Thermal behaviour and ventilation efficiency of a low-cost passive solar energy efficient house

ARTICLE IN PRESS

Renewable Energy 33 (2008) 1959–1973 www.elsevier.com/locate/renene

Thermal behaviour and ventilation efficiency of a low-cost passive solar energy efficient house Golden Makakaa,, Edson L. Meyerb, Michael McPhersona a

North West University, Private Bag X2046, Mafikeng 2735, South Africa Institute of Technology, University of Fort Hare, Private Bag X1314, Alice 57000, South Africa

b

Received 28 August 2007; accepted 29 November 2007 Available online 28 January 2008

Abstract Low-cost houses in South Africa are characterized by poor craftsmanship and design with no regard to energy efficient passive solar design features resulting in high electrical energy consumption, uncomfortable indoor thermal environment and poor ventilation efficiency. Incorporating energy efficiency passive solar design strategies and correct material choice can significantly reduce energy consumption in buildings. This paper presents the effects of fly ash to brick properties, thermal comfort analysis and the impact of the ventilation components to the indoor environment of a low-cost energy efficient passive solar house constructed using fly ash bricks. The addition of fly ash to clay improved brick properties. The house was monitored for a period covering all the South African seasons. In summer, the passive solar house was found to be thermally comfortable for 66% of the period monitored, while for winter it was about 79%. Windows were found to have a higher impact on the ventilation rates than doors. The indoor carbon dioxide concentration monitored over night was found to be 0.248%, which is less than the maximum range limit of 0.500%. The performance of the house was seen to depend on how the occupants operate the house. The house was found to create a thermally comfortable indoors environment and experienced minimal temperature and humidity swings, with a better performance in winter than summer. r 2007 Elsevier Ltd. All rights reserved. Keywords: Thermal comforts; Energy efficient; Low-cost; Passive solar; Ventilation rate; Fly ash

1. Introduction The purpose of buildings is to provide shelter and comfortable living conditions. Most of the low-cost houses constructed under the South African government Reconstruction and Development Programme (RDP) housing scheme are characterized by poor indoor air quality, highenergy consumption and poor thermal performance. The building materials are substandard, with high water absorption, high thermal conductivity and low compressive strength. This results in cracking walls and high indoor temperature swings [1]. The house design has no regard to passive solar design features. Fig. 1 shows the typical lowcost houses (RDP).

Corresponding author.

E-mail address: [email protected] (G. Makaka). 0960-1481/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2007.11.014

There is need to improve the disadvantageous brick properties so as to improve the thermal efficiency of the low-cost houses. About 17% of the South Africa electrical energy is used in residential buildings in trying to keep the indoor environment within the comfort levels. Simple design interventions such as north orientation (in southern hemisphere) and correct material choice can significantly reduce energy consumption by buildings. South Africa is one of the countries in the world that receives very high solar radiation. The present architecture needs to take advantage of this high solar radiation in heating the indoor environment in winter. Prevailing winds need to be taken advantage of in controlling the indoor thermal comfort and air quality thus minimizing energy consumption. Passive solar energy efficient strategies are an immediate and long-term solution to the energy problem facing South Africa [2]. However, good the house design could be, correct operation of the

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Fig. 1. Low-cost houses (RDP).

house plays central role in achieving a comfortable indoor environment. Air temperature and relative humidity have significant impact on peoples’ perception of thermal comfort. Relative humidity below 30% may cause dry skin, eye irritation and/or respiratory problems. Relative humidity above 60% may provide an environment conducive to the growth of mold, mildew, and dust mites, and cause allergic reactions [3]. The South African Residential Building Code (SABC) recommends the indoor temperature range of 16–28 1C and relative humidity range of 30–60% [4]. Human activities such as cooking, bathing, breathing, and maintaining houseplants introduce indoor pollutants [5]. It is therefore necessary to evaluate the indoor air quality and improve the daily management of the ventilation components. The SABC recommends an average natural infiltration rate of 0.35 m3/h m2 (air change rate) and an indoor carbon dioxide concentration less than 0.500% [4]. This paper presents the thermal behaviour, the effects of fly ash on brick properties and the impact of the ventilation components on the ventilation rate in a passive solar energy efficient house constructed in Somerset East in the Eastern Cape Province in South Africa. Somerset East is located at 321420 S latitude and 251330 E longitude at an altitude of 790 m. It experiences a subtropical type of climate with long hot summer months and moderate sunny winter months with annual daily average of 7–8 sunshine hours [6]. For the evaluation of the ventilation efficiency, the tracer gas (carbon dioxide) method was used. 2. Thermal comfort One of the approaches to analyse thermal comfort is through the adaptive method, and it rest on the observation that people in daily life are not passive in relation to their environment. If a change occurs such as to produce discomfort in ways which tend to restore their comfort [7–9]. The adaptive opportunity [10] may be provided, for instance, by fans or openable windows in summertime or by temperature controls in winter. During summer months

most of the low-cost houses in South Africa are freerunning (i.e. not heated or cooled), and the indoor temperatures changes according to the outdoor temperature. Humphreys [11] investigated thermal neutrality of the human body. It is defined as the temperature at which the person feels thermally neutral ‘‘comfortable’’. Using the available data from more than 30 comfort surveys from the world, Humphreys proposed a series of simple correlations for thermal comfort prediction. The data was statistically analysed by using regression analysis. Humphreys showed that 95% of the neutral temperature is associated with the variation of outdoor mean temperature. Other workers have since found similar results (e.g. [12]). For free running buildings, i.e. not heated or cooled, the regression equation is approximated by [13] T c ¼ 0:534T m þ 11:9,

