Economic feasibility of passive ambient comfort in Baja California dwellings

Building and Environment 37 (2002) 993 – 1001

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Economic feasibility of passive ambient comfort in Baja California dwellings % Miguel Angel Porta-G%andara ∗ , Eduardo Rubio, Jos%e Luis Fern%andez Centro de Investigaciones Biologicas del Noroeste, S.C. P.O. Box 128, La Paz 23000 BCS, Mexico Received 10 December 2000; received in revised form 4 April 2001; accepted 3 September 2001

Abstract The economic evaluation of some passive thermal comfort techniques is performed in order to compare long-term energy savings. A direct comparison is made of vernacular architecture, based on adobe walls, against modern, concrete brick building of low-income family housing in tropical, dry-climate conditions in La Paz, Baja California Sur, Mexico. The expected energy requirements of each type, for the same comfort level, are calculated by means of a calibrated mathematical model, and present value of each option is obtained by conventional means using 10% interest over 15 years. The results indicate that, in cases as those analyzed, the use of c 2002 Elsevier vernacular passive techniques is more comfortable and economic than present light buildings by a very wide margin.  Science Ltd. All rights reserved. Keywords: Passive energy savings; Vernacular architecture; Adobe walls

1. Introduction Engineering in these modern days must be as harmless as possible to the environment. Engineering systems are often designed and built in accordance with high level speci
energy demand, which in turn means that smaller facilities are needed. Savings in energy can then be directed towards improving the viability of insulation techniques, by means of regional programs. Other authors have proposed roof insulation with the purpose of improving comfort [2], which in turn results in increased productivity and wellbeing, notwithstanding energy savings, which in South Africa alone, have been calculated as high as 3000 GWh in winter heating loads. The use of highly reHecting materials on the roof, another passive cooling technique, may reduce the solar gains. It has been shown that the addition of certain pigmentation to roof paint, such as barium sulfate, can improve the performance of the paint coatings as selective infrared radiators ◦ [3], dropping roof temperatures by more than 3 C. More sophisticated techniques, such as radiative solar collectors, can be used to reduce interior temperatures in buildings [4]. However, the application of radiative cooling in buildings still requires improved radiator design. Several authors have coined the term of ‘solar assisted architecture’ [5], a concept that has been around for many centuries, where human health and comfort are prime design targets. There is growing evidence of integration of building form and material using direct solar power to engineer acceptable internal microclimates. As has been shown, the indiscriminate use of active equipment, such as air

c 2002 Elsevier Science Ltd. All rights reserved. 0360-1323/02/$ - see front matter
PII: S 0 3 6 0 - 1 3 2 3 ( 0 1 ) 0 0 0 8 5 - 3

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M.A. Porta-Gandara et al. / Building and Environment 37 (2002) 993–1001

Nomenclature A Cp F H h k L M Nu q Q Ra T t U

heat transfer area, m2 speci
conditioners, can result in health hazards. Solar chimneys are proposed to provide comfort while reducing indoor contaminants. Nevertheless, some of these techniques demand a better knowledge of local climatic and weather conditions. In this context, the basic concept of comfort is frequently questioned, since some societies tolerate high temperature and humidity better than others do [6]. Hence, the integrated treatment of indoor climate control for comfort [7] has been proposed. The prevailing view of the conventional heating, ventilating and air conditioning (HVAC) technology is unsatisfactory. It operates in a wasteful manner, is energetically entropic, and contributes signi
Greek letters     

absorptance, dimensionless thermal expansion coe=cient, K −1 kinematic viscosity, m2 s−1 emittance, dimensionless Stefan–Bolzman constant, W m−2 K −4

Indices c i r s v

convection internal radiation building materials window pane

external blinds and windows placed behind the plane of the facade, which can reduce direct heat gain up to 27% [11]. Extensive work has been done in the process of reducing solar gains through roofs in hot weather [12], taking advantage of shadowing alone and of creating a chimney e?ect with the buoyancy forces in the con
M.A. Porta-Gandara et al. / Building and Environment 37 (2002) 993–1001

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United States

Mexico

La Paz

Fig. 2. Conventional housing built with hollow concrete blocks.

Fig. 1. La Paz, BCS, Mexico.

comfort. Special mention will be made of the use of locally built adobe bricks, which contribute heavily to comfort in terms of its high thermal capacity. As mentioned before, thermal mass can reduce peak cooling load [17] and indoor air temperature swings. This work aims at contributing to the designer task of optimizing thermal mass in buildings with a long-term commitment to comfort and energy savings. Hence, an economic appraisal is made of the energy savings, and therefore environmental bene
Fig. 3. Vernacular architecture built with adobe bricks.

