enough to interface with the other building elements and be able to adapt to different buildings. Combination of heating and cooling systems is able to harness the solar energy throughout the year in countries with hot and cold seasons, whereas hybrid of solar active and passive technologies would improve the system efficiency and cost effectiveness. Hence, limitations of the technologies are overcome by each other's advantages and making the overall solar heating and cooling system feasible, more marketable and increase the public acceptance.