(1)

where Tc is the predicted indoor comfort temperature and Tc is monthly mean outdoor temperature. Another method of assessing the thermal comfort is to use the predicted mean vote (PMV) and the predicted percentage dissatisfied (PPD) equations [14]. ISO 7730 norm suggest that PPD must not exceed 10%, i.e. [0.5oPMVo0.5]. PMV and PPD provide a measure of the likely response of people to the thermal comfort. The predicted mean vote is an index that ranges from 3 (representing a response of very cold) through 0 (representing a thermally neutral response) to +3 (representing a response of very hot). PPD provides the information as to whether the environment is likely to be acceptable, and PMV informs what the problem is, whether it is too hot or too cold. The PMV is based on the calculation of energy balances in human body and it takes into account, on the one hand, human activity, metabolic rate, clothing, and thermal resistance, and on the other hand, air temperature, average radiate temperature, air speed and steam pressure. The percentage of people living under discomfort conditions (PPD) can be estimated by the equation [15]: PPD ¼ 100  95 eð0:03353PMV

4

þ0:2179PMV2 Þ

(2)

ARTICLE IN PRESS G. Makaka et al. / Renewable Energy 33 (2008) 1959–1973

with PMV being given as:

1961

Table 1 Guide values for CO2 concentration

PMV ¼ ð0:303 e0:036M þ 0:028ÞfðM  W Þ  3:05  103 ½5733  6:99ðM  W Þ  Pa 

Concentration (ppm)

Condition

 0:42½ðM  W Þ  5815  1:7  105 ð5867  Pa Þ

40,000 5000 400

Proportion in exhaled human breath (20 l/h) Limit of CO2 concentration within a space Fresh, natural ambient air

 0:0014Mð34  ta Þ  3:96  108  f cl ½ðtcl þ 273Þ4  ðtr þ 273Þ4   f cl hc ðtcl  ta Þg, ð3Þ 2

where M, metabolic rate (W/m ); W, external work (W/m2 of the body area); Pa, partial water pressure (Pa); ta, air temperature; fcl, ratio of clothed surface to nude surface area; tr, mean radiant temperature; and hc, convective heat transfer coefficient. 3. Airflow rates Air may enter a measured zone not only directly from outdoors, but also from neighbouring zones, whose CO2 concentration may differ from that of the outdoor air. These inter-zone airflows may influence the CO2 concentration in the measured zone [16]. The concept of ‘‘equivalent outdoor airflow rate’’ is introduced to offset this inconvenience. It corresponds to the outdoors airflow rate that would result in the same CO2 concentration in the measured room without inter-zone airflows. An adult person produce on average (i.e. quiet or doing light work, about 100 W metabolic rate) about 20 l/h carbon dioxide. At steady state, assuming that occupants are the only CO2 sources, the equivalent airflow rate per person, Qe, is related to CO2 concentration (Ci indoors and C0 outdoors) by [17] Qe ¼

S , Ci  C0

(4)

where S is the CO2 source strength, i.e. about 20 l/h. Assuming a steady-state (constant carbon dioxide concentration), Eq. (3) can be used to asses the equivalent outdoor airflow rate per person. Another way is to use the CO2 concentration when there is no CO2 source in the building. The concentration decays to the background concentration due to the dilution with the outdoor airflow. If there is good mixing and for a constant outdoor airflow rate, the CO2 concentration decays exponentially and at any time t, is given as: C ¼ C 0 emt ,

(5)

where m is the air exchange rate, C0 is the initial concentration above the background concentration. Taking logarithms both sides of Eq. (2) and differentiating with respect to time the air exchange rate can be approximated by the following expression: DlnðCÞ ¼ mDt.

(6)

Plots of Dln(C) against Dt will a produce a straight line whose gradient is equal to the air exchange rate. If the

Fig. 2. Passive solar energy efficient house.

outdoor airflow rate is not constant, which is mostly the case, the decrement calculated from two measurements of concentration taken at times t1 and t2 provides an unbiased estimate of the average equivalent outdoor specific airflow. 4. Indoor air quality The amount of CO2 indoor determines the air quality and its concentration indicate the ventilation rate within a room. The time it takes for a room to reach equilibrium depends on the number of people in a room, the volume of the room and the ventilation rate within the room. If the room is poorly ventilated and has very low occupant densities, it may take a number of hours before the equilibrium level is reached. Table 1 gives a guide to the carbon dioxide concentration levels. Outdoors levels of CO2 are relatively constant and range between 350 and 600 ppm; inside levels will never be below the outside level [18]. The amount of CO2 in a room can give an indication of the number of persons within a room. 5. Description of the house The house floor plan measured 6880 mm  6580 mm. An open plan layout was adopted to optimize natural ventilation as mechanical ventilation systems were avoided to keep the running cost of the house low. Fly ash brick wall and concrete floor was used as the thermal mass. The roof was split into two, the lower and upper roof. The lower roof faces north, while the higher roof faces south. This was done in order to insert clerestory windows making it possible to direct solar radiation to the desired rear zone (floor and southern wall) and to maximize day lighting,