During the
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Building an adobe brick house begins with digging a trench. In a concrete brick house, this trench would normally be
Table 1 Capital costs of alternative dwellings (US dollars as of 31 July, 2000) Cost element

Concrete block dwelling (alternative a)

Adobe dwelling (alternative b)

Soil preparation Walls Roof Capital cost

30 3500 2500 6030

60 2910 2500 5570

These followed the same estimation procedure they would otherwise use if they were to write a formal proposal. The results of the fundamental issues in
M.A. Porta-Gandara et al. / Building and Environment 37 (2002) 993–1001

Global solar radiation, W/m²

Spring

Summer

1000

1000

800

800

600

600

400

400

200

200

0

0

5

10

15

0

20

0

5

10

Autumn 1000

800

800

600

600

400

400

200

200

0

5

10

15

20

15

20

Winter

1000

0

997

15

0

20

0

5

10

Time after sunrise (h) Fig. 4. Horizontal global solar radiation as function of time after sunrise for the four seasons.

45

4

Spring

2

40

0 0

35

6

12

18

24

4

Summer Summer

25 Autumn 20 Spring 15

Wind speed (m/s)

Temperature (˚C)

30

2 0

0

6

12

18

4

24

Autumn

2 0 0

6

12

18

24

Winter 10

4

0 0

Winter

2

5

0

5

10

15

20

0

6

12

18

24

Time after sunrise (h)

Time after sunrise (h)

Fig. 5. External ambient temperature as function of time after sunrise for the four seasons.

variability natural in these situations. A full year of weather measurements was thus analyzed to detect the most meaningful set of variables to feed the mathematical model. The resulting representative set of boundary conditions was elaborated, as shown in Figs. 4 – 6. These variables are taken as representative for the 45 days before and after the computed four critical dates, and was therefore the basis for the derived economic considerations.

Fig. 6. Wind speed as function of time after sunrise for the four seasons.

The computation yields the basic results shown in Fig. 7a, b. In Fig. 7a, the inside temperature is shown to vary along a typical spring day in the adobe option. For the calculations, the added heat sink of a 400 W air conditioner was included in the heat balance. Di?erent set points selected for comfort result in very di?erent energy consumption. Energy requirement for air conditioning is obtained from ASHRAE [20] standards. It can also be seen that, in the adobe house, ◦ the set point of 28 C does not require any energy expen◦ diture. At set point 27 C, energy consumption is minimal,

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M.A. Porta-Gandara et al. / Building and Environment 37 (2002) 993–1001 Set point 26 ˚C

24˚C, 16.74 MJ/day

24.1

100

24 23.9

90 0

25˚C, 8.76 MJ/day 6

12

18

24

80

Temperature (˚C)

24.9

0

26˚C, 3.96 MJ/day 6

12

18

24

26 25.8 25.6 27 26.8 26.6 26.4 26.2 27 26.8 26.6 26.4 26.2

(a)

0

0

27˚C, 0.42 MJ/day 6

28˚C, 0 MJ/day

12

18

24

6

12

18

24

6

12

18

24

Air conditioner energy usage (MJ/day)

25 24.95

70

60

50

40

Block

30

20 0

10

Time after sunrise (h) 24˚C, 30.24 MJ/day

28

Adobe

0 0

50

100

150 200 Day of the year

26

250

300

350

24 28

0

25˚C, 21.84 MJ/day 6

12

18

24

0

26˚C, 18.72 MJ/day 6

12

18

24

26

Fig. 8. Air conditioner energy usage as function of the day of the year for adobe and block brick dwellings.

Temperature (˚C)

24 28

Spring 60

26 24 28

0

27˚C, 12.66 MJ/day 6

12

18

24

6

12

18

24

6

12

18

24

50

24 0 28

28˚C, 4.8 MJ/day

26 24

(b)

0

Time after sunrise (h)

Fig. 7. (a) Adobe house inside temperature (spring) and energy usage as function of time after sunrise for di?erent set points. (b) Concrete block house inside temperature (spring) and energy usage as function of time after sunrise for di?erent set points.

required only to limit the peak room temperature in the early night. Further reduction in set point temperature result in very marked increase in energy demand. The coolest tem◦ ◦ perature analyzed, that of 24 C, which is 2 C colder than the expected comfort temperature for the corresponding climate, requires more than four times the energy expenditure. The nature of the air conditioner control system plays a very important role in room temperature stability and, ultimately, in energy requirements. For clarity, the same calculations were run for the concrete wall house, as included in Fig. 7b. The air conditioner system employed for this calculation was retained from the preceding exercise, with 400 W power. It can be seen that the air conditioning system is too small for this house, and a ◦ set point of 27 C is needed to achieve the control purpose. Hence, to achieve the control over lower room temperatures, equivalent to the adobe brick house, a larger air conditioner would be required. A comparison between energy consumption for adobe and block brick dwellings, for the full year, can be
Air conditioner energy usage (MJ/day)

26

40 Block

30

20

Adobe

10

0 23.5

24

24.5

25 25.5 26 Set point temperature (˚C)

26.5

27

27.5

Fig. 9. Air conditioner energy usage as function of set point temperature for both dwelling options.

critical date. Fig. 8 shows how daily energy requirements vary for each option. Given the local weather, both choices require no air conditioning during winter. Energy demand is larger in autumn than in spring, and summer peak consumption in the concrete block house requires nearly
M.A. Porta-Gandara et al. / Building and Environment 37 (2002) 993–1001 16 Set point, 24˚ C 14 25

Daily operating time (h)

12 26 10

8

6

4 27 2

0

0

100

200

300 400 Air conditioner nominal load (W)

500

600

700

Fig. 10. Daily operating time as function of air conditioner nominal load for spring (adobe option) for di?erent set points and for various nominal loads.

Finally, a step toward economic appraisal is drawn in Fig. 10, where the chosen date of Spring and the adobe option are retained to explore daily operating time of the air conditioner, in order to guarantee a given set point room temperature. The horizontal axis corresponds to the air conditioner nominal load. It is clearly appreciated that a larger system will always require less operating time. The left and right limits of each curve indicate the minimal and maximal air conditioner nominal capacity that can be applied to the conditions given in the small house. As can be inferred, these nominal capacity limits vary along the year, and presumably, when system operating policies vary, such as further shading, human occupancy, ventilating and so forth. The product of nominal load and operating time yields the energy requirement, and therefore the operating cost. It must not be ignored that there may be an important ef
4. Air conditioning cost calculations The economic evaluation is performed from the previous results. For this calculation, a full appraisal of each building was made. It was found that the thermal capacity of the inside walls (as opposed to outside walls) was minimal, hence its participation in the calculations was not considered. The results are equal to supposing that inside walls in the dwelling are made of concrete block or other lighter building material. The resulting consumption of energy for air conditioning alone, and an appreciation of the di?erence between alternatives, has been shown in Fig. 10.

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The cost of energy can be calculated as follows, based on current costs and prevalent tari?s in the Baja region. The average kW h would cost about 0.1 (all costs are quoted in American dollars as of August 2000). Air conditioning equipment is associated with a SEER factor (the conversion factor between heat extracted from the room and the electrical power needed) of 10. The e?ective cost of energy is then obtained by multiplying the thermal load that must be removed, in MJ, by 3:413×10−2 , in dollars. The yearly cost is thus the product of the heat that needs to be removed daily, calculated as indicated, added over the whole year and multiplied by the number of years the calculation is relevant. In the resent work, the period of 15 years was preferred since this is the payback period for urban housing of the working class, who are the most negatively a?ected by raises in housing and energy costs. The resulting
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M.A. Porta-Gandara et al. / Building and Environment 37 (2002) 993–1001

massive walls, and running costs, represented by large energy bills. The case analyzed is a typical situation where low initial cost, based on the o=cial perception of the needs of the less well o?, result in enormous operating costs. This conventional wisdom is Hawed since it is found that the initial cost of adobe and concrete block walls is very similar. The extra cost of urbanization, resulting from the thicker walls, was not addressed in the present paper, since this cost must not necessarily be translated to the user. This extra cost is very small compared to the extra generating and distribution capacity needed to meet the power peak. Further work must address the economic e?ect of further reducing energy consumption by incorporating more advanced passive techniques, hence increasing embodied energy resources. Acknowledgements The authors appreciate the
thermal balance are: for the window pane, dTv = Qv − As Usv (Tv − Ts ) − Av Uvi (Tv − Ti ) Mv Cpv dt − Av Uva (Tv − Ta ):

(A.1)

Similarly, for the building materials, dTs = Qs − Av Usv (Ts − Tv ) − As Usi (Ti − Ts ) Ms Cps dt − As Usa (Ts − Ta ):

(A.2)

And
(A.4)

In Eq. (A.4), the term with subindex r refers to radiation, and the one with c, to convection. Both are very sensitive to temperature. The radiation term is usually approximated by (see Du=e and Beckman [21]) hrjk = F(Tj2 + Tk2 )(Tj + Tk );

(A.5)

where  stands for the Stefan Boltzman constant,  is the thermal absorptance,  is thermal emittance and F is a geometric shape factor, in such a way that radiative heat transfer can be linearized, i.e., radiation heat transfer per unit area is calculated as: qrjk = hrjk (Tj − Tk ):

(A.6)

No generally acceptable coe=cients are available for this application. However, it is possible to employ adapted expressions for heat transfer by natural convection in closed cavities, such as can be found in Thomas [23], making use of the adequate aspect-ratio relationships. For the convection term,
n H Nu k k m = CRaL hcjk = : (A.7) L L L In this case, the convective heat transfer coe=cient is sensitive to the characteristic distance L, conductivity k of air at the mean temperature inside the envelope, Rayleigh number Ra, aspect ratio H=L; constant C and powers m and n are adjusted to experimental results. With the aid of experimental data, the following values apply to the examples below: for Uvi and Usi , hr = 0, and hc values are calculated with Eq. (7) using C = 0:2; m = 0:28; n = 0; H = 2:4; and L = 12 for Uvi and L = 5 for Usi .