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thus minimizing the use of artificial day lighting during the day. Fig. 2 shows the completed passive solar energy efficient house. 6. Methodology 6.1. Simulation and thermal performance The passive solar house was designed and modelled using AUTO-CAD software. The model was then exported to ECOTECT and DOE for indoor temperature simulation. Bricks of different mixing proportions of fly ash to clay were moulded, dried for 3 weeks and fired up to a temperature of about 1300 1C in a high oven. Bricks properties (water absorption, thermal conductivity and compressive strength) were then measured. To monitor the thermal performance parameters of the house, a number of sensors were installed and these included thermocouples, HMP50 temperature–humidity probes, and model 03001-wind sentry anemometer and vane, and pyranometer. Twenty-six thermocouples were installed to map-out the indoor temperature distribution pattern. For the measurement of indoor surface wall temperatures two thermocouples were installed, one on the north and another on the south wall while four thermocouples were installed outdoor to measure surface wall temperatures, one on each of the four walls. The indoor and outdoor surface wall thermocouples were placed at the centre of the walls. A wind anemometer and a vane, temperature–humidity probe, and a LI-COR pyranometer were placed on top of the roof. A second temperature humidity probe was placed in the centre of the house at 2 m above the floor. All sensors were then connected to a CR1000 datalogger, powered by a 12 V battery and charged by a 20 W solar panel. 6.2. Ventilation rate Tracer gas tests were used to measure air change rates. Carbon dioxide was injected into the house and its concentration monitored over time to determine how quickly the gas dissipates through the house’s envelope. Non-dispersive infra-red absorbance (NDIR) gas sensors were used to monitor indoor carbon dioxide concentration. The carbon dioxide sensor was placed at height of about 0.45 cm above the floor. To investigate the effects of each of the ventilation components, i.e. windows and doors, the ventilation rate tests were carried out in four phases. 6.2.1. Phase one Carbon dioxide gas was injected into the house when all doors and windows were closed and a fan was switched on for 5 min to mix the air in the house. The use of the fan was intended to evenly distribute the initial tracer dose throughout the room. Windows and doors were then opened and carbon dioxide concentration was recorded at

a time interval of 1 min until a constant carbon dioxide concentration was achieved. 6.2.2. Phase two The procedure of phase one was repeated but keeping the doors closed and the windows opened and carbon dioxide concentration readings taken at 1 min interval. 6.2.3. Phase three The procedure of phase one was repeated but doors were opened and windows closed and the carbon dioxide concentration recorded at a time interval of 1 min. 6.2.4. Phase four The procedure cited in phase one was repeated but all windows and doors were closed and carbon dioxide concentration recorded at 1 min interval. In all the above cases it was not possible to inject equal amounts of the tracer gas as the equipment used could not allow the measurement of the amount of gas injected. 6.3. Measurement of air quality Maintenance of indoor air quality (IAQ) through the correct management of the ventilation components of the house is of prime importance. To maintain good indoor air quality, it is essential to provide outside air to dilute indoor air pollutants and exhaust the contaminants along with moisture and odours. To assess the indoor air quality, the waste case was considered, that is, the most likely time when the indoor air is mostly contaminated is at night when all the designed ventilation components would be closed and all the occupants being indoor. Carbon dioxide concentration was monitored over the whole night and readings were recorded at 1 min interval. 7. Results 7.1. Brick properties Materials play an important role in determining the indoor thermal behaviour. South Africa’s energy sector is almost entirely coal-based and the country produces about 30 million tons of ash annually. Eskom derives nearly 90% of its power from coal-fired electric power stations and it faces the fly ash waste disposal management. One of the ways to put this waste into use is to use it in brick making. The properties of fly ash bricks depend mainly on two factors: (i) the energy content of fly ash used and (ii) the chemical composition of fly ash. Table 2 shows the chemical composition of fly ash used to make the bricks. Other constituents include FeO, Na2O, K2O and unburnt carbon. The South African fly ash has high-energy content (carbon), which makes it excellent in brick making. High-energy content fly ash has a great influence on thermal conductivity and reduces brick firing period. Chemical composition of the fly ash and the temperature

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Table 2 Chemical composition of fly ash (%) Sulphate (as SO4)

Phosphate (as PO4)

Silicate (SiO2)

Calcium (Ca)

Magnesium (Mg)

Potassium (K)

Aluminium (Al)

0.2

0.1

20.9

2.15

0.12

2.68

19.6

Water absorption (%)

25 20 15 10 5 0 0

10

20

30

40 50 Fly ash (%)

60

70

80

Fig. 3. Variation of water absorption with amount of fly ash.

attained during firing determine water absorption, compressive strength, thermal conductivity and colour of the brick. Water is associated with the deterioration processes affecting masonry materials. Its presence within the interior pore structure of masonry can result in physical destruction if the material undergoes wet/dry or freeze/thaw cycles. The freeze/thaw process is particularly damaging if the masonry material has high water absorption. The water absorption results in high expansion and contraction thus weakening the brick. Fig. 3 shows the variation of water absorption, while Fig. 4 shows the variation of compressive and thermal conductivity with the amount of fly ash. From Figs. 3 and 4, it can be observed that a mixing proportion of 50% fly ash to 50% clay produced a brick with the desired properties; low thermal conductivity (0.0564 W/mK), low water absorption (8.4% by weight) and high compressive strength (12 MPa). According to the South African Building Code (SABC), the brick water absorption must be less than 20% by weight. From Fig. 3, it is clear that the addition of 20% fly ash by volume reduced the water absorption by 32%, while the addition of 50% fly ash lowers the water absorption by 62%. The mixing proportion of 50% fly ash to 50% clay resulted in 93% reduction in thermal conductivity as compared to a pure clay brick. The fly ash bricks were found to be light in weight (density: 400–1190 kg/m3) making it easier to transport. As the carbon in the brick burns, the trace elements melts thus sintering the brick forming a ceramic like material and at the same time creating small unconnected cavities giving the brick effective heat insulating properties and a reduction in permeability and porosity. Fig. 5 shows the melting point temperatures of the trace elements in the fly ash.

The reduction in permeability and porosity implies the reduction in freezing/thawing damage of the brick since there will be minimal amount of water in the brick. As the content of fly ash increases beyond 50%, the amount of unburnt carbon increases and upon burning a significant proportion of the brick will burn. In this case the trace metallic elements are insufficient to bind the remaining proportion thereby creating connected cavities that result in high water absorption and low compressive strength. Since fly ash is locally available (from boilers and electrical power plants), local brick makers can make improved quality bricks (without any additional cost) resulting in houses with thermally comfortable indoor environment and a reduction in energy consumption. The inclusion of fly ash in the brick making sector will create employment thus improving the living standard of the poor and also a reduction in embodied energy in house construction. 7.2. Predicted performance of the passive solar energy efficient house EcotectTM and DOE Building Design Software were used to simulate the thermal performance of the passive solar house (PSH). Weather data, i.e. temperature, relative humidity, diffuse solar radiation, global radiation, wind speed, wind direction, cloud cover and average rainfall data was entered. Material properties were also specified and other parameters like wall thickness, slab and glass window thickness, etc., were also entered. These parameters were varied optimizing the material combination performance. The 2004 weather data obtained from the South African weather station was used in the simulation. Fig. 6 shows the simulated results of the PSH. From the simulated results, the indoor temperature of the passive solar house was found to be within the limits of the comfort levels but with a delayed response to the outdoor temperature. With reference to Fig. 6, the PSH was found to have an average thermal time delay of 3 h and an average decrement factor of 0.67. Indoor temperature was found to be within the comfort levels for 100% according to ECOTECT simulation results and 98% for DOE simulation results for the total period (380 h) tested while the outdoor temperature was out of the comfort levels for 42% of the period with 26% being above the upper comfort level (28 1C) and 16% below the lower comfort level (16 1C). Using ECOTECT the maximum indoor temperature attained was about 27.3 1C and a minimum of 17.4 1C giving an average temperature swing of 9.9 1C. In case of DOE, the maximum temperature

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14000000

1 Thermal conductivity Compressive strength

0.9 0.8

10000000

0.7 0.6

8000000

0.5 6000000

0.4 0.3

4000000

0.2 2000000

Thermal conductivity (W/ mk)

Compressive strength (Pa)

12000000

0.1

0

0 0

10

20

30

40 50 Fly ash (%)

60

70

80

Fig. 4. Variation of thermal conductivity and compressive strength with fly ash.

7.3. Measured thermal performance

Al K

Elements

Mg Ca SiO2 PO4 SO4 0

100

200

300

400

500

600

700

800

900

Melting temperature (°C)

Fig. 5. Melting points of the trace in fly ash.

45 40 Temperature (°C)

35 30 25 20 15 10

Tout Upper bound temperature Tin (ecotect)

5

Lower bound temperature Tin (DOE)

0 0

50

100

150 200 250 Time (hours)

300

350

400

Fig. 6. Indoor temperature simulations (ECOTECT and DOE).

attained was about 29 1C and a minimum temperature of about 15.7 1C, giving a temperature swing of 13.3 1C, while the average outdoor temperature swing was about 30.7 1C. With reference to Fig. 6, ECOTECT and DOE simulation results shows a small differences in peak and minimum values.

7.3.1. Temperature and humidity The passive solar house (PSH) was monitored for a period covering all the South African seasons. At the time of measurement, four people were staying in the house, i.e. an elderly couple, an 8-year toddler and an unemployed middle aged man. The occupants were observed to have a number of visitors, especially during the weekends, and at times church services were conducted in the house. The indoor temperature at a given outdoor temperature was found to be a distribution rather than a single value. Fig. 7 shows the indoor and outdoor temperature and relative humidity distribution for the entire period monitored. The indoor temperature followed the outdoor temperature fluctuations but attaining higher peak temperatures and with thermal time delay of 3 h. The mean peak indoor and outdoor temperature difference for the entire monitored period was about 4 1C/day, while the mean peak indoor and outdoor relative humidity difference was about 22%/day. The fact that the indoor temperature followed the distribution of the outside temperature suggests that the indoor temperature is influenced more by the outside temperature rather than by the heating or cooling within the house. Using Humphreys model (Eq. (1)), the indoor environment was found to be thermally comfortable for 72% of the period monitored. The indoor relative humidity followed the fluctuations of the outdoor relative humidity, with the outdoor relative humidity being higher than the indoor relative humidity. Maximum relative humidity occurred in the morning at around 07:00 h when all the occupants would be indoor and it would be the time when they prepared breakfast and minimum relative humidity occurred in the afternoon at around 15:30 h. According to the thermal performance characteristics, two periods were analyzed in detail. The first period (22 February to 22 March) was chosen because it presented

ARTICLE IN PRESS G. Makaka et al. / Renewable Energy 33 (2008) 1959–1973

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90

120

80

100

70

80

60

60

50

40

40

20

30

0

20

-20

10

-40

0

Relative humidity (%)

Temperature (°C)

Daily indoor relative humidity Daily average outdoor relative humidity Daily mean outdoor temperature

-60 21- 13- 2- 22- 12- 1- 21- 11Feb Mar Apr Apr May Jun Jun Jul

31- 20- 9- 29- 19- 8Jul Aug Sep Sep Oct Nov

Fig. 7. Mean indoor and outdoor temperature and relative humidity.

seasonal temperatures typical for summer time. The second one (13 June to 13 July) was chosen because of extreme low temperatures typical for winter time. The thermal behaviour of the house during the first period (summer) is presented in Fig. 7. During the summer period, the general thermal behaviour of the house was strongly influenced by the life-style of the occupants. During this period, the indoor temperature was generally higher than the outdoor temperature by a mean value of 6.5 1C. The maximum attained indoor temperature was 38.8 1C while for the outdoor it was 34.3 1C. High temperatures were noted during cooking periods as the stove would be in use and more people would be indoor for meals. The heat generated by the indoor electrical equipment was trapped indoor as windows were permanently closed, thus reducing ventilation rates. This resulted in the indoor temperature being higher than the outdoor temperature. The minimum indoor temperature recorded was 16.6 1C while for the outdoor it was 8.1 1C. The indoor experienced a mean temperature swing of 11.0 1C while the outdoor mean temperature swing was 16.1 1C. According to the design of the house, the west door and window, the south and east windows, including four of the clerestory windows need to be opened to obtain optimal ventilation rate. From the statistics of the hourly indoor temperatures during the summer period analysed with Humphreys comfort model (Eq. (1)), it was observed that about 66% of the hourly data, of the passive solar house was thermally comfortable, implying that heating or cooling was not necessary for 66% of the time. The indoor relative humidity followed the outdoor relative humidity fluctuations but with a time delay of 30 min. The outdoor relative humidity was found to have higher amplitudes, and the indoor relative humidity was within the comfort levels (30–60%) for 75% of the time. The maximum outdoor relative humidity was about 97% while for the indoor it was 74%. The mean outdoor and indoor relative humidity difference was about

30%. The indoor relative humidity swing was 31% while for the outdoor it was 56%. At night temperatures were found to drop to about 13 1C, making it possible to use nighttime ventilation to pre-cool the floor and walls. In the morning windows can be closed to prevent hot day air from entering the house. The cool walls and floor can then maintain a comfortable indoor environment for a significant period of the day. It was noted that the age of the occupants affected significantly the operation of the house, as the clerestory windows were never opened. Fig. 9 shows the thermal behaviour for the second period (13 June 2006 to 13 July 2006), representing winter. This period (13 June 2006 to 13 July 2006) was characterized by low outdoor temperatures. The outdoor attained a minimum temperature of about 1.6 1C, while the corresponding indoor minimum temperature was 9.3 1C. This suggests that the house can be freeze resistant. The mean indoor temperature swing was about 10.1 1C while for the outdoor it was 15.1 1C. The outdoor temperature never exceeded 27.0 1C. From 20:00 h, the indoor temperatures were found to be generally lower than the lower comfort level but at this time the occupants would have gone to bed, thus eliminating the need for heating. Using Humphreys model (Eq. (1)) to the winter statistical hourly indoor temperature, it was observed that about 79% of the hourly data, the passive solar house thermally comfortable. The indoor relative humidity was found to follow the outdoor relative humidity but with a time delay of 3 h, which is six times longer compared to the time delay in summer. The mean indoor and outdoor relative humidity swings were found to be 26.6% and 57%, respectively. Comparing Figs. 8 and 9, the house was found to perform 14% better in winter than in summer. The summer thermal performance can be improved through the correct house operation management. In winter, the mean maximum indoor temperature was within the comfort levels, while in summer it was above the upper bound comfort level by 5 1C. However, in winter the mean

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Indoor temperature Indoor humidity

Outdoor temperature Outdoor humidity

100

110 Summer

90

90

80 Temperature (°C)

50

60

30

50 40

10

30

Relative humidity (%)

70 70

-10 20 -30

10 0 22-Feb 25-Feb 28-Feb 03-Mar 06-Mar 09-Mar 12-Mar 15-Mar 18-Mar 21-Mar

-50

Fig. 8. Summer: temperature and humidity response.

Indoor temperature Indoor humidity

100

Outdoor temperature Outdoor humidity

110

Winter

90

90

80

50

60

30

50 40

10

30

Relative humidity (%)

Temperature (°C)

70 70

-10 20 -30

10 0 13-Jun 16-Jun 19-Jun 22-Jun 25-Jun 28-Jun

-50 1-Jul

4-Jul

7-Jul

10-Jul

13-Jul

Fig. 9. Winter: temperature and humidity evolution.

minimum indoor temperature was lower than the lower bound comfort levels by 3 1C, while in summer the mean indoor temperature was within the comfort levels. 7.3.2. Influence of wind speed and direction on indoor environment Fig. 10 shows the wind speed and direction variation. From Fig. 10 it can be observed that Somerset East experiences W(6017151)N winds with an average velocity of 0.67 m/s. The correct operation of the ventilation components help in regulating the indoor temperature and humidity, thus maintaining the indoor environment within the comfort levels. The highest wind speed of about 12.33 m/s was recorded on 2 May 2006 at around 9:00 a.m. The high air speed facilitates high heat losses and prevailing winds need to be

taken advantage of in heat dissipation during summer. From Fig. 10, it was observed that wind speed had a significant influence on the indoor temperature. High wind speeds were found to correspond to low indoor temperatures. It was observed that through proper house management of the ventilation components, the indoor temperature could be regulated to meet the occupants’ requirements. 7.3.3. Influence of solar radiation on indoor environment Fig. 11 shows the variation of the indoor and outdoor temperatures and solar radiation. From Fig. 11, it can be seen that the indoor and outdoor temperatures follows the fluctuations of the solar radiation, with the outdoor temperatures responding faster than the indoor temperature.

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Wind speed

Wind direction

Mean outdoor temperature

Mean indoor temperature

1967

300

Temperature (°C)

200 40 100 30

0

20

-100

10 0 12-Feb

Wind direction (degrees)

50

-200

4-Mar

24-Mar

13-Apr

3-May

23-May

12-Jun

2-Jul

-300 22-Jul

Fig. 10. The influence of wind on indoor temperature.

2000 Indoor Temperature Outdoor Temperature Solar radiation

1800 1600

100 Temperature (°C)

1400 80

1200 1000

60

800 40

600

Solar radiation (W/m 2)

120

400 20 200 0 22-Feb

0 24-Feb

26-Feb

28-Feb

2-Mar

4-Mar

6-Mar

8-Mar

Fig. 11. Solar radiation and temperature.

When the solar radiation attained a peak value, the indoor peak temperature was delayed approximately by 4.5 h while the outdoor peak temperature delayed by 15 min. This implies that the solar intensity can be an indicator for predicting the opening and closing of windows to maintain the indoor temperature within the comfort levels. Peak indoor relative humidity occurred at night when all the occupants would be indoors and when all doors and windows were closed. With reference to Fig. 11 and considering the day 26 February 2006, the rates of temperature increase and decrease were calculated. It was found that the rates of indoor and outdoor temperature increase and decrease were different indicating different response to the solar radiation. The outdoor temperature increased at 3.5 1C/h, while the indoor temperature increased at 2.9 1C/h. The rate of outdoor temperature decrease was found to be 5.2 1C/h while for the indoor it was 3.21C/h.

7.3.4. Ventilation rate Figs. 12 and 13 illustrate the tracer gas concentration profiles measured for different ventilation component configurations, that is opening and closing of doors and windows. The average indoor and outdoor temperatures during these tests were Tin=20 1C and Tout=17 1C, and an average wind speed of 0.5 m/s blowing from W(6017151)N. Fig. 12 shows the tracer gas concentration variation when both windows and doors were open (configuration I). Results indicate that the concentration decays exponentially to the background concentration within a period of 18 min. Assuming that the west window and door are the only paths through which the westerly winds enter the house, then the mass airflow rate through p the door and window is approximated by ffiffiffiffiffiffiffiffiffiffiffi ffi _ ¼ C d A 2rDP. For wide-open windows and doors, the m opening area is the sum of the windows and doors areas, which gives 2.06 m2, and taking the discharge coefficient

ARTICLE IN PRESS G. Makaka et al. / Renewable Energy 33 (2008) 1959–1973

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Carbon dioxide concentration (%)

0.4 0.35 0.3 0.25 Background concentration

0.2 0.15 0.1 0.05

Configuration I

Configuration II

R2 = 0.9807

R2 = 0.9243

2

6

0 0

4

8 10 Time (min)

12

14

16

18

Fig. 12. Tracer gas concentration decay for configurations I and II.

Carbon dioxide concentration (%)

0.6 0.5 0.4

Configuration IV

R2 = 0.9978

0.3 0.2

Configuration III

0.1

R2 = 0.994

Background concentration

0 0

10

20

30

40 Time (min)

50

60

70

80

Fig. 13. Tracer gas concentration decay for configurations III and IV.

Cd ¼ 0.6, average air density to be 1.2 kg/m3 and an average indoor and outdoor pressure difference of 4 Pa, the average mass airflow was found to be approximately 3.83 kg/s. From Fig. 12 (configuration II), it was observed that the closing of doors significantly reduced the carbon dioxide concentration decay rate implying a reduction in the ventilation rates achieved when both doors and windows were open. Opening windows and closing doors reduced the mass flow rate to 3.16 kg/s (i.e. a reduction of 17%). This means doors play a significant role in the ventilation of the PSH. Fig. 13 illustrates the decay of the tracer gas concentration for configurations III and IV, that is for open doors and closed windows, and for when both windows and doors were closed, respectively. Comparing configurations I and III, it was found that the opening of doors and closing windows reduced the mass flow rate from 3.83 to 0.67 kg/s (i.e. a reduction of 82%). Configuration IV produced the minimum tracer gas concentration decay

rate. It took approximately 69 min for the tracer gas to decay to the background concentration. When both doors and windows were closed, the infiltration and exfiltration airflow was through the unintended gaps, such as gaps between floor and door, roof and wall. With reference to Figs. 12 and 13 and taking a reference initial tracer gas concentration of 0.28%, the time taken for the tracer gas concentration to decay to the background concentration varied depending on the type of the ventilation components in use. Table 2 summarizes the time taken for the carbon dioxide concentration to decay from 0.28% to the background concentration. From Table 2, it can be seen that windows have a higher ventilation effect (shorter decay time) than doors. However, this depends on the wind speed and direction and the orientation of the ventilation component. Somerset East experience W(6017151)N prevailing winds, and when windows are open and doors closed, the west side windows capture the prevailing winds which then escape through the east and south windows, and to a lesser

ARTICLE IN PRESS G. Makaka et al. / Renewable Energy 33 (2008) 1959–1973

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0 -0.2

Ln (C)

-0.4 -0.6

R2 = 0.8321

-0.8 -1 All doors and windows opened

-1.2 -1.4 -1.6 14:34

14:35

14:36

14:38 Time

14:39

14:41

14:42

Fig. 14. Logarithmic graph of concentration: all doors and windows closed.

0 -0.1

R2 = 0.9976

All windows open and doors closed

Ln (C)

-0.2 -0.3 -0.4 -0.5 -0.6 -0.7 09:50

09:51

09:53

09:54

09:56 09:57 Time

09:59

10:00

10:01

10:03

Ln (C)

Fig. 15. Logarithmic graph of concentration: windows open and doors closed.

0 -0.05 -0.1 -0.15 -0.2 -0.25 -0.3 -0.35 -0.4 -0.45 09:53 09:56 09:59

R2 = 0.8903

All doors open and windows closed

10:01 10:04 10:07 10:10 10:13 Time

10:16 10:19 10:22

Fig. 16. Logarithmic graph of concentration: doors open and windows closed.

0 -0.05

R2 = 0.9342

Ln (C)

-0.1 -0.15 -0.2 All doors and windows closed

-0.25 -0.3 11:45

11:52

12:00

12:07 Time

12:14

12:21

Fig. 17. Logarithmic graph of concentration: all doors and windows closed.

12:28

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extent through the north side windows. This gives an effective controllable air inflow and outflow by adjusting the opening area of windows. 7.3.5. Air quality Figs. 14–17 show the graphs of ln(Cn) versus time for different ventilation component configurations. For each period, the initial and final times were determined and a normalized concentration, Cn, was calculated for each measurement time: Cn ¼

CðtÞ  Cð0Þ Cð0Þ  C 0

(7)

where C(0) is the initial concentration at the beginning of the decay period. A base outdoor concentration C0 was determined from the minimum values at the end of long decay periods and it was seen to be about 0.234%. This outdoor concentration was first deducted from the carbon dioxide concentration to get the increase resulting from the instant of injection. The air change rate, which is the slope of the line that represents ln(Cn) versus time was calculated for each graph, and the results are summarized in Table 3. In this table, the confidence intervals were calculated from the dispersion of the concentration measurements around the regression line, using 0.1% probability (99.9% confidence). When all the designed ventilation components were closed the least air exchange rate of 0.2970.03 h1 was observed. The equivalent outdoor airflow rates for each ventilation component configurations were calculated by multiplying the air exchange rate by the house volume. The house has a volume of 34.56 m3 and an envelope area of 61.1 m2. Assuming that the flows are due to the envelope leakage, the specific leakage rate was obtained by dividing the flow rate by the envelope area and results are summarized in Table 4. The ventilation rate greatly affects the indoor thermal environment, as the in-coming air carries with it thermal energy. If the outdoor temperature is higher than the

Table 5 Ventilation heat flow rate for different ventilation components configuration

Table 3 Decay periods for different ventilation components status Ventilation components state

Decay period (min)

All All All All

14 17 18 69

doors and windows open doors closed and windows open windows closed and doors open windows and doors closed

indoor temperature, and as the outdoor airflows indoors, the tendency is to raise the indoor temperature. The rate of ventilation heat flow is approximated by equation: Qv ¼ 1200V_ DT. Taking the mean indoor–outdoor temperature difference DT to be 7 1C and the ventilation rates from Table 3, the ventilation heat flow rates were calculated. Table 4 shows the summary of results for the rate of ventilation heat flows for different ventilation component configurations. Depending on the indoor and outdoor temperature difference, configuration I (doors and windows open) which has the highest rate of ventilation heat flow, can result in excess heat gains or loses. However, adjusting the effective open areas of the ventilation components can regulate the heat gain/loss, thus keeping the indoor environment within the comfort levels Table 5 shows the summary of the rates of ventilation heat flows for different ventilation components configurations. It must be noted that the rate of ventilation heat flow for configuration IV is through the unintended ventilation path ways, i.e. through gaps between doors and floors, etc., since all designed ventilation components were closed. Fig. 18 shows the moving average of the tracer gas concentration, while Fig. 19 shows the variation of wind speed and carbon dioxide concentration. It is worth to mention that on the day of taking measurement it was drizzling with a mean indoor and outdoor temperature difference of about 7 1C. The mean wind speed was about 0.6 m/s and blowing from W601N. The drizzling forced occupants to get indoor early. From Fig. 16, it can be seen that before 5:30 p.m. the carbon dioxide concentration was almost constant and slightly greater than the background concentration by about 0.009%. At 5:30 p.m., the concentration abruptly increased to 0.245% and remained constant for about 1:30 h. It can be observed that as from 5:45 p.m., the carbon dioxide concentration abruptly increased to 0.248% and

I II III IV

Ventilation component configuration

Rate of ventilation heat flow (J/s)

Doors Doors Doors Doors

772 140 68 24

and windows open closed and windows open open and windows closed and windows closed

Table 4 Specific airflow rates calculated from the various CO2 concentration decays

I II III IV

Ventilation component status

Specific air exchange rate (h1)

Equivalent outdoor air flow rate (m3/h)

Specific leakage rate (m3/h m2)

Doors Doors Doors Doors

9.5870.04 1.7470.02 0.8470.04 0.2970.03

331.0871.38 60.1370.69 29.0371.38 10.0271.04

5.42 0.98 0.48 0.16

and windows open closed and windows open open and windows closed and windows closed

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100 0.25

Carbon dioxide concentration

90

70

0.245

60 50 0.24 40

Indoor temperature

Indoor humidity

Outdoor temperature

Outdoor humidity

30 0.235

Temp (°C)./ Humidity (%)

Carbon dioxide concentration (%)

80

20 10

0.23 4:19 PM

0 6:43 PM

9:07 PM

11:31 PM

1:55 AM

4:19 AM

6:43 AM

9:07 AM

Fig. 18. Indoor air quality.

0.251

1.8

0.25 Carbon dioxide concentration (%)

0.249

1.4 Wind speed (m /s)

0.248 1.2

0.247

1

0.246

0.8

0.245 0.244

0.6

0.243 0.4

0.242

0.2 0 3:21 PM

Carbon dioxide concentration (%)

1.6

0.241 5:45 PM

8:09 PM

10:33 PM 12:57 AM

3:21 AM

5:45 AM

0.24 8:09 AM

Fig. 19. Wind speed and carbon dioxide concentration.

then levels out but some fluctuations. This sharp increase might have been attributed to the closing of the remaining open windows and probably with all the occupants getting indoors resulting in more carbon dioxide production. It was also the time when the occupants started to prepare for supper, resulting higher production of carbon dioxide. The fluctuations might be due to frequent opening and closing of doors. Two major drops in carbon dioxide concentration can be noticed, one occurring as from 9:50 to 10:30 p.m. and the other from 01:50 to 4:15 a.m. A prolonged opening of some doors might have caused the drop in carbon dioxide concentration. The ventilation rate depends on the wind speed and direction, and the fluctuations in

wind speed and directions results in the smaller fluctuation amplitudes in carbon dioxide concentration. From Fig. 19, it can be noted that the two drops in carbon dioxide correspond to a sudden drop in wind speed. At 6:00 a.m., the school child started preparing to go to school and the opening of doors result in the drop in the carbon dioxide concentration. The occupants are of old age and they wake up late at around 8:00 a.m. and this is noticed in the sudden drop in the carbon dioxide concentration when they open most of the ventilation components. According to the South African Indoor Air Quality Code, the indoor carbon dioxide concentration should be less than 0.500%. With reference to Fig. 19, the indoor

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carbon dioxide concentration never exceeded the maximum limit of 0.500%. This implies the passive solar house has a well-ventilated design, which does not require any mechanical device for ventilation to maintain the air quality within the recommended range. Using Eq. (1), i.e. Qe ¼ nS=ðC i  C 0 Þ, and taking S ¼ 20 l/h, Ci ¼ 0.248, C0 ¼ 0.234 and n ¼ 5 (number of occupants), the equivalent outdoor airflow rate per person was found to be 7.142 m3/(h person). 8. Feedback from the occupants The occupants have an experience of the thermal comfort of the Reconstruction and Development Programme (RDP) houses and the shacks. They say the passive solar house is much comfortable than the RDP houses and shacks. They claim that they no longer use the heater or fans that they used when staying in the shack to maintain the indoor environment within the comfort levels. While staying in the shacks, they usually use coal to warm the indoor environment in winter. Even visitors who are currently staying in RDP houses express the same sentiments and they wish if the local authority can adopt such a design in constructing RDP houses. For the evaluation of the thermal comfort, the predicted mean vote (PMV) and predicted percentage dissatisfied (PPD) were calculated. With reference to Eqs. (1) and (2), PMV was found to be 0.65, and substituting in Eq. (2), one obtains: 4

PPD ¼ 100  95 eð0:03353PMV þ0:2179PMV ¼ 100  95 eð0:033530:65

4

2

Þ

þ0:21790:652 Þ

 14%. This implies that 14% of the occupants will feel discomfort while 86% will feel thermally comfortable. This confirms the sentiments of the beneficiaries of the passive solar house and of visitors that the PSH offer a more comfortable indoor thermal environment.

the saved money in energy consumption can therefore be used to buy other essentials, such as clothing and food. Mismanagement of building component (inadequate use of doors and windows) greatly affected the thermal performance of the building. The indoor temperature was observed to be influenced by a number of factors including occupants’ behaviour, thermal inertia of the building and climate factors. The indoor temperature followed the outdoor temperature implying that the heating or cooling system had minimal influence on the indoor temperature. However, no heating or cooling system was used. The thermal performance of the building was 14% better in winter than in summer and wind speed and direction was observed to have to a significant impact on the indoor thermal environment. The indoor temperature was observed to follow the solar radiation intensity but with a time delay of 15 min. The ventilation rate was found to depend on the ventilation component in use, windows were found to have a higher ventilation effect than doors. Correct opening and closing of windows can regulate the air infiltration thus controlling the indoor air quality. The carbon dioxide concentration measured over night when most of the ventilation components were closed and all occupants were indoors never exceeded the recommended limit of 0.500%, but only attained a maximum of 0.248%. The house has therefore a good indoor air quality. The monitored results suggest that the building behaviour cannot be restricted to construction issues only, i.e. useful inferences cannot be done from building technology only. Simulation results shows that the thermal efficiency of the house is 36% much higher than the measured thermal efficiency for summer and 22% as compared to winter thermal performance. This implies that the house performed 14% better in winter. The PSH beneficiaries expressed satisfaction on the thermal comfort of the house. PPD was calculated and was found to be 14%, implying that 86% of the occupants will express satisfaction on the thermal condition of the indoor environment. References

9. Conclusions Energy efficient passive solar house can be one of the long and immediate solutions to the energy crisis facing South Africa and many other African countries. National electrical power shedding can be minimized as the building energy consumption is reduced. These houses can result in the reduction of environmental pollution, as the use of fossil fuels would be reduced. The addition of fly ash to clay improves brick properties (a reduction in thermal conductivity and water absorption, an increase in compressive strength). The inclusion of fly ash in brick making by local brick makers can result in job creation, thus empowering and improving the living standards of the poor. The construction of the low-cost houses can result in reduction in energy consumption, and

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ARTICLE IN PRESS G. Makaka et al. / Renewable Energy 33 (2008) 1959–1973 [10] Baker NV, Standeven MA. A behavioural approach to thermal comfort assessment in naturally ventilated buildings. In: Proceedings CIBSE national conference. Eastbourne, Chartered Institute of Building Services Engineers, London; 1996. p 76–84. [11] Humphreys MA. The influence of season and ambient temperature on human clothing behaviour. In: Fanger PO, Valbjorn O, editors. Indoor climate. Copenhagen: Danish Building Research; 1979. [12] de Dear RJ, Brager GS. Thermal comfort in naturally ventilated buildings: revisions to ASHRAE Standard 55. Energy Build 2002;34(6):549–61. [13] Humphreys MA, Nicol JF. Understanding the adaptive approach to thermal comfort. ASHRAE Trans 1998;104(1):991–1004.

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[14] Olesene BW. A better way to predict comfort. ASHRAE J, August 2004, p. 20–26. [15] Markov D. Practical evaluation of the thermal comfort parameters. Bulgaria: Technical University of Sofia; 2006. [16] Roulet CA. Simple and cheap air change rate measurement using CO2 concentration decays. Air Infiltration and Ventilation Centre technical note; 1991. p. 34. [17] Smith PN. Determination of ventilation rates in occupied buildings from metabolic CO2 concentrations and productions rates. Build Environ 1988;23:2. [18] Penman R. Experimental determination of airflow in a naturally ventilated room using metabolic CO2. Build Environ 1992;17:4.