M.A. Porta-Gandara et al. / Building and Environment 37 (2002) 993–1001

Finally, the required temperature-dependent air transport properties were evaluated at mean air temperature in each time interval by the following expressions, which are valid ◦ ◦ between 2 C and 77 C. Temperature in the equations must be expressed in K. For thermal di?usivity,  = 1:534 × 10−3 T − 0:2386 (m2 s−1 × 10−4 ). For kinematic viscosity,  = 0:1016T − 14:8 (m2 s−1 × 10−6 ). For thermal conductivity, k = 7:58 × 10−5 T + 3:5 × −3 10 (W m−1 K −1 ). And for the thermal expansion coe=cient,  = T −1 (K −1 ). References [1] Mathews EH, Kleingeld M, Taylor PB. Estimating the electricity savings e?ect of ceiling insulation. Building and Environment 1999;34:505–14. [2] Taylor PB, Mathews EH, Kleingeld M, Taljaard GW. The e?ect of ceiling insulation on indoor comfort. Buildings and Environment 2000;35:339–46. [3] Orel B, Klanjsek M, Krainer A. Radiative cooling e=ciency of white pigmented paints. Solar Energy 1993;50(6):477–82. [4] Erell E, Etzion Y. Radiative cooling of buildings with Hat-plate solar collectors. Building and Environment 2000;35:297–305. [5] Kumar S, Sinha S, Kumar N. Experimental investigation of solar chimney assisted bioclimatic architecture. Energy Conversion and Management 1997;39(5=6):441–4. [6] Mallick F. Thermal comfort and building design in tropical climates. Energy and Buildings 1996;23:161–7. [7] Mahdavi A, Kumar S. Implications of indoor climate control for comfort, energy and environment. Energy and Buildings 1996;24:167–77. [8] Yang KH, Su CH. An approach to building energy savings using the PMV index. Building and Environment 1997;32(1):25–30. [9] Holtz R, Hourigan A, Sloop R, Monkman P, Krarti M. E?ects of standard energy conserving measures of thermal comfort. Building and Environment 1997;34(1):31–43.

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[10] Sekhar SC. Higher space temperatures and better thermal comfort — a tropical analysis. Energy and Buildings 1995;23:63–70. [11] Salazar JH. Solar performance and shadow behaviour in buildings. Case study with computer modelling of building in Loranca, Spain. Building and Environment 1998;33(2–3):117–30. [12] Morillon D, Samano DA, Avila FE, Fernandez JL. Applied Energy 1997;57(1):1–11. [13] Serres L, Trombe A, Conilh JH. Study of coupled energy savings systems sensistivity factor analysis. Building and Environment 1997;32(2):137–48. [14] Labaki LC, Kowaltowski D. Bioclimatic and vernacular design in urban settlements of Brazil. Building and Environment 1998;33(1):63–77. [15] Kying-Hoi L, Dong-Wook H, Ho-Jin L. Passive design principles and techniques for folk houses in Cheju Island and Ullung Island of Korea. Energy and Buildings 1996;23:207–16. [16] Malama A, Sharples S. Thermal performance of traditional and contemporary housing in the cool season of Zambia. Building and Environment 1997;32(1):69–78. [17] Balaras CA. The role of thermal mass on the cooling load of buildings. An overview of computational methods. Energy and Buildings 1996;24:1–10. [18] New Mexico Adobe, Rammed earth building code, Part V engineering regulations — Quality and Design of Materials of Construction. General Construction Bureau, Santa Fe, New Mexico 87504. [19] Porta MA, Rubio E, Fern%andez JL, G%omez V. Heat transfer in dwelling buildings. Proceedings of the ISES Millenium Solar Forum 2000, 17–24 September 2000. Mexico City, Mexico. [20] ANSI=ASHRAE Standard 55-1992, “Thermal environmental conditions for human occupancy”. American Society of Heating, Refrigerating and Air Conditioning Engineers. [21] Du=e JA, Beckman WA. Solar engineering of thermal processes. New York: John Wiley & Sons, 1991. pp. 824 –7. [22] Watmu? JH, Charters WWS, Proctor D. Solar and wind induced external coe=cients for solar collectors, Internationale Heliotechnique, Vol. 2, 1977. p. 56. [23] Thomas LC. Heat transfer. Englewood Cli?s, NJ: Prentice Hall, 1992. p. 